Monitoring and Control

Transcription

25
MONITORING AND CONTROL
TOM RITZDORF
To obtaining quality deposits it is important to monitor and
control electrochemical deposition (ECD) processes. This is
also essential for an automatic plating system designed to
minimize operator attention to production operations. A
skilled and knowledgeable operator can maintain a reasonable plating quality with little supporting equipment. However, trends to more automation to reduce costs with higher
processing speeds, application of plating to higher value
products, and assurance of quality plating make it necessary
to utilize many monitoring and control facilities.
Electroplating process results are dependent on process
stability and control and on the chemical composition of the
plating bath. This makes it important to monitor the concentration of the various species in the bath. At a minimum, it
is usually important to monitor the concentration of the metal
ions and the supporting electrolyte. It may also be necessary
to monitor constituents that interact in trace amounts or
organic additives that modify the behavior of the process.
Electroless plating, as described elsewhere, is an autocatalytic process initiated by a catalyst such as palladium and
then catalyzed by the deposited metal itself [1]. Electrochemically, the system is quasi-stable, as it must be to
work. This necessitates a closer control of bath chemistry
than that required for electroplating solutions. Frequent
electroless bath analysis and maintenance are necessary,
especially when fast plating is involved, due to the speed
with which the chemistry changes. Fast electroless metal
deposition or poor bath control can result in spontaneous
bath decomposition, causing metal particles to form and strip
the solution of all metallic ions. Manual bath sampling and
analysis are often not practical. Machines have been developed which automatically take samples and analyze and
reconstitute the bath chemistry [2]. These will be discussed
in Section 25.2.11.
This chapter has three sections: process monitoring, bath
constituent concentration monitoring and replenishment,
and product monitoring. The first section focuses on sensors
and techniques for monitoring the plating and associated
processes and how to use these to increase the automation
level of plating equipment. Because the concentration
monitoring and control of chemistries used in the plating
processes are so important and may be handled by equipment separate from the electroplating equipment in some
cases, these topics are given their own section. Finally, a
short section on product monitoring and quality control is
included, although the reader should understand that product monitoring should be dictated more by the product
requirements than by the process used to produce it. This
section is meant merely as a reference to some of the
common monitoring methods in use and how they are
applied to control plating processes.
25.1
PROCESS MONITORING
Plating processes, whether electrolytic or electroless, are
utilized for many varied purposes. These may be for the
manufacture of simple and cheap parts or for the precise
manufacturing of extremely high-tech or one-of-a-kind components. In any case, decisions regarding how much effort to
put in to controlling the process parameters and their impact
on the process must be made based on economics, product
volume, and expected results.
An electroplating line consists of a complex set of
equipment, including motors, tanks, water rinses, product
transport equipment, filters, heaters, electrical power
supplies, process monitoring devices, process control
devices, and the like. Electroplating process conditions
Modern Electroplating, Fifth Edition Edited by Mordechay Schlesinger and Milan Paunovic
Copyright Ó 2010 John Wiley & Sons, Inc.
527
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to be monitored and controlled in a typical line may
include:
Solution level
Solution temperature
Solution flow and agitation
Solution chemistry
. Metal concentration(s)
. pH
. Specific gravity
. Additive concentration(s)
. Impurity levels
Current at each electrochemical step
Charge passed
Cell voltages
Line speed
Blow-off air pressure
Exhaust vacuum
Methods of monitoring these parameters will be discussed in
the following sections.
The designer of modern electroplating equipment has an
ever-growing array of devices for monitoring and controlling
almost every aspect of the process available to him or her.
Plating machines can run automatically at high speeds, make
adjustments, and signal an operator if help is needed. The
capital investment and maintenance cost must be justified
by an increased rate of production while maintaining or
improving product quality. With the level of control and
automation available today, completely automated equipment can be designed to handle almost any ECD task.
Semiconductor and Microelectronic Processing Equipment Equipment that is designed for semiconductor processing or for making devices such as MEMS (microelectromechanical systems) and heads for magnetic recording is
usually subjected to more demanding requirements than
other plating equipment. This is due to the stringent requirements for contamination control in the thin-film manufacturing technologies used to make these devices as well as the
high value of the products being produced.
Semiconductor equipment is highly automated, capable
of processing expensive parts with little operator intervention, and is subject to industry-specific manufacturing specifications [3–7]. These specifications are the result of many
years’ worth of learning regarding what materials are compatible with semiconductor manufacturing and what may
lead to particle or chemical contamination, requirements
from insurance companies, and what is required to interface
the equipment to automated data and product handling in an
automated semiconductor fabrication facility. Communication between the automated processing equipment control
computer and the fab computer network is usually handled
automatically and provides for the transfer and storage of
large amounts of data regarding the process flow and processing of the wafers used to manufacture microelectronic
devices. These data-handling requirements necessitate the
incorporation of relatively powerful computers and the ability
to work with host fab automation and control networks.
In addition, these systems may include high-energy particulate air (HEPA) or ultralow particulate air (ULPA) filters
and air ionizers to condition the environment around the
wafer-handling area. Some systems may also include
vision systems for verification of proper equipment operation [8, 9]. This extensive level of control requires many
components. Most of the components can be implemented
as simple controls or as part of a complex and automated
plating system.
25.1.1
Solution Level
The liquid level or volume in the tanks used for plating is
important to control for several reasons. Mechanically, enough solution must be kept in the tank to be able to operate
pumps, cover any components or sensors that are meant to be
immersed, and perform the plating operations themselves.
Maintaining a relatively constant solution level is also important as it relates to controlling the concentrations of the
constituents of the solution. In addition, it is important to
be able to sense low level or absence of liquid in cases where
electrical immersion heaters are used in order to prevent the
possibility of fires. Liquid-level sensors are usually used to
maintain solution level with replenishment of water.
The solution level in a tank can be monitored simply with a
float containing a magnet that operates reed switches: one for
each level in the tank to be detected. Other types of sensors
that may be used to detect liquid levels include capacitive,
pressure sensing, optical, and ultrasonic devices. Load cells
may also be used in some cases to determine solution volume.
Some of these may be used on the exterior of the plating tank
(depending on materials of construction), making them less
prone to attack by the typically harsh chemicals used in a
plating process. There are also several types of level sensors
which may be designed to provide an analog output representing the distance to the plating solution, as opposed to a simple
detector that indicates that solution is present at a certain
location. Plating tanks that have a high concentration of
electrolyte can form salt deposits on a float causing it to become immobile. This may be avoided by directing the replenishment water at the float area. Crystallization interfering
with sensor operation, reliability in harsh chemical environments, and signal-to-noise ratios under practical operating
conditions are all factors that should be considered when
choosing solution-level sensors. It is also important to consider
the failure mode of normally open and normally closed sensors
and the impact of a failure on the equipment and process.
PROCESS MONITORING
One common exception to using automatic solution-level
control is for precious metal plating tanks. Platers can be
concerned about the reliability of an automatic solution-level
system for a precious metal plating tank for fear of overflow
and losing gold or platinum down the drain. Instead of
automatic addition of water to gold and other precious metal
plating tanks, an audio alarm can sound so the operator can
adjust the level manually.
25.1.2
Solution Temperature
Solution temperature can be monitored with a thermocouple,
thermistor, RTD (resistance temperature detector), or silicon
integrated circuit device. Many sensors for plating parameters are temperature sensitive and the output signal must
be compensated with an accurate temperature measurement.
This is often done with an on-board silicon integrated device
that senses temperature and corrects the parameter
measurement [10].
25.1.3
Solution Flow
Rapid solution flow around the cathode is a common method
of providing agitation to enable higher plating speeds. Large
tanks for barrel or rack plating improve agitation at the
cathode by moving the cathode or sparging solution or air
from the bottom of the tank upward. In a strip plater that uses
a line of small plating cells and reservoir tanks underneath,
solution is pumped into the cells at rates sufficient to replenish the metal ion content but also, perhaps more importantly,
to produce rapid agitation around the strip to enable highspeed plating. Semiconductor or microelectronic plating
systems typically use some combination of solution flow,
wafer rotation, and mechanical agitation systems to provide
fluid agitation at the cathode surface.
For rate of flow, a rotameter, paddle wheel magnetic
device, pressure differential, ultrasonic, or electromagnetic
sensor can be used. It is relatively simple to buy or make a
rotameter that provides output(s) indicating the actual solution flow by incorporating magnetic, optical, or capacitive
sensors. There are also numerous manufacturers who produce flow sensors using some of the other technologies
mentioned that are useful for plating applications. In any
case, it is useful to have some understanding of the technology utilized and how the results may be impacted by viscosity, specific gravity, solution conductivity, or the presence
of bubbles entrained in the solution. It is also important to
note how the various flow sensors are calibrated and that the
calibration may need to be repeated if the solution properties
are changed.
25.1.4
Plating Current and Cell Voltage
The plating current is typically controlled during an industrial plating process because it has a direct relationship with
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the deposition rate and because it is not as sensitive as the cell
voltage to changes in the physical system. The current at each
electrochemical step can be displayed on an ammeter. It is
also easily monitored by measuring the voltage drop across
a resistor in series with the plating cell. That voltage can be
used in an electronically controlled power supply to maintain
the current at a constant value. An open circuit or power
supply failure in the system can be detected by a programmable logic controller (PLC) or computer to stop a plating
line and sound an alarm. Many industrial plating power
supplies, or rectifiers, include automatic monitoring and
control of output currents and/or voltages, along with the
ability to set tolerance limits that will trigger alarms if
exceeded. These alarms can be used to stop processing or
to prevent additional product from being started, depending
on the severity of the condition encountered.
Monitoring of electroplating cell voltages is necessary to
detect cell variations such as electrical shorts or opens
between the anode and cathode, contamination or polarization of the anode, or changes in solution concentrations.
Shorting can occur in electropolishing cells of strip platers
with close anode–cathode spacing when the strip is a copper
alloy. Copper powder is produced; it collects at the cathode
and eventually forms a conducting path between the anode
and cathode. Methods of automatically removing the copper
powder are discussed in Chapter 24. Solution changes that
affect the conductivity will also impact the cell voltage, as
will any extra resistance caused by corrosion of electrical
components or passivation of the anode or cathode surface.
25.1.5
Timers, Ammeters, and Coulometers
Timers with ammeters were the first means of monitoring the
amount of metal electroplated. In many shops they are the
only instruments needed to produce an acceptable product.
The first automatic control device was the ampere-hour
(or ampere-minute) meter that stops plating when a preset
number of coulombs has passed through the cell. Simple
automatic plating machines rely on timers to advance product
through processing steps. Coulometer control of plating steps
is commonly included in power supplies and rectifiers designed for electroplating.
25.1.6
Sensors
A typical plating tank will include several different kinds of
sensors (Fig. 25.1). It is important to choose sensors carefully
with consideration given to the purpose of the sensor and its
interaction with other components of the system. The impact
of the chemistry (or chemistries) used on the operation of
each sensor may also be an important consideration, and
contact or noncontact versions of several sensing systems
may be available. It is important to keep in mind that any time
a device with electrical connections is added to a tank
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FIGURE 25.1 Typical sensor assembly on a plating tank cover.
Left to right: Fluid level, temperature, pH, and conductivity.
containing plating chemistry there is potential for electrical
interactions that may impact the process results, at a minimum upon failure of the sensor.
Advanced sensors coupled with modern control technology revolutionized automatic manufacturing [11–13].
Sensors play a vital role in providing information about
plating machines and processes that is necessary for automatic monitoring and control [14]. There are three basic
sensor types: on–off, analog, and digital. The on–off type can
be considered a form of digital sensor without quantitative
information. For many situations, an on–off sensor is sufficient. For example, when a take-up reel of strip becomes full
or a payoff reel becomes empty, a simple on–off signal is
adequate. Analog sensors provide quantitative information
that can be read on a meter, but for automatic control it is
usually necessary to convert the signal from analog to digital
for electronic processing. Many sensors produce a digital
signal directly. One example is a photoelectric sensor that
generates pulses at a rate proportional to a plating line speed.
Electronic processing then converts pulse rate to line speed
which can be read on a display or fed into an electronic
motor-speed controller.
An integrated system of sensors tied to an electronic
controller is required to free an operator from machine
watching and fine tuning critical process variables. Sensor
technology has developed rapidly in recent years, and we
now have a vast array of commercially available sensors,
many of which are useful for plating applications. In fact the
number of sensors available for some applications is so large
that one must study the features of each sensor in order to
make the best choice. For example, a survey showed there
were 144 manufacturers of level sensors making over 20
different types [15].
There are several things to consider when choosing a
sensor to monitor plating variables. The important items are
accuracy, reliability, sensor life, cost, signal conditioning,
and maintenance. The accuracy required depends on the
process or machine function. Solution temperatures may
need to be controlled to 10 C or to 0.2 C depending on
the process. Cumulative variables such as plating current,
plating efficiency, and line speed need to be controlled so the
overall accuracy is within acceptable limits. For example, if
each of these three parameters is allowed to vary as much as
1%, then the plating thickness can vary as much as 3%.
Plating machine sensors usually must operate in a hostile
environment with corrosion, fumes, and electrical noise
present. For high reliability, the sensor design and choice
of construction materials must be considered carefully. The
sensor should hold its calibration as long as possible to
minimize recalibration. The only assurance that a sensor is
working properly is to either check it frequently or build in
a self-checking system utilizing a sensor-within-sensor together with special electronics. These are called smart
sensors, although sometimes there are other features included
such as having the sensor directly provide a control signal to
the line. Smart sensors often can be reprogrammed by
an external computer. A schematic of a smart sensor and
how it may interact in an automatic plating system is shown in
Figure 25.2
Sensor life depends not only on how well it is designed and
made but also on how it is used and maintained. A glass pH
electrode, for example, once put into use must be kept wet for
reliable operation. If removed from plating solution, it should
be stored in water or buffer solution until returning to the
bath. A float-level sensor in a plating solution can become
inoperative if plating salts crystallize out on the float, causing
it to stick. The problem may be avoided if water added to
maintain the level is put in at the float. This removes the
tendency for salts to form at the float. This is just one example
of how the environment in a plating system may impact the
operation of sensors that are meant to control its operation.
How much one should pay for a sensor depends on the
value of the variable being sensed and the product being
processed. Temperature sensors are relatively cheap and are
used wherever necessary without much thought of expense.
On-line plating thickness monitors are expensive, but they
can save many times their cost annually by avoiding excessive gold plating or production of scrap due to underplating.
Sensor signal conditioning involves converting the electrical signal into a form that can be displayed in engineering
units such as grams per liter of metal in solution. A raw signal
may be conditioned using electronic hardware or computer
software or a combination of the two. If only a few sensor
systems are involved, hardware instead of software is usually
the most cost-effective choice. Software can be developed
so that a computer can convert raw sensor signals to a form
useful for process control, but this tends to be more costly.
PROCESS MONITORING
531
SMART SENSOR
CHECKING SENSORS
SENSOR
MICROPROCESSOR
CONTROL DEVICE
PROCESS
COMPUTER
FIGURE 25.2
25.1.7
Smart sensor concept for monitoring with local control.
Line Speed
Line speed determines the dwell time of the product in each
processing step in a strip plating application. In plating cells
it can determine the plating thickness. Big machines that
electrogalvanize steel strip monitor zinc thickness with an
X-ray gauge that provides a feedback signal to a computer for
line speed adjustment to maintain a specified zinc thickness [16]. Punched parts that interrupt light periodically are
used to control strip speed with a photo-optical system that
produces an electrical pulse as each segment moves by. An
electronic pulse counter is calibrated to display line speed
and provide feedback to control line speed.
25.1.8
Rinse Quality
In some cases, it makes sense to measure the quality of the
rinse processes after chemical steps. This can be done by
measuring the resistivity of the water that is used to rinse the
parts. As chemicals are removed and the rinse water becomes
less contaminated, the resistivity of the rinse water will
increase. This can be especially useful when deionized water
is used for the rinse operation. It is important to pay close
attention to the system design, though, to ensure that contaminated rinse water is not held in the system where it will
produce a high reading even after the parts are thoroughly
rinsed. This can be a little tricky, since the resistivity probes
normally used must be kept immersed at all times.
25.1.9
Solution Blow-Off
Solution drag-out as parts leave a processing cell can be
minimized by blowing air at the part in the right direction as
parts leave the tank. Large mill plating machines for electrotinning steel strip can drag out 30 gallons of solution per hour
unless special measures are taken to remove as much solution
as possible [16]. Standard shop floor compressors should not
be used for blow-off air, since the air usually contains oil.
A good roof air blower is preferred; its pressure should be
monitored to ensure that adequate solution blow-off occurs.
25.1.10
Fume Exhaust
A reliable fume exhaust vacuum system is important to the
health of plating shop workers and to provide a warning of
faults that will lead to equipment and part corrosion. A sensor
placed in the exhaust plenum detects the level of exhaust
which is necessary to remove chemical fumes. One type of
vacuum sensor placed in the exhaust plenum works by
monitoring the cooling of a heated leg of a Wheatstone
bridge. If the exhaust air stops or decreases to an unacceptable level, the sensor can trigger an alarm.
25.1.11 Programmable Logic Controllers
and Computers
Sensors are the “eyes and ears” of a modern automatic
monitoring and control system but PLCs and computers are
the “brains” that assimilate sensor information with programmed instructions that initiate the control function.
The first automatic plating machines used timers to advance product through a line. Next a control system based
on punched tape and photocells was developed that operated
relays in the proper sequence to control a plating line.
General Motors pioneered a wired relay logic system for
machine control. Modification of that system by rewiring
proved too time consuming and costly. Electronics came to
the rescue when the PLC was developed in 1969. Since then
PLCs have become more sophisticated with computerlike
features to monitor and control many machine operations
including electroplating.
While PLCs are still superior for direct machine monitoring and control, computers are better at processing data
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and communicating with other plant operations. Specialty
computers have been developed for direct interface with
machines for monitoring and control. Fuzzy logic control
is a new technique that mimics human reasoning [17].
25.1.12
Robots and Handling Systems
Robots are used in many automated plating systems. These
handling and automation systems require their own monitoring and control devices. These devices may include such
components as position sensors and encoders and vision
systems. Automation and robotics are involved subjects
which are not covered in detail here. The interested reader
may find any number of references to robotic systems. When
automating electrochemical processing systems with robotics, it is extremely important to remember that the environment will be corrosive due to the chemicals involved.
25.2 BATH CONSTITUENT CONCENTRATION
MONITORING AND REPLENISHMENT
The composition of electroplating solutions has changed
very little in recent years with the exception of high-speed
plating where higher metal content and special additives are
used and new solutions designed to be more compatible with
health and environmental requirements. Analytical techniques have been continuously developed and improved to
enable more precise or more automated control, however.
Plating baths can be analyzed by evaluating their deposit
properties under a specific set of conditions, by analyzing the
concentrations of specific chemical species, or by analyzing
the effect of a chemical species or group of species on the
performance of the plating bath. Each of these methods has
its own advantages and drawbacks, and oftentimes combinations of these analytical approaches are utilized as specific
situations dictate.
Analysis of deposit properties may take place using a
special plating apparatus or by plating a special part that is
used for material property analysis, which may be destructive. This is considered a “qualification” test, and some of
the testing techniques considered useful are described in the
third section of this chapter. Special plating cells have been
developed that can be used for plating bath analysis.
Sometimes these require the qualitative judgment of
an experienced technician in order to understand the relationship of the deposit produced to the chemistry in the
plating bath.
Wet chemical methods of analysis have been developed to
determine the composition of most plating solutions [18, 19].
Instrumentation for rapid chemical analysis of inorganic and
organic species has become highly automated, providing
accurate and rapid analysis. The advantage of on-line monitoring and control of plating solutions is that deviations from
optimum concentration ranges can be corrected as soon as
set limits are exceeded. Of course, this level of automatic
monitoring comes at a cost.
Plating solution chemistry is usually monitored off-line
by taking a solution sample and performing an analysis at the
machine or in a chemical analysis laboratory. Solution
adjustments are then made as necessary. Metal ion concentration, supporting electrolytes, additives, and pH are most
often monitored. Metal ion concentrations are measured
by colorimetry, polarography, or ion-selective electrodes.
Automated versions of these methods have been developed
for on-line analysis of many metals including nickel, copper,
lead–tin, tin–silver, and gold [20–22]. On-line methods may
involve extracting a sample from the bath and performing
analysis or incorporating a sensor in the plating bath.
There is a fundamental question, especially when measuring bath additives, about the preference to measure the
specific concentration of each chemical species in the bath or
whether to monitor the effect of the chemicals that influence
the performance characteristics of the plating process. While
most scientists would prefer to know the exact concentration
of each chemical in the system, it is often extremely difficult
to quantitate the concentrations of similar organic compounds that might have a range of molecular weights, for
instance. This problem becomes aggravated as the bath is
used and some of the additives become oxidized and/or
reduced or simply react to produce other compounds. Once
you take into account complexation with metal ions, equilibrium balances, and the buildup of contaminants as parts are
processed, it is sometimes much easier and more advantageous to consider the effect of these chemicals on the
electrochemical activity, for instance [23]. Most electrochemical analysis methods simply measure the impact of
the additives on the bath activity as being representative of
the additive concentration. The impact of these decisions
becomes especially important as the bath chemistry ages and
impurities or additive breakdown products build up in the
bath. Solution impurity buildup sometimes can be ignored,
such as when drag-out removes enough solution to keep the
impurity level low or when the impurity has little or no effect
on the deposit or bath analysis.
25.2.1
Plating Test Cells
Test cells have been used for evaluating plating chemistry
for many years [24]. These cells can be used in a development environment to understand effects of varying operating conditions on deposit properties or they can be used in a
production environment to understand how a plating bath is
performing compared to expected results. Also, they can
provide a convenient means to separate the performance of
the plating chemistry from effects produced by the equipment that is used for the process. A sample of the plating
bath can be removed, tested in the test cell, then discarded or
BATH CONSTITUENT CONCENTRATION MONITORING AND REPLENISHMENT
TABLE 25.1
Electrochemical Test Cells
Hull cell
Rotating cylindrical
Hull cell
Haring–Blum cell
Stress cells
Rotating disk
electrode (RDE)
Rotating ring disk
electrode
(RRDE)
Electrochemical
quartz crystal
microbalance
(EQCM)
Additive concentration effects, varying CD
and agitation
Additive concentration effects, varying CD
and agitation
Throwing power
Deposit stress as function of varying CD and
thickness
Polarization, limiting current density, etc.
Impact of electrochemical reactions on bath
chemistry
Deposit efficiency, absorption vs. applied
potential
added back to the plating tank. Table 25.1 lists some
common types of electrochemical test cells and their
typical uses.
Hull Cell The Hull cell is a plating test cell that was
designed to allow a quick analysis of the “health” of the
plating chemistry and to allow the effect of additions to the
bath to be easily scaled up to an industrial system [25–27]. An
example is shown in Figure 25.3. The Hull cell is a trapezoidal plating cell that utilizes a cathode placed diagonal to
the anode to cause a variation of current density across the
cathode surface. The Hull cell is usually used with a magnetic
stir bar on the bottom, which results in stronger agitation
at the bottom of the cathode panel than at the top. This allows
a panel to be plated that shows qualitatively the effects of
current density and agitation variations on the deposit properties (usually deposit morphology or brightness). Small
additions of additives can be made to the chemistry in the
Hull cell, which usually contains 267 mL of plating bath.
Once the desired performance is produced on the Hull cell
panel, the additions to the plating tank can be quickly
calculated since each gram added to the 267-mL Hull cell
is equivalent to 0.5 oz/gal. There are many variants of Hull
cells with different agitation mechanisms or temperature
control capability, such as air-agitated or hanging Hull cells
and Gornall cells, but they all work in generally the same
way [25–27].
Rotating Cylindrical Hull Cell The rotating cylindrical
Hull cell is a modification to the Hull cell concept that is
especially useful for development of new chemistry and
processes, as it allows for precisely controlled uniform
agitation with the current density variation typical of a
standard Hull cell. This cell essentially makes use of a
rotating-disk electrode rotator system to control the rotation
of a shaft that incorporates a cylindrical cathode. This
arrangement provides uniform and controllable mass transfer
across the cathode surface. The placement of the rotator with
respect to the anode provides for variations in current density
across the surface of the cathode [28, 29].
Haring–Blum Cell The Haring–Blum cell is a test cell that
is used to measure the throwing power of a plating process.
It is a 1000-mL cell that is rectangular and contains two
cathodes and one anode. The cathodes are placed at different
distances from the anode. Usually, one cathode is five times
farther from the anode than the other. The ratio of the
thickness deposited on each cathode can be used to calculate
the throwing power of the system [30–32]. See Figure 25.4.
Stress Cells There are several types of stress analysis cells
available. Usually, they utilize a beam-bending technique
to calculate the stress of the film deposited on a cantilever
cathode. This can be done by using two beams and measuring
how far apart from each other they bend or by using a sort of
modified Hull cell test with tabs that bend in one direction as
metal is deposited on one side [33, 34].
Cathode
Anode
Bus bar
FIGURE 25.3
Hull cell.
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FIGURE 25.4
Haring throwing-power box.
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Recently, several researchers have begun using in situ
stress measurement during electroplating experiments by
incorporating a laser deflection measurement system as part
of an electroplating cell [35–37]. This is a powerful technique
because it has the capability to measure the stress in situ as
the film is being deposited. This technique provides information related to the incremental stress added at each stage
of the deposition, and it can also measure changes associated
with holding the sample at open-circuit potential. With the
right apparatus, the impact of monolayer adsorption on the
cathode surface can be seen [36].
Electrochemical Quartz Crystal Microbalance There are
specialized electrochemical measurement systems called
electrochemical quartz crystal microbalances, or EQCMs,
that utilize a quartz crystal as a working electrode [38–40].
The quartz crystal is caused to vibrate due to the piezoelectric
effect and its oscillation frequency is monitored, along with
the plating current and potential as a function of time. The
oscillation frequency is a function of the mass of the crystal
and the deposits on it, and the instruments are typically
sensitive enough to measure submonolayer adsorption of
organic materials on the electrode surface or micrometerthick deposits of metals. The Sauerbrey equation can be used
to calculate the mass of a deposit on the crystal, which can be
converted to thickness if the density of the deposit is known:
2
2ff
Df ¼ mF
Zq
sample. Fiber optics has made small colorimeters practical
for automatic analysis of many plating solution species [45].
A light source and absorbance detector can be placed remotely from the analysis cell via a bundle of flexible glass or
plastic pipe fibers. The optimum wavelength of light to be
used for a given species is best determined from a spectrographic scan over a wide range of wavelengths, Awavelength
at or near a strong absorption peak is selected and a calibration curve based on Beer’s law is prepared. The unknown
can then be measured and its absorbance compared to the
calibration curve to determine the concentration of the
constituent of interest. UV/visible photometric analysis
works well for the transition metals. Some examples and
the wavelengths that are useful for analysis are shown in
Table 25.2. For example, Cu(II) in electroless plating baths
is determined at 620 nm wavelength. Cobalt(II) in hard gold
baths may be analyzed by first oxidizing it with hypochlorite
to Co(III). Cobalt(III) forms a pink complex with ethylenediaminetetraacetic acid (EDTA) which can be analyzed at
520 nm wavelength [46]:
A¼elc
where A ¼ absorbance
e ¼ molar absorptivity
l ¼ path length or cell length
c ¼ concentration
25.2.3
where Df ¼ frequency shift of the loaded crystal
ff ¼ fundamental frequency
Zq ¼ acoustic impedance of the material (8.8 106
kg m2 s1 for AT-cut quartz)
mF ¼ mass per unit area
The EQCM is a powerful tool for understanding the reactions that occur on a working electrode during a potential
sweep or for understanding the cathode efficiency of a
deposition. These instruments are typically used in process
development or troubleshooting of electrochemical
processes [41, 42].
Other Cells There are several other kinds of electrochemical cells that can provide useful information in the proper
circumstances. These include cells such as jiggle cells and
bent-cathode test cells [43, 44].
25.2.2
Colorimetry or Photometry
Photometric analysis techniques are fast, simple, and relatively cheap. Additionally, these techniques can be easily
automated, potentially without involving extraction of a bath
Beer’s law
Electrochemical Analysis
Additives in plating baths are important to achieving the
desired physical properties of many electrodeposits. Additive
concentrations must be maintained within a specific range for
best results. Techniques used for monitoring additive concentrations include cyclic voltametric stripping (CVS) [47–53],
polarography [54], and other electrochemical methods. The
method used must be able to detect the active species in the
presence of degraded nonactive material and/or impurities.
Polarography Polarography using a dropping mercury
electrode (DME) has made considerable progress over the
past 50 years as a laboratory technique for chemical analysis
of plating solutions (Table 25.3), but it was not used in the
manufacturing environment until about 30 years ago, mainly
because of the sensitivity of the electrode to manual handling.
When manual handling is eliminated by automation, the
DME can be used as a reliable and reproducible sensor in
automatic analyzers and controllers in industrial applications [20, 21].
Cyclic Voltammetric Stripping CVS is used to determine
the concentration of organic additives such as suppressors,
TABLE 25.2
Absorbed and Perceived Colors
Absorbed Wavelength (nm)
Absorbed Color
Perceived (Transmitted) Color
Metal Complexes
310
314
400
437
450
480
500
512
530
548
570
600
650–730
Ultra-violet
Violet
Violet
Blue
Indigo
Blue
Blue-green
Yellow/green
Green
Yellow
Yellow-green
Orange
Red
Pale yellow
Yellow
Green-yellow
Orange
Yellow
Orange
Red
Violet/red
Purple
Violet
Dark blue
Blue
Green
[Co(CN)6]3
Pd-NH3-complex
brighteners, and levelers from the effect the additive exerts
on the electrodeposition rate at a given potential. The
potential of a rotating platinum electrode is cycled at a
constant sweep rate in a bath sample so that a small
amount of metal is deposited on the electrode and then
stripped off by anodic dissolution (Fig. 25.5). The charge
required to strip the metal is related to the additive
concentration using predetermined calibration curves, as
in Figure 25.6. By using this electrochemical approach
coupled with titrations of certain bath components or other
suppressors and accelerators, a reasonable correlation can
be made to the additives in the electroplating bath. Com-
TABLE 25.3
535
Co-NH3-complex
[Co(NH3)5H2O]3+ [Ti(H2O)6]2+
CoCl2
KMnO4, Cu(ED)22+
Cu-(ED)2-complex
Cu2+
Ni2+
puterized instruments have been developed that automatically determine additive concentrations [23]. CVS has
become a very common method due to its relative simplicity and the availability of automated equipment to
perform this analysis. One aspect of CVS analysis that is
both an advantage and a drawback is that this technique
provides a single numeric value that is used to monitor the
characteristics of the plating bath. All of the information
contained in the current values as the potential is dynamically scanned is lumped into a single number that is
affected by all the bath constituents and contaminants at
that point in time.
Examples of Polarographic Analysis of Several Plating Baths
Polarography Can Determine These:
In These Baths
Major Components
Trace Metals
Organic Additives
1. Zinc Sulfate
Zinc
o-Chlorobenzaldehyde
2. Palladium
3. Gold(I) cyanide
Palladium, chloride
Gold, free cyanide
4. Watts’ nickel
Nickel, chloride,
boric acid
Copper, cadmium,
arsenic
Tin
Cadmium,
cobalt, copper,
zinc, iron, tin,
chromium
—
5. Electroless copper
6. Copper sulfate
Copper,
formaldehyde
Copper
—
7. Solder bath
8. Brass
9. Nickel–cobalt
Lead, Sn(II)
Copper, zinc
Nickel, cobalt
Sn(IV)
Lead, arsenic
—
—
Hydroquinone
Saccharin,
o-Benzaldehyde
sulfonic acid
Mercaptobenzothiazole
Thiourea and
thiourea derivatives
536
TABLE 25.4
Manufacturers of Plating Bath Analysis Systems
Company
Technique
ECI
Metrohm
Ancosys
Technic
ATMI/Semitool
Dionex
Applikon
Ebara-Udylite
CVS, photometry, titration
CVS, titration
CVS, photometry, HPLC, titration
CVS, polarography
Chronopotentiometry
HPLC
Titration
that utilizes a small electrode as part of a probe that can
be placed in a plating tank has been developed by Technic [58–60]. This has the benefits of an electrode probe which
can be placed in the plating bath and which can run multiple
analysis cycles without the need for withdrawing a chemical
sample from the tank. This analyzer usually uses harmonic
analysis of the plating response, which has been shown to be
indicative of certain components of the plating bath. The
method development seems to be a bit more empirical and the
operating window of these systems can be narrower than with
other techniques, but it is an operator-friendly system and is
simple to use.
25.2.4
Chronopotentiometry and Chronoamperometry These
techniques have also been used to analyze the additive
concentrations in plating baths [55, 56]. A version of a
chronopotentiometric technique that has been commercialized is referred to as pulsed cyclic galvanostatic analysis
(PCGA) [57]. Figure 25.7 shows an example current pulse
train and a series of potential response curves that may be
generated as the suppressor concentration in a copper plating
bath is increased. A set of response curves such as this can be
used to generate a calibration curve similar to the one in
Figure 25.6, which is then used to determine the additive
concentration of an unknown bath. Advantages of these
techniques include that they gather more information that
is related to the state of the chemical constituents and
they separate the dynamic nature of the process from the
steady-state bath behavior (electrochemical activity). This
allows the operator to try to better differentiate between the
effects of multiple components of the chemistry.
Real-Time Analyzer, or RTATM An analysis technique
based on combinations of these electrochemical techniques
FIGURE 25.5
Ion-Selective Electrodes
Ion-selective electrodes have been developed which can
monitor the concentration of a number of anions and cations [61–64]. A list of commercially available ion-selective
electrodes is given in Table 25.5. The membrane can be glass,
as for a pH electrode which measures H þ concentrations, or
liquid or solid material. Some ion-selective electrodes have
limited lifetimes such as the CN electrode. Only periodic
analysis is practical with a water soak in between applications. However, this problem can be minimized with a welldesigned sampling and flushing system. Care should be
exercised in selecting the most appropriate ion-specific
electrode or oxidation–reduction potential (ORP) electrode
for a particular application. Flow-through cells using ionselective electrodes have been used for continuous on-line
monitoring of copper in electrowinning solutions [65].
Solution pH A pH electrode monitors hydrogen ion activity. Therefore it directly measures the acidity or alkalinity of
a plating bath. All plating baths work best over a specific,
sometimes narrow, pH range. pH sensing and control systems
CVS plot from copper sulfate plating bath.
537
Of course, hydrogen ion concentration can also be measured by acid/base titration. This technique has been used in
several automated bath analysis systems for plating bath
analysis and control.
25.2.5
FIGURE 25.6
CVS calibration curve.
are commercially available. pH electrodes are also used to
detect the concentration of certain compounds in plating
baths by measuring solution before and after a reagent
addition. For example, in electroless copper plating the
amount of formaldehyde present can be determined by
measuring the pH of a sample of solution before and after
fixed amounts of H2SO4 and Na2SO3 are added. This method
was used by Photocircuits and McDermid for automatic
analysis and control instruments for electroless copper plating processes.
The standard glass electrode is a reliable pH electrode as
long as it is kept wet. It cannot be immersed in strong alkali,
fluoride or fluoborate solutions for very long before the glass
is etched away. However, it can be used to measure pH in
those solutions for a reasonable period of time if the electrode
is rinsed and stored in distilled water after each quick pH
measurement.
FIGURE 25.7
High-Performance Liquid Chromatography
High-performance liquid chromatography (HPLC) is another
method that has been used to monitor the concentrations of
plating bath constituents, especially organic additives. HPLC
uses a pump-driven eluent stream (usually acidic) which
carries the sample into a separation column containing a resin
material designed to adsorb components of the bath with
varying affinity. The resin material is chosen to retain the
components of the sample in the column for differing amounts
of time, dependent on their molecular weight and chemical
interactions with the resin, which results in their separation
into discrete bands before entering the detector. These bands
are then detected by UV/VIS absorption, by conductivity, or
possibly using an electrochemical detector. The output from
this separation and detection is a chromatogram which is
recorded with respect to elapsed time. As with the other
techniques already discussed, the concentration of the samples
can be determined by comparing their chromatogram peak
heights or areas with those generated during calibration runs.
HPLC and ion chromatography techniques are most often
used in off-line applications [66]. They involve relatively
high capital expenditure, and the maintenance requirements
tend to be intensive with these systems, which has limited
their use in automatic on-line analysis [23]. These techniques
provide a measurement of the concentrations of certain
constituents of the plating bath, as opposed to the impact
they have on the electrochemical activity. This makes the
methods a more direct measurement of the chemistry, which
makes it important to be able to separate the constituents and
Current and potential plots from PCCA analysis.
538
FIGURE 25.8 Mass spectroscopic analysis of fresh and used accelerator in copper plating bath.
identify the electroactive species as well as to quantitate them
appropriately.
25.2.6
Mass Spectroscopy
Mass spectroscopy (MS) methods are based on separating
atoms or compounds based on their mass. Mass spectrometers usually require some level of ionization, which may
involve an inductively coupled plasma source (ICP-MS) or
an electrospray ionization system. These systems can provide an impressive amount of information related to the
chemical species present and their amounts, but they tend
to be expensive and require significant data interpretation and
maintenance.
A wide strip electrotinning line built by British Steel in
Wales incorporates a fully automatic analytical system to
monitor solutions along the line [67]. Samples are taken
automatically from cleaning, pickling, and plating tanks by
taking a chemical sample from each tank and conveying it to
a remote analytical station. There a robot prepares the
samples for analysis by an ICP mass spectrometer. Results
are fed back to the control station where the operator makes
the necessary adjustments.
Recently, an automated bath analysis system has been
developed using mass spectroscopy (Fig. 25.8) [1, 68, 69].
This system was designed as a process control system for
plating baths, but it has the additional capability of determining
an entire spectrum of components in a plating bath, including
TABLE 25.5
Electrodes
Commercially Available Ion-Selective
Lower
Detection
Membrane Limit (M)
Ion
Hþ
Na þ
Kþ
Kþ
BF
4
NO
3
Cl
Ca2þ
Water
hardness
F
Cl
Br
I
S
CN
Ag þ
Cd2þ
Pb2þ
Cu2þ
Principal
Interferences
Glass
Glass
Glass
Liquid
Liquid
Liquid
Liquid
Liquid
Liquid
1014
106
104
104
104
104
104
104
103
None below pH 13
Hþ , Agþ
Hþ , Na þ , Ag þ
NH4þ , H þ
NO
3 , Br , CIO4 , I
I , Br
I, NO
3 , Br
2þ
2þ
Zn , Fe , Pb2þ , Cu2þ , I
Zn2þ , Fe2þ , Pb2þ , Cu2þ
Solid
Solid
Solid
Solid
Solid
Solid
Solid
Solid
Solid
Solid
106
5 101
5 101
5 101
1017
106
2017
107
107
108
OH at high pH
S, I, Br, CN
I, S, CN
S
None
S, I
Hg2þ
Agþ , Cu2þ , Hg2þ
Agþ , Cu2þ , Hg2þ
Agþ , Hg2þ
separating species that are similar but have slightly different
molecular weights. This system, from Metara, provides volatilization through an electrospray system which keeps most of
the constituents relatively intact for measurement purposes.
25.2.7
Atomic Absorption Spectrometry
Atomic absorption (AA) spectrometry is similar to photometry, but it utilizes an atomized sample of the liquid being
analyzed and usually requires dilution of a bath sample
before measurement. Atomization may be provided using
a flame, an ICP source, or a graphite furnace. A narrowwavelength spectrum source such as a hollow cathode lamp
or a laser is used to excite the electrons of the atomized
sample to higher energy orbitals characteristic of the metal
being measured by the absorption of the light from the
illumination source. The absorption is measured and used
to determine the amount of that metal present
Atomic absorption analysis is fairly reliable and cheap but
requires quite a bit of maintenance. Therefore, AA techniques have been used in off-line analysis in chemical laboratories but have not been used much in automated plating
bath control.
25.2.8
X-Ray Fluorescence
X-ray fluorescence (XRF) analysis is discussed in Section 25.3
with respect to product monitoring, but it can also be used as
539
a technique to monitor the metal concentrations in plating
baths. It is a relatively straightforward technique as long as
the fluid handling is managed and is a good way to measure
the ratios of metal ion concentrations in alloy plating baths.
XRF is an expensive technique, though, and care must be
taken to avoid errors due to drift of the X-ray source or
detector. The frequent calibration requirements and expense
have prevented widespread use of XRF for solution analysis,
although industrial systems for this purpose have been
built [70].
25.2.9
Total Organic Carbon Analysis
Total organic carbon (TOC) analysis is sometimes used to
provide a measurement of all the organic materials added to
a plating bath. This usually includes additives such as grain
refiners, brighteners, and surfactants as well as any other
organic materials that are carried into the bath on the substrates being plated. TOC values do not include dissolved
CO2 or carbonates from inorganic sources. These are removed from the bath sample by purging prior to the TOC
analysis. The TOC level is determined by oxidizing the
sample using chemical, electrochemical, or photochemical
means. The evolved carbon dioxide can then be measured
with a CO2 detector. TOC is a good way to evaluate the total
amount of organic material introduced into the bath over its
life and may be used to monitor the end of life of a plating
bath due to buildup of organic additives and their breakdown
products or other materials such as masking agents or
photoresist components that may be leached out of the
samples being processed.
Organic impurities can be removed by activated carbon
treatment when necessary. However, this usually strips the
bath of all organic components so additives need to be
replenished afterward. Harmful impurities that are known
to build up in a plating solution should be monitored routinely. Often the first sign of an excessive impurity level is
off-color plating or a film roughness and morphology
changes. Water for rinsing often needs to be free of organic
and inorganic impurities as well [71].
25.2.10
Other Analysis Methods
Specific Gravity Specific gravity may be monitored to
maintain a concentration range for sufficient solution conductivity or to detect excessive buildup of salts. A simple
manually operated hydrometer can be used. A more elaborate
automatic device uses a radio-frequency oscillator with a
loop probe exposed to the solution. The oscillator frequency
shifts in proportion to the specific gravity.
Conductivity A conductivity measurement is sometimes
used to determine the presence or absence of inorganic
impurities. A good strategy to minimize the amount of
540
WASTE
DISPOSAL
PUMP
ELECTROPOLISH
RINSE
D.I.
WATER
D.I.
WATER
PUMP
PUMP
PUMP
ACID
DIP
RINSE
NICKEL
PLATE
RINSE
GOLD
STRIKE
HARD
GOLD
RINSE
PUMP
FINAL
RINSE
RESERVOIR TANKS
FIGURE 25.9
Water management system for a strip plater.
high-quality rinse water required and volume of wastewater
produced is to use the good water first where impurities can
be most harmful, for example, after nickel plating. When the
rinse water conductivity reaches a set level, that water can be
transferred back to the preceding rinses that do not require
water of such high quality. This is illustrated in Figure 25.9
for a strip plater involving electropolishing, nickel plating,
and gold plating. High-conductivity water is used only to
maintain water levels in reservoir tank rinses after nickel and
hard gold plating. When the conductivity in the nickel water
rinse drops below a preset limit, water is pumped out of the
rinse after electropolishing to waste disposal. That water is
replaced by water from the rinse after acid pickling. Next
water from the nickel rinse replenishes the acid rinse, and
finally new water restores the rinse after nickel. The process
continues until the conductivity of the nickel rinse water
reaches a preset value. A similar feedback system is used in
the gold plating side. The gold strike solution is operated at
about 65 C (150 F) and loses water rapidly through evaporation into the exhaust. A low solution level activates a
pump to transfer rinse water after hard gold to the gold strike
reservoir. The final rinse water replenishes the preceding
rinse. Fresh deionized (DI) water restores the level in the final
rinse. In addition to conserving water, this system keeps the
gold in rinses moving back to the plating solutions. A gold
reclaim plating cell in the rinse tank after hard gold plating
can also be used to reclaim gold.
ALARMS
SENSORS
SIGNAL
CONDITIONING
INTERFACE
PLATING
PROCESS
CONTROL
DEVICES
25.2.11 Automatic Plating On-Line Monitoring
and Control
Automatic electroplating may utilize on-line monitoring
and control. The technology has developed rapidly with the
availability of modern equipment, including sensors and
electrical and electronic devices, and better mechanical designs [72–75]. Continuous-flow analysis of electroplating
solutions can be fully automated [65, 76, 77]. Palladium,
copper, nickel, tin, silver, iron, chromium, persulfate, peroxide, and chlorate are among substances that have been
determined automatically.
Figure 25.10 illustrates how the various parts of an
automatic electroplating process monitoring and control
system interact. Sensors provide information about the plating process. After signal conditioning, information is sent to
a PLC or a computer that is programmed to control the
system. Actual conditions can be analyzed and stored and/or
displayed. Deviations from the programmed limits will
activate control devices such as motors, valves, and power
supplies to bring the plating process back in control. If the
operator must be involved, an audio alarm sounds. When
there are multiple alarms, each condition may have a different audio sound so the operator can quickly recognize the
type of problem or a status display will show each alarm
condition. Completely automatic analysis and control systems have also been developed which do not require any
operator intervention during normal operation.
DISPLAY
PLC or
COMPUTER
DATA
PROCESSING
& STORAGE
Motor
Valves
Power supplies
FIGURE 25.10 Schematic of an automated electroplating process monitoring and control system.
541
FIGURE 25.11 Automated chemical analysis and replenishment system to support semiconductor copper plating tools.
Since the mid-1990s completely automated copper
analysis and replenishment systems have been used with
semiconductor plating equipment for copper damascene
interconnect applications (Fig. 25.11). The analysis systems
usually use a titration cell for determining copper and acid
concentration, titration or an ion selective electrode for ppmlevel chloride determination, and titration combined with
CVS or chronopotentiometry for analysis of organic additives such as suppressor, accelerator, and leveler. These
systems typically make use of slipstreams that are recirculated from the plating equipment through the analyzer. The
analysis system utilizes a block of valves to periodically
divert a small sample from the slipstream to the analysis
vessel, where it is analyzed and then sent to waste recovery.
As explained earlier, electroless plating chemistries are
not very stable and require frequent bath analysis and monitoring. Electroless copper plating parameters that need
monitoring and control are temperature, pH, copper concentration, formaldehyde (reducing agent), and cyanide ion.
Accurate temperature monitors/controllers are readily available. The pH value is measured with a standard glass
electrode. Hydroxide solution is metered in as required to
maintain pH. Copper concentration can be monitored colorimetrically or by polarography. Formaldehyde may be measured by adding 0.045 M sulfuric acid to a bath sample to
reduce the pH to about 9.5. When a 0.05 M sodium sulfite
solution (also at pH 9.5) is added, the resulting pH is
proportional to the formaldehyde content. Polarography can
be used to measure both formaldehyde and cyanide bath
concentrations [20].
Several on-line electroless copper plating analyzers/controllers have been developed for copper plating of printed
circuit boards [2, 20, 21, 78]. They are used to monitor and
control baths that deposit a thin layer of copper in through
holes prior to electroplating the thicker copper. Automatic
analyzers or controllers are essential when the entire copper
circuit is built up by electroless plating.
In 1971 Photocircuits developed its Mark VI automatic
electroless copper plating bath analyzer to monitor and
control its CC-4 electroless copper plating bath. Later
McDermid developed a similar automatic bath analyzer
which is shown schematically in Figure 25.12 [78]. A single
peristaltic pump moves a sample and the reagents through
the analyzer. The pH is adjusted, then the copper ion concentration is measured spectrophotometrically. A formaldehyde reagent is added next. The resulting pH is proportional
to the formaldehyde concentration. The controller activates
replenishment pumps to restore the copper ion and the
formaldehyde concentrations to their optimum levels.
Bell Laboratories machines utilized polarography to analyze for Cu2þ , HCHO, and CN [20]. With the solution
still, polarographic currents are measured at 1.0 V for
Cu2þ , 1.8 V for formaldehyde, and 0.25 V for free
cyanide. This is illustrated in Figure 25.13. The machine
automatically averaged 10 measurements of each component
for better accuracy. The block diagram in Figure 25.14 shows
how the various parts interact. The actual machine
(Fig. 25.15) was built in two parts: one for wet chemistry
on the left and the electronics on the right. The two units
could be separated by several hundred feet if necessary. One
advantage of the separation was that the wet chemistry
cabinet could be hosed down occasionally. Electroless copper solution flows through the flow gauge at the left continuously. Periodically a sample is taken into the analysis cell
along with some NaOH reagent. The pH is measured, and
dropping mercury polarograph measurements are made.
After electronic processing the results of analysis are displayed. If chemical additions are required, replenishment
pumps are turned on. A complete analysis can be made
every four minutes.
542
FIGURE 25.12 Schematic of McDermid electroless copper plating bath controller.
FIGURE 25.13 Polarographic analysis of electroless copper
plating bath controller.
FIGURE 25.14 Electroless copper plating bath monitoring and
control system.
MONITORING AND CONTROL OF FINISHED PRODUCT
543
usually done on a statistical sample for economic reasons.
The advantage of on-line inspection is that every part may be
tested. If plating is out of specification, either plating conditions can be changed automatically or an operator can be
alerted for a manual correction.
25.3.1
FIGURE 25.15 Electroless copper plating bath automatic analyzer and controller: (A) mercury reservoir, (B) analysis cell,
(C) plating solution flowmeter, (D) motorized syringes, (E) chart
recorder, (F) analysis display meters, (G) pH monitor, (H) set point
controls, (I) process controller, (J) polarograph, (K) viscosity
monitor, and (L) power source.
25.3 MONITORING AND CONTROL
OF FINISHED PRODUCT
There are at least as many ways to measure the results of a
plating process as there are products produced using plating
processes. Ultimately, the monitoring of a product produced
using a plating process or of the film plated should be done
based on an understanding of the properties of the deposit that
are important to that product. Having said that, we will review
several types of measurements that are typically used for
measuring the deposit quality of plated films. These are
meant to be examples of common measurements and should
be adjusted as necessary to ensure that a particular deposit
meets the specifications demanded of it for a particular
application. There is extensive literature regarding inspection of manufactured parts and quality control and inspections. These can be used to set up a statistically sound
monitoring program to monitor the quality of parts being
produced and to improve the product quality or reduce the
variability. Typical electrodeposit properties inspected are
thickness, color, brightness, hardness, magnetic properties,
and alloy composition. Automatic instruments are available
for off-line and on-line measurements. Off-line inspection is
Thickness
Various wet chemical methods are available for measuring
electrodeposit thickness: coulometric, dropping test, and
spot test [79]. A disadvantage of these tests is that the part
used generally becomes scrap.
A nondestructive thickness test is usually preferred. There
are several methods available, and most have been automated
as stand-alone instruments or incorporated into on-line monitoring and control plating systems [80, 81]. The methods
used include beta backscatter, X-ray fluorescence, sheet
resistance (eddy current or four-point probe), and magnetic
field measurements. Deposited metals tested by these methods are shown in Table 25.6. Profilometry can also be used to
measure deposit thickness when selective deposition is used
or when part of the deposit can be etched away.
In general, there are extremely sensitive instruments
that have been developed for the microelectronics industry
for measuring the properties of deposits produced on semiconductor wafers or similar substrates. These instruments
are commonly automated wafer handling systems that are
completely computer controlled and are capable of intensive
data manipulation. While these systems are far different from
the typical thickness measurement tools described in the
sections that follow, they are increasingly becoming important in applications involving electrochemical deposition for
microelectronic applications. Some of these thickness measurement techniques are then described under Profilometry,
Optical Methods, and Laser-Acoustic Methods. These are
examples of some of the systems that have been developed
for microelectronics applications, where the substrate is very
well controlled and where extreme precision is required of
the deposits on them. Other new analytical techniques are
being developed at a rapid rate, specifically for the microelectronics industry.
Coulometric Method The coulometric method is based
on Faraday’s law: One gram equivalent weight of metal is
stripped away from an anode for every 96,500 coulombs of
TABLE 25.6 Nondestructive Electroplated Deposit
Thickness Tests
Method
Metals Tested
Beta backscatter
X-ray fluorescence
Eddy current
Au, Ag, Cd, Cr, Cu, Ni, Zn, alloys
Au, Ag, Cd, Cr, Cu, Ni, Zn
Cu, Zn, Cd, Ni, Cr
544
electricity passed through the cell. Four parameters must be
controlled: surface area, current, time, and anode (deposit)
dissolution efficiency. At 100% anode efficiency, the deposit
thickness is calculated from the formula
10
Thickness ¼ eit
Ad
where e ¼ electrochemical equivalent, g/A-s1
i ¼ constant current, A
t ¼ time, s
A ¼ area, cm2
d ¼ metal density
It is necessary to use a specific electrolyte for each metal
deposit and substrate [82]. The electrolyte must not chemically attack the plated film. An automated instrument can
detect the endpoint by sensing a voltage change when the
substrate metal is exposed. The method is capable of an
accuracy of 10% of the true value. One advantage of the
method is its ability to measure combination deposits such as
copper–nickel–chromium.
Dropping Test The dropping test is very simple. It is
performed by dropping chemical etch solution on a particular
spot at a rate of about 100 drops per minute. The operator
observes the time it takes to expose the base metal. The test is
calibrated using a sample with a known thickness of plated
metal. For consistent results, the drop size, temperature, and
etch solution must be controlled. Operator skill is an important factor in achieving reproducible results with an accuracy
of 15% [83].
Spot Test This simple test was developed as a rapid and
inexpensive test for chromium deposits on nickel or stainless
steel. A drop of hydrochloric acid is placed inside a ring
of wax on the part to be tested. Hydrochloric acid attacks
chromium with the evolution of hydrogen gas bubbles.
Gassing stops when all the chromium is dissolved. The time
of dissolution is proportional to the chromium thickness. The
test is calibrated with a known chromium-plated thickness
specimen. An accuracy of 20% is obtained for deposits up
to 1.2 mm thick [84].
Beta Backscatter The radioactive decay of certain isotopes
produces beta rays, which are fast-moving electrons. When
these are directed at an electrodeposit, some are slowed and
change direction out of the deposit. The number of these
backscattered electrons is proportional to the number of
atoms per unit area and their atomic number. The backscatter
is measured with a Geiger–Muller tube counter. The isotope
used is chosen based on its maximum beta-ray energy and
half-life. As the deposit thickness increases, the energy
needed to penetrate the deposit increases. Metals with higher
atomic numbers require higher energies for the same thickness. Instruments designed to measure metal deposit thickness utilize a Geiger–Muller tube electron counter and, with
a built-in microprocessor, compute the thickness. The device
is also capable of feedback current control of on-line plating
systems [85].
X-RayFluorescenceandX-RayReflectometry This method
is similar to beta backscatter in that a radiation is directed at
the deposit and the energy emitted is measured. The method
uses X rays produced by an X-ray tube that is specific to the
deposit metal. The radiation emitted from the deposit is
proportional to the deposit thickness. The system may be
calibrated with a known thickness and composition standard for additional accuracy. The instrument provides the
thickness measurement which can be used to control the
plating process.
An XRF thickness monitor is used to control the thickness
of zinc plating on steel mill strip by a feedback signal to a
computer that adjusts the line speed in order to maintain a
specified zinc thickness [16]. Emissions are specific for each
metal, so alloy compositions can be determined. Metal
deposit thickness in the range of 0.25–10 mm can be measured depending on the metal. Measurement accuracy is
10% with proper calibration [86].
Precision XRF measurement equipment has been developed for the microelectronics industry that provides much
better precision than the simple instruments described above.
These systems are capable of measuring with greater accuracy and potentially utilizing spot sizes as small as 50 mm.
They can be used to measure thickness or composition of
an alloy deposit. In some cases, XRF has been combined with
X-ray reflectance (XRR) in order to provide precision thickness measurement of extremely thin metal films on flat
substrates. XRR uses glancing angles close to the critical
angle at which total external reflection occurs for a given film
in a q/2q scan mode [87–89]. The value of the critical angle
can be used to determine the density of the film being
measured. At angles above the critical angle, the X rays
penetrate the film and are reflected from the top and bottom
surfaces of the film, giving rise to interference fringes which
can be used to determine the film thickness based on the
angular separation between the fringes. These systems are
capable of measuring metal film thickness as thin as 2 nm,
with accuracy on the order of 1 A [90, 91]. XRR does not
work well beyond about 200 nm, so XRF techniques are
employed for thicker films. XRR techniques can also provide
information related to film roughness.
Eddy Current (Sheet Resistance) Eddy current thickness
gauges are electromagnetic instruments designed to measure
a change in impedance of a coil that induces an eddy current
into the plated metal. The phenomenon is based on the
difference between the electrical conductivity of the basis
metal and the deposit that produces the change in impedance.
The thickness measurement is performed with a probe which
is positioned perpendicular to and sometimes in contact with
the surface at the point of measurement. Thickness gauge
instruments are available with a digital display, memory, and
computer-prompted calibration procedure. The measurement accuracy is 10% to the true thickness in the range
of 5–50 mm. Factors that influence measurement accuracy
are surface contours, surface roughness, and type of plating
process that can influence deposit conductivity [92].
High-precision specialized sheet resistance measurement
equipment based on eddy current probes or four-point electrical contact is also available for the semiconductor and
microelectronics industry [89, 93]. These instruments provide a very common method of determining the thickness of
metal films based on measuring the sheet resistance.
Magnetic Method The magnetic method utilizes the magnetic influence of the electrodeposit for measurement. Two
types of probes are in use: (1) a mechanical device that
measures the influence of the plated metal on the attractive
force between a magnet and the base material and (2) an
electromagnetic probe that measures the influence of the
plated metal on the reluctance of a magnetic flux through
the deposit and base metal. Three types of deposits can be
measured with the second probe: (1) nonmagnetic deposits
on ferromagnetic base metals, (2) nickel deposits on ferromagnetic base metals, and (3) nickel deposits on nonmagnetic base materials.
Both probe types require perpendicular positioning on
the surface. The magnet probe works by measuring the force
required to pull it from the surface. The electromagnetic
probe measures the reluctance difference with and without
the plated metal. The use of both probes requires a calibration, and averaging several measurements improves the
accuracy. Commercial instruments utilizing the electromagnetic probe are available with microprocessors that can
automate the system by averaging data, display results, store
or transmit information, and provide on-line monitoring and
control [94, 95].
Profilometry Profilometry is a convenient method for measuring the plated thickness of features deposited through an
insulating mask such as photoresist on a flat substrate. It can
also be utilized when a portion of the deposited film is etched
away after the deposition. Typically, a profilometer makes
use of a very small needlelike stylus that is moved across the
sample as its vertical position is monitored. The height of the
sample can then be plotted against the lateral dimension to
provide an image of the sample surface. Once the image is
leveled, it can be used to provide information such as
maximum deposit thickness, average deposit thickness, or
deposit roughness.
545
Variations of stylus profilometry have been developed
using other measurement techniques. Systems for measuring
semiconductor wafers and similar substrates have been
developed based on optical profilometry and atomic force
microscopy (AFM). Each technique has its own advantages
regarding sample area or length, measurement time, and
height variation that can be measured. In the end, though,
each of these techniques provides a two- or three-dimensional image of the surface profile that can be used to provide
thickness profiles and roughness information. The microprocessors that are used to control these systems usually have
enough computing power to manipulate the information and
provide statistical details or to identify features that do not fit
the expected profile for quality control or yield management
purposes.
Optical Methods Optical profilometers have been developed, as mentioned above. These typically use an optical
lever to measure changes in step height of a sample. In
addition, interferometric techniques have been used to
measure height variations on flat substrates. These can be
monochromatic light measurements or broad-spectrum measurements using phase shift interferometry or vertical scanning interferometry, respectively.
Optical profilers can be used to do the same job as stylus
profilometers, although they more commonly produce a
three-dimensional image of a surface by scanning the laser
measurement point or by measuring a broad area of the
sample as in interferometry. Interferometry systems are very
useful for measuring large areas quickly, such as measuring
the height of electroplated solder bumps on a wafer for flipchip attachment. These systems rely on intensive computing
power and calculations and are very good at identifying
small numbers (ppm level) of features out of many that
do not have the expected height. Examples of optical
measurement systems for wafer applications are Rudolph
Technology’s NSX systems, Zygo’s NewView interferometers, KLA-Tencor’s MicroXam, and Veeco’s Wyko
instruments.
Laser-Acoustic Methods Laser-based optoacoustic methods have been developed recently for measuring film thickness of opaque films such as metals. There are two main
techniques that have been commercialized as automated
systems for measuring metal film thickness of semiconductor
wafers or other flat substrates. Both techniques are capable
of measuring film thickness with a high degree of precision
using a small spot size and are capable of measuring the
thickness of multiple films in a stack. The two techniques
make use of probing and excitation lasers and mathematical
models of the optical and/or acoustical properties of the film
stacks in order to provide film thickness measurements. One
method uses a sonar technique, and the other uses surface
acoustic waves to determine the film thickness [89].
546
The optoacoustic technique that is known as picosecond
ultrasonic laser sonar uses a very short laser pulse to
thermally excite the sample surface. This launches an
acoustic wave down into the film stack, which is reflected
back to the surface at each material interface. As the sonic
wave bounces through the film stack producing acoustic
reflections, a probe laser detects the vibrations at the sample
surface. A model based on the sonic velocity in each material
and the expected film thickness is used to determine the
actual film thickness in the stack. This method was commercialized by Rudolph Technologies as its MetaPULSE
metrology tools.
The surface acoustic wave technique uses two probing
lasers to produce an optical grating that launches an acoustic
wave in the film stack. The time-dependent diffraction of
light is then analyzed with a physical model of the acoustic
wave to calculate the film thickness. This technique detects
surface acoustic waves traveling in the plane of the film
being measured. This technique has been commercialized
by Philips Analytical and is available from Semilab AMS
(Advanced Metrology Systems) in its AMS 3300 metrology
system.
Both the Rudolph and Philips metrology systems rely on
mathematical models of the physical system being analyzed
and its acoustical properties. This makes them sensitive to
variations in roughness at the film interfaces and density
variations as well as defects within the small spot size being
measured. On the other hand, they are high-resolution techniques that can be very useful with the proper understanding
of the sample being analyzed and its relationship to the
measurement configuration.
25.3.2
FIGURE 25.16 Schematic of an on-line gold plating color
monitor.
color. The result can be displayed on a meter and/or fed into
an automatic process control system to correct off-color
plated gold.
Color
The color of electrodeposits has an esthetic value particularly
on jewelry, home appliances, autos, and other decorative
objects. Even on parts that are hidden from view when
assembled into an object, an attractive appearance is considered a quality factor. Off-color plating or variable plating
color often is considered a sign of a process problem.
Generally, monitoring and control of color are done by visual
inspection by an operator but on-line automatic inspection of
color is feasible for many plating lines. An optical fiber
system was developed to monitor the color of gold plating on
a moving strip, shown schematically in Figure 25.16 [96].
An optical fiber bundle is positioned over a gold-plated strip.
The bundle is divided into three parts. Light is directed into
one arm that illuminates the moving strip. Reflected light
from the gold surface passes into two separate optical fiber
bundles. The light from one passes 700–800 nm through an
optical filter that serves as a standard. The other light path
passes 450–575 nm through the optical filter. An electronic
comparator determines any deviation from the optimum
25.3.3
Brightness
Reflectivity is a common method used to measure deposit
brightness, especially in the semiconductor industry. Reflectivity measurements are simply quantitative measurements
of the amount of light reflected from a given surface area. The
instruments used to measure reflectivity must usually be
calibrated to a reference standard, and the values reported
are relative to the reference. In industrial applications, deposit brightness may be judged qualitatively by simply
deciding if the deposit is bright and shiny and has the desired
surface appearance.
Bright plating for decorative applications usually requires
closer monitoring and control of the bath chemistry. Additives that produce brightening, usually organic, can become
depleted or degraded by oxidation at the anode or from
atmospheric exposure. Additive depletion is easily corrected
by adding more additives, and this can be done by metering it
in on the basis of plating time (coulombs) and/or elapsed
time. Brightener concentration also may be controlled by
547
chemical analysis, plating tests (Hull cell), or CVS, as
described in Section 25.2 [47, 48].
be affected by the film thickness or the edges of a patterned
structure that is being used for measurements.
25.3.4
25.3.6
Roughness
Roughness of the plated surface is a measure of the quality of
the plating process that is also related to the deposit brightness. A deposit that has a rough morphology will tend to be
dull or matte in finish, rather than bright. Roughness can be
measured quantitatively, or it can be gauged qualitatively
by looking at optical or scanning electron microscopy (SEM)
photomicrographs. The roughness of a deposit will typically
have several dimensional scales and may be related to the
grain size or the crystal structure of the deposit, as seen in
Figure 25.20. In some cases, the deposit roughness may
increase, become nodular, or even produce dendritic structures as the plating rate approaches the limiting current
density of the system.
The deposit morphology may also be affected by other
structures on the surface, such as underlying topography or
masking materials such as photoresist. These effects can be
used as indicators of changes in the process or chemistry or
can be used to gauge how close a process is to the edges of
its process window.
There has also been some work done on correlating the
roughness evolution as a function of thickness for plated
films. These data provide information related to the nucleation size and density and the film growth.
Film stress can be a very important parameter to monitor due
to the possibility of creating problems with the part after
plating. Excessive film stress can create problems with
adhesion which lead to delamination or it can cause cracking
or lead to excessive etching of the film when it is exposed to
chemistry. Plating test cells for monitoring stress were discussed in Section 25.2.1. Film stress can also be measured on
the part after the electroplated deposit is produced. In the
semiconductor industry, global film stress is measured by
evaluating the wafer bow before and after plating a film on
one side. The radius of curvature, or deflection at a given
distance from the center, is measured and Stoney’s equation
is used to determine the stress of the thin film that was added
to the substrate [97]. The reaction of the relatively thick
substrate to the deposited film thickness produces curvature
changes, as shown in Figure 25.17.
The film stress may also be estimated from XRD measurements. These measurements are essentially a measure of
the strain of individual crystals of the deposit taken from the
measurement of their lattice parameter, so they may not
correlate to the global film stress measured using the wafer
bow or bending beam methods.
25.3.7
25.3.5
Hardness
Electroplating hardness is an important property to control in
applications where minimum wear is required. There are
many test methods for hardness available, but all are done
off-line. Most testing is done with indentation instruments
of somewhat different designs. The various types are Brinell,
Rockwell, Vickers, Solersoope, Knoop, and Tukon. An
indenter is used to form an indentation in the deposit using
a known force, and the size of the indentation is measured to
determine the hardness of the material. Care must be taken
to ensure that the size of the indentation is small enough not to
Film Stress
Grain Size
The grain size of an electrodeposited film is important in
determining the mechanical properties of the material. Of
course, grain size relates to the hardness of the material. It has
also been shown that the grain size of electroplated copper
films has a strong impact on the resistivity of these deposits [98]. There has been quite a volume of work presented
on the characterization of grain size changes that occur in
these films after deposition at room temperature or at elevated
temperatures [99–109]. The grain size changes have been
related to changes in resistivity, hardness, crystal texture, and
in-film incorporation of organic additives.
FIGURE 25.17 Effects of film stress.
548
FIGURE 25.18 Grains in electrodeposited copper film.
The grain size of an electrodeposited film may be estimated from XRD data or it may be measured directly from
an image that highlights the individual grains. Software
may be used to identify individual grains and create grain
size distribution plots, or standard methods may be used to
estimate average grain sizes based on counting the number of
grain boundaries intersecting a line of a given length [110].
Recently, focused ion beam (FIB), systems have been developed that utilize a focused beam of gallium ions to cut
through thin films. The gallium beam may also be used to
image the sample, and the channeling depth variation of the
gallium ions into grains with varying orientation provides
excellent contrast, as seen in Figure 25.18.
25.3.8
X-Ray Diffraction and Crystal Texture
X-ray diffraction (XRD) analysis can be useful in understanding the characteristics of a plated film. XRD analysis
can provide estimates of grain size and lattice constant as
well as information related to the crystallographic orientation
or texture of the grains of a deposit. Theta-Two theta scans
and rocking curves provide standard plots that are used for
these purposes. Complete XRD pole figures may also be used
to get a more complete understanding of the orientation of
the crystal lattice.
The texture of plated films can be important in that it may
affect the magnetic properties, the interactions with other
films in a stack, or the mechanical or electrical properties of
the film.
25.3.9
Recrystallization Rate
There has been an enormous amount of information published in the last decade regarding “self-annealing” or recrystallization of electrodeposited metal films, especially
copper [99–108]. These publications focus on grain growth,
FIGURE 25.19 Effect of electroplated copper film recrystallization of sheet resistance.
549
reduction of resistivity, changes in film stress, and changes in
crystallographic texture (see Fig. 25.19). There is also information relating to the evolution of organic contaminants
that have been incorporated in the film and the coalescence of
dislocations to form microvoids in the film.
Grain recrystallization and grain growth in plated films are
driven by an excess of grain boundary energy. This process is
accompanied by the diffusion of contaminants that have been
incorporated in the film during deposition and migration of
dislocations as well as stress changes in the film. These four
processes are all occurring together, so the behavior can be
somewhat complicated. Understanding these processes and
noting differences in the extent to which they occur and their
rates in plated films can be useful in understanding differences in plated grain size, dislocation density, formation of
voids, incorporated contaminants, and film stress.
Some of the first work done on the “self-annealing” effect,
or grain growth at room temperature, dealt with grain growth
in mechanically worked sheets of copper [111–114]. The
mechanical properties and XRD analysis of these metal
sheets were used to characterize the effects of recrystallization and grain growth. Obviously, this effect has been known
for a long time, although it has been studied most extensively
in the last 10 years as electroplated copper has been used in
semiconductor devices. The behavior of copper films has
been shown to relate to the electromigration resistance and
stress migration induced voiding that can cause reliability
failures in these devices.
measurement of the surface roughness with a profilometer
and calculating one of the many roughness parameters that
are commonly used. There is even an example of an in-line
automated reflectometer that monitors the color or the
roughness of plated gold, as shown in Figure 25.16.
Roughness is an important deposit parameter that represents how the process is performing. The roughness of a
plated film is determined by the plating rate, the waveform
used for plating, additives in the chemistry (brighteners,
levelers, grain refiners, etc.), contaminants in the chemistry,
and cleanliness of the substrate before plating. If the desired
deposit is bright and shiny, the roughness should be minimized. This may require the use of grain refiners and leveling
agents and sometimes requires leveling of roughness that
may already exist on the surface. On the other hand, sometimes a “matte” finish is desired, so a controlled amount of
roughness is used to produce this surface finish. It may even
be desirable to produce a rough film in order to improve
mechanical adhesion or maximize surface area for some
other purpose. In any case, it is important to understand
the process variables that impact deposit roughness and
to control them in order to minimize variations over the
long term.
Morphological variations are sometimes associated with
masked features on a plated part. These may be due to simple
current crowding effects or they may be due to interactions
of the masking with additives in the plating chemistry [115].
These effects can be seen in Figure 25.20.
25.3.10
25.3.11
Surface Morphology
The surface morphology of plated films can be important for
several reasons. The morphology can include local surface
roughness as well as effects that relate to plating within
masked patterns. Measurement of the morphology of plated
films will depend on the application and may range from an
optical inspection, or imaging with a microscope or SEM, to
Surface and Compositional Analysis
Of course, the composition of the electroplated deposit is
usually an important parameter to measure. There are many
methods available for measuring the bulk composition or
surface composition of a deposit, some of which are destructive to the sample being measured. The composition may also
be inferred by measuring some other property of the deposit,
FIGURE 25.20 Morphology of through-mask electrodeposits.
550
such as magnetic moment, magnetostriction, or melting
temperature. These methods are particularly useful for alloy
deposits.
X-ray fluorescence is one of the most common methods
of compositional analysis used for measuring electroplated
alloy composition. It works well for measuring heavy elements like metals in quantities above 0.5% and has the
additional benefit of providing thickness measurements
for films less than 10 mm in thickness. XRF is a nondestructive method and can be adapted to work for many different
plating applications. Equipment ranges from expensive
equipment adapted for measuring films on semiconductor
wafers or similar substrates, as described above, to simple
hand-held instruments that are designed as portable thickness
gauges [86].
Destructive methods for composition analysis usually
involve dissolving the film in an acid and measuring the
quantity of each metal in the solution. These methods work
best for compositional analysis of alloy deposits. The most
common methods in this category include AA analysis and
ICP-MS.
A method somewhat similar physically to beta backscatter
metrology is Rutherford backscatter spectrometry RBS. This
method uses positively charged ions to bombard the sample
and measures the backscattered ions. By measuring the
energy loss of the backscattered electrons, the composition
and thickness of thin films can be measured. This is a more
involved method than beta backscatter thickness measurements that is useful for surface analysis [116].
When it is important to measure very small quantities
of contaminants in plated films or to measure light elements,
it is usually necessary to use some sort of surface analysis
technique. The drawback is that these techniques usually
measure only very near the surface of the film although they
can be very sensitive. In order to measure below the surface of
the film or to remove surface contamination before measuring, it may be important to use argon sputtering to remove
the surface layers or to sputter through the film and make
measurements in a depth profile. The most common methods
used in this category include Auger electron spectroscopy
(AES), electron spectroscopy for chemical analysis or X-ray
photoelectron spectroscopy (ESCA or XPS), and secondary
ion mass spectroscopy (SIMS).
25.3.12
plated feature from the surface. The force required before
failure is monitored, and the failure mode can be inspected to
differentiate between interfacial failures, brittle bulk material failure, and ductile bulk material failure.
Wire pull testing can also be used to understand the
mechanical strength of an interconnect feature with forces
perpendicular to the substrate surface. In this method, a wire
is bonded to the pad or feature to be tested and it is pulled
straight away from the surface. Again, the force is measured
and the failure mode or interface can be determined by
optical inspection or compositional analysis of the failed
interface. Variants of these tests can be devised to monitor the
most important failure mode for a given application.
25.3.13
X-Ray Transmission Imaging
X-ray transmission imaging has been used, especially for
detecting voids or similar defects, in quality control of
electronic components such as plated wiring or solder bumps
on boards or semiconductor wafers or chips. This technique
is analogous to medical X rays in that the X rays are passed
through the sample and the image shows the intensity of the
X rays that pass through the sample. Voids are seen as light
spots in the image. This can be an important technique for
inspecting solder bumps after reflow, because it is possible
that voids may be created during the reflow process, especially if the amount of organic materials incorporated into the
film is high. See Figure 25.21.
Similar to medical X-ray imaging, systems are now being
made that use X rays for tomographic imaging. By utilizing
the relative rotation of the sample and X-ray detector, threedimensional tomographic images may be created and used to
visualize a sample. These can be useful for viewing multiple
Shear Testing or Pull Testing
Mechanical testing can be an important part of monitoring
plated deposit performance. Shear strength testing is a
method that can be used to measure the mechanical properties of a deposit or to monitor for interfacial adhesion
problems. This is a common technique used for monitoring
electronic components such as solder bumps or other electrical interconnect features. It is a destructive method (at least
for the feature being tested) which involves shearing off a
FIGURE 25.21 Transmission X-ray image of solder bumps after
reflow.
REFERENCES
FIGURE 25.22 Transmission X-ray image of through-silicon vias.
levels of electrical interconnections on an electronic part.
With improved X-ray sources and detectors, the resolution of
these systems continues to be improved. See Figure 25.22.
25.4
SUMMARY
The electroplating industry has at its disposal an extensive
amount of information and technology for monitoring and
controlling all aspects of the plating process. Sensors have an
important role in monitoring the status of each important
parameter that must be controlled for successful plating. The
design and engineering of individual sensor devices have
improved greatly in recent years. Many incorporate a microprocessor for signal conditioning, and some now can also
carry out control of certain plating conditions directly without relying on a PLC or computer. As new phenomena and
technology have been converted into practical cost-saving
devices for monitoring and controlling electroplating parameters and systems, the industry has adopted them. Electronic devices, computers in particular, have revolutionized
the conversion of sensor signals to a useful form for monitoring and control. Cheap computers have made it easy and
cost effective to automate plating systems. Further, plating
information can be gathered and analyzed quickly to ensure
that quality is maintained at all times. Automation of electroplating utilizing monitoring and control has resulted in
lower manufacturing costs while improving quality and
reproducibility of processing.
Constituent monitoring and control for solutions used for
electrochemical deposition have improved dramatically as
well. The emphasis has been on providing rapid analysis
while reducing the chemistry usage and increasing the level
of automation and control of the chemical control system.
Electrochemical analysis techniques have been widely used,
551
and other analytical techniques have been integrated into
on-line analysis systems as well. The level of understanding
of what is happening to the bath chemistry over time and how
to control it has been continuing to improve.
Finally, the number of choices for measurement and
monitoring of the finished product has continued to grow.
Our ability to measure material properties with more accuracy and to ever-finer levels of resolution allows us to
improve our understanding of the materials we are producing. While there has been a large number of measurement
techniques discussed, there are many other techniques that
were glossed over or not discussed at all. The appropriate
techniques to monitor deposits for a particular application
must be explored, adapted, and sometimes invented in order
to ensure that the important deposit properties are being
monitored to the level required to ensure product performance. The more we learn, the more we find that there is a lot
more to be learned in order to completely understand our
plating processes and the deposits we are creating.
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