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Characterization of filters used in
recirculated buffered oxide etch baths
Joseph Zahka, Ven Anantharaman, Mark Carroll, Karim Vakhshoori
Millipore Cop., Bedford, Massachusetts
The physical, performance,and cost characteristicsof
f i b for recirculatingetch baths are discussed. However,
to choose the proper filter for a productionoperation,
knowledgeof the etching application and practical experience ate also needed.
n ideal chemical filter should provide total retention and
complete cleanliness; no particles should appear downstream,
and there should be no extractables. The filter should behave
like an open pipe (i.e., no pressure drop). Its initial cost should
be low; it should not degrade or "plug in" the chemical and so
should not need to be changed.In real world applications, some
of these characteristics are mutually exclusive and tradeoffs must
be made. Particle retention and flow rate, for instance, are in
contlic~the more retentive the fiiter, the lower the flow rate. The
desire for low cost conflicts with the high expense of filter materials required to achieve low extractables in aggressive chemical
environments. To achieve downstream cleanliness, filters must
be manufactured in clean environments and cleaned in postmanufacture pmcesses, which also add to cost. This article examines these issues and describes the testing of filtration devices for
recirculated buffered oxide etch (BOE) applications. Here, flow rate
and freedom from particles are very important and, due to the high
surface tension and low gas solubility of many etch bath formulations,fdter wettability is also signifcant.
Figure 1.Recirculation etch bath system.
BOE solutions are used extensively in silicon processing to remove
oxide layers while leaving Si unaffected. A traditional BOE bath
consists of 7:l 40% ammonium fluoride, 49% HF [l].More recent
fondations use lower concentrationsof ammonium fluoride, which
results in an unsaturated solution with lower surface tension [21.
Bufferedoxide etching is often accomplished in recirculating etch
baths (REB) (Fig. 1). Fluid in the tank flows over a weir into the
suction of a pump (either a singlestagecentrifugal pump or a positive displacement diaphragm pump), through a particle-removing filter, back into the tank through eductors in the bottom of
the tank, and then up past the wafers to complete the loop.
Depending on the application, some properties of filters are
more critical than others. For BOE REBs, flow rate, extractables,
and particle retention are very important, but filter wettability is
also significant, as mentioned above. In the sections that follow,
filter properties are described in the order of importance for the
REB application. The laboratory tests used to quantify each
property were performed on three filters designed specifically
for etch bath applications (Table 1).
June 1993 Solid State Technology 63
ed. Filter C has about one-half the pressure drop of Filter A.
Flow rate through a filter affects its particle removal rate. At high
flows, there is faster bath tumover and increased particle removal.
High flows also increase agitation and eliminate dead zones in
the bath. Flow is controlled by the resistance of the piping, eductors, and filter [3J.Reducing filter resistance increases flow, espeto pipe resistance.
cially if filter resistance is
In the current work, deionized (DI) water was circulated through
the filter to measure flow rate. DI water was used rather than
BOE solution. However, membrane filter samples were tested with
BOE to ensure that the flow rate was proportional to viscosity
with no additional flow loss due to other effects.The results of pressure drop testing are shown in Fig. 2. Pressure drop versus flow
is shown for all the filters tested (housing resistance is included).
Pressure drops at 10 gpm water flow are given in Table 2. Note that
Filter C has about one-half the pressure drop of Filter A. Because
all these filters have such low resistance, the effect on bath performance is not significant. When Filter A replaced Filter C in a
7:l BOE with a single-stage centrifugal pump, flow decreased only
from 6.0 to 5.5 gpm.
W b l e S
In BOE applications, a bath volume of about 20 liters was run in
a totally recirculated mode. Extrdctables from the filter cartridge
remain in the bath until the chemical is hanged (typically once a
Solid State Technology June 1993
Figure 3.Resistivity and TOC flushup of Filter A. Both parameters
TOC Analyzer Model A-100 with one
week). If extrdctablesare inithfly high or there is continuous e m c tion at a low rate, recirculation exacerbates the problem.
DI water was chosen as one extraction solution because BOE
solutions are also aqueous-based and the low interferenceof water
with analytical techniques allows for low-level measurement. A
second, more aggressive extraction sohtion, 10% HCI, was selected to identlfy the potential for release of metallics from the filter.
Extractables were measured by soaking the filter cartridgesin a liter
of DI water at room temperature for four hours. Concentrations
of ionic species were measured with ion chmmatography.The filters
were then soaked in 10??HCI for about 16 hours, and Cu, Fe, and
Al concentrations were measured with atomic absorption. Table 3
shows results for static extraction. The three filter types released
approximately the same extmctables. As-shipped chemical cleanliness appears adequate.
Table 4. Downstream cleanliness summary time
to reach background at steady flow
Effect of pulsing
slight increase initially
no effect after 4th pulse
slight increase initially
no effect after 4th pulse
all pulses (1.5 to 2.5 log)
Notes: Downstream cleanliness measured with PMS M50
Background = 5 particles (>0.05pm ) per milliliter.
Steady flow at 12 liter/min for 120 min.
Pulsed flow at 15-minintervals for 120 min.
Figure 4. Latex bead retention of etch bath filters. Monolayer coverage is 0.1%. Filters A and B showed equivalent retention. Filter C had
poorer retention, with the curve shifted by 0.1 pm. Published filter
retention ratings of 0.1 pn and 0.2 pn do not correlate with these
hard particle retention results.
The dynamic flusliup of Filter A is recorded in Fig. 3. The
resistivity and TOC of 111 water downstream of a filter as it is initially flushed with clean DI water is measured with an Anatel
TOC analyzer. Resistivity peifonnance is good, and the TOC recovery is excellent.
Particle retention was tested by challenging filter rneinbranes
with a monolayer of latex b e d s of fixed size [41. These lxads were
stabilized with 0.1% Triton X-100. After the challenge, the filters
were flushed with surfactant, and the bead concentration in solution was determined by nephelometry (a method for comparing
tlie brightness of light passing through a solution or suspension
with light passing through a stanciard solution). Beads passing
the initial challenge and the flush are sunmed and used to calculate total passage and retention. The results of separate tests on a c h
fdter for each size of bead are presented in Fig. 4.Filters A and B
have better retention than Filter C.
Retention can also be characterized by alcohol porosinietry [51,
a test in which air flow through a dry membrane is compared to
air flow through an alcohol-wetted membrane over a range of pres
sures. The alcohol stays in the pores of the wet filter at low pres
sures due to capillary forces but is forced from the pores at higher
pressures. The results of such tests indicate the distribution o f
pore size in the filter. The bubble points of Table 2 correspond
well to the hard particle retention data of Fig. 4. A lower bubble
point corresponds to greater porosity and, hence, lower retention.
Membranes are difficult to wet spontaneously with 4Wi NH/,Fsince
it is a nearly saturated solution with a surface tension of 95 dynes/cm
[61; some membrane filters actually dewet in concentrated baths
of NH4F. Dewetting, which causes a loss in flow and an increase
in particle counts on wafers, can be e l h a t e d by using a BOE sohition with lower surface tension and higlier g~ssolubility,i.e., a sohtion with a lower concentration of amnioniiiin fluoride.
Wettability was measured by placing a drop of 40% NH4F on the
surface of the fdter and recording the time to wetting. Wettzability
was also measured for NaCl solutions of various concentrations
Solid State Technology June I993
(surfwe tensions). The Same inembranes used for downstream
cleanliness and flow tests were used for wettability tests. They
had been water wet and dried at 60°C.
A sumniary of filter wettability is given in Table 2. Filters A
and C were spontaneously wettable with 40??ammonium fluoride.
Filter 13 would not even wet with water. The flushing and drying
may have had a deleterious effea on the wettability of Filter 13.
Clean water was circulated through a new filter, and downstream
particles were measured using PMS optical particle counter models
M50 and HSLIS, which are capable of detecting particles greater
than 0.05 pn and 0.10 pm, respectively. Initially, a steady flow ot
12 l i t e r / n ~was run for 120 inin. Then the flow was pulsed niomentarily (no flow for 2 s) at 15-min intervals for the next 120 min.
Filter A reached background counts within 90 inin after staring tlie test. Pulsing had only a slight effect on patticle levels. Counts
returned to the background level of tlie optical particle counter
within the fourth pulse. The performance of Filter B was siniihr
to Fdter A. Filter C, on the other hand, was very dirty. I’article counts
never reached tlie Ixseline, and particle spikes were evident at each
pulse throughout the test. A second Filter C was tested with the
sanie results, but a third Filter c was slightly cleaner. “Particles”generated by Filter C were not easily removed by the 0.1-pii PTFE prefilter in the cleanliness system. The water in the tank had to be
changed before the particle counts would return to the baseline.
Table 4 suinnmrizes the data.
The REI3 application does not stress cartridge filters to their limits.
The typical single-stage centrifugal pump produces a pressure
differential of less than 20 psig across the fdter. Furthennore, aninioniuni fluoride and hydrofluoric acid are not aggressive toward most
pbstic nwcrials, and fdters must have only n ~ mphysical
to have acceptable performance.
One of each type of filter was subjected t o an integrity test that
finds defects that may exist in the filter cartridge structure. Since
filters were only exposed t o DI water at pressure differentialsof less
than 20 psig, failure would indicate serious strength issues.
Filters A and I3 were integral. Filter C failed integrity because o f
a defect Where the lneInbl-dne Was Sealed to the e I l d Cap.
Two Filter A cartridges were tested for graded failure [71.This test
exposes filters to increasing pressure differentials in the fonmrct
and reverse directions and t o pulsing conditions. The cartridge is
continidly monitored t o determine the point o f failure. The car-
Figure 5. Filter A aidwater diffusion curves. The data at 15, 20, and
25 psig is the average of fourteen cartridges. The remaining data is
for two cartridges.
tridges survived pressure pulsing at 100 psig in the forward direction and a reverse pressure of 75 psig before failing at a 110 psig
pressure differential in the forward direction. Such strength performance exceeds the needs of REBs.
Membrane strength was also evaluated with a Mullens burst
test [SI. In this case a rubber bladder expands under a piece of membrane that stretches until it ruptures. The bladder pressure at rupture
indicates the strength of the membrane. PTFE membranes generally reach a yield point before they rupture; then yield pressure is
recorded. As indicated in Table 2, the membrane in Filter C is
very weak. This is one of the reasons why the cartridge has poor
strength. Filter B is also relatively weak but probably strong enough
for the application.
It is worthwhile to conduct integrity tests before installing filters
in a REB El. An integrity test may indicate filter failure and the potential for particle passage. For REB applications, in which particles
that pass through a small filter defect will be removed in the next
tumover of the bath, integrity tests are not critical; here integrity
testing need only detect a major failure of the filter. While particle
monitoring of the test wafer surface can serve the same purpose,
integrity tests have the advantage of specifically identfying the Filter
as the source of particle problems.
Integrity testing involves wetting the Filter with water and subjecting it to upstream air pressure. The pressure is increased, and
the flow of air through the filter is measured. Excessive air flow,
beyond difFusive flow through the membrane, indicates a hole in
the filter device.
All filters were tested in a water-wet condition. After flushing and
drying, the Filter B membrane had a water bubble point of only
15 psig. This is lower than predicted by the mean IPA bubble point,
probably due to poor wetting of the filter. Filter C was not integral after flow testing, but the defect could be detected with an
integrity test.Figure 5 plots average values of diffusive air flow versus
air pressure for water-wetted Filter A cartridges.
Price and cost
The most obvious cost in the REB application is the purchase price
of the filter, but factors such as filter lifetime affect cost. Filter replacement has a labor cost associated with it. In REB applications,
filter lifetimes can vary from one week to more than one year.
Solid State Technology June 1993
To the IC manufacturer who is using filters, yield and throughput can be the real determinants of filter cost. A filter that has no
extractables and quickly removes particles from the bath has cost
benefits compared to a filter with poorer performance. A true
measure of cost would account for all economic and functional
aspects of the filter, but price is often substituted for cost because
it is easy to measure. In noncritical applications, price may be
near the top of the attribute list. In critical applications, where a
slight improvement in the REB perfonnance has potential for improving yield, price may move to the bottom of the attribute list.
Table 2 shows that Filters B and C have the same price, while
Filter A is substantially more expensive. There is insufficient
information to estimate total lifetime cost for the filters.
Knowledge of the intended application is required to determine the
important performance attributes for a filter (Table 5). For REB applications, Filter flow rate, extractables, and retention are among the
most important characteristics. Of the three filter types developed
for the REB application and discussed in the present work, no single
type was best in all categories. Thus, the filter user’s experience
is required to subjectively rank and weigh each performance
This paper was f m presented in May 1992 at the IESAnnual Meeting
in Nashville, TN.
1. W. Kern, C. Deckert. ChemicalEtcbing, Thin Film Processes, Chap. 6, pp. 401-
496, Acad. Press Inc., 1978.
2. T. Ohmi et al., “Optimization of the Wet Process by Controlling Composition,
Reaction Products, Crystal Deposition, and Wettability,” Nikkei Microdevice,
3. J. Zahka, D. Grant, C. Myhaver, ”Modeling of Particle Removal from a Circulating Etch Bath,” in Particles in Gases and Liquids 2, Detection, Characterization, andcontrol, K.L. Mittal, ed.,Plenum Press: New York, NY, p. 367 (1990).
4. MilliporeTest Method 0001143TM, “Membrane Characterizationby Polystyrene
(PSL) Latex Bead Challenge.”
5. T.D. Brock, “Membrane Filtration: A User’s Guide and Reference Manual,” p.
46, Science Tech Inc.
6. R. Matthews, J. Zahka, “Optimization of Recirculated Etch Baths,” presented
at SEMICON/Europa 91 Technical Conference, Zurich, Switzerland, March
7. Millipore Test Method 0001720TM, “Fluorogard Plus Hydraulic Stress Test,”
8. “Standard Test Method for Bursting of Paper,” ASTM Designation: D 774-67,
1986 Annual Book of ASTM Standards, Sect. 15, vol. 15.09, p. 151, ASTM.
continued on page 71
ZAHKAreceived his B.S. degree from
MIT and his M.S. degree from RPI in Chemical
Engineering in 1970and 1971,respectively. He
is a consulting engineer in the Microelectronics Applications Department of Millipore’s
Process Group. He has worked for Millipore
for thirteen years, applying products to the
microelectronics,pharmaceutical, and medical
received the B.S. degree in
Chemical Engineering from the University of
New Hampshire in 1982. He is now a senior
development engineer in the Microelectronics
Div. of Millipore Corp., responsible for new
product development in the liquid fdtrationarea.
Prior to joining Millipore in 1986, Carroll was
producVprocess engineer for Sprague Electric
Co., where he was responsible for the design
and manufacture of solid state devices.
received the M.S. degree
in in Analytical Chemistryfrom Indian Institute
of Technology, India, in 1972, and the Ph.D.
in Chemistry (spectroscopy) from the University of Toledo, OH, in 1983. He joined Millipore’s Analytical ChemistryDept. in 1984,specializing in trace contaminant characterization and analysis of organics and inorganics in
water and chemicals
received the associates
degree in Mechanical Engineering and the B.S.
degree in Petrochemical Engineering from
Louisiana Tech University in 1983. He is an
applications engineer in the Microelectronics
Applications Dept. at Millipore Corp., responsible for evaluating the performance of Millipore and competitive products used in the
June 1993 Wid State Technolcgy 71