Oxidation

The silicon substrate and
adding to it—Part 2
 Describe the processes of
 Oxidation, both
 dry oxidation and
 wet oxidation
 Evaporation, both
 resistive reheating and
 e-beam
 Sputtering,
 DC,
 RF,
 reactive, and
 magnetron
 Chemical vapor deposition (CVD)
 Electrodeposition
 Spin casting
 Wafer bonding
 Calculate
 relative thicknesses of added oxide layers to original
wafer thickness, and
 oxide thickness as a function of time and vice versa
 Compare and contrast the advantages and disadvantages of
evaporation versus sputtering
 Give the relative advantages and disadvantages of CVD
compared to PVD
Adding layers to the silicon substrate
thin film
thin film
Bulk micromachining
Surface micromachining
Adding layers to the substrate
Many different methods
• Epitaxy—growing an additional
crystalline layer of Si on top of an
existing wafer
− Has same crystalline orientation of
underlying Si (unless it is on top of
an amorphous substrate, in which
case it is polycrystalline)
− Has different dopant type and
concentration
− Uses?
• Oxidation—chemical reaction of Si
with O2 to form layer of amorphous
silicon dioxide (SiO2)
Physical vapor deposition
• Evaporation
(PVD)
• Sputtering
• Chemical vapor deposition (CVD)
• Electrodeposition
• Spin casting
• Wafer bonding
Oxidation
Chemical reaction of Si with O2 to form layer of amorphous silicon dioxide (SiO2)
•
•
•
•
•
Called “oxide layer” or just “oxide”
Uses?
Thin layers < 100 nm
Thick layers 100 nm – 1.5 μm
Use of furnaces at high
temperatures, ~800°-1200°C
Oxidation furnaces
A schematic diagram of a typical oxidation furnace
“Bubblers” (bubble = burbuja) are
used for wet oxidation.
Wet oxidation vs. dry oxidation
Oxidation can be dry or wet.
Dry oxidation:
Si + O2 →
Dry oxidation creates a very high quality (de
calidad alta) oxide, but it takes a long time.
Wet oxidation:
Si + H2O →
Wet oxidation creates a lower quality (de menos
calidad) oxide, but it is fast.
¿Cuál se usaría para estructuras? ¿para “sacrificial layer? ¿por qué?
Oxidation
xadd
xox
Te toca a ti
A 150-mm (6 inch) diameter silicon wafer requires a
0.8-μm thick layer of oxide as a sacrificial layer. If
the wafer is originally 650 mm thick, how much
thicker is the wafer after oxidation? How much of
the wafer has been “used up” (se ha sido gastado) to
create the oxide later?
xadd
 0.54
xox
Respuesta:
• 0.43 μm thicker (total thickness
= 650.43 μm)
• 0.37 μm of wafer “used up”
Oxide thickness
How can you tell how thick your oxide layer is?
(www.filmetrics.com)
 Look at the color!
(onlinelibrary.wiley.com)
(www.cleanroom.byu.edu)
Oxidation kinetics
The Deal-Grove model of oxidation kinetics is the most widely used model to
predict oxide thickness as a function of time.

A 
4B
xox   1 
(t   )  1
2
2 
A

A and B depend on
• Temperature
• Wafer type; i.e., (100) or (111)
Depends on native oxide thickness
What do you think the model is based on?
Te toca a ti
• Sketch (don’t plot) the general shape of the oxide thickness as a
function of time. Why does it look this way?
• Approximate what the function is for very long times. (Es decir,
t >> τ)
xox  B(t   )
The Deal Grove model
Short time approximation comes from
a series expansion:
short time approx.
B
xox  (t   )
A
Linear rate constant
long time approx.
Deal Grove model for wet oxidation of (100) Si at 1000°C
¡Más te toca a ti!
• Approximate how long it takes to
grow 1 μm of oxide at 1000°C for
(100) silicon using wet oxidation.
• Compare your result to the long time
approximation.
Respuestas: 4.47 hr, 3.13 hr
¡Aun más te toca a ti!
Repeat the last problem for (111) Si. That is,
• Approximate how long it takes to grow 1 μm of
oxide at 1000°C for (111) silicon using wet
oxidation.
• Compare your result to the long time
approximation.
Respuestas: 3.93 hr, 3.13 hr
Oxidation for (111) Si is faster:
( B / A) (111)
( B / A) (100 )
Why?
 1.68
¡Mucho tiempo significa muchísimo tiempo!
Physical vapor deposition
Physical vapor deposition (PVD) − a purified, solid material is vaporized and then
condensed onto a substrate in order to form a thin film.
Evaporation
PVD
shadow
Sputtering
thin film
target
source
PVD is called a line-of-sight method.  Shadowing
Vacuums
PVD requires the use of a vacuum.
Write down some reasons why you think a vacuum is necessary for PVD.
• Vaporized atoms do not run into other gas atoms
• Need a vacuum to create a vapor out of the source material
• Vacuum helps keep contaminants from being deposited on the
substrate
Vacuum fundamentals
Vacuum means pressure less than atmospheric pressure.
Standard unit is a torr:
1 atm = 1.01325×105 Pa = 760 torr
Pressure ranges for various vacuum regions
Creating a vacuum
Vacuum pumps
Pressure ranges for various vacuum regions
A rotary vane pump
High vacuum pumps
Turbopump
Diffusion pump
Cryopump
Vacuum systems
In what order would you operate
the pumps and open and close
valves to create a high vacuum in
the vacuum chamber?
Typical vacuum system setup in a PVD system
1.
Close Hi-vac and foreline
valves
2.
Run the “rough pump” to
lower chamer to low
vacuum
3.
Close rough valve
4.
Open foreline valve
5.
Open Hi-vac valve
6.
Run Hi-vac pump
Vacuum theory and relationships
The ideal gas equation
PV  Nk b T
Boltzmann´s constant
Mean free path
kb = 1.381×10-23 J/K
V
kbT


2 N
2P
σ is the interaction cross section.
• ~ probability of interaction between particles
• dimensions of area
Te toca a ti
Estimate the number of molecules of air in a 1 cm3 volume at room temperature
and the two pressures given. Also calculate the mean free path.
a. P = 1 atmosphere
b. P = 1×10-7 torr.
Take the interaction cross section to be σ = 0.43 nm2
Useful information:
− kb = 1.381×10-23 J/K
− 1 atmosphere = 760 torr
− torr = 133 Pa
Respuestas:
a. 2.50×1019 molecules, 66 nm
b. 3.29×109 molecules, 500 m
Now estimate how many molecules are in a thumbprint.
. Thermal evaporation
substrate
Flux, F: (molecules leaving source)/(area×time)
F
Pv (T )
2Mkb T
Requirements for evaporated materials:
• Pv must be > background vacuum pressure, ~ <
10-2 torr < Pv < 1
to vacuum
pump
source
• Elements or simple oxides of elements
• 600°C < T < 1200°C
• Examples Al, Cu, Ni, ZiO
• No heavy metals; e.g. Pt, Mo, Ta, and W
Resistive heating vs. e-beam evaporation
evaporant
resistive heater
Evaporation by resistive heating
e-beam evaporation
Shadowing
shadow
thin film
target
source
Shadowing
Arrival rate A, (incident molecules)/(area×time)
cos  cos
A
F
2
d
Compare to view factors in radiation heat transfer
(radiación térmica)
1
cos 1 cos  2
F 12  
dA2 dA1
2
A1 A1 A2
d
Shadowing
cos  cos
A
F
2
d
t1 cos 1

t 2 cos  2
Step coverage
Te toca a ti
Aluminum is evaporated onto a silicon substrate at a
rate of 0.5 nm/s according to the evaporator. For the
geometry shown in the figure, estimate the thickness of
aluminum on surfaces (1), (2), (3) , and (4) after one
hour.
Respuestas: t1=1.56 μm, t2=1.64 m
(1)
(2)
54.7°
source
(3)
(4)
30°
Shadowing
How do you think you might reduce shadowing and therefore increase step coverage?
• Rotate the wafers as the deposition is taking place  planetary
wafer rotators
• Heat the wafers to allow the deposited material to flow
• Or don’t! Sometimes you can use shadowing to make structures
you want 
Lift-off
Sputtering
substrate
substrate
Ar+
source
Evaporation
Sputtering
source
DC sputtering
Source is not a “point” but a parallel
plate.
Source material must be conductive
Typical DC sputtering configuration
Other sputtering techniques
RF (radio frequency)
Sputtering
Reactive sputtering
Magnetron
sputtering
• Applies an AC voltage to
target at frequencies > 50
Hz
• Target does not need to
be conductive
• Chamber walls also
sputtered
• Reactive gas (such as O2)
added to chamber
• Reacts with target,
products forming the
deposited materials
• Products can be
deposited on surfaces
other than the substrate
• Reduction in sputtering
rates typically seen
• Addition of magnets
behind target keep
electrons from travelling
too far
• Increased ionization at
cathode
• Leads to higher yields
Magnetron sputtering

  
F  q ( E  v  B)
Magnetron principle
Comparison of evaporation and sputtering
Evaporation
• Limited to lighter elements and simple
compounds
• Low energy ions/atoms (~0.1 eV)
• High purity thin films
• Less dense films, large grain size,
adhesion problems (problemas de pegar)
• Requires a high-vacuum
• Directional
 can use for lift-off
• Components evaporate at different rates
 composition of deposited film is
different than source
Sputtering
• Virtually anything can be sputtered
• High energy ions/atoms (~1-10 eV)
• Gas atoms implanted in films  lower
purity
• Dense films, smaller grain size, good
adhesion
• Can use a low vacuum ~10-2 to 10-1 torr
• Poor directionality
 good step coverage
• Components deposited at similar rates
Chemical vapor deposition
Chemical Vapor Deposition (CVD)
Common way to deposit
polycrystalline silicon thin films (often
called simply “poly”
Using silane:
SiH4 → Si +
Using trichlorosilane:
Basic chemical vapor deposition process
HSiCl3 → Si +
Chemical vapor deposition
Silicon dioxide (SiO2) thin films
Using silane:
SiH4 + O2 →
Using dichlorosilane and nitrous oxide:
SiCl2H2 + N2O → SiO2
Uses?
Silicon nitride (Si3N4) thin films
• Insulator
• Structural layer
• Chemical barrier
Using silane:
SiH4 +
NH3 → Si3N4
Using dichlorosilane:
SiCl2H2 + NH3 → Si3N4 +
Comparison of PVD and CVD
PVD
• Evaporation is limited to certain materials.
Sputtering has yield problems.
• Generally no hazardous byproducts
• Lower temperatures
• Requires a high-vacuum
• Directional
 can use for lift-off
CVD
• Preferred method for
• polysilicon layers and
• silicon nitride
• Hazardous byproducts
• Often requires high temperatures (~500°850°C)  Cannot deposit on top of many
metal layers
• Requires a high-vacuum (LPCVD is most
common)
• Poor directionality
 good step coverage
Other deposition methods
Electrodeposition (electroplating)
-
+
Cu
Metal
e-
SO42-
Cu2+
• Often used to deposit metals and magnetic materials
• Inexpensive and easy (barato y fácil)
• Surface quality usually worse than PVD (higher
roughness)
• Uniformity can be an issue
Spin casting
Material is
• dissolved in solution,
• poured onto wafer, and
• the wafer is spun to distribute the solution
across surface
• Wafer is then baked to remove the solvent,
leaving behind the thin film.
Also called simply “spinning”
Used for polymers, piezoelectric materials, and
is the standard method of applying
photoresist.
Wafer bonding
Most commonly used in packaging rather than in creating MEMS structures themselves.
• Use of adhesives and solders
• Thermal bonding
• Anodic bonding
Generic anodic bonding setup
Thermally induced stress can be an
issue, leading to fracture.