1. The silicon substrate and adding to it—Part 2
Describe the processes of
Calculate
Oxidation, both
relative thicknesses of added oxide layers to original wafer thickness, and
dry oxidation and
wet oxidation
oxide thickness as a function of time and vice versa
Evaporation, both
Compare and contrast the advantages and disadvantages of evaporation versus sputtering
resistive reheating and
e-beam
Give the relative advantages and disadvantages of CVD compared to PVD
Sputtering,
DC,
RF,
reactive, and
magnetron
Chemical vapor deposition (CVD)
Electrodeposition
Spin casting
Wafer bonding

2. Adding layers to the silicon substrate
thin film
thin film
Bulk micromachining
Surface micromachining

3. Adding layers to the substrate
Many different methods
Evaporation
Epitaxy—growing an additional crystalline layer of Si on top of an existing wafer
Sputtering
Chemical vapor deposition (CVD)
Has same crystalline orientation of underlying Si (unless it is on top of an amorphous substrate, in which case it is polycrystalline)
Electrodeposition
Spin casting
Wafer bonding
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
(PVD)
Ask students for uses of epitaxy (ans: create p-n junctions, create piezoresistitity, etc.
Ask students to give uses of evaporation and sputtering. Why is it called PVD?
What is electrodeposition?
What substance that you have already seen is spin casting used for?
Why might you want to nond wafers to each other?

4. Called “oxide layer” or just “oxide” Uses? Thin layers < 100 nm
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
Use as a hard mask, electrical insulation (isolator), sacrificial layer, structural layer

5. Oxidation furnaces
A schematic diagram of a typical oxidation furnace
“Bubblers” (bubble = burbuja) are used for wet oxidation.

6. Oxidation can be dry or wet.
Wet oxidation vs. dry oxidation
Oxidation can be dry or wet.
Dry oxidation:
Si + O2 →
Wet oxidation:
Si + H2O →
Dry oxidation creates a very high quality (de calidad alta) oxide, but it takes a long time.
Wet oxidation creates a lower quality (de menos calidad) oxide, but it is fast.
Have students balance chemical reactions:
Si + O2  SiO2
Si +2H2O  SiO2 + 2H2
¿Cuál se usaría para estructuras? ¿para “sacrificial layer? ¿por qué?

7. xadd xox Oxidation Te toca a ti Respuesta:
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?
Respuesta:
0.43 μm thicker (total thickness = μm)
0.37 μm of wafer “used up”

8. How can you tell how thick your oxide layer is?  Look at the color!
Oxide thickness
How can you tell how thick your oxide layer is?
 Look at the color! (
(onlinelibrary.wiley.com) (

9. Depends on native oxide thickness
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 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?
Native oxide thickness is ~25 nm. Deal Grove does not work well for under 25 nm.
τ found by setting xox = xi with t=o and solving for τ.
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)

10. Short time approximation comes from a series expansion:
The Deal Grove model
Short time approximation comes from a series expansion:
short time approx.
Linear rate constant
long time approx.
¡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.
Go to BYU site and show the online calculator.
Deal Grove model for wet oxidation of (100) Si at 1000°C
Respuestas: 4.47 hr, 3.13 hr

11. Repeat the last problem for (111) Si. That is,
¡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.
Oxidation for (111) Si is faster:
Why?
Respuestas: 3.93 hr, 3.13 hr
¡Mucho tiempo significa muchísimo tiempo!

12. 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.
Spray
paint
Evaporation
PVD
shadow
thin film
Sputtering
target
Why is it called “physical”?
source
PVD is called a line-of-sight method.
 Shadowing

13. 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

14. Vacuum fundamentals
Vacuum means pressure less than atmospheric pressure.
Standard unit is a torr:
1 atm = ×105 Pa = 760 torr
Pressure ranges for various vacuum regions

15. Creating a vacuum Vacuum pumps A rotary vane pump
Pressure ranges for various vacuum regions
A rotary vane pump

16. High vacuum pumps Turbopump Diffusion pump Cryopump
In turbopumps a gas molecule will randomly enter the turbo pump and be trapped between a rotor and a stator. When the gas molecule eventually hits the spinning underside of the rotor, the rotor imparts momentum to the gas molecule, which then heads towards the exhaust.
Cryopump

17. Typical vacuum system setup in a PVD system
In what order would you operate the pumps and open and close valves to create a high vacuum in the vacuum chamber?
Close Hi-vac and foreline valves
Run the “rough pump” to lower chamer to low vacuum
Close rough valve
Open foreline valve
Open Hi-vac valve
Run Hi-vac pump
Typical vacuum system setup in a PVD system

18. Vacuum theory and relationships
The ideal gas equation
Boltzmann´s constant
kb = 1.381×10-23 J/K
Mean free path
σ is the interaction cross section.
~ probability of interaction between particles
dimensions of area

19. Take the interaction cross section to be σ = 0.43 nm2
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.
P = 1 atmosphere
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:
2.50×1019 molecules, 66 nm
3.29×109 molecules, 500 m
Now estimate how many molecules are in a thumbprint.

20. . Thermal evaporation substrate
Flux, F: (molecules leaving source)/(area×time)
Requirements for evaporated materials:
Pv must be > background vacuum pressure, ~ < torr < Pv < 1
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
to vacuum pump
source

21. Evaporation by resistive heating
Resistive heating vs. e-beam evaporation
evaporant
resistive heater
Evaporation by resistive heating
e-beam evaporation

22. Shadowing shadow thin film target source Spray paint
Why is it called “physical”?
target
source

23. Shadowing
Arrival rate A, (incident molecules)/(area×time)
Compare to view factors in radiation heat transfer (radiación térmica)

24. Shadowing 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.
(1)
(2)
(3)
Note that the angle w, surface 4 is negative & therefore the thickness is O. Also note that we have ignored the d’ term assuming that the deposition rate given in the problem would be for9 surface that looks like surface I in the first figure.
54.7°
(4)
source
30°
Respuestas: t1=1.56 μm, t2=1.64 m

25. 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

26. Sputtering substrate substrate Ar+ source source Evaporation

27. Source material must be conductive
DC sputtering
Source is not a “point” but a parallel plate.
Source material must be conductive
Typical DC sputtering configuration

28. Other sputtering techniques
RF (radio frequency)
Sputtering
Applies an AC voltage to target at frequencies > 50 Hz
Target does not need to be conductive
Chamber walls also sputtered
Reactive sputtering
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
Magnetron
sputtering
Addition of magnets behind target keep electrons from travelling too far
Increased ionization at cathode
Leads to higher yields

29. Magnetron sputtering
Magnetron principle

30. Comparison of evaporation and sputtering
Limited to lighter elements and simple compounds
Virtually anything can be sputtered
Low energy ions/atoms (~0.1 eV)
High energy ions/atoms (~1-10 eV)
High purity thin films
Gas atoms implanted in films  lower purity
Less dense films, large grain size, adhesion problems (problemas de pegar)
Dense films, smaller grain size, good adhesion
Requires a high-vacuum
Can use a low vacuum ~10-2 to 10-1 torr
Directional
Poor directionality
 can use for lift-off
 good step coverage
Components evaporate at different rates  composition of deposited film is different than source
Components deposited at similar rates

31. 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:
HSiCl3 → Si +
SiH4 → Si + 2H2
HSiCl3 → Si + HCl + Cl2
Basic chemical vapor deposition process

32. Chemical vapor deposition
Silicon dioxide (SiO2) thin films
Using silane:
SiH4 + O2 →
Using dichlorosilane and nitrous oxide:
SiCl2H2 + N2O → SiO2 
Silicon nitride (Si3N4) thin films
Using silane:
SiH4 + NH3 → Si3N4
Using dichlorosilane:
SiCl2H2 + NH3 → Si3N4 +
Uses?
Insulator
Structural layer
Chemical barrier
SiH4 + O2 → SiO2 + 2 H2
SiCl2H2 + 2 N2O → SiO2 + 2 N2 + 2 HCl
3 SiH4 + 4 NH3 → Si3N4 + 12 H2
3 SiCl2H2 + 4 NH3 → Si3N4 + 6 HCl + 6 H2

33. Comparison of PVD and CVD
Evaporation is limited to certain materials. Sputtering has yield problems.
Preferred method for
polysilicon layers and
silicon nitride
Generally no hazardous byproducts
Hazardous byproducts
Lower temperatures
Often requires high temperatures (~500°- 850°C)  Cannot deposit on top of many metal layers
Requires a high-vacuum
Requires a high-vacuum (LPCVD is most common)
Directional
Poor directionality
 can use for lift-off
 good step coverage

34. Electrodeposition (electroplating)
Other deposition methods
Electrodeposition (electroplating)
- + Cu
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
Metal e-
Cu2+
SO42-

35. 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.

36. Use of adhesives and solders Thermal bonding Anodic bonding
Wafer bonding
Most commonly used in packaging rather than in creating MEMS structures themselves.
Use of adhesives and solders
Thermal bonding
Anodic bonding
Thermally induced stress can be an issue, leading to fracture.
Generic anodic bonding setup