This document discusses various thin film processing techniques used in microsystems technologies. It covers topics such as microfabrication techniques including photolithography, etching, and bonding. It also discusses additive thin film deposition methods like oxidation, chemical vapor deposition, physical vapor deposition, sputtering and evaporation. Subtractive thin film processes like wet and dry etching of materials are also covered. Sacrificial layer processes, electrodeposition, electroforming and other inorganic material processes are summarized as well.

1. Microsystems Technologies
Selected traditional micromachining
photolithography and mask design, wet and dry bulk etching,
bonding, thin film deposition and removal, metallization,
sacrificial processes, other inorganic processes
Polymer techniques
thick-film polymers, stamping, soft lithography and
micromolding, stereolithography, LIGA

2. Thin-Film Processing: Introduction
Subtractive processes
Wet etching
insulators
metals
insulators
metals
Additive processes
Oxidation of silicon
Chemical vapor depostion (CVD)
LPCVD/PECVD
oxide, nitride, polysilicon
epitaxy
Physical vapour deposition
sputtering
evaporation
Physical deposition:
spin-on-glass
other spin-on
Combined processes
Sacrificial processes
Example uses of thin film
processes in bio-microsystems
Masking
Channel fabrication and coating
Metal electrodes, routing
Active films for actuation (valves,pumps)
Sealing
Doping
ion implantation
diffusion
We will only look at a few most
relevant ones.
Dry etching

3. Thin-Film Processing:Additive Processes
Oxidation of Silicon
“Bare” silicon “always” has ~15-20 Å of “native” SiO2
Two types: wet and dry oxidation
Dryoxidation Wetoxidation
Si + O2 SiO2 Si + 2H2O SiO2 + 2H2
Wet oxidation is faster, but dry oxidation gives a more dense film
Addition of boron or phosphorous:
borosilicate glass (BSG)
phosphosilicate glass (PSG)
Uses: etch mask
Both processes
consume silicon!
surface passivation
insulation and isolation (e.g., under electrodes)
microelectronics uses (e.g., gate oxide)

4. Thin-Film Processing: Additive Processes
Oxidation growth over time
Dry oxidation Wet oxidation
Provided for your reference only, although note practical times,
thicknesses, and use of high temperatures!

5. Thin-Film Processing:Additive Processes
Chemical vapor deposition: CVD
Formation of film on substrate by reaction of vapor
phase chemicals;the higher the temp ೦the
higher the deposition rate
Typically deposited materials:
silicon dioxide (including low-temp, or LTO), doped
for PSG, BSG, and PBSG
silicon nitride
poly and amorphous silicon
silicon carbide
metal

6. Thin-Film Processing:Additive Processes
CVD steps
1.
2.
3.
4.
5.
Gases are introduced into reaction chamber
Gas species are moved to substrate
Reactants are adsorbed onto the surface
Film-forming chemical reactions
Desorption and removal of by-products

7. Thin-Film Processing: CVD
CVD: film quality issues
1. Stoichiometery: exact composition of the film
2. Purity of film and contamination,
3. particulatesUniformity and thickness
4. Conformality and step coverage
5. Pin holes and cracks
6. Adhesion of film
7. Stress in film
8. Density

8. Thin-Film Processing:Additive Processes
APCVD (AP for atmospheric pressure)
Poor step coverage
Used for low-temperature oxide (LTO)
LPCVD (LP for low pressure)
0.1 to 1 Torr pressure
Medium temperatures >500 ◌C
Good quality films
Good step coverage
PECVD (PE for plasma enhanced)
Low temperature operation
Fast deposition
Less dense films, contamination
Good step coverage

9. Thin-Film Processing:Additive Processes
CVD Chemistry
Silicon dioxide
(tetraorthosilicate, or TEOS)
APCVD is typically for LTO, where T <=500C
LPCVD: typical deposition rate: 0.01 μm/min
typically performed at ~450 C, except TEOS (~700 C)
PECVD:
typical deposition rate: 0.03 μm/min
typically performed at ~200 C
In-situ doping:
add phosphine PH3 for PSG, diborane B2H6 for BSG

10. Thin-Film Processing: CVD
Silicon dioxide comparison chart

11. Processing:Additive Processes
CVD Chemistry
Silicon nitride
3 +4 6HCl + 6H2
Si3N4 stoichiometric;
in practice, Si/N ratio
varies from 0.75 (N
rich) to 1.2 (Si rich).

12. Thin-Film Processing: Additive Processes
SiH4
Si + 2H2
LPCVD:
~600-700 C (~0.2-1Torr) 20-100% silane (SiH4)
~growth rate several to tens of nm/min
grain size dependent on growth temperature
in situ doping: B2H6, B/Si ~2.5x10-3; dep. rate arsine
AsH3, PH3; dep. rate
Polysilicon

13. Thin-Film Processing:Additive Processes
Polysilicon continued
Annealing stress out
•
•
•
Anneal at 900-1150 C for a few hours
Or use rapid thermal anneal
Use doped glass either side never one side only (or nonuniform
stress – doped glass used for fast sacrificial process)
PECVD
lower temperature (Al compatible)
amorphous or very small grains
laser annealing to increase grain size

14. Thin-Film Processing: Additive Processes
Other films deposited by CVD
Silicon carbide (SiC) PECVD
SiH + CH4 ೦SiC + 4H2
Polycrystalline diamond
Parylene polymer PECVD
Metals; e.g., tungsten
-2WF6 + 3SiH4 ೦2W + 3SiF4 + 6H2
Silicon epitaxy (single crystal silicon)

15. Thin-Film Processing:Additive Processes
Evaporation
Main types: thermal, ebeam, RF
Material heated to gaseous state
High vacuum conditions
Highly directional:
anisotropic arrival
geometric shadowing
source
Thermal evaporation
Uses W, Ta, or Mb filaments to heat source, Tmelt<1800 ◌C
Typical
Typical
Au,
Sb,
dep.rate:1 – 20 Å/sec
materials:
Ag, Al, Sn, Cr,
Ge, In, Mg, Ga (mean free path)
CdS, PbS, NaCl
KCl, CdSe, AgCl (pressure)

16. Thin-Film Processing:Additive Processes
Ebeam evaporation
Uses a stream of high-energy electrons (5-30 keV) to evaporate source
material from crucible
Can evaporate nearly any material
Typical deposition rate: 10 – 100 Å/sec
Typical materials (in addition those thermally evaporated):
Ni, Pt, Ir, Rh, Ti, V, Zr, W, Ta, Mo,Al2O2, SiO2, SnO2, TiO2, ZrO2

17. Thin-Film Processing:Additive Processes
Sputtering
High energy plasma particles dislodge
atoms from source
Purely physical process
Low-medium vacuum (~10 mTorr)
+ Easy to deposit alloys
+ Step coverage good
(argon gas
at 10mT)
-
-
Problem with stoichiometry, impurities
Need large target for uniform
thickness over large substrates
Deposition rates vary significantly
Some materials degrade with ion
bombardment (e.g., organics)
-
-
Examples: TiNi, SiO2, Ti, Pt, Ag,
W, Si, teflon, almost
anything!

18. Thin-Film Processing:Additive Processes
Comparision of step coverages
(electron-cyclotron resonance)
(although lose some substrate)
(high energy sputtering)

19. Thin-Film Processing: Subtractive Properties

20. Thin-Film Processing: Subtractive Properties
Thin film etching: Kirt Williams’
Wet and dry etchants
Includes silicon,
insulators, and
common metals
Details of mask selectivity
(UC Berkeley) table
(Both table and paper
posted on Web CT)

21. Thin-Film Processing: Subtractive Properties
Thin film wet etching examples
Silicon dioxide
Hydrofluoric acid (HF) based
Si02 + 6HF H2SiF6 +
2H20 "Concentrated" HF is usually
49%
Buffered oxide etch (B0E) often used for constant etch rate
(contains small amount of NH3F to control pH)
Selective to silicon, but will fluorinate silicon surface
Dopants affect etch rate (e.g., PSG etches faster)
Etches Al; using very high (>73%) conc. HF will minimize
Silicon nitride
Phosphoric acid (H3P04), usually "hot" (almost boilingS)
Selectivity with Si02 is 40:1
HF can also be used, but it is very slow

22. Thin-Film Processing: Subtractive Properties
More thin film wet etching examples
Polysilicon
Same as for single crystal silicon already discussed
Generally etches faster (increased etching at grain boundaries)
Aluminum
Strong
2Al
2Al
acids or bases, e.g.
+ 6Na0H 2Na3Al03 + 3H2
+ 6HCl 2AlCl3 + 3H2
Very common etch is acetic acid: nitric acid: phosphoric acid
(in volume percentage 20:3:77) at elevated temp. (>40C)
Gold
Aqueous KI3 at 20 – 50C
Aqua regia (nitric acid: HCl in
Platinum
Aqua regia (nitric acid: HCl in
1:3 ratio)
1:3 ratio)

23. Thin-Film Processing: Subtractive Properties
Thin film dry etching examples:
lots of different chemistries
just realize that all will be done
Silicon dioxide
CF4 (Freon 14) + 10% 02, CHF3 (Freon
C2F6 (Freon 116), or C3F8 (Freon 118)
Silicon nitride
using RIE
23),
CF4 (Freon 14) + 4% 02, CHF3 (Freon
C2F6 (Freon 116), or SF6 + He
Silicon carbide
SF6, CF4, or NF3
Polysilicon
Single crystal silicon etchants
23),
Doping can affect etch rate (e.g., p-polysilicon
faster in chlorine plasmas)
etches

24. Sacrificial Processes
Simple single level polysilicon example
“structural” material
“sacrificial” material
“release” etch
Polysilicon typically 1 - 3 μm, although thicker has been tried
Sacrificial layer typically 0.5 - 2 μm
Using PSG enables equal doping from both sides, minimizes
stress gradients and bending in structural polysilicon

25. Sacrificial Processes
Multiple level polysilicon example: hinges

26. Sacrificial Processes
Example material combinations
(for sacrificial)
opposing
forces
structure (Potential problem: stiction)
Surface tension forces pull structure liquid
to surface during drying forces
surface
tension
Structures are welded to
including capillary
substrate by a
condensation,
number of forces,
electrostatic forces,
van der Walls, hydrogen bonding, etc.
substrate

27. (Sacrificial Processes)
Solutions to stiction problem
Physical structures (e.g., dimples)
that reduce droplet area
Avoid liquid vapor meniscus
supercritical drying (C02)
vapor phase sacrificial etch
(e.g., vapor HF, 02)
HF water methanol liquid C02
C02 goes liquid to supercritical to gas
hydrophilic θ<90◌
Alter surface wettability hydrophobic θ>90◌
hydrophobic teflon-like films
hydrophobic self-assembled
monolayers

28. 0ther Processes for Inorganic Materials
Porous silicon
Photopatternable glass (Foturan™)
Physical micromachining:
Precision mechanical micromachining
Abrasive powder blasting
*Ultrasonic micromachining
Laser micromachining (additive and subtractive methods)
Focused ion beam (FIB) etching
Micro electro discharge machining (micro EDM)
*Planarization (Chem-mechanical-polishing, or CMP)
Electrochemical based processes:
Electrodepositing
*metals
other materials (including photoresist)
*Electroforming
*LIGA(convential x-ray/PMMAbased)

29. 0ther Inorganics Processing
Ultrasonic micromachining
Ultrasonic vibrations are delivered to a tool that machines the
substrate; combined with an abrasive slurry
Vibrations usually actuated piezoelectrically
Machined area becomes counterpart of tool
piezoceramic
Example materials: silicon, glass, alumina, ceramics
Example application: drilling through-holes in glass
glass
vibration

30. 0ther Inorganics Processing
Planarization (CMP)
Used to obtain planar surfaces
Necessary to avoid large step heights with a number of layers, or if
subsequent processing otherwise requires flat surface
Downward force plus chemical slurry
Example materials: metals, silicon, dielectrics
(including some polymers)
Mostly used for multi-layer sacrificial processes

31. Electrodeposition
Basics
Electrochemical method of coating substrate with metal (can
also electrodeposit other materials, e.g. photoresist) Substrate is
metallized and/or masked to define regions for electroplating
and held at negative potential (cathode)
Counter electrode in solution is held at positive potential (anode)
Electrolyte solution contains a reducible form of ion for metal Requires
inexpensive, readily available equipment
Example metals: Cr, Fe, Co, Ni, Cu, Zn, Ru, Rh, Pd, Ag, Cd, In, Sn,
Ir, Pt, Au, Pb; hundreds of alloys (e.g., NiFe "permalloy")

32. (Electrodeposition)
Tips
Typically need a few to a few hundred mA/cm2
Aseed layer may be required for conduction or adhesion
Clean substrates, solutions, containers, etc. as even low
impurity levels will result in poor films
Good mask (resist) adhesion is required
Current density must be uniformly distributed
Influencing factors
Current density
Nature of anions or cations in the solution
Bath composition and temperature
Solution concentration
Power supply current waveform (e.g., DC, pulsed)
Presence of impurities
Physical and chemical nature of electroplating surface

33. Example materials (and recipes)
Copper 10 mA/cm2
CuS04
 Cu2+ S04
+ 2-
Cu2+ + 2e- Cu
recipe: copper sulfate (CuS04•5H20) 120g/l
sulfuricacid(H2S04) 100g/l
100 mA/cm2
AuCN + CN-Gold
- (fast equilibrium reaction)
Au(CN)2
AuCN + e- Au + CN- (slower charge
cyanide (KAu(CN)2)
transfer)
20g/l
150g/l
40g/l
recipe: potassium gold
potassium citrate (K3C6H507•H20)
potassium phosphate (HK204P)
Nickel 100 mA/cm2
NiS04
 Ni2+ S04
+ 2-
Ni2+ + 2e- Ni
recipe: nickel sulfate (NiS04•6H20)
nickel chloride (NiCl2•6H20)
boric acid (B(0H)3)
330g/l
45g/l
38g/l
Electrodeposition

34. Electrodeposition
Electroless plating
No electrical contacts needed
Reducing agent (part that gives up electron) is chemical
Common reducing agents:
alkaline borohydrides
alkaline diboranes
formaldehyde
hypophosphorous acid
Example: Au
potassium hydroxide (K0H)
pottasium cyanide (KCN)
potassium borohydride (KBH4)
potassium gold cyanide KAu(CN)2
11.2g/l
13g/l
26.1g/l
5.8g/l
Can plate insulators
Uniform coverage, especially on parts with multiple faces

35. Electroforming
Process in which templates are formed in metal (or another
material) and plating is used to replicate them
Template is either peeled away or sacrificially etched
Patterning Blanket
electroplate
Pattern
metal #2
needed
Remove
of mold as metal #1
metal #1 (mold) metal #2
Process utilizing electroforming,planarization,
and sacrificial etching is (still) available commercially:
http://www.microfabrica.com/ (really cool; look up website!)

36. Thank You
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