1. Thin Film Deposition
Quality – composition, defect density, mechanical and electrical properties
Uniformity – affect performance (mechanical , electrical)
Thinning leads to  R
Voids: Trap chemicals lead to cracks (dielectric) large contact resistance and sheet resistance (metallization)
AR (aspect ratio) = h/w  with  feature size in ICs.
Plummer et al.

2. Chemical Vapor Deposition
Methods of Deposition:
Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD: evaporation, sputtering)
Cold wall reactor
Atmospheric Pressure : APCVD
Cold wall reactors (walls not heated only the susceptor)
Flat on the susceptor
Low pressure: LPCVD – batch processing.
Hot wall reactor
Plummer et al.

3. Plasma Enhanced Chemical Vapor Deposition(PECVD)
Used when :
Low T required (dielectrics on Al, metals) but CVD at decreased T gives increased porosity, poor step coverage.
Good quality films – energy supplied by plasma increases film density, composition, step coverage for metal decreases but WATCH for damage and by product incorporation.
13.56 MHz
P  50 mtorr - 5 torr
Plasma: ionized excited molecules, neutrals, fragments, ex. free radicals  very reactive the Si surface enhanced  increase deposition rates
Ions, electrons, neutrals = bombardment
Outgassing , peeling , cracking stress. °C
Plummer et al.

4. Physical Vapor Deposition (PVD) – no chemical reactions
(except for reactive sputtering)
Evaporation
Advantages:
Little damage
Pure layers (high vacuum)
Disadvantages:
Not for low vapor pressure metals
No in-situ cleaning
Poor step coverage
Very low pressure (P < 10 –5 torr) - long mean free path.
purer – no filaments, only surface of the source melted
X-rays generated  trapped charges in the gate oxides  anneal it !
Plummer et al.

5. Evaporation Step Coverage Poor :
Partial Pressure of the source (target)
Needed for reasonable v  m/min
No alloys – partial pressure differences
Use separate sources and e-beam  
Step Coverage Poor :
Long mean free path (arrival angle not wide = small scattering) and low T (low energy of ad-atoms)
Sticking coefficient high T) no desorption and readsorption  poor step coverage
Heating can increase Sc but may change film properties (composition, structure)
Sticking coefficient
Plummer et al.

6. Higher pressures 1 –100 mtorr ( < 10-5 torr in evaporation)
Sputter Deposition
Higher pressures 1 –100 mtorr ( < 10-5 torr in evaporation)
Major Technique in Microelectronics for:
DC Sputtering (for metal)
Alloys (TiW, TiN etc)
good step coverage
controlled properties
Conductive
Al, W, Ti
Ar inert gas at low pressure.
No free radicals formed by Ar (ex. O, H ,F as was for PECVD)
Plummer et al.

7. - + RF Sputter Deposition Dielectric
13.6 MHz RF coupled capacitively to plasma
DC sputter cannot be used for dielectrics
secondary e-
plasma extinguished (VZ )
More e-
on the walls -
charge built-up
potential VP
potential
@ the target ( area)
e- charge
faster, smaller
several 100V
tenths of volts A1 A2
wafers
= NON-CONDUCTING A2
Oscillating (with RF) e-
ionization yield
pressure
can be used
large A1 area +
magnet e- trajectory Magnetron Sputter Deposition have better ionization yields
deposition rates (10-100X)
better film quality (Ar needed)
use in DC & RF ( heating of the target since I )
Plummer et al.

8. Polysilicon - Very Widely Used in MEMS
Low T gives more amorphous layers
As, P the grain boundaries
r ( B does not ! )
columnar structure
625°C
As & P deposition rate of poly – Si use doping after poly deposition
B Vpoly - Si
Plummer et al.

9. Various Aspects of Deposition Processes
C. Liu

10. Etching
DRY ETCH
ANISOTROPIC
Plummer et al.

11. Etching Profiles PR Mask Lateral etching chemical good selectivity
poor selectivity
Rounded & sloped PR
Required for scaled down devices
Plummer et al.

12. Wet Etching – Isotropic Etch
Plummer et al.

13. Plasma Etching
Replace wet processes in VLSI – directional etching, faster, (less) selective but does not degrade PR adhesion as some wet steps do.
MEMS use plasma etching widely (deep etch, highly anisotropic)
Parallel plate system
Reactive chemical components
Ionic components !
Low pressure
1mtorr-1torr
As in CVD & or sputtering (here RF electrode was much smaller and neutral gas Ar)
Plummer et al.

14. + O2 F recombination with CF3 CF4 F etch rates
Physical Etching
Ion bombardment
Degrades selection
= sputter etch
Chemical Etching
Isotropic arrival angle
volatile
Cl+
Low sticking coefficient
ISOTROPIC ETCH
+ O F recombination with CF CF F etch rates
@ small amounts of O2 but
@ large O2 etch rates decreases+oxidation of Si takes place
Free radicals : S
But in practice S is low
( F - Si)
Plummer et al.

15. Ion-Enhanced Etching No plasma Sputtering
Chemical component selectivity
Physical component anisotropy
Etching
Enhancement by ions
Role of ions:
Adsorption, Reaction, Formation of byproducts, and their removal
Polymer formation on all walls but removed at the bottom by bombardment
volatility of byproducts
Plummer et al.

16. Anisotropic Etch Fast formation of the polymer Slow polymer formation
May contain byproducts of etching, various layers including resist

17. MEMS call for optimization of cross-reactivity of various materials (layers) and processes

18. Silicon-Based MEMS Processes
Bulk micromachining (historically the first): silicon substrate is the main active part of the MEMS structures
Etch silicon
Wafer bonding
Oxide growth
or nitride deposition
(if needed)
Strip PR
Wafer thinning
by Chemical Mechanical Polishing to leave a thin membrane
Make piezoresistors (deposition, patterning, doping) to measure stress (use Wheastone bridge etc.)
Expose PR
Develop PR
Si etched
Oxide etch Or nitride if used as a mask for Si etching
C. Liu

19. Bulk Micromachining
Fabrication of pressure sensors seen in cross-sections
Membrane made of poly-Si, Si-nitride, or of oxide but also from polymers
C. Liu

20. Surface Micromachining
Historically - the later process. Relies on the sacrificial layers deposited and etched selectively
etching
C. Liu

21. LIGA process
Three dimensional metallic and polymer structures 500µm deep (up to 6cm?!) require deep etching, molding, plating etc.
LIGA=X-ray Lithography, electroplating (galvo) and injection molding (abformung) and damascene processes are widely used. Now UV-LIGA is used more frequently.
500-60,000µm
LIGA integration with CMOS via: Post-processing approach
Preprocessing approach
Side-by-side processing
C. Liu

22. New Materials and Fabrication Processes
Materials: Silicon was the main material but others are also widely used
Polymers as active structures: optical transparency, biocompatibility
Polymers as protection and sealing layers
High T and corrosive operation conditions (silicon carbide, diamond, nitrides …)
Other semiconductors (optical operation)
Processes: traditional IC fabrication and other complementary/new processes (for nanoscale dimensions) and/or complementary materials
Self assembly
New lithography processes (molding, imprints …)