1. 7. Surface Micromachining Fall 2013 Prof. Marc Madou MSTB 120

2. Surface Micromachining
Basic Process Sequence (poly-Si) n
Blanket n+ diffusion of Si substrate (ground plane)
Passivation layer (e.g. SiO2 , Si3N4 , LPCVD Si3N4 on top of SiO2)
Opening up the passivation layer for contacts (observe color change or hydrophobic/hydrophilic behavior):
wet (BHF)
dry (SF6)
Strip resist in piranha (adds some oxide in the window)
Short BHF etch to remove thin oxide n+ n
tSiO2 Lm
Lm + 2tSiO2
Photoresist
Si3N4

3. Surface Micromachining
PSG
Deposition of a base, spacer or sacrificial layer-phosphosilicate glass (PSG)-CVD
Densification at 950 °C for min in wet oxygen
Base window etching in BHF for anchors
Structural material deposition e.g. poly-Si (doped or undoped) from (CVD at about 600°C , 73 Pa and 125 sccm (standard centimeter cube per minute); at about 100Å/min) e.g.
Anneal of the poly-Si at 1050°C for 1 hour to reduce stress in the structure
nitride
Basic Process Sequence (poly-Si).
Structural layer

4. Basic Process Sequence (poly-Si)
Surface Micromachining
Doping of poly-Si: in-situ, PSG sandwich and ion implantation
Patterning of structural material e.g. RIE in , say, CF4-O2
Release step, selective etching of spacer layer e.g. in diluted HF
nitride Ri Rm RS
Basic Process Sequence (poly-Si)

5. Generic principle of surface micromachiningAl Si
Sacrificial layer definition
Etch access
Polyimide diaphragm
deposition Si
Releasing diaphragm:
phosphoric/acetic acid/nitric acid (PAN) Si
Generic principle of surface micromachining

6. Surface Micromachining
LPCVD of poly-Si
Hot wall, horizontal reactor
Reaction rate controlled--at lower pressures and well controlled temperatures (100 to 200 wafers)
Poly-Si deposits everywhere requiring periodic cleaning (e.g. every 20 runs if each run deposits 0.5 µm)
Visit: and

7. Surface Micromachining
Stiction
Stiction during release:
Surface tension during drying pulls movable members together (See also room temperature bonding of Si to Si and glass to Si)
Solutions:
Stand-off bumps
Sacrificial polymer
Sacrificial poly-Si links to stiffen the structures
HF vapor
Freeze-drying water/methanol mixtures
Super critical cleaning
Stiction after release:
Hydrophobic monolayers
Rough surfaces
Bumps

8. Surface Micromachining
Control of film stress
With L=150 µm and W=t=2 µm, fo=10 to 100 kHz.
Annealing at high temperature ( °C)
Fine-grained tensile vs large grained compressive
Doping elements
Sandwich doping and annealing.
Vary material composition e.g Si rich Si3N4
In PECVD: change the RF power and frequency
In sputtering: gas pressure and substrate bias

9. Surface MicromachiningY
Control of film stress
Folding flexures makes the resonant frequency independent of the residual stress but warpage becomes an issue
Corrugated structural members (see above) X

10. Surface Micromachining
Sealing processes
Microshells a wafer level packaging strategy
Thin gaps (e.g. 100 nm) are etched out and then sealed:
Reactive sealing by oxidation
LPCVD deposition

11. Surface Micromachining
Comparison of CMOS and Surface Micromachining
Critical Process Temperatures for Microstructures
- Junction migration at 800 to 950°C
- Al interconnect suffers at °C
- Topography
IC compatibility

12. Poly-Si surface micromachining modifications: porous poly-Si
Just like we can make porous Si from single crystal Si we can do the same with poly-Si (low currents densities in highly concentrated HF)
Applications:
Channels parallel to a flat surface (switch from porous to polishing and back--chambers with porous plugs)
Enclosed chambers (blisters of free poly-Si)
Hollow resonators (higher Q)
CVD poly-Si
CVD Si3N4
CVD Si3N4

13. Poly-Si surface micromachining modifications: hinged poly-Si
Make structures horizontally and erect them on a poly-Si hinge (probe station)---rigid structures (Prof. Pister, UCB)
Polyimide hinges also have been made ( butterfly wing)---movable structures
polyimide hinge (E= 3 GPa)
poly-Si hinge (E= 140 GPa)

14. Poly-Si surface micromachining modifications:hinged poly-Si
Micromachined integrated optics for free space interconnections
Pister et al., UCB

15. Poly-Si surface micromachining modifications: thick poly-Si and HEXSIL
Thick poly-Si--10 µm in 20 ‘ with SiH2Cl2 at 1000°C has become possible (low tensile stress)
HEXSIL (Dr. Keller, UCB):
Deep dry etching of trenches in SCS (e.g. 100 µm deep)-short isotropic etch to smooth the walls
Deposition of sacrificial and structural materials (undoped, doped poly-Si and metal e.g. Ni)
Demolding by etching away the sacrificial material

16. Poly-Si surface micromachining modifications: HEXSIL
HEXSIL tweezers
Membrane filter with stiffening rib
Dr. Keller, UCB

17. Poly-Si surface micromachining modifications: SIMOX
Types of Silicon On Insulator (SOI) processes:
SIMOX (Separated by IMplanted OXygen)
Si fusion bonded wafers
Zone-melt recrystallized polysilicon (ZMR)
Poly-Si surface micromachining modifications: SIMOX

18. Non-poly-Si surface micromaching.
Surface Micromachining
Polyimide: e.g. SRI flat panel display
UV depth lithography
AZ-4000 (high viscosity, many layers)
SU-8 (IBM)
Non-poly-Si surface micromaching.