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

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

Surface Micromachining PSG Deposition of a base, spacer or sacrificial layer-phosphosilicate glass (PSG)-CVD Densification at 950 °C for 30-60 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

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)

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

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: http://mems.eeap.cwru.edu/shortcourse/partII_2.html and http://www-mtl.mit.edu:800/htdocs/tutorial.html

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

Surface Micromachining Control of film stress With L=150 µm and W=t=2 µm, fo=10 to 100 kHz. Annealing at high temperature (900-1150°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

Surface Micromachining Y 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

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

Surface Micromachining Comparison of CMOS and Surface Micromachining Critical Process Temperatures for Microstructures - Junction migration at 800 to 950°C - Al interconnect suffers at 400-450 °C - Topography IC compatibility

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

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)

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

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

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

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

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.