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Surface Micromachining Dr. Marc Madou, Fall 2012, UCI Class 10
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Blanket n + diffusion of Si substrate (ground plane) Passivation layer (e.g. SiO 2, Si 3 N 4, LPCVD Si 3 N 4 on top of SiO 2 ) Opening up the passivation layer for contacts (observe color change or hydrophobic/hydrophilic behavior): –wet (BHF) –dry (SF 6 ) Strip resist in piranha (adds some oxide in the window) Short BHF etch to remove thin oxide n+n+ n n+n+ n t SiO2 LmLm Lm + 2t SiO2 Photoresist Si 3 N 4 Surface Micromachining Basic Process Sequence (poly-Si)
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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 PSG Structural layer nitride Surface Micromachining Basic Process Sequence (poly-Si).
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Basic Process Sequence (poly-Si) Doping of poly-Si: in-situ, PSG sandwich and ion implantation Patterning of structural material e.g. RIE in, say, CF 4 -O 2 Release step, selective etching of spacer layer e.g. in diluted HF RSRS RiRi RmRm nitride Surface Micromachining
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Generic principle of surface micromachining Si Al Etch access Si Sacrificial layer definition Releasing diaphragm: phosphoric/acetic acid/nitric acid (PAN) Polyimide diaphragm deposition Surface Micromachining
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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/shortcou rse/partII_2.html and http://www- mtl.mit.edu:800/htdocs/tutorial.html http://mems.eeap.cwru.edu/shortcou rse/partII_2.htmlhttp://www- mtl.mit.edu:800/htdocs/tutorial.html Surface Micromachining
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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
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Control of film stress With L=150 µm and W=t=2 µm, f o =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 Si 3 N 4 In PECVD: change the RF power and frequency In sputtering: gas pressure and substrate bias Surface Micromachining
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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) Y X Surface Micromachining
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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
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IC compatibility 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 Surface Micromachining
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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 Si 3 N 4 CVD poly-Si CVD Si 3 N 4 Surface Micromachining
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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) Surface Micromachining
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Poly-Si surface micromachining modifications:hinged poly-Si Pister et al., UCB Micromachined integrated optics for free space interconnections Surface Micromachining
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Poly-Si surface micromachining modifications: thick poly-Si and HEXSIL Thick poly-Si--10 µm in 20 ‘ with SiH 2 Cl 2 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 Surface Micromachining
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Poly-Si surface micromachining modifications: HEXSIL Dr. Keller, UCB Membrane filter with stiffening rib HEXSIL tweezers Surface Micromachining
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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) Surface Micromachining
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Non-poly-Si surface micromaching. Polyimide: e.g. SRI flat panel display UV depth lithography –AZ-4000 (high viscosity, many layers) –SU-8 (IBM) Surface Micromachining Capp Spindt
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