Surface Micromachining Dr. Marc Madou, Fall 2012, UCI Class 10

 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)

 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 PSG Structural layer nitride Surface Micromachining Basic Process Sequence (poly-Si).

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

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

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: rse/partII_2.html and mtl.mit.edu:800/htdocs/tutorial.html rse/partII_2.htmlhttp://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, f o =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 Si 3 N 4  In PECVD: change the RF power and frequency  In sputtering: gas pressure and substrate bias Surface Micromachining

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

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

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

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

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

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

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

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

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

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