MEMS Fabrication Cristina Rusu Imego AB 2011-02-21 2011-02-21 – Cristina Rusu MEMS • Semiconductors as mechanical materials • Bulk micromachining – Dry etching – Wet etching • Surface micromachining – MUMPs • Polymer MEMS • Wafer bonding 2011-02-21 – Cristina Rusu Technology: Micromachining Micro El ec tro Mec hani cal S yst ems (MEMS) Micro System Technology (MST) . Fabrication process similar to that used to make computer chips (Integrated Circuits) •Cappgable of High Precision • Can Operate at High Volumes • Produces Parts at Low Cost . Silicon is… • Extremely pure • Compatible with electronics • Suitable for micro-scale production … and it has outstanding mechanical properties 2011-02-21 – Cristina Rusu MEMS vs CMOS • CMOS compatible processes – No Au, no alkali metals (K, Na, ..) – Limited thermal budget (After doping) 2011-02-21 – Cristina Rusu Semiconductors as mechanical materials • First paper: ”Silicon as a mechanical material” (Kurt Petersen, 1978) – Stiffness: Young’s modulus of Si (130 GPa) close to that of steel – No plastic deformation – (Almost) no fatigue • Other semiconductor materials that are used as mechanical materials: – GaAs, InP , ... 2011-02-21 – Cristina Rusu Other MEMS materials • Polymers – Direct patternable: • UV: SU-8, Polyimide, BCB SU-8 SU-8 • Synchrotron X-ray: PMMA – Etchable • Polyimide, BCB – Moldable: • COC, PDMS, PMMA, Parafin – Evaporable • Parylene • Ceramics – Glass: P yrex, B orofl oa t, Quart z – LTCC PMMA 2011-02-21 – Cristina Rusu Aspect ratio = ratio of the depth to the width of hole / structure 2011-02-21 – Cristina Rusu MEMS • Semiconductors as mechanical materials • Bulk micromachining – Dryyg etching – Wet etching • Surface micromachining – MUMPs • Polymer MEMS • Wafer bonding 2011-02-21 – Cristina Rusu Bulk micromachining • Dry etching – Deep reactive ion etching (DRIE) – Inductively coupled plasma (ICP): The Bosch process • Wet etching – Isotropic (HNA) – Anisotropic (KOH, TMAH, ...) 2011-02-21 – Cristina Rusu DRY Etching - principle Reactant Products Mask Bombardment Impulse transfer Physical etching Film/substrate (a) Reactant Products Mask Adsorption Desorption Chemical etching Reaction (b) Reactant Products Mask Adsorption Desorption Ion-enhanced reaction Synergetical (c) 2011-02-21 – Cristina Rusu Chemical: isotropic etching E.g. XeF2 or SF6 2011-02-21 – Cristina Rusu Physical: tapered etching 2011-02-21 – Cristina Rusu Physical: tapered etching 2011-02-21 – Cristina Rusu Synergetical: vertical etching 2011-02-21 – Cristina Rusu Synergetical: vertical etching 2011-02-21 – Cristina Rusu Typical etching 2011-02-21 – Cristina Rusu The Bosch process 2011-02-21 – Cristina Rusu Cryogenic DRIE • Principle – SF6/O2 plasma – At cryogenic temperatures (T < -100 C), a passivating SiOxFy layer forms on top of the silicon surface – sputtered away from horizontal surfaces by directional ion bombardment. – thickness of the passivation layer is mainly determined by the O2 flow rate (more O2, more passivation) • Superior sidewall quality 2011-02-21 – Cristina Rusu http://www.clarycon .com/etch_mech_pic .html 2011-02-21 – Cristina Rusu Artifacts in dry etching Notching (ion trajectory distortion RIE lag or ARDE & chemical etching) Aspect ratio dependent etching Faceting, Ditching (Trenching) and Redeposition2011-02-21 – Cristina Rusu Advanced dry etching (1) 2011-02-21 – Cristina Rusu Advanced dry etching (2) 2011-02-21 – Cristina Rusu Typical RIE Gases Typical etch rate Material Typical etchant Typical mask (µm/min) SF6 ~ 3 - 8(DRIE)8 (DRIE) Si Photo resist, SiO2, Al BCl3 + Cl2 ~ 0.5 SiO2 CF4 ~ 0.02 Photo resist, Al Si3N4 CHF3 ~ 0.1 - 0.2 Photo resist, Al GaAs CCl2F2 + O2 ~ 0.2 Ni, Al, Cr SiC SF6 ~ 0.2 - 0.5 Photo resist, Al Al Cl2 ~ 0.3 Photo resist Au CCl2F2 ~ 0.05 Photo resist 2011-02-21 – Cristina Rusu Wet etching • Isotropic etching – Same etch rate in all directions – Lateral etch rate is about the same as vertical etch rate – Etch rate does not depend upon the orientation of the mask edge • Anisotropic etching – Etch rate depends upon orientation to crystalline planes – Lateral etch rate can be much larger or smaller than vertical etch rate, depending upon orientation of mask edge to crystalline axes – Orientation of mask edge and the details of the mask pattern determine the final etched shape • Can be very useful for making complex shapes • Can be very surprising if not carefully thought out • Only certain “standard” shapes are routinely used • MhhMuch cheaper th thdan dry et thithiching techniques • Higher safety risk for lab personnel: bases & acids instead of confined plasma 2011-02-21 – Cristina Rusu Crystal planes in silicon • Silicon: Face Centered Cubic (FCC) [100] [111] [010] [001] 2011-02-21 – Cristina Rusu Anisotropic wet etching - orientation dependent etching Si 2011-02-21 – Cristina Rusu <100> 2011-02-21 – Cristina Rusu Si 2011-02-21 – Cristina Rusu <011> 2011-02-21 – Cristina Rusu Anisotropic wet etching: AFM tips resistors Tip connection 2011-02-21 – Cristina300 μ Rusum KOH • Comparatively safe and non-toxic • High crystal plane selectivity • Limited SiO 2 selectivity • Not CMOS compatible: potassium (K) • Careful cleaning can allow KOH-etched wafers (Piranha cleaning) in not too picky CMOS facilities 2011-02-21 – Cristina Rusu Tetra-Methyyy()l Ammonium Hydroxide (TMAH) • CMOS compatible • Lower crystal plane selectivity: (111):(011):(100) 1:60:20 • High selectivity towards SiO2 • Pooso,coison, corros ive 2011-02-21 – Cristina Rusu Crystal alignment • Identifying the correct crystal alignment – Flat alignment: ±1º (standard) – Test etch + alignment – Alignment forks (Vangbo and Bäcklund): ±0.05º 2011-02-21 – Cristina Rusu Misalignment in orientation dependent etching Wafer flat <011> <100> <111> 2011-02-21 – Cristina Rusu Misalignment in orientation dependent etching Wafer flat 5o 2011-02-21 – Cristina Rusu Misalignment in orientation dependent etching Wafer flat 45o 2011-02-21 – Cristina Rusu Alignment forks (Vangbo & Bäcklund) 2011-02-21 – Cristina Rusu Corner compensation structures 2011-02-21 – Cristina Rusu Solution: corner compensation structures 2011-02-21 – Cristina Rusu Simulation software • Cellular automata -based simulation – 3D continuous • Intellisuite AnisE •Fast • Does not simulate surface roughness – Monte Carlo • CoventorWare: Etch3D • Advantage: precise • Slow, heavy on resources (memory, cpu) 2011-02-21 – Cristina Rusu AnisE 2011-02-21 – Cristina Rusu CoventorWare Etch3d 1406µm 575µm 700µm 575µm 140µm 500µm 575µm 290µm 2011-02-21 – Cristina Rusu MEMS • Semiconductors as mechanical materials • Bulk micromachining – Dry etching – Wet etching • Surface micromachining – Stiction – Lithophraphy – MUMPs • Polymer MEMS • Wafer bonding 2011-02-21 – Cristina Rusu Evaporation Drying - Stiction 2011-02-21 – Cristina Rusu Stiction = Big problem in MEMS Capillary force greater than structural stiffness • The microstructures may remain stuck to substrate even after dry . • Cause: solid bridging, van der Waals force, electrostatic force, hydrogen bonding, etc 2011-02-21 – Cristina Rusu Supercritical Drying Evaporation Drying Material Tc (ºC) Pc (atm) Pc (psi) Water 374 218 3204 Methanol 240 80 1155 CO231731073 Sublimation Drying Vapour phase Etching T-butyl alcohol – freezes at 26 ºC P-dichlorobenzene – freezes at 56 ºC Anhydrous HF vapour avoiding liquid-gas transition 2011-02-21 – Cristina Rusu Stiction Reduction Strategies Reduce Adhesion Area • dimples • surface roughening • low surface-energy coatings Integrate supporting microstructures • increase tolerance of capillary forces Examples: • microtethers •microfuses •sacrifici al suppor ting layers (ex. p ho tores is t) • coat devices with low surface-energy films 2011-02-21 – Cristina Rusu Lithography issues • MEMS: often ”large” height differences – Spray coating – Proximity exposure Still lower resolution 2011-02-21 – Cristina Rusu Surface micromachining, e.g. polyMUMPs • Cost per submission is $3,200/academic, $4,500/commercial – 1cm2 die area per submission – 15 identical dice returned (~$2/mm2) • Dicing, bonding, HF release are all available for additional cost • Parameterized and static design cells are free online • Design services are available for additional cost • 2-5 weeks time to evaluate/test chips and revise design for next scheduled run 2011-02-21 – Cristina Rusu polyMUMPs process flow 2011-02-21 – Cristina Rusu polyMUMPs process flow 2011-02-21 – Cristina Rusu polyMUMPs process flow 2011-02-21 – Cristina Rusu Example – IR microspectrometer 2011-02-21 – Cristina Rusu Different MUMPs processes • PolyMUMPs – 8 lithography levels , 7 physical layers – 3 Poly layers – 1 Metal layer • SOIMUMPs – 10 or 20 µm structure layer – Double-sided pattern/etch – 2 Metal layers • MetalMUMPs – 10 lithography layers – Thick electroplated Ni (18-22 µm) Source: MEMSCAP 2011-02-21 – Cristina Rusu MEMS • Semiconductors as mechanical materials • Bulk micromachining • Surface micromachining • Polymer MEMS • Wafer bonding 2011-02-21 – Cristina Rusu Polymer MEMS • Fabrication methods • Polymers – Parylene – PDMS – Paraffin – Polyimide, BCB – SU-8 – PMMA – ... 2011-02-21 – Cristina Rusu Polymer fabrication methods (1) Injection moulding Hot embossing Casting 2011-02-21 – Cristina Rusu Polymer fabrication methods (2) Stereolithography Ink jet printing 2011-02-21 – Cristina Rusu Parylene • Poly-para-xylylene • Vapor-phase deposition – Low-tempera ture process ( <100 ºC) – Very conformal (~100mbar) • Advantages: – Low surface roughness – Stress free – Excellent dielectric breakdown properties <1µm – Pinhole