MEMS Fabrication Cristina Rusu Imego AB 2011-02-21
MEMS Semiconductors as mechanical materials Bulk micromachining Dry etching Wet etching Surface micromachining MUMPs Polymer MEMS Wafer bonding
Technology: Micromachining Micro Electro Mechanical Systems (MEMS) Micro System Technology (MST) Fabrication process similar to that used to make computer chips (Integrated Circuits) Capable 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
MEMS vs CMOS CMOS compatible processes No Au, no alkali metals (K, Na,..) Limited thermal budget (After doping)
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,...
Other MEMS materials Polymers Direct patternable: UV: SU-8, Polyimide, BCB Synchrotron X-ray: PMMA Etchable Polyimide, BCB Moldable: COC, PDMS, PMMA, Parafin Evaporable Parylene Ceramics Glass: Pyrex, Borofloat, Quartz LTCC SU-8 SU-8 PMMA
Aspect ratio = ratio of the depth to the width of hole / structure
MEMS Semiconductors as mechanical materials Bulk micromachining Dry etching Wet etching Surface micromachining MUMPs Polymer MEMS Wafer bonding
Bulk micromachining Dry etching Deep reactive ion etching (DRIE) Inductively coupled plasma (ICP): The Bosch process Wet etching Isotropic (HNA) Anisotropic (KOH, TMAH,...)
Reactant DRY Etching - principle Products Mask Bombardment Impulse transfer Physical etching (a) Film/substrate Reactant Products Mask Chemical etching Adsorption Reaction Desorption Reactant Products (b) Mask Adsorption Desorption Ion-enhanced reaction Synergetical (c)
Chemical: isotropic etching E.g. XeF 2 or SF 6
Physical: tapered etching
Physical: tapered etching
Synergetical: vertical etching
Synergetical: vertical etching
Typical etching
The Bosch process
Cryogenic DRIE Principle SF 6 /O 2 plasma At cryogenic temperatures (T < -100 C), a passivating SiO x F y 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 O 2 flow rate (more O 2, more passivation) Superior sidewall quality
http://www.clarycon.com/etch_mech_pic.html
Artifacts in dry etching Notching (ion trajectory distortion & chemical etching) RIE lag or ARDE Aspect ratio dependent etching Faceting, Ditching (Trenching) and Redeposition
Advanced dry etching (1)
Advanced dry etching (2)
Typical RIE Gases Material Typical etchant Typical etch rate (µm/min) Typical mask Si SF 6 BCl 3 + Cl 2 ~ 3-8(DRIE) ~ 0.5 Photo resist, SiO 2, Al SiO 2 CF 4 ~ 0.02 Photo resist, Al 2 4 Si 3 N 4 CHF 3 ~ 0.1-0.2 Photo resist, Al GaAs CCl 2 F 2 + O 2 ~ 0.2 Ni, Al, Cr SiC SF 6 ~ 0.2-0.5 Photo resist, Al Al Cl 2 ~ 0.3 Photo resist Au CCl 2 F 2 ~ 0.05 Photo resist
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 Much cheaper than dry etching techniques Higher safety risk for lab personnel: bases & acids instead of confined plasma
Crystal planes in silicon Silicon: Face Centered Cubic (FCC) [100] [111] [010] [001]
Anisotropic wet etching - orientation dependent etching Si
<100>
Si
<011>
Anisotropic wet etching: AFM tips resistors Tip connection 2011-02-21 Cristina 300 μm Rusu
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
Tetra-Methyl Ammonium Hydroxide (TMAH) CMOS compatible Lower crystal plane selectivity: (111):(011):(100) 1:60:20 High selectivity towards SiO 2 Poison, oso,corrosiveos
Crystal alignment Identifying the correct crystal alignment Flat alignment: ±1º (standard) Test etch + alignment Alignment forks (Vangbo and Bäcklund): ±0.05º
Misalignment in orientation dependent etching <011> Wafer flat <100> <111>
Misalignment in orientation dependent etching Wafer flat 5 o
Misalignment in orientation dependent etching Wafer flat 45 o
Alignment forks (Vangbo & Bäcklund)
Corner compensation structures
Solution: corner compensation structures
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)
AnisE
CoventorWare Etch3d 1406µm 575µm 700µm 575µm 140µm 500µm 575µm 290µm
MEMS Semiconductors as mechanical materials Bulk micromachining Dry etching Wet etching Surface micromachining Stiction Lithophraphy MUMPs Polymer MEMS Wafer bonding
Evaporation Drying - Stiction
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
Evaporation Drying Supercritical Drying Sublimation Drying Material Tc (ºC) Pc (atm) Pc (psi) Water 374 218 3204 Methanol 240 80 1155 CO2 31 73 1073 Vapour phase Etching T-butyl alcohol freezes at 26 ºC P-dichlorobenzene freezes at 56 ºC Anhydrous HF vapour avoiding liquid-gas transition
Stiction Reduction Strategies Reduce Adhesion Area dimples surface roughening low surface-energy coatings Integrate supporting microstructures increase tolerance of capillary forces Examples: microtethers microfuses sacrificial i supporting layers (ex. photoresist) t) coat devices with low surface-energy films
Lithography issues MEMS: often large height differences Spray coating Proximity exposure Still lower resolution
Surface micromachining, e.g. polymumps Cost per submission is $3,200/academic, $4,500/commercial 1cm 2 die area per submission 15 identical dice returned (~$2/mm 2 ) 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
polymumps process flow
polymumps process flow
polymumps process flow
Example IR microspectrometer
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
MEMS Semiconductors as mechanical materials Bulk micromachining Surface micromachining Polymer MEMS Wafer bonding
Polymer MEMS Fabrication methods Polymers Parylene PDMS Paraffin Polyimide, BCB SU-8 PMMA...
Polymer fabrication methods (1) Injection moulding Hot embossing Casting
Polymer fabrication methods (2) Stereolithography Ink jet printing
Parylene Poly-para-xylylene Vapor-phase deposition Low-temperature t process (<100ºC) Very conformal (~100mbar) Advantages: Low surface roughness Stress free Excellent dielectric breakdown properties <1µm Pinhole free for film thicknesses >0.5µm Low autofluorescence Biocompatible Adhesion: Silanization recommended for Si, Si 3 N 4, SiO 2 and Al surfaces O 2 plasma treatment recommended for polymer surfaces No preparation required for Cr, Au, Ti Microfabrication Etched using O 2 and CF 4 atmosphere
PDMS Polydimethylsiloxane Silicone Not allowed at MC2 Biocompatible Can be spun or poured on mold Easily bonded to PDMS, Si, SiO 2 using O 2 plasma activation Low cost
Paraffin Properties Low thermal conductivity Low electrical conductivity Low chemical reactivity High thermal expansion during phase change High boiling point Applications Microactuators
Polyimide & Benzo cyclobutene (BCB) PI: Kapton Spin-on Patterning Photopatternable (negative) storage temperatures < -25ºC Etchable (O 2 plasma) Advantages Chemical & thermal stability Low water uptake Biocompatibility (PI) Multilayer deposition
SU-8 Negative photoresist (UV) High aspect ratios (>18) can be obtained Higher aspect ratios (>60) with synchrotron X-ray Can be used as mold or as structural material
PMMA Polymethylmethacrylate (Plexiglass) X-ray patternable Aspect ratios: 50-500 (freestanding-supported) when applied in LIGA 100-3000 µm thick (Also used as e-beam resist, but a slightly lower thicknesses) LIGA
Other materials Hydrogels Good for chemical sensing applications Biodegradable materials Polyglycolic acide (PGA), polylactic acid (PLDA),... Fabrication methods: imprinting, hot embossing, stereolithography, laser micromachining
MEMS Semiconductors as mechanical materials Bulk micromachining Dry etching Wet etching Surface micromachining MUMPs Polymer MEMS Wafer bonding