MEMS devices: How do we make them? A mechanism Gear chain Hinge Gear within a gear Sandia MEMS

Basic MEMS materials Silicon and its derivatives, mostly Micro-electronics heritage Si is a good semiconductor, properties can be tuned Si oxide is very robust Si nitride is a good electrical insulator Substrate Cost Metallization Machinability Silicon High Good Very good Plastic Low Poor Fair Ceramic Medium Glass

Surface micromachining How a cantilever is made: http://www.darpa.mil/mto/mems

One can make devices as complex as one wishes using deposition and micromachining processes http://mems.sandia.gov/

Any MEMS device is made from the processes of deposition and removal of material e.g. a state-of-the art MEMS electric motor www.cronos.com

The History of MEMS Y.C.Tai, Caltech

Isotropic Anisotropic Bulk micromachining Wet Chemical etching: Masking layer Bulk Si Bulk Si Isotropic Anisotropic

Bulk micromachining Dry etching Ions: Reactive ion etching (RIE), focused ion beams (FIB) Laser drilling: using high powered lasers (CO2/YAG) Electron-beam machining: sequential slow

Wet Etching: Isotropic atomic layer by atomic layer removal possible Isotropic etching: Hydrofluoric + nitric + acetic acids (HNA) Bulk Si Chemical reaction: Si + 6 HNO3+6 HF H2SiF6 + HNO2 + H2O + H2 Principle: HNO3 (Nitric acid) oxidizes Si  SiOx HF (Hydrofluoric Acid) dissolves SiOx Acetic acid/water is a diluent

This implies preferential/anisotropic etching is possible Anisotropic etching, due to the Silicon crystal structure - Diamond cubic crystal structure X Y Z Different planes of atoms in a Silicon crystal have different densities of atoms (111) (100) (110) (111) This implies preferential/anisotropic etching is possible

Applications: Anisotropic Etching Aligning fibers Inkjet printers fiber

Wet etching: Anisotropic Etching (100) (110) (100) (111) Bulk Si Bulk Si Chemical recipes: EDP (Ethylene diamine, pyrocatechol, water) [NH2(CH2)2NH2, C6H4(OH)2] - low SiO2 etch rate, - carcinogenic KOH (Potassium hydroxide), - high <110> / <111> and <100>/ <111> selectivity ( ~ 500) - high SiO2 etching TMAH (Tetra-methyl Ammonium Hydroxide: (CH3)4NOH) - Low SiO2 and SixNy etch rate - smaller <100> / <111> selectivity

Comparison of wet chemical etches Etchant Typical etching conditions Anisotropic <100>/<111> etching ratio Etch rate of masking layers EDP 50-115 oC 20-80 mm/hr 10-35 SiO2(2 Å/min) SiN(1 Å/min) KOH 50-90 oC 10-100 mm/hr 100-400 TMAH 60-90 oC 10-60 mm/hr 10-20 Reference: “Etch rates for Micromachining Processing” - K. R. Williams, IEEE Journal of MEMS, vol. 5, page 256, 1996.

Sensors based on (100) preferential etching Honeywell sensor

Micro-fluidic channels based on (110) preferential etching

MEMS Process Sequence Slide courtesy: Al Pisano

Surface micromachining How a cantilever is made: http://www.darpa.mil/mto/mems Sacrificial material: Silicon oxide Structural material: polycrystalline Si (poly-Si) Isolating material (electrical/thermal): Silicon Nitride

MEMS Processing Oxidation of Silicon  Silicon Oxide (Sacrificial material) Dry Oxidation: flowing pure oxygen over Si @ 850 – 1100 oC (thin oxides 1- 100 nm, high quality of oxide) Uses the Deal-Grove Model: xoxide = (BDGt)1/2 Temperature (oC) BDG (mm2/ hour) 920 0.0049 1000 0.0117 1100 0.027

MEMS Processing Oxidation of Silicon  Silicon Oxide (Sacrificial material) Wet Oxidation: uses steam for thicker oxides (100nm – 1.5 mm, lower quality) Temperature (oC) BDG (mm2/ hour) 920 0.203 1000 0.287 1100 0.510 Higher thicknesses of oxide: CVD or high pressure steam oxidation

Silicon oxide deposition LTO: Low Temperature Oxidation process For deposition at lower temperatures, use Low Pressure Chemical Vapor Deposition (LPCVD) SiH4 + O2  SiO2 + 2 H2 : 450 oC Other advantages: Can dope Silicon oxide to create PSG (phospho-silicate glass) SiH4 + 7/2 O2 + 2 PH3  SiO2:P + 5 H2O : 700 oC PSG: higher etch rate, flows easier (better topography) SiH4 + O2 425-450 oC 0.2-0.4 Torr

Case study: Poly-silicon growth SiH4 by Low Pressure Chemical Vapor Deposition T: 580-650 oC, P: 0.1-0.4 Torr Effect of temperature Amorphous  Crystalline: 570 oC Equi-axed grains: 600 oC Columnar grains: 625 oC (110) crystal orientation: 600 – 650 oC (100) crystal orientation: 650 – 700 oC Amorphous film 570 oC Crystalline film 620 oC Kamins,T. 1998 Poly-Si for ICs and diplays, 1998

Poly-silicon growth Mechanisms of grain growth: Strain induced growth Temperature has to be very accurately controlled as grains grow with temperature, increasing surface roughness, causing loss of pattern resolution and stresses in MEMS Mechanisms of grain growth: Strain induced growth - Minimize strain energy due to mechanical deformation, doping … - Grain growth  time 2. Grain boundary growth - To reduce surface energy (and grain boundary area) - Grain growth  (time)1/2 3. Impurity drag - Can accelerate/prevent grain boundary movement - Grain growth  (time)1/3

Grains control properties Mechanical properties Stress state: Residual compressive stress (500 MPa) - Amorphous/columnar grained structures: Compressive stress - Equiaxed grained structures: Tensile stress Thick films have less stress than thinner films ANNEALING CAN REDUCE STRESSES BY A FACTOR OF 10-100 Thermal and electrical properties Grain boundaries are a barrier for electrons e.g. thermal conductivity could be 5-10 times lower (0.2 W/cm-K) Optical properties Rough surfaces!

Silicon Nitride (for electrical and thermal isolation of devices) r: 1016 W cm, Ebreakdown: 107 kV/cm Is also used for encapsulation and packaging Used as an etch mask, resistant to chemical attack High mechanical strength (260-330 GPa) for SixNy, provides structural integrity (membranes in pressure sensors) Deposited by LPCVD or Plasma –enhanced CVD (PECVD) LPCVD: Less defective Silicon Nitride films PECVD: Stress-free Silicon Nitride films SiH2Cl2 + NH3 x SiH2Cl2 + y NH3  SixNy + HCl + 3 H2 700 - 900 oC 0.2-0.5 Torr

Depositing materials PVD (Physical vapor deposition) Sputtering: DC (conducting films: Silicon nitride) RF (Insulating films: Silicon oxide) http://web.kth.se/fakulteter/TFY/cmp/research/sputtering/sputtering.html

Depositing materials PVD (Physical vapor deposition) Evaporation (electron-beam/thermal) Commercial electron-beam evaporator (ITL, UCSD)

Electroplating Issues: Micro-void formation Roughness on top surfaces Courtesy: Jack Judy Issues: Micro-void formation Roughness on top surfaces Uneven deposition speeds Used extensively for LIGA processing e.g. can be used to form porous Silicon, used for sensors due to the large surface to volume ratio

Depositing materials –contd.- Spin-on (sol-gel) e.g. Spin-on-Glass (SOG) used as a sacrificial molding material, processing can be done at low temperatures Dropper Si wafer

Surface micromachining - Technique and issues - Dry etching (DRIE) Other MEMS fabrication techniques - Micro-molding - LIGA Other materials in MEMS - SiC, diamond, piezo-electrics, magnetic materials, shape memory alloys … MEMS foundry processes - How to make a micro-motor

Surface micromachining Carving of layers put down sequentially on the substrate by using selective etching of sacrificial thin films to form free-standing/completely released thin-film microstructures http://www.darpa.mil/mto/mems HF can etch Silicon oxide but does not affect Silicon Release step crucial

Release of MEMS structures A difficult step, due to surface tension forces: Surface Tension forces are greater than gravitational forces ( L) ( L)3

Release of MEMS structures To overcome this problem: Use of alcohols/ethers, which sublimate, at release step Surface texturing Supercritical CO2 drying: avoids the liquid phase Si substrate Cantilever 35oC, 1100 psi

A comparison of conventional vs. supercritical drying

Reactive Ion Etching (RIE) DRY plasma based etching http://www.memsguide.com Deep RIE (DRIE): Excellent selectivity to mask material (30:1) Moderate etch rate (1-10 mm/minute) High aspect ratio (10:1), large etch depths possible

Deep Reactive Ion Etching (DRIE) A side effect of a glow discharge  polymeric species created Plasma processes: Deposition of polymeric material from plasma vs. removal of material Usual etching processes result in a V-shaped profile Bosch Process Alternate etching (SF6) +Passivation (C4F8) Bowing: bottom is wider Lag: uneven formation

Gas phase Silicon etching Room temperature process No surface tension forces No charging effects Isotropic XeF2 BrF3 Developed at IBM (1962) Developed at Bell labs (1984) 2 XeF2 + Si  2 Xe + SiF4 4 BrF3 + 3 Si  2 Br2 + 3 SiF4 Cost: $150 to etch 1 g of Si $16 for 1 g of Si Etching rate: 1-10 mm/minute

Micro-molding For thick films (> 100 mm) HEXSIL/PDMS, compatible with Bio-MEMS C. Keller et al, Solid state sensor & actuator workshop, 1994 - loss of feature definition after repeated replication - Thermal and mechanical stability

(LIthographie, Galvanoformung, Abformung) LIGA (LIthographie, Galvanoformung, Abformung) For high aspect ratio structures Thick resists (> 1 mm) high –energy x-ray lithography ( > 1 GeV) Millimeter/sub-mm sized objects which require precision Electromagnetic motor Mass spectrometer with hyperbolic arms

Technology Comparison Bulk vs. Surface micromachining vs. LIGA Capability Bulk Surface LIGA Max. structural thickness Wafer thickness < 50 mm 500 mm Planar geometry Rectangular Unrestricted Min. planar feature size 2 depth < 1 mm < 3 mm Side-wall features 54.7o slope Limited by dry etch 0.2 mm Surface & edge definitions Excellent Adequate Very good Material properties Very well controlled Well controlled Integration with electronics Demonstrated Difficult Capital Investment Low Moderate High Published knowledge Very high