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

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