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

2. 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 SubstrateCostMetallizationMachinability Silicon HighGoodVery good Plastic LowPoorFair Ceramic MediumFairPoor Glass LowGoodPoor

3. Materials in MEMS Dominant: SEMICONDUCTORS (Silicon centric) MEMS technology borrows heavily from the Si micro-electronics industry The fabrication of MEMS devices relies on the processing of Silicon and silicon compounds (silicon oxide, nitride …) METALS: used in electrical contacts (Al,Cu), magnetic elements (Ni, NiFe) POLYMERS: used as sacrificial layers, for patterning (photoresist/polyimide)

4. Making MEMS Planar technology, constructing components (MEMS & electronics) on initially flat wafers > Wafer level processes > Pattern transfer Introduction to Micro-machining - Wet and Dry etching - Bulk and surface micro-machining What kinds of materials are used in MEMS? -Semiconductors - Metals - Polymers

5. Photolithography Photoresist Silicon substrate MASK Light Deposit Metal Silicon substrate MASK Light Positive photoresist Negative photoresist

6. -Deposit and remove materials precisely to create desired patterns The photo-lithography process J. Judy, Smart Materials & structures, 10, 1115, 2001 Positive Negative Remove deposit and etch

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

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

9. 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

10. The History of MEMS Y.C.Tai, Caltech

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

12. Bulk micromachining Dry etching Ions : Reactive ion etching (RIE), focused ion beams (FIB) Laser drilling: using high powered lasers (CO 2 /YAG) Electron-beam machining: sequential  slow

13. Wet Etching: Isotropic atomic layer by atomic layer removal possible Isotropic etching: Hydrofluoric + nitric + acetic acids (HNA) Bulk Si Si + 6 HNO 3 +6 HF  H 2 SiF 6 + HNO 2 + H 2 O + H 2 Chemical reaction: Principle: HNO 3 (Nitric acid) oxidizes Si  SiO x HF (Hydrofluoric Acid) dissolves SiO x Acetic acid/water is a diluent

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

15. Applications: Anisotropic Etching fiber Aligning fibers Inkjet printers

16. Wet etching: Anisotropic Etching (100) (110) (100) (111) Chemical recipes: EDP (Ethylene diamine, pyrocatechol, water) [NH 2 (CH 2 ) 2 NH 2, C 6 H 4 (OH) 2 ] - low SiO 2 etch rate, - carcinogenic KOH (Potassium hydroxide), - high / and / selectivity ( ~ 500) - high SiO 2 etching TMAH (Tetra-methyl Ammonium Hydroxide: (CH 3 ) 4 NOH) - Low SiO 2 and Si x N y etch rate - smaller / selectivity Bulk Si

17. Comparison of wet chemical etches EtchantTypical etching conditions Anisotropic / etching ratio Etch rate of masking layers EDP 50-115 o C 20-80  m/hr 10-35SiO 2 (2 Å/min) SiN(1 Å/min) KOH 50-90 o C 10-100  m/hr 100-400 SiO 2 (2 Å/min) SiN(1 Å/min) TMAH 60-90 o C 10-60  m/hr 10-20 SiO 2 (2 Å/min) SiN(1 Å/min) Reference: “Etch rates for Micromachining Processing” - K. R. Williams, IEEE Journal of MEMS, vol. 5, page 256, 1996.

18. Sensors based on (100) preferential etching Honeywell sensor

19. Micro-fluidic channels based on (110) preferential etching

20. MEMS Process Sequence Slide courtesy: Al Pisano

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

22. MEMS Processing Oxidation of Silicon  Silicon Oxide (Sacrificial material) Dry Oxidation: flowing pure oxygen over Si @ 850 – 1100 o C (thin oxides 1- 100 nm, high quality of oxide) Uses the Deal-Grove Model: x oxide = (B DG t) 1/2 Temperature ( o C)B DG  m 2 / hour  9200.0049 10000.0117 11000.027

23. Wet Oxidation: uses steam for thicker oxides (100nm – 1.5 mm, lower quality) Temperature (oC)BDG (mm 2 / hour) 9200.203 10000.287 11000.510 Higher thicknesses of oxide: CVD or high pressure steam oxidation Oxidation of Silicon  Silicon Oxide (Sacrificial material) MEMS Processing

24. Silicon oxide deposition For deposition at lower temperatures, use Low Pressure Chemical Vapor Deposition (LPCVD) SiH 4 + O 2  SiO 2 + 2 H 2 : 450 o C Other advantages: Can dope Silicon oxide to create PSG (phospho-silicate glass) SiH 4 + 7/2 O 2 + 2 PH 3  SiO 2 :P + 5 H 2 O : 700 o C PSG: higher etch rate, flows easier (better topography) SiH 4 + O 2 425-450 o C 0.2-0.4 Torr LTO: Low Temperature Oxidation process

25. Case study: Poly-silicon growth - by Low Pressure Chemical Vapor Deposition - T: 580-650 o C, P: 0.1-0.4 Torr Effect of temperature Amorphous  Crystalline: 570 o C Equi-axed grains: 600 o C Columnar grains: 625 o C (110) crystal orientation: 600 – 650 o C (100) crystal orientation: 650 – 700 o C SiH 4 Amorphous film 570 o C Crystalline film 620 o C Kamins,T. 1998 Poly-Si for ICs and diplays, 1998

26. Poly-silicon 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: 1.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

27. 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!

28. Silicon Nitride  Is also used for encapsulation and packaging  Used as an etch mask, resistant to chemical attack  High mechanical strength (260-330 GPa) for Si x N y, 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 (for electrical and thermal isolation of devices)  10 16  cm, E breakdown : 10 7 kV/cm SiH 2 Cl 2 + NH 3 x SiH 2 Cl 2 + y NH 3  Si x N y + HCl + 3 H 2 700 - 900 o C 0.2-0.5 Torr

29. 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

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

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

32. 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 Si wafer Dropper

33. 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

34. 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

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

36. Release of MEMS structures To overcome this problem: (1)Use of alcohols/ethers, which sublimate, at release step (2)Surface texturing (3)Supercritical CO 2 drying: avoids the liquid phase 35 o C, 1100 psi Si substrate Cantilever

37. A comparison of conventional vs. supercritical drying