Wireless Sensor Nodes

Transcription

1 2.008 Design & Manufacturing II Spring 2005 Sang-Gook Kim MEMS, Tiny Products Wireless Sensor Nodes Millennial's i-bean nodes Intel, Berkeley Crossbow Technology Low Power Consumption Energy density, weight Storage Harvesting Portable Energy Research Energy Research Ensuring security & diversity of supply Promoting sustainable development Conversion efficiency $$$

2 Portable Energy Wireless Sensing Mobile Computing Mobility? Piezo MEMS energy harvesting 1 µw 1 mw Energy Harvesting Modified with SOURCE: Carlin, Richard, ONR, Electric Power Sources for the Navy and Marine Corps, Presented at the Navy Grand Challenges Workshop, November 16-18, Harvesting Ambient Vibration Energy By Thin Film Piezo Inter-digitated Electrode t=1µm E PZT ZrO 2 membrane L=170µm MIT PMPG (Piezoelectric Micro Power Generators) Y. Jeon, R. Sood, and S.G. Kim, Hilton Head Conference 2004, and will appear Sensors and Actuators A Physical

3 Device Fabrication Si (a) Films deposition (CVD, spinning) Interdigitated Electrode (Ti/Pt) (c) Films patterning (RIE) Proof mass (b) Top electrode deposition (e-beam) and then lift-off PZT (Pb(Zr,Ti)O 3 ) ZrO 2 Membrane (d) Proof mass (SU-8, spinning) Device Fabrication (e) Cantilever release (XeF 2 etcher) (f) Packaging & PZT poling

4 Power Generation First mode of resonance at 13.9 khz with 14 nm amplitude Rectifying the AC signal, then store the charge Variation of load resistance Schottky diodes: smallest forward voltage drop 10 nf mylar cap: low leakage current Load voltage (V c ) 2.28V Source current (µv) 0.4µA Time (sec) Time (sec) Structural Health Monitoring

5 Research/Educational Opportunities Monitoring of Large-scale Remote Systems Gas line monitoring Underground beacons Measuring rotational machines Smart bearings Helicopter blades Structural health monitoring Multi-functional structures Process Flow Wafers Photo resist coating Pattern transfer Photo resist removal Devices Deposition Lithography Etch Oxidation Sputtering Evaporation CVD Sol-gel Epitaxy Wet isotropic Wet anisotropic Plasma RIE DRIE

6 Deposition processes Chemical CVD(Chemical Vapor Deposition) Thermal Oxidation Epitaxy Electrodeposition Physical PVD Evaporation Sputtering Casting Thermal Oxidation Silicon is consumed as the silicon dioxide is grown. Growth occurs in oxygen and/or steam at C. Compressive stress ~2um films are maximum practically. Simple, easy process for electrical insulation, intentional warpage, etc. O 2 Silicon SiO 2 Silicon

7 Thermal Oxidation Oxidation can be masked with silicon nitride, which prevents O 2 diffusion Silicon nitride Silicon SiO 2 Chemical Vapor Deposition Thermal energy to dissociate gases and deposit thin films on surfaces, high productivity, better step coverage low pressure (LPCVD), atmospheric pressure (APCVD), plasma enhanced (PECVD), horizontal, vertical LPCVD pressures around 300mT (0.05% atmosphere) Moderate Temperatures 450 o C SiO 2 : PSG, LTO o C polysilicon > o C Si x N y SiH4 + NH3 Very dangerous gases Silane: SiH4 Arsine, phosphine, diborane: AsH 3, PH 3, B 2 H 6

8 Physical Vapor Deposition Evaporation Evaporated metals in a tungsten crucible Aluminum, gold, Pt, W Evaporated metals and dielectrics by electron-beam or resistance heating Typically line-of-sight deposition Very high-vacuum required to prevent oxidation, load lock Shadowing E-beam evaporator Physical Vapor Deposition Sputtering Sputtered metals and dielectrics Argon ions bombards target Ejected material takes ballistic path to wafers Typically line-of-sight from a distributed source Requires high vacuum, but low temperature RF sputter Evaporation vs. Sputtering

9 Spin Casting Viscous liquid is poured on center of wafer Wafer spins at 1,000-5,000 RPM for ~30s Baked on hotplates o C for s Application of etchants and solvents, rinsing Deposition of polymers, sol-gel PZT dispenser PR wafer ω vacuum chuck level out slow coat spin down t Deposition Issues - Compatibility Thermal compatibility Thermal oxidation and LPCVD films Thermal oxidation and LPCVD vs. polymers (melting/burning) and most metals (eutectic formation, diffusion) Topographic compatibilitiy Spin-casting over large step heights Deposition over deep trenches-key hole

10 Deposition Issues - Conformality A conformal coating covers all surfaces to a uniform depth A non-conformal coating deposits more on top surfaces than bottom and/or side surfaces Conformal Non-conformal Non-conformal Photo 1. Cracking of sol-gel deposited PZT after 650C firing Photo 2. Poor step coverage and high stress evolved at corner of a step

11 Process Flow Wafers Devices Photo resist coating Pattern transfer Photo resist removal Deposition Lithography Etch Oxidation Sputtering Evaporation CVD Sol-gel Epitaxy Wet isotropic Wet anisotropic Plasma RIE DRIE Etching Wet Dry Isotropic silicon etchants HNA ( poly-etch ) -wet Mix of HF, nitric acid (HNO 3 ), and acetic acids (CH 3 COOH) Difficult to control etch depth and surface uiformity XeF 2 -dry gas phase, etches silicon, polysilicon Does not attack SiO2, SiNx,metals, PR

12 Anisotropic wet etching Many liquid etchants demonstrate dramatic etch rate differences in different crystal directions <111> etch rate is slowest, <100> fastest Fastest: slowest can be more than 100:1 KOH, EDP, TMAH most common anisotropic silicon etchants Potasium Hydroxide (KOH), Tetramethyl Ammonium Hydroxide (TMAH), and Ethylene Diamine Pyrochatecol (EDP) KOH Etching Etches PR and Aluminum instantly (100) to (111) 100 to 1 etch rate V-grooves, trenches Concave stop, convex undercut CMOS incompatible Masks: SiO 2 : for short period Si x N y : Excellent heavily doped P ++ silicon: etch stop <111> <100> a 0.707a 54.7 Silicon Substrate

13 Anisotropic wet etching When a (100) wafer with mask features Oriented to <110> direction is placed in an anisotropic etchant. A square <110> oriented mask feature results in a pyramidal pit. Anisotropic wet etching When a (100) wafer with mask features Oriented to <110> direction is placed in an anisotropic etchant. A square <110> oriented mask feature results in a pyramidal pit.

14 V-grooved Optical Bench w R A B B (100)silicon θ h A Etch mask Top-view A-A cross section Dry etching RIE (reactive ion etching) Chemical & physical etching by RF excited reactive ions Bombardment of accelerated ions, anisotropic SF 6 Si, CHF 3 oxide and polymers Anisotropy, selectivity, etch rate, surface roughness by gas concentration, pressure, RF power, temperature control Plasma etching Purely chemical etching by reactive ions, isotropic Vapor phase etching Use of reactive gases, XeF2 No drying needed sticktion

15 DRIE (Deep RIE) Alternating RIE and polymer deposition process for side wall protection and removal Etching phase: SF6 /Ar Polymerization process: CHF3/Ar forms Teflon-like layer Invented by Bosch, process patent, to 4 µm/min -selectivity to PR 100 to 1 Scalloping and Footing issues of DRIE Top wafer surface cathode Top wafer surface anode Scalloped sidewall Tip precursors <100 nm silicon nanowire over >10 micron gap 1 µm Footing at the bottom of device layer

16 Deep Reactive Ion Etch STS, Alcatel, Trion, Oxford Instruments Most wanted by many MEMS students High aspect ratio 1:30 Easily masked (PR, SiO2) Wet or dry? Wet Low resolution feature size Low cost Undercut for isotropic Wider area needed for anisotropic wafer etch Sticking Dry High resolution feature size Expensive Vertical side wall Avoid sticking

17 Etching Issues - Anisotropy -Structural layer -Sacrificial layer mask An-isotropic Isotropic Etching Issues - Selectivity Selectivity is the ratio of the etch rate of the target material being etched to the etch rate of other materials Chemical etches are generally more selective than plasma etches Selectivity to masking material and to etch-stop is important Mask target Etch stop

18 Process Flow Wafers Photo resist coating Pattern transfer Photo resist removal Devices Deposition Lithography Etch Oxidation Sputtering Evaporation CVD Sol-gel Epitaxy Wet isotropic Wet anisotropic Plasma RIE DRIE Lithography (Greek, stone-writing ) Pattern Transfer Appication of photosensitive PR Optical exposure to transfer image from mask to PR Remove PR - binary pattern transfer

19 Photo resist Spin coat phto-resist rpm, sec Viscosity and rpm determine thickness Soft bake ->alignment ->exposure Develop PR after exposure Hardbake dispenser PR Positive Negative ω wafer vacuum chuck level out slow coat spin down t Photomasks Master patterns to be transferred Types: Photographic emulsion on soda lime glass (cheap) Fe 2 O 3 or Cr on soda lime glass Cr on quartz (expensive, for deep UV light source) Polarity Light field: mostly clear, opaque feature Dark field: mostly opaque, clear feature

20 Types of Aligner Contact Proximity Projection Reduction ratio 1:1 Array of the same pattern Stepper Double sided Alignment

21 Pattern transfer by lift-off Micromirror Arrays XGA 1024 X ,432 pixels VGA 640 X ,200 pixels Human Hair 50 µm

22 TMA(Thinfilm Micromirror Array) Light 50 µm Mirror Piezoelectric Actuator Active Matrix Mirror Array on Piezoelectric Actuator Array Daewoo Electronics Evolution of TMA Pixels Optical Efficiency(%) st 1st 2nd 2nd 3rd 3rd Time from the start of project, year Daewoo Electronics

23 Clean Room Gowning with bunny suit Class 1, 10, 100 A salt grain on a chip Class of clean rooms Class 1 means one speckle of 0.5 µ partical in one ft 3. Class 10, 100, 1000 HEPA filter, AHU

24 Air Filters HEPA (High Efficiency Particulate Air) filters High efficiency, low p, good loading characteristics Glass fibers in a paper like medium 97% retainment of incident particles of 0.3 µm or larger Class of clean rooms Class 0.5 µ particle Temp tolerance RH tolerance $/ft 2 10,000 10,000 +/- 3 o F +/- 5% $ ,000 1,000 +/- 2 o F +/- 5% $ /- 1 o F +/- 5% $ /- 0.5 o F +/- 3% $ /- 0.3 o F +/- 2% $10,000

25 Particles class 0.1µ 0.2µ 0.3µ 0.5µ 5.0µ N/A N/A 100 N/A N/A 1000 N/A N/A N/A Federal Standard 209; Number of particles per cubic foot Toxicity TLV (Threshold Limit Value) Upper limit material concentration that an average healthy person can be exposed without adverse effects, ppm or mg/m 3 Notorious Poisons CO (100 ppm), CO 2 (5000 ppm), HCN (110 ppm), H 2 S (10 ppm) SO 2 (5 ppm), NH 3 (50 ppm) Arsenic trioxide AS 2 O 3 (0.1g fatal) Hg (0.1 ppm via skin contact) All material are toxic in sufficient quantity, 5g caffein is fatal.

26 Design Domains Design is a mapping process From What to How Small scale systems design What How