Techniques for Miniaturization of Circuits and Systems for Wireless Sensing

1. Techniques for miniaturization of circuits and systems for wireless sensing Brian OtisWireless Sensing LabSeattle, WA, USAbotis@ee.washington.edu

2. Vision • Existing technologies • How do we get there? • Circuit techniques • Energy harvesting techniques • Integration techniques

3. Vision: autonomous sensing • Miniaturized devices (a few mm3) • Extremely inexpensive • Frequent radio contact with peersand with basestation • Periodic sensing of environmental parameters (temperature, light, pressure, acceleration etc.) • Flexible deployment in wide variety of biological, manufacturing, or environmental monitoring applications

4. Vision: autonomous sensing • Miniaturized devices (a few mm3) • Extremely inexpensive • Frequent radio contact with peersand with basestation • Periodic sensing of environmental parameters (temperature, light, pressure, acceleration etc.) • Flexible deployment in wide variety of biological, manufacturing, or environmental monitoring applications Critical challenges: miniaturization of - RF Link- Reference clock generation- Power sources

5. RF Link: existing designs won’t work – why? 1.They are too large. Traditional architectures require multiple off-chip components, high die area, and a large quartz crystal resonator. 2.They consume too much power. Bluetooth & Zigbee (the “low power” standards) consume > 20mW. This eliminates the possibility of energy harvesting. 3. They require high-end processes and high transistor counts. ~2cm

6. What about RFID? • Case study: Hitachi m-chip • (150x150x7.5)mm3 (168e-6 mm3) • Si Density r=2330kg/m3 mass of one chip = 0.393 mg (small) • Millions of die/wafer • < $0.10 US (cheap) • Interrogator output power: 0.3W • Range: 450mm (limited capabilities) M. Usami et. al, ISSCC 2006

7. Case Study: Hitachi RFID chip Power harvesting Frequency reference harvesting(100kHz clock) • Power is extracted from incoming RF energy • External antenna (few cm) • Ideal for embedding in secure documentation M. Usami et. al, ISSCC 2006

8. RFID Interrogators Power dissipation >1W Cost >$100 US Provides two critical functions that are currently impossible to generate on-chip: • Accurate quartz-based frequency reference • Power source

9. RFID summary • RFID chips can be made extremely small and cheap • These are radios that harvest their power from an incoming RF signal. RF power falls off quadratically (at best) with distance, resulting in high interrogator power and very short range. 3. There is little energy available for sensing or computation. 4. They cannot form peer-to-peer networks.

10. Research Goal Self-contained wireless sensing systems that can be fabricated exclusively with thin-film processing techniques. This should include: Peer-to-peer Wireless links Computation/Data Storage Chemical/biological Sensors Electrical Sensor Interfaces Energy/Power Source

11. Three steps to autonomy • Generate accurate frequency reference locally • Generate power locally • Develop circuit design techniques for reducing computing/sensing/communication power consumption

12. RF MEMS: path to ultra-small radios? On-Chip Inductors (Q ~10) MEMS Resonators(Q~1000) 100mm ~300mm • MEMS resonators have significantly higher Q than on-chip inductors • Possibility for elimination of quartz resonators • MEMS sensing capabilities

13. System proof-of-concept Can we design an entire low-power radio link using MEMS resonators as a frequency reference? Case Study: 2GHz transceiver for wireless sensors Goal: Use matching RF MEMS resonators on the transmit and receive paths to define carrier frequency

14. 1mm3, 2GHz super-regenerative transceiver 1mm CMOS BAW 2mm • No external components (inductors, crystals, capacitors) • 0.13um CMOS • Operates above transistor fT Total Rx: 380uW Range: 30m Datarate: 50kbps B. Otis et al., IEEE ISSCC 2005

15. Three steps to autonomy • Generate accurate frequency reference locally • Generate power locally • Develop circuit design techniques for reducing computing/sensing/communication power consumption

16. antenna PV cell Energy Harvesting Extracting energy from the environment to power the electronics reduces maintenance costs and increases capabilities Bottom line: -Approximately 100uW/cm3 available(but efficiency decreases as volume shrinks)-Power consumption of electronics determines wireless sensor volume and capabilities

17. Thermoelectric energy harvesting Why thermoelectric? Large, stable temperature gradients often exist in ubiquitous sensing applications Monolithic, solid state, possibleto integrate with circuitry • How does it work? • Converts thermal gradient to electric potential via Seebeck effect • Thermocouples connected in series as a thermopile increases voltage (and resistance) • Radioisotope powered TEGs widely used in space missions Work-in-progress: • SOI-based mTEG • p,n silicon thermoelements • Floating membrane increases thermal isolation

18. Three steps to autonomy • Generate accurate frequency reference locally • Generate power locally • Develop circuit design techniques for reducing computing/sensing/communication power consumption-> example: sensor ID generation

19. Inexpensive, low power sensor identification 10101111 00110101 • Wireless sensor network addressing • Object identification for Radio Frequency ID (RFID) tags • Wafer and process tracking of individual chips for failure analysis • Tracking for implantable electronics devicesCan we extract a unique digital fingerprint from process variations? 0111001

20. ID Generating Circuit Requirements • ID circuit must generate a digital output • ID code must be repeatable and reliable over supply, temperature, aging and thermal noise • The ID code length and stability must allow positive unique identification of each die • Low power consumption, no calibration

21. voltage (V) A B A B time(s) Proposed Idea: positive feedback ID generation • Each ID cell: cross-coupled gates used to amplify transistor mismatch • Evaluation period Node A and B will split due to transistor mismatch • Readout period  Digital-level output will be obtained directly at ID node

22. Chip Implementation • 128 ID generators – 140nW @ 1V • Technology: 0.13m CMOS • Provides stable fingerprint with extremely high probability of correctchip identification Su, Holleman, Otis, IEEE ISSCC 2007

23. 500um Conclusions 1. Wireless sensor scaling is constrainedby energy source, antenna dimensions, and frequency reference 2. Self-contained wirelesssensors less than 1mm3 are on the horizon 3. Future chips will include circuitry, EM elements, MEMS structures, sensors, and power generation 4. Interdisciplinary collaboration is critical to focus our efforts on relevant sensing problems