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Techniques for miniaturization of circuits and systems for wireless sensing. Brian Otis Wireless Sensing Lab Seattle, WA, USA [email protected] Vision Existing technologies How do we get there? Circuit techniques Energy harvesting techniques Integration techniques.

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slide1

Techniques for miniaturization of circuits and systems for wireless sensing

Brian OtisWireless Sensing LabSeattle, WA, [email protected]

slide2

Vision

  • Existing technologies
  • How do we get there?
    • Circuit techniques
    • Energy harvesting techniques
    • Integration techniques
slide3

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
slide4

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

slide5

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

slide6

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

slide7

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

slide8

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
slide9

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.

slide10

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

slide11

Three steps to autonomy

  • Generate accurate frequency reference locally
  • Generate power locally
  • Develop circuit design techniques for reducing computing/sensing/communication power consumption
slide12

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
system proof of concept
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

1mm 3 2ghz super regenerative transceiver
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

slide15

Three steps to autonomy

  • Generate accurate frequency reference locally
  • Generate power locally
  • Develop circuit design techniques for reducing computing/sensing/communication power consumption
slide16

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

thermoelectric energy harvesting
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
slide18

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
inexpensive low power sensor identification
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

id generating circuit requirements
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
proposed idea positive feedback id generation

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
chip implementation
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

slide23

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

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