Brian OtisWireless Sensing Lab

Seattle, WA, USAbotis@ee.washington.edu

Techniques for miniaturization of circuits and systems for

wireless sensing

• Vision

• Existing technologies

• How do we get there?

– Circuit techniques

– Energy harvesting techniques

– Integration techniques

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

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

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

What about RFID?

• Case study: Hitachi -chip

• (150x150x7.5)m3 (168e-6 mm3)

• Si Density =2330kg/m3

mass of one chip = 0.393 g (small)

• Millions of die/wafer

• < $0.10 US (cheap)

• Interrogator output power: 0.3W

• Range: 450mm (limited capabilities)

M. Usami et. al, ISSCC 2006

Case Study: Hitachi RFID chip

• Power is extracted from incoming RF energy

• External antenna (few cm)

• Ideal for embedding in secure documentation

M. Usami et. al, ISSCC 2006

Power harvesting

Frequency reference harvesting(100kHz clock)

RFID Interrogators

Provides two critical functions that are currently impossible to generate on-chip:

1. Accurate quartz-based frequency reference

2. Power source

Power dissipation >1WCost >$100 US

RFID summary

1. RFID chips can be made extremely small and cheap

2. 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.

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

Three steps to autonomy

1. Generate accurate frequency reference locally

2. Generate power locally

3. Develop circuit design techniques for reducing computing/sensing/communication power consumption

100m

On-Chip Inductors (Q ~10) MEMS Resonators(Q~1000)

~300m

• MEMS resonators have significantly higher Q than on-chip inductors

• Possibility for elimination of quartz resonators• MEMS sensing capabilities

RF MEMS: path to ultra-small radios?

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

1mm3, 2GHz super-regenerative transceiver

• No external components (inductors, crystals, capacitors)

• 0.13um CMOS

• Operates above transistor fT

2mm

Total Rx: 380uW

Range: 30m

Datarate: 50kbps

1mm

BAWCMOS

B. Otis et al., IEEE ISSCC 2005

Three steps to autonomy

1. Generate accurate frequency reference locally

2. Generate power locally

3. Develop circuit design techniques for reducing computing/sensing/communication power consumption

Bottom line:

-Approximately 100uW/cm3 available(but efficiency decreases as volume shrinks)

-Power consumption of electronics determines wireless sensor volume and capabilities

Energy Harvesting

Extracting energy from the environment to power the electronics reduces maintenance costs and increases capabilities

PV cell

an

tenna

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

Thermoelectric energy harvesting

Why thermoelectric?• Large, stable temperature gradients

often exist in ubiquitous sensing applications

• Monolithic, solid state, possibleto integrate with circuitry

Work-in-progress:• SOI-based TEG

• p,n silicon thermoelements

• Floating membrane increases thermal isolation

Three steps to autonomy

1. Generate accurate frequency reference locally

2. Generate power locally

3. Develop circuit design techniques for reducing computing/sensing/communication power consumption-> example: sensor ID generation

Inexpensive, low power sensor identification

• 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 devices

Can we extract a unique digital fingerprint from process variations?

10101111 00110101

0111001

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

• 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

B A

A

B

time(s)

volta

ge

(V)

Chip Implementation

• 128 ID generators – 140nW @ 1V• Technology: 0.13m CMOS• Provides stable fingerprint with extremely high probability of correct

chip identification Su, Holleman, Otis, IEEE ISSCC 2007

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

500um

4. Interdisciplinary collaboration is critical to focus our efforts on relevant sensing problems