Prof. Brian Otis, Prof. Babak ParvizUniversity of Washington

Electrical Engineering DepartmentSeattle, WA, USA

Miniaturized Systems for Wireless Monitoring

• Project goals

• Challenges & the state-of-the-art

• Our progress since last year:

– Digital sensor identification test chip– Low frequency on-chip system timer– Thermal energy harvesting

Goals: wireless monitoring

• Example: Structural monitors, fluid conductivity, flow rate, temperature, etc, at arbitrary locations

• Unrealistic to position thousands of complete sensing and chemical analysis units

?

• Use large number of inexpensive wireless monitors

• Network density provides high spatial resolution and robustness

Challenges: cost, size, reliability

• Necessary to integrate many different technologies in a very small volume:

– Various sensors– RF Transceiver/antenna– Microprocessor– Power storage/conversion– Quartz frequency reference

• Typically done with surface-mount PCB

– Too large for microscale sensing– Prohibitively expensive for ubiquitous deployment

State-of-the-art: Passive tags

• RFID (example: Hitachi μ-chip)

• No battery or power supply

• (150x150x7.5)μm3 (168e-6 mm3)

• Si Density ρ=2330kg/m3

∴ mass of one chip = 0.393 μg (small)

• Millions of die/wafer

• < $0.10 US (cheap)

• Interrogator output power: 0.3W

• Range: 450mm (limited capabilities, no sensing)

M. Usami et. al, ISSCC 2006

State-of-the-art: Active tags

Gen 1Softbaugh MSP board

Gen 3Gen 2Traditional Analog Transponder

• Peer-to-peer communication possible• Transceivers require multiple off-chip components, increasing size• Power consumption is too high, requiring large batteries

(typically 2 AA batteries) and frequent replacement• Limited sensing functionality Bowen, Burt

Room for improvement

• New techniques for miniaturizing RF transceivers

• Reduce the power consumption to shrink power supply

• Low power sensor interface circuitry

• Low cost self-assembly on arbitrary substrates

• Means of augmenting battery power through energy harvesting

In no particular order…

Self-assembled microsystems

1. Integrate circuitry, sensors, and power generationon arbitrary substrates

2. Inexpensive self-assembly process with modular architecture allows user customization

3. Ultra-low power RF subsystems that operate in peer-to-peer mesh network or RFID interrogation modes

Progress update: sensor identification

Digital IC fingerprinting

• Thousands of identical, inexpensive sensors

• How to differentiate?

• Take advantage of IC process variations to extract a unique digital fingerprint from each chip

1010111100110101

0111001

?

Lofstrom, ISSCC 2000

Latch-Based ID Cells

• Output of the circuit depends on random mismatch

B A

A

B

time(s)

volta

ge (V)

• Random process variation “freezes” a unique ID into circuit

• Use many (say, 128) ID cells to create digital fingerprint

Su, Holleman, Otis, JSSC 2008

ID Output Codes• Vanishingly small probability of chip misidentification with

billions of fabbed chips

Combination SRAM/ID prototype• The latch is very similar to an SRAM unit

cell• Can we reuse ID circuit for memory

storage?• Will the data we store in SRAM

affect the ID over time?

Ying Su

Results-Averaged ID Output

• Averaged ID output over 18 chips for each individual SRAM cell– No noticeable artifacts exist, indicating that gradient, shadowing and

edge effects have been suppressed.

The nodes spend most of their time sleeping…

How do we wake them up?

Motivation• Only need to

sense/compute/communicate every second or so

• Need a slow clock (approx 1 Hz) to act as a heartbeat to wake up the system periodically

• Trivial to make on-chip clocks that run from 100kHz to 1GHz, but difficult to make clocks slower…

J. Bowen

Relaxation oscillator using gate leakage

1. Replace the resistor in a standard relaxation oscillator with a thin-oxide transistor

2. The gate leakage of the tunneling transistor simulates a very large resistor

3. All gates except the tunneling device have thick gate oxides

Y. Lin et. al, IEEE CICC 2007.

1 Hz timer schematic

Oscillator layout – 90nm CMOS

1mm

Consumes 30um x100umof die area

Consumes <1nW from a 0.35V supply

Ryan Ricchiuti

Results: 24hr stability

Far inferior to quartz, but quite good (in power and stability) compared to ring-oscillator based solutions

How do we power these systems?Towards energy harvesting for autonomous

sensors

Good candidates for wireless process monitors: solar & thermal.

Energy harvesting

Energy scavenging eliminates maintenance costs and allows reduced energy storage volume

S. Roundy, B. Otis, Y.H. Chee, J. Rabaey, P. Wright, IEEE ISLPED 2003

Thermal power supply footprint

• System specifications:

- 1cm2 footprint- 10uW average power- First step: supply power

to a custom sensor interface preamplifier IC

Thermal power supply prototype

1cm

1mm

4-channel low power preamp

Output w/ voltage converter

Summary

1. We are developing a self-assembled system architecture to bridge the gap between passive and active wireless tags

2. We have demonstrated prototypes of a few key technologies:• SRAM/digital chip ID• Fully integrated, sub-uW system timer• Miniaturized thermal powersource• Low power temp sensor IC

3. The next step: self-assembly of wireless temp sensor onto a plastic substrate