A Photoacoustic Gas Sensing Silicon Microsystem Per Ohlckers*, **Alain M. Ferber, Vitaly K. Dmitriev and Grigory Kirpilenko* *Fifty-four point Seven,

A Photoacoustic Gas Sensing Silicon Microsystem

Per Ohlckers*, **Alain M. Ferber*, Vitaly K. Dmitriev*** and Grigory Kirpilenko***

*Fifty-four point Seven, Forskningsveien 1, 0314 Oslo, Norway, [email protected]

**University of Oslo, 0316 Oslo, Norway

***Patinor Coatings, 103460 Zelenograd, Moscow, Russia

”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 > Slide 2

Outline

Motivation: Microsystem technology can give cost effective gas sensors with high performance

Description of the 54.7 photoacoustic gas sensing technology

Design and technology for the infrared emitterDesign and technology for the silicon

microphonePreliminary experimental resultsConclusions, further work and

acknowledgements


Motivation:

• Microsystem technology can give cost effective photoacoustic gas sensors with high performance

– Batch organised manufacture for low cost– Silicon micromachining for high performance and small size– Piezoresistive microphone for high-sensitivity sensing of the

photoacoustic signal– Multistack wafer anodic bonding to produce the hermetic target

gas chambers– etc

• The start-up microsystem company 54.7 started its operation on September 1, 1999, with its first venture to commercialise this patented scheme for photoacoustic gas sensing modules using microsystem technology


Technology of 54.7• The 54.7 Photoacoustic Gas Sensing Technology

– Using a silicon micromachined acoustic pressure sensor with an enclosed cavity with the gas species to be measured as a selective filter. This intellectual property is protected with 3 patents.


Technology of 54.7, continued

• Absorbed modulated IR radiation is converted into acoustic signal in a sealed gas chamber

The photoacoustic principle

Window

Microphone ~ Pressure sensor

Gas

ModulatedIR source


Conventional Photoacoustic Gas Sensor

• Well known with high performance at high cost

IR-filterMicrophone

IR-window

Mirror

Microphone

Display Lock-inamplifierOscillatorPower

supply

PulsedIR source

Valve

Valve

Pump

•


Photoacoustic Technology of 54.7

• Increased amount of target gas present in the absorption path gives a correspondingly decreasing photoacoustic response in the sealed target gas chamber due to the transmission loss

• Explain better! Include absorption lines etc!!!

Pressure sensor (microphone)

Optical window

Sealed target gas chamber

Read out electronics Absorption path

Modulated IR emitter


Photoacoustic Response

• Decreasing PA signal with increasing gas concentration in absorption path. Here shown at 8 HZ modulation.

-20 0 20 40 60 80 100 120 140 160 180

0

50

100

150

200

250O

utp

ut v

olta

ge f

rom

am

plif

ier

[mV

]

time [ms]

PA-signal

Emitter voltage

Emitterradiation

Response without gas in absorption path


The Diamond-like Thin Film/Silicon Micromachined IR Emitter

• Manufactured by Patinor Coatings– Based upon Diamond-Like Carbon (DLC) thin film heating resistor on silicon micromachined diaphragm structure:

1: Bonding pads 2&3: SiO2 4: Si3N4 5: DLC film– Using a CVD process to deposit the DLC thin film– Pulse modulated high speed broad band grey body IR emission– Working temperaure about 700-800 C– High reliability


CVD Process for the IR Emitter

• Silicon-organic liquid (C2H5)3SiO[CH3C6H5SiO]3Si(CH3)3 (PPMS) is used as a plasma-forming substance of the open plasmatron

• Doping by molybdenum is done during plasma deposition process wafer by magnetron sputtering of a MoSi2 target in argon atmosphere

• Pressure is about 510-2 Pa, the magnetron current is about 2 A, the plasmatron arc discharge current is about 6 A

• By changing those deposition parameters it is possible to modify the resistance of the IR emitters


Principle of a Microsystem based Photoacoustic Gas Sensing Cell (Early Prototype)

• The photoacoustic sensing microsystem is enabled by packaging a silicon micromachined acoustic pressure sensor chip in a transistor package

10.0 mm

TO-header

IR radiation

4.0 mm

Silicon micromachinedacoustic pressure

sensor chip

Target gas

WindowAbsorptionchamber

Transistor cap


Principle of the Silicon Microphone used in the Gas Sensing Cell (Early Prototype)

• Integrated pressure equalising channel• The diaphragm can have a centre boss structure to increase linearity

Target gas

Al coatingPressure equalising channel

TO-header

Piezo resistors

Sensor chip

Support chip

Window


Silicon Microphone Prototype (Q3/2000)

• Designed by SINTEF and 54.7

• Piezoresistive with centre boss structure

• Manufactured by SensoNor with their Europractice/NORMIC multiproject wafer foundry services


Silicon Microphone Prototype: Design and Process

• Piezoresistive with centre boss structure– Chip size is 6 mm x 6 mm. Diaphragm diameter is 2 mm

• SensoNor/NORMIC process: Process E/ MPW : Combined Diaphragm- and Mass-Spring-based Piezoresistive Sensor Process

– 3 micrometer epitaxial layer– 2-level etch stop using anisotropic TMAH process with electrochemical etch stop at 3 and 23 micrometers– Buried piezoresistors with 480 Ohm/square sheet resistance– Anodic bonded triple stack glass-silicon-glass structure

Glass top chip

Si diaphragm chip

Glass bottom chip


The 54.7 photoacoustic gas sensing cell design (Q4/2000)

• Cell with silicon or electret microphone– Electret microphones model 9723 from Microtronic used in present prototypes

IR-emitter Microphone

Perforated aluminum tube

IR window or filter

Thermopile or pyroelectric IR reference sensor

90 mm

Target gas

6mm IR radiation Absorption path


Sensor Module Design Q4/2000

• Sensor module with the gas sensing cell mounted on a surface mount printed circuit board with analog and digital electronics for monitoring, control and interface

• Size approximately 70mm x 20mm x 10mm


Preliminary Test of Silicon Microphone versus Electret Microphone

• Comparable signal-to-noise performance

0.00

0.01

0.01

0.02

0.02

0.03

0.04

0.04

0.05

0.05

0.06

0.07

0.07

0.08

0.08

0.09

0.10

0.10

Time (s)

Rel

am

pli

tud

eElectretmicrophone

Siliconmicrophone


Test of the DLC IR Emitters

• Power efficiency about 0.1


IR Emitters: Radiation Spectrum

Useful IR spectrum from around 1 to around 10 micrometers


Main characteristics of the IR Emitters• Resistance value: Nominal 55, from 35 to 125 Ohms• Supply voltage: From 5 up to 12 V• Power consumption: 0.5 – 1.0 W• Maximum frequency modulation of the emitted

energy: 200 Hz (~100% modulation at 10 Hz)• Working temperature of film resistor: 500-800 oC,

with header temperature not exceeding 70 oC• Warm-up time: < 30 s• The emissivity factor of the emitting surface: ~0.8• Emitting efficiency (=3-14 micrometers): ~10%• Life time: Mean Time Between Failure (MTBF) of

more than 25 000 hours (more than 3 years)


Preliminary experimental results of CO2 module prototype

Graph of 15 hours measurement (one sample per minute) Lab test: Increased CO2 at start and at inspection. Resolution around 0.3 ppm. Accuracy around ±10ppm?

0 200 400 600 8000.986

0.988

0.99

0.992

0.994

0.996

0.998

1

Temp

Vref

Vref-temp-c

Vg

Vg-temp-c

Vg-temp-ref-c

0.002 approximately:25 ppm CO2

1 oC


Conclusions, further work and acknowledgements• The concept is promising for commercialisation

• Low cost, high selectivity, and high sensitivity can be achieved– Example: CO2 measured with around 10 ppm accuracy and 0.3 ppm resolution

• Potential show stoppers• Long term drift and thermal effects

– Example: Some thermal effects are yet to be understood and minimised

• Further work• Long term stability need to be verified further• Thermal effects will need to be investigated, reduced and compensated• Low cost microsystem production technology need to be further developed

• Many thanks to my coauthors• Dr. Martin Lloyd of Farside Technology is thanked for his

contribution on the digital electronics and the software• Dr. Henrik Rogne and Dag T. Wang of SINTEF are acknowledged

for the design of the silicon microphone

Documents

A Photoacoustic Gas Sensing Silicon Microsystem Per Ohlckers*, **Alain M. Ferber*, Vitaly K. Dmitriev*** and Grigory Kirpilenko*** *Fifty-four point Seven,

A Photoacoustic Gas Sensing Silicon Microsystem Per Ohlckers*, **Alain M. Ferber, Vitaly K. Dmitriev and Grigory Kirpilenko* *Fifty-four point Seven,