1. 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,
**University of Oslo, 0316 Oslo, Norway
***Patinor Coatings, Zelenograd, Moscow, Russia

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 emitter
Design and technology for the silicon microphone
Preliminary experimental results
Conclusions, further work and acknowledgements
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

3. 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
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

4. 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.
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

5. Technology of 54.7, continued
The photoacoustic principle
Window
Microphone ~ Pressure sensor
Gas
Modulated
IR source
Absorbed modulated IR radiation is converted into acoustic signal in a sealed gas chamber
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

6. Conventional Photoacoustic Gas Sensor
Power
Lock-in
Oscillator
supply
Display
amplifier
Valve
Pulsed
Microphone
IR source •
Mirror
Microphone
IR-filter
IR-window
Valve
Pump
Well known with high performance at high cost
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

7. 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!!!
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

8. Photoacoustic Response
Response without gas in absorption path
Emitter voltage
250
Emitter radiation
200
PA-signal
150
Output voltage from amplifier [mV]
100 50
-20 20 40 60 80
100
120
140
160
180
time [ms]
Decreasing PA signal with increasing gas concentration in absorption path. Here shown at 8 HZ modulation.
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

9. 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 C
High reliability
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

10. 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 510-2 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
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

11. Silicon micromachined
Principle of a Microsystem based Photoacoustic Gas Sensing Cell (Early Prototype)
10.0 mm
Silicon micromachined
acoustic pressure
sensor chip
4.0 mm
Transistor cap
Target gas
TO-header
Absorption
Window
chamber
IR radiation
The photoacoustic sensing microsystem is enabled by packaging a silicon micromachined acoustic pressure sensor chip in a transistor package
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

12. Principle of the Silicon Microphone used in the Gas Sensing Cell (Early Prototype)
Piezo resistors
Pressure equalising channel
Al coating
Sensor chip
Support chip
Target gas
TO-header
Window
Integrated pressure equalising channel
The diaphragm can have a centre boss structure to increase linearity
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

13. 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
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

14. 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
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

15. The 54.7 photoacoustic gas sensing cell design (Q4/2000)
90 mm
IR-emitter
IR window or filter
Microphone
6mm
IR radiation Absorption path
Thermopile or pyroelectric IR reference sensor
Target gas
Perforated aluminum tube
Cell with silicon or electret microphone
Electret microphones model 9723 from Microtronic used in present prototypes
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

16. 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
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

17. Preliminary Test of Silicon Microphone versus Electret Microphone
Comparable signal-to-noise performance
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

18. Test of the DLC IR Emitters
Power efficiency about 0.1
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

19. IR Emitters: Radiation Spectrum
Useful IR spectrum from around 1 to around 10 micrometers
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

20. 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: 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 hours (more than 3 years)
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

21. Preliminary experimental results of CO2 module prototype
Temp 1
Vref
0.998
Vref-temp-c
0.996 Vg
0.994
Vg-temp-c
0.992
Vg-temp-ref-c
0.99
0.002 approximately:
25 ppm CO2
1 oC
0.988
0.986
200
400
600
800
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?
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >

22. 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
”A Photoacoustic Gas Sensing Silicon Microsystem” Presented at Transducers 2001 >