A Photoacoustic Gas Sensing Silicon Microsystem

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, Per.Ohlckers_at_fys.uio.no
• University of Oslo, 0316 Oslo, Norway
• Patinor Coatings, 103460 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

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

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.

5
Technology of 54.7, continued

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

6
Conventional Photoacoustic Gas Sensor
Power
Lock-in
Oscillator
Display
supply
amplifier
Valve
Microphone
Pulsed
IR source




Mirror
Microphone
IR-filter
IR-window
Valve
Pump

• Well known with high performance at high cost

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!!!

8
Photoacoustic Response
Response without gas in absorption path
Emitter voltage
250
Emitterradiation
200
PA-signal
150
Output voltage from amplifier mV
100
50
0
-20
0
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.

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 structure1 Bonding pads 23 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

10
CVD Process for the IR Emitter

• Silicon-organic liquid (C2H5)3SiOCH3C6H5SiO3Si(C
  H3)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

11
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

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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

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

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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

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

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

17
Preliminary Test of Silicon Microphone versus
Electret Microphone

• Comparable signal-to-noise performance

18
Test of the DLC IR Emitters

• Power efficiency about 0.1

19
IR Emitters Radiation Spectrum

• Useful IR spectrum from around 1 to around 10
  micrometers

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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 lt 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)

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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
0
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?

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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