Miniaturisation and Integration of a
Cantilever based Photoacoustic Sensor
into Micro Micromachined Device
M.F. Bain1, N. Mitchell1, B.M. Armstrong1, J.
Uotila2, I. Kauppinen2, E. Terray3, F. Sonnichsen3
and B. Ward4
1 NISRC School of Electronics, Elec Eng and Comp Sci Queen’s University of Belfast
2 Gasera Ltd Finland, 3 Woods Hole Oceanographic Institute, 4 Dep of Physics NUI Galway
Oct 2011
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Introduction
• Cantilevers and Photoacoustic Gas
Sensors (PAS)
• Motivation for PA cell Miniaturisation
• Fabrication of µPAS device
• Experimental
• Results and Analysis
• Further Work
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Photoacoustic Gas Sensors
Highly sensitive Photoacoustic (PA) Gas Sensor
Cantilever deflection is measured by laser interferometery
focused at the cantilever tip. Sensitivity of 0.001Å
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PA Cell miniaturisation
• In conventional spectroscopy sensitivity
decreases with dimensions.
• Photoacoustic spectroscopy response is
enhanced as the volume decreases.
• Using MEMS technology to incorporate
the cantilever and gas cavities into one
structure.
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µPAS Cell: Proposed device
Cavity dimensions: ~1mm wide,
12mm long, 250µm deep.
Quartz
Cantilever dimensions: ~ 500µm
wide, 500µm length and various
thickness.
Excitation laser inlet defined
1877nm for CO2
Cantilever
Gas inlet
laser
Quartz window allows deflection
measurements using
interferometer
Cavity
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Gas inlet/outlet vias to be etched
through the substrate.
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Fabrication: Cavity Substrate
(a)
Silicon Substrate
(a) The gas inlet/outlet through holes are
initially defined with a dry etch (depth
~300µm)
(b) the second etch defines the PA cell
cavity, approximately 12mm long 1mm
wide and ~250µm deep. The gas
inlet/outlet meander and the laser inlet
are also defined at this stage.
(b)
(c)
Gas inlet
(c) plan view of etched cavity substrate.
The substrate is still robust enough to be
subjected to chemical cleaning.
Cavity
Laser inlet
Gas outlet
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Fabrication: Cavity Substrate
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Fabrication: Cantilever Substrate
(d)
(d) SOI substrate defines the thickness
of the cantilever. BOX thickness also
important
SOI Substrate
(e)
(e) the cantilever is defined in the SOI
substrate prior to bonding. Defining the
cantilever length, width and gap size, .
SOI Substrate
length
Width
Oct 2011

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Fabrication: Bonded Structure
(f)
(f) the two substrates are bonded such
that the cantilever is positioned over the
cell cavity using an EV bond aligner.
(g)
Oct 2011
IR picture of bonded interface. Typical
yield on bonded devices is 11/12 or 12/12.
(g) the cavity behind the cantilever is
defined and acts as a balance cell.
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Fabrication: Bonded Structure
X section shows the gas meander and
PA cell.
Plan view micrograph of cantilever.
Talysurf image of cantilever.
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Fabrication: Final Structure
(g)
Laser inlet Cavity
Gas inlet
Cantilever
(g) plan view of device. µPAS devices of
thickness 4, 6.5, 10 and 15µm were
successfully fabricated.
Gas outlet
(h)
(h) the device is sealed by electrostatic
bonding to a quartz substrate. The
quartz substrate/window will allow
deflection detection by interferometery.
Device should be very leak tight.
Chips were successfuly bonded to a Si
substrate
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Experimental
Test jig for the µPAS allows N2
pressurization of device through the gas
vias and cavity.
µPAS device mounted and clamped to
prevent leaks.
N2 pressure controlled and monitored.
Vent
Pressure
Sensor
ATM
N2
Regulator
µPAS Test
jig
Measurement of cantilever shape using
white light interferometery.
Fringes show the cantilever is inplane
with the SOI surface.
Fringes show the cantilever is deflected
occurs due to N2 pressure
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Results and Analysis
µPAS devices of thickness 4, 6.5, 10 and
15µm were successfully fabricated. At rest
deflection was measured. (L-0.5, W-0.5mm)
Deflection vs cantilever thickness
defection (nm)
500
400
300
The µPAS devices were subjected to a range
of pressures and deflection was measured.
200
100
0
0
5
10
SOI thickness (um)
Oct 2011
15
20
3L3
F
Deflection,  calculations  
2 EWt 3
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P
F
A
Results and Analysis
A SOI substrate (4µm thick) was bonded to a cavity substrate. This produced a
diaphragm structure over the PA cell. The cantilever substrate 4µm is also
thick, allowing a direct comparison between the diaphragm and cantilever
structures over the same pressure range.
0.25
2.5
4.5
The cantilever is an order of magnitude more sensitive than the diaphragm
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Future work
• Insertion of laser to excite specific gases
and measure using interferometer
• Reference cells fill with specific gas at the
bonding level
• Multiple cantilevers for reference and
increased sensitivity
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Acknowledgements
Financial support of the National Science
Foundation (USA)
Science Foundation of Ireland
Dept of Education and Learning (NI)
Questions?
Oct 2011
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