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Oct 2011 ECS Boston 220
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 ECS Boston 220
Introduction
• Cantilevers and Photoacoustic Gas Sensors (PAS)
• Motivation for PA cell Miniaturisation
• Fabrication of µPAS device
• Experimental
• Results and Analysis
• Further Work
Oct 2011 ECS Boston 220
Photoacoustic Gas Sensors
Cantilever deflection is measured by laser interferometery focused at the cantilever tip. Sensitivity of 0.001Å
Highly sensitive Photoacoustic (PA) Gas Sensor
Oct 2011 ECS Boston 220
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.
Oct 2011 ECS Boston 220
Cavity dimensions: ~1mm wide, 12mm long, 250µm deep.
Cantilever dimensions: ~ 500µm wide, 500µm length and various thickness.
Excitation laser inlet defined 1877nm for CO2
Gas inlet/outlet vias to be etched through the substrate.
Quartz window allows deflection measurements using interferometer
µPAS Cell: Proposed device
Quartz
Cantilever
Cavity
Gas inlet
laser
Oct 2011 ECS Boston 220
Fabrication: Cavity Substrate(a) The gas inlet/outlet through holes are initially defined with a dry etch (depth ~300µm)
(a)
Silicon Substrate
(b)(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.
Cavity
Gas inlet
Gas outlet
Laser inlet
(c)(c) plan view of etched cavity substrate. The substrate is still robust enough to be subjected to chemical cleaning.
Oct 2011 ECS Boston 220
Fabrication: Cantilever Substrate
(d) SOI substrate defines the thickness of the cantilever. BOX thickness also important
(d) SOI Substrate
(e) the cantilever is defined in the SOI substrate prior to bonding. Defining the cantilever length, width and gap size, .(e) SOI Substrate
length
Width
Oct 2011 ECS Boston 220
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.
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.
(g)
Oct 2011 ECS Boston 220
Fabrication: Bonded StructureX section shows the gas meander and PA cell.
Plan view micrograph of cantilever.
Talysurf image of cantilever.
Oct 2011 ECS Boston 220
Fabrication: Final Structure
Cavity
Gas inlet
Gas outlet
Laser inlet
(g)
Cantilever(g) plan view of device. µPAS devices of thickness 4, 6.5, 10 and 15µm were successfully fabricated.
(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
Oct 2011 ECS Boston 220
ExperimentalTest jig for the µPAS allows N2 pressurization of device through the gas vias and cavity.
µPAS device mounted and clamped to prevent leaks.
N2 Regulator
VentPressure Sensor
µPAS Test jig
ATM
N2 pressure controlled and monitored.
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
Oct 2011
Results and AnalysisDeflection vs cantilever thickness
0
100
200
300
400
500
0 5 10 15 20
SOI thickness (um)
def
ecti
on
(n
m)
µPAS devices of thickness 4, 6.5, 10 and 15µm were successfully fabricated. At rest deflection was measured. (L-0.5, W-0.5mm)
FEWt
L3
3
2
3
A
FP Deflection, calculations
The µPAS devices were subjected to a range of pressures and deflection was measured.
ECS Boston 220
Oct 2011 ECS Boston 220
Results and Analysis
The cantilever is an order of magnitude more sensitive than the diaphragm
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
Oct 2011 ECS Boston 220
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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