Miniaturisation and Integration of a Cantilever based Photoacoustic Sensor into Micro Micromachined Device M.F. Bain 1 , N. Mitchell 1 , B.M. Armstrong 1 , J. Uotila 2 , I. Kauppinen 2 , E. Terray 3 , F. Sonnichsen 3 and B. Ward 4 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

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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 0.001

Å

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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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Quartz Cantilever laser Cavity

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µPAS Cell: Proposed device

Gas inlet Cavity dimensions: ~1mm wide, 12mm long, 250µm deep.

Cantilever dimensions: ~ 500µm wide, 500µm length and various thickness.

Excitation laser inlet defined 1877nm for CO 2 Gas inlet/outlet vias to be etched through the substrate.

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Quartz window allows deflection measurements using interferometer

(a) (b) (c)

Fabrication: Cavity Substrate

Silicon Substrate Gas inlet (a) The gas inlet/outlet through holes are initially defined with a dry etch (depth ~300 µm) (b) the second etch defines the PA cell cavity, approximately 12mm long 1mm wide and ~250 µm deep. The gas inlet/outlet meander and the laser inlet are also defined at this stage.

(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) SOI Substrate (e) Width SOI Substrate length  (d) SOI substrate defines the thickness of the cantilever. BOX thickness also important (e) the cantilever is defined in the SOI substrate prior to bonding. Defining the cantilever length, width and gap size,  .

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(f)

Fabrication: Bonded Structure

(f) the two substrates are bonded such that the cantilever is positioned over the cell cavity using an EV bond aligner.

(g) 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 15 µm 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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N 2

Experimental

Test jig for the µPAS allows N 2 pressurization of device through the gas vias and cavity.

µPAS device mounted and clamped to prevent leaks.

N 2 pressure controlled and monitored.

Regulator Vent Pressure Sensor ATM 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 N 2 pressure

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500 400 300 200 100 0 0

Results and Analysis

Deflection vs cantilever thickness

µPAS devices of thickness 4, 6.5, 10 and 15 µm were successfully fabricated. At rest deflection was measured. (L-0.5, W-0.5mm) The µPAS devices were subjected to a range of pressures and deflection was measured.

5 10

SOI thickness (um)

15 20 Deflection,  calculations   3

L

3 2

EWt

3

F P



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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?

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