Oct 2011ECS Boston 220 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

Oct 2011ECS 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 2011ECS 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 2011ECS 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 2011ECS 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 CO 2 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 2011ECS 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 2011ECS Boston 220 Fabrication: Cavity Substrate

Oct 2011ECS 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 2011ECS 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 2011ECS Boston 220 Fabrication: Bonded Structure X section shows the gas meander and PA cell. Plan view micrograph of cantilever. Talysurf image of cantilever.

Oct 2011ECS 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 2011ECS Boston 220 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. N2N2 Regulator Vent Pressure Sensor µPAS Test jig ATM N 2 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 N 2 pressure

Oct 2011 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,  calculations The µPAS devices were subjected to a range of pressures and deflection was measured. ECS Boston 220

Oct 2011ECS 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

Oct 2011ECS 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

Acknowledgements Financial support of the National Science Foundation (USA) Science Foundation of Ireland Dept of Education and Learning (NI) Questions? Oct 2011ECS Boston 220