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Proceedings Paper

Low-cost uncooled microbolometers for thermal imaging
Author(s): Niclas Roxhed; Frank Niklaus; Andreas C. Fischer; Fredrik Forsberg; Linda Höglund; Per Ericsson; Björn Samel; Stanley Wissmar; Anders Elfving; Tor Ivar Simonsen; Kaiying Wang; Nils Hoivik
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Paper Abstract

Cost efficient integration technologies and materials for manufacturing of uncooled infrared bolometer focal plane arrays (FPA) are presented. The technology platform enables 320x240 pixel resolution with a pitch down to 20 μm and very low NETD. A heterogeneous 3D MEMS integration technology called SOIC (Silicon-On-Integrated-Circuit) is used to combine high performance Si/SiGe bolometers with state-of-the-art electronic read-out-integrated-circuits. The SOIC integration process consists of: (a) Separate fabrication of the CMOS wafer and the MEMS wafer. (b) Adhesive wafer bonding. (c) Sacrificial removal of the MEMS handle wafer. (d) Via-hole etching. (e) Via formation and MEMS device definition. (f) Sacrificial etching of the polymer adhesive. We will present an optimized process flow that only contains dry etch processes for the critical process steps. Thus, extremely small, sub-micrometer feature sizes and vias can be implemented for the infrared bolometer arrays. The Si/SiGe thermistor is grown epitaxially, forming a mono-crystalline multi layer structure. The temperature coefficient of resistance (TCR) is primarily controlled by the concentration of Ge present in the strained SiGe layers. TCR values of more than 3%/K can be achieved with a low signal-to-noise ratio due to the mono-crystalline nature of the material. In addition to its excellent electrical properties, the thermistor material is thermally stable up to temperatures above 600 °C, thus enabling the novel integration and packaging techniques described in this paper. Vacuum sealing at the wafer level reduces the overall costs compared to encapsulation after die singulation. Wafer bonding is performed using a Cu-Sn based metallic bonding process followed by getter activation at ≥350 °C achieving a pressure in the 0.001 mbar range. After assembling, the final metal phases are stable and fully compatible with hightemperature processes. Hermeticity over the product lifetime is accomplished by well-controlled electro-deposition of metal layers, optimized bonding parameters and a suitable bond frame design.

Paper Details

Date Published: 13 May 2010
PDF: 10 pages
Proc. SPIE 7726, Optical Sensing and Detection, 772611 (13 May 2010); doi: 10.1117/12.855752
Show Author Affiliations
Niclas Roxhed, KTH - Royal Institute of Technology (Sweden)
Frank Niklaus, KTH - Royal Institute of Technology (Sweden)
Andreas C. Fischer, KTH - Royal Institute of Technology (Sweden)
Fredrik Forsberg, KTH - Royal Institute of Technology (Sweden)
Linda Höglund, Acreo AB (Sweden)
Per Ericsson, Acreo AB (Sweden)
Björn Samel, Acreo AB (Sweden)
Stanley Wissmar, Acreo AB (Sweden)
Anders Elfving, SensoNor AS (Norway)
Tor Ivar Simonsen, SensoNor AS (Norway)
Kaiying Wang, Vestfold Univ. College (Norway)
Nils Hoivik, Vestfold Univ. College (Norway)

Published in SPIE Proceedings Vol. 7726:
Optical Sensing and Detection
Francis Berghmans; Anna Grazia Mignani; Chris A. van Hoof, Editor(s)

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