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Improvements of a digital 25 µm pixel-pitch uncooled amorphous silicon TEC-less VGA IRFPA with massively parallel Sigma-Delta- ADC readout

: Weiler, D.; Ruß, M.; Lerch, R.G.; Yang, P.; Bauer, J.; Heß, J.; Kropelnicki, P.; Vogt, H.


Andresen, B.F. ; Society of Photo-Optical Instrumentation Engineers -SPIE-, Bellingham/Wash.:
Infrared technology and applications XXXVII : 25 April 2011, Orlando, Florida, USA
Bellingham, WA: SPIE, 2011 (Proceedings of SPIE 8012)
ISBN: 978-0-8194-8586-1
Paper 80121F
Conference on Infrared Technology and Applications <37, 2011, Orlando/Fla.>
Conference Paper
Fraunhofer IMS ()
infrared detector; IRFPA; VGA; uncooled microbolometer; amorphous silicon; Sigma-Delta-ADC on chip

This paper presents the improvements of an advanced digital VGA-IRFPA developed by Fraunhofer-IMS. The uncooled IRFPA is designed for thermal imaging applications in the LWIR (8 .. 14 µm) range with a full-frame frequency of 30 Hz and a high sensitivity with NETD < 100 mK @ f/1. The microbolometer with a pixel-pitch of 25 µm consists of amorphous silicon as the sensing layer. The structure of the microbolometer has been optimized for a better performance compared to the 1st generation IRFPA1. The thermal isolation has been doubled by increasing the length and by decreasing the width of the legs. To increase the fill-factor the contact areas have been reduced. The microbolometers are read out by a novel readout architecture which utilizes massively parallel on-chip Sigma-Delta-ADCs. This results in a direct digital conversion of the resistance change of the microbolometer induced by incident infrared radiation. Two different solutions for the vacuum package have been developed. To reduce production costs a chip-scale-package is used. This vacuum package consists of an IR-transparent window with antireflection coating and a soldering frame which is fixed by a wafer-to-chip process directly on top of the read substrate. An alternative solution based on the use of a standard ceramic package is utilized as a vacuum package. This packaging solution is used for high performance applications. The IRFPAs are completely fabricated at Fraunhofer-IMS on 8" CMOS wafers with an additional surface micromachining process.