Low--‐cost uncooled infrared imaging sensor using mems and a modified standard cmos process

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2013
Gülden, Mehmet Ali
The thesis presents a monolithically integrated low-­‐cost uncooled infrared imaging sensor using a MEMS process and a modified standard CMOS process. The designed sensor has an image format of 160×120 with a pixel pitch of 40 μm. The sensor is implemented with microbolometers that sense the infrared radiation in the 8-­‐12 μm spectral band, where the sensing elements in each pixel are formed with CMOS diodes to sense the temperature variation in the pixel by monitoring the change in the forward bias voltage of diodes. The pixels have multiple serially connected diodes to increase the temperature sensitivity. The number of serially connected diodes in each pixels can be configured as 4, 5, and 6 with simple modification in the CMOS metal layers, at the expense of using higher supply voltages required to bias the pixel array. The current sensor design has 6 serially connected diodes in the pixel, requiring a 7 V supply voltage in the pixel array. The sensor has been implemented using a 1 μm SOI-­‐CMOS process, with a slight process modification, where the pixel area is processed using a higher grade photolithography process allowing a minimum feature size of 0.5 μm. The smaller feature size in the pixel area allows obtaining better thermal isolation of the suspended bridges that hold the diodes through support arms. Suspended bridge like structures are obtained with a simple post-­‐ CMOS etching process, which does not require any critical lithography steps. Hence, microbolometer fabrication cost can be decreased drastically, making these microbolometers potentially suitable for newly emerging various cost-­‐effective infrared imaging applications, such as automotive, advanced presence detection, security, and consumer electronics. The designed infrared imaging sensor has a system-­‐on-­‐chip (SoC) architecture, and it runs on a single 7 V supply voltage. The designed sensor has both analog and digital integrated circuit modules, typically running at 5 V supply voltage except the pixel array. The designed imaging sensor is highly programmable, where timing of the critical digital signals and level of analog voltage and current biases can be programmed using programmable on-­‐chip digital controllers and digital-­‐to-­‐analog converter (DAC) based voltage and current biasing circuits. The chip has also an integrated output buffer to drive high capacitance external loads. The infrared imaging sensor developed in this thesis has been fabricated in a 1 μm SOI-­‐CMOS process using a wafer-­‐level test run with a slight modification to allow fabrication of pixels with 0.5 μm minimum features size in the pixel area. Fabricating the imaging sensor at wafer-­‐level allows also using conventional wafer-­‐level microfabrication and MEMS processing steps, avoiding any difficulties that may arise due to use of die-­‐level processing. The fabricated imaging sensor measures 10.5 mm×11.0 mm, where the readout circuitry is integrated in two parts at the top and the bottom of the chip. In this way, the pixel array is placed in the middle of the chip, hence it is possible to add a rectangular ring structure around the pixel array, which can later be used as a bonding surface required by several wafer-­‐level vacuum packaging processes. The fabricated infrared imaging sensor is tested in detail to verify its functionality and to perform its characterization. The sensitivity and DC responsivity of single pixel test structures having 6 serially connected diodes are measured to be -­‐7.05 mV/K and 18,194 V/W, respectively. The overall output noise of the readout circuit is measured as 124 μVrms. The integrated detector noise of the single pixel is measured as 5.19 μVrms in a 4 kHz bandwidth, resulting in an expected NETD value of 840 mK for the imager with an f/1 optics.

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Citation Formats
M. A. Gülden, “Low--‐cost uncooled infrared imaging sensor using mems and a modified standard cmos process,” M.S. - Master of Science, Middle East Technical University, 2013.