A low cost uncooled infrared microbolometer focal plane array using the CMOS n-well layer

Abstract

This paper reports a low-cost, 256-pixel uncooled infrared microbolometer focal plane array (FPA) implemented using a 0.8 /spl mu/m CMOS process where the n-well layer is used as the active microbolometer material. The suspended n-well structure is obtained by simple front-end bulk etching of the fabricated CMOS dies, while the n-well region is protected from etching by electrochemical etch-stop technique within a TMAH solution. Electrical connections to the suspended n-well are obtained with polysilicon interconnect layer instead of aluminum to increase the thermal isolation of the pixel by an order of magnitude. Since polysilicon has very low TCR and high resistance, the effective TCR of the pixel is reduced to 0.34%/K, even though the n-well TCR is measured to be 0.58%/K. A 16/spl times/16 pixel array prototype with 80 /spl mu/m/spl times/80 /spl mu/m pixel sizes has successfully been implemented. The pixel resistance measurements show that pixels are very uniform with a nonuniformity of 1.23%. Measurements and calculations show that the detector and the array provide a responsivity of 1200 V/W, a detectivity of 2.2/spl times/10/sup 8/ cm/Hz/sup 1/2//W, and a noise equivalent temperature difference (NETD) of 200 mK at 0.5 Hz frame rate with fully serial readout scheme. This performance can be further increased by using other advanced readout techniques, therefore, the CMOS n-well microbolometer approach seems to be a very cost-effective method to produce large focal plane arrays for low-cost infrared imaging applications.