Current production testing of CMOS imager sensors is mainly based on capturing images and detecting failures by image processing with special algorithms. The fault coverage of this costly optical test is not sufficient given the quality requirements. Studies on devices produced at large volume have shown that Horizontal Fixed Pattern Noise (HFPN) is one of the common image failures encountered on products that present fault coverage problems, and this is the main cause of customer returns for many products. A detailed analysis of failed devices has demonstrated that HFPN failures arise from changes of electronic circuit topology in pixel addressing decoders or the metal lines required for pixel powering and control. These changes are usually due to the presence of spot defects, causing some pixels in a row to operate incorrectly, leading to an HFPN failure. Moreover, defects resulting in partially degraded metal lines may not induce image failure in limited industrial test conditions, passing the optical tests. Later, these defects may produce an image failure in the field, either because the capture conditions would be more stringent, or because the defects would evolve into catastrophic faults due to electromigration. In this paper, we have first enhanced the HFPN detection algorithm in order to improve the fault coverage of the optical test. Next, a built-in self-test structure is presented for the on-chip detection of catastrophic and non-catastrophic defects in the pixel power and control lines.

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ISBN : 978-1-4799-0039-8International audienceInterconnection lines in the sensors of CMOS imagers are used for pixel bias, addressing and readout. Catastrophic faults in these lines can cause parts of the pixel matrix to operate incorrectly and produce image defects like residual stripes and bands in images. These kinds of image defects are often difficult to remove by the image processing correction algorithm, and they are clearly visible as a sort of noise pattern. Among the defects in the pixel array, these catastrophic faults have most important influence on yield. In addition, partially degraded metal lines cannot be detected on todays' standard industrial testers for image sensors. These defects can evolve into catastrophic faults and they are the main cause of customer returns for many products. This paper proposes two built-in self-test (BIST) solutions to catch these defects in the pixel array, taking into account the industrial test constraints, namely increase of fault coverage, decrease of test time and test cost minimization

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