A Dual-mode Large-arrayed CMOS ISFET Sensor for Accurate and High-throughput pH Sensing in Biomedical Diagnosis

Goal: The existing ISFET based DNA sequencing detects hydrogen ions released during polymerization of DNA strands on microbeads, which are scattered into microwell array above the ISFET sensor with unknown distribution. However, false pH detection happens at empty microwells due to cross-talk from neighbouring microbeads. In this paper, a dual-mode CMOS ISFET sensor is proposed to have accurate pH detection towards DNA sequencing. Methods: Dual-mode sensing, optical and chemical modes, is realized by integrating CMOS image sensor (CIS) with ISFET pH sensor, and is fabricated in standard 0.18μm CIS process. With accurate determination of microbead physical locations with CIS pixel by contact imaging, the dual-mode sensor can correlate local pH for one DNA slice at one location-determined microbead, which can result in improved pH detection accuracy.

Moreover, towards high-throughput DNA sequencing, a correlated double sampling (CDS) readout that supports large array for both modes is deployed to reduce pixel-to-pixel non-uniformity such as threshold voltage mismatch. Results: The proposed CMOS dual-mode sensor is experimentally examined to show a well correlated pH map and optical image for microbeads with a pH sensitivity of 26.2mV/pH, a fixed pattern noise (FPN) reduction from 4% to 0.3%, and a readout speed of 1200 frames/second (fps). Conclusion: A dual-mode CMOS ISFET sensor with suppressed FPN for accurate large-arrayed pH sensing is proposed and demonstrated with state-of-the-art measured results towards accurate and high-throughput DNA sequencing. Significance: The developed dual-mode CMOS ISFET sensor has great potential for future personal genome diagnostics with high accuracy and low cost.

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