Evolution of Scientific Image Sensor ICs - Past, Present, and Future
Channel: IEEE Buenaventura Section
Date: 10/12/2021

Initial advancement of image sensor IC technology from the 1970s to 1990s focused on improving signal to noise ratio or detector limited performance. This was achieved by extending integration time of each pixel to the complete available exposure time and reducing readout noise below the detector dark noise. First phase of pixel miniaturization helped convert scanning imagers into time-delay integration (TDI) scanners and eventually into staring arrays, thus extending their integration time. The read noise was reduced through input circuit innovations such as source follower circuit, capacitive trans-impedance amplifier (CTIA), gate-modulation (GM), direct-injection (DI), buffered direct-injection (BDI) circuit and other more advanced circuits.

The second wave of image sensor IC innovation in the last two decades focused on developing large formats with smaller pixels. To regain low noise performance on large focal plane arrays (image sensor sensors) without compromising on frame rates the analog ICs needed to be converted into digital ICs. Digital ICs started from having digitizer (ADC) per chip in late 1990s, to ADC per column in 2000s and eventually per pixel ADC were developed. Other innovative IC technology was also demonstrated in the past two decades. These included multi-color pixels, pixels with intensity and ranging capabilities, ultra-low noise astronomy pixels supporting very long integration, and rad-hard by design ICs for space. These image sensor IC innovations of the past have been enabled by shrinking CMOS design rules and other CMOS process improvements.

Current wave of imaging IC innovation is focusing on utilizing the per pixel ADC capability to develop high dynamic range (HDR) imagers and further shrinking of the pixel size. As in the past the focus was to achieve background limited SNR performance, currently the focus is on achieving diffraction limited spatial resolution for all the bands. Visible pixels are reaching 1/2 micron size, near infrared (NIR) pixels have reached 1-micron dimensions, SWIR pixels are being developed for 1-micron to 3-micron pitch, MWIR pixel for 3-micron to 8-micron pitch and LWIR pixels for 8-micron to 12-micro pitch. In addition to interesting optical challenges these small pitch imagers are pushing the IC technology to achieve the same charge density and similar low noise circuitry as that of the much larger pixels of previous generation ICs. The latest CMOS technology innovations are being employed again to achieve the performance required from the current ICs. These include, stacked high-density high-K dielectric capacitors, trench capacitors, deep sub-micron processes and 3D-IC integration to stack multi-layer ICs. This presentation delves into the past innovations and what the future of image sensors entails.

ABOUT THE SPEAKER
Atul Joshi is the CEO and chief innovation officer of SAAZ Micro Inc that he founded in 2012. His image sensor research is published in over 100 international publications, he has over 50 inventions, and he has 6 active US patents. Atul has received various awards including the prestigious R&D100 award and his team also received the Wall Street Journal’s award for most innovative semiconductor product of the year. During his graduate and post-graduate research at UC Berkeley Atul designed particle detector ICs used at CERN for the discovery of the Higgs Boson. After academic research Atul joined Rockwell International’s imaging group (now Teledyne Imaging) where he developed some of the first commercial CMOS imaging systems-on-a-chip and also the largest format infrared sensors and cameras used in aerospace. Currently he is developing novel sensors and systems for industrial, airborne and space imaging.