Charge Quantizing CCDs Optimized for Astronomy

Spectroscopy is a key tool used in many branches of science and industry. Light emitted in specific colors can identify the elements present, their relative concentration, temperature or even the velocity of the source. However, the signals are weak, and the electrical noise added by the sensor itself can limit what can be measured. This electrical noise cannot be eliminated, but in sensors called “Charge Couple Devices” (CCD) the electrical charge generated by the signal can be measured many times and averaged until the exact number of electrons in the pixel is determined. Unfortunately, this technique greatly increases the time to readout a complete image. This team will demonstrate the ability to readout an image in a few minutes instead of hours. The image sensor will be designed by the vendor Semiconductor Technology Associates (STA) to have 4096x4096 pixels, with several hundred optimized outputs operating in parallel. Caltech will develop compact electronics located near the sensor to digitize the many output signals. These sensors will be manufactured at Microchip Technology silicon foundry in Arizona and will subsequently become commercially available. These CCDs will typically improve the sensitivity of spectrographs used in astronomy by a factor of two to four. Similar gains will be possible in low light applications in chemistry, biology, and medicine. Caltech will involve undergraduate students in the testing of electronics and detectors. This team will produce an n-channel Quantizing CCD (qCCD) in a commonly used 4K x 4K format (split frame transfer) with 128 floating gate amplifiers per extended serial register. A new differential version of the standard Floating Gate Amplifier (FGA) will halve the readout time for any given channel count and pixel sampling rate. The Differential FGA output will first be optimized in numerical simulations using a commercial software package from SILVACO and then validated by testing a range of transistor geometries and doping profiles. A new mounting package to support the high pin count will connect by flex circuit to a compact electronics module designed to support 128 differential channels. This will perform digital differential averaging close to the detector to greatly reduce the number of signal connections needed through the vacuum wall. The 4K x 4K qCCD will be manufactured using conventional epitaxial n-channel technology at an onshore foundry. The move from traditional 6” wafers to 8” wafers will increase the number of devices per wafer from one to four. Backside processing and thinning will be optimized to maximize UV and blue response. This program will include a demonstration on a new spectrograph being deployed at Palomar Observatory. The creation of a 4Kx 4K blue optimized CCD that achieves charge quantization in under 2 minutes should open a pathway to very significant sensitivity gains at astronomical telescopes. This project will result in large-format image sensor manufacturing in the USA again, and it will promote global competitiveness of USA-based small businesses engaged in semiconductor design and fabrication. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.