Proceedings Article | 15 September 2009
KEYWORDS: Charge-coupled devices, Silicon, Imaging arrays, Sensors, Quantum efficiency, Ultraviolet radiation, Back illuminated sensors, Semiconducting wafers, CMOS sensors, Image sensors
In this paper, we report the latest results on our development of delta-doped, thinned, back-illuminated CMOS imaging
arrays. As with charge-coupled devices, thinning and back-illumination are essential to the development of high
performance CMOS imaging arrays. Problems with back surface passivation have emerged as critical to the prospects
for incorporating CMOS imaging arrays into high performance scientific instruments, just as they did for CCDs over
twenty years ago. In the early 1990's, JPL developed delta-doped CCDs, in which low temperature molecular beam
epitaxy was used to form an ideal passivation layer on the silicon back surface. Comprising only a few nanometers of
highly-doped epitaxial silicon, delta-doping achieves the stability and uniformity that are essential for high performance
imaging and spectroscopy. Delta-doped CCDs were shown to have high, stable, and uniform quantum efficiency across
the entire spectral range from the extreme ultraviolet through the near infrared. JPL has recently bump-bonded thinned,
delta-doped CMOS imaging arrays to a CMOS readout, and demonstrated imaging. Delta-doped CMOS devices exhibit
the high quantum efficiency that has become the standard for scientific-grade CCDs. Together with new circuit designs
for low-noise readout currently under development, delta-doping expands the potential scientific applications of CMOS
imaging arrays, and brings within reach important new capabilities, such as fast, high-sensitivity imaging with parallel
readout and real-time signal processing. It remains to demonstrate manufacturability of delta-doped CMOS imaging
arrays. To that end, JPL has acquired a new silicon MBE and ancillary equipment for delta-doping wafers up to 200mm
in diameter, and is now developing processes for high-throughput, high yield delta-doping of fully-processed wafers
with CCD and CMOS imaging devices.
