In this work we describe efforts to reduce the read noise in fully depleted, scientific charge-coupled devices (CCDs). The read noise is proportional to the total capacitance at the floating-diffusion node. Reductions in the capacitance at the floating diffusion are accomplished by implementing a direct contact between the output transistor, polysilicon-gate electrode and the floating diffusion. We have previously reported promising results for this technology that were measured on small-format CCDs with 4-channel readout where each channel had a different output transistor geometry. In this work we present the results of the use of this technology on 12 and 16-channel, large-format CCDs in order to determine the reproducibility of the process. The contact size for this work is two microns by two microns, and projection lithography was used to print the contacts. We have also utilized selective wafer-stepper lithography to generate contacts that are one micron on a side. We also describe efforts in the device design of the output transistor to further reduce the noise.
We describe work at Lawrence Berkeley National Laboratory (LBNL) to develop enhanced performance, fully
depleted, back-illuminated charge-coupled devices for astronomy and astrophysics. The CCDs are fabricated on
high-resistivity substrates and are typically 200–300 μm thick for improved near-infrared response. The primary
research and development areas include methods to reduce read noise, increase quantum efficiency and readout
speed, and the development of fabrication methods for the efficient production of CCDs for large focal planes.
In terms of noise reduction, we will describe technology developments with our industrial partner Teledyne
DALSA Semiconductor to develop a buried-contact technology for reduced floating-diffusion capacitance, as well
as efforts to develop ”skipper” CCDs with sub-electron noise utilizing non-destructive readout amplifiers allowing
for multiple sampling of the charge packets. Improvements in quantum efficiency in the near-infrared utilizing
ultra-high resistivity substrates that allow full depletion of 500 μm and thicker substrates will be described, as
well as studies to improve the blue and UV sensitivity by investigating the limits on the thickness of the back-side
ohmic contact layer used in the LBNL technology. Improvements in readout speed by increasing the number of
readout ports will be described, including work on high frame-rate CCDs for x-ray synchrotrons with as many as
192 amplifiers per CCD. Finally, we will describe improvements in fabrication methods, developed in the course
of producing over 100 science-grade 2k × 4k CCDs for the Dark Energy Survey Camera.
We describe the design and optimization of low-noise, single-stage output amplifiers for p-channel charge-coupled
devices (CCDs) used for scientific applications in astronomy and other fields. The CCDs are fabricated on highresistivity,
4000-5000 Ω-cm, n-type silicon substrates. Single-stage amplifiers with different output structure
designs and technologies have been characterized. The standard output amplifier is designed with an n+ polysilicon
gate that has a metal connection to the sense node. In an effort to lower the output amplifier readout
noise by minimizing the capacitance seen at the sense node, buried-contact technology has been investigated. In
this case, the output transistor has a p+ polysilicon gate that connects directly to the p+ sense node. Output
structures with buried-contact areas as small as 2 μm × 2 μm are characterized. In addition, the geometry of the
source-follower transistor was varied, and we report test results on the conversion gain and noise of the various
amplifier structures. By use of buried-contact technology, better amplifier geometry, optimization of the amplifier
biases and improvements in the test electronics design, we obtain a 45% reduction in noise, corresponding to
1.7 e- rms at 70 kpixels/sec.
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