CFHT Information Bulletin, number 38, First Semester 1998



The CFH12k CCDs

Tim Abbott, CFHT. (tmca@cfht.hawaii.edu)

This new mosaic camera is currently under construction in Gerry Luppino's lab at IfA, University of Hawaii at Manoa. The dewar has been built and leak tested and the cold-surface will begin fabrication in January 1998. The camera will incorporate 12 MIT, Lincoln Labs 2k x 4k 15 micron CCDs arranged in a mosaic of two devices by four to produce overall image dimensions of 8,192 by 12,288 pixels.

The CCDs themselves will be derived from a development effort led by Gerry Luppino and Barry Burke of Lincoln Labs and funded by a consortium comprising CFHT, IfA, ESO, AAO, Keck and Subaru. CFHT has bought IfA's share of the first four wafer runs. The first 11 devices, of which CFHT will receive four, have undergone preliminary testing. Some remarkable CCDs are to be found among these - at least one has shown a read noise of less than one electron, a first for the CCD industry, many have almost perfect charge transfer efficiency, and the first device selected for the CFH12k has only four traps and no hot columns! Some general characteristics of the devices tested so far are:

Horizontal CTE

0.99999 - 1.0

Vertical CTE

0.999995 - 0.999999

Full well

106,000 - 176,000 electrons

Dark current at -110 ºC

1.8 - 5.8 electrons per pixel per hour

Readout noise at 4µs/sample

2-5.5 electrons/pixel

Of course, these devices are not without problems, the backside passivation technology involves laser annealing of a boron implant. Lincoln labs have experienced difficulties with the uniformity of the annealing process and a regular pattern of high and low quantum efficiency is seen in the flat field (Figure 34). This pattern hasan amplitude which increases towards the blue and with decreasing device temperature. Lincoln Labs have given a high priority to solving this problem and have already reduced its effect considerably.

The simple, single-layer SiO2 anti-reflection coating used on the first devices limits the blue sensitivity (Figure 35). Later devices will have a multi-layer coating tuned for better blue response.

Devices have been made on two types of silicon: standard EPI and high-resistivity. The latter are so-called deep-depletion devices wherein the charge collection fields of the pixels extend deeper into the CCD, and so signal electrons generated by more deeply penetrating red photons are accumulated and the red efficiency is enhanced. However, high resistivity silicon is brittle and hard to handle. Thus only one wafer run was dedicated to this technology and the yield has been less impressive than the EPI runs.

The CFH12k camera will see first light at CFHT prime focus late in the 1998I semester. Please see http://www.cfht.hawaii.edu/Instruments/Imaging/CFH12k/@ to follow the progress of this project.

(All test data are courtesy of Gerry Luppino, IfA and Richard Stover and Mingzhi Wei of Lick Observatory.)





Editor: Dr. T. M. C. Abbott, tmca@cfht.hawaii.edu
Copyright © 1998, CFHT Corporation. All rights reserved.