Space Based Imagers

Mature silicon processing technology enabled the production of very high performance CCDs for space based applications. A space based planet finder for the NASA Kepler Discovery mission requires a 2200x1024 pixel imager with 27 micron pixels [4]. A large focal plane of 46 backside-illuminated imagers fills the requisite optical field. Figure 2 shows STA manufactured initial prototypes of the CCD.

Figure 2. (left) Kepler 2Kx1K and (right) FAME 2Kx4K CCDs.

A somewhat larger 2048x4064 imager with a 15 micron pixel was produced for the Naval Observatories FAME program. This device incorporated a novel input serial register at the top of the CCD to allow for independent metered background charge to each column (see Fig. 2). This serves to mitigate radiation damage effects over long space missions by filling the radiation induced traps.

The excellent yield and performance of these devices demonstrates significant improvement in silicon material quality and processing cleanliness.

2.3 Orthogonal Transfer CCD

As silicon processing technology has improved, so has device capability. An example is the orthogonal transfer CCD [5]. This device requires 4 levels of polysilicon to achieve charge transfer in arbitrary directions. The additional complexity enables performance of astronomical tip-tilt correction directly within the CCD. In addition, on chip logic selects individual sub-arrays for readout.

Figure 3 shows the focal plane, whish consists of an 8*8 array of sub-arrays, each with 480*494 12 micron pixels. The CCD has 8 separate outputs. A column of 8 sub-arrays can be read out one by one through one of the outputs by setting the appropriate control lines. An important advantage of 64 separate sections for a large CCD is the inherent fault tolerance. If a section has a serious defect it can be deselected with a control signal. The benefit is a higher effective yield. A catastrophic defect in a normal 4K*4K CCD would disqualify the entire device. In an Orthogonal Transfer CCD this would mean only a single missing section of 64, and the device could still be utilized.

OTCCD SubArray 8x8 Array of OTCCDs

Figure 3. Orthogonal Transfer CCD configuration.

OTCCD SubArray 8x8 Array of OTCCDs

Figure 3. Orthogonal Transfer CCD configuration.

2.4 STA0500A 4k Imager

STA utilizes 150 mm wafers for CCD fabrication. There are significant advantages of these wafers. Our group originally fabricated 4K*4K imagers on 100 mm wafers at Ford Aerospace with only marginal yield of less than 10%. It is only possible to fit a single device on a wafer of this size (see Fig. 1). In conjunction with the Imaging Technology Laboratory (ITL) at the University of Arizona [6], we have designed a new 4K*4K imager incorporating new process technology tools and an increased wafer size. Figure 4 demonstrates the advantages of such an approach.

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UA Foundry Run

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UA Foundry Run

Figure 4. STA0500A wafer delineation.

The wafer mask was designed to include several different types of CCDs needed for various UA projects, including 800^1200 and 2688x512 spectroscopic format CCDs. These are updated designs of detectors produced many years ago and fabricated at Ford Aerospace by the author. The devices were designed by Dr. John Geary of the Smithsonian Astrophysical Observatory. A single wafer includes two 4Kx4K CCDs, four 2688x512 CCDs, four 1200x800 CCDs, four 512x1024 frame transfer guider CCDs, eight 128x128 adaptive optics CCDs and several four side buttable demonstration CCDs. It is a significant cost benefit to fabricate a variety of CCDs on a single wafer. The cost of fabricating a 100 mm wafer is identical to the cost of a 150 mm wafer. Producing two 4Kx4K devices on a single wafer results in one half the fabrication cost. It also reflects higher yields because the proportion of good silicon area to edge is higher. DC yields of 4Ks on 150 mm wafers routinely exceed 60%. Functional yield is also very high. A recent run yielded 15 astronomy grade (the highest) 4K CCDs out of 48, a 30% yield.

Once a fabrication run of 24 wafers is completed, STA performs DC parametric tests and nominal functional tests. The best wafers are selected and shipped to ITL for further low temperature testing and selection of candidates for thinning. Figure 5 shows a completed 4K in its dewar ready for mounting on the Magellan telescope. The results for 4K thinned devices have been excellent. Output noise is routinely ~2.8 electrons at 50kHz for the four outputs. HCTE and VCTE are >0.999998. For the multi-pinned phase mode of operation full well charge capacity is >80,000 electrons. Dark current in non-MPP mode is ~10 e-/pixel/hour at -100C.

Figure 5. Backside thinned 4K CCD for Carnegie Magellan Telescope.

Maximum quantum efficiency is obtained by backside thinning and applying a suitable anti-reflection coating for the optical region of interest. Figure 6 shows the QE curve for a "blue" optimized 4K CCD.

Figure 6. 4K QE at -115°C optimized for blue response.

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