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The work being undertaken by Mike Lesser at Steward promises to be crucial in the task of obtaining high DQE Loral foundry CCDs. The goal is to back-thin devices from their wafer runs and provide options for packaging single and buttable devices. The thinning process Mike has developed works well and so the expectation of obtaining thinned devices appears to be good.
To date Mike has successfully thinned several types of single detectors, for example he had recently delivered some devices to NOAO. However he has not yet delivered any functional thinned, buttable devices but is actively working on this aspect at present. After thinning the silicon, Mike has to Indium bump-bond the inverted CCD to a substrate. The problem arises in ensuring that all the Indium bumps make proper contact with the substrate- an operation which has so far not met with complete success. Once this problem has been solved, there will be a backlog of devices waiting to be thinned and packaged.
After successful thinning and bump-bonding the next important step in producing a device with the best UV response, is to apply an appropriate surface treatment. Mike has developed an oxide flash gate technique which achieves a high response when used with a UV flood to charge the back-side. This technique has now been refined to the stage where the UV response can be maintained reliably.
Together with the oxide-flash gate, Mike has also investigated the use of novel anti-reflection (AR) coatings to optimise the DQE; some of these results show peak responses of (see figure 1).
All thinned CCDs exhibit some level of fringing at red wavelengths and this is a problem Mike is aware of in the devices he is fabricating. Our view is that any progress he can make in reducing this will be useful but would not preclude procurement of his devices.