Introduction

Although most CCDs are used for multi-band imaging or variable dispersion spectrographs, there are some fixed-format spectrographs where it can be useful to match the CCD peak response to the wavelength being measured. The performance of CCDs in such instruments can be improved by adjusting the AR coating thickness across the device to match the wavelength projected by the spectrograph [1]. This paper describes the results of depositing a graded thickness coating of hafnium oxide on large format detectors.

A graded-AR coating has been previously been supplied on CCD25-20 sensors for the MERIS/ENVISAT ESA satellite. The Meris coating was of tungsten oxide and was deposited on each individual packaged device. This material has some losses at UV wavelengths.

The preferred material for single layer AR coatings on CCDs operating from ultraviolet to infrared wavelengths is hafnium oxide. The deposition conditions (higher temperature) and the available deposition geometry (whole wafers) make the production of graded thickness coatings more difficult to realise. Development work has been carried out at e2vtechnologies to allow a graded thickness of hafnium oxide to be deposited onto large format CCDs.

2. AR-COATING DEPOSITION

After preliminary trials, full-sized 5" wafers were coated with a graded thickness, and used to manufacture CCD44-82 devices (2K*4K) for evaluation. A proprietary coating process was used to achieve the graded thickness deposition. The deposited coating was designed to provide minimum reflectivity (maximum QE) ranging from 300 nm to 1000 nm at the two extremes of the CCD length (61 mm). The coating was applied to standard silicon wafers (backthinned to 16 ^m) and to deep depletion ones (thinned to 40 ^m). An image of the chip is shown in Fig. 1.

Figure 1. Backside surface of CCD44-82 showing graded coating along the major axis.

Note. See colour section ofproceedings for colour copy offigure.

Figure 1. Backside surface of CCD44-82 showing graded coating along the major axis.

Note. See colour section ofproceedings for colour copy offigure.

The coating thickness was measured by ellipsometry (Geartner Scientific Corp., model-L117). The coating was slightly thinner than target, but the gradient was very close to the intended design. Note that the thickness varies non-linearly, since refractive index is non-linear, especially at blue wavelengths. We anticipate that with our improved understanding of the coating process, we will be able to achieve coatings very close to the design intention.

The following sections describe performance measurements, some measurements were taken before the device was sawn from the wafer, and others taken on the completed device.

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