OPTIMIZING CCDS FOR SPECTROGRAPHS

MICHAEL ANDERSENDivision of Astronomy, University of Oulu, Finland

e-mail: michael.anderson@oulu.fi

(Received 16 June 2000; accepted in revised form 7 December 2000)

Abstract. For a spectrograph giving a fixed format spectrum, the quantum efficiency (QE) can beoptimized for the different wavelengths across the CCD. It is shown that a slight modification ofthe conventional single layer anti-reflection coating can give major improvements in QE for suchinstruments, while at the same time minimizing problems with fringing and stray light from theCCD.

Keywords: instrumentation: spectrographs – detectors: – CCDs

1. Introduction

In recent years, CCDs with peak quantum efficiency (QE) indistinguishable from100% have appeared. The successful manufacture of these devices has been pos-sible due to near perfect QE pinning at all optical wavelengths and the applicationof a HfO2 single layer anti-reflection coating (Lesser, 1990; Lesser & Iyer 1998).For wavelengths shorter than 800nm, the QE is completely limited by optical lossesin the coating of the CCD, while at longer wavelengths the increasing transparencyof silicon leads to the fall-off of the QE, i.e. the QE excluding optical losses, isessentially 100% at all wavelengths (see Figure 1).

While CCDs with near optimum QE are now available, some problem areas stillremain for thinned back-side illuminated devices. Possibly the most important ofthese is fringing at near IR wavelengths, arising from multiple reflections at thefront and back of the epitaxial silicon. The wavelengths at which the fringing prob-lem occurs can be shifted into the near IR by using thicker high resistivity epitaxialsilicon (Stover, 1998; Groom, 1999). But the problem remains. Another problem isthe very high reflectivity of the CCD surface, as compared to optical components(typically one order of magnitude difference). The ghost images resulting from thisreflection is a major challenge to optical designers.

2. The gradient thickness AR coating

In modern high resolution échelle spectrographs, where high stability of the wave-length calibration is desired, the spectral format on the CCD is usually fixed.

Experimental Astronomy 11: 81–83, 2001.© 2001 Kluwer Academic Publishers. Printed in the Netherlands.

82 M. I. ANDERSEN

Figure 1. Measured quantum efficiency on a Loral/Lesser Cat-C 2k3eb CCD. The dashed curve isa model QE curve, corresponding to 16 micron epitaxial silicon and a single layer coat centered on435nm. The measured QE is consistent with all losses being purely optical.

Likewise, it is possible to design a prism cross-dispersed low resolution spectro-graph having a fixed spectral format. If designed for working in first, second andthird order of diffraction, the full spectral range detectable by the CCD may berecorded, using a moderately long slit.

These applications open an opportunity to optimize the anti- reflection coatingindividually for each spectral order, by changing the thickness of the coating acrossthe CCD. This can be accomplished by moving a slow shutter across the CCDduring the coating process, while monitoring the deposition of the coating. Thiswill enhance the QE at all wavelengths, while almost eliminating the problemswith fringing and reflection from the CCD. Figure 2 shows the QE for a gradientthickness single layer coating, as compared to a constant thickness single layercoating. The fringing amplitude is reduced from above 20% to less than 2%.

In high resolution èchelle spectrographs there are two main sources of stray light(uncontrolled photons): Scattered light from the èchelle grating, and light reflectedfrom the CCD. The first problem can be largely solved by proper baffling. Thisleaves reflection on the CCD surface as the main problem, effectively preventingvery high fidelity spectra to be obtained. With the gradient thickness coating the re-flectivity is reduced by more than an order of magnitude, making the CCD surfaceoptically comparable to lens surfaces. At the same time, the fringing amplitude isalso reduced by an order of magnitude.

OPTIMIZING CCDS FOR SPECTROGRAPHS 83

Figure 2. Model of CCD QE with a constant thickness and gradient thickness single layeranti-reflection coating. 12µ of epitaxial silicon in double pass is assumed.

3. Conclusions

We are building large telescopes and equipping them with expensive spectrographswith the purpose of observing fainter and more distant objects. Yet, we are throwingaway about 1/5 of the photons at the surface of our detector. By applying a gradientthickness coating to the CCD for dedicated spectrographs, it is possible to obtainnear perfect QE for all wavelengths, while at the same time solving problems withfringing and ghost images.

References

Groom, D. E. et al. 1999, ‘Quantum efficiency of a back-illuminated CCD imager: An opticalapproach’, Proc SPIE Vol. 3649, pp. 13–18.

Lesser, M. P. 1990, "CCD thinning, coating, and mounting research for astronomy", in ASP Conf.Vol. 8, pp. 65-75, Ed. G. H. Jacoby.

Lesser, M. P., Iyer, V. 1998, "Enhancing back-illuminated performance of astronomical CCDs", inProc. SPIE Vol. 3355, pp. 446-456.

Stover, R. J. et al. 1998, "High performance CCD on high-resistivity silicon", in Proc. SPIE Vol.3505, pp. 13-18.