Fully Depleted, 300 mm Thick CCD Image Sensors
With Applications in the x-ray, UV, Visible and Near IR Regions

S.E. Holland, D.E.Groom, M.E. Levi, S. Perlmutter
Lawrence Berkeley National Laboratory
University of California, Berkeley 94720

R.J. Stover, M. Wei
University of California Observatories/Lick Observatory
University of California, Santa Cruz 95064

Abstract

We are developing large-format, high-quantum-efficiency charge coupled devices (CCD's) for astronomy and astrophysics applications. The CCD's have high quantum efficiency at red and near -infrared wavelengths due to an active thickness of 300 mm, which also results in good efficiency for low-energy x rays.  The CCD's are operated back illuminated with good blue response, and the self-supporting 300 mm thick substrate eliminates the high costs associated with the thinning process required for conventional back-illuminated CCD's.   Point spread function is determined by diffusion of carriers in an electric field and is well-understood and characterized.   The p-channel CCD's are fabricated on high-resistivity substrates and are expected to have improved radiation resistance to charged particles due to the near-elimination of phosphorus in the CCD channel region.  The relatively large thickness results in increased sensitivity to cosmic ray and background radiation, and these effects are under investigation.

I. Introduction

We are developing CCD image sensors for use in astronomy and astrophysics [1,2].  The CCD’s are a spinoff of detector technology for high energy physics that is based on fully-depleted p-i-n diodes fabricated on high-resistivity silicon.  Conventional CCD processing is employed but with high-resistivity starting material.  A thin backside contact structure allows for rear illumination with good quantum efficiency in the blue [3], while the nominal 300 mm thickness yields good red and near-IR quantum efficiency as well as improved sensitivity to x rays.  The backside contact also acts as an electrode through which a bias voltage is applied to fully deplete the substrate, in contrast to deep-depletion CCD’s developed for x-ray astronomy that rely on a combination of vertical clock potentials and high-resistivity substrates for depletion [4,5].  Fringing due to multiply-reflected light in the red, a problem in thinned CCD’s, is eliminated.  Radiation hardness is expected to be good due to the small concentration of phosphorus in the CCD channel.   Drawbacks of such a thick device include enhanced sensitivity to cosmic rays and background radiation as well as a larger volume for dark current generation.

II. Technology

The CCD’s are fabricated in a standard single-metal, triple-polycrystalline silicon technology.  The starting material is ~ 10,000 W-cm, n-type silicon.  The crystalline orientation is <100>.  Low dark current is achieved by the deposition of a back-side layer of in-situ (phosphorus) doped polycrystalline silicon near the beginning of the process [6].  The role of this layer is to trap fast diffusing metallic contaminants that would otherwise result in high dark current.

The gate dielectric consists of 50 nm each of Si3N4 and SiO2.  The p-channel region is implanted with boron, followed by a lower-dose, mini-channel implant in the serial registers.  Polycrystalline silicon gate electrodes are formed by plasma etching in Cl2/HBr/O2 mixtures with high selectivity to the underlying Si3N4 gate dielectric.  The thick (~ 1 mm) polycrystalline silicon back-side layer is removed after all high temperature processing and replaced by a much thinner layer (~ 20 nm) in order to permit back illumination with good quantum efficiency in the blue.  A two-layer anti-reflection (AR) coating is sputtered onto the back side of the wafer.  This coating consists of indium tin oxide (ITO) and SiO2.

The CCD’s described in this work are fabricated at the Lawrence Berkeley National Laboratory (LBNL) MicroSystems Laboratory.

III. Results

The technology has been demonstrated on devices as large as 2048 x 2048 (15 x 15 mm pixels) [7].  Figure 1 shows back-illuminated 2k x 2k test pattern images and quantum efficiency measured at Lick Observatory.  The AR coating was optimized for the near IR.  Measured noise is about 4 electrons rms at a sample-to-sample time of 8 ms, and dark current is roughly 20-40 electrons per pixel per hour.  Charge transfer efficiency (CTE) is > 0.999995.

A concern for a thick CCD such as this is degradation in spatial resolution (point spread function) due to diffusion of photo-generated charge.  This has been analyzed experimentally and theoretically [8,9].  For the fully depleted case, the spatial resolution is determined by the transit time of carriers in the electric field.  For the asymptotic case of a constant field, the rms standard deviation s is given by

1) 	s = [( 2 kT)/(qVappl)]1/2 zsub

where kT/q is the thermal voltage and Vappl is the voltage drop across the drift region of thickness zsub.  Vappl consists of the substrate bias voltage minus the potential a small distance away from the potential wells.  The latter is an average value generated by the vertical clock levels and is negative.  At a temperature of –120C and substrate bias voltage of 30V, the charge diffusion is about 10 mm rms, comparable to a thinned CCD with a 10 mm undepleted region at the back surface [9].

IV. Work in progress

We are presently fabricating 2048 x 4096 CCD’s (15 x 15 mm pixels) for spectroscopic applications at the Keck Telescope.  In addition, smaller pixel CCD’s (12 mm and 10.5 mm) of interest to the SuperNova Acceleration Probe project [10] are fabricated on the same wafers.

For space-based applications, radiation hardness is of paramount importance.  Typically n-channel CCD's suffer CTE degradation in space due to proton-generated phosphorus-vacancy pairs in the CCD channel [11].  The CCD described here is p-channel with phosphorus concentration in the channel roughly 5 orders of magnitude smaller than that in a conventional n-channel CCD.  Hence we expect improved radiation hardness, and plan to study this via proton irradiation at the LBNL 88” Cyclotron.

Degradation in image quality due to cosmic rays is also a concern due to the thickness of the CCD.  High-energy cosmic rays will generate minimum-ionizing tracks in the CCD’s, generating on average 80 electron-hole pairs per mm traveled. Cosmic rays at incident angle q will generate an ionizing track that will project to [(T/p) tan q] pixels where T is the wafer thickness and p is the pixel pitch.  Background radiation of other types, such as Compton electrons from gamma rays, will also degrade image quality.  Although a tradeoff exists between thickness needed for near-IR quantum efficiency and resulting degradation due to cosmic rays, the tracks generated by cosmic rays are unambiguous and can be dealt with by off-line processing.

Studies to separate the various components of background radiation the CCD detects are underway at LBNL’s Low Background Facilities [12].  CCD’s have been tested 180 m underground in lead vaults with greatly reduced cosmic ray flux, as well as in low terrestrial background facilities where cosmic rays and radiological contaminants in the dewar and packaging materials dominate.  These studies should prove valuable in terms of particle identification and shielding requirements.  In addition the facilities are useful for the qualification of low-background dewar and packaging materials.

V.	Acknowledgements

This work was supported by the U.S. Department of Energy under contract No. DE-AC03-76SF00098, by the National Science Foundation (US) under grant NSF/ATI 9876605 and by the National Aeronautics and Space Administration under grant NRA-99-01-SPA-040.

References

1)	S. E. Holland et al, IEDM Technical Digest, pp. 911-914, 1996.

2)	R.J. Stover et al, Proc. SPIE, 3019, pp. 183-188, 1997.

3)	S.E. Holland, N.W. Wang, and W.W. Moses, IEEE Trans. Nucl. Sci., 44(3), pp. 443-447, 1997.

4)	B.E. Burke et al, IEEE Trans. Elec. Dev., 44(10), pp. 1633-1642, 1997.

5)	P.S. Heyes, P.J. Pool, R. Holton, Proc. SPIE, 3019, pp. 201-209, 1997.

6)	S. Holland, Nucl. Instrum. Meth. A, 275, pp. 537-541, 1989.

7)	R.J. Stover et al, to appear in Proc. 4th ESO Workshop on Optical Detectors for Astronomy, Garching, Germany.

8)	S.E. Holland et al, 1997 IEEE Workshop on Charge-Coupled Devices and Advanced Image Sensors, Bruges, Belgium, 1997.

9)	D.E. Groom et al, to appear in Proc. 4th ESO Workshop on Optical Detectors for Astronomy, Garching, Germany.

10)	M.E. Levi et al, these proceedings.

11)	J. Janesick. T. Elliot, and F. Pool, IEEE Trans. Nucl. Sci., 36(1), pp. 572-578, 1989.

12)	http://user88.lbl.gov/lbf/index.htm

Figure 1.  Test pattern images taken with a back-illuminated 2048 x 2048 CCD at –120C.  Measured quantum efficiency and (1 – Reflectivity) versus wavelength [7].  CTIO T2k is a thinned CCD (approximately 20 mm thick).