Electron-Multiplying CCDs for Future Space Instruments
ABSTRACT
The rapid proliferation of Electron Multiplying Charge Coupled Devices (EMCCDs) in recent years has revolutionized low light imaging applications. EMCCDs in particular show promise to enable the construction of versatile space astronomy instruments while space-based observations enable unique capabilities such as high-speed accurate photometry due to reduced sky background and the absence of atmospheric scintillation. The Canadian Space Agency is supporting innovation in EMCCD technology by increasing its Technology Readiness Level (TRL) aimed at reducing risk, cost, size and development time of instruments for future space missions. This paper will describe the advantages of EMCCDs compared to alternative low light imaging technologies. We will discuss the specific issues associated with using EMCCDs for high-speed photon counting applications in astronomy. We will show that a careful design provided by the CCD Controller for Counting Photons (CCCP) makes it possible to operate the EMCCD devices at rates in excess of 10 MHz, and that levels of clock induced charge and dark current are dramatically lower than those experienced with commercial cameras. The results of laboratory characterization and examples of astronomical images obtained with EMCCD cameras will be presented. Issues of radiation tolerance, charge transfer efficiency at low signal levels and life time effects on the electron-multiplication gain will be discussed in the context of potential space applications.
1. INTRODUCTION
Scientific low light imaging applications such as astronomical spectroscopy, faint object photometry, coastal surveillance and molecular imaging drive optical sensor system requirements towards the detection of ever smaller signals at increasing pixel rates. Since their invention in 1969, the Charge-Coupled Devices (CCDs) have become the detector of choice for high quality low light imaging in a wide range of fields. They have the advantages of excellent resolution, 100% fill factor, greater than 90% peak quantum efficiency, excellent charge transfer efficiency and very low dark signal with sufficient cooling.1 However, the performance of conventional CCDs has always been limited by readout noise inherent in the output amplifier of the device. To minimize read noise the readout speed has to be relatively slow in the order of a few seconds to read the image. In the electron multiplying CCD (EMCCD) device technology developed in the 1990s an additional gain register is incorporated in the architecture through which the signal passes as it is read out of the CCD array.2 Because the signal is amplified prior to readout this effectively reduces the magnitude of the amplifier noise and improves significantly the signal-to-noise ratio (SNR) without restrictions of slow readout times. In addition to all advantages of the CCD technology, EMCCDs are able to achieve sub-electron read noise at high frame rates allowing single-photon detection. 3,4 The EMCCDs are now available commercially from E2V Technologies Ltd. and from Texas Instruments. In spite of the reported improvements in performance, the EMCCDs have not been widely used in space instrumentation. In this paper we review the advantages of EMCCDs in comparison alternative low light imagers and discuss the specific issues relating to their operation and potential applications. The results of laboratory characterization and astronomical testing are presented to demonstrate the superior performance of this low light imaging technology.
2. LOW LIGHT IMAGING SENSORS
2.1. Scientific CCDs
The state-of –the-art scientific-grade CCDs are available in a number of distinct technologies such as front-illuminated CCDs, thinned back-illuminated CCDs and back-illuminated deep-depletion CCDs. In a traditional front-illuminated CCD light passes through the polysilicon gates that define each pixel and generates electric charge in the collecting well when pixels are electrically biased. Due to reflection and absorption losses in the poly-gate structure quantum efficiency (QE) of front-illuminated devices is only about 50%. To improve QE, the Si substrate material can be uniformly removed to attain approximately 10 to 15 μm thickness so that an image can be focused directly onto the photosensitive area of the CCD without absorption losses in the gate structure. Compared to front-illuminated CCDs, these thinned back-illuminated devices have a higher QE across the visible spectrum with peak QE >90%. To further improve QE, especially for near-infrared (NIR) imaging and x-ray applications, the CCD are manufactured from a high-resistivity silicon with a thickness ranging from 50 to 300 μm in order to produce a “deeper” depletion region (i.e., larger active photosensitive volume). This architecture allows longer-wavelength photons to interact within the depletion region layer as opposed to merely penetrating it, ultimately helping to increase QE in the NIR spectral region.
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