Modification of the Surface Band-Bending of A Silicon CCD for Low-Energy Electron Detection
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ABSTRACT Silicon CCDs have limited sensitivity to particles and photons with short penetration depth, due to the surface depletion caused by the inherent positive charge in the native oxide. Because of surface depletion, internally-generated electrons are trapped near the irradiated surface and therefore cannot be transported to the detection circuitry. This deleterious surface potential can be eliminated by low-temperature molecular beam epitaxial (MBE) growth of a delta-doped layer on the Si surface. This effect has been demonstrated through achievement of 100% internal quantum efficiency for UV photons detected with delta-doped CCDs. In this paper, we will discuss the modification of the band bending near the CCD surface by lowtemperature MBE and report the application of delta-doped CCDs to low-energy electron detection. We show that modification of the surface can greatly improve sensitivity to lowenergy electrons. Measurements comparing the response of delta-doped CCDs with untreated CCDs were made in the 50 eV-1.5 keV energy range. For electrons with energies below 300 eV, the signal from untreated CCDs was below the detection limit for our apparatus, and data are presented only for the response of delta-doped CCDs at these energies. The effects of multiple electron hole pair (EHP) production and backscattering on the observed signals are discussed. INTRODUCTION Imaging systems for low energy particles generally involve the use of microchannel plate electron multipliers followed by position sensitive solid state detectors, or phosphors and position sensitive photon detectors. These systems work well and can process up to 106 electrons/sec., however, the spatial resolution of these compound systems is considerably less than that of a directly imaged CCD. Also, these systems have difficulties with gain stability and they require high voltages. The present large format of CCDs, up to 4000x4000 pixels, could represent a major advance for the imaging of low energy particles. CCDs exhibit a highly linear response which is advantageous for quantitative detection applications. The full well capacity of buried channel CCDs corresponds to a collected electron density of about 1011 electrons/cm 2 , which together with the low readout noise, gives CCDs a large dynamic range. Charge. coupled devices (CCDs) are high resolution imaging devices which are typically nchannel fabricated in a p-type substrate and frontside, or processed-side, illuminated. Incident radiation is required to penetrate the CCD polycrystalline silicon gates (typically -5000 A) before being able to generate electron-hole pairs (EHP) in the pixel. This configuration makes radiation of low penetration depth undetectable. One attempt to eliminate this problem involves turning the chip around in order to illuminate from the back side, thus eliminating attenuation due to the CCD processed layers. Backside illumination requires removal of the thick p+ substrate in order to bring the exposed back surface in close proximity to the intended frontside potentia
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