Low-energy electron detection with delta-doped CCDs

Abstract

Delta-doped CCDs have achieved stable quantum efficiency, at the theoretical limit imposed by reflection from the Si surface in the near UV and visible. In this approach, an epitaxial silicon layer is grown on a fully-processed CCD using molecular beam epitaxy. During the silicon growth on the CCD, 300/o of a monolayer of boron atoms are deposited nominally within a single atomic layer, resulting in the effective elimination of the backside potential well.In this paper, we will briefly discuss delta-doped CCDs and their application of to low-energy electron detection. We show that modification of the surface this way can greatly improve sensitivity to low-energy 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. LOW-ENERGY PARTICLE DETECTION AND CCDsImaging systems for low-energy particles generally involve the use of microcbannel 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 charge-coupled device (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 10 11 electrons/cm 2 , which together with the low readout noise, gives CCDs a large dynamic range.Frontside illumination of CCDs makes radiation of low penetration depth undetectable, because incident radiation is required to penetrate the CCD po;ycrystalline Si gates (-5000 A),. 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 potential well. However, thinning the CCD by chemically removing the substrate is not sufficient to obtain high quantum efficiency, because positive charge in the native oxide traps electrons generated near the back surface of the CCD. Termination of a Si surface with Si(h leads to depletion of carriers at the surface, and in p-type Si the band bending due to surface depletion serves to create a surface potential well for electrons. Thi...

Solid‐state curved focal plane arrays realized with simple approaches for compact and high‐performance optical systems

“…CFPAs' enabling significant miniaturization and improvement of optical systems could lead to achieving advanced scientific objectives in future space missions. Since our first demonstrations of CFPAs (Nikzad et al 2003), interest in CFPAs has significantly increased across applications and spectral range. CFPAs have enabled ultra‐compact cameras with high image quality in consumer products, for example, by Sony (May 2021) in such applications as mobile phone cameras and vehicle cameras.…”

“…Solid‐state CFPAs enable compact, high‐efficiency, less complex optical systems with reduced aberration and distortion; these detectors do not require high‐voltage power supplies, they are reliable, they provide a higher degree of freedom for the designer, they are curved as needed to match the specific curvature dictated by the specific application, they can be adjustable for potential use in adaptive optics, and the curvature of the focal surface is independent of the VLSI fabrication process, making this approach low cost and practical. Our first demonstration of CFPAs were presented in 2000s (Nikzad et al 2003). Those results and work by other groups since then (DARPA 2019; HRL 2020; Shah 2012; Swain & Mark 2004) have further emphasized the importance and impact of CFPAs.…”