Articles you may be interested inResponse of a delta-doped charge-coupled device to low energy protons and nitrogen ions Rev. Sci. Instrum. 77, 053301 (2006); 10.1063/1.2198829 Substrate preparation and low-temperature boron doped silicon growth on wafer-scale charge-coupled devices by molecular beam epitaxy
Delta-doped CCDs, developed at JPL's Microdevices Laboratory, have achieved stable 100% internal quantum efficiency in the visible and near UV regions of the spectrum. In this approach, an epitaxial silicon layer is grown on a fully-processed commercial CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30% of a monolayer of boron atoms are deposited on the surface, followed by a 15 A silicon layer for surface passivation. The boron is nominally incorporated within a single atomic layer at the back surface of the device, resulting in the effective elimination of the backside potential well. The measured quantum efficiency is in good agreement with the theoretical limit imposed by reflection from the Si surface. Enhancement of the total quantum efficiency in the blue visible and near UV has been demonstrated by depositing antireflection coatings on the delta-doped CCD. Recent result4 on antireflection coatings and quantum efficiency measurements are discussed. . IntroductionHigh resolution charge-coupled devices (CCDs) with high UV quantum efficiency (QE) have many applications in space and ground-based astronomy. The short absorption length of photons in the frontside structure of the CCD makes frontside-illuminated CCDs unresponsive in the UV. Two possible ways of making UVresponsive frontsideilluminated CCDs have been demonstrated: One is through structural modification, i.e., virtual phase CCDs1, and the other is the by using a phosphor to convert UV into visible light? e.g., lumogen.coated frontside CCDs. While backside-illuminated, thinned CCDs offer the possibility of obtaining high UV quantum efficiency, detecting UV photons in a silicon CCD is complicated by the short absorption length of UV photons in silicon (e.g., 4 nm absorption length at 270 nm) and the existence of a backside potential well (caused by positive charge at the interface between Si and SiOJ. Treating the back surface of the CCD by negative-surface charging (i.e., UVflooding, bias flashgating) or ion implantation, has yielded reasonable or high UV quantum efficiency. However, these treatments suffer variously from problems of yield, response stability, hysteresis, and long-term reliability. Stability of the quantum efficiency has great impact on ground and space-based astronomy. A device with stable quantum efficiency is particularly important in space-based astronomy where renewal of the back surface treatment (e.g., by exposing the device to intense UV light) is not an attractive option.Molecular beam epitaxy (MBE) promises a significant enhancement of imaging device technology. Nanometer-scale dopant profiles, not accessible with ion implantation or diffusion processes, could expand the range of the performance of existing devices.Using MBE, delta-doped CCDs with 100% internal quantum efficiency in the visible and 0-8194-1493-X/94/$6.00 SPIE Vol. 2198 / 907 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 06/25/2016 Terms of Use: http://spiedigitallibrary.org/ss/TermsOfUse.aspx
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 commercial CCD using molecular beam epitaxy. During the silicon growth on the CCD, 30% of a monolayer of boron atoms are deposited nominally within a single atomic layer, resulting in the effective elimination of the backside potential well. These devices are highly uniform and have exhibited long-term stability. To achieve significantly higher total quantum efficiency, antireflection layers can be directly deposited on the device. This was demonstrated in the 250-400 nm region. Ultraviolet detection with CCDsDetection of ultraviolet light has numerous strategic and ground and space-based scientific applications.Useful information can be obtained, for example, from the detection of gamma-band radiation of nitrogen oxide (190-280 nm) formed in the high temperature shock layer of hypervelocity vehicles.1 The large format and low noise of charge-coupled devices (CCDs) are ideal for these applications. However, the detection of ultraviolet light in Si CCDs has been a long-standing problem, due to the short absorption length of UV photons in silicon. The absorption depth problem is ifiustrated by Fig. 1, which gives the photon absorption length in crystalline silicon versus wave1enth. Note that the absorption depth drops to a minimum of 40 A at about 270 nm, and is less than 100 A over the range of wavelengths from 60 nm to 400 nm.The highest possible quantum efficiency (QE) is obtained by backside-illumination of thinned CCDs. However, positive charge trapped at the SiJSiO2 interface of a bare silicon surface forms a backside potential well that traps photoelectrons generated near the back surface. For untreated thin CCDs this results in poor and unstable UV quantum efficiency. This backside potential typically extends 0.5 p.m into the silicon lattice preventing detection of photoelectrons produced within that region. Treating the back surface of the CCD by negative surface charging (i.e., UV-flooding, bias flash-gating) or ion implantation, has yielded reasonable or high UV quantum efficiency.2'3 However, these treatments suffer variously from problems of yield, response stability, hysteresis, and long-term reliability. Stability of the quantum efficiency has great impact on ground and space-based applications. Stable quantum efficiency of the device is particularly important in space-based applications where renewal of the back surface treatment (e.g., by exposing the device to intense UV light) is not an attractive option.In this paper, we describe the growth of delta-doped silicon on commercial CCDs using molecular beam epitaxy (MBE), and the resulting enhancement of the UV quantum efficiency. Deposition of antireflection coatings on delta-doped CCDs is discussed. The characteristics of the modified CCDs, such as the uniformity and stability of the quantum efficiency, are described.
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