We have demonstrated a wafer-scale back-illumination process for silicon Geiger-mode avalanche photodiode arrays
using Molecular Beam Epitaxy (MBE) for backside passivation. Critical to this fabrication process is support of the thin
(< 10 μm) detector during the MBE growth by oxide-bonding to a full-thickness silicon wafer. This back-illumination
process makes it possible to build low-dark-count-rate single-photon detectors with high quantum efficiency extending
to deep ultraviolet wavelengths. This paper reviews our process for fabricating MBE back-illuminated silicon Geigermode
avalanche photodiode arrays and presents characterization of initial test devices.
We present a unique hybridization process that permits high-performance back-illuminated silicon Geiger-mode
avalanche photodiodes (GM-APDs) to be bonded to custom CMOS readout integrated circuits (ROICs) - a hybridization
approach that enables independent optimization of the GM-APD arrays and the ROICs. The process includes oxide
bonding of silicon GM-APD arrays to a transparent support substrate followed by indium bump bonding of this layer to
a signal-processing ROIC. This hybrid detector approach can be used to fabricate imagers with high-fill-factor pixels and
enhanced quantum efficiency in the near infrared as well as large-pixel-count, small-pixel-pitch arrays with pixel-level
signal processing. In addition, the oxide bonding is compatible with high-temperature processing steps that can be used
to lower dark current and improve optical response in the ultraviolet.
Dark current for back-illuminated (BI) charge-coupled-device (CCD) imagers at Lincoln Laboratory has historically been higher than for front-illuminated (FI) detectors. This is presumably due to high concentrations of unpassivated dangling bonds at or near the thinned back surface caused by wafer thinning, inadequate passivation and low quality native oxide growth. The high dark current has meant that the CCDs must be substantially cooled to be comparable to FI devices. The dark current comprises three components: frontside surface-state, bulk, and back surface. We have developed a backside passivation process that significantly reduces the dark current of BI CCDs. The BI imagers are passivated using molecular beam epitaxy (MBE) to grow a thin heavily boron-doped layer, followed by an annealing step in hydrogen. The frontside surface state component can be suppressed using surface inversion, where clock dithering reduces the frontside dark current below the bulk. This work uses surface inversion, clock dithering and comparison between FI and BI imagers as tools to determine the dark current from each of the components. MBE passivated devices, when used with clock dithering, have dark current reduced by a factor of one hundred relative to ion-implant/laser annealed devices, with measured values as low as 10-14 pA/cm<sup>2</sup> at 20°C.
The Extreme-Ultraviolet Variability Experiment (EVE) is a component of NASA's Solar Dynamics Observatory (SDO)
satellite, aimed at measuring the solar extreme ultraviolet (EUV) irradiance with high spectral resolution, temporal
cadence, accuracy, and precision. The required high EUV quantum efficiency (QE), coupled with the radiation dose to
be experienced by the detectors during the five year mission (~1 Mrad), posed a serious challenge to the charge-coupled
device (CCD) detectors. MIT Lincoln Laboratory developed the 2048 × 1024 pixel CCDs and integrated them into the
detector system. The devices were back-side thinned and then back surface passivated using a thin, heavily boron-doped
silicon layer grown by molecular beam epitaxy (MBE) at less than 450°C. Radiation-hardness testing was performed
using the Brookhaven National Laboratory's National Synchrotron Light Source (BNL/NSLS). The MBE-passivated
devices were compared against devices with back surfaces passivated with a silver charge chemisorption process and an
ion-implant/furnace anneal process. The MBE devices provided both the highest QE at the required (-100°C) operating
temperatures, and superior radiation hardness, exceeding the goals for the project. Several flight-ready devices were
delivered with the detector system for integration with the satellite.