We present a detailed design study for a novel solid-state focal plane array of silicon avalanche photodiodes (APDs) using an advanced silicon-on-sapphire substrate incorporating an antireflective bilayer consisting of crystalline aluminum nitride (AlN) and amorphous, non-stoichiometric, silicon rich, silicon nitride (a-SiNX<1.33) between the silicon and sapphire. The substrate supports electrical and optical integration of a nearly 100% quantum efficiency, silicon APD capable of operating with wide dynamic range in dual linear or Geiger-mode, with a gallium nitride (GaN) laser diode in each pixel. The APD device and epitaxially grown GaN laser are fabricated within a crystallographically etched silicon mesa. The high resolution 27 μm emitter-detector pixel design enables single photon sensitive, solid-state focal plane arrays (FPAs), with passive and active imaging capability in a single FPA. The square 27 μm emitter-detector pixel achieves SNR>10 in active detection mode for Lambert surfaces at 20,000 m.
There is a growing need in scientific and industrial applications for dual-mode, passive and active 2D and 3D LADAR
imaging methods. To fill this need, solid-state, single photon sensitive silicon avalanche photodiode (APD) detector
arrays offer high sensitivity and the possibility to operate with wide dynamic range in dual linear and Geiger-mode for
passive and active imaging. To support the fabrication of large scale, high quantum efficiency and high resolution
silicon avalanche photodiode arrays and other advanced solid-state optoelectronics, a novel, high transmittance, backilluminated
silicon-on-sapphire substrate has been developed incorporating a single crystal, epitaxially grown aluminum
nitride (AlN) antireflective layer between silicon and R-plane sapphire, that provides refractive index matching to
improve the optical transmittance into silicon from sapphire. A one quarter wavelength magnesium fluoride
antireflective layer deposited on the back-side of the sapphire improves optical transmittance from the ambient into the
sapphire. The magnesium fluoride plane of the Si-(AlN)-sapphire substrate is bonded to a fused silica wafer that
provides mechanical support, allowing the sapphire to be thinned below 25 micrometers to improve back-illuminated
optical transmittance, while suppressing indirect optical crosstalk from APD emitted light undergoing reflections in the
sapphire, to enable high quantum efficiency and high resolution detector arrays.
There is a growing need in scientific research applications for
dual-mode, passive and active 2D and 3D LADAR
imaging methods. To fill this need, an advanced back-illuminated silicon avalanche photodiode (APD) design is
presented using a novel silicon-on-sapphire substrate incorporating a crystalline aluminum nitride (AlN) antireflective
layer between the silicon and R-plane sapphire. This allows integration of a high quantum efficiency silicon APD with a
gallium nitride (GaN) laser diode in each pixel. The pixel design enables single photon sensitive, solid-state focal plane
arrays (FPAs) with wide dynamic range, supporting passive and active imaging capability in a single FPA. When (100)
silicon is properly etched with TMAH solution, square based pyramidal frustum or mesa arrays result with the four mesa
sidewalls of the APD formed by (111) silicon planes that intersect the (100) planes at a crystallographic angle, φ
c = 54.7°. The APD device is fabricated in the mesa using conventional silicon processing technology. The GaN laser
diode is fabricated by epitaxial growth inside of an inverted, etched cavity in the silicon mesa. Microlenses are
fabricated in the thinned, and AR-coated sapphire substrate. The APDs share a common, front-side anode contact, and
laser diodes share a common cathode. A low resistance (Al) or (Cu) metal anode grid fills the space between pixels and
also inhibits optical crosstalk. SOS-APD arrays are flip-chip
bump-bonded to CMOS readout ICs to produce hybrid
FPAs. The square 27 μm emitter-detector pixel achieves SNR > 1 in active detection mode for Lambert surfaces at
1,000 meters.
We present a detailed design study for a novel solid-state focal plane array of silicon avalanche photodiodes (APDs), with each detector in the array capable of operating with wide dynamic range in linear or Geiger-mode. The detector array is based on back-illuminated, crystallographically etched, (100) epitaxial silicon on R-plane sapphire substrates using 27 µm pixels, although other sizes could be supported as well. Analysis of the APD pixel design, which includes the photoelectron noise, dark current noise, and electronic circuit noise, shows the capability of imaging with a signal-to-noise ratio greater than 7 at −30 °C and ultralow illuminance of 10−4 lux. The detector array design will enable large focal planes, capable of imaging with high sensitivity and high speed, over a wide range of natural illumination conditions from direct sunlight to a cloudy moonless night.
There is a growing need in industrial and scientific research applications for very wide dynamic range, high sensitivity imaging methods. To fill this need, an advanced back-illuminated avalanche photodiode (APD) design is presented based on crystallographically etched (100) epitaxial silicon on R-plane sapphire (SOS), enabling large single photon sensitive, solid-state focal plane arrays with very wide dynamic range. The design will enable reliable and low cost fabrication. When (100) silicon is properly etched with KOH:IPA:H2O solution through a thermally grown oxide mask, square based pyramidal frustum or mesa arrays result with the four mesa sidewalls of the APD formed by (111) silicon planes that intersect the (100) planes at a crystallographic angle of 54.7 degrees. The APD is fabricated in the silicon mesa. Very wide dynamic range operation at high quantum efficiencies exceeding 50% for 250 nm<lambda<400 nm and exceeding 80% for 400 nm<lambda<700 nm, is enabled by a carefully optimized p-type boron spatial impurity profile, fabricated by high temperature diffusion. Monte Carlo modeling confirms the majority of electron-hole pairs are photogenerated outside of the narrow device multiplication region, resulting in a very low APD excess noise factor.
There is a growing need in space and environmental research applications for dual-mode, passive and active 2D and
3D ladar imaging methods. To fill this need, an advanced back-illuminated avalanche photodiode (APD) design is
presented based on crystallographically etched (100) epitaxial silicon on R-plane sapphire (SOS), enabling single photon
sensitive, solid-state focal plane arrays (FPAs) with wide dynamic range, supporting passive and active imaging
capability in a single FPA. When (100) silicon is properly etched with KOH:IPA:H2O solution through a thermally
grown oxide mask, square based pyramidal frustum or mesa arrays result with the four mesa sidewalls of the APD
formed by (111) silicon planes that intersect the (100) planes at a crystallographic angle, Φc = 54.7°. The APD device is
fabricated in the mesa using conventional silicon processing technology. Detectors are back-illuminated through light
focusing microlenses fabricated in the thinned, AR-coated sapphire substrate. The APDs share a common, front-side
anode contact, made locally at the base of each device mesa. A low resistance (Al) or (Cu) metal anode grid fills the
space between pixels and also inhibits optical cross-talk. SOS-APD arrays are indium bump-bonded to CMOS readout
ICs to produce hybrid FPAs. The quantum efficiency for the square 27 µm pixels exceeds 50% for 250 nm < λ < 400
nm and exceeds 80% for 400 nm < λ < 700 nm. The sapphire microlenses compensate detector quantum efficiency loss
resulting from the mesa geometry and yield 100% sensitive-area-fill-factor arrays, limited in size only by the wafer diameter.
Brian Aull, Andrew Loomis, Douglas Young, Alvin Stern, Bradley Felton, Peter Daniels, Debbie Landers, Larry Retherford, Dennis Rathman, Richard Heinrichs, Richard Marino, Daniel Fouche, Marius Albota, Robert Hatch, Gregory Rowe, David Kocher, James Mooney, Michael O'Brien, Brian Player, Berton Willard, Zong-Long Liau, John Zayhowski
Lincoln Laboratory has developed 32 x 32-pixel ladar focal planes comprising silicon geiger-mode avalanche photodiodes and high-speed all-digital CMOS timing circuitry in each pixel. In Geiger mode operation, the APD can detect as little as a single photon, producing a digital CMOS-compatible voltage pulse. This pulse is used to stop a high-speed counter in the pixel circuit, thus digitizing the time of arrival of the optical pulse. This "photon-to-digital conversion" simultaneously achieves single-photon sensitivity and 0.5-ns timing. We discuss the development of these focal planes and present imagery from ladar systems that use them.
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