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.
Geiger-mode avalanche photodiodes (APDs) can convert the arrival of a single photon into a digital logic pulse. Arrays of APDs can be directly interfaced to arrays of per-pixel digital electronics fabricated in silicon CMOS, providing the capability to time the arrival of photons in each pixel. These arrays are of interest for "flash" LADAR systems, where multiple target pixels are simultaneously illuminated by the laser during a single laser pulse, and the imaging array is used to measure range to each of the illuminated pixels. Since many laser radar systems use Nd:YAG lasers operating at 1.06 um, we have extended our earlier work with silicon-based APDs by developing arrays of InGaAsP/InP APDs, which are efficient detectors for near-IR radiation. 32x32 pixel arrays, with 100-um pixel pitches, are currently being successfully used in demonstration systems.
A stepper operating at the 193-nm wavelength has been constructed for use in the development of resist processes. The stepper lens has a 4-mm field diameter and a 0.33 NA. The stepper uses an unnarrowed ArF excimer laser as the light source, and uses diffractive lenslet arrays to transform the low divergence excimer beam into a suitable pupil fill. The stepper is routinely used for resist studies and has been used to pattern lines and spaces as small as 0.15 ?m.
The local modification of an integrated circuit (IC) requires in general the availability of three generic processes. First, a method for cutting conductors must be provided. Second, a process for depositing new conductors must be available. Finally, a means of opening via holes through the chip passivation to the underlying conductors is needed; this operation enables newly deposited conductors to make connections to the existing circuit elements, and also provides probe access to facilitate testing of the circuit.
Polyalkylsilynes have been used as resists for 193-nm projection lithography. These resists can be either wet developed using toluene or dry developed using HBr reactive ion etching (RIE). Wet development relies on crosslinking via intermolecular Si-O-Si bond formation to reduce solubility (negative tone) whereas the dry development relies on photo-oxidation to induce etch selectivity (also negative tone). The sensitivity in either case ranges from 20 to 200 mJ/cm2 and depends on the resist formulation. The best resist compositions are those that contain predominantly small (n-butyl) aliphatic pendant groups rather than large (cyclohexyl, phenyl) pendant groups. Using a 0.33 NA catadioptric lens with a phase mask, equal line-and-space features as small as 0.15 micrometers have been printed and transferred through 1.0 micrometers of planarizing layer (aspect ratio > 6) using oxygen RIE.
A silylation process for novolac-based resins was developed which
results in positive-tone imaging. This process is based on 193nminduced
crosslinking followed by a low temperature silylation step.
Novolac resin without diazoquinone additives may also be used as
positive-tone resists. Typical conditions were exposure to
dimethylsilyldimethylainine vapor at 10 Torr for 1 minute at 100 °C.
This incorporates silicon in the upperniost 100 to 1000 nn of the film,
depending on the resist. Etch selectivities in a 10 rnTorr oxygen
reactive ion etching plasma with a bias voltage of -200 V were typically
30:1. Resolution below 0.3 m has been demonstrated with this