Fabrication of low leakage-current backside illuminated photodiodes
Ultra-low leakage current backside-illuminated semiconductor photodiode arrays are fabricated using a method of formation of a transparent, conducting bias electrode layer that avoids high-temperature processing of the substrate after the wafer has been gettered. As a consequence, the component of the reverse-bias leakage current associated with strain, crystallographic defects or impurities introduced during elevated temperature processing subsequent to gettering can be kept extremely low. An optically transparent, conductive bias electrode layer, serving as both an optical window and an ohmic backside equipotential contact surface for the photodiodes, is fabricated by etching through the polysilicon gettering layer and a portion of the thickness of heavily-doped crystalline silicon layer formed within, and near the back of, the substrate during the gettering process.
1 . A backside illuminated diode, comprising:
a single piece, homostructural substrate, having a first conductivity type, and formed of a semiconductor material having a first portion, and a second portion near a backside of the semiconductor substrate, said second portion being an optically substantially transparent conductive crystalline portion adapted for receiving a bias potential and for receiving incominq light; and
a plurality of gate structures, having a second conductivity type opposite to said first conductivity type, and formed in said first portion of said homostructural substrate, thereby forming a PN junction in said first portion.
2 . A diode as in claim 1 , wherein said second portion has a thickness greater than 0.25 μm.
3 . A diode as in claim 1 , further comprising an anti-reflection coating, coated on said backside.
4 . A diode as in claim 3 , wherein said anti-reflection coating is formed of a dielectric material.
5 . A diode as in claim 4 , wherein said anti-reflection coating is formed of multiple layers of dielectric materials.
6 . A diode as in claim 1 , wherein said first conductivity type is an N conductivity type.