Etch induced damage for different processes

Etch induced damage for different processes - HgCdTe

Material Name: HgCdTe
Record No.: 7
Primary Chemical Element in Material: Hg
Sample Type: Layer
Uses: Etching
Etchant Name: None
Etching Method: Dry etching
Etchant (Electrolyte) Composition: No data
Procedure (Condition): No data
Note: Etch induced damage
While developing a dry plasma etching technology for HgCdTe, there are certain aspects of the materials that need to be taken care of:
. conductivity type conversion,
. stoichiometry changes and creation of defects,
. surface roughness,
. polymer deposition.
The low damage threshold of HgCdTe is due to weak Hg-Te bond and low volatility of CdTe component. A physical component in dry etching is required for anisotropic mesa profile and achieving a stoichiometric surface. A certain level of ion induced reaction is necessary for etching HgCdTe to overcome low volatility of Cd and assist in desorption of etch products form the surface. Ion bombardment during dry etching can modify the electrical and optical properties of HgCdTe.

Both ion milling and RIE cause damages to HgCdTe during etching. Ion milling of HgCdTe results in creation of extensive structural defects, type conversion of p-type HgCdTe extending to large distances (~200 µm) for short process times and produces long-range isotropic damage in n-type HgCdTe. Reactive ion etching of HgCdTe using CH4/H2 discharge, is done at the process pressures of 100-400 mTorr and ion energies are > 100 eV. These systems induce type conversion and damages in the processed devices, particularly of p-type material.

A study of spatial changes in electrical characteristics of HgCdTe photoconductive (n-type) and photovoltaic (p-type) fabricated by RIE system was performed by Smith et al. and compared to wet chemical processing using Br2/methanol. They performed laser beam induced current (LBIC) measurements to characterize electrically active impurities/defect clusters, material in-homogeneities, junctions etc. The comparison results are reported in Table 1. Wet processed devices show a superior performance (responsivity, detectivity and noise) to those that have undergone dry plasma etching. However, the RIE induced type conversion in extrinsically doped p-type and intrinsically doped n-type HgCdTe can be removed by low temperature mercury annealing.

ECR etching of HgCdTe using low energy Ar ion bombardment has a sputter component at high DC bias values and results in increased Hg removal. The damage depth due to sputtering has been estimated to <10 nm at operating bias voltage. RHEED, LEED analysis of ECR etched (211) surface showed a crystalline surface unlike the amorphous surface observed in III-V semiconductors, but a twinned and faceted surface was observed, all other defects recombine to give a crystalline surface. Planar surfaces of etched HgCdTe have been analysed for changes in stoichiometry and transport properties. But the properties of sidewall of the mesa may be different and are difficult to characterize. Mesa sidewall damage, n-type doping variations, introduction of additional minority carrier recombination centres etc. can degrade detector performance. Reverse bias I-V characteristics of 30 µm unit cell HgCdTe diodes of various trench geometries were measured at 78 K. The results indicate that diodes exhibit a good performance (break down voltage of ~0.7 V) with high impedance. The more narrow trenches or trenches that have been etched to a greater depth with etch lag effects, show a degradation in I-V characteristics with smaller breakdown voltages and increased reverse current.
The noise performance of the dry processed detectors (any technique), as seen from Table 1, is lower than wet etched devices. However, dry processing, particularly HDP etching is capable for reducing pixel pitch and dramatically improving fill factors needed for high-density detector arrays. Hence for a given unit cell design parameters (etch depth, width, mesa profile and process time), the dry etch process has to be optimized to give high performance diodes.
Reference: V. SRIVASTAV, R. PAL, and H.P. VYAS, Overview of etching technologies used for HgCdTe, OPTO-ELECTRONICS REVIEW 13(3), 2005, pp. 197-211.

Table 1: Etch induced damage for different processes.