INVESTIGATION OF AMORPHOUS HYDROGENATED Si AS A RESIST FOR VACUUM-COMPATIBLE LITHOGRAPHY OF HgCdTe/CdTe FILMS

Abstract

The vision of achieving a completely in-vacuum process for fabricating HgCdTe Infrared detector arrays is contingent on the availability of a vacuum-compatible lithography technology. One such technology for vacuum-lithography involves the use of amorphous hydrogenated Si (a-Si:H) as a dry photoresist. The basic concept has recently been demonstrated whereby a-Si:H resists were deposited via plasma enhanced chemical vapor deposition (PECVD), and then patterned using an excimer laser. The patterns were then hydrogen plasma developed to remove unirradiated areas. Finally, an Ar/H2 electron cyclotron resonance (ECR) plasma was used to transfer patterns to underlying Hg1-xCdxTe film layers.

This thesis presents a continued investigation of a-Si:H as a resist material wherein the resists are deposited using an Ar-diluted silane precursor. To determine the best conditions for the technique, the effects of different laser fluences, and exposure environments were studied. Analysis via transmission electron microscopy (TEM) reveals that the excimer-exposed surfaces are polycrystalline in nature, indicating that the mechanism for pattern generation in this study is based on melting and crystallization of the exposed areas. To reduce undesirable surface roughness induced by laser irradiation, a step-wise crystallization/dehydrogenation technique is demonstrated. Fundamental aspects of pattern transfer (via ECR plasma etching) to CdTe and HgCdTe films are also demonstrated, where etch selectivities of 8:1 and 16:1 (respectively) are observed. These values represent a significant improvement to etch selectivities obtained using commercially available organic resists. To address concerns regarding possible damage to HgCdTe caused by the a-Si:H dry lithography process, preliminary studies were carried out using double-crystal rocking curve X-Ray diffraction and high-resolution TEM. The results indicate no evidence of microstructural damage to the HgCdTe film. Other characterization techniques used throughout this thesis include Fourier transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), and stylus profilometry.

The implementation of the a-Si:H dry lithography process represents a crucial step toward achieving totally integrated fabrication of HgCdTe IR detector arrays. In addition this lithography technique is both low temperature and contamination-free, so that other semiconductor microfabrication processes could potentially benefit from its use.