Processing and characterization of a-Si:H photoresists for a vacuum-compatible photolithography process

Abstract

The vision of achieving a completely in-vacuum process for fabricating HgCdTe detector arrays is contingent on the availability of a vacuumcompatible photolithography technology. One such technology for vacuum photolithography involves the use of amorphous-hydrogenated Si (a-Si:H) as a photoresist. In this work, we deposit a-Si:H resists via plasma-enhanced chemical-vapor deposition (PECVD) using an Ar-diluted silane precursor. The resists are then patterned via excimer laser exposure and development etched in a hydrogen plasma where etch selectivities between unexposed and exposed regions exceed 600:1. To determine the best conditions for the technique, we investigate the effects of different exposure environments and carry out an analysis of the a-Si:H surfaces before and after development etching. 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. To demonstrate pattern transfer, underlying CdTe films were etched (after development of the resist) in an electron cyclotron resonance (ECR) plasma, where etch selectivities of approximately 8:1 have been achieved. The significance of this work is the demonstration of laser-induced poly-Si as an etching mask for vacuum-compatible photolithography.

Effects of a-Si:H resist vacuum-lithography processing on HgCdTe

“…A single 65 mJ/cm 2 pulse from the KrF laser (l 5 248 nm) melts the a-Si:H near surface, which crystallizes to form poly-Si. 2,3 The samples were then transferred back to the PECVD chamber, where the a-Si:H layer was developed in a hydrogen plasma to completely etch the unirradiated areas of the resist. Parameters during hydrogen plasma development were fixed at 10 W input rf power, 50°C substrate temperature, and an overall pressure of 1 torr.…”

“…A vacuumcompatible technology for etching mesas in HgCdTe epilayers has been previously demonstrated, whereby amorphous hydrogenated silicon (a-Si:H) was utilized as a dry photoresist. [1][2][3] In contrast to conventional wet-chemical techniques, the essential processing steps, including resist deposition, patterning, development, and pattern transfer (mesa-etching), were all carried out within a vacuum environment. Low-temperature resist deposition (and removal) via plasmaenhanced chemical vapor deposition (PECVD) ensures that thermal degradation of HgCdTe and/or other underlying film layers can be avoided.…”

“…Previous work has demonstrated that poly-Si patterns can be generated on the a-Si:H surface by exposure to excimer laser irradiation. 2,3 Pattern development was carried out in a hydrogen plasma (within the PECVD chamber) where etch selectivities over 1000:1 for aSi:H and the irradiated surfaces have been demonstrated. The resulting a-Si:H masks have been used to create mesas in underlying HgCdTe or CdTe epilayers by etching in an electron cyclotron resonance (ECR) plasma reactor.…”

“…2,5 This system is a critical step toward achieving a waferin, array-out, all-dry fabrication process for HgCdTe detector arrays. The potential advantages of a complete high vacuum processing line includes less wafer handling, lower surface contamination, and higher manufacturing yield.…”

Ultra-fast Laser Synthesis of Nanopore Arrays in Silicon for Bio-molecule Separation and Detection

“…[10] Laser-amorphized Si can be selectively removed from the crystalline matrix because amorphous silicon etches 500 times faster than crystalline silicon in hydrogen plasma. [11] For pore synthesis, critical issues are thermal energy transport and atomic mobility at interfaces at very short times in the Si crystal lattice, since these define the size and shape of the pore. A landmark paper by Liu et al demonstrated that for a single pulse of 20 ps 532 nm laser energy incident on a (111) crystalline Si surface, there exists a critical threshold energy density of 0.2 J/cm 2 , necessary to induce an amorphous phase transition.…”