Deposition and Etching of Conformal Boron Films for Neutron Detector Applications
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Deposition and Etching of Conformal Boron Films for Neutron Detector Applications Nicholas LiCausi1, Justin Clinton2, Yaron Danon2, James J.-Q. Lu1 and Ishwara B. Bhat1 1
Electrical, Computer and Systems Engineering Department, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, U.S.A. 2
Mechanical, Aerospace and Nuclear Engineering Department, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY 12180, U.S.A. ABSTRACT In this work, the deposition of boron using low pressure chemical vapor deposition (LPCVD) has been investigated on planar and deep reactive ion etched (DRIE) Si substrates. Deposition rate and conformal coverage have been studied. Additional studies of “dry” RIE etching and “wet” chemical etching of the deposited boron films are presented. Deposition rates as high as 1 µm/hr and conformal coverage ratios of ~80% have been achieved. Etching rates for various methods studied range widely from 0.35 µm/hr to 1.2 µm/min. INTRODUCTION There has been a significant research effort surrounding the detection of thermal neutrons for the discovery of nuclear materials. Typically these detectors work on the principle of converting spontaneously or induced fission thermal neutrons into alpha-particles and 7Li ions, which are then sensed by a Si p-n junction. The conversion of neutrons to these particles is necessary because the neutron is charge-neutral and cannot be sensed by the standard p-n junction. To detect thermal neutron, a converter material with high thermal neutron cross-section is desirable; typically 10B or 6LiF is used [1,2]. 10B has been selected in this work because it has a higher cross-section and therefore can result in higher detection efficiency. Boron is also compatible with the device fabrication process and thus can simplify the fabrication of the detector device. To maximize the probability that an incident neutron can interact with a 10B atom, the 10B layer must be very thick (45 µm). However, for the generated alpha-particles to reach the p-n junction, the 10B layer must be thin (2-3 µm); otherwise, the alpha-particles can be reabsorbed by the 10B. To meet this contradicting thickness requirement, we proposed to use 10B filled high aspect-ratio holes and trenches with continuous Si p-n junctions on all areas [3,4]. A schematic representation of this detector cross-section is shown in figure 1. Thorough GEANT4 simulations were carried out in our earlier studies which determined that the optimal detector geometry is comprised of a honeycomb-like structure with hexagonal holes that have a flat-to-flat diameter of 2.8 µm and 1.0 µm Si walls between these holes [3]. Hole-depths as deep as 50 µm were experimentally studied. Due to the very high aspect-ratio of these features and the availability of processing equipment/materials, low pressure chemical vapor deposition (LPCVD) was selected for the boron deposition. It is believed that LPCVD can be tailored to deposit continuous, conformal films unlike other deposition techniques [5]. This is necessary to obtain void-free h
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