Improved Fabrication Technique for Microstructured Solid-State Neutron Detectors
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1164-L06-01
Improved Fabrication Technique for Microstructured Solid-State Neutron Detectors S.L. Bellinger, W.J. McNeil, D.S. McGregor S.M.A.R.T. Laboratory, Mechanical and Nuclear Engineering Dept., Kansas State University, Manhattan, KS 66506, U.S.A. ABSTRACT Microstructured semiconductor neutron detectors have superior efficiency performance over thin-film coated planar semiconductor detectors. The microstructured detectors have patterns deeply etched into the semiconductor substrates subsequently backfilled with neutron reactive materials. The detectors operate as pn junction diodes. Two variations of the diodes have been fabricated, which either have a rectifying pn junction selectively formed around the etched microstructures or have pn junctions conformally diffused inside the microstructures. The devices with the pn junctions formed in the perforations have lower leakage currents and better signal formation than the devices with selective pn junctions around the etched patterns. Further, pulse height spectra from conformally diffused detectors have the main features predicted by theoretical models, whereas pulse height spectra from the selectively diffused detectors generally do not show these features. The improved performance of the conformal devices is attributed to stronger and more uniform electric fields in the detector active region. Also, system noise, which is directly related to leakage current, has been dramatically reduced as a result of the conformal diffusion fabrication technique. A sinusoidal patterned device with 100 μm deep perforations backfilled with 6LiF was determined to have 11.9 ± 0.078% intrinsic detection efficiency for 0.0253 eV neutrons, as calibrated with thin-film planar semiconductor devices and a 3He proportional counter. INTRODUCTION Compact neutron detectors with high counting efficiency and low voltage operation can be fabricated by etching micro-cavity patterns into a semiconductor diode [1]. When these microscopic patterns are filled with a neutron conversion material such as 10B or 6LiF, the energetic reaction products in the form of charge particles can be captured within a silicon diode. The deep trenches backfilled with neutron reactive material increase the neutron absorption efficiency, and for devices with narrow trenches, the probability of registering the energetic reaction products is increased [1-3]. Overall, the neutron detection efficiency is significantly increased over that of a thin-film neutron detector [3-8]. Construction of the detectors has many challenges, which include minimizing the leakage current while achieving a good signal to noise ratio. Deeply etched structures are easily fabricated with inductively-coupled plasma etching technology (Fig. 1) [4,6,9]. However, leakage current and noise are serious issues when introducing the deep vertical structures into a semiconductor device, such as a pn junction diode radiation detector. Leakage current contributes to electronic noise and reduces the ability to detect small signals. Hence, much work has
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