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Novel High-Radiopurity Doped Amorphous Silicon Resistors for Low-Background Detectors

A. Anker, P. C. Rowson, K. Skarpaas, S. Tsitrin, I. J. Arnquist, L. Kenneth S. Horkley, L. Pagani, T. D. Schlieder, E. van Bruggen, P. Kachru, A. Pocar, N. Yazbek

TL;DR

This work addresses the need for ultra-low-background spacer/resistors in the nEXO LXe Time Projection Chamber by developing doped amorphous silicon resistors on fused silica substrates that also serve as structural field-cage elements. The approach combines LPCVD silicon deposition, end metallization, and phosphorus doping to achieve resistances in the range of 0.1–10 GΩ at 165 K, with radiopurity at or below ppt levels for U and Th when production controls are followed. A key finding is that 0.7 sccm SiH4/PH3 doping yields resistivities and layer thicknesses that produce the desired resistance at cryogenic temperatures, and that radiopurity can meet nEXO requirements, with UV reflectivity around 48% at 175 nm offering modest light-collection benefits. The work also documents practical fabrication steps, a notable negative TCR, light-sensitivity considerations, and emphasizes HV testing and mechanical integration as areas for further development. The methods and results point to broader applicability in other low-background detectors and substrate choices, including sapphire, with continued optimization for production-scale deployment.

Abstract

We present the results of a study of lightly doped amorphous silicon used as a resistive medium for high-radiopurity resistors in nuclear and particle physics research instrumentation. Prototypes are produced for a Time Projection Chamber design for the nEXO neutrinoless double-beta decay search experiment that meet requirements for ultra-high radiopurity, good mechanical, cryogenic and high voltage performance, as well as useful vacuum ultraviolet (VUV) reflectivity. Further study is warranted to refine production methods and to confirm that the technology used here is useful for more general applications.

Novel High-Radiopurity Doped Amorphous Silicon Resistors for Low-Background Detectors

TL;DR

This work addresses the need for ultra-low-background spacer/resistors in the nEXO LXe Time Projection Chamber by developing doped amorphous silicon resistors on fused silica substrates that also serve as structural field-cage elements. The approach combines LPCVD silicon deposition, end metallization, and phosphorus doping to achieve resistances in the range of 0.1–10 GΩ at 165 K, with radiopurity at or below ppt levels for U and Th when production controls are followed. A key finding is that 0.7 sccm SiH4/PH3 doping yields resistivities and layer thicknesses that produce the desired resistance at cryogenic temperatures, and that radiopurity can meet nEXO requirements, with UV reflectivity around 48% at 175 nm offering modest light-collection benefits. The work also documents practical fabrication steps, a notable negative TCR, light-sensitivity considerations, and emphasizes HV testing and mechanical integration as areas for further development. The methods and results point to broader applicability in other low-background detectors and substrate choices, including sapphire, with continued optimization for production-scale deployment.

Abstract

We present the results of a study of lightly doped amorphous silicon used as a resistive medium for high-radiopurity resistors in nuclear and particle physics research instrumentation. Prototypes are produced for a Time Projection Chamber design for the nEXO neutrinoless double-beta decay search experiment that meet requirements for ultra-high radiopurity, good mechanical, cryogenic and high voltage performance, as well as useful vacuum ultraviolet (VUV) reflectivity. Further study is warranted to refine production methods and to confirm that the technology used here is useful for more general applications.
Paper Structure (13 sections, 1 equation, 9 figures, 10 tables)

This paper contains 13 sections, 1 equation, 9 figures, 10 tables.

Table of Contents

  1. Introduction
  2. nEXO Motivated Design
  3. Amorphous and Polycrystalline Silicon: Initial R&D at SNF
  4. Experimental setup at SLAC
  5. Phosphorous Doped Silicon R&D at MBNL
  6. Low Temperature testing
  7. Fabrication of spacer/resistors
  8. Measurement of the thermal coefficient of resistivity (TCR)
  9. Photosensitivity
  10. Comment on High Voltage testing
  11. Radiopurity testing at PNNL
  12. UV Reflectivity Studies at UMASS
  13. Conclusions and General Applications

Figures (9)

  • Figure 1: Rendering of the nEXO field cage. The spacer/resistors have a silvery finish, while the ordinary fused silica or sapphire spacers are transparent and reveal the tensioning rods (here shown with a contrasting amber color) that pass through them. The copper field shaping rings are aluminized.
  • Figure 2: Illustration of the thermal coefficient of resistivity (TCR) at near room temperature of a single SNF-fabricated undoped aSi prototype (selected from Run 2, 530° C deposition). The fitted TCR corresponds to $1.9 \times 10^{-2} \rm { ~per~degree ~K}.$
  • Figure 3: Metal testing box for spacer/resistors with custom spring fixturing to make contact under compression.
  • Figure 4: Segment of a diced wafer (a "strip") with gold plating on the ends and dimensions 10 cm x 0.5 cm.
  • Figure 5: Fabrication drawing for the MBNL spacer/resistors. The chamfer at the ends, done by hand for the SNF parts, is designed to improve the integrity of the metallization coating.
  • ...and 4 more figures