6:1 aspect ratio silicon pillar based thermal neutron detector filled with B10

Abstract

Current helium-3 tube based thermal neutron detectors have shortcomings in achieving simultaneously high efficiency and low voltage while maintaining adequate fieldability performance. By using a three-dimensional silicon p-i-n diode pillar array filled with boron-10 these constraints can be overcome. The fabricated pillar structured detector reported here is composed of 2μm diameter silicon pillars with a 4μm pitch and height of 12μm. A thermal neutron detection efficiency of 7.3+∕−0.6% and a neutron-to-gamma discrimination of 105 at 2V reverse bias were measured for this detector. When scaled to larger aspect ratio, a high efficiency device is possible.

Development of 10B2O3 processing for use as a neutron conversion material

“…To achieve the aspect ratios required, we employ nLOF 2035 negative photoresist, which produces a mask of ~2.8 μm in thickness. Etching is done in an STS DRIE Bosch system, using a process recipe composed of alternating fluorobutante (C 4 F 8 ) sidewall passivation and sulfur hexafluoride (SF 6 ) silicon etch steps [8]. Erosion of the photoresist mask is a potentially serious problem if the selectivity is not carefully controlled; an overall etch selectivity of >25:1 must be achieved to produce silicon pillar arrays of greater than 75 μm.…”

“…Common solid source deposition techniques, such as sputtering and e-beam evaporation, generally do not achieve a reasonable fill due to pinching off or shadowing effects at the tops of the pillars. While gaseous chemical vapor deposition is possible and has produced excellent results for our 10 B detectors [5][6][7][8][9][10], material precursors and deposition equipment can be expensive. As an alternative, we have developed processing capabilities to deliver 10 B 2 O 3 via a solution-based process to achieve near 100% fill.…”

“…Our group has been developing a 3-dimenstional (3d) Si based "Pillar Detector," first reported in 2005 by Nikolic et al [8] with 10 B as the thermal neutron conversion material. This approach relies on pure 10 B deposited by low pressure chemical vapor deposition (LPCVD ) [10], and has achieved detectors with 20% intrinsic detection efficiency [9], with the possibility to increase to greater than 50%.…”

“…These include boron-10-( 10 B) lined gaseous tubes, boron-10-triflouride ( 10 BF 3 ) -filled tubes, lithium-6-( 6 Li) doped scintillators, and other semiconductor based platforms utilizing 6 Li [1][2][3][4], 10 B or 10 B containing compounds [5][6][7][8][9][10] such as boron nitride, boron carbide [11] and boron phosphide. Our group has been developing a 3-dimenstional (3d) Si based "Pillar Detector," first reported in 2005 by Nikolic et al [8] with 10 B as the thermal neutron conversion material.…”

Planarization of high aspect ratio p-i-n diode pillar arrays for blanket electrical contacts

“…We are currently investigating three dimensional P-I-N Si diode pillar arrays filled with isotopically enriched 10 Boron for thermal neutron detection. 7 In order to realize functionality of our devices, as well as similar three dimensional structures for other uses, it is necessary to make an electrical contact to the top of structure, which is the p+ layer of the diode. Achieving a blanket, conformal coating can be difficult due to topography across the device structure.…”