High-efficiency detectors for nuclear radiations are important in various applications such as nuclear medicine, security and industrial imaging systems, personnel and area monitoring. Solid state semiconductor-based detectors owing to their remarkable advantages such as small size, compactness, low operating voltage, low power consumption, and low cost are being increasingly used over conventional detectors (gas proportional counters or scintillation detectors).
A pillar semiconductor neutron detector is being developed (Figure 1) to eliminate the geometrical constraint which limits the efficiency to <5% in the planar configuration. In such 3-D configuration, alpha converting material (10B) fills the high aspect ratio vertical trenches etched in Silicon P-I-N detector. This ensures a very high probability of charged particles coming from 10B-neutron reaction to penetrate the P-I-N detector due to the close proximity and thus significantly improves the neutron detection efficiency.
The key process in detector fabrication includes etching of high aspect ratio vertical trenches in Silicon using Reactive Ion Etching (RIE). After etching, Plasma Enhanced Chemical Vapor Deposition (PECVD) is used for conformal filling of BxC as a converter material in high aspect ratio trenches using Ortho-Carborane (O-C2B10H12) precursor. However, during the deposition of BxC in the trenches, excess BxC deposits on the top of pillars. Since, the pillars are P-I-N, the excess surface layer is further removed by fluorine based chemistry to enable metal contact to the p-type Silicon.
In order to fabricate the detector, we have indigenously designed and developed Inductively Coupled Plasma- Reactive Ion Etching (ICP-RIE) system for etching with etch rate of 1.12 µm/min. In addition, PECVD system was designed and developed indigenously in such a way that the BxC deposition is carried out with substrate biasing. The precursor for BxC deposition namely Ortho- Carborane (O-C2B10H12) is a solid material and it sublimes at 70-90°C. Thus, a bubbler assembly is developed to sublime Ortho- Carborane and vapors are carried to the PECVD chamber using a carrier gas.
We have demonstrated a single step process for conformal deposition of Boron Carbide (BxC) with 94.25% fill factor and the high conformal coverage ratio of 88.25% in high aspect ratio (>6:1) trenches (Figure 6 and 7). We have successfully etched excess BxC from the top of the pillar exposing underlying Silicon (Figure 8) with the selectivity of 4:1 against Silicon.
The results are very promising for fabrication of neutron detector with high detection efficiency. This work is funded by Board of Research in Nuclear Science (BRNS), Department of Atomic Energy (DAE), Gov. of India.