Junction Counters Produced by Irradiation of Silicon with Dopant Ions

Abstract

Silicon P-N junctions fabricated using kilovolt energy phosphorus and boron ions are described. The dopant ions are implanted through a previously applied layer of sputtered quartz, resulting in a conductive silicon layer of 0.1 to 1.0 μ thickness beneath the passivating quartz. Advantages of counters constructed in this manner are resistance to abrasion and ambient vapors similar to that of diffused counters, and uniform, thin dead layers smaller than those of typical diffused counters. In addition, temperatures used in fabrication are lower than those used during diffusion, so that the lifetime of high resistivity silicon should not be affected. Best values obtained with phosphorus ions implanted through 1200 angstroms of quartz are a dead layer of 0.16 microns combined with a surface resistance of 600 ohms. One-half inch active diameter counters implanted with boron ions through 1200 angstroms of quartz have yielded a breakdown voltage of 1050 volts, and resolution when cooled to 120°K of 80 keV from 70 to 400 volts, for 5.4 MeV alpha particles, with a calculated depletion depth of 1.1 mm at the latter voltage. Implantation through a mask has been used to produce ten counters on a single slice for use in the focal plane of a magnetic spectrograph. The counters are 11 mm in length and 0.9 mm in width, with 0.1 mm separation between implanted areas. Resistance between counters is about 6 megohms, enabling negligible crosstalk, and optimum resolution is 120 keV at 100 volts bias, with a depletion greater than 0.4 mm.