Junction Counters Produced by Ion Implantation Doping

Boron ions of 500 Kev and 100 Kev energy (0.7 microns and 0.14 microns range respectively) from a Van de Graaff accelerator have been used to fabricate 1/2 - inch diameter junction counters by bombarding the surface of etched 5000 Ω-cm and 10, 000 Ω-cm n-type silicon slices. After being passed through an analyzing magnet to obtain a spectroscopically pure beam, the ions were scanned by electrostatic deflection plates to obtain uniform irradiation of the samples. Boron concentration could be varied as a function of depth in the crystal by programmed rotation of the slices about an axis perpendicular to the beam. After post-irradiation annealing at 300°C to remove radiation damage, evaporated aluminum contacts were applied to the front and electroless nickel to the back. Initial investigation resulted in diodes with leakage as low as 10 μa at -100 volts and reverse breakdown voltage greater than -400 volts. The thinnest dead layer was 0.2 microns. Pulse height spectra taken with Po210 alpha particles (5.3 Mev), in general contained large numbers of counts at low pulse heights, presumably due to carrier trapping in radiation-induced crystal defects. The best counter had a resolution of 50 Kev with insignificant low energy noise.