Bobbie D. Stone

Detection and Identification of Potential Impurities Activated by Neutron Irradiation of Czochralski Silicon

A worst-case experiment was carried out wherein a matrix of Czochralski-grown silicon crystals irradiated to the five ohm-cm level was analyzed for radioactive isotopes that might result from activation of impurities initially present in the crystals. A computer study was used to predict the nuclides most likely to occur. Results from the two methods of analysis—radioactivity decay rate and gamma spectroscopy—were consistent with each other. Methods of measurements, calibrations and calculations to assure that radioactivity levels do not exceed the Nuclear Regulatory Commission exempt concentrations standards are presented.

Large Scale Production of NTD Silicon in the United States

The growth of the demand for NTD float zone silicon in the U.S. is traced in general terms with a discussion of device use patterns. A brief survey of types of irradiation facilities in the U.S. is outlined including an indication of commercial capabilities. Published licensed requirements of the U.S. Nuclear Regulatory Agency are discussed including monitoring of irradiated material for residual radioactivity, personnel protection measures, control of effluent levels, inventory monitoring and associated documentation Product specifications and tolerances are discussed with emphasis on the role played by the starting material. Analysis of the possible sources of inaccuracies is made.

Calibration of the Photoluminescence Technique for Determination of Phosphorus in Silicon by Neutron Transmutation Doping

A rapid accurate method for characterizing polycrystal silicon for residual electrically-active impurities has been developed based on the simultaneous determination of boron, phosphorus, arsenic and aluminum in the 10-10 atoms/atom range in silicon monocrystal by photoluminescence. The method was originally calibrated on the basis of resistivity, but this leads to a relatively large uncertainty, especially for donor impurities. This paper describes the recalibration of the method for phosphorus by NTD using a carefully controlled neutron fluence. The agreement between the three methods of measuring impurity content are presented and the statistical reproducibility of the method is discussed.

Neutron Transmutation Doping of Silicon Slices

A process for irradiating float zone crystal in slice form to reduce irradiation charges is described. Results indicate that breakage losses are a maximum of 8% and may be as low as 3%. In this process, wherein ingot identity is maintained, targeting precision and resistivity uniformity are equivalent to rod doped NTD crystals. Lifetime measured on 6mm slugs handled along with the slices are significantly lower than those observed on rods, but a 100 µ second minimum specification is still possible at resistivities above 25 ohm cm. Cosmetic stains on as-cut slices were observed, but techniques for avoiding them were devised. A qualitative cost comparison of the two doping methods is made.