Calibration of Phosphorus Implantation Dose in Silicon by Radiochemical Neutron Activation Analysis
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Calibration of Phosphorus Implantation Dose in Silicon by Radiochemical Neutron Activation Analysis Rick L. Paul and David S. Simons Chemical Science and Technology Laboratory National Institute of Standards and Technology Gaithersburg, MD 20899 ABSTRACT A radiochemical neutron activation analysis (RNAA) procedure has been developed, critically evaluated, and shown to have the necessary sensitivity, chemical specificity, and matrix independence to certify phosphorus at ion implantation levels. 32P, produced by neutron capture of 31 P, is chemically separated from the sample matrix, and measured using a beta proportional counter. Analysis of 12 pieces of a silicon wafer, implanted at a nominal dose of 8.5 x 1014 atoms·cm-2, yielded a phosphorus concentration of (8.30 ± 0.16) x 1014 atoms·cm-2 (expanded uncertainty), in agreement with a consensus value of (8.23 ± 0.33) x 1014 atoms·cm-2 (1s uncertainty) determined from a round robin SIMS investigation. The relative standard deviation of < 2 % is suitable for certification of a Standard Reference Material. INTRODUCTION The U.S. semiconductor industry relies on secondary ion mass spectrometry (SIMS) for characterization of the depth distribution of dopants such as boron, arsenic, and phosphorus in silicon. To assist the industry in achieving high accuracy measurements, Standard Reference Materials (SRMs) of boron and arsenic implants in silicon have been developed by NIST as SIMS calibration standards.1,2 Plans are underway to develop a phosphorus implant in silicon SRM, to be certified by radiochemical neutron activation analysis (RNAA). RNAA was chosen because other techniques lack the necessary sensitivity, chemical specificity and matrix independence to measure phosphorus at implantation levels. Preliminary measurements were carried out on six pieces of a phosphorus-implanted silicon wafer previously used in a round-robin study of SIMS measurement repeatability.3 The measurements yielded a mean phosphorus concentration and standard deviation of (8.35 ± 0.20) x 1014 atoms·cm-2, in agreement with both the nominal implantation dose (8.5 x 1014 atoms·cm-2) and a consensus value of (8.23 ± 0.33) x 1014 atoms·cm-2 (1s uncertainty) determined from the SIMS investigation. In the current investigation, we set out to modify the initial procedure in order to improve precision and minimize uncertainties. EXPERIMENTAL Measurements were carried out on 12 pieces of the same phosphorus-implanted silicon wafer analyzed in the initial investigation. Samples were prepared for RNAA by cutting the silicon into 1 cm x 1 cm squares using a wafer saw. A micrometer was then used to measure the length and width of each piece to an accuracy of 0.0001 cm. The average surface area of the 12 samples was 0.9845 ± 0.0002 cm2. Blanks were prepared from two different silicon wafers which contained no implanted phosphorus. Blanks are necessary in order to correct the 32P produced from neutron capture by the implanted phosphorus (near surface layer), from 32P produced by neutron capture by phosphorus
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