Impurity Depth Profiles by Microanalysis Techniques

A method for determining the depth profile of aluminum atoms near the surface of SiO2 has been demonstrated by means of a sharp 27Al(p,7)28Si resonance. The proton beam from the NRL 5-MV Van de Graaff is passed through two beam-energy analyzers and allowed to fall upon the specimen located in the well of a sodium-iodide gamma-ray detector. The gamma-ray yield is measured as the beam energy is varied in small steps throughout a neighborhood in the vicinity of the resonance energy. The shape of the resulting yield curve depends upon the impurity (implanted) atom profile, the resonance shape, the beam energy distribution, the mean energy loss of the protons as they penetrate the specimen, and the fluctuations about this mean. All factors except the impurity atom concentration depth profile are assumed known and the concentration profile is extracted from the data by iterative syntheses of the yield curve shape and comparison by means of a computer. Depth resolution of the order of 100 A is practicable near the bombarded face of the specimen. The method is capable of an absolute measurement of the number of impurity atoms but that has not been demonstrated in this work. The tesults have been compared with those from a Cameca ion-beam mass spectrometer.