Ion implantation for threshold control in COSMOS circuits

The use of ion implantation for close threshold control of N-MOS and P-MOS transistors has been studied from an experimental and theoretical viewpoint. Experimental determinations of ion-implanted diffusion profiles, sheet resistivities, and threshold voltages for boron, phosphorus, and arsenic implantations are reported for doses in the range from 1 × 10¹¹to 1 × 10¹⁴ions/cm². Care has been taken to ensure accuracy of implanted dose in the 35-150 keV range in order to permit direct comparison of experiments with theory. Satisfactory agreement is observed between experiment and computer predictions using only the implantation dose and energy. The analytical model tested assumed outdiffusion in a neutral ambient from an implanted Gaussian layer. Experimental tests of the calculation are made for thresholds, sheet-resistance profiles, and amounts of dopant lost to the oxide masking layer. The theoretical model has been used as a basis for threshold control. The application of these principles is related to the practical fabrication of COSMOS circuits with close threshold control through suitable selection of starting substrate material and implantation conditions.