Process Techniques and Radiation Effects in Metal-Insulator Semiconductor Structures

In the search for a means to minimize the radiation sensitivity of MOS transistors, a study was made on the interrelation between sensitivity to space radiation in MOS devices and the method of fabricating the gate oxide film of these devices. Both MOS capacitors and transistors were studied, using automatic plotting of capacitance-voltage and channel current gate voltage characteristics. For both types of device, the oxides studied were fabricated under varied but closely controlled conditions. Both special laboratory furnaces and production type of furnaces were used to grow the gate oxide. As a result of these experiments, the following conclusions were reached. (1) Gate oxide process techniques vary in their effect on the sensitivity of MOS structures to high-energy radiation by an order of magnitude. (2) The reproducibility of the degree of sensitivity to radiation is greater when steam-growth of oxides is used, rather than dry-oxygen growth. (3) In general, the nearer the prebombardment C-V characteristic of an MIS insulator is to the ideal (charge-free) charateristic, the less sensitive is the MIS device to radiation. (4) A key experiment was the successful "hardening", by a factor of 2 to 3 in the voltage shift, of dry oxide layers by the addition of a coating of phosphorus containing glass. (5) Several other process parameters (e. g. crystallographic orientation of the silicon) can be ruled out as to their influence on radiation sensitivity. (6) The present levels of radiation sensitivity in mass-produced devices can be reduced by changes in process technique.