A self-aligned nitrogen implantation process (SNIP) to minimize field oxide thinning effect in submicrometer LOCOS

A self-aligned nitrogen implantation process (SNIP) utilizing low-energy and high-dose molecular nitrogen ions has been developed to minimize the field oxide thinning effect in submicrometer local oxidation of silicon (LOCOS) isolation. Molecular nitrogen ions with a dosage of 2.5×10¹⁶ cm^-2 were implanted at 20 keV into large isolation regions to selectively form a thin nitridelike layer which can effectively retard the thermal oxidation of silicon. Self-aligned spacers were developed to shield small-isolation regions from the nitrogen implantation. The oxidation rate in small-isolation regions was therefore not affected. The final field oxide thickness became more uniform for various isolation dimensions across the wafer. The device characteristics of the n- and p-MOSFET with the SNIP were similar to those of devices with the conventional LOCOS process. An increase in the magnitude of field threshold voltages at submicrometer isolation regions was measured for both n- and p-channel parasitic field-effect transistors with the SNIP. A minimal reduction in field oxide thickness of less than 10% and an acceptable field threshold voltage magnitude of higher than 7.5 V were achieved for an isolation width as narrow as 0.5 μm