The effect of copper contamination on thin gate oxide integrity

Abstract

Contamination of silicon with trace amounts of copper during processing can adversely affect the gate oxide integrity of integrated circuits, resulting in substantial yield loss and reliability problems. Because of increased use of copper as interconnect material, cross-contamination during the production process must be avoided. Establishing adequate protocols for wafer handling and tool use requires understanding of the mechanisms by which copper affects the gate oxide and knowledge of acceptable contamination limits. The effect of copper contamination on thin gate oxide integrity was studied by purposely introducing small amounts of copper in controlled contamination experiments. Copper contamination causes gate oxide defects by precipitating near the Si/SiO₂ interface when the silicon is cooled from the processing temperature. The concentration at which defects start to appear on flat capacitors decreases with decreasing oxide thickness. When copper contamination from a contaminated ammonium hydroxide peroxide mixture cleaning solution occurs, deposition of more than 7 x 10¹⁰ atoms/cm² is required before the gate oxide integrity of 3 nm oxides is affected. Such levels are not readily exceeded in state of the art integrated circuit fabrication facilities using high-grade chemicals. Copper deposition from contaminated hydrofluoric acid (HF) solutions results in oxide lifetime degradation for 3 nm oxides even when less than 1 x 10¹⁰ atoms/cm² is deposited. The copper catalyzes hydrogen evolution, which in turn causes dissolution, and thus roughening, of the silicon surface. Roughening of the silicon is responsible for the reduced oxide quality. Thus, removal of the copper after contamination has occurred does not lead to recovery of the oxide quality. A copper contaminated silicon surface, which is immersed in uncontaminated HF, will also have reduced oxide lifetime. Precipitation occurs more readily at field overlap regions, since these regions have a large amount of mechanical stress, which getters contaminants and allows precipitates to nucleate and grow more easily. Field overlap precipitates result in a large field overlap-related defect density when copper contamination occurs. Copper contamination from contaminated HF also results in bumps in an oxide that is subsequently grown on the surface. These local regions of thicker oxide can lead to threshold voltage non-uniformity.