Silicon nitride is commonly etched by hot orthophosphoric acid. Hot diluted hydrofluoric acid is hereby used as an alternative. Nonetheless, in presence of silicon surfaces, some corrosion has been evidenced, degrading significantly active areas during the STI (Shallow Trench isolation) integration. Oxygen in hot deionized water or hot HF generates this corrosion and selecting a relevant chemical oxide before dispensing hot diluted HF is key in solving the concern.
Orthophosphoric acid is the worldwide standard chemical to quickly and selectively etch thick silicon nitride films towards silicon oxide. Along the semiconductors node evolution, new demands have arisen to etch few nanometers thick SiN films, with much better defectivity than H3PO4. Several alternatives have been developed during the past few years. Among them, a hot very diluted HF process was developed in the early 2000’s. This paper deals with several applications where significant benefits are demonstrated compared with H3PO4: first a Stress Proximity Technic successfully adopted into production mode, secondly a SiN films patterning thanks to a softmask. Although this process benefits from high selectivity, low cost of ownership, good defectivity, its within wafer uniformity still remains poor due to the actual hardware.
For 28 nm and beyond, severe specifications in terms of dimensions and materials integrity still drive further cleaning process improvements. As the global “HF budget” drastically decreases with interconnections dimensions, HF solution dilution and process time both decreased stepwise. However, very short recipes with process time shorter than 15s start to suffer from lack of robustness, in particular for the monitoring of inline parameters such as flow-rates and temperature. In this paper, we highlighted that a first matching of silicon oxide consumption was usefull to select temperature and concentration range for the diluted HF solution. High dilution ratio, and “room temperature” (20 °C) were then selected. Variations in cleaning efficiency were analyzed as regard with electrical defects density at three metals levels, then the use of 0.025 %wt. HF, 20 °C, 40 s. was pointed out as the more promising solution for process of record replacement. Process robustness, i.e. inline monitoring data collection and uniformity on wafer should thus be improved thanks to this longer process time and a lower process temperature.
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