At FSI’s recent Knowledge Services™ Seminar series Enrico Bellandi described the success he and his colleagues at Numonyx had in reducing the defectivity and improving the yield of their cobalt salicide process using the ZETA® spray cleaning system. Reduction of contact and gate conductor resistance through self-aligned silicide formation, also known as “salicide,” is a well-known integrated circuit manufacturing module. A critical step in this module is the selective removal of unreacted metal after the silicide is formed by an annealing step. For flash memory manufacturing it is especially critical to remove any residual metal from the nitride spacer on the side of the gate stack.
Any metal remaining there can cause electrical shorts and leakage between the source/drain and the gate conductor, and cobalt removal presents unique challenges. Using the ZETA® spray tool, Bellandi optimized a sequence of sulfuric acid/hydrogen peroxide mixture (“SPM” or “piranha”) followed by ammonium hydroxide/hydrogen peroxide mixture (“APM” or “SC1”) that significantly improved the process yield. He attributed the improvement to a reduction in particle defects and a decrease in residual cobalt levels on the nitride spacers.
The key attribute of the metal stripping process is its ability to selectively remove the unreacted metal without attacking the silicide or other materials exposed to the cleaning process. However, it is also critical that the process etches sufficient nitride from the surface of the spacer to undercut and remove any residual cobalt. In addition, the process must minimize particulate defects resulting from redeposition of removed material. Process optimization consists of balancing these requirements to deliver low, repeatable contact resistance with minimal yield loss to shorts, caused by unreacted cobalt or particulate contamination, and leakage, caused by residual cobalt diffused into the nitride spacer. Ultimately process performance must also be balanced by cost of ownership considerations that include operating costs, such as chemical consumption, flexibility, and the tool’s ability to perform other cleaning processes without cross contamination.
Bellandi’s presentation focuses on Numonyx’s migration from a conventional SC1/SC2 process to Piranha/SC1 chemistry and the optimization of the new process. It includes careful quantitative characterization of defectivity and decontamination performance versus numerous process variables including chemical composition and temperature, reporting improved selectivity to CoSi2 and SiO2, reduced residual Co levels through increased Si3N4 etching, and the elimination of CoSi residue particles. The report also documents dramatic reductions in defectivity and yield losses from shorts and leakage. Finally, it attributes reduced chemical consumption and multiple process capability to the ZETA® system’s proprietary fresh-mixed chemical spray technology. The full presentation is available to FSI customers and can be requested on the FSI web site at http://www.fsi-intl.com
Figure 1. Lower yield loss (higher yield) attributed to ZETA® SPM SC1 process
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