Effects of megasonic power in DIW rinsing on particle removal efficiency and defect density
Abstract- As the IC fabrication feature size gets smaller, the particle removal process becomes a critical step. It was proven that megasonic in SC-1 chemical is effective in removing surface contaminants but might inflicting structure damage [1-2] especially for geometry smaller than 130nm technology. Since Moore’s law states that the number of transistor on an IC chip will be double every two years, the problem will only get larger . Therefore, it is important to optimize the current RCA cleaning step in order to improve the particle removal efficiency and at the same time preventing pattern damage. Effects of megasonic power in both SC-1 and DIW in term of particle removal efficiency and defect density will be studied. Experimental result for different megasonic power using blanket and pattern wafers will be presented. Results indicate that additional megasonic cleaning in DIW after SC-1 chemistry drastically improves particle removal efficiency, and with the use of appropriate megasonic power in both SC-1 and DIW, it is possible to maintain the particle removal efficiency and simultaneously, cause no pattern damage.
Particulate surface contamination is a major cause of yield loss in semiconductor . Particles easily introduced by chemistry during processing. SC-1 chemical used in RCA clean is already contaminated with particles of submicron size. Chemicals of the grade need in ULSI processing were tested for particulate by Frederick W. Kern and found that ammonium hydroxide has the highest particle count as compared to other chemistry such as hydrochloric . In addition, the same chemical is used repetitively for several batches and this can become the source of contamination since the residue of organic will build up in the fluid and might re-deposit back on other surface . It is impossible for a foundry to change the practice of current RCA cleaning thoroughly as this will involves extremely high cost with new tool purchasing. Consequently, QDR after SC-1 bath is a good position to remove the particles besides rinsing off the chemical residue. QDR is a quick dump rinse process where the DIW is dumped every cycle without recycling. No cross contaminantion will occur. Particle removal efficiency can be further enhanced with megasonic cavitation besides the bubbling and showering effect in QDR tank. The cost of megasonic unit installation in QDR tank will be much cheaper as compared to new tool purchasing. The evaluation result of megasonic power in both SC-1 and QDR in term of particle removal efficiency and degree of structural damage will be shown in this paper.
Particle removal efficiency and degree of seriousness in structural damage were studied at 50, 100 and 150Watt at 950 kHz (megasonic range) in both SC-1 and QDR tank as refer to the DOE (Design of Experiment) table, Table 1.1. The experiments were performed using a commercially megasonic cleaning system installed in fully automated DNS wet station. Bare silicon wafers used for particle removal efficiency evaluation were 125 mm p-type of 7-10 (-cm resistivity. These wafers were pre-contaminated with a short dip of hydrofluoric acid in a resist grade tool to create a hydrophobic surface to attract particles. Particle contamination levels before and after cleans were monitored using a KLA-Tencor Surfscan SP1 at a threshold diameter of 0.06µm and 0.13µm. Wafers were randomly picked as pre count will impact the particle removal efficiency result. Short loop wafers were used for defect density evaluation. Wafers were processed from first oxidation until gate poly etch but skipping implantation steps. 0.13µm technology is chosen for the study as this technology currently encounters yield impact with reduction of megasonic power and simultaneously more susceptible to pattern damage. All the cleaned...
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