Sony Researches Removal Efficiency of SC-1 Cleaning
Re-evaluating immersion-type wet cleaning.
Maria A. Lester, Associate Editor -- Semiconductor International, 4/1/1998
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| 1. Removal efficiency may depend on the total amount of impurities brought into the solution. (Source: Sony Corp.) |
Researchers from Sony Corp. (Atsugi, Japan) recently tested the influence of initial wafer cleanliness on metal removal efficiency in an RCA Standard Clean I solution NH4OH/H2O2 /H2O. The experiments used an SC-1 solution spiked with various contamination levels and controlled levels of Fe and Cu at 1012 - 1014 atoms/cm2. Then one, three, six or 25 contaminated wafers with the same concentration level of Fe or Cu were simultaneously immersed in the ultrapure SC-1 solution. After each experiment, the solution was disposed and a new solution was prepared. The experiment revealed that concentrations after cleaning depend on the initial wafer cleanliness, the number of wafers treated at the same time and the total amount of impurities brought into the solution, as shown in Figure 1. This implies that the level of contamination after treatment depends on the total amount of impurities brought into the solution.
To prevent contamination, the researchers recommended the following procedures:
- Brush or water-jet scrubbing of wafer back surfaces before immersion-type cleaning.
- Scrubbing of wafer front surfaces between each dirty wafer processing step and the subsequent immersion-type cleaning.
- Adding cheating agents to the solution to prevent metal absorption onto silicon surface.
- Disposing of chemical solutions after each cleaning cycle.
Filtration of Large Particles in CMP Slurries
One of the challenges in CMP processes is the reduction of microscratches on polished wafers. Among many parameters in a CMP process, large particles (i.e., >1 µm) in slurries can be an important contributor to microscratching. Implementation of a proper filtration strategy can remove these undesirable large particles without affecting slurry performance. The challenge lies in detecting the large particles in a slurry with high solids concentration. Researchers at Millipore Corp. (Bedford, Mass.) recently developed a method for on-line or off-line particle counting to determine the large particle concentration in slurries.
Large particles can be found in both oxide and metal slurries, as shown in Figure 2. These particles were captured on a 3 µm Isopore PCTE membrane. Conventional particle size distribution analyzers that are due to the extremely higher concentration of smaller particles cannot quantify the concentration of large particles. The method used by Millipore to detect large particles is based on a commercially available optical particle counter (OPC). To bring the particle concentration to the OPC's working range, the slurry samples have to be diluted by an appropriate and particle-free diluent. By minimizing the chemistry shock to the slurry, the dilution step should not create or reduce the particle counts in the sample. Therefore, the particle counting system consists of an OPC and the components for the delivery and mixing of filtered diluent and slurry sample.
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| 2. Method for in-line/off-line large particle counting in slurries. (Source: Millipore Corp.) |
Millipore is further improving the procedures of the particle counting method. According to Lin, "There are many different types of slurries with different chemistries. We have selected appropriate diluents and developed procedures for different groups of slurries. Close collaborations with slurry manufacturers and end-users have helped us in developing particle monitoring procedures and applications knowledge for new slurries."
