Specific Process Knowledge/Wafer cleaning/RCA: Difference between revisions
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RCA1 is a powerful oxidizer and will thus remove organic contaminants. The ammonium hydroxide in RCA1 also has good solvating power as it can complex metals such as Cu, Ag, Ni, Co and Cd into solution. RCA2 forms soluble complexes with heavy metals and also shows cleaning action similar to that of piranha solutions. Furthermore, solutions of hydrochloric acid, which is a major constituent of RCA2, has proven effective at removing alkali metal ions. RCA2 is also effective at removing metal hydroxide traces that cannot be removed in ammonia-containing solutions such as RCA1. These include Al(OH)<sub>3</sub>, Fe(OH)<sub>3</sub>, Mg(OH)<sub>2</sub> and Zn(OH)<sub>2</sub>. It is important that the chemicals used are very clean. RCA mixtures are therefore only used shortly after mixing and for a limited number of samples.<br> | RCA1 is a powerful oxidizer and will thus remove organic contaminants. The ammonium hydroxide in RCA1 also has good solvating power as it can complex metals such as Cu, Ag, Ni, Co and Cd into solution. RCA2 forms soluble complexes with heavy metals and also shows cleaning action similar to that of piranha solutions. Furthermore, solutions of hydrochloric acid, which is a major constituent of RCA2, has proven effective at removing alkali metal ions. RCA2 is also effective at removing metal hydroxide traces that cannot be removed in ammonia-containing solutions such as RCA1. These include Al(OH)<sub>3</sub>, Fe(OH)<sub>3</sub>, Mg(OH)<sub>2</sub> and Zn(OH)<sub>2</sub>. It is important that the chemicals used are very clean. RCA mixtures are therefore only used shortly after mixing and for a limited number of samples.<br> | ||
Since RCA1 can oxidize silicon to silicon dioxide, an HF dip is sometimes used between RCA1 and RCA2. This will also remove contaminants that can be trapped in the formed oxide. This step can also be omitted, since RCA1 removes the native oxide and then re-oxidizes the surface. This in itself has a cleaning action. It is generally not advised to do an HF dip after RCA-2 if the samples will be processed by wet chemical processes afterwards. This is because the hydrogenated surface will be extremely reactive and therefore attract contaminants from both the air and processing liquids. In this case it is better to continue processing with the passivated oxide surface. | Since RCA1 can oxidize silicon to silicon dioxide, an HF dip is sometimes used between RCA1 and RCA2. This will also remove contaminants that can be trapped in the formed oxide. This step can also be omitted, since RCA1 removes the native oxide and then re-oxidizes the surface. This in itself has a cleaning action. It is generally not advised to do an HF dip after RCA-2 if the samples will be processed by wet chemical processes afterwards. This is because the hydrogenated surface will be extremely reactive and therefore attract contaminants from both the air and processing liquids. In this case it is better to continue processing with the passivated oxide surface. RCA clean is often done prior to furnace processes, an in these cases an HF dip after RCA-2 makes sense. Ideally the wafers should be transferred to the furnace immediately after RCA cleaning, because the samples will be very sensitive to recontamination as mentioned above.<br> | ||
As mentioned above a crucial part of the RCA cleaning procedure is the oxidation by H<sub>2</sub>O<sub>2</sub> at elevated temperatures. Therefore, the lifetime of RCA1 and RCA2 solutions is limited to about 1 hour after preparation since heating to 70<sup>o</sup>C because H<sub>2</sub>O<sub>2</sub> decomposes at this temperature. | As mentioned above a crucial part of the RCA cleaning procedure is the oxidation by H<sub>2</sub>O<sub>2</sub> at elevated temperatures. Therefore, the lifetime of RCA1 and RCA2 solutions is limited to about 1 hour after preparation since heating to 70<sup>o</sup>C because H<sub>2</sub>O<sub>2</sub> decomposes at this temperature. | ||