Specific Process Knowledge/Wafer cleaning/RCA: Difference between revisions

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RCA cleaning

The RCA cleaning procedure was originally developed by the Radio Corporation of America (RCA) as part of manufacturing electron tube components. This process has since been adapted to silicon semiconductor processing. RCA is a two-step process (RCA1 and RCA2, sometimes referred to as SC-1 and SC-2 (Standard Clean)), and RCA1 should always be made before RCA2 because removing organic contaminants is a requirement before metal contaminants can be removed effectively.

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)₃, Fe(OH)₃, Mg(OH)₂ and Zn(OH)₂. 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.

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.

As mentioned above a crucial part of the RCA cleaning procedure is the oxidation by H₂O₂ at elevated temperatures. Therefore, the lifetime of RCA1 and RCA2 solutions is limited to about 1 hour after preparation since heating to 70^oC because H₂O₂ decomposes at this temperature.

RCA1 contains: H₂O, NH₄OH and H₂O₂ (5:1:1). It is used for removal of light organics, particles and metals.
RCA2 contains: H₂O, HCl and H₂O₂ (5:1:1). It is used for removal of heavy metals, metal hydroxides, and residual metals including alkali metals.

You can find the APV for the RCA bench here

RCA procedure

RCA1: 10 min
DI water rinsing (dumping three times)
Optional: HF: 30 sec (you may avoid this step in case of a very thin oxide (0-200 Å) as the top layer)
DI water rinsing (bubbler)
RCA2: 10 min
DI water rinsing (dumping three times)
Optional: HF: 30 sec (avoid it if you have a very thin oxide (0-200 Å) as the top layer)
DI water rinsing (bubbler)

For procedure details please look in the user manual in LabManager.

The user manual, user APV and contact information can be found in LabManager: RCA info page in LabManager

Overview of RCA process data

	RCA1	RCA2	HF
General description	Used for removal of light organics, particles and desorption of trace metals (Au, Ag, Ni, Cd, Zn, Co, Cr, etc).	Used for removal of alkali ions, metal hydroxides (of Al, Fe, Mg, Zn) and residual trace metals (e.g. Cu and Au).	Used for removal of oxide generated in RCA1 and RCA2
Chemical solution	H₂O, NH₄OH(25-29%) and H₂O₂(30%) (5:1:1)	H₂O, HCl(37%) and H₂O₂(30%) (5:1:1)	5% HF
Process temperature	70-80 ^oC	70-80 ^oC	Room temperature
Process time	10 min.	10 min.	30 sec.
Life time of the chemical solutions	Can only be heated one time. When hot: it lasts for ~1h	Can only be heated one time. When hot: it lasts for ~1h	~2 months
Allowed materials	Silicon Silicon oxide Silicon nitride Quartz wafers/fused silica	Silicon Silicon oxide Silicon nitride Quartz wafers/fused silica	Silicon Silicon oxide Silicon nitride
Batch size	1-25 2",4" or 6" wafers at a time	1-25 2",4" or 6" wafers at a time	1-25 2",4" or 6" wafers at a time
Size of substrate	4"-6" wafers For 2" wafers please contact Danchip staff	4"-6" wafers For 2" wafers please contact Danchip staff	4"-6" wafers For 2" wafers please contact Danchip staff

@@ Line 8: / Line 8: @@
 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. For furnace processes, an HF dip after RCA-2 makes more sense (but ideally the wafers should be transferred to the furnace processes immediately).<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. 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.