Wafer Cleaning Explained!

Wafer Cleaning

Silicon Wafer

• Wafer cleaning process is an extremely important step in IC manufacturing as it directly affects the yield of the ICs.
• It is essential to clean silicon wafer to remove both organic and inorganic residues incorporated during wafer preparation.
• Wafer cleaning consists of several steps that include repeated De-ionized (DI) water rinse.
• Wafer cleaning usually includes:

1. -Solvent clean on a silicon wafer.
2. -DI water rinse.
3. -RCA clean.
4. -DI water rinse.
5. -Dip into HF solution
6. -DI water rinse.
7. -Dry.

RCA CLEANING PROCESS: - (SC-1, Oxide, SC-2)

[The cleaning steps outlined below must be followed consecutively in order to achieve the best results.  You should prepare all baths prior to starting the cleaning process. ]

 STANDARD CLEAN No.1 (SC-1):-

Chemicals and mixing ratio:

1. 5 parts Di water
2. 1 part Ammonium Hydroxide (NH4OH – 28%)
3. 1 part Hydrogen Peroxide (H2O2 – 30%)

Equipment/Apparatus:

1. Wet bench
2. Chemical tank  (unless heated solution is desired)
3. Wafer(s) holder
4. Beaker (optional)

Process:

1. Add 100ml of ammonium hydroxide to tank.
2. Add 100ml of hydrogen peroxide to tank.
3. Add 500ml of DI water to a tank.
4. Place wafer(s) into tank and hold for 5 minutes or until metals have been removed.
5. Rinse with DI water tank for 3 minutes while flushing tank.
6. Immediately place wafers into HF bath for the oxide clean.
   
   [Note: If heated bath is desired, submerge wafers in holder with wafers in cold solution and place the beaker on hot plate.]
   
7. Heat solution to 75 to 80°C and maintain temp.
8. Place wafer(s) into solution and hold for 5 minutes or until metals have been removed. 
9. Remove wafer(s) and place it immediately in DI water tank and rinse for 3 minutes while flushing tank.
10. Transfer wafer(s) directly into oxide clean bath.

Clean-up:

1. Use the chemical aspirator to dispose of chemical waste after the solution has cooled down.
2. Rinse out the chemical tank with DI water and aspirate to remove rinsing water.
3. Rinse, dry and put away all chemical mixing containers.
4. Wipe down all working surfaces on a wet bench.
5. Turn off the air and DI water supply to the wet bench.

 OXIDE CLEAN:-

Chemicals and mixing ratio

1. 1 part Hydrofluoric Acid (HF – 49%)
2. 30 parts DI water

Equipment/Apparatus :

1. Propylene beaker (DO NOT use glass beaker for HF chemical processing)
2. Wafer(s) holder

Process:

1. Pour DI water into the beaker.
2. Pour acid into beaker and mix.
3. Place wafer(s) in solution for 15 seconds. Exposed silicon (but not SiO2) should repel HF solution.
4. Rinse in DI water tank for 30 seconds to remove HF solution (this minimizes re-growth of a hydrous oxide film).
5. Transfer wafer assembly immediately, without drying, into hot SC - 2 solution.

 STANDARD CLEAN No.2 (SC-2):– desorption of ionic and heavy metal atomic contaminants

Chemicals needed and mixing ratio

1. 6 parts DI water
2. 1 part Hydrogen Peroxide (H2O2 – 30%)
3. 1 part Hydrochloric Acid (HCL – 37%)

Equipment/Apparatus:

1. Wet bench
2. Use chemical tank
3. Wafer holder
4. Beaker (optional)

Process:

1. Pour chemicals into the chemical tank.  Be sure to add acid to water and not vice versa.
2. Submerge wet wafers from HF clean into the bath for 10 minutes or until metals have been dissolved.
3. Remove wafers from bath and immediately place wafers in dirty DI rinse tank while flushing tank for 5 minutes.
4. Rinse wafers in clean DI rinse tank for 30 seconds.
5. Dry wafers with an N2 gun.
   [Note: If the heated bath is desired, submerge wafers in the holder with wafers in cold solution and place the beaker on the hot plate. ]
   
6. Heat solution to 75 to 80°C and maintain temp.
7. Place wafer(s) into solution and hold for 10 minutes or until metals have been dissolved. 
8. Remove wafer(s) and place it immediately in DI water tank and rinse for 5 minutes while flushing tank.
9. Rinse wafers with clean DI rinse tank for 30 seconds.
10. Dry with the N2 gun.

Clean-up:

1. Use the chemical aspirator to dispose of chemical waste after the solution has cooled down.
2. Rinse out the chemical tank with DI water and aspirate to remove rinsing water.
3. Rinse, dry and put away all chemical mixing containers.
4. Wipe down all working surfaces on a wet bench.
5. Turn off the air and DI water supply to the wet bench.

 Steps to be followed:

• Clean the wafer ultrasonically in tri-chloro-ethylene for 5 minutes.
• Explanation: The wafer is dipped in tri-chloro-ethylene for removal of fingerprints or another heavy residue present on the wafer. (This step is omitted these days).
• Clean the wafer meticulously in acetone.
• Explanation: The wafer is then dipped in acetone that removes the TCE residue and acts as a further cleaning solvent.
• Rinse the wafer in running DI water for about 5 minutes.
• RCA Cleaning is done as explained above.
• Finally, the wafer is kept for drying.

  ~Jay Mehta

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Jay Mehta.

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