Si wafer cleaning


INRF application note

Process Name:



Mark Bachman, Spring 2002.  Adapted for AMPEL ANF, M. Beaudoin, April 2014.


Silicon wafers are cleaned by a solvent clean, followed by a dionized water (DI) rinse, followed by an RCA clean and DI rinse, followed by an HF dip and DI rinse and blow dry. Parts of this (RCA, HF dip) are dangerous wetbench processes and requires qualification for dangerous wetbench processes. The use of dangerous chemicals requires that the user may not perform the dangerous parts of the process alone (buddy system).

Time Needed:

This process takes one hour to complete in total.

Materials Needed:
  • Acetone
  • Methanol
  • Ammonium hydroxide
  • Hydrogen peroxide
  • Dilute (2%) hydrofluoric acid
  • Pyrex bath containers
  • Neoprene gloves
  • Hot plate

Solvent clean

Setup time for this process is about 5 minutes. This process takes about 15 minutes to complete.  Solvents can clean oils and organic residues that appear on glass surfaces. Unfortunately, solvents themselves (especially acetone) leave their own residues. This is why a two-solvent method is used. Pour the acetone into a glass container. Pour the methanol in a separate container. Place the acetone on hot plate to warm up (do not exceed 55°C). Place the silicon wafer in the warm acetone bath for 10 minutes. Remove and place in methanol for 2–5 minutes. Remove and rinse in DI water (DI water rinse is optional). Blow dry with nitrogen. Pour the used acetone and methanol in the non-halogenated solvent waste container (red can in the yellow cabinet). Do not pour any solvents down the drain.

RCA-1 clean
Setup time for this process is about 5 minutes. This process takes about 20 minutes to complete. RCA clean is used to remove organic residues from silicon wafers. In the process, it oxidizes the silicon and leaves a thin oxide on the surface of the wafer. The general recipe for RCA-1 cleanser is: 5 parts water (H2O), 1 part 27% ammonium hydroxide (NH4OH), 1 part 30% hydrogen peroxide (H2O2).

Here’s how to prepare it.

  • 325 ml DI water
  • 65 ml NH4OH (27%)
  • 65 ml H2O2 (30%)

Put 325 ml DI water in a Pyrex beaker, add 65 ml NH4OH (27%) and then heat to 70±5° C on hot plate. Remove from hot plate and add 65 ml H2O2 (30%). Solution will bubble vigorously after 1–2 minutes, indicating
that it is ready for use. Soak the silicon wafer in the solution for 15 minutes. When finished, transfer the wafer to a container with overflowing DI water from a tap to rinse and remove the solution. After several water changes, remove the wafer under flowing water. (Still water surface can contain organic residue that will redeposit on the
wafer surface when removing wafer.) To dispose of the RCA-1 solution, dilute with cold water then pour down the drain with plenty of cold water to flush. Old RCA-1 cleaning solution cannot be used since it loses its effectiveness in 24 hours at room temperature (30 minutes at 70°C).

HF dip
Setup time for this process is about 5 minutes. This process takes about 5 minutes to complete. Hydrofluoric acid (HF) is used to remove native silicon dioxide from wafers. Since it acts quickly, one needs to only expose the wafer for a short time (“dip”). HF is a dangerous chemical and protective gear must be worn when using it, in particular,
apron, acid (orange) gloves and eye protection, including full face shield, must be worn. A prepared solution should already be available to you (eg BOE).  If not, here’s how to prepare a 2% solution.

  • 480 ml water
  • 20 ml HF (49%)

Don all protective gear. Add 480 ml water to polypropylene beaker, then add 20 ml HF. Never use a glass beaker with HF since HF attacks glass. Label the bottle “2% HF solution—Dangerous acid!”, then add your name and the date. Soak the wafer for 2 minutes in this solution. Remove the wafer and rinse in running DI water. Check for hydrophobicity by performing a wetting test. Pour a little DI water on the surface. If the water beads up and rolls off, the surface is hydrophobic and water will not wet it. Since oxide is hydrophilic and pure silicon is hydrophobic, a non-wetting surface is clean of oxides. Blow dry with nitrogen and store in a clean, dry environment. The 2% solution may be saved for other cleaning (it does not lose its effectiveness). To dispose the 2% HF solution,
pour into the waste BOE and HF labeled waste container located under the wetbench in the same secondary container where HF is stored. Be sure to wash your acid gloves and return to their proper location when finished.

Safety & Emergency:

All ANF safety and procedural regulations must be followed.  Acetone and methanol are flammable liquids. Handle with care. Do not let the solvent exceed 55°C. Do not store the solvents near the hotplate or any other source of heat. Hydrogen peroxide is an explosive chemical. Keep it away from the solvents. Never leave the RCA process unattended. Hydrofluoric acid (HF) is an extemely toxic and dangerous acid. Use of HF requires at least one other person in the clean room (buddy system). HF should be handled in a laminar flow bench, using the apron, nitrile gloves inside the acid (orange) gloves and eye protection, including full face shield. Any small spills should be wiped up immediately with wipes and rinsed. Rinse the wipes in the sink under running DI water and dispose of in the waste container. DO NOT LEAVE the etchant unattended.

In case of exposure to HF Seek medical attention immediately! For skin exposure, flush immediately with water for 5 minutes followed by liberal application of calcium gluconate gel to the skin. Remove all clothing that are exposed before and while flushing with water. For eye exposure, flush the eyes with water three times, 5 minutes each. Irrigate the eye repeatedly with 500-1000 ml of a 1% calcium gluconate solution applied through a syringe. Call for prompt emergency room transport. Apply ice-water compresses during transport. In case of large spill, EVACUATE THE CLEANROOMS IMMEDIATELY and activate the evacuation alarm.

  • Kevin M. Walsh, University of Louisville Standard Operating Procedures,
  • Prudent Practices in the Laboratory, National Research Council, 1995.
  • W. Kern and J. Vossen, Thin Film Processes, Academic Press: New York, 1978, Ch V-1.
  • W. Kern, Ed., Handbook of Semiconductor Cleaning Technology, Noyes Publishing: Park Ridge, NJ, 1993, Ch 1.

The following checklist is designed to aid the researcher when performing this process.

Solvent clean

  • Prepare solvent baths: acetone and methanol
  • Prepare RCA bath: 5 parts water (H2O), 1 part 30% hydrogen peroxide (H2O2), 1 part 27% ammonium hydroxide (NH4OH).
  • Soak wafer in warm acetone (10 minutes)
  • Soak wafer in methanol (5 minutes)
  • DI rinse (optional) and blow dry
  • Clean up, dispose wastes

RCA #1 clean

  • Soak wafer in RCA bath at 70 °C for 15 minutes
  • DI rinse and blow dry
  • Clean up, dispose wastes

HF dip

  • Neoprene gloves required for HF dip
  • Dip wafer in 2% HF solution, 2 minutes
  • Wettability test
  • DI rinse/blow dry
  • Clean up, dispose wastes