Substrate Cleaning
a) Substrate Handling
Care must be taken when handling wafers not to scratch or contaminate them. A clean set of plastic or stainless-steel tweezers should be used when handling the wafers. The wafers should never be handled by hand or gloves as this will introduce contaminants to the substrate. Although scratching the substrate is unavoidable when handling the wafer with tweezers, using plastic tweezers will reduce the amount of scratching. Make an effort to handle the substrates by the edge of the wafers to limit scratching to the edges.
b) Substrate Cleaning
Semiconductor wafers are subjected to physical handling during cutting, lapping, polishing, and packaging. This leads to molecular contamination, much of which is of unknown origin. Included in the list of contaminants are airborne bacteria, grease and wax from cutting oils and from physical handling, abrasive particulate (usually silicon carbide, alumina, and diamond dust) from the grinding and sawing operations, as well as a variety of plasticizers which come from the containers and wrapping which the wafers are shipped. Removal of these contaminants is the first step in wafer cleaning, and is usually done by rinsing in hot organic solvents such as acetone, accompanied by mechanical scrubbing, ultrasonic agitation, or by compressed gas jets.
Further cleaning of wafers must be done after each processing step in the fabrication sequence, and especially before each high-temperature operation. It is desirable that the cleaning procedures give a surface that is reproducible and stable. It is essential that the results of a particular operation be predictable each time that operation is performed. Depending on the particular processing step, the three types of cleanings may be used and these are explained in the next three sections.
c) Substrate Degreasing
- Place the wafers in acetone for 5 minutes. Use petri dish labeled “acetone”.
- Rinse the wafers in DI water for 1 minute. Use petri dish labeled “DI-1”.
- Place the wafers in methanol for 5 minutes. Use petri dish labeled “methanol”.
- Rinse the wafers in flowing DI water for 10 minutes. Use the carrier.
- Dry the wafers with nitrogen.
d) Organic Contaminant Removal
A fresh batch of “piranha” solution must be prepared for this procedure. The piranha solution is prepared by adding 2 parts of H2SO4 to 1 part of H2O2. Caution must be taken when preparing piranha as the reaction is exothermic and causes the solution to reach a temperature of 100 to 150 °C. Wear the appropriate PPE (acid gloves, face shield, acid apron) when preparing piranha. Make sure to use the funnel when pouring chemicals into smaller beakers. Rinse the funnel between every use. Only use glass or Teflon containers since Piranha can melt and even attack plastic containers.
When preparing the piranha solution, always add the peroxide to the acid very slowly. Note: an explosion could occur if the H2O2 concentration is ≥50%, therefore the peroxide must always be added to the sulfuric acid.
Preparation of Piranha
- Don acid gloves, face shield, and acid apron. The order in which the chemicals below are mixed is critical.
- Measure 2 parts of H2SO4with the beaker labeled “H2SO4” and pour it into the Piranha mixing container.
- Measure 1 part of H2O2with the beaker labeled “H2O2” and then slowly pour it into the Piranha mixing container. (The solution will give off vapor and get up to 150 °C).
Solution Container |
Maximum Wafers |
Total Volume (mL) |
H2SO4 (mL) |
H2O2 (mL) |
Petri Dish |
1 |
90 |
60 |
30 |
Piranha Tank |
25 |
3900 |
2600 |
1300 |
Procedure Day-3: Organic Contaminant Removal
- Dip the wafers in fresh piranha solution for 8 minutes at 100 to 150 °C.
- Rinse the wafers in running DI water for 10 minutes.
- Dry the wafers with nitrogen. (Water should stick to the clean wafers – hydrophilic.)
Procedure: Disposal of Piranha
- Don acid gloves, face shield, and acid apron.
- When temperature is <40 °C, use aspirator to dispose piranha from tank.
- Rinse the tank and aspirator with at least 2000 mL of DI.
e) Native Oxide Removal
Hydrofluoric acid solutions are normally used for etching silicon dioxide. Caution must be taken when working with hydrofluoric acid. Wear the appropriate PPE when working with hydrofluoric acid.
The HF solution used in this procedure is called buffered oxide etch or BOE. This solution consists of 6 parts of 40% NH4F to 1 part of 50% HF or 6:1 BOE. Note that a Teflon beaker must be used since BOE and HF etches glass.
When the native oxide has been removed completely, the HF solution (and DI) will shed cleanly off the wafer (i.e. the wafer becomes hydrophobic).
Procedure Day-3: Native Oxide Removal
- Don acid gloves, face shield, and acid apron.
- Pour BOE into a plastic container labeled “BOE”.
- Dip the wafers slowly in buffered oxide etch (BOE) solution for 60 seconds.
- Rinse the wafers in running DI water for 10 minutes. Use the carrier. (Check that the water sheds off the wafer – hydrophobic).
Procedure: Disposal of BOE
- Don acid gloves, face shield, and acid apron.
- Prepare a plastic waste container.
- Pour the BOE into the plastic waste container.
f.) RCA Standard Cleans: SC-1 and SC-2
This procedure consists of SC-1 (high pH) and SC-2 (low pH) cleanings. Properly dispose of all acid waste.
Procedure Day-4: Preparation of SC-1 Solution
- Heat the hot plate to 80 ºC.
- Don acid gloves, face shield, and acid apron.
- Pour 5 parts of DI into the container labeled “SC-1”.
- Pour 1 part of H2O2into the container labeled “SC-1”.
- Slowly pour 1 part of NH4OH into the container labeled “SC-1”.
- Heat the SC-1 solution on the hot plate for 5 min.
Amounts for 5:1:1 DI:H2O2:NH4OH Solution
Solution Container |
Maximum Wafers |
Total Volume (mL) |
DI (mL) |
H2O2 (mL) |
NH4OH (mL) |
Petri Dish |
1 |
70 |
50 |
10 |
10 |
Plastic Box |
25 |
2200 |
1500 |
350 |
350 |
Procedure: SC-1 Clean
- Dip the wafers in the SC-1 solution for 10 minutes.
- Rinse the wafers in running DI water for 10 minutes.
Procedure: Disposal of SC-1 Solution
- Don acid gloves, face shield, and acid apron.
- Use aspirator to remove SC-1 solution from container.
- Rinse the container and aspirator with at least 2000 mL of DI.
Procedure: Preparation of SC-2 Solution
- Heat the hot plate to 80 ºC.
- Don acid gloves, face shield, and acid apron.
- Pour 6 parts of DI into the container labeled “SC-2”.
- Pour 1 part of H2O2into the container labeled “SC-2”.
- Slowly pour 1 part of HCl into the container labeled “SC-2”.
- Heat the SC-2 solution on the hot plate for 5 min.
Amounts for 6:1:1 DI:H2O2:HCl Solution
Solution Container |
Maximum Wafers |
Total Volume (mL) |
DI (mL) |
H2O2 (mL) |
NH4OH (mL) |
Petri Dish |
1 |
80 |
60 |
10 |
10 |
Plastic Box |
25 |
2200 |
1650 |
275 |
275 |
Procedure: SC-2 Clean
- Dip the wafers in the SC-2 solution for 10 minutes.
- Rinse the wafers in running DI water for 10 minutes. Use the carrier.
- Dry the wafers with nitrogen.
Procedure: Disposal of SC-2 Solution
- Don acid gloves, face shield, and acid apron.
- Use aspirator to remove SC-2 solution from container.
- Rinse the container and aspirator with at least 2000 mL of DI.