The purpose of the RCA clean is to remove organic contaminants (such as dust particles, grease or silica gel) from the wafer surface; then remove any oxide layer that may have built up; and finally remove any ionic or heavy metal contaminants.
The RCA clean procedure should be performed immediately prior to any crucial step, especially those involving high temperatures.
For safety reasons, all activities in the RCA clean procedure should be performed in a Clean Station Work Area which is located under an exhaust fume hood.
The RCA clean proceedure consists of the following steps:
Chemicals used in the RCA clean procedure are stored underneath the work area. It is important that acids and solvents always be stored separately.
Fresh chemical solutions should be mixed each time an RCA clean is performed.
All solutions should be made at the same time to minimize delays in processing.
First we prepare the Organic solution.
Use the deionized water gun to place 1000 ml of water in a graduated cylinder. Pour this into the quartz vat used for the Organic Solution.
To this, add 200 ml hydrogen peroxide. Then add 200 ml ammonium hydroxide. Now turn on the hot plate under the vat. Set the temperature to 350 degrees C. The solution will need to heat to 80 degrees C. This will take about 15 minutes.
Next, we will prepare the Ionic Solution.
It is important to use different containers when preparing the solutions in order to avoid cross-contamination.
Use the deionized water gun to place 1000 ml of water in the graduated cylinder. Pour this into the quartz vat used for the Ionic Solution.
To this, add 200 ml hydrogen peroxide. Then add 200 ml hydrochloric acid. Now turn on the hot plate under the vat. Set the temperature to 350 degrees C. The solution will need to heat to 80 degrees C. This will take about 15 minutes.
Finally, we will prepare the Oxide Stripping solution.
When mixing solutions, it is important to remember to ALWAYS add acid to the solution and NEVER to pour the solution into the acid! Otherwise, the acid may splatter.
In our lab, for safety reasons this solution is always prepared by the laboratory instructor.
Use the deionized water gun to place 2000 ml of water in the left side of the polyproylene vat.
Pour into this 40 ml of hydroflouric acid.
Turn on the deionized water flow valve to the bubbler rinse station. Be careful that the valve is not opened too far, as this could cause overflowing of the station.
Turn on the nitrogen flow to the rinse station. When correctly adjusted, there should be a gentle bubbling of nitrogen through the rinse station.
First the wafer are submerged in the Organic Solution for 15 minutes.
Then the wafers are placed in Bubbler Rinse for 5 minutes.
The Organic clean phase removes surface contaminents such as dust, grease and silica gel from the wafer.
Next, the wafers are submerged in the Oxide Stripping solution for 15 seconds.
Then, the wafers are immediately placed into the Bubbler Rinse, where they are agitated for 30 seconds.
The oxide clean removes any thin oxide layer which may have formed on the wafer surface.
It is important that the wafers remain in the Bubbler Rinse no more than 30 seconds as this could cause a new oxide layer to form.
The wafers are submerged in the ionic solution for 15 minutes.
Then, the wafers are placed in Bubbler Rinse for 5 minutes.
The ionic clean removes any heavy metal ions from the surface of the wafer.
Before leaving the clean station, the teflon carrier containing the wafers is placed in a black box and covered. This prevents the wafers from being re-contaminated.
The laminar flow of nitrogen under the clean station fume hood makes this area cleaner than the general lab area.
Wafers should always be handled with teflon tweezers which will not scratch them or leave deposits.
First the wafers are loaded into the dryer.
Then the door is shut, and the start button is pressed.
Following the dry, the wafers are ready for the next process step.
It is important that whatever this next step is be performed immediately to ensure that the wafers are as clean as possible.