Piranha clean 1. Fill two rinse beakers with DI water. 2. Place magnetic stir bar in bottom of third beaker. 3. Place beaker on hot plate/stirrer. 4. 5:1 H SO : H O Piranha solution 2 4 2 2 a. Pour sulfuric acid into beaker, enough to cover the top of the wafer in Teflon dipper completely when immersed (Do not put wafer in the solution yet!). For 125 mm crystallization dish use about 100 mL H SO 2 4. o b. Heat sulfuric acid to approximately 80 C. c. Measure out H O in a graduated cylinder so that it is 1/5 the volume 2 2 of the H SO . For 125 mm crystallization dish use about 20 mL H O 2 4 2 2. d. Very slowly add the H O to the H SO . 2 2 2 4 NOTE: H O reacts with H SO exothermically. The solution will 2 2 2 4 start to bubble and heat up. e. Turn on stirrer. Stir the mixture slowly. If the H O floats on top of the 2 2 H SO , use a stirring rod to mix the two. BE CAREFUL, this will 2 4 cause the two to react more quickly. o 5. Heat solution to 100 to 110 C. The solution should begin to bubble o vigorously as it approaches 100 C. If this does not happen wait a little longer and/or add more H O . 2 2 NOTE: The solution can heat up very quickly, even when you think nothing is happening. Watch it carefully so it does not overheat. If it does overheat, turn off the hot plate or, if safe, remove the beaker from the hot plate until it cools down. o 6. Keep solution between 100 to 110 C for the entire clean. 7. Immerse wafer in H SO : H O for 10 minutes. 2 4 2 2 8. Carefully remove wafer from solution. Be careful of dripping. 9. Immerse wafer in rinse beaker with running DI water for 5 minutes. 10. Immerse wafer in second rinse beaker with running DI water for 5 minutes. 11. Air dry wafer. 12. Piranha solution can be used many times a. If the solution is still hot and bubbling, just continue using it to clean additional wafers. b. If the solution has cooled and is not more than 12 hour old, it can be refreshed by heating and adding H O until it starts to react again. 2 2 Thin Oxide Film Removal Dip in 10:1 H2O: HF until front and back of wafer dewet. Pulling the wafer from the water leaves an absolutely water free surface with only a drop or two on the lowest edge. If this test fails, reclean. RCA Clean – optional RCA-1 - Removes organic residues • 5 H2O (DI Water) • 1 NH4OH (Ammonium Hydroxide) • 1 H2O2 (Hydrogen Peroxide) 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%). 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 wafersurface when removing wafer.) RCA-2 - Removes metallic residues • 6 H2O (DI Water) • 1 HCl2 (Hydrochloric Acid) • 1 H2O2 (Hydrogen Peroxide) Put 300 ml DI water in a Pyrex beaker, carefully add 50 ml HCl and then heat to 70±5° C on hot plate. Remove from hot plate and add 50 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 10 minutes. When finished, remove the wafer and rinse with clean DI water.