Solvents are effective at removing organic contamination from sources such as finger oils, vacuum greases, etc. Heated solvents are not required to remove these types of hydrocarbon oils based on XPS studies of surface carbon contamination. The following general procedure will remove finger grease contamination.
- Acetone in ultrasonic for > 2 minutes. (ultrasonic optional, will facilitate particle removal)
- Methanol in ultrasonic for > 2 minutes. (ultrasonic optional, will facilitate particle removal)
- Running DI water for 0.5 minute.
- Blow dry with N2 gun.
Oxygen atoms/ions/radicals can be used to “burn” off organic residues left on wafer surfaces due to resist cross-linking or polymerization. We have UV-ozone treatment, oxygen plasma etching, and oxygen reactive ion etching here at UCSB.
- UV-Ozone: This is the most gentle technique. Place samples in the system and turn on for 1-30 minutes. Many have seen very good ohmic contacts using this technique just before metal lift-off.
- Plasma etching: In this technique, oxygen radicals, atoms and ions are excited in a plasma using a capacitively coupled 30 kHz RF system. There may be some damage of the surface due to ion bombardment. This will depend on power, time, and material being exposed to the plasma.
PEII etcher at 300 mT O2, 100 W for 15-30 seconds to remove residual organics. Note that this will leave a native oxide on the surface that may need to be removed. For stubborn organic films, higher powers up to 300 W and times up to 10 minutes or more may be required. Nomarski-based microscopy is most effective for seeing residual material. Remember that higher powers will also lead to thicker oxides on the surface and may result in ion damage to the surface.
- Oxygen RIE: In this technique, oxygen radicals, atoms and ions are excited in a plasma using a capacitively coupled 13.56 MHz RF system. The samples are placed on the cathode of the system and a DC bias develops between the plasma and cathode. This is the most aggressive of the three techniques since the energetic oxygen ions are bombarding the surface. You should reserve this only for the most stubborn films, such as fluorinated halocarbon-like compounds or polymers.
RIE1, RIE3, or RIE5 may be used for this technique. Use 100-400 V of bias at pressures of 10-50 mT for several minutes.
Resist Strippers / Concentrated Developers:
Resist strippers have been designed to remove photoresist compounds but can also be effective at removing some organic contamination such as polymers. We have a variety of strippers in the laboratory and you should look at the resist materials page and resist removal page in the lithography section of this web site’s process page. Be sure to check for material compatibility of your samples with the strippers.
Strong acid/base mixtures are also effective at organic removal. Before using etchants make sure that your substrate or other materials on the wafer will not be attacked by the etchant.
- RCA-1 Clean: NH4OH:H2O2:DI 1:1:5 at 80°C for > 5 minutes. Finish with a DI rinse. (This is the first step in the RCA clean process designed in the early 1970s.)
- Pirhana etch: H2SO4:H2O2 from 3:1 to 4:1 ratios at 120°C for 10 minutes. Finish with a DI rinse.
Reference: See Chapter 2 of “ULSI Technology” by C.Y. Chang and S.M. Sze for further information.