Specific Process Knowledge/Wafer cleaning/7-up & Piranha

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Cleaning of wafers or masks

Cleaning of wafers or masks can be done in dedicated baths ("Wafer clean" and "Mask clean" respectively). They are both located inside "Wet bench 03: Wafer and mask cleaning" and both contain concentrated sulfuric acid to which a little ammonium persulfate is added before use. This creates very reactive peroxide species that remove organic traces very efficiently. Historically this mixture has also been referred to as "7-Up" becauses it fizzes and bubbles similar to the 7-Up beverage.

Samples not allowed or not compatible with these baths can be cleaned manually in a beaker filled with the so called "Piranha" mixture using one of the acids/bases fume hoods. This mixture consists of concentrated sulfuric acid and hydrogen peroxide forming very aggressive and reactive peroxide species (as in 7-Up) which is very effective at removing organic contaminents and to some extent some metal ions.

Always use one of these cleaning procedures after KOH etch or hot phosphoric acid etch (Nitride etch) to remove alkali ions before further processing. 7-up and Piranha are always used as cleaning solutions after stripping resist if you have resist on your samples. Cleaning samples with resist coatings or significant amounts of other organic substances can be very dangerous, because 7-Up and Piranha are so aggressive. These solutions are intended for removing TRACES of organic matter. Adding large amounts of organics can lead to explosive reactions!

'Wafer clean' bath in cleanroom D3

'Mask clean' bath in cleanroom D3

Fume hood 01 and 02 in cleanroom D3

User manuals, risk assessments and contact information can be found on the equipment Info-pages in LabManager:
Wafer Cleaning Info page in LabManager
Mask Cleaning Info page in LabManager
Fume hood 01 Info page in LabManager
Fume hood 02 Info page in LabManager

Comparing the 7-up cleans and Piranha clean

	Wafer Cleaning	Mask Cleaning	Piranha (fume hood)
General description	Cleaning of wafers using the dedicated bath in Wet bench 03.	Cleaning of masks using the dedicated bath in Wet bench 03.	Cleaning of wafers using a beaker in a fumehood in cleanroom B1 or D3. Used for glass wafers or wafers with metal or other materials that you are not allowed to put in the Wafer or Mask Cleaning bath.
Chemical solution	Sulfuric acid (98%) and ammonium persulfate	Sulfuric acid (98%) and ammonium persulfate	Sulfuric acid (98%) and hydrogen peroxide (30%) in the ratio 4:1. First add H2SO4 into a glass beaker then add H2O21.
Process temperature	80°C	80°C	~70-80°C. The chemicals will heat up to working temperature during mixing, therefore be careful!1.
Process time	10 min.	10 min.	10 min.
Allowed materials	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Quartz/fused silica	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Quartz/fused silica Pyrex Soda lime glass Wafers with Cr	All materials allowed in cleanroom (in beaker).
Batch size	1-25 4" or 6" wafers	1-25 4" or 6" wafers or 1-4 5" masks	1-5 4" wafer at a time
Size of substrate	4-6" wafers	4-6" wafers or 5" masks	All sizes that can fit into the beaker in a dedicated holder

Note 1: In preparing a solution involving an acid, always add the acid last. The exception to this rule is Piranha, in which case you add H₂O₂, which is a very strong oxidant, to H₂SO₄, which is a very strong acid. This is done because it is potentially explosive and at the very least will cause the solution to become very warm.