Specific Process Knowledge/Wafer cleaning

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Cleaning of wafers

During processing it is sometimes mandatory, necessary or just recommended to clean the wafers or the photo lithographic masks. This can be done by different cleaning procedures depending on what the wafers have been exposed to, and where they are going to be further processed. In the comparison table below is listed different cleaning procedures available at nanolab.

Wafers that go into the high temperature furnaces

As a general rule all wafers that go into the high temperature furnaces need a full RCA clean directly before the furnace step.

Wafers that have been in KOH baths (Si Etch 1, 2 and 3), KOH BHF baths or 'Nitride etch: H3PO4' (warm phosphoric acid)

Wafers that have been etched in KOH are often etched in warm phosphoric acid afterwards to strip the silicon nitride etch mask. Therefore, wafers that have been processed in KOH baths, KOH BHF baths or phosphoric acid nitride etch baths often need at minimum a 7-up or Piranha clean before they can be further processed (in order to remove KOH etch residuals and metal ions).

Items that have been outside the cleanroom

Before bringing samples into the cleanroom you have to get approval from Nanolab. Send an e-mail to nanolabsupport@nanolab.dtu.dk.

When you have an approval the items that have been outside the cleanroom should always be cleaned in soap and ultra sound followed by a 7-up or Piranha clean before entering the cleanroom.

Masks

Masks that have become dirty by dust particles or resist residues have to be cleaned in the 7-up (mask) bath before use.

Before entering specific process equipment

Sometimes the wafers need a certain cleaning process before they can enter specific process equipments due to the risk of cross contamination. Please consult the cross contamination scheme to see whether your wafers need a cleaning before the next processing sequence.

Comparison of Wafer Cleaning Methods

	RCA	Wafer/Mask Clean baths & Piranha	5% HF	IMEC	Soap Sonic	Post CMP Cleaner
Generel description	Two step process to remove traces of organics and metals	Removes traces of organics and alkali ions	Removing native oxide	Removing dust, traces of organics and alkali ions and slightly polish the surface. Make the surface hydrophillic	Removing dust and particles	Removing slurry residues from the CMP
Purpose	Mandatory prior furnace processes	When needed and always after KOH etch and Nitride etch in Phosphoric acid. Sometimes mandatory before next processing step.	Optional during RCA cleaning	Recommended cleaning and treatment before wafer bonding	Mandatory cleaning of very dirty items that enters the cleanroom. Should always be followed by a piranha clean.	Recommended cleaning after CMP
Substrate size	#1-25 2", 4" and 6" wafers	#1-25 2", 4" and 6" wafers	#1-25 2", 4" and 6" wafers	#1-25 2" and 4" wafers	All sizes that can go into the bath	#1 20x20mm sample as well as 2", 4" and 6inch wafers
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, other glass and Silicon wafers with Cr ONLY allowed in Mask Cleaning bath or beaker in fumehood	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxy-nitride Quartz/fused silica Resists (depending on bath) Pyrex (depending on bath)	Silicon Poly Silicon Silicon Oxide Silicon Nitride Silicon Oxynitride Quartz/fused silica Pyrex	All materials	Silicon Poly Silicon Silicon Oxide Quartz/fused silica