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 |
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