Removal Silicon Nitride by Phosphoric Acid H3PO4.
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Silicon Nitride Removal Si₃N₄

In the semiconductor manufacturing process a silicon nitride, Si₃N₄ layer with a pattern is used as a mask to grow field oxide, SiO₂ that displays the negative pattern. The nitride pattern is then removed in a wet chemical batch process using hot phosphoric acid, H₃PO₄.

Targets of this process are:

• To remove the nitride completely;
• To leave the thin layer of gate oxide or pad oxide under it as intact as possible;
• Only remove a small and controlled amount of the field oxide;

To achieve this, it is crucial to control the etch rate, process time and selectivity as precise as possible.

The process time is easily controlled by using automatic wet stations. It is however a little more difficult to maintain a good and repeatable etch rate and sufficient selectivity. It requires quite some effort in measuring, software and adjustment.

The important parameters for the etch rate, or etching speed, are concentration and temperature of the phosphoric acid.

The selectivity is the ratio of etch rate in the material that you want to etch to the etch rate in the material that you don't want to etch. Typical for our case our case: nitride etch rate : oxide etch rate which is about 9:1 in 160°C H₃PO₄.

As higher temperature increases the etch rate but decreases the selectivity, 160°C is a good compromise.

The chemical reaction:

Si₃N₄ + 6 H₂O 3 SiO₂ + 4 NH₃↑

Interesting, that the phosphoric acid only acts as a catalyst and is not actually consumed. Can it be used forever?

Our ZEUS/J.E.T. wet benches offer a repeatable and precise nitride removal process. technology

Crystec Technology Trading GmbH will be pleased to further discuss details with you.
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