A friend asked: What should be done when a photomask gets dirty? How often should it be cleaned, and what is the principle behind the cleaning?
Why does a photomask (reticle) get dirty?
Taking projection lithography machines and contact lithography machines as examples (see previous article: “What are Contact/Proximity/Projection/Stepper Lithography Machines?”):
In projection lithography machines, although the photomask does not come into direct contact with the wafer, photoresist and other substances can volatilize and condense on the surface of the mask, leading to contamination.
In contact lithography machines, the photomask touches the wafer, which is coated with photoresist. After each contact, a small amount of photoresist remains on the mask. As usage increases, the contamination becomes more severe.
What is the impact of a dirty photomask on the lithography process?
Contaminants on the photomask can cause diffraction or obstruction of light as it passes through the mask, leading to pattern distortion on the wafer and resulting in defects.
What solution is used to clean the photomask?
If it’s not particularly dirty, acetone and IPA can be used for cleaning.
If it is quite dirty, a strong cleaning with H2SO4 + H2O2 can be used, which is very effective at removing organic substances.
What machine is used for cleaning?
There are specialized photomask cleaning machines, and major domestic manufacturers of wet processing equipment offer them. The equipment is not particularly complex.
How often is the photomask cleaned?
Cleaning frequency varies depending on the requirements of different wafer fabs. Generally, a thorough cleaning is done once a week. Some fabs perform a light cleaning after completing the lithography process for each batch of products.
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