A Guide to Understanding Resist Materials for Lithography Resist Adhesion

Lithography resists are most commonly used in the semiconductor industry and are crucial in advancing chip technology. For that reason, it is very important to understand the basics of litho resists.

There are many types of materials available for Lithography resist adhesion. However, each type of resist is characterized and classified by the following:

• Resist material availability: A resist is often available in either a liquid or dry film form.

• Resist thickness per layer/coating: Resist thickness per coating can be as thin as 5nm for a EUV litho process to 70-80nm thick for the current immersion lithography processes.

• Resist sensitivity/operating wavelength: Resist material has a certain sensitivity to a specific wavelength of light. The most common type of litho resists available today is “dry” i-line and KrF resists.

• Resist patterning: A resist can be used to transfer a pattern from an etch resist, or it can be used as an etch mask itself (i.e., positive tone lithography resists).

• Resist pattern transfer: A resist can be a positive or negative tone material (if used as an etch mask), or it can be used to “transfer” the pattern onto the substrate (i.e., act as an etch resist).

• Cohesion strength of resist film: The cohesive strength of a resist film is directly related to the dried or cured state of a resist. Good lithography resist will have strong adhesion to the substrate and low outgassing rates.

• Resist pattern fidelity: Multi-level features need very high fidelity resists for them to be etched properly. For example, currently, state-of-the-art immersion lithography resists pattern features as small as 45nm half-pitch.

• Formulation of resist: Resist chemistries, formulations and materials can be a positive or negative tone (material used for the etch process), or it can be a direct pattern transfer material (i.e., the dry film resists).

What are the main materials used?

HSQ hydrogen silsesquioxane resist is one of the most used resists today. It is a negative tone resist, which has good adhesion to both silicon dioxide and nitride substrates. It is chemically inert and provides high line edge roughness (LER) after post-exposure bake (PEB).

Diphenyl sulfone-based resist is another commonly used lithography resist adhesion material. It has slightly worse adhesion vs. HSQ hydrogen silsesquioxane resist, but it can be developed using a different solvent mixture and developer combination (i.e., KOH).

These are the most commonly used Lithography resist adhesion materials available on the market today. As technology advances and chips become smaller and thinner, it is important to understand that lithography resists will continue to be a crucial step in advancing chip fabrication.