Semiconductor processing involves numerous critical steps, and one of the most important is photoresist stripping. This step requires careful attention because it ensures that the temporary masking layer used during patterning is fully removed without damaging the underlying structures and without leaving any residue behind.
What is Photoresist Stripping?
Photoresist stripping is the process of removing the photoresist material from the wafer after it has served its purpose. Stripping can be performed in a single step or multiple steps, depending on the resist type and process requirements.
To define the appropriate stripping method, we first need to understand the type of photoresist used. There are two main categories:
- Positive Photoresist:
- Decomposes upon exposure to UV light, becoming more soluble in solvents like acetone or NMP (1-methyl-2-pyrrolidone).
- The exposed regions are chemically altered, allowing them to be dissolved by the developer solution.
- Commonly used for high-resolution patterning where long-term adherence, chemical resistance, mechanical strength, and thermal stability are not critical.
- Negative Photoresist:
- Undergoes polymerization or cross-linking when exposed to UV light, becoming less soluble.
- Only the unexposed regions are removed during development.
- Favored for applications requiring lower resolution but high durability in terms of adhesion, chemical resistance, mechanical strength, and thermal stability.
Regardless of the type, photoresists are used as temporary masks during structuring steps and must eventually be removed completely.
Key Factors Affecting Photoresist Removal
It’s important to recognize that photoresist removal is a chemical process—whether done through wet or dry etching. The effectiveness of stripping is influenced by several factors, including:
- Bake temperature and duration
- Photoresist film thickness and uniformity
- Substrate topography
- Device dimensions
- Time the resist remains on the wafer
- Surface preparation before photoresist application
Since the photoresist initially coats the entire wafer, any variation in film uniformity can impact stripping efficiency and completeness.
Stripping Methods
There are two main techniques used to strip photoresist:
- Dry Etch (Plasma-Based Removal):
- Uses reactive plasmas tailored to the resist’s chemical composition.
- Equipment parameters and gas mixtures are optimized to maximize removal rate and efficiency.
- However, plasma can sometimes damage device surfaces or introduce unwanted electrical charges, so careful control is essential.
- Wet Etch (Chemical Removal):
- Involves immersing or spraying the wafer with a chemical solution that dissolves the resist.
- Often used when dry etching poses risks to the device or when the process flow restricts plasma use.
One thing is certain: regardless of the technique used or the type of photoresist chosen, the complete removal of the photoresist from the wafer surface—as well as any small detached residues (‘little skins’)—is essential to avoid any impact on subsequent device fabrication steps.
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