Photoresist in Semiconductor Manufacturing
Photoresist, a crucial component in semiconductor manufacturing, is a radiation-sensitive compound that can be classified into two types: positive and negative. The classification is based on how they respond to radiation during the manufacturing process.
- Behavior: In positive photoresists, the exposed regions become more soluble, making them easier to remove during the development process.
- Resulting Patterns: The patterns formed in positive photoresists mirror those on the mask used in the fabrication process.
- Composition: Positive photoresists consist of three key components—photosensitive compounds, a base resin, and an organic solvent.
- Process: Before exposure, the photosensitive compound is insoluble in the developer solution. Upon exposure, radiation is absorbed in the exposed areas, inducing a change in the chemical structure of the photosensitive compound. This change makes it soluble in the developer solution. After development, the exposed areas are removed, leaving behind the desired pattern.
- Behavior: Exposed regions in negative photoresists become less soluble, resulting in patterns that are the reverse of those on the mask.
- Composition: Negative photoresists are composed of polymers combined with a photosensitive compound.
- Process: Upon exposure, the photosensitive compound absorbs optical energy and undergoes a chemical reaction, initiating a polymer crosslinking reaction. This crosslinking causes the polymer molecules to become insoluble in the developer solution. After development, the unexposed areas are removed, leaving behind the inverse of the mask patterns.
- Drawback: A major drawback of negative photoresists is the swelling of the resist mass during development due to the absorption of the developer solvent. This swelling limits the resolution of negative photoresists, impacting their ability to reproduce fine details.
Understanding the characteristics and behaviors of positive and negative photoresists is crucial in the precise formation of semiconductor patterns during the photolithographic process, a foundational step in semiconductor manufacturing. The choice between positive and negative photoresists depends on the specific requirements of the semiconductor fabrication process and the desired outcome for the integrated circuits.
Steps in Transferring IC Patterns from Mask to Silicon Wafer
Figure 1 outlines the sequential steps involved in transferring integrated circuit (IC) patterns from a mask to a silicon wafer, specifically one with a SiO2 insulating layer on its surface.
- Cleanroom Preparation: The process begins in a cleanroom illuminated with yellow light, as photoresists are insensitive to wavelengths greater than 0.5 µm. This controlled environment is crucial for maintaining the purity of the semiconductor fabrication process.
- Adhesion Promoter Application: To ensure proper adhesion of the resistor, an adhesion promoter, often hexamethylene–disiloxane (HMDS) for silicon ICs, is applied to the wafer’s surface.
- Resist Coating: Liquid resist is applied to the wafer’s center, and the wafer is rapidly rotated to spread the resist uniformly. The spin speed typically ranges from 1000 to 10,000 rpm, resulting in a uniform film approximately 0.5–1 µm thick. The thickness is directly related to the resist’s viscosity.
- Soft-Baking: The wafer undergoes a “soft-bake” process at temperatures between 90 to 120°C for 60–120 seconds. This step removes solvent from the photoresist and enhances adhesion to the wafer.
- Mask Alignment and Exposure: The wafer is aligned with the mask in an optical lithographic system, and the resist is exposed to ultraviolet light.
- Positive Photoresist Development: If a positive photoresist is used, the exposed resist is dissolved in the developer solution. Photoresist development involves flooding the wafer, followed by rinsing and drying.
- Postbaking: Postbaking at approximately 100–180°C may be necessary to further enhance resist adhesion to the substrate.
- Insulation Layer Etching: The wafer is placed in an ambient that selectively etches the exposed insulation layer while preserving the resist.
- Resist Stripping: The resist is stripped using solvents or plasma oxidation, leaving behind an insulator image mirroring the opaque image on the mask.
- Application in Subsequent Processing: The resulting insulator image can serve as a mask for subsequent processing steps, such as ion implantation, allowing precise doping of the exposed regions for the creation of semiconductor devices.