Sample interview questions: Can you discuss your knowledge of the principles behind optical lithography?
Sample answer:
Optical Lithography: Principles and Applications
Optical lithography is a fundamental process in modern semiconductor fabrication, used to define circuit patterns on a silicon wafer. It involves directing ultraviolet (UV) light through a patterned mask to selectively expose and modify a photosensitive resist layer on the wafer. The exposed resist regions are then selectively removed, etching the underlying silicon to create the desired circuit features.
Key Principles:
- Resolution: The minimum feature size that can be defined using optical lithography is determined by the wavelength of the UV light source and the numerical aperture (NA) of the optical system. Shorter wavelengths and higher NAs yield higher resolution.
- Depth of Field (DOF): The DOF refers to the vertical range over which the projected image remains sharp. It is inversely proportional to the NA and the wavelength of the light source.
- Exposure Dose: The amount of UV exposure required to properly expose the resist depends on the resist sensitivity and the optical properties of the wafer and mask.
- Resist Properties: The photosensitive resist used in optical lithography exhibits a change in solubility upon exposure to UV light. Exposed regions become more soluble, enabling their selective removal during development.
Challenges and Advancements:
- Diffraction Effects: Diffraction of UV light around the edges of features limits resolution and pattern fidelity. Techniques such as phase-shifting masks and edge correction are employed … Read full answer