Describe in detail how to structure a chromium micro-pattern on a silicon wafer using photolithography and evaporation techniques.(5 Punkte/Points)
1. Cleaning: Remove organic\inorganic contamination 2. Preparation: Heat wafer to remove moisture & apply adhesion promoter typically HMDS 3. Photoresist application: Spin coat wafer with liquid resist. 4. Pre-bake: Bake coated wafer to remove the solvent in the resist.5. Exposure: Photoresist is exposed to a intense light patterned by the mask 6. Post-exposure bake: Reduce mechanical stress caused by pre-bake and exposure 7. Development: Remove soluble resist. Typically developer bath or spray 8. Hard bake: Solidifies the remaining photoresist, make more durable 9. Additive or subtractive step: Etch, electroplate, coat, etc... 10. Removal of photoresist: Resist stripper (chemical bath) & etch resist (plasma)
Explain and sketch how to avoid misalignment of consecutive photolithography steps. 2P
Skizze S.2
Give three plausible reasons for patterning errors in lithography. (1 Punkt/Point)
● mechanical contact between mask and resist may cause debris ● damages mask will be reproduced in subsequent exposures ● gap between mask and substrate will cause image of the mask edge on the substrate surface to be blurred and diffuse (diffraction)
Explain and sketch how the gray levels are being achieved in the example given in the lecture using the grey-scale lithography technique.(2 Punkte/Points)
● By modulating the intensity of light through a gray-scale optical mask, a positive photoresist is partially exposed to different depths ● The intensity is modulated by the ratio between opaque and transparent area on the mask, while using a projection lithography system ● Sub-resolution pixels block a fraction of the incident light and cause diffraction as the light passes between themSkizze: S.3
Explain what the "contrast" of a photoresist is. Explain the choice of photoresists for the gray-scale lithography using a projection lithography system. 2P
Contrast is a measure of how sensitive the photoresist is to changes in the exposure dose. The contrast of a photoresist is defined as the slope of the development rate vs. log(exposure dose) curve. Low contrast photoresists are preferred because they yield more available ray levels (wider operating window) Thicker photoresists are preferred because the gray levels are more pronounced and a larger margin of error is allowed to the process
There is one important material property in the EUV wavelength range which is influencing the design of optics for EUV lithography systems. What is this property? Explain how the optics of the EUV lithography systems is addressing this issue. 2p
Property: Absorption - very strong in all materials, even gases Solution: ● no refractive optics can be used ● EUV imaging systems are reflective systems 3 10.8.2021 ● however, EUV reflectivity of individual materials is very low ● distributed Bragg reflectors consisting if multi-layer thin coatings to achieve high reflectivities