1. C05 - 1 Virginia Tech Effect of Resist Thickness Resists usually do not have uniform thickness on the wafer –Edge bead: The build-up of resist along the circumference of the wafer - There are edge bead removal systems –Step coverage Centrifugal Force

2. C05 - 2 Virginia Tech Effect of Resist Thickness The resist can be underexposed where it is thicker and overexposed where it is thinner –This can lead to linewidth variations Light intensity varies with depth below the surface due to absorption where  is the optical absorption coefficient Thus, the resist near the surface is exposed first –We have good fortune. There is a process called bleaching in which the exposed material becomes almost transparent i.e.,  decreases after exposure to light - Therefore, more light goes to deeper layers

3. C05 - 3 Virginia Tech C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. 27(23), Dec. 1, 1988, pp. 4913-4919.  exposed = B and  unexposed = A+B

4. C05 - 4 Virginia Tech

5. C05 - 5 Virginia Tech Photoresist Absorption If the photoresist becomes transparent, and if the underlying surface is reflective, reflected light from the wafer will expose the photoresist in areas we do not want it to. However, this leads to the possibility of standing waves (due to interference), with resultant waviness of the developed resist We can solve this by putting an antireflective coating on the surface before spinning the photoresist  increases process complexity

6. C05 - 6 Virginia Tech Standing Waves due to Reflections

7. C05 - 7 Virginia Tech Standing Waves Due to Reflections http://www.lithoguru.com/scientist/lithobasics.html

8. C05 - 8 Virginia Tech (a) (b) (c) Diffusion during a post-exposure bake (PEB) is often used to reduce standing waves. Photoresist profile simulations as a function of the PEB diffusion length: (a) 20nm, (b) 40nm, and (c) 60nm. http://www.lithoguru.com/scientist/lithobasics.html Removal of Standing Wave Pattern

9. C05 - 9 Virginia Tech Mask Engineering There are two ways to improve the quality of the image transferred to the photoresist –Optical Proximity Correction (OPC) –Phase Shift Masks (PSM) We note that the lenses in projections systems are both finite and circular Most features on the mask are square We lose the high frequency components of the pattern We thus lose information about the “squareness” of the corners

10. C05 - 10 Virginia Tech Mask Engineering The effects are quite predictable We can correct them by adjusting feature dimensions and shapes in the masks

11. C05 - 11 Virginia Tech Mask Engineering

12. C05 - 12 Virginia Tech Phase Shift Masks In a projection system, the amplitudes of the diffracted light at the wafer add –Closely spaced lines interact; the intensity at the wafer is smeared If we put a material of proper index of refraction on part of the mask, we can retard some of the light and change its phase by 180 degrees –Properly done, the amplitudes interfere The thickness of the PS layer is n is the index of refraction of the phase shift material

13. C05 - 13 Virginia Tech Phase Shift Masks (PSM) Intensity pattern is barely sufficient to resolve the two patterns.