1. 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. 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. aexposed = B and aunexposed = A+B
C. A. Mack, “Absorption and exposure in positive photoresist”, Appl. Opt. 27(23), Dec. 1, 1988, pp

5. 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. Standing Waves due to Reflections

7. Standing Waves Due to Reflections

8. Removal of Standing Wave Pattern
                                                                                                                       
(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.

9. 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. Mask Engineering The effects are quite predictable
We can correct them by adjusting feature dimensions and shapes in the masks

11. Mask Engineering

12. 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. Phase Shift Masks (PSM)
Intensity pattern is barely sufficient to resolve the two patterns.

14. Scanning Projection Aligners
The wafer is simultaneously scanned, thus printing the mask on the wafer
Systems of the type shown on the next page are cost effective, but they must use 1:1 masks
The concept is that it is easier to correct for aberrations in small regions than a large area
Integration of a focusing laser and vertical positioner allowed adjustment of imaging plane to maximize resolution
Technology became obsolete as wafer size increased and linewidths became smaller

15. Scanning Projection Printer

16. Step and Repeat Projection Systems
Steppers expose a limited portion of the wafer at a time
Features on the masks (reticules) are 4-5X the size of the features exposed on the wafer
Steppers also allow better alignment because they align on the exposure field rather than for the entire wafer
Integration of a focusing laser and vertical positioner
Laser is also used to read information that is scribed on wafer prior to processing

17. Projections Systems Key features of steppers include
Kohler illumination
Off-axis illumination
Kohler illumination focuses the light at the entrance pupil of a projection lens, rather than on the photoresist
This setup allows the projection lens to capture the diffracted light from any features on the mask

18. Kohler Illumination

19. Off-Axis Illumination
By changing the angle of incidence of the light on the mask, we also change the angle of the diffracted light
Although some of the diffracted light is lost in this scheme, much of the higher order diffraction is captured

20. Off-Axis Illumination

21. Step and Scan Aligners

22. Step and Scan

24. DNQ/Novolac Resist Process
Hexamethyldisilane (HMDS) is often used as an adhesion promoter
As a liquid, drops are deposited on the wafer and then spread by spinning at 3000 – 6000 rpm for 30 s
Sometime HMDS is applied from the vapor
The surface chemistry is that the silane end of the molecule bonds with the Si while the other end bonds with the resist
Resist is then spun on
immediately following HMDS

25. DNQ/Novolac Resist Process
Prebake is usually done on a hot plate at 90—100oC
Infrared or microwave heating can also be used
This step:
Evaporates the last of the solvent
Solvent content in the photoresist film decreases from 25% to 5%
Adhesion is improved because heat strengthens the bonds between the resist and HMDS
Stresses in the resist caused by spinning are thermally relieved
The resist flows slightly

26. DNQ/Novolac Resist Process
Exposure times and source intensity are reciprocal—one can reduce exposure times with more intense sources
Exposure time is increase by increasing the bake temperature (due to decomposition of the PAC and thus decreased sensitivity)
The postexposure bake is often done before development because the PAC can diffuse and this will eliminate the standing wave pattern
The developer is a basic solution such as TMAH, NaOH, or KOH and is applied by immersion, or spraying

27. DNQ/Novolac Resist Process
Rinsing in H2O stops the development process
The rate for developing depends strongly on temperature, developer concentration, and the exposure and bake procedures
The chemistry is the dissolution of the carbolic acid
The final step is postbake (typically 10—30 min at 100—140 C)
This hardens the resist and improves etch resistance
The resist flows a bit during the process, and all remaining solvents are driven off
Many of the steps described above are done in a single system called a wafer track system

28. DNQ/Novolac Resist Process
A SITE system ESVG 86 track and coat system

29. Measurement Methods Measurements are made to determine
Mask Features and defects
Resist Patterns
Etched Features
Alignment Accuracy (x-y-theta)

30. Measurement of Mask Features and Defects
Because almost all production now uses reducing steppers, a defect in a mask will produce a defect in every die
Therefore the mask must be “perfect”
Because of the complexity of the masks, the inspection must be fully automated
manual observation under a microscope is not possible
The process is illustrated in the next slide

31. Mask Inspection System

32. Measurement of Mask Features and Defects
Light is passed through the mask and collected by an image recognition system
Solid state detectors are used to collect the light
The information is compared against the database of the mask design or with an identical mask
The inspection process is more difficult if the mask contains OPC (optical proximity correction) or is a PSM (phase shift mask)
Often, defects found in this process can be corrected
Lasers can burn off excess Cr
Adding Cr to clear areas is harder
Done using chemical vapor deposition

33. SEM Measurement

34. State-of-the-Art Capable of exposing down to ~ 10nm E-beam lithography
X-ray lithography
Extreme UV lithography
E-beam and EUV are performed under vacuum
Throughput is very slow
New resist families are required
Most are very difficult to remove after use
Research needed on mask material for x-ray and EUV
Glass absorbs
Thickness of metal needed to block x-rays is very thick (20-50mm)