1. II-Lithography Fall 2013 Prof. Marc Madou MSTB 120

2. Content Lithography definitions Resist tone
Introduction to the lithography process
Surface Preparation
Photoresist Application
Soft Bake
Align & Expose
Develop
Hard Bake
Inspection
Etch Layer or Add Layer
Resist Strip
Final Inspection
Clean- Room, Wafer Cleaning
CD and Tg
Making a Mask
Moore’s ‘Law’

3. Photolithography -- Definitions
Photolithography is used to produce 2 1/2-D images using light sensitive photoresist and controlled exposure to light.
Microlithography is the technique used to print ultra-miniature patterns -- used primarily in the semiconductor industry. 3 3 3 3 3 3 4 4 4 3 3

4. Photolithography is at the Center of the Wafer Fabrication Process
Photolithography -- Definitions *
Thin Films
Implant
Diffusion
Etch
Test/Sort
Polish
Photo
Patterned wafer
Photolithography is at the Center of the Wafer Fabrication Process 4 4 4 4 4 4 5 5 5 4 4

5. Resist Tone
Negative: Prints a pattern that is opposite of the pattern that is on the mask.
Positive: Prints a pattern that is the same as the pattern on the mask. 10 10 10 10 10 10 11 11 11 10 10

6. Resist Tone Negative Lithography
photoresist
oxide
silicon substrate
Ultraviolet Light
Exposed area of photoresist
Shadow on photoresist
Chrome island on glass mask
Areas exposed to light become polymerized and resist the develop chemical.
Island
silicon substrate
oxide
photoresist
Window
Resulting pattern after the resist is developed.
Negative Lithography 11 11 11 11 12 12 12 11 11

7. Resist Tone Positive Lithography
photoresist
silicon substrate
oxide
Ultraviolet Light
Areas exposed to light become photosoluble.
Chrome island on glass mask
Island
Shadow on photoresist
Window
photoresist
Exposed area of photoresist
oxide
silicon substrate
Resulting pattern after the resist is developed.
Positive Lithography 12 12 11 12 12 12 13 13 13 12 12

8. Resist Tone

9. Resist Tone
Photoresist profiles
Overcut (LIFT-OFF)
Vertical
Undercut

10. Resist Tone Photoresist profiles Overcut (LIFT-OFF) Vertical Undercut
Dose : High
Developer: Low
Dose : Medium
Developer: Moderate
Dose : Low
Developer: Dominant

11. Ten Basic Steps of Photolithography
Introduction to the Lithography Process
1. Surface Preparation
2. Photoresist Application
3. Soft Bake
4. Align & Expose*
5. Develop
6. Hard Bake
7. Inspection
8. Etch
9. Resist Strip
10. Final Inspection
Ten Basic Steps of Photolithography
This following slides addresses Engineering Technology Content Standard 3 and 4 on Manufacturing and Materials.
* Some processes may include a Post-exposure Bake 13 13 14 14 14 13

12. 1. Surface Preparation (HMDS vapor prime)
Dehydration bake in enclosed chamber with exhaust
Clean and dry wafer surface (hydrophobic)
Hexamethyldisilazane (HMDS)
Temp ~ °C
Time ~ 60 sec. 14 13 14 14 15 15 15 14

13. 1. Surface Preparation (HMDS vapor prime)

14. 1. Surface Preparation (HMDS vapor prime)

15. 2. Photoresist Application
Wafer held onto vacuum chuck
Dispense ~5ml of photoresist
Slow spin ~ 500 rpm
Ramp up to ~ rpm
Quality measures:
time
speed
thickness
uniformity
particles & defects
vacuum chuck
spindle
to vacuum pump
photoresist dispenser 15 14 15 15 16 16 16 15

16. 2. Photoresist Application
Resist spinning thickness T depends on:
Spin speed
Solution concentration
Molecular weight (measured by intrinsic viscosity)
In the equation for T, K is a calibration constant, C the polymer concentration in grams per 100 ml solution, h the intrinsic viscosity, and w the number of rotations per minute (rpm)
Once the various exponential factors (a,b and g) have been determined the equation can be used to predict the thickness of the film that can be spun for various molecular weights and solution concentrations of a given polymer and solvent system

17. 2. Photoresist Application

18. 2. Photoresist Application

19. 3. Soft Bake Partial evaporation of photo-resist solvents
Improves adhesion
Improves uniformity
Improves etch resistance
Improves linewidth control
Optimizes light absorbance characteristics of photoresist 16 15 16 16 17 17 17 16

20. 4. Alignment and Exposure
UV Light Source
Mask
Resist l
Transfers the mask image to the resist-coated wafer
Activates photo-sensitive components of photoresist
Quality measures:
linewidth resolution
overlay accuracy
particles & defects 17 16 17 17 18 18 18 17

21. 4. Alignment and Exposure
Alignment errors (many different types)
Mask aligner equipment
Double sided alignment especially important in micromachines

22. 4. Alignment and Exposure

23. 4. Alignment and Exposure

24. 4. Alignment and Exposure
Contact printing
Proximity printing
Self-aligned (see next)
Projection printing : R = 2bmin = 0.6/NA ~ ~

25. 4. Alignment and Exposure

26. 4. Alignment and Exposure
The defocus tolerance (DOF)
Much bigger issue in miniaturization science than in ICs
Optics_Reference_Guide.html

27. 4. Alignment and Exposure

28. 5. Develop
Soluble areas of photoresist are dissolved by developer chemical
Visible patterns appear on wafer
windows
islands
Quality measures:
line resolution
uniformity
particles & defects
vacuum chuck
spindle
developer
dispenser
to vacuum pump 18 18 18 17 19 19 19 18

29. 6. Hard Bake Evaporate remaining photoresist Improve adhesion
Higher temperature than soft bake 19 18 19 19 20 20 20 19

30. 7. Development Inspection
Optical or SEM metrology
Quality issues:
particles
defects
critical dimensions
linewidth resolution
overlay accuracy 20 19 20 20 21 21 21 20

31. 8. Plasma Etch-Or Add Layer
Selective removal of upper layer of wafer through windows in photoresist: subtractive
Two basic methods:
wet acid etch
dry plasma etch
Quality measures:
defects and particles
step height
selectivity
critical dimensions
Adding materials (additive)
Two main techniques:
Sputtering
evaporation
Plasma
CF4 21 20 21 21 22 22 22 21

32. 8. Plasma Etch-Or Add Layer

33. 9. Photoresist Removal (strip)
No need for photoresist following etch process
Two common methods:
wet acid strip
dry plasma strip
Followed by wet clean to remove remaining resist and strip byproducts O2
Plasma 22 21 22 22 23 23 23 22

34. 10. Final Inspection Photoresist has been completely removed
Pattern on wafer matches mask pattern (positive resist)
Quality issues:
defects
particles
step height
critical dimensions 23 22 23 23 24 24 24 23

35. Clean-rooms, Wafer Cleaning
Yellow light and low particle size/density curves
Cleaning steps
RCA1-peroxides and NH3-removes organics
RCA2-peroxide and HCl-removes metals
Dry vs. wet cleaning
Supercritical cleaning-no liquid phase

36. Clean-rooms, Wafer Cleaning

37. Clean-rooms, Wafer Cleaning

38. Clean-rooms, Wafer Cleaning

39. Clean-rooms, Wafer cleaning
Yield is the reason for the clean-rooms-the smaller the features the more important the cleanroom
In the future people will work outside the cleanroom and only wafers will be inside the clean environment
At universities, modularity (many different materials and processes) is more important than yield

40. CD and Tg
CD (e.g. 90 nm) i.e. critical dimension (the smallest feature made in a certain process)
Glass transition temperature, above Tg the resist picks up dirt quite readily and the profile might get degraded

41. Making a Mask
Software Mask

42. Moore’s ‘Law’
Observation and self fulfilling prophecy --not a physical law
Is it running out of steam?