Lithography-Part 1 Dr. Marc Madou, Winter 2011 UCI Class 2

1. Lithography-Part 1 Dr. Marc Madou, Winter 2011 UCI Class 2

2. 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 Content

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.

4. Photolithography is at the Center of the Wafer Fabrication Process

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.

6. Negative Lithography

7. Positive Lithography

8. Resist Tone

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

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

11. This following slides addresses Engineering Technology Content Standard 3 and 4 on Manufacturing and Materials.This following slides addresses Engineering Technology Content Standard 3 and 4 on Manufacturing and Materials.

12. 1. Surface Preparation (HMDS vapor prime) Dehydration bake in enclosed chamber with exhaust Clean and dry wafer surface (hydrophobic) Hexamethyldisilazane (HMDS) Temp ~ 200 - 250�C Time ~ 60 sec.

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 ~ 3000 - 5000 rpm Quality measures: time speed thickness uniformity particles & defects

16. 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

20. 4. Alignment and Exposure Transfers the mask image to the resist-coated wafer Activates photo-sensitive components of photoresist Quality measures: linewidth resolution overlay accuracy particles & defects

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

22. 4. Alignment and Exposure

23. 4. Alignment and Exposure

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

25. 4. Alignment and Exposure

26.  The defocus tolerance (DOF) Much bigger issue in miniaturization science than in ICs

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

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

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

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

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

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?