II-Lithography Fall 2013 Prof. Marc Madou MSTB 120

1. II-LithographyFall 2013Prof. Marc MadouMSTB 120

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 • 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.

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

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

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

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. 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 * Some processes may include a Post-exposure Bake

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

13. 1. Surface Preparation (HMDS vapor prime)

14. 1. Surface Preparation (HMDS vapor prime)

15. 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 photoresist dispenser vacuum chuck to vacuum pump spindle 2. Photoresist Application

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

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

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 http://www.newport.com/tutornew/optics/ Optics_Reference_Guide.html

27. 4. Alignment and Exposure

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

29. 6. Hard Bake • Evaporate remaining photoresist • Improve adhesion • Higher temperature than soft bake

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

31. 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 CF4 Plasma 8. Plasma Etch-Or Add Layer

32. 8. Plasma Etch-Or Add Layer

33. 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 9. Photoresist Removal (strip)

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

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?