Advanced Manufacturing Choices

1. Advanced Manufacturing Choices ENG 165-265 Spring 2017, Class 6 Photolithography

2. Content • CD and Tg • Making a Mask • Moore’s ‘Law’ • 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 Stripping • Final Inspection • C-MEMS • LIGA • Clean- Room, Wafer Cleaning

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 -- Definitions 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. Ultraviolet Light Chrome island on glass mask Exposed area of photoresist Window photoresist Shadow on photoresist photoresist oxide oxide silicon substrate silicon substrate Resist Tone Negative Lithography 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. 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

9. HMDS 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.

10. 1. Surface Preparation (HMDS vapor prime)

11. 1. Surface Preparation (HMDS vapor prime)

12. photoresist dispenser vacuum chuck to vacuum pump spindle 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

13. 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

14. 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

15. UV Light Source Mask l Resist 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

16. 4. Alignment and Exposure • Alignment errors (many different types) • Mask aligner equipment (3 major types-see next slide) • Double sided alignment especially important in micromachines

17. 4. Alignment and Exposure

18. 4. Alignment and Exposure • Contact printing • Proximity printing • Self-aligned • Projection printing : R = 2bmin = 0.6/NA =k1λ/NA

19. 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

20. 4. Alignment and Exposure

21. Photolithography-DOF • The defocus tolerance (DOF) • Much bigger issue in miniaturization science than in ICs A small aperture was used to ensure the foreground stones were as sharp as the ones in the distance. What you need here is a use a telephoto lens at its widest aperture.

22. Photolithography-DOF

23. developer dispenser vacuum chuck to vacuum pump spindle 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

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

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

26. CF4 Plasma 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

27. 8. Plasma Etch-Or Add Layer

28. O2 Plasma 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

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

30. Two variation on the lithography process: 1.C-MEMS Process, 2. LIGA (a) Spin-coating photoresist SU-8 Si (b) UV exposure (c) Developing (d) Pyrolysis Carbon post SU-8 post UV light Mask • Park B, Taherabadi L, Wang C, Zoval J, Madou M. Electrical properties and shrinkage of carbonized photoresist films and the implications for carbon microelectromechanical systems devices in conductive media. Journal of the Electrochemical Society 2005;152(J136). • Ranganathan S, McCreery R, Majji S, Madou M. Photoresist-derived carbon for microelectromechanical systems and electrochemical applications. Journal of the Electrochemical Society 2000;147(1):277-82. • Wang C, Jia G, Taherabadi L, Madou M. A novel method for the fabrication of high-aspectratio C-MEMS structures. Journal of Microelectromechanical Systems 2005;14(2):348-58

31. Two variation on the lithography process: 1.C-MEMS Process, 2. LIGA

32. 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

33. Clean-rooms, Wafer Cleaning

34. 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

35. 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

36. Making a Mask • Software Mask

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