1. Advanced Manufacturing Choices ENG 165-265 Spring 2015, Class 6 Photolithography 6/9/2015

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 C-MEMS LIGA Clean- Room, Wafer Cleaning 6/9/2015 CD and T g 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. Photolithography is at the Center of the Wafer Fabrication Process * Thin Films Implant Diffusion Etch Test/Sort Polish Photo Patterned wafer 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. Negative Lithography Island silicon substrate oxide photoresist Window Areas exposed to light become polymerized and resist the develop chemical. Resulting pattern after the resist is developed. photoresist oxide silicon substrate Ultraviolet Light Exposed area of photoresist Shadow on photoresist Chrome island on glass mask Resist Tone

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

8. Ten Basic Steps of Photolithography 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 * Some processes may include a Post-exposure Bake Introduction to the Lithography Process

9. 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. HMDS

10. 1.Surface Preparation (HMDS vapor prime) 6/9/2015

11. 1.Surface Preparation (HMDS vapor prime) 6/9/2015

12. 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 vacuum chuck spindle to vacuum pump photoresist dispenser

13. 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,  the intrinsic viscosity, and  the number of rotations per minute (rpm) Once the various exponential factors ( ,  and  ) 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 6/9/2015 2.Photoresist Application

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. 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 UV Light Source Mask ResistResist

16. 4.Alignment and Exposure Alignment errors (many different types) Mask aligner equipment Double sided alignment especially important in micromachines 6/9/2015

17. 4.Alignment and Exposure 6/9/2015

18. 4.Alignment and Exposure Contact printing Proximity printing Self-aligned Projection printing : R = 2b min = 0.6  /NA 6/9/2015

19. The defocus tolerance (DOF) Much bigger issue in miniaturization science than in ICs 6/9/2015 http://www.newport.com/tutornew/optics/ Optics_Reference_Guide.html 4.Alignment and Exposure

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21. Photolithography-DOF u The defocus tolerance (DOF) u 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. 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 to vacuum pump vacuum chuck spindle developer dispenser

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. 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 CF 4

27. 8.Plasma Etch-Or Add Layer 6/9/2015

28. 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 O2O2 O2O2 Plasma

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 30 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 SU-8 Si (a) Spin-coating photoresist Carbon post SU-8 post (c) Developing (d) Pyrolysis Mask (b) UV exposure UV light

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 NH 3 - removes organics ▫RCA2-peroxide and HCl- removes metals Dry vs. wet cleaning Supercritical cleaning-no liquid phase 6/9/2015

33. Clean-rooms, Wafer Cleaning 6/9/2015

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 6/9/2015

35. CD and T g CD (e.g. 90 nm) i.e. critical dimension (the smallest feature made in a certain process) Glass transition temperature, above T g the resist picks up dirt quite readily and the profile might get degraded 6/9/2015

36. Making a Mask Software Mask 6/9/2015

37. Moore’s ‘Law’ Observation and self fulfilling prophecy --not a physical law Is it running out of steam? 6/9/2015