ME 598 – Lecture 2 - Photolithography

1. Thin Films Polish Patterned wafer Etch Photo Diffusion Test/Sort Implant * ME 598 – Lecture 2 - Photolithography

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

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

4. Photoresist Side Profiles Overcut (aka lift-off): Used for material deposition after photolithography step • Vertical (aka normal): • Used for material removed after photolithography step • Undercut: • Typical result when trying to achieve highest resolution

5. Development Profiles PR PR Substrate Substrate Normal Development Incomplete Development PR PR Substrate Substrate Under Development Over Development

6. Resolution • Critical Dimension (CD) refers to a specific “minimal” feature size and is a measure of the resolution of the lithographic process • Theoretical limitations can be calculated based on experimental measures (wavelength, gap size, PR thickness…)

7. AZ 1518 Exposure Dose 1 μm trench 2 μm mask lines 270 mJ/cm2 250 mJ/cm2 310 mJ/cm2 290 mJ/cm2

8. UV light  Mask 3 Resist thickness z s b Z 2 2 Wafer (S + ) R = Bmin = Shadow Printing Resolution Shadow printing (Contact and Proximity) • Factors that affect resolution • Separation between the wafer and the mask • Nonuniformities in wafer/mask flatness • Debris on mask or PR layer • Size of the PR edgebead • Photoresist thickness • Exposure wavelength • = wavelength s = separation z = resist thickness What happens when s goes to 0 (contact printing)? What happens when s>>z (proximity printing)? What is the difference between AZ 1518 and AZ 4620?

9. Edgebead?

10. Dynamic Spin Rate – why? Spin rate Time

11. Wavelength Source? I G H

12. Resolution Limitation?

13. Standing Wave Effect on Photoresist l/nPR Photoresist Substrate Overexposure Underexposure

14. Exposure Intensity

15. Phase Shift Masks • Extends resolution capability of current optical lithography • Takes advantage of the wave nature of light • PSM changes the phase of light by 180° in adjacent patterns leading to destructive interference rather than constructive interference • Improves MTF of aerial image on wafer. Making k1 smaller.

16. Optical Proximity Correction • Optical Proximity Correction (OPC) can be used to compensate somewhat for diffraction effects. • Sharp features are lost because higher spatial frequencies are lost due to diffraction. These effects can be calculated and can be compensated for. This improves the resolution by decreasing k1.

17. Optical Proximity Correction

18. Immersion Photolithography • NA is determined by the acceptance angle of the lens and the index of refraction of the medium surrounding the lens. The physical limit for an air based system is clear, but what if a medium with a higher index of refraction is substituted for air? Microscopy has for years used oil between the lens and the sample being viewed for resolution enhancement and it is somewhat surprising that the semiconductor industry has taken this long to seriously consider the merits of replacing air with an alternative.

19. Immersion Photolithography • The medium between the lens and the wafer being exposed needs to have an index of refraction >1, have low optical absorption at 193nm, be compatible with photoresist and the lens material, be uniform and non-contaminating. Surprisingly, ultrapure water may meet all of these requirements. Water has an index of refraction n = 1.47, absorption of <5% at working distances of up to 6mm, is compatible with photoresist and lens and in it’s ultrapure form is non-contaminating.

20. Use Water for Immersion Lithograph? • Index of refraction of air? 1.000 • Index of refraction of glass? 1.517 • Index of refraction of water? 1.332 website simulator: http://interactagram.com/physics/optics/refraction/

21. Immersion Photolithography what does immersion lithography do to DOF? http://spie.org/x8368.xml?ArticleID=x8368

22. Anyone Decided? Topic can NOT be your thesis work. • MEMS batteries (micro-batteries) • MEMS gas turbine • MEMS microphone or speaker • MEMS super-capacitor • Micro-robot or MEMS walker • Biosensing methods for microfluidics • Microfluidic cell sorting methods

23. Why will we be using an SEM? • Because the wavelength of the visible light, which is from 3900 Å (violet) to 7500 Å (red), is larger than many feature aspects. We need smaller wave length microscope such as electron microscope to investigate the small feature size pattern.

24. Quotes • Listen, opportunity sometimes knocks very softly. • Strive for excellence, not perfection. • Be tough minded, but tenderhearted. • Never underestimate your power to change yourself. • The race is not always to the swift, but to those who keep on running