1. NANOSCALE LITHOGRAPHY MICHAEL JOHNSTON 4/13/2015

2. ABSTRACT AND OUTLINE Nanoscale Lithography is an ever growing fabrication process due to technology demands. We are continuously striving to increase the number of transistors on a chip to increase performance. The drive for smaller and faster technologies has caused the development of fabrication techniques that allow us to work at the nanoscale feature size. A few lithography techniques will be explored to show how this whole process works. The processes I will cover include Photo, Electron Beam and X-Ray lithography. These forms of lithography share a common process of preparation exposure and development while fabricating wafers. Nanoscale Lithography is an ever growing fabrication process due to technology demands. We are continuously striving to increase the number of transistors on a chip to increase performance. The drive for smaller and faster technologies has caused the development of fabrication techniques that allow us to work at the nanoscale feature size. A few lithography techniques will be explored to show how this whole process works. The processes I will cover include Photo, Electron Beam and X-Ray lithography. These forms of lithography share a common process of preparation exposure and development while fabricating wafers. Key Terms: Photolithography, Electron Beam Lithography, X-Ray Lithography, Photo Resist, Electron Scattering, Photo Mask, Ionization Key Terms: Photolithography, Electron Beam Lithography, X-Ray Lithography, Photo Resist, Electron Scattering, Photo Mask, Ionization Overview(What, Why, Nano) Photolithography Electron Beam Lithography X-Ray Lithography Conclusion References Concept Check

3. WHAT IS LITHOGRAPHY!? The process of transferring or printing a pattern from one medium to another The process of transferring or printing a pattern from one medium to another Johann Alois Senefelder (1796), used ink on limestone Johann Alois Senefelder (1796), used ink on limestone

4. TECHNOLOGY TRENDS Moore’s Law Moore’s Law Smaller, Faster, Cheaper Smaller, Faster, Cheaper Performance Performance

5. NANOSCALE LITHOGRAPHY Using Lithographic tools for fabrication of any structures having feature size of less than 100 nm Using Lithographic tools for fabrication of any structures having feature size of less than 100 nm Human hair is ~80,000 nm thick Human hair is ~80,000 nm thick Downscale feature size, high throughput and quality Downscale feature size, high throughput and quality

6. FORMS OF LITHOGRAPHY Photo Photo Electron Beam Electron Beam X-Ray X-Ray Projection Printing Direct Writing Proximity Printing

7. PHOTOLITHOGRAPHY UV light passes through focusing lenses and masks to react with a photoresist to form patterns on a wafer UV light passes through focusing lenses and masks to react with a photoresist to form patterns on a wafer Projection Printing Technique Projection Printing Technique Limited by diffraction limit Limited by diffraction limit

8. PHOTOLITHOGRAPHY PROCESS General Steps: General Steps: 1. Substrate preparation 2. Photoresist 3. Alignment and Masking 4. Development

9. PHOTOLITHOGRAPHY PROCESS 1. SUBSTRATE PREPARATION Use modern clean rooms with robotic wafer cleaning systems Surface Impurities are removed by wet chemical treatment Baked to remove moisture 2. PHOTORESIST Apply a layer of resist that will react to the light Evenly coated by spin coating

10. PHOTOLITHOGRAPHY PROCESS ALIGNMENT AND MASKING Wafer is carefully aligned using automated mechanical process Mask is aligned over wafer with desired pattern DEVELOPMENT Developed using liquid solvent Wet or Dry Etching performed on substrate Photo Resist strip

11. ELECTRON BEAM LITHOGRAPHY (EBL) Highly focused electron beam is exposed to a resist material that modifies the solubility of the resist for development Highly focused electron beam is exposed to a resist material that modifies the solubility of the resist for development Allows accuracy down to as small as 10nm dimensions Allows accuracy down to as small as 10nm dimensions Nanoscale design is done on computers and the pattern is written on a chip using highly precise mechanical devices Goals of EBL writing are highly accurate and reliable pattern writing

12. LIMITING FACTORS IN EBL Quality of the Electron Optics Choice of Substrate, Resist, Developer Electron Beam Energy Level and Dose Development Time and Temperature Electron Scattering

13. ELECTRON SCATTERING Mutual repulsive forces from the electrons charges-worse at higher currents and lower energy levels Mutual repulsive forces from the electrons charges-worse at higher currents and lower energy levels Gas Scattering-reduced by using a vacuum Gas Scattering-reduced by using a vacuum Forward and Back Scattering Forward and Back Scattering Ionization (Secondary Electron Generation) Ionization (Secondary Electron Generation)

14. ELECTRON SCATTERING Forward Scattering- electrons entering resist undergo low energy inelastic collisions which deflect the electrons slightly causing the beam to widen Back Scattering-electrons pass through the resist into substrate and undergo enough collisions to deflect and reemerge into the resist causing proximity issues

15. EBL RESISTS Positive Resist: Converts from low to high solubility when exposed to electrons Positive Resist: Converts from low to high solubility when exposed to electrons PMMA (poly-methyl methacrylate)- longer polymer chain which is broken up into smaller more soluble chains PMMA (poly-methyl methacrylate)- longer polymer chain which is broken up into smaller more soluble chains Negative Resist: Converts to low solubility when exposed to electron beam HSQ (hydrogen silsesquioxane)- undergoes polymer cross-linking to form longer less soluble chains

16. DEVELOPMENT After exposure, resist is immersed in a liquid solvent to dissolve the fragments (positive) or non cross-linked molecules (negative). Temperature and duration are a large factor here Cold treat PMMA for higher resolution

17. EBL PROCESS PARAMETERS

18. ELECTRON ENERGY LEVEL Inelastic cross sectional area decreases proportionately to electron energy increasing Inelastic cross sectional area decreases proportionately to electron energy increasing In positive resists this causes fewer chain scissions per electron resulting in a lower sensitivity In positive resists this causes fewer chain scissions per electron resulting in a lower sensitivity Electrons with higher energy levels undergo less forward scattering, resulting in a narrower electron beam Higher energy electrons penetrate deeper into the substrate causing proximity issues

20. X-RAY LITHOGRAPHY Parallel Proximity Printing Parallel Proximity Printing X-Ray lithography uses ultra thin masks ( <2 micro) X-Ray lithography uses ultra thin masks ( <2 micro) X-Rays pass directly through mask and onto wafer X-Rays pass directly through mask and onto wafer Shorter wavelength than UV (0.4- 4nm) Shorter wavelength than UV (0.4- 4nm)

21. CHALLENGES OF X-RAY LITHOGRAPHY The thin masks are prone to deform due to small stresses The thin masks are prone to deform due to small stresses Masks deformation is huge in XRL because of the direct mapping Masks deformation is huge in XRL because of the direct mapping X-Rays cannot be focused through a lens X-Rays cannot be focused through a lens

22. CONCLUSION PHOTO UV Light Limited by UV Wavelength Projection Printing ELECTRON BEAM Direct Writing Expensive Highly Accurate Low Throughput Primarily Used to develop masks for X- Ray X-RAY Proximity Printing EfficientAccurate Expensive/Fragile Masks

23. REFERENCES Stepanova, Maria, and Steven Dew. Nanofabrication: Techniques and Principles. Wien: SpringerWienNewYork, 2012. Print. Jackson, M. J. "Microfabrication Using X-Ray Lithography" Micro and Nanomanufacturing Springer, 2007. Print. H. J. Levinson, Principles of Lithography, Second Edition, SPIE Press, Bellingham, WA (2005). Available at: http://spie.org/x33182.xml Pictures: Pictures: “Senefelder” http://en.wikipedia.org/wiki/Alois_Senefelder#/media/File:Senefelder.jpg “Senefelder” http://en.wikipedia.org/wiki/Alois_Senefelder#/media/File:Senefelder.jpghttp://en.wikipedia.org/wiki/Alois_Senefelder#/media/File:Senefelder.jpg “Moore’s Law” http://www.cuug.ab.ca/~branderr/eeepc/016_moores_law_intel.html “Moore’s Law” http://www.cuug.ab.ca/~branderr/eeepc/016_moores_law_intel.htmlhttp://www.cuug.ab.ca/~branderr/eeepc/016_moores_law_intel.html “Hair” http://waynesword.palomar.edu/lmexer1.htm “Hair” http://waynesword.palomar.edu/lmexer1.htmhttp://waynesword.palomar.edu/lmexer1.htm “E-Beam Lithography” http://www.ece.umd.edu/class/enee416/GroupActivities/Ebeam.pdf “E-Beam Lithography” http://www.ece.umd.edu/class/enee416/GroupActivities/Ebeam.pdfhttp://www.ece.umd.edu/class/enee416/GroupActivities/Ebeam.pdf

24. CONCEPT CHECK Describe the basic lithographic process Describe the basic lithographic process What types of resist are used, and what are the differences between them? What types of resist are used, and what are the differences between them? What are the three lithographic processes and what type of printing do they perform? What are the three lithographic processes and what type of printing do they perform? What is forward and back scattering and how do they effect the fabrication process? What is forward and back scattering and how do they effect the fabrication process? What is a drawback of EBL? What is a drawback of EBL?