NANOSCALE LITHOGRAPHY MICHAEL JOHNSTON 4/13/2015.
Post on 24-Dec-2015
222 Views
Preview:
Transcript
NANOSCALE LITHOGRAPHYMICHAEL JOHNSTON
4/13/2015
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.
• 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
WHAT IS LITHOGRAPHY!?
• The process of transferring or printing a pattern from one medium to another
• Johann Alois Senefelder (1796), used ink on limestone
TECHNOLOGY TRENDS
• Moore’s Law
• Smaller, Faster, Cheaper
• Performance
NANOSCALE LITHOGRAPHY
• Using Lithographic tools for fabrication of any structures having feature size of less than 100 nm
• Human hair is ~80,000 nm thick
• Downscale feature size, high throughput and quality
FORMS OF LITHOGRAPHY
• Photo
• Electron Beam
• X-Ray
• Projection Printing
• Direct Writing
• Proximity Printing
PHOTOLITHOGRAPHY
• UV light passes through focusing lenses and masks to react with a photoresist to form patterns on a wafer
• Projection Printing Technique
• Limited by diffraction limit
PHOTOLITHOGRAPHY PROCESS
• General Steps:
1. Substrate preparation
2. Photoresist
3. Alignment and Masking
4. Development
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
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
ELECTRON BEAM LITHOGRAPHY (EBL)
• 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
• 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
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
ELECTRON SCATTERING
• Mutual repulsive forces from the electrons charges-worse at higher currents and lower energy levels
• Gas Scattering-reduced by using a vacuum
• Forward and Back Scattering
• Ionization (Secondary Electron Generation)
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
EBL RESISTS
• 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
• Negative Resist: Converts to low solubility when exposed to electron beam
• HSQ (hydrogen silsesquioxane)- undergoes polymer cross-linking to form longer less soluble chains
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
EBL PROCESS PARAMETERS
ELECTRON ENERGY LEVEL
• 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
• 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
X-RAY LITHOGRAPHY
• Parallel Proximity Printing
• X-Ray lithography uses ultra thin masks ( <2 micro)
• X-Rays pass directly through mask and onto wafer
• Shorter wavelength than UV (0.4-4nm)
CHALLENGES OF X-RAY LITHOGRAPHY
• The thin masks are prone to deform due to small stresses
• Masks deformation is huge in XRL because of the direct mapping
• X-Rays cannot be focused through a lens
CONCLUSION
PHOTOUV Light
Limited by UV Wavelength
Projection Printing
ELECTRON BEAMDirect Writing
Expensive
Highly Accurate
Low Throughput
Primarily Used to develop masks for X-Ray
X-RAYProximity Printing
Efficient
Accurate
Expensive/Fragile Masks
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:
• “Senefelder” http://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
• “Hair” http://waynesword.palomar.edu/lmexer1.htm
• “E-Beam Lithography” http://www.ece.umd.edu/class/enee416/GroupActivities/Ebeam.pdf
CONCEPT CHECK
• Describe the basic lithographic process
• 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 is forward and back scattering and how do they effect the fabrication process?
• What is a drawback of EBL?
top related