Transcript Nanoscale lithography

NANOSCALE LITHOGRAPHY
MICHAEL 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
2. PHOTORESIST
• Use modern clean rooms with robotic • Apply a layer of resist that will
wafer cleaning systems
• Surface Impurities are removed by
wet chemical treatment
• Baked to remove moisture
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 (0.44nm)
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
PHOTO
ELECTRON BEAM
X-RAY
UV Light
Direct Writing
Proximity Printing
Limited by UV Wavelength
Expensive
Efficient
Projection Printing
Highly Accurate
Accurate
Low Throughput
Expensive/Fragile Masks
Primarily Used to develop masks for XRay
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?