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?