Lithography In the Top-Down Process – New Concepts

• Learning Objectives– To identify issues in current photolithography– To quantify the needs of nanomanufacturing– To define improvements in photolithography– To explore new lithography processes– To define the limitations of these new

processes in top-down nanomanufacturing

What are the limitations of current photolithography processes?

• Light sources from traditional mercury vapor lamps have little “deep UV” spectra

• Finer feature sizes require shorter wavelength sources

• Photoresist must be sensitive to appropriate wavelengths of light

• Lenses and optical components have limited numeric aperture

Why Shorter Wavelengths?

• Minimum Feature Sizes are dictated by the following relationship:

F = K (λ/NA)

Where F = Feature Size in nM

λ = Wavelength (nM)

K = Process Constant

NA = Numeric Aperture

Shorter Wavelength Sources

• Replace the mercury vapor lamp an excimer (exciplex) laser source with shorter wavelength emission– ArF – 193 nM – Shorter wavelength than so-called

“deep UV” peak of 248nM

– F2 Laser – Low output but at 157nM

• Matching photoresist that is sensitive to this spectra is also required.

• Laser sources under development – 13.5 nM! (extreme UV or EUV range)

Numerical Aperture

• Light passing through the mask will be subject to diffraction. The numerical aperture of the lens used determines its capability to bring the diffracted pattern into a single point of focus.

• NA = n sin θ where n = index of refraction of the media in which the lens is working (air) and θ is the angular spacing between objects making up the image

Numerical Aperture (2)

• sin θ = 1.22 λ/D where θ is the angular spacing between objects and D = diameter of the lens

• A larger diameter lens helps, but is difficult to manufacture

• Depth of Field Issues limit the use of larger diameter as a solution

Increased NA reduces depth of field

Improving the Index of Refraction

Improving the Photomask

• Sharp edges in photomasks are not well reproduced as feature sizes shrink

• Optical proximity correction techniques put borders on corners and edges to correct for this

Practice Questions

1. What limits feature sizes in photolithography?

Click once for each question.

Wavelength of the light source used

2. What effect causes blurring in photomasks?Diffraction of the light source

3. What is the limitation that occurs when numerical aperture is increased?

Depth of field is decreased

Numerical aperture of the lens

Alternative Exposure Methods – Electron Beam Lithography

• Use of exposure sources other than UV light has been studied for some time.

• An electron beam is exceptionally “narrow”, and does not require a mask

• λ=h/P

Phase Shift Mask

Electron Beam Lithography (2)

• E-beam lithography also serves as a tool for mask making

• Throughput is not an issue in this case, since the masks are made once, and used many times.

• Sub-50 nM feature sizes are possible

X-Ray Lithography

• Synchrotron radiation sources can be used

• Masks use “absorber” materials on a membrane

• X-rays pass through the membrane

• PMMA photoresists can be used

X-Ray Lithography Issues

• Spacing, mask dimension, and wavelength are critical

So-called “sweet spot” will provide small feature size for a given wavelength exposure and defined mask feature

Nano-Imprint Lithography

• Concept – To use a “stamp” of precise dimension to create features in resist

• Advantages– High Throughput– No issues in diffraction– No secondary emission– Can be carried out in non-vacuum

environment

Nano-Imprint Lithography (2)

• T-NIL (Thermal Nano-Imprint Lithography)– PMMA resist similar to that

used in X-ray lithography is spin coated onto surface

– Stamp is pressed into contact with surface

– Substrate is heated to glass temperature of resist

– Pressure is applied to “stamp” imprint

– Substrate cools and stamp can be removed

http://www.nilt.com/Default.asp?Action=Details&Item=219

UV-Nano Imprint Lithography

• UV-NIL (Ultra-violet Nano-Imprint Lithography)– A UV sensitive resist is

used– The stamp must be

UV-transmissive– UV light is applied

through the stamp– Pressure is applied to

“stamp” imprint

Issues in Nanoimprint Lithography

• Alignment of layers can be more difficult than with projection lithography

• “Proximity effect” of having large stamp areas near small features may cause uneven feature sizes

• Residual layer thickness and profile may vary

• Patterned areas may “stick” to stamp

AFM Probe Lithography

• An atomic force microcscope cantilever writes a pattern in resist– Extremely precise– “Scratching the

surface”

http://www.pacificnanotech.com/afm-modes_active-modes.html

AFM “Dip Pen” Lithography

• An atomic force microcscope cantilever writes a pattern in resist– Extremely precise– “Scratching the

surface”

http://www.pacificnanotech.com/afm-modes_active-modes.html