Transcript Lecture 13 - University of Alabama

Optical Lithography
Lecture 13
G.J. Mankey
[email protected]
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Lithography
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The transfer of a pattern to a material or patterning of a material by a printing
process.
In optical lithography, specific areas of a layer of photosensitive resist or
photoresist are illuminated through a mask.
The illuminated resist either becomes soluble in developer (positive resist) or
insoluble (negative resist).
The patterns can then be transferred to the substrate using subtractive or additive
processes.
We perform mainly single-layer processes to pattern materials.
For multiple layer processes, the successive layers must be aligned relative to one
another using a contact mask aligner or projection stepper.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Positive Resist Processing
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The layer of resist is exposed in
specific areas through a mask.
Development washes away
exposed resist.
In the additive process, material is
deposited through the holes in the
resist.
For a subtractive process, material
is removed by ion milling through
the holes in the resist.
In the final step, resist is removed.
UV Light
mask
photoresist
substrate
Exposure
Developing
Deposition
Ion Milling
Additive
Subtractive
Finished Product
Finished Product
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Negative Resist Processing
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The layer of resist is exposed in
specific areas through a mask.
Baking helps to harden the exposed
resist.
A second exposure exposes the soft
resist.
Development washes away soft resist.
Resist remains in the areas initially
exposed through the mask.
Exposure
UV Light
mask
photoresist
substrate
Baking
2nd Exposure
Developing
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Resolution Limit
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The minimum resolution of an optical element is given by the Raleigh criterion:
resolution  k1
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l
NA
k1 is a constant that depends on the thickness and index of refraction of the
photoresist (~0.6).
l is the wavelength of the radiation used to expose the resist.
NA is the numerical aperture of the focusing optical elements (~0.5).
The basic rule of thumb is that the minimum line width achievable is basically
equal to the wave length of the incident radiation.
We use the i-line from a Hg arc lamp which has a 365 nm wavelength.
Shorter wavelengths are available, with the current limit below 200 nm.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Photoresist Coating
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Photoresist is coated onto the substrate using a spinner, which rotates the
substrate up to 10,000 rpm while resist is placed on the surface using a
dropper.
We normally use Shipley positive resist.
The thickness of resist is a function of concentration (viscosity) and rotation
speed.
A set of spin-curves should first be made to find the parameters for producing
resist coating within the desired range of thickness and uniformity.
Typical resist thickness is of the order of 100 nm, and the ellipsometer is the
ideal tool for determining thickness.
Resist is usually prebaked for a short period of time prior to exposure through
a mask.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Resist Exposure
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The resist is exposed though the mask using the mask aligner.
The vacuum contact method is best since it insures a good contact between mask and
photoresist.
The light intensity should first be checked using the photometer.
Light intensity is controlled by adjusting the discharge current in the lamp.
Exposure time is controlled by the aligner, and a series of exposures is necessary to
optimize exposure time to achieve the desired resolution and aspect ratio.
Array masks are available with lines and bars from tens of micrometers wide down to
bars and dots 0.7 micrometers in size.
Resist is then developed by immersing the substrate into developer solution.
After a hard bake, the resulting resist pattern should be measured using the AFM.
The optimized process can then be applied to additive or subtractive materials
processing.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center
Process Parameters
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The process parameters should be optimized to produce the desired resist patterns:
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Resist concentration.
Spinner speed and time.
Pre-exposure bake temperature and time.
Exposure intensity and time.
Developer concentration and time.
Post developing hard bake.
Resulting pattern
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Edge and corner definition.
Pattern replication.
Resolution / minimum feature size.
Aspect ratio.
Center for Materials for Information Technology
an NSF Materials Science and Engineering Center