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Resolution enhancement techniques:Mask engineering or wavefront
engineering
Sharp features are lost because higher frequencies are lost dueto diffraction. These effects are calculable and can be
compensated for.
The resolution of an optical system can be improved by
increasing the numerical aperture and reducing the wavelength.
Increasing the numerical aperture and reducing the wavelength,
however, decrease the depth of the focus. Further reduction in
the wavelength requires the development of new optical
systems and resist compositions.
It is known that in the sub 0.5m range, a perfect
image on the mask can, from diffraction effect, resultin a distorted pattern in the resist.
OPC mask attempt to reverse the situation by havinga distorted image on the mask that is design to,produce a perfect image on the resist. A computer isused to analyze exposure process conditions.
However, the use of OPC are so difficult that they are
unlikely to be implemented on a large scale in thenear future.
Resolution enhancement techniques:(1) Optical proximity corrections
(OPC)
Resolution enhancement techniques:
OPC
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Resolution enhancement techniques:
(2) Optical Phase Shifting
Diffraction problem could
be more pronounced as twomask patterns get closertogether
OPS uses phase shifting tosharpenprinted images.
These techniques can allowexisting exposure tools to
be used in manufacturing atleast one more technologygeneration.
Unresolved pattern
(3)
When the angle of
illumination and the
angle of diffraction
are well matched, theamount of light
diffracted can be
enhanced and the
contrast of the image
improved.
(4) Off-axisIlluminationtechnique
Next Generation lithographicmethods
Why is optical lithography so widely used and what makes it such a
promising method?
It has high throughput, good resolution, low cost and ease in operation.
However, due to deep submicron IC process requirements, optical
lithography has limitation that have not yet been solved.
Therefore, it is required to find alternatives to optical lithography. The
possible promising techniques are:
Electron beam lithography
Extreme Ultraviolet Lithography
X-ray lithography
Ion beam lithography
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Schematic of an
electron beam
lithography machine.
Advantage:Generation of submicronResist geometriesGreater depth of focusDirect patterning on aSemiconductor withoutUsing a mask.Currently EBL is theTechnology of choice forMask generation due to
Its ability to accuratelydefine small features.
In raster scan system
The beam scan sequentially
over every possible location(pixel) on the mask and is
Turned off where no exposure
Is required.
In a vector scan system
The beam is directed only to the
Requested pattern features
and jumps from features to
Features.
(a) Raster scan writing scheme. (b) Vector scan writingschemes. (c) Shapes of electron beam: round, variable, andcell projection.
Issue associated with EBL: Proximity effect
In EBL scattering causes the
electron beam to broaden and
expose a large volume of resist
then expected.
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Scattering with angular limitation projectionelectron beam lithography (SCALPEL)
This approach has not yet been in a full scale manufacturing environment, butIt appears to have significant promise for future lithography needs.
Schematic representation of an extreme ultraviolet (EUV)lithography system.
Challenges:
EUV is strangely absorbed
In all materials.
Lithography process must be performed in vacuum
Mask blank must also be multilayer coated to minimize
Its reflection.
Schematic representation of a proximity x-ray
lithography system.
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The primary factor limiting resolution inoptical lithography is diffraction. However,
because of advancement in excimer lasers,photoresist chemistry and resolutionenhancement techniques , optical lithographywill remain the main stream technology , atleast to the 100nm generation.
Although all non optical lithography techniques have100 nm or better resolution , each process has itsown limitations:Proximity effect in electron beam lithographyMask blank production difficulties in EUVlithographyMask fabrication complexity in X-ray lithographyRandom space charge effect in ion beam lithography