resolution enhancement techniques

7/29/2019 resolution enhancement techniques

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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

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resolution enhancement techniques