resolution enhancement techniques.pptx

7/27/2019 Resolution enhancement techniques.pptx

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RESOLUTION ENHANCEMENT

TECHNIQUES IN OPTICAL

LITHOGRAPHY

Presented ByPrashant Kumar

M.TECH, 1ST

YearNanoscience & Technology


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What Is Resolution or Resolving

Power ? A measure of smallest separation at which an optical

instrument can observe two neighboring objects as

two separate objects.

So for the better picture quality it is required have

minimum value of resolution.


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The Rayleigh Criterion

Rayleigh's criterion states that two similar diffractionpatterns can just be separated if the first zero of onepattern falls on the central peak of the other.


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Resolution Enhancement

Techniques(RET) Sharp features are lost because higher frequencies are lost due

to 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 opticalsystems and resist compositions.

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Resolution Enhancement

Techniques

Where,k1 = Process Complexity Factor = The vacuum wavelength of the

lithographic imaging toolNA = Numerical Aperture

K1 depends on the details of the imaging process


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Types Of RETOptical Proximity Correction

Spatial FilteringOff Axis Illumination

Phase Shift Mask


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Optimal Proximity Correction(OPC)

It is known that in the sub 0.5m range, a perfectimage on the mask can, from diffraction effect, resultin a distorted pattern in the resist.

OPC mask attempt to reverse the situation by having adistorted image on the mask that is design to, producea perfect image on the resist. A computer is used toanalyze exposure process conditions.

However, the use of OPC are so difficult that they areunlikely to be implemented on a large scale in the nearfuture.

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Optimal Proximity Correction

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Optimal Proximity Correction(OPC)


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Photolithography- OPC

Optical Proximity Correction (OPC) can be used tocompensate somewhat for diffraction effects.

Sharp features are lost because higher spatial frequenciesare lost due to diffraction.

These effects can be calculated and can be compensatedfor. This improves the resolution by decreasing k

1

.

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Photolithography- OPC


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Spatial Filtering

Spatial filter modulates the amplitude and phase of alight wave at its frequency domain, which can improveimage resolution and depth of focus of the imaging

system. From the theory of Fourier optics, optical imaging can

be mathematically expressed as a Fouriertransformation process, i.e. a mask pattern is

transformed by a lens from object space to image spaceat the lens pupil plane where the light intensitydistribution through the pattern mask becomesamplitude and phase distribution.


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Spatial Filtering

Optical diffractometer set for image reconstruction and filtering. Filterscan be introduced in the transform plane in order to alter the image.


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Spatial Filtering If an image is sharp and there is a loss of details, it

would be rich in high frequency components in itsfrequency spectrum at the pupil plane.

The concept of spatial filtering is to partially deter thelow frequency components while enhancing the highfrequency components by using a spatial filter at thelens pupil plane, which can improve sharpness of animage.


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Spatial Filtering

The microscope objective lens produces a Fourier transform of thelaser beam in the back focal plane. The pinhole aperture then only allows thecentral maximum to propagate through the optical system. Lens 1 transforms thelight passing through the pinhole into a plane parallel beam.


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Spatial Filtering They are basically circular plates with three zones.

Each zone has certain transparency and phasemodulation to the incoming light wave as shown in

figure below. The filter 1 has only the variation in transmission at

each zone an filter 2 has not only the transmissionvariation but also 180 degree phase difference in the

central zone. Computer simulations of the two spatial filters have

indicated improvements in both image contrast anddepth of focus.


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Filter 1 Filter 2

Structure of two spatial Filters


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Limitations of Spatial FilteringAlthough the spatial filtering technique can improve

the quality of optical lithography, it is fairly difficult toimplement the technique, because the filter has to beinserted into lens pupil plane.

Modification of lens column is necessary.

Recently a new fractional filtering approach may

provide hope to avoid the difficulty because afractional filter can be placed at any position on anoptical path.


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References1. Micro-Nanofabrication By Zheng Cui

2. Fundamental Principle of optical lithography ByChris Mack

3. www.google.co.in


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THANK YOU

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