Electron projection lithography

Electron projection lithography

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  ELECTRON PROJECTION LITHOGRAPHY
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  Contents Disadvantages Advantages Mask construction Patterning techniques Working Introduction
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  Introduction • Electron projectionlithography is a technique in which electrons are used to transfer a pattern to the substrate. A mask is used to project an image to all or a large portion of the pattern. . The substrate is coated with a film (or a resist) and a beam of electrons is scanned across a patterned mask and the desired pattern is obtained by selectively removing either exposed or non-exposed regions of the resist.
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  Working • The electronbeam is ejected from a thermionic filament source and focused on the mask membrane with magnetic lenses. • Mask is a solid membrane with holes in it representing the pattern. • The electron beam is absorbed in the solid parts and passes through the holes thus imparting the pattern on to the beam.
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  Patterning Techniques • Twotechniques possible for patterning on the wafer for electron projection lithography (EPL): 1. A mask is placed in close proximity to the wafer and scanned with an electron pencil beam of few mm in diameter. 2. The mask is irradiated with border beam and a lens is used to project the image of the mask onto the wafer.
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  Mask construction There aretwo approaches to the mask construction for electron projection lithography: Scattering stencil Continuous membrane
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  Scattering stencil • Scatteringstencil masks use a membrane which is thin enough to absorb only a small fraction of beam energy as it passes through (2 micron thick Si has been used). Electron beam energies of about 100 KeV are typically used. Electrons passing through the membrane are scattered through a small angle as they pass through but not absorbed. An aperture further on the electron optical system stops the scattered electrons from reaching the substrate (for most part).The membrane is patterend by etching holes in the membrane.
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  Continuous membrane • Themask is composed of two layers,a thin membrane of low atomic number material, which scatter the coming electrons to very few extent . Very next to it a thin layer of high atomic number material (e.g. tungsten) is placed, which will scatter the electron more strongly than the membrane. After passing through the mask contrasts is produced in the image with an aperture at the back focal plane of imaging system which stops the scattered beam and allow the unscattered beam to pass.
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  Applications •Electron projection beamis widely used in the field of electronic industry i.e. to fabricate circuits with greater accuracy.
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  Advantages Low current density, Resolutionof 100nm is possible.
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  Disadvantages Electrons absorbed atthe stencil will deposit a significant amount of energy causing it to heat up which leads to the distortion. As beam current is increased to increase exposure rate, electron-electron scattering tends to degrade the resolution of the image.