Electron beam lithography Electron beam lithography (often abbreviated as e-beam lithography) is the practice of scanning a beam of electrons in a patterned fashion across a surface covered with a film (called the resist),[1] ("exposing" the resist) and of selectively removing either exposed or non-exposed regions of the resist ("developing"). The purpose is to create very small structures in the resist that can subsequently be transferred into another material for a number of purposes, for example for the creation of very small electronic devices. The primary advantage of electron beam lithography is that it is one of the ways to beat the diffraction limit of light and make features in the nanometer regime. This form of maskless lithography has found wide usage in mask-making used in photolithography, low-volume production of semiconductor components, and research & development. On the other hand, the key limitation of electron beam lithography is throughput, i.e., the very long time it takes to expose an entire silicon wafer or glass substrate. A long exposure time leaves the user vulnerable to beam drift or instability which may occur during the exposure. Also, the turn-around time for reworking or re-design is lengthened unnecessarily if the pattern is not being changed the second time. Electron Beam Lithography Systems Electron beam lithography systems used in commercial applications are dedicated e-beam writing systems that are very expensive (>$4M USD). For research applications, it is very common to convert an electron microscope into an electron beam lithography system using a relatively low cost accessory (<USD 100k). Such converted systems have produced linewidths of ~20 nm since at least 1990, while current dedicated systems have produced linewidths on the order of 10 nm or smaller. Electron beam lithography systems can be classified according to both beam shape and beam deflection strategy. Older systems used Gaussian-shaped beams and scanned these beams in a raster fashion. Newer systems use shaped beams, which may be deflected to various positions in the writing field (this is also known as vector scan). Electron sources Lower resolution systems can use thermionic sources, which are usually formed from LaB6. However, systems with higher resolution requirements need to use field emission sources, such as heated W/ZrO2 for lower energy spread and enhanced brightness. Thermal field emission sources are preferred over cold emission sources, in spite of their slightly larger beam size, because the former offer better stability over typical writing times of several hours. Lenses Both electrostatic and magnetic lenses may be used. However, electrostatic lenses have more aberrations and so are not used for fine focusing. There is no current mechanism to make achromatic electron beam lenses, so extremely narrow dispersions of the electron beam energy are needed for finest focusing. Stage, stitching & alignment Typically, for very small beam deflections electrostatic deflection 'lenses' are used, larger beam deflections require electromagnetic scanning. Because of the inaccuracy and because of the finite number of steps in the exposure grid the writing field is of the order of 100 micrometre - 1 mm. Larger patterns require stage moves. An accurate stage is critical for stitching (tiling writing fields exactly against each other) and pattern overlay (aligning a pattern to a previously made one). Electron beam write time The minimum time to expose a given area for a given dose is given by the following formula: Dose * exposed area = beam current * exposure time = total charge of incident electrons For example, assuming an exposure area of 1 cm2, a dose of 10-3 Coulombs/cm2, and a beam current of 10-9 Amperes, the resulting minimum write time would be 106 seconds (about 12 days). This minimum write time does not include time for the stage to move back and forth, as well as time for the beam to be blanked (blocked from the wafer during deflection), as well as time for other possible beam corrections and adjustments in the middle of writing. To cover the 700 cm2 surface area of a 300 mm silicon wafer, the minimum write time would extend to 7*108 seconds, about 22 years. This is a factor of about 10 million times slower than current optical lithography tools. It is clear that throughput is a serious limitation for electron beam lithography, especially when writing dense patterns over a large area. E-beam lithography is not suitable for high-volume manufacturing because of its limited throughput. The serial nature of electron beam writing makes for very slow pattern generation compared with a parallel technique like photolithography (the current standard) in which the entire surface is patterned at once (1X optical steppers only, 4X or 5X steppers take proportionally longer). To pattern a single wafer with an electron beam lithography system for sub-100 nm resolution, it would typically take days, compared to the few minutes it would take with a photolithography system. Even an optical maskless lithography tool is much faster than an electron beam used for the same purpose. Defects in electron-beam lithography Despite the high resolution of electron-beam lithography, the generation of defects during electron-beam lithography is often not considered by users. Defects may be classified into two categories: data-related defects, and physical defects. Data-related defects may be classified further into two sub-categories. Blanking or deflection errors occur when the electron beam is not deflected properly when it is supposed to, while shaping errors occur in variable-shaped beam systems when the wrong shape is projected onto the sample. These errors can originate either from the electron optical control hardware or the input data that was taped out. As might be expected, larger data files are more susceptible to data-related defects. Physical defects are more varied, and can include sample charging (either negative or positive), backscattering calculation errors, dose errors, fogging (long-range reflection of backscattered electrons), outgassing, contamination, beam drift and particles. Since the write time for electron beam lithography can easily exceed a day, "randomly occurring" defects are more likely to occur. Here again, larger data files can present more opportunities for defects. (下文略) http://en.wikipedia.org/wiki/E-beam_lithography -- ◣ ◢ .. ◥S --

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