E-beam lithography (EBL)

About this technique

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Electron beam (or e-beam) lithography (EBL) is a technique that enables the user to create extremely small structures on the surface of a sample, often down to the 10s of nm scale. It is usually applied in the field of microelectronics, often to develop semiconductor components on the sub-micrometre scale. The principle of the technique involves scanning an electron beam in a predetermined pattern across the surface covered with a thin film (normally referred to as the “resist”). The electron beam selectively removes the resist creating patterns that can subsequently be transferred to the substrate, such as Si wafers. During the exposure process, secondary electrons are generated by the forward scattering and back scattering of the incident electron beam, causing the exposed regions of the resist material to change its solubility and hence allow for its selective removal (or the selective removal of unexposed areas) in the subsequent development processes. Because EBL has much higher resolution than conventional optical lithography, it is often used for photomask fabrication and nanoscale device prototyping in research. It is possible to achieve sub 100 nm patterning of silicon wafers.

The throughput and the sensitivity of EBL exposure are mainly affected by the frequency of beam blanking and beam settling of the control system, where a beam blanker and pattern generator controls the movement and dwelling of the beam on the polymer surface.

The substrates can be metallic, semiconducting or insulating wafers, but must be flat and with maximum dimensions of 20 x 20 mm. These substrates should be spin-coated with an electron-sensitive polymer resist e.g. PMMA (positive tone), HSQ (negative tone) or a variety of other commercial resist formulations. The samples should be prepared at low contamination level and annealed in vacuum oven.

Patterns designs should be created in GDS-II or BMP formats and the dimensions should not exceed the exposure field. Commercial software and freeware exists for creating pattern designs. A maximum field of 1 x 1 mm can be achieved although there is generally a loss of resolution for larger write fields in order to maintain reasonable write times. Field stacking is possible however.

Depending on the type of polymer resist used, a post-exposure bake may be required. The development normally involves the dipping of sample in aqueous or solvent-based developers followed by rinsing with deionised water or IPA, and drying in N₂. Determination of the optimum protocols and developers may require experimentation, although parameters can often be obtained from the literature or resist manufacturers.