Grayscale Lithography without a Mask

Furthermore, Electron beam writers can also be used for maskless grayscale lithography. A Vistec SB-254 is available at Fraunhofer ENAS for this application. This is a shaped beam tool with an acceleration voltage of 50 kV. In contrast to traditional Gaussian beams, this type of system offers increased throughput, as primitive shapes (rectangles, triangles) are exposed in one shot instead of requiring a complete raster exposure.

In combination to grayscale lithography with the i-line stepper, this also makes it possible to flexibly implement new designs with a higher resolution requirement. In accordance with the principle of “rapid prototyping”, new design variations can be quickly implemented and produced with electron beam lithography. All common 3D structures are possible here, including pyramids or lenses, for example. The disadvantage of this process is its low writing speed, making it unsuitable for the production of large quantities.

Electron beam lithography involves writing directly into the photoresist. The intensity of the beam can be modulated locally in order to adjust the exposure dose delivered per shot. As the resist thickness after development depends on the exposure dose, different grayscale levels can be produced in the photoresist in this way. For this, the contrast curve of the resist has to be know in detail, such that different dose levels can be mapped to the resulting resist height precisely. Special data preparation of the layouts is required for preparing the exposure and the fracturing of layouts, which can be tuned to the specific constraints of the layout and application. After exposure, the photoresist is developed and, depending on the type of resíst used (positive or negative), either the exposed or unexposed areas are removed respectively. In further process steps such as etching or deposition, the final 3D structures are then created using the finished resist mask.