Improved Grayscale Lithography

Commercially available laser lithography systems have been available for several years. One such system manufactured by Heidelberg Instruments can be used to produce masks for lithography or to directly pattern photoresist using either a 3 micron or 1 micron beam. These systems are designed to operate using computer aided design (CAD) mask files, but also have the capability of using images. In image mode, the power of the exposure is based on the intensity of each pixel in the image. This results in individual pixels that are the size of the beam, which establishes the smallest feature that can be patterned. When developed, this produces a range of heights within the photoresist which can then be transferred to the material beneath and used for a variety of applications. Previous research efforts have demonstrated that this process works well overall, but is limited in resolution and feature size due to the pixel approach of the exposure. However, if we modify the method used, much smaller features can be resolved, without the pixilation. This is achieved by utilizing multiple exposures of slightly different CAD type files in sequence. While the smallest beam width is approximately 1 micron, the beam positioning accuracy is much smaller, with 40 nm step changes in beam position based on the machine's servo gearing and optical design. When exposing in CAD mode, the beam travels along lines at constant power, so by automating multiple files in succession, and employing multiple smaller exposures of lower intensity, a similar result can be achieved. With this line exposure approach, pixilation can be greatly reduced. Due to the beam positioning accuracy of this mode, the effective resolution between lines is on the order of 40 nm steps, resulting in unexposed features of much smaller size and higher resolution.