A new patterning method using electron beam direct write (EBDW) was developed at Fraunhofer CNT for a holography demonstrator for SeeReal Technologies GmbH, Dresden. The project included data preparation of the customer-specific layout, the development of the complete resist process including coating, baking and developing the wafer, resist patterning using electron beam technology and a final optical inspection. Thus the advantages of EBDW could be fully utilized. Optional design changes could be implemented very fast. Maskless electron beam lithography was the preferred method for this rapid prototyping project for a
component of a holographic projection display prototype.

Contrary to other projects, which require high resolution of structures, one of the challenges of this project was to pattern 8 inch glass substrates with a transparent conductive oxide (TCO) (figure 1). New alignment and height mapping concepts for the electron beam were established to overcome difficulties with the transparent stack and substrate. Thereby, the dimension of the smallest TCO electrode was in the lower wave length range of light spectrum. For establishing a proper working resist process stack, compositions were changed and varied facing material delamination and degradation (oxidation). To provide conductivity and to avoid short cuts of the electrodes, high demands in a defect-free process were required.

First analyses of appearing defects were done so far and processes were changed and optimized regarding defect density. The etch process for the display component is also very challenging. In general, display components with structures smaller than 1 μm could not be etched with a wet etch process so far. Due to the fact that Fraunhofer CNT is not able to run 8 inch etch processes, the component was etched by another project partner with an RIE dry etch process. The study was realized successfully and a follow up project with SeeReal was agreed upon.

Electron beam lithography at a glance:
• Litho character: line/space structures
• Critical dimensions: line/spaces: 500 nm/250 nm until 2 μm/2 μm
• Stack: TCO/SiO2 on 8 inch glass substrate
• Resist: thickness 160 nm, tonality negative, chemically amplified