Ion Beam Technology for Slanted Surface Relief Gratings

Augmented Reality (AR) describes a technology where computer-generated elements are displayed in real-life surroundings. Over recent years, augmented and mixed-reality devices have gained a lot of interest, predominantly relying on lightweight, close-to-eye displays in so-called AR glasses.

To produce these displays, you need surfaces with slanted gratings, which are used to couple light from the source into the waveguide and out of it towards the eye.

The production of these surface relief gratings plays a crucial role in nano-structuring today. These gratings can either be etched directly into the glass, or they can be etched into a master (stamp) used to produce the display via e. g. nanoimprint lithography. Achieving optimal coupling effect, brightness, and efficiency requires slanted structures. For advanced grating designs, variations in tilt angle and, potentially, etch depth are necessary.

Reactive Ion Beam Etching (RIBE) developed as the preferred technology for etching slanted relief gratings due to its unique ability to regulate gas mixture, ion energy, ion current, substrate angle, and even substrate temperature.  For varying slant angles and different etching depths across the substrate, the technology of Reactive Ion Beam Trimming (RIBT) is needed.

Figure 1 and 2 show SEM images of slanted structures for AR applications with different angles of incidence and aspect ratios, each etched into a silicon wafer using the scia Trim 200. Necessary adjustments to the bottom geometry can be achieved by adding reactive gases.