Slanted gratings with varying slant angle by localized reactive ion beam processing

Augmented and mixed reality applications have recently gained large interest. In order to fabricate lightweight near eye displays surface relief gratings are used for coupling the light from the source into the light guide and out of the light guide towards the eye. To suppress higher diffraction orders and thus maximize the light yield those gratings are slanted. In order to achieve high field of view variation of the slant angle is desirable yet rather difficult to achieve. Ion beam trimming (IBT) is a method well established in MEMS and sensor manufacturing to locally modify the thickness of a layer or substrate in order to achieve a desired target topography. This method has been adapted towards fabrication of surface relief gratings with both varying slant angle and varying trench depth. The optically transparent substrate (e.g. SiO2) had been covered with a hard mask providing the basic grating geometry such as period and duty cycle. Localized ion beam etching was then applied to etch the trenches. Varying the ion irradiation dose (by means of dwell time variation) sets the trench depth on a local level. The angle of incidence of the ion beam directly transfers into the substrate as the slant angle and can be varied independently of the dwell time providing the unique possibility to achieve a continuous variation of the slant angle across one die. To avoid redeposition of the sputtered trench material to the grating side walls the etching was carried out in the reactive ion beam etching (RIBE) regime. Furthermore the choice of the process parameters (process gas composition, ion energy) allows for tuning the selectivity between hard mask and target material such that high aspect ratio trenches are possible while keeping the hard mask thin avoiding shadowing of neighboring structural features.