Tuning of width to period ratio of synchrotron gratings by reactive ion beam etching

The fabrication of synchrotron gratings requires high precision in lithography and structure transfer. The challenge with binary profiles lies in a specific groove width to period ratio (w/p-ratio) and a defined profile depth. This study shows how reactive ion beam etching (RIBE) can be used to change the w/p-ratio after lithography. More precisely, there was a variation in the w/p-ratio of the resist profiles from the center to the edge of the grating area, which meant that the grating will not deliver the required efficiencies after the transfer into the silicon substrate. For the adjustment of the resist profiles a broad oxygen ion beam from a Kaufman-type ion source was used, whereby the difficulty was to increase the w/p-ratio to varying degrees depending on the position on the sample. This step already led to a varying etching of the silicon substrate and the formation of a silicon oxide layer with a varying thickness, which made the subsequent structure transfer very difficult. For the structure transfer, a RIBE process with a mixture of trifluoromethane and oxygen was used again with the aim of achieving a homogeneous profile depth of 8 nm over the whole grating area. How this was done exactly is explained in the following sections.