Blanks need attention too!
In addition to mirrors that we use to direct and focus our beams of synchrotron light, we also need high precision gratings to diffract the light. The blank substrates for these gratings are coming from Zeiss, but the rest of the production is happening here at HZB.
A few days ago, I received the blanks for the EMIL-beamline and before we can move to the next steps I have to measure them to make sure they fulfill all the specifications. It’s like with everything else: in a long production process, errors tend to add up, so they have to be minimised every step of the way. In the case of the gratings: if the substate is not good, you will very likely run into a lot more trouble later.
So first, I have to determine the roughness of the surfaces. The blanks are highly polished blocks of silicium. Any unevenness must be less than 0,2 nm rms.
Then, as the surfaces are not entirely flat but curved in a specific way, I measure the slope profile and the rating.
Step one is done in our cleanroom lab with a white light interferometer. For step two I bring the gratings over to our optics lab and measure on the NOM (Nanometer Optical Component Measuring Machine).