High Resolution, High Efficiency SiN ZPs

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The motivation for designing the new SiN optics range, was the need for a ZP that could image samples heated to a few hundred degrees only a few millimetres away (focal length, Water Window).

In this set up, conventional Gold or Nickel ZPs on SiN substrates would typically last only a few hours before de-lamination and ultimate destruction of the optic! As expected, our Tungsten  ZPs on SiN substrates performed much better, because of the close thermal matching between the tungsten zones and the SiN substrates (See table). However, it became clear that a completely monolithic optic would be even better, and the obvious starting point was the SiN films that we use as substrates. The strategy was to start with a sufficiently thick SiN film, and simply etch the grooves of the optic at sufficient depth to obtain the required Diffraction Efficiency (DE).

It is crucial to use LOW STRESS (or Silicon Rich) films rather than stoichiometric films!

The films we use have a composition closer to Si4N3 rather than Si3N4, which is the standard composition of SiN films!

For operation in the Water Window, we usually start with 500 nm film thickness, and etch to a depth of 400 or 450 nm, leaving a residual 100 or 150 nm SiN as a "substrate" respectively.  For such deep grooves there are obvious limits to the minimum zone widths that can be achieved, but we have successfully managed to fabricate ZPs  with 30 nm outermost zones, with obvious improvements in Diffraction Efficiency over more conventional zone materials.

In some cases, where the residual stress in the film is close to zero, we are able to etch right through the film, essentially creating a “Free Standing" optic. Of course in such cases, one has to create a support structure to hold the optic together, and the loss of area by the support structure could be comparable to the gain in efficiency from the extra film thickness. Because of this, for operation above ~280 eV, it is usually better to use 450 nm grooves with 50 nm residual “substrates”. For operation below 280 eV (and especially at 100 eV or less) the absorption by SiN is so severe, that the free standing solution is clearly advantageous, despite the possible area loss in the support structure!

Some examples of SiN optics can be seen here