Revised vacuum platform

[NemoAndrea]

For the temporary stage (PI M-525.2MO), I had to redesign the vacuum sample stage.

The principle is the same, but I have opted for an acrylic top surface for decreased surface roughness. Frosted acrylic is purely for aesthetic reason and probably slightly increases roughness; but it works well enough in practice.

The same basic vacuum pump is connected to the stage. In this case this piece is printed out of PLA, and seals against the acrylic with a basic rubber gasket.

The PLA part is printed with 4 walls in the hope that this helps with sealing. I think the sealing could be further improved by having a barb fitting with O-ring to really seal it. The hole that the barbed fitting threads into is deliberately left small so that the brass piece really cuts into the PLA and makes a half-decent seal. But could certainly be improved.

It holds a 200um thick 4-inch Siliconwafer quite securely on the stage with the weak vacuum pump. Quite a large force has to be applied for the wafer to start slipping on the surface. The vacuum is strong enough to support the (admittedly light) wafer against gravity.

The design files were added in openMLA/medjed-assembly@faaa49a in case you want to have a look.