Why small samples can crash the DWL write head
The pneumatic (air-gauge) autofocus sits a few mm away from the laser spot. If it leaves the sample, it reads the chuck instead and drives the head down, straight into the sample.
Top view: drag the sample or the write head
Side view through the air gauge (vertical not to scale)
1. How it focuses
Air flows out of the 4 mm gauge opening. The back-pressure tells the system how far the head is from the surface, and it keeps that gap (working distance ≈100 µm) constant, also following wafer bow.
2. What goes wrong
Past the sample edge the air escapes freely. The pressure drops, so the system thinks the head is too high and lowers it by roughly the sample thickness. A 525 µm sample against a 100 µm gap: the head hits the sample.
3. Why 1 inch is marginal
On a 25.4 mm sample the edge is only 12.7 mm from the centre. Add the gauge offset, the write area and a 2-3 mm misplacement, and the gauge is off the sample.
4. What to do
Centre small samples carefully, keep the write area well inside, and keep the write area ≥ 6 mm from every edge (SOP/CR/003). Samples must be larger than 1 inch, square or round, and go on the 4-inch square holder.
From SOP/CR/003 v7.0: working distance 100 µm, write area ≥ 6 mm from the sample edge, minimum distance to plate edge 5 mm, samples larger than 1 inch. The nozzle body (≈35 mm) and the gauge opening (≈4 mm) were measured on the nozzle. The gauge-to-laser offset is an assumption, so set it to the real value. Sample thickness and gap are exaggerated vertically.