Design of a machine for the universal non-contact measurement of large free-form optics with NM uncertainty

4.3. Metrology system Design

The metrology loop should measure the position of the probe relative to the product within the measurement plane. The probe R and Z-positions should is measured with a maximum uncertainty of about NM, the product R and Z-position to About 5 nm and the tilt to about 0.1μrad. Figure 5.4 shows, the setup in which, the probe position may be measured. There is several configurations possible, with variations in mirror positions etc. but the principle remains the same.

To measure the probe position, A heterodyne interferometer Beam (1) is delivered to a non-polarizing Beamsplitte r  (2), for instance with folding mirrors attached to the stages (not shown). From here the beams travel to the polarizing beamsplitters (3). Here, one polarization direction of the beam travels straight through, and the other part was deflected to The r Eference Mirror (5). The beam is reflected and passes AΛ/4 Plate (4) for The second time, causing it to pass through the Beamsp Litter. With a lens (7) The Beam is today focused on the center of The reflectingψ-axis Rotor (8). After reflection, it passes ANOTHERΛ/4 Plate (4)  for the second time, which now causes it to join the othe R Beam again, resulting in interference between the. With mirrors, the beams (ten) can now is transported to the receivers. basically, one part of the beam travels St Raight through Beamsplitter (3), while The other part makes a detour between The reference mirror and theψ-axis rotor, resulting in OPD between the beams. This is a direct measurement of the probe displacement in R and z-direction with respect to the metrology frame have been achieved, free of Abbe errors. All critical stage Errors is now measured and can thus is corrected for in the data-processing. As mentioned before, Theψ- angle of the probe is a second order error and would be measured with an encoder on theψ- Axis.

the product is assumed to being rigidly attached to the spindle rotor. The Blockhead 10R spindle is specified at <20 nm axial and radial error motion and <0.1μrad tilt error motion. The synchronous part is calibrated; The asynchronous part can is measured with capacitive probes measuring to a calibrated edge on the rotor. This, a short metrology loop between probe and product have been created, in which all the critical positioning ER Rors is measured  (Figure 5.5). The metrology frame is a shown schematically in this figure; It is closed at the bottom for stability in future designs.

When measuring a free-form, for instance a toroid of which the surface varies between the continuous and the dotted line O F Figure 5.5, the only moving parts in the machine is the continuously rotating spindle and the focusing part of the prob E. Since this focusing part is very light, there'll be is very little dynamical errors in the system, which'll enable High measurement speeds. Most of the vibrations that remain'll be measured and can thus is corrected for, which'll be further explained in the Dynamic analysis of the Section 6.5.

To the above metrology loop design, the beam delivery path and beam shielding from environmental disturbances would be adde D in the coming period. The spindle measurement method and metrology frame design would also be continued.

Design of a machine for the universal non-contact measurement of large free-form optics with (nm) uncertainty