The NANOMEFOS non-contact measurement machine for freeform optics
A separate metrology system measures the displacement of the probe relative to the product, in the plane of motion of the probe.This way most of the critical position errors of the motion system are cancelled from the metrology loop.To measure the displacement of the -axis rotor (1 in Fig. 22)relative to the metrology frame (2), an interferometry system is applied. Hereto the -axis rotor has been made reflective uch that the measurement beam (3) can be focused onto it by a lens (4).The beam layout is such that the measurement beam also reflects on a reference mirror (5), directly measuring the displacement between the -axis rotor and the reference mirror. A similar setup is employed in horizontal direction (6). Reaction forces of the probe servo system (7) on the -axis bearing, or pressure variations in the stage bearings, etc. will cause displacement of the probe. With this setup, the position errors of the whole motion system in r- and z-direction are measured at the -axis rotor, and can be compensated for in the data-processing. To determine the position of the spindle rotor (8) relative to the metrology frame, capacitive probes (9 in Fig. 22) measure the spindle error motion in the sensitive directions. With this metrology loop concept, all six sensitive directions of Fig. 6. are measured directly. When measuring a freeform surface, only the probe focusing mechanism is moving dynamically while measuring a circular track. The static and dynamic displacements of probe and product that occur during this measurement are recorded by the metrology system and can be compensated for in the (off-line) data-processing. Fig. 23 shows the final design of the metrology system, and its main components.
4.1. Interferometry system
The interferometry system measures the probe position, and comprises the laser, the R-stage optics and Z-stage optics of Fig. 23.The beam path is shown schematically in Fig. 24. The heterodyne interferometer beam comes in from the top. At the polarizing beam splitter (PBS). The out-of-plane polarization component passes straight into the pickup (PU) and the in-plane component is reflected towards the reference mirror (RM). After reflection it passes a quarter wave plate (QWP1) for the second time, such that the beam travels through the PBS and QWP2. The cylinder lens (CL) focuses the beam onto the centre line of the cylindrical mirror on the -axis. After reflection, the beam passes QWP2 for the second time, and thus joins the first beam towards the PU. This way, the displacement of the -axis is easured directly relative to the reference mirror, while it can translate as well as rotate. A similar setup is applied in vertical (z) direction, as already schematically depicted in Fig. 22. This setup allows the probe to be
oriented perpendicular to surfaces with large slopes, without the loss of accuracy described in Section 2.1. In Fig. 25 the beam layout is shown that supplies the interferometer laser to the 3 interferometer axes, around the structure of the R- and Z-stage and the -axis. Note that the assembly is viewed from the backside for clarity. The laser (1) is aligned to the R-stage,on which also the R-stage optics assembly (2) is mounted. When the R-stage is repositioned (and thus also the Z-stage and -axis),the R-stage optics assembly moves away from or towards the laser at a constant distance under the horizontal reference mirror (3). At the R-stage optics, part of the beam is split off and directed towards the Z-stage optics (4) via a periscope. This allows the Z-stage optics assembly to move vertically relative to the R-stage optics assembly. Behind the -axis rotor, a part of the beam is directed into the rotor towards the non-contact probe. The rest proceeds towards the R-interferometer.All components that require accurate alignment are mounted to elastic alignment mechanisms. These are in turn attached to separate sub-frames to isolate them from possible deformations in the motion system to which it is attached. The interferometry system assembly is shown in Fig. 26, with the -axis (1), R-interferometer (2) and Z-interferometer (3) and their respective optics (4 and 5).
