Interferometer measures both sides of disk

An interferometer designed for memory-disk metrology evaluates surface parallelism and flatness of both sides of the disk in sequence, without any operator activity. Developed by engineers at Wyko (Tucson, AZ), the system consists of an automatic disk-handling system and a dual-path Fizeau interferometer. Data acquisition and analysis for a double-sided disk requires only 5 s.

Current metrology systems require the operator to manually insert and align the disk, measure one side, remove the disk, and perform the whole sequence over again to evaluate the other side of the disk. In the Wyko system, metrology takes place on both sides of the disk in situ. The operator loads a cassette of disks into the unit, and the automatic handler lifts a disk and places it onto the disk hub, a mount based on an optical flat that permits metrology of both sides of the system and avoids mechanical distortions that would be introduced by edge mounting. The mounting flat ensures gross disk alignment. If local tilts in the disk produce too high a fringe density in isolated regions of the disk, an automatic alignment feature adjusts disk hub tip and tilt to provide the best possible fringe density across the disk before a second measurement.

An 80-mW diode-pumped Nd:YAG laser operating CW at 1064 nm provides illumination. A beamsplitter sends the beam down two separate orthogonal optical paths, one for the top side of the disk, one for the bottom. Turning mirrors at 45° bring the beams collinear again, after which they pass through collimating optics. The system is configured as a phase-shifting Fizeau interferometer.

The disk itself is sandwiched between two reference flats, one for each optical path. Piezoelectric transducers on the flats apply a phase shift to the measurement wavefront; phase shifts can be introduced into the two beam paths independently. The system measures the two surfaces of the disk in sequence, blocking the beam for the nonactive side during data acquisition. A 736 × 480-pixel CCD camera captures the fringe pattern, which is digitized and analyzed by computer.

In addition to surface quality, the system can evaluate absolute thickness variation across the disk and wedge. The system can characterize disks as large as 130 mm, with a maximum surface slope of 0.6 µm/mm and maximum wedge of 0.32 µm/mm. Throughput per hour runs up to 380 disks.