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XYPREC and XYGEPREC

In response to an increasing number of applications demanding unprecedented accuracy and precision for beam measurement, Photon Inc. offers two ultra-high precision BeamScan scan heads - models XYPREC and XYGEPREC. These models were developed after rigorous studies into improving and optimizing every tolerance involved in the manufacture of the BeamScan.

After a careful analysis of the available materials and their mechanical properties we concluded that aluminum, which is widely used in lasers and optics instruments, proved to be incapable of the precise machining and metal stability required for an ultra high precision instrument. Of the many materials we examined we found stainless steel proved to be the most appropriate material because of its mechanical stability. Therefore, the first modification was the use of stainless steel. Every component in the high precision head is precisely machined out of ASTM 303 stainless steel.

BeamScan XYPREC and XYGEPREC Scan Heads These precision instruments are for measuring very small beams, as small as a few microns (µm) in diameter. In this region, the ability to control the precise location of the measurement plane becomes critical. Because the depth of focus of a diffraction-limited beam is roughly the beam diameter squared, a 6-µm beam, for example, will have a depth of focus of only 36 µm. Consequently, if the measurement plane is not precisely known, the measurement may not be at the actual waist of the beam, and thus lead to measurement error.

Photon conducted a comprehensive examination of the optimum way to improve the precise location of the measurement plane and the level of confidence in that location. This task was additionally complex because six different mechanical tolerances contributed to the measurement: axial distance to the slit; axial difference between the X and the Y slit; motor jitter; perpendicular positioning of the rotating hub and the precise mounting of the slits. To achieve the highest possible precision, these tolerances had to be reduced to an absolute minimum.

The dimensions that are measured and controlled are as follows:

• Stainless steel drum is hand lapped until the X and the Y slit planes are flat, parallel and symmetric about the central axis of the hub to better than 2.5 µm.
• Slit selection and installation is precisely controlled. We start with two matched slits and we mount the slits so as to have a negligible effect on the measurement plane.
• Motor mount parallelism to datum plane.
• Motor barrel to motor mount interface.
• Motor shaft centering and parallelism to the motor barrel.
• Motor shaft to rotating drum interface.

The result is a scan head where the measurement plane is controlled to an absolute dimension of better than 7 µm or optionally 3.8 µm. This instrument is then measured on a coordinate measuring machine (CMM) where the actual distance from the measurement plane to the datum is measured and reported. The CMM has an absolute accuracy of 2.5 µm. Therefore, the measurement plane is known to within ±5 µm or optionally ±3 µm.

The Problem:
A scan head with uncontrolled tolerances can produce angular errors. Over the 3.5 mm slit height, the maximum error could be 50 µm between Datum 1 and Datum 2.