MFA-400

MFA-400™

Mid-spatial Frequency Metrology

Low to mid-spatial frequencies are a critical quality to account for in the production of ultrahigh precision optics, including the optics used in lithography systems. Unacceptable levels of mid-spatial frequencies will produce undesirable flaring in the final system image. Traditionally however, measuring these frequencies, particularly on an aspheric surface, has been challenging both in terms of achieving high accuracy results, but also measuring the surface quickly enough to meet the turn-around times required in a production environment.

To help meet the needs of one of our lithography customers, QED developed the MFA-400™ metrology system to solve this challenging problem. The MFA-400™ is capable of measuring low-to-mid spatial frequencies (0.33 to 5 cycles per mm) on steep aspheres, bridging the spatial frequency gap between figure measuring interferometry and interference microscopes.

Based on the subaperture stitching technology developed for the SSI, the MFA-400™ combines an optimized interferometer with specially designed transmission spheres, integrated onto a stitching platform (based again on the SSI.) As with QED's other metrology products, the MFA-400™ software fully automates the measurement process, and ensures high accuracy measurements with automatic calibration of system errors. Unique to the MFA-400™, its software features the ability to generate power spectral density (PSD) maps. PSD maps can be automatically calculated and displayed using the data specified.

The MFA-400™ can accommodate spherical to very steep aspheric optics up to 400mm in diameter with unprecedented low levels of uncertainty.

MFA-400 Performance: Reproducibility
In a paper¹ presented at OSA's Optical Fabrication and Testing conference in Rochester, NY in October 2006, our customer reported on the results achieved using the MFA-400™. In the example below, the MFA-400™ demonstrated the ability to achieve reproducibility of (2σ) 0.09 nm RMS for steep aspheres, surpassing the target of 0.2 nm. An aspheric part with a base radius of curvature of 357 mm was measured with f/12.3 diverger TS and 3 (5 phase average for each) data sets were gathered for the same part. In the example on the left, both the top and bottom results are stitched data sets, taken separately.

MFA-400 Performance: Cross-test
Cross tests were then conducted to estimate uncertainty. As shown in the bottom example, the same aspheric part was evaluated by comparing the stitched results generated by using 2 different transmission spheres (f/14.9 and f/28.3). Both TS reference surfaces were calibrated by modified random averaging method. The cross test difference was found to be 0.18 nm RMS for steep asphere, exceeding the desired target accuracy of 0.2 nm.

How it works 
The MFA-400™ operates using the same platform and concept as the SSI®. That is, it measures many subapertures of the surface and stitches the results together into a lattice to synthesize a full-aperture map. An optimized interferometer with specially designed transmissions sphere is combined with QED's automated 6-axis computer-controlled workstation. The MFA-400™ is self calibrating for errors of motion, geometric distribution and reference wave-and automatically generates the lattice of subapertures needed for full coverage of the aspheric surface.

The lens to be measured is placed on the interferometer. An arbitrary set of regions, each 10 mm in diameter is determined. For each of these regions, a subaperture lattice map is automatically generated and the fringes are nulled. QED's powerful stitching engine then generates and compiles the data for each of the regions and 3D phase maps are produced.

More information
Please contact us for a copy of this technical paper and/or for more information on using stitching metrology to measure mid-spatial frequency.