We developed a nano-profiler to determine the shape from slope distribution data for highly accurate free-form surface shape measurement; for example, high-precision optical systems are required in the x-ray and semiconductor fields. An accuracy of ±0.2 µrad is required to achieve a shape measurement accuracy of 30 nm. The angle of the rotary stage is controlled by the rotary encoder, and the calibration curve of the rotary encoder is guaranteed by the national standard machine. However, the error associated with assembling the rotary encoder to the device is not included in the national standard machine's calibration data. Therefore, we propose a method that combines self-calibration with six encoder heads and calibration with a national standard machine. Using this method reduces the extent of calibration by the national standard machine and minimizes the influence of the assembly error. Furthermore, to verify whether the calibration in the proposed method is appropriate, a new encoder for evaluation was installed and evaluated. The results revealed that the influence of the assembly error was reduced to the minimum, and the difference in the calibration between the encoder for evaluation and the national standard machine was 0.027 µrad.

High-precision free-form surface mirrors are required for synchrotron radiation facilities in the scientific field and semiconductor lithography systems in the industrial field. Previously, we developed a nano-profiler with the goal of achieving a measurement accuracy of 30 nm. The nano-profiler scanned and measured the slope angle of the surface to be measured with laser light and calculated the shape from the angle information. By driving the optical head and surface to be measured with four rotation axes and one translation axis, the surface could be scanned while keeping the optical path length constant. Although the rotation axis was controlled by a high-precision rotary encoder, pitching and yawing errors occurred in the translation axis. In this study, we attempt to eliminate the error of pitching and yawing from the conventional measurement operation of four axes of rotation and one axis of translation to the drive of only four axes of rotation. If the translation drive is eliminated, the optical path length will not be constant, and the sensitivity of the light-receiving element will change. Therefore, we propose a new method to calibrate the sensitivity of the receiving element and perform a comparative measurement with the conventional measurement method. Comparing the measured shapes obtained by both measurement results, it was found that the shapes had a maximum peak to valley difference of 6.2 nm. Thus, the proposed novel measurement method allows a significant reduction in pitching and yawing errors.

No abstract

Unlike interferometers that obtain data from the entire sample, three-dimensional (3D) measuring instruments acquire data from multiple points. A nano-profiler measures slopes at different points on the surface to determine the sample shape, while a coordinate measuring machine (CMM) utilizes contact or non-contact height displacement probes for taking measurements. Therefore, the 3D measurement time increases if the area to be measured is wide. This leads to a reduced measurement accuracy owing to environmental changes. To reduce the 3D measurement time, a nano-profiler that rotates measured objects at a constant velocity and quickly scans each measurement point by following a spiral trajectory was designed. The developed nano-profiler measures angular distribution instead of height distribution. This makes it possible to reduce the drive error during rotation. Compared to an interferometer, the measurement time was reduced from 163 to 17 min with the nano-profiler and a 10 nm peak-to-valley difference was achieved. The slope measurement with spiral scanning was confirmed to be a quick and accurate shape measurement technique that can be used while manufacturing high-precision optical components.

Improving the optical performance and functions of mirrors and lenses requires manufacturing pattern shapes such as those of microlens arrays. In this context, various measuring devices have been developed to accurately measure the depth of shapes with a microscopic interferometer. However, with an interferometer, the number of pixels of a light-receiving element, such as a charge-coupled device, is limited; furthermore, when measuring a pattern shape, the measurement field of view becomes narrow. Thus, measuring a wide range of shapes is not possible. By contrast, if the device can arbitrarily increase the number of measurement points, such as in the case of a three-dimensional measuring machine, in principle, a pattern shape can be measured, while ensuring the measurement capability for a wide range of shapes; however, in this case, the measurement time increases. A nano-profiler previously developed by the authors can measure the shape and slope angle distribution of a surface with a laser beam. In this study, the measurement program was changed to realize continuous high-speed scanning at 1 kHz, and the number of measurement points was increased to verify the compatibility between fine pattern shape measurements and diverse shape measurements. The results measured by the nano-profiler were compared with those obtained using microscopic and Fizeau-type interferometers, which can measure the pattern and overall shapes, respectively. In comparison, the difference in pattern depth and overall shape was 2.1 and 2.9 nm PV, respectively.