Dongmei Ma

Ultra-high accuracy point diffraction interferometer: development, acccuracy evaluation and application

Phase-shifting Point Diffraction Interferometer (PSPDI) utilizing nearly perfect spherical wavefront diffracted by a pinhole as reference wavefront, which diminishes the influence of reference optics used in traditional interferometers, has been developed with high accuracy, repeatability and reproducibility. Accuracy of PSPDI is mainly limited by the quality of diffracted reference wavefront. We analyze the quality of diffracted reference wavefront by using of Rayleigh- Sommerfeld diffraction theory and performed FDTD numerical simulation. Based on analysis, we have developed a phase-shifting point diffraction interferometer. Ultra-precise pinhole alignment technical, high stable mount, high stable testing environment and error source insensitive data processing algorithm was used to achieve high stability and accuracy. Via accuracy evaluation, a deep sub-nanometer system error of developed PSPDI is obtained. A cross comparison of PSPDI measurement and measurement of another kind of interferometer was done, and the difference was 0.16nmRMS. The developed PSPDI has been applied in spherical mirror testing and EUV projection objective testing.

An in situ method for diagnosing phase shifting interferometry

Current diagnosing phase shifting interferometry is a time and funds consuming process. Hence a brief and effective method is necessary to satisfy the real-time testing. In this paper, mathematical solutions for errors were deduced from the difference of intensity patterns. Based on the diversity of error distributions, an effective method for distinguishing and diagnosing the error sources is proposed and verified by an elaborative designed simulation. In the actual comparison experiment, vibration, phase-shift error and intensity fluctuation were imposed to demonstrate this method. The results showed that this method can be applied into the real-time measurement and provide an in situ diagnosing technique.

Point diffraction interferometry based on the use of two pinholes

Point diffraction interferometer (PDI) has become the high degree of accuracy device. In the optical wavefront testing the measurement accuracy is much higher than 1.0 nm RMS. In the paper there is presented a new version of PDI with two independently controlled beams using a pinhole plate with two pinholes as a beam coupler instead of a single-mode fiber or single-pinhole plate. Theoretical analysis of the pinhole diffraction wavefront and double pinholes diffraction interference is given. The PDI is used to investigate an interferometer reference lens and compare measurement results. The device can test high NA, the interference is obtained in circularly polarized light, and fringe contrast is adjustable to measure surfaces with different reflectance. The measurement repeatability now has been sub-nm RMS (measured NA = 0.33). The experiment result provides guarantee for the measurement in the high degree of accuracy. In the double pinholes PDI, generating two ideal spherical waves through two pinholes, one wave is as the reference wavefront for interference test, another ideal wavefront is reflected to the pinhole plate by the test mirror, and the tested wavefront and reference wavefront bring interference. Advantages of such arrangement of the PDI are: high maximum numerical aperture (NA = 0.55), distinct fringe patterns of high contrast, high accuracy of surface figure testing and wave-front repeatability RMS error 0.3 nm.

Effect of the edge roughness of the pinhole in point diffraction interferometer on light diffraction

The quality of the reference wave front in point diffraction interferometer (PDI) is mainly determined by pinhole diameter, pinhole edge roughness and so on. The edge roughness of an actually electric beam etched pinhole is determined by least square fitting method. The Gaussian noise with zero means and σ root mean square (RMS) is added to a perfect pinhole to model the edge roughness pinhole. Based on Rayleigh-Sommegeld diffraction formula, the quality of the far field wave front diffracted by a rough edge pinhole is analyzed in detail. Pinhole edge roughness mainly causes trefoil and coma aberrations in diffracted wave front. For pinholes with diameters from 400 nm to 1000 nm, when the edge roughness σ are 0 nm, 15 nm and 30 nm, the RMS deviation of the diffracted wave fronts are in the order of 10 -8 λ, 10 -4 λ and 10 -3 λ, respectively. The results show that pinhole edge roughness has a significance infection on wave front errors, while it has little to do with the intensity distribution in the diffracted wave front. The edge roughness of the reality pinhole used in PDI is 2.37 nm, and the wave front errors of the wave front diffracted from this pinhole can reach 0.08 nm RMS.