To explore the potential of achieving low-stress and high-reflectance Mo/Si multilayers deposited by conventional magnetron sputtering with bias assistance, we investigated the effects of varying Ar gas pressure, substrate bias voltage and bias-assisted Si ratio on the stress and EUV reflectance of Mo/Si multilayers. To reduce the damage of ion bombardments on Si-on-Mo interface, only final part of Si layer was deposited with bias assistance. Bias voltage has strong influence on the stress. The compressive stress of Mo/Si multilayers can be reduced remarkably by increasing bias voltage due to the increase of Mo-on-Si interdiffusion and postponement of Mo crystallization transition. Properly choosing gas pressure and bias-assisted Si ratio is critical to obtain high EUV reflectance. Appropriately decreasing gas pressure can reduce the interface roughness without increasing interdiffusion. Too much bias assistance can seriously reduce the optical contrast between Mo and Si layers and lead to a remarkable decrease of EUV reflectance. Thus, by appropriately choosing gas pressure, bias voltage and bias-assisted Si ratio, the stress values of Mo/Si multilayers can be reduced to the order of -100 MPa with an EUV reflectance loss of about 1%.
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.
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.