At-wavelength interferometry for extreme ultraviolet lithography

The phase-shifting point diffraction interferometer (PS/PDI) has recently been developed to address the problem of at-wavelength metrology of extreme ultraviolet (EUV) optical systems. Although extremely accurate, the fact that the PS/PDI is limited to use with coherent EUV sources, such as undulator radiation, is a drawback for its widespread use. An alternative to the PS/PDI, with relaxed coherence requirements, is lateral shearing interferometry (LSI). Here we describe various LSI implementations and demonstrate the use of a cross-grating, carrier-frequency configuration to characterize a large-field 4×-reduction EUV lithography optic. The results obtained are directly compared with PS/PDI measurements.

The phase-shifting point diffraction interferometer (PS/PDI) is so far the most accurate measurement tool in atwavelength interferometry of projection optics for extreme ultraviolet lithography (EUVL). The complicate interrelationships between configuration parameters of PS/PDI call for an optimization to achieve high accuracy of PS/PDI. In this paper, a novel system-level modeling approach is proposed to optimize the parameters of PS/PDI designed for visible light (λ=632.8nm) concept proof experiment. The optimal reference pinhole size selection is performed by modeling pinhole spatial filtering effect using Diffraction-Based Beam Propagation (BPR) module of CODE V and in house software. The result shows that various pinhole diameters ranging from 1.6um to 2.2um should be used in our PS/PDI experiment. The test window size and grating duty cycle optimization, which is based on the spatial frequency domain analysis of PS/PDI, is conducted by modeling the entire PS/PDI system using Physical Optics Propagation (POP) module of Zemax and in house software. The optimal window size is approximately 62um for a given window-pinhole separation of 63.3um. The optimal duty cycle of grating is calculated to be 83% to obtain the maximum fringe contrast of 0.879.

At-wavelength interferometric characterization of a new 4×-reduction lithographic-quality extreme ultraviolet (EUV) optical system is described. This state-of-the-art projection optic was fabricated for installation in the EUV lithography Engineering Test Stand (ETS) and is referred to as the ETS Set-2 optic. EUV characterization of the Set-2 optic is performed using the EUV phase-shifting point diffraction interferometer (PS/PDI) installed on an undulator beamline at Lawrence Berkeley National Laboratory’s Advanced Light Source. This is the same interferometer previously used for the at-wavelength characterization and alignment of the ETS Set-1 optic. In addition to the PS/PDI-based full-field wave front characterization, we also present wave front measurements performed with lateral shearing interferometry, the chromatic dependence of the wave front error, and the system-level pupil-dependent spectral-bandpass characteristics of the optic; the latter two properties are only measurable using at-wavelength interferometry.

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.

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.

Purpose: The aim of this study was to evaluate the distribution of wavefront aberrations in Indian children with refractive errors and to determine the correlation between the degree of refractive error and aberration. Materials and Methods: A prospective study was conducted on 216 eyes of 108 consecutive children attending the outpatient clinic. Based on refractive error, the children were allocated to five groups: “Emmetropic,” “hyperopic,” “simple astigmatism,” “compound astigmatism,” and “myopic” group. Lower order aberrations (LOAs) and higher order aberrations (HOAs) were analyzed within and between the groups. Results: Mean age of the children was 10.9 years. Of the total 216 eyes, 40 eyes had emmetropia, 52 had simple astigmatism, 54 had compound astigmatism, 43 had myopia, and 27 had hyperopia. In all the groups, increase in spherical equivalent was associated with increase in LOAs of defocus and astigmatism and the root mean square (RMS) of total HOAs. In comparison with the “emmetropic” group, all other groups showed a significant increase in LOAs (P < 0.001) and RMS of total HOAs (P < 0.001), with the “compound astigmatism” group showing higher values. Spherical aberrations were found to be associated with simple astigmatism, compound astigmatism (P = 0.008), and myopia (P = 0.041). Statistical analysis was done using analysis of variance applying Bonferroni correction factor and Pearson's correlation coefficient. Conclusions: Ametropic eyes are associated with HOAs. Indian children were found to have higher values of aberrations when compared to other races. Practitioners may need to factor this while treating children for refractive errors.

Objective To study the impact on human visual function after viewing 2D/3D television; to explore the corresponding relationship between visual fatigue and human wavefront aberrations. Methods Thirty healthy adults were recruited for this research study. They watched the same film on 2D and 3D television but they wore two types of polarized 3D glasses for the 3D task-type A and type B (the optical index of type A was better than type B). Wavefront aberrations of the subjects was recorded before and after the visual task, included the modulation transfer function (MTF), root mean square (RMS) of higher order aberrations amplitude, etc. A single factor analysis of variance was used to compare values between the groups. Results The MTF variation was maximized when watching 3D television with type B glasses (3D-B), so the MTF tended to diminish after watching the film. The differences between 2D, 3D-A and 3D-B condition were significant (P<0.05). In the three test conditions, there were significant differences in RMS3 and RMS4 comparisons between 3D-A and 3D-B condition (F=0.293, 0.044, P<0.05). Vertical coma variations under the 2D/3D conditions were negative, and there were significant differences between 2D, 3D-A and 3D-B condition (F=0.025, P<0.05). Conclusion The stimulation of 2D/3D film can lead to changes in visual quality, which improves the index change in wavefront aberrations. This can be used in the future for evaluating visual comfort when viewing 3D television. Key words: Wave-front aberration; Visual quality; Visual fatigue

Force and vibration generated in apical direction by three endodontic files of different kinematics during simulated canal preparation: An in vitro analytical study.

The performance of the recently developed EUV phase-shifting point diffraction interferometer (PS/PDI) depends heavily on the characteristics of the grating beamsplitter used in the implementation. Ideally, such a grating should provide throughput of better than 25% and diffraction efficiency, defined as the ratio of the first-diffracted-order power to the zero-order power, variable in the range from approximately 10 to 500. The optimal method for achieving these goals is by way of a phase grating. Also, PS/PDI system implementation issues favor the use of transmission gratings over reflection gratings. Here, the design, fabrication, and characterization of a recently developed transmission phase grating developed for use in EUV interferometry is described.

Three-Dimensional Quantitative Comparative Analysis of Trapezial-Metacarpal Joint Surface Curvatures in Human Populations

The aim of the present research is to calculate the Relative Accuracy (R.A) and Root Mean Square Error (RMSE) for the location of Technical Institute, Mosul in Nineveh Governorate, Iraq. at the northern part of Mosul city by using surveying Instruments (GPS 1230 , Total Station 06 & Level NAK2), to calculate the coordinates and elevations of (16) Ground Control Points which covered the study area. The field data, remote sensing techniques and GIS software (Arc Map GIS10.3) are used to check the digital map of study area that helped the administrator to take the correct decision, determining positions of the future expansionist for buildings, roads, measuring lengths, areas and there coordinates, directions as well as the total costs estimation needed for any development from the final map. The gradient percentage between the highest and lowest points is calculated to be (9.954%). Relative accuracy for the based measured points (R.A) = 1:60000), and Root Mean Square error in Easting coordinates (RMSE) = ± 0.102m, in Northing coordinates (RMSN) = ± 0.096m, the Root Mean Square for the resultant (RMSR) = ± 0.099m, and Root Mean Square for Elevations ( RMSRL)= ± 0.005m. We Matched and correlated (10) measured points from study area with their graphic data positions on the aerial photo, determining the offsets for position and elevations as follows: (RMSE = ± 0.076m), (RMSN = ± 0.125m), (RMSR = ± 0.088m), and (Root Mean Square for Elevations = RMSRL= ± 0.045m). This research enable us after checking the results to produce new maps from Aerial photograph or Digital Image to use it for land uses and future extensions in whole study area.

Radar antennas in the open air are affected by dynamic wind loading during azimuthal scanning. In this state, the acquisition and prediction of the aerodynamic load is the focus of the structure design for the antenna. We performed wind tunnel force tests on flat plate antennas with different aspect ratios. The changes in mean, maximum, and root mean square values of the aerodynamic coefficients relative to the antenna aspect ratio and reduced frequency are given for the first time. The effects of the antenna aspect ratio and reduced frequency on aerodynamic characteristics are analyzed. The experimental results indicate that the magnitude of the azimuthal moment is related to the antenna aspect ratio and the reduced frequency. It is worth noting that the antenna aspect ratio has a significant impact on the mean, maximum, and root mean square values of dynamic aerodynamic coefficients. The mean, maximum, and root mean square values of the rolling moment coefficient, azimuth moment coefficient, and pitching moment coefficient monotonously decrease with the increase in aspect ratio. In addition, the influence of reduced frequency on the dynamic aerodynamic coefficient is closely related to the antenna aspect ratio. Under the same antenna aspect ratio, increasing the reduced frequency will increase the mean, maximum, and root mean square values of the aerodynamic coefficient. For flat plate antennas with different aspect ratios, at the condition of increasing the aspect ratios, the mean, maximum, and root mean square values of aerodynamic coefficients do not change significantly with the change of reduced frequency.

In order to measure the instantaneous wavefront of large aperture optical elements, a method based on the structure of oblique incidence of reflective shearing point diffraction interferometer is proposed. A lateral displacement between the reference wavefront and the test wavefront is formed after passing this structure. The shear of two beams introduces linear spatial carrier frequency to the point diffraction interferogram. After receiving a good contrast interferogram, wavefront phase is retrieved by Fourier transform (FT) automatically to realize the dynamic measurement of instantaneous wavefront. The optical path is up to 20 m, so the air current is a significance factor to the result. Besides, because of the air current, the system itself can be seen as a instantaneous wavefront happening and measurement of large aperture optical elements. The results indicate that the root mean square value is in accord with that acquired by SID4 wavefront sensor (less than 1/50λ), so about the repeated accuracy. The method proposed can be applied in high resolution and accuracy measurement of instantaneous wavefront.

Soil Roughness Measuring System Combined With Image Processing

In order to improve the measuring accuracy of the visible light phase-shifting point diffraction interferometer (PS/PDI) for the extreme ultraviolet lithography (EUVL) aspheric mirrors, the main measuring errors will be discussed in this paper. At first, the elementary configuration and measuring principle of the visible light phase-shifting point diffraction interferometer are introduced briefly, then the different errors which are possible to affect the measuring result are summed up, the errors include PZT phase-shifting error, detector nonlinearity error, detector quantization error, wavelength instability error and intensity instability error of the laser source, vibration error, air refractivity instability error and so on. Through detailed analysis and simulation, the magnitude of these errors can be obtained. By analysing the reasons which cause these errors and the relationship between these errors and interferometer configuration parameters, some methods are put forward to avoid or restrain these errors accordingly.