Chunshui Jin

High-efficiency multilayer-coated ion-beam-etched blazed grating in the extreme-ultraviolet wavelength region

We describe a simple method to fabricate blazed gratings used in the extreme ultraviolet wavelength region. The method uses an argon and oxygen mixed-gas ion beam to directly etch the grating substrate through a rectangular profile photoresist grating mask. With this method the etched grating groove profile can be well controlled. An Mo/Si multilayer-coated specimen with a blaze angle of 1.9 degrees was fabricated and measured. At an incident angle of 10 degrees and a wavelength of 13.62 nm, the diffraction efficiency of the negative second order reaches 36.2%.

Broadband multilayer-coated normal incidence blazed grating with 10% diffraction efficiency through the 13-16 nm wavelength region

Diffraction gratings used in extreme UV are typically coated with periodic multilayer thin films. These coatings have a small bandwidth, thus leading to a narrow usable spectral region of multilayer gratings. Well-designed aperiodic multilayer coatings could provide high reflectivity over a much broader wavelength region, so they could broaden the usable spectral region of multilayer gratings. We designed and deposited an aperiodic Mo/Si multilayer coating onto a blazed grating substrate. At an incidence angle of 10°, the -2nd-order diffraction efficiency of the multilayer grating is ~10% through the wavelength range of 13-16 nm.

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.

Control of lateral thickness gradients of Mo–Si multilayer on curved substrates using genetic algorithm

An inversion method based on a genetic algorithm has been developed to control the lateral thickness gradients of a Mo-Si multilayer deposited on curved substrates by planar magnetron sputtering. At first, the sputtering distribution of the target is inversed from coating thickness profiles of flat substrates at different heights. Then, the speed profiles of substrates sweeping across the target are optimized according to the desired coating thickness profiles of the primary and secondary mirrors in a two-bounce projection system. The measured coating thickness profiles show that the non-compensable added figure error is below 0.1 nm rms, and the wavelength uniformity across each mirror surface is within ±0.2% P-V. The inversion method introduced here exhibits its convenience in obtaining the sputtering distribution of the target and efficiency in coating iterations during process development.

Influence of substrate temperatures on the properties of GdF 3 thin films with quarter-wave thickness in the ultraviolet region

High-quality coatings of fluoride materials are in extraordinary demand for use in deep ultraviolet (DUV) lithography. Gadolinium fluoride (GdF3) thin films were prepared by a thermal boat evaporation process at different substrate temperatures. GdF3 thin film was set at quarter-wave thickness (∼27  nm) with regard to their common use in DUV/vacuum ultraviolet optical stacks; these thin films may significantly differ in nanostructural properties at corresponding depositing temperatures, which would crucially influence the performance of the multilayers. The measurement and analysis of optical, structural, and mechanical properties of GdF3 thin films have been performed in a comprehensive characterization cycle. It was found that depositing GdF3 thin films at relative higher temperature would form a rather dense, smooth, homogeneous structure within this film thickness scale.

Low-stress and high-reflectance Mo/Si multilayers for EUVL by magnetron sputtering deposition with bias assistance

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%.

Low-stress and high-reflectance Mo/Si multilayers for extreme ultraviolet lithography by magnetron sputtering deposition with bias assistance.

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 a bias-assisted Si ratio on the stress and extreme ultraviolet (EUV) reflectance of Mo/Si multilayers. To reduce the damage of ion bombardments on an Si-on-Mo interface, only the final part of the 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 a 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 a 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%.

Control of lateral thickness gradients of EUV/soft x-ray multilayer on curved substrates

To meet the requirements of wavelength matching and figure preservation for EUV multilayer optics, study of precise control of the lateral thickness gradients of multilayer was performed. The distribution of the magnetron sputtering source was derived by fitting the coating thickness profiles of flat substrates sweeping across the source with constant velocity at different heights using genetic algorithm. Then, genetic algorithm was also used in finding the proper speed profiles for the desired thickness profiles. By the method mentioned above, extremely precise control of the lateral thickness gradients of multilayer on curved substrates was realized.

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.