Real space soft x-ray imaging at 10 nm spatial resolution.

Using Fresnel zone plates made with our robust nanofabrication processes, we have successfully achieved 10 nm spatial resolution with soft x-ray microscopy. The result, obtained with both a conventional full-field and scanning soft x-ray microscope, marks a significant step forward in extending the microscopy to truly nanoscale studies.

Microscopy of extreme ultraviolet lithography masks with 13.2 nm tabletop laser illumination.

We report the demonstration of a reflection microscope that operates at 13.2 nm wavelength with a spatial resolution of 55+/-3 nm. The microscope uses illumination from a tabletop extreme ultraviolet laser to acquire aerial images of photolithography masks with a 20 s exposure time. The modulation transfer function of the optical system was characterized.

Extreme-ultraviolet lensless Fourier-transform holography.

We demonstrate 100-nm-resolution holographic aerial image monitoring based on lensless Fourier-transform holography at extreme-UV (EUV) wavelengths, using synchrotron-based illumination. This method can be used to monitor the coherent imaging performance of EUV lithographic optical systems. The system has been implemented in the EUV phase-shifting point-diffraction interferometer recently developed at Lawrence Berkeley National Laboratory. Here we introduce the idea of the holographic aerial image-recording technique and present imaging performance characterization results for a 10x Schwarzschild objective, a prototype EUV lithographic optic. The results are compared with simulations, and good agreement is obtained. Various object patterns, including phase-shift-enhanced patterns, have been studied. Finally, the application of the holographic aerial image-recording technique to EUV multilayer mask-blank defect characterization is discussed.

Tolerancing of diffraction-limited Kirkpatrick-Baez synchrotron beamline optics for extreme-ultraviolet metrology.

The recent interest in extreme-ultraviolet (EUV) lithography has led to the development of an array of at-wavelength metrologies implemented on synchrotron beamlines. These beamlines commonly use Kirkpatrick-Baez (K-B) systems consisting of two perpendicular, elliptically bent mirrors in series. To achieve high-efficiency focusing into a small spot, unprecedented fabrication and assembly tolerance is required of these systems. Here we present a detailed error-budget analysis and develop a set of specifications for diffraction-limited performance for the K-B optic operating on the EUV interferometry beamline at Lawrence Berkeley National Laboratory's Advanced Light Source. The specifications are based on code v modeling tools developed explicitly for these optical systems. Although developed for one particular system, the alignment sensitivities presented here are relevant to K-B system designs in general.

Transmission phase gratings for EUV interferometry.

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.

At-wavelength, system-level flare characterization of extreme-ultraviolet optical systems.

The extreme-ultraviolet (EUV) phase-shifting point-diffraction interferometer (PS/PDI) has recently been developed to provide high-accuracy wave-front characterization critical to the development of EUV lithography systems. Here we describe an enhanced implementation of the PS/PDI that significantly extends its measurement bandwidth. The enhanced PS/PDI is capable of simultaneously characterizing both wave front and flare. PS/PDI-based flare characterization of two recently fabricated EUV 10x-reduction lithographic optical systems is presented.

Phase-shifting point-diffraction interferometry at 193 nm.

Phase-shifting point-diffraction interferometry at the 193-nm wavelength suitable for highly accurate measurement of wave-front aberration is introduced. The interferometer preserves the advantages of the previously described extreme-ultraviolet phase-shifting point-diffraction interferometer but offers higher relative efficiency. Wave-front measurement of an imaging system, operating at the 193-nm wavelength, is reported. Direct measurement of the refractive-index change in a deep-ultraviolet radiation-damaged fused-silica sample is also presented as an application.

Dual-domain point diffraction interferometer.

The phase-shifting point diffraction interferometer has recently been developed and implemented at Lawrence Berkeley National Laboratory to meet the significant metrology challenge of characterizing extreme ultraviolet projection lithography systems. Here we present a refined version of this interferometer that overcomes the original design's susceptibility to noise attributed to scattered light. The theory of the new hybrid spatial- and temporal-domain (dual-domain) point diffraction interferometer is described in detail and experimental results are presented.

Extreme-ultraviolet phase-shifting point-diffraction interferometer: a wave-front metrology tool with subangstrom reference-wave accuracy.

The phase-shifting point-diffraction interferometer (PS/PDI) was recently developed and implemented at Lawrence Berkeley National Laboratory to characterize extreme-ultraviolet (EUV) projection optical systems for lithography. Here we quantitatively characterize the accuracy and precision of the PS/PDI. Experimental measurements are compared with theoretical results. Two major classes of errors affect the accuracy of the interferometer: systematic effects arising from measurement geometry and systematic and random errors due to an imperfect reference wave. To characterize these effects, and hence to calibrate the interferometer, a null test is used. This null test also serves as a measure of the accuracy of the interferometer. We show the EUV PS/PDI, as currently implemented, to have a systematic error-limited reference-wave accuracy of 0.0028 waves (lambda/357 or 0.038 nm at lambda = 13.5 nm) within a numerical aperture of 0.082.

Ensemble-averaged Shack-Hartmann wave-front sensing for imaging through turbid media.

A technique is described for ensemble-averaging the light wave emerging from a turbid medium, enabling the recovery of optical information that is otherwise lost in a speckle pattern. The technique recovers both an amplitude and a phase function for a wave that has been corrupted by severe scattering, without the use of holography. With the phase estimated, an ensemble-averaged field is constructed that can be backprojected to form an image of the object obscured by the scattering medium. Experimental results suggest that the technique can resolve two object points whose signals are unresolved on the exit surface of a diffuser.

Time-gated ensemble-averaged imaging through highly scattering media.

A previously described ensemble-averaged imaging method [Opt. Lett. 21, 1691 (1996)] is extended by its combination with holographically implemented time-gated imaging. This combined method is shown to extend the effectiveness of the ensemble-averaged method by permitting imaging through thicker diffusers. Experimental results are presented.

Analysis of the confined-reference coherence-encoding method for image transmission through optical fibers.

Performance of the confined-reference coherence-encoding method for two- and three-dimensional complex image-data transmission through optical fibers is analyzed. Both acousto-optic and phase-only spatial light modulator implementations are considered. The signal-to-noise ratio, data rate, and probability of bit errors are issues that are discussed. Simulation results are presented.

Direct three-dimensional image transmission through single-mode fibers with monochromatic light.

Direct three-dimensional image transmission through one single-mode or multimode fiber is demonstrated. Image transmission is carried out with a grating interferometer under monochromatic, spatially incoherent illumination.

Ensemble-averaged imaging through highly scattering media.

A method for imaging through highly scattering media is described that consists of forming a multiplicity of holograms and performing an extensive averaging process. This process produces an estimate of the phase distribution across the exiting surface of the medium. This information is combined with the available magnitude data to form an ensemble-averaged wave front that can be backprojected to form an image of absorbers within or behind the scattering medium.

Resolution-enhanced detection of moving objects embedded within scattering media using time-gated speckle methods.

The holographic first-arriving-light method in combination with the speckle differencing method is used to provide resolution-enhanced detection of moving objects embedded in scattering media. Results show that the first-arriving-light technique provides significant resolution improvements over standard speckle differencing.

Noise analysis for the holographic first-arriving-light technique.

A noise analysis is performed on an electronic holography first-arriving-light system. Analytical expressions for the signal-to-noise ratio caused by the dominant noise terms are derived. The effect of various system parameters on the signal-to-noise ratio is explored; numerical and experimental examples are given.

Analysis of direct three-dimensional image transmission through optical fibers by the use of coherence methods.

The use of a grating interferometer under spatially incoherent illumination for direct three-dimensional image transmission through optical fibers is analyzed. The issues of resolution, image depth, and signal-to-noise ratio are addressed. Experimental results are presented.

Motion-resolved imaging of moving objects embedded within scattering media by the use of time-gated speckle analysis.

Spectral analysis of a time-evolving speckle pattern is used to provide motion-resolved detection of moving objects embedded within scattering media. Results show that the ability to detect small nonstationary scattering objects and to discriminate between objects moving at different rates is greatly enhanced.

Detection of moving objects embedded within scattering media by use of speckle methods.

Speckle-pattern subtraction methods are used for the detection of moving objects embedded in scattering media. Results show that the ability to detect small nonstationary objects is greatly enhanced.