Chang

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.

Tunable coherent radiation in the soft X-ray and extreme ultraviolet spectral regions

Undulator radiation, generated by relativistic electrons traversing a periodic magnet structure, can provide a continuously tunable source of very bright and partially coherent radiation in the extreme ultraviolet (EUV), soft X-ray (SXR), and X-ray regions of the electromagnetic spectrum. Typically, 1-10 W are radiated within a 1/N relative spectral bandwidth, where N is of order 100. Monochromators are frequently used to narrow the spectral bandwidth and increase the longitudinal coherence length, albeit with a more than proportionate loss of power. Pinhole spatial filtering is employed to provide spatially coherent radiation at a power level determined by the wavelength, electron beam, and undulator parameters. In this paper, experiments are described in which broadly tunable, spatially coherent power is generated at EUV and soft X-ray wavelengths extending from about 3 to 16 nm (80-430-eV photon energies). Spatially coherent power of order 10 /spl mu/W is achieved in a relative spectral bandwidth of 9/spl times/10/sup -4/, with 1.90-GeV electrons traversing an 8-cm period undulator of 55 periods. This radiation has been used in 13.4-nm interferometric tests that achieve an rms wavefront error (departure from sphericity) of /spl lambda//sub euv//330. These techniques scale in a straightforward manner to shorter soft X-ray wavelengths using 4-5-cm period undulators at 1.90 GeV and to X-ray wavelengths of order 0.1 nm using higher energy (6-8 GeV) electron beams at other facilities.

Single-element objective lens for soft x-ray differential interference contrast microscopy

High-resolution soft x-ray differential interference contrast (DIC) imaging was demonstrated through the use of a single-element objective, the XOR pattern, in a full-field soft x-ray microscope. DIC images of the magnetic domains in a 59 nm thick amorphous Gd25Fe75 layer were obtained and magnetic phase contributions were directly imaged. With its elemental, chemical, and magnetic specificity, compatibility with various sample environments, and ease of implementation, we expect this soft x-ray DIC technique to become one of the standard modes of operation for existing full-field soft x-ray microscopes.

Direct measurement of index of refraction in the extreme-ultraviolet wavelength region with a novel interferometer.

To the best of our knowledge, the first direct measurement of the dispersive part of the refractive index is performed at extreme-ultraviolet (EUV) wavelengths, where absorption is higher as compared with hard-x-ray and visible wavelengths. A novel diffractive optical element that combines the functions of a grating and a zone plate is fabricated with Fourier optical techniques and employed here for the first time at EUV/soft-x-ray wavelengths. Both the real and the imaginary parts of the complex refractive indices are measured directly by this technique without recourse to Kramers-Kronig transformations. Data for Al and Ni in the vicinity of their L and M edges, respectively, are presented as first examples of this technique.

Diffractive optical elements based on Fourier optical techniques: a new class of optics for extreme ultraviolet and soft x-ray wavelengths

A diffractive optical element, based on Fourier optics techniques, for use in extreme ultraviolet/soft x-ray experiments has been fabricated and demonstrated. This diffractive optical element, when illuminated by a uniform plane wave, will produce two symmetric off-axis first-order foci suitable for interferometric experiments. The efficiency of this optical element and its use in direct interferometric determination of optical constants are also discussed. Its use in direct interferometric determination of optical constants is also referenced. Its use opens a new era in the use of sophisticated optical techniques at short wavelengths.

Analysis of illumination coherence properties in small-source systems such as synchrotrons

Modern synchrotron beamlines often take the form of critical illumination systems, where an incohrent source of limited spatial extent is re-imaged to an experimental plane of interest. Unique constraints of synchrotron sources and beamline, however, may preclude the use of the simple Zernike approximation for calculating the object-image coherence relationship. Here, we perform a rigorous analysis of the object-image coherence relationship valid for synhrotron beamlines. The analysis shows that beamline aberrations have an effect on the coherence properties. Effects of various low-order aberrations on the coherence properties are explicitly studied.

The EUV Phase-Shifting Point Diffraction Interferometer

The extreme ultraviolet (EUV) phase-shifting point diffraction interferometer (PS/PDI) was developed and implemented at Lawrence Berkeley National Laboratory to meet the significant measurement challenge of characterizing EUV projection lithography optics. The PS/PDI has been in continuous use and under ongoing development since 1996. Here we describe recent improvements made to the interferometer, and we summarize metrology results from state-of-the-art 10×-reduction EUV projection optics.

Direct index of refraction measurements at extreme-ultraviolet and soft-x-ray wavelengths

Coherent radiation from undulator beamlines has been used to directly measure the real and imaginary parts of the index of refraction of several materials at both extreme-ultraviolet and soft-x-ray wavelengths. Using the XOR interferometer, we measure the refractive indices of silicon and ruthenium, essential materials for extreme-ultraviolet lithography. Both materials are tested at wavelength (13.4 nm) and across silicons L2 (99.8 eV) and L3 (99.2 eV) absorption edges. We further extend this direct phase measurement method into the soft-x-ray region, where measurements of chromium and vanadium are performed around their L3 absorption edges at 574.1 and 512.1 eV, respectively. These are the first direct measurements, to our knowledge, of the real part of the index of refraction made in the soft-x-ray region.

Direct index of refraction measurement of silicon and ruthenium at EUV wavelengths

The use of coherent radiation from undulator beamlines has been used to directly measure the real and imaginary parts of the index of refraction of several metals1. Here we extend the same interferometric technique to slightly higher energies, and measure the indices of refraction of silicon and ruthenium, essential materials for extreme ultraviolet (EUV) lithography. Both materials are tested at-wavelength (13.4 nm.) Silicon is also measured about its L2 (99.8 eV) and L3 (99.2 eV) absorption edges. This measurement technique is currently being expanded further to soft X-ray wavelengths.

Grain orientation mapping of passivated aluminum interconnect lines with X-ray micro-diffraction

A micro x-ray diffraction facility is under development at the Advanced Light Source. Spot sizes are typically about 1-μm size generated by means of grazing incidence Kirkpatrick-Baez focusing mirrors. Photon energy is either white of energy range 6–14 keV or monochromatic generated from a pair of channel cut crystals. Laue diffraction pattern from a single grain in a passivated 2-μm wide bamboo structured Aluminum interconnect line has been recorded. Acquisition times are of the order of seconds. The Laue pattern has allowed the determination of the crystallographic orientation of individual grains along the line length. The experimental and analysis procedure used is described, as is the latest grain orientation result. The impact of x-ray micro-diffraction and its possible future direction are discussed in the context of other developments in the area of electromigration, and other technological problems.