Xenon capillary discharge extreme-ultraviolet source emitting over a large angular range.

We describe a capillary discharge source configuration, allowing for collection of extreme-ultraviolet (EUV) radiation at large off-axis angles, without the need for an EUV window. Operating with xenon gas, the source emits intensely within the EUV spectral region at 11.3 and 13.5 nm. When coupled with a high-collection-efficiency optical system, this source may be suitable for a number of high-average-power EUV imaging applications.

High-Temperature Lithium Metal-Vapor Capillary Discharge Extreme-Ultraviolet Source at 13.5 nm.

We describe a high-temperature lithium extreme-ultraviolet (EUV) source based on a capillary discharge configuration that was developed for operating with metal vapors. The source produces narrow-band emission at 13.5 nm in the EUV spectral region, with emission intensity proportional to the lithium-vapor density. At an operating temperature of 725 degrees C, our measurements showed that, on axis, the source generated approximately 0.2 (mJ/2pi sr)/pulse at 13.5 nm.

Intense xenon capillary discharge extreme-ultraviolet source in the 10-16-nm-wavelength region.

We have observed intense extreme-ultraviolet emission, within the 10-16-nm-wavelength range, emitted by a xenon capillary discharge plasma. Within a 0.3-nm bandwidth centered at 13.5 nm the axial emission intensity was comparable with that from the brightest laser-produced plasma sources, and a similar intensity was measured at approximately 11.3 nm. This source could thus be suitable for extreme-ultraviolet imaging applications, such as extreme-ultraviolet lithography.

Intense plasma discharge source at 13.5 nm for extreme-ultraviolet lithography.

We measured an emission of 6 mJ/pulse at 13.5 nm produced by the Li(2+) Lyman-? transition excited by a fast capillary discharge, using a lithium hydride capillary. 75% of the energy emanated from a spot size of 0.6 mm. The emission is narrow band and would thus be useful in extreme-ultraviolet lithography imaging systems that use Mo:Si multilayer mirrors. The output within the bandwidth of Mo:Si mirrors was comparable with that of a laser-produced plasma (LPP), and the wallplug efficiency of 0.1% was nearly an order of magnitude better than that of a LPP.

Production of spectrally narrow soft-x-ray radiation through the use of broadband laser-produced plasma sources and multilayer-coated reflecting optics.

We describe a special filter design that produces spectrally narrow soft-x-ray radiation by using a broadband laser-produced plasma source and multilayer-coated reflecting optics. Calculations for the design were carried out at several laser-produced plasma-source temperatures and various multilayermirror combinations with and without a soft-x-ray filter. We determined that the best arrangement for a laser-produced plasma source consists of two multilayer mirrors and one soft-x-ray filter for each temperature investigated.

Velocity characterization of particulate debris from laser-produced plasmas used for extreme-ultraviolet lithography.

Debris from laser-produced plasmas created with solid Sn and Au targets has been characterized according to speed and particulate size. Conditions for the experiments were those appropriate for producing an optimum laser-produced plasma emission at 13.5 nm for use in extreme-ultraviolet lithography. Results in the form of histogram data show that the speed distribution of the debris particulates is quite varied and in general exhibits an upper limit of ~640 m/s. In the case of Sn a peak in the velocity distribution is observed near 300 m/s. Small particulates, of the order of 1 µm or less, constitute the majority of the particulate emission in both materials. The implications for debris reduction based on the measurements are also discussed.

Characterization and control of laser plasma flux parameters for soft-x-ray projection lithography.

Laser plasmas are intrinsically an attractive soft-x-ray source for projection lithography. Compact, flexible, and small enough to be dedicated to a single installation, they offer an alternative to costly multi-installation synchrotron sources. For laser plasmas to provide ideal sources of soft x rays for projection lithography, their properties must be tuned to optimize several critical parameters. High x-ray conversion in the spectral band relevant to projection lithography is obviously required and has already received the attention of several studies. However, other features, such as the spectral content and direction of the x-ray emission, the plasma and particulate emission, the technology of the target, and efficient laser design, must also be optimized. No systematic study of all these features specifically for projection lithography has yet been made. It is our purpose to optimize these parameters in a coordinated approach, which leads to the design of a source that satisfies all the demanding requirements of an operating lithographic installation. We make an initial investigation of the plasma and particle emission of plasmas that have previously been shown to be good x-ray converters to the 13-nm band. The importance of the results reported may well force new approaches to the design of laser plasma soft-x-ray sources for projection lithography.

Introduction to special issue of Applied Optics on soft-x-ray projection lithography.

This special issue contains a collection of papers describing results that were presented at the Second Topical Meeting on Soft-X-Ray Projection Lithography sponsored by the Optical Society of America and held 6-8 April 1992 in Monterey, California, along with several additional papers submitted after that meeting. These papers are being published in this collection to make them readily available to a larger audience than would normally occur with a proceedings and also to take advantage of the critical review process.

Investigation of distortion and damage of molybdenum†silicon multilayer reflective coatings with high-intensity ultraviolet radiation.

Studies are performed to determine an upper limit on the optical damage threshold of a soft-x-ray molybdenum-silicon multilayer reflective coating by the use of a 308-nm, 15-ns pulse from a Xe-Cl excimer laser in order to simulate the potential damage induced by the x-ray flux from a pulsed laser-produced plasma. Experimental results yield a value of 0.26 J/cm(2) to produce visible signs of damage as determined by optical microscopy. Experiments are conducted first on silicon, as a reference point of a bulk material, and then applied to molybdenum-silicon in an effort to facilitate a theoretical comparison between a simple and a more complicated material. Theoretical predictions are in reasonable agreement with experimental results, but suggest that a lower value of 0.085 J/cm(2) might cause significant thermal-induced damage.

Soft-x-ray projection imaging with a 1:1 ring-field optic.

A molybdenum/silicon multilayer-coated 1:1 ring-field optic with a numerical aperture of 0.0835 is used to carry out soft-x-ray projection imaging with undulator radiation at 12.9 nm. An ideal optic of this type should be able to image 0.1-µm features with a contrast exceeding 90% at this wavelength. The useful resolution of our ring-field optic is experimentally found to be approximately 0.2 µm, probably because of the presence of substrate figuring errors.

Soft-x-ray projection lithography: printing of 0.2-microm features using a 20:1 reduction.

We demonstrate nearly diffraction-limited printing of 0.2-microm features, using soft x rays of approximately 36-nm wavelength. An open-stencil transmission mask with minimum features of 4 microm was imaged by a twentyfold-reduction Schwarzschild-type objective onto silicon wafers coated with various e-beam resists. Implications for soft-x-ray projection lithography are discussed.

Simple efficient travelin-wave excitation of short-wavelength lasers using a conical pumping geometry.

A conically shaped pumping geometry can produce an efficient burst of laser radiation, without the need for an optical cavity, by restricting amplified spontaneous emission losses to a small region near the apex of the cone. Requirements on the active medium and on the size and intensity of the pumping source to make such a burst laser are derived. We calculate that a 15-mJ pulse of energy at 37.2 nm at an efficiency of 0.15% can be extracted from sodium vapor photoionized with radiation from a 1.06-microm-laser-produced plasma using this pumping geometry.

Population inversions in C IV in a recombing theta-pinch plasma.

Population inversions on the 5g-4f and 4f-3d transitions of C IV at 253.0 and 116.9 nm have been observed in the recombination phase of a theta-pinch plasma initially containing 14 mTorr of acetylene and 0.5 mTorr of hydrogen gas. Population ratios of 17:11:1 for the 5g, 4f, and 3d levels were deduced from measured absolute line intensities of the 5g-4f and 4f-3d, and 3d-2p transitions at 253.0, 116.9, and 38.4 nm. The resulting gain-length product for a single pass through the 23-cm-long plasma column, based on the measured Doppler-broadened linewidth, is 2.3% at 116.9 nm and 5.8% at 253.0 nm.

Stimulated emission in the ultraviolet by optical pumping from photoionization-produced inner-shell states in Cd(+).

Stimulated emission on three UV transitions in Cd(+) has been observed by transferring population from inner-shell d-electron states, populated by photoionization, to outer-shell p-electron states using the output from a narrow-frequency dye laser. The use of similar techniques in other elements could eventually result in a number of new lasers in the UV and VUV.

Gain scaling of short-wavelength plasma-recombination lasers.

The theoretical Z(7.5) dependence of the small-signal gain of plasma-recombination lasers on the ionization stage Z of the laser species implies a straightforward development of short-wavelength lasers but appears to be significantly larger than the experimentally achieved gain scaling. A new analysis, which incorporates a simple experimental parameter representing the efficiency of populating the upper level at a given plasma electron density, results in a gain scaling with ionization state (Z(4.5)) that is more consistent with experiments and also reveals a need for better experimental control of the ion-production and electron-ion-recombination processes when one attempts to make such lasers.

High-gain inner-shell photoionization laser in Cd vapor pumped by soft-x-ray radiation from a laser-produced plasma source.

A soft-x-ray-pumped inner-shell photoionization laser has been produced in Cd vapor at 4416 and 3250 A. A gain of 5.6 cm(-1) has been measured at 4416 A, and a reasonably high-energy storage of 0.2 mJ/cm(3) in the upper laser states has been obtained.

Recombination laser transitions in expanding plasmas of Mg, Ca, Cu, Zn, Ag, Cd, In, Sn, Pb, and Bi.

Fifty new laser transitions in the visible and near infrared in ten metal vapors have been observed in the recombination phase of the expanding plasmas produced by a segmented-plasma laser device. Several of the strong visible transitions might be attractive for applications requiring simple lasers that operate at high repetition rates with pulse energies of the order of 0.1 mJ. All the transitions reported here suggest candidates for short-wavelength recombination lasers when isoelectronically scaled to higher ion stages.

Recombination lasers in Nd and CO(2) laser-produced cadmium plasmas.

This is the first report of a recombination laser that has been produced in laser-vaporized target material. Inputlaser-pulse energies as low as 0.5 mJ at 1.06 microm and 5 mJ at 10.6 microm have resulted in laser oscillation in Cd I at 1.40, 1.43, 1.45, and 1.64 microm in approximately 1-cm-radius cadmium plasmas. This laser appears to have the potential for high efficiency. In addition, scaling of this plasma-recombination process to higher-ionization stages should yield shortwavelength lasers.