William T. Silfvast

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.

High-power plasma discharge source at 13.5 nm and 11.4 nm for EUV lithography

An intense pulsed capillary discharge source operating at 13.5 nm and 11.4 nm, suitable for use in conjunction with Mo:Si or Mo:Be coated optics, has produced an average power of approximately 1.4W within a 0.3 nm emission bandwidth from the end of the capillary when operated at a repetition rate of 100 Hz. The source is comprised of a small capillary discharge tube filled with xenon gas at low pressure to which electrodes are attached at each end. When a voltage is applied across the tube, an electrical current is generated for short periods within the capillary that produces highly ionized xenon ions radiating in the EUV. Issues associated with plasma bore erosion are currently being addressed from the standpoint of developing such a source for operation at repetition rates of greater than 1 kHz.

Intense EUV incoherent plasma sources for EUV lithography and other applications

Intense visible and ultraviolet sources, both incoherent and coherent, have been used in a variety of commercial applications over the years. Perhaps two of the most far-reaching applications are in the areas of microlithography and materials processing. In microlithography, the mercury vapor discharge lamp has provided the illuminating flux for microlithography machines for over 20 years. More recently, excimer lasers are playing an increasing role in this field. In materials processing, because of flux requirements that will be discussed later, sources have been largely restricted to lasers. The available lasers cover a wide range of wavelengths and pulse durations and have become major industrial tools for a broad spectrum of applications. This paper will point out the role that intense extreme ultraviolet incoherent (nonlaser) sources might play in the future, in that they may be able to provide similar intensities to those presently provided only by lasers, but in a much simpler, more efficient way and, in semi systems, at a potentially much lower cost.

Laser‐produced plasmas for soft x‐ray projection lithography

Laser‐produced plasmas are one of the most likely sources to be used for soft x‐ray projection lithography. The characteristics of these sources are described in terms of the expected radiation efficiency within the illumination bandwidth of a lithographic system. Measurements of the plasma particulate emission are described and techniques for interdicting this emission before it reaches the illumination optics are discussed. The laser requirements are obtained for a lithographic system producing a wafer rate of 60, 6 in. wafers per hour.

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.

Comparative extreme ultraviolet emission measurements for lithium and tin laser plasmas

Detailed spectroscopic studies on extreme UV emission from laser plasmas using tin and lithium planar solid targets were completed. At 13.5 nm, the best conversion efficiency (CE) for lithium was found to be 2.2% at intensities near 7 x 10(10) W/cm(2). The highest CE measured for tin was near 5.0% at an intensity close to 1 x 10(11) W/cm(2).

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.

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.

Short‐wavelength annular‐field optical system for imaging tenth‐micron features

We have found that the 1:1 Offner annular‐field system can provide high‐resolution imaging in the extreme ultraviolet (XUV) when used at numerical apertures (NA’s) around 0.1. For example, when illuminated with radiation at wavelengths shorter than 15 nm, a 0.0835‐NA system with a 233.6‐mm‐diam primary mirror should be able to image 0.1‐μm lines and spaces in a 100‐micron‐wide, 50‐mm‐radius ring‐shaped field at high contrast.

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.