X-ray Lithography Source Research Articles

Lattice design considerations for compact storage rings

The use of superconducting bending magnets in compact electron storage rings for generation of synchrotron radiation imposes additional design constraints caused by the nonlinear and non-isomagnetic field of the magnets. A symplectic third order beam optics calculation including real fields and chromatic effects is performed on the proposed Karlsruhe compact synchrotron light source for deep-etch X-ray lithography. The results show that satisfactory dynamic apertures and beam sizes and divergences can be found. With the magnet design used, there remains even a certain flexibility to adapt the lattice to different lithographic requirements.

Laser plasma x-ray sources for microlithography

A plasma produced by laser irradiation of solid targets is a promising candidate as an efficient x-ray lithography source. In order to design a practical laser created x-ray source, it is necessary to study the factors affecting the x-ray emission. For this purpose, we investigate both theoretically and experimentally the influence of the laser target parameters on the x-ray emission in different spectral ranges for two laser wavelengths (λ=1.06 μm, λ=0.26 μm). From these results and considering mask transmission, resist sensitivities, and wafer throughput, we establish the characteristics of the laser required as an x-ray lithography source with performance acceptable for industrial applications.

Properties of a laser-plasma x-ray source for x-ray lithography

We have used an advanced, near diffraction limited Nd:YAG laser, operated at 1.06 μm and employing a zig-zag slab amplifier, to generate x rays from the laser induced plasmas of a number of target materials. The x-ray conversion efficiencies were studied as a function of target material and laser parameters. We have measured the x-ray spectra, for x-ray energies above about 1 keV, and used them to evaluate this source with regard to x-ray lithography. A number of target material, resist, and mask combinations appear suitable for a lithography system capable of 0.25-μm minimum feature size.

Compact superconducting SR ring for X-ray lithography

A compact synchrotron radiation ring based on a new injection method has been designed as a light source for X-ray lithography, and is now being constructed. This machine consisting of a superconducting weak-focusing single-body magnet is 3 m in outer diameter and 2.2 m in height. The injection method uses half-integer resonance to inject the high-energy and high-intensity electron beams into this small ring of 1 m orbit diameter. The beam injected at 150 MeV is accelerated up to 650 MeV, while the magnetic field is excited from 1.0 to 4.3 T at a rate of 0.02 T/s. Betatron tunes are changed dynamically during this acceleration period. A small cavity of two λ/4 coaxial resonators supplies the acceleration voltage of 120 kV. A vacuum chamber of a hybrid structure contains a beam duct and cryopanels. The vacuum pressure of the duct is 6 × 10 −10 Torr. The beam intensity is 300 mA and its lifetime is longer than a day.

Generation of soft x rays using a rare gas-hydrogen plasma focus and its application to x-ray lithography

A conventional plasma focus device was used to produce a hot, dense plasma which emits radiation strongly in the soft x-ray region. We showed experimentally that a plasma produced from a mixture of hydrogen and a rare gas such as neon, argon, or krypton is an effective source of a characteristic x ray of the rare gas. Resist exposures and pattern printings were made to evaluate the feasibility of this device as a source for soft x-ray lithography.

A plasma x-ray source for x-ray lithography

Employing a gas-‘‘puff’’ z-pinch plasma, a high brightness and compact plasma x-ray source has been constructed by using a high-repetition-rate discharge and a new x-ray extraction method. The high-repetition-rate discharge (3 Hz) is attained by using a fast-acting gas valve, capable of operating stably under gas plenum pressures below 300 Torr. This low gas pressure discharge makes possible high-conversion efficiency of electrical input to x-rays, highly reliable plasma pinching, and a drastic reduction of debris resulting from electrode vaporization. An axial x-ray exposure is required to reduce the blur in pattern replication caused by the penumbral effect. To carry out the axial exposure, the high-energy particles emitted from the plasma are removed using a plasma reflector plate and magnet. The x rays are efficiently extracted through a window highly transparent to x rays. With this x-ray source, 9–14 Å x rays with 200 J per pulse (600 W average power) are obtained using Ne gas plasma in a 3 Hz operation.

Laser-plasma sources for x-ray lithography

The status of characterization and engineering of laser-heated plasma sources for x-ray lithography is reviewed.

Plasma focus as a radiation source for x-ray lithography

The soft x-ray emission of a modified plasma focus device, which can be operated with neon gas filling, was investigated with respect to x-ray lithography. An increase of the emitted x-ray energy by more than two orders of magnitude compared to that of conventional hydrogen plasma foci has been achieved. First test exposures were performed with an SEL-N resist.

High brightness laser/plasma source for high throughput submicron x-ray lithography

Laser irradiation of solid targets has been demonstrated to produce an extremely intense source of soft x rays. Conversion efficiencies of greater than 10% emission of over 1 keV radiation into 2π sr have been demonstrated with subjoule 1.06 μ laser pulses. This high conversion efficiency is achieved by using subnanosecond pulse lengths, tight focusing, and shaped pulses. A laser capable of producing the requisite high quality laser pulses at 100 Hz repetition rates has been constructed using newly developed slab techniques to maintain good beam quality at high average powers. A windowless target chamber has been designed to mate to x-ray stepper/aligners which are now being built. The present laser will permit average x-ray intensities of 2 mW/cm2 to be realized behind current (boron nitride) x-ray masks with negligible penumbral effects. This is an order of magnitude higher than possible with conventional electron beam impact sources and, with the present or upgraded laser system, should assist the commercialization of x-ray steppers.

Design parameters for a small storage ring optimized as an x-ray lithography source

This paper examines the design parameters for a ‘‘compact storage ring’’ which is matched well to x-ray lithographic requirements, but is as small as possible. This calculation uses a model of a lithographic system which obtains its input parameters from a technology of mask, resist and beam line based on the IBM program at the Brookhaven National Laboratory vacuum ultraviolet electron storage ring. Based on this lithographic system, we model exposure throughput as a function of storage ring parameters to understand which storage ring designs provide adequate but not excessive soft x-ray flux in the lithographically important region. Our scan of storage ring sources will cover a wide range of energies and magnetic fields, to permit consideration of superconducting as well as more standard strong- or weak-focusing designs. Furthermore, we will show that the results of the calculations presented here can be scaled in a simple way to cover a wide range of x-ray lithography system assumptions.

Enhanced brightness x-ray source

An improved source for x-ray lithography is described which is capable of significantly enhanced brightness through an improved cooling system. In the normal Gaines source which has an inverted cone shaped anode, the cone walls are straight, and the water flows in a narrow channel coaxial with the anode. Anode cooling occurs with water in the nucleate boiling regime, and maximum heat flux is set by the critical density of vapor bubbles at which coalescence occurs. By providing the flow of water around the anode with a concave upstream curvature, huge inertial forces create instantaneous phase separation. As a result the power density at which burnout occurs is substantially increased and moreover, significantly stabilized. In addition the use of a shaped anode allows for a self-guiding jet with no channel, which eases the mechanical design considerably. Measurements of the anode temperature excursion indicate a temperature rise of 450 °C with 6.2 kW in a 3 mm spot (25 kW/cm2). It is anticipated that fluxes two to three times higher may be accommodated. The Pd Lα line output (2.84 keV) was measured as a function of the incident power. It is found that Pd Lα generation does not follow the common power law for Kα radiation of most materials. The behavior can be explained in terms of the shallow incident beam angle and radiation take off angle. The absolute conversion efficiency of Pd Lα was measured at 20 keV and found to be 34 μW/W-sr in this gun configuration.

Experiments on X-ray lithography using synchrotron radiation from the VEPP-2M storage ring

During the last few years the electron-positron storage ring VEPP-2M of the Institute of Nuclear Physics in Novosibirsk has been used as an efficient source for X-ray lithography in the intermediate wavelength range (λ ∼ 10 A ̊ ) . Selection of the required part of the SR spectrum is done by means of a mask substrate fabricated from silicon of 2 μm thickness; Al windows can be installed before the X-ray mask. Exposures can be made in vacuum or in helium. The lithographic system can be used to fabricate one-layer devices on 100 mm diameter wafers with a production rate of 10–12 wafers per hour. The influence of the geometric conditions and of various temperature modes of exposure on the precision of replication from the mask onto the resist film have been investigated. Parameters of several X-ray and electron resists are studied. A lithographic system for fabricating multilayer devices is being developed. At present near to the storage ring a special room with clean-room environment is being built. Two SR beam lines come into it. The first beam line brings SR from the bending magnet. In the future the second beam line will bring SR from the special “lithographic wiggler”. In this room all operations on wafer preparation, subsequent processing steps of resist and masked surfaces after X-ray lithography and control of the patterns obtained will be carried out.

Evaluation of the gas puff z pinch as an x-ray lithography and microscopy source

Soft x rays (100–10 000 eV), due to their short wavelength (0.1–10 nm) can play an important role in high resolution microscopy and lithography. The gas puff Z pinch is an intense source of soft x rays. Calorimeter and x ray diode measurements showed that 10% of the stored electrical energy was converted to radiation in the range of 1–10 nm. Commercial photoresist polymethyl methacrylate (PMMA) and some new resists—CR 39, nitrocellulose, were exposed to the pinch radiation. The developed images on the resists have been studied with a scanning electron microscope. The resolution was found to be source limited, but a simple modification can improve the resolution by more than an order of magnitude.

Prospects for high-brightness x-ray sources for lithography

The x-ray source is a critical part of any x-ray lithography system since the source characteristics directly affect resolution, alignment, and throughput. Conventional electron bombardment sources, using either stationary or rotating anodes, are now in wide use. However their poor efficiency, low brightness, and uncollimated nature makes them doubtful candidates for high throughput lithographic systems. Several possible alternative sources have been discussed in recent years. They fall into two broad categories: accelerated charged particle beams and high temperature plasmas. The first category includes the synchrotron and related devices such as wigglers. Although these are large and costly installations, they are excellent sources of soft x-rays and have many advantages as lithographic sources. The second category contains many different systems capable of producing dense, high temperature plasmas. Some of these have been shown to have promise as compact, efficient, high brightness x-ray sources. Devices in each of these two categories will be discussed and the relative advantages and disadvantages of each as a source for x-ray lithography will be outlined.

A new transmission target for the x-ray lithography source

A new transmission target that is suitable as an x-ray lithography source is described. This target is a large stationary plane-source with a nearly parallel electron beam impinging on it. Problems due to mechanical vibration and irregular discharge encountered with conventional sources are eliminated. A prototype machine, comprising the electron optics and this 10×10 cm target, has been developed. Some excellent replication results were obtained from mask patterns with such exposure times as 6 min and 1.8 h.

X-ray lithography source using a stationary solid Pd target

This paper describes a relatively high power density (10–15 kW/cm2) x-ray source for x-ray lithography applications. The source utilizes a stationary solid Pd target cooled by a high velocity water flow. This source has been operated for several months in a shop environment. No degradation in performance has been observed.

Application of synchrotron radiation to x-ray lithography

Synchrotron radiation from the German electron synchrotron DESY in Hamburg has been used for x-ray lithography. Replications of different master patterns (for magnetic bubble devices, Fresnel zone plates, etc.) were made using various wavelengths and exposures. High-quality lines down to 500 Å wide have been reproduced using very soft x rays. The sensitivities of x-ray resists have been evaluated over a wide range of exposures. Various critical factors (heating, radiation damage, etc.) involved with x-ray lithography using synchrotron radiation have been studied. General considerations of storage ring sources designed as radiation sources for x-ray lithography are discussed, together with a comparison with x-ray tube sources. The general conclusion is that x-ray lithography using synchrotron radiation offers considerable promise as a process for forming high-quality-submicron images with exposure times as short as a few seconds.

Optimized source for x-ray lithography of small area devices

Optimized source for x-ray lithography of small area devices

Efficient continuum sources for x-ray lithography

Efficient x-ray sources whose dominant output is from the x-ray continuum are examined experimentally for use in x-ray lithography. At 10 kV with a 6-μm Mylar mask and 50-μm Be window, a Au continuum source is more than 3 times as efficient as the Al Kα source. Submicrometer resolution can be obtained with x-ray continuum sources.

Optimized source for x-ray lithography of small area devices

The design and performance of an optimized soft-x-ray source using an air-cooled Al target are described in this paper. The technical considerations leading to the choice of mask material, source wavelength, vacuum enclosure, and electron optics are also described. Although the authors have previously shown the need for large, powerful x-ray sources, the source described here is intended for exposure of small-area surface-acoustic-wave (SAW) devices with submicrometer dimensions. Since distortion over large areas need not be controlled, the x-ray source is limited to 1 mm in diameter and the electron beam power to 250 W. A 25-μm-thick Be vacuum window was developed which made it possible to use the Al Kα x-ray wavelength with a thin Si mask and yet maintain the exposure area in He at atmospheric pressure. The custom-designed electron gun, exposure monitor, and thermal control system are also described. Results are reported on the replication of a SAW device pattern in PMMA resist with 0.5-μm linewidths.