A high intensity laser beam focused onto a surface is a source of soft X-rays that is both simple and elegant. In the past the high peak power laser sources were limited in repetition rate and average power. Thus the laser produced plasma was not considered useful in applications where average power is important such as lithography. However, even single pulse, low average power, laser plasma soft X-ray sources have been used for lithography studies. 1-4 In this talk we describe advances in laser sources that may lead to laser plasma soft X-ray average power levels that are adequate for production X-ray lithography. Our work in this area was motivated by the recent advances in high peak and average power laser sources, particularly the slab geometry solid state laser devices. 5,6 We were also motivated by Grobman's pro7posal that synchrotron radiation could be used for production X-ray lithography. We were curious as to whether a laser plasma soft X-ray source could be competitive with the synchrotron source. Our study showed that a laser source of 100-W average powqr could produce soft X-rays at the same level as the synchrotron source proposed by Grobman.8 However, the laser source must provide high peak power pulses to drive the plasma efficiently. Using the slab geometry it is possible to construct a 100-W average power, high peak power Nd:Glass laser source. The laser source must meet a set of engineering criteria such as size, reliability and efficiency in addition to criteria required to efficiently drive the plasma such as high peak and average power, excellent spatial mode properties and proper wavelength. The design of the laser source must also reflect the application of the soft X-rays to lithography, microscopy or spectroscopy. We briefly describe the slab geometry laser concept and then proceed to consider the plasma properties. We conclude by comparing various laser sources for driving the plasma and compare the plasma soft X-ray source to a conventional rotating anode source.
This paper presents a survey of the lithography-relevant properties of various x-ray sources, such as x-ray tubes, storage rings, and plasma sources. Based on this, the projects which have been started within the Berlin lithography group are summarized briefly. The main activities are concentrated on the development of a low-cost compact storage ring, designa-ted "COSY", which is an ironless monolithic storage ring with superconducting coils, with a critical wavelength of 12 A, and a current of several 100 mA. The first prototype construc-ted at BESSY will be ready for tests at the end of 1985. In addition, initial experiments concerning a laser-induced plasma source, carried out together with PTB, are described briefly.
Multi-million degree plasmas produced at the foci of powerful lasers efficiently emit short intense pulses of soft x-rays. Recent x-ray measurements obtained with laser-repetition rates up to 10 Hz are reviewed. High average-power solid and gas laser systems with powers sufficient for x-ray generation are now being designed. New laser-plasma x-radiation sources should find extensive use in research and industry. Demonstrated and expected uses of x-rays from laser-heated plasmas are summarized.
The properties of phase and amplitude modulating zone plates have to date been examined by various investigators interested in the performance of these structures in the soft x-ray range. With only few exceptions, however, these investigations have focussed on computing the performance of pre-determined zone plate geometries, with small attention being paid to the question of establishing the optimum geometry for maximization of the diffracted power. In view of the fact that maximization of the diffracted power is an extremely important goal in the soft x-ray range, especially.with regard to future developments in x-ray laser tech-nology and x-ray microscopy, we have undertaken a comprehensive analysis of this important problem, and in this paper we discuss our analytical approach and solutions. The results are of importance to workers in the field of diffraction optics. Portions of this work were performed at the Stanford Synchrotron Radiation Laboratory, which is supported by the Department of Energy.
A beamline for making X-ray lithography exposures using synchrotron radiation has been built and is now in routine operation at Brookhaven National Laboratory. The beamline, its optics, and its control system are described, and results are presented showing the intensity and uniformity of the radiation at the wafer. Results of exposures in a variety of resists are shown and discussed.
An in-vacuum lithography system is under development on beam line 111-3 at the Stanford Synchrotron Radiation Laboratory (SSRL). The beam line accepts 0.6 mrad of radiation from a bending magnet of the SPEAR storage ring and provides a beam of about 2 X 8 mm2 at the exposure station. Facilities for exposing photoresist with either filtered or monochromatic radiation are being developed in an effort to characterize the spectral sensitivity of state of the art photoresists. Design details and initial results are presented in which submicron features were printed on silicon wafers using boron nitride supported gold ab-sorber masks.
Wiggler and undulator insertion devices are very powerful sources which allow new approaches in the design of synchrotron x-ray lithogra-phy systems. The principal advantage of insertion devices is that they can be used as narrow bandwidth high throughput lithography sources at any wavelength. For a wiggler the increased flux can be continuously traded off against beam current, horizontal beam angle and bandwidth. The undulator offers several watts of power in a narrow 1% bandwidth and has a very compact beam (0.05 mr) which must be scanned both horizontally and vertically. Insertion" devices could thus be used in an evolutionary way with conventional x-ray tube technologies or in a revo-lutionary way at high contrast very soft x-ray wavelengths. A systems design approach is used to identify feasible options for wiggler and undulator beam lines for x-ray lithography in the 0.5-0.2 11,μm linewidth region over 5 cm by 5 cm fields. Typical parameters from the Wiggler and Undulator in the Advanced Light Source designed at the Lawrence Berkeley Laboratory are used as examples. Moving from the conventional wavelengths of 4-9 A to very soft wavelengths around 15 A is shown to be very promising. The mask absorber thickness can be reduced a factor of three so that 0.2 Am features can be made with a 1:1 mask aspect ratio. The mask heating rnited exposure time is also reduced a factor of three to 3 sec/cm . However, extremely thin beam line windows (1/4 mil Be) and mask supports (1 Am Si) must be used. A wiggler beam line design using a small slit window at a scanning mirror appears feasible. A unconventional, windowless differentially pumped beam line with dual deflecting mirrors could be used with an undulator source.
The design of an electron storage ring for Industrial X-ray Lithography is examined and a parameter optimization carried out based on a model lithography system in use at the National Synchrotron Light Source at Brookhaven National Laboratory. In this optimization the potential use of superconducting or permanent magnet wigglers is considered. The basic parameters and geometry of a number of "t(exposure) = 5 sec., X (operating) = 8.26 A" devices are presented.
High collimation of synchrotron radiation perpendicular to the orbital plane of the 'electrons in a storage ring results in only a small exposed stripe on the wafer. Typically, the height ranges between 0.5 and 1 cm. Two methods of enlarging the exposure field, dealt with in this paper, are the application of plane scanning X-ray mirrors and the stimulation of electron beam oscillations in the ring. Due to the considerable decrease in reflectivity with decreasing wavelength and increasing mirror angle, the usable reflection angles are limited to the grazing-incidence region below 2° in the case of X-ray lithography. The pure Si02-mirror (Zerodur) used in the experiments has reflectivities above 80 % at up to 1.5°. For the wavelengths in question (--1 nm). The theoretical values of the absorbed energy in the resist, depending on the glacing angle, have been compared with experimental results. From this dependence, the velocity profile of the mirror movement for attaining a homogeneous exposure can be derived. The preliminary experiments concerning surface degradation due to cracked hydrocarbons indicate that this factor is not as severe as previously expected in the case of the soft X-rays used in lithography. The alternative method of enlarging the exposed area, by stimulating electron oscillations in the storage ring, has also been investigated. The initial results show a practically-homogeneous exposure area 3 cm in height. A simple improvement in the control electronics for the additional steering magnets, which is being made at the moment, will at least double this area. Then, the exposable step-and-repeat field will amount to 6 x 6 cm2 with this specific method, which does not require any special arrangements in the beam line.
An oscillating grazing incidence mirror can be used to achieve vertically uniform illumination with a synchrotron radiation source. To increase the intensity it has been suggested previously to use a figured mirror rather than a plane mirror so that light from a larger horizontal angle can be directed into the exposure area. This paper presents results of ray-tracing calculations that show how much the intensity can actually be increased while maintaining good horizontal uniformity. Cylindrical, parabolic and elliptical mirrors are investigated and geometrical parameters are optimized. It is shown further that with an ellipsoid the throughput of a step-and-repeat camera is almost independent of the exposure field size. The parameters of the IBM X-ray lithography beamline are discussed.
With design rules for highly integrated circuits approaching or even attaining the sub-micron range, a mask aligner which is not working near its resolution limit is needed in order to provide enough process latitude. A candidate for this kind of machine is an x-ray stepper operating at a safe gap between mask and wafer and using a synchrotron radiation source. A mechanical stepping system, a gap setting mecnanism and an autoalignment scheme, in addition to the adaptation to synchrotron sources are described. An outlook on the economical advantages of this kind of lithography versus optical steppers is given.
An automatic X-ray alignment/exposure system developed for VLSI application involving printed linewidths in the one micron and submicron range is described. This paper focuses on the automatic alignment system designed to meet the specific overlay requirements for such applications. Other key elements of this fully automated, high throughput wafer exposure system are briefly presented.
The fabrication of masks used in storage ring X-ray lithography is described. These masks consist of a gold absorber electroplated over a substrate formed by a thin boron-doped silicon membrane covered by a layer of polyimide. Measurement of the properties of the materials which are of concern in this application is also described.
Since the initial demonstration of X-ray lithography (D. L. Spears and H. I. Smith Electronics Lett. 8 102-104 (1972)) there has been considerable development in masks, sources, and resists. The need for close matching of all elements of the system has been recognized and several alternatives are being pursued including electron beam impact anode sources, pulsed plasma sources, and electron storage ring sources. An advantage of the storage ring source is a combination of highly collimated and intense radiation: this combination relaxes some of the constraints on a resist and process which are inherent in the anode or plasma source systems. For example, the past emphasis for X-ray resists has been on increasing sensitivity with requirements of 1-10mJ/cm2 doses to provide sufficient wafer writing throughput. Sensitivity is no longer an overriding concern if storage ring and to some extent plasma sources are developed: storage ring sources for step and repeat are feasible with 100-300mJ/cm2 resist sensitivities. A survey of resists which have been used for X-ray exposure is made with the advantages of the storage ring source in mind: materials are evaluated primarily on the basis of sensitivity and RIE resistance. Results are shown from the IBM lithography beam line at the National Synchrotron Light Source at Brookhaven National Laboratory which demonstrate some of the capabilities for single layer resists. Finally, some future directions are suggested for resist development to improve storage ring resists.
This paper discusses the capabilities of a vector scan electron-beam system as an X-ray mask writer for pattern geometries at and below one-half micron. The noise level in the deflection system has been reduced to an RMS value of 150 A over a 0.5 mm deflection field, thus making our exposure system usable in the one-quarter micron regime. Pattern geometries below 2000 A have been fabricated on a thin membrane. Drift compensation techniques, implemented in software, have reduced placement errors over the entire mask to less than 700 A. Accomplishments in the areas of noise reduction, bandwidth error compensation, system resolution, and improvements in pattern placement accuracy are discussed.