There continues to be a common belief that the registration of single photographic grains or emission of single photo electrons at a time validates the assertion that the interference and diffraction patterns are built through the contribution of individual photons (hν). A careful analysis of the past literature indicates that these experiments actually were not able to ascertain that one photon at a time interacted with the photo detector. This paper reviews a series of experiments carried out during the early eighties, which suggest that the simultaneous presence of multiple photons (multiple units of hn) makes possible the registration of a single photographic blackening spot or the emission of a single photoelectron. The congruency with the paradigm of "wave-particle duality" is now better maintained by assuming that the photons, after they are emitted and then propagate from the source, develop the "bunching" property, which we proposed as a "photon clump" in 1985 and explained with a plausible extension of the Heisenberg's Uncertainty Principle.
For a number of years, ALFT Inc. has developed a laboratory soft X-ray source for the many analyses that are now being carried out at synchrotron locations. It is a Vacuum Spark source (VSX). The source is a pulsed point plasma emitting pulses of soft X-rays around 1 keV or 10 Angstroms wavelength. The average power is 728 mW in the VSX 700 machine. In terms of photon flux, it emits an average of 1015 photons/sec, with peak flux of 1018 photons/sec. Being a thermal source, the radiation is divergent. Because of its divergence, the source can be used along different directions, 4 beams being now part of the present commercial machine. This laboratory tool has been thoroughly developed to the point that it reliably runs continuously without deterioration of power or spectral purity. The plasma can be generated from among many elements, copper and tungsten being two of the presently used. A more powerful source, the VSX Z10, of 10 Watts average power, has been also developed by ALFT Inc. and has now reached the prototype stage. It is again a thermal divergent plasma source emitting in excess of 1016 photons/sec with peak flux of 1024 photons/sec. Sources in the 50 Watts range are now being considered for further development.
The new development in ALFT soft X-ray point source VSX-400 consists mainly of an improvement of the nozzle design to reduce the source size, as well as the introduction of a novel trigger system, capable of triggering the discharge hundreds of million of times without failure, and a debris removal system. Continuous operation for 8 hours at 20 kHz allows us to achieve 400 mW of useful soft X-ray radiation around 1 nm wavelength. In another regime of operation with a high energy machine, the VSX-Z, we have been able to achieve consistently 10 J of X-rays per pulse at a repetition rate that can reach 1 Hz with an input electrical energy of approximately 3 kJ and an efficiency in excess of 10-3.
The vacuum spark X-ray source is a device operating at high frequency, of the order of several kilohertz, where the X- rays are emitted in small dose in each pulse. At ALFT, we are developing such X-ray sources for lithography in a series of models called VSX. Continuous operation at 10 kHz, full voltage (14~15 kV) for more than 9 hours has been routinely obtained with a carefully designed trigger and a movable anode. The X-ray output is 275 (mu) J/pulse at the source with a capacitance of 13 nF. An extendibility study has been carried out focused mainly on debris management and heat dissipation. The study evaluated the possibility of extending the machine to the 150 W X-ray output level required by the semiconductor industry within the operating regime of 40 kHz and anode moving at speed up to 11.2 m/sec.
The Vacuum Spark X-ray Source (VSX) embodies a miniature, high-repetition rate discharge emitting radiation in a broad spectrum optimized to emit soft X-rays with about 10 + wavelength. We have obtained over 100 (mu) J of energy per pulse in a narrow bandwidth centered around 7 Angstroms and pulse width less than 30 ns. The system operates with an ALFT proprietary pulser that yields a power output of 1 to 10 Watts of X-rays. A proprietary nested cone collimator is used to optimize the collection of the solid angle and direct parallel radiation towards the wafer. We predict this design will yield a throughput of 5 to 20 WLPH on 200 mm wafers. Presently, we are validating results obtained in the 1 - 5 WLPH range. ALFT's goal is to create an X-ray Point Source suitable for Next Generation Lithography. To date, two prototypes have been created to validate our theories and guide future development. The latest prototype has been designed to accept modifications to allow us to meet our 2001 performance goals. This year we achieved two major breakthroughs. We created a unique regenerating anode design that will enable thermal and debris management at the desired output levels, and we designed a trigger that ensures reliable discharge at repetition rates above 5 kHz. Following our previous SPIE Conference reports (SPIE Volume 3676-1, P. 410), the progress on the VSX Source is described here. Moreover, our technical roadmap is presented and the anode and trigger designs are disclosed.
The VSX is essentially a miniature discharge capable of emitting soft X-ray radiation. Because the radiation is emitted in small dose in each spark, it is necessary to repeat the phenomenon at high frequency in order to achieve the industrial throughput requirement. Relying on a X-ray collimator (either nested cones or capillary), the point radiation source can deliver a high quality beam suitable for microlithography. Current study shows that the X-ray energy per pulse can be 113 (mu) J or higher with a pulse width (FWHM) of less than 50 ns. A 30 kHz operation in burst mode has been realized. The optimum conditions of the VSX operation have been carefully studied and are reported here. The lifetime can be as high as tens of hours in the low frequency operation or 100 million shots and this figure can be greatly improved. Pinhole camera images show that the source size is less than 1 mm. The throughput of exposing 1 field per hour or more is readily achievable with current parameters and can be greatly improved with the higher operating frequency. The Spherical Pinch is a modified (theta) -pinch with high efficiency in the EUV region of spectrum. Working with different gases, the SPX machine (up to 33 kJ input energy) is optimized for EUV emission and can be fitted in an advanced EUV lithography system. The point source requirements are indeed met by the SPX for such system.
The technology of x-ray/EUV point plasma sources is competing with the multiple beam synchrotrons as radiation sources for submicron lithography. The company ALFT has been doing research and development on two plasma point sources for several years now. They are the vacuum spark (VSX) and spherical pinch (SPX) technologies. Both have a long history of previous research to support the contention that are well qualified to be converted into technological tools for the manufacturing of the next generation of IC chips. The VSX is essentially a miniature discharge capable of emitting soft x-ray radiation. Because the plasma emits a small dose of x- ray in each spark, it is necessary to repeat the phenomenon at high frequency in order to meet the requirement for microlithography. The SPX is mainly a strong source of EUV/X-ray radiation that operates at a frequency of one hertz or more.
The vacuum spark is an excellent source of pulsed X-rays (also called flash X-rays) suitable for high speed photography. In the vacuum spark concept a capacitor is discharged through two properly shaped electrodes, made of selected materials, in a vacuum. This high current (over 1 kA) discharge produces intense pulsed hard X-rays with a pulse width of about 10 ns (FWHM, Full Width at Half Maximum). The measured source size by pinhole photography is smaller than 0.5 mm. Efforts have been made to reduced the total inductance (below 200 nH) and to use a relatively small capacitor (just a few nF), so as to increase the X-ray intensity. A vacuum spark X-ray source (VSX I) has been under routine operation at ALFT and has logged over 1,200 shots during X-ray tests carried out with Los Alamos National Laboratory. The radiation head was designed and built by ALFT and the remaining components are all commercial, off-the-shelf products. An external signal of 10 V, 1 ns rise time and 500 ns width triggers the machine at rep-rates up to 10 Hz, and higher rep-rate operation of the vacuum spark is being studied at ALFT.
Spherical pinch and vacuum spark have been pursued by Advanced Laser and Fusion Technology, Inc. for a number of years as candidates for point radiation source needed in microlithography. In the spherical pinch electrical energy is used to generate spherical imploding shock waves that compress a performed plasma into small (diameter < 1.0 mm) radiation source. The temperature of the central plasma can be high enough for emission of broadband radiation from the UV to the soft X-ray region of the spectrum. In the vacuum spark a small capacitor (a few nF) is discharged through two properly shaped electrodes in a vacuum. During the discharge 'hot spots' (minute high temperature plasmas) are formed in the vicinity of the anode and intense pulsed soft X-rays can be generated around the characteristic lines of the electrode materials. High rep-rate operation of the vacuum spark is necessary to provide sufficient dosage for microlithography.
In this paper we describe the performances of two kinds of high-flux radiation sources that have been developed at Advanced Laser and Fusion Technology, Inc. The first kind is the spherical pinch which exploits the principle of spherical convergence of strong shock waves in noble gases to generate a hot plasma at the center of a spherical vessel. The temperature of the central plasma can be high enough for emission of broadband radiations from the UV to the soft X-ray region of the spectrum. The second kind is the vacuum spark in which a capacitor is discharged through two properly shaped electrodes in a high vacuum. During the discharge minute spots of hot plasmas are formed on or around the electrodes and strong line radiation (characteristic of the electrode materials) can be generated in the soft X-ray region. High repetition rate operation of the vacuum spark may lead to the dosage required by the submicron lithography technology.
We have developed two sources of high flux radiation: one, designated as SPX II, exploits the principle of spherical convergence of a strong shock wave in a noble gas to generate a plasma at the center of a sphere hot enough to emit broadband radiation from the UV to the soft x-ray region of the spectrum; the other is based on the principle of vacuum spark, whereby accelerated electrons in a strong electric field in vacuum bombard an aluminum anode surface, leading to K-alpha line emission (1.7 keV). Typical measured outputs from the SPX II are: 186 W in the visible range (3500 - 2800 angstrom); 754 W in the UV range (2800 - 1800 angstrom); 650 W in the deep UV (1800 - 600 angstrom), and 5 W in the soft x ray (15 - 10 angstrom) for an operating repetition rate of the machine of 1 Hz. Typical measured output from the vacuum spark source is 30 W in the K-alpha line (7 angstrom) for a few kilowatt machine. The above are all powers measured beyond the pertinent filter used to select the radiation of interest, such as beryllium, aluminum, or other filters.
Study of soft x-ray (0.8 - 10 keV) output from the SPX II, a spherical pinched plasma radiation source, is presented. The soft x-ray output increases with the discharge voltage. The output is also a function of the gases used and pressures. A simple analytic model was developed which can scale the imploding wave velocity with respect to the discharge voltage and the gas density. Experiments to measure the imploding wavefronts were carried out. The predictions of the model agree well with the measured results in a wide range of the parameter space.
The concept of generating soft X-rays using a Spherical Pinch source was presented at the 1991 and 1992 SPIE conferences. In this paper we present the electrical specifications, plasma and radiation characteristics, system operation and maintenance for the SPX II prototype machine. In addition, we present the X-ray output power levels and times of exposure to attain a given dose level for different input electrical energies of the machine. Finally, we provide a status report on the construction of a new upscale version (SPX III) of the Spherical Pinch X- ray source which is being built as an industrial prototype for application in X-ray microlithography.
KEYWORDS: X-rays, Plasma, Deep ultraviolet, X-ray lithography, Spherical lenses, Ultraviolet radiation, Energy efficiency, Prototyping, Lithography, Energy transfer
The concept of X-ray generation in a Spherical Pinch machine was presented at the 1991 SPIE conference (Proc. SPIE, Vol. 1465, p. 318, 1991). The machine is a pulsed power device which drives strong spherically imploding shock waves to create a highly compressed hot plasma of small size which acts like a blackbody radiator. At the present time the machine is operating at an input electrical energy of 18 kJ and is capable of reaching 34 kJ. An efficiency of about 20% has been measured for the transfer of electrical energy to the plasma. The radiation output of the machine ranges over a broad spectrum which peaks near the deep UV (100 eV - 150 eV). Therefore the machine is useful for applications such as UV and deep UV lithography, in addition to soft X-ray lithography. The dosimetry analysis shows that about 100 mJ/cm2 of UV and deep UV and about 3 - 5 mJ/cm2 of soft X-rays per single discharge is available at a distance of 20 cm from the source. The radiation output is now being optimized in order to reach the design level of 10 mJ/cm2/discharge in the soft X- ray region of the blackbody spectrum. The advantages of the machine are, besides compactness and cost effectiveness, relatively long pulse duration (approximately 10 microsecond(s) ec) and low peak power at the mask and wafer. The machine is capable of operating at a repetition rate of 0.1 Hz. A new machine with higher input energy and repetition rate is being designed and constructed and is expected to generate about 12 mJ/cm2/discharge in the soft X-ray region of the spectrum at a distance of 30 cm from the source.
A new type of plasma x-ray source has been developed by applying the spherical pinch concept of imploding shock waves to produce a dense plasma hot enough to emit soft x-rays. Very intense x-rays in a few keV energy (5-10 angstroms) is radiated from a small plasma volume as microsecond pulses. SPX II is a prototype machine designed to demonstrate the engineering feasibility of the spherical pinch scheme. It is expected to generate at least 10 mJ/cm2 of usable soft x-rays for microlithography.
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