Patent Number: 054266866
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention concerns the generation of x-rays, and particularly time-resolved x-rays having nanosecond and shorter duration. The present invention particularly concerns x-ray sources for lithography, and especially sources providing an energetic flux of hard x-ray radiation over a spatially extended area. 2. Background of the Invention The present invention generally relates to the production of x-ray radiation, particularly time-resolved pulses of x-ray radiation, and particularly relates to the production of x-ray radiation over a spatially extended area. 2.1 Time-Resolved X-ray Sources The earliest attempts to produce time-resolved x-rays employed mechanical shutters that moved in front of x-ray sources. For example, transmission of x-rays through x-ray transparent apertures within a rotating apertured disk that was otherwise opaque to x-rays permitted the generation of millisecond x-ray pulses. These millisecond x-ray pulses were too slow to permit the study by x-ray diffraction of any type of molecular phenomena such as reaction, melding, dissociation, or vibration. Millisecond x-ray pulses were, however, sometimes sufficient to permit observation of certain biological phenomena, although not normally at the biomolecular level. Davanloo et al., Rev. Sci. Instrum. 58:2103-2109 (1987) reported constructing an x-ray source capable of producing x-ray pulses of nanosecond (ns) duration. That x-ray source utilized (i) a low impedance x-ray tube, (ii) a Blumlein power source, and (iii) a commutation system for periodically applying power from the Blumlein power source to the x-ray tube. The system yielded 140-mW average power in 15 ns pulses of radiation near 1 .ANG.. That device, and others based on Blumlein-generators, suffers from (i) low repetition rates in the range of 100 hertz, (ii) prospective inability to produce pulses shorter than about 15 nsec, and (iii) low energy efficiency on the order of 25%. The durability in operational use of Blumlein-based sources of x-ray flashes is also uncertain. More recently, Science News, Vol. 134, No. 2: pp. 20 (1989) reported that scientists at Cornell University and the Argonne National Laboratory have developed a device, called an undulator, capable of producing x-ray pulses one-tenth of a billionth of a second (100 picoseconds) in duration. The undulator utilized synchrotron radiation from fast-moving charged particles in an electron storage ring. Because electron storage rings are typically large and expensive, the ring used at Cornell being one half-mile in diameter, the production of bright x-ray flashes by such means is distinctly not adaptable to the scale and budget of a typical materials or biological laboratory. X-rays have been produced using plasma sources that are energized by lasers. In laser plasma x-ray sources, either a pulsed-infrared (IR) laser or a ultraviolet (UV) excimer laser is used with pulse widths varying from less than 10 picoseconds to 10 nanoseconds. The laser beam is focused on a target where it creates a plasma having a sufficiently high temperature to produce continuous and characteristic x-ray radiation. Major disadvantages of laser plasma x-ray sources include (i) a diffuse, non-point, area of x-ray emission (ii) low efficiency (iii) low repetition rate. 2.2 X-ray Sources for Lithography Since the seminal paper by Henry Smith appeared in 1972, the achievement of economical x-ray lithography has been rather elusive. During the intervening years, however, considerable progress in many areas has been made, including development of masks, resists and registration capabilities. Three main classes of x-ray sources are considered as a possible choice for lithography. Those are electron impact tubes, laser-based plasmas, and synchrotrons. Progress has been made in each of these sources, particularly in laser-driven plasma x-ray sources. Efforts in Japan have been devoted to the development of compact, high density synchrotrons. Even today, each of these sources has its limitations for a practical system. The most intense sources are the synchrotrons, but so far their price, size and complexity make them prohibitive for use in a production line. Electron impact tubes are the simplest and cheapest sources. However, their effectiveness is best only in the hard x-ray region. For high current output electron impact tubes must be pulsed because of the extreme heat generated on the anode by electron impact on the anode. Laser driven x-ray sources have started to appear and show promise. The requirements for a practical x-ray source for lithography are dependent on development of the other two critical components of the lithographic process--mask and resist. Most of the research and development for x-ray sources is centered in the 0.4-5 nm wavelength range where suitable resists are available. Use of still harder x-rays, 0.1-1.0 nm, would bring additional benefits, such as the possibility of ultrasensitive microsensors for medical and technological applications and, of course, higher resolution lithography permitting a denser layout of semiconductor components. The present invention will be seen to be concerned with the generation of x-ray pulses for lithography in a manner that is believed to provide several distinct advantages over previous x-ray sources. 2.3 Photoemissive Sources of Electrons By way of background to the present invention, Lee, et al., in Rev. Sci. Instrum., 56:560-562 (1985) described a laser-activated photoemissive source of electrons. In the laser-activated photoemissive electron source a photocathode is illuminated with high intensity laser light as a means of generating numerous electrons by the photoelectric effect. The electrons emitted from the photocathode are focused in an electrical field, typically produced by electrodes in an electron-gun configuration, in order to produce a high intensity electron beam. 2.4 Rectification of Ultrashort Optical Pulses to Produce Electrical Pulses By way of further background to the present invention, the rectification of ultrashort optical pulses in order to generate electrical pulses having durations and amplitudes that are unobtainable by conventional electronic techniques is described by Auston, et al, in the Annl. Phys. Lett., 20:398-399 (1972). Electrical pulses on the order of 4 amperes in 10 picoseconds are generated by rectification of 1.06 micrometer optical pulses in a LiTaO.sub.3 crystal doped with approximately 2.24% Cu (LiTaO.sub.3 :Cu.sup.++). A doped transmission line, having an absorption coefficient of 60 cm.sup.-1 and a thickness of 0.2 mm, is bonded with a thin epoxy layer to an undoped crystal in the form of a TEM electro-optic transmission line of 0.5.times.0.5-mm cross-sectional area. Current pulses are generated by absorption in this transducer of single 1.06 micrometer mode-locked Nd: glass laser pulses, typically of duration 3-15 psec and with an energy of approximately 1 mJ. The electro-optic transmission line, or switch, operates to conduct current during the presence of laser excitation by action of the macroscopic polarization resulting from the difference in dipole moment between the ground and excited states of absorbing Cu.sup.++ impurities. Effectively, the electric-optic transmission line, or switch, has a very great number of charge carriers, and is a very good conductor, during the presence of laser excitation. During other times it is a semiconductor and does not conduct appreciable current. The excited-state dipole effect of the transmission line, or switch, is exceptionally fast, on the order of 1 or 2 psec or less. SUMMARY OF THE INVENTION The present invention contemplates a compact, high-intensity, inexpensive, reliable, tunable, high-intensity pulsed x-ray (PXR) light source where copious electrons are efficiently produced at a photocathode by the photoelectric effect and then, having been efficiently produced, effectively accelerated and focused in a strong electric field to impinge upon a desired area of an anode, thereby to produce bright x-ray light by bremstrahlung. In various embodiments an x-ray source in accordance with the present invention can produce pulsed x-ray radiation that is any one or ones of (i) very short (typically 20 ps), (ii) very bright (typically 6.2.times.10.sup.6 cm.sup.-2 sr.sup.-1 at the Ka wavelength (1.54 .ANG.), and/or (iii) very hard (typically 0.1-1 micrometer wavelength). X-ray source in accordance with the present invention are effectively applied in the areas of crystallography, spectrography, and especially lithography. Particularly for lithography applications, a compact wide-area x-ray source can produce from 1 to 40 mW/cm.sup.2 x-ray radiation flux (depending upon the duration and repetition rate of the laser pulses) uniformly over an area (typically circular in shape) that is as large as 20 cm.sup.2. Such an energetic high-intensity pulsed hard x-ray flux over such a large area is manifestly suitable for the masked exposure of photoresists in the production of semiconductors: the x-ray source, mask, resist and semiconductor substrate are placed tight together in simple close contact--obviating any need for focusing. An x-ray source in accordance with the present invention has (i) a laser for producing a laser beam (a beam of laser light), and (ii) an electron source means, preferably photoemissive, that is capable of producing electrons in response to illumination by the laser beam and which is positioned for illumination by the laser beam. The x-ray source also includes (iii) a high voltage means energized to generate an electric field for accelerating, as an electron beam, the electrons produced by the impinging of the laser beam on the electron source means, and (iv) an electron beam target means positioned to intercept the accelerated electrons.(electron beam) in order to produce x-rays in response thereto. Preferably, the x-ray source further includes a high voltage switching means selectively operable to energize the high voltage means for a selected period of time for accelerating the electron beam during the selected time period to produce an x-ray pulse. Preferably, the high voltage switching means comprises an electrical switch selectively operable to selectively energize the high voltage means in response to, and in synchronism with, the laser beam pulses. In another preferred embodiment, the x-ray source further includes a means for producing the laser beam as pulses in substantial temporal synchronization with the energization of the high voltage means. In still another preferred embodiment, the x-ray source includes a field electrode means disposed between the electron source means and the electron beam target means for substantially suppressing the electron beam in response to deenergization of the high voltage means. Preferably, the field electrode means comprises an electrode positioned closer to the electron source means than to the electron beam target means. In embodiments containing an electrode, it is preferred that the x-ray source further include a means for negatively voltage biasing the electrode relative to the electron source means for substantially maintaining the electrons produced by the electron source means in a region between the electrode and the electron source means in response to deenergization of the high voltage means. A still further preferred embodiment of the x-ray source of this invention includes a means for directing the laser beam pulses onto a scattering sample (a sample for scattering the x-ray radiation) for energizing the scattering sample substantially simultaneously with illumination of the sample by the x-ray radiation. Still another preferred embodiment of the x-ray source of this invention includes an x-ray switch means for switching x-rays received from the electron beam target means to produce an x-ray pulse. Preferably, the x-ray switch means comprises an apertured plate, such as a rotating plate, movable to selectively and alternately occlude and to pass the x-rays through an aperture for producing an x-ray pulse. In one preferred x-ray source in accordance with the present invention, the electron source means comprises a photocathode, the electron beam target means comprises an anode, and the high voltage power supply is connected between the photocathode and the anode for generating the electric field used for accelerating the electrons produced by the photocathode as an electron beam that impinges the anode to produce the x-rays. In another embodiment, the present invention contemplates a source of x-ray radiation comprising a laser source of laser light, a chamber evacuated to a high vacuum, a photocathode within the chamber for emitting electrons in response to illumination thereof by the laser light, an anode within the chamber spaced apart from the photocathode, and a high voltage source for electrically biasing the anode to a high voltage relative to the cathode for accelerating electrons emitted from the cathode as an electron beam to impinge upon the anode and to produce x-ray radiation. Preferably, a high voltage switch is connected to the high voltage source, the photocathode and the anode, for selectively biasing the anode with high voltage relative to the cathode in synchronization with the illumination of the photocathode by the pulses of laser light. Preferably, the high voltage switch is selectively operable for switching the biasing of the anode in response to and in synchronization with the pulses of laser light. Preferably, the high voltage switch comprises a semiconductor switch responsive to the pulses of laser light. Preferably, the source of x-ray radiation of this invention further includes (i) a grid electrode within the chamber between the anode and the photocathode, and (ii) a voltage source for electrically biasing the grid electrode with a voltage, lower than the high voltage, for jointly limiting the drift of the emitted electrons under the space charge effect to a region of the chamber proximate the anode when the anode is not electrically biased with the high voltage, and (iii) a high voltage switch connected to the high voltage source and the photocathode for selectively applying the high voltage between the anode and the photocathode to produce pulses of emitted electrons accelerated from the photocathode through the grid electrode to impinge the anode, producing pulses of x-ray radiation. The present invention still further contemplates an improvement to the photocathode element of the laser-activated, photoemissive, electron source. A metal, is preferably deposited on, or is alternatively mixed in bulk with, a semiconductor. The metal is preferably tantalum (Ta), copper (Cu), silver (Ag), aluminum (Al) or gold (Au) or oxides or halides of these metals, and is more preferably tantalum. The depositing is preferably by sputtering or annealing, and is preferably by annealing. The semiconductor is preferably cesium (Cs) or cesium antimonide (Cs.sub.3 Sb) or gallium arsenide (GaAs), and is more preferably cesium antimonide. A photocathode so formed exhibits efficient electron emission by the photoelectric effect and improved longevity. In another embodiment, the present invention contemplates a method of producing x-ray radiation comprising illuminating a photocathode in a high vacuum with laser light, preferably at intermittent intervals, in order to produce electrons therefrom by the photoelectric effect and accelerating the produced electrons in a high voltage electric field to impinge on an anode in the high vacuum to produce x-ray radiation. In an embodiment particularly suited for use in x-ray lithography the x-ray source of the present invention includes a laser light generator for producing laser light illumination over a spatially extended area and a spatially extended photoemitter means intercepting the laser light illumination over the spatially extended area in order to produce electrons by the photoelectric effect over the same spatially extended area. A high voltage source generates an electric field for accelerating the produced electrons as a wavefront of electrons, the wavefront again occurring over the spatially extended area. A spatially extended metal foil is positioned to intercept the wavefront of electrons over the spatially extended area of such wavefront, and, responsively to this interception, for producing x-rays. The x-rays so produced over a spatially extended area are particularly useful for lithography, including in the masked exposure of photoresist upon a semiconductor substrate where the substrate, photoresist, and mask are tight against (i.e., at a separation that is typically .ltoreq.5 micrometers) the metal foil. These and other aspects and attributes of the present invention will become increasingly clear by reference to the following drawings and accompanying specification.