Patent Number: 059129395
Section: description

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to FIG. 1, the preferred embodiment of this microfluoroscope uses a laser-produced plasma as the source of soft x-rays. This type of source is preferred to other types of plasma sources due to its simplicity, reliability, high repetition rate, consistent location of the plasma from shot-to-shot, and small source size. For generating soft x-rays, a spot on the target T in a vacuum chamber V is illuminated by a high-power pulsed laser beam L. The laser itself and the vacuum pump for evacuating the chamber are not shown. The laser beam is focused onto the target surface by a lens Z which illuminates the target through vacuum window W.sub.L. Of course, the focusing lens could be situated inside the vacuum chamber. The vacuum environment is necessary to prevent electrical breakdown of the air by the focused laser beam, as well as to prevent absorption of soft x-rays by gas. It is possible to operate a laser-plasma source in a partial vacuum, especially with helium as the gas. The high power density of the beam on the target creates an expanding plasma P which emits radiation X, which includes soft x-rays. One of the convenient features of laser-plasma sources is that the laser optical path stays clean, even though plasma debris lands on the optic surfaces exposed to the plasma. The clean optical path is due to the continuous ablation of condensed debris material on the optical surfaces under the high laser power. For producing x-rays in the water window range, a target irradiance of 10.sup.12 -10.sup.13 Watts/cm.sup.2 is optimal. Following previous designs, the target is preferably a rotating cylinder mounted on a motor M. The motor drives the target cylinder on a helical thread so that a fresh surface is exposed for each shot, or for a fixed number of shots. Therefore, a helical pattern of small craters K is created on the target surface. This allows the target to last for a large number of shots before it has to be replaced. Other target geometries such as wires or tapes have been used advantageously. There has been some investigation of using gas targets which have the advantage of not producing a shower of condensable plasma debris. There are other types of plasma sources that include: gas-puff z-pinches, electron beam/plasma interaction sources, and dense plasma focus devices, which can also be used by this technique. While all of these sources produce copious amounts of soft x-rays, they have features which make them generally not as attractive as the laser-produced plasma source. Regarding the required laser, it is instructive to consider what power level is required to achieve the desired target irradiance of 10.sup.12 -10.sup.13 Watts/cm.sup.2. A common choice for the laser type is a Q-switched Nd:YAG laser. With a typical mid-size laser having a pulse length of 5 nsec and pulse energy of 0.5 Joules, the peak power is 10.sup.8 Watts. To achieve 10.sup.12 W/cm.sup.2, a focal spot size of .apprxeq.110 microns is required. This is easily achieved with a single-mode laser and a low-cost focusing lens. It is desirable to use the lowest power, and therefore the least expensive laser possible. The output of the laser can be reduced from the above parameters by achieving a smaller focal spot. Other common lasers used for generating x-ray emitting plasmas are Nd:glass, and excimer lasers. Due to the plasma debris and vacuum environment of the source, it is not necessarily desirable to have the microfluoroscope screen F positioned in close proximity to the plasma. If the source is placed some distance away, the radiation flux will decrease by the inverse-square law if no optics are used to redirect the diverging radiation. Therefore, a relay optic of some sort is desirable to focus the source on the fluorescent screen to maintain a reasonable radiation flux. In this embodiment, a glass capillary tube C is used to transmit x-rays from the plasma to the fluorescent screen. Because x-rays have an index of refraction slightly less than unity in all materials, they will be reflected by total external reflection at grazing incidence angles. Therefore, a hollow glass capillary tube functions as an x-ray guide, similar to a solid fiber optic with visible light. A typical capillary inside-diameter range is 100-500 microns. The distance of the capillary entrance from the plasma is typically a few centimeters. There are other types of relay-optics which can be used with this invention which use grazing incidence optics such as toroidal mirrors, or normal-incidence multilayer mirrors. The glass capillary optic has the advantage that it can be replaced very inexpensively after it becomes coated with too much plasma debris material. The extremely hot plasma of the source emits a wide spectrum of radiation which ranges from the infrared to the soft x-ray range. It is necessary to remove all of the photons which are not in the energy range desired for the optimal imaging of the specimen. This is to prevent poor contrast, large diffraction blurring, unnecessary radiation exposure, and heating of the specimen. This can be accomplished by placing a thin-film filter 3 in the optical path between the plasma and the fluorescent screen. Although it is shown in the air gap G, the filter could be placed in other locations. It should be understood that more sophisticated optics which utilize x-ray monochromators for achieving tunable and narrow-band radiation could be used instead of simple filter elements. These optics would be much more complicated and costly than simple thin-film filters. It is very desirable to have the fluorescent screen F outside of the vacuum environment of the target chamber. To achieve this, a thin window W.sub.x which supports 1 atmosphere pressure and is reasonably transparent to the soft x-rays is used to seal the target chamber. A good choice for this window is silicon nitride (Si.sub.3 N.sub.4). This material can support an atmospheric pressure differential on a window several tenths of a millimeter across, when only 1000 .ANG. thick. A window of this thickness will transmit well in most of the water window range. The gap G between the thin window and the fluorescent screen should be small due to the high absorption of soft x-rays by air. In some parts of the water window, the 1/e attenuation length is below 1 mm. This gap can be lengthened appreciably by replacing the air with a helium atmosphere. Due to the limited resolution of microfluoroscopy, it is sometimes desirable to use conventional x-ray contact microscopy for higher resolution imaging. A photoresist-coated substrate R is shown which can replace the fluorescent screen. If the specimen S is placed directly on the fluorescent screen, it will be difficult to move it onto a photoresist for subsequent imaging without damaging it. However, it is possible to have the specimen supported on a very thin film (not shown) which would allow the specimen to be lifted from the fluorescent screen, moved onto the photoresist surface, and exposed with the plasma soft x-rays X. The fluorescent screen F, or the optional photoresist R, is mounted on a scanner J for aligning a feature of interest to the x-ray beam and objective lens O. The objective lens is part of an optical microscope Y, which includes an eyepiece I for direct viewing of the fluorescent screen's output light U with the observers eye E. The microscope is focused on the front surface of the fluorescent screen 1 by viewing it through the back surface 2. It will be understood that the eyepiece and direct viewing could be replaced with several options (not shown) such as a television camera, an image intensifier tube, an ultraviolet image converter tube, an ultraviolet-to-visible-phosphor screen, a photographic camera, or some other sort of image recording device. Electronic recording devices could be interfaced with a computer for image processing. Referring to FIG. 2, a close up view of the fluorescent screen region of the instrument is shown. There are several choices for the fluorescent screen. Shown in the figure is a standard phosphor screen, which is composed of a phosphor powder layer H deposited onto a thin transparent substrate D. An optional thin metal coating A, such as several hundred Angstroms of aluminum, is shown over the phosphor layer. A specimen S is placed directly on the metal film or is positioned in close proximity. The metal coating is used to block any stray light from direct fluorescence of the specimen. The metal layer will also reflect the phosphor's fluorescent light U traveling away from the objective lens O back toward it to increase the signal. Standard phosphor powder fluorescent screens can be used with this technique, but the grain size must be extremely small. Transparent, vapor-deposited phosphor materials are a better choice for the phosphor layer since they form grainless films, although their efficiency is not as good as the standard powder screens. Another choice for the phosphor layer is an organic scintillator layer which can be spin-coated onto substrates. These organic compounds are more susceptible to radiation damage degradation, but this is not an important issue for this application, since the screens can be frequently replaced. Another possibility is a single-crystal scintillator screen, such as cerium doped YAG or YAP (not shown). In the case of single-crystal scintillators, there is not a separate substrate, rather the whole crystal is fluorescent. Fortunately, the soft x-rays are attenuated extremely rapidly in the crystal, so that all the fluorescence is generated in a very thin surface layer, and there is not a great deal of out-of-focus fluorescent light. The thickness of the fluorescent screen should be very thin to allow the close approach of the objective lens O of an optical microscope to the front surface of the phosphor screen 1 from the back surface 2 The objective lens must be corrected for any spherical aberration caused by the thickness of the screen. This is easily provided if the screen has the same optical thickness as a microscope cover glass, and a standard cover-glass-corrected objective is used. In the standard operation of the microscope, a specimen S is placed directly onto the front surface 1 of the screen F. An alternate specimen mounting arrangement is to have the specimen S supported on a very thin film N, such as carbon, which allows the specimen to be removed from the fluorescent screen. It is desirable to achieve the highest resolution possible with the optical microscope used to view the screen. The resolution of an optical microscope is given by: EQU .delta..apprxeq..lambda./2 NA where .lambda. is the wavelength of the light, and NA is the numerical aperture of the objective lens. The NA of a lens is given by: EQU n sin .phi. where n is the index of refraction of the medium between the objective and the object, and .phi. is the half angle of the light cone collected by the objective lens. Therefore, it is desirable to use the highest NA objective possible, and to use a fluorescent screen with the shortest possible emission wavelength. For direct viewing of the screen by eye, it is obviously necessary to use visible light. If the objective is coupled to an ultraviolet sensitive device--such as a television camera, image intensifier, or image converter tube--then the short wavelength limit will be determined by the transmission of the optics or the response of the imaging device. If easily visible 5000 .ANG. blue-green fluorescent light is used with a 1.4 NA oil-immersion objective, the resolution limit will be approximately 1800 .ANG.. By using ultraviolet emitting fluorescent screens and high-quality ultraviolet optics, it should be possible to increase the resolution to 1000 .ANG. or better. The shortest usable wavelength would be achieved by using an objective lens with all reflective optics which can operate well into the vacuum ultraviolet region. The limitation would then be the availability of short wavelength emitting phosphor materials, and the absorption of the ultraviolet fluorescence by the phosphor substrate. Referring to FIG. 3, an arrangement for performing both microfluoroscopy and standard light microscopy with the same relatively standard optical microscope is shown. When performing microfluoroscopy imaging in this embodiment, the optical microscope Y' has a movable light condenser-optic Q which is removed from its normal place below the microscope sample stage 4. The microscope is mounted above a laser-plasma x-ray source B, similar in construction to that shown in FIG. 1. One difference in the x-ray source shown here from FIG. 1 is a lengthened x-ray guide tube C with attached thin window W.sub.x, which is extended upwards to closely approach the fluorescent screen F mounted on the microscope sample stage. The optical microscope is mounted on a linear slide bearing 5, so that it can be slid into place over the x-ray guide tube when the condenser optics are removed. Microfluoroscopy is performed when the system is in this configuration. For normal light microscopy operations (often on the same specimen), the microscope is slid away from the guide tube, and the condenser Q is replaced into its normal position below the microscope stage. Although shown with the x-ray source positioned below the microscope, an inverted microscope--which is often advantageous for biological applications--could be constructed. In this case, the plasma source and condenser optics would be positioned above the sample stage and objective lenses. Another possible configuration which would not require a removable condenser would be to have the x-ray source mounted to the side of the microscope and have the x-rays traveling horizontally. A multilayer mirror (not shown) mounted above the condenser would then be used to reflect the x-rays 90.degree. and upward to the fluorescent screen. The multilayer mirror would also act as a monochromator. A microfluoroscope/optical microscope combination instrument as described here could use more sophisticated light optics for performing confocal, phase contrast, fluorescence, interference, or other advanced light microscopy techniques. Referring to FIG. 4, a miniaturized laser plasma source is placed directly onto the specimen stage 4 of a conventional optical microscope Y which has objective lenses O for viewing the fluorescent screen F. In this embodiment, the vacuum chamber V is reduced in height to fit between the microscope's condenser optics and the fluorescent screen. Often the plasma source is placed directly on the microscope sample stage. A small diameter cylindrical target T is positioned in the small vacuum chamber. Of course, as in the previous embodiments, other target geometries are possible. The distance between the laser-produced plasma P, and the x-ray transmissive window W.sub.X is typically less than 2 cm. The laser beam L enters the vacuum chamber through a window W.sub.L and is typically deflected downward by a mirror or prism 5. The beam is focused onto the target by a lens Z. Unlike the previous microfluoroscope embodiments, there are no relay optics to collect the diverging x-rays X. Instead, the plasma acts as a point source, and the close proximity of the source to the screen assures an adequate flux. By using a small target-to-fluorescent-screen distance, a lower energy laser 5 can be used if the focal spot is made small enough. For example, a target irradiance of 10.sup.12 W/cm.sup.2 can be achieved with a 5 nsec laser pulse of 20 mJ if the focal spot is reduced to 23 microns. Such lasers are very compact and relatively inexpensive. Due to the smaller energy of laser pulse (and plasma), the thin window W.sub.X can survive the close proximity of the plasma, although it will need periodic replacement as it gets coated with plasma debris. Although the laser-produced plasma is the preferred embodiment for the radiation source, it is possible to envision other miniaturized plasma sources such as hot electrical sparks. With proper design, it is possible to have visible light from condenser optics Q pass through the vacuum chamber to allow the almost simultaneous viewing of the specimen by light microscopy. As in the previous embodiment of FIG. 3, other specialized types of optical microscopes can be used such as confocal, phase contrast, fluorescence, interference, or other. The use of an inverted microscope geometry is also quite feasible, and would reduce some of the size constraints of the vacuum chamber. The reader will understand that the specimen is located as before; between the x-ray source and the screen. It is not shown in FIG. 4.