SCANNING TRANSMISSION ELECTRON MICROSCOPE WITH VARIABLE AXIS OBJECTIVE LENS AND DETECTIVE SYSTEM
Not applicable. Not applicable. Not applicable. Not applicable. The present invention generally relates to the technical field of charged particle microscope, and more particularly, to a scanning transmission electron microscope (STEM) for examining biological and cryogenic specimens. The STEM has a variable axis objective lens and a variable axis collection lens, between which is the specimen to be inspected. The axis of the collection lens varies along with the variation of the objective lens axis in a coordinated manner. The STEM of the invention exhibits technical merits such as large scanning field, high image resolution across the entire scanning field, and high throughput, among others. The present invention can also find applications in various microscopes using other charged particles, for example, proton, positively or negatively charged atoms, positive ions such as Gallium ions and Helium ions, and positively or negatively charged molecules. In a charged-particle microscope (CPM), an imaging beam of charged particles is directed onto a sample from an illuminator. In a transmission-type CPM (TCPM), a detector is used to intercept a flux of charged particles that traverse the sample, generally with the aid of an imaging system that is used to focus (part of) said flux onto the detector. Such a TCPM can be used in scanning mode (STCPM), in which case the beam of charged particles from the illuminator is scanned across the sample, and the detector output is recorded as a function of scan position. In addition to imaging, a CPM may also have other functionalities, such as performing spectroscopy, examining diffractograms, performing (localized) surface modification (e.g. milling, etching, and deposition), etc. An illuminator refers to a particle-optical column comprising one or more electrostatic and/or magnetic lenses that can be used to manipulate a “raw” charged-particle beam from a source (e.g. a Schottky source or ion gun), serving to provide it with a certain focus or deflection and/or to mitigate one or more aberrations therein. An illuminator can be provided with a deflector system that can be invoked to cause the beam to perform a scanning motion across the sample under investigation. Electrons, because of their wave-particle duality, can be accelerated to have picometer wavelength and focused to image in real space. It is now possible to image with high resolution, reaching the sub-Angstrom scale. Well-known electron microscopes include Transmission Electron Microscope (TEM), Scanning Electron Microscope (SEM), Scanning Transmission Electron Microscope (STEM), and “dual-beam” tools (e.g. a FIB-SEM), which additionally employ a “machining” Focused Ion Beam (FIB), allowing supportive activities such as ion-beam milling or Ion-Beam-Induced Deposition (IBID). In a TEM, the electron beam used to irradiate a sample will generally be of significantly higher energy than in the case of a SEM (e.g. 300 keV vs. 10 keV), so as to allow its constituent electrons to penetrate the full depth of the sample. A sample investigated in a TEM will also generally need to be thinner than that investigated in a SEM. In traditional electron microscopes, the imaging beam is “on” for an extended period of time during a given imaging capture; however, electron microscopes are also available in which imaging occurs on the basis of a relatively short “flash” or “burst” of electrons, which approach is particularly useful when a user is attempting to image moving samples or radiation-sensitive specimens. Scanning transmission electron microscope (STEM) is operated under a principle similar to scanning electron microscope (SEM). A primary beam is emitted from an electron source and focused by objective lens on a specimen which is about 100 nm in thickness. Deflectors drive the focused primary beam to scan on the specimen, and the electrons that have penetrated through the specimen are collected by a detector to generate an image. With reference to Primary beam landing on filmy specimen is focused by objective lens 105. Then transmission electron beam 109 In the prior art, the scanning field is limited below several micro meters (um) at high resolution image mode. For example, conventional STEM usually has 0.5 um×0.5 um maximum scanning field with 0.5 nm resolution, or 2 um×2 um maximum scanning field with 2 nm resolution. In high resolution STEM, primary electron beam is intensely focused on specimen. The focus spot size is usually in the magnitude of several nanometers, or even several angstroms. Usually, a specimen is placed in the focus field of the objective lens (in-lens type). This design of lens shows relatively small aberration coefficients. U.S. Pat. Nos. 7,285,776, 7,355,177, 7,459,683 and 7,745,787 disclose an in-lens type objective lens for STEM. Thin specimen is placed between upper pole piece and lower piece of objective lens, and the specimen is immersed in the magnetic field of the objective lens. The in-lens type structure ensures a small spot size on specimen, in other words, it ensures a high resolution image. The size of the scanning field has a great impact on the throughput of STEM. At the same scanning speed and beam current, the larger the scanning field, the higher the throughput. Compared to STEM with large scanning field, STEM with small scanning field demands more time in manipulating the specimen stage (e.g. moving and stopping) in order to change the area of interest (or observed portion) in the specimen under inspection. Mechanical manipulation of specimen stage takes much longer time than electron beam scanning. In traditional in-lens objective lens structure, specimen is immersed in focus field to ensure high resolution, but the scanning field is very small. Since the focus magnetic lens is very close to specimen, at the short focal length, the off-axis aberrations such as coma, chromatic aberrations and distortion increase quickly, and in proportional to the distance from center optical axis. To achieve approximate resolution between center and scanning field edge, the primary beam must be deflected only near the optical axis, and thus cover a small scanning field. Traditional in-lens type STEMs usually have several micrometers or several hundred nanometers scanning field at high resolution (several nanometers or several angstroms spot size). U.S. Pat. No. 4,544,846 teaches a variable axis immersion electron lens (VAIL) projection system. A deflector having a designed field coupled to focus field can shift the optical axis of objective lens. When the axis is shifted to the same position and direction as the scanning beam, the off-axis aberrations are eliminated, and a small spot size is obtained similar to that with the center optical axis. U.S. Pat. No. 6,392,231 discloses another VAIL system, called swing axis immersion electron lens (SAIL). The SAIL is used to achieve a large scanning field in SEM. A deflector having a designed field coupled to focus field can swing the optical axis of objective lens. When the axis is swing to the same position and direction as the scanning beam, the off-axis aberrations are eliminated and a similar spot size is obtained as center optical axis can. Yan Zhao et al. have attempted to use the variable axis objective lens concept in SEM and electron beam lithography. They have also proposed on how to make different types of variable axis system by using different types of coupling conditions between deflectors and objective lens. For details, see Yan Zhao et al. “Comparative study on magnetic variable axis lenses using electrostatic and magnetic in-lens deflectors”, Proceedings of the SPIE, Volume 3777, 1999, p. 107-114; as well as Yan Zhao et al. “Variable axis lens of mixed electrostatic and magnetic fields and its application in electron-beam lithography systems” Journal of Vacuum Science & Technology, B 17(6), November/December 1999, p. 2795-2798. When the primary beam is focused on a specimen in an immersion field, electrons that transmit the thin film specimen is focused and projected on a detector. The transmission electrons carry the information about the structure and materials contrast of specimen. The detector is usually divided into bright-field detective portion and dark-field detective portion to catch the transmission electron with different angular ranges. The bright-field detector collects the direct transmission electrons and the dark-field detector mainly collects the scatter transmission electrons taking atomic number signal in large angular range. In STEM, the transmission electrons from the scanning field edge have a different position on detector compared to center transmission electrons on optical axis. Thus the transmission electrons from scanning field except the center cannot be projected on detector circle symmetrically as the center transmission electrons. The bright-field and dark-field detectors cannot catch the signal transmission electrons precisely corresponding to transmission angle, so the image quality and contrast at the edge of the scanning field is worse than scanning center. It is impossible to obtain a high resolution image in large scanning field. To resolve the problem, international application publication WO2012/009543 discloses a de-scan deflective system that is put below the specimen in a STEM to correct the projection trajectory of transmission electrons. This de-scan deflective system eliminate the difference of projective location of transmission electrons on detector. But compared with the deflectors which are coupled with focus field in variable axis lens system, the deflector in WO2012/009543 can only correct the transmission electrons from relative small scanning field. Thus, there is a need to enlarge the scanning field of STEM, and in the meanwhile, to maintain a high resolution image. For example, a trend in recent years is using an electron microscope to generate high resolution image for 3D reconstruction of tissue volume in biological research. However, the throughput of convectional scanning transmission electron microscope is not satisfactory, because of the small scanning field and low scanning speed. Advantageously, the STEM of the invention can acquire high resolution images in large area and at a high speed. One aspect of the present invention provides a scanning transmission electron microscope (STEM) comprising the following components: (1) an electron source for emitting a primary electron beam; (2) a detector for receiving the electron beam, wherein a reference axis is defined by the straight line connecting the electron source and the detector; (3) a specimen plane located between the electron source and the detector, wherein the reference axis is perpendicular to the specimen plane; (4) a first redirector that redirects the electron beam to a path not in alignment with the reference axis; (5) a lens module comprising a variable axis objective lens and a variable axis collection lens, between which is the specimen plane. The variable axis objective lens is located between the electron source and the specimen plane for focusing the electron beam redirected by said first redirector to a focusing spot on the specimen plane. The variable axis collection lens is located between the specimen plane and the detector for collecting the electron beam that has passed through the specimen plane; and (6) a second redirector that redirects the electron beam that has been collected by the variable axis collection lens back to a path in alignment with the reference axis, before the beam reaches the detector. Another aspect of the present invention provides an electron microscope. The microscope includes an electron source for emitting a primary electron beam; a detector for receiving and detecting the electron beam; a specimen plane located between the electron source and the detector, wherein at least a portion of the electron beam can pass through the specimen plane; and a variable axis collection lens located between the specimen plane and the detector for collecting the electrons that have passed through the specimen plane. One of the advantages associated with the STEM of the present invention is that its scanning field is increased, but not at the cost of sacrificing the image resolution. In other words, the invention can meet two requirements at the same time, large scanning field and high image resolution. Typical embodiments of the invention employ a variable axis objective lens for primary scanning beam and a variable axis collection lens system for correcting transmission signal beam. The above features and advantages and other features and advantages of the present invention are readily apparent from the following detailed description of the best modes for carrying out the application when taken in connection with the accompanying drawings. The present invention is illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings and in which like reference numerals refer to similar elements. All the figures are schematic and generally only show parts which are necessary in order to elucidate the invention. For simplicity and clarity, elements shown in the figures and discussed below have not necessarily been drawn to scale. Well-known structures and devices are shown in simplified form such as block diagrams in order to avoid unnecessarily obscuring the present invention. Other parts may be omitted or merely suggested. In the following description, for the purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It is apparent, however, to one skilled in the art that the present invention may be practiced without these specific details or with an equivalent arrangement. Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this present invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. Hereinafter, a schematic cross-sectional view of the scanning transmission electron microscope (STEM) according to an embodiment of the present invention will be described with reference to With reference to A thin sheet specimen to be examined is located between electron source 11 and detector 19, and is represented as the specimen plane 14. The physical arrangement of electron source 11, detector 19 and specimen place 14 is such that reference axis 13 is perpendicular to the specimen plane 14. Reference axis 13 passes through a reference point O1 on the specimen plane 14. In Upon emitted out from the electron source 11, and in the absence of any influence from a redirector, electron beam 12 always travels, or propagates, in a path in alignment with reference axis 13, and axis 12 Referring to Variable axis objective lens 42 is located between the electron source 11 and the specimen plane 14 for focusing the electron beam 12 redirected by said first redirector 21 to a focusing spot on the specimen plane. Variable axis collection lens 44 is located between the specimen plane 14 and the detector 19 for collecting the electron beam that has passed through the specimen plane 14. In preferred embodiments, the variable axis objective lens 42 and the variable axis collection lens 44 remain symmetrical about the specimen plane 14. As a result, variable axis 42 Referring back to Referring back to One of the advantages associated with the STEM of the invention is that the transmission electron beam from different scanning position will have the same or similar radial emission angle when being projected on the detector 19. The term “axial” in the application means “in the optical axis direction of a round or multi-pole lens”, while the term “radial” means “in a direction perpendicular to the optical axis”. Redirector 31 can eliminate the positional effect on reception/detection of transmission electrons. The scanning field of primary beam 12 can be enlarged dramatically by adjusting first redirector 21 and variable axis objective lens 42 coordinately in raster-scanning process, and high image resolution can still be maintained. When redirector 21 redirects or deflects primary beam 12 at a large scanning field edge, off-axis aberrations will be significantly decreased because primary beam 12 can enter into the region near axis 44 For simplicity and clarity of illustration, other parts in the STEM are omitted in the figures. These parts may be, for example, electron optics, condenser lens, various control units, image aperture, correctors such as stigmators, image formation unit, and other optical components. A stigmator can compensate the beam shape of the primary electron beam that is incident onto the specimen plane 14. Other components in the STEM may include a retarding electrode below the variable axis objective lens and having an opening aligned with the optical axis for the primary electron beam passing through; and a specimen stage below the retarding electrode and supporting the specimen. It should be appreciated that a specimen with certain thickness, as conceptualized and represented as specimen plane 14, is typically placed on a stage system (not shown) for adjusting the specimen height to the focused plane of electron beam 12, and for moving observed area of specimen. For example, the stage system may include a supporting stand for sustaining the weight of the system; a z-stage providing a degree of freedom in vertical direction respect to the ground; and an X-Y stage for a degree of freedom in the two horizontal direction respect to the ground. In an example, the specimen is located between an upper pole piece and a lower pole piece of the magnetic lens field generator 41, as known to a skilled artisan in the field, to make an immersion objective lens system. An immersion objective lens ensures a small focused spot on specimen plane 14 because the spherical aberration coefficient and chromatic aberration coefficient are smaller than the objective lens field far away from the specimen plane. As previously mentioned, variable axis objective lens 42 focuses electron beam 12 that has been redirected by first redirector 21 to a focusing spot on specimen plane 14. The focusing spot may be a round dot with a dimension of e.g. 1 or 2 nm. However, the focusing spot may also take other shape such as oval. The central point of the focusing spot is conceptually defined as focusing point O2. As shown in The maximum scanning field of the STEM according to the invention can be 500 um×500 um, 250 um×250 um, or 100 um×100 um scanning field with a resolution small than 5 nm, or smaller than 2 nm. For example, the STEM of the invention may have a scanning field of 40 um×40 um with a 2-nm resolution in one setting; and a scanning field of 10 um×10 um with a 0.5-nm resolution in another setting. In non-limiting examples, first redirector 21 generates a first deflective field, second redirector 31 generates a second deflective field, but none of said two fields has an overlap with the magnetic field of lens module 15. For example, first redirector 21 may include a pre-scan deflective system that works with the first in-lens deflector 43 for scanning the primary electron beam on the specimen plane 14. In other words, first redirector 21 and the first in-lens deflector 43 can be adjusted or tuned in a coordinated fashion to scan electron beam 12 across a target area on the specimen plane 14. The deflective field of the pre-scan deflective system has no overlap with variable axis objective lens 42, let alone variable axis collection lens 44. In embodiments of the invention, the pre-scan deflective system includes 1, 2, 3, 4 or more deflectors. By the same token, an example of second redirector 31 may include an after de-scan deflective system that works with the second in-lens deflector 45 for correcting the transmission electron beam through the specimen back to the center axis of detector (part of reference axis 13). Similarly, the deflective field of after de-scan deflective system has no overlap with variable axis collection lens 44, let alone variable axis objective lens 42. In embodiments of the invention, the after de-scan deflective system includes 1, 2, 3, 4 or more deflectors. In a specific embodiment, the pre-scan deflective system includes two deflectors for pre-deflecting the primary electron beam on an axis perpendicular to specimen plane 14 or parallel to (but not overlaps with) reference axis 13. These two deflectors can work with the first in-lens deflector 43 for laterally or parallel moving (like “sliding” on specimen plane 4) the central axis of the objective lens 42, as described above. The after de-scan deflective system may also have two deflectors for correcting the transmission electron beam back to the central axis of detector 19, which coincides with reference axis 13. In another specific embodiment, the pre-scan deflective system includes only one deflector for pre-deflecting the primary electron beam along a swinging axis inclined to specimen plane 14. This single deflector can work with the first in-lens deflector 43 for swinging the central but variable axis 42 In non-limiting examples, detector 19 may have (A) a disk-shaped bright-field detective area in the center to catch only electrons that have directly irradiated onto, and transmitted through, the specimen; and (B) a ring-shaped dark-field detective area outside the disk to catch electrons that have scattered-in and transmitted through the specimen. In some embodiments, the bright-field detective area and dark-field detective area are located on different height to enhance receiving efficiency. When the primary beam 12 is focused on a specimen in an immersion field, electrons that transmit the thin film specimen is focused and projected on detector 19. The transmission electrons carry the information about the structure and materials of the specimen. The detector is usually divided into bright-field detective portion and dark-field detective portion to catch the transmission electron with different angular ranges. The bright-field detector collects the direct transmission electrons and the dark-field detector mainly collects the scatter transmission electrons taking atomic number signal in large angular range. In the STEM of the invention, the transmission electrons from the entire scanning field can be projected on detector 19 circle-symmetrically as transmission electrons from point O1. The bright-field and dark-field detectors can therefore catch the signal transmission electrons precisely corresponding to transmission angle, so the image quality and contrast at the edge of the scanning field is as good as that of the scanning center O1. A high resolution image in large scanning field can therefore be obtained. As an advantage of the invention, the transmission electron beam 12 according to the radial angle as detected by detector 19 is independent of (not affected by) the location of the focusing spot or focusing point O2 on specimen plane 14. In other words, the detector for receiving the transmission electron beam according to the radial angle has nothing to do with the scanning position on the specimen plane. In a group of embodiments featured by a moving (or “sliding”) objective lens as described above, the first redirector 21 may include two or more deflectors for redirecting electron beam 12 to a path perpendicular to the specimen plane 14. The first in-lens deflector 43 has a coupling field with the magnetic field of the objective lens 42 for moving (or “sliding on plane 14”) the central axis 42 First redirector 21 in this example includes a pre-scan deflective system 204 as shown in Lens module 15 in this example may comprise magnetic lens 205, first in-lens deflector 212 With reference to Under a working principle similar to the variable axis objective lens above specimen 206, the variable axis collection lens below specimen 206 is used to collect, and correct the trajectory of, transmission electron beam 209 from large scanning field edge. With reference to If central optical axis 203 D (z) is the in-lens deflective field of 212 In another group of embodiments featured by a “swing” objective lens, the first redirector 21 comprises a single deflector for redirecting the electron beam to a path not “substantially perpendicular” (or inclined) to specimen plane 14. The first in-lens deflector has a coupling field with the magnetic objective lens field located above the specimen plane for swinging the central axis thereof, so that said central axis is substantially in alignment with the redirected electron beam. The second in-lens deflector has a coupling field with the magnetic collection lens field located below the specimen plane for swinging the central axis thereof, so that said central axis is substantially in alignment with the electron beam when the beam exits from the collection lens. The second redirector 31 comprises a single deflector that redirects the electron beam to a path in alignment with the reference axis before the beam reaches the detector. In lens module 15 of STEM 300, first in-lens deflector 312 The coupling condition between the deflective field of deflectors 312 In summary, the present invention can provide a large scanning field with high resolution image in STEM. A detailed image in large area on specimen can be obtained at one time. In other words, the scanning process can be completed using a single and continuous scanning operation. As a merit, the throughput of STEM can be significantly improved. The invention provides a scanning transmission electron microscope (STEM) having both large scanning field and high image resolution. The sample is typically put into an immersion focal field. A variable axis objective lens (VAL) including a convergent field and a deflector above the sample can focus and scan a primary electron beam on the sample, to obtain a large scanning field with low off-axis aberrations. Another variable axis lens (VAL) including a focused field and a deflector below the sample can converge and correct the transmission electrons back to central axis of the STEM, to ensure the quality of transmission image. The invention improves the throughput of STEM at high resolution by utilizing large scanning ability of a variable axis objective lens combined with a variable axis collection lens. As briefly mentioned above, the present invention can, on one hand, enlarge the STEM scanning field, while on the other, maintain high image resolution. In an exemplary embodiment, the lens module 15 of the invention may include a variable axis objective lens for primary scanning beam and a variable axis collection lens (or projection lens) for transmission signal beam. It should be appreciated that other configurations of lens module 15 are also possible. The variable axis objective lens may include an immersion objective lens and a coupled deflector. Thin film specimen is put between upper pole piece and lower pole piece of a magnetic lens, at or near the maximum field plane, to ensure small spot size of focused primary beam so as to achieve high image resolution. A specially designed deflector has a coupled field with the objective lens is able to shift the center optical axis of the objective lens. When the primary electron beam is deflected by the pre-scanning deflector(s) to off-axis position, the coupled deflector shift the axis of objective lens to eliminate the off-axis chromatic aberrations and/or coma aberrations, or reduce them to a near zero level. So the primary beam can scan over a much larger field in the STEM of the invention without degraded image resolution caused by off-axis aberrations at scanning field edge. Furthermore, a de-scan variable axis detective system is provided in this invention. This equipment ensures electron beam which has been scanned away from center axis in large field can be detected appropriately after passing through specimen as the center electron beam. A deflector coupled with the focus field of the collection lens below the specimen has the ability to shift the center optical axis of the collection lens, in a manner similar to variable axis objective lens, to eliminate off-axis aberrations of the signal electrons passed though specimen. The transmission electron beam is focused by the variable axis collection lens below the specimen, then de-scanned back to center axis, and at last detected by a transmission electron detector. The detector is composed of a bright-field detective area and a dark-field detective area. The transmission electron beam from the central area of the scanning field, and the beam from the edge or peripheral area, are detected according to transmission angle with the same effect by bright-field and dark-field areas. Such same effect provides similar image resolution and similar contrast ratio all over the large scanning field. In contrast, conventional STEMs cannot provide an image resolution and contrast in the scanning edge, same as or similar to those in the scanning center, because the influence of different transmit position in the scanning field is not compensated. In accordance with the inventions, relative larger scanning field and high resolution image are obtained in STEM without the negative influences caused by off-axis aberrations and scanning positions. STEM of the invention can scan a much larger field of specimen in each time and can accomplish a higher throughput, as compared to conventional STEMs. Various embodiments of the present invention are described above in detail. However, those skilled in the art should understand that various modifications, combinations or sub-combinations may be made to these embodiments without departing from the principles and spirits of the present invention and such modifications should fall within the scope of the present invention. The present invention provides a scanning transmission electron microscope (STEM). In the STEM, a specimen is sandwiched between a variable axis objective lens and a variable axis collection lens. The axis of the collection lens varies along with the variation of the objective lens axis in a coordinated manner. The STEM of the invention exhibits technical merits such as large scanning field, high image resolution across the entire scanning field, and high throughput, among others. 1. A scanning transmission electron microscope (STEM) comprising:
an electron source for emitting a primary electron beam; a detector for receiving the electron beam, wherein a reference axis is defined by the straight line connecting the electron source and the detector; a specimen plane located between the electron source and the detector, wherein the reference axis is perpendicular to the specimen plane; a first redirector that redirects the electron beam to a path not in alignment with the reference axis; a lens module comprising a variable axis objective lens and a variable axis collection lens, between which is the specimen plane, wherein the variable axis objective lens is located between the electron source and the specimen plane for focusing the electron beam redirected by said first redirector to a focusing spot on the specimen plane, and wherein the variable axis collection lens is located between the specimen plane and the detector for collecting the electron beam that has passed through the specimen plane; and a second redirector that redirects the electron beam that has been collected by the variable axis collection lens back to a path in alignment with the reference axis, before the beam reaches the detector. 2. The STEM according to 3. The STEM according to 4. The STEM according to 5. The STEM according to 6. The STEM according to 7. The STEM according to 8. The STEM according to 9. The STEM according to 10. The STEM according to 11. The STEM according to 12. The STEM according to 13. The STEM according to 14. The STEM according to 15. The STEM according to 16. The STEM according to 17. The STEM according to 18. The STEM according to 19. The STEM according to 20. An electron microscope comprising:
an electron source for emitting a primary electron beam; a detector for receiving the electron beam;
a specimen plane located between the electron source and the detector, wherein at least a portion of the electron beam can pass through the specimen plane; and a variable axis collection lens located between the specimen plane and the detector for collecting the electrons that have passed through the specimen plane.CROSS-REFERENCE TO RELATED U.S. APPLICATIONS
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REFERENCE TO AN APPENDIX SUBMITTED ON COMPACT DISC
FIELD OF THE INVENTION
BACKGROUND OF THE INVENTION
SUMMARY OF THE INVENTION
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT











