Abstract:
A sample holder assembly includes a sample tray, a base plate, a stage mount, and a calibration standard mounted onto the stage mount. Three mating structures on the bottom of the base plate mate with corresponding structures on a stage mount that is attached to the sample stage of the SEM. An optional contacting conductor provides electrical contact between the stage mount and the base plate so that charge generated on the sample by the electron beam can leave the sample through the sample conductive layer to the sample tray, to the base plate, to the stage mount, and through the grounded stage.

Description:
TECHNICAL FIELD OF THE INVENTION 
       [0001]    The present invention relates to a sample holder for mineralogical samples for x-ray spectroscopic analysis. 
       BACKGROUND OF THE INVENTION 
       [0002]    Mineral analysis systems, such as the Qemscan and MLA from FEI Company, have been used for many years to determine minerals present in mines in order to determine the presence of valuable minerals. Such systems direct an electron beam toward the sample and measure the energy of X-rays coming from the material in response to the electron beam. One such process is called “energy dispersive x-ray analysis” or “EDS,” which can be used for elemental analysis or chemical characterization of a sample. 
         [0003]    In EDS analysis, a high-energy beam of charged particles such as electrons or protons, or a beam of X-rays, is focused into the sample being studied to stimulate the emission of X-rays from the sample. The energy of the X-rays emitted from a specimen is characteristic of the atomic structure of the elements making up the specimen. By measuring the number and energy of the X-rays emitted from a specimen using an energy-dispersive spectrometer and comparing the measured spectra to a library of reference spectra of known compositions, the unknown elemental composition of the specimen can be determined. EDS analysis, especially when coupled with back-scattered electron (BSE) analysis, can also be used to quantify a wide range of mineral characteristics, such as mineral abundance, grain size, and liberation. Mineral texture and degree of liberation are fundamental properties of ore and drive its economic treatment, making this type of data invaluable to geologists, mineralogists, and metallurgists who engage in process optimization, mine feasibility studies, and ore characterization analyses. 
         [0004]    Mineral analysis systems of this type are also used in the oil and gas industry, as well as mines. Drill cuttings (drill bit-induced rock chips) and diamond drill cores can be analyzed to allow geologists to determine the exact nature of the material encountered during drilling, which in turn allows more accurate predictions as to the material still ahead of the drill, thus reducing risk in exploration and production. During drilling, a liquid referred to as “mud” is injected into the well to lubricate the drill and return the cuttings out of the well. A sample can be taken from the mud that includes cuttings from the drill. Great importance is often placed on documenting cuttings and cores as accurately as possible, both at the time of drilling and post-drilling. Characterizing down-hole lithological variation in a reservoir sequence is a critical requirement in exploration wells and production wells, and mineralogical and petrographic studies underpin the fundamental understanding of reservoir and seal characteristics. Traditional optical, scanning electron microscope (SEM), electron probe microanalysis (EPMA), and X-ray diffraction (XRD) analysis methods are well established and widely used within the industry. 
         [0005]    Samples for use in analytical instruments such as Qemscan and MLA systems are prepared so that the material to be analyzed is presented to the instrument as a flat, carbon coated surface within a sample block, typically 30 mm in diameter. Material to be analyzed, such as material retrieved from a mine, is carefully sampled from the mine, crushed, and mixed with epoxy in a mold. The sample is cured and then the sample block is removed. The sample block is ground and polished to expose the interior of some of the particles and to produce a smooth surface. The surface is coated with a carbon film to form a conductive coating to prevent electrical charging by the electron beam. 
         [0006]    The sample block is then placed into a sample holder and clamped in place. Exchange of the older style sample holders requires operator skill and an understanding of the mating surfaces, careful alignment conducted by eye and, in some instances, use of a tool. Manually aligning by operator eye can be difficult and is a frequent source of error. If the sample holder is not seated and aligned correctly, which can only be confirmed by completing system set up, the whole process may need to be redone. That is, the beam is turned off, the vacuum chamber vented, and the sample holder removed and re-installed. A less experienced operator may fail to recognize that the sample holder is misaligned and make faulty measurements, losing many hours of work. 
         [0007]    Once the sample holder block is correctly positioned, the calibration points need to be re-entered into the software by the operator using both the SEM software controls and manual manipulation of the sample stage. This operation requires a clear understanding of the set up process and the knowledge and ability to complete a stage rotation alignment. The process relies on operator skill and is not readily automated. It would be preferable to have a system that is fast, repeatable, does not require a skilled operator, and is susceptible to automation. 
       SUMMARY OF THE INVENTION 
       [0008]    An object of the invention is to provide a sample holder that facilitates mineralogy applications in the field. 
         [0009]    In a preferred embodiment, a sample holder assembly includes a sample tray, a base plate, a stage mount, and a calibration standard mounted onto the stage mount. Three mating structures on the bottom of the base plate mate with corresponding structures on a stage mount that is attached to the sample stage of the SEM. An optional contacting conductor provides electrical contact between the stage mount and the base plate so that charge generated on the sample by the electron beam can leave the sample through the sample conductive layer to the sample tray, to the base plate, to the stage mount, and through the grounded stage. 
         [0010]    The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]    For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
           [0012]      FIG. 1  is an exploded view of a sample holder assembly embodying the present invention. 
           [0013]      FIG. 2  shows a detail of the sample tray component of the sample holder assembly of  FIG. 1 . 
           [0014]      FIG. 3  shows a top view of the base plate of the sample holder assembly of  FIG. 1 . 
           [0015]      FIG. 4  is an exploded view showing the sample holder assembly being placed onto the stage mount. 
           [0016]      FIG. 5  is a flow chart showing the preferred steps of using an embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0017]    Charged particle beam systems for mineral analysis are preferably rugged for use in the field near a mine or a well site. SEMS intended for use in the field are preferably adapted to be used by less skilled technicians and for automation. Such design attributes are also beneficial for SEMS used in laboratories. The alignment of the sample holder is preferably simple, precise, quick, and easily automated. 
         [0018]    A common issue in automated mineralogy is the inability of the beam to automatically return to previously set calibration points for automated system calibration and to return to sample locations for measurement after a sample exchange. If the calibration standard is positioned in the sample holder and removed with the sample holder, the sample holder must be carefully aligned in on the sample stage so that the calibration standard is positioned in a known location with respect to the beam. If the alignment is not correct, the beam will impact at a different point on the calibration sample each time, which could result in erroneous calibration and measurements. 
         [0019]    A preferred robust sample holder system of the present invention ensures precise and repeatable sample and calibration standard positioning and provides that the calibration and sample locations cannot be influenced by differences between operator skill levels. A preferred embodiment allows for greater accuracy and speed in manual operation and for automation by improving the ease of use of the sample holder. Preferred embodiments provide an easier, faster sample exchange process with precise repeatable locating of calibration standards and samples without operator influence on positioning at sample exchange. 
         [0020]    In preferred embodiments, the stage-to-sample holder assembly interface uses complementary aligning structures, such as ball and cone locating interfaces, on the sample holder assembly and the stage mount to locate and orient the sample holder relative to the stage or a stage mount mounted onto the stage. The calibration standard remains on the SEM stage as the sample holder is removed and replaced. The calibration standard provides the operator with a visual locator for correct orientation of sample holder assembly. 
         [0021]      FIG. 1  shows an exploded view of a preferred sample holder assembly  100  including a sample tray  102 , a base plate  104 , and a knob  106  that secures the sample tray  102  to the base plate  104  by screwing onto shaft  106  extending from the sample tray  102 . Multiple sample blocks  108  (one shown) are positioned at the six holes  110  in sample tray  102  and are secured between the sample tray  102  and the base plate  104  when the knob  106  is threaded onto a post  112  extended from sample tray  102  through base plate  104 . Base plate  104  includes conical indentation  120  to mate with corresponding mating structures on a stage mount as described below. Knob  106  allows sample holder assembly  100  to be assembled rapidly by an operator without the use of tools, such as screwdrivers. 
         [0022]      FIG. 2  shows a detail of the edge of the hole in sample tray  102 . Each sample hole  110  includes a counterbore  204  that provides an indentation  206  that positions the sample block  108  and provides a lip  208  having a diameter smaller than that of the sample block to prevent the sample block from passing through the hole.  FIG. 3  shows a top view of the base plate  104 , showing springs  302  that press the sample blocks  108  against the lip  208  to orient the sample at a known and repeatable position in relation to the base plate. The springs or other biasing means ensures the sample surfaces are flat, normal to the beam, and held at a known working distance from the column, as well as to ensure a good electrical contact to allow electrical charges to drain from the sample block  108  to the sample tray  102 . 
         [0023]      FIG. 4  shows sample holder assembly  100  (without sample blocks) being positioned onto stage mount  402 . Stage mount  402  is secured to a moveable stage (not shown) for a charged particle beam system. A calibration standard holder, such as a calibration column  404 , is secured to the stage mount  402 . Hemispherical structures  406  mounted onto stage mount  402  provide mating structures for the conical indentations in the bottom of the base plate  104 . Biasing means, such as leaf springs  408 , provide electrical contact between the stage mount  402  and the sample holder assembly  100 . The tension in leaf springs  408  is sufficient to provide electrical contact, but not sufficient to prevent seating of the conical indentations of sample holder  100  fully onto the hemispherical mating surfaces of stage mount  402 . Sample holder assembly  100  rests on stage mount  402  without being clamped during operation, with the weight alone of sample holder assembly  100  maintaining the contact between the aligning structures in the sample holder assembly and the aligning structures in the stage mount, therefore maintaining the sample holder in the proper position and orientation. Aperture  410  accommodates knob  106  ( FIG. 1 ) protruding from sample holder assembly  100 . 
         [0024]      FIG. 5  is a flow chart showing a method of using a sample holder. The sample holder assembly is loaded by turning the sample tray upside down in step  502  and in step  504 , the sample blocks are placed facing down into the sample tray with the sample at one or more of the hole locations. Each hole location includes a counterbore that provides an indentation that positions the sample block and provides a lip having a diameter smaller than that of the sample block to prevent the sample block from passing through the hole. The bottom plate is then placed over the sample tray in step  506 . The bottom plate includes a biasing means, such as a spring, at each of the sample block locations to press the sample block against the lip, thereby positioning the top of the sample block at a consistent, known height above the bottom of the sample holder assembly, which assists in rapidly focusing the electron beam. By pressing the sample block into the lip, the spring also ensures a good electrical contact between the conductive top of the sample block and the sample tray. 
         [0025]    The base plate is then secured against the sample tray in step  508 , for example, by threading a knob nut onto a shaft extending from the sample tray extending through the base plate. The knob can be easily threaded onto and off of the shaft of the sample tray to rapidly change sample blocks by hand, without the use of tools. Other types of quick clamping devices may also be used to secure the sample tray to the base plate. 
         [0026]    The base plates include three conical indentations. The indentations are preferably manufactured separately and pressed into the base plate. The stage mount includes three hemispherical structures that mate with the three conical indentations on the bottom of the base plate. In step  510 , the SEM is opened to provide access to the stage mount. In step  512 , the sample holder assembly is set onto the stage mount, with the calibration cylinder fitting into a notch in the sample holder assembly to provide rough positioning of the sample holder assembly, with the rough positioning being sufficiently close so that the hemispherical structures on the stage mount will self align with the conical indentation in the conical indentations to produce a fine alignment. The aligning structures on the base plate and stage mount preferably constrain the sample assembly in six degrees of freedom. The orientation and height of the sample holder assembly, as well as the position, is determined by the aligning structures. Thus, precise positioning facilitates automation by facilitating automatic focusing to the known height. As will be recognized, the use of three conical indentations and three hemispherical structures overconstrains the sample holder assembly in three dimensions. The use of identical indentation and hemispherical structure reduces manufacturing costs, while the overconstraint does not decrease the precision to below an acceptable level. Maintaining the calibration standard in the sample chamber facilitates automation by providing a consistent position for the calibration standard, which position does not change as the samples are loaded and unloaded. 
         [0027]    Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions, and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.