Abstract:
We have developed a particular combination of elements and devices which enables the portability of a scanning electron microscope (SEM). In particular the combination enables a small size, typically less than about 50 liters, a manageable weight, typically less than about 15 kg, and a low power requirement, typically less than about 100 W, which permits operation using power supplied from a portable source such as a battery. Higher performance versions may exhibit increased volume in the range of about 150 liters, increased weight, in the range of 45 kg, and a power requirement in the range of 300 W. The higher performance version of the portable scanning electron microscope may be portable with the assistance of a dolly (rolling cart) or with the assistance of attached wheels and pulling appendage.

Description:
This application is a continuation application of U.S. application Ser. No. 10/453,117, filed Jun. 2, 2003, which issued as U.S. Pat. No. 6,897,433 on May 24, 2005. 
    
    
     FIELD OF THE INVENTION 
     In general, the present invention relates to a portable scanning electron microscope which is sufficiently small in size and light weight that it can be transported to and used at locations which have not been possible with present day scanning electron microscopes. Further, the invention enables the inspection of samples which will not fit into a standard vacuum chamber. 
     BRIEF DESCRIPTION OF THE BACKGROUND ART 
     The electron microscope was developed to enable imaging of objects and substrates which are too small to be imaged by light microscopes, due to the length of the light wave. The first electron microscopes were developed in the early 1930&#39;s and were limited in their resolution by problems such as specimen heating and specimen damage due to high electron energy exposure in general. Considerable work was needed to produce a proper condenser, polepieces for objective and projective, as well as airlocks for specimens and photoplates. 
     Today, the scanning electron microscope is widely used, mainly for the study of surfaces as well as transparent specimens. Two major applications for the scanning electron microscope are analytical inspection and lithography. 
     Krans et al., in U.S. Pat. No. 6,218,664 B1, issued Apr. 17, 2001, describe a scanning electron microscope (SEM) provided with an electrostatic objective and an electrical scanning device. The design for the particle-optical apparatus disclosed includes a particle source for producing a primary beam of electrically charged particles which travel along an optical axis of the apparatus towards a substrate/specimen to be irradiated. The primary beam is focused using electrostatic electrodes, to provide a focus point which is in the vicinity of the specimen to be exposed to radiation. A beam deflection system located between the source of the primary beam and the electrostatic electrodes is used to deflect the primary beam so that tile beam can be rapidly scanned over the surface which is to be analyzed. The detection means has a longitudinal axis which is essentially perpendicular to the longitudinal axis of the source of the primary beam, which travels through a bore present in the detection means. The design is said to provide advantages in tents of reducing imaging error regardless of the magnitude of the scanning motion of the primary beam. 
     U.S. Pat. No. 6,320,194 B1 to Khursheed et al. describes a “portable” high resolution scanning electron microscope column using permanent magnet electron lenses.  FIG. 3  in the &#39;194 patent illustrates the SEM column, which includes a condenser lense which provides demagification of the electron beam using two cylindrical shaped permanent magnets. The condenser lense has a center bore for passage of an electron beam from an electron beam gun. Two cylindrical coils are positioned around a pole piece cylinder, through which the primary electron beam passes. Electric current is passed through the coils for the purpose of adjusting the level of demagnification/condensing of the primary electron beam&#39;s spot size. The SEM column also includes an objective lens. The objective lens includes a tapered objective lens pole piece structure including a cylinder having a bore through which the electron beam passes. The objective lens is also designed to generate the magnetic field primarily through use of a permanent magnet or magnets which are positioned as far away from the pole piece as is practical. The permanent magnet is used in combination with a tuning coil which is used to adjust the focus of tie beam on the specimen. The description in the Khursheed et al. patent illustrates a compact electron optics, but does not address the mobility of the electron microscope system which is significantly affected by elements other than the electron optics. The mobility of the system as a whole is particularly important for use at locations which are difficult to access. 
     In an article entitled “Electrostatic einzel lenses with reduced spherical aberration for use in field-emission guns”, J. Vac. Sci. Technol. 15 (3), May/June 1978, G. H. N. Riddle describes focusing properties and aberration coefficients for electrostatic einzel lenses suitable for use as preaccelerator lenses in field-emission electron guns. Various lens shapes are analyze(, and asymmetric designs with conical central electrodes are found to have reduced spherical aberration. A lens shape with optimized geometry is found to have a spherical aberration coefficient of less than six times the working distance from the lens to the focal point. The article describes trade-offs between spherical and chromatic aberration, depending on factors such a beam current required, the current/solid angle which is drawn from the emitter, and the voltage spread in the beam. The latter factor is said to depend primarily on emitter temperature, which to a large extent, is determined by vacuum characteristics at the tip of the emitter. Again, this reference focuses on miniaturized electron optics but does not describe an entire system Which is sufficiently mobile to be independently used at locations which are difficult to access. 
     In a presentation at the 1999 NASA/JPL Workshop on Miniature Vacuum Pump Technology, John L. Callas of the Jet Propulsion Laboratory described capabilities for a scanning electron microscope which might be used to sample and analyze specimens from a surface on the planet Mars. A comparison was made of a system which included emerging microcolumn technology and more standard SEM technology. However, the overall system requirements, such as power supply and pump down requirements for this system were considerably different from the requirements for most applications useful on earth. In addition the sample stage described was inadequate for use in a portable scanning electron microscope. 
     SUMMARY OF THE INVENTION 
     We have developed a particular combination of elements and devices which enables the portability of a scanning electron microscope (SEM). In particular the combination enables a small size, typically less than about 50 liters, a manageable weight, typically less than about 15 kg, and a low power requirement, typically less than about 100 W, which permits operation using power supplied from a battery source. Higher performance versions way exhibit increased weight, which is typically less than about 45 kg, and these may be portable with the assistance of a dolly (rolling cart) or with the assistance of attached wheels and pulling appendage. 
     The portable SEM makes use of low beam energies, in the range of about 1 keV to 10 keV, more typically in the range of about 1 keV to 5 keV, and even more typically within a range of about 1 keV to 3 keV, since this latter range is more advantageous because of smaller size and weight requirements. These low beam energies allow analytical imaging of, for example, dielectrics without charging effects. The low beam energies also reduce damage to biological samples analyzed using the portable scanning electron microscope. The typical magnification of the portable SEM enables imaging of feature sizes as small as about 50 nm for a back pack-sized SEM and about 10 nm for a dolly SEM. This makes the portable SEM useful for analysis of metallurgical grain boundaries. 
     The basic optical system consists of a source for generating charged particles. These particles are focused onto a specimen by focusing elements and then are deflected by scanning elements. During analysis of a surface, secondary species generated upon contact of the primary charged particles with the contact surface, backscattered species, or a combination thereof are collected by a detector. The scan is generated by a system of electronics that coordinate the generation of the scan signals and the collection of the signal from the detector. The scan is then sent to a user interface. 
     The charged particles may also be used to do lithography. In this case, the charged particle beam can be placed at a particular, desired location on a substrate surface using software, where a blanker or beam defocus is used while tile beam is moved from one location to another, to expose a resist which is sensitive to charged particles. 
     The main attributes of the design which enable the portable SEM are: use of a minimum number of focusing elements to reduce the necessary electronics; avoidance of current drawing elements (all elements are permanent magnetic and/or electrostatic) to reduce the size of a portable power source, for example batteries; an electron optical system which is made small, by way of example using micromachined (for example, MEMS) elements, so that the entire optical system exhibits a footprint which is less than about 4 square centimeter, typically less than about 1 square centimeter, and exhibits a length which is less than about 15 centimeters, typically less than about 10 cm. Reduction in the optical system volume reduces the amount of vacuum pumping necessary, thereby reducing both power consumption and vacuum pump size and weight. 
     The sample may be presented on a sample stage which is a modular unit attached to another module housing the electron optical system. In the alternative, the sample may be a surface to which a suction-sealed housing is attached, with the electron optical system module attached to the suction-sealed housing. When the sample stage modular unit is used, the sample size may be as large as about 3 cm×3 cm, having a thickness up to about 2 cm. 
     However, when the suction-sealed housing is used, there is no limit on sample size, only a limit on the surface area of the sample which can be viewed at one time. For example, the wing of a Boeing 747 or Airbus 340 could be the sample, and the suction sealed housing could be attached to a point of interest on the wing. The dimensions of the sample which can be analyzed without moving the suction-sealed housing are limited by the maximum scan field of either the electron optics or a mechanical stage which moves the electron column with respect to the suction-sealed housing (rather than moving the sample). The mechanical stage for moving the electron column may be similar to the mechanical stage shown for moving the sample in  FIG. 4 . 
     For a portable SEM which can fit into a back pack, the maximum scan field is typically about 1 mm by 1 mm, which can be extended by operation of the mechanical stage to about 3 cm by 3 cm. When the scan field is the same for the dolly SEM as for the back pack SEM, an improvement by a factor of about 25 in the resolution can be achieved for the dolly SEM due to the availability of improved electron optics which is permitted by a size and weight increase. When the suction-sealed housing is attached to a surface of tile sample itself, tile surface roughness and curvature of the surface must be considered in determining the mechanism necessary to seal against the sample surface. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a cross-sectional schematic of a basic optical system of the kind useful in the portable SEM of the present invention, including but not limited to the primary electron beam source, combination electrostatic/magneto static optics, scanning deflection elements, integrated low-energy secondary and backscattered electron detector, sample stage, vacuum system, and a user interface (display). 
         FIG. 2  shows a cross-sectional schematic of one embodiment of a basic electron optical system which meets the combinational requirements necessary for a portable SEM. 
         FIG. 3A  shows a cross sectional schematic of one embodiment of a portable SEM of the present invention used with a sample stage, where the sample stage is in the open position. 
         FIG. 3B  shows a cross-sectional schematic of the  FIG. 3A  portable SEM, with the sample stage in the closed position. 
         FIG. 4  shows a cross sectional schematic of one embodiment of a sample stage  400  which may be used as part of the portable SEM. The electro optical system is present in the tube/pipe area  430  shown attached to the sample stage  400 . 
         FIG. 5A  shows a basic cross-sectional schematic of a suction-sealing housing which employs a bellows. 
         FIG. 5B  shows the suction-sealing housing of  FIG. 5A , sealed against a sample prior to the application of significant vacuum to the interior of the housing. 
         FIG. 5C  shows the suction-sealing housing of  FIG. 5A , sealed against a sample after the application of high vacuum to the sample surface. 
         FIG. 6  shows an assembly of one embodiment of a suction-sealing housing which employs a bellows, in combination with SEM elements. 
         FIG. 7A  shows a basic cross-sectional schematic of a suction-sealing housing which employs a flexible sealing lip. 
         FIG. 7B  shows the suction-sealing housing of  FIG. 5A , sealed against a sample prior to the application of significant vacuum to the interior of tile housing. 
         FIG. 7C  shows the suction-sealing housing of  FIG. 5A , sealed against a sample after the application of high vacuum to the sample surface. 
         FIG. 8  is a block diagram which shows the various elements of a portable SEM and the manner in which these elements relate to each other. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As a preface to the detailed description, it should be noted that, as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. 
     As mentioned in the Background Art section above, the present invention pertains to a portable SEM which may either be of a size and weight that it can be carried in a back pack or which may be of a size and weight that can be moved about on a dolly. The dolly-portable SEM may be used for inspection at manufacturing sites where space is constrained or may be loaded into the back of all off road vehicle or pickup trick and taken where needed, provided the landscape topography can be traversed by the dolly. In particular the combination of elements, devices and modules which make up the portable backpack SEM enable a small size, typically less than about 50 liters, a manageable weight, typically less than about 15 kg, and a low power requirement, typically less than about 100 W, which permits operation using power supplied from a battery source. The dolly-portable SEM is capable of providing greater performance, but exhibits somewhat increased size, typically less than about 100 liters, and increased weight, typically less than about 30 kg at a power requirement of about 200 W. 
     Tile portable SEM makes use of low beam energies, in the range of about 1 keV to 10 keV, more typically in the range of about 1 keV to about 5 keV, with a range of about 1 keV to 3 keV being more advantageous because of smaller size and weight requirements. These low beam energies allow analytical imaging of, for example, dielectrics without charging effects. The low beam energies also reduce damage to biological samples analyzed using the portable scanning electron microscope. The typical magnification of the portable SEM enables imaging of feature sizes as small as about 50 nm for the backpack SEM and about 10 nm for the dolly SEM. This makes the portable SEM useful for analysis of metallurgical grain boundaries. 
       FIG. 1  shows a basic optical system of the kind which may be used for the portable SEM. The basic optical system  100  comprises a source for generating charged particles  102 , which particles are focused onto a specimen  108  via focusing elements  104  and particle deflecting-scanning elements  106 . Secondary species which are generated in/at the surface of specimen  108  and backscattered species leave the surface and travel to a detector  110 . The surface of specimen  108  is scanned according to a pattern controlled by an electronics system  112  that coordinates the generation of a scan signal  116  to deflecting-scanning elements  106  with the collection of tile signal  118  from the detector  110 . The output from the scanning operations is then sent to a user interface  114  which is typically a display. 
       FIG. 2  shows a cross-sectional schematic of one embodiment of a basic optical system  200  which meets the combinational requirements necessary for a portable SEM. The main attributes of the design which enable the portable SEM are: use of a minimum number of focusing elements to reduce the necessary electronics (especially power supply); avoidance of current drawing elements (all elements are either permanent magnet and/or electrostatic) to reduce the size of the battery source required to power the portable SEM; an electron optical system which is made small, by way of example using MEMS elements, so that the entire optical system typically exhibits a footprint which is less than about 1 square centimeter and less than about 10 cm in length. Reduction in the optical system volume reduces the amount of vacuum pumping necessary, thereby reducing power consumption and size. The basic optical system  200  is a three element charged particle optical system of the kind proposed in the Riddle article which was mentioned in the Background Art. The basic optical system  200  includes a source  202  for generating charged particles, where primary charged species (not shown) are accelerated from a cathode (not shown) within the source  202  by an extraction element  2   a . Extraction element  2   a  is part of a lense  3 , where accelerated primary charged species are focused by an asymmetric focusing element  2   b  and accelerated to their final kinetic energy by an anode  2   c . The asymmetric lens design as illustrated by the Riddle article has been shown to form an optical system with minimal chromatic and spherical aberrations. The beam of primary charged species (not shown) is scanned using a post lens electrostatic octopole deflector  6   b . Optionally, a pre-correction deflector  6   a  can be included for centering, astigmatism correction and/or dynamic focus. This allows the scan signals (not shown) and the correction signals (not shown) to be separated. The post lense deflector  6   b  can be used to make the corrections referred to with deflector  6   a ; however, this complicates the electronics required to produce the signals. An electrostatic octopole deflector of the kind referred to with respect to the post lense deflector  6   b  is described in an article by H. S. Gross, F. E. Prins, and D. P. Kern entitled “Fabrication and characterization of an array of miniaturized electrostatic multipoles”, Microelectronic Engineering 41/42 (1998) 489–492. The post lense deflector  6   b  need not be an electrostatic octopole deflector, but may be, for example a twenty pole (icosopole) electrostatic deflector of the kind having 4 signal wires. With the appropriate selection of the electrode widths of tile electrostatic twenty pole deflector, the number of required signal wires can be reduced to four, making this design competitive with the electrostatic octopole. The advantage of the electrostatic twenty pole deflector is that less aberration occurs and only half of the signal wires are required than for the octopole. The disadvantage is that with only 4 signal wires astigmatism correction is no longer possible. An optimized system employs a combination of two deflectors, a pre-correction deflector  6   a  which is an octopole to correct astigmatism, and a post lense deflector which is a twenty pole beam deflector. 
     Secondary charged species (not shown) are generated within the surface of specimen  10  in response to the impingement of charged species (not shown). The secondary species generated within the surface of specimen  10  can be accelerated by a collection grid  8   a  to a detector  8   b . The collection grid  8   a  typically has, for example, a voltage in tile range of −100V to +100V applied. The collection grid  8   a  either attracts or repels secondary species, depending on the voltage applied, but has very limited effect on primary or backscattered species. 
     Examples of detectors suitable for this system are scintillation detectors and PIN diode detectors, both of which are commonly available within the art. For example, Hamamatsu Corporation, having offices in Bridgewater, N.J., provides both types of detectors. Suitable sources of charged particles can be obtained from FEI Corporation having offices in Hillsboro, Oreg. or from Denka Corporation, having offices in New York, N.Y. Further additional elements may be obtained from FEI Corporation or from Phillips Electronics, having offices at Sunnyvale, Calif. A roughing (for example, a diaphragm) pump is available from Rietschle Thomas Sheboygan, having offices in Sheboygan, Wis.; a high vacuum (For example, a turbomolecular) pump is available from Pfeiffer Vacuum Technology, having offices in Asslar, Germany; an ultra high vacuum (for example, an ion getter) pump is available from Varian Corporation, having offices in Palo Alto, Calif.; a customized (for electron lenses) high voltage power supply is available from Applied Kilovolts, having offices in Portslade, Sussex, UK; and one recommended embodiment sample stage is available from Thermionics Corporation, having offices in Port Townsend, Wash. 
     A further enhancement to the design shown in  FIG. 2  is to include magnetic elements  4   a  which are typically composed of a magnetic material embedded into the lense  3  design. A permanent magnet  4   b  is situated between the elements  4   a . This creates a static magnetic focusing element. The electrostatic element  2   b  then acts as a weak focusing element with the magnetic portion of the lense  3  providing the majority of tile focus. 
     The main attributes of the electron optical system shown in  FIG. 2  which make the design suitable for a portable SEM are: 1) A minimum number of focusing elements, reducing the necessary electronics; 2) No current drawing elements. All of the elements are either permanent magnet and/or electrostatic. The deflection elements are also electrostatic. This reduces the power consumption necessary for operation, thereby reducing the size of any battery operation; 3) The assembly is highly compact. The electron optical system can be made extremely small using either conventional machining or MEMS elements. The entire volume of the electron optical system can be reduced to only a few cubic centimeters. This limits the amount of vacuum pumping necessary, thereby reducing power consumption and size. 
     The sample to be examined using the portable SEM may be presented on a sample stage which is a modular unit attached to another module housing the electron optical system. In the alternative, the sample may be a surface to which a suction-sealed housing is attached, with the electron optical system module attached to the suction-sealed housing. When the sample stage modular unit is used, the sample size may be about 3 cm×3 cm having a thickness up to about 2 cm. 
     However, when the suction-sealed housing is used, there is no limit on sample size, only a limit on tile surface area of the sample which can be viewed at one time. For example, the wing of a Boeing 747 or Airbus 340 could be the sample, and the suction-sealed housing could be attached to a point of interest on the wing. The dimensions of the sample which can be analyzed without moving the suction-sealed housing are limited by the maximum scan field of a combination of the electron optics and a mechanical stage which moves the electron column with respect to the suction-sealed housing (rather than moving the sample). The mechanical stage for moving the electron column may be similar to the mechanical stage shown for moving the sample in  FIG. 4 . 
     For a portable SEM which can fit into a back pack, the maximum scan field is typically about 1 mm by 1 mm, which can be extended by operation of the mechanical stage to about 3 cm by 3 cm. When the scan field is the same for the dolly SEM as for the back pack SEM, an improvement by a factor of about 25 in the resolution can be achieved for the dolly SEM due to the availability of improved electron optics which is permitted by a size and weight increase. When the suction-sealed housing is attached to a surface of the sample itself, the surface roughness and curvature of the surface must be considered in determining the mechanism necessary to seal against the sample surface. 
       FIGS. 3A and 3B  show a cross sectional schematic of one embodiment of a portable SEM  300  of the present invention. This embodiment is a “vacuum chamber” concept employing a sample stage which includes a quick access door. The portable SEM  300  includes the vacuum chamber  302  which surrounds the entire SEM  300  when the quick access door  319  is closed. The quick access door  319  and sample stage  303  are shown in the open position in  FIG. 3A  and in the closed position in  FIG. 3B . In the sample-load position, the open position, the sample stage  312  surface  311  which is to contact the sample (not shown) is facing upward and in position for sample loading. In tile sample exposure position, the closed position, the sample stage  312  surface  311  which contacts the sample is facing the scanner/detector  328 . 
     The vacuum chamber  302 , as shown in  FIG. 3B , encompasses an ultra high vacuum pump  304 , a filament/emitter/suppressor  306 , a lens  308 , a high vacuum valve  313 , a scanner/detector  310 , and sample stage  312 . With reference to  FIG. 3B , the sample stage  312  can be moved in the z direction using positioning knob  326  and in the x direction using positioning knob  324 . The vacuum chamber  302  is pumped down through outlet (also inlet)  328  by a fore pump (not shown) and a high vacuum pump (not shown). The particular combination of pumps depends on the ultimate vacuum to be achieved. By using a staged combination of a roughening pump and a high vacuum pump, the maximum vacuum obtainable is in the range of about 10 −6  Torr. By using a staged combination of a roughening pump, a high vacuum pump and an ultra high vacuum pump, the maximum vacuum obtainable is in the range of about 10 −9  Torr. Examples of the kinds of pumps which are in the size and weight categories useful in the present invention include, but are not limited to, a roughening diaphragm pump 7106Z available from Rietschle Thomas Sheboygan, Sheboygan, Wis.; a high vacuum turbo molecular pump TPD 011 available from Pfeiffer Vacuum Technology of Asslar, Germany; and an ultra high vacuum miniature ion getter pump available from Varian Corporation of Palo Alto, Calif. 
     As shown in  FIG. 3B , the high vacuum valve  313  is in an open position, and has been opened using a bellows feed through attached to a turning knob or a motor  316  which acts against the spring loading on high vacuum valve  313 , which spring loading provides a normally closed position. Mechanical aperture  315  protects the electron optics system. The quick access door  319  is held in locked position by knob  320  which is typically a knurled knob. 
     The vacuum chamber  302 , as shown in  FIG. 3A  includes the same elements as those shown in  FIG. 3B , but is shown with the quick access door  319  open and the sample stage  312  in the loading position. With reference to  FIG. 3A , a vacuum which may have been generated during previous operations is released through outlet (also inlet)  328  to permit opening of quick access door  319 . The knob  320  has been adjusted to permit opening of the quick access door  319 , which opens on hinge  318 . When access door  319  is in the open position, the sample stage  312  can be moved in the x direction using positioning knob  326  and in the z direction using positioning knob  324 . A sample (not shown) can easily be placed on surface  311  of sample stage  312 . The high vacuum valve  313  is in the normally closed position. 
       FIG. 4  shows a cross sectional schematic of a second embodiment manual sample stage  400  which may be used as part of the portable SEM. The electron optical system (not shown) is present in the metal tube area  430  shown attached to the sample stage  400 . The chamber housing  402  of sample stage  400  typically has an outer diameter in the range of about 6.5 centimeters, although this chamber may be larger depending on other elements used in the SEM. The manual sample stage  400  is designed to be particularly compact and light weight. The sample stage  400  includes an x-axis drive  410 , a y-axis drive  408 , and a z axis drive  406 . The opening to the x-axis drive includes lightweight linear bellows feedthroughs  429  of the kind shown on the y-axis drive  408 , each bellows feedthrough having travel capabilities in the range of about ±15 mm. The bellows feed through is a robust and maintenance free feed through device. 
     The manual sample stage  400  is attached to the metal tube area  430  via a hinge and lock (not shown) which are located at a position  424  where sample stage  400  is attached to metal tube area  430 . An O-ring seal  426  is present between sample stage  400  and metal tube area  430 . A sample  422  to be scanned is mounted on a mounting plate  420  designed to meet requirements of the particular application of the portable SEM. A plunger  418  is used to move the sample mounting plate  420  along the z-direction. The sample  422  size is typically less than about 3 cm by about 3 cm and less than about 2 cm thick, by way of example and not by way of limitation. Yoke  416  is used to attach a feed through from the x-axis drive  410  to cube  414 . A similar yoke is used to attach a feed trough from the y-axis drive  408  to aluminum cube  414 . Guide rod  427  from the y axis direction and a similar rod (not shown) from tile x axis direction are used to maintain the sample in a level position with respect to the electron optics. An XYZ spider and hub assembly  412  are used to limit tile travel in the z direction in response to knob  406 . Vacuum pump down and release occur through aperture opening  428 . Aperture opening  428  also acts to protect the electron optics during operation of the portable SEM. Primary charged species and secondary generated species (not shown) pass through aperture opening  428  from the electron optical system (not shown) present within metal tube area  430 . A vacuum pumping system (not shown) is in communication with the opening  425  to metal tube area  430 . 
       FIG. 5A  shows a basic cross-sectional schematic of a suction-sealing housing  500  which employs a bellows  504 . The suction-sealing housing  500  is used against a surface of a sample or against a platform surface on which a sample is present. The electron optical system used in combination with the suction-sealing housing  500  may be attached to a wall  503  of main housing structure  502  or may be present within an interior  505  of main housing structure  502 . Main housing structure  502  is attached to a bellows assembly  504  which includes an upper attachment flange  507 , a bellows  509 , a bellows footing  506 , and a bellows sealing surface  508 , which is typically formed from a relatively soft sealing material such as a silicone. The attachment is made using a rigid fastener  510 , which may be a machine screw, for example. Sealing structures  512 , an O-ring for example, but not by way of limitation, seal the upper attachment flange  507  of bellows assembly  504  against main housing structure  502 . Sealing structures  514  seal the main housing structure  502  against the upper surface  521  of sample  520 , as shown in  FIG. 5C . A low vacuum, in the range of about 2 Torr is pulled  519  through opening  511  in main housing structure  502 . A high vacuum, in the range of about 10 −5  Torr is pulled  518  through opening  513  of main housing structure  502 . Tile vacuum pumps (not shown) used to produce a low vacuum at outer sealed locations  524  and an high vacuum at inner sealed location  526 , as shown in  FIG. 5C  are of the kind known in the art and are selected to be small in size and light in weight, such as those previously described herein. 
       FIG. 5B  shows the suction sealing housing  500  sealed against upper surface  521  of sample  520  prior to the application of high vacuum to the assembly  530 , wherein bellows  504  is not yet fully compressed, and sealing structure  514  is not yet in contact with upper surface  521  of sample  520 .  FIG. 5C  shows the suction sealing housing  500  sealed against the upper surface  521  of sample  520  after the application of high vacuum to the assembly  530 , wherein bellows  509  is fully compressed. And sealing structure  514  is sealed against upper surface  521  of sample  520 . The pressure within suction sealing housing  500  is staged so that a low vacuum is present at outer locations  524 , in the range of about 2 Torr; and, a high vacuum is present at the inner location  526 , in the range of about  10   −5  Torr. 
       FIG. 6  shows a cross-sectional schematic of one embodiment of an assembly  600  of a suction-sealing housing  630  with attached SEM elements. Assembly  600  includes, not by way of limitation, a second stage vacuum pump which is a high vacuum pump or ultra high vacuum pump  602 , which may be a turbo molecular pump, or a combination of a turbo molecular pump with a miniature ion getter pump, respectively. The second stage vacuum pump  602  is sealed against a column tube  604  which provides an exterior housing for electron optics housing  605 . Column tube  604  is sealed against a suction-sealing housing  630  via an upper seal  607  secured by a clamping means  606 . Suction sealing housing  630  includes, not by way of limitation, an outer sealing surface area  617  and an inner sealing surface area  615  where a seal is created against an upper surface  621  of a sample  620  using a relatively soft material such as a soft VITON®. The outer sealing mechanism includes, but is not limited to, a structure  627  which includes a base sealing structure  632 , a flexible ring  634  and an upper sealing stricture  636 , all of which are constructed from a flexible material. The base sealing structure  632  and upper sealing structure  636  are preferably constructed from a compressible and tacky material. Interior to structure  627  is a rigid cone-shaped torus  614  having an upper pressure transfer foot  610  and a lower transfer foot  622 . Interior to cone-shaped torus  614  is an inner high vacuum chamber  625  which is constructed using, not by way of limitation, an upper sealing housing portion  624  which is in contact with upper seal  607 , a stiff bellows  618  which is sealed against upper sealing housing portion  624  and a lower sealing housing portion  626  which is sealed against stiff bellows  618  and which is in contact with a second base sealing structure  628 . Upper sealing housing portion  624  and lower sealing housing portion  626  are constructed from a rigid material, while inner second base sealing structure  628  is constructed from a soft sealing material such as the VITON® previously mentioned. The first stage lower vacuum is created between rigid ring  614  and inner high vacuum chamber  625  in the area marked  611  via application of a low vacuum roughing pump (not shown) attached to roughing pump line  608 . The second stage high vacuum is created in inner high vacuum chamber  625  in the area marked  613  via application of the vacuum pump  602 . 
       FIGS. 7A–7C  show a basic cross-sectional schematic of a suction-sealing housing which employs a flexible sealing lip. The basic structure of the suction-sealing housing is very similar to that shown in  FIGS. 5A–5C . 
     The suction-sealing housing  700  is used against a surface of a sample or against a platform surface on which a sample is present. The electron optical system used in combination with the suction-sealing housing  700  may be attached to a wall  703  of main housing structure  702  or may be present within an interior  705  of main housing structure  702 . A flexible seal lip  704  is attached to an attachment flange  707  which is fastened to main housing structure  702  via a fastener  710 . Sealing structures  712 , an O-ring for example, but not by way of limitation, seal the upper attachment flange  707  against main housing structure  702 , as shown in  FIGS. 7B and 7C . Sealing structures  714  seal the main housing structure  702  against the upper surface  721  of sample  720 , as shown in  FIG. 7C . A low vacuum, in the range of about 2 Torr is pulled  719  through opening  711  in main housing structure  702 . A high vacuum, in the range of about 10 −5  Torr is pulled  718  through opening  713  of main housing structure  702 . The vacuum pumps (not shown) used to produce a low vacuum at outer sealed locations  724  and an high vacuum at inner sealed location  726 , as shown in  FIG. 7C  are of the kind known in the art and are selected to be small in size and light in weight, such as those previously described herein. 
       FIG. 7B  shows the suction-sealing housing  700  sealed against upper surface  721  of sample  720  prior to the application of high vacuum to an assembly  730  which includes the sample surface.  FIG. 7C  shows the suction sealing housing  700  sealed against the upper surface  721  of sample  720  by contact with flexible seal lip  704  after the application of high vacuum to the assembly  730 , where sealing structure  714  is sealed against upper surface  721  of sample  720 . The flexible seal lip  704  is typically constructed from a soft material which preferably exhibits a tacky surface. A polysilicone works well for this application. The pressure within suction sealing housing  700  is staged so that a low vacuum is present at outer locations  724 , in the range of about 2 Torr; and, a high vacuum is present at the inner location  726 , typically in the range of about 10 −5  Torr. 
       FIG. 8  is a block diagram  800  which shows the various elements of a portable SEM and the manner in which these elements relate to each other. The basic elements of the portable SEM are shown within a vacuum chamber  830 , where the basic elements of the portable SEM include, but are not limited to: an ultra high vacuum pump  812 , a filament/emitter/suppressor  814 , electron lens  816 , ultra high vacuum valve  818 , stigmator  820 , electron scanner  822 , and electron detector  824 ; vacuum gauge  826 , high vacuum pump  825  and fore pump  827  are in communication with vacuum chamber  830  in the manner shown in block diagram  800 . An up-to-air valve with filter combination  828  and quick access door/sample positioning stage  823  are also in communication with vacuum chamber  830  through an aperture  821 . In this  FIG. 8 , the up-to-air valve is directly connected to the sample positioning stage, whereas, in  FIGS. 3A and 3B , the up-to-air valve is connected indirectly through orifice  328 . The power to the system is supplied either via a power source  804  such as a battery, or from an electrical outlet  802 , depending on availability. The power is applied to a low voltage power supply (which includes a battery charger)  806  which is controlled by an analog-digital/digital-analog converter  808  which is used in conjunction with numerous other devices as shown on block diagram  800 . For example, but not by way of limitation, the analog-digital/digital-analog converter  808  is used in conjunction with a display  840 ; various high voltage power supplies  810 ; a video card interface to the display  840 ; various pumps used in the system such as a fore pump  827 , a high vacuum pump  825 , the ultra high vacuum pump  812 ; ultra high vacuum valve  818 ; and electron detector  824 . The video card  838  interfaces with the scanning electronic  836  which is in communication with stigmator  820  and electron scanner  822 . Video card  838  also interfaces with low noise amplifier  834  which is in communication with electron detector  824 . Signals from video card  838  transmit through the analog-digital/digital-analog converter  808  to display  840 , which is typically a laptop computer. 
     The portable SEM of tile present invention is operated in the manner of existing large scale SEMs and one skilled in the art would be able to operate the portable SEM with minimal experimentation to arrive at appropriate operating conditions for the particular application. 
     The above described preferred embodiments are not intended to limit the scope of the present invention, as one skilled in the art can, in view of the present disclosure, expand such embodiments to correspond with the subject matter of the invention claimed below.