Abstract:
A specimen enclosure assembly ( 100 ) for use in an electron microscope and including a rigid specimen enclosure dish ( 102 ) having an aperture ( 122 ) and defining an enclosed specimen placement volume ( 125 ), at electron beam permeable, fluid impermeable, cover ( 114 ) sealing the specimen placement volume ( 125 ) at the aperture ( 122 ) from a volume outside the enclosure and a pressure controller communicating with the enclosed specimen placement volume ( 125 ) and being operative to maintain the enclosed specimen placement volume ( 125 ) at a pressure, which exceeds a vapor pressure of a liquid sample ( 123 ) in the specimen placement volume ( 125 ) and is greater than a pressure of a volume outside the enclosure, whereby a pressure differential across the cover ( 114 ) does not exceed a threshold level at which rupture of the cover ( 114 ) would occur

Description:
REFERENCE TO CO-PENDING APPLICATIONS  
       [0001]     Applicant hereby claims priority of Israel Patent Application Serial No. 150056, filed on Jun. 5, 2002, entitled “Low Pressure Chamber for Scanning Electron Microscopy in a Wet Enviiomnment”.  
       FIELD OF THE INVENTION  
       [0002]     The present invention relates to specimen enclosures for scanning electron microscope (SEM) inspection systems and more particularly to fluid specimen enclosures.  
       BACKGROUND OF THE INVENTION  
       [0003]     The following patent documents are believed to represent the current state of the art: 
        U.S. Pat. Nos. 4,071,766; 4,720,633; 5,250,808; 5,326,971; 5,362,964; 5,417,211; 4,705,949; 5,945,672; 6,365,898; 6,130,434; 6,025,592; 5,103,102; 4,596,928; 4,880,976; 4,992,662; 4,720,622; 5,406,087; 3,218,459; 3,378,684; 4,037,109; 4,448,311; 4,115,689; 4,587,666; 5,323,441; 5,811,803; 6,452,177; 5,898,261; 4,618,938; 6,072,178; 4,929,041 and 6,114,695.        
 
       SUMMARY OF THE INVENTION  
       [0005]     The present invention seeks to provide apparatus and systems for enabling scanning electron microscope inspection of fluid containing specimens.  
         [0006]     There is thus provided in accordance with a preferred embodiment of the present invention a specimen enclosure assembly for use in an electron microscope and including a rigid specimen enclosure dish having an aperture and defining an enclosed specimen placement volume, an electron beam permeable, fluid impermeable, cover sealing the specimen placement volume at the aperture from a volume outside the enclosure and a pressure controller communicating with the enclosed specimen placement volume and being operative to maintain the enclosed specimen placement volume at a pressure which exceeds a vapor pressure of a liquid sample in the specimen placement volume and is greater than a pressure of a volume outside the enclosure, whereby a pressure differential across the cover does not exceed a threshold level at which rupture of the cover would occur.  
         [0007]     In accordance with another preferred embodiment of the present invention the pressure controller includes a passageway communicating with the enclosed specimen placement volume. Preferably, the passageway includes a tube having a lumen whose cross section is sufficiently small as to maintain the pressure, which exceeds the vapor pressure of the liquid sample in the specimen placement volume and is greater than the pressure of the volume outside the enclosure, for a time period of at least 15 minutes. Additionally, the tube communicates with a fluid reservoir.  
         [0008]     In accordance with yet another preferred embodiment of the present invention the specimen enclosure assembly also includes a liquid ingress and egress assembly permitting supply and removal of liquid from the enclosed specimen placement volume. Preferably, the liquid ingress and egress assembly includes at least two tubes.  
         [0009]     There is also provided in accordance with another preferred embodiment of the present invention a specimen enclosure assembly for use in an electron microscope and including a rigid specimen enclosure dish having an aperture and defining an enclosed specimen placement volume, an electron beam permeable, fluid impermeable, cover sealing the specimen placement volume at the aperture from a volume outside the enclosure and a liquid ingress and egress assembly permitting supply and removal of liquid from the enclosed specimen placement volume.  
         [0010]     Preferably, the liquid ingress and egress assembly includes at least two tubes.  
         [0011]     There is further provided in accordance with yet another preferred embodiment of the present invention a scanning electron microscope assembly including a scanning electron microscope defining an examination volume, a specimen enclosure assembly disposed in the examination volume and including a rigid specimen enclosure dish having an aperture and defining an enclosed specimen placement volume, an election beam permeable, fluid impermeable, cover sealing the specimen placement volume at the aperture from a volume outside the enclosure and a pressure controller communicating with the enclosed specimen placement volume and being operative to maintain the enclosed specimen placement volume at a pressure which exceeds a vapor pressure of a liquid sample in the specimen placement volume and is greater than a pressure of a volume outside the enclosure, whereby a pressure differential across the cover does not exceed a threshold level at which rupture of the cover would occur.  
         [0012]     In accordance with another preferred embodiment of the present invention the pressure controller includes a passageway communicating with the enclosed specimen placement volume. Preferably, the passageway includes a tube having a lumen whose cross section is sufficiently small as to maintain the pressure, which exceeds the vapor pressure of the liquid sample in the specimen placement volume and is greater than the pressure of the volume outside the enclosure, for a time period of at least 15 minutes. Additionally, the tube communicates with a fluid reservoir.  
         [0013]     In accordance with yet another preferred embodiment of the present invention the scanning electron microscope also includes a liquid ingress and egress assembly permitting supply and removal of liquid from the enclosed specimen placement volume. Preferably, the liquid ingress and egress assembly includes at least two tubes.  
         [0014]     There is also provided in accordance with yet another preferred embodiment of the present invention a specimen enclosure assembly for use in an electron microscope and including a fluid reservoir, a plurality of rigid specimen enclosure dishes, each having an aperture and defining an enclosed specimen placement volume, the plurality of rigid specimen enclosure dishes communicating with the fluid reservoir, an electron beam permeable, fluid impermeable, cover sealing each of the specimen placement volumes at the apertures from a volume outside each of the enclosures, and a pressure controller communicating with the fluid reservoir and being operative to maintain the enclosed specimen placement volumes at a pressure which exceeds a vapor pressure of a liquid sample in the specimen placement volumes and is greater than a pressure of a volume outside the fluid reservoir, whereby a pressure differential across the covers does not exceed a threshold level at which rupture of the covers would occur.  
         [0015]     In accordance with another preferred embodiment of the present invention the pressure controller includes a passageway. Preferably, the passageway includes a tube having a lumen whose cross section is sufficiently small as to maintain the pressure, which exceeds the vapor pressure of the liquid sample in the specimen placement volume and is greater than the pressure of the volume outside the plurality of enclosures, for a time period of at least 15 minutes. Additionally, the tube communicates with the fluid reservoir. Additionally or alternatively, the specimen enclosure assembly also includes a liquid ingress and egress assembly permitting supply and removal of liquid from the enclosed specimen placement volume. Preferably, the liquid ingress and egress assembly includes at least two tubes.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     The present invention will be understood and appreciated more fully from the following detailed description, taken in conjunction with the drawings in which:  
         [0017]      FIG. 1  is a simplified sectional illustration of a specimen enclosure assembly constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0018]      FIG. 2  is a simplified sectional illustration of a specimen enclosure assembly constructed and operative in accordance with another preferred embodiment of the present invention;  
         [0019]      FIG. 3  is a simplified sectional illustration of a specimen enclosure assembly constructed and operative in accordance with yet another preferred embodiment of the present invention;  
         [0020]      FIG. 4  is a simplified sectional illustration of a multiple specimen enclosure assembly constructed and operative in accordance with a preferred embodiment of the present invention;  
         [0021]      FIG. 5  is a simplified pictorial and sectional illustration of a scanning electron microscope including the specimen enclosure assembly of  FIG. 1 ;  
         [0022]      FIG. 6  is a simplified pictorial and sectional illustration of a scanning electron microscope including the specimen enclosure assembly of  FIG. 2 ;  
         [0023]      FIG. 7  is a simplified pictorial and sectional illustration of a scanning electron microscope including the specimen enclosure assembly of  FIG. 3 ; and  
         [0024]      FIG. 8  is a simplified pictorial and sectional illustration of a scanning electron microscope including the multiple specimen enclosure assembly of  FIG. 4 .  
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0025]     Reference is now made to  FIG. 1 , which is a simplified sectional illustration of a specimen enclosure assembly  100  constructed and operative in accordance with a preferred embodiment of the present invention. As seen in  FIG. 1 , the specimen enclosure assembly  100  comprises a specimen enclosure dish  102  seated in a container  104 .  
         [0026]     Specimen enclosure dish  102  preferably is formed of a ting  106  having a generally central aperture  108 . Ring  106  is preferably formed of PMMA (polymethyl methacrylate), such as Catalog No. 692106001000, commercially available from Irpen S. A. of Barcelona, Spain, and preferably defines a specimen placement enclosure with a volume of approximately 20 microliters and a height of approximately 2 mm. The specimen enclosure dish  102  is seated in a recess  109  formed in a top of the container  104 .  
         [0027]     An O-ring  110  is preferably disposed between ring  106  and an interior surface  112  of container  104 .  
         [0028]     An electron beam permeable, fluid impermeable, cover  114  is placed on specimen enclosure dish  102  against and over central aperture  108 .  
         [0029]     The electron beam permeable, fluid impermeable, cover  114  preferably comprises a polyimide membrane, such as Catalog No. LWN00020, commercially available from Mox-tek Inc. of Orem, Utah, U.S.A. Cover  114  is adhered, as by an adhesive, to a mechanically supporting grid  116 , which is not shown to scale, such as Catalog No. 2007N or Catalog No. 2005N, which is commercially available from Structure Probe Inc. of 569 East Gay Street, West Chester, Pa., U.S.A. A preferred adhesive is commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A., identified by Catalog No. NOA61. The electron beam permeable, fluid impermeable, cover  114  is also adhered to ring  106 , preferably by an adhesive, such as Catalog No. NOA61, commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A.  
         [0030]     A top element  118  is provided to retain the specimen enclosure dish  102  in container  104 . Top element  118  is preferably formed as a ring having a generally central aperture  122  and is attached to container  104  by any conventional means, such as by screws (not shown).  
         [0031]     A specimen  123 , typically containing cells  124  in a liquid medium  125 , is typically located within the specimen enclosure dish  102 , lying against the electron beam permeable, fluid impermeable, cover  114 . Examples of specimens containing liquid may include cell cultures, blood and bacteria. It is noted that the liquid  125  in specimen  123  does not flow out of the specimen enclosure dish  102  due to surface tension.  
         [0032]     Container  104  defines a fluid reservoir  126  containing at least one fluid. The fluid preferably comprises a liquid  128 , such as water or specimen liquid. The liquid  128  in fluid reservoir  126  is provided to supply the specimen enclosure dish  102  with vapor, such as water vapor, so as to prevent evaporation of the specimen liquid  125  by permitting vapor flow into specimen enclosure dish  102  through aperture  108 .  
         [0033]     A pressure controller assembly  130  is operative to maintain the specimen enclosure dish  102 , during microscopic inspection, generally over a time duration in a range of several minutes to several hours, typically a time period of at least 15 minutes, at a pressure which exceeds a vapor pressure of the specimen  123  and is greater than a pressure of a volume outside the specimen enclosure assembly  100 , whereby a pressure differential across the electron beam permeable, fluid impermeable, cover  114  does not exceed a threshold level at which rupture of cover  114  would occur.  
         [0034]     The pressure controller assembly  130  preferably comprises a tube  132 , such as Catalog No. MF34G-5 or Catalog No. MF28G-5, commercially available from World Precision Instruments Inc. of 175 Sarasota Center Boulevard, Sarasota, Fla., U.S.A., and a tube housing  134 .  
         [0035]     Tube  132  is inserted into an aperture  136  formed in a wall of container  104  above a surface of the liquid  128  in the fluid reservoir  126 . Tube  132  is sealingly attached to the container wall so that fluid flows from container  104  only through the tube  132 . It is a particular feature of the present invention that the tube  132  has a lumen with a cross section sufficiently small, preferably of a diameter in a range of 50 to 150 micrometers, to provide for relatively slow dissipation of pressure from the specimen enclosure assembly  100 .  
         [0036]     Reference is now made to  FIG. 2 , which is a simplified sectional illustration of a specimen enclosure assembly  200  constructed and operative in accordance with another preferred embodiment of the present invention. As seen in  FIG. 2 , the specimen enclosure assembly  200  comprises a specimen enclosure dish  202  seated in a container  204 .  
         [0037]     Specimen enclosure dish  202  preferably is formed of a ring  206  having a generally central aperture  208 . Ring  206  is preferably formed of PMMA (polymethyl methacrylate), such as Catalog No. 692106001000, commercially available from Irpen S. A. of Barcelona, Spain, and preferably defines a specimen placement enclosure with a volume of approximately 20 microliters and a height of approximately 2 nun. The specimen enclosure dish  202  is seated in a recess  209  formed in a top of the container  204 .  
         [0038]     An O-ring  210  is preferably disposed between ring  206  and an interior surface  212  of container  204 .  
         [0039]     An electron beam permeable, fluid impermeable, cover  214  is placed on specimen enclosure dish  202  against and over central aperture  208 .  
         [0040]     The electron beam permeable, fluid impermeable, cover  214  preferably comprises a polyimide membrane, such as Catalog No. LWN00020, commercially available from Moxtek Inc. of Orem, Utah, U.S.A. Cover  214  is adhered, as by an adhesive, to a mechanically supporting grid  216 , which is not shown to scale, such as Catalog No. 2007N or Catalog No. 2005N, which is commercially available from Structure Probe Inc. of 569 East Gay Street, West Chester, Pa., U.S.A. A preferred adhesive is commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A., identified by Catalog No. NOA61. The electron beam permeable, fluid impermeable, cover  214  is also adhered to ring  206 , preferably by an adhesive, such as Catalog No. NOA61, commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A. A top element  218  is provided to retain the specimen enclosure dish  202  in container  204 . Top element  218  is preferably formed of as a ring having a generally central aperture  222  and is attached to container  204  by any conventional means, such as by screws (not shown).  
         [0041]     A specimen  223 , typically containing cells  224  in a liquid medium  225 , is typically located within the specimen enclosure dish  202 , lying against the electron beam permeable, fluid impermeable, cover  214 . Examples of specimens containing liquid may include cell cultures, blood and bacteria.  
         [0042]     Container  204  contains at least one fluid. The fluid preferably comprises a liquid  228 , such as water or specimen liquid. Liquid  228  typically fills container  204  and specimen enclosure dish  202 , as shown in  FIG. 2 , or fills part of container  204 , similar to reservoir  126  shown in  FIG. 1 . The liquid  228  in container  204  is provided to supply the specimen enclosure dish  202  with vapor, such as water vapor, so as to prevent evaporation of the specimen liquid  225 .  
         [0043]     A fluid reservoir  230  contains at least one fluid in addition to the fluid contained in the specimen enclosure assembly  200 . The fluid preferably comprises a liquid  232 , such as water or specimen liquid. Preferably, fluid reservoir  230  has a larger internal volume than specimen enclosure assembly  200 . A lid  234  covers fluid reservoir  230  and is attached to fluid reservoir  230  by any conventional means, such as by screws (not shown). The liquid  232  in fluid reservoir  230  is provided to supply the specimen is enclosure assembly  200  with additional vapor, such as water vapor, in addition to the container liquid  228 , so as to Anther prevent evaporation of the specimen liquid  225 , by permitting vapor flow into specimen enclosure dish  202  through a fluid passageway  240 .  
         [0044]     The fluid passageway  240  comprises a conduit  242  having a first end portion and a second end portion, designated by reference numerals  244  and  246  respectively. First end portion  244  is inserted into an aperture  248  formed in a wall of container  204  and second end portion  246  is inserted into an aperture  250  formed in a wall of fluid reservoir  230 .  
         [0045]     A pressure controller assembly  260  is operative to maintain the specimen enclosure dish  202 , during microscopic inspection, generally over a time duration in a range of several minutes to several hours, typically a time period of at least 15 minutes, at a pressure which exceeds a vapor pressure of the specimen  223  and is greater than a pressure of a volume outside the specimen enclosure assembly  200 , whereby a pressure differential across the electron beam permeable, fluid impermeable, cover  214  does not exceed a threshold level at which rupture of cover  214  would occur. Additionally, the fluid in fluid reservoir  230  is provided to further maintain the pressure within the specimen enclosure assembly  200 , as described hereinabove, during microscopic inspection.  
         [0046]     The pressure controller assembly  260  preferably comprises a tube  262 , such as Catalog No. MF34G-5 or Catalog No. M28G-5, commercially available from World Precision Instruments Inc. of 175 Sarasota Center Boulevard, Sarasota, Fla., U.S.A., and a tube housing  264 .  
         [0047]     Tube  262  is inserted into an aperture  266  formed in the fluid reservoir wall above a surface of the liquid  232  in the fluid reservoir  230 . Tube  262  is sealingly attached to the fluid reservoir wall so that fluid flows from fluid reservoir  230  only through the tube  262  and fluid passageway  240 . It is a particular feature of the present invention that the tube  262  has a lumen with a cross section sufficiently small, preferably of a diameter in a range of 50 to 150 micrometers, to provide for relatively slow dissipation of pressure from the fluid reservoir  230 .  
         [0048]     Reference is now made to  FIG. 3 , which is a simplified sectional illustration of a specimen enclosure assembly  300  constructed and operative in accordance with yet another preferred embodiment of the present invention. As seen in  FIG. 3 , the specimen enclosure assembly  300  comprises a specimen enclosure dish  302  seated in a container  304 .  
         [0049]     Specimen enclosure dish  302  preferably is formed of a ring  306  having a generally central aperture  308 . Ring  306  is preferably formed of PE (polymethyl methacrylate), such as Catalog No. 692106001000, commercially available from Irpen S. A. of Barcelona, Spain, and preferably defines a specimen placement enclosure with a volume of approximately 20 microliters and a height of approximately 2 mm. The specimen enclosure dish  302  is seated in a recess  309  formed in a top of the container  304 .  
         [0050]     An O-ring  310  is preferably disposed between ring  306  and an interior surface  312  of container  304 .  
         [0051]     An electron beam permeable, fluid impermeable, cover  314  is placed on specimen enclosure dish  302  against and over central aperture  308 .  
         [0052]     The electron beam permeable, fluid impermeable, cover  314  preferably comprises a polyimide membrane, such as Catalog No. LWN00020, commercially available from Moxtek Inc. of Orem, Utah, U.S.A. Cover  314  is adhered, as by an adhesive, to a mechanically supporting grid  316 , which is not shown to scale, such as Catalog No. 2007N or Catalog No. 2005N, which is commercially available from Structure Probe Inc. of 569 East Gay Street, West Chester, Pa., U.S. A preferred adhesive is commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A., identified by Catalog No. NOA61 The electron beam permeable, fluid impermeable, cover  314  is also adhered to ring  306 , preferably by an adhesive, such as Catalog No. NOA61, commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A. A top element  318  is provided to retain the specimen enclosure dish  302  in container  304 . Top element  318  is preferably formed as a ring having a generally central aperture  372  and is attached to container  304  by any conventional means, such as by screws (not shown).  
         [0053]     A specimen  323 , typically containing cells  324  in a liquid medium  325 , is typically located within the specimen enclosure dish  302 , lying against the electron beam permeable, fluid impermeable, cover  314 . Examples of specimens containing liquid may include cell cultures, blood and bacteria.  
         [0054]     Container  304  contains a liquid  328 , such as water or specimen liquid, filling container  304  and the specimen enclosure dish  302 .  
         [0055]     A fluid reservoir  330  contains at least one fluid in addition to the fluid contained in the specimen enclosure assembly  300 . The fluid preferably comprises a liquid  332 , such as water or specimen liquid. Preferably, fluid reservoir  330  has a larger internal volume than specimen enclosure assembly  300 . A lid  334  covers fluid reservoir  330  and is attached to fluid reservoir  330  by any conventional means, such as by screws (not shown). The liquid  332  in fluid reservoir  330  is provided to supply the specimen enclosure assembly  300  with additional vapor, such as water vapor, in addition to the container liquid  328 , so as to further prevent evaporation of the specimen liquid  325 , by permitting vapor flow into specimen enclosure dish  302  through a fluid passageway  340 .  
         [0056]     The fluid passageway  340  comprises a conduit  342  having a first end portion and a second end portion, designated by reference numerals  344  and  346  respectively. First end portion  344  is inserted into an aperture  348  formed in a wall of container  304  and second end portion  346  is inserted into an aperture  350  formed in a wall of fluid reservoir  330 .  
         [0057]     A pressure controller assembly  360  is operative to maintain the specimen enclosure dish  302 , during microscopic inspection, generally over a time duration in a range of several minutes to several hours, typically a time period of at least 15 minutes, at a pressure which exceeds a vapor pressure of the specimen  323  and is greater than a pressure of a volume outside the specimen enclosure assembly  300 , whereby a pressure differential across the electron beam permeable, fluid impermeable, cover  314  does not exceed a threshold level at which rupture of cover  314  would occur, Additionally, the fluid in fluid reservoir  330  is provided to further maintain the pressure within the specimen enclosure assembly  300 , as described hereinabove, during microscopic inspection.  
         [0058]     The pressure controller assembly  360  preferably comprises a tube  362 , such as Catalog No. MF34(G-5 or Catalog No. MF28G-5, commercially available from World Precision Instruments Inc. of 175 Sarasota Center Boulevard, Sarasota, Fla., U.S.A., and a tube housing  364 .  
         [0059]     Tube  362  is inserted into an aperture  366  formed in the fluid reservoir wall above a surface of the liquid  332  in the fluid reservoir  330 . Tube  362  is sealingly attached to the fluid reservoir wall so that fluid flows from fluid reservoir  330  only through the tube  362  and fluid passageway  340 . It is a particular feature of the present invention that the tube  362  has a lumen with a cross section sufficiently small, preferably with a diameter in a range of 50 to 150 micrometers, to provide for relatively slow dissipation of pressure from the fluid reservoir  330 .  
         [0060]     Specimen enclosure assembly  300  is preferably provided with a liquid ingress and egress assembly  370  so as to permit supply and removal of liquid from the specimen enclosure assembly  300  to an environment outside a SEM enclosure wall, here designated by reference numeral  372 . Liquid ingress and egress assembly  370  preferably comprises an inlet conduit  374  and an outlet conduit  376  attached to specimen enclosure assembly  300 .  
         [0061]     Reference is now made to  FIG. 4 , which is a simplified sectional illustration of a multiple specimen enclosure assembly constructed and operative in accordance with a preferred embodiment of the present invention. As seen in  FIG. 4 , the multiple specimen enclosure assembly is comprised of a plurality of individual specimen enclosure assemblies  400 .  
         [0062]     Each specimen enclosure assembly  400  comprises a specimen enclosure dish  402  seated in a container  404 .  
         [0063]     Specimen enclosure dish  402  preferably is formed of a ring  406  having a generally central aperture  408 . Ring  406  is preferably formed of PEA (polymethyl methacrylate), such as Catalog No. 692106001000, commercially available from Irpen S. A. of Barcelona, Spain, and preferably defines a specimen placement enclosure with a volume of approximately 20 microliters and a height of approximately 2 mm. The specimen enclosure dish  402  is seated in a recess  409  formed in a top of the container  404   
         [0064]     An O-ring  410  is preferably disposed between ring  406  and an interior surface  412  of container  404 .  
         [0065]     An electron beam permeable, fluid impermeable, cover  414  is placed on specimen enclosure dish  402  against and over central aperture  408 .  
         [0066]     The electron beam permeable, fluid impermeable, cover  414  preferably comprises a polyimide membrane, such as Catalog No. LWN00020, commercially available from Moxtek Inc. of Orem, Utah, U.S.A. Cover  414  is adhered, as by an adhesive, to a mechanically supporting grid  416 , which is not shown to scale, such as Catalog No. 2007N or Catalog No. 2005N, which is commercially available from Structure Probe Inc. of 569 East Gay Street, West Chester, Pa., U.S.A. A preferred adhesive is commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A., identified by Catalog No. NOA61. The electron beam permeable, fluid impermeable, cover  414  is also adhered to ring  406 , preferably by an adhesive, such as Catalog No. NOA61, commercially available from Norland Products Inc. of Cranbury, N.J., U.S.A.  
         [0067]     A top element  418  is provided to retain the specimen enclosure dish  402  in container  404 . Top element  418  is preferably formed as a ring having a generally central aperture  422  and is attached to container  404  by any conventional means, such as by screws (not shown).  
         [0068]     A specimen  423 , typically containing cells  424  in a liquid medium  425 , is typically located within the specimen enclosure dish  402 , lying against the electron beam permeable, fluid impermeable, cover  414 . Examples of specimens containing liquid may include cell cultures, blood and bacteria. It is noted that the liquid  425  in specimen  423  does not flow out of the specimen enclosure dish  402  due to surface tension.  
         [0069]     The multiple specimen enclosure assembly, as shown in  FIG. 4 , also comprises a fluid reservoir  430  containing at least one fluid. The fluid preferably comprises a liquid  432 , such as water or specimen liquid. The liquid  432  in fluid reservoir  430  is provided to supply each specimen enclosure assembly  400  with vapor, such as water vapor, so as to prevent evaporation of the specimen liquid  425  by permitting vapor flow into individual specimen enclosure assemblies  400  through apertures  434  formed on a bottom surface of containers  404 .  
         [0070]     A lid  440  defines an array of specimen enclosure assembly support locations  442 . Each specimen enclosure support location  442  is preferably defined by a recess  444  arranged to receive specimen enclosure assemblies  400 . Specimen enclosure assemblies  400  are sealingly attached to lid  440 , by any conventional means, such as by screws (not shown), so as to prevent dissipation of fluid from lid  440 .  
         [0071]     Lid  440  covers the fluid reservoir  430  and is attached to fluid reservoir  430  by any conventional means, such as by screws (not shown).  
         [0072]     A pressure controller assembly  460  is operative to maintain, during microscopic inspection, generally over a time duration in a range of several minutes to several hours, typically a time period of at least 15 minutes, each specimen enclosure dish  402  at a pressure which exceeds a vapor pressure of the liquid specimen  423  and is greater than a pressure of a volume outside the specimen enclosure assembly  400 , whereby a pressure differential across the electron beam permeable, fluid impermeable, cover  414  does not exceed a threshold level at which rupture of cover  414  would occur.  
         [0073]     The pressure controller assembly  460  preferably comprises a tube  462 , such as Catalog No. MF34G-5 or Catalog No. MF28G-5, commercially available from World Precision Instruments Inc. of 175 Sarasota Center Boulevard, Sarasota, Fla., U.S.A., and a tube housing  464 .  
         [0074]     Tube  462  is inserted into an aperture  466  formed in a wall of fluid reservoir  430  above a surface of the liquid  432 . Tube  462  is sealingly attached to the fluid reservoir wall so that fluid flows from fluid reservoir  430  only through the tube  462 . It is a particular feature of the present invention that the tube  462  has a lumen with a cross section sufficiently small, preferably with a diameter in a range of 50 to 150 micrometers, to provide for relatively slow dissipation of pressure from the specimen enclosure assembly  400 .  
         [0075]     Reference is now made to  FIG. 5 , which is a simplified pictorial and sectional illustration of a SEM including the specimen enclosure assembly of  FIG. 1 . As seen in  FIG. 5 , the specimen enclosure assembly, here designated by reference numeral  500 , is engaged with a pressure controller assembly, here designated by reference numeral  502 . Specimen enclosure assembly  500  and pressure controller assembly  502  are shown positioned on a stage  504  of a SEM  506 .  
         [0076]     Reference is now made to  FIG. 6 , which is a simplified pictorial and sectional illustration of a SEM including the specimen enclosure assembly of  FIG. 2 . As seen in  FIG. 6 , the specimen enclosure assembly, here designated by reference numeral  600 , is engaged with a fluid reservoir, here designated by reference numeral  602 , via a fluid passageway  604 . A pressure controller assembly, here designated by reference numeral  608 , is engaged with fluid reservoir  602 . Specimen enclosure assembly  600  and fluid reservoir  602  are shown positioned on a stage  610  of a SEM  612 .  
         [0077]     Reference is now made to  FIG. 7 , which is a simplified pictorial and sectional illustration of a SEM including the specimen enclosure assembly of  FIG. 3 . As seen in  FIG. 7 , the specimen enclosure assembly, here designated by reference numeral  700 , is engaged with a fluid reservoir, here designated by reference numeral  702 , via a fluid passageway  704 . A pressure controller assembly, here designated by reference numeral  708 , is engaged with fluid reservoir  702 . Specimen enclosure assembly  700  and fluid reservoir  702  are shown positioned on a stage  710  of a SEM  712 . An inlet conduit  720  and an outlet conduit  722  are attached to the specimen enclosure assembly  700 .  
         [0078]     Reference is now made to  FIG. 8 , which is a simplified pictorial and sectional illustration of a SEM including the multiple specimen enclosure assembly of  FIG. 4 . As seen in  FIG. 8 , the multiple specimen enclosure assembly, here designated by reference numeral  800 , is shown positioned on a stage  802  of a SEM  804 . A pressure controller assembly, here designated by reference numeral  808 , is engaged with the multiple specimen enclosure assembly  800 .  
         [0079]     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention includes both combinations and subcombinations of the various features described hereinabove as well as modifications and variations thereof as would occur to a person of skill in the art upon reading the foregoing specification and which are not in the prior art