Patent Publication Number: US-2023154724-A1

Title: Pole piece for a transmission electron microscope

Description:
FIELD 
     The disclosure relates to a pole piece for an electron microscope, and in particular a Transmission Electron Microscope (TEM). The disclosure also relates to a pole piece for use in charged particle devices, such as electron-beam lithography instruments (EBL), e-beam sterilisation instruments, or e-beam 3D-printers and the like. 
     BACKGROUND 
     An electron microscope is a microscope that uses a beam of accelerated electrons as a source of illumination. As the wavelength of an electron can be up to 100,000 times shorter than that of visible light photons, electron microscopes have a higher resolving power than light microscopes and can reveal the structure of smaller objects. 
     Electron microscopes are used to investigate the ultrastructure of a wide range of biological and inorganic specimens including microorganisms, cells, large molecules, biopsy samples, metals, and crystals. Industrially, electron microscopes are often used for quality control and failure analysis. 
     There are a number of different types of electron-beam imaging or lithography instruments. One example is a transmission electron microscope (TEM). 
       FIG.  1    is a schematic diagram of a TEM  1  which shows its basic features. It comprises an electron source  3 , a condenser system  5  which focuses the electron beam from the source, a specimen stage  7  where the specimen is held, an objective lens  9  which collects and focuses electrons which have been transmitted through the sample and an imaging lens which expands the beam onto an imaging device  10 . The elements are all typically enclosed in a vacuum chamber. A TEM uses a high voltage to create an electron beam which is produced by an electron gun. The electron beam is accelerated by an electric field to a desired energy. 
     The electromagnetic lens is usually made of a solenoid coil nearly surrounded by ferromagnetic materials designed to concentrate the coil&#39;s magnetic field into a precise, confined shape. When an electron enters and leaves this magnetic field, it converges around the curved magnetic field lines in a manner similar to an optical converging lens. A magnetic lens&#39;s focusing power can be changed by adjusting the current passing through the coils. This allows a lens system, between the source and the sample (the “condenser lens” system) to produce a parallel beam over 1 millimetre in diameter, a tightly focused beam smaller than an atom, or anything in between. 
     The magnetic field produced for the lens should be radially symmetrical, as deviation from the radial symmetry of the magnetic lens will causes aberrations, such as chromatic or spherical aberration. Electron lenses are generally manufactured from iron, iron-cobalt or nickel cobalt alloys, such as permalloy. These are selected for their magnetic properties, such as magnetic saturation, hysteresis and permeability. 
     An example cross-section of part of a TEM  11  is shown in  FIG.  2   . This part receives an incident electron beam from a condenser lens. The electromagnet comprises a yoke  13  and coils  21 . The sample space comprises a pole gap  19  which is bounded by pole piece  17  mounted on the pole  15 . The axially extending space  23  defines a bore which accommodates the beam-path. 
     Control of the strength of the magnetic fields is achieved by varying the current in the coils and shapes the electron beam. 
     The pole piece is a magnetic pole made of a soft magnetic material (for example pure iron, Permandur, Hiperco50 or similar material) which concentrates the magnetic flux produced by the electromagnet and is guided by the yoke to produce a strong magnetic field in a narrow gap in the pole piece. Pole pieces are known for a long time in electron lens systems, for example various pole piece designs are disclosed in U.S. Pat. Nos. 3,324,433; 2,749,464; 2,472,315; 2,754,443 and 2,418,432, however none of these pole pieces are suitable for incorporation in modern electron microscopes. 
     Other patent publications including GB2161019; US20110012018 and JP2000156191 disclose pole pieces exclusively for scanning electron microscopes (SEM) and the type of lens that is used is a ‘pinhole’ or ‘snorkel’ lens. Such pole pieces can be interchangeable and are found not to be very practical. For example, GB2161019 shows interchangeable modules that need the column to be vented to atmosphere to make the replacement. This is undesirable and not possible for TEM microscopes without dismantling and reassembling. 
     The pole gap largely dictates the final shape of the magnetic field incident upon the sample. The geometry of an installed pole piece may be optimised for different purposes and is selected when an instrument is purchased. This configuration will then typically remain for the entire lifetime of the microscope (perhaps 20 years). 
       FIG.  3    shows an example of a pole-piece  31 , known in the art for use in a TEM microscope, which comprises an upper pole  33  positioned towards the electron source and condenser lens of the microscope and a lower pole  35 . The upper pole  33  and lower pole  35  are hollow and frustoconical in shape with the poles pointing towards one another. The smaller, lower pole  35  engages with the remainder of the iron magnetic circuit via its base portion, while the upper (larger) pole  33  mates with a base portion  34 . The opposing adjacent planes of the upper and lower pole are separated by a fixed gap  37 . The upper and lower poles  33 ,  35  are held at a fixed separation by a supporting frame  32  made from some non-magnetic material. This frame may include ports  39  for various access requirements. 
       FIG.  4    is a known graph  41  which plots the spherical and chromatic aberration coefficients  43  (the two dominant optical defects in these systems) against the focal length  45  of various available pole-pieces. Because of unavoidable physical limitations to the shaping of magnetic fields, the optical quality of electron lenses scales with their focal length which itself scales with the pole-piece gap size. 
     For example, a microscopist purchasing a new state-of-the-art TEM (at a cost of around €4-5 M) may select a small-gap ultra-high-resolution pole-piece (UHR), an intermediate resolution (HR) pole-piece, or a so-called ‘analytical’ (ARP) with a larger gap. Selection of the configuration of a pole piece may be made to, for example, provide maximum image resolution (a smaller gap size and focal length) or to maximum chemical mapping sensitivity (using a larger gap size to allow closer approach of spectrometers or mirrors), but these features of the output cannot both be maximised for a single pole piece configuration. As a result, an Ultra-high-resolution (UHR) pole-piece is required for the microscope to deliver on its ultimate resolution potential but this severely limits the potential for x-ray spectroscopic collection. The analytical-resolution pole-piece (ARP) on the other hand allows for better chemical spectroscopy and greater tilting or a wider range of in-situ experiments, but at the cost of some lost resolution. 
     In principle it would be possible for a customer to request the manufacturer to exchange the pole-piece on a modern-generation TEM, but this can incur a down-time of around one month per change and great associated expense. 
     It is an object of the present invention to create a pole piece for a transmission electron microscope which addresses the above problems. 
     SUMMARY OF THE INVENTION 
     In accordance with a first aspect of the invention there is provided, as set out in the appended claims, a pole piece for an electron microscope. 
     In one embodiment the pole piece comprises: 
     an upper pole piece, containing a first pathway for an electron beam, 
     a lower pole piece which is coupled to the upper pole piece and which contains a second pathway operatively connected to the first pathway, the upper pole piece and lower pole piece being separated by a gap between the first pathway and the second pathway, 
     wherein the pole piece comprises a mechanism which can extend or reduce the distance between the upper pole piece and the lower pole piece by changing the distance between the first pathway and the second pathway. 
     An important advantage of the pole piece of the present invention is that the spacing between the upper and lower pole piece can be adjusted whilst under vacuum using the mechanism. This provides a major advancement for TEM and STEM microscopes as the invention provides for easy adjustment of pole pieces that heretofore was not possible. Additionally, different samples can be introduced for inspection using the microscope with minimum effort. 
     In at least one embodiment, the mechanism adjusts the position of the upper pole piece. 
     In at least one embodiment, the mechanism moves the upper pole piece towards or away from the lower pole piece whilst maintaining a common axis. 
     In at least one embodiment, the upper pole piece comprises an outer part with a concentrically mounted inner part. 
     In at least one embodiment, the inner part is coupled to the mechanism and is moveable to extend or reduce the distance between the upper pole piece and the lower pole piece. 
     In at least one embodiment, the mechanism comprises a bearing mounted between a sleeve, the sleeve formed from an inner concentric surface of the outer part and an outer concentric surface of the concentrically mounted inner part. 
     In at least one embodiment, the bearing is actuated by a rotatable cam with the bearing acting as the follower which experiences linear motion. 
     In at least one embodiment, the bearing and sleeve have cooperating threads which allow the bearing to be rotated and moved in linearly. 
     In at least one embodiment, the mechanism comprises: 
     a spacer which couples the outer part of the upper pole piece to the lower pole piece; 
     an anulus rotatably mounted in a substantially circular channel between the outer part and the inner part of the upper pole piece and connected to an actuator wherein rotation of the annulus causes the actuator to move the inner part to extend or reduce the distance between the upper pole piece and the lower pole piece. 
     In at least one embodiment, the actuator comprises an inclined ratchet and pawl mechanism. 
     In at least one embodiment, the mechanism is rotatable in one direction, clockwise or anticlockwise. As the mechanism only rotates in one direction means that pole piece ‘locks’ into position without any external clamping or braking required. 
     In at least one embodiment, the inclined ratchet comprises a stepped surface of the outer part of the upper pole piece and the pawl comprises an engaging lower surface of the annulus. 
     In at least one embodiment, the stepped surface comprises a bottom surface of the channel and the pawl comprises a lower surface of the annulus. 
     In at least one embodiment, the height of the steps on the stepped surface define discrete values of the size of the distance between the upper pole piece and the lower pole piece and therefore, the gap between the first channel and the second channel. 
     In at least one embodiment, the annulus includes three pawls. An advantage of having three pawls is that it produces a stable and repeatable positioning to adjust the distance between the upper and lower pole piece. 
     In at least one embodiment, the step heights define a gap between the upper pole piece end and lower pole piece end of 1.5 mm, 4.0 mm and 6.5 mm respectively. 
     In at least one embodiment, there are five steps which define a gap between the upper pole piece end and lower pole piece end in increments of 1.25 mm from a smallest gap of 1.5 mm to a largest gap of 6.5 mm. Suitably five steps can be provided spaced at 1.5 mm, 2.75 mm, 4.0 mm, 5.25 mm, and 6.5 mm. 
     In at least one embodiment of the present invention, the inner part of the upper pole piece comprises a hollow centred generally cylindrical body having a flange for engagement with the anulus at one end with a frustoconical second end which narrows to meet the pole end of the lower pole piece. 
     In at least one embodiment, a drive mechanism is used to rotate the annulus. 
     In at least one embodiment, the drive mechanism comprises a set of gear teeth mounted on its outer circumference of the annulus which are operatively connected to one or more cog which couples the annulus to a drive shaft. 
     In at least one embodiment, the drive shaft is manually operable. 
     In at least one embodiment, the drive shaft is machine operable. 
     In at least one embodiment, a feedback mechanism is provided to note the position of, or number of revolutions of, the drive shaft such that the separation of the upper and lower pole pieces is known. Suitably the distance between the upper and pole piece can be visually displayed. The visual display can be a numerical value, text or colour/symbolic indicator. 
     In at least one embodiment, a switch/trigger/sensor monitors the position of the upper and/or lower pole piece. 
     In at least one embodiment, access is provided for a camera or similar device to observe the position of the upper and/or lower pole piece. 
     In one embodiment there is provided a pole piece for a transmission electron microscope, the pole piece comprising:
         an upper pole piece, containing a first pathway for an electron beam,   a lower pole piece which is coupled to the upper pole piece and which contains a second pathway operatively connected to the first pathway, the upper pole piece and lower pole piece being separated by a gap between the first pathway and the second pathway, characterised in that:       

     the pole piece comprises a mechanism which can extend or reduce the distance between the upper pole piece and the lower pole piece by changing the distance between the first pathway and the second pathway, wherein, the mechanism comprises a spacer which couples the outer part of the upper pole piece to the lower pole piece; an anulus rotatably mounted in a substantially circular channel between the outer part and the inner part of the upper pole piece and connected to an actuator wherein rotation of the annulus causes the actuator to move the inner part to extend or reduce the distance between the upper pole piece and the lower pole piece. 
     In one embodiment there is provided a transmission electron microscope comprising a pole piece as claimed in any of appended claims  1  to  20 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be more clearly understood from the following description of an embodiment thereof, given by way of example only, with reference to the accompanying drawings, in which: 
         FIG.  1    is a schematic diagram of a TEM as generally known; 
         FIG.  2    is a schematic cross-section of a symmetrical objective-lens of a TEM including a known pole piece; 
         FIG.  3    is a perspective view of a known fixed gap pole piece; 
         FIG.  4    is a graph which plots the aberration coefficient  43  against the focal length of the spherical and chromatic aberrations; 
         FIGS.  5   a  and  5   b    are schematic representations of a first embodiment of the present invention; 
         FIG.  6    is a cut away perspective view of a second embodiment of the present invention; 
         FIG.  7    is a perspective view of the second embodiment of the present invention; 
         FIG.  8    is a perspective view of the second embodiment of the present invention; 
         FIGS.  9   a  to  9   c    are side views of the second embodiment of the present invention which show differing gap heights; 
         FIG.  10    is a graph which plots optical axis distance against magnetic flux density for a 1.5 mm pole piece gap; 
         FIG.  11    is a graph which plots optical axis distance against magnetic flux density for a 4 mm pole piece gap; and 
         FIG.  12    is a graph which plots optical axis distance against magnetic flux density for a 6.5 mm pole piece gap. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
     The present invention provides a pole piece for a transmission electron microscope (TEM) which allows the pole piece gap, that is, the gap between the upper pole piece and the lower pole piece to be adjusted without the cost in time and money of disassembling the TEM to replace the pole piece with one having a different sized pole piece gap. Moreover the pole piece of the present invention can work in a vacuum when incorporated into a transmission electron microscope. In other words the upper pole piece and the lower pole piece can be adjusted in a vacuum. 
       FIGS.  5   a  and  5   b    are schematic diagrams which show a pole piece  51  which has an upper pole piece (UPP)  53  and a lower pole piece (LPP)  55 . Pathway inside the UPP  53  illustrates the cavity running the length of the UPP at its centre where the magnetic field is concentrated. The pathway and magnetic field focusses the electrons as they travel from the TEM&#39;s electron source to its specimen stage (not shown). A similar pathway is shown in the LPP for electrons once transmitted from the specimen towards the objective and imaging lenses. 
     In this example of the present invention a mechanism for linear movement of the UPP  53  comprises a carriage or bearing  71  which is positioned between an outer part  61  of the UPP  53  and an inner part  59 . The inner part  59  and the outer part  61  are concentric, the centre of the pathway being aligned with the centre of the UPP  53 . The bearing is mounted in a sleeve  69  which forms an inner concentric surface of the outer part  61 . A spacer (not shown) fixedly can connect the outer part  61  of the UPP  53  to the LPP  55  in practice. The inner part  59  is not so connected and may move linearly as shown by arrow  58  to and from the pole end of the LPP  55 . 
     The carriage or bearing  71  may be actuated in a number of ways, for example a small cam may be mounted on the bearing which may be resiliently mounted such that rotation of the cam will cause linear movement of the bearing which will move the inner part  59  of the UPP  53  linearly as shown by arrow  58 . Alternatively, the bearing  71  and sleeve  69  may have cooperating threads which allow the bearing to be rotated and moved in direction  58 . 
       FIGS.  6  to  9    show a second and another embodiment of the present invention. In  FIGS.  6  to  9    reference the same numerals are used to describe the same features when illustrated in different drawings. 
       FIGS.  6  to  9    show a pole piece  81  which has an upper pole piece (UPP)  93  and a lower pole piece (LPP)  85 . Pathway  84  inside the UPP illustrates the cavity running the length of the UPP at its centre where the magnetic field is concentrated. A similar pathway  86  is shown in the LPP for electrons once transmitted from the specimen towards the objective and imaging lenses. Pathways  84  and  86  are accurately aligned to allow the electrons to pass through to an objective lens (not shown). 
     In this example, the UPP comprises an outer part  89  and an inner part  91 . The outer pole part  89  is supported by a spacer  113  which connects it to the LPP  85 . The inner part  91  comprises a hollow centred generally cylindrical body having a flange  105  for engagement with a coupling ring or anulus at one end with a frustoconical second end which narrows to meet the pole end of the lower pole piece  85  at the gap  90 . The cylindrical surface of the inner part  91  is slidable connected to the outer part  89  at surface  93 . 
     The coupling ring couples the inner part  91  to a drive mechanism. The outer circumference of the ring has gear teeth  107  which are operatively connected to toothed cogs  109  which, in turn are connected to a gear shaft  117 , worm gear  115  and drive shaft  111 . It will be appreciated that only one of the three cogs  109  can be is connected to the drive shaft. The other two cogs are free rotating and are used to stabilise the toothed ring. Any combination of cogs can be used 
     The coupling ring has a tiered bottom surface and is located in a channel  88  which comprises a tiered upwardly pointing surface  98  of the outer UPP  89 . The top tier of the upwardly pointing surface comprises an inclined stepped surface, that is, the size of the step increases around the circumference of the channel. The cooperative engagement between the tiered bottom surface of the ring and the steps acts to change the distance between the inner UPP  91  and the LPP  85  upon rotation of the ring. The stepped shape also acts to prevent contra-rotation of the ring, in other words, the inner UPP  91  is engaged on an inclined ratcheting plane so as to lift when rotated. This lifting is then released and the inner portion locks into a new raised position. 
     The design achieves the required rotation and lifting of the inner UPP using the gear shaft  117  to translate mechanical motion via the worm gear  115  from the external drive shaft  111 . A feedback mechanism (not shown) can be provided to cooperate with the drive shaft  111  or gear shaft  117  to note the position of, or number of revolutions of, the drive shaft such that the separation of the upper and lower pole pieces is known. Suitably the distance between the upper and pole piece can be visually displayed. The visual display can be a numerical value or colour indicator. For example, a numerical counter can be used to display the position of the upper and lower pole pieces with respect to each other. A switch/trigger can be provided to monitor the position of the upper and/or lower pole piece. It will be appreciated that the pole piece can be dimensioned to allow access by a camera or other viewing device to visually inspect the pole piece or visually display the physical position of the pole piece within a microscope. 
       FIGS.  9   a  to  9   c    show an enlargement of the central portion of the adjustable pole-piece  81  as shown in the embodiment of  FIGS.  6  to  8    with similar reference numerals. 
       FIG.  9   a    shows the pole piece  81 , with an inner UPP  91 , ratchet surface in the channel of the outer UPP  89 . Pathways  84  and  86  are accurately aligned to allow the electrons to pass through to the objective lens. In this example, the mechanism moves the inner UPP to set a gap  119  of 1.5 mm to the LPP  85 . 
       FIG.  9   b    shows the pole piece  81 , with an inner UPP  91 , ratchet surface in the channel of the outer UPP  89 . Pathways  84  and  86  are accurately aligned to allow the electrons to pass through to the objective lens. In this example, the mechanism moves the inner UPP to set a gap  121  of 4.0 mm to the LPP  85 . 
       FIG.  9   c    shows the pole piece  81 , with an inner UPP  91 , ratchet surface in the channel of the outer UPP  89 . Pathways  84  and  86  are accurately aligned to allow the electrons to pass through to the objective lens. In this example, the mechanism moves the inner UPP to set a gap  123  of 6.5 mm to the LPP  85 . 
       FIG.  10    is a graph  131  of optical axis distance  133  against magnetic flux density  135  for a 1.5 mm pole piece gap. The curve  137  is a gaussian curve with a single peak. The intensity distribution as a function of the distance from the optical axis  139  is also shown. 
       FIG.  11    is a graph  141  which plots optical axis distance against magnetic flux density for a 4 mm pole piece gap. The curve  143  is a gaussian curve with a narrow double peak. The intensity distribution as a function of the distance from the optical axis  149  is also shown. 
       FIG.  12    is a graph  151  which plots optical axis distance against magnetic flux density for a 6.5 mm pole piece gap. The curve  153  is a gaussian curve with a wide double peak. The intensity distribution as a function of the distance from the optical axis  159  is also shown. 
     In the context of the present invention the terms ‘first pathway’ and ‘second pathway’ is used to describe a bore or channel through and between the pole pieces that allows correct operation of an electron microscope and understood by a person skilled in the art of electron microscopes. 
     In general, a single peak with a sharp Gaussian distribution of magnetic flux density with respect to optical axis distance would be considered to be an ideal lens, whereas at the more expanded setting the optical quality is poorer but allows a spectrometer to be closer to the sample or larger sample tilts. 
     In the specification the terms “comprise, comprises, comprised and comprising” or any variation thereof and the terms include, includes, included and including” or any variation thereof are considered to be totally interchangeable and they should all be afforded the widest possible interpretation and vice versa. 
     The invention is not limited to the embodiments hereinbefore described but may be varied in both construction and detail.