NL2034496B1 - Electro-magnetic lens for charged particle apparatus. - Google Patents

Abstract

The invention relates to an electro-magnetic lens for a charged-particle apparatus, such as the lens for an electron microscope. As known to the skilled artisan in an electron microscope a beam of electrons propagates around a particle-optical axis and the magnetic field of the lens, interacting with said beam, should show a high degree of rotational symmetry around the particle-optical axis as otherwise a deflection of the beam occurs, or aberrations such as astigmatism, coma and other aberrations occur. Inventors realized that the high degree of rotational symmetry is only needed close to the axis (near the inner pole shoe 108) and further from the axis less symmetry is needed (outer enclosure 116-i). A design using a square outer enclosure was made and used, showing satisfactory results. By forming the squares of the outer enclosure from rectangular plates much material is saved, resulting in a cheaper product.

Description

Electro-magnetic lens for charged particle apparatus.

Technical field of the invention.

[0001] The invention relates to an electro-magnetic lens with a particle-optical axis, the lens for focusing an energetic beam of charged particles, the lens comprising: e An electro-magnetic coil, e A yoke, the yoke comprising two polepieces made of magnetically soft material the polepieces showing rotational symmetry around a particle-optical axis, the two polepieces separated by a gap.

[0002] The invention further relates to particle-optical probe forming apparatuses equipped with one or more lenses.

Background of the invention.

[0003] As known to the person skilled in the art electro-magnetic lenses are used in Scanning Electron Microscopes (SEMs), Transmission Electron Microscopes (TEMs), and in Scanning Transmission Electron Microscopes (STEMs), Electron

Probe Micro-Analysers (EPMAs), etc.. These instruments have in common that a beam of energetic charged particles is manipulated (focused, imaged).

[0004] An electro-magnetic lens comprises an electric coil through which current flows, a yoke made of a magnetically soft material for guiding the magnetic field generated by the electric coil, and a gap where the magnetic field interacts with the beam of energetic particles (electrons, ions) resulting in a focussing of said beam.

[9005] It is noted that a magnetically soft material in this respect is a ferro- or ferri- magnetic material, mostly a ferro-magnetic material, typically comprising iron and/or cobalt and/or nickel, showing a high relative permeability Hr and a low hysteresis. Also important for such a material is the magnetic field at which saturation occurs

[0006] As known to the skilled artisan the beam of charged particles typically propagates around a particle-optical axis and the magnetic field interacting with said beam should show a high degree of rotational symmetry around the particle- optical axis as otherwise a deflection of the beam occurs, or aberrations such as astigmatism, coma and other aberrations occur. The yoke and polepieces are made of a magnetically soft material showing little hysteresis, and the saturation of the material(s) is chosen according to the maximum magnetic flux density B occurring in the materials.

[0007] As an example, a Scanning Electron Microscope (a SEM) typically comprises an electron source, one or more lenses, and a specimen holder in a specimen chamber. In use the electron source and the specimen chamber are placed in an evacuated area, evacuated typically to a pressure of 10 Pa or less.

The beam of electrons is swept over a part of the specimen using deflectors, and detectors detect secondary and/or backscattered electrons.

Ina TEM the beam of electrons is transmitted through a thin sample and an interference image is made.

In a STEM a probe of electrons is swept over a thin sample and the electrons transmitted through the sample are detected.

In an EPMA a probe of electrons is imaged and swept over a sample and X-rays emitted by the sample are detected and analysed.

In other instruments ions are focused by the lens. It is noted that, as the charge- to-mass ratio e/m is smaller, magnetic lenses are less effective and more often electro-static lenses are used for the focusing of ions.

[0008] To avoid aberrations to occur the yoke, comprising one or more polepieces centred around the particle-optical axis and parts further removed from the particle-optical axis (for example surrounding the electric coil) a high degree of rotational symmetry is typically used. If a hole perpendicular to the particle-optical axis is made in the lens, for example for mounting a specimen holder or a detector, then typically three corresponding holes at 90° are made to restore symmetry as much as possible and introduce a high order multipole (in the example an octupole).

[0009] It is noted that, when using the phrase hole, the hole may be plugged by a non-magnetic material, for example aluminium, so that vacuum integrity is maintained.

[0010] It is noted that soft ferro-magnetic material is often quite expensive and often thermal treatment is needed to improve its magnetic properties.

[0011] A drawback of a highly symmetrical yoke is that it necessitating expensive parts made of forged iron, sintered iron, iron made of solid cylinders, etc., making lenses expensive parts. The outer diameter of the yoke is large, typically between cm and 30 cm, and if this is manufactured from a solid cylinder much material 10 is wasted.

[0012] A partial solution for this is, for example, described in Japanese Patent

Application publication JPS59123147A to Nippon Electron Optics Laboratory. This application describes that round rings of magnetic material are formed and stacked and form the outer diameter of the lens. The total cost of the rings is less than the cost of a cylinder of comparable material.

[0013] However, the price of the yoke hinders the production of a low-cost lens, and thus of a low-cost SEM, TEM, STEM, EPMA or other probe forming apparatuses.

[0014] The invention intends to avoid, or at least lessen, this drawback and enable the design and production of a low-cost SEM, EPMA, etc..

Summary of the invention.

[0015] To that end the lens according to the invention is characterized in that the yoke further comprises: e two radial plates made of magnetically soft material, each radial plate in magnetic contact with a corresponding polepiece, the radial plates extending perpendicular to the particle-optical axis, e three or more substantially isosceles trapezoidal separation plates made of magnetically soft material, the separation plates magnetically surrounding the coil, two opposing sides of each separation plate substantially in magnetic contact with other separation plates, the three or more separation plates thus forming a magnetically closed path around the coil, the other two opposing sides of each separation plate in contact with the radial plates.

[0016] By energizing the coil, a magnetic field H is formed between the polepieces.

[0017] Inventors found that, although a high degree of rotational symmetry of the polepieces is needed, other parts of the yoke do not need to have the same high degree of symmetry. If three separation plates of identical shape are used, a small sextupole field, also known as a hexapole field, is superimposed on the (rotationally symmetric) magnetic field in the gap. If more separation plates are used, higher order fields (octupole, decapole, etc.) are superimposed on the magnetic field in the gap. However, as the order of the superimposed field is high, the effect of a small high order field becomes negligible. This is because the field of the sextupole scales with r3 with r the distance from the particle-optical axis.

Therefore, the aberrations introduced by a hexapole field, or fields with an even higher number of poles, such as octupole fields, decapole fields, etc., are small or even negligible.

It is noted that a multipole with number n gives a field that scales with r*2 with r the distance from the particle-optical axis.

[0018]lt is noted that in many materials the relative permeability Hr depends on the direction in which the material is rolled, forged, etc. Preferably the relative permeability Hr is constant in all directions perpendicular to the particle-optical axis.

[0019] By forming the separation plates from rectangular plates much material is saved, resulting in a cheaper product.

[0020] In an embodiment the substantially isosceles trapezoidal separation plates are substantially rectangular separation plates.

[0021] Although the separation plates may be isosceles trapezoidal separation plates, rectangular plates are easier to implement. This also implies that they are oriented perpendicular to the radial plates, and thus parallel to the particle-optical axis. Due to manufacturing constraints the coil preferably has a constant inner- and outer-diameter.

[0022] By forming the separation plates from rectangular plates much material is saved, resulting in a cheaper product.

[0023] In another embodiment the number of separation plates equals four. 5 [0024] Four separation plates result in a simple design and easy manufacturing.

The four separation plates form a square box in which the coil is placed. By forming the separation plates from rectangular plates much material is saved, resulting in a cheaper product.

[0025]In yet another embodiment the separation plates have an identical shape and are made of material with identical magnetic properties

[0026] If the separation plates have different shapes and/or are made of a material with different magnetic properties, for example different coercivity (resulting in a different hysteresis), relative permeability Hr, etc., additional aberrations are added as the symmetry of the lens is compromised: the symmetry does not only require a symmetric form of the yoke, but also a symmetric magnetic behaviour (Hr, hysteresis and saturation).

[0027] In still another embodiment at least one of the polepieces shows a truncated cone facing the gap.

[0028] As known to the skilled person, polepieces with truncated cones often show good particle-optical properties, such as low aberrations etc.

[0029] In yet another embodiment the radial plates have a square outline.

[0030] Especially when using four separation plates, this is most convenient size.

But also when using another numbers of separation plates, such as when using six or eight plates, square radial plates may be most convenient in terms of manufacturability.

[0031]In still another the two polepieces are made of material with identical magnetic properties.

[0032] Although the polepieces may be made of materials with different magnetic properties, the use of one magnetic material enables the use of polepieces manufactured of one rod of (magnetically soft) material, or even to make the polepieces identical and thus interchangeable, reducing the cost of the polepieces and thus the complete lens.

[0033] In yet another embodiment the first and the second radial plates are made of material with identical magnetic properties.

[0034] The use of one magnetic material enables the use of radial plates manufactured of one large plate of (magnetically soft) material, or even to make the radial plates identical and thus interchangeable, reducing the cost of the radial plates and thus the complete lens.

[0035] In still another embodiment the radial plates and the separation plates are made of material with substantial identical magnetic properties.

[0036] The use of one magnetic material for the radial plates and the separation plates enables manufacturing from one large plate of (magnetically soft) material, reducing the cost of the radial plates and thus the complete lens.

[0037]1t is noted that ideally the radial plate shows a constant Hr in all directions perpendicular to the particle-optical axis, and that that is not needed for the separation plates (preferably a constant u parallel to the particle-optical axis).

Therefore, a solution using two different materials may be preferred.

[0038] In yet another embodiment the separation plates are equipped to hook into each other, thereby forming a magnetically closed path around the coil.

[0039] Although it is feasible to screw the plates onto each other, it is also possible to equip the plates with indents such, that it is possible to hook them into each other. This is especially easy when the number of plates equals four. An example will be given further in this document in a detailed description of figure 3.

[0040] In still another embodiment the radial plates show trenches in which the separation plates rest.

[0041]By making trenches in the radial plates where the separation plates rest, the rigidity of the lens is slightly improved and, more important, the magnetic path is improved, as less unwanted interstices between the two occur.

[0042] In yet another embodiment the gap is formed by a hollow cylinder of non- magnetizable material.

[0043] By forming the gap from non-magnetic material, the size of the gap between the polepieces is well-defined and may, when needed and with proper care, be vacuum tight.

[0044] In still another embodiment in the polepieces and in the gap an evacuated vacuum tube is placed.

[0045] Placing a vacuum tube in the polepieces is a simple way to reduce the number of vacuum seals. The use of a vacuum tube may be combined with the earlier mentioned use of a hollow cylinder of non-magnetizable material to define the size of the gap.

[0046] In an aspect of the invention a probe forming apparatus equipped with a lens according to any of the preceding claims.

[0047] The probe forming apparatus may be a SEM, a TEM, a STEM or an EPMA.

[0048] It is noted that in a TEM no focal point is formed on the sample, but the sample is irradiated with a highly coherent beam, which enables foci at other points and thus the forming of a probe.

[0049] In a further aspect the apparatus is equipped with at least two adjacent lenses, the two lenses sharing a common radial plate.

[0050] A probe forming apparatus, such as an electron microscope, typically has more than two lenses. Adjacent lenses can then share a common radial plate.

[0051]lt is noted that, if the magnetic polarity of the lenses is chosen properly, the magnetic flux through the shared radial plate is less than the sum of the flux when no radial plates are shared, and thus the thickness of the shared plate can be equal to that of a not shared plate, or even be thinner.

[0052]In yet another aspect the apparatus is equipped with at least two adjacent lenses, the polepiece of one lens forming an integral part with a polepiece of the other lens.

[0053] By integrating two polepieces, a further reduction of parts and thus price is realized.

Brief description of the drawings.

[0054] The invention is now elucidated using figures, in which identical reference signs indicate corresponding features. To that end:

Figure 1 schematically shows a cross-section of a lens according to the invention,

Figure 2 schematically shows a cross-section of the lens shown in figure 1 along line AA’

Figure 3 schematically shows a separation plate equipped to be hooked into another separation plate,

Figure 4 schematically shows two lenses with a common radial plate and with two pole pieces that are an integral part,

Detailed description of the invention.

[0055] Figure 1 schematically shows a cross-section of a lens 100 according to the invention. The particle-optical lens is rotationally symmetric around a particle- optical axis 102. An electric coil 104 generates a magnetic field when energized.

The thus generated magnetic flux is, via polepieces 106 and 108, guided to a gap 110, where it causes a rotationally symmetric magnetic field on and around the axis 102. The radial plates 112 and 114, together with the separation plates 116-I also help guide the magnetic flux generated by the coil.

[0056] The particle-optical beam, for example a beam of electrons, passes along the axis through the bore in the polepieces. The radial component in the gap forces off-axial particles, or at least their wave function, toward the axis.

[0057] The field in the gap 110 must be rotational symmetric because only then no deflection, astigmatism, coma or other aberrations occurs for an axial beam. In traditional lenses all parts are therefore formed as rotationally symmetric elements. Inventors realized that, although symmetry at the polepieces is important, a non-symmetry removed from the axis does not or hardly hinder the symmetry of the field at the axis. If, for example, four separation plates are used (instead of a tubular segment surrounding the coil as used in traditional lenses), the asymmetry is negligible. Fabricating a lens with radial plates and separation plates that take the form of flat plates results in a much-reduced price.

[0058] As a material for such plates relatively cheap RVS430 plates can be used, or (a stack of plates of) SiFe as used in, for example, mains transformers.

For the polepieces a much higher quality material such as iron, nickel-iron or cobalt-iron, can be used that should be heat-treated to get optimal magnetic properties.

[0059] For more information about magnetic materials, heat treatments, etc. the person skilled in the art is referred to, for example, “Weichmagnetische

Werkstoffe”, Richard Boll, ISBN 3-8009-1546-4.

[0060] It is noted that the line AA’ is used to define the cutline used to show a cross-section in figure 2.

[0061]Figure 2 schematically shows a cross-section of the lens shown in figure 1 along line AA’.

[0062] In figure 2 a cross-section is shown of a lens with four separation plates 116-1, 116-2, 116-3 and 116-4. Shown is the bore 202 in polepieces 108.

[0063] It is noted that here a lens with four separation plates is shown, introducing a small eight-fold asymmetry (octupole), but that a higher number of plates can be used, introducing higher order asymmetry and thus lower aberration.

[0064] The separation plates can be screwed together, glued together, etc.

Preferably a small overlap between the plates exists to avoid that any magnetic gap due to small mechanical inaccuracies leads to magnetic asymmetry. Also, and sometimes more important, to avoid that external fields can enter the area enclosed by the four separation plates. The latter can occur when the lens is exposed to for example a radial magnetic field stemming from e.g., 50Hz/60Hz transformers, the field otherwise deflecting the beam.

[0065] It is noted that the coil can be wound on a non-magnetic former, for example a plastic or aluminium former. Also cooling can be integrated in the former and/or the windings of the coil, for example water cooling.

[0066] Figure 3 schematically shows a view of a separation plate that can be hooked into other separation plates.

[0067] The separation plate shows two extensions 302 with a trench 304, the trench having a length d equal to half the length D of the separation plate. The width £ of the trench is identical to, or slightly more, than the thickness of the separation plates.

[0068] By inserting the trench of another separation plate into the trench, the separation plates at right angles to each other, the plates hook into each other and are magnetically connected. The end of the trenches join each other, as the length d of each trench equals half of the length D of the separation plates. This can be done for all four separation plates, so that the coil is surrounded by four plates hooked into each other and forming an interconnected magnetic body.

[0069] Figure 4 schematically shows a cross-section of two lenses with a common radial plate and two polepieces made from one integral part.

[0070] The figure shows two lenses 402, 404 centered around particle-optical axis 102. It is noted that typically two or more lenses are used to manipulate a beam of particles in such a way that the beam current and the focal position (along the particle-optical axis) can be varied independently.

[0071] Lens 402, comprising radial plate 420 and 422, pole pieces 410 and 416A, coil 418 and separation plates 426-i. Polepieces 410 and 412A are separated by a non-magnetic spacer 416, for example an aluminium or copper spacer. The lens 402 further comprises a coil 430.

[0072] Likewise lens 404 comprises radial plate 422 and 424, pole pieces 412B and 414, coil 432 and separation plates 428-i. Polepieces 412B and 414 are separated by a non-magnetic spacer 418, for example an aluminium or copper spacer.

[0073] Because of the vacuum to which the inner bore of this part is exposed, the part is preferably not made of a plastic, although O-rings 438-i made of an elastomer usable for vacuum sealings, such as Viton™ can be used between polepiece and spacer to form a high-vacuum seal. Another solution is to insert a non-magnetic tube in the bore, for example a tube made of a non-magnetic steel, and only evacuate the inside of the tube.

[0074] To clamp the radial plates together several rods 434, preferably four, can be used with nuts 436-k. Washers (not shown) can be used to limit the force.

These4 rods are preferably placed at the corners of the radial plates.

[0075] It is noted that as the magnetic field in the shared radial plate 422 is the result of the two magnetic fields generated by the two coils 430 and 432. If the fields have in the shared radial plate the same orientation, they will add up, if they have opposite orientation, the resultant field will be less than the field generated by each of the coils. The skilled person knows that the lens action is independent on the direction of the magnetic field, and both orientations are possible.

However, the relative magnetic susceptibility H: of the material of the shared radial plate is a function of the magnetic field, and thus there is some cross-talk between the lenses: without changing the coil current of one lens the strength of that lens is influenced by a change of strength of the other lens.

[0076] It is noted that the size of the spacers 416 and 418 need not be identical but are governed by the design of the lens. The same holds for the form of the polepieces 410, 412A, 412B and 414 near the spacers.

Claims (16)

Conclusions. 1. An electromagnetic lens (100) with a particle-optical axis (102), the lens for focusing an energetic beam of charged particles, the lens of which consists of: e An electromagnetic coil (104), e a yoke, the yoke consisting of two pole pieces (106, 108} made of magnetically soft material, the pole pieces exhibiting rotational symmetry about the particle-optical axis (108), the two pole pieces separated by a gap (110), Characterized in that the yoke further comprises two radial plates (112, 114) of magnetically soft material, each radial plate being in magnetic contact with a corresponding pole piece (106, 108), the radial plates being perpendicular to the particle-optical axis (102), three or more substantially isosceles trapezoidal separator plates (116-i) made of magnetically soft material, the separator plates magnetically around the coil (104), two opposite sides of each separator plate (116-1) largely in magnetic contact with other separator plates (116-j, 116- k), the three or more separator plates thus form a magnetically closed path around the coil (104), the other two opposite sides of each separator plate in contact with the radial plates (114, 118). 2. The lens of claim 1 wherein the substantially isosceles trapezoidal separation plates (116-i) are essentially rectangular separation plates (124-i). 3. The lens according to any of the foregoing claims in which the number of dividing plates (116-i, 202-i) is four. 4. The lens according to any of the foregoing claims in which the separating plates (116-i, 202-i} have an identical shape and are made of material with identical magnetic properties. 5. The lens of any preceding claim wherein at least one of the pole pieces (106, 108) has a truncated cone facing the aperture (110). 6. The lens of any preceding claim wherein the radial plates (112, 114) have a square outline. 7. The lens of any preceding claim wherein the two pole pieces (106, 108) are made of material with identical magnetic properties. 8. The lens of any preceding claim wherein the first and second radial plates (112, 114) are made of material having identical magnetic properties. 9. The lens of any preceding claim wherein the radial plates (112, 114) and the separator plates (116-i, 202-i) are made of material with substantially identical magnetic properties. 10. The lens of any of the foregoing statements in which the separator plates (116-i, 202-i) are arranged to interlock, forming a magnetically closed path around the coil (104). 11. The lens according to any of the foregoing statements in which the radial plates (112, 114) have slots in which the separator plates (116-i, 202-i) rest. 12. The lens according to any of the foregoing claims wherein the opening is formed by a hollow cylinder of non-magnetizable material. 13. The lens according to any of the preceding claims in which an evacuated vacuum tube is placed in the pole pieces and in the opening. 14. A probe forming instrument equipped with a lens according to any of the foregoing claims. The probe forming instrument of claim 14 equipped with at least two lenses (402, 404} according to any one of claims 1-13, wherein the two lenses share a common radial plate (422). 16. The probe forming instrument of claim 14 or claim 15 equipped with at least two lenses (402, 404) according to any one of claims 1 to 13, wherein the pole piece (412A) of one lens (402) forms an integral part with a pole piece (412B) of the other lens (404).