Classifications

• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/04—Arrangements of electrodes and associated parts for generating or controlling the discharge, e.g. electron-optical arrangement, ion-optical arrangement
  • H01J37/10—Lenses
  • H01J37/12—Lenses electrostatic
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J37/00—Discharge tubes with provision for introducing objects or material to be exposed to the discharge, e.g. for the purpose of examination or processing thereof
  • H01J37/02—Details
  • H01J37/20—Means for supporting or positioning the objects or the material; Means for adjusting diaphragms or lenses associated with the support
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/04—Means for controlling the discharge
  • H01J2237/045—Diaphragms
  • H01J2237/0451—Diaphragms with fixed aperture
  • H01J2237/0453—Diaphragms with fixed aperture multiple apertures
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/04—Means for controlling the discharge
  • H01J2237/049—Focusing means
  • H01J2237/0492—Lens systems
  • H01J2237/04924—Lens systems electrostatic
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/10—Lenses
  • H01J2237/12—Lenses electrostatic
  • H01J2237/1205—Microlenses
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/10—Lenses
  • H01J2237/12—Lenses electrostatic
  • H01J2237/121—Lenses electrostatic characterised by shape
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/153—Correcting image defects, e.g. stigmators
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/26—Electron or ion microscopes
• • H—ELECTRICITY
  • H01—ELECTRIC ELEMENTS
  • H01J—ELECTRIC DISCHARGE TUBES OR DISCHARGE LAMPS
  • H01J2237/00—Discharge tubes exposing object to beam, e.g. for analysis treatment, etching, imaging
  • H01J2237/30—Electron or ion beam tubes for processing objects
  • H01J2237/317—Processing objects on a microscale
  • H01J2237/3175—Lithography

Definitions

• the present invention relates to a charged particle beam system such as an electron beam exposure system, scanning and non-scanning electron microscopes, and the like.
• the current invention presents a microlens array for generating a plurality of focused beamlets or focusing beams with different incident angles in a charged particle beam exposure or imaging apparatus with zero field curvature, minimized geometrical aberrations such as coma and astigmatism, comprising a current limiting aperture, generating a plurality of charged particle beamlets, a lens array aligned with the current limiting aperture, for focusing all the beamlets into a flat image plane. It solves the problem of how to generate a plurality of focussed beamlets with a minimum of aberrations in a diverging beam system, such as in multi-beamlet inspection systems, in particular desired at a limited source to target distance such as in a multi beamlet, maskless lithography system.
• JP60031225, JP60039828 and J.Vac.Sci.Technol.B 4(5), Sep/Oct 1986 an electron matrix lens with reduced aberrations is disclosed where the centre of a current limiting aperture is shifted from the optical axis of a lens associated with said aperture, to an optimum position.
• the position of the current limiting aperture, which is fabricated in a separate plate, is chosen such that a virtual aperture, which is symmetrical along the optical axis, is in a position causing the total aberration for off- axial lenses to be minimal.
• Patent publication JP60042825 discloses a correction means for the field curvature for each lens, by changing the focus using a correction lens matrix.
• the astigmatism becomes dominant with increasing incident angle and eventually restricts the maximum incident angle to less than 30mrad, thus the throughput of the system is limited.
• the throughput of the system is also restricted due to a small filling factor of the lens that is allowed.
• a lens array forming a plurality of focused beamlets from a diverging broad beam is disclosed.
• Figure 1OA thereof discloses a schematic of an example of the lens array, but does not indicate what the position of the electrode must be with respect to the beam.
• the lens is, in a particular embodiment, concaved with respect to the source so that the off-axial beamlets can pass the lens along the optical axis.
• drawbacks include that the image plane for all the beamlets is present in a concaved surface with respect to the source.
• a difficulty exists in the alignment between the lens array and a "spatial filter", in fact the current limiting aperture structure, which in this publication is taught to be on a planar surface.
• a further objective of the invention is to minimize the image aberrations for the realised lens array.
• Another objective of this invention is to improve the resolution of charged particle beam systems departing from a source with at least one diverging charged particle beam. Again another objective of this invention is to improve the throughput of such charged particle beam systems.
• Yet another object of this invention is to control the uniformity of the beams.
• the present invention relates to charged particle optical system as defined in claim 1.
• the dimension of the effective electric field height is reduced by a factor of thousand relative to the prior art micro lens structures, resulting in strongly reduced chance of effectively having a beamlet passing through a lens part with strongly sub- optimal conditions with respect to image aberrations.
• optimise the measure according to the invention will be performed by dimensioning the lens with changes within order of the new lens and effective field size, i.e. will remain within the order of ultimate smallness as claimed, totally different in effect and basic principle than the measure that has been taken with the present invention.
• the invention in a further elaboration thereof also relates to an apparatus for generating a plurality of focused charged particle beamlets or focusing beams with different incident angles in charged particle beam systems, comprising: a) " a current limiting aperture array located either before or after the lens array, to split up the diverging charged particle beam into a plurality of charged particle beamlets; b) a lens array comprising a plurality of lenses to focus the beamlets with different incident angles into a plane; c) the above said current limiting aperture being aligned with the lens in the lens array for a beamlet with a particular incident angle such that the virtual aperture is symmetrical along the optical axis and is located optimised in the lens with respect to resulting aberrations, e.g. by a, though not necessarily, centred location.
• the current limiting aperture is aligned with each lens in the lens array.
• the current limiting aperture is according to invention preferably fabricated on the same plate as either the first or the last lens electrode, but can be on a separate plate.
• the current limiting aperture is positioned in a field free region, while the diameter, more in general the size, largeness or magnitude of use surface area, of the current limiting aperture may change for homogeneity of beam currents, in particular as a function of its distance to the centre of the lens array.
• the lens array comprises of two planar electrodes, which are in a separation of less than a few tens of microns.
• the two electrodes are aligned with respect to each other.
• the bore diameters, in general the bore size, in the two electrodes are the same, and smaller than the thickness of the electrodes to limit the lens field from penetrating deep into the lens holes, alternatively denoted lens apertures.
• the lens size, in the specific embodiment of a cylindrical opening, the diameter increases for off-axial lenses for field curvature correction.
• the lens array comprises of a single planar electrode, with at least two, preferably three macro-electrodes facing the lens holes.
• the first electrode has the same or higher potential as the aperture lens electrode, while the second electrode has a higher potential than the first electrode, and the third electrode has a lower potential than the second electrode.
• the diameter of the said aperture lens is smaller than the thickness of the said lens electrode to limit the lens field from penetrating deep in to the lens holes.
• the strength of the off-axial aperture lens in the aperture lens array is made weaker by having the field in front of the said aperture lens weaker than that of the central lens. In this way, by using the larger focal length at the off axial lenses, the field curvature is corrected, i.e. the image plane is brought into a planar surface.
• the lens diameter preferably increases for off-axial lenses for field curvature correction.
• the lens array comprises three planar electrodes and the opening angle limiting aperture is made on a separate plate.
• the three electrodes are aligned in such a way that the centre of the beam with a specific incident angle passes through the centre of each electrode.
• the size of the lens hole cross section e.g. expressed by a diameter, is preferably made larger for off-axial lenses for field curvature correction.
• the lens holes are made elliptical for correction of astigmatism. It is in this respect to be noted that in principle, most if not all of the features described as embodiments or not in this document, may be combined.
• a micro-lens array with limited lens field in case of a two-electrode lens array, the separation between two electrodes is less than a few tens of microns, and the lens bore diameter, in case of an elliptical shape, the smallest diameter, is smaller than the thickness of the lens electrode; in case of aperture lens array, the lens diameter is smaller than the thickness of the electrodes.
• the third order aberrations especially coma and astigmatism, will be minimized for off-axial beamlets.
• the first order field curvature due to a longer objective distance for off-axial beamlets is compensated by increasing the radius of the off-axial lens holes, so that each lens of the lens array focuses a beamlet at the same image plane
• the field curvature can be corrected by adding three macro electrodes facing the lens aperture, with the potential of the first macro electrode the same or higher as that of the aperture lens array.
• This configuration leads to a curved equal-potential plane in front of the aperture lens array.
• the curvature of the equal-potential plane leads to a smaller aperture lens strength for the off-axial lenses than that of the central lens, and in this way, the field curvature can be corrected.
• the apparatus mentioned above can be either an aperture lens with shifted current limiting apertures, a two electrode microlens array made of a SOI wafer or a two electrode microlens array by bonding of two wafers.
• a three-electrode lens array can be used for generating a plurality of beamlets or focusing beams with different incident angles, where the lens electrodes are skewed in such a way that the centre of the beam passes through the center of each electrode.
• the field curvature may be corrected by increasing lens radius for off-axial lens in the lens array.
• Elliptical lens holes may be used to correct astigmatism.
• the current limiting aperture is made on a separate plate.
• the current limiting apertures said in above definitions and the first or last lens electrode are made of one piece . of wafer, the alignment is done with optical lithography.
• the current limiting apertures mentioned in the above definitions is in a field free region by limiting the lens bore diameter smaller than the electrode thickness. The diameter of the current limiting aperture may change for homogeneity of beamlet currents.
• Figure 1 is a schematic illustration of a charged particle or light optic beam passing a lens system with a lens and a beam current limiting opening;
• Figure 2 illustrates a two-electrode micro lens, here created by bonding of two wafers, and showing a shifted apertures
• Figure 3 represents an alternative to the embodiment of figure 2, by having an electrode produced of an SOI wafer;
• Figure 4 schematically illustrates two equivalent integrated aperture lenses with shifted current limiting aperture;
• Figure 5 and 6 illustrate embodiments of an aperture lens array combined with three macro lenses, fig. 6 thereby illustrating the working of equipotential lines, and the requirement to consider the same at the location of a lens array;
• Figure 7 illustrates a micro-einzel-lens with shifted electrodes
• Figure 8 illustrates a micro-einzel-lens array with field curvature correction means
• Figure 9 illustrates the difference in size and configuration between a prior art lens and a lens in accordance with the invention.
• Figure 1 shows an alignment of a current limiting aperture CLA, normally part of an array of such apertures CLA and included in an aperture plate AP, and a lens 2.
• the current limiting aperture CLA of the lens is in such a position, that for a beam 3, such as a charged particle beam, with a certain incident angle ⁇ , the virtual aperture VA is located in a centre part of the lens 2, and symmetrical along the optical axis OA of the lens.
• the middle i.e. centre of the lens means the geometry centre of the lens.
• the middle of the lens is at the middle of two electrodes and the in case of three electrode lens, the middle of the lens is at the middle of the central electrode, and in case of an aperture lens, the middle of the lens is at the end plane of the aperture.
• the image further shows an image plane IP.
• lens structures 5, 6, in short also denoted lenses.
• the filling factor for the present two-electrode lenses can be 85% and even higher, without significant increase in coma, astigmatism and field curvature.
• These lens structures come with aperture plates AP, which are here integrated with one electrode, thereby forming a so-called integrated aperture lens array.
• the two electrodes are aligned with each other.
• the lenses show cylindrical holes Hl, of which the diameter dl is smaller than the thickness De of the first electrode.
• the effect of such measure being that the effective electric field Ef, illustrated in figure 9, of the lens is prevented from penetrating deep into the lens holes Hl.
• the current limiting aperture CLA is fabricated on the first lens electrode, and is positioned in a field free region.
• the radius of off-axial lens hole Hl increases in order to correct the first order field curvature, and, surprisingly, the radius variation appears to have a large effect on the lens strength, more than enough to correct the first order field curvature. By doing so, the sizes of the images formed by the lens array are equal to each other, and without inducing further aberrations.
• FIG 4 a schematic of an aperture lens is shown, where the aperture plate AP with current limiting aperture CLA and the lens holes Hl are made of a piece of wafer.
• the current limiting aperture CLA is in such a position that the virtual aperture VA is at the end plane of the electrode E and symmetric along the optical axis.
• the lens bore diameter dl is smaller than the thickness De of the electrode in order to limit
• FIG. 4 a pertaining lens field Ef from penetrating deep into the lens holes Hl.
• the right hand side of figure 4 indicates an equivalent version wherein the sequence of lens opening or electrode and the current limiting aperture array is inversed with respect to the direction of an incident beamlet.
• an example of an aperture lens array here an integrated aperture lens array IAL is shown, with three macro electrodes MEl, ME2 and ME3 facing the aperture lens, i.e. all of the beams that are passed through the aperture lens array, pass the set of macro-electrodes MEl, ME2 and ME3, in the central part thereof.
• the second electrode ME2 is at a higher potential V2 than Vl, i.e. V2 > Vl.
• the figure indicates equi-potential lines EPL for this alternative embodiment.
• Aperture lenses are here formed at the lower end of a bore or lens hole Hl. It may be clear form the illustration that with the equipotential lines locally being closer to one another and to the aperture lens AL, the central aperture lens, or lenses as the case may be in larger embodiments, is respectively are stronger than that of relatively off-axial aperture lenses. Thus the central aperture lens is stronger than the off-axial lenses, which phenomenon is here used for field curvature
• Figure 6 provides a functionally equivalent example of a largely inversed arrangement, wherein the current limiting apertures are located downward from the macro-lenses ME1-ME3 in view of the main direction of the beamlets 3, and with appropriately adapted voltages.
• the arrangement may be set into a zero strength mode and a non-zero strength mode of operation.
• the non-zero mode is here illustrated and provides for a corresponding focusing effect as is the case in the figure 5 arrangement, while in the non-depicted zero strength mode, the field is solely applied for generating a plurality of focused beams with to function with field curvature correction.
• Figure 6 furthermore illustrates the effect of equipotential lines in a said three macro-lens lens structure in slight more detail, and also shows the electric field effect of the macro lenses ME1-ME3. The figure in particular illustrates how the equipotential lines are most close to one another in the most centered lens holes, and somewhat further away from one another at the off-axial lens holes.
• Figure 7 shows a 3-electrode lens, comprising a current limiting aperture CLA made in a separate aperture plate AP, and 3 electrodes Esl-Es3.
• the current limiting aperture CLA is aligned to the three electrodes Esl-Es3 in such way that the virtual aperture VA is in the centre part of the middle electrode and symmetrical along the optical axis of each lens.
• the three electrodes E1-E3 are skewed in such a way that the centre of the beam passes through the center of each electrode hole Hl.
• the 3-electrode lens array is shown with lens radius increasing for off-axial lenses, so as to correct field curvature.
• the images of each lens are by this lens structure projected onto a flat image plane.
• elliptical lens holes Hl may according to the invention be used to correct for leftover astigmatism.
• Figure 9 provides an illustration of difference in size and configuration of a prior art lens structure, e.g. as in the earlier cited J.Vac.Sci. Technol.B 4(5), Sept/Oct 1986, in the upper figure part, relative to that of the structure according to the invention, represented on scale in the encircled figure part, and provided as an exploded view in the lower figure part. Also from the latter is evident that the largest dimension dl in a cross section of a lens hole HL is equivalent and preferably smaller than the thickness of a lens electrode, while the lens field is confined to within the thickness of the electrode.

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• Chemical & Material Sciences (AREA)
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• Electron Beam Exposure (AREA)

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• 2007
  • 2007-07-13 CN CN200780028626.1A patent/CN101496129B/zh active Active
  • 2007-07-13 EP EP20110183872 patent/EP2405459A1/de not_active Withdrawn
  • 2007-07-13 KR KR1020097003925A patent/KR101464388B1/ko not_active IP Right Cessation
  • 2007-07-13 EP EP11183870A patent/EP2405458A1/de not_active Withdrawn
  • 2007-07-13 JP JP2009521713A patent/JP5340930B2/ja active Active
  • 2007-07-13 EP EP20070768909 patent/EP2050118A1/de not_active Withdrawn
  • 2007-07-13 WO PCT/NL2007/000180 patent/WO2008013442A1/en active Application Filing
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