Inspecting system for particle-optical imaging of an object, deflection device for charged particles and method for operating the same

The invention relates to an examination system for the two-dimensional imaging of structures of an object by means of charged particles, such as electrons, on a receiver, such as a camera. The structures examined can in particular be those which are conventionally examined by electron microscopy, backscattering electrons, secondary electrons, transmission electrons or photoelectrons emerging from the object being observed. The invention is not intended to be limited to these types of electrons; in particular, it is also intended to observe ions emerging from the object.

The invention further relates to a deflection device for rays of charged particles, which can be used in particular in the examination system for examining the object. However, the use of the deflection device should not be restricted to this application. The invention further relates to a method for operating the deflection device.

One possible area of ​​application of the examination system is in methods for producing miniaturized components, and in particular in the localization of defects on the components during manufacture and on masks used for this, such as lithography masks.

No. 5,578,821 discloses an examination system in which an electron beam is focused on a point (pixel) of the object to be examined and backscattered electrons, secondary electrons and transmission electrons emerging from the object are registered by corresponding detectors. In this case, a deflector device is provided in order to shift the location at which the focused beam strikes the object, and the respective detectors receive all backscattering, secondary and transmission electrons integrally regardless of the location shifting. The detectors themselves do not work in a spatially resolving manner, but it is possible to obtain a spatially resolved image of the structures of the object if the intensity registered by a detector is assigned to the point determined by the deflector device at which the beam hits the object. The beam is then successively deflected by actuating the deflector device to the different locations (pixels) of the object, the measured intensities assigned to the different locations are registered, and the spatial image of the object is composed of the successively registered intensities. This process is very time consuming.

US 6,087,659 discloses an examination system in which a spatially extended field on the object is irradiated with primary electrons and in which secondary electrons emerging from the object are imaged on a spatially resolving detector. In contrast to the system shown in US Pat. No. 5,578,821, this means that sequential scanning of pixels by means of a deflector device is not necessary; rather, a large number of pixels can be detected simultaneously. However, with a desired high magnification of the image, the field imaged on the detector is too small to be able to image an object of greater spatial extent at once. A mechanical displacement table is therefore provided for the object in order to displace it relative to the illuminated field, so that a large number of images of the step-wise displaced object can be recorded one after the other in time, in order to finally depict the image by combining the several images entire property. This is again time-consuming and also requires a sliding table, the mechanical precision of which corresponds approximately to the desired resolution of the image.

Accordingly, it is an object of the present invention to propose an examination system for imaging an object, with which spatially extended objects can be imaged comparatively quickly.

It is also an object of the invention to propose such an examination system in which the requirements for a mechanical displacement of the object relative to the examination system are reduced.

In a first aspect, the invention provides an examination system for imaging an object which can be arranged in an object plane and which comprises a spatially resolving detector on which charged particles which emerge from the object in a spatially extended field are imaged.

A first deflector device is also provided, which provides a changeable deflection field in order to shift the area which is imaged onto the detector by means of the charged particles in the plane of the object. The first deflector device serves to guide the charged particles entering the first deflector device from different directions, depending on the displacement of the area in the object plane, through an essentially fixed, predetermined beam cross section.

The particle beam is always directed onto the same section of the detection surface of the detector, regardless of the position of the imaged area in the object plane. The projection location of the imaged area on the detection surface does not shift if the area that is imaged on the detector is shifted. In particular, the particle beam emitted by the object always passes through the fixed beam cross section, essentially in the same direction, for example parallel to an optical axis of the system, regardless of the position of the region.

After passing through this beam cross section, which is independent of the displacement of the imaged area in the object plane, the charged particles optionally pass through optics to increase the magnification of the image and finally strike the spatially resolving detector. Because the area of ​​the object plane imaged on the detector can be moved, it is possible to spatially image structures of comparatively large objects without having to move the object mechanically relative to the examination system.

The examination system further comprises an illumination device in order to supply the object with energy in such a way that charged particles emerge from locations of the object, these emerging particles then being subsequently fed to the detector in order to obtain the image of the object. The energy can be supplied to the object in the form of charged particles, in particular electrons, or electromagnetic radiation (light). Accordingly, the lighting device then comprises a light source, such as a laser, or a particle source, in particular an electron source. The particles emerging from the object due to the supply of energy are preferably photoelectrons or backscattered electrons, secondary electrons or transmission electrons.

The inspection system further includes a controller to control the first deflector device such that the area of ​​the object imaged on the detector is changed, the controller being further provided to control the illumination device to the limited field in which energy is supplied to the object to move together with the area imaged on the detector in the plane of the object. The illuminated field preferably coincides essentially with the area imaged on the detector.

Energy is therefore not supplied to the entire object. Rather, in view of the lowest possible thermal load on the object, only one area of ​​the same energy is supplied. This additionally achieves a reduction in the particle flow, which, for example, reduces space charge effects. If the illuminated field and the imaged area coincide, then the thermal load on the object is not greater than is absolutely necessary in view of the creation of the image of the object.

A first focusing lens device, which has the effect of a focusing lens on the particles emerging from the object, is preferably provided between the object and the location in the beam path at which the charged particles running towards the detector pass through the fixed beam cross section. With regard to a sufficient magnification of the image, this focusing lens device is to be arranged relatively close to the object. To avoid imaging errors, when using a conventional rotationally symmetrical magnetic lens, the bore diameter should be chosen to be relatively small, so that when the round lens with a small bore diameter is arranged close to the object, the area that is imaged on the detector is not so far in the plane of the object deflectable, as would be desired with regard to imaging large-area objects without mechanical movement thereof relative to the examination system. For this reason, lens devices are preferably used as the first focusing lens device, which are different from the magnetic circular lens, as can be used in the other components of the examination system, such as a re-magnification optics in front of the detector.

A preferred possible embodiment of the first focusing lens device, which provides its focusing deflection field over a relatively large area transverse to the beam direction, is described in DE 196 34 456 A1, the disclosure of this document being incorporated fully into the present description by reference. This focusing lens device comprises a cylindrical lens and a quadrupole lens arranged stationary to the cylindrical lens. The axis of the cylindrical lens is arranged transversely to the beam direction and focuses the beam in a direction transverse to the axial direction of the cylindrical lens. A major axis of the quadrupole lens substantially coincides with the axis of the cylindrical lens, and the field of the cylindrical lens and the field of the quadrupole lens act together to focus on charged particles that pass through the lens assembly.

A further preferred embodiment of the first focusing lens device is described in DE 199 44 857 A1, the disclosure of which is fully incorporated into the present application by reference. The focusing lens device disclosed there also comprises a cylindrical lens, the axis of which extends transversely to the beam direction. A quadrupole field is also superimposed on the deflection field provided by the cylinder lens, which, however, is not stationary but can be displaced in the direction of the cylinder axis. For this purpose, the lens arrangement comprises a plurality of electrodes which are arranged in pairs opposite one another along the cylinder axis. Voltages can be selectively applied to the electrodes in such a way that they provide an electric field with quadrupole symmetry at a selectable point along the cylinder axis. When using such a first focusing lens device, the control is also provided so that the location of the quadrupole field just provided is arranged approximately centrally with respect to a bundle of charged particles which emanate from the area imaged on the detector with its corresponding deflection in the plane of the object.

It is also preferred to design the first focusing lens device as a variable axis lens. Such a lens is described in the article "MOL (" Moving Objective Lens "), Optic 48 (1977), pages 255ff. By E. Goto et al. Or in US 4,376,249.

A preferred embodiment of the first focusing lens device as a magnetic lens with a variable axis is described in the patent application DE 100 44 199.8 of the applicant and corresponding European application No. 01121374.1 and is also incorporated into the present application by reference. This focusing lens device comprises a magnetic lens which provides a static rotationally symmetrical deflection field, and furthermore a coil arrangement which is arranged within the rotationally symmetrical magnetic field and generates a dipole field of adjustable size there. The superimposition of the rotationally symmetrical magnetic field and the dipole field, the field direction of which is oriented transversely to the symmetry axis of the magnetic lens and the beam direction, leads to a likewise rotationally symmetrical focusing field, the symmetry axis of which, however, is shifted with respect to the symmetry axis of the static lens depending on the strength of the dipole field. The strength of the dipole field and thus the displacement of the effective optical axis of the focusing lens device is preferably also adjusted by the control in such a way that the axis displacement takes place together with the displacement of the illuminated field and the area of ​​the object imaged on the detector in the object plane.

The control is preferably carried out in such a way that the optical axis of the focusing lens field provided by the first focusing lens device intersects the area imaged on the detector, in particular centrally.

The examination system is preferably designed in such a way that backscatter electrons or / and secondary electrons and / and transmission electrons and / or photoelectrons emerging from the object are imaged on the corresponding detector.

In order to image photoelectrons onto the detector, the illumination device then comprises a photon source in order to illuminate the field in the object plane. The photons emitted by the photon source are preferably directed towards a movable deflection device which reflects the photons onto the object. The movement of the deflection device is controlled by the control in order to achieve the deflection of the illuminated field in the object plane by changing the orientation of the deflection device.

In order to image transmission electrons onto the detector, the examination system comprises an electron source and an aperture, in order to form a beam from the electrons emitted by the source, which beam illuminates the field in the object plane. Furthermore, a second deflector device is provided in order to deflect the electron beam transversely to its beam direction and thus to achieve the displacement of the illuminated field in the object plane. The first deflector device and the detector are then arranged on a side of the object opposite the source with respect to the object plane, in order to detect the transmission electrons passing through the object.

To detect backscattered electrons or secondary electrons, the source and the detector are arranged on a side of the object that is the same with respect to the object plane. The first deflector device is then preferably used not only to shift the area from which emerging electrons are imaged onto the detector in the object plane, but also to shift the field illuminated by the electrons emerging from the source in the object plane. The first deflector device is then penetrated by the electron beam extending from the source to the object in one direction and by the electron beam extending from the object to the detector in the other direction. The electrons running from the source to the object generally have a greater kinetic energy than the electrons running from the object to the detector. The deflector device is preferably designed such that it provides an essentially identical deflection angle for the electrons running in both directions.

Such a deflection device forms a second aspect of the present invention.

For this purpose, the invention provides a deflection device which provides an electrical and a magnetic deflection field in a spatial volume, the directions of movement of the electrons and the field directions of the electrical and magnetic fields being oriented in pairs approximately orthogonally to one another.

The electrical deflection field can be provided, for example, by an electrode arrangement, and the magnetic deflection field can be provided, for example, by an electric conductor winding arrangement. A controller is then provided according to the invention which adjusts currents through the conductor winding arrangement and voltages applied to the electrode arrangement such that the electrons running from the source to the object and the electrons running from the object to the detector move on essentially the same or similar trajectories .

In order to precisely deflect the electrons moving in opposite directions through the deflection device, the deflection device according to a further aspect of the invention is designed in such a way that it comprises a plurality of rings arranged at an axial distance from one another and made of a material with high magnetic permeability and the current conductor turns each encompass at least one of the rings. The material with high magnetic permeability is a material whose permeability is significantly greater than that of the vacuum.

In order to avoid eddy currents, this material is preferably essentially electrically non-conductive or an electrical insulator. An example of such a material is a ferrite.

The electrodes of the electrode arrangement are preferably arranged radially inside the rings and the current conductor windings wound thereon. In a preferred embodiment, when the charged particles are guided in a vacuum, it is provided that the current conductor winding arrangement is arranged outside a corresponding vacuum jacket and the electrode arrangement is arranged radially inside the vacuum jacket.

In order to change the deflection angle provided for the electrons by the deflection device, the control changes the strengths of both the electrical and the magnetic field in the deflection volume. The change in the field strengths is preferably carried out in such a way that the ratio between the strength of the magnetic field and the strength of the electrical field is constant regardless of the deflection angle. Preferably, this ratio is substantially equal to the difference in the velocities of the electrons traveling from the source to the object and from the object to the detector as they pass the deflection device divided by the product of these two speeds.

Embodiments of the invention are explained in more detail below with reference to drawings. Here shows:<dl id="dl0001"><dt>Figure 1</dt><dd>1 shows a schematic representation of an embodiment of the examination system according to the invention, in which secondary electrons are observed,</dd><dt>Figure 2</dt><dd>2 shows a schematic representation of the function of an embodiment of a deflection device according to the invention, which can also be used in the examination system of FIG. 1,</dd><dt>Figure 3</dt><dd>2 shows a perspective illustration of the deflection device explained in FIG. 2,</dd><dt>Figure 4</dt><dd>3 shows a representation of the arrangement of current conductor turns in the deflection device shown in FIG. 3,</dd><dt>Figure 5</dt><dd>3 shows a spatially broken up representation of a focusing lens that can be used in the examination system of FIG. 1,</dd><dt>Figure 6</dt><dd>5 shows a schematic illustration to explain the focusing lens of FIG. 5,</dd><dt>Figure 7</dt><dd>a variant of the focusing lens shown in Figures 5 and 6,</dd><dt>Figure 8</dt><dd>another variant of the focusing lens shown in FIGS. 5 and 6,</dd><dt>Figure 9</dt><dd>a potential curve provided by the focusing lens of FIG. 7,</dd><dt>Figure 10</dt><dd>7 shows a detailed illustration of the focusing lens of FIG. 7,</dd><dt>Figure 11</dt><dd>7 shows a control diagram for the focusing lens of FIG. 7,</dd><dt>Figure 12</dt><dd>1 shows a schematic representation of a further embodiment of the examination system according to the invention, in which backscattered electrons are observed,</dd><dt>Figure 13</dt><dd>2 shows a schematic illustration of a variant of the embodiment shown in FIG. 12,</dd><dt>Figure 14</dt><dd>1 shows a schematic representation of a further embodiment of the examination system according to the invention, in which transmission electrons are observed,</dd><dt>Figure 15</dt><dd>a schematic representation of a variant of the examination system shown in Figure 14 and</dd><dt>Figure 16</dt><dd>is a schematic representation of a further embodiment of the examination system according to the invention, in which photoelectrons are observed.</dd></dl>

FIG. 1 shows a schematic representation of a first embodiment of an examination system 1 according to the invention. The examination system 1 is used to image structures of an object 3, such as a semiconductor wafer 3 or other miniaturized structures, such as a lithography mask or also a biological sample. The structures of the object 3 are examined by imaging secondary electrons emerging from the object 3 onto a spatially resolving detector 5. The detector 5 is spatially resolved in two dimensions in that it has a multiplicity of image points (pixels) which are arranged in a two-dimensional matrix. Each pixel collects readable information that represents an electron intensity striking the pixel in an exposure time interval.

The detector is preferably a component which is able to integrate an intensity distribution of charged particles hitting a detection surface in two-dimensional, spatially resolved fashion. The information detected in this way can be output for further processing to an external processing device, which for example can also be part of a control system that controls the entire system. In particular, the detector can be one or more CCD chips.

Electron-optical components are provided between the object 3 and the detector 5, which image a spatially limited rectangular field 7 in the plane of the object 3 on the detector 5. The imaging takes place in such a way that secondary electrons emerging at different locations in the field 7 meet different pixels of the detector 5. Structures of the object 3, which differ with regard to the intensity of emitted secondary electrons, can thus be represented in a pictorial manner via the detector 5.

In Figure 1, the trajectories 9, 10 of two secondary electrons, which emerge from one location of the field 7 at different angles are symbolically represented. The secondary electrons emerging from object 3 with a kinetic energy between 0 eV and approximately 5 eV are first accelerated to a kinetic energy of approximately 20 keV. The acceleration takes place through an electric field, which is provided between the object 3 and an electrode 13 arranged in front of the object. After the acceleration, the electrons pass through a focusing lens doublet, which comprises a focusing lens 15 near the object and a further focusing lens 17. The electrode 13 cooperates with the focusing lens 15 close to the object and forms with it a first focusing unit of the doublet, the second focusing unit of which is formed by the further focusing lens 17.

After passing through the doublet, an image enlargement takes place in a schematically illustrated post-enlargement optics 19, which can contain one or more particle-optical lenses and other components before the electrons strike the detector 5.

The focusing lenses 15, 17, the re-magnification optics 19 and the detector 5 are centered with respect to a main axis 21 of the examination system. However, the field 7, which is electron-optically imaged on the detector 5, can be displaced with respect to the main axis 21 in an x ​​direction transverse to the main axis 21. For this purpose, two deflectors 23 and 24 are arranged between the two focusing lenses 15 and 17. FIG. 1 shows a situation in which the center of the field 7 imaged on the detector 5 is displaced from the main axis 21 by a distance M in the x direction. After passing through the focusing lens 15 close to the object, the trajectories of the electrons extend parallel to the z-axis or main axis 21. The deflector 24 arranged closer to the object 3 causes the trajectories to be deflected by an angle β such that the electrons move in the direction of the main axis 21 run. Thereupon, the deflector 23 arranged further away from the object 3 causes the electrons to be deflected by an angle β such that the electrons continue to run parallel to the z-axis. The deflection M of the field 7 away from the main axis 21 is thus given by the distance between the two deflectors 23, 24 from one another and the deflection angle β provided by the two deflectors 23, 24.

A controller 25 is also provided, which controls the deflectors 23 and 24 in order to set the deflection angle β and thus the deflection M of the field 7 away from the main axis 21. The angles β are set in such a way that the secondary electrons, after passing through the deflector 23 which is remote from the object 3, pass through a beam cross section which is independent of the deflection M and is fixedly positioned with respect to the examination system. This beam cross section, which is independent of the deflection M, is identified in FIG. 1 by the reference symbol 27 and is centered with respect to the main axis 21. After passing through the beam cross section, which is identified by reference numeral 27 and is independent of the deflection, the electrons continue to the detector in a beam path which is also independent of the deflection M. In the embodiment shown in FIG. 1, this further beam path toward the detector is also centered with respect to the main axis 21. However, it is also possible to provide beam deflectors and other components, such as energy filters, which guide the beam away from the main axis 21 between the cross section 27, which is independent of the deflection, and the detector 5.

In order to detach secondary electrons from the object 3 in the field 7 imaged on the detector 5, this field 7 is illuminated with primary electrons. A primary electron beam 29 is formed by an electron gun 31, a magnetic lens 33 and a beam shaping diaphragm 35. The kinetic energy of the primary electrons of the beam 29 is approximately 22 keV.

The primary electron beam 29 is superimposed by means of a beam combiner 37 with the secondary electron beam running towards the detector 5. The beam combiner 37 is arranged in the beam path of the secondary electrons between the focusing lens 17 and the re-magnification optics 19 and is penetrated in a straight line by the secondary electrons. The beam combiner 37 deflects the primary electrons 29 entering the beam combiner 37 at an angle to the main axis 21 in such a way that the primary electrons move along the main axis 21 in the direction of the object 3 after passing through the beam combiner 37.

After the beam combiner 37, the primary electrons pass through the focusing lens 17 and the beam cross section 27, which is independent of the deflection M, and then through the deflector 23, which deflects it away from the main axis 21 by an angle β. Subsequently, the primary electrons are deflected by the deflector 24 by an angle β toward the main axis 21, so that they finally run parallel to the main axis 21 again, although the primary electron beam through the two deflectors 23, 24 is also by the distance M from the main axis 21 was moved away. The primary electrons then pass through the focusing lens 15 and are braked to a kinetic energy of approximately 2 keV by the electrical field provided by the electrode 13 before they strike the object 3 in the field 7 and detach the secondary electrons there.

The value of the kinetic energy of about 2keV for the generation of the secondary electrons given here is an exemplary value. The primary electrons can also be braked to other energies, for example to energies in a range from 100 eV to 5 keV.

The deflectors 23, 24 together provide the same displacement M transversely to the z-axis or main axis 21 for both the primary electrons and the secondary electrons. For this purpose, the two deflectors 23, 24 have a symmetrical structure and mode of operation, which are explained in FIGS. 2 to 4.

2, the primary electrons 29 enter the deflector 23 from the left, and the secondary electrons 9, 10 enter the deflector from the right . The deflector 23 provides in a volume 37 an electric field generated by electrodes 38 and a magnetic field, which is identified by symbols 40. The directions of the electric field and the magnetic field are aligned orthogonally to one another and to the z direction.

After passing through the deflector 23, the direction of movement of the electrons has an angle β with respect to their direction of movement before entering the deflector, which angle is given by the following equation for non-relativistic calculations:<maths id="math0001" num=""><math display="block"><mrow><mtext>β = </mtext><mfrac><mrow><mtext mathvariant="italic">e</mtext><mtext>·</mtext><mtext mathvariant="italic">l</mtext><mtext>·(</mtext><msub><mrow><mtext mathvariant="italic">E</mtext></mrow><mrow><mtext mathvariant="italic">x</mtext></mrow></msub><mtext> - </mtext><msub><mrow><mtext mathvariant="italic">B</mtext></mrow><mrow><mtext mathvariant="italic">y</mtext></mrow></msub><mtext> · </mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">e.g.</mtext></mrow></msub><mtext>)</mtext></mrow><mrow><msubsup><mrow><mtext mathvariant="italic">mV</mtext></mrow><mrow><mtext mathvariant="italic">e.g.</mtext></mrow><mrow><mtext>2</mtext></mrow></msubsup></mrow></mfrac><mtext>,</mtext></mrow></math><img file="EP1280184A2_D0001.tif" /></maths> in which<dl id="dl0002" compact="compact"><dt>e</dt><dd>the elementary charge</dd><dt>m</dt><dd>the electron mass</dd><dt>l</dt><dd>the length of the field volume 37 in the z direction</dd><dt>E_x</dt><dd>the strength of the electric field,</dd><dt>B_y</dt><dd>the strength of the magnetic field and</dd><dt>V_e.g.</dt><dd>the speed of the electrons in the z direction</dd></dl> indicates.

The deflection angles β and -β for the primary electrons and the secondary electrons are the same if the electrical field strength and the magnetic field strength approximately meet the following relation:<maths id="math0002" num=""><math display="block"><mrow><mtext mathvariant="italic">B</mtext><mtext>(</mtext><msub><mrow><mtext mathvariant="italic">E</mtext></mrow><mrow><mtext mathvariant="italic">x</mtext></mrow></msub><mtext>)=</mtext><mfrac><mrow><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">e.g.</mtext></mrow></msub><msub><mrow><mtext>​</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> - </mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">e.g.</mtext></mrow></msub><msub><mrow><mtext>​</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow><mrow><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">e.g.</mtext></mrow></msub><msub><mrow><mtext>​</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> · </mtext><msub><mrow><mtext mathvariant="italic">V</mtext></mrow><mrow><mtext mathvariant="italic">e.g.</mtext></mrow></msub><msub><mrow><mtext>​</mtext></mrow><mrow><mtext>2</mtext></mrow></msub></mrow></mfrac><mtext> · </mtext><msub><mrow><mtext mathvariant="italic">E</mtext></mrow><mrow><mtext mathvariant="italic">x</mtext></mrow></msub><mtext>,</mtext></mrow></math><img file="EP1280184A2_D0002.tif" /></maths> in which<dl id="dl0003" compact="compact"><dt>V_z1</dt><dd>the speed of the primary electrons and</dd><dt>Vz2</dt><dd>the speed of the secondary electrons</dd></dl> indicates.

The relationship between the electrical and magnetic field strengths given above applies approximately. When taking into account relativistic terms and higher-order terms, which arise, for example, when considering edge or stray fields of the electric or magnetic field, deviations from this can occur.

With this choice of the relationship between the electrical and magnetic field strengths, it is thus possible for the primary electrons and the secondary electrons to be deflected by the deflector 23 by oppositely equal angles and thus their trajectories overlap.

The deflectors 23, 24 each have one or more ferrite rings 43 and ceramic rings 45 of the same diameter, which are stacked on top of one another alternately in the center of the main axis 21, the ends of the stack in the z direction being each formed by a ceramic ring 45. The ceramic rings 45 arranged at the ends of the stack each have a plurality of bores arranged distributed in the circumferential direction of the rings for the passage of current conductor turns 47. Each current conductor turn 47 extends parallel to the main axis 21 and encompasses several of the ferrite rings 43. A distribution of the current lead turns 47 in the circumferential direction of the rings 43, 45 can be seen in FIG. 4. The angles θ occurring here₁ to θ₇ have the following values: θ₁= 21.6 °, θ₂= 41.6 °, θ₃= 47.6 °, θ₄= 62.4 °, θ₅= 68.4 °, θ₆= 78.5 ° and θ₇= 84.5 °. These angles are chosen such that the magnetic field generated by the current conductor windings 47 is an essentially homogeneous magnetic field oriented in the y direction.

Radially inside the rings 43, 45 and the windings 47 are eight partially cylindrical electrodes 38 arranged uniformly distributed around the circumference, to which voltages can be applied in such a way that they produce the essentially homogeneous and x-direction oriented electric field in the volume 37 provide.

A deflection of the beam in the y direction can also be generated with the electrodes 38 in order to also slightly deflect the illuminated field on the object in the y direction, as will be explained in the following.

The controller 25 adjusts the voltages applied to the electrodes 38 and the currents flowing through the windings 47 in such a way that the electrical field generated in the volume 37 and the magnetic field generated there satisfy the above-mentioned relation.

The focusing lens 15 close to the object is shown in perspective and schematically in FIG. This comprises two flat diaphragm electrodes 51, 53, which are oriented orthogonally to the z direction and are arranged at a distance from one another in the z direction. Each of the diaphragm electrodes 51, 53 has a slot 55 which extends in the x direction and through which the primary electrons and the secondary electrons pass. If an electrical voltage is present at the electrodes 51, 53, these each generate an electrical field with a field line profile, as is shown schematically in FIG. 6a. Such an electric field acts on the electrons passing through the electrodes 51, 53 like a cylindrical lens.

In the z-direction between the two electrodes 51, 53 there are two parallel rows of finger electrodes 57, the rows extending in the x-direction and the primary electrons and the secondary electrons passing between the two rows. The entirety of the finger electrodes 57 forms an electrode arrangement, hereinafter referred to as comb electrode 58. Controlled by the controller 25, a separately adjustable voltage can be applied to each finger electrode 57. A voltage pattern is applied to the finger electrodes 57 in such a way that the electric field generated by the finger electrodes 57 has approximately quadrupole symmetry, as shown in FIG. 6b. By controlling the finger electrodes 57 appropriately, the controller 25 can shift a main axis 59 of the quadrupole field in the x direction.

A superposition of the cylindrical lens field according to FIG. 6a and the quadrupole field according to FIG. 6b thus acts on the electrons passing through the focusing lens 15. This superposition of the electric fields corresponds to a round lens field with an axis of symmetry 59, as is shown schematically in FIG. 6c.

This means that the lens 15 has an effect like a focusing round lens on the electrons passing through it. Here, however, the axis of symmetry 59 of the round lens array can be displaced in the x direction via the controller 25.

In operation, the controller 25 applies a voltage pattern to the finger electrodes 57 such that the axis of symmetry 59 of the round lens field is displaced from the main axis 21 by the same amount M as the center of the field 7 of the object 3 imaged on the detector 5 the effect of the focusing lens 15 on the secondary electrons emerging from the field 7 is essentially independent of the deflection M of the illuminated field 7. As a result, imaging errors, such as those produced by conventional round lens fields for beam paths running away from the axis of symmetry of the round lens field, are largely avoided.

The lens 15 explained in connection with FIGS. 5 and 6 can also be used in other applications that are different from electron microscopic applications. These can be all applications in which a focusing or / and deflecting effect is to be exerted on a beam of charged particles, which in particular also includes ions, an axis center of this focusing effect being displaceable transversely to the beam direction. These are, for example, also applications in lithography processes, where structures of a mask are transferred to a radiation-sensitive layer by means of charged particles. Applications are also envisaged in which quadrupole fields are generated through the comb aperture along the slot direction at a plurality of locations which are arranged at a sufficient distance from one another, so that a focusing effect can be exerted at a plurality of locations. The lens can thus focus a plurality of spaced apart beams of charged particles simultaneously.

Here, in particular, the same potentials can be applied to the aperture electrodes 51 and 53, so that the charged particles have the same kinetic energy in the z direction before and after they pass through the lens. However, it is also possible to put the two diaphragm electrodes 51 and 53 at different potential, so that the charged particles passing through the lens 15 are accelerated or decelerated by the lens. In both cases it is possible for the four electrodes to average, ie apart from the quadrupole potential which they provide, have a potential which is equal to the potential of the aperture electrode 51 and / or the aperture electrode 53, or that the finger electrodes 57 provide an average potential which is different from the potential of the aperture electrodes 51 and 53 and which overall is in particular higher than the highest of the two diaphragm electrodes 51, 53 and this can also be lower than the lowest potential of the two diaphragm electrodes 51, 53.

In the application explained in connection with FIG. 1, the lens 15 with its arrangement of diaphragm electrodes and finger electrodes is arranged directly in front of the object 3. It is then desirable here if, in addition to its focusing function, the lens 15 also performs the function of accelerating the secondary electrons emerging from the object 3 or the corresponding deceleration of the primary electrons. In such a case, the lens 15 then acts as an immersion lens and its diaphragm electrodes 51 and 53 must be at different electrical potentials. Furthermore, it is desirable in this case if there is already a certain potential difference between the object 3 and the aperture electrode 53 facing the object, so that the secondary electrons, which sometimes exit the object 3 with very low kinetic energy, are already accelerated on their way to the lens 15 ( Suction of the secondary electrons). However, since the electric field that initially accelerates the slow secondary electrons is primarily provided by the aperture electrode 53 facing the object, the cylindrical lens field of the aperture electrode 53 focusing in the y direction acts more intensively on the initially slow electrons than the quadrupole field of the finger electrodes 57 focusing in the x direction subsequently acts on the already faster electrons. Thus, when the lens 15 shown in FIG. 5 is operated as an immersion lens, the combined action of the cylindrical lens field and the quadrupole field is disturbed such that these two fields are experienced as a whole by the particle beam passing through the lens as a round lens.

Variants of the focusing device shown in FIG. 5, which take this problem into account in order to achieve a more precise round lens effect, are shown schematically in section in FIGS. 7 and 8. Their components, which correspond to the components of FIG. 5 in terms of function and structure, are identified by the same reference numerals as in FIG. 5, but with an additional letter to distinguish them.

A focusing lens 15f shown in FIG. 7 comprises two diaphragm electrodes 51f and 53f arranged at a distance from one another in the z direction, and a comb lens 58f with a plurality of finger electrodes not shown in FIG. A further aperture electrode 101 is arranged between the aperture electrode 53f close to the object 3f and the object. All of the electrodes 51f, 58f, 53f and 101 have an elongated opening or a slot 55f for the passage of the primary or secondary electrons. They have a width y in the y direction, that is to say transversely to the direction of extent of the slots 55f₁ of, for example, 4 mm. In the x direction, ie in the direction of extension of the slots 55f, they have a length of, for example, 50 mm. In an embodiment of such dimensions, the aperture electrode 101 close to the object 3 has a distance z in its area close to the opening₁ from the object 3f of 2 mm and is at an electrical potential of 1 kV with respect to the object 3f. The diaphragm electrode 53f following in the z direction of the diaphragm electrode 101 is at a distance z from the diaphragm electrode in its region close to the opening₂ in the z direction of 4 mm and is at an electrical potential of 15 kV. The comb lens 58f is a clear distance z from the aperture electrode 53f₃ of 5 mm and is at an electrical potential of 20 kV. Your finger electrodes extend in the z direction over a length z₄ of 3 mm. The diaphragm electrode 51f, which is arranged at a distance from the object 3f, then has a clear distance z from the comb electrode 58f₅ of 4 mm and is at an electrical potential of 15 kV. The diaphragm electrodes 51f, 53f and 101 are each made of a sheet metal with a thickness of 0.5 mm.

FIG. 9 shows, with a curve 103, the course of the electrical potential, which the lens 15f provides along the z-axis, starting from the object 3f, in arbitrary coordinates. In FIG. 9, the first location derivative of the potential curve 103 is also entered as curve 105 and the second location derivative of the same as curve 107. In a region 109, the potential initially rises linearly, so that the emerging secondary electrons are accelerated uniformly. The focusing cylindrical lens field acts in an area 111 in the center of the lens 15f, as can be seen from the upward-facing belly of the first derivative 105 of the potential profile. Between the regions 109 and 111, however, the electrodes provide a region 113 in which the potential curve 103 does not rise sharply and forms a plateau. In this region 113, the first derivative 105 of the potential profile has a belly hanging downward, so that a defocusing cylindrical lens field acts on the electrons there. This defocusing cylindrical lens field serves to reduce the above-described total cylindrical lens effect compared to the quadrupole field which can be provided by the comb aperture 58f, so that the weight between the total cylindrical lens effect and the quadrupole lens effect also comes close to a round lens effect in the case of operation of the lens 15f as an immersion lens . In the case of the lens 15f shown in FIG. 7 in the z direction, this favorable field profile is due to the two electrodes 53f and 101 arranged at a distance from one another.

A focusing lens 15g shown in FIG. 8 has a structure that is similar to the lens shown in FIG. In contrast to this, however, no additional diaphragm electrode close to the object is provided. Rather, a diaphragm electrode 53g close to a comb lens 58g in the z-direction to an object 3g is not made from a thin sheet metal, but rather has a trapezoidal shape shown in the sectional illustration in FIG. 8 with an increased thickness z on both sides of an opening 55g₂ on. In a possible design of the embodiment of the lens 15g, the dimensions entered in FIG₁, e.g.₂, e.g.₃, e.g.₄, e.g.₅ and y₁ same as the corresponding dimensions of Figure 7. Here, the aperture electrode 53g close to the object 3g is at a potential of 1 kV, the comb lens 58g is at an average potential of 20 kV and an aperture electrode 51g away from the object 3g is at a potential of 15 kV .

A similar defocusing cylindrical lens field can be provided with the thickened diaphragm electrodes 53g close to the object, as by the two electrodes 101 and 53f of the embodiment shown in FIG.

FIG. 10 shows a detailed illustration of the comb screen 58f used in the embodiment according to FIG. 7. Finger electrodes 57f are designed as identical plates which are arranged next to one another in the x direction, with adjacent finger electrodes 57f being separated by plates 105 made of ceramic insulator material 105. The electrode plates 57f have a thickness x₁ of 1 mm, while the insulator plates 105 each have a thickness x₂ of 0.5 mm. In the z direction, the finger electrode plates 57f and the insulator plates 105 extend over a height z₄ of 4 mm. In a direction towards the opening 55f, the electrode plates 57f stand in relation to the insulator plates 105 into the opening 55f by a distance y₂ of 2.5 mm in front.

FIG. 11 explains how different potentials can be applied to the finger electrodes of the embodiments shown in FIGS. 7 and 8, so that the comb diaphragms provide the quadrupole field.

In the embodiment of FIG. 11, a separate driver 109 is assigned to each finger electrode 57f in order to apply a desired voltage to the respective finger electrode 57f. The individual drivers 109 are in turn controlled by a superordinate focusing lens control 25f in order to shift the voltage pattern generated by the drivers 109 in the x direction if the round lens effect is to be shifted during operation of the lens 15f. In particular, this shift of the quadrupole field in the x direction is also possible continuously by changing the voltages applied to the finger electrodes. Furthermore, the shift can also take place step by step, for example by switching a constant voltage pattern applied to the finger electrodes from one finger electrode group to another finger electrode group.

As an alternative to the configuration of the focusing lens 15 explained in connection with FIGS. 5 to 11, the lens with a variable axis based on E. Goto can also be used, as has already been explained above.

Because of the different kinetic energies or velocities of the primary electrons and the secondary electrons, it is possible that the deflectors 23, 24 provide the same deflection angle β or deflections M away from the main axis 21 for both types of electrons. However, the focusing lens 15 and also the focusing lens 17 for the two types of electrons have different focusing effects or different focal lengths due to the difference in their kinetic energy. The focusing lenses 15 and 17 are then adapted such that they are optimized with regard to a focusing effect for the secondary electrons in order to achieve the best possible resolution of the optical image of the field 7 on the detector 5. This adjustment of the focusing lenses 15 and 17 is then not optimal for the primary electrons for illuminating the field 7. However, this can be accepted, since precise imaging of the diaphragm 35 onto the plane of the object 3, for example, is not necessary only for illuminating the field 7.

With the examination system 1, a spatial image of structures of the object 3 can be obtained as follows. The maximum deflection M of the field away from the main axis 21 is limited by the length of the slot 55 in the x direction and by the maximum beam offset that can be generated by the deflectors. The deflectors 23, 24 and the focusing lens 15 are first set by the control in such a way that the field 7 deflected to the maximum in the (-x) direction is imaged on the detector 5. The detector 5 is exposed to secondary electrons until an image with sufficient contrast can be read out, which is stored. The deflection M is then reduced by a value which corresponds to the expansion of the field 7 in the x direction, and a renewed recording of the secondary electrons with the detector is obtained, which is also stored. This process is repeated step by step until the field 7 is arranged with its maximum deflection in the (+ x) direction. The object 3 is then mechanically displaced in the y direction relative to the examination system by means of a drive (not shown in the figures), namely by a distance which corresponds to the extent of the field 7 in the y direction. Thereupon, images M are again recorded with the camera 5 with the respective different deflections. When the entire surface of the object is scanned in this way, the images are combined to obtain the spatial image of the structures of the object.

It is also possible to design the deflectors 23, 24 such that in addition to the deflection of the field 7 in the x direction, they can also produce a deflection of the field in the y direction. Here, the maximum deflection in the y direction is less than the maximum deflection in the x direction, in such a way that, with the comparatively small deflections in the y direction, an image of sufficient quality of the illuminated field 7 on the detector is achieved. It is then possible to continuously mechanically shift the object 3 in the y direction relative to the examination system and to adapt the illuminated field to the mechanical displacement of the object 3 relative to the examination system with the moving object in the y direction.

With a primary electron current of 3.2µA and an expansion of the field 7 in the plane of the object of 100µmx100µm, a current of approximately 2.6µA ​​of secondary electrons can be generated. If the camera has a resolution of 1,000 by 1,000 pixels, the spatial resolution of the examination system is 100 nm. If around 2,000 electrons are to hit each pixel of the camera per image, the examination system can measure 0.2 cm²/ sec or 720cm²/ h of the object surface can be scanned.

Variants of the invention are explained below. Components which correspond to components of the embodiments explained in FIGS. 1 to 11 in terms of their function and structure are designated with the same reference numerals as in FIGS. 1 to 11, but are provided with an additional letter to distinguish them.

An examination system 1a, shown schematically in FIG. 12, serves to image backscattered electrons which emerge from an object 3a to be examined in a field 7a onto a spatially resolving detector 5a. As in the embodiment described above, the field 7a imaged on the detector 5a can also be deflected with respect to a main axis 21a (deflection M).

Since backscattered electrons have a significantly greater kinetic energy than secondary electrons, a separate electrode for accelerating the backscattered electrons (compare electrode 13 in FIG. 1) is not absolutely necessary. However, a corresponding electrode can also be provided if necessary to increase the kinetic energy of the backscattered electrons.

After leaving the surface of the object 3a, the backscattering electrons first pass through a focusing lens 15a close to the object, then two deflectors 24a, 23a, the deflection angles of which are set such that the backscattering electrons emerging from the field 7a have a fixed beam cross section regardless of the deflection M of the field 7a Pass through 27a. After passing through the fixed beam cross section 27a, the backscattered electrons pass through a further focusing lens 17a and a beam splitter or beam combiner 37a, which serves to superimpose the beam of backscattered electrons with a beam of primary electrons 29a, which is formed by an electron gun 31a, a focusing lens 33a and an aperture 35a . The current of the primary electron beam is, for example, 10 μA, and the kinetic energy is, for example, 10 keV.

After the beam combiner 37a, the primary electrons successively pass through the focusing lens 17a, the two deflectors 23a, 24a and the focusing lens 15a. The deflectors 23a and 24a are in turn set such that they also provide such a deflection M for the primary electrons that the area on the object 3a illuminated by the primary electrons essentially coincides with the field 7a imaged on the detector 5a.

Since the backscattered electrons have a relatively broad energy distribution, an image-preserving energy filter 67 is provided between the beam combiner 37a and a re-magnification optics 19a in order to select electrons from a smaller energy band from the backscattered electrons, which are then enlarged with the optics 19a and imaged on the detector 5a. The energy filter 67 comprises a plurality of sector magnets 69 and is constructed in such a way that the geometric mapping of the field 7a onto the detector 5a is not disturbed by the filter 67. An example of such an energy filter 67 is described in the applicant's EP 0 218 920 B1. The disclosure of this document is incorporated by reference in its entirety in the present application.

The focusing lens 15a close to the object 3a can have a construction as was explained in the preceding exemplary embodiment in connection with FIGS. 5 to 11. However, it is also possible to use a lens with a variable axis, as it goes back to E. Goto.

An examination system 1b shown schematically in FIG. 13 is constructed essentially similarly to the examination system shown in FIG. It is also used to image backscattered electrons, which emerge from a field 7b that can be displaced in the plane of the object 3b, onto a detector 5b. In contrast to the embodiment shown in FIG. 12, however, two separate deflectors are not provided here in order to generate the variable deflection M of the field 7b to be imaged. Only one deflector 23b is provided, and the function of the second deflector is taken over by a focusing lens 15b close to the object 3b. A suitable construction of the focusing lens 15b is disclosed in DE 196 34 456 A1 and comprises an electric or magnetic cylindrical lens field, on which a stationary electric or magnetic quadrupole field is superimposed.

An examination system 1c shown schematically in FIG. 14 serves to observe transmission electrons emerging from a field in the plane of an object 3c on a detector 5c. The object 3c is a lithography mask for imaging a structure 75 on a wafer in a lithography process. The field imaged on the detector 5c by means of the transmission electrons can be shifted in the plane of the object (deflection M).

To form a beam of primary electrons 29c for illuminating the field to be imaged on the detector 5c in the object plane, an electron gun 31c, an aperture 35c and a focusing lens 33c are provided. After the focusing lens 33c, the primary electron beam passes successively through two deflectors 23c and 24c which are arranged at a distance from one another and deflect the beam by the same angle β in each case in order to generate the deflection M. The primary electron beam 29c is then focused by a focusing lens 15c onto the object 3c in order to illuminate the field to be imaged there. The focusing lens 15c is a lens with a variable optical axis, as it goes back to E. Goto. For this purpose, it comprises a round lens 77 for providing a focusing magnetic field rotationally symmetrical with respect to a main axis 21c and coils 78, 79 for generating a magnetic dipole field. The current through the dipole coils 78, 79 can be adjusted by a control 25c in such a way that the superimposition of the stationary round lens field and the dipole fields leads to the effect of a round lens field, the axis of symmetry 59c of which is likewise shifted away from the main axis 21c by the amount M. 14, a dynamic focus coil is shown in FIG. 14, which is also controlled by the control 25c in order to optimize the effect of the focusing lens 15c with different deflections M.

In order to trace the transmission electrons passing through the object 3c when the field is deflected onto the main axis 21c or a fixed beam cross section denoted by 27c in FIG. 14, which is independent of the deflection M, a beam guide with components that are symmetrical to the ones is provided for the transmission electrons Components of the beam guide for deflecting the primary electrons are constructed. The components for the beam guidance of the transmission electrons are designated by the same reference numerals as the corresponding components for the beam guidance of the primary electrons, but additionally provided with a '. This beam guide thus comprises a focusing lens 15c 'close to the object with a variable axis and two deflectors 24c' and 23c '.

In the beam path following the deflection-independent fixed beam cross-section 27c, a re-magnification optics 19c with two round lenses 83, 84 are provided before the transmission electrons strike the detector 5c.

It can be seen from the beam path for the transmission electrons shown in FIG. 14 how an object size 85 is imaged via an intermediate image 86, 87 into an image 88 on the detector 5c.

As an alternative to the focusing lenses 15c or 15c 'close to the object 3a with a variable optical axis, one or both of these lenses can be replaced by a lens, as was explained above in connection with FIGS. 5 to 13.

An examination system 1d shown schematically in FIG. 15 is constructed similarly to the examination system shown in FIG. 14. It serves to image a field that can be deflected with respect to a main axis 21d in the plane of an object 3d onto a detector 5d. In contrast to the two separate deflectors provided in the embodiment according to FIG. 14 for generating the deflection of a primary beam 29d from the main axis 21d, only a separate deflector 23d is provided here, and the function of the second deflector is provided by a focusing lens 15d close to the object 3d provided. The one for returning the transmission electrons to the main axis 21d or towards a beam cross-section 27d that is independent of the deflection M, its components are constructed symmetrically with regard to its components to the beam guidance for the primary electrons, and the components provided for this purpose, namely the focusing lens 15d 'and the deflector 23d' are provided with corresponding reference numerals but with an additional ' Mistake.

An examination system 1e shown schematically in FIG. 16 serves to image photoelectrons, which emerge from a spatially limited field 7e on the surface of an object 3e to be examined, onto a detector 5e. The field 7e to be imaged can be deflected with respect to a main axis 21e or a fixed beam cross section 27e (deflection M). The electron-optical system for guiding the photoelectrons emerging from the object 3e to the detector 5e is constructed similarly to the system for guiding the backscattered electrons to the detector explained in connection with the embodiment shown in FIG. 7, although an electrode 13e is provided for to accelerate the photoelectrons after they exit object 3e.

The deflection M is generated by two deflectors 23e and 24e which are arranged at a distance in the direction of the main axis 21e, and a focusing lens 15e close to the object is constructed as a lens with a variable axis and comprises dipole coils 78e and 79e as well as dynamics controlled as a function of the deflection M. Focus coils 77e.

Furthermore, an image-preserving energy filter 67e is provided in order to cut the energy spectrum of the photoelectrons, which is supplied to a post-magnification optical system 19e. The energy filter 67e comprises a plurality of sector magnets, the photoelectrons being guided away from the main axis 21e by the sector magnet 69e of the energy filter 67e, into which they first enter.

The field 7e to be imaged is illuminated with photons to generate the photoelectrons by means of a light source 91 which generates a light beam 93 which is directed onto a deflecting mirror 95 arranged on the main axis 21e. The deflecting mirror 95 is arranged at a point on the main axis 21e at which the photoelectron beam has already been deflected away from the main axis 21e by the first sector magnet 69e.

The deflecting mirror 95 can be pivoted by means of a drive 97 controlled by the control 25e. The controller 25e controls the pivoting of the deflection mirror 95 as a function of the deflection M of the field 7e imaged on the detector 5e in such a way that the imaged field 7e is always illuminated with photons from the light source 91. Here, the area illuminated with photons in the plane of the object 3e essentially corresponds in terms of its geometry to the field 7e, which is electron-optically imaged on the detector 5e.

As an alternative to the design of the focusing lens 15e close to the object 3e as a lens with a variable axis, it is also possible to use such a lens as the focusing lens 15e, as was explained in connection with FIGS. 5 to 11.

Furthermore, it is also possible not to use two separate deflectors 23e and 24e arranged one after the other to provide the deflection M, but only to provide one separate deflector and to provide the function of the second deflector through the focusing lens 15e, as was explained in connection with FIG. 8 Embodiment has been shown.

In the embodiments explained above, in which secondary electrons and backscattered electrons are imaged on the detector, the deflector device is traversed by both the primary electrons and the secondary or backscattered electrons. This places special demands on the deflector device, since both the primary electrons and the secondary or backscattered electrons should experience essentially the same deflections. In the embodiments in which transmission electrons and photoelectrons are imaged on the detector, on the other hand, the requirements for the deflector device are lower, since only the electrons running from the object to the detector are to be deflected in a defined manner. Deflectors can then also be used, which only provide a changeable electric field or a changeable magnetic field.

In the above-described embodiments, which work with photoelectrons, a pivoting mirror was used in order to displace the field illuminated with photons in the object plane. As an alternative to this, other measures can also be used to deflect the deflectable photons. An example of this is an acousto-optical modulator.

In connection with FIGS. 5 to 11, embodiments of the first focusing lens have been described, which are particle-optical. Focus device work with a comb screen. Regardless of the use of this focusing device in the examination system described above, it is also contemplated to make modifications to it. For example, it is provided that the diaphragm closest to the object, which was previously described as a simple slit diaphragm, is designed as a comb diaphragm, so that this comb diaphragm can also be used to intervene in addition to the potential curve of the focusing device and this intervention also along the slit direction can be designed relocatable. Likewise, it is also possible to design one or even both of the panels 51 and 53 adjacent to the comb panel 58 in FIG. 2 itself as a comb panel.