ES2222793B1 - WIEN TYPE MONOCROMATOR WITH OPENING CORRECTION. - Google Patents

Abstract

Monochromator type Wien with correction of aberrations. The invention improves the two most characteristic parameters of Wien filters: energy resolution and spatial dispersion. It consists of two identical filters, each formed by twelve poles that extend parallel to the optical axis. Each pole is constructed with a magnetic conductive material. A potential is applied and magnetized with a current calculated to achieve a configuration of fields such that the electric and magnetic dipole components fulfill the Wien condition. In each pole, the potentials and currents suitable for creating electric and magnetic quadripolar fields are also excited, which, combined with the previous ones, allow a great control of the total force acting on the charges and correct the aberrations inherent in the system. Modifying the radius of the diagonal poles adds another control factor. By adjusting the excitations and said radio simultaneously, the resolution in energy is improved in tens or hundreds compared to conventional Wien filters. The use of a second filter identical to the first through which the beam is passed once selected in energy, significantly reduces the size of the beam.

Description

Monochrome type wien with correction of aberrations.

Technical Field of the Invention

The invention presented herein falls within the field of optoelectronics devices specifically able to select those particles charged that part of a beam with a predetermined energy U _ {0}. It is related to Wien filters (filters \ vec {E} \ times \ vec {B}), spin spinners, energy analyzers, ion separators and the like.

The invention also relates to an electron microscope that uses the wien monochromators.

State of the art

The operation of the device is based on the creation of a region where electric and magnetic fields act perpendicular to each other and to the direction of advance of the beam. The magnitudes of the fields must be such as to cause the cancellation of the electric and magnetic forces for particles with energy U _ {0} to the flow through the device when entering a velocity parallel to the axis of the same (condition Wien).

This type of monochromators (or energy filters) has been used over the last hundred years, being subject to various improvements in order to optimize its performance. One of its main problems was that Wien's condition was only strictly fulfilled in the central region of the filter but not at its ends where the edge effects for the electric and magnetic field were very different, and therefore the electric and magnetic forces associated with them differed appreciably. The solution to this problem was addressed by TT Tang [TT Tang, Optik 74 (1986) 51-56] and H. Rose [H. Rose, Optik 77 (1987) 26-34] proposing a multipolar configuration using a magnetic material conductor both to generate the magnetic field \ vec {B} (surrounding the corresponding pole with a winding through which an electric current is passed) and to create the electric field \ vec {E} (applying a suitable potential to it), perpendicular to \ vec {B}. The distribution of both fields in this configuration is due to the same differential equation and therefore its modules will fulfill the Wien condition at every point in space: E = \ nu_ {0} B , where \ nu_ {0} is the axial velocity of the corresponding particle energy U _ {0}.

However, this improvement does not solve a problem that is inherent in the operation of the filter when used with the aforementioned dipole fields plus an electric quadripolar component, added to achieve the convergence of the beam at the output of the filter. Thus, starting from a point beam at the entrance and launching the loads with a transverse velocity component according to different azimuthal angles (between 0 ° and 360 °) superimposed on the nominal axial speed \ nu_ {0}, the beam does not converge at exit at a point but occupies a certain region of space when a cut is made in a plane perpendicular to the optical axis [K. Tsuno and J. Rouse, J. Electron Microsc. 45 (1996) 417-427.]. The shape that this cross section of the beam takes is called the aberration figure and accounts for the spatial dispersion suffered by the particle beam selected by the filter.

The characterization of any device used in Electronic Optics requires two clearly differentiated stages: 1) calculation of field distribution; 2) determination of the trajectories that the charged particles follow under the action of said fields. To obtain the distribution of fields, the authors have developed a version of the Boundary Element Method (BEM ) method because it is the one that best suits the simulation of the contours of the poles used in this design. In addition, algorithms can be optimized to achieve high accuracy in the results.

The components of the force acting on an electron of mass m , charge- e , which crosses the filter with velocity v (\ nu_ {x}, \ nu_ {y}, \ nu_ {z}) and acceleration a ( a_ {x }, a_ {y}, a_ {z} ), are given by:

F_ {x} = ma_ {x} = -e \ \ {E_ {x} + \ nu_ {y} B_ {z} - \ nu_ {z} B_ {y} \},

F_ {y} = ma_ {y} = -e \ \ {E_ {y} + \ nu_ {z} B_ {x} - \ nu_ {x} B_ {z} \},

F_ {z} = ma_ {z} = -e \ \ {E_ {z} + \ nu_ {x} B_ {y} - \ nu_ {y} B_ {x} \},

E x, E y, E z being the components of the electric field and B x, B y, B z those of the magnetic field. There is a great variety of numerical techniques for the integration of these equations. In this case an algorithm of the type has been used
Dormand-Prince 5th order, which offers good efficiency and meets stable precision requirements
acids.

Making a very precise simulation of the operation of the filter of K. Tsuno and J. Rouse using these algorithms [G. Martínez and K. Tsuno, Ultramicroscopy 93 (2002) 253-261], the inventors of this patent demonstrated that the origin of the problem is that the particles (electrons in this case) released with transverse speeds such that they approach the poles at those that apply a positive potential, increase their axial velocity, while those thrown in the opposite direction suffer a decrease in said velocity component when passing through the filter. The result is that the magnetic force acting on them is slightly different from the electric force, forcing them to cut the shaft at different points.

In the same study, the inventors propose as a solution the use of a double filter, that is, to place two identical configurations in series so that at the exit of the first filter the selection is made in energy and at the exit of the second one the beam is almost totally corrected in terms of spatial dispersion. The designs analyzed reduce the size of the aberration figure by a factor greater than ten, which is useful for many applications. The use of the double filter had already been established in the 70s but not with the optimization of the configuration of 12 magnetic conductor poles [WHJ Andersen. JB Le Pole, J. Phys. E: Sci. Instrum. 3 (1970) 121-126].

Although the mounting of a double filter greatly improves the performance of the monochromator, there are applications for which higher values are required not only for spatial resolution but also for energy resolution . This is the case of the latest generation microscopes in which an resolution power of 1 Å or less is attempted. Therefore, the purpose of the modifications that have been incorporated into the device and which are presented in the following section are essentially these two improvements.

Explanation of the invention. Wien type monochromator with aberration correction

First we describe the geometry of the device and then we will go on to detail the way in which excite the poles to generate the field distribution desired.

Figure 1 shows a longitudinal section of the monochromator consisting of two Wien filters placed consecutively along the optical axis (Z axis). The components of said filters (poles) have symmetry of order 12 with respect to the optical axis. The plane R represented schematically by a dashed line, is also a reflection plane of the system. By means of the groove located in said plane the selection of the part of the beam carrying the nominal energy U 0 is made. The drivers located at the ends have the mission of shielding the fields created by other components (eg retarding lenses, accelerators, etc.) that are found before or after the
device.

Figure 2 shows the cross section of one of the filters (XY plane) that are part of the monochromator. This consists of twelve poles numbered correlatively from 1 to 12 and arranged concentrically around the axis. They must be built with a magnetic material and a good conductor so that they can simultaneously generate electric and magnetic fields. The angle that covers each pole in the azimuthal direction is \ phi and the spacing between consecutive poles \ Delta \ phi. In the radial direction, the poles extend from an inner radius r to an outer radius r e large enough to accommodate the windings through which the currents that magnetize the material will circulate ( r e \ approx 5 r ).

In the present invention, the value of r varies slightly from one pole to another so that:

r = r 0 at poles 1, 3, 4, 6, 7, 9 10, 12

r = r 0 + δ at poles 2, 5, 8, 11

In more detail Figure 3 shows how the poles are excited to generate the electric dipole field, E {1}, the magnetic dipole field, B _ {1}, and the electric quadrupole, E {2}. Applying a potential V {1}, the 1-2-11-12 poles and a potential - V _ {1} to the poles 5-6-7-8 a field E {1} creates relatively uniform the central region, directed towards the negative x . Moreover, a current I passing _ {1}, for winding coils around the N poles 2-3-4-5 and the same winding in reverse on 8-9-10-11 poles, a field B _ {1} obtained with a similar distribution lines to E _ {1} yet in the negative direction of the Y axis Finally, applying a potential V {2} to 1 to 12 poles, 6 -7, and - V _ {2} to 3-4, 9-10 poles one quadrupole electric distribution type is useful for controlling the convergence of the beam direction . Details of the behavior of the excited monochromator with these fields only will be given later ( reference monochromator ).

In order to improve the performance of the system it is essential to add a magnetic quadrupole component. In Figure 4 the magnitudes are indicated and signs of the currents needed to generate a field B {2} with (that improves performance) with similar distribution to that created by the electric potential V {2} but lines which of Field are orthogonal to the quadripolar electric. They are wound currents nI _ {2} on the poles 1, 3, 7 and 9; currents - nI2 over 4, 6, 10 and 12; currents 2 nI 2 over 2, 8 and currents -2 nI 2 over 5, 11.

Description of the figures

The invention is accompanied by a series of figures, which illustratively represent the following:

Figure 1.- Diagram of the longitudinal section of the Monochromator

Figure 2.- Diagram of the cross section of a 12-pole filter

Figure 3.- Way to excite the dipole fields electric and magnetic and the quadripolar electric field.

Figure 4.- Way to excite the quadripolar field magnetic.

Figure 5.- Paths through the monochromators for electrons released with azimuthal angle 0 = 0 ° and half-opening α = 11 °. The dashed lines correspond to trajectories in the reference monochromator and the continuous lines to trajectories in the model to be patented.

Figure 6.- Projection in the plane Y-Z trajectories through monochromators for electrons released with azimuth angle \ varphi = 0 ° and half-opening α = ± 1 °.

Figure 7.- Projection in the XZ plane of trajectories through the monochromators for electrons released with azimuthal angle \ varphi = 0 ° and semi-opening α = \ pml °. The projections are identical for the two values of α. The dashed line corresponds to trajectories in the reference monochromator and continues to trajectories in the model to be patented.

Figure 8.- Aberration figure in the R plane calculated for the reference monochromator .

Figure 9.- Aberration figure in the R plane calculated for the monochromator of the invention.

Figure 10.- Idem figure 9 with the X axis scale modified

Figure 11.- Aberration figure at the system output calculated for the reference monochromator .

Figure 12.- Figure of aberration at the exit of the system calculated for the monochromator of the invention.

Embodiment of the invention

The improvements introduced in the invention are illustrated by an example. For this, the results will be compared with those obtained in the reference monochromator mentioned above.

The geometry data for the reference monochromator , according to figures 1 and 2 (all values expressed in mm except those corresponding to angles) are:

H = 40; r = r 0 = 5; r e = 25; d = 5; w = 10; r p = 2; ph = 24 °; Δ \ phi = 6 °.

Values excitations, they appropriate to select an electron beam with energy U _ {0} = 1000 eV, are:

V 1 = 696.020 V; V 2 = 65.4906 V; NI _ {1} = 120,563 ampervueltas.

The geometry of the model to be patented differs from the anterior only within the inner radius of the poles, so that It is built with the following provision:

• r = r 0 = 5 at poles 1, 3, 4, 6, 7, 9, 10, 12

r r = r 0 + δ = 5 + 0.0574 at poles 2, 5, 8, 11

Excitations values, appropriate to select an electron beam with energy U _ {0} = 1000 eV and including a quadrupole magnetic component are:

V1 = 702.707 V; V 2 = 190.925 V; NI _ {1} = 121,721 ampervueltas; nI _ {2} = 12.9345 ampervueltas.

The figures to be commented below show electron paths calculated on both Monochromators They are the ones used to characterize the called opening geometric aberrations. The excitations are have chosen so that there is total symmetry in the cuts with the Z axis. All of them start from the same point as located on said axis at the entrance of the device (left side in the figures) and at a distance of 30 mm from the center of the first filter. They all carry the same initial axial velocity \ nu_ {z0} = 1.876 x 10 7 ms -1 and the transverse velocity is \ nu_ {t0} = \ pm3.275 \ times10 <5> ms <-1>, which corresponds at a half-opening angle? =?. The trajectories are integrate along the double filter and cut the shaft at a distance 30 mm from the center of the second filter. The plane where an intermediate image is formed coincides with the reflection plane R and it is also 30 mm from the center of the first and second filters. He total travel along the Z axis is therefore 4 \ times30 = 120 mm

Figure 5 shows the trajectories calculated when the electrons are released with the initial conditions mentioned in the XZ plane (azimuth angle \ varphi = 0º). The dashed lines correspond to paths calculated in the reference monochromator and the continuous lines have been calculated in the model object of the invention. It is clear that the aberration of the original system has been corrected where the paths with positive initial transverse velocity cross the axis after the R plane and those that begin with negative transverse velocity cross before the R plane. In the new model both paths are cut in said plane.

Figure 6 shows the projection of the trajectories in the YZ plane that have been launched with the same initial conditions as in the previous case, except that the azimuth angle is now var = 90 °. Projections agree for the two systems are compared and both give an intermediate image in R. It plane noted that this is achieved by adjusting the value of the potential created by the quadrupole component E {2} in the monochromator reference while in the new E must be set {2} and B {2} simultaneously. The trajectories shown in Figure 7 are, in our opinion, the most important when judging the improvements introduced in the system object of this patent. They are the projections in the XZ plane of the trajectories of particles thrown with var = 90 °; in the dashed line it is seen that at the time of forming an image in the R plane it is not in a null value but about 2.5 \ num is separated. while for the corrected system the value is zeroed. In addition, and given the symmetry imposed, when leaving the second filter it also falls to a null value.

The effect of the modifications can also be clearly seen in the aberration figures obtained for the reference monochromator and the new one. Figures 8 and 9 show the cross sections of the beam in the R plane for the first and second configuration respectively. In the reference monochromator it is seen that the center of the beam is outside the axis and that the diameter of the envelope is about 25 \ num. Figure 9 clearly illustrates the effect of the correction carried out in the XZ and YZ planes: the beam is centered with respect to the optical axis, makes it very narrow in the direction of the X axis and shortens it in that of the Y axis. it is best seen in figure 10 where the same points are shown as in figure 9 but with different scales on both axes.

We can take advantage of the aberration figures shown to clarify the concept of "energy resolution", E res, of the monochromator. If instead of the nominal energy U 0, the beam enters with energy U 0 - Δ U 0, the paths follow paths slightly different from those drawn in Figures 5-7 and suffer a deviation towards the positive x ; Similarly, the paths with energy U 0 + Δ U 0 form their aberration figure on the side of the negative x . For E {res} minimum value of \ Delta U _ {0} is calculated figure whose aberration has no intersection with the corresponding U _ {0} and from it defined

E_ {res} = \ frac {\ Delta U_ {0}} {U_ {0}}

Given the characteristics of the device, it is Of course, the modifications introduced allow us to improve substantially the energy resolution of the new Monochromator

Finally, Figures 11 and 12 show the cross sections of the beam at the exit of the monochromator, When the second image is formed. If figures 8 and 11, the corrective effect of a double filter can be seen mentioned in the previous section, that is, you can reduce the beam size by a factor greater than 100 by placing a second filter in series with the first and excited with the same values of current and voltage (which greatly simplifies its assembly). This beneficial effect is still maintained for the filter of the invention but only in the direction of the Y axis, since the correction in the X axis it was already carried out in the first filter in the most optimal possible. If you compare figures 11 and 12, you can observe an improvement in the spatial dispersion of the new filter, with a reduction in size by a factor 2 in the X direction and by 5 in the Y direction.

It should be noted that the case shown is just an example of how the device can work. Thus, trajectories have been shown for semi-opening angles of 1 °; however, high resolution microscopes work at angles of 0.5 ° and lower. We have verified that in all cases, and with small readjustments of the excitations, there is a behavior similar to that presented in this report. In addition, the resolution in energy increases and the beam size at the output decreases as the angle α decreases. As an example, let's say that for a beam with U 0 = 1000 eV, the energy resolution goes from a value E res = 2.75 x 5 - 5 for α = 1 ° to a value E res = 2.0 x -6 for α = 0.4 °. These amounts represent an increase in factors of 38 and 160, respectively, with respect to the E res values calculated for the reference monochromator .

The same can be said of the assigned values at the radius of the poles and the separation between them (data geometric). If they change within reasonable margins, adjusting the excitations to the new data you can get the correction of aberrations that we have presented here, or in a enough degree to get a very behavior improved.

When machining, assembling, and aligning Monochromator elements are inevitable to occur deviations with respect to the values used in the design. At technical language is spoken of "mechanical aberrations" and can Degrade image quality appreciably. However, in the Nowadays laboratories can have a machinery for the mechanization of the parts controlled by computer and that ensure accuracies below \ num in lengths of several mm Techniques have also progressed in recent years alignment with the use of the laser. So that you can appreciate the corrective effect of aberrations presented in this memory, it is necessary to carry out the construction of the monochromator using the highest quality standards possible.

Claims (8)

1. A wien monochromator with aberration correction characterized in that it comprises: two identical Wien filters placed along an optical axis substantially parallel to the Z axis and with an R plane perpendicular to said axis with respect to which the two filters are arranged symmetrically (reflection plane). 2. Monochromator according to claim 1, characterized in that for a particle beam launched from the optical axis near its entrance and according to different azimuthal angles, it allows: - form a first image in the plane of R reflection, centered on the optical axis and with minimal dispersion on the X axis, which results in resolutions in the beam energy far superior to that of other Wien filters without correction of aberrations - form a second image at the exit of the device with a spatial dispersion substantially less than in other Wien filters without correction of aberrations. 3. Monochromator, according to claims 1 and 2, characterized in that each of the filters is a configuration of magnetic conductors capable of producing at least one electric dipole field in the direction of the X axis and a magnetic dipole field in the direction of the Y axis , consisting of: 12 poles that extend substantially parallel to the optical axis presenting a symmetry of order 12 in around said axis, and whose input and output faces are in the X-Y plane. 4. Monochromator, according to the preceding claim, characterized in that the inner radius of the poles varies according to the position they occupy. 5. Monochromator, according to the preceding claims, characterized in that each of the filters contains the excitations added to produce an electric quadrupole field with symmetry with respect to the XZ and YZ planes, and a magnetic one with symmetry around the planes formed by the axes X = ± Y and the Z axis, the field lines of both quadripolar components being perpendicular to each other. 6. Monochromator, according to previous claims, characterized in that the design of the filter poles contains the modification of the inner radius of the poles whose average planes are located at an azimuth angle of 45 °, 135 °, 225 ° and 315 ° with respect to to the XZ plane. The value δ of the increase in radius with respect to that of the remaining poles, r 0, must be such that 0.008 δ / r 0 ≤ 0.016 to obtain an estimated improvement over others filters without correction of aberrations. 7. Monochromator, according to previous claims, wherein the filter is characterized by having a ratio δ / r 0 and being excited with the potentials and currents such that completely cancel out the geometric opening aberrations of a beam of charged particles, in the XZ and YZ planes and consequently minimize said aberrations in the rest of the beam. 8. An electron microscope characterized in that it contains: - A source of electrons to generate and accelerate an electron beam - The monochromator of claims 1 to 7.