Abstract:
An improved column for a charged particle beam device is constituted by, among other things, deflectors for scanning the beam over the specimen, for aligning the beam with regard to the objective and for compensating aberrations caused by the objective. Thereby, the total number of electrode arrangements and/or coil arrangements that are used for the deflectors and that are independently controllable, is 8 or less.

Description:
FIELD OF THE INVENTION 
     The invention relates to an apparatus for the examination of specimen with a beam of charged particles. In particular, this invention relates to a miniaturized column for a charged particle beam device. Furthermore, this invention relates to a deflector for a charged particle beam device. 
     BACKGROUND OF THE INVENTION 
     Charged particle beam devices, such as scanning, transmission or micro-probe apparatuses to quote only a few, are powerful instruments which permit the observation, characterization and modification of heterogeneous organic and inorganic materials and their surfaces. In these instruments, the area to be examined (modified) is irradiated with a charged particle beam, which may be static or swept in a raster across the surface of the specimen. Depending on the specific application, the charged particle beam is more or less focused and the kinetic energy of the particles can vary considerably. 
     The types of signals produced when the charged particles impinge on a specimen surface include e.g. secondary electrons, backscattered electrons, Auger electrons, characteristic x-rays, and photons of various energies. These signals are obtained from specific emission volumes within the sample and can be used to examine many characteristics of the sample such as composition, surface topography, crystallography, etc. 
     Lately, attempts have been made to miniaturize charged particle beam devices. Several of these devices could then be grouped together to simultaneously examine or modify larger areas of the specimen or they could be installed in process lines with tight space restrictions. Furthermore, since spherical and chromatic aberrations of particle beam devices scale proportional to their geometrical dimensions, as long as the potential remains constant, miniaturized devices would be able to deliver higher spatial resolution and high beam current in a given spot size. 
     In general, most of the present charged particle devices are between 0,5 and 1,2 meters high with an average diameter of about 15 cm-40 cm. Distinct from that, developers are aiming at producing beam devices which are smaller than 10 cm with an average diameter of about 4 cm. However, since modem charged particle beam apparatuses are complex technical instruments with sophisticated vacuum systems, alignment mechanism and electronic control units, their geometrical dimensions can not simply be shrinked proportionally, even so this is attempted wherever possible. 
     For forming of the particle beams in the particle beam columns electromagnetic lenses and electromagnetic multipoles are used. Lenses are axially symmetric electromagnetic fields used for focusing of the beam. Electromagnetic multipoles generate static deflecting fields (deflectors) for correction of the beam path through the electromagnetic lenses and for positioning the beam at the specimen, dynamic deflecting fields used for scanning the beam over the specimen and quadrupole fields (stigmators) used for compensation of the aberrations arising from the deviations of the lenses from the axial symmetry. Each particle beam instrument contains usually at least one multipole for alignment, one multipole for astigmatism correction, one multipole for beam shift at the specimen and one multipole for beam scanning at the specimen. Each multipole usually consists of eight electrodes or coils. This can result in large amount, for example 30 to 40, of independent voltages and currents that have to be supplied to the particle beam column. 
     In a single standard commercial column the number of the control signals required to operate the device does not present a limiting factor. In miniaturised columns and column arrays, however, the large number of the voltages and currents that have to be supplied to each column presents a major problem. Especially, the complexity of the electrical connections required to control every column in a column array increases the cost of an column array significantly. Furthermore, the complexity of the circuits required to control and to drive the large number of the voltages and currents also leads an significant increase of the cost of a column array. 
     SUMMARY OF THE INVENTION 
     The present invention provides an improved column for a charged particle beam device, especially for miniaturized charged particle beam device. Furthermore, the present invention provides an improved deflector for a charged particle beam device. According to one aspect of the present invention, there is provided a column for a charged particle beam device as specified in independent claim  1 . According to a further aspect of the present invention there is provided a deflector for a charged particle beam device as specified in independent claims  17  and  19 . Further advantageous, features, aspects and details of the invention are evident from the dependent claims, the description and the accompanying drawings. The claims are intended to be understood as a first non-limiting approach of defining the invention in general terms. 
     The present invention provides an improved column for a charged particle beam device. The column comprises deflectors for scanning the beam over the specimen, for aligning the beam with regard to the objective and for compensating aberrations caused by the objective. Thereby, the total number of electrode arrangements and/or coil arrangements that are used for the deflectors and that are independently controllable, is 8 or less. This results in a reduction of 50% in the number of signals, that have to be supplied to column in order to control the direction of the beam, compared to the best column known to the inventor. Accordingly, the complexity of the wiring needed to supply these signals the column is reduced considerably. Furthermore, the complexity of the driving circuits is also reduced. 
     According to a further aspect of the present invention, a deflector for a charged particle beam device is provided. The deflector, according to one embodiment, comprises four electrode arrangements wherein each electrode arrangement consists of three single electrodes, each electrode having the shape of a ring segment, and wherein the four electrode arrangements are positioned along a ring in a manner that between each pair of electrodes from one electrode arrangement an electrode from another electrode arrangement is located. The deflector, according to a further embodiment, comprises four coils wherein two coils are positioned along a first ring and two coils are positioned along a second ring, which is concentric with the first ring and has a larger diameter than the first ring, in a manner that when viewed from the center of the rings every coil positioned on the first ring overlaps with the two coils positioned on the second ring. These improved deflectors have the advantage that they provide a high degree of homogeneity in the electric, magnetic deflecting field, respectively. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some of the above indicated and other more detailed aspects of the invention will be described in the following description and partially illustrated with reference to the figures. Therein: 
     
       
         
               
               
             
           
               
                   
               
             
             
               
                 FIG. 1 
                 shows schematically a column according to a first 
               
               
                   
                 embodiment according to the present invention, 
               
               
                 FIGS. 2A, B 
                 show schematically the deflectors used in the embodiment 
               
               
                   
                 shown in FIG. 1, 
               
               
                 FIGS. 3A, B 
                 show schematically further deflectors that can be used in 
               
               
                   
                 the embodiment shown in FIG. 1, 
               
               
                 FIG. 4 
                 shows schematically a deflector according to a further 
               
               
                   
                 embodiment according to the present invention, 
               
               
                 FIG. 5 
                 shows schematically a deflector according to a still further 
               
               
                   
                 embodiment according to the present invention, 
               
               
                 FIG. 6 
                 shows schematically a column according to a further 
               
               
                   
                 embodiment according to the present invention, and 
               
               
                 FIGS. 7A-D 
                 show schematically the deflectors used in the embodiment 
               
               
                   
                 shown in FIG. 6. 
               
               
                   
               
             
          
         
       
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     An embodiment according to the invention is shown schematically in FIG.  1 . The charged particle beam device  1  comprises a top cover plate  2  to which a charged particle source  3  is attached. In electron beam devices, electron sources such as Tungsten-Hairpin guns, lanthanum-Hexaboride Guns, Field emission guns etc. can be used. The invention, however, is not limited to electron sources; it can be used together with all kinds of charged particle sources. In the embodiment shown in FIG. 1 an extractor  4 A and a suppressor  4 B is arranged below particle source  3 . The extractor  4 A which is set on an accelerating potential accelerates the charged particles coming from the source. Contrary to that, the suppressor  4 B which is arranged between the accelerator  4 A and the source  3  is set on a suppression potential to limit the number of particles being pulled out of the source  3 . This way the beam current is prevented from becoming to high. 
     After the beam of charged particles  5  has been formed and left the source  3 , the deflectors  30  and  40  are used to control and to refine the charged particle beam before it hits the specimen  8 . The particular arrangement of these components is shown in FIGS. 2A and 2B. In order to shape the beam  5  one or more condenser lenses (not shown) can be used. The beam  5  then enters the electrostatic objective lens  10  which is used to focus the beam  5  onto the specimen  8 . In the present example the electrostatic objective lens  10  comprises three electrodes  10 A,  10 B, and  10 C each having the form of a flat ring. 
     When the particles of beam  10  strike the surface of specimen  8 , they undergo a series of complex interactions with the nuclei and electrons of the atoms of the specimen. The interactions produce a variety of secondary products, such as electrons of different energy, X rays, heat, and light. Many of these secondary products are used to produce the images of the sample and to collect additional data from it. A secondary product of major importance to examination or the image formation of specimens are secondary electrons that escape from the specimen  8  at a variety of angles with relatively low energy (3 to 50 eV) The secondary and the back scattered electrons reach the detector  16  and are measured. By scanning the electron beam over the specimen and displaying/recording the output of the detector  16  an image of the surface of the specimen  8  is formed. 
     FIGS.  2 A,B show schematically the deflectors  30  and  40  used in the column shown in FIG.  1 . The deflectors  30  and  40  are electrostatic deflectors which influence the beam of charged particle by a static electric field. Each of the deflectors comprises  4  electrodes, electrodes  31 ,  33 ,  35 ,  37  and  41 ,  43 ,  45 ,  47 , respectively, which are positioned along a ring centered around the beam of charged particles. In a cross-section normal to the beam of charged particles all the electrodes  31 ,  33 ,  35 ,  37  and  41 ,  43 ,  45 ,  47  exhibit the shape of a ring segment, as can be seen from FIGS. 2A, B. In order to influence the propagation of the charged particle beam, every electrode is independently controllable, in the sense that the potential of every electrode can be selected independently from the potentials present on all the other electrodes. 
     In order to provide a predetermined potential on a selected electrode, every electrode comprises a connection  32 ,  34 ,  36 ,  38  and  42 ,  44 ,  46 ,  48 . The connections  32 ,  34 ,  36 ,  38  and  42 ,  44 ,  46 ,  48  are used to supply the corresponding voltages to the selected electrodes. During the operation of the column shown in FIG. 1 the deflectors  30  and  40  cooperate in order to scan the beam of charged particles over the specimen, to align the beam with regard to the objective lens axis and to compensate aberrations caused by the objective. In order to compensate aberrations (especially astigmatism) caused by the objective (stigmation), for example, a voltage +V 1  is supplied to the electrodes  31  and  35  of deflector  30  whereas the voltage −V 1  is supplied to the electrodes  33  and  37  of deflector  30 . However, in general the field of a 4-pole is not sufficient to compensate astigmatism caused by the objective. Therefore, a voltage +V 2  is supplied to the electrodes  41  and  45  of deflector  40  whereas the voltage −V 2  is supplied to the electrodes  43  and  47  of deflector  40  . Due to fact that the two deflectors  30  and  40  are rotated with regard to each other (in this example by 45° degrees), the astigmatism can be compensated in every chosen plane normal to the plane of the drawing. By varying V 1  and V 2  independently from each other every plane orientation can be selected. 
     In order to align the beam of charged particles with regard to the objective or to shift the beam of charged particles with regard to the objective, for example, a voltage +U x  and a voltage −U x  is supplied to the electrodes  35  and  31  of deflector  30 , respectively. Furthermore, a voltage −k x U x  is supplied to the electrodes  43  and  45  and a voltage +k x U x  is supplied to the electrodes  41  and  47  of deflector  40 , respectively. Thereby, k x  is a constant depending on the angle of rotation between the two the deflectors  30  and  40  and the desired path of the charged particle beam in x-direction. By using this arrangement, the charged particle beam can be guided through the two deflectors as shown in FIG. 1A which corresponds to a situation where the beam exhibits an initial misalignment. Obviously, by supplying the voltages +U y  to the electrodes  33 , −U y  to the electrodes  37 , −k y U y  to the electrodes  41  and  43  and +k y U y  to the electrodes  45  and  47  the same can be achieved for the y-direction. 
     In order to scan the beam of charged particles over the specimen, the same voltage arrangement that has been used in order to align the beam can be used. Only the values of the voltages (e.g. V x  and V y ) and the values of the constants (e.g. c x  and c y ) are different. During the operation of the deflectors all these voltages (V 1 , V 2 , U x,  U y , V x  and V y ) are used simultaneously, so that, in general, every electrode may have a potential that is different from the potentials of all the other electrodes. Thereby, the signals controlling the scanning, alignment and stigmation are mixed already before the final amplifier (not shown) so that the electronic circuits are kept relatively simple and only the resulting voltage is supplied to each electrode. Accordingly, the two deflectors are controlled via only 8 connections ( 32 ,  34 ,  36 ,  38  and  42 ,  44 ,  46 ,  48 ). 
     In order to further reduce the number of necessary connections to the deflectors, two of the electrodes of each deflector  30  and  40 , e.g. electrodes  35  and  37  of deflector  30  and electrodes  41  and  43  of deflector  40 , may be kept on a fixed potential, e.g. ground potential. Accordingly, no signals controlling the potentials of the electrodes  35 ,  37 ,  41  and  43  have to be provided and the corresponding wiring can be omitted. In such an embodiment the number of electrodes that are independently controllable is reduced to  4 . 
     The deflectors  30  and  40  shown in FIGS. 2A and B are electrostatic deflectors which influence the beam of charged particle by a static electric field. As an alternative magnetic deflectors  50  and  60  as shown in FIGS. 3A and B can also be used. The deflectors  50  and  60  comprises  4  coils, coils  51 ,  53 ,  55 ,  57  and  61 ,  63 ,  65 ,  67 , respectively, which are positioned along a ring centered around the beam of charged particles. In a cross-section normal to the beam of charged particles all the coils  51 ,  53 ,  55 ,  57  and  61 ,  63 ,  65 ,  67  exhibit the shape of a ring segment, as can be seen from FIG. 3A, B. In order to influence the propagation of the charged particle beam, every coil is independently controllable, in the sense that the current through every coil can be selected independently from the currents flowing through the other coils. 
     In order to provide a predetermined current flowing through a selected coil, every coil comprises two connections ( 52 ,  54 ,  56 ,  58  and  62 ,  64 ,  66 ,  68 ) which are used to supply a corresponding current to the selected coil. When one replaces voltages by currents basically all what has been said with regard to electrostatic deflectors  30  and  40  remains true for the magnetic deflectors  50  and  60 . Accordingly, deflectors  50  and  60  can also be controlled by only 8 independent signals, in order to scan the beam of charged particles over the specimen, to align the beam with regard to the objective and to compensate aberrations caused by the objective. 
     FIG. 4 shows schematically a deflector according to a further embodiment according to the present invention. The magnetic deflectors  50  and  60  as shown in FIGS. 3A and B exhibit a magnetic field that shows a high degree of homogeneity in the vicinity of the charged particle beam. However, in order to improve the homogeneity of the magnetic field, the magnetic deflector  70  as shown in FIG. 4 is provided. The deflector  70  comprises four coils  71 ,  73 ,  75 , and  77 . Thereby, two coils  71  and  75  are positioned along a first ring and two coils  73  and  77  are positioned along a second ring, which concentric with the first ring and has a larger diameter than the first ring. The four coils are positioned in a manner that when viewed from the center of the rings every coil  71  and  75 , positioned on the first ring, overlaps with the two coils  73  and  77 , positioned on the second ring. The coils  71 ,  73 ,  75 , and  77  are designed so that the angle covered by a coil is about 120° degrees. The magnetic deflector  70  exhibits a magnetic field that shows a very high degree of homogeneity in the vicinity of the charged particle beam. 
     FIG. 5 shows schematically a deflector according to a still further embodiment according to the present invention. The electrostatic deflectors  30  and  40  as shown in FIGS. 2A and B exhibit an electrostatic field that shows a high degree of homogeneity in the vicinity of the charged particle beam. However, in order to improve the homogeneity of the electrostatic field, the electrostatic deflector  80  as shown in FIG. 5 is provided. The deflector  80  comprises four electrode arrangements  81 ,  83 ,  85 , and  87 . Thereby, each electrode arrangement  81 ,  83 ,  85 , and  87  consists of three single electrodes ( 81   a,    81   b,    81   c,    83   a,    83   b,    83   c,    85   a,    85   b,    85   c,    87   a,    87   b  and  87   c ), each electrode ( 81   a,    81   b,    81   c,    83   a,    83   b,    83   c,    85   a,    85   b,    85   c,    87   a,    87   b  and  87   c ) having the shape of a ring segment. The four electrode arrangements  81 ,  83 ,  85 , and  87  are positioned along a ring in a manner that between each pair of electrodes from one electrode arrangement an electrode from another electrode arrangement is located. 
     The electrodes of one electron arrangement, for example the electrodes  81   a,    81   b,  and  81   c  of electrode arrangement  81 , are always kept on the same potential. Accordingly, only one connection  82  is necessary to supply the corresponding voltage to the electrodes ( 81   a,    81   b,  and  81   c ). As can be seen from FIG. 5, one of the three electrodes (for example  81   a ) of a electrode arrangement (for example 81) is at least twice the size of the other electrodes ( 81   b  and  81   c ) of the electrode arrangement. The electrostatic deflector  70  exhibits an electrostatic field that shows a very high degree of homogeneity in the vicinity of the charged particle beam. 
     A further embodiment according to the invention is shown schematically in FIG.  6 . This column  100  is similar to that of FIG. 1, except for the following. The column contains four deflectors  110 ,  120 ,  130  and  140  which positioned along the path of the charged particle beam. The corresponding deflectors are shown in FIGS. 7A to  7 D. 
     The deflectors  110 ,  120 ,  130  and  140  are electrostatic deflectors which influence the beam of charged particle by a static electric field. Each of the deflectors comprises only two active electrodes  111 ,  115 ,  121 ,  125 ,  131 ,  135 ,  141  and  145 , respectively, which are positioned along a ring centered around the beam of charged particles. In a cross-section normal to the beam of charged particles all the active electrodes  111 ,  115 ,  121 ,  125 ,  131 ,  135 ,  141  and  145  exhibit the shape of a ring segment covering an angle of about 120°, as can be seen from FIGS. 7A to  7 D. In addition to the active electrodes each of the deflectors  110 ,  120 ,  130  and  140  comprises two inactive electrodes  200  which are kept on fixed potential, e.g. the column potential. In order to influence the propagation of the charged particle beam, again every electrode is independently controllable, in the sense that the potential of every electrode can be selected independently from the potentials present on all the other electrodes. 
     In order to provide a predetermined potential on a selected electrode, every electrode comprises a connection  112 ,  116 ,  122 ,  126 ,  131 ,  136 ,  141  and  146 , respectively. The connections  112 ,  116 ,  122 ,  126 ,  131 ,  136 ,  141  and  146  are used to supply the corresponding voltages to the selected electrodes. During the operation of the column shown in FIG. 6 the deflectors  110 ,  120 ,  130  and  140  cooperate in order to scan the beam of charged particles over the specimen, to align the beam with regard to the objective and to compensate aberrations caused by the objective. In order to compensate aberrations caused by the objective (stigmation), for example, a voltage +V 1  is supplied to the electrodes  111  and  115  of deflector  110  whereas the voltage −V 1  is supplied to the electrodes  131  and  135  of deflector  130 . Furthermore, a voltage +V 2  is supplied to the electrodes  121  and  125  of deflector  120  whereas the voltage −V 2  is supplied to the electrodes  141  and  145  of deflector  140  . Due to fact that the deflectors  110 ,  120 ,  130  and  140  are rotated with regard to each other (in this example by 45° degrees), the aberrations can be compensated in every chosen plane normal to the plane of the drawing. 
     In order to align the beam of charged particles with regard to the objective or to shift the beam of charged particles with regard to the objective, for example, a voltage −U x  and a voltage +U x  is supplied to the electrodes  111  and  115  of deflector  110 , respectively. Furthermore, a voltage −k 1   x U x  is supplied to the electrodes  121  and  125  and a voltage +k 2   x U x  is supplied to the electrodes  141  and  145  of deflector  140 , respectively. Thereby, k 1   x  and k 2   x  are constants depending on the angles of rotation between the deflectors  110 ,  120  and  140  and the desired path of the charged particle beam in x-direction. Obviously, by supplying the voltages +U y  to the electrodes  131 , −U y  to the electrodes  135 , −k 1   y U y  to the electrodes  121  and  125  and −k 2   y U y  to the electrodes  141  and  145  the same can be achieved for the y-direction. 
     In order to scan the beam of charged particles over the specimen, the same voltage arrangement that has been used in order to align the beam can be used. Only the values of the voltages (e.g. V x  and V y ) and the values of the constants (e.g. c 1   x , c 2   x , c 1   y  and c 2   y ) are different. During the operation of the deflectors all these voltages (V 1 , V 2 , U x , U y , V x  and V y ) are used simultaneously, so that, in general, every electrode may have a potential that is different from the potentials of all the other electrodes. Again, the signals controlling the scanning, alignment and stigmation are mixed already before the final amplifier (not shown) so that the electronic circuits are kept relatively simple and only the resulting voltage is supplied to each electrode. Accordingly, the four deflectors are controlled via only 8 connections. 
     The deflectors  110 ,  120 ,  130  and  140  shown in FIGS. 7A to  7 D are electrostatic deflectors which influence the beam of charged particle by a static electric field. As an alternative magnetic deflectors, each having two independently controllable coils can also be used.