Abstract:
A retarding field analyzer uses the existing components of a charged particle beam system eliminating the need for inserting a separate retarding field analyzer device. Using components of the existing column reduces the time required to analyze the beam. Using the imaging capabilities of the existing column facilitates alignment of the beam with the analyzer.

Description:
This application claims priority from U.S. Prov. Appl. No. 61/718,684, filed Oct. 25, 2012, which is hereby incorporated by reference. 
     TECHNICAL FIELD OF THE INVENTION 
     The present invention relates to charged particle beam devices and more particularly to a retarding field analyzer for analyzing charged particle beams. 
     BACKGROUND OF THE INVENTION 
     For charged particle beams such as electron beam systems and focused ion beam (FIB) systems, beam quality is very important to imaging performance. In particular the uniformity of particle kinetic energy is an important quality. Ideally, all particles in the beam have the exact same kinetic energy. Particles having different energies focus at different points, thereby enlarging the spot size of the beam on the work piece and reducing resolution. This is referred to as chromatic aberration. In order to evaluate the uniformity of the energies of the particles in the beam, it is important to be able to measure the distribution of particle kinetic energy in the beam. It can also be important to determine the absolute energy of the particles in the beam. The absolute energy and the energy distribution are typically measured by a retarding field analyzer (RFA). 
     To measure the beam energy and/or energy distribution, the RFA is temporarily inserted into the charged particle beam system downstream of the focusing column. The retarding field strength is increased incrementally, and the beam current that passes through the retarding field after each incremental field strength increase is measured. Eventually, the incremented retarding field will be sufficiently strong to stop all particles in the beam from passing through the field. RFAs typically include a filter lens or a grid to which a voltage is applied to produce the retarding field. Using an RFA requires mounting additional equipment after the focusing column, which entails additional costs. The RFA requires additional vacuum electrical feed-throughs and wiring to connect the additional equipment, extra power supplies that are highly regulated, and some type of beam detector. Significant time is required to place the RFA into operation. 
     Because the beam energy at the retarding element is very low, the beam is easily disturbed, making precise beam alignment important to reduce measurement error. RFAs typically direct the beam into a Faraday cup and do not have imaging capability, so it can be difficult to verify that the beam is accurately aligned with the RFA. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a retarding field analyzer for a charged particle beam system. 
     The present invention uses existing elements of a charged particle beam system to provide retarding field energy analysis. The focusing lens of the optical column is used to provide a retarding field analyzer integral to the column. The imaging capability of the column can be used to align the beam for analysis in some embodiments. Beam current after the retarding element can be measured, for example, using a current meter connected to a Faraday cup, by observing the gray level of an image of a work piece, or by measuring stage current. 
     The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows a schematic view of a charged particle beam system according to an embodiment of the present invention; 
         FIG. 2A  is a flow chart showing the steps of one method for aligning a column before performing retarding field analysis.  FIG. 2B  shows the steps of employing a retarding field analyzer in accordance with one embodiment of the invention; 
         FIG. 3  shows a simulation of a charged particle beam in a retarding field analyzer embodying the invention; 
         FIG. 4  shows the electrical potential in a lens from the optical center line radially outward to the lens element for different values of voltages applied to the lens for one embodiment of the invention; 
         FIG. 5  shows, for one lens design, the difference between the electrical potential at the center of the beam (r=0) and at the beam radius, as a function of the radius of the beam radius entering the lens; 
         FIG. 6  shows, for lens bores having radii between 0 mm (a metal plate with no bore) and 2 mm, the depth of the potential well as determined by a model of a particular lens; and 
         FIG. 7  shows a plot of gray level (GL) versus retarding potential, as well as a derivative of the plot. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Definitional note: when referencing the charged particle beam device, the terms “before” and “after” relate to the direction of particle travel, so that if element B is positioned after element A, the particles pass element A prior to passing, or otherwise encountering B. 
     One embodiment of the present invention comprises a focused ion beam (FIB) system  100 , having a retarding field analyzer (RFA) integral with its focusing column, and a method of using the same Skilled persons will recognize that the invention is generalizable to other focused ion beam (FIB) systems, electron beam systems, such as electron microscopes, and other charged particle beam devices. Referring to  FIG. 1 , a FIB  100  system includes an ion source  112 , such as a liquid metal ion source or a plasma ion source, producing an ion beam  114 . FIB system  100  includes an optional first electrostatic focusing lens  115  (referred to as “lens 1”), including an upper lens element  115 A, a center lens element  115 B, and a lower lens element  115 C and a second electrostatic focusing lens  116  (referred to as “lens 2”) including an upper lens element  116 A, a center lens element  116 B, and a lower lens element  116 C. 
     The upper lens element  116 A, and lower lens elements  115 C and  116 C, are typically maintained at ground potential, with the middle lens elements  115 B and  116 B can be either positive or negative with respect to ground; thereby creating the electrical equipotential lines  118 A and  118 B as shown. Lens element  115 A is at a potential equal to the beam voltage minus the extraction voltage, typically 30,000 V minus 10,000 V, or 20,000 V. Upper lens element  116 A and lower lens elements  115 C and  116 C may be set at potentials other than ground in some embodiments. A precisely adjustable, highly stable power supply  120  sets the voltage for the middle lens element  116 B. In some embodiments, the power supply  120  is capable of producing a higher voltage and higher precision than a typical power supply in a charged particle beam system without an integrated RFA. An aperture  117  limits the size of beam  114 . Deflectors  122  are used to direct the beam, often in a raster pattern, to process or form an image of a target. Deflectors  122  are also used to position the beam on the target electronically, whether or not the beam is rastering. The steering deflectors are generally dipole units, one for x and one for y deflection, although octupole assemblies that deflect the beam simultaneously in x and y could be used. 
     A stage  130  is adapted to support a sample  131 . A secondary electron detector  132  receives secondary electrons emitted from the sample or other targets in response to ion or electron bombardment, thereby permitting an image to be formed by associating the strength of the secondary electron current with the position of the beam in the raster scan, and forming an image in which the gray level at each point corresponds to the secondary electron current. A Faraday cup  134  is located near the edge of stage  130 , and is electrically connected to a sensitive current meter  138 , for measuring the current of charged particles entering into the Faraday cup  134 . The current meter  138  and its manner of connection to Faraday cup  134  are preferably such that current variations of less than two picoamperes or more preferably, less than one picoampere can be measured. In one preferred embodiment current variations of less than 100 femtoamperes can be measured. To reduce noise, the current meter is positioned close to the Faraday cup  134 , which limits resistance and avoids induced voltages, and the electrical connection between the current meter and the Faraday cup  134  is preferably via a triaxial cable  136  or other low-noise connector. 
     The charged particle beam column of  FIG. 1  is typically a conventional electrostatic charged particle beam column. Preferred embodiments of the invention use the existing focusing lens of the charged particle beam column to provide the retarding field of the retarding field analyzer, thereby obviating the need for reconfiguring the column and adding additional equipment. In some embodiments, the conventional lens power supply may be modified to provide a higher voltage and/or high resolution. The resolution of the energy analyzer depends in part on the minimum size of voltage increment that can be provided by the power supply. The desired resolution for a specific implementation typically depends on the energy spread of the beam being measured. Different types of ion sources have different energy spreads. For example, a typical energy spread of a liquid metal ion source is about 5 eV, a typical energy spread of a plasma ion source is about 20 eV, and a typical energy spread of a magneto-optical trap ion source (MOTIS) can be 0.100 eV or less. An energy analyzer preferably has a resolution of at least one twentieth of the energy spread. That is, an energy analyzer used in a column having a liquid metal ion source will preferably have a resolution of better than 0.25 eV and require a power supply able to increment voltage in steps of 0.25 V or less. An energy analyzer used in a column having a plasma ion source will preferably have a resolution of better than 1.0 eV and require a power supply able to increment voltage in steps of 1.0 V or less. An energy analyzer used in a column having a MOTIS will preferably have a resolution of better than 0.005 eV and require a power supply able to increment voltage in steps of 0.005 V. In less demanding application, a voltage increment size of one tenth or smaller of the beam energy may be acceptable, while in other application, an increment size of one thirtieth or smaller of the beam energy spread may be desired. 
     For example, the power supply may provide voltage setting in increments of less than about 1.0 V, such as less than about 0.5 V, less than about 0.3 V, or less than about 0.1 V, to provide higher resolution measurements of the beam energy. The smaller incremental voltage steps may require a more accurate digital-to-analog (DAC) converter, such as a one or more DACs using a total of at least 16 bits, for example, an 18 bit DAC or a 32 bit DAC. Some embodiments use a stack of more than one DAC, for example, with one 16 bit DAC covering 0-35 kV and another 16 bit DAC controlling the same power supply over a 1 kV range, to get the resolution desired for energy analysis. 
     In some embodiments, the accelerating voltage of the charged particle beam may be reduced during retarding field analysis so that the required voltage of the center lens element is within the capability of an existing power supply. The accelerating voltage may also be reduced so that the required voltage of the center lens element is not so great as to cause arcing within some focusing lens systems having a smaller lens element gaps. The longitudinal energy spread of the beam typically does not vary significantly as the accelerating voltage or column length is changed, so the longitudinal energy spread determined at lower beam energy can be used to determine the energy spread of the beam at the higher, operating beam energy. 
     The center lens element typically has a bore diameter of, for example, about 5 mm. The beam diameter is typically about 0.1 mm at the lens bore. The electrical potential within the lens element will not be the same as the voltage applied to the lens element and the electrical potential will not be uniform within the lens bore. To determine the energy of the beam, it is necessary to determine the value of the retarding field at the position in the lens where the beam passes. The field within the lens can be determined by simulations. The results of some simulations are shown in  FIGS. 4-6  below, which are described in more detail below. To determine the energy spread of the beam, rather than the absolute value of the energy, it is unnecessary to determine precisely the electrical potential along the optical axis. Determining the absolute energy calibration is useful, for example, when determining the energy of a beam from a plasma source in which the beam energy is not identical to the voltage applied to the system components. 
     Before measuring the energy of the beam, it is necessary to align the beam.  FIG. 2A  is a flowchart of the steps used to align the beam. Precise alignment of the beam with the retarding lens is important, because as the voltage applied to the center lens element increases, the energy of the charged particles at the center lens element decreases. When the beam energy is very small, the charged particles can be easily deflected by being off-axis in the lens focusing field. For that reason, it can be important to have the beam properly centered on the retarding field, so that the energy analysis will not be compromised by entering the lens retarding field off-axis. 
     Beam alignment for an RFA typically comprises two parts: normal alignment of the lens as shown in box  202  and alignment of the lens near the cut-off potential, as shown in box  204 . In some embodiments, the optical column may be so well-aligned mechanically that part of the alignment process can be skipped, particularly the alignment of the beam near the cut-off potential. In step  206 , the sample stage is moved so that an observable feature is within the raster pattern of the charged particle beam. The image is formed using secondary electron detector  132  and with the focusing column operating in its normal mode, e.g., a beam landing energy of about 30 keV for a FIB system. The voltage on center lens element  116 B may be, for example, about 18,000 V, and upper lens element  116 A and lower lens element  116 C are typically grounded. For example, the image formed may be of the top of Faraday cup  134 . When the beam is scanned over the hole in the top of the Faraday cup by deflectors  122 , the beam enters the cup during a portion of the scan. Secondary electrons generated at the bottom of the cup will not escape the cup and will therefore not be detected by detector  132 . The part of the raster scan during which the beam enters the hole in the Faraday cup will therefore appear black. When the beam hits the metal top of the Faraday cup, secondary electrons will be generated and will be detected—the top of the cup will therefore appear lighter. The image formed by the beam rastering over the top of the cup will therefore be a lighter rectangle having a black hole near the center. 
     In step  208 , the focusing voltage on the center lens element  116 B is “wobbled,” that is, varied slightly, perhaps 10V-15V, and the image is observed. If the image is found to shift laterally in decision block  210 , the alignment of the column is adjusted in step  212 , mechanically by adjusting the ion source and/or lens positions, or electronically, by shifting the beam. The voltage is again wobbled, and the image observed to see whether or not it shifts in decision block  210 . The sequence of adjusting and observing is continued until the image no longer shifts laterally when the focusing voltage is wobbled, which indicates the column is aligned and normal alignment is complete. 
     After the normal alignment, the column is aligned again near the cut-off potential, i.e., with the center element of the lens near the beam energy, as shown by the steps in block  204 . In step  220 , lens  115  is adjusted to provide a collimated beam  114 . In step  222 , an aperture  117  having the minimum diameter necessary to pass adequate current for energy analysis is inserted into the beam path. The combination of aperture diameter and the voltage on lens element ( 115 B) also determines the maximum diameter of the beam envelope entering lens  116 , which determines the energy resolution as described below and shown in  FIG. 5 . In step  224 , the voltage on center element  116 B is adjusted to be near the accelerating voltage, but to still allow all charged particles to pass. In step  226 , the focusing voltage on the center lens element  116 B is “wobbled,” and the image is observed. If the image is found to shift laterally in decision block  228 , the alignment of the column is adjusted in step  230  electronically, by shifting the beam. The voltage is again wobbled, and the image observed to see whether or not it shifts in decision block  228 . The sequence of adjusting and observing is continued until the image no longer shifts laterally when the focusing voltage is wobbled, which indicates the column is aligned near the cut-off voltage. After the beam is aligned at normal operating voltage and near the cut off voltage, the retarding field analysis is performed as indicated in step  232 . 
     The focusing voltage on lens element  116 B is slowly adjusted so that it starts to cut off the beam current. In this condition, if the beam is scanned, there will not be a focused image, but rather a large white round blur. As the focus voltage on lens element  116  gradually increases and cuts off the beam, the white round blur shrinks in size and gradually disappears. This occurs because the beam is scanning over the saddle point in the lens and the saddle cutoff is seen in the image. If the round blur shrinks symmetrically, the beam is aligned with the saddle point in the lens. If not, then the beam needs to be redirected to fulfill that condition, usually by means of the deflection assembly  122 , which consists of beam steering plates as well as beam deflection (rastering and patterning) plates. 
       FIG. 2B  shows the steps of performing a retarding field analysis in accordance with one embodiment of the invention. In step  240 , the stage is moved so that the center axis of Faraday cup, or any other device used in the determination of the beam current, is under the charged particle beam. In step  242 , the rastering is ceased. In step  244 , the voltage on the center lens element  116 B is adjusted to a voltage near the accelerating voltage, but still allowing all charged particles to pass. The beam passes approximately through the center of the lens  116 , and the potential in the center of the lens will be somewhat less than the voltage applied at lens element  116 B. For example, if a voltage of 30,000 V is placed on center lens elements  116 B, the potential along the optical axis in the center of lens element  116 B may be closer to 29,700 V. Because the energy spread in the nominal 30 keV beam is typically less than 100 V, essentially all the charged particles in the beam will pass through the lens when 30,000 V is applied to the center lens element. 
     In step  246 , the beam current is measured using a Faraday cup and current meter  138 , or any other type of measurement that provides information about the beam current, such as gray level or stage current.  FIG. 7 , described in more detail below, shows beam current determined by gray level. In decision block  248 , the system determines whether or not the beam current is zero. If the beam current is zero, that is, if all the charged particles are stopped by the retarding field, then the data collection for the analysis is complete. The current at different voltage levels on center lens element  116 B can then be compared to determine the beam energy or beam energy spread. If the current is not zero, the voltage on the center element  116 B is incremented in step  250  and the current is measured again in step  246 . The increment used will depend on the desired resolution for the energy analysis and on the resolution of the voltage supply. For example, the increment can be 1.0 V, 0.5 V, 0.3 V or 0.1V. A plot of the current against voltage on the center lens element  116 B can be expected to be in a shape similar to an error function. 
       FIG. 3  shows modeling of a charged particle beam passing through the center lens element  116 B at a voltage slightly below the cutoff voltage.  FIG. 3  shows lens elements  116 A,  116 B, and  116 C, as well as the field lines  302 . The voltage applied to the center electrode element  116 B during retarding field analysis is much higher than the voltage applied to lens element  116 B during normal focusing operation. The high voltage causes a beam crossover  304  positioned within center lens element  116 B and second cross over  306  at the image plane. 
     The performance of an energy analyzer can be assessed in terms of a variety of evaluative merits. One most notable evaluative merit is the resolution versus acceptance size. Accepting a large beam size leads to lower resolution and accepting a smaller beam size leads to a higher resolution. In a preferred embodiment, the RFA is operated within a set of conditions in which the beam size entering the RFA results in the desired resolution. Most electrostatic charged particle beam lenses contain a saddle field such that the axial potential is less than the voltage applied to the lens element.  FIG. 3  shows a potential drop between the middle lens element  116 B and the center of the optical axis at  304 . For example, for the condition of a middle element  116 B 5 mm long with a bore radius of 2 mm, the maximum axial potential is 29,989.85 V from the applied voltage to the middle lens element of 30,302.5 V. 
     Determining the potential within the lens, one must be concerned not only with the voltage in the center of the lens along the lens axis, but also with the off-axis variation of potential.  FIG. 4  shows the electrical potential within the lens, at points extending radially outward from the lens axis to the lens element, for different values of voltage applied to the center lens element. From  FIG. 4  the off-axis potential at the peak of the saddle field for different voltages can be seen, specifically 30,000 V; 30,100 V; 30,200 V; and 30,300 V, applied to the middle lens element. 
     The change in beam energy across the lens bore causes the resolution of the analyzer to be limited by the beam diameter because charged particles at the edges of the beam will see a different retarding potential than charged particles in the center of the beam.  FIG. 5  shows, for one lens design, the difference between the saddle point electrical potential at r=0 and at the saddle point beam radius, as function of the radius of the beam entering the lens. From  FIG. 5  one can determine the maximum allowable beam radius entering the lens for a given beam energy resolution for a lens geometry having middle element 5 mm long with a 2 mm radius bore. For example, a 100 μm radius beam cannot have its energy resolution determined to better than 1.05 V. Similarly, a beam entering the lens with a 50 μm radius cannot be energy resolved to less than 0.4 V. Similarly, it can be understood that for a beam entering the lens off-axis, the RFA resolution is reduced by the potential offset shown in  FIG. 5 . Hence it is desirable to assure good beam alignment using the alignment procedures described above or other alignment methods. 
       FIG. 6  shows how the depth of the saddle field varies with the diameter of the lens bore. Specifically,  FIG. 6  shows the difference between the voltage applied to the lens element and the axial potential in the saddle field (defined as the depth of the potential well) versus the lens bore radius for a 5 mm long lens element  116 B. It can be seen that for this particular lens design, the maximum axial potential is approximately 300 V less than the voltage applied to the lens element for a lens bore that is 2 mm in radius. 
     As shown by  FIGS. 4 ,  5  and  6 , the beam coming into lens 2 is preferably carefully aligned and focused so that the charged particles pass through the lens near its center to minimize the effects of the axial variations in the electric field. 
     Rather than measuring the current using Faraday cup  134  and current meter  138 , the beam could be directed to a sample, such as a semiconductor wafer, and the beam current can be inferred from the secondary electron current generated by the beam. The secondary electron current can be detected as stage current or as gray level of a secondary electron image.  FIG. 7  shows a plot of the measured gray level (filled rectangles) versus the voltage on second lens element  116 B. As shown in  FIG. 7 , as the voltage on lens element  116 B increases, the gray level decreases to zero, with the data points approximating an error function  402 .  FIG. 7  also shows the derivative (empty rectangles) of the measured gray level, the derivative approximating a Gaussian function  404 . The details of the energy spread which are of interest are not only the width of the function but the tails. In the derivative (empty rectangles), one can see an asymmetric energy tail. In another embodiment, the beam current is determined by measuring the stage current when the stage is electrically biased to suppress secondary electrons emission. The stage current will then represent the primary beam current only. 
     Embodiments of the invention can be used, for example, to evaluate design changes to a FIB device. For example, if an energy filter were to be added to a FIB, in order to achieve greater uniformity of ion kinetic energy values, then the RFA-integrated focus column  116  can be used to measure the beam improvement resulting from this design change. 
     Embodiments of the invention allow the energy or energy spread of the beam to be measured in-situ, that is, without removing the emitter from the column and without opening the column to add additional measuring equipment. The energy or energy spread can be analyzed, and then the charged particle beam system can be used to process a sample, without having to open the vacuum chamber to remove or reconfigure equipment. Embodiments of the present invention can therefore be used to test the operation of the emitter, either periodically to monitor the operation and aging of the emitter, or for trouble shooting if a problem is suspected. For example, if FIB device  100  has a heated ion source, such as liquid metal ion source (LMIS) or a single element heated ion source, the RFA-integrated focus column  116  can be used to periodically evaluate the beam  114  to ensure that liquid metal evaporation, over time is not causing overheating by the heating element, thereby causing changes in the characteristics of beam  114 . The same process can be used for evaluating the operation of a plasma ion source, such as the inductively coupled plasma source described in U.S. Pat. No. 7,241,361. The invention can be used for verifying or tuning an in-column energy filter to the desired beam energy spread. 
     While the embodiment above describes increasing the voltage on the lens from a voltage at which essentially all the charged particles pass through the lens to a voltage at which essentially no charged particles pass through the lens, the voltage could start out with essentially no particles passing through the lens, and then the voltage could be decreased incrementally until essentially all particles pass through the lens. In some embodiments, the voltage need not be scanned over the full range between full current and no current, but can be scanned over a partial range. Also, the voltage need not be incremented discretely, but could be varied in a continuous fashion. 
     Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.