Abstract:
A non-dispersive electrostatic energy analyzer for electrons and other charged particles having a generally coaxial structure of a sequentially arranged sections of an electrostatic lens to focus the beam through an iris and preferably including an ellipsoidally shaped input grid for collimating a wide acceptance beam from a charged-particle source, an electrostatic high-pass filter including a planar exit grid, and an electrostatic low-pass filter. The low-pass filter is configured to reflect low-energy particles back towards a charged particle detector located within the low-pass filter. Each section comprises multiple tubular or conical electrodes arranged about the central axis. The voltages on the lens are scanned to place a selected energy band of the accepted beam at a selected energy at the iris. Voltages on the high-pass and low-pass filters remain substantially fixed during the scan.

Description:
GOVERNMENT INTEREST 
     This invention was partially developed under NASA Contract No. SBIR NNC04CA20C. The government may have certain rights in the invention. 
    
    
     FIELD OF THE INVENTION 
     The invention relates generally to analyzers for charged particle beams. In particular, the invention relates to electrostatic energy analyzers of electrons or other charged particles. 
     BACKGROUND ART 
     Charged particle energy analyzers, also called spectrometers, are used in many scientific and technical applications in which the energy distribution of charged particles such as electrons are measured. Such uses include characterizing the composition and other properties of materials in which the electron energy needs to be measured, for example, X-ray photoelectron spectrometers and electron spectrometers, and secondary ion spectrometers. Similar spectroscopes, such as secondary ion spectrometers, have been applied to other charged particles, such as energetic ions. Many scientific experiments require accurate measurement of the energy distribution of charged particles. 
     The performance of a charged particle energy analyzer, of which an electron energy analyzer is but one example but the most prevalent one, is gauged by several conflicting characteristics. It needs to have a narrow resolution over a reasonably large energy band and the selected energy should be easily tuned. Its resolution needs to be stable and not require repeated calibration. The energy analyzer needs to have a high detection efficiency, which results in a high throughput of analyzed samples. Of especial importance in material characterization in which secondary electrons or ions are emitted over a wide angle from the material being probed, the energy analyzer should have a wide aperture and a wide acceptance angle to thereby increase the collection efficiency. A typical requirement of a commercial electron energy analyzer is that it be able to analyze 10 to 20% of the electrons emitted from the material and to distinguish electrons whose energies differ by as little as 0.1%. 
     Commercial energy analyzers should be rugged, small, easy to operate, and relatively inexpensive. If these commercial characteristics can be improved, materials analysis equipment can more readily find acceptance in production environments, such as in-line processing monitors in the semiconductor industry. Such characteristics are also important for remote operation, such as the search for life on Mars. For space applications, an energy analyzer needs to be lightweight, a characteristic also desired for other applications. 
     Dispersive energy analyzers rely upon electrostatic or magnetic deflection of the charged particles to select the energy of the particle to be detected. Although effective at very high resolution, dispersive energy analyzers tend to be large and have relatively small acceptance apertures, which result in a low measurement throughput. On the other hand, non-dispersive energy analyzers typically rely upon serially arranged low-pass and high-pass energy filters to allow only the particles within a selected energy band to reach the detector. A low-pass filter passes particles having energies below a cutoff energy and blocks those above. A high-pass filter passes particles having energies above another cutoff energy and blocks those below. It is understood that the cutoff energy need not represent a sharp discontinuity in the transmission factor, which may vary somewhat gradually across the cutoff energy. 
     Two of the present inventors disclose a compact non-dispersive energy analyzer for analyzing the energy of electrons in the range of a few electron volts (eV) to a few keV in U.S. patent application Ser. No. 10/961,631, filed Oct. 8, 2004 and published as U.S. Patent Application Publication 2005/0045832 A1, incorporated herein by reference. This energy analyzer includes an initial low-pass filter followed by a high-pass filter, both of which incorporate biased electrical grids through which the charged particles of the proper energy may pass. In particular, the low-pass filter includes a curved grid which together with a similarly curved electrode in back of it reflects the low-energy electrons into a collimated beam, which then passes through a planar high-pass grid filter. The energy overlap of the low-pass and high-pass filters determines the overall pass band of the energy analyzer, which is tuned to provide an energy spectrum. 
     The described energy analyzer provides superior performance. However, we now believe that its fabrication is overly complex particularly because of the curved grid, which should be large and ellipsoidally shaped. Further, the preferred embodiments include an entrance section arranged along an axis generally perpendicularly to the axis of the rest of the cylindrically shaped chamber so that the overall size and weight of the analyzer are increased, thereby decreasing the usefulness of the design for space applications. The reference also describes a coaxial design, but this design requires the electron source, typically a sample being irradiated by probe particles or radiation, to be inserted into the middle of the high-vacuum coaxial analyzer. Such a sample insertion is disadvantageous for remote high-throughput operation as required for a space application or even for an industrial application. In any case, a sample apparatus located in the beam path between the low-pass and high-pass filters is bound to absorb some of the desired back-reflected electrons and reduce the throughput of the analyzer. 
     Tepermeister et al. disclose a coaxial two-section analyzer in “Modeling and construction of a novel electron energy analyzer for rapid x-ray photoelectron spectroscopy spectra acquisition,”  Review of Scientific Instrumentation , vol. 62, no. 8, August 1992, pp. 3828-3834. However, the Tepermeister design includes two large curved grids between its two sections and does not control the energy of the particles incident on the first section and does not focus them before entering the first section. Thus, the Tepermeister analyzer is considered to be large, difficult to build, and provide low throughput. 
     A compact, economical, and efficient charged particle analyzer is thus still needed for many applications both in the laboratory and commercial production line and in demanding space applications. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, a charged particle energy analyzer, for example, an electron energy analyzer, includes a coaxial set of electrostatic optics including a series of coaxial electrodes forming walls of the analyzer. The electrodes may have tubular or conical shapes. A charged particle detector detects the intensity of charged particles passed by the low-pass filter. 
     In one aspect of the invention, the charged particle analyzer includes a high-pass electrostatic filter followed by a low-pass electrostatic filter. The high-pass filter may include a plurality of differentially biased wall electrodes. Preferably, one or more biased planar grids separate the two filters. 
     An electrostatic lens may be placed between the source of the charged particles, such as electrons. The lens, which may include a plurality of differentially biased wall electrodes, advantageously includes elements including a biased iris which controls the energy of the charged particle entering the high-pass filter. Preferably, the lens selectively controls, e.g. reduces, the energy of the charge particle entering the high-pass filter to a substantially constant input energy such that the energy spectrum may be scanned substantially within the lens while the downstream elements process charged particles of substantially the same energy and the voltages of their electrostatic elements are not substantially changed during the scan. That is, the energy scanning is preferably performed in the lens with a selected amount of acceleration or retardation so that the selected energy band enters the filters at a substantially fixed energy. 
     In one design based on coaxial electrodes, the particle detector is placed within the low-pass filter and accepts only charged particles entering it from the downstream side. 
     The lens, which may be composed of coaxial electrodes similar to those of the filters, may focus the charged particles through an iris at the input to the filters. The lens may include at its input a curved mesh having a concave side facing the source of charged particles. The mesh shape is preferably aspheric and more preferably ellipsoidal. 
     A dual screen comprising two grids may separate the high-pass and low-pass filters. In normal operation, both grids are biased to substantially the same voltage, which may be that of the adjacent coaxial electrodes. However, in a calibration mode, the first grid is biased more negative (for electrons) than the second grid so that no charged particles within the passband pass the screen according to the design. Nonetheless, those charged particles are detected at the output of the low-pass filter, which detected particles represent spurious signals or noise. The spurious spectrum is subtracted from the spectrum detected in normal mode to optimize the resolution of the analyzer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of an electron energy analyzer, which is an embodiment if the invention. 
         FIG. 2  is a cross-sectional view of an electron analyzer similar to that of  FIG. 1  but more clearly illustrating the electrode and other structure. 
         FIGS. 3 and 4  are alternative embodiment of a textured surface on the walls of the low-pass filter. 
         FIG. 5  is a partially sectioned orthographic view of an operational analyzer system. 
         FIG. 6  is a schematic illustration of the electrical circuitry associated with the analyzer in one embodiment of the invention. 
         FIG. 7  is a schematic illustration of the effect of differential biasing of the two grids between the high-pass and low-pass filters. 
         FIG. 8  is a graph showing the resultant spectra in the biasing of the two grids. 
         FIG. 9  is a graph of the transmission coefficients associated with different aspects of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A first embodiment of a coaxial electron energy analyzer  10  of the invention is schematically illustrated in the cross-sectional view of  FIG. 1 . The analyzer  10  is generally circularly symmetrical shaped about a central axis  12 . The entrance end of the analyzer  10  is positioned next to a sample  14  which emits electrons in the eV to low-keV energy range. The sample  14  is preferably also located on the central axis  12  but it may be inclined to accommodate the probe beam which excites the electrons from the sample  14 . The entrance end includes a curved input grid  16 , preferably aspherically shaped and more preferably ellipsoidally shaped, which accepts electrons from the sample  14  within an acceptance half angle α and, in cooperation with potentials on other lens electrodes of an electrostatic input lens  20 , focus them on the plane of an iris  22 . The aspheric grid  16  may be ellipsoidally shaped by hydraulically compressing an 80-mesh stainless steel screen mesh sandwiched between aluminum foil and copper disks against a concave ellipsoidal mold while holding the outer periphery of the screen sandwich above the lip of the mold. Although it is not required, the aspheric grid  16  is held at the same potential as the sample  14  so that the electrons leaving the sample  14  are in a field-free region. 
     The aspheric grid  14  is mounted on and electrically connected to a conically shaped end electrode  18  of the electrostatic lens  20 , which is coaxial about the central axis  12 . Several differentially biased coaxial electrodes of either tubular or conical shape, to be illustrated in detail later, form the side walls of the lens  20 . The biased iris  22  has a central aperture  24  on which the lens  20  including the curved input grid  16  focuses the electron trajectories. In one implementation, the iris  22  is biased such that an electron leaving the sample  14  at any energy selected for analysis between 50 and 1500 eV exits the aperture  24  at a fixed energy for the selected analysis energy of 1000 eV in the standard mode and between 200 and 500 eV, for example, 333 eV, in the high-resolution mode. That is, the lens  20  may act as either a retarding or an accelerating lens depending upon the biasing of the different electrodes in the lens  20  so that the electrons enter the following sections within standard bandpass energies of those filters. 
     The electrons passing through the aperture  24  enter an electrostatic high-pass filter  26 , which is coaxial about the central axis  12  and has a narrow entrance end having a diameter of that of the exit end of the lens  20  but then flaring to a wider exit end. Differentially biased conical or tubular (round) electrodes, to be illustrated in more detail later, form the side walls of the high-pass filter  26 . A biased dual screen  28  is placed at the wider exit end of the high-pass filter  28  perpendicular to the central axis  12 . Advantageously, the large dual screen  28  may be planar, greatly simplifying the design and fabrication of the analyzer. The first grid in the dual screen  28  in typical operation is biased at nearly the same voltage as the last electrode in the high-pass filter  26 . 
     The electrodes in the high-pass filter  26  are biased to retard the energy of the electrons so that all those below the cutoff energy of the high-pass filter  24  (approximately 1000 eV at its input in the low-resolution mode and about 1 or 2 eV or even less at its output) have insufficient energy to reach the dual screen  28  and are reflected from it. Those electrons having energy greater than the cutoff energy pass through the dual screen  28 . Those having only slightly more energy approach the dual screen  28  at nearly normal angles at pass through it perpendicularly. 
     The second grid  74  is not required for normal operation in which the two grids are held at the same potential. However, the two grids  66 ,  72  allow spurious electrons to be canceled by reference to a calibration run. Specifically, if in a calibration mode the first grid is set to a voltage above the selected bandpass while the second grid is set to the normal voltage at the bottom of bandpass, no electrons passing the first grid should be within the selected bandpass. Any electrons which are nonetheless detected in the calibrating mode are spurious. The detected intensity in the calibration mode represents background signal, which can be subtracted from the detected intensity in the normal mode when the two grids are held at the same potential, thereby sharpening the bandpass. 
     Those electrons above the cutoff energy enter an electrostatic low-pass filter  30  which has a diameter equal to that of the exit end of the high-pass filter  26 . The low-pass filter  30  includes a series of coaxial electrodes along its sidewalls and back wall. These electrodes deflect low-energy electrons toward the electron detector  34  while allowing higher-energy electrons to strike either the electrodes or the front housing of the detector  34 . Also, a central nose electrode  32  in front of an electron detector  34  deflects low-energy electrons away from the central axis  12 . The detector  34  is located so that only low-energy electron that are deflected from the electrodes of the low-pass filter  30  enter it. The low-energy electrons for the most part need to reverse directions to reach the sensitive part of the detector  34  located at the downstream side of the detector  34 . As a result, the low-pass filter  30  also acts as an electrostatic reflecting lens. 
     In one implementation, the sample  14  is separated from the annular rim of the grid  16  by 0.85″ (22 mm) and α=25° for an analysis area on the sample having a 3.5 mm diameter. The tubular diameter of the lens  20  is about 2.5″ (64 mm), the diameter of the low-pass filter  30  is about 5.9″ (150 mm), and the total length is about 15.4″ (390 mm). 
     A more detailed structural view of one design of the analyzer  10  is shown in cross section in  FIG. 2 . The lens  20  includes a conically shaped electrode  40  on which a flange  42  of the aspheric mesh  16  is mounted and is electrically connected. The lens  20  further includes a combined conical and tubular electrode  44 , tubular electrodes  46 ,  48  and the independently biased iris  22 . All the electrodes are coaxial about the central axis  12 . The lens  20  tends to focus every electron trajectory  50  having the desired bandpass energy through the aperture  24  of the iris  22 . 
     The high-pass filter  26  includes a tubular entry electrode  52 , conical electrodes  54 ,  56 ,  58 ,  60  of increasing diameters, and a tubular exit electrode  64 , all coaxial about the central axis  12 . A first grid  66  of the dual screen  28  is mounted on a flange  68  on the back of the exit electrode  64  and electrically connected to it. 
     The low-pass filter  30  includes a tubular entry electrode  70  partially inside of which is disposed a separately biasable band-shaped grid electrode  72 . A second grid  74  of the dual screen  28  is mounted on a flange  76  on the front of the entry electrode  70  and is electrically connected to it. The low-pass filter  30  further includes a can-shaped electrode  78  forming the part of the sidewall and part of the back wall of the low-pass filter  30 . A circular back electrode  80  fits within an aperture in the can-shaped electrode  78  and includes a projection  82  towards the detector  34 . The back electrode  80 , the projection  82  in back of the detector  34 , and the nose electrode  32  in front of the detector  34  may be commonly biased. All the electrodes in the low-pass filter  30  are coaxial about the central axis  12  and are biased to optimize the reflected electrons within the passband and also to absorb those of higher energy. 
     The detector  34 , which may be in the form of two micro-channel plates (MCPs), is covered by a wire mesh on the back of a detector housing  86 , which is supported by multiple legs  88  on the back electrode  80 . Electrical lines for the detector  34 , the detector housing, and the nose electrode  32  are led through the interior of the legs  88 . The detector mesh may be held at about 5V to attract low-energy electrons while the detector housing is held at the potential of the second flat grid  74 . Only the electrons having energies less than about 1 eV when they pass through the flat grids  66 ,  74  pass through the wire mesh covering the detector  34 . Other electrons within the low-pass filter  30  strike and are absorbed by the other surfaces. 
     Two potential problems of higher-energy electrons striking the walls of the low-pass filter  30  is that they simply reflect rather than be absorbed or that they emit secondary electrons of lower energy which are then detected out of band. These problems can be reduced by a corrugated electrode structure illustrated in the cross-sectional view of  FIG. 3 . An electrode  200  or other wall of the filter is formed with teeth  202  and intervening grooves  204  on the side facing the interior of the filter and extending in the direction perpendicular to the illustration. The pitch and depth of the teeth  202  and grooves  204  is on the order of 1 mm, for example, 0.2 to 5 mm, and an aspect ratio of the grooves of at least 1. Preferably, the width of the grooves  204  is greater than that of the teeth  202 . Thereby, when a primary electron  206  strikes the sidewall of the groove  204 , its reflected trajectory will be towards other walls of the corrugated electrode  200  or secondary electrons  208  are likely to be emitted at angles such that they are absorbed by other walls within the grooves  204 . That is, neither the reflected primaries nor the secondaries are likely to reenter the body of the filter to be detected as low energy electrons. The wall structure is not limited to the illustrated rectangular corrugation. For example, as illustrated in the cross-sectional view of  FIG. 4 , an electrode  210  may be formed with a serrated edge having triangular protrusions  212 , preferably having the pitch and depth previously mentioned and preferably having acute apexes  214  with angles of less than 90° and preferably less than 60°. Other shapes are possible. The corrugated texturing produced by the teeth  202  and grooves  204  or triangular protrusions  212  result in a textured surface having portions of differing heights. The corrugated texturing is advantageously applied to the electrodes  70 ,  72 ,  78 , and  80  within the low-pass filter  30  in an axisymmetric pattern about the central axis  12 . 
     Returning to  FIG. 2 , the incident electron trajectory  50  is schematically illustrated as terminating either in a low-energy reflected trajectory  90  from the back of the high-pass filter  26  or a high-energy trajectory  92  absorbed by one of electrodes of the low-pass filter  30  or the detector housing  86 . Only a passband trajectory  94  incident on the mesh of the detector  34  is detected. 
     The analyzer  10  is preferably enclosed in a magnetic shroud to exclude any extraneous magnetic field from affecting the low-energy electron trajectories. For similar reasons, all screws and other analyzer parts should be non-magnetic. The very low electron energies require that the analyzer be enclosed in a non-magnetic, e.g. aluminum, vacuum housing maintained at a very low pressure, for example, no greater than 10 −8  ton. The sample  14  may be inserted within the vacuum housing and then the entire vacuum housing pumped down to the requisite pressure. Alternatively, an electron transmissive vacuum window described by Bryson et al in U.S. Pat. No. 6,803,570 may be interposed between the entry end of the analyzer  10 , specifically the aspheric grid  16 , and the sample held at a somewhat higher pressure. 
     An analyzer system  110  illustrated in the cutaway orthographic view of  FIG. 5  includes a vacuum housing  112  pumped by a turbo pump  114 . A magnetic shield  116  is interposed between the vacuum housing  112  and an aluminum shell on which the analyzer electrodes are supported and accurately aligned. Multiple electrical vacuum feedthroughs  118  provide biasing power to the electrodes and the detector  34  and pass out the detected signal. The vacuum housing  112  and magnetic shield  116  as well as the analyzer  10  itself are generally coaxial about the central axis. In the illustrated embodiment, the sample  14  is fixed to a pedestal  120  inside the vacuum housing  112  but the analyzer system  110  may be adapted to other types of sample handling or external sample chambers. This embodiment is designed for x-ray photoelectron spectroscopy (XPS) so two X-ray sources  122 ,  124  may irradiate the sample  14  to produce photoelectrons whose energy is analyzed by the energy analyzer  10 . The analyzer system  110  may be adapted to other types of excitation sources such as high-energy electron guns. 
     One embodiment of the electrical circuitry associated with the analyzer  10  is schematically illustrated in  FIG. 6 . A computer controlled DC power supply  130  operating, for example, between 0.3 and 1.6 kV controls the scan voltage delivered to the lens  20  through a resistor chain. A variable resistor  132  controls the focusing. A floating DC power supply  136  for the high-pass filter  26  applies a fixed between the iris  22  and the dual screen  74  of either its 200V or 1000V outputs connected through a toggle  138  to the output of the scan voltage supply  130  so that electrons passing through the iris  22  with a voltage less than 200 or 1000 eV cannot pass through the screen  74 , but higher-energy electrons can. The toggle  138  determines whether the analyzer is being operated with low or high resolution. The voltage across the high-pass power supply  136  is distributed to the electrodes of the high-pass filter  26  through another resistor chain. Another DC power supply  140 , for example, operating at 6V, controls the voltage applied to the low-pass filter electrodes distributed by a third resistor chain and to the detector  34 . Another toggle  142  operating in tandem with the first toggle  138  determines the resolution. The voltage supplied to the dual screen  74  may be the same between the two screens or, as will be explained with reference to  FIG. 7 , differential voltages may be applied between them. 
     A high-voltage DC detector power supply  146 , for example, operating at 2 kV but referenced to the low-pass power supply  140 , powers the photo-multiplier tube. The electron signal is tapped from the line between the detector power supply  146  and the micro-channel plate detector  34  and is led to a controller  150  in the exterior through an AC coupling capacitor  152 . The controller  152  controls the scan of the lens power supply  130  and hence the energy scan of the electron analysis and stores the electron current measured by the detector  34  as a function of the scanning voltage to produce the desired energy spectrum. The controller  152  is typically a computer and includes a memory  154  which contains the control program and settings for the analyzer and which records the values of detected current in synchronism with the variation of the scan voltage of the lens power supply  130  to thereby produce an energy spectrum of the charged particles. However, other memory devices are possible to record the data including visual spectrum displays and strip recorders. 
     The dual screen  74  is at a potential at which electrons in the passband have an energy between 0 and 1 eV as they enter the low-pass filter  30 . The high-pass filter  26  is advantageously operated at a fixed energy (1000 eV in the standard mode) so the electrons entering the high-pass filter  26  are within a fixed energy band, for example, 1000 to 1001 eV. The desired energy translation is accomplished by tying the negative side of the negative side of the high-pass power supply  136  to the dual screen  74  and the positive side to the iris  22 . 
     As was previously briefly explained, the low-pass filter  30  is not completely effective and for one reason or another high-energy electrons may reach the detector  34 . As illustrated in  FIG. 7 , if a toggle  156  is set to select a tie connection so that both grids  66 ,  74  of the dual screen  28  are equally biased from an input terminal, an electron beam  160  passing through the dual screen  28  consists of most of the electron above the energy E 1  at the lower edge of the passband. A measured transmission spectrum  162  is represented in  FIG. 6 . However, if the toggle  156  selects a voltage supply  158  to negatively bias the first grid  66  with respect to second grid  74  during a calibration mode to reflect energies within the passband so that an incident beam  164  should be totally reflected. Nonetheless, the combination of the detected desired lower-energy electrons and the detected higher-energy electrons, for instance some that are not rejected in the low-pass filter  30 , and secondary electrons emitted from the electrodes in the low-pass filter  30 , produce a measured transmission spectrum  166  during the calibration mode. Accordingly, in a calibration mode, the two grids  66 ,  74  are differentially biased and a electron spectrum is measured by scanning the voltages on the lens  20 . The differential biasing, which may be accomplished by applying different voltages to the exit electrode  66  of the high-pass filter  26  and to the input electrode  70  of the low-pass filter  30 , may be somewhat more than the expected resolution of the analyzer, that is, about 1V or perhaps twice that. Other means of selective differential biasing are possible including separate power supplies or a selected element in a resistive chain. The measured spectrum is a background or noise spectrum. In the normal mode, the grids  66 ,  74  are commonly biased. Values of the calibration spectrum are stored and then subtracted, for example, in the controller  150  from corresponding values on the spectrum measured in the normal mode to produce a corrected spectrum. 
     The graphs of  FIG. 9  display the calculated transmission or throughput for the filters of an analyzer of the invention with the understanding that the lens accelerates or retards sample electrons of the desired energy to about 1000 eV and presents them to the sequentially arranged filters. The first, high-pass filter, as shown in spectrum  170 , passes virtually no electrons up to an energy of 999 eV. Thereafter, the transmission coefficient rapidly rises over about 1 eV to near unity. On the other hand, the second, low-pass filter, as shown in spectrum  172 , passes most electrons up to just above 999 eV. Thereafter, the transmission coefficient falls over about 1 or 2 eV to nearly zero. The total transmission, which is the product of the two spectra  170 ,  172 , is shown in spectrum  174 . The transmission FWHM passband is about 0.9 eV. However, if the background spectrum is subtracted from the total spectrum, the correct transmission, shown by a corrected spectrum  176 , shows a passband of about 0.7 eV. 
     Although the invention has been developed as an electron analyzer, with proper scaling the invention may be applied to energy analyzers of other charged particles, such as positively charged ions. 
     The analyzer of the invention is capable of relatively high resolution in a small and lightweight structure. Nonetheless, the sensitivity or throughput may be ten times greater than that of the conventional analyzer in a laboratory-quality XPS. The coaxial design reduces the complexity and fabrication costs and also reduces the weight and size of the analyzer. Nonetheless, the analyzer can be made relatively rugged.