Abstract:
An electron multiplier having an access for allowing a beam to pass is presented. The electron multiplier collects particles traveling back along the beam and is capable of collecting the particles arbitrarily close to the beam. The electron multiplier includes at least two plates having secondary electron emitting surfaces, the at least two plates being separated by a small distance. The electron multiplier has a beam access through the at least two plates. Particles enter the electron multiplier in a direction opposite that of propagation of the beam and impact a secondary electron emitting surface, thereby being captured between the top plate and the bottom plate. In some embodiments of the invention, the electron multiplier is segmented so that azimuthal distributions of the particles can be determined. In some embodiments, the electron multiplier includes a stack of electron emitting surfaces arranged so that an angular distribution of the particles can be determined.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to electron multiplier devices and, in particular, to electron multiplier devices for detection of particles emitted from a surface as a result of an incident beam impacting the surface. 
     2. Related Art 
     Electron multipliers are useful tools for various applications, including the detection of photons, electrons, ions and heavy particles. Such detectors are utilized in various spectroscopic techniques, including Auger electron spectroscopy (AES), x-ray photoelectron spectroscopy, ultraviolet photoelectron spectroscopy, and electron energy loss spectroscopy. Further, electron multipliers may be utilized for detection of secondary and back-scattered electrons in scanning electron microscopes, focused ion-beam tools, or e-beam lithography tools. 
     In general, electron multipliers have had two configurations, the channeltron multiplier or multi-channel plate multiplier. FIG. 1 shows a parallel plate electron multiplier  100  as described in numerous publications, including S. Suzuki and T. Konno, “A Computer Simulation Study On the Detection Efficiencies of Parallel-Plate Electron Multipliers,” Sci. Instrum. 66 (6), p. 3483-87 (June, 1995); and L. P. Andersson, E. Grusell and S. Berg, “The Parallel-Plate Electron Multiplier,” J. Phys. E: Sci. Instrum., Vol. 12, p. 1015-22 (1979). 
     Electron multiplier  100  includes secondary emitting surfaces  101  and  102 , deposited on glass plates  111  and  112 , respectively, and separated by a spacing  104 . A voltage V d  is applied along the length of electron multiplier  100  so that electrons entering at an open end  105  are accelerated along the length of electron multiplier  100  away from open end  105 . When the electron collides with one of secondary emitting surfaces  101  and  102 , multiple secondary electrons are emitted. The secondary electrons are then accelerated along electron multiplier  100  and themselves may collide with one of secondary emitting surfaces  101  and  102 . On each collision of an electron with sufficient kinetic energy with one of emitting surfaces  101  or  102 , further electrons are emitted. By repeated collisions of electrons with secondary emitting surfaces  101  and  102 , an output pulse containing a very large number of electrons is emitted from electron multiplier  100 . 
     The output pulse is received by collector  103  located on the side of electron multiplier  100  opposite from open end  105 . Typically, collector  103  is held at an elevated voltage from the voltage of that end of electron multiplier  100 . The output pulse is detected by detection circuitry  106  coupled to collector  103 . The gain of electron multiplier  100  depends on the voltage V d  applied across electron multiplier  100 , the secondary emission properties of secondary emitting surfaces  101  and  102 , and the physical dimensions of electron multiplier  100 . 
     In some electron multipliers such as electron multiplier  100 , further voltages are applied to either end of one of secondary emitting surfaces  101  and  102 . In such cases, electric fields can be created that are not parallel with the length of electron multiplier  100 , thereby enhancing collisions with one of secondary emitting surfaces  101  or  102 . Further, collector  103  may be tilted (i.e., the collector surface may not be perpendicular to the surfaces of secondary emitters  101  and  102 ) in order to further enhance collection of output pulses of electrons and to further supply a component of the electric field not parallel with electron multiplier  100 . 
     Some electron multipliers may be constructed from a glass tube coated with a secondary emitting surface. The resulting multiplier, in principle, operates as described above for parallel plate electron multiplier  100  except that, instead of parallel plate secondary emission surfaces, the secondary emission surface is cylindrical in shape. The tubular channeltron multiplier has the advantage that, because of its tubular nature, it can be shaped into loops and spirals that reduce its overall size without affecting the overall length of the multiplier. 
     However, each of these multipliers are difficult to use in certain environments. For example, in some instances, such as in lithography or in electron microscopes, it is difficult to detect reflected electrons that are close to an incident electron beam. In some applications, it is desirable to collect electrons from as close to the incident beam as possible. With electron multiplier  100  or the tubular channeltron multiplier, positioning of the opening surface can be difficult. 
     Therefore, there is a need for an electron multiplier that is easily constructed, of small size, and capable of monitoring the particles close to a beam incident on a surface that emanate from the surface. 
     According to the present invention, an electron multiplier capable of detecting particles such as, for example, ions, photons, or electrons, traveling close to an incident beam is presented. The electron multiplier includes a top plate and a bottom plate separated by a small gap. Each of the top plate and the bottom plate includes an access through which an incident beam can pass. The accesses of the top plate and the bottom plate are aligned so that the incident beam can pass through the electron multiplier. Particles traveling close to the incident beam, and in a direction opposite that of propagation of the incident beam, can enter the electron multiplier between the top plate and the bottom plate and thereby be detected. 
     The top and bottom plates each have a secondary electron emitting surface. The secondary electron emitting surface of each of the top and bottom plate emit electrons when the surface is impacted with a particle of sufficient energy. Further, each of the top and bottom plates are resistive so that a current can flow through them. Finally, in most embodiments, the top and bottom plates provide structural support for the secondary emitting surfaces. 
     In some embodiments, the top and bottom plates can be a single material, for example lead oxide glass, bismuth oxide glass, or iron borate glass. These materials are resistive, provide a secondary emitting surface, and provide structural support. In another embodiment, each of the top and bottom plates can include secondary electron emission layers, for example CVD diamond or an alkali halide, deposited on a resistive layer, for example a metal or low resistance semiconducting layer, deposited on a structural substrate, such as glass. 
     In one embodiment, the top plate and the bottom plate have an annular geometry. The top secondary emitting surface has an outside radius and an access with an inside radius, and the bottom secondary surface has an outside radius and an access with an inside radius. In another embodiment, the secondary electron emitting surface of the top and bottom plates have annular geometry. The access allows an incident beam to pass through the top plate, with the top secondary emitting surface, and the bottom plate, with the bottom secondary emitting surface, without impacting the electron multiplier. 
     The references to top and bottom or up and down in this disclosure is with reference to the direction of propagation of an incident beam. Bottom or down refers to a direction closest to a surface on which the incident beam is incident. Top or up refers to the opposite direction from bottom or down. 
     In most embodiments the outside radius of the top secondary emitting surface and the outside radius of the bottom secondary emitting surface are about the same. In some embodiments the inside radius of the access of the top is less than the inside radius of the access of the bottom secondary emitting surface so that particles (e.g., electrons, ions, or photons) are easily captured into the multiplier. 
     The access through the annular geometry, through which a beam can pass, has the inside radius of the top secondary emitting surface at the top plate and the inside radius of the bottom secondary emitting surface at the bottom plate of the electron multiplier. In most embodiments, the access is arranged to be at the center of the annular geometry of the electron multiplier. 
     Voltages applied to electrodes coupled to the top plate and the bottom plate at the inside and outside radiuses of the top secondary emitting surface and the bottom secondary emitting surface provide a radial electric field in the annular electron multiplier. A collector is arranged around the outside radius of the top secondary emitting surface and the bottom secondary emitting surface to collect any burst of electrons emitted from the electron multiplier. 
     In some embodiments of the invention, the annular collector is segmented. Each segment of the annular collector is coupled to detection electronics in order to measure the angular distribution of particles received into the multiplier. Further, in some embodiments the secondary electron emitting surfaces of the top and bottom plates are segmented in order to enhance the measurements of the azimuthal distribution. Embodiments that segment the secondary electron emitters may have the advantage of reducing distortions of measurements of the angular distribution. 
     In some embodiments of the invention, the electron multiplier includes several stacked plates, each with secondary electron emitting surfaces, each separated by a small distance. Each of the stacked plates has an access with a different inner radius and are arranged in order of the size of the inner radius. In most embodiments, the plate with the largest inner radius is on the bottom (i.e., closest to the surface on which the incident beam impacts) and the plate with the smallest inner radius is on the top. Plates having inner radiuses of various sizes are arranged accordingly between the top plate and the bottom plate. Embodiments of this type provide the ability to measure the angular distribution of particles emanating from the surface. Those particles having the smallest angle from the incident beam impact on the top secondary emitting surface while those particles with larger angles impact on other secondary emitting surfaces of the stacked plates in the electron multiplier. 
     In some embodiments of the invention, the resistivity of the plates can be varied radially in order to affect the electric field. Further embodiments may radially adjust the separation between plates, causing the secondary electron emitting surfaces to be, for example, conically or terraced shaped. Further, the electric field may be tilted (i.e., have components perpendicular to the radial direction) by adjusting the voltages supplied to the various electrodes and the bias voltage of the annular collector and tilt of the annular collector. 
     These embodiments, along with others, are further discussed below with reference to the following figures. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1 shows a cross-sectional diagram of an electron multiplier. 
     FIG. 2 a  shows a cross-sectional diagram of an electron multiplier according to the present invention. 
     FIG. 2 b  shows a cross-sectional diagram of a plate utilized in an electron multiplier as shown in FIG.  2 A. 
     FIGS. 3 a  and  3   b  show top and bottom planar views of an electron multiplier according to the present invention. 
     FIGS. 4 a  and  4   b  show top and bottom planar views of an electron multiplier according to the present invention constructed on a glass substrate. 
     FIG. 4 c  shows a cross-sectional view of the electron multiplier shown in FIGS. 4 a  and  4   b.    
     FIGS. 5 a  and  5   b  each shows a plan view of another embodiment of an electron multiplier having a segmented collector electrode. 
     FIG. 6 shows a cross-sectional diagram of a stacked electron multiplier according to the present invention. 
     FIG. 7 shows a cross-section view of an electron microcolumn having an electron multiplier according to the present invention. 
    
    
     DETAILED DESCRIPTION 
     FIG. 2 a  shows a cross-sectional view of an electron multiplier  200  according to the present invention. Electron multiplier  200  includes a top plate  201  having an access  213  with an inside radius r 1  and having an outside radius r 2 , a bottom plate  202  having an access  214  with an inside radius r 3  and having an outside radius r 4 . Top plate  201  and bottom plate  202  are separated by a distance s, forming a separation space  208 , and arranged such that access  213  is aligned with access  214 . 
     An electrode  203  is located around access  213  of top plate  201  and an electrode  205  is located along the outside radius of top plate  201 . An electrode  204  is located around access  214  of electron emitter  202  and electrode  206  is located around the outside radius of electron emitter  202 . Collector  207  is positioned around electron multiplier  200  at a radius about that of electrodes  205  and  206 . A line  215 , which defines the path of an incident beam through electron multiplier  200 , passes through both access  213  and  214 . 
     In some embodiments of the invention, the separation between top plate  201  and bottom plate  202 , s(r), varies as a function of radius from line  215 . In those embodiments, plates  201  and  202  can, for example, be conically shaped or terraced. Further, in some embodiments, the resistivity ρ(r) of plates  201  and  202  may vary as a function of radius. Also, in some embodiments collector  207  may be “tilted” at an angle θ with respect to a normal to a plane of electron multiplier  200 , providing a component of the electric field that is perpendicular to the radial direction in multiplier  200 . 
     Additionally, the separation s can be maintained with spacers  220  that are placed to hold plates  201  and  202  apart. In some embodiments, collector  207  forms a portion of spacer  220 . 
     The inside walls and outside walls of top plate  201  and bottom plate  202  need not be circular. They can be of any shape and are described as circular here for demonstrative purposes only. One skilled in the art will recognize that other geometries (e.g., square or elliptical) are within the scope and spirit of this disclosure. 
     FIG. 3 a  shows a plan view of the top of electron multiplier  200 . Top plate  201  in the embodiment shown in FIG. 3 a  is an annular surface having access  213  with inner radius r 1  and having outer radius r 2 . Electrode  203  is coupled to top plate  201  at radius r 1  around access  213  and electrode  205  is coupled around top plate  201  at radius r 2 . Collector  20  electrode  207  is arranged around top plate  201  in order to collect the output burst of electrons from electron multiplier  200 . In some embodiments, top plate  201  may extend beyond collector  207  so that collector  207  functions as part of spacer  220  (FIG. 2 a ) or spacer  220  may be inserted outside the radius of collector  207 . 
     FIG. 3 b  shows a plan view of the bottom of electron multiplier  200 . Bottom plate  203  in FIG. 3 a  is an annular surface having access  214  with inner radius r 3  and having outer radius r 4 . In most embodiments, outer radius r 2  of top electron emitter  201  is about r 4 . In some embodiments inner radius r 1  of access  213  is less than that of access  214 , r 3 . Therefore, FIG. 3 b  also shows top plate  201  and electrode  203 . Electrode  204  is coupled to bottom plate  202  at radius r 3  and electrode  206  is coupled to bottom plate  202  at radius r 4 . 
     Plates  201  and  202  (FIG. 2 a ) each have a secondary electron emitting surface, surfaces  210  and  211  respectively, capable of emitting one or more electrons when impacted with particles of sufficient energy. Each of plates  201  and  202  also provides structural support for electron multiplier  200 . Further, plates  201  and  202  are conducting so that current can flow through plates  201  and  202 . 
     FIG. 2 b  shows a portion of a plate  230  such as top plate  201  or bottom plate  202 . Plate  230  includes structural portion  231 , a resistive portion  232 , and a secondary electron emitting surface  233 . Plate  230  is oriented in electron multiplier  200  such that secondary emitting surface  233  is on the inside. For example, in FIG. 2 a , top plate  201  includes structural portion  209  and secondary emitting surfaces  210 ; bottom plate  202  includes structural portion  212  and secondary emitting surface  211 . 
     In some embodiments, plate  230  may be a single material that include structural portion  231 , resistive portion  232 , and secondary emitting surface  233 . For example, plate  230  may be lead oxide glass, bismuth oxide glass, or iron borate glass. In some embodiments, plate  230  may include separate layers for one or more of structural portion  231 , resistive portion  232 , and secondary electron emitting surface  233 . For example, secondary electron emitting surface  233  may be a diamond layer or an alkali halide layer. Resistive portion  232  may be a metal layer or a low resistivity semiconducting material (such as Si) or an insulating high resistivity material (such as Silicon Nitride). 
     In one embodiment, separation space  208  may be filled with glass spheres. The glass spheres may be, for example, of lead oxide glass, bismuth oxide glass, or iron borate glass. The glass spheres, then, provide secondary electron emitting surface  233  and spacer  220  (FIG. 2 a ). One advantage of having secondary electron emitting surface  210  and  211  in FIG. 2 a  being glass spheres is that ion feedback can be reduced. 
     A radial electric field is created within electron multiplier  200  by applying voltages at electrodes  203 ,  205 ,  204  and  206 : A voltage V 1  is applied to electrode  203 ; a voltage V 2  is applied to electrode  205 ; a voltage V 3  is applied to electrode  204 ; and a voltage V 4  is applied to electrode  206 . An anode voltage V A  is applied to collector  207 . 
     Many embodiments of electron multiplier  200  include two annular plates  201  and  202  separated by a small distances. The two annular plate  201  and  202  have similar outer diameters but different inner diameters at access  213  and  214 , respectively. Annular plate  202 , having a larger inner diameter than annular plate  201 , is arranged to be closer to the source of particles to be detected. In operation, the particles to be detected will pass through access  214  of annular plate  202  and a significant number of them strike the secondary electron emitting surface of top plate  201 . If the particle is of sufficient energy, multiple secondary electrons may be emitted by secondary electron emitter  201  as a result of the particle&#39;s impact. 
     Voltage V 2  is greater than voltage V 1  and voltage V 4  is greater than voltage V 3 , creating electric fields having radial components. Electrons are multiplied as they collide with the secondary electron emitting surfaces of plates  201  and  202  and are accelerated radially toward collector  207 . Collector  207  is biased with a voltage V A  that is greater than the voltages V 2  and V 4  so that it attracts electrons. Detection electronics  220  detects bursts of electrons at collector  207 . 
     The annular geometry of embodiments of electron multiplier  200  is highly suited for detection of secondary and back-scattered electrons in, for example, a scanning electron microscope or e-beam lithography tool. Specifically, detector  200  can be a thin package with an input area arbitrarily close to an input beam along line  215 . The thickness of multiplier  200  reduces the column length occupied by the detector. This reduction in thickness is particularly important in micro-columns, in columns where electron multiplier  200  is placed between deflectors and the final lens, and in systems with an external electron multiplier  200  where a short working distance is desired. Moving the entrance of the detector close to the optic axis facilitates detection of secondary and back-scattered electrons that are inherently close to the axis or that have been collimated by a lens or an extraction field. In some embodiments, collector  207  is segmented and discrete electronics  220  is provided for each segment, facilitating the measurement of the azimuthal distribution of the incoming particles. In some embodiments, electron multiplier  200  may be completely segmented (i.e., secondary electron emitting surfaces of plates  201  and  202  may be segmented) in order to improve the measurement of the azimuthal distribution of the incoming particles. 
     Additionally, unlike channeltrons or other parallel plate multipliers, the resistance, size, shape and spacing of plates  201  and  202  may be easily changed, allowing for optimization of multiplier gain, maximum output current and bandwidth. 
     The annular geometry of this embodiment of electron multiplier  200  also yields other inherent advantages. Assuming a uniform resistivity of plates  201  and  202 , the resistance per unit of radial distance will be higher near line  215  and lower at the outer radius. Therefore, there will be a stronger electric field near the incident beam, resulting in the gain being larger during the earlier stages of multiplication. High gain at the early stages of multiplication is important for lowering the statistical contributions to the signal to noise ratio. As the pulse propagates radially outward, the field will decrease, and the charge will spread more in the azimuthal direction, decreasing the charge density and delaying the onset of saturation effects. If plates  201  and  202  are fabricated using a thin film emitter material (dynode) and an insulator (support portion  231  in FIG. 2 b ), the film thickness could be varied to further optimize the electric field. The dynodes, which may include resistive portion  232  and secondary electron emitting surface  233  (FIG.    2   b), can then be patterned to make discrete sectors that might confine the current pulses, resulting in more accurate measurements of the azimuthal distribution. 
     The gain of electron multiplier  200  can be optimized by adjusting the outer radiuses of the secondary electron emitting surfaces of plates  201  and  202 . Additionally, the separation between plates  201  and  202  s(r) can be varied to compensate for the radial field dependence. Plates  201  and  202  can, for example, then be conical in shape or terraced. Also, the radial resistance ρ(r) of each of electron emitters  201  and  202  can be varied to provide a more uniform radial electric field. Additionally, the electric field can be tilted at an angle with respect to plates  201  and  202  to achieve the same gain one would achieve with an optimum plate separation. This tilt may be varied in the radial direction with independent biasing of the four electrodes  201 ,  205 ,  204  and  206 . Tilting collector  207  at an angle can also contribute normal components of the electric field. 
     In one particular embodiment, r 1  is about 0.68, r 2  is about 2.5 mm, s(r) is about 0.60 mm, r 3  is about 1.25 mm, and r 4  is about 2.5 mm. With V 4  about 0 V, V 2  about 2000V, V 3  about 0 V, and V 4  2000V electron multiplier  200  has a gain of about 2100. 
     FIGS. 4 a  and  4   b  show bottom and top planar views, respectively, of an electron multiplier  400  according to the present invention. FIG. 4 c  shows a cross-sectional view of election multiplier  400  along direction a as indicated in FIGS. 4 a ,  4   b , and  4   c . Electron multiplier  400  includes plate  401   a  having secondary electron emitting surface  402  (FIG. 4 a ) and plate  401   b  having secondary electron emitting surface  408  (FIG. 4 b ). 
     In FIGS. 4 a  and  4   c , plate  401   b  and secondary electron emitting surface  402  has an access  416  with an inner radius that is larger than that of access  417  of plate  401   a  and secondary electron emitting surface  408 . Electrode  404 , which is coupled to the inside radius of secondary electron emitting surface  402 , is electrically coupled to a pad  405 . Voltage V 3  can be applied to electrode  404  through pad  405 . Access  407 , in FIG. 4 a  a small pie-shaped area where secondary electron emitting surface  402  is not deposited, provides the ability to electrically couple electrode  404  with pad  405 . Electrode  403  is coupled around the outer radius of electron emitting surface  402  and is electrically coupled to pad  406 . A voltage V 4  can be applied to electrode  403  through pad  406 . 
     In FIGS. 4 b  and  4   c , secondary electron emitting surface  408 , and plate  401   a , has an access  417  with an inner radius that is smaller than that of secondary electron emitter  402 . Additionally, secondary electron emitting surface  408  and secondary electron emitting surface  402  are arranged coincidentally so that their centers are at the same position, providing access to an incident beam traveling through electron multiplier  400  normal to the surface of plates  401   a  and  401   b.    
     In most embodiments, access  407  also aligns with access  413 . Electrode  409  is coupled along the inside radius of electron emitting surface  408  and is electrically coupled to a pad  415  through access  413 . A voltage V 1  can be applied to electrode  409  through pad  415 . Electrode  410  is coupled around the outside radius of electron emitting surface  408  and is electrically coupled to pad  414 . A voltage V 2  can be applied to electrode  410  through pad  414 . 
     Collector  411  is arranged around the outside of electron emitting surface  408  and  402  and is electrically coupled to pad  412 . A bias voltage can be applied to collector  411  through pad  412 . Furthermore, burst signals indicating detection of particles is measured by detection electronics at pad  412 . Furthermore, as is shown in FIG. 4 c , collector  411  can also provide spacing between plates  401   a  and  401   b.    
     The embodiment shown in FIGS. 4 a ,  4   b , and  4   c  can be fabricated using micromachining techniques. Electrodes such as electrodes  404 ,  403 ,  409 ,  410  and collector  411 , interconnects, resistive layers secondary emitting surfaces  402  and  408  of plates  401   a  and  401   b , respectively, can be patterned lithographically on insulating substrates. 
     Plates  401   a  and  401   b  include a substrate  430  and  431 , respectively, which can be an insulator. Secondary electron emitting surfaces  402  and  408  are then deposited on substrates  430  and  431 , respectively. Electron emitting surfaces  402  and  408  may include a resistive layer and a secondary electron emitting layer or the resistive layer may also serve as a secondary emitting layer. 
     FIGS. 5 a  and  5   b  show planar views, respectively, of another electron multiplier  500  according to the present invention. Electron multiplier  500  includes a collector  504  having N collector segments  504 - 1  through  504 -N. Electron multiplier  500  further includes a secondary electron emitting surface  501  (FIG. 5 a ) having an access  510  with an inner radius and having an outer radius. Electrode  502  is arranged around the inner radius of secondary electron emitting surface  501 . Electrode  503  is arranged around the outer radius of secondary electron emitting surface  501 . Secondary electron emitting surface has an access  511  with inner radius and an outer radius. Electrode  506  is arranged around the inside of secondary electron emitting surface  505 , and electrode  507  is arranged around the outside of secondary electron emitting surface  505 . As has been previously described, secondary electron emitting surfaces  501  and  505  are separated by a small separation, which may vary as a function of radius. Secondary electron emitting surfaces  501  and  505  are portions of plates  520  and  530 , respectively, as has been previously discussed, plates  520  and  530  include a structural portion and resistive portion as well as secondary emitting surfaces  501  and  505 . 
     Collector  504  is segmented into collector segments  504 - 1  through  504 -N in order to allow measurements of the azimuthal distribution of particles entering electron multiplier  500 . Each of collector segments  504 - 1  through  504 -N can be separately biased and monitored by detection electronics. Measurement of an electron burst on one of collector segments  504 - 1  through  504 -N, then, is indicative of a particle entering electron multiplier  500  in a direction toward that one of collector segments  504 - 1  through  504 -N. The number of collector segments N can be as large as needed to yield a desired resolution for measurements of the azimuthal distribution of incoming particles. 
     In addition to collector  504  being segmented, electron multiplier  500  may be completely segmented into segments  500 - 1  through  500 -N. Segmenting electron multiplier  500 , for example, will reduce cross-talk between segments and improve the precision of the azimuthal distribution measurements. Further, segmenting electron multiplier  500  allows for independent control over each of segments  500 - 1  through  500 -N. 
     Each of segments  500 - 1  through  500 -N is arranged to include a corresponding one of collector segments  504 - 1  through  504 -N, respectively. In FIG. 5 a , secondary electron emitting surface  501  includes electron emitting surface  501 - 1  through  501 -N aligned with collector segments  504 - 1  through  504 -N. Electrode  502  is arranged around the inner radius of secondary electron emitting surface  501 . A separate voltage can be applied to each segment of electrode  502 , electrodes  502 - 1  through  502 -N. Electrode  503  is arranged along the outer radius of electron emitting surface  501 . A separate voltage can also be applied to each electrode segment  503 - 1  through  503 - 2  of electron emitter  503 . 
     FIG. 5 b  shows a corresponding planar view of a bottom portion of electron multiplier  500 . Secondary electron emitting surface  505  includes secondary electron emitting surface segments  505 - 1  through  505 -N. Secondary electron emitting surface segments  505 - 1  through  505 -N are arranged such that electron emitting surface segment  505 - 1  is directly below, and separated from, electron emitting surface segment  501 - 1  of FIG. 5 a . Electrodes  506 - 1  through  506 -N are arranged along the inner radius of secondary electron emitting surfaces  505 - 1  through  505 -N, respectively. Additionally, electrodes  507 - 1  through  507 -N are arranged along the inner radius of secondary electron emitting surfaces  505 - 1  through  505 -N, respectively. 
     Therefore, electron multiplier segment  500 -j, an arbitrary one of electron multiplier segments  500 - 1  through  500 -N, includes secondary electron emitting surface  501 -j and secondary electron emitting surface  505 -j directly beneath secondary electron emitting surface  501 -j. Electrodes  502 -j and  503 -j are arranged along the inside and outside respectively, of secondary electron emitting surface  501 -j. Electrodes  506 -j and  507 -j are arranged along the inside and outside respectively, of secondary electron emitting surface  505 -j. Electron multiplier segment  500 -j is aligned with collector segment  504 -j. Electrons entering electron multiplier segment  500 -j are accelerated radially towards collector segment  504 -j. Each collision with one of secondary electron emitting surfaces  501 -j or  505 -j results in a multiplication of electrons so that collector segment  504 -j can receive a pulse of electrons in response to the incoming electron. 
     FIG. 6 shows a cross-sectional view of a stacked electron multiplier  600 . Electron multiplier  600  includes plates  601 - 1  through  601 -K. Each of plates  601 - 1  through  601 -K has an access  607 - 1  through  607 -K with inner radius r I   (1)  through r I   (K) , respectively. Further, each of plates  601 - 1  through  601 -K has an outer radius r o   (1)  through r o   (K) , respectively. Plates  601 - 1  through  601 -K may each include resistive portions, structural portions, as well as a secondary electron emitting surfaces. In FIG. 6, each of electron emitters  601 - 2  through  601 -(K- 1 ) include structural and resistive portions sandwiched between two secondary electron emitting surfaces. Additionally, in some embodiments the resistivity of each of plates  601 - 1  through  601 -K may vary as a function of radial distance from a common line  606 . 
     Plates  601 - 1  through  601 -K are stacked such that the centers of access  607 - 1  through  607 -K lie along common line  606 . In most cases, common line  606  is normal to the surfaces of plates  601 - 1  through  601 -K. In many embodiments, plates  601 - 1  through  601 -K are stacked in order of increasing radius such that r I   (1)  is the smallest radius and r I   (K)  is the largest radius (i.e., r I   (1) &lt;r I   (2) &lt;•••&lt;r I   (K) ). Further, in many embodiments the outer radii of plates  601 - 1  through  601 -K are substantially the same (i.e., r o   (1) ≈r o   (2) ≈•••≈r o   (K) ). 
     Plates  601 - 1  through  601 -K are separated by a small distance, which may not be the same for each sequential pair of plates  601 - 1  through  601 -K. Further, the separation between pairs of plates  601 - 1  through  601 -K may vary as a function of radius from common line  606 . The separation between adjacent plates forms a spacing  605 - 1  through  605 -(K 1 ). Plates  601 - 1  through  601 -K in many embodiments are structurally separated by spacers (not shown), which may include collectors  602 - 1  through  602 -(K- 1 ). 
     Collectors  602 - 1  through  602 -(K- 1 ) are arranged to collect bursts of electrons from between each sequential pair of plates. For example, collector  602 - 1  is arranged in spacing  605 - 1 . Further, electrodes  604 - 1  through  604 -K are arranged around the inside radius of the secondary electron emitting surfaces of plates  601 - 1  through  601 -K, respectively, and electrodes  603 - 1  through  603 -K are arranged around the outside of the secondary electron emitting surfaces of plates  601 - 1  through  601 -K, respectively. Radial electric fields for accelerating electrons towards each of collectors  602 - 1  through  602 -(K- 1 ) are arranged by applying the appropriate voltages to electrodes  603 - 1  through  603 -K and  604 - 1  through  604 -K. 
     By stacking plates to form electron multiplier stacks (e.g., the combination of plates  601 - 1  and  601 - 2  forms an electron multiplier; the combination of plates  601 - 2  and  601 - 3  forms an electron multiplier, as does any consecutive pair of plates  601 - 1  through  601 -K), the angle of a particle emitted from a surface relative to the incident beam can be measured. In most embodiments, the incident beam is along common line  606  and propagates from plates  601 - 1  through  601 -K. The stacked electron multiplier  600  can also be segmented, as described for embodiments of electron multiplier  500  of FIGS. 5 a  and  5   b , to further measure the azimuthal distribution of particles reflected from a surface impacted by the incident beam. 
     FIG. 7 shows in a cross-sectional view an electron multiplier  712  according to this invention mounted within a micro-column  700 . Micro-column  700  is contained within an evacuated chamber (not shown). The substrate of photocathode  710  may suffice as a vacuum window allowing a laser light source onto the irradiation regions of photocathode  710  or alternatively photocathode  710  may be fully enclosed in the vacuum chamber. Electron beams  711  are emitted from the emission region of photocathode  710  and are accelerated through anode  701 . Anode  701  is held at a voltage of from one kilovolt to several tens of kilovolts over that of the photoemitters in photocathode  710 . Limiting aperture  702  blocks a portion of beams  711  which have a larger emission solid angle than desired. Deflector  703  allows the image of the emission regions contained in electron beams  711  to be laterally shifted. Einzel lens, having electrodes  704 ,  705 , and  706 , focuses and demagnifies the image onto target  707 . Target  707  may be any surface impacted by electron beam  711 , including either a semiconductor wafer or a mask blank for electron beam lithography. 
     Beam  713  is reflected or emitted from target  707  in response to electron beam  711 . Electron multiplier  712  passes electron beam  711  to target  707  and, as has been described above with relation to FIGS. 2 through 6, captures particles from beam  713 . Electron multiplier  712  may be segmented as is described with FIGS. 5 a  and  5   b , in which case the azimuthal distribution of radiation emitted from surface  707  can be measured. Further, electron multiplier  712  may be stacked as in FIG. 6, in which case the angular distribution from incident beam  711  can be measured. Further, electron multiplier  712  may be placed at any position along microcolumn  700 . 
     The above described examples are demonstrative only. Variations that are obvious to one skilled in the art fall within the scope of this invention. As such, this application is limited only by the following claims.