WO2001084590A2

WO2001084590A2 - Method and apparatus for imaging a specimen using indirect in-column detection of secondary electrons in a microcolumn

Info

Publication number: WO2001084590A2
Application number: PCT/US2001/012172
Authority: WO
Inventors: Stuart L. Friedman
Original assignee: Etec Systems, Inc.
Priority date: 2000-05-04
Filing date: 2001-04-12
Publication date: 2001-11-08
Also published as: WO2001084590A3; TW508615B; AU2001255373A1

Abstract

In a scanning electron microscope microcolumn which may be used in inspection of articles, a target is used to convert secondary electrons emitted by the specimen into tertiary electrons. These tertiary electrons are then detected by a conventional electron detector. Because the shape and dimensions of the target are relatively unconstrained in comparison with the shape and dimensions of the electron detector, an effective detection of the secondaries passing close to the optical axis of the apparatus can be achieved. The shape and material of the target may be chosen to optimize the detection efficiency of the system.

Description

METHOD AND APPARATUS FOR IMAGING A SPECIMEN USING INDIRECT IN-COLUMN DETECTION OF SECONDARY ELECTRONS IN A

MICROCOLUMN

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention generally relates to the design of microcolumns for scanning electron microscopes used in lithography and inspection of articles. In particular, the invention relates to design of microcolumns which utilize indirect detection of secondary electrons.

2. Description of the Related Art

In a conventional scanning electron microscope (SEM) a specimen under investigation is irradiated using a primary electron beam. The interaction of the primary electron beam with the specimen causes the latter to emit secondary electrons with kinetic energies of up to 50 eN. The secondary electrons emitted by the specimen typically carry information about the topographical structure of the specimen. The interaction of the primary electron beam with the specimen also causes the emission of a second class of electrons, called backscattered electrons. The backscattered electrons have energies ranging from 50 eN and up to the kinetic energy of the electrons in the primary electron beam, and carry information about the material composition of the specimen. The secondary and backscattered electrons emitted by the specimen are collected using an electron detector. The electron detector collects the electrons emitted by the specimen and generates an output electrical signal representative of the cumulative charge of the collected electrons, multiplied by the amplification factor of the detector. The electrical signal produced by the electron detector is used in creating an image of the specimen. Depending on the nature of the electrons (secondary or backscattered) used in the imaging process, the created image is indicative of either the topographic or the material structure of the specimen. After the image of the irradiated spot of the specimen is generated using the secondary and/or backscattered electrons, the specimen is moved with respect to the irradiating primary electron beam so that the electron microscope can create an image of the next spot. It should be noted that in scanning electron microscopes the scanning of the specimen with the electron beam can be accomplished either by moving the specimen with respect to the stationary electron beam, or by moving the electron beam with respect to the stationary specimen. In some designs, both the specimen and the beam are moved. In the vast majority of such applications the electron beam moves over a fine region (from less than a micron to tens of microns), while the specimen itself moves over a coarser region (from a few tens of microns to many millimeters or even centimeters).

Unfortunately, the secondary electrons emitted by the specimen typically have a wide kinetic energy distribution. Therefore, the amount of time required for various electrons to reach the electron detector of the column can vary substantially. A wide disparity in the arrival times of the secondary electrons from the specimen results in a decrease in scanning speed of the electron microscope, because the microscope has to "wait" for the slowest electrons emitted by the irradiated spot of the specimen to reach the detector, before the microscope can move on to scan the next spot. Because the width of the collection time distribution is proportional to the length of the electron's travel path, it is highly advantageous to have a microscope column with small linear dimensions. Such microscope columns have been developed and are known in the art as microcolumns.

A typical scanning electron microscope microcolumn 10 is shown in Fig. 1. A primary electron beam 1 produced by the primary beam source S enters the microcolumn 10 from the left and passes through a middle channel 2 thereof. The passing electron beam 1 irradiates a spot on the specimen 3. A set of electron lenses 6 is used to "focus" the primary electron beam 1 on the specimen 3, which may be an article to be inspected. Secondary electrons 4 emitted by the specimen 3 travel back into the column 10, where they are detected by means of an electron detector disposed at either position 5 or position 15.

To decrease the secondary electron collection times, the electron detector can be placed close to the specimen 3, preferably between the specimen 3 and the electron lens 6, at the location designated by numeral 15 in Fig. 1. In this configuration of the microcolumn based scanning electron microscope, the secondary electrons 4 emitted by the specimen 3 are detected directly by a microchannel plate electron multiplier (MCP), or any other electron detector 15 located between the focussing electron lens 6 and the specimen 3. While such a line of sight electron detecting scheme minimizes the electron collection times, it suffers from two significant problems. First, the electron detector 15 is very close to the specimen 3 and is subject to contamination from specimen outgassing and poor specimen chamber vacuum. Second, in the microcolumn 10 the primary electron beam 1 passes through the center of the electron detector 15. Because it is difficult to fabricate an MCP with an active area within a short distance of the primary electron beam axis, the central cone of electrons emitted by the specimen is not detected.

One way to avoid the detector contamination problem is to place the electron detector inside the microcolumn, preferably at location 5 in Fig. 1. An in-lens (or in- column) detector would suffer less contamination from the specimen and would permit shorter working distances. However, achieving large collection efficiencies with an in- column detector remains challenging. Also, a relatively long distance between the specimen and the in-column detector can lead to an unacceptable spread in transit times for the electrons corresponding to a given image pixel.

It may be possible to address the issue of near axis detection using some kind of reflection technique. This kind of approach would differ from the indirect detection scheme, described below. That is, instead of detecting the secondaries and backscattered electrons emitted by the specimen near the optic axis, the reflector would move the in- lens secondaries away from the optic axis. For an electron detector with a fixed central aperture, spreading the electrons out radially would increase the collection efficiency. It also would spread the signal over a larger area of the detector, reducing saturation effects. Unfortunately, reflecting the electrons requires reducing their energy to almost zero near the detector. Decelerating the electrons increases the electron transit times. In some circumstances, it also may increase the spread in the arrival times of the electrons (though it also may actually decrease the spread in some cases). Decelerating the electrons also increases the energy dependence of the detector output signal.

Yet another approach, indirect detection of secondary electrons in scanning electron microscopes, has been used for some time. For example, a standard technique for detecting backscattered electrons in a scanning electron microscope is to collect the tertiaries generated when the backscattered electrons strike various components of the microscope. In particular, the Opal CD SEM, described in U.S. Patent Nos. 5,466,940 and 5,644,132, uses a conical aperture placed in the center of its in-column secondary electron detector for a hybrid direct/indirect electron detection. A portion of the backscattered electrons, which pass close to the optical axis of the system, hits the conical aperture, producing tertiary electrons. Another portion of the backscattered electrons, traveling remotely from the optical axis, directly hits the surface of the electron multiplier away from the conical aperture. The system disclosed in the above U.S. patents uses a hybrid direct/indirect electron detection scheme in a conventional electron microscope column setting.

As can be appreciated from the foregoing discussion, it would be highly advantageous to have an improved microcolumn that would achieve a efficient detection of secondary and backscattered electrons passing close to the optical axis of the electron microscope and minimize the spread in the collection times of the detected electrons.

Even though the hybrid direct/indirect detection scheme has been used in conventional electron microscope columns, the conventional approaches have failed to address the problem of indirect detection of the electrons in a microcolumn.

SUMMARY OF THE INVENTION It is a main feature of the invention, in overcoming the above shortcomings of known approaches, to provide an improved microcolumn that would achieve a highly efficient detection of secondary and backscattered electrons passing close to the optical axis of the electron microscope and minimize the spread in the collection times of the detected electrons. To provide the above and other features and realize the benefits and advantages of the invention, a method and system for indirect detection of secondary electrons in a microcolumn are provided.

According to the inventive method, a specimen is induced to emit secondary electrons. This can be accomplished by irradiating the specimen with a primary electron beam. However, the exact manner in which the specimen is induced to emit secondary electrons is not critical to the present invention and, therefore, other methods for inducing secondary electron emission can be used. The inventive method also involves providing a target in the electron microscope microcolumn. The secondary electrons incident on the target produce tertiary electrons, which, in turn, are used to obtain information regarding the specimen. To this end, the tertiary electrons can be detected by an electron detector disposed in, or about the microcolumn. If the secondary electrons are emitted by the specimen as a result of the irradiation of the specimen with the beam of primary electrons, the target is preferably provided with a central opening through which the beam of primary electrons passes towards the specimen. Such opening can have a size slightly exceeding the size of the cross-section of the primary electron beam. Typically, the size of the opening will be between 100 and 400 microns.

It is another feature of the invention that substantially all detected electrons are indirectly produced on the target.

It is yet another feature of the invention that the shape and the material of the target are optimized to provide for effective operation of the microcolumn. To this end, the target can be textured, have an angled structure, a conical structure, a sawtooth shape, or a rough surface. The material of the target can be specially chosen to maximize the yield of tertiary electrons produced thereon, or the target can be coated with such a material. The target can be produced by micromachining or otherwise. It is yet another feature of the invention that the target is arranged such that a normal to its surface points away from a dead region of a detector for detecting the tertiary electrons.

Another aspect of the invention is an apparatus for imaging a specimen by means of a scanning electron microscope microcolumn. The inventive apparatus comprises an electron beam generation and transport system for providing a primary electron beam. The primary electron beam irradiates the specimen and induces it to emit secondary electrons. The secondary electrons strike a target and produce tertiary electrons. The inventive apparatus also includes an electron detector for detecting the tertiary electrons to obtain information regarding the specimen. The inventive apparatus can be further provided with a set of electrostatic electron lenses for focusing the primary electron beam on the specimen. The apparatus can be also provided with a cylindrical electrode for shielding the primary electron beam and the secondary electrons from a bias applied to an input surface of the electron detector.

It is a feature of the invention that the electron detector is disposed between the target and the specimen, while the electron lenses are placed between the electron detector and the specimen. BRIEF DESCRIPTION OF THE DRAWINGS

These and other features and benefits of the invention will be readily appreciated in the light of the following detailed description of preferred embodiments thereof, given by way of example only with reference to the accompanying drawings wherein: Fig. 1 shows a schematic diagram of a scanning electron microscope microcolumn.

Fig. 2 shows a simulation of the secondary electron trajectories in the inventive microcolumn.

Fig. 3 shows a simulation of the tertiary electron trajectories in the inventive microcolumn.

Fig. 4 shows an embodiment of the inventive microcolumn.

Fig. 5 shows another embodiment of the inventive microcolumn.

Fig. 6 shows yet another embodiment of the inventive microcolumn.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Preferred embodiments of the present invention will now be described with reference to the attached drawings, wherein identical elements are designated with like numerals.

The inventive indirect in-column detection scheme offers a number of clear and distinct advantages over other detection schemes. In the context of this disclosure, the indirect electron detection scheme is defined as one wherein the secondary or backscattered electrons from the specimen first strike a target rather than an active detector. The secondary electrons _λ generated at the target ("tertiary" electrons) are then collected using any conventional, active electron detector. The inventive detection method is indirect because the conventional electron detector never sees the electrons from the specimen, but detects only the electrons generated on the target. A target is any solid object, the size and shape of which are relatively unconstrained. The target is passive and will introduce very little gain (less than 10) in the number of the electrons. The active detector, which is used to detect tertiary electrons produced on the target, is an MCP or any other electron multiplier. The active electron detector can also be a photodetector such as a photomultiplier tube (PMT) or a photodiode. In contrast to a passive target, the size and shape of the active detector have numerous constraints relating to the detector's function and manufacturability. The gain of the active electron detector can be as high as 10⁶.

One of the primary benefits of the inventive detection scheme stems from the fact that, unlike the conventional electron detector, there are very few constraints on the geometry of the target. For this reason, the target can have a substantially small central aperture and secondary electrons traveling close to the optical axis of the microcolumn can strike it. In addition, the tertiary electrons incident on the active detector can be spread over a large active area, limiting the saturation and aging effects in the detector that would otherwise occur if a narrow beam of secondaries and backscattered electrons struck the detector directly.

Fig. 2 shows simulation results of secondary electron trajectories in the inventive microcolumn, demonstrating the feasibility of the inventive indirect electron detection scheme. In the example shown, over 60% of the secondaries 4 emitted by the specimen 3 with energy of 5 eN pass through electrostatic electron lenses 6. The lenses may be positioned in an einzel mode. The electrons 4 are accelerated in the electric field from the lenses 6 to the target 7. Approximately 10% of the secondary electrons 4 pass through the aperture 2 in the target 7, and 50% of the secondary electrons hit the target 7. The equipotential lines are designated in the figure by the numeral 11.

In the simulation shown in Fig. 3, 5 eN electrons are launched from various points on the target 7 to represent tertiary electrons emitted when the secondaries from the specimen hit the target. The vast majority of these tertiary electrons are accelerated into the electron multiplier or scintillation detector 5. In the embodiment shown, a cylindrical electrode 9 shields electrons (the primary electron beam and the secondary electrons) traveling along the optical axis from the voltage bias applied to the input surface of the electron detector 5. Further optimization of both the geometry and the applied voltages can be used to increase the fraction of the secondary electrons 4 striking the target 7 as well as the number of tertiaries 8 reaching the electron detector 5.

For example, the biasing potentials can be applied to the components of the microcolumn in a manner shown in Fig. 3. This figure shows a voltage source NS1 connected between the specimen 3 and the surface of the target 7. The potential difference Nl created by this voltage source is responsible for accelerating the secondary and backscattered electrons before they strike the surface of the target 7. It will be undoubtedly appreciated by those of skill in the art, that to achieve the aforementioned acceleration, the potential of the target 7 should be positive with respect to the potential of the specimen 3. i On the other hand, the second voltage source NS2 generating the potential difference N2 applied between the target 7 and the electron detector 5 is responsible for directing the tertiary electrons 8 towards the electron detector 5 and accelerating the tertiary electrons 8. It will be clear to persons skilled in the art that in the exemplified configuration, the potential of the electron detector 5 should be positive with respect to the potential of the target 7.

A number of advantages of the inventive indirect detection scheme can be seen from the above two figures. First of all, secondary electrons near the optical axis can now be efficiently detected. In the above discussion, provided merely by way of example, the primary electron beam passes through aperture 2 (which as an example, may be 400 microns in diameter) made in the target electrode. This aperture can easily be reduced to, for example, 100 microns in diameter, allowing detection of electrons down to a distance of 50 microns from the optical axis. In contrast, it is difficult to fabricate a conventional active detector with such a small central aperture. MicroChannel plates, which are especially desirable because of their low noise and high bandwidth, are not available with an active area within 1 mm of the axial hole.

Second, even if one could obtain a detector with a near axis active area and use it as a direct detector by placing it in the position occupied by the target 7, the entire secondary electron current 4 would hit a only small area of the detector. The large current density in the detector would lead to saturation effects and would decrease the lifetime of the detector. On the other hand, in the inventive microcolumn shown in Fig. 3, the tertiary electrons 8 are spread over a larger area of the detector 5. Thereby, the current density in the detector is decreased.

The composition and configuration of the target 7 should be optimized to increase the yield of the tertiary electrons 8. The chosen target material should have a high secondary yield at an energy consistent with the optical design of the system. The secondaries from the specimen can easily be accelerated to, for example, 50 eN or even a few hundred eN. Any of the metals or compounds used in conventional multiplier dynodes, multichannel plates, or channeltrons are likely candidates. The target itself can be fabricated from such a material, or the material can be applied as a coating to a substrate such as a silicon membrane. The topography of the target also can be modified to increase the yield of the tertiary electrons. A coating can be deposited onto the surface of the target so that its roughness increases. Alternatively, the surface of the target can be roughened by sputtering or etching. With a rough target surface, the electrons would be incident at the target at larger angles with respect to the local surface normal, increasing the tertiary yield. The surface of the target 7 can also be shaped or micromachined for a similar effect, as shown in the embodiments of Figs. 4-6.

The target's topography can also help in directing the tertiaries away from the detector's axis and toward the active area of the detector. It should be noted that the tertiary electrons are emitted by the target consistent with a cos(Θ) intensity distribution, with respect to the normal of the local surface plane. Here, Θ is the angle between the normal of the local surface plane and the direction of the electron emission. Therefore, more tertiary electrons will hit the detector 5 if the local normal to the target's surface does not point toward the dead center of the detector 5. For example, the sawtoothed (Figs. 4 and 5) and conical (Fig. 6) target surface shapes will aid in directing the tertiaries towards the active area of the detector 5. The local surface normal of roughened or fingerlike surfaces created by sputtering, etching, or deposition will also face away from the optical axis.

Another benefit of the inventive indirect detection scheme stems from the fact that the secondaries and backscattered electrons 4 emitted by the specimen 3 must be accelerated inside the microcolumn 10 so that they strike the target 7 with sufficient kinetic energy to generate the tertiary electrons 8. The requisite acceleration of the secondary electrons is achieved by creating an appropriate electric field inside the microcolumn 10. The aforementioned accelerating electric field in the microcolumn can be produced by biasing the electrostatic lenses 6 and target 7, such as to create a suitable electrical potential difference between the electrostatic lenses 6 and the target 7. Similarly, a potential difference created between the target 7 and the electron detector 5 would effectuate the acceleration of the tertiary electrons 8 towards the electron detector 5. This potential difference can be achieved by suitably biasing the electron detector 5 with respect to the target 7.

The acceleration of the secondary electrons increases the electrons' kinetic energy E and decreases the ratio ΔE/E of the electrons' kinetic energy dispersion to the value of their kinetic energy. A decrease in this ratio results in a decrease of the dispersion of the secondary electrons' collection times. Therefore, the inventive microcolumn overcomes the time-of-flight problems seen in other in-lens detection techniques.

One such problem is the long time that slow moving secondary electrons will need to get to an in-lens detector. At a data rate of 100 Megapixels/second, the scanning electron microscope's imaging time per pixel is only 10 ns. Therefore, the transit times of electrons with kinetic energies between 0 and 20 eN can be large in comparison with this imaging time. For instance, in a reflection detection scheme discussed above the secondaries emitted with E = 10 eN can have a time of flight of 2.5 ns, while secondaries with E = 2 eN can have a time of flight of 4.4 ns. Accelerating the secondary electrons reduces this transit time. While the data acquisition system can compensate in part for a long transit time, it cannot correct for a closely related problem: the transit time dispersion.

If the secondary electrons emitted by a single spot of the specimen arrive at the electron detector over a time interval longer than the pixel dwell time, image resolution and signal-to-noise ratio (SΝR) will be degraded. The spread in the secondary electrons' arrival times results from the spread in the energies of the emitted secondary electrons. Therefore, decreasing the ΔE/E ratio by accelerating the electrons (increasing E) will decrease the spread in the arrival times. Third, any variation in trajectories of the secondaries with different energies can induce unwanted contrast. For instance, if the specimen 3 charges in a non-uniform manner, the detected signal could vary, even though the yield of the secondary electrons at the specimen does not vary. By lowering the ΔE/E ratio in the region near the electron detector 5, the detection efficiency can be made less sensitive to the inherent energy distribution of the emitted secondary electrons. Additionally, fast moving secondary electrons will be influenced less by stray external fields, limiting another source of unpredictable variations in the detected signal.

The specimen 3 may be an article to be inspected. In a semiconductor fabrication context, the specimen 3 may be a semiconductor wafer or reticle. Fine and coarse scanning motion of a beam with respect to the wafer or reticle are well known to those skilled in the art, and need not be detailed here. While the invention has been described herein with respect to preferred embodiments, it will be readily appreciated by those skilled in the art that various modifications in form and detail may be made therein without departing from the scope and spirit of the invention.

Claims

WHAT IS CLAIMED IS:

1. A method for imaging a specimen in a scanning electron microscope microcolumn, comprising:

(a) inducing said specimen to emit electrons; (b) providing a target in said electron microscope microcolumn, said emitted electrons incident on said target producing tertiary electrons; and (c) using said tertiary electrons to create an image of said specimen.

2. The method of claim 1, wherein said emitted electrons comprise secondary electrons and said image is indicative of a topographic structure of said specimen.

3. The method of claim 1, wherein said emitted electrons comprise backscattered electrons and said image is indicative of a material structure of said specimen.

4. The method of claim 1, further comprising accelerating said emitted electrons before their incidence on said target.

5. The method of claim 4, wherein said accelerating comprises providing an electrical field in said scanning electron microscope microcolumn.

6. The method of claim 1, wherein said specimen is a semiconductor wafer.

7. The method of claim 6, further comprising (d) using said image to inspect said wafer for manufacturing defects.

8. The method of claim 1, wherein said specimen is a reticle.

9. The method of claim 8, further comprising (d) using said image to inspect said reticle for manufacturing defects.

10. The method of claim 1, wherein (a) comprises irradiating said specimen with a beam of primary electrons.

11. The method of claim 10, wherein said beam of primary electrons passes towards said specimen through a central opening in said target. i

12. The method of claim 11, wherein a size of said central opening is substantially a size of a cross-section of said beam of primary electrons.

13. The method of claim 11, wherein said central opening has a diameter of 100-400 microns.

14. The method of claim 1, wherein (c) comprises detecting said tertiary electrons by an electron detector, and wherein substantially all electrons detected by said electron detector are said tertiary electrons produced on said target.

15. The method of claim 1, wherein said target is a textured target.

16. The method of claim 1, wherein said target is conically shaped.

17. The method of claim 1, wherein said target is sawtooth shaped.

18. The method of claim 1, wherein said target comprises a material providing a high yield of said tertiary electrons.

19. The method of claim 1, wherein said target has a rough surface.

20. The method of claim 1, wherein said target has a coating comprising a material providing a high yield of said tertiary electrons.

21. The method of claim 1, wherein said target is arranged such that a normal to a surface of said target points away from a dead region of a detector for detecting said tertiary electrons.

22. A method for imaging a specimen by means of a scanning electron microscope microcolumn, said method comprising: (a) irradiating said specimen with a primary election beam and inducing said specimen to emit electrons; (b) producing tertiary electrons using a target in said electron microscope microcolumn, said emitted electrons being incident on said target; and (c) detecting said tertiary electrons with an electron detector to create an image of said specimen.

I

23. The method of claim 22, wherein (a) comprises passing said primary electron beam through an opening in a central portion of said target.

24. The method of claim 22, wherein (c) comprises disposing said electron detector between said target and said specimen.

25. The method of claim 22, wherein (a) further comprises providing an electron lens for focusing said primary electron beam on said specimen.

26. The method of claim 25, wherein said electron lens is disposed between said electron detector and said specimen.

27. The method of claim 25, wherein said electron lens is disposed between said electron detector and said specimen.

28. The method of claim 22, further comprising accelerating said tertiary electrons before their detection by said electron detector.

29. The method of claim 28, wherein said accelerating comprises providing an electrical field in said scanning electron microscope microcolumn.

30. An apparatus for imaging a specimen by means of a scanning electron microscope microcolumn, said apparatus comprising: (a) an electron beam generation and transport system for providing a primary electron beam; said primary electron beam irradiating said specimen and inducing said specimen to emit electrons; (b) a target; said emitted electrons incident on said target producing tertiary electrons; and (c) an electron detector for detecting said tertiary electrons to create an image of said specimen.

31. The apparatus of claim 30, wherein said electron beam generation and transport system comprises an electron lens for focusing said primary electron beam on said specimen.

32. The apparatus of claim 30, further comprising an electrode for shielding said primary electron beam and said emitted electrons from a bias applied to an input surface of said electron detector.

33. The apparatus of claim 32, wherein said shielding electrode has a cylindrical shape.

34. The apparatus of claim 32, wherein said electron detector has a circular opening in a central portion thereof, said shielding electrode being disposed within said opening, and said primary electron beam and said emitted electrons passing inside said electrode.

35. The apparatus of claim 30, wherein said target has an opening in a central portion thereof through which said primary electron beam passes towards said specimen.

36. The apparatus of claim 35, wherein a size of said central opening is substantially a size of a cross-section of said primary electron beam.

37. The apparatus of claim 35, wherein said opening has a diameter between 100 and 400 microns.

38. The apparatus of claim 30, wherein substantially all electrons detected by said electron detector are said tertiary electrons produced on said target.

39. The apparatus of claim 30, wherein said target is a textured target.

40. The apparatus of claim 30, wherein said target is conically shaped.

41. The apparatus of claim 30, wherein said target is sawtooth shaped.

42. The apparatus of claim 30, wherein said target comprises a material providing a high yield of said tertiary electrons.

43. The apparatus of claim 30, wherein said target has a rough surface.

44. The apparatus of claim 30, wherein said target has a coating comprising a material providing a high yield of said tertiary electrons. i

45. The apparatus of claim 30, wherein said electron detector is disposed between said target and said specimen.

46. The apparatus of claim 31 , wherein said electron lens is disposed between said electron detector and said specimen.

47. The apparatus of claim 31, wherein said electron lens and said target are biased to provide an acceleration of said emitted electrons before their incidence on said target.

48. The apparatus of claim 30, wherein said electron detector and said target are biased to provide an acceleration of said tertiary electrons before their detection by said electron detector.