Abstract:
One embodiment relates to a high-resolution Auger electron spectrometer in a scanning electron beam apparatus. An electron source generates a primary electron beam, and an immersion objective lens is configured to focus the primary electron beam onto a surface of a target substrate. A Wien filter is configured within the immersion objective lens and to deflect and disperse secondary electrons from the surface. A position sensitive detector is configured to receive the secondary electrons so as to detect an Auger electron spectrum. A first electron-optical lens may be positioned after the Wien filter so as to transfer a minimal-dispersion plane to an aperture plane. A second electron-optical lens may be positioned after the aperture so as to transfer a virtual focused-spectrum plane to a detector plane. Other embodiments, aspects and features are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   The present application claims the benefit of U.S. Provisional Patent Application No. 60/881,417, entitled “High-Resolution Auger Electron Spectrometer”, filed Jan. 19, 2007, by inventor Alexander J. Gubbens, the disclosure of which is hereby incorporated by reference in its entirety. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present disclosure relates to Auger electron spectrometry and to electron beam apparatus. 
   2. Description of the Background Art 
   When an electron is emitted from a core level of an atom, leaving a vacancy, an electron from a higher energy level may fall into the lower-energy-level vacancy. This results in a release of energy either in the form of an emitted photon or by ejecting another electron. Electrons ejected in this manner are called Auger electrons. 
   Conventional Auger electron spectrometers include the hemispherical analyzer, the cylindrical mirror analyzer, and the hyperbolic field analyzer. The hemispherical analyzer and the cylindrical mirror analyzer are serial spectrometers where the spectrometer is scanned in order to collect a complete spectrum in a serial fashion. The hyperbolic field analyzer is an example of a parallel spectrometer where a complete spectrum is acquired in parallel fashion. 
   Auger electron spectrometers traditionally are incorporated into only non-immersion type scanning electron microscopes (SEMs). In such SEMs, the magnetic and/or electrostatic focusing fields are fully contained or nearly fully contained in the objective lens, and the sample is placed in a field-free or nearly field-free region below the lens. In such a spectrometer, the Auger electrons travel in substantially straight lines from the sample, and the spectrometer&#39;s collection efficiency is determined largely by the solid angle of the spectrometer&#39;s entrance aperture and the spectrometer&#39;s take-off angle with respect to the sample surface. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a cross-sectional diagram depicting a magnetic immersion objective lens for a scanning electron microscope. 
       FIG. 2  is a schematic diagram of an immersion objective lens having additional components for high-resolution electron spectrometry in accordance with an embodiment of the invention. 
       FIG. 3  is a graph showing the small or negligible energy dispersion at a plane near the middle of the Wien filter which may be mapped onto an aperture plane in accordance with an embodiment of the invention. 
       FIG. 4  is graph showing energy dispersion at a plane below a Wien filter which may be mapped onto a detector plane in accordance with an embodiment of the invention. 
   

   SUMMARY 
   One embodiment relates to a high-resolution Auger electron spectrometer in a scanning electron beam apparatus. An electron source generates a primary electron beam, and an immersion objective lens is configured to focus the primary electron beam onto a surface of a target substrate. A Wien filter is configured within the immersion objective lens and to deflect and disperse secondary electrons from the surface. A position sensitive detector is configured to receive the secondary electrons so as to detect an Auger electron spectrum. A first electron-optical lens may be positioned after the Wien filter so as to transfer a minimal-dispersion plane to an aperture plane. A second electron-optical lens may be positioned after the aperture so as to transfer a virtual focused-spectrum plane to a detector plane. Other embodiments, aspects and features are also disclosed. 
   Other embodiments, aspects and features are also disclosed. 
   DETAILED DESCRIPTION 
   There is an increasing need for high-resolution scanning electron microscopes (SEMs) in all areas of development and manufacture of micro-electronic and opto-electronic components. High-resolution scanning electron microscopes are useful so as to visually evaluate sub-micrometer structures. High-resolution SEMs may be used to identify deviations from standard patterns and to acquire and evaluate topographical data such as heights, widths or angles of inclination. 
   Unfortunately, conventional scanning electron microscopes do not have the required resolution of a few nanometers unless very high landing energies above about 10 kiloelectronVolts (keV) are used which may cause resist structures and integrated circuits to be damaged and non-conductive or high resistant specimens to be disadvantageously charged. 
   Low-Voltage SEMs with Immersion Objective Lenses 
   Low-voltage SEMs avoid the above-discussed problem of damaging specimens which is problematic in higher-voltage SEMs. It is advantageous to use an immersion objective lens in a low-voltage SEM because immersion objective lenses tend to have superior spatial resolution performance. Immersion objective lenses immerse the sample in a magnetic and/or decelerating electrostatic field. 
     FIG. 1  is a cross-sectional diagram depicting a magnetic immersion objective lens  100  for a scanning electron microscope. As shown, a primary electron beam  101  travels down an optical axis and through the objective lens  100  to become focused upon the surface of a target substrate. A magnetic pole piece  102  of the objective lens  100  is configured about the optical axis, with a gap  105  extending away from the optical axis. The pole piece  102  is configured about an electromagnetic device  104  so as to generate a magnetic field which immerses the target substrate. The pole piece  102  is further configured at a high voltage potential. 
   Incompatibility of Immersion Lenses with Conventional Auger Electron Spectrometry 
   Because immersion lenses immerse the sample in a magnetic field and/or a decelerating electrostatic field, there is no field-free or nearly field-free region below the lens. As such, the Auger electrons cannot travel in substantially straight lines from the sample to an Auger spectrometer located outside the SEM column. Because of this, Auger spectrometry is typically performed with non-immersion lenses, and in the case of low voltage operation this unfortunately, limits the Auger spectrometry to lower spatial resolutions. 
   High-Resolution Auger Electron Spectrometer 
   The present disclosure enables a high-resolution Auger electron spectrometer. This is accomplished by incorporating the Auger spectrometer functionality into an immersion-type SEM in a way that does not interfere with or reduce the high-resolution performance of the SEM. An SEM configured in accordance with an embodiment of the invention may be used not only for high-resolution defect location and classification, but also for elemental analysis at a high spatial resolution (for example, at a nanometer level resolution). 
     FIG. 2  is a schematic diagram of an immersion objective lens system  200  having additional components for high-resolution Auger electron spectrometry in accordance with an embodiment of the invention. The additional components include a Wien filter  202 , electron-optical lenses  206  and  210 , an aperture  208 , and a position-sensitive detector  212 . 
   As shown in  FIG. 2 , secondary electrons (including Auger electrons)  204  are emitted from the sample  106  (for example, a manufactured semiconductor wafer, or other sample). These secondary electrons  204  spiral along the magnetic flux lines up the bore of the lens. Although in the strictest sense secondary electron means those electrons emitted from the sample with and energy equal to or less than 50 eV of energy, here secondary electrons cover all emitted electrons, including secondary, backscattered and Auger electrons. 
   In accordance with an embodiment of the invention, a Wien filter  202  located in the objective lens deflects the secondary electrons  204  off-axis (i.e. off the optical axis of the column). The Wien filter  202  also introduces an energy dispersion in the deflected secondary electrons  204 . In alternate embodiments, another type of deflection device besides a Wien filter may be utilized. Other types of deflection devices include, for example, electrostatic deflectors. However, a Wien filter advantageously does not disturb the primary electron beam. 
   In one implementation, the Wien filter  202  may deflect the secondary electrons by about 6 degrees off-axis, where other conditions include a 5 keV primary beam energy, a 3 keV landing energy, and no electrostatic field between the magnetic immersion objective lens and the sample (such as, for example, a semiconductor wafer). Such conditions may be accomplished by having the electron gun (not shown) at −5 kilovolts, and the objective lens and the sample at −2 kiloVolts. Advantageously, such conditions minimize charging on a sample by having little or no electrostatic field between the sample and the objective lens. 
   In further accordance with an embodiment of the invention, the deflected secondary electrons have their trajectories transformed by pre-aperture electron-optics  206 . The pre-aperture electron optics  206  focuses a minimal-dispersion plane near the center of the Wien filter  202  to the plane of the aperture  208  (i.e. to the aperture plane). The purpose of the aperture is to limit the polar angles of the electrons in order to improve the final energy resolution in the Auger spectrum. In one implementation, the pre-aperture electron optics  206  may be implemented with one or more round lenses. The minimal-dispersion plane and the function of the aperture  208  are described further below in relation to  FIG. 3 . 
   In further accordance with an embodiment of the invention, after passing through the aperture, the secondary electrons have their trajectories transformed by post-aperture electron-optics  210 . The post-aperture electron optics  210  transfers a virtual focused-spectrum plane below the Wien filter  202  to the plane of the position-sensitive detector  212  (i.e. to the detector plane). The position-sensitive detector  212  may comprise a scintillator in combination with any multichannel detector suitable for spectroscopy, for example, charge coupled device (CCD) arrays, CMOS sensors, photodiode arrays and the like. In one implementation, the post-aperture electron optics  210  may be implemented with quadrupole lenses, preferably with separate control of x and y magnifications. The virtual focused-spectrum plane is described further below in relation to  FIG. 4 . 
     FIG. 3  is a graph showing the small or negligible energy dispersion at a minimal-dispersion plane near the middle of the Wien filter  202  which may be mapped onto the aperture plane in accordance with an embodiment of the invention. The graph in  FIG. 3  is generated by a simulation which extrapolates back Auger electron ray traces through the Wien filter  202  to the minimal-dispersion plane at the middle of the Wien filter  202 . The conditions for the simulation included a 5 keV primary beam energy, a 3 keV landing energy, and no electrostatic field between the magnetic immersion objective lens and the wafer sample. 
   Shown in  FIG. 3  are circular patterns corresponding to Auger electrons at different energies emitted from a sample wafer at a polar angle of one degree and azimuthal angles ranging from 0 degrees to 360 degrees. As seen by the locations of the center of the circular patterns, the energy dispersion is small (near zero) at the minimal-dispersion plane. 
   The varying radii of the circular patterns indicates that the aperture  208  limits Auger electrons of different energies to different polar angles. This means that the collection efficiency of the high-resolution Auger spectrometer will vary across the energy spectrum. In accordance with an embodiment of the invention, this variation may be characterized and divided out prior to display and analysis of the Auger electron spectra. 
   Although an aperture may be placed, in theory, at this minimal-dispersion plane in the middle of the Wien filter  202 , doing so would disadvantageously interfere with the functioning of the Wien filter as well as the primary electron beam and with the secondary electrons in a normal imaging mode. Hence, in accordance with an embodiment of the invention, the pre-aperture electron optics  206  is positioned and configured to transfer this minimal-dispersion plane to an aperture plane outside the Wien filter  202  (see aperture  208  in  FIG. 2 ). 
     FIG. 4  is graph showing energy dispersion at a focused-spectrum plane below the Wien filter  202  which may be mapped onto the detector plane in accordance with an embodiment of the invention. The graph in  FIG. 4  is also generated by a simulation. Here, the simulation extrapolates back Auger electron ray traces through the Wien filter  202  to a virtual focused-spectrum plane below the Wien filter  202 . The conditions for the simulation are the same as those used to generate  FIG. 3 . 
   Shown in  FIG. 4  are positions of the Auger electrons where the positions are dispersed depending on the energy of the Auger electrons. As seen, the dispersion is approximately along a line, though the magnitude of the dispersion has a non-linear dependence on energy. 
   In accordance with an embodiment of the invention, the post-aperture electron optics  210  is positioned and configured to transfer the virtual focused-spectrum plane to the detector plane (see position-sensitive detector  212  in  FIG. 2 ). This function likely necessitates the separate control of x and y magnification. Hence, separate x and y quadrupole electron lenses may be used optionally in combination with rotationally symmetric electron lenses. 
   In accordance with an embodiment of the invention, because the magnitude of the dispersion is non-linear in relation to energy, this non-linearity would be characterized and transformation of the data would be made to properly stretch out the spectrum prior to display and analysis. 
   CONCLUSION 
   The present application discloses a technique for incorporating a fast, high-resolution Auger spectrometer into a high-resolution, low voltage, immersion lens SEM. Simulations generated by applicants show that such a spectrometer design provides an energy resolution ranging from about 1 eV at low Auger energies to about 10 eV at 2 keV Auger energy for a net collection efficiency of about 0.1%. Because the collection efficiency varies approximately linearly with energy resolution, a 2 times lower energy resolution may be traded off for a 4 times higher collection efficiency, for example. 
   In accordance with one embodiment, the Wien filter  220  may be operated in two settings. In one setting, the Wien filter  220  may deflect the secondary electrons in one direction towards a conventional secondary electron detector of the SEM. In the other setting, the Wien filter  220  may deflect the secondary electrons in an opposite or a different direction towards the Auger spectrometer. In accordance with an alternate embodiment, the function of the conventional secondary detector may be incorporated into the Auger spectrometer, such that only one operational setting for the Wien filter is needed. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. Specific dimensions, geometries, and lens currents of the immersion objective lens will vary and depend on each implementation. 
   The above-described invention may be used in an automatic inspection system and applied to the inspection of wafers, X-ray masks and similar substrates in a production environment. While it is expected that the predominant use of the invention will be for the inspection of wafers, optical masks, X-ray masks, electron-beam-proximity masks and stencil masks, the techniques disclosed here may be applicable to the high speed electron beam imaging of any material (including perhaps biological samples). 
   In the above description, numerous specific details are given to provide a thorough understanding of embodiments of the invention. However, the above description of illustrated embodiments of the invention is not intended to be exhaustive or to limit the invention to the precise forms disclosed. One skilled in the relevant art will recognize that the invention can be practiced without one or more of the specific details, or with other methods, components, etc. In other instances, well-known structures or operations are not shown or described in detail to avoid obscuring aspects of the invention. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those skilled in the relevant art will recognize. 
   These modifications can be made to the invention in light of the above detailed description. The terms used in the following claims should not be construed to limit the invention to the specific embodiments disclosed in the specification and the claims. Rather, the scope of the invention is to be determined by the following claims, which are to be construed in accordance with established doctrines of claim interpretation.