Abstract:
One embodiment relates to a scanning electron beam apparatus having curved electron-optical axes. An electron gun and illumination electron optics are configured to generate a primary electron beam along a first axis. Objective electron optics is configured about a second axis to receive the primary electron beam, to focus the incident electron beam onto the substrate, and to retrieve an emitted beam of scattered electrons from the substrate. Detection electron optics is configured about a third axis to receive the emitted beam and to focus the emitted beam onto a detector. A beam separator is coupled to and interconnecting the illumination electron optics, the objective electron optics, and the detection electron optics in such a way that there is a same angle between the first and second axes as between the second and third axes. A beam deflector is configured to controllably scan the primary electron beam across the substrate and to de-scan the emitted electron beam. Other embodiments are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
   The present application claims the benefit of U.S. Provisional Patent Application No. 60/736,439, entitled “Scanning Electron Microscope with Curved Axes,” filed by Marian Mankos and Kurt Weiner on Nov. 14, 2005. 

   GOVERNMENT LICENSE RIGHTS 
   The U.S. Government has a paid-up license in this invention and the right in limited circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DAAD19-00-3-1001 awarded by the Defense Advanced Research Projects Agency. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to scanning electron beam apparatus and methods for inspection or defect review of substrates, such as semiconductor wafers and masks. 
   2. Description of the Background Art 
   Scanning electron beam systems are utilized for the inspection and defect review of semiconductor substrates and for other purposes. A conventional scanning electron beam system utilizes a straight optical axis down an electron-optical column. The signal electrons are generally detected with either an annular detector, a combination of a Wien filter and an off-axis detector, or an Everhart-Thornley detector (secondary electrons and no extraction field only). 
   With a straight optical axis, the detection electron-optics overlaps with the illumination electron-optics. This overlap often disadvantageously limits the detection functionality, efficiency and flexibility. 
   For example, with an annular detector, it is difficult to detect all the signal electrons under all imaging conditions, where the imaging conditions may include various landing energies, extraction field strengths, and Wehnelt electrode biases. This difficulty reduces the detection efficiency and limits throughput of an inspection system. 
   As another example, a Wien filter used to deflect the secondary electrons may induce aberrations of the primary beam. Such aberrations reduce the system resolution. In addition, the beam deflection tends to increase the region across which the signal electron move in the detector plane. This larger region in the detector plane results in a need for a larger area detector which limits the detection speed. 
   In addition, with a straight optical axis, it is often difficult to discriminate the signal electrons based on their energy. In other words, it is often difficult to implement an energy filter with the conventional geometry. 
   SUMMARY 
   One embodiment relates to a scanning electron beam apparatus having curved electron-optical axes. An electron gun and illumination electron optics are configured to generate a primary electron beam along a first axis. Objective electron optics is configured about a second axis to receive the primary electron beam, to focus the incident electron beam onto the substrate, and to retrieve an emitted beam of scattered electrons from the substrate. Detection electron optics is configured about a third axis to receive the emitted beam and to focus the emitted beam onto a detector. A beam separator is coupled to and interconnecting the illumination electron optics, the objective electron optics, and the detection electron optics in such a way that there is a same angle between the first and second axes as between the second and third axes. A beam deflector is configured to controllably scan the primary electron beam across the substrate and to de-scan the emitted electron beam. 
   Other embodiments are also disclosed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic diagram depicting a scanning electron beam apparatus having a curved optical axis for incident and scattered electron beams in accordance with an embodiment of the invention. 
       FIG. 2  depicts the scanning electron beam apparatus of  FIG. 1 , where the apparatus operates in a spectral integration mode in accordance with an embodiment of the invention. 
       FIG. 3A  is a schematic diagram depicting a scanning electron beam apparatus having a curved optical axis and an aperture positioned to selectively detect backscattered electrons in accordance with an embodiment of the invention. 
       FIG. 3B  is a schematic diagram depicting a scanning electron beam apparatus having a curved optical axis and an aperture positioned to selectively detect secondary electrons in accordance with an embodiment of the invention. 
       FIG. 4  is a schematic diagram depicting a scanning electron beam apparatus configured in an energy spectrum mode in accordance with an embodiment of the invention. 
       FIG. 5  is a schematic diagram depicting a scanning electron beam apparatus configured in a dark field imaging mode in accordance with an embodiment of the invention. 
       FIG. 6  is a schematic diagram depicting a scanning electron beam apparatus configured in a diffraction mode in accordance with an embodiment of the invention. 
       FIG. 7  is a schematic diagram depicting a scanning electron beam apparatus where advantage is taken of the direct line of sight to the substrate in accordance with an embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   Scanning Electron Beam Apparatus with Curved Optical Axes 
     FIG. 1  is a schematic diagram depicting a scanning electron beam apparatus  100  having curved optical axes for primary and scattered electron beams in accordance with an embodiment of the invention. The apparatus may be utilized, for example, to inspect a substrate or for defect review. The apparatus  100  is configured so as to have curved, not straight, optical axes for the primary and scattered electron beams. This is achieved by use of a magnetic prism array  114  or similar beam separator device. 
   As seen in  FIG. 1 , the electron-optical system is composed of four subsystems: the illumination optics (including the lens of the electron gun  110  and first transfer lens  112 ), the objective optics (including the objective lens  118 ), the detection optics (including the second transfer lens  126 ), and the beam separator or beam splitter (comprising, for example, the magnetic prism array  114 ). In  FIG. 1 , the illumination and detection optics are oriented along a horizontal optical axis, while the objective optics is oriented along a vertical optical axis. 
   The optical axis for the primary electron beam begins at the electron gun  110  and extends in a line through the illumination optics until reaching the magnetic prism array  114 . Within the magnetic prism array  114 , the optical axis for the primary electron beam bends by ninety degrees or thereabout so as to be oriented towards the objective optics. Upon exit of the magnetic prism array  114 , the optical axis for the primary electron beam extends in a line through the objective optics until reaching the substrate  122 . 
   The optical axis for the scattered electron beam begins at the substrate  122  and extends in a line through the objective optics until reaching the magnetic prism array  114 . Within the magnetic prism array  114 , the optical axis for the scattered electron beam bends so as to be oriented towards the detection optics. Upon exit from the magnetic prism array  114 , the optical axis for the scattered electron beam extends in a line through the detection optics until reaching the detector  128 . 
   One specific embodiment of the beam separator comprises a magnetic prism array  114 , for example, such as the one disclosed in U.S. Pat. No. 6,878,937, which is entitled “Prism array for electron beam inspection and defect review” and which issued Apr. 12, 2005 to inventor Marian Mankos. The disclosure of the aforementioned patent is hereby incorporated by reference. Per that disclosure, the magnetic prism array  114  may comprise a compact, double focusing magnetic prism array. The prism array  114  may be configured to include a central magnetic section, an inner magnetic section outside the central section, and an outer magnetic section outside the inner section. Each magnetic section may be configured so as to provide a uniform magnetic field of different strength and/or length. This allows obtaining stigmatic focusing and makes the prism array  114  behave as a round lens, which simplifies set-up, alignment and operation of the apparatus. 
   The electron gun  110  for generating the primary electron beam may comprise, for example, a thermionic field emission (Schottky) gun. The electron gun  110  may include a gun lens (shown in the figure). Electrons emitted from the gun  110  are collimated and are focused at an achromatic plane in the prism array  114 . The magnetic prism array  114  deflects the trajectory of the primary electron beam by ninety degrees. The objective lens  118  is configured so as to decelerate and refocus the primary electron beam onto a spot on the surface of the substrate  122 . The magnetic deflection system  116  is configured to scan the primary electron beam spot in a direction perpendicular to the movement of the wafer stage  204 . An extraction electrode  120  may be configured to extract secondary electrons from the substrate surface. 
   The scattered electron beam includes electrons emitted over a range of solid angles with energies ranging from near zero (secondary electrons  136 ) to the landing energy (backscattered electrons  130 ). Electrons with intermediate energies include Auger electrons  134  and also electrons that suffered from plasmon losses  132 . 
   The scattered electrons are accelerated and collimated by the objective lens  118 , and de-scanned by the deflection system  116 . The magnetic prism array  114  deflects the trajectory of the primary electron beam by approximately ninety degrees (for the elastically backscattered electrons  130 ) or more (for the lower energy electrons). 
   Per the example embodiment shown in  FIG. 1 , the transfer lens  126  in the detection optics may be configured such that the scattered electrons of various energies are focused to the detector  126 . Data collection circuitry coupled to the detector  128  is configured in synchronization with the deflection system  116  so as to be able to form a two-dimensional image of the data collected from the scanned area. 
   Spectral Integration Imaging Mode 
     FIG. 2  depicts the scanning electron beam apparatus of  FIG. 1 , where the apparatus operates in a spectral integration mode in accordance with an embodiment of the invention. As shown in  FIGS. 1 and 2 , the detector  128  may comprise a small area detector, and the detection optics may be configured such that all or practically all of the scattered electrons converge upon the small area detector. In other words, as indicated by the shading of the scattered electron beam in  FIG. 2 , the scattered electrons of various energies are focused by the second transfer lens  126  to the detector plane, and hence the integrated spectrum of the scattered electrons is detected. 
   Hence, in this spectral integration imaging mode, the detected signal includes the elastically backscattered electrons  130 , the electrons that suffered plasmon losses  132 , the Auger electrons  134 , and the secondary electrons  136 . Advantageously, this mode provides both high detection efficiency (due to the detection of all or mostly all of the signal electrons) and high detection speed (due to the smaller detector area). 
   Selected Energy (Energy Filtered) Imaging Mode 
   In accordance with other embodiments of the invention, the energy dispersion caused by the prism array  114  may be used to advantageously discriminate the scattered electrons based on their energy. In other words, the prism beam splitter  114  may be utilized to provide energy-filtering functionality. Embodiments using this energy-filtering functionality so as to selectively image backscattered or secondary electrons are now discussed in relation to  FIGS. 3A and 3B . 
     FIG. 3A  is a schematic diagram depicting a scanning electron beam apparatus  300  having a curved optical axis and an aperture  302  positioned to selectively detect backscattered electrons  130  in accordance with an embodiment of the invention. The aperture  302  may be positioned as shown in the energy-dispersive plane at the second transfer lens  126 . The backscattered electrons  130  pass through the aperture  302  to the detector  128 , while electrons with lower energies (for example, the electrons that suffered plasmon losses  132 , the Auger electrons  134 , and the secondary electrons  136 ) are blocked by the aperture  302  and so are not detected. 
     FIG. 3B  is a schematic diagram depicting a scanning electron beam apparatus  350  having a curved optical axis and an aperture  352  positioned to selectively detect secondary electrons  136  in accordance with an embodiment of the invention. The aperture  352  may be positioned as shown in the energy-dispersive plane at the second transfer lens  126 . The secondary electrons  136  pass through the aperture  352  to the detector  128 , while electrons with other energies (for example, the Auger electrons  134 , the electrons suffering plasmons losses  132 , and the backscattered electrons  130 ) are blocked by the aperture  352  and so are not detected. 
   Energy Spectrum Imaging Mode 
     FIG. 4  is a schematic diagram depicting a scanning electron beam apparatus  400  configured in an energy spectrum mode in accordance with an embodiment of the invention. The apparatus  400  may be configured with a linear detector  402  which includes a linear array of detection elements. Alternatively, a detector with a two-dimensional array of detection elements may be used to similar effect. The array of detection elements may be utilized to provide the energy spectrum (i.e. N(E) in the upper left corner of the figure) of the scattered electrons. For example, as shown in  FIG. 4 , the backscattered electrons  130  impinge upon one section of the linear detector  402 , while the secondary electrons  136  impinge upon a different section of the linear detector  402 . The second transfer lens  404  may be configured so as to achieve such energy dispersion at the detector plane. 
   Dark Field Imaging Mode 
     FIG. 5  is a schematic diagram depicting a scanning electron beam apparatus  500  configured in a dark field imaging mode in accordance with an embodiment of the invention. The apparatus  500  is configured to be able to generate an image based on selected diffracted electrons  504 , rather than based on the specularly scattered electrons  502 . For example, aperture  506  may be positioned as shown in the diffraction plane (same as energy-dispersive plane) at the second transfer lens  126 . The diffracted electrons  504  selected by the aperture  506  pass through to the detector  128 , while specularly scattered electrons  502  are blocked by the aperture  506  and so are not detected. The second transfer lens  126  focuses the selected electrons onto the detector  128  for dark field imaging. 
   Alternatively, the aperture  506  may be positioned so as to select the specular electrons  502  while blocking the diffracted electrons  504 . This would provide a bright field imaging mode. 
   Diffraction Mode 
     FIG. 6  is a schematic diagram depicting a scanning electron beam apparatus  600  configured in a diffraction mode in accordance with an embodiment of the invention. The second transfer lens  604  of the apparatus  600  is configured so as to generate a diffraction pattern  606  at the plane of the detector  602 . The detector  602  may comprise a two-dimensional CCD (charge coupled device) array or other two-dimensional array. In this embodiment, the whole energy diffraction pattern  606  may be acquired at once for every position on the substrate  122 , and the two-dimensional data may be processed using a computer. 
   The two-dimensional diffraction pattern  606  may include a superposition of a first diffraction pattern  608  from the backscattered electrons and a second pattern  610  from the secondary electrons. The second pattern  610  is shifted in position relative to the first diffraction pattern  608  due to the energy dispersion between the backscattered electrons  130  and the secondary electrons  136 . 
   Direct Line of Sight 
   An additional aspect of the geometry discussed herein is that it allows a direct line of sight of the substrate  122 . The direct line of sight is available from the side of the prism array  114  which is directly opposing the substrate  122 . 
     FIG. 7  is a schematic diagram depicting a scanning electron beam apparatus  700  wherein advantage is taken of the direct line of sight to the substrate  122  in accordance with an embodiment of the invention. As illustrated in  FIG. 7 , additional components may include, for example, a flood gun  702 , a laser  704 , a semitransparent mirror  706  and a CCD camera  708 . The flood gun  702  may be configured to transmit a flood beam through the prism array  114  onto the surface of the substrate  122 . Light from the laser  704  may be reflected by the semi-transparent mirror  706  onto the surface of the substrate  122 . The laser light may also be transmitted through the semi-transparent mirror  706  to the CCD camera  708 . Such additional components may be used for wafer height determinations and for other purposes. 
   CONCLUSION 
   A prism-based SEM (scanning electron microscope) with curved optical axes is disclosed herein. The detection optics is separated from the illumination optics. This improves the detection efficiency and flexibility and allows straightforward implementation of desirable imaging modes. Such imaging modes include energy filtering modes and dark field modes. 
   In accordance with certain embodiments of the invention, the area of the detector exposed by signal electrons may be advantageously reduced to less than one square millimeter. Hence, faster detectors with small detection areas may be employed. 
   In the selected energy imaging mode, a specific part of the energy spectrum (for example, secondary electrons, or backscattered electrons, or chemical-composition specific Auger electrons) may be included or excluded from the detected signal. Such an imaging mode may potentially increase contrast and thus improve defect detection sensitivity. 
   When the whole spectrum is acquired for each pixel in accordance with the energy spectrum imaging mode, a series of parallel images may be generated, each image with different energy characteristics. This opens up a new dimension for analyzing the data so as to increase defect detection sensitivity. 
   The dark field imaging and diffraction modes may be readily implemented so as to provide further information relating to a substrate being inspected or under defect review. In addition, due to the direct line of sight available in the disclosed geometry, coaxial light optics and a flood gun may be readily included. 
   The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. The above-described invention may be used in an automatic inspection or review system and applied to the inspection or review of optical or X-ray masks and similar substrates in a production environment. 
   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.