Abstract:
The present invention includes an electron beam device for examining defects on semiconductor devices. The device includes an electron source for generating a primary electron beam, wherein the total acceleration potential is divided and is provided across the ground potential. Also included is at least one condenser lens for pre-focusing the primary electron beam, an aperture for confining the primary electron beam to ameliorate electron-electron interaction, wherein the aperture is positioned right underneath the last condenser lens, and a SORIL objective lens system for forming immersion magnetic field and electrostatic field to focus the primary beam onto the specimen in the electron beam path. A pair of grounding rings for providing virtual ground voltage potential to those components within the electron beam apparatus installed below a source anode and above a last polepiece of the SORIL objective lens.

Description:
FIELD OF THE INVENTION 
       [0001]    The invention relates to an electron beam inspection system and specifically the present invention relates to a low landing energy, high resolution low current system for electron beam inspection. 
       BACKGROUND OF THE INVENTION 
       [0002]    An electron beam inspection system has many applications during semiconductor chip fabrication process. For example, besides using the system in process wafer inspection, photomask inspection, linewidth, other critical dimension of device structures may also be measured with the electron beam system. However, as the linewidth shrinks from the order of micrometer to the order of nanometer there is, more and more demand for a higher resolution and lower landing energy electron beam system for semiconductor related observation. 
       SUMMARY OF THE INVENTION 
       [0003]    The present invention includes an electron beam system for a next generation inspection tool with a higher resolution and lower landing energy by reducing chromatic aberration of the objective lens and having less electron-electron interaction within the primary electron beam. 
         [0004]    The present invention includes an electron beam device for examining defects on semiconductor devices. The device includes an electron source for generating a primary electron beam, wherein the total acceleration potential is divided and is provided across the ground potential. Also included is at least one condenser lens for pre-focusing the primary electron beam, an aperture for confining the primary electron beam to ameliorate electron-electron interaction, wherein the aperture is positioned right underneath the last condenser lens, and a SORIL objective lens system for forming immersion magnetic field and electrostatic field to focus the primary beam onto the specimen in the electron beam path. A pair of grounding rings for providing virtual ground voltage potential to those components within the electron beam apparatus installed below a source anode and above a last polepiece of the SORIL objective lens. There is at least one pair of deflectors for deflecting the primary electron beam over the specimen surface to form a scanning pattern. A detection system is provided for detecting signal electrons and a stage holds the specimen to be inspected. 
         [0005]    In order to ameliorate the chromatic aberration of the electron beam apparatus, one embodiment disclose an acceleration voltage potential higher than other conventional potential. The acceleration potential is applied in a special way directly across the cathode and the anode. 
         [0006]    The other embodiment disclose a method to ameliorate the electron-electron interaction or the Boersch effect within the primary beam by position the aperture right beneath the condenser lens. 
         [0007]    Another embodiment disclose a method to reduce micro-arcing during operation by lowering the stage floating voltage respect to ground potential. For example, the retarding potential set on the operation stage is down to −6 kV. 
         [0008]    According to an embodiment of the present invention, the scintillator-photomultiplier combination detector for detecting the secondary electrons is sitting on +4 kV potential, in order to have 10 kV bias for the best detecting result. 
         [0009]    And yet another embodiment of the present invention disclose a second method to ameliorate the electron-electron interaction within the primary beam by reducing electron interaction time. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         [0010]      FIG. 1  is a schematic diagrammatic representation of a charged particle apparatus according to an embodiment of the present invention. 
           [0011]      FIG. 2  is a diagrammatic representation of the axial potential distribution in voltage of a conventional column design. 
           [0012]      FIG. 3  is a diagrammatic representation of the axial potential distribution in voltage of a SORIL system disclosed by Chen et al. 
           [0013]      FIG. 4  is a diagrammatic representation of the axial potential distribution in voltage according to an embodiment of the present invention. 
           [0014]      FIG. 5  is a schematic diagrammatic representation of a charged particle apparatus with energy elevator, according to an embodiment of the present invention. 
           [0015]      FIG. 6  is a diagrammatic representation of the axial potential distribution in voltage according to an embodiment with energy elevator of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0016]    The present invention relates generally to an electron beam inspection system. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiment and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0017]    Reference will now be made in detail to specific embodiments of the invention. Examples of these embodiments are illustrated in accompanying drawings. While the invention will be described in conjunction with these specific embodiments, it will be understood that it is not intended to limit the invention to these embodiments. On the contrary, it is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention as defined by the appended claims. In the following description, numerous specific details are set forth in order to provide a through understanding of the present invention. The present invention may be practiced without some or all of these specific details. In other instances, well known process operations are not been described in detail in order not to unnecessarily obscure the present invention. 
         [0018]    The present invention relates to an electron beam inspection system. More specifically, embodiments of the present invention are utilized with Swinging Objective Retarding Immersion Lens (SORIL) system to provide a low landing energy, high resolution low current system for electron beam inspection. More specifically, the present invention not only can be used on wafer inspection, but also on mask inspection and after development inspection (ADI). However, it would be recognized that the invention has a much broader range of applicability. 
         [0019]    As mentioned above, the present invention a SORIL system disclosed by, for example, U.S. Pat. No. 6,392,231 is utilized to examine the specimen. The SORIL system includes a magnetic lens to generate a magnetic field in the vicinity of the specimen for focusing the electron beam on the specimen. The magnetic lens has a central bore through which electron beam travel toward the specimen. The SORIL system also includes an electrode to provide a retarding electric field to convert the high energy electron beam to a lower energy electron beam before strike onto the specimen surface to minimize damage to the specimen. Moreover, the SORIL system includes a plurality of deflecting unites situated along the beam axis for deflecting the electron beam to allow scanning of the specimen. At least one of the deflecting units is located in the retarding field to provide a relatively large deflecting field to reduce the time to fully examine the specimen. To describe the features of the present invention in more detail, refer to the following description. 
         [0020]      FIG. 1  is a diagrammatic representation of an electron beam apparatus according to an embodiment of the present invention. The electron beam apparatus  100  includes an electron beam source  102 , a cathode  105 , an anode  106 , a condenser lens  107 , an aperture  108 , and a SORIL objective lens system  104 . Beam axis  109  is defined as the line connecting the electron beam source  102  and the specimen  122 , which is the primary electron beam travel during inspection. Within the SORIL lens  104 , around the beam axis  109 , there are a plurality of deflection units  120  and ExB filter  121 . The ExB filter  121  aligns or directs the secondary electron  140  (or signal electron) beam emanating from the specimen surface to detection system  124 . The detection system  124  of the present invention is a scintillator-photomultiplier combination detector. This type of detector has higher sensitivity and lower noise to the secondary electrons  140  (or signal electrons) than a semiconductor type detector. However, the detecting system can also perform side view of the specimen when a segment detector is equipped onto the electron beam system as disclosed, for example, by Wang et al. in U.S. Patent Application 11/668846. 
         [0021]    It is calculated by Chen et al. that in order to obtain a high resolution for an objective lens, relatively high electron beam energy, for example 15 keV or more, primary electron beam source is required. Applying such a high energy electron beam directly to the specimen may induce radiation damage on the substrate material, which consequently leads to a reliability problem in the emerging, manufacturing and production of integrated circuits. Therefore, a retarding field is generally equipped with the electron beam system to reduce the landing energy of the primary electron beam. Using a low energy primary electron beam source is a way to avoid the material damage induced reliability issue. However, lower electron beam energy will limits the spatial resolution by worse chromatic aberration of the objective lens and by more electron-electron interaction within the beam. Accordingly, both electron-electron interaction (space-charge effect) and chromatic aberration can ameliorated by using a higher energy electron beam. 
         [0022]      FIGS. 2 ,  3 , and  4  are diagrams that illustrate axial electron potential energy of a conventional electron beam system, an electron beam system with SORIL lens and the present invention, respectively. Both of the two prior art electron beam source diagrams ( 2  and  3 ) do not have a beam energy that is as high as the present invention. Unlike the prior art, a system and method in accordance with the present invention does not use an extractor between the cathode  105  and anode  106  of the electron beam source  102 . An extractor is designed to pull electrons out of cathode during the first generation of electron beam source. Conventionally, the electron acceleration potential is applied to the electrons in two steps with an extractor in between the cathode and anode. In the first step between the cathode and the extractor, about 3 to 5 kV are applied to pull electrons out. The second step applies the rest of the potential between the extractor and anode to accelerate electrons pulled out in the first step. The benefit of apply acceleration potential through the extractor is that it can keep stable the emission current of electron source stable. Another benefit of which is that process can accurately control the total acceleration potential applied on electrons with a control circuit setup between the circuit of extractor and anode. 
         [0023]    Although the presence of extractor will provide a bigger total emission under a given emission angular intensity than without an extractor, it will also induce more electron-electron interaction in the meantime. As mentioned above, more electron-electron interaction is bad for chromatic aberration. The system has several features that allow for improved operation that will be described in detail hereinafter. 
       Direct Application of Acceleration Potential 
       [0024]    The electron beam acceleration potential is directly applied across the cathode  105  and anode  106  no extractor is located in between. The electron beam acceleration potential, for example 16 keV, is applied to the electrons in such way as −8 kV on the cathode (Vk) and +8 kV on the anode (VA). After the primary beam  110  exit anode  106 , the axial potential is dropped to virtual ground by a pair of grounding rings  111 . The pair of grounding rings  111  provides a virtual ground voltage potential (Vg) to those components within the electron beam apparatus that installed below the source anode and above the last polepiece of SORIL objective lens. The primary beam electrons will then travel with a lower speed and energy for example, 8 keV. The primary beam  110  is then passed through the condenser lens  113  to pre-focus the beam and form a beam crossover if necessary. Following the condenser lens  113 , an aperture  114  is set to confine the primary beam diameter to a range, for example, from 20 micrometer to 200 micrometer before passing through the SORIL  104  objective lens system. The SORIL  104  system also is at virtual ground (Vg). The benefit of operating at virtual ground is that the electrostatic operated deflecting units  120  can provide a fast response and larger field of view during the inspection process. The specimen  122  is charged to, for example, −6 kV (Vs) to provide a retarding electric field. The retarding electron field will decelerate the primary electron beam  110  before it impact with the specimen  122  of lower landing energy, for example 2 keV in the present experience. 
       Provided High Energy 
       [0025]    As mentioned above, higher beam energy may ameliorate the chromatic aberration of the apparatus. The conventional and the SORIL electron beam both designed an over 10 kV electron acceleration voltage across the cathode (Vk, minus potential) and the anode (VA, ground potential). Such a high potential (over 10 kV) respect to the ground potential is very easy to induce micro-arcing during inspection process between the short distance between the polepiece of the objective lens and sample stage. In the present invention, the high acceleration voltage (16 kV) between the cathode and the anode is divided across the ground potential, for example −8 kV on cathode and +8 kV on anode. The present design reduces the voltage floating relative to the ground potential (−8 kV and +8 kV). On the sample stage with a retarding field the voltage is further reduced to −6 kV. Thus decreases the micro-arcing probability during operation. 
       Minimize Micro-Arcing Problem 
       [0026]    An ameliorating electron-electron interaction or Boersch effect within the electron beam is disclosed utilizing this system. The present invention design set the aperture  114  is provided beneath the condenser lens  113  to trim down the excess electrons in the early stage. 
       Placing Detector at a Positive Potential 
       [0027]    A detecting condition for the scintillator-photomultiplier detector  124  is disclosed utilizing this system. A scintillator-photomultiplier detector  124  is a very sensitive detector for detecting secondary electrons. However, the detector requires 10 kV bias to accelerate the secondary electrons  140  emanating from the specimen  122  surface to get the best result. For example, a specimen should be exposed to a retarding field of −6 kV, therefore, the detector  124  is designed to operate +4 kV (Vsed) for the best detecting result. 
       Energy Elevator 
       [0028]    Another way to ameliorate the electron-electron interaction by shorten the time that electrons travel together is disclosed utilizing this system. It is well-known that electrons in a higher voltage potential electric field travel faster than which in a lower voltage potential electric field. An energy elevator  210  illustrated in  FIG. 5 , is designed for to keep the primary beam electrons in high energy condition before aperture. In this embodiment, the pair of grounding rings  111  provides virtual ground potential (Vg) to those components within the electron beam apparatus installed below the source anode and above the last polepiece of SORIL objective lens.  FIG. 6  is a schematic diagrammatic representation of the axial potential distribution in voltage according to the embodiment with the energy elevator  210 . 
         [0029]    Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the appended.