Abstract:
A substrate inspection method includes:
       generating an electron beam and irradiating the electron beam as a primary electron beam to a substrate as a specimen;   inducing at least any of a secondary electron, a reflected electron and a backscattering electron which are emitted from the substrate receiving the primary electron beam, and magnifying and projecting the induced electron as a secondary electron beam so as to form an image of the secondary electron beam; a trajectory of the primary electron beam and a trajectory of the secondary electron beam having an overlapping space and space charge effect of the secondary electron beam occurring in the overlapping space,   detecting the image of the secondary electron beam to output a signal representing a state of the substrate; and   suppressing aberration caused by the space charge effect in the overlapping space.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
   This application claims benefit of priority under 35 USC §119 to Japanese Patent Application No. 2004-159089, filed on May 28, 2004, the contents of which are incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a substrate inspection method, a method of manufacturing a semiconductor device and a substrate inspection apparatus. 
   2. Related Art 
   In recent years, methods of inspecting defects in semiconductor patterns by utilizing electron beams have been proposed. For example, there is disclosed in Japanese Patent Laid-Open No. 7 (1995)-249393 a method which includes: generating a rectangular electron beam as a primary electron beam by an electron irradiator and irradiating a specimen with the electron beam; then enlarging and projecting an image of secondary electrons, reflected electrons and backscattered electrons generated according to the change of the shape/properties/potential of the specimen surface, as a secondary electron beam, on an electron detector by a mapping projection optical system; and obtaining an image of the specimen surface. In addition to the method, another method has been proposed in Japanese Patent Laid-Open No. 11 (1999) -132975, by which a primary electron beam is deflected by a Wien filter as an electron beam deflector so as to be nearly perpendicularly incident on a specimen surface, and also a secondary electron beam is made to travel straight through the same Wien filter so as to enter a mapping projection optical system. 
   In the apparatus disclosed in Japanese Patent Laid-Open No. 11 (1999)-132975, the Wien filter is used to form a Koehler illumination system in order to make the primary electron beam perpendicularly incident on the specimen surface. Specifically, the primary electron beam entering the Wien filter is deflected by the Wien filter so that the focus of the primary electron beam coincides with the focus of the secondary beam between the Wien filter and a cathode lens. Further, in order to minimize the effect of deflection aberration in the Wien filter, an image of the secondary electron beam is once formed on the center surface of the Wien filter. 
   However, the apparatus disclosed in Japanese Patent Laid-Open No. 11 (1999)-132975, in which there exists space where the primary electron beam overlaps with the secondary electron beam between the Wien filter and the specimen, has a problem in that the aberration of the secondary electron beam is increased by the significant influence of the electron-electron interaction, i.e. the so-called space charge effect due to the high electron density in this overlapping space. In particular, the current density of the secondary electron beam is the highest at the focus position of the secondary electron beam, and the focus position of the primary electron beam where the current density of the primary electron beam is the highest is also exists at the same position, as a result of which the space charge effect of the secondary electron beam becomes most significant in the above described overlapping space. 
   BRIEF SUMMARY OF THE INVENTION 
   According to a first aspect of the present invention, there is provided a substrate inspection method comprising: 
   generating an electron beam and irradiating the electron beam as a primary electron beam to a substrate as a specimen; 
   inducing at least any of a secondary electron, a reflected electron and a backscattering electron which are emitted from the substrate receiving the primary electron beam, and magnifying and projecting the induced electron as a secondary electron beam so as to form an image of the secondary electron beam; a trajectory of the primary electron beam and a trajectory of the secondary electron beam having an overlapping space and space charge effect of the secondary electron beam occurring in the overlapping space, 
   detecting the image of the secondary electron beam to output a signal representing a state of the substrate; and 
   suppressing aberration caused by the space charge effect in the overlapping space. 
   According to a second aspect of the present invention, there is provided a method of manufacturing a semiconductor device comprising a substrate inspection method including: 
   generating an electron beam and irradiating the electron beam as a primary electron beam to a substrate as a specimen; 
   inducing at least any of a secondary electron, a reflected electron and a backscattering electron which are emitted from the substrate receiving the primary electron beam, and magnifying and projecting the induced electron as a secondary electron beam so as to form an image of the secondary electron beam; a trajectory of the primary electron beam and a trajectory of the secondary electron beam having an overlapping space and space charge effect of the secondary electron beam occurring in the overlapping space, 
   detecting the image of the secondary electron beam to output a signal representing a state of the substrate; and 
   suppressing aberration caused by the space charge effect in the overlapping space. 
   According to a third aspect of the present invention, there is provided a substrate inspection apparatus comprising: 
   an electron beam irradiator which generates an electron beam and which irradiates the electron beam as a primary electron beam to a substrate as a specimen; 
   a detector with a detecting surface to detect at least any of a secondary electron, a reflected electron and a backscattering electron which are emitted from the substrate receiving the primary electron beam, the detector outputting a signal representing a state of the substrate; 
   a mapping projection optical system which induces at least any of the secondary electron, the reflected electron and the backscattering electron so as to magnify and project the induced electron as a secondary electron beam, and which forms an image of the secondary electron beam on the detecting surface of the detector; a trajectory of the primary electron beam and a trajectory of the secondary electron beam having an overlapping space and space charge effect of the secondary electron beam occurring in the overlapping space, and 
   an aberration suppressor which suppresses aberration caused by the space charge effect in the overlapping space. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
       FIG. 1  is a block diagram schematically showing a configuration of a substrate inspection apparatus according to a first embodiment of the present invention; 
       FIG. 2  is a perspective view showing a specific configuration of a Wien filter provided for the substrate inspection apparatus shown in  FIG. 1 ; 
       FIG. 3  is an illustration of an operation principle of the Wien filter shown in  FIG. 2 ; 
       FIG. 4  is an illustration of an operation principle of the Wien filter shown in  FIG. 2 ; 
       FIG. 5  is an electron beam trajectory figure explaining a substrate inspection method according to the first embodiment of the present invention; 
       FIG. 6  is a block diagram schematically showing a configuration of a substrate inspection apparatus according to a second embodiment of the present invention; 
       FIG. 7  is an electron beam trajectory figure explaining a substrate inspection method according to the second embodiment of the present invention; 
       FIG. 8  is a perspective view showing a round hole electrode provided for the substrate inspection apparatus shown in  FIG. 6 ; and 
       FIG. 9A  and  FIG. 9B  are a top view and side view of the round hole electrode provided for the substrate inspection apparatus shown in  FIG. 6 , respectively. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   (1) First Embodiment 
     FIG. 1  is a block diagram schematically showing a configuration of a substrate inspection apparatus according to a first embodiment of the present invention. As will be described in further detail below, the present embodiment is characterized in that a deflector  68  is arranged between multistage quadrupole lenses  15  and a Wien filter  41 , and in that the deflector  68  controls a trajectory of a primary electron beam Bp so as to shrink space where the primary electron beam Bp overlaps with a secondary electron beam Bs. 
   The substrate inspection apparatus  1  shown in  FIG. 1  comprises a primary optical system  10 , a secondary optical system  20 , an electron detecting unit  30 , the Wien filter  41 , a stage  43 , various control units  16 ,  17 ,  51  to  56 ,  70 , an image signal processing unit  58 , a host computer  60  and a display unit  62 . 
   The primary optical system  10  includes an electron beam gun  11  and the multistage quadrupole lenses  15 . The electron beam gun  11 , which corresponds to e.g. electron beam irradiating means, has a LaB6 line cathode  112  having a rectangular electron emitting surface with the long axis of 100 to 700 μm and the short axis of 15 μm, a Wehnelt electrode  114 , an anode  116  to derive an electron beam and a deflector  118  to adjust an optical axis. The LaB6 line cathode  112 , the Wehnelt electrode  114 , the anode  116  and the deflector  118  are connected to the control unit  16 , and an acceleration voltage, an ejecting current and an optical axis Ap of the primary electron beam Bp are controlled by a signal from the control unit  16 . The quadrupole lenses  15  are connected to the quadrupole lenses control unit  17 . The primary electron beam Bp emitted from the line cathode  112  is converged by the quadrupole lenses  15  controlled by a signal from the quadrupole lenses control unit  17 , and is incident obliquely on the Wien filter  41 . The primary electron beam Bp is deflected in the direction perpendicular to a specimen S by the Wien filter  41 , and then subjected to a lens action by a cathode lens  21  in the secondary optical system  20  so as to be irradiated perpendicularly to the specimen S. 
   The specimen S is placed on the top surface of the stage  43 , which is arranged such that a negative voltage can be applied to the specimen S by the stage voltage control unit  51 . This arrangement is to reduce the incidence damage of the specimen S caused by the primary electron beam Bp, and to increase the energy of the secondary electron beam Bs consisting of secondary electrons, reflected electrons and backscattered electrons which are generated according to the change of the shape/properties/potential of the specimen S surface by the irradiated primary electron beam Bp. 
     FIG. 2  shows a specific configuration of the Wien filter  41 , the operation principle of which is described briefly with reference to  FIG. 3  and  FIG. 4 . As shown in  FIG. 2 , the fields of the Wien filter  41  are formed so as to make the electric field E and the magnetic field B orthogonal to each other in a plane CSw perpendicular to an optical axis As of the secondary optical system, and functions to allow only those electrons among the incident electrons which meet the Wien condition of qE=vB (q: electron charge, v: velocity of electron advancing straight), to advance straight. As shown in  FIG. 3 , in the inspection device  1 , the force FB of the magnetic field and the force FE of the electric field acts on the primary electron beam Bp in the same direction so that the primary electron beam Bp is deflected so as to be perpendicularly incident on the specimen S. On the other hand, as shown in  FIG. 4 , the FB and the FE act on the secondary electron beam Bp in the reverse direction and the Wien condition of FB=FE is still established, as a result of which the secondary electron beam Bs advances straight to enter the secondary optical system without being deflected. 
   Returning to  FIG. 1 , the secondary optical system  20 , which corresponds to e.g. mapping projection means, includes the cathode lens  21  which is a rotationally symmetric electrostatic lens, a second lens  22 , a third lens  23  and a numerical aperture  24  and a field aperture  26 , which apertures are arranged between the second and third lenses. The cathode lens  21 , the second lens  22 , the third lens  23 , are connected to the secondary optical system lens control units  52 ,  54 ,  55 , respectively, and the secondary electron beam Bs is enlarged and projected by control signals outputted from these control units so that an image of the secondary electron beam is formed on a detecting surface of a MCP (Micro Channel plate) detector  31 . 
   The electron detecting unit  30 , which corresponds to e.g. detecting means, includes the MCP detector  31 , a fluorescent plate  32 , a light guide  33  and an imaging element  34 , such as CCD (Charged Coupled Device). The secondary electron beam Bs incident on the MCP detector  31  is amplified by the MCP and irradiated to the fluorescent plate  32 . A fluorescent image generated by the fluorescent plate  32  is detected by the imaging element  34  via the light guide  33 . A signal outputted from the imaging element  34  is processed by the image signal processing unit  58  and transmitted to the host computer  60  as image data. 
   The host computer  60  is connected to the various control units  16 ,  17 ,  51  to  56 ,  70 , and controls the overall apparatus via these control units. The host computer  60 , which is also connected to the image signal processing unit  58  and the display unit  62 , receives the image data from the image signal processing unit  58  and stores the image data in an image memory (not shown), and displays the image of the secondary electron beam in the display unit  62 , and also performs a defect detecting processing, etc. by means of image processing corresponding to an inspection object. 
   In this apparatus, a Koehler illumination system is formed to make the primary electron beam Bp perpendicularly incident on the specimen S. Heretofore, in order to form the Koehler illumination system, a measure has been used to make a focus FPp of the primary electron beam coincide with a focus FPs of the secondary electron beam. The measure can be realized by deflecting the primary electron beam Bp with the Wien filter  41  so that the focus FPp of the primary electron beam coincide with the focus FPs of the secondary electron beam between the Wien filter  41  and the cathode lens  21 . 
   However, between the Wien filter  41  and the specimen S there exists space where the trajectory of the primary electron beam Bp overlap with the trajectory of the secondary electron beam Bs (see  FIG. 5 ). In this space, the influence of the electron-electron interaction, i.e. the so-called space charge effect becomes significant due to the high electron density in this space, thereby causing a problem that the aberration of the secondary electron beam Bs is increased. In particular, since the current density of each beam is the highest in any of the focus position FPp of the primary electron beam and the focus position FPs of the secondary electron beam, in the case where the focus FPp of the primary electron beam is made to coincide with the focus FPs of the secondary electron beam, the aberration of the secondary electron beam Bs is significantly increased by the synergistical action of the space charge effect. In order to reduce the aberration of the secondary electron beam Bs, it is most effective to shrink the space charge effect in the focus position FPs of the secondary electron beam. 
   The substrate inspection apparatus  1  according to the present embodiment controls deflection of the primary electron beam Bp by means of the deflector  68  arranged between the multistage quadrupole lenses  21  and the Wien filter  41 , and thereby suppresses the increase of aberration due to the space charge effect of the secondary electron beam Bs without considerably impairing the Koehler illumination for the primary electron beam Bp. The deflector  68  is connected to the deflection control unit  70 , and the deflection control unit  70  is further connected to the host computer  60 . The host computer  60  generates a control signal for deflecting the primary electron beam Bp so that the space charge effect in the focus position FPs of the secondary electron beam is reduced and thereby the resolution of the secondary electron beam image is improved, and transmits the control signal to the deflection control unit  70 . The deflection control unit  70  receives the control signal from the host computer  60 , and applies a deflection voltage corresponding to the control signal to each electrode of the deflector  68 , whereby the deflector  68  forms a deflection field to deflect the primary electron beam Bp. When the primary electron beam Bp is deflected excessively, however, the center of illumination area on the specimen S is largely deviated from the optical axis As of the secondary optical system, so that the primary electron beam Bp is not irradiated sufficiently to the intersection of the optical axis As of the secondary optical system and the surface of the specimen S and to the vicinity of the intersection, where the lowest aberration is expected. As a result, sufficient quantity of the secondary electrons/reflected electrons/backscattering electrons are not emitted from the intersection of the optical axis As of the secondary optical system and the surface of the specimen S and from the vicinity of the intersection, so that the S/N of signals outputted from the imaging element  34  is lowered so as to deteriorate the inspection image. The host computer  60  performs control to optimize the deflection of the primary electron beam Bp within a range in which these adverse effects cause no problem in the inspection. 
   Thus, as in the electron beam trajectory figure shown in  FIG. 5 , the host computer  60  performs the deflection control of the primary electron beam Bp so as to make a focal surface FS 1  of the primary electron beam Bp coincide with a focal surface FS 2  of the secondary electron beam Bs, and to prevent the focus position FPs of the secondary electron beam where the current density of the secondary electron beam is the highest in the above described overlapping space from coinciding with the focus position FPp of the primary electron beam where the current density of the primary electron beam Bp is the highest in the overlapping space. Thereby, the increase of aberration of the secondary electron beam Bs due to the space charge effect can be effectively suppressed without considerably impairing the Koehler illumination for the primary electron beam Bp. In the present embodiment, the overlapping space of the primary electron beam Bp and the secondary electron beam Bs is shrunk by controlling the trajectory of the primary electron beam Bp, but the embodiment is not limited to the case, it is also possible to shrink the above described overlapping space by controlling the trajectory of the secondary electron beam Bs. 
   (2) Second Embodiment 
     FIG. 6  is a block diagram schematically showing a configuration of a substrate inspection apparatus according to a second embodiment of the present invention, and  FIG. 7  is an electron beam trajectory figure explaining a substrate inspection method according to the present embodiment of the present invention. A substrate inspection apparatus  3  according to the present embodiment is characterized in that rotationally symmetric round hole electrodes  72 ,  82  which are arranged between the Wien filter  41  and the cathode lens  21  and which are connected to power supplies  74 ,  84 , respectively, are further provided, and in that the secondary electron beam Bs is accelerated in the overlapping space of the primary electron beam Bp and the secondary electron beam Bs, by an acceleration electric field formed by the electrodes  72 ,  82 . 
     FIG. 8  is a perspective view showing the electrode  72 .  FIG. 9A  shows a top view of the electrode  72  and  FIG. 9B  shows a side view along the line A-A in  FIG. 9A . As typically shown in  FIG. 8 ,  FIG. 9A  and  FIG. 9B , the electrodes  72 ,  82  are provided at the center thereof with a round hole for allowing the passage of the secondary electron beam Bs, respectively. 
   Returning to  FIG. 6 , the electrode voltage applying power supplies  74 ,  84  are further connected to the host computer  60 . The host computer  60  generates a control signal to form the acceleration electric field to accelerate the secondary electron beam Bs, and supplies the signal to the power supplies  74 ,  84 . The electrode voltage applying power supplies  74 ,  84  apply voltages in accordance with the control signal from the host computer  60  to the electrodes  72 ,  82 , so as to accelerate the secondary electron beam Bs between the electrodes  72 ,  82 . In order to form the acceleration electric field for the secondary electron beam Bs, for example, it is only necessary that a positive voltage is applied to the electrode  72  and a positive voltage lower than the positive voltage applied to the electrode  72  or a negative voltage or 0 voltage is applied to the electrode  82 . 
   Since the current density is the highest at the focus position of the secondary electron beam Bs, in the case where the focus position of the secondary electron beam Bs is included in the space where the secondary electron beam Bs is accelerated, the increase of aberration of the secondary electron beam Bs due to the space charge effect can be effectively suppressed without impairing the Koehler illumination system for the primary electron beam Bp. In the present embodiment, as shown in the electron beam trajectory figure in  FIG. 7 , the electrode  72  and the electrode  82  are arranged so that the focal surface FS 1  of the primary electron beam coincides with the focal surface FS 2  of the secondary electron beam, that the focus position FPp of the primary electron beam where the current density of the primary electron beam Bp is the highest in the overlapping space of the primary electron beam Bp and the secondary electron beam Bs, coincides with the focus position FPs of the secondary electron beam where the current density of the secondary electron beam Bs is the highest in the above described overlapping space, and that the focal surface FS 2  of the secondary electron beam exists between the electrode  72  and the electrode  82 . 
   (3) Method of Manufacturing Semiconductor Device 
   The use of the above described substrate inspection method in the manufacturing process of a semiconductor device enables a semiconductor device to be inspected with high sensitivity and to thereby be manufactured in a short TAT (Turn Around Time) and with a high yield.