Patent Publication Number: US-11651929-B2

Title: Charged particle source and charged particle beam device

Description:
TECHNICAL FIELD 
     The present invention relates to a charged particle source that emits charged particles. 
     BACKGROUND ART 
     A charged particle source is used in a charged particle beam device such as a scanning electron microscope (SEM). The SEM is a device that performs measurement and inspection of a sample shape using an image obtained by scanning a sample with a focused electron beam. With complexity and miniaturization of a semiconductor device in recent days, especially in the SEM for semiconductor device inspection, there has been a strong demand for high resolution, improved throughput of measurement and inspection, and the like. In order to improve inspection sensitivity and length measurement capability of the SEM under a high current density condition, it is effective to narrow an energy width of particles emitted from an electron source and reduce an electron source diameter. 
     The following Patent Literature 1 takes “providing an electron source capable of preventing an increase in a surplus current and extraction voltage as compared with those in the related art and reducing an energy width in use with a relatively high angular current density” as a problem to be solved, and describes a technique of “an electron source provided with a supply source of an element for reducing a work function of a high-melting point metal on a needle made of a single crystal of the high-melting point metal with an axial orientation of &lt;100&gt;, in which an acuminate portion at a tip end of the needle has a shape including a conical portion (A), a columnar portion (B) following the conical portion (A), and a conical portion (C) following the conical portion (B) with a forefront end portion in a spherical shape, a conical angle (θ) of the conical portion (A) is 25 degrees or less; a curvature radius (r) at a spherical surface portion of a tip end of the conical portion (C) is 1.0 to 2.5 μm; and a ratio (r/L) of the curvature radius (r) of the spherical surface portion at the tip end of the conical portion (C) to a distance (L) from a boundary between the conical portion (A) and the columnar portion (B) to the tip end of the conical portion (C) is 0.1 to 0.3″ (see Abstract). 
     The following Patent Literature 2 takes “implementing an electron emitting element and an electron gun having a high luminance and a narrow energy width, and implementing an electron microscope and an electron beam drawing device which are equipped with the electron emitting element and the electron gun and have a high luminance and a high resolution” as a problem to be solved, and discloses a technique of “considering emitting electrons from the vicinity of a five-membered ring of a closed structural region in an electron emitting element using a tubular substance having carbon as a main component, providing a large-diameter tube as necessity in order to reduce energy dispersion effect by space electron repulsion by widening an interval within the five-membered ring, and implementing a large-diameter tube structure with a stable structure by reducing dynamic distortion of a cap structure in the invention” (see Abstract). 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1: JP-A-2005-339922 
         PTL 2: JP-A-2008-305597 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     Patent Literature 1 aims to reduce an energy width of particles under a high current density condition with a configuration in which an electron source including a conical portion and a columnar portion. On the other hand, as to an electron source shape, since an emitter shape is discontinuous in the vicinity of an intersection of a spherical surface portion and the columnar portion, an electric field at a position deviated from a central axis of the electron source is non-uniform. When electrons are emitted from the off-axis position, a non-uniform electric field intensity acts on the emitted electrons with aberration, and an electron source diameter increases. Since the increase in the electron source diameter leads to an increase in a beam diameter, spatial resolution decreases. In other words, although the energy width of particles from the electron source described in the same literature is narrowed, it is difficult to prevent a decrease in the inspection sensitivity and length measurement capability of an SEM, especially under a high current density condition. 
     Patent Literature 2 describes a method for stabilizing a tube structure in a case where a carbon nanotube is used as an electron source. However, in the same literature, an electric field in the vicinity of the electron source is not particularly considered. Therefore, according to the technique described in the same literature, it is not necessarily clear whether the inspection sensitivity and the length measurement capability of the SEM can be prevented from being reduced. 
     The invention has been made in view of the above problems, and an object of the invention is to provide a charged particle source in which energy dispersion of an emitted charged particle beam is small even under a high-angle current density condition and a large charged particle current can be stably obtained even with a small light source diameter. 
     Solution to Problem 
     The charged particle source according to the invention has a spherical virtual cathode surface from which charged particles are emitted, and a virtual cathode surface for charged particles emitted from a first position on a surface of an emitter tip end and a virtual cathode surface for charged particles emitted from a second position on the surface of the emitter tip end match each other. 
     Advantageous Effect 
     According to the charged particle source in the invention, electric field intensity distribution in the vicinity of an emitter tip end is uniform over a wide range. Accordingly, energy dispersion of an emitted charged particle beam is prevented to be small, and a large charged particle current can be stably obtained even with a small light source diameter. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a side view showing an example of a tip end shape of a charged particle source  1  in the related art. 
         FIG.  1 B  is a view showing electric field intensity distribution in the vicinity of the charged particle source  1  shown in  FIG.  1 A . 
         FIG.  2 A  is a side view showing another example of a tip end shape of the charged particle source  1  in the related art. 
         FIG.  2 B  is a view showing electric field intensity distribution in a case where an elevation angle α is set to 90 degrees in the shape example of  FIG.  2 A . 
         FIG.  3    is a side view showing a tip end shape of the charged particle source  1  according to a first embodiment. 
         FIG.  4    is a view showing electric field intensity distribution in the vicinity of the charged particle source  1  according to the first embodiment. 
         FIG.  5    shows a simulation result of electric field intensities on a surface of the charged particle source  1 . 
         FIG.  6    is a graph showing a correspondence relationship between an energy width (ΔE) of electrons and a radius R of a spherical surface  4 . 
         FIG.  7    is a view showing a tip end shape of the charged particle source  1  according to a third embodiment. 
         FIG.  8    is a view showing emitted charged-particle trajectories in the vicinity of an extraction electrode  20 . 
         FIG.  9    is a diagram of a configuration of a charged particle beam device  100  including the charged particle source  1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     &lt;Charged Particle Source in Related Art&gt; 
       FIG.  1 A  is a side view showing an example of a tip end shape of a charged particle source  1  in the related art. The charged particle source  1  includes a tip end portion with a spherical surface shape at a tip end of an emitter needle  7 . The tip end portion has a radius R with a central point  5  as a center. An intersection  9  between a ridge line of the emitter needle  7  and a spherical surface of the tip end portion is at a position having an elevation angle α with respect to an optical axis  3 . An angle θ between the ridge line of the emitter needle  7  and the optical axis  3  may be referred to as a cone angle. When heat or a high electric field is applied to the tip end portion of the emitter needle  7 , electrons are emitted in a direction of the optical axis  3 . 
     In the charged particle source  1  in the related art, when the elevation angle α is smaller than 90 degrees, the electric field intensity distribution in the vicinity of the tip end portion is not uniform as described below. In particular, the electric field intensity distribution becomes non-uniform toward the vicinity of the intersection  9 . As a result, electrons emitted from a portion away from the optical axis  3  are not on so-called paraxial trajectories and would receive off-axis aberration. Accordingly, a light source diameter of a charged particle beam increases and thus spatial resolution of the charged particle beam decreases. In order to reduce the off-axis aberration, it is necessary to reduce an acceptance angle of the charged particle beam (described later), but in this case, no large charged particle current can be obtained. A large current can be obtained even when the acceptance angle of the charged particle beam is small if an angular current density is large, but energy dispersion of an emitted charged particle beam becomes large when the angular current density is increased at the small acceptance angle. 
     The tip end shape as shown in  FIG.  1 A  has the following problems: properties of emitted charged particles become unstable due to reasons such as (a) the electric field intensity distribution in the vicinity of the emitter tip end is non-uniform, and (b) the tip end shape becomes thermodynamically unstable and changes easily since a region where the tip end shape is uniform is small. 
     In particular, in an SEM for semiconductor inspection, high-speed scanning of electron beams is effective as a solution for improving an inspection speed. In order to obtain a high-quality image, the charged particle source is required to operate under a high-angle current density condition. However, since an energy width of the electron beams increases and the light source diameter increases under the high-angle current density condition for the above reasons, the spatial resolution of the SEM may decrease and inspection sensitivity and dimensional measurement capability may decrease. 
       FIG.  1 B  is a view showing electric field intensity distribution in the vicinity of the charged particle source  1  shown in  FIG.  1 A . An electric field generated by an extraction electrode (not shown) is generated in the vicinity of the tip end of the charged particle source  1 . Intensity and distribution of the electric field are represented by an equivoltage surface  16 . Since the elevation angle α is smaller than 90 degrees, a tangent line  13  of the spherical surface at the intersection  9  intersects with the optical axis  3 . In such a shape, the electric field intensity distribution in the vicinity of the spherical surface  4  becomes non-uniform. This indicates that the equivoltage surface  16  is not parallel to the spherical surface  4  and the emitter needle  7 . 
       FIG.  1 B  shows emission points S 1  to S 3  of charged particle beams on the spherical surface  4 . The charged particle beams from the emission points have corresponding trajectories  17 . Intersections of ridge lines of trajectories  17  can be regarded as positions where the charged particles are geometrically emitted. These virtual emission points constitute a virtual cathode surface  10 . In the tip end shape as shown in  FIG.  1 B , the virtual cathode surface  10  has an elliptical surface shape extending in a direction perpendicular to the optical axis  3 . Then, a distance between the emission point S 1  and the virtual cathode surface  10 , a distance between the emission point S 2  and the virtual cathode surface  10 , and a distance between the emission point S 3  and the virtual cathode surface are different from each other, and thus the emission points S 1  to S 3  are not equivalent. Although adjacent emission points, such as the emission points S 1  and S 2 , can be regarded as approximately equivalent, a range that can be regarded as such is fairly small. Therefore, a permissible elevation angle β that can be regarded as equivalent to properties of the charged particle beam is also small. 
     It is assumed that a planar virtual light source  11  is disposed at a center of the virtual cathode surface  10 . Downstream of the charged particle source  1  (space between the charged particle source  1  and a sample) provided an opening plate used to narrow the charged particle beam. Although the charged particle beam is emitted from each position on the surface of the spherical surface  4 , the charged particle beam with which the sample is irradiated can be narrowed by adjusting an opening and narrowing the acceptance angle. When the ridge line of the trajectory  17  of the charged particle beam to be accepted (passing through the opening plate) is extended to the virtual light source  11 , both ends of the virtual light source  11  (i.e., a size of the virtual light source  11 ) are determined according to the acceptance angle. When the acceptance angle is increased in order to obtain a large charged particle current (e.g., the charged particle beam from the emission point S 3  in  FIG.  1 B  is accepted), the size of the virtual light source  11  becomes large. An increase in a light source size means a decrease in the spatial resolution of the charged particle beam. 
       FIG.  2 A  is a side view showing another example of a tip end shape of the charged particle source  1  in the related art. In  FIG.  2 A , an emitter tip end portion is divided into a spherical surface portion and a flat portion (or a columnar portion). The flat portion intersects with the emitter needle  7  at an intersection  9 ′. The elevation angle α is the same as that in  FIG.  1 A . Although the charged particle source  1  of  FIG.  2 A  is the same as that of  FIG.  1 A  in terms of electron emission operation, the electric field intensity distribution in the periphery of the emitter tip end portion is different from that of  FIG.  1 A , and electron emission properties are also different from those of  FIG.  1 A . 
       FIG.  2 B  is a view showing electric field intensity distribution in a case where the elevation angle α is set to 90 degrees in the shape example of  FIG.  2 A . When the elevation angle α is set to 90 degrees, the tangent line  13  is parallel to the optical axis  3 . Even in this case, especially in the vicinity of the intersection  9 ′, the electric field intensity distribution becomes non-uniform as shown by the equivoltage surface  16 . Further, similarly to  FIG.  1 B , the virtual cathode surface  10  is not a spherical surface but an elliptical surface shape. Therefore, the same problem also occurs in  FIG.  2 B . 
     First Embodiment 
       FIG.  3    is a side view showing a tip end shape of the charged particle source  1  according to the first embodiment of the invention. In  FIG.  3   , the intersection  9  where a ridge line of the emitter needle  7  intersects with the spherical surface  4  is disposed at a position where the elevation angle α exceeds 90 degrees. Other configurations are the same as in  FIG.  1 A . 
       FIG.  4    is a view showing electric field intensity distribution in the vicinity of the charged particle source  1  according to the first embodiment. In  FIG.  4   , a shape of the equivoltage surface  16  is relatively uniform even in the vicinity of the intersection  9 . In other words, a region where the equivoltage surface  16  is parallel to the spherical surface  4  is wider than that in  FIGS.  1 B and  2 B . At least in a region on a tip end side with respect to the central point  5 , the equivoltage surface  16  is parallel to the spherical surface  4 . Since the elevation angle α exceeds 90 degrees and the intersection  9  is disposed at a position away from the emitter tip end, it is considered that interference between an electric field in the vicinity of the spherical surface  4  and an electric field in the vicinity of the emitter needle  7  is weakened. 
     When a virtual cathode surface of the emission point S 1  is equal to a virtual cathode surface of the emission point S 2 , S 1  and S 2  are equivalent. The same applies to S 3 . This condition is satisfied when electric field intensities at the emission points S 1  to S 3  are uniform (the equivoltage surface  16  is parallel to the spherical surface  4 ). Therefore, since the emission points that can be regarded as equivalent can be assured to be wider, the permissible elevation angle β can be made larger by separating the intersection  9  from the emission points S 1  to S 3  as much as possible (the elevation angle α may be enlarged as much as possible). In addition, a range in which the virtual cathode surface  10  can be regarded as the spherical surface becomes wider. 
     A range of the permissible elevation angle β can be described as follows. An angle between the tangent line  13  and the optical axis  3  is α−90 degrees. If non-uniformity of the electric field does not occur in the vicinity of the intersection  9 , charged particles emitted from emission points at the angle of α−90 degrees as viewed from the central point  5  are equivalent to charged particles emitted from the emission point S 1 . In fact, since the electric field becomes slightly non-uniform in the vicinity of the intersection  9 , an angle of an emission point equivalent to S 1  is not exactly α−90 degrees. However, as the elevation angle α is larger than 90 degrees, a range of emission points that can be regarded as equivalent to S 1  becomes wider, and thus the permissible elevation angle β also increases. 
     When the permissible elevation angle β is larger, the acceptance angle of the charged particle beam can be increased. This is because the virtual cathode surface  10  has a wide range of emission points that can be regarded as the spherical surface (S 1  to S 3  in  FIG.  4    can be regarded as equivalent). Therefore, a large charged particle current can be obtained while reducing the size of the virtual light source  11 . 
       FIG.  5    shows a simulation result of electric field intensities on a surface of the charged particle source  1 . It is assumed that the electric field is measured at a measurement position  19  on the spherical surface  4 . An angle γ formed by the optical axis  3  and a straight line connecting the central point  5  and the measurement position  19  is referred to as an off-axis angle. In an upper part of  FIG.  5   , a horizontal axis represents the off-axis angle γ and a vertical axis represents a normalized electric field intensity. Situations in which α&lt;90 degrees correspond to the charged particle sources in the related art. In this case, it can be seen that the electric field intensity rapidly increases as the off-axis angle γ increases. On the other hand, in cases where α&gt;90 degrees corresponding to the first embodiment, the electric field intensity slightly decreases as the off-axis angle γ increases, but an amount of change is small. Therefore, it can be said that a range in which the electric field in the vicinity of the emitter tip end can be regarded as uniform is wider than that in the related art. 
     Overview of First Embodiment 
     In the charged particle source  1  according to the first embodiment, the virtual cathode surfaces  10  of the emission points S 1  to S 3  on the surface of the spherical surface  4  match each other. Accordingly, a range of charged particle emission points that can be regarded as equivalent can be widely assured, and thus a large charged particle current can be obtained while reducing the size of the virtual light source  11 . In addition, since it is not necessary to increase the angular current density in order to obtain a large current, an energy width of the charged particle beam can be reduced. 
     In the charged particle source  1  according to the first embodiment, the region where the electric field intensity distribution in the vicinity of the emitter tip end is uniform is wide (as described in  FIG.  5   , the off-axis angle γ where the electric field on the surface of the spherical surface  4  can be regarded as the same is large), and a spherical surface region of the emitter tip end is large. Accordingly, a shape of the emitter tip end is thermodynamically stabilized. As a result, there is an advantage that properties of the charged particles emitted from the charged particle source  1  are also stabilized. 
     Second Embodiment 
       FIG.  6    is a graph showing a correspondence relationship between an energy width (ΔE) of electrons and a radius R of the spherical surface  4 . Here, it is assumed that the charged particle source  1  emits an electron beam, and the angular current density is 400 μA/sr. A structure of the charged particle source  1  is the same as that of the first embodiment, but a suitable range of the radius R will be discussed in the second embodiment. 
     In the related art, in an SEM for high resolution observation, the charged particle beam is narrowed down with a small current, and thus the angular current density is small enough and is approximately 150 μA/sr or less. If a large current is obtained in order to obtain high throughput, it is necessary to increase the angular current density. However, as the angular current density increases, the energy dispersion increases. As shown in  FIG.  6   , ΔE decreases as the radius R increases, and thus it is desirable to increase the radius R to a certain extent. 
     In the SEM in the related art, when the angular current density is 150 μA/sr, the energy dispersion ΔE is controlled to be, for example, 0.6 to 0.7 eV. According to  FIG.  6   , it can be seen that the radius R needs to be approximately 1 μm or more in order to obtain the same level of energy dispersion ΔE as in the related art even when the angular current density is 400 μA/sr. By adjusting the radius R of the spherical surface  4  at the emitter tip end to this range, it is possible to prevent spread of the energy width even in a high angular current density. 
     Third Embodiment 
       FIG.  7    is a view showing a tip end shape of the charged particle source  1  according to the third embodiment of the invention. An upper part of  FIG.  7    is a side view, and a lower part of  FIG.  7    is a front view. The charged particle source  1  according to the third embodiment has five facets (flat surfaces) on the spherical surface  4  in addition to the shape described in the first embodiment. A facet  18 - 1  is perpendicular to the optical axis  3 . Facets  18 - 2  and  18 - 4  are perpendicular to a first straight line (upper-lower line in the lower part of  FIG.  7   ) that is perpendicular to the optical axis  3 . Facets  18 - 3  and  18 - 5  are perpendicular to a second straight line (left-right line in the lower part of  FIG.  7   ) that is perpendicular to both the optical axis  3  and the first straight line. When the emitter needle  7  is formed of, for example, a tungsten single crystal, the direction of the optical axis  3  can be regarded as a crystallographic axis orientation of &lt;100&gt;. 
       FIG.  8    is a view showing emitted charged-particle trajectories in the vicinity of an extraction electrode  20 . When a voltage is applied to the extraction electrode  20 , charged particles are emitted from each of the facets  18 - 1  to  18 - 5 . The charged particles emitted from the facet  18 - 1  mainly pass through a hole of the extraction electrode  20  on the optical axis  3  and travel toward a sample. The charged particles emitted from the facets  18 - 2  to  18 - 5  mainly collide with the extraction electrode  20 . 
     When the atmospheric gas or the like in the periphery of the extraction electrode  20  adheres to the extraction electrode  20 , the atmospheric gas is discharged from the extraction electrode  20  when a charged particle beam device  100  is operating, and a vacuum degree in the periphery of the charged particle source  1  decreases to hinder the operation. This may affect, for example, properties of the charged particle beam. By applying the charged particles emitted from the facets  18 - 2  to  18 - 5  to the extraction electrode  20 , it is possible to promote the discharge of the adhering gas. That is, the periphery of the charged particle source  1  can be quickly returned to a high vacuum state. As a result, there is an advantage that the operation of the charged particle source  1  is stabilized. 
     Since the facets  18 - 2  to  18 - 5  are disposed at positions away from the optical axis  3  and are oriented in a direction orthogonal to the optical axis  3 , most of the charged particles emitted from the facets  18 - 2  to  18 - 5  collide with the extraction electrode  20  at positions away from the optical axis  3 . Therefore, even if secondary electrons are generated from the extraction electrode  20  through charged particle collision, the secondary electrons do not pass through the hole on the optical axis  3  or be directed to a sample side. That is, it is possible to prevent background noise of an observation image caused by the secondary electrons. 
     Fourth Embodiment 
       FIG.  9    is a diagram of a configuration of the charged particle beam device  100  including the charged particle source  1 . The charged particle beam device  100  can be, for example, a scanning electron microscope. The charged particle source  1  has been described in any one of the first to third embodiments. A charged particle beam extracted from the charged particle source  1  by the extraction electrode  20  is focused by an objective lens  24  and emitted onto a sample  25 . An intermediate lens  22  is disposed between the objective lens  24  and the extraction electrode  20 . The intermediate lens  22  is used to control a current amount of the charged particle beam emitted onto the sample  25 , an opening angle of the charged particle beam on the sample  25 , and the like. Deflectors  21  and  23  individually deflect the charged particle beam. 
     The charged particle beam device  100  according to the fourth embodiment can obtain a large current while reducing the energy width of the charged particle beam due to action of the charged particle source  1 . In addition, the properties of the charged particle beam are also stable. 
     Modifications of Invention 
     The invention is not limited to the embodiments described above and includes various modifications. For example, the above embodiments have been described in detail for easy understanding of the invention, and are not necessarily limited to those having all configurations described above. Furthermore, a part of a configuration of one embodiment can be replaced with a configuration of another embodiment, and a configuration of another embodiment can be added to a configuration of one embodiment. In addition, a part of a configuration of each embodiment can be added, deleted, or replaced with another configuration. 
     The charged particle source  1  according to the invention can be used as, for example, an ion source in a focused ion beam device or an electron source in a scanning electron microscope. The charged particle source  1  can be a thermal field emission electron emitting source or a cold cathode electron source. 
     When the cone angle θ increases, a voltage of the spherical surface  4  and a voltage of a ridge line portion of the emitter needle  7  interfere with each other in the vicinity of the intersection  9 , and the electric field intensity distribution is non-uniform. Therefore, it is desirable that the cone angle θ is as small as possible. It is preferable to set approximately θ≤5 degrees. θ=0 degree (i.e., the ridge line portion of the emitter needle  7  is parallel to the optical axis  3 ) may be satisfied. Further, θ&lt;0 degree (i.e., a shape in which the emitter needle  7  gradually becomes thicker toward the tip end) may be satisfied. 
     REFERENCE SIGN LIST 
     
         
         
           
               1 : charged particle source 
               3 : optical axis 
               4 : spherical surface 
               5 : central point 
               7 : emitter needle 
               9 : intersection 
               10 : virtual cathode surface 
               11 : virtual light source 
               13 : tangent line 
               16 : equivoltage surface 
               17 : trajectory 
               20 : extraction electrode 
               100 : charged particle beam device