Abstract:
One embodiment disclosed relates to a Wien filter for a charged-particle beam apparatus. The charged-particle beam is transmitted through the Wien filter in a first direction. A magnetic field generation mechanism is configured to generate a magnetic field in a second direction which is perpendicular to the first direction, and an electrostatic field generation mechanism is configured to generate an electrostatic field in a third direction which is perpendicular to the first and second directions. The field generation mechanisms are further configured so as to have an offset between the positions of the magnetic and electrostatic fields along the first direction. Another embodiment disclosed relates to a Wien filter type device wherein the magnetic force is approximately twice in strength compared to the electrostatic force. Other embodiments are also disclosed.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to Wien filters, such as those used in charged-particle beam systems and for other purposes. 
   2. Description of the Background Art 
     FIG. 1  illustrates the operating principle of the Wien filter. This type of velocity (momentum) filter uses crossed electric and magnetic fields to exert opposing forces on charged particles passing through the filter. An X-Y-Z orthogonal coordinate system  8  is shown. 
   A substantially homogeneous magnetic field  10  of magnitude B is directed parallel to the Y-axis in the negative direction. A substantially homogeneous electric field  12  of magnitude E is directed parallel to the X-axis, also in the negative direction. A beam of charged particles  14  is directed initially (before encountering the fields  10  and  12 ) parallel to the Z-axis in the positive direction, and through the electric and magnetic fields. The fields  10  and  12  are positioned in space such that along trajectory  14 , the magnitudes of fields  10  and  12  are always in the same proportion, rising from initial values of zero Gauss (G) and zero Volts/centimeter (V/cm) to some well-defined maximum values, then decreasing back to zero G and zero V/cm again. 
   Fields  10  and  12  apply forces to the beam of particles  14 . The first Equation  16  expresses the force on beam  14  due to electric field  12 . This electric force is in the −X direction for positive (+) particles as shown by force vector  18  in  FIG. 1 . The second Equation  20  expresses the force on beam  14  due to magnetic field  10 . This magnetic force is proportional to the vector cross-product of the velocity v of each charged particle in beam  14  and the strength, B, of magnetic field  10 . In this case, this magnetic force is in the +X direction for positively-charged particles as shown by force vector  22  in  FIG. 1 . 
   A conventional Wien filter is configured such that the force vectors  18  and  22  are equal in magnitude. As such, the electric and magnetic forces  18  and  22  will cancel each other for a charged particle traveling in one direction along the z-axis, while the electric and magnetic forces  18  and  22  will add together to a larger force (double the individual forces) for the same charged particle traveling in the opposite direction along the z-axis. This is due to the fact that the direction of the magnetic force  22  depends on the direction of the velocity vector  14  of the particle, while the direction of the electric force  18  is independent of the velocity vector  14 . For example, consider the specific case where the charged particles are electrons. The electric and magnetic forces will cancel each other for electrons going in the negative z direction, while the electric and magnetic forces will add together for electrons going in the positive z direction. 
     FIGS. 2A and 2B  show a typical design for the magnetic field pole pieces  40  and  42 , and electric field pole pieces  44  and  46 , of a conventional Wien filter. In  FIG. 2A , the magnetic field lines are shown as they extend through the magnetic material of pole pieces  40  and  42  (both at zero volts) and between them in the space  48 , through which beam  14  passes.  FIG. 2B  shows the electric field lines which result when voltages +V and −V are applied to electric pole pieces  44  and  46 , respectively. 
   As pointed out in the present disclosure, one disadvantageous aspect of a conventional Wien filter is the chromatic aberration that is induced. Charged particles in a beam of different speeds are deflected to different angles by a conventional Wien filter. In other words, the change in trajectory caused by a conventional Wien filter depends on the energy of the charged particle. 
   It is desirable to improve charged-particle beam apparatus. It is also desirable to improve Wien filters. In particular, it is desirable to reduce the chromatic aberration caused by Wien filters. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is diagram of the operating principle of a conventional Wien filter. 
       FIGS. 2A and 2B  are cross-sectional diagrams of a typical conventional Wien filter, showing magnetic and electric fields, respectively. 
       FIG. 3A  is a schematic diagram depicting a first configuration for a Wien filter in accordance with an embodiment of the invention. 
       FIG. 3B  provides a table showing chromatic dispersion as a function of the relative strengths of the two magnetic deflection coils in an example apparatus in accordance with en embodiment of the present invention. 
       FIG. 4  is a schematic diagram depicting a second configuration for a Wien filter in accordance with another embodiment of the invention. 
       FIG. 5  is a schematic diagram of a charged-particle apparatus with a conventional Wien filter configuration. 
       FIG. 6  schematic diagram of a charged-particle apparatus with a Wien filter configuration in accordance with an embodiment of the invention. 
   

   SUMMARY 
   One embodiment of the invention pertains to a Wien filter for a charged-particle beam apparatus. The charged-particle beam is transmitted through the Wien filter in a first direction. A magnetic field generation mechanism is configured to generate a magnetic field in a second direction which is perpendicular to the first direction, and an electrostatic field generation mechanism is configured to generate an electrostatic field in a third direction which is perpendicular to the first and second directions. The field generation mechanisms are further configured so as to have an offset between the positions of the magnetic and electrostatic fields along the first direction. Advantageously, the offset between the positions of the magnetic and electrostatic fields along the first direction may be configured such that a chromatic dispersion at a target plane is minimized 
   Another embodiment disclosed relates to a Wien filter type device wherein the magnetic force is approximately twice in strength compared to the electrostatic force. The device may be configured to deflect an incident beam of charged particles towards a target substrate and to deflect a scattered beam from the substrate to a detector. Advantageously, the device may be configured to reduce or eliminate the chromatic aberration caused by the device. 
   Other embodiments are also discussed. 
   DETAILED DESCRIPTION 
     FIG. 3A  is a schematic diagram depicting a first configuration  300  for a Wien filter in accordance with an embodiment of the invention. In this embodiment, the first-order chromatic aberration may be reduced or eliminated by offsetting the effective position of the magnetic field with respect to the electrostatic field. 
   In  FIG. 3A , the z-axis is shown as going downwards within the page. The trajectory of the charged particles entering the Wien filter is along the z-axis. The Wien filter configuration  300  is such that the electrostatic field is perpendicular to the plane of the page in the approximate area  302  indicated near the center of  FIG. 3A . 
   Unlike the conventional Wien filter, the Wien filter configuration  300  of  FIG. 3A  includes two pairs of magnetic deflection coils  304  and  306 , each pair of coils being offset relative to the average z-position of the electrostatic field  302 . One pair of coils  304  is offset in a negative z-direction relative to the average position of the electrostatic field  302 , and the other pair of coils  306  is offset in a positive z-direction relative to the average position of the electrostatic field  302 . Each pair of coils is configured so as to generate a magnetic field in a direction perpendicular to the electrostatic field and perpendicular to the z-axis. As in the conventional Wien filter, the ratio of the electrostatic field strength (E) to the magnetic field strength (B) is equal to, or approximately equal to, the speed (v) of the charged particles In other words, E/B=v. 
   By adjusting the electric current running through one pair of coils relative to the current running through the other pair of coils, the z-position offset of the total effective magnetic field may be controllably changed. The specific currents to run in each pair of coils may be determined so as to minimize the chromatic aberration for the specific implementation. 
     FIG. 3B  includes a table showing chromatic dispersion as a function of the relative strengths of the two pairs of magnetic deflection coils in an example apparatus in accordance with en embodiment of the present invention. The first two columns give the relative strengths (RS 1  and RS 2 ) of the two pairs of coils (named Coil  1  and Coil  2 ). The third column shows the transverse chromatic aberration coefficient for the Wien filter (|C TW |). The fourth column shows the resultant first-order chromatic dispersion at the target plane (δr 1 ) in nanometers. As shown in the table, the transverse chromatic aberration coefficient and the first-order chromatic dispersion at the target plane are dramatically reduced (by a factor of more than a hundred) to a negligible amount when the relative strengths of coils  1  and  2  are 0.556 and 0.444, respectively. 
     FIG. 4  is a schematic diagram depicting a second configuration for a Wien filter in accordance with an embodiment of the invention. Similar to the embodiment discussed in relation to  FIG. 3A , the embodiment of  FIG. 4  provides for the reduction or elimination of the first-order chromatic aberration. In the embodiment of  FIG. 4 , this is accomplished by offsetting the effective position of the electrostatic field with respect to the magnetic field. 
   In  FIG. 4 , the z-axis is shown as going downwards within the page. The trajectory of the charged particles entering the Wien filter is along the z-axis. The Wien filter configuration  400  is such that the magnetic field is perpendicular to the plane of the page in the approximate area  402  indicated near the center of  FIG. 4 . 
   Unlike the conventional Wien filter, the Wien filter configuration  400  of  FIG. 4  includes two pairs of electrostatic deflection plates  404  and  406 , each pair of plates being offset relative to the average z-position of the magnetic field  402 . One pair of plates  404  is offset in a negative z-direction relative to the average position of the magnetic field  402 , and the other pair of plates  406  is offset in a positive z-direction relative to the average position of the magnetic field  402 . Voltages are applied across each pair of plates so as to generate an electrostatic field in a direction perpendicular to the magnetic field and perpendicular to the z-axis. 
   By adjusting the voltage difference applied to one plate relative to the voltage difference applied to the other plate, the z-position offset of the total effective electrostatic field may be controllably changed. The specific voltage differences to be applied may be determined so as to minimize the chromatic aberration for the specific implementation. 
   In other embodiments, other configurations may be used to offset the electrostatic and magnetic fields relative to each other so as to achieve the desired reduction in chromatic aberration. For example, while the embodiment of  FIGS. 3A  is advantageously easy to adjust electronically, an alternate embodiment may utilize two pairs of pole pieces to generate the magnetic field, rather than two pairs of coils. As another example, a single pair of pole pieces (or coils) may be used, and the offset between magnetic and electrostatic fields may be accomplished by mechanical positioning so as to offset the pole pieces (or coils) from the electrostatic plates. Other embodiments may utilize various alternate ways to achieve the offset between magnetic and electrostatic fields. 
     FIG. 5  is a schematic diagram of a charged-particle apparatus  500  with a conventional Wien filter configuration. This is a typical arrangement of a Wien filter  504  as used in an electron microscope or similar charged-particle beam apparatus. In this conventional arrangement, the Wien filter  504  is configured with an E/B ratio of v/c, where E is the magnitude of the electrostatic field strength, B is the magnitude of the magnetic field strength, v is the speed of the incident charged particles. 
   The incident beam  502  of charged particles is directed at a perpendicular angle towards the target substrate  506 , and the incident beam  502  passes through the Wien filter  504 , typically un-deflected, on its way to impinging upon the substrate  506 . Due to the interaction between the incident beam  502  and the substrate  506 , a scattered beam  508  of charged particles is generated and travels back through the Wien filter  504 . In this case, since the velocity of the scattered beam  508  is opposite in direction to the velocity of the incident beam  502 , the scattered beam  508  does not pass un-deflected through the Wien filter  504 . Instead, the Wien filter  504  deflects the scattered beam by an angle α up . By such deflection, the scattered beam  508  is separated from the incident beam  502 , such that the scattered beam  508  may travel to a detector. 
   Results have been determined for an apparatus  500  with a conventional Wien filter configuration as in  FIG. 5 . These results assumed an electric field strength E=220 volts/cm, and a magnetic field strength B=EG/v=5.25 gauss. It is calculated that the deflection angle for the scattered beam α up =5.80 degrees. The calculated chromatic spread for a typical TFE electron source is Δα=8.7×10 −5  degrees, which is significant. 
     FIG. 6  schematic diagram of a charged-particle apparatus  600  with a modified Wien filter configuration in accordance with an embodiment of the invention. This arrangement of a modified Wien filter  604  may be called an “achromatic” arrangement and may be used in an electron microscope or similar charged-particle beam apparatus. Here, the modified Wien filter  604  is configured such that the electrostatic field is only one half the strength of the electrostatic field in the conventionally-configured Wien filter  604 . In other words, the modified Wien filter  604  is configured with an E/B ratio of v/2. 
   Here, the incident beam  602  of charged particles is directed at a non-perpendicular angle α down  towards the target substrate  606 . The incident beam  602  enters the modified Wien filter  604  and is deflected such that the beam  602  may impinge at a perpendicular angle upon the substrate  606 . Due to the interaction between the incident beam  602  and the substrate  606 , the scattered beam  608  of charged particles is generated and travels back through the modified Wien filter  604 . In this case, the modified Wien filter  604  deflects the scattered beam by an angle α up . By such deflection, the scattered beam  608  is separated from the incident beam  602 , such that the scattered beam  608  may travel to a detector. 
   Results have been determined for an apparatus  600  with the Wien filter configuration discussed above in relation to  FIG. 6  in accordance with an embodiment of the invention. These results assumed an electric field strength E=220 volts/cm, and a magnetic field strength B=2E/v=10.49 gauss. It was calculated that the deflection angle for the primary beam is α down =2.90 degrees, and that the deflection angle for the scattered beam α up =8.70 degrees, such that the beam separation=α down −α up =11.60 degrees. Advantageously, the calculated chromatic aberration Δα=0 degrees. This is so because the B=2E/v condition eliminates the first order chromatic aberration of the Wien filter but leaves a non-zero net deflection of the beam. This net deflection can practically be accommodated by many different means. One is presented in  FIG. 6  where the incoming beam  602  is mechanically positioned at an angle with respect to the final imaging elements of the system. In other manifestations it is possible to electronically correct (deflect or pre-deflect) the incoming beam to ensure the path along the optical axis of the remaining optical elements. 
   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 wafers, optical masks, X-ray masks, electron-beam-proximity masks and stencil 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.