Abstract:
One embodiment relates to an objective lens utilizing magnetic and electrostatic fields which is configured to focus a primary electron beam onto a surface of a target substrate. The objective lens includes a magnetic pole piece and an electrostatic deflector configured within the pole piece. An electrostatic lens field is determined by the pole piece and the electrostatic deflector, and the electrostatic lens field is configured by adjusting offset voltages applied to plates of the electrostatic deflector. Other embodiments, aspects and features are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of U.S. Provisional Patent Application No. 60/881,319, entitled “Objective Lens With Deflector Plates Immersed In Electrostatic Lens Field”, filed Jan. 19, 2007, by inventors Alexander J. Gubbens and Ye Yang, the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to electron beam apparatus, including scanning electron microscopes and the like, and method of operating such apparatus. 
     2. Description of the Background Art 
     There is an increasing need for high-resolution scanning electron microscopes (SEMs) in all areas of development and manufacture of micro-electronic and opto-electronic components. High-resolution scanning electron microscopes are useful so as to visually evaluate sub-micrometer structures. High-resolution SEMs may be used to identify deviations from standard patterns and to acquire and evaluate topographical data such as heights, widths or angles of inclination. 
     Unfortunately, conventional, non-immersion, scanning electron microscopes do not have the required resolution of a few nanometers unless very high landing energies above about 10 kilo-electronVolts are used which may cause resist structures and, integrated circuits to be damaged and non-conductive or high resistant specimens to be disadvantageously charged. 
     It is highly desirable to improve electron beam apparatus, including scanning electron microscopes and the like, and methods of operating such apparatus. 
     SUMMARY 
     One embodiment relates to an objective lens utilizing magnetic and electrostatic fields which is configured to focus a primary electron beam onto a surface of a target substrate. The objective lens includes a magnetic pole piece and an electrostatic deflector configured within the pole piece. An electrostatic lens field is determined by the pole piece and the electrostatic deflector, and the electrostatic lens field is configured by adjusting offset voltages applied to the plates of the electrostatic deflector. 
     Another embodiment relates to a method of focusing an electron beam onto a target specimen. Offset voltages are applied to a main deflector so as to fine tune an electrostatic lens field, where the electrostatic lens field is determined by the offset voltages on the deflector and a high voltage on a magnetic pole piece surrounding the deflector. 
     Another embodiment relates to an electron beam apparatus. The apparatus includes at least an electron source, a magnetic immersion objective lens, and an electrostatic deflector. The electron source is configured to generate a primary electron beam, and the magnetic immersion objective lens is configured to focus the primary electron beam onto a surface of a target substrate. The electrostatic deflector is configured within the pole piece of the magnetic immersion objective lens. An electrostatic lens field is determined by the pole piece and the electrostatic deflector, and the electrostatic lens field is configured by adjusting offset voltages applied to plates of the electrostatic deflector. 
     Other embodiments, aspects and features are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram of an immersion objective lens having electrostatic deflector plates positioned therein, where a grounded shunt is configured at the bottom of the deflector plates. 
         FIG. 2  is a schematic diagram of an immersion objective lens having electrostatic deflector plates deeply positioned therein, without a grounded shunt configured at the bottom of the deflector plates. 
         FIG. 3  is a table of simulation results showing performance parameters of a magnetic immersion objective lens having electrostatic deflector plates positioned therein, where a grounded shunt is configured at the bottom of the deflector plates. 
         FIG. 4  is a table of simulation results showing performance parameters of an immersion objective lens having electrostatic deflector plates deeply positioned therein, without a grounded shunt configured at the bottom of the deflector plates. 
     
    
    
     DETAILED DESCRIPTION 
     Immersion Objective Lenses 
     Low-voltage scanning electron microscopes (SEMs) often use immersion objective lenses because they tend to have superior resolution performance. Immersion objective lenses immerse the sample in a magnetic and/or decelerating electrostatic field. The highest resolution low voltage SEMs use both magnetic and electrostatic immersion. The magnetic immersion allows for very small working distances and small aberration coefficients. The use of electrostatic immersion to retard the primary electron beam just prior to the substrate further reduces the aberration coefficients and thereby further increases the resolution. 
     The landing energy of the primary electron beam is determined by the potential difference between the electron gun&#39;s cathode and the subsrate. The retarding field at the substrate is determined by the potential difference between the substrate and the objective lens pole piece above it. 
     To allow independent control of the landing energy and the retarding field, the objective lens pole piece(s) are typically biased at a high voltage potential (greater than 30 V in magnitude). The substrate potential is then set to determine the landing energy and the objective lens pole piece potential is next set to determine the strength of the retarding electrostatic field. 
     Scanning of the primary electron beam is conventionally done by means of a multipole deflector located on the inside of the objective lens near the tip. Electrostatic deflection is often preferred over magnetic deflection because it does not suffer from hysteresis as magnetic deflectors do and because it is easier to make work well at high speeds. 
     Conventional electrostatic deflectors use shunt electrodes on both sides to accurately define and terminate the axial extend of the deflecting field. The DC value of the electrostatic deflector and shunts are typically held at ground potential to allow for simpler driving electronics. This then necessarily introduces an electrostatic lens field between the objective lens pole piece at high voltage and the deflector at DC ground. 
     In the case of magnetic immersion, the secondary electron detection is typically through the objective lens. This is because the secondary electrons are strongly captured by the magnetic field and spiral up along the optical axis through the bore of the lens. At the point where the magnetic field decays on the inside of the lens, the secondary electrons are no longer captured and “spill out” (i.e. travel along divergent paths), making it difficult to collect and detect them. 
     It is therefore desirable to have an electrostatic lens on the inside of a magnetic immersion objective lens. Such an electrostatic lens on the inside of a magnetic immersion objective lens may advantageously collimate the secondary electrons and transport them more efficiently to the secondary detector located elsewhere in the electron beam column. 
     Magnetic Immersion Objective Lens Having an Electrostatic Deflector Plates with a Bottom Shunt Positioned Therein 
       FIG. 1  is a schematic diagram of a magnetic immersion objective lens  100  having electrostatic deflector plates  108  positioned therein, where a grounded shunt  110  is configured at the bottom of the deflector plates  108 . This diagram shows a cross-sectional view of the objective lens  100 . 
     As shown, a primary electron beam travels down an optical axis  101  and through the objective lens  100  to become focused upon the surface of a target substrate. A magnetic pole piece  102  of the objective lens  100  is configured about the optical axis  101 , with a gap  103  extending away from the optical axis  101 . The pole piece  102  is configured about an electromagnetic device  104  so as to generate a magnetic field which immerses the target substrate. The pole piece  102  is further configured at a high voltage potential. 
     As further shown, electrostatic deflector plates  108  are configured within the pole piece  102 . By controllably varying the voltages applied to the electrostatic deflector plates  108 , the primary beam may be controllably deflected so as to be scanned over an area of the target substrate. 
     In this case, a top shunt  106  and a bottom shunt  110  are provided above and below the deflector plates  108 . The top shunt  106  may comprise, for example, a conical shunt. The bottom shunt  110  may comprise, for example, a conical or straight shunt. The shunts  106  and  110  are further configured to be grounded (i.e. conductively connected to an electrical ground). 
     An electrostatic lens may be defined between the grounded bottom shunt electrode  110  and the high-voltage objective lens pole piece  102 . This electrostatic lens may be configured so as to collimate secondary electrons emitted from the target substrate. 
     Unfortunately, applicants have determined that the accurate alignment of the bottom shunt electrode  110  and the pole piece  102  is needed for high resolution performance of the objective lens  100  shown in  FIG. 1 . This requirement drives mechanical tolerances that are on the order of micrometers and adds substantially to the manufacturing cost of the magnetic lens. 
     Magnetic Immersion Objective Lens Having an Electrostatic Deflector Plates Positioned Lower in the Lens 
       FIG. 2  is a schematic diagram of an immersion objective lens  200  having electrostatic deflector plates  208  deeply positioned therein, without a grounded shunt configured at the bottom of the deflector plates  208 . This diagram shows a cross-sectional view of the objective lens  200 . 
     As shown, a primary electron beam travels down an optical axis  201  and through the objective lens  200  to become focused upon the surface of a target substrate. A magnetic pole piece  202  of the objective lens  200  is configured about the optical axis  201 , with a gap  203  extending away from the optical axis  201 . The pole piece  202  is configured about an electromagnetic device  204  so as to generate a magnetic field which immerses the target substrate. The pole piece  202  is further configured at a high voltage potential. 
     As further shown, electrostatic deflector plates  208  are configured within the pole piece  202 . By controllably varying the voltages applied to the electrostatic deflector plates  208 , the primary beam may be controllably deflected so as to be scanned over an area of the target substrate. 
     In this case, a top shunt  206  is provided above the deflector plates  208 , but there is no bottom shunt (in contrast to  FIG. 1 ). The top shunt  106  may comprise, for example, a conical shunt. 
     Here, an electrostatic lens field  210  may be defined between the electrostatic deflector plates  208  and the high-voltage objective lens pole piece  202 . Advantageously, applicants have determined that, unlike the electrostatic lens field  110  of  FIG. 1 , the electrostatic lens field  210  of  FIG. 2  does not require highly-accurate alignment of the deflector plates  208  and the high-voltage objective lens pole piece  202 . This is because the ends of the deflector plates  208  are immersed in the electrostatic field  210 . 
     Because the ends of the deflector plates  208  are immersed in the electrostatic field  210 , the electrostatic field  210  may be controllably adjusted by applying DC voltages to all of the deflector plates  208 . These DC voltages may be used to properly align the electrostatic field  210  within the objective lens  200 . While the DC voltages applied to all of the deflector plates  208  may be adjusted to change the alignment of the electrostatic field  210 , it does not affect the scanning of the electron beam which is controlled by the AC voltage (i.e. the time-varying voltage signal) applied to one or more of the deflector plates  208 . 
     Applicants have determined that the lens configuration of  FIG. 2  allows for significantly greater mechanical tolerances for the same high resolution performance. The greater mechanical tolerances are enabled by the advantageous feature that the shape and alignment of the electrostatic lens field  210  may be controllably varied and fine-tuned by applying the DC voltages to the deflector plates  208 . 
     As a result, the mechanical tolerances of the objective lens may be relaxed while the resolution performance may be maintained or even improved. Relaxation of the mechanical tolerances greatly reduces manufacturing cost. 
     Simulation Results 
       FIG. 3  is a table of simulation results showing performance parameters of a magnetic immersion objective lens having electrostatic deflector plates positioned therein, where a grounded shunt is configured at the bottom of the deflector plates (like the objective lens  100  of  FIG. 1 ). In contrast,  FIG. 4  is a table of simulation results showing performance parameters of an immersion objective lens having electrostatic deflector plates deeply positioned therein, without a grounded shunt configured at the bottom of the deflector plates (like the objective lens  200  of  FIG. 2 ). 
     Note that these simulations included both octupole and quadrupole electrostatic lens characteristics. The octupole and quadrupole are shifted downward (closer to the target substrate) for the simulation of  FIG. 4  in comparison to the simulation of  FIG. 3 . As indicated, the primary beam energy is 2,000 electron Volts, and the landing energy is 1,000 electron Volts, for both of these simulations. 
     Comparing the results shows that removing the bottom shunt and moving the deflectors down increases the scan sensitivity. The higher scan sensitivity results in an approximately 35% reduction in scan voltages required. Moreover, the scan aberrations are reduced by a factor of 2 or 3. This is advantageously accomplished while the secondary electron collection efficiency is unaffected (as shown by a separate simulation). 
     The simulations show that leaving out the bottom shunt and immersing the electrostatic deflector plates in the electrostatic lens field not only allows the mechanical tolerances of the objective lens to be reduced, it actually also increases the scanning performance of the electron microscope. 
     CONCLUSION 
     As integrated circuits continue to get smaller and smaller with the progression down Moore&#39;s curve, the resolution requirements on critical dimension and review SEMs continue to increase. Increasing resolution requirements impose tighter and tighter mechanical tolerances on the lens design of immersion objective lenses. The present application discloses methods and apparatus to introduce an additional degree of freedom that may be utilized to ensure optimal alignment and resolution performance. This advantageously reduces required mechanical tolerances and improves the manufacturability of a combined magnetic/electrostatic objective lens for scanning electron microcopy. 
     The above-described diagrams are not necessarily to scale and are intended be illustrative and not limiting to a particular implementation. Specific dimensions, geometries, and lens currents of the immersion objective lens will vary and depend on each implementation. 
     The above-described invention may be used in an automatic inspection system and applied to the inspection of wafers, X-ray masks and similar substrates in a production environment. While it is expected that the predominant use of the invention will be for the inspection of wafers, optical masks, X-ray masks, electron-beam-proximity masks and stencil masks, the techniques disclosed here may be applicable to the high speed electron beam imaging of any material (including perhaps biological samples). 
     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.