Patent Publication Number: US-2020278524-A1

Title: Optical objective lens

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. application 62/564,983 which was filed on Sep. 28, 2017, and which is incorporated herein in its entirety by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure generally relates to an optical objective lens assembly used in a charged particle beam inspection system. 
     BACKGROUND 
     A charged particle beam based microscope, such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM), is capable of providing image resolution down to less than a nanometer, and thus serves as a practicable tool for inspecting samples having a feature size that is sub-100 nanometers. The charged-particle-beam based microscope focuses and scans a beam of charged particles on a sample and detects secondary particles reflected from the sample. An optical imaging system is usually included in the charged-particle beam-based microscope to allow visual inspection of a sample. 
     SUMMARY 
     According to some embodiments of the disclosure, an objective lens for forming an image of an object is provided. The objective lens includes, sequentially from an image side to an object side, a first lens group having negative refractive power, and a second lens group having positive refractive power. 
     According to some embodiments of the disclosure, an optical imaging system used in a charged particle beam inspection system is provided. The optical imaging system includes: an illumination module configured to project a first light beam to an object; an objective lens configured to receive a second light beam reflected from the object and form an image of the object; and a detection module configured to detect the image of the sample. The objective lens includes, sequentially from an image side to an object side, a first lens group having negative refractive power and a second lens group having positive refractive power. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate several embodiments. 
         FIG. 1  is a schematic diagram of an exemplary charged particle-beam inspection system with an exemplary optical imaging system consistent with some disclosed embodiments. 
         FIG. 2  is a schematic diagram of a sectional view of an exemplary objective lens  200 , consistent with some disclosed embodiments. 
         FIG. 3  is a graph showing longitudinal aberration of an objection lens in an exemplary embodiment. 
         FIG. 4  is a graph showing Fast Fourier transform (FFT) modulation transfer function (MTF) of an objection lens in an exemplary embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Reference will now be made in detail to the example embodiments, which are illustrated in the accompanying drawings. Although the following embodiments are described in the context of utilizing electron beams, the disclosure is not so limited. Other types of charged particle beams can be similarly applied. 
     The disclosed embodiments provide an objective lens to be used in an optical imaging system in a charged particle beam inspection system. The objective lens includes, sequentially from an image side to an object side, a first lens group having negative refractive power, and a second lens group having positive refractive power. An effective focus length (EFL) of some embodiments of the first lens group ranges from −51 mm to −35 mm, and an EFL of some embodiments of the second lens group ranges from 26 mm to 30 mm. The objective lens described herein provides a longer working distance and a larger field of view. 
       FIG. 1  is a schematic diagram of an exemplary charged particle beam inspection system  100  with an exemplary optical imaging system  120  consistent with some disclosed embodiments. Charged particle beam inspection system  100  includes a charged particle beam column  110  for irradiating a charged particle beam (e.g., electron beam) on an inspected sample  102 , and optical imaging system  120  for obtaining an image of sample  102 . Optical imaging system  120  includes an illumination module  130 , a detection module  140 , and an objective lens  160 . Illumination module  130  is configured to project a light beam to sample  102 . Objective lens  160  is configured to conjugate a light beam reflected from sample  102 . Detection module  140  includes a detector  142 , a tube lens  146 , a beam splitter  148 , and a vacuum window  150 . Detection module  140  is configured to detect an image of sample  102  based on the light beam reflected from sample  102 . Charged particle beam column  110 , objective lens  160 , and sample  102  are disposed in an inspection chamber  170 . 
     When illumination module  130  emits a light beam along an X-direction parallel to a surface of sample  102 , part of the light beam is reflected by beam splitter  148  downwards along a Z-direction perpendicular to the surface of sample  102 . The light beam is conjugated by objective lens  160  onto a surface of sample  102 . At the sample surface, the light beam is reflected by sample  102 . The reflected light beam is conjugated by objective lens  160  towards detection module  140  to form an image of sample  202 . In detection module  140 , the light beam passes through vacuum window  150  and beam splitter  148 . Tube lens  146  magnifies the image of sample  102  formed by the light beam. The light beam then transmits to detector  142 , which detects the image of sample  102 . 
     Illumination module  130  may include an illumination source that emits the light beam. The illumination source may be a white light source which has a wide wavelength range. However, such a wide wavelength range of the light source will incur chromatic aberration easily. Therefore, in some exemplary embodiment of the present disclosure, the illumination source is a narrow-band light source that emits a light beam having a narrow wavelength distribution in the range of, for example, 495 nm to 572 nm. For example, the narrow-band light source can be a light emitting diode (LED). The narrow-band light source can effectively reduce or even eliminate the chromatic aberration. Consequently, it is desirable for objective lens  160  to be suitable for the narrow-band light beam. 
     During operation of charged particle beam inspection system  100 , inspection chamber  170  is evacuated to place sample  102 , charged particle beam column  110 , and objective lens  160  under vacuum conditions. Therefore, it is desirable to configure objective lens  160  to be suitable for operation under vacuum conditions. 
     In addition, charged particle beam column  110  may include electronic components that are applied with high voltages (e.g., 30 kV) in order to incur electric breakdown in order to generate the charged particle beam. In order to prevent the negative effect of the high voltages on objective lens  160 , objective lens  160  is usually arranged in a relatively large distance from sample  102 . Therefore, it is desirable for objective lens  160  to have a long working distance and large field of view (FOV). 
       FIG. 2  is a schematic diagram of a sectional view of an exemplary objective lens  200 , consistent with some disclosed embodiments. Objective lens  200  can be used as objective lens  160  in optical imaging system  120  in charged particle beam inspection system  100  of  FIG. 1 . Objective lens  200  receives light from an object  230  (e.g., sample  102 ) and forms an image  240  of object  230 . Hereinafter, the side where object  230  is disposed is referred to as an “object side,” and the side where image  240  is formed is referred to as an “image side.” 
     As illustrated in  FIG. 2 , objective lens  200  includes a first lens group G 1  and a second lens group G 2  sequentially arranged from the image side to the object side. First lens group G 1  of the example of  FIG. 2  provides negative refractive power and consists of a first lens unit L 1 . The effective focal length (EFL) of first lens group G 1  ranges from −51 mm to −35 mm. Second lens group G 2  provides positive refractive power and includes a second lens unit L 2 , a third lens unit L 3 , and a singlet lens element L 4  sequentially arranged from the image side to the object side. The EFL of second lens group G 2  ranges from 26 mm to 30 mm. The total EFL of objective lens  200  ranges from 22 mm to 27 mm. 
     First lens unit L 1  is a doublet cemented lens that provides negative refractive power. First lens unit L 1  consists of a first lens element L 11  and a second lens element L 12  sequentially arranged from the image side to the object side and are joined together. First lens L 11  is a biconcave lens having a concave image surface  201  that faces image  240 . Second lens L 12  is a meniscus lens having a convex image surface  202  that faces image  240  and a concave object surface  203  that faces object  230 . Surfaces  201 ,  202 , and  203  provide negative, positive, and negative refractive powers, respectively. The EFL of first lens unit L 1  ranges from −51 mm to −35 mm. 
     Second lens unit L 2  is a doublet cemented lens that provides positive refractive power. Second lens unit L 2  includes a third lens element L 21  and a fourth lens element L 22  sequentially arranged from the image side to the object side and are joined together. Third lens element L 21  is a biconcave lens having a concave image surface  204  that faces image  240 . Fourth lens element L 22  is a biconvex lens having a convex image surface  205  that faces image  240  and a convex object surface  206  that faces object  230 . Surfaces  204 ,  205 , and  206  provide positive, negative, and positive refractive powers, respectively. The EFL of second lens unit L 2  ranges from 74 mm to 369 mm. 
     Third lens unit L 3  is a doublet cemented lens that provides positive refractive power. Third lens unit L 3  includes a fifth lens element L 31  and a sixth lens element L 32  sequentially arranged from the image side to the object side and are joined together. Fifth lens element L 31  is a biconvex lens having a convex image surface  207  that faces image  240  and a convex image surface  208  that faces object  230 . Sixth lens element L 32  is a meniscus lens having a convex object surface  209  that faces object  230 . Surfaces  207 ,  208 , and  209  provide positive, negative, and positive refractive powers, respectively. The EFL fL 3  of third lens unit L 3  ranges from 86 mm to 185 mm. 
     Singlet lens element L 4  is a meniscus lens that provides positive refractive power. Singlet lens element L 4  has a convex image surface  210  and a concave object surface  211 . Both surfaces  210  and  211  provide positive refractive powers. The EFL fL 4  of singlet lens element L 4  ranges from 49 mm to 66 mm. 
     When objective lens  200  is used in combination with a tube lens (e.g., tube lens  146 ) with having an EFL of 200 mm, objective lens  200  can provide magnification ranges from 7.4× to 9.1×. The magnification of objective lens  200  can be further extended to a range from 9.1× to 12.1× by scaling the whole objective lens set. Objective lens  200  can be infinity corrected, and can have a working distance (WD) ranging from 35 mm to 40 mm, while keeping its parfocal length unchanged at 95 mm. Objective lens  200  can achieve a field of view (FOV) of at least 2.4 mm. 
     Table 1 summaries optical parameters of objective lens  200 , according to an exemplary embodiment. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
               
               
                   
                 Radius of 
                   
                 Refractive 
                 Abbe&#39;s 
                   
               
               
                 Surface 
                 curvature R 
                 Surface space 
                 index 
                 number 
               
               
                 No. 
                 (mm) 
                 (mm) 
                 nd 
                 Vd 
                 Lens No. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 201 
                 —19.284 
                 4.800 
                 1.689 
                 31.185 
                 L11 
               
               
                 202 
                 25.845 
                 5.000 
                 1.923 
                 18.895 
                 L12 
               
               
                 203 
                 503.910 
                 6.302 
               
               
                 204 
                 —75.000 
                 5.000 
                 1.854 
                 40.599 
                 L21 
               
               
                 205 
                 47.877 
                 8.000 
                 1.713 
                 53.939 
                 L22 
               
               
                 206 
                 —29.419 
                 8.139 
               
               
                 207 
                 79.625 
                 8.000 
                 1.594 
                 67.327 
                 L31 
               
               
                 208 
                 —24.927 
                 4.300 
                 1.755 
                 27.546 
                 L32 
               
               
                 209 
                 —98.282 
                 1.000 
               
               
                 210 
                 32.856 
                 8.690 
                 1.729 
                 54.669 
                 L4 
               
               
                 211 
                 212.344 
               
               
                   
               
            
           
         
       
     
     In objective lens  200  of the exemplary embodiment described in Table 1, surfaces  201 - 211  provide negative, positive, negative, positive, negative, positive, positive, negative, positive, positive, positive refractive powers, respectively. The EFL of first lens group G 1  and first lens unit L 1  is 35.794 mm. The EFL of second lens group G 2  is 26.949 mm. The EFL of second lens unit L 2  is 81.474 mm. The EFL of third lens unit L 3  is 121.079 mm. The EFL of singlet lens element LA is 51.385 mm. The total EFL of objective lens  200  of Table 1 is 24.188 mm and the numerical aperture (NA) is 0.26. The WD is 35.769 mm. The parfocal length is 95 mm. 
       FIG. 3  is a graph showing longitudinal aberration of objective lens  200  in the exemplary embodiment of Table 1. In the graph, the horizontal axis represents a focus position, and the vertical axis represents a pupil radius. The solid curve  310  represents the longitudinal aberration for a light having a wavelength of 435 nm. The dotted curve  320  represents the longitudinal aberration for a light having a wavelength of 465 nm. The dashed curve  330  represents the longitudinal aberration for a light having a wavelength of 495 nm. According to  FIG. 3 , the spherical aberration of objective lens  200  of Table 1 is well corrected at 0.55 pupil and 0.82 pupil. At 0 and 1.0 pupils, the residual aberration is about 10 μm. 
       FIG. 4  is a graph showing Fast Fourier transform (FFT) modulation transfer function (MTF) of objective lens  200  in the exemplary embodiment of Table 1. In the graph, the horizontal axis represents spatial frequency, and the vertical axis represents a MTF value. Curve  410  represents the tangential (T) MTF for the 0.0 degree field point; curve  412  represents the sagittal (S) MTF for the 0.0 degree field point; curve  420  represents the tangential (T) MTF for the 2.0 degree field point; curve  422  represents the sagittal (S) MTF for the 2.0 degree field point; curve  430  represents the tangential (T) MTF for the 2.8 degree field point; and curve  432  represents the sagittal (S) MTF for the 2.8 degree field point. The 2.8 degree field point corresponds to the 1.2 mm field point. According to  FIG. 4 , the MTF of objective lens  200  of Table 1 has been well corrected to coincide with a diffraction limited MTF. At the spatial frequency of 600 cycles per mm, the MTF value for the three field points are almost kept as high as 0.3. 
     According to the above disclosed embodiments, objective lens  200  includes, sequentially from the image side to the object side, first lens group G 1  having negative refractive power and second lens group G 2  having positive refractive power. An EFL of first lens group G 1  ranges from −51 mm to −35 mm, and an EFL of second lens group G 2  ranges from 26 mm to 30 mm. Objective lens  200  has a long working distance and large field of view, and thus is suitable for use in optical imaging system  120  in charged particle beam inspection system  100 . 
     The embodiments may further be described using the following clauses: 
     1. An objective lens for forming an image of an object, comprising, sequentially from an image side to an object side: 
     a first lens group having negative refractive power; and 
     a second lens group having positive refractive power. 
     2. The objective lens of clause 1, wherein an effective focus length (EFL) of the first lens group ranges from −51 mm to −35 mm, and an EFL of the second lens group ranges from 26 mm to 30 mm.
 
3. The objective lens of any one of clauses 1 and 2, wherein the first lens group comprises a first doublet cemented lens consisting of, sequentially from the image side to the object side, a first lens and a second lens that provide negative refractive power in total,
 
     the first doublet cemented lens has first through third surfaces arranged sequentially from the image side to the object side, and 
     the first through third surfaces provide negative, positive, and negative refractive powers, respectively. 
     4. The objective lens of any one of clauses 1 through 3, wherein the second lens group comprises, sequentially from the image side to the object side, a second doublet cemented lens L 2 , a third doublet cemented lens L 3 , and a singlet lens L 4 .
 
5. The objective lens of clause 4, wherein the second doublet cemented lens consists of, sequentially from the image side to the object side, a third lens L 21  and a fourth lens L 22  that provide positive refractive power in total,
 
     the second doublet cemented lens L 2  has fourth through sixth surfaces arranged sequentially from the image side to the object side, 
     the fourth through sixth surfaces provide positive, negative, and positive refractive powers, respectively. 
     6. The objective lens of clause 5, wherein an EFL of the second doublet cemented lens ranges from 74 mm to 369 mm.
 
7. The objective lens of any one of clauses 4 through 6, wherein the third doublet cemented lens L 3  comprises, sequentially from the image side to the object side, a fifth lens L 31  and a sixth lens L 32  that provide positive refractive power in total,
 
     the third doublet cemented lens L 3  has seventh through ninth surfaces arranged sequentially from the image side to the object side, 
     the seventh through ninth surfaces provide positive, negative, and positive refractive powers, respectively. 
     8. The objective lens of clause 7, wherein an EFL of the third doublet cemented lens ranges from 86 mm to 185 mm.
 
9. The objective lens of any one of clauses 4 through 8, wherein the singlet lens L 4  provides
 
     positive refractive power, the singlet lens L 4  has tenth and eleventh surfaces arranged sequentially from the image side to the object side, 
     both of the tenth and eleventh surfaces provide positive refractive powers. 
     10. The objective lens of clause 9, wherein an EFL of the singlet lens ranges from 49 mm to 66 mm.
 
11. The objective lens of any one of clauses 1 through 10, wherein a total EFL of the objective lens ranges from 22 mm to 27 mm.
 
12. The objective lens of any one of clauses 1 through 11, wherein when the objective lens is used in combination with a tube lens having an EFL of 200 mm, magnification of the objective lens ranges from 7.4× to 9.1×.
 
13. The objective lens of any one of clauses 1 through 11, wherein when the objective lens is used in combination with a tube lens having an EFL of 200 mm, magnification of the objective lens ranges from 9.0× to 12.1×.
 
14. The objective lens of any one of clauses 1 through 13, wherein a working distance of the objective lens ranges from 35 mm to 40 mm.
 
15. The objective lens of any one of clauses 1 through 14, wherein a parfocal length of the objective lens is about 95 mm.
 
16. The objective lens of any one of clauses 1 through 15, wherein a field of view of the objective lens is about 2.4 mm.
 
17. An optical imaging system used in a charged particle beam inspection system, comprising:
 
     an illumination module configured to project a first light beam to an object; 
     an objective lens configured to receive a second light beam reflected from the object and form an image of the object; and 
     a detection module configured to detect the image of the sample, 
     wherein the objective lens comprises, sequentially from an image side to an object side: 
     a first lens group having negative refractive power; and 
     a second lens group having positive refractive power. 
     18. The system of clause 17, wherein the illumination module includes a narrow-band light source having a wavelength range from 495 nm to 572 nm.
 
19. The system of any one of clauses 17 and 18, wherein an effective focus length (EFL) of the first lens group ranges from −51 mm to −35 mm, and an EFL of the second lens group ranges from 26 mm to 30 mm.
 
20. The system of any one of clauses 17 through 19, wherein the first lens group comprises a first doublet cemented lens consisting of, sequentially from the image side to the object side, a first lens and a second lens that provide negative refractive power in total,
 
     the first doublet cemented lens has first through third surfaces arranged sequentially from the image side to the object side, and 
     the first through third surfaces provide negative, positive, and negative refractive powers, respectively. 
     21. The system of any one of clauses 17 through 20, wherein the second lens group comprises, sequentially from the image side to the object side, a second doublet cemented lens L 2 , a third doublet cemented lens L 3 , and a singlet lens L 4 .
 
22. The system of clause 21, wherein the second doublet cemented lens consists of, sequentially from the image side to the object side, a third lens L 21  and a fourth lens L 22  that provide positive refractive power in total,
 
     the second doublet cemented lens L 2  has fourth through sixth surfaces arranged sequentially from the image side to the object side, 
     the fourth through sixth surfaces provide positive, negative, and positive refractive powers, respectively. 
     23. The system of clause 22, wherein an EFL of the second doublet cemented lens ranges from 74 mm to 369 mm.
 
24. The system of any one of clauses 21 through 23, wherein the third doublet cemented lens L 3  comprises, sequentially from the image side to the object side, a fifth lens L 31  and a sixth lens L 32  that provide positive refractive power in total,
 
     the third doublet cemented lens L 3  has seventh through ninth surfaces arranged sequentially from the image side to the object side, 
     the seventh through ninth surfaces provide positive, negative, and positive refractive powers, respectively. 
     25. The system of clause 24, wherein an EFL of the third doublet cemented lens ranges from 86 mm to 185 mm.
 
26. The system of any one of clauses 21 through 25, wherein the singlet lens LA provides positive refractive power,
 
     the singlet lens L 4  has tenth and eleventh surfaces arranged sequentially from the image side to the object side, 
     both of the tenth and eleventh surfaces provide positive refractive powers. 
     27. The system of clause 26, wherein an EFL of the singlet lens ranges from 49 mm to 66 mm.
 
28. The system of any one of clauses 17 through 27, wherein a total EFL of the objective lens ranges from 22 mm to 27 mm.
 
29. The system of any one of clauses 17 through 28, wherein when the objective lens is used in combination with a tube lens having an EFL of 200 mm, magnification of the objective lens ranges from 7.4× to 9.1×.
 
30. The system of any one of clauses 17 through 28, wherein when the objective lens is used in combination with a tube lens having an EFL of 200 mm, magnification of the objective lens ranges from 9.0× to 12.1×.
 
31. The system of any one of clauses 17 through 30, wherein a working distance of the objective lens ranges from 35 mm to 40 mm.
 
32. The system of any one of clauses 17 through 31, wherein a parfocal length of the objective lens is about 95 mm.
 
33. The system of any one of clauses 17 through 31, wherein a field of view of the objective lens is about 2.4 mm.
 
     While the present invention has been described in connection with various embodiments, other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.