Abstract:
A magnetic lens focuses a charged particle beam generated by an instrument to a very small spot for deriving characteristics of a sample. A magnetic flux pattern is created which provides improved high resolution. The lens includes a polepiece with an inner yoke, an outer yoke and a winding. A lens outer pole is secured to the outer yoke and includes a first surface having a first opening defined therein positioned such that the beam passes therethrough. A lens inner pole is secured to the inner yoke and includes a second surface having a second opening defined therein aligned with the first opening, but with a smaller inner diameter.

Description:
FIELD OF THE INVENTION 
     The invention is directed to a magnetic lens for focusing a charged particle beam generated by an instrument to a very small spot for deriving characteristics of a sample and, in particular, to create a magnetic flux pattern which provides improved high resolution. 
     BACKGROUND OF THE INVENTION 
     Various instruments are known which rely on interaction of charged particles from a sample to derive characteristics of the sample. Examples of such instruments are an electron microscope and a focused ion beam microscope. A focused beam of charged particles is also used in a machine for conducting electron beam lithography. 
     For facilitating the description of the present invention, it will be explained in connection with a scanning electron microscope (“SEM”). However, it should be understood that the invention is not limited to an SEM and can be applied by one with ordinary skill in the art to instruments and machines such as those mentioned above which require a focused beam of charged particles. 
     An SEM operates by generating a primary scanning electron beam that impacts a sample, a surface of which is being imaged. As a result, backscattered and secondary electrons are emitted from the sample surface and collected by a detector which is arranged near the surface of the sample. The detector generates a signal from the electron emission collected from the sample surface as it is exposed to the electron beam. The signal from the detector is used to display an image of the surface on a video screen. 
     A typical arrangement of the main components of an SEM is schematically shown in FIG.  1 . Electron source  2  generates an electron beam  3  which is directed through aligned openings at opposite ends of tube  4  toward sample  5 . Detector  6  collects electrons emitted from sample  5 . Beam  3  passes through opening  8  in detector  6 . Beam  3  is controlled by stigmation coils  7 , alignment coils  9 , scan coils  11   a  and  11   b,  and lens  13 . The function of these components is well known. Briefly, stigmation coils  7  are used to correct the shape of the beam. Alignment coils  9  are used to align the beam through the tube  4 . Scan coils  11   a  and  11   b  deflect electron beam  3  in two directions, respectively, such as along an x-direction and a y-direction in a plane perpendicular to the beam direction. SEM&#39;s can contain more than one of any of these components. 
     Electromagnetic lens  13  is provided for focusing of the beam  3  to a very small spot to enable high resolution imaging. One type of lens  13  is an immersion lens. U.S. Pat. No. 5,493,116 discloses an immersion lens, and that lens is shown schematically in FIGS. 1 and 2 hereof. It includes a toroidal, channel-shaped magnetic polepiece  14  with a lens inner pole  15  and a lens outer pole  17 , and a winding  19  inside the channel. 
     One characteristic of an SEM lens is its electron-optical working distance (“E.O.”). The E.O. refers to the distance between the surface plane of sample  5  and a plane corresponding to a region of maximum flux density of the lens. The region of maximum flux density for lens  13  is located at plane  22 . The E.O. is described as being slightly negative by approximately −1 mm, so that the plane of sample  5  is above the plane  22 . This configuration is alleged to have the beneficial result of considerably increasing the collection efficiency of low-yield backscattered electrons because electrons are swept by this slightly negative E.O. onto the detector (or detectors), such as the electron shown as having an initial trajectory along path  20 , which is at a significant angle from normal, but is deflected and reaches the detector via deflected trajectory  21  (see FIG.  2 ). 
     A shortcoming of this prior art approach, however, is that the magnetic field, as shown in FIG. 2, interacts with the sample and anything below the sample in the SEM if they have magnetic properties, such as the x-y stage (not shown) which is used to move the sample to its desired scanning position relative to the electron beam. Such interaction causes the field to become distorted. In fact, it is not as shown in FIG. 2, and this deteriorates the resolution achievable with the instrument. In addition, the flux below the sample serves no useful purpose, but power is consumed to generate it. Power used to create this flux generates heat which then has to be conducted away from the coil winding  19 . Furthermore, aberrations in generating the small spot can be minimized by creating a magnetic flux pattern which has a concentration of magnetic field near the sample. Since this prior art approach does not produce such a field, higher aberration coefficients can be expected. 
     A pinhole lens is another type of magnetic lens known in the prior art for focusing a charged particle beam. In contrast to the immersion lens, the bulk of the magnetic field generated by a pinhole lens is above the sample (i.e. it has a positive E.O.). A shortcoming of this lens is that it has a high focal length which interferes with attaining a high resolution. Also, on-axis and near on-axis electrons cannot pass through this field and, therefore, the detector must be positioned below the lens. This further increases the focal length and exacerbates the difficulty in attaining high resolution. Moreover, a detector located in that position can collect only electrons which are substantially off-axis, thereby losing the other electrons. 
     SUMMARY OF THE INVENTION 
     One object of the invention is to provide a magnetic lens which produces improved focusing of a beam of charged particles. 
     Another object of the invention is to provide a magnetic lens which produces improved high resolution imaging. 
     A further object of the invention is to provide a charged particle lens for imaging which exhibits reduced aberration coefficients. 
     Yet another object of the invention is to provide a magnetic lens which does not waste power. 
     Still another object of the present invention is to provide a magnetic lens having a magnetic field which does not interact strongly with the sample and things below the sample. 
     One other object of the invention is to create desirable lens properties for a magnetic lens while allowing emitted electrons to efficiently reach the detector. 
     These and other objects are attained in accordance with one aspect of the present invention directed to a magnetic lens for an instrument which directs a charged particle beam toward a sample. A polepiece includes an inner yoke, an outer yoke and a winding. A lens outer pole is secured to the outer yoke and includes a first surface having a first opening defined therein positioned such that the beam passes therethrough. A lens inner pole is secured to the inner yoke and includes a second surface having a second opening defined therein aligned with the first opening, but with a smaller inner diameter. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic cross-section of a prior art SEM. 
     FIG. 2 shows an expanded view of the lens from FIG. 1 with a depiction of the magnetic flux pattern it generates. 
     FIG. 3 shows a cross-section taken along lines III—III in FIG. 4 of a lens in accordance with the invention. 
     FIG. 4 shows a cross-sectional view taken along line IV—IV in FIG.  3 . 
     FIG. 5 shows an expanded view of a portion of the lens from FIG.  3 . 
     FIG. 6 shows a portion of the lens from FIG. 3 with a depiction of the magnetic flux pattern it generates. 
     FIG. 7 is a view similar to FIG.  5  and including a depiction of equipotential lines for the magnetic flux pattern of FIG.  6 . 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 3 shows a cross section of a lens  30  in accordance with the present invention. Magnetic lens  30  has a toroidal, channel-shaped magnetic polepiece  40 . Polepiece  40  has an inner yoke  42 , an outer yoke  44 , and a winding  46  inside the channel. The manner with which these components are mounted within the SEM is well known and, thus, no details are deemed necessary. 
     Turning now to the specific features of the lens which embody the principles of the present invention, the inner and outer yokes  42  and  44  are provided with poles  60  and  50 , respectively that project toward sample  5  and serve to create a magnetic flux pattern which focuses beam  3  to a very small spot as it impacts the sample. In particular, and with reference to FIGS. 3 and 4, attached to the bottom end  51  of outer yoke  44  is lens outer pole  50 . Pole  50  has a cylindrical outer part  55  and a substantially flat, horizontal inner part  56  lying in a plane which is substantially parallel to sample  5 . A flange  59  is at the inner periphery of part  55 . Flange  59  facilitates alignment of the part during assembly. In addition, it serves to support seal  70 . The O-rings  71  in seal  70  seal the interior of the lens in order to maintain a vacuum, as is well known, without requiring the space occupied by winding  46  to be included in the vacuum. Holes  52  are provided in outer part  55  for attachment of pole  50  to yoke  44  by screws (not shown). A centrally located circular opening  57  is defined in inner part  56  by edge  58 . Edge  58  is tapered in a manner described in detail below. Pole  50  can be made of any material with magnetic properties sufficient to carry the flux required for operating the lens. 
     Attached to the bottom end  51   a  of inner yoke  42  is lens inner pole  60  which has a flange  61  at the upper, outer periphery of a substantially flat, horizontal outer part  62  which lies in a plane substantially parallel to sample  5 . Flange  61  facilitates alignment of the part during assembly. In addition, it serves to support seal  70 . Holes  63  are formed through part  62  to attach pole  60  to yoke  42  by screws (not shown). Horizontal part  62  also includes holes  65  which secure a detector (not shown) in position. The inner part  66  of pole  60 , beginning at the I.D.  64  of outer part  62  angles down toward sample  5  and has a central circular opening  67  defined by edge  68 . Opening  67  is smaller than and concentric with opening  57 . The angled inner part  66  tapers down to edge  68 . Inner part  66  is tapered in a manner described in detail below. Edge  68  lies in a plane above inner part  56  of pole  50 , as further explained below. Circumferential gaps  74  are provided to permit air to pass out during pumpdown while creating a vacuum. Also, the gaps  74  accommodate wiring for the detector. The gaps should be arranged symmetrically to avoid creating aberrations, although some departure from perfect symmetry can be tolerated. 
     FIG. 5 provides more detail regarding the lens outer pole  50  and lens inner pole  60 . The I.D.  64  of pole  60  is selected to enable all electrons emitted through a wide angle from normal (i.e. the on-axis direction) to reach the detector. The I.D. of opening  67  and the angle for the cone of inner part  66  of pole  60  are selected based on this consideration of enabling emitted electrons to reach the detector. Another way of describing this physical arrangement is that the virtual extension of the inner surface of part  66  along line  66   a  reaches the area of sample  5  which is being imaged by the beam. 
     Once these dimensions and configurations are established for lens inner pole  60 , lens outer pole  50  is configured to create a magnetic field by suitable positioning and sloping relative to pole  60 . More specifically, pole  50  cannot be so close to pole  60  as to extinguish the magnetic field. A certain gap must be provided. Since inner part  66  angles down toward sample  5 , the taper of the upper surface of edge  57  and the taper of the lower surface of inner part  66  form a uniform gap of a selected distance between poles  50  and  60 . However, the gap need not be uniform because the uniformity of the gap has little effect on the flux pattern which influences the beam. Also, the extension of the tapered surface of edge  58  with virtual lines, as shown in FIG. 5, reaches approximately the same spot on the sample as virtual extension  66   a.    
     The diameter of opening  67  must be smaller than the diameter of opening  57  so that the flux pattern emanates down toward sample  5 . Opening  67  of pole  60  is located in a plane above opening  57  of pole  50 . This results in raising the region of maximum flux density created by the lens. The diameter of opening  67  affects the focal length and the field profile generated by the lens. Thus, a larger diameter produces a longer focal length, and vice versa. Also, increasing this diameter while keeping the diameter of opening  57  unchanged raises the region of maximum flux density. 
     FIG. 6 shows the magnetic flux pattern produced by lens  30 , and how that flux pattern is positioned relative to sample  5 . A plane  78  corresponding to a region of maximum flux density is created by the lens. Plane  78  should be between the bottom of the lens (e.g. the bottom surface of part  56 ) and the sample surface. This creates a positive E.O. in that the plane of sample  5  is below the plane  78 . The plane  78  can be raised when interaction of the magnetic field emanating from the lens with the sample and/or materials below the sample is undesirable. However, by adjusting the size and positioning of the lens components, plane  78  can also be lowered, even to the extent of a negative E.O., when that is found desirable, such as to control the path of electrons emitted from the sample. 
     This configuration according to the invention has several beneficial results. The magnetic field reaches sample  5  to effectively focus the beam, but only an insignificant portion of the field extends below the sample. This avoids interaction of the field with samples and components below the sample which could distort the field and adversely affect the resolution. Also, the magnetic flux pattern has a concentration of magnetic field near the sample. FIG. 7 depicts the equipotential lines  80  of the flux which demonstrate a well behaved flux pattern that leads to low aberration coefficients. Such a field can be created using less power which effectively reduces the generation of heat. 
     Although a preferred embodiment of the present invention has been discussed in detail above, various modifications thereto will be readily apparent to anyone with ordinary skill in the art. For example, dimensions such as height and angles can be changed. Scaling of the dimensions is also possible. The planes of  56 ,  62  need not be flat nor parallel to sample  5 . Also, well known electrostatic techniques can be applied in combination with the magnetic lens properties described herein. These and all other such variations are intended to fall within the scope of the present invention as defined by the following claims.