Abstract:
One embodiment relates to a device that senses alignment and height of a work piece. The device may include both an alignment sensor and a height sensor. The alignment sensor generates a first illumination beam that illuminates an alignment mark on the work piece so as to create a first reflected beam, and determines the alignment of the work piece using the first reflected beam. The height sensor generates a second illumination beam that is directed to a surface of the work piece at an oblique angle so as to form a second illumination spot and images the second illumination spot to determine the height of the work piece. Other embodiments, aspects and features are also disclosed.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
     The present application claims the benefit of provisional U.S. Patent Application No. 61/888,385, filed Oct. 8, 2013 by inventors Mark McCord and Joe Di Regolo, the disclosure of which is hereby incorporated by reference. 
    
    
     GOVERNMENT LICENSE RIGHTS 
     This invention was made with Government support under Agreement No. HR0011-07-9-007 awarded by DARPA. The Government has certain rights in the invention. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Technical Field 
     The present disclosure relates to apparatus and methods for alignment and height sensing. 
     2. Description of the Background Art 
     A conventional lithographic process includes the patterned exposure of a resist allowing portions of the resist to be selectively removed, thereby exposing underlying areas for selective processing, such as etching, material deposition, ion implantation and the like. Typically, lithographic processes utilize ultraviolet light for selective exposure of the resist. 
     In addition, charged particle beams (e.g., electron beams) have been used for high resolution lithographic resist exposure. The use of e-beam based lithography systems allows for relatively accurate control of the electron beam at relatively low power and relatively high speed. Electron beam lithographic systems may include electron-beam direct write (EBDW) lithography systems and electron beam projection lithography systems. 
     SUMMARY 
     One embodiment relates to a device that senses alignment and height of a work piece. The device may include both an alignment sensor and a height sensor. The alignment sensor generates a first illumination beam that illuminates an alignment mark on the work piece so as to create a first reflected beam, and determines the alignment of the work piece using the first reflected beam. The height sensor generates a second illumination beam that is directed to a surface of the work piece at an oblique angle so as to create a second reflected beam and detects the second reflected beam to determine the height of the work piece. 
     Another embodiment relates to a method of determining an alignment and height of a work piece. A first illumination beam is generated. The first illumination beam is shaped to form a first illumination spot on the work piece. The first illumination spot may advantageously have a shape comprising a long axis of illumination that is parallel to a long dimension of a rectangular alignment mark on the work piece and a short axis of illumination that is parallel to a short dimension of the rectangular alignment mark. A first reflected beam is obtained from the first illumination spot, and the first reflected beam is used to determine the alignment. A second illumination beam is generated and directed to a surface of the work piece at an oblique angle such that a second illumination spot is formed on the surface of the work piece. The second illumination spot is imaged so as to determine a height of the surface. 
     Another embodiment relates to an apparatus for electron-beam lithography. The apparatus includes a plurality of electron-beam lithography columns spaced at a pitch in at least one dimension. The apparatus further includes a plurality of alignment sensors for determining an alignment of the plurality of electron-beam lithography columns. Each alignment sensor of the plurality of alignment sensors generates a first illumination beam that illuminates an alignment mark on a wafer so as to create a first reflected beam, and determines the alignment of the wafer using the first reflected beam. The apparatus further includes a plurality of stages that are movable in the one dimension on which the plurality of sensors are mounted, wherein the plurality of stages are spaced at the pitch in the one dimension. 
     Another embodiment relates to an alignment sensor. An illumination system generates a shaped illumination beam and focuses the shaped illumination beam to form an illumination spot on an alignment mark on the work piece, wherein the illumination spot has a shape comprising a long axis of illumination that is parallel to the long dimension of a rectangle of the alignment mark and a short axis of illumination that is parallel to the short dimension of the rectangle of the alignment mark. A detection system detects a reflected beam from the illumination spot to determine an alignment of the work piece. 
     Another embodiment relates to a method of sensing a height of a work piece. An illumination beam is generated and directed to a surface of the work piece at an oblique angle such that an illumination spot is formed on the surface of the work piece. A reflected beam from the illumination spot is detected so as to obtain an imaged spot. A derivative signal of the imaged spot is generated, and a threshold is applied to the derivative signal to construct a spot outline. A center location of the spot outline is determined, and the center location is used to determine the height of the work piece. 
     Other embodiments, aspects and features are also disclosed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  is an optical ray diagram showing an illumination beam of an alignment sensor in accordance with an embodiment of the invention. 
         FIG. 1B  is an optical ray diagram showing a reflected beam of the alignment sensor in accordance with an embodiment of the invention. 
         FIG. 1C  is an optical ray diagram showing the illumination and reflected beams of the alignment sensor in accordance with an embodiment of the invention. 
         FIG. 2  is an optical ray diagram of a height sensor in accordance with an embodiment of the invention. 
         FIG. 3  depicts an asymmetric spot image, a conventionally-determined spot center, and a correctly-determined spot center in accordance with an embodiment of the invention. 
         FIG. 4  depicts a combined alignment and height sensor in accordance with an embodiment of the invention. 
         FIG. 5  is a schematic diagram of an arrangement of lithography columns that utilizes the combined alignment and height sensor in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     Accurate and precise alignment of a work piece is often highly desirable or necessary in manufacturing and other processes. In an exemplary application, the work piece may be a semiconductor wafer on which integrated circuits are being manufactured. In this context, the alignment sensor may be an optical sensor that may use imaging, diffraction or optical interference to determine the relative position of a mark on the wafer. 
       FIG. 1A  is an optical ray diagram showing an illumination beam of an alignment sensor  110  that forms an illumination spot on an alignment mark  152  on a work piece  150  in accordance with an embodiment of the invention. As shown, the illumination path of the alignment sensor  110  may include an illumination source  112 , illumination lens (or lenses)  114 , an illumination aperture  116 , a beamsplitter  118 , and an objective lens  120 . 
     An illumination beam  111  may be generated using the illumination source  112 , the illumination lens (or lenses)  114 , and the illumination aperture  116 . The illumination beam  111  may be reflected or partially reflected by the beam splitter  118  towards the objective lens  120 . The objective lens  120  focuses the illumination beam  111  to form a beam spot image on the surface of the work piece  150 . 
     In an exemplary implementation, the alignment mark  152  may be rectangular in outline shape with a smaller dimension limited to less than the width of the scribe line between dies on a semiconductor wafer being manufactured. Alignment marks  152  may be on both horizontal and vertical scribe lines. For an alignment mark on a horizontal scribe line, the smaller dimension of the rectangular outline of mark  152  is in the vertical dimension. For an alignment mark on a vertical scribe line, the smaller dimension of the rectangular outline of the  152  is in the horizontal dimension. 
     The illumination generated by the source  112  may be white light, monochromatic light from a laser or diode, or a multispectral illumination. While conventional alignment sensors use a circularly symmetric illumination spot, the presently-disclosed alignment sensor may use an elliptical or rectangular illumination spot so as to provide an improved signal-to-noise ratio in accordance with an embodiment of the invention. The smaller dimension of the ellipse or rectangle is in the same dimension as the smaller dimension of the rectangular outline of the alignment mark  152 . Such an elliptical or rectangular illumination spot may be used to maximize illumination on the mark (so as to maximize signal strength) while minimizing illumination outside the mark (which would contribute to noise). 
     In one embodiment, the elliptical or rectangular illumination spot may be generated using a cylindrical lens in the illumination optics  114 . In another embodiment, the elliptical or rectangular illumination spot may be generated using a shaped illumination aperture  116  at an image plane. In another embodiment, the elliptical or rectangular illumination spot may be generated using an illumination source  112  that is shaped. 
     When alignment marks  152  are implemented on both horizontal and vertical scribe lines between integrated circuit dies, a mechanism for rotating the spot axis by ninety degrees may be employed. For example, if a cylindrical lens in the illumination optics  114  is used to generate the elliptical or rectangular illumination spot, then the cylindrical lens may be rotated by ninety degrees. If a shaped illumination aperture  116  is used to generate the elliptical or rectangular illumination spot, then the shaped illumination aperture  116  may be rotated by ninety degrees. If a shaped illumination source  112  is used to generate the elliptical or rectangular illumination spot, then the shaped illumination source  112  may be rotated by ninety degrees. Alternatively, instead of rotating the spot axis by ninety degrees, a fixed illumination spot with a cross pattern or an L-shaped pattern may be used. Another embodiment, instead of rotating the spot axis, may have two shaped lenses, apertures or light sources at two orientations, that are swapped electrically (light sources) or mechanically (lenses, apertures or light sources). 
       FIG. 1B  is an optical ray diagram showing a reflected beam of the alignment sensor  110  in accordance with an embodiment of the invention. As depicted, the reflected beam  121  is formed from the light reflected or diffracted from the alignment mark  152  on the work piece  150 . 
     The reflected beam  121  may be focused by the objective lens  120  and transmitted or partially transmitted through the beam splitter  118 . A first focusing lens  122  focuses the reflected beam  121  transmitted through the beam splitter  118  onto a reference grating  124 . 
     The reference grating  124  diffracts the light so as to form a diffracted beam  125 . The diffracted beam  125  is focused by a second focusing lens  126  onto a detector  128 . The alignment mark and reference grating is coordinated such that the signal from the detector  128  indicates alignment of the illumination beam relative to the alignment mark. 
       FIG. 1C  is an optical ray diagram showing the illumination beam  111  and the reflected beam  121  of the alignment sensor  110  in accordance with an embodiment of the invention. As shown, the beam splitter  118  allows for the optical paths of both the illumination and reflected beams to pass through the objective lens  120 . 
     In accordance with an embodiment of the invention, improved performance may be realized in at least three ways. First, the alignment sensor  110  may provide an illumination spot sufficient to illuminate the entire alignment mark  152  at once. This advantageously assists in averaging out errors from line edge roughness and random placement errors in mark fabrication. Second, the illumination spot may be scanned over the wafer in either horizontal or vertical directions. This allows for the reading of alignment marks placed in either horizontal or vertical scribe lines while the stage is moving in only one direction. Third, if local optical power density is a limitation, then performance of the alignment sensor  110  may be improved by increasing the optical power. This reduces shot noise effects and the relative impact of detector electronic noise. 
     In some applications, instead of being statically illuminated, the alignment mark  152  may be scanned by an illumination spot that moves relative to the work piece  150 . The relative motion may be due to movement of the wafer stage relative to the alignment sensor  110 . 
     For a round illumination spot, the alignment mark  152  should be scanned parallel to its long axis to maximize the signal collection time for best mark detection accuracy. However, for an illumination spot that is shaped asymmetrically (for example, as an ellipse, rectangle, cross shape, or L shape) to match the shape and size of the alignment mark  152 , the mark may be scanned in either direction with equivalent or near equivalent accuracy. 
     In another embodiment, instead of the reflected beam  121  of the alignment mark  152  being diffracted through a reference grating as shown in  FIGS. 1A-1C , the alignment mark  152  may be imaged on a two-dimensional detector (such as a 2D CCD array). 
       FIG. 2  is an optical ray diagram of a height sensor  200  in accordance with an embodiment of the invention. The height sensor  200  may be used to determine the distance of the work piece  150  relative to the height sensor  200  (in the vertical direction if the height sensor  200  is vertically above the work piece  150 ). 
     As depicted, the height sensor  200  may include an illumination source  202 , an illumination lens  204 , a detection lens  206 , and a detector  208 . The illumination source  202  generates an incident light beam that is focused by the illumination lens  204  onto a spot on the surface  210  of the work piece  150 . The incident light beam may impinge on the surface  210  at a glancing or oblique (non-normal) incident angle and be reflected from the surface  210  at a corresponding reflection angle. The reflected light is focused by the detection lens  206  onto the detector  208 . 
     In accordance with an embodiment of the invention, the detector  208  may be a two-dimensional (2D) detector that is position sensitive in the two dimensions. For example, the detector  208  may be a 2D charge-coupled device (CCD) imaging sensor. A windowing capability of the 2D detector may be utilized so as to provide a higher frame rate by ignoring detector pixels outside the window, where the window is set based on the known information about the location, size and shape of the spot  302 . 
     In a conventional apparatus, such a 2D detector is prone to erroneous readings in the presence of pre-existing patterns formed on the wafer. This is due to the pre-existing patterns distorting the intensity and symmetry of the reflected beam. Errors due to either the variations in wafer reflectivity or diffraction effects caused by the pre-existing patterns cause an apparent shift in the wafer height reading. 
     The present disclosure provides a solution that provides an accurate wafer height reading, even in the presence of a pre-existing pattern on the wafer. As an example, consider the asymmetric spot image  302  shown on the left in  FIG. 3  as may be formed on the surface  210  of the work piece  150  by the height sensor  200 . As discussed above, the asymmetric spot image  302  may be due to a wafer pattern effect. 
     A conventional determination of a spot center  304  of an asymmetric spot image  302  is shown in the middle in  FIG. 3 . The spot center  304  is offset from the correct center of the spot because the asymmetric intensity of the spot  302 . In this case, since the intensities are higher on the left side of the spot  302 , the spot center  304  is shifted to the left from the correct center. 
     One method for determining the correct center  308  of the spot  302  is illustrated on the right side of  FIG. 3  in accordance with an embodiment of the invention. In this method, edge detection may be used to determine a spot outline  306 . If the detector signal is analog, then the edge detection may involve applying a derivative-forming circuit to the detector signal so as to generate a derivative signal. If the detector signal is digital, then the edge detection may involve digitally generating the derivative signal. The derivative signal is indicative of “edges” that are potential locations of the spot outline  306 . Other algorithms may also be used to estimate the true location of the spot (i.e. the location of the spot in the absence of the pattern noise). A threshold may be applied to the derivative signal so as to construct the spot outline  306 . The knowledge of the true size and shape of the spot  302  may be utilized in constructing the spot outline  306 . A correct center of the spot  302  may be determined by locating a center  308  of the spot outline  306 . 
     The height of the work piece may then be determined from the location of the center  308 . For example, the closer the center  308  is to the illumination lens  204 , the higher is the height of the work piece. On the other hand, the closer the center  308  is to the detection lens  206 , the lower is the height of the work piece. 
     In an alternate embodiment, a one-dimensional (1D) detector may be used for the detector  208 , such as a 1D CCD or CMOS imaging sensor. While more accurate height sensing may be performed with a 2D detector, using a 1D detector advantageously provides a higher frame rate and uses less system bandwidth. For a 1D detector, derivative signal indicates edge points that are potential locations for the end points (extremities) of the spot  302 , and the threshold application may be used to select the end points of the spot  302  (i.e. the “outline” of the spot in 1D) from the edge points. The correct center  308  of the spot  302  may then be determined to be at a middle point between the end points. 
     In another possible detector configuration, two apertures at the illumination image plane that are spaced along the motion axis of the sensor may be monitored by a detection system. The detection system may be implemented, for example, as two position-sensitive detectors, or two bi-cell detectors, or a CCD or CMOS imaging sensor. The spacing of the two apertures may be set to be sufficiently far apart to reduce any underlying pattern dependent diffraction from changing the energy spread across the spaced apertures. For example, an underlying pattern may be a patterned layer underlying a resist layer. Provided that the optical configuration of the height sensor  200  is telecentric, the spacing and magnification should remain constant as the wafer position changes or tilts. 
     A polarizing filter in the illumination arm and/or projection arm of the height sensor  200  may be provided to reduce the effects of underlying patterns (for example, patterned layers underlying a resist surface). The polarizing filter does this by increasing the reflectivity of the “S” polarization at the resist surface. In addition, the angle of incidence to the wafer may also be reduced to improve the reflectivity of the resist surface so as to reduce the effects of the underlying patterns. 
       FIG. 4  depicts a combined alignment and height sensor  400  in accordance with an embodiment of the invention. The combined sensor  400  may be operated to determine both alignment and height at a same time. 
     As shown, the combined sensor  400  includes components of both the alignment sensor  100  and the height sensor  200 . In order to minimize space, cost, and maximum accuracy, the optics of the height sensor  200  and the optics of the alignment sensor  100  may share the same optical axis. The optical axis of the height sensor  200  is depicted in  FIG. 2 , and the optical axis of the alignment sensor is depicted in  FIG. 1B . Optical components may or may not be shared, depending upon the optical and mechanical design. 
     In one embodiment of the combined alignment and height sensor  400 , one or more filter may be added to the optics for the alignment sensor  100  and/or the height sensor  200 . The filters may be advantageously used to prevent the light from the height sensor from degrading the alignment signal and also to prevent the light from the alignment sensor from degrading the height sensor reading. 
       FIG. 5  is a schematic diagram of an arrangement of lithography columns  504  that utilizes multiple sensors  508  in accordance with an embodiment of the invention. As depicted, various components may be mounted on a metrology frame  502 . The metrology frame  502  may be used to maintain position and alignment of the components. 
     In one embodiment, the multiple sensors  508  may be used for alignment and so each sensor  508  may include components of the alignment sensor  100  described above. As depicted, an illumination beam  510  (in the plane of the page) from each of sensor  508  may be deflected 90 degrees by an angled reflector  512  (so as to be perpendicular to the plane of the page) to an alignment mark on a semiconductor wafer on which circuitry is being manufactured. 
     The multiple sensors  508  may be mounted on a movable stage  506 . Each stage  506  may move in one-dimension so as to adjust a position of a sensor  508 . In particular, the sensors  508  may be moved so as to adjust the pitch between the sensors  508  (in the vertical direction in  FIG. 5 ). In one implementation, the pitch between the sensors  508  may match the die (or “street”) pitch on a silicon wafer being manufactured. With the pitches matching, all the sensors  508  may read alignment marks at a same time. This advantageously reduces an overall measurement time. 
     Because the position of the alignment sensors must be precisely known in all axes, capacitive or interferometric sensors may be used to accurately measure the position of each sensor  508 . The sensor position measurements may include measurements in the x, y, and z axes and of tip and tilt, as may be needed. 
     While  FIG. 5  shows one linear array of sensors  508  in a first dimension that matches the pitch of the lithography columns  504  in that first dimension, another embodiment of the invention may have a second linear array of sensors  508  in a second (perpendicular) dimension that matches the pitch of the lithography columns  504  in that second dimension.