Abstract:
Methods and apparatus relating to electromagnetic beam (e.g., laser beam) conditioning are described. In an embodiment, electromagnetic beam conditioning may be performed utilizing reflectors to temporally differentiate electromagnetic beam subsections and sub-beams resulting in reduced spatial coherence of the beam. Other embodiments are also described.

Description:
FIELD 
     The subject matter described herein generally relates to electromagnetic beam conditioning. In one embodiment, some of the techniques described herein may be utilized to reduce the spatial coherence of an electromagnetic beam. 
     BACKGROUND 
     When manufacturing integrated circuit (IC) devices, a laser beam may be used during the manufacturing process to inspect media used in IC fabrication for defects. Generally, a laser beam is a highly coherent beam of radiation. Laser beams may be used to generate an image of the media under inspection. An object with a rough surface (e.g., media used in IC fabrication), when illuminated by a laser beam, may exhibit a speckled appearance. This speckled appearance may cause the media image to appear noisy and distorted. Hence, the presence of speckle in the inspection image may result in an ineffective inspection process. To this end, reducing the amount of speckle encountered in the inspection process may be essential to the effectiveness of the inspection process. 
     SUMMARY 
     In accordance with some embodiments, techniques for conditioning an electromagnetic beam (such as a laser beam) are described. In an embodiment, electromagnetic beam conditioning may be performed to reduce the spatial coherence of an electromagnetic beam. 
     In one embodiment, an apparatus may include a plurality of plane reflectors to temporally differentiate X-directed subsections of an electromagnetic beam (e.g., a continuous wave laser beam or a pulse laser beam). Additionally, the apparatus may include a second set of reflectors to temporally differentiate Y-directed subsections resulting in an output beam comprising a plurality (e.g., columns and rows) of temporally differentiated sub-beams. 
     In one embodiment, an apparatus may include a plurality of corner cube reflectors to temporally differentiate column subsections of an electromagnetic beam (e.g., a continuous wave laser beam or a pulse laser beam). Additionally, the apparatus may include right angle reflectors to temporally differentiate row sub-beams of the subsections resulting in an output beam comprising a plurality (e.g., columns and rows) of temporally differentiated sub-beams. 
     In another embodiment, a method may reflect an electromagnetic beam off of a plurality of corner cube reflectors and right-angle reflectors (or plane mirrors) to temporally differentiate sub-beams of the electromagnetic beam. Additionally, the electromagnetic beam may be reflected off of the plurality of corner cube reflectors and right-angle reflectors to generate a plurality of sub-beams of unequal sizes. 
     Additional advantages, objects, and features of embodiments of the invention are set forth in part in the detailed description which follows. It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of embodiments of the invention, and are merely intended to provide an overview or framework for understanding the nature and character of embodiments of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings are included to provide further understanding of embodiments of the invention, illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items. 
         FIG. 1  illustrates a diagram of an electromagnetic beam conditioning system, according to an embodiment employing plane reflectors. 
         FIG. 2  illustrates a corner cube reflector that may be utilized in various embodiments of the invention. 
         FIG. 3  illustrates a right-angle reflector that may be utilized in various embodiments of the invention. 
         FIG. 4  is a flow diagram of a method to condition an electromagnetic beam to reduce spatial coherence, according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In the following description, numerous specific details are set forth in order to provide a thorough understanding of embodiments of the invention. Embodiments of the invention may be practiced without some or all of these specific details. In other instances, well known process operations have not been described in detail in order not to unnecessarily obscure embodiments of the invention. 
     Also, reference in the specification to “one embodiment” or “an embodiment” means that a particular feature, structure, or characteristic described in connection with the embodiment is included in at least an implementation. The appearances of the phrase “in one embodiment” in various places in the specification may or may not be referring to the same embodiment. 
       FIG. 1  illustrates a diagram of an electromagnetic beam conditioning system  100  in accordance with an embodiment of the invention. In various embodiments, the system  100  may be used to condition an electromagnetic beam (which may be also referred to herein more as a “beam”, “laser” or “laser beam”) in order to reduce spatial coherence, such as discussed further herein with reference to  FIGS. 1-4 , for example. 
     As shown in  FIG. 1 , the system  100  may include a plurality of plane reflectors  104  to reflect a plurality of subsections of the electromagnetic input beam  102 . The plurality of plane reflectors  104  may be spaced along a first axis of the electromagnetic beam  102  such that each plane reflector  104  may reflect a subsection of the beam. For example, the plane reflectors  104  may be arranged to reflect vertical slices (parallel to the y-axis) of the electromagnetic beam  102  side by side but at different depths along the z-axis so that each plane reflector  104  reflects a subsection or vertical slice of the electromagnetic beam  102  so as to delay each subsection with respect to the other subsections. In another embodiment, the system  100  may include a plurality of corner cube reflectors  202  in place of the plane reflectors where the corner cube reflectors may be hollow corner cube reflectors. For example, a hollow corner cube reflector may be viewed as a corner that has been cut out of a box where each of the three sides of the box at the corner may be flat reflectors. More specifically, a hollow corner cube reflector may have three mutually perpendicular surfaces and a hollow center. In another embodiment, the system  100  may include a plurality of corner cube reflectors  202  where the corner cube reflectors may be solid corner cube reflectors. For example, a solid corner cube reflectors may have the same properties as a hollow corner cube reflector but may be a solid prism instead of a hollow reflector. More specifically, a solid corner cube reflector may be a prism with three mutually perpendicular surfaces and a hypotenuse face. In yet another embodiment, the system  100  may include a plurality right-angle reflectors  304  where the flat reflectors are replaced by right-angle reflectors. 
     The system  100  may additionally include a plurality of reflectors (right angle prisms, corner cubes or flat mirrors)  106  to reflect a plurality of sub-beams (horizontal slices) of each of the plurality of subsections. The plurality of plane reflectors  106  may be spaced along a second axis of the electromagnetic beam  102  such that each plane reflector  106  may reflect its sub-beam delaying this sub-beam relative to the other sub-beams. For example, the plane reflectors  106  may be arranged along the Y-axis to reflect a horizontal slice of the electromagnetic beam  102  top to bottom but at different depths along the z-axis of the electromagnetic beam such that each plane reflector&#39;s reflected horizontal slice of the electromagnetic beam is delayed by a different amount with respect to all the other rows. In an embodiment, the system  100  may include plane reflectors, corner cube reflectors or right-angle reflectors  106  and plane reflectors, corner cube reflectors or corner cube reflectors  104  in any combination. For example, the reflectors may be positioned such that the electromagnetic beam  102  reflects off of the right-angle reflectors  106  first and the corner cube reflectors  104  second. In another embodiment, the system  100  may include right-angle reflectors  106  and corner cube reflectors  104  that may be positioned such that the electromagnetic beam  102  reflects off of the corner cube reflectors  104  first and the right angle reflectors  106  second. In another embodiment, the system  100  may include corner cube reflectors  104  and right angle reflectors  106  that may be spaced to produce plurality of sub-beams of unequal size  108 ,  110 ,  112 . For example, the plurality of sub-beams may be collectively viewed as a grid (e.g., columns and rows) of individual beams where each beam in the grid may be of a different size and shape than the other beams in the grid in order to produce sub-beams of uniform power, such as discussed with reference to  FIG. 4 . In another embodiment, the system  100  may include a beam generator to generate an electromagnetic beam. 
       FIG. 2  is an illustration of a corner cube reflector  200 , according to an embodiment. In one embodiment, the reflector of  FIG. 2  may be a component discussed with reference to  FIGS. 1 and 4 . In an embodiment, the corner cube reflector  200  may be made from three flat mirrors so that the reflections occur at the surfaces or it may alternatively be made from a solid piece of material with the reflections occurring due to total internal reflection. A corner cube reflector  202  may be a reflector with three mutually perpendicular surfaces and a hypotenuse face. The nature of the reflector may be such that a beam entering the corner cube reflector is reflected off of the three perpendicular surfaces of the corner cube reflector and is reflected back along a path parallel to the path that the beam took to enter the corner cube reflector. 
       FIG. 3  is an illustration of a right-angle reflector  300 , according to an embodiment. In one embodiment, the reflector of  FIG. 3  may be a component discussed with reference to  FIGS. 1 and 4 . As shown in  FIG. 3 , a beam  302  may be reflected by a right angle reflector  304  to generate a reflected beam  306 . 
       FIG. 4  illustrates a flow diagram of a method  400  to condition an electromagnetic beam to reduce spatial coherence. In one embodiment, various operations discussed with reference to  FIG. 4  may be performed by some of the components discussed with reference to  FIGS. 1 ,  2 , and  3 . 
     Referring to  FIGS. 1-4 , at an operation  402 , a source beam (e.g., an electromagnetic beam) is input (e.g., directed to a plurality of reflectors). At an operation  404 , the electromagnetic beam may be reflected off of the plurality of column reflectors spaced along a first axis (x-axis) to separate the electromagnetic beam into subsections. The column reflectors may also be spaced along the beam&#39;s direction of travel thereby creating temporally differentiated beam subsections. For example, given five reflectors the electromagnetic beam may be split along the x-axis to create five individual columns. Furthermore, because each of theses columns may be reflected by reflectors at different Z positions along the beam path each reflection may be separated in time with respect to the others. The time delay may be a function of the Z spacings of the reflector and the speed of light (which is about 1 ns per foot). Spacing the reflectors along the z-axis of the electromagnetic beam creates five columns separated by few nano-seconds time from the adjacent column. 
     In an embodiment, the column reflectors may be spaced non-equidistant from each other along the first axis (X-axis) creatin unevenly sized subsections  108  thereby compensating for the low intensity of the beam along the outer edge of a nonuniform electromagnetic beam. 
     Referring to  FIGS. 1-4 , at an operation  406 , the electromagnetic beam is reflected off of the plurality of row reflectors spaced along a second axis (y-axis) creating sub-beams from each of the row reflectors. The row reflectors are spaced along the beam&#39;s direction of travel thereby creating temporally differentiated sub-beams from each of the row reflectors. For example, given five reflectors the electromagnetic beam may be split along the y-axis to create five individual rows. By appropriately spacing the reflectors along the z-axis of the beam each reflection may be delayed by a time proportional to the separation of the mirrors and the speed of light. In an embodiment, the row reflectors may be spaced non-equidistant from each other along the second (Y-axis) axis creating unevenly sized sub-beams  110  thereby compensating for the low power of the beam along the outer edge of the electromagnetic beam. 
     Referring to  FIGS. 1-4 , at an operation  408 , the subsections generated at the operation  404  and the sub-beams generated at the operation  406  may generate a plurality of temporally differentiated sub-beams. For example, the plurality of sub-beams may be viewed as a grid (e.g., columns and rows) of individual sub-beams where each sub-beam represents a portion of the original electromagnetic beam  111 . Furthermore, each sub-beam may be delayed from other sub-beams in time (e.g., the sub-beam may be delayed relative to the sub-beam next to it). Additionally, the grid may comprise M columns and N rows of sub-beams where the delay between each column of sub-beams may be N coherence lengths and the delay between each row of sub-beams may be one coherence length. The coherence length may be the difference in propagation distance of two sub-beams from a coherent source (e.g., the electromagnetic beam) where the interference, as a function of Z between the two sub-beams is reduced by a specified degree. 
     In some embodiments, at operation  408 , the subsections generated at the operation  404  and the sub-beams generated at the operation  406  may generate a plurality of temporally differentiated sub-beams of unequal cross-sectional areas  112 . For example, in order to maximize the light throughput, the electromagnetic beam (which may have a Gaussian intensity distribution) may not uniformly illuminate the input aperture of the beam conditioner. One drawback may be that the parts of the beam that have low power (integrated intensity) may not contribute the same to the speckle reduction as those from the areas of high power. To obtain relatively even contributions, the input beam could be specified to have a flat intensity profile. This may be accomplished by expanding the beam so that the middle of the Gaussian beam is used. This may however reduce the efficiency of the module. Instead, the grid of M columns and N rows may have a non-uniform spacing. Sub-beams farther from the center may have larger cross-sectional areas and therefore may contain more light than they would if the grid were uniform. This may make each X/Y sub-beam have equal power and thus an equal contribution to the speckle reduction. 
     Although embodiments have been described in language specific to structural features and/or methodological acts, it is to be understood that claimed subject matter may not be limited to the specific features or acts described. Rather, the specific features and acts are disclosed as sample forms of implementing various embodiments. While the invention has been described above in conjunction with one or more specific embodiments, it should be understood that the invention is not intended to be limited to one embodiment. The invention is intended to cover alternatives, modifications, and equivalents as may be included within the spirit and scope of the invention, such as those defined by the appended claims.