Abstract:
In one embodiment, an interferometer system comprises two unequal path interferometers assemble comprising; a first reference flat having a first length L 1  in a first dimension, a second reference flat having a second length L 2  in the first dimension, a cavity D 1  defined by a distance between the first reference flat and the second reference flat, a wafer holder to receive an object in the cavity such that an optical path remains open at an outer annual area between the first reference flat and the second reference flat and at least one wafer holder motor coupled to the wafer holder such that an object may be tilted in the cavity as to allow for measurements of local areas of interest, and a radiation targeting assembly to direct a collimated radiation beam to the interferometer assembly, a radiation collecting assembly to collect radiation received from the interferometer assembly, and a controller comprising logic to; vary a wavelength of the collimated radiation beam, record interferograms formed by a plurality of surfaces, extract phases of each of the interferograms for each of the plurality of surfaces to produce multiple phase maps, determine each map from its corresponding interferogram, determine from each map local areas of interest with high slopes, tilt the wafer holder to allow measurement of the high slope areas of interest, and process measurement that covers the entire surface of an object including high slope areas.

Description:
BACKGROUND 
       [0001]    This application relates to radiation-based inspection techniques and more particularly to interferometric profilometry systems and methods which may be used to measure the shape, and thickness variation of a wafer with high slopes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0002]      FIG. 1  is a schematic illustration of an interferometer assembly according to embodiments. 
           [0003]      FIG. 2A  is a flowchart illustrating operations of a method which may be used to measure the shape and thickness of a wafer with high slopes according to an embodiment. 
           [0004]      FIG. 2B  is a flowchart illustrating operations of a method which may be used to measure the shape and thickness of a wafer with high slopes according to an embodiment. 
           [0005]      FIG. 3  is a schematic illustration of an integrated visible pilot beam for non-visible interferometric device according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0006]    Described herein are exemplary systems and methods which may be used to measure the surface height on both sides and the thickness variation of a wafer with high surface slopes. In the following description, numerous specific details are set forth in order to provide a thorough understanding of various embodiments. However, it will be understood by those skilled in the art that the various embodiments may be practiced without the specific details. In other instances, well-known methods, procedures, components, and circuits have not been described in detail so as not to obscure the particular embodiments. 
         [0007]    Embodiments described herein may be used in conjunction with two unequal path length interferometers (such as Fizeau interferometers), contemporaneously acquiring two sets of intensity frames that record interferograms generated with wavefronts reflected from both sides of a wafer surfaces and from the reference flats. These intensity frames may be acquired sequentially, by changing the wavelength in a measurement system. The wavelength can be changed mechanically, or, preferably, with a tunable laser light source. As used herein, the phrase contemporaneous events refers to events which happen within a reasonable time period of one another, given the technical circumstances. As used herein, the term “contemporaneous” should not be construed to mean “simultaneous.” Embodiments of an interferometer assembly for contemporaneous acquisition of multiple sets of intensity frames are described in U.S. Pat. No. 6,847,459 to Freischlad, et al., entitled METHOD AND APPARATUS FOR MEASURING THE SHAPE AND THICKNESS VARIATION OF POLISHED OPAQUE PLATES, the disclosure of which is incorporated herein by reference in its entirety. 
         [0008]    In general, the methods described herein take advantage of the fact that the high spatial slope of the optical path difference (OPD) between a wafer surface and a reference flat can be locally nulled or reduced by changing tilt of the wafer. This implies that the interferogram moves around on the imaging plane or the measuring area of the wafer surface shifts if the tilt of the wafer changes. This is because an interferogram only appears at the area where spatial slope of OPD is small and the measurement is only accomplished at the area where the interferogram is visible. A map of partial wafer surface may be achieved from each measurement at a tilt position of the wafer. Multiple such maps that correspond to different parts of the wafer surface may be required to measure a wafer with high surface slopes. 
         [0009]    In general, the system and method of an embodiment of the invention may be able to produce a height map that covers a wafer surface with high slopes by combining multiple maps of partial wafer surface measurements. 
         [0010]    In general, the system and method of a preferred embodiment of the invention may be able to construct the surface height maps on a part of or whole wafer on both sides simultaneously and may be able to measure thickness variation of a wafer without errors results from the cavity path difference. 
         [0011]    In general, the system and method of an embodiment of the invention employs two reference flats that are larger in size than the measuring wafer. While the center part of reference flats forms the interferograms with the wafer surfaces, their outside annular area generates the interferogram of themselves. In some embodiments, the invention is not only is able to determine the location of a testing wafer very precisely by finding out its shade on the reference flats but also is able to monitor the relative tilt change of reference flats with every measurement. Consequently, it is capable of measuring the wafer edge location without the influence of the surface slopes at the wafer edge thereby significantly increases the measurement repeatability. 
         [0012]      FIG. 1  is a schematic illustration of an interferometer assembly according to embodiments. In some embodiments, the unequal path interferometer may be a Fizeau interferometer. In some embodiments, the unequal path interferometer may be a Twyman-Green interferometer. An overview of an embodiment of the invention is shown in  FIG. 1 . For wafer measurement, a wafer  160  may be placed in a cavity in the center between two improved Fizeau interferometers  120  and  140 , such that both wafer sides  161  and  162  are minimally obscured by the holding devices  172 ,  174 . The interferometers  120  and  140  may operate in the following way: light is emitted from an illuminator  110  along fibers  122 ,  142 , reflected at a polarizing beam splitter  126 ,  146  and passes through a quarter-wave plate  128 ,  148  aligned at 45 degrees to the polarization direction of the polarizing beam splitter  126 ,  146 . Two multimode optic fibers  142  and  122  collect the light from the illuminator  110  and carry it to the two source locations  144  and  124  of the two interferometer channels  140  and  120 . The light is circularly polarized after the quarter-wave plate. This beam then propagates to the lens  130 ,  150 , where it is collimated with a beam diameter larger than the wafer diameter. The collimated beam then falls on the reference flat  132 ,  152 , where part of the light is reflected at the reference surface, and another part is transmitted. The central part of the transmitted beam is reflected at the test surface  161 ,  162 ; and the outer part of the transmitted beam travels on to the opposite reference flat  152 ,  132 , where it is reflected at the reference surface  153 ,  133 . 
         [0013]    The light reflected at the wafer surface  161 ,  162  constitutes the wafer test beam. The light reflected at the opposite reference surface  133 ,  153  constitutes the cavity ring test beam; and the light reflected at the reference surface constitutes the reference beam. All three reflected beams travel back through the reference flat  152 ,  132  and through the collimator lens  150 ,  130  to the quarter-wave plate  148 ,  128 . After the quarter-wave plate, the beams are linearly polarized with the plane of polarization of the reflected beams rotated 90 degrees compared to the outgoing beams. When the reflected beams reach the beam splitter  146 ,  126 , they are transmitted and directed to an imaging lens  154 ,  134 , which relays the beams to a detector  156 ,  136 , where the interference patterns between the test beams (reflected from the wafer  160 ) and the reference beams occur. 
         [0014]    The detector  156 ,  136  may consist of a video camera, the signal of which is digitized and further processed in a computer  158 ,  138 . The computers  158  and  138  of each interferometer channel are connected for data exchange and synchronization. Alternatively, one common computer could be used to receive the camera signals of both channels. Computers  158 ,  138  are further discussed with reference to  FIG. 3 . 
         [0015]    The data acquisition is now described in more detail for interferometer channel  140 . The second interferometer channel  120  behaves in an equivalent way. The two reference surfaces  153 ,  133  and the wafer  160  are substantially parallel. Thus, the interference pattern appears on detector  156 . There is a central area of interference fringes imposed on the wafer surface, generated by interference of the wafer test beam, with the reference beam. In addition, there is an area surrounding the wafer  160  with interference fringes generated by the cavity ring test beam and the reference beam. Depending on the slopes of the wafer surfaces, there may be a zone without any interference fringes. This zone without fringes is caused by the test beam being reflected at such high angles that it does not reach the camera. 
         [0016]    In some embodiments, wafer holding devices  172 ,  174  may be actively tilted by holding device motors  170 ,  176  in a controlled way between data acquisitions to allow measurements of local areas on the wafer with high slopes. By way of example and not limitation an area may be considered to have a high slope when a surface slopes greater than wavelength divided by twice w, where w is the pixel width. The wafer tilting may be performed to null the local high slopes in a region of interest of the wafer, which corresponds to a certain sensing area of the detectors  156 ,  136 . In such embodiments, predictions may be made as to what tilt may yield a complete dataset covering the full area of the wafer. Therefore, a wafer with high surface slope may be measured by an adequate sequence of tilts of the wafer. In some embodiments, the wafer holding devices  172  and  174  are a right body or can not move relatively. In such embodiments, the wafer holding devices  172  and  174  are always tilted together by either the holding device motors  170  or  176 . This allows for the free shape of the wafer to be more accurately measured. In some embodiments, any suitable wafer holding device may be used, with a preferred holder being a vertical wafer pallet minimizing the stress on the wafer to be held. In such embodiments, this may allow for the free shape of the wafer to be more accurately measured. In operation, the wafer holding device may be implemented by using two computer controlled piezo-actuated flexures allowing the angles to be precisely controlled. 
         [0017]    In order to obtain height maps from the interference patterns, a phase-shifting data acquisition method is applied to extract the interferometric fringe phase. In some embodiments, extracting phases of each of the interferograms may be effected by a computer. In some embodiments, recording the multiple optical interferograms may be effected by means of a CCD camera. 
         [0018]      FIG. 2A  is a flowchart illustrating operations of a method which may be used to measure the shape and thickness of a wafer with high slope areas according to an embodiment. At operation  201 , an interferometer may be initiated. In some embodiments, a test object such as, but not limited to, a wafer may be placed in the interferometer. By way of example and not limitation, the interferometer may be a Fizeau interferometer, a Twyman-Green interferometer, or the like. At operation  206 , coherent light may be supplied to a test object. In some embodiments, the coherent light may be supplied by a tunable laser or the like. At operation  211 , an interferometer may record interference patterns. In some embodiments, recording the multiple optical interferograms may be effected by means of a CCD camera. At operation  216 , interferograms may be extracted from data recorded by the interferometer. In some embodiments, extracting phases of the interferograms from each channel of the interferometer may be effected by a computer. In some embodiments, the process of extracting phases may be affected by two computers. 
         [0019]    In some embodiments, analysis of these interferograms allows for the determination of various information, such as but not limited to, local areas of interest on the surface of the object that have high slopes. These areas of high slope do not produce height or thickness information as the reflected light returns at such an angle as it is not received by the detectors. At operation  226 , the object surface is analyzed to determine in there are any areas of high slope which may require further analysis to determine their properties. If at operation  226 , there are local areas of interest for additional analysis, then at operation  231  the wafer may be titled by the wafer holding devices. In operation, predictions may be made as to what tilt sequence may yield a complete dataset covering the full area of the wafer. 
         [0020]    Once the wafer has been tilted to the determined angle, additional interferograms are produced through repeating operations  206  to  216  until there are no longer unmeasured high slope areas. If at operation  226 , there are no areas that may require additional analysis due to high slopes, then at operation  236  complete parameter maps of the object surface may be produced. In some embodiments, a computer may stitch together all partial surfaces that are taken at different tilts and partially overlap each other and thereby produce a measurement that covers the entire object surface. 
         [0021]    Additional information relating to the wafer properties may by obtained through analysis of phase maps obtained from the interferograms. By way of example and not limitation, arbitrarily identifying the phase of the interferogram formed by the front reference plate and the front of the wafer surface as A, and the phase of the interferogram formed by the back reference flat and the back surface of the wafer as B, and the phase of the interferogram formed by the cavity of the front reference flat and the back reference flat as C, then the surface parameters may be determined as follows: A corresponds to the front surface height of the wafer, B corresponds to the back surface height of the wafer, C−(A+B) corresponds to the thickness variation in the wafer. 
         [0022]      FIG. 2B  is a flowchart illustrating operations of a method which may be used to measure the shape and thickness variation of a wafer according to an embodiment. At operation  205  the laser is activated at a first wavelength. In operation, the laser  110  generates electromagnetic radiation in a range of wavelengths to interferometer  120 ,  140 . 
         [0023]    At operation  210  radiation reflected is captured. In some embodiments, radiation reflected is captured as the wavelength of radiation is changing. The reflected radiation is directed by interferometer  120 ,  140 , contemporaneously, multiple interferograms to detector  136 ,  156  (e.g., a CCD camera or other suitable recording planes). In some embodiments, contemporaneous events may be defined as events that occur within a reasonable time period of one another, given the technical circumstances. The detector  136 ,  156  may include a frame grabber for storing images; alternatively, the computer  138 ,  158  may be configured to provide this function. In any event, the images obtained by the detector  136 ,  156  are supplied to the computer  138 ,  158  for processing to produce the desired profiles in a suitable form for immediate display, or storage for subsequent utilization. At operation  215  interference patterns in the reflected radiation are captured. 
         [0024]    If, at operation  220 , the amount of data acquired is not sufficient, then control passes to operation  225  and the wavelength of the radiation generated by laser  110  is changed. In some embodiments, if the amount of data acquired in not sufficient, the control passed to operation  225  to keep changing its wavelength. For example, the wavelength may be increased or decreased by a predetermined amount. Control then passes back to operation  210  and the reflected radiation is captured. Operations  210 - 225  are repeated until an adequate number of data samples are acquired, whereupon control passes to operation  230  and one or more phases of interferograms are extracted from the data collected. In some embodiments, a control passed to operation  225  to stop its wavelength changing while another control passed to operation  230 . In some embodiments, the phases of interferograms may be extracted and stored. 
         [0025]    At operation  235  one or more parameters are determined from the phases obtained in operation  230 . By way of example and not limitation, arbitrarily identifying the phase of the interferogram formed by the front reference plate  133  and the front  161  of the wafer  160  surface as A, and the phase of the interferogram formed by the back reference flat  153  and the back surface  162  of the wafer  160  as B, and the phase of the interferogram formed by the cavity of the front reference flat  133  and  153  as C, then the surface parameters may be determined as follows: A corresponds to the front surface height of the wafer  160 , B corresponds to the back surface height of the wafer, C−(A+B) corresponds to the thickness variation in the wafer  160 . 
         [0026]    If at operation  240 , there are local areas on the object that have unmeasured parameters due to the high slope of the object at those locations, then at operation  245  the object may be tilted. Operations  205  through  235  may then be performed until all local areas of interest with high slopes have been sufficiently mapped. 
         [0027]    If at operation  240 , there are no additional local areas of interest with high slope, then at operation  250  parameter maps for the entire object may be produced. In some embodiments, a computer may stitch together all partial surfaces that overlap each other and thereby produce a measurement that covers the entire object surface. 
         [0028]      FIG. 3  is a schematic illustration of one embodiment of a computing system which may be used to implement the computer  138 ,  158  of  FIG. 1 . The computer system  300  includes a computer  308  and one or more accompanying input/output devices  306  including a display  302  having a screen  304 , a keyboard  310 , other I/O device(s)  312 , and a mouse  314 . The other device(s)  312  can include a touch screen, a voice-activated input device, a track ball, and any other device that allows the system  300  to receive input from a developer and/or a user. The computer  308  includes system hardware  320  and random access memory and/or read-only memory  330 . A file store  380  is communicatively connected to computer  308 . File store  380  may be internal such as, e.g., one or more hard drives, or external such as, e.g., one or more external hard drives, network attached storage, or a separate storage network. 
         [0029]    Memory  330  includes an operating system  340  for managing operations of computer  308 . In one embodiment, operating system  340  includes a hardware interface module  354  that provides an interface to system hardware  320 . In addition, operating system  340  includes one or more file systems  350  that manage files used in the operation of computer  308  and a process control subsystem  352  that manages processes executing on computer  308 . Operating system  340  further includes a system call interface module  342  that provides an interface between the operating system  340  and one or more application modules  362 . 
         [0030]    In operation, one or more application modules and/or libraries executing on computer  308  make calls to the system call interface module  342  to execute one or more commands on the computer&#39;s processor. The system call interface module  342  invokes the services of the file system(s)  350  to manage the files required by the command(s) and the process control subsystem  352  to manage the process required by the command(s). The file system(s)  350  and the process control subsystem  352 , in turn, invoke the services of the hardware interface module  354  to interface with the system hardware  320 . 
         [0031]    The particular embodiment of operating system  340  is not critical to the subject matter described herein. Operating system  340  may be embodied as a UNIX operating system or any derivative thereof (e.g., Linux, Solaris, etc.) or as a Windows® brand operating system. 
         [0032]    In some embodiments, computer system  300  includes one or more modules to implement hybrid database query caching. In the embodiment depicted in  FIG. 3 , computer system  300  includes a surface analysis module  362  which implements the operations described with reference to  FIG. 2A  and  FIG. 2B . 
         [0033]    While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. While the invention has been particularly shown and described with reference to a preferred embodiment and various alternate embodiments, it will be understood by persons skilled in the relevant art that various changes in form and details can be made therein without departing from the spirit and scope of the invention. 
         [0034]    Thus, 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 the claimed subject matter.