Abstract:
Apparatus for quantitatively measuring the curvature and/or relative tilt of large surfaces wherein a small array of parallel laser beams, each separated by a known distance, reflect from the surface of a sample and fall upon a feedback controlled front-surface steering mirror to a detector that measures both the change in separation of the reflected beams and the spatial translation of the entire array on the detector. The sample surface is translated beneath or in front of the fixed laser array by means of a computer controlled stage or other apparatus to create a 1-dimensional line scan or 2-dimensional map of both bow and relative tilt of the sample surface. A computer-driven, feedback-controlled steering mirror compensates for varying sample tilt by precisely realigning the reflected laser array onto the detector as the sample is translated. The apparatus also utilizes a laser with intensity feedback control to continuously optimize the reflected laser power for varying surface reflectivity as the sample is translated. This combination provides a means to quantitatively measure curvature and relative tilt of sample areas much larger than the actual laser beam array size.

Description:
PRIORITY CLAIM 
   This application claims the benefit of U.S. Provisional Application No. 60/474,648, filed Jun. 2, 2003. 

   FIELD OF THE INVENTION 
   The invention pertains to devices and methods for measuring the surface curvature and/or tilt of a surface utilizing detection and analysis of an array of laser beams directed to said surface. 
   BACKGROUND OF THE INVENTION 
   Certain materials, particularly semiconductor wafers, must be treated, manufactured, handled and otherwise processed with a very high level of precision. Various processing conditions can result in the creation of substantial stresses within the materials, and accordingly, evaluation of the surface curvature, tilt and roughness is useful in analyzing the potential for stress-related failure of such components during the manufacturing process. While measurement of the curvature of such surfaces for the purpose of determining stress levels has been attempted in a number of different forms, there is substantial room for improvement in existing techniques. 
   Typical of the current technology are the measurement apparatus and associated techniques disclosed in U.S. Pat. No. 5,912,738, issued to Chason, et al., and disclosing an apparatus for measuring the curvature of a surface using a plurality of parallel light beams which reflect from a surface and are directed to a detector to measure the separation of the reflected beams of light. 
   Additional methodologies are disclosed in U.S. Pat. No. 4,929,846, issued to Mansour, and U.S. Pat. No. 4,291,990, issued to Takasu. Further, an understanding of mechanical stresses in epitaxial films is discussed by Schell-Sorokin and Tromp, in Physical Review Letters, Vol. 64, number 9, Feb. 26, 1990. 
   Existing technology is adequate, in some circumstances, on smaller samples, and in conditions where it is not necessary to measure a selected area of a larger surface. However, even with the existing technology, there are serious limitations in that the existing technology is incapable of measuring the degree of tilt of a target surface, and there are no provisions in the current art for monitoring the measurement process and providing appropriate feedback. Likewise, current technology relies on large single arrays of parallel beams which are expensive to produce and require a corresponding increases in size of associated equipment to generate the necessary arrays. 
   Our invention overcomes these and other limitations in existing art, by providing a device which works well on large surfaces using a small array, incorporates unique feedback control, allows the measuring of a plurality of selected areas utilizing a repositionable sample stage, and which is capable of evaluating tilt and roughness as well as curvature. 
   BRIEF SUMMARY OF THE INVENTION 
   The invention is a device and method for quantitatively measuring the curvature and/or relative tilt of large surfaces, such as semiconductor wafers, utilizing a small array of ordered light beams, each separated by known distance, which reflect from the surface of a sample and are directed to a feedback controlled front surface steering mirror to a detector which is capable of measuring both the change in separation of the reflected beams and the spacial translation of the entire array on the detector. The information from selected areas of the sample may be collated into a continuous set of measurements across the entire surface of the sample. 
   The sample surface is translated beneath or in front of a fixed laser array by means of a computer-controlled stage or other apparatus which is capable of moving the sample surface in various axes to create a one-dimensional line scan or a two-dimensional map of both the curvature and relative tilt of the sample surface. A computer-driven, feedback controlled steering mirror compensates for varying sample tilt by precisely realigning the reflected laser array onto the detector as the sample is translated. The invention also utilizes a laser with intensity feedback control to continuously optimize the reflected laser power for varying surface reflectivity as the sample is translated. This combination provides a means to qualitatively measure curvature and relative tilt of sample areas much larger than the actual laser beam array size. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a schematic view of the invention showing the interrelation of the components, the laser beam, and the electronic circuits. 
       FIG. 1A  is a simplified schematic view of the sample carrier showing the axis of movement of the sample. 
       FIG. 2  is a top perspective view of the optical portions of the invention showing their relative placement on the optical stage. 
       FIG. 3  is a perspective view of optical stage and the scanning stage of the invention showing the respective placement of their components and the placement of each stage. 
       FIG. 4A  is a graph of the typical data output obtained from the application of the inventive method and apparatus to a sample. 
       FIG. 4B  is a graph of data generated from 10 successive applications of the invention to a sample. 
       FIG. 5  is a flow chart demonstrating the measurement process showing the interrelation of the laser control, mirror control, detector exposure control, control of the scanning stage and feedback. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The apparatus and method described herein will be best understood by reference to  FIGS. 1-5 . The principal optical elements of the system are found in  FIG. 2 . The principal scanning elements are shown in  FIG. 3 . 
   The structure and layout of the various elements of the optical stage  2  will be best understood by reference to  FIG. 1  and  FIG. 2 . The principal elements of the optical stage  2  are mounted to the interior and exterior of enclosure  40 , which, in turn, is mounted to a supporting surface  30 , such as a table, positioning jig, or similar structure designed to provide a secure substrate to which to mount the optical stage enclosure  40 . It is highly desirable to insure that all elements of the system are as immobile as possible, and immune from any degree of external vibration to insure that the laser beam array later described is not subject to imparted motion by virtue of vibration in the environment. The enclosure  40  is preferably of lightweight but rigid metallic material, such as aluminum or steel or alloys thereof. Enclosure  40  may be permanently or removably secured to supporting surface  30 . Enclosure  40  comprises a base  42 , a plurality of side walls  44  and a cover (not shown), which, when affixed to the side walls  42  creates a substantially light-proof and dust-proof enclosure  40 . The various sub-components of the optical stage  2  are, as will be described, affixed to the enclosure  40 . 
   In the present embodiment, a laser source  10  is affixed to the exterior of one of the side walls  44  of enclosure  40 . It will be appreciated, however, that by changing the size of enclosure  40 , sufficient interior volume of enclosure  40  may be provided to permit laser source  10  to be mounted to the interior of enclosure  40  if desired. Laser source  40  may be one of a wide variety of commercially available laser light sources, typically those utilizing laser diodes or helium neon laser technology, which, when appropriately powered, provides a continuos wave laser light output in the form of a beam of collimated light which laser beam  12  may be visible or invisible, depending on the desired wavelength. Pulsed lasers may also be used. The wavelength of laser light may be selected to optimize performance of the invention depending on the type of material to be measured. The laser beam  12  so generated is then presented as an input to optical fiber  50 , the input end of which is preferably encased in an optical fiber guide  52  to provide support and protection for the optical fiber  50 . The optical fiber  50  is flexible, allowing it to be easily routed from the optical fiber guide  52  to the next element of the optical stage  2 . The output end of the optical fiber  50  is secured to an optical fiber output support  56  which secures and positions the output end of the optical fiber  50  in fixed relationship to the enclosure  40  and the remaining optical components of the optical stage  2 . Positioned adjacent the optical fiber output support  56  is a first etalon  60  which is secured to the enclosure  40  utilizing first etalon support  62 . Again, the first etalon  60  and support  62 , like all elements the optical stage  2  are firmly secured through their various supports to the enclosure  40  to provide a highly secure platform for each element of the optical stage  2 . First etalon  60  separates the laser beam  12  into a one dimensional array  13  of parallel laser beams which are then directed to a second etalon  64  which is secured to second etalon support  66 . Second etalon transforms the one dimensional laser beam array  13  into a two dimensional laser beam array  14 . The number of individual elements for each dimension of the laser beam array is determined by the type of first etalon  60  and second etalon  64  selected, as will be described further herein. Etalons  60  and  64  may be solid, air-spaced or computer-controlled optically encoded types. And, while conventional etalons are preferred as the means for creating the parallel beam arrays, the arrays may also be created by contoured lens optics, periodic or quasi-periodic grating optics. 
   In the preferred embodiment, the laser beam array  14  so generated is directed onto the surface of a first fixed alignment mirror  80 , which, in turn, is securely mounted to the first fixed alignment mirror support  82  which is affixed to the enclosure base  42 . The array  14  from first fixed alignment mirror  80  is directed to sample  20 . Sample  20  is typically a semiconductor wafer undergoing testing by the apparatus and method of the invention. Semiconductor wafer  20  may exhibit various surface irregularities, and the laser beam array  14  above described is analyzed to ascertain its reflected characteristics from the surface of the sample  20 . The laser beam array  14  is directed to a target area  16  on the surface of sample  20 . The laser beam array  14  so directed is then reflected back to the optical stage  2 . The sample  20  may be positioned by virtue of the various elements of the scanning stage  4  which will be described herein. The reflected array  22  is directed by fixed alignment mirror  80  to a steering mirror  90 . Steering mirror  90  is mounted to the base  42  of enclosure  40  by virtue of a steering mirror mount  92  which is provided with steering mirror servos  94 . Steering mirror mount  92  may also consist of a rotational portion attached to a fixed portion to allow mount  92  to be rotated under servo control. Steering mirror servos  94  permit the steering mirror  90  to be articulated and aimed, allowing the reflected array  22  to be controllably directed to laser beam array detector  26 . Laser beam array detector  26  is secured to the base of enclosure  42  by a detector support  28 . Detector  26  is preferably in the form of a CCD array-type detector, which is capable of providing a precise map of the reflected array  22  as presented to the internal sensing array of the CCD detector. The output from the detector  26  is then transmitted to the computers and controllers which will be described herein. 
   With reference now to  FIG. 3 , the various components of the scanning stage  4  will be understood. In one embodiment of the invention, the scanning stage  4  is secured adjacent to and below the optical stage  2 . This permits the entire scanning stage  4  to be secured to the floor  122  of the facility in which the invention is utilized, or to the base of the structure to which the optical stage  4  is mounted. The use of floor-mounting is often preferable to insure that the scanning stage  4  is subjected to the minimum amount of environmental vibration. 
   In this embodiment, the scanning stage  4  utilizes a primary support  120  which may be in the form of a plate or bracket which may be secured to the floor  122  of the facility in which the invention is used, the primary support  120  providing the primary base on which the remaining elements of scanning stage  4  are mounted. The sample  20  is secured to a sample holder (not shown), which, in turn, is secured to a z translation carrier  118 . Z translation carrier  118  is provided with a servo (not shown) which permits rotation of the z translation carrier. Z translation carrier, in turn, is mounted to a y translation carrier  116 , which, in turn, is provided with y translation carrier servo assembly  117  which permits movement of the entire z translation carrier in the y axis as shown in  FIG. 1A . Mounted to the y translation carrier  116  is the x translation carrier  114 , which, in turn, is provided with x translation carrier servo assembly  115 . The various servo assemblies so described include servo motors to permit the various carrier so described to be moved in the x, y and z (rotational) axis as shown in  FIG. 1A . In this fashion, the sample  20  may be selectively positioned so that any specific target area  16  of the sample  20  can be presented to the laser beam array  14  generated by the optical stage  2 . 
   All of the movement of the servo motors for the sample carriers  114 ,  116  and  118 , as well as the servos  94  for the multi-axis steering mirror  90  are controlled by and provide feedback to a multi-action motion controller circuit  100 . This circuit contains the necessary drivers to provide appropriate power and signals to the various servo motors to selectively position the x translation carrier  114 , y translation carrier  116  and z translation carrier  118 , as well as the steering mirror  90 , and further provides the necessary circuitry to sense the positions of each of said servo motors, and to transmit those positions to a computer  102 , which is typically in the form of a preprogrammed micro controller or a conventionally available personal computer. The computer  102  is further connected to the laser source  10  and the detector  26 . Computer  102  can accordingly control the output of the laser source  10  while simultaneously monitoring the output of the detector  26 . Wiring harnesses  128  and electrical connectors  130  are utilized to interconnect the circuit  100 , computer  102 , the various servo motors, the laser source  10  and the detector  26 , and enable feedback to be obtained from the various components as to both position and intensity of the laser beam array  14  as it is presented to the sample  20  and reflected back as reflected array  22  to the detector  26 . 
   The elements of optical stage  2  and scanning stage  4  derive their power from a conventional electrical power source (not shown) utilized to activate the various servos, and to energize the laser source  10 , the detector  26 , the multi-action motion controller  100  circuitry and the computer  102 . 
   A laser light source  10  of conventional manufacture provides the primary laser beam  12  for the invention. Controllable laser light output from the laser source  10  is directed through an optical fiber  50 , which, in turn, terminates at a laser output where the laser beam  12  is then presented to a first etalon  60 , the output of which is presented to a second etalon  64  resulting in the emission from the second etalon  64  of a two-dimensional laser beam array  64  of a plurality of parallel laser light beams. The dimension of the array  14  is typically 3×3, 4×4, 5×5 or 6×6, however, any dimension may be used within the limits of the size of the optics incorporated in the device. The laser beam array  14  is reflected from fixed alignment mirror  80  to a multi-axis steering mirror  90 . The orientation of the steering mirror  90  is controlled by a plurality of servos  94  to position the reflected array  22  onto the internal sensing elements of the detector  26 . 
   The steering mirror  90  is controlled by a multi-axis motion controller circuit  100 , which, in turn, derives its input from a computer  102  which communicates with both the laser source  10  and the detector  26 . 
   The process of positioning the sample will be appreciated by reference to  FIG. 3 , which shows the scanning stage  4  and a sample  20  disposed thereon. It will be appreciated from  FIG. 3  that the scanning stage  4  is movably mounted on a support  120  which permits the sample  20  to be moved in the x-axis and y-axis under the control of servo assemblies  115 ,  117 , affixed to the support. In addition, the sample  20  is mounted on a rotational carrier  118  which allows z-axis translation of the sample  20  as well. 
   The measuring process is performed by monitoring the sample  20  as the array  14  of parallel laser beams is directed onto the sample  20 . Under the direction of the computer  102  and the controller  100 , the position of the two-dimensional laser array  14  may be steered to a particular position of the sample  20 , in much the same fashion as an electron beam may be steered across the face of a cathode ray tube. 
   The intensity and orientation of the reflected laser beams provides accurate determination of surface qualities, including surface film thickness, growth rate and optical constants. The laser array  14  also provides information on stress, and can be used for curvature profiling. 
   The laser array  14  as detected is analyzed by a series of algorithms to analyze radius of curvature, tilt and other topographical qualities as to any segment on the sample being measured. 
   Utilizing advanced data acquisition routines in software, the output so analyzed may be selectively filtered, sampled and externally triggered. The software is capable of generating real time output in the form of both video display terminal output as well as printed data charts which will be useful in analyzing stresses, strains, radius of curvature and other variables associated with the sample. 
     FIG. 5  provides a flow chart of the measuring routine in detail. The routine begins with the placement  200  of a new wafer on the scanning stage of the invention. The next step involves setting the laser output and detector exposure to preset values at process step  202 . 
   The computer is then instructed to initiate the “spot find” subroutine process  204 . This computer program subroutine analyzes the detector  26  output to determine whether or not the array  14  is being presented to the detector  26 . If no array  14  is located, the routine flow proceeds to process  206 , which involves resetting the laser source to 100% power. Following this step, the routine returns to the “spot find” process  204 . 
   Assuming that the “spot find” process  204  is successful in verifying the presence of the array  14  on the detector  26 , the next step in the routine is to center the array  14  in the center of the CCD array of the detector  26  within a predetermined tolerance. This centering process  208  is performed utilizing the steering mirror and its associated servos. If the process  208  is unsuccessful in being able to center the array  14 , the measuring routine is terminated at process  210  based on the presumed failure of either the wafer or one or more aspects of the hardware of the optical stage or the scanning stage. 
   If the array centering process  208  is successful, the routine then continues to process step  212 , wherein the intensity of the laser output is optimized by analyzing the intensity of the individual elements of the array  14  on the internal sensing elements of the detector  26 . If any portion of the CCD sensing elements is saturated, the laser power is incrementally lowered until no area of the detector sensor elements is saturated. 
   After optimizing the laser power intensity, the next process step  214  is a recentering of the array  14  again using the steering mirror  90 . The process steps  204 ,  208 ,  212  and  214  are repeated until the optimal intensity of the laser power has been reached and the array is centered in the CCD detector array. Once these conditions have been met, the array  14 , now presumed to be perfectly centered and at the correct intensity, is evaluated by comparison to a previously established reference file, and the variation in position of the actual array elements is compared to a stored standard in process  216 . This data is then recorded. If for any reason process step  216  fails to produce a stable intensity, the routine terminates at process step  220 . If, on the other hand, adequate array stability is obtained, the scanning stage of the invention is instructed to increment the position of the wafer to a new physical location, and the entire above routine is repeated until the requisite number of measurements has been taken. The routine then moves to the reposition process  221  to reposition the sample. 
   Should the “spot find” subroutine of process  206  fail, the routine branches to process step  222 , which sets and locks the laser power to a maximum, and thereafter to process step  224 , which effectively doubles the detector  26  exposure time. The routine then calls the “spot find” process  228  to determine whether or not the array  14  can be detected by the detector  26 . If not, the routine branches to process step  224  to again double the exposure time. Following the process step  224 , the detector exposure time is tested at decision process  226  to determine if the maximum exposure time has been reached and an array  14  has still not been located in the detector  26 . 
   Assuming that the “spot find” process  228  is successful in validating the presence of the array  14  on the detector, the next step in the process  230  centers the array  14  to within a predetermined tolerance. If the centering process  230  is unsuccessful, the measurement is determined to be unsuccessful, and the routine fails at process step  232 . If, on the other hand, the center array process  230  is successful, the routine continues to process step  234  wherein the exposure time of the detector is adjusted to eliminate over-saturation of any portion of the detector  26  array. If the optimization process  234  is not successful, the routine fails at process step  236 . If the intensity of the array  14  can be optimized at process step  234 , the routine continues to process step  238  wherein the array  14  is recentered, again utilizing the steering mirror  90 . Once the intensity in position of the array  14  has been confirmed, the routine continues to process step  240  wherein the position of the array elements is compared to the known standard, and the variation between the two is reported and the data stored. Once this routine has been completed, the next process step  242  is to reposition the location of the array on the surface of the wafer and return to the “spot find” subroutine process at process  228 . 
   The variations in the curvature of the surface thus obtained are shown in graphic representation in  FIG. 4A . The x axis of the graph designates the position of the sampling array across a range of separate sampling points linearly spaced across the surface of a sample wafer. The y-axis of the graph charts the dimensional curvature. As can be seen from  FIG. 4B , which represents 10 successive scans taken across the same portions of a sample wafer the near-perfect overlap demonstrates a high degree of repeatability of results utilizing our invention.