Abstract:
An addressable micromirror array is employed in conjunction with circuit topology navigation software to rapidly wavelength sample selected measurement points in an integrated circuit region.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application Ser. No. 61/197,265, filed Oct. 23, 2008 entitled METROLOGY OF THIN FILM DEVICES USING AN ADDRESSABLE MICROMIRROR ARRAY, by Edgar Genio, et al. 
     
    
     BACKGROUND 
       [0002]    Fabrication of thin film products such as microelectronic integrated circuits is enhanced by periodic measurements of,key characteristics of the product during the fabrication process, enabling on-going process adjustments to enhance quality and yield. A prominent characteristic to be measured is thin film thickness at or around a specific location or a specific circuit element. Measurements of such characteristics as film thickness are best made by analyzing the wavelength spectrum of light reflected from the feature or location of interest on the workpiece or wafer. How to infer a measurement of a quantity such as film thickness from the wavelength spectrum is known. Many measurements may be desired during the processing of each individual wafer, so that the time required to perform each measurement reduces productivity. Such measurements must be made at predetermined precise locations (i.e., at user-selected devices in the integrated circuit, for example). Optical apparatus employed to capture a wavelength spectrum reflected from a specific or user-selected device or feature in the integrated circuit must be accurately focused on the exact location of that device or feature. The problem is that the movement or re-directing of the optical apparatus from one selected device to the next in the integrated circuit requires a significant amount of time. The movement must be precise and each selected feature must be located within an array of hundreds of thousands of features included in the integrated circuit. 
         [0003]    One way this can be accomplished is to capture a digitized planar spatial image of a larger region of the integrated circuit that is most likely to contain the selected feature or device. This larger region may be a die or a portion of a die, and the precise location of the selected feature within the region is as yet unknown. Special pattern recognition algorithms are then employed to analyze the planar spatial image of the integrated circuit using the circuit design layout used to fabricate the integrated circuit. This analysis produces the exact location in the image of the selected circuit feature or device. This location may be specified as an exact X-Y location or a picture element (pixel) in the digital image. The optics is then used re-positioned to focus reflected light from the exact location discovered by the pattern recognition algorithm onto a diffraction grating. The spectrum of light emitted by the diffraction grating forms a wavelength-distributed intensity pattern along an axis of the grating, and this intensity pattern is focused onto a line sensor such as a charge coupled device (CCD) line imager. The output of the imager provides the reflection spectrum from the selected feature. Special wavelength analysis algorithms are employed to analyze this spectrum and infer from it a measured characteristic of the selected feature, such as thin film thickness for example. A limitation of this approach is that the mechanical re-positioning of the optics to each precisely determined location on the wafer is time consuming and must be performed for each successive measurement. 
         [0004]    Another more sophisticated way in which thin film measurements at user-selected locations may be performed is to employ a spectral mapping and analysis of the entire region containing the user-selected feature. This latter approach eliminates the need to mechanically re-position the optics after capturing the image of the larger region. Specifically, the wavelength spectrum of each pixel of a large region most likely to contain the user-selected circuit feature is first obtained. Each row of pixels in the spatial image is passed through a line spectrometer grating whose output is focused on a CCD line sensor, producing columns of intensity values sorted by wavelength. This involves mapping each row of pixels in the spatial image into plural columns (one for each spatial image pixel) of spectral intensity values. Special algorithms analyze the spectra of all the pixels in the image of the large region and note contrasts in wavelength responses between different spatial regions. These contrasts point to boundaries between adjacent regions each containing common circuit features that differ from the common circuit features of the adjoining region. The locations of these boundaries may be correlated to the circuit design layout used to fabricate the integrated circuit. This correlation provides a precise mapping of locations in the image of the large region of the integrated circuit to features in the circuit design layout. From this mapping, the location of the user-selected feature or device is immediately deduced, identifying the exact pixel in the image of this feature. The wavelength spectrum of that pixel was previously obtained during the prior acquisition of the wavelength spectra of all pixels in the image of the large region. Therefore, the spectra of the identified pixel is simply fetched and provided for use by a special wavelength analysis algorithms to analyze this spectrum and infer from it a measured characteristic of the selected feature, such as thin film thickness for example. While this second approach eliminates the need for any mechanical repositioning of the optics or to focus the optics on any particular pixel, it is limited because the initial step of processing an array of wavelength spectra of all pixels in the image of the large region is computational intensive and represents a very large burden. 
         [0005]    What is needed is a way of rapidly measuring plural user-selected circuit features on a wafer without having to re-position optics to each feature location and without imposing a large computational burden. 
       SUMMARY 
       [0006]    A wafer metrology system includes a camera and an addressable micromirror array both focused on a wafer. The system performs a measurement at a selected location or point on the surface of a semiconductor wafer having thin film features formed in accordance with an integrated circuit design. The system acquires a two-dimensional spatial image of a region of the wafer surface containing the selected point. The system processes the spatial image of the region with reference to the integrated circuit design and with reference to the selected location, to determine at least one pixel of the spatial image containing or adjacent the selected location. The system focuses an addressable micromirror array onto the region whereby individual micromirrors of the array receive light from corresponding individual areas of the selected region, the selected areas corresponding to respective pixels of the spatial image. The system correlates the one pixel with at least one of the micromirrors of the array and orients the at least one micromirror to reflect light from the wafer surface to a wavelength separation element to generate a wavelength-dispersed image. The system directs the remaining micromirrors to not reflect light from the wafer surface to the wavelength separation element. A spectral image processor processes the wavelength dispersed image to deduce the value of a selected characteristic at the selected location of the surface of the wafer. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    So that the manner in which the exemplary embodiments of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings. It is to be appreciated that certain well known processes are not discussed herein in order to not obscure the invention. 
           [0008]      FIG. 1A  illustrates a wafer metrology system in accordance with one embodiment. 
           [0009]      FIG. 1B  is an exploded view corresponding to a portion of  FIG. 1A . 
           [0010]      FIG. 2  is an enlarged view corresponding to a portion of  FIG. 1A  including the wafer. 
           [0011]      FIG. 3  is a block diagram illustrating a hierarchy of processors in the system of  FIG. 1A . 
           [0012]      FIG. 4  is a flow diagram illustrating a method in accordance with an embodiment. 
           [0013]      FIG. 5  depicts a wafer metrology system in accordance with a related embodiment. 
       
    
    
       [0014]    To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements and features of one embodiment may be beneficially incorporated in other embodiments without further recitation. It is to be noted, however, that the appended drawings illustrate only exemplary embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments. 
       DETAILED DESCRIPTION 
       [0015]    Referring to  FIGS. 1A and 1B , a workpiece such as a semiconductor wafer  102  is supported on a pedestal or table  104 . An addressable digital micromirror array  106  faces the wafer  102 . The micromirror array  106  may be a Digital Micromirror Device (DMD) by Texas Instruments Incorporated, and is available through Digital Light Innovations (DLI), 3201 Industrial Terrace, Suite 120, Austin, Tex. 78758. The micromirror array  106  has an array  108  of many micromirrors  110 , the micromirrors being closely spaced so as to provide roughly 70% area coverage. The micromirror array  106  further includes a mirror control layer  112  behind the micromirrors  110 , the control layer  112  consisting of actuators  114  depicted in the exploded partial view of  FIG. 1B  for controlling the orientation of each one of the micromirrors  110  individually. The actuators  114  may, for example, be electronic or electrooptical devices, each actuator  114  individually coupled to (or integrated with) a corresponding one of the micromirrors  110 . The micromirrors  110  may be arranged generally in a plane in periodically spaced rows and columns, the actuators  114  in the control layer  112  being similarly arranged. The actuators  114  are individually addressable, for example by row and column decoders  120 ,  122 , respectively, so that the orientation of each micromirror  110  may be individually controlled. 
         [0016]    The micromirror array  106  reflects light from a selected area  130  on the wafer  102  to a wavelength separation element  132 . The wavelength separation element  132  may be a diffraction grating or a prism, for example, and will be referred to hereinafter as a grating. A focus element (e.g., a lens assembly)  134  focuses light reflected from the selected area  130  onto the micromirror array  106 , so that (as indicated in  FIG. 1B ) light from each image element or pixel  136  in the selected area  130  is directed to a corresponding one of the micromirrors  110 . Each micromirror  110  is initially oriented to direct that light to the same point  132   a  on the grating  132 . The light incident on the point  132   a  of the grating  132  is converted to a line image or spectrum of intensities distributed by wavelength, this line image being captured by an optical sensor  140  which may be a CCD line imager. The spectrum or line image captured by the line imager  140  is output to a spectrum image processor  142 . The spectrum image processor  142  employs conventional algorithms to measure a characteristic (such as thin film thickness) from the spectrum or wavelength distribution of intensities represented by the output of the line sensor  140 . 
         [0017]    A camera vision system or microscope  150  consisting of a lens system  152  and a two-dimensional CCD image sensor  154  has a field of view focused on the selected area  130  of the wafer  102 . The two-dimensional spatial image of the selected area  130  is captured by the image sensor  154  and fed as data to an in-image navigation processor  156  having a memory  158  containing data representing the circuit design layout of the integrated circuitry on the surface of the wafer  102 . The in-image navigation processor  156  is capable of identifying the precise location of a user-selected circuit feature within the two-dimensional image of the selected area  130  captured by the microscope. Specifically, the in-image navigation processor  156  can identify the particular one of the pixels  136  representing the selected area  130  that contains the user-selected circuit feature. The in-image navigation processor  156  may employ geometric pattern recognition software sold by Cognex Corporation of Natick, Mass. The camera vision system  150  may be obtained from Cognex Corporation. 
         [0018]    In one embodiment, the micromirror array  106  may be controlled by a control processor  160 . The control processor  160  may also control the optical apparatus of the camera vision system  150  (specifically, the lens system  152 ) and the focusing lens system  134 , in order to ensure that both the camera vision system  150  and the micromirror array  106  are focused on the same selected area  130  of the wafer  102 . Once the in-image navigation processor  156  identifies the particular one of the pixels  136  containing the user-selected circuit feature, the control processor  160  uses this information to direct all of the micromirrors  110  away from the grating  132  except for the one micromirror focused on the pixel identified by the in-image navigation processor  156 . For example, in  FIG. 1B , the pixel  136 ′ may be the one identified by the in-image navigation processor  156 . In this case, the control processor  160  sends commands through the row and column decoders  120 ,  122  ( FIG. 1A ) to direct all the micromirrors  110  away from the grating  132  with the exception of the micromirror  110 ′ that is focused on the pixel  136 ′. The control processor  160  then directs the spectrum image processor  142  to fetch the spectrum image data from the line sensor  140  and process that data to compute the desired measured quantity such as film thickness. 
         [0019]    No movement of optical lens assemblies is required once the image of the desired selected area  130  has been acquired by the camera vision system  150  and by the micromirror array  106 . Many different circuit features selected by the user within the area  130  may be measured or analyzed in rapid succession without any mechanical movement of lenses or optical assemblies. The only motion required is performed by micromirror actuators  114 , which are virtually instantaneous compared to the slow time response of actuators required to move optical lens assemblies. Moreover, spectral decomposition and wavelength-based image processing of the entire selected area  130  is not required, thereby minimizing the computational burden of each measurement. 
         [0020]      FIG. 2  illustrates how the control processor  160  may position the lens assemblies  152  and  134  to inspect a selected area  130  lying within one of many die  103  into which the surface of the wafer  102  is divided, each die constituting a single integrated circuit. After measurements have been taken at all the user-selected circuit features in a particular selected area  130 , the control processor  160  may manipulate the lens assemblies  134 ,  152  (using an actuator apparatus  164 ) to direct the camera vision system  150  and the micromirror array  106  to a different area  130 ′ for a new series of measurements at various user-selected features within the new area  130 ′. The new area  130 ′ may be adjacent the prior area  130  or may be located in a completely different or opposite region of the wafer from the prior area  130 . This movement between successive large areas  130 ,  130 ′ involves a relatively slow movement of the micromirror array  106  and camera vision system  150  and/or the large lens assemblies  134 ,  152 . In comparison, the action of the micromirror actuators  114  to position the various micromirrors  110  toward or away from illuminating the grating  132  is nearly instantaneous. 
         [0021]      FIG. 3  is a block diagram depicting the command hierarchy among the processors  142 ,  156  and  160 . As indicated in  FIG. 3 , the in-image navigation processor  156  is furnished with a two-dimensional image of the selected area  130  (from the camera vision system  150 ) as well as the circuit design layout data (from the memory  158 ) defining the topology of the integrated circuit features on the wafer  102 . In response to identification of a user-selected circuit feature, the processor  156  identifies the pixel in the two-dimensional image corresponding to the location of that feature and transmits this pixel identification to the control processor  160 . The control processor  160  commands the micromirror array  106  to disable all the micromirrors  110  with the exception of the one micromirror corresponding to the identified pixel. The control processor  160  then commands the spectrum image processor  142  to capture the spectrum image form the grating  140  and process the captured image to perform the desired wave-length computed measurement. 
         [0022]      FIG. 4  is a block flow diagram depicting a method in accordance with one embodiment. The method depicted in  FIG. 4  may be carried out, for example, by the control processor  160 . Referring to  FIG. 4 , the user may define the location of the large region  130  that contains one or more user-selected circuit features or points at which measurements are to be made, such as measurements of film thickness for example (block  210  of  FIG. 4 ). In addition, the user may specify one or more circuit features or point locations within the large region  130  at which measurements are to be made (block  215  of  FIG. 4 ). The control processor  160  then commands the camera vision system  150  to direct its field of view to coincide with the large region  130  specified by the user (block  220  of  FIG. 4 ). The control processor also commands the addressable micromirror array  106  to direct its field of view to coincide with the large region  130  (block  230  of  FIG. 4 ). At this time, there is a one-to-one correspondence between image locations or pixels in the digital image acquired by the camera vision system  150  and respective micromirrors  110  in the micromirror array  106 . Design data defining the circuit design topology of the wafer  102  is provided to the in-image navigation processor  156  (block  235  of  FIG. 4 ). The control processor  160  then directs the camera vision system  150  to capture a two-dimensional spatial image of the large region  130  (block  240  of  FIG. 4 ) and directs the in-image navigation processor  156  to process the two-dimensional spatial image of the large region  130  to find the one pixel in the image containing a first one of the user-selected circuit features (block  245  of  FIG. 4 ). The control processor  160  fetches the identity of that one pixel and correlates it to a particular one of the micromirrors  110  (block  250  of  FIG. 4 ). The control processor  160  then sends appropriate commands to the micromirror array  106  (e.g., to the row and column decoders  120 ,  122 ) to leave only the particular one micromirror oriented to direct light to the grating  132  while directing all the other micromirrors  110  away from the grating  132  (block  260 ). This creates a spectral (wavelength-dispersed) image at the line sensor  140 . The control processor  160  then directs the spectral image processor  142  to process the spectral image captured by the line sensor  140  to perform a measurement of a characteristic such as thin film thickness at the user-selected circuit feature (block  270  of  FIG. 4 ). 
         [0023]    The operations of blocks  245 ,  250 ,  260  and  270  may be repeated for successive user-defined circuit features contained within the large region  130 . 
         [0024]    In one mode, the pixel size in the two-dimensional digital image captured by the camera vision system  150  and the micromirror size and spacing may both be so fine that a given user-selected circuit feature may occupy a neighborhood of adjacent pixels. In this case, the control processor  160  may enhance signal-to-noise ratio by enabling light from the corresponding group of adjacent micromirrors  110  to direct light to the grating  132 , so that the single grating point  132   a  receives a sum of light from the group of pixels/micromirrors  110 . The signal-to-noise ratio of the resulting spectral image created by the grating  132  and captured by the line sensor  140  is enhanced in proportion to the number of contributing pixels or micromirrors. 
         [0025]      FIG. 5  depicts another embodiment in which the micromirror array  106  and the camera vision system  150  are directed to a selected area  130  of the wafer  102  by moving the wafer rather than moving the optical components such as the camera vision system  150  and the micromirror array  106 . In the embodiment of  FIG. 5 , orthogonal gantry rails  310 ,  315  support the wafer table  104  and provide two-dimensional shifting of the wafer table  104  under control of respective X-stage and Y-stage actuators  320 ,  325 . The control processor  160  may govern the actuators  320 ,  325 . 
         [0026]    While the foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof, and the scope thereof is determined by the claims that follow.