Abstract:
A spectroscopic instrument for conducting multi-wavelength, multi-azimuth, multi-angle-of-incidence readings on a substrate, the instrument having a broadband light source for producing an illumination beam, an objective for directing the illumination onto the substrate at multiple azimuth angles and multiple angels-of-incidence simultaneously, thereby producing a reflection beam, an aperture plate having an illumination aperture and a plurality of collection apertures formed therein for selectively passing portions of the reflection beam having desired discreet combinations of azimuth angle and angle-of-incident, a detector for receiving the discreet combinations of azimuth angle and angle-of-incident and producing readings, and a processor for interpreting the readings.

Description:
FIELD 
       [0001]    This invention relates to the field of integrated circuit fabrication. More particularly, this invention relates to multi-angle spectroscopic reflectometry as used in the metrology of integrated circuit structures. 
       INTRODUCTION 
       [0002]    Spectroscopic tools, including both reflectometers and ellipsometers, have been widely used in the metrology and process control of integrated circuit fabrication processes. As the term is used herein, “integrated circuit” includes devices such as those formed on monolithic semiconducting substrates, such as those formed of group IV materials like silicon or germanium, or group III-V compounds like gallium arsenide, or mixtures of such materials. The term includes all types of devices formed, such as memory and logic, and all designs of such devices, such as MOS and bipolar. The term also comprehends applications such as flat panel displays, solar cells, and charge coupled devices. 
         [0003]    Spectroscopic tools have been used, for example, to measure the thicknesses and critical dimension of samples such as film stacks and circuit structures. The measurements taken by these tools are based on measuring the diffraction of light through the material and geometric structures of the sample. Thus, as the dimensions of these structures become smaller (such as thickness and line width), the amount of diffraction that they create becomes very small. Therefore, it has become more difficult to measure these smaller structures and, at the same time, maintain the desired measurement precision and tool-to-tool uniformity. 
         [0004]    Modern spectroscopic tools try to overcome these issues by measuring a sample at multiple wavelengths and multiple discrete angles. However, because at least one of the available wavelengths or the available angles are limited, it is difficult to set the wavelengths and measurement angles at values that produce the optimum measurement sensitivity as determined by a model-based analysis, given the fact that this optimum combination of wavelength and angle varies from sample to sample. 
         [0005]    What is needed, therefore, is a system that reduces problems such as those described above, at least in part. 
       SUMMARY OF THE CLAIMS 
       [0006]    The above and other needs are met by a spectroscopic instrument for conducting multi-wavelength, multi-azimuth, multi-angle-of-incidence readings on a substrate, the instrument having a broadband light source for producing an illumination beam, an objective for directing the illumination onto the substrate at multiple azimuth angles and multiple angels-of-incidence simultaneously, thereby producing a reflection beam, an aperture plate having an illumination aperture and a plurality of collection apertures formed therein for selectively passing portions of the reflection beam having desired discreet combinations of azimuth angle and angle-of-incident, a detector for receiving the discreet combinations of azimuth angle and angle-of-incident and producing readings, and a processor for interpreting the readings. 
         [0007]    In this manner, the instrument is able to quickly take several readings at a desired combination of wavelengths, azimuth angles, and angles-of-incidence, simultaneously if desired. Thus, the aperture plates can be set up to capture the set of readings that is most sensitive to the substrate being measured, thereby increasing the accuracy of measurement. Further, the instrument can be quickly reconfigured for different readings on different substrates. 
         [0008]    In various embodiments, the aperture plate is a set of aperture plates that can be used either individually or in combination to select different discreet combinations of multiple azimuth angles and multiple angles-of-incident. In other embodiments, the aperture plate is electronically configurable to produce different discreet combinations of multiple azimuth angles and multiple angles-of-incident. In some embodiments, the detector receives the discreet combinations of azimuth angle and angle-of-incident simultaneously. In other embodiments, the detector receives the discreet combinations of azimuth angle and angle-of-incident serially. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0009]    Further advantages of the invention are apparent by reference to the detailed description when considered in conjunction with the figures, which are not to scale so as to more clearly show the details, wherein like reference numbers indicate like elements throughout the several views, and wherein: 
           [0010]      FIG. 1  depicts a composited multi-angle spectroscopic tool according to multiple embodiments of the present invention. 
           [0011]      FIG. 2  depicts an aperture plate according to a first embodiment of the present invention. 
           [0012]      FIG. 3  depicts an aperture plate according to a second embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Various embodiments of the present invention describe a spectroscopic tool (reflectometer or ellipsometer) that can measure a sample at any continuously-variable composite angle within the entire half-hemisphere (angle of incidence of from about zero to about ninety degrees, azimuth angle of from −180 to 180 degrees, with respect to the measurement spot in the plane of the sample), and at more than one composite angle simultaneously or sequentially, where composite angle refers to the combination of incidence and azimuth. 
         [0014]    These embodiments conduct measurements at each of the angles corresponding to a small collection aperture, which removes the integration of multiple rays in model-based analysis, leading to significant improvements in measurement speed and throughput, while maintaining the small measurement spot that is desired in integrated circuit fabrication processes. 
         [0015]    With reference now to  FIG. 1 , there is depicted a multi-angle spectroscopic tool  100  according to several composited embodiments of the present invention. Thus, any given embodiment might not have all of the structures depicted in  FIG. 1 . Instead,  FIG. 1  depicts the various essential and optional elements that could be present in the different embodiments. 
         [0016]    Light  102  from a broadband light source  104  is directed to an objective lens  106  to illuminate the sample  108  at spot  110 . A two-lens objective  106  is depicted in  FIG. 1 , but other objective lens  106  designs are also contemplated. The objective lens  106  illuminates the sample  108  from substantially any continuously-variable composite angle with a cone of light. The light  112  that is reflected from the sample  108  is collected by the objective  106 , and directed to a spectrometer  114  for measurement. A beam splitter  126  allows the light paths  102  and  112  to be along a common path  128  between the beam splitter  126  and the sample  108 . 
         [0017]    A polarizer  118  is optionally placed in the illumination path  102  in some embodiments, enabling polarized spectroscopic reflectometry measurements, in which the polarization state in the collection path  112  is determined by the angular position of the plane of incidence in reference to the illumination polarization state. To explicitly define the polarization state in the collection path  112 , another polarizer, referred to as the analyzer  120 , is optionally placed in the collection path  112 . In alternate embodiments, a single polarizer  122  is placed in the common path  128 , in place of the polarizer  118  and the analyzer  120 , in a so-called double-pass configuration. Other embodiments include a waveplate  124  disposed in one or both of the illumination path  102  and the collection path  112 , or alternately in the common path  128 . These elements  118 ,  120 ,  122 , and  124  can either be fixed or rotating, in virtually any combination. 
         [0018]    An aperture plate  116  selectively blocks and transmits the desired fan of both illumination light  102  and reflected light  112 . In some embodiments, the aperture plate  116  is disposed either adjacent or within the aperture in the objective  106 . Embodiments of the aperture plate  116  are depicted in greater detail in  FIGS. 2 and 3 .  FIG. 2  depicts an illumination aperture  202  that passes desired portions of the illumination beam  102  (as a component of the composite beam  128 ) and collection apertures  204  disposed in rows and columns in the aperture plate  116  that pass desired portions of the reflection beam  112  (as a component of the composite beam  128 ). Points within the collection apertures  204  that lie along a circle that is concentric with the axis of the reflection beam  112  all have substantially identical angles of incidence. Points within the collection apertures  204  that lie along a common radial line extending out from the axis of the reflection beam  112  all have substantially identical azimuth angles. The collection apertures  204  can be fashioned with both illumination apertures  202  and collection apertures  204  configured so as to simultaneously receive with at sensor  114  a plurality of broadband readings from a plurality of azimuth angles and angles of incidence. By fashioning the collection apertures  204  as very small openings, the integration of rays from too broad a fan can be removed from the model-based analysis, thus leading to significant improvements in measurement speed and throughput, while maintaining the small measurement spot required in integrated circuit fabrication processes. The numbers and sizes of the apertures  204  are by way of example only. 
         [0019]    In some embodiments, the film stack to be analyzed by the tool  100  is mathematically modeled prior to measurement, and it is determined which composite angle or set of composite angles offers the greatest sensitivity. An aperture plate  116  is then fabricated with collection apertures  204  specifically located for this set of composite angles (for example), where the different composite angles in the set are either serially selectable, as depicted in the example of the aperture plate  116  of  FIG. 2 , or are simultaneously investigated, as depicted in the example of the aperture plate  116  of  FIG. 3 . Additional standard collection apertures  204  could also be added to such custom aperture plates  116 , so that the aperture plate  116  could be used for generic measurements. 
         [0020]    Different aperture plates  116  can be fabricated as optimized for different film stacks, and then swapped out of the tool  100  as desired. In other embodiments the collection apertures  204  of the aperture plate  116  are configurable such as by processor-driven motorized mechanical or electronic shuttering means (such as a liquid crystal panel) that can be reconfigured as to the number, size, and placement of the collection apertures  204  in the aperture plate  116 . 
         [0021]    Different embodiments use various means to receive, either sequentially or simultaneously, the reflected light  112  from the different composite angles. Various embodiments of these means are described below. 
         [0022]    Option 1: Select one composite angle at a time using an aperture plate  116  configured to do such, and the spectrometer  114  produces a signal from the light  112  gathered from this composite angle only. Accessing additional composite angles is conducted sequentially thereafter by moving or replacing the aperture plate  116 . 
         [0023]    Option 2: Select multiple angles (such as three angles) at the same time with an aperture plate  116  configured to do such, use optics to shift the multiple reflection beams  112  into different parts of the spectrometer  114 , which detects the signals simultaneously. 
         [0024]    Option 3: Select multiple angles (such as four) at the same time with an aperture plate  116  configured to do such, use optics to shift the multiple reflection beams  112  into multiple spectrometers  114  (such as four, to continue the example) to detect the signals corresponding to the multiple composite angles simultaneously. 
         [0025]    Option 4: Use optics, such as micro-electronic-mechanical system minors, to partition the beam  112  or direct selected portions of the beam  112  through the aperture plate  116 . 
         [0026]    The foregoing description of embodiments for this invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Obvious modifications or variations are possible in light of the above teachings. The embodiments are chosen and described in an effort to provide illustrations of the principles of the invention and its practical application, and to thereby enable one of ordinary skill in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. All such modifications and variations are within the scope of the invention as determined by the appended claims when interpreted in accordance with the breadth to which they are fairly, legally, and equitably entitled.