Abstract:
A method for measuring a characteristic of a substrate, including directing an incident beam at an inspection grid of points on the substrate, receiving the reflected beam with a position sensitive detector, measuring the displacement of the reflected beam from its expected location, compiling a database of the displacement measurements, examining the database for effects of a pattern induced anomaly in the displacement measurements, producing an adjusted database, and deriving the characteristic of the substrate from the adjusted database. Thus, pattern induced errors from the displacement measurements are corrected. In this manner, problems with interpreting the reflection angles of a beam in substrate stress analysis equipment are overcome where distortions in the reflection angles are caused by deposition patterns on the substrates.

Description:
FIELD 
   This invention relates to the field of integrated circuit fabrication and inspection. More particularly, this invention relates to the measurement of surface characteristics of substrates. 
   BACKGROUND 
   Integrated circuits are manufactured by processes such as sputtering, ion implantation, and chemical vapor deposition that create successive layers of thin films of electrically conducting, electrically non conducting, and electrically semiconducting materials on a substrate. Techniques such as those involving photo resistive chemicals, masks, ultraviolet light, and etching are used to create multiple layers of these materials in patterns that create the electronic pathways that form an integrated circuit. Typically, hundreds of identical integrated circuits are created simultaneously in a geometrically repetitive pattern on a single substrate. Often these processes are conducted at high temperatures and other process conditions that induce mechanical stress in the substrates. These stresses can eventually cause cracking, delaminating, voiding, and other defects in the integrated circuits. Such defects may not become apparent until much later in the manufacture of the integrated circuit, so it is important to measure the stress levels at successive stages in the fabrication process and cull defective substrates before investing more resources to complete their fabrication. 
   Excessive stresses in the thin films distort the flatness of the substrate. Various techniques have been developed over the years to measure this distortion and use that information to compute the residual stress. Most of these techniques employ a laser beam that impinges the surface of the substrate and deposited materials. The expected location of the reflected beam is calculated based upon the geometry of the apparatus. The displacement of the actual reflected beam from its expected location is used to map the topography of the substrate and deposited materials. Differences detected between successive maps of topographical information are typically input into the well known “Stoney&#39;s equation” to calculate the induced stress. A significant problem with this process is that patterns in the deposited materials distort the reflection angle of the laser beam, making it difficult to interpret the reflection displacement data. 
   What is needed is a system to overcome the problems with interpreting the reflection angles of a laser beam in substrate stress analysis equipment where distortions in the reflection angles are caused by deposition patterns on the substrates. 
   SUMMARY 
   The above and other needs are met by a method in one embodiment of this invention for measuring a characteristic of a substrate, including directing an incident beam at an inspection grid of points on the substrate, receiving the reflected beam with a position sensitive detector, measuring the displacement of the actual landing location of the reflected beam from its expected landing location, compiling a database of the displacement measurements, examining at least one displacement measurement in the database for effects of a pattern induced anomaly in the displacement measurements, selectively correcting the displacement measurements to produce an adjusted database, and deriving the characteristic of the substrate from the adjusted database. Thus, pattern induced errors from the displacement measurements are corrected. In a most preferred embodiment the correction is achieved by deriving the tilt at an inspection point and subtracting the tilt at one inspection point from the tilt at a comparable point in an adjoining pattern. 
   In an alternate embodiment, the method includes directing an incident beam at a inspection grid of points on the substrate where the points are selected so that the reflected beam from each point in the inspection grid will have minimal distortion, receiving the reflected beam with a position sensitive detector, measuring the displacement of the reflected beam at each point from its expected location, and deriving the characteristic, such as topography, of the substrate from the displacement measurements. 
   In some embodiments the method includes adjusting the incident beam with a collimating lens such that incident beams are parallel for at least two points on the inspection grid. In some embodiments the incident beam is comprised of more than one wavelength and in some embodiments the incident beam is multiplexed between two or more wavelengths. In some embodiments the method for directing the incident beam between points on the patterned substrate comprises redirection by a galvo driven mirror, and in other embodiments the method for directing the incident beam between points on the patterned substrate comprises redirection by an acousto optic modulator. 
   In some embodiments the position sensitive detector is a segmented detector, in other embodiments the position sensitive detector is an imaging detector, and in another embodiment the detector is a current sharing position sensitive detector. 
   According to another aspect of the invention there is described an apparatus for measuring a characteristic of a substrate. A beam generator produces an incident beam, a scanner directs the path of the incident beam onto a substrate such that the incident beam strikes inspection points on a inspection grid, where the spacing of the inspection points along a first axis of the inspection grid is a submultiple of a repeat dimension of the pattern in that axis. A modulator causes the incident beam to strike the substrate when the incident beam is pointed at each inspection point. A position sensitive detector receives the reflected beam and produces an electronic signal representing the displacement of the reflected beam from its expected location. A controller collects the data on the displacement of the reflected beam from its expected location, and compiles a database of displacement measurements. The repeat dimension of the pattern along the first axis is used to analyze the displacement measurements in the database for pattern induced anomalies in the displacement measurements. The controller corrects the displacement measurements if the errors exceed a defined threshold, to produce an adjusted database, and outputs data defining the characteristic of the substrate. The controller computes the difference between data values corresponding to two equivalent points on the pattern. These equivalent points are separated by one repeat distance. The difference values form a new database from which the curvature can be extracted. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     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: 
       FIG. 1  is a schematic view of a mapping system illustrating the path of a beam through the apparatus. 
       FIG. 2  is a schematic view of a mapping system illustrating multiple beam paths. 
       FIG. 3  is a schematic view of a mapping system illustrating the effect of an anomaly in the surface of the substrate on the path of the beam. 
       FIG. 4  depicts a complete measuring system that incorporates a controller attached to the mapping system. 
   

   DETAILED DESCRIPTION 
   With reference now to  FIG. 1 , there is depicted a laser  10  which produces an incident laser beam  20 . Means such as a beam modulator  15  preferably turns off and turns on the incident laser beam  20 . In some embodiments the laser  10  is a diode laser which is modulated directly by its excitation current and no separate modulator  15  is used. A vertical scanner  30  and horizontal scanner  40  typically combine to provide a mechanism for directing the incident laser beam  20  across the surface of a substrate  60 . In many embodiments the substrate  60  is a wafer used in the fabrication of integrated circuits. For example, substrates formed of a monolithic semiconducting material such as a group IV material like silicon or germanium or a group III–IV material like gallium arsenide are preferably used. In many instances the substrate  60  has at least one thin film coating (not shown) on its surface, and the term “substrate” as used herein is intended to connote both bare substrates  60  and substrates  60  with thin film coatings. 
   In some embodiments a collimating lens  50  is used to direct the incident laser beam  20  on parallel paths toward the substrate  60  regardless of the position of the vertical scanner  30  and the horizontal scanner  40 . The incident laser beam  20  is reflected from the surface of the substrate  60  as a reflected laser beam  70 . A focusing lens  80  is preferably used to direct the reflected laser beam  70  toward the center of a position sensitive detector  90 . In a preferred embodiment the position sensitive detector  90  detects the location of the reflected laser beam  70  in two dimensions. In some embodiments a segmented detector, such as a quadrant detector, is used as the position sensitive detector  90 . The position sensitive detector  90  can be, for example, a charge coupled device. 
     FIG. 2  depicts the light paths of several rays of light  21 – 24  that vary in location depending upon the position of the vertical scanner  30  and horizontal scanner  40 . For example, when horizontal scanner  40  is in one configuration, incident laser beam  20  is directed along incident ray  21 , where it is reflected from the surface of substrate  60  as reflected ray  71 . When horizontal scanner  40  is in a different configuration, incident beam  20  is directed along incident ray  22 , where it is reflected from the surface of substrate  60  as reflected ray  72 . Similarly, incident ray  23  is reflected as reflected ray  73  and incident ray  24  is reflected as reflected ray  74 . 
     FIG. 3  illustrates the path  22  of incident ray  20  when it is directed at a spot on the substrate  60  where there is an anomaly on the surface of the substrate  60 . Without the anomaly the reflected ray  72  would be directed along virtual path  77  to the center of the position sensitive detector  90 . However, because of the anomaly the reflected ray  72  strikes the position sensitive detector  90  off center. 
     FIG. 4  schematically illustrates a most preferred embodiment where a computer  110  is connected by a detector interface cable  100  to the position sensitive detector  90 . The position sensitive detector  90  uses the detector interface cable  100  to send electronic signals to the computer  110 . The electronic signals from the position sensitive detector  90  convey the location where the reflected ray  72  has struck the position sensitive detector  90 . The amount of variance, and optionally the direction of variance, from the center of the position sensitive detector  90  is typically recorded by the computer  110 , and subsequently herein that information is referred to as a displacement measurement. 
   Successive signals conveying the location where a reflected laser beam  70  strikes the position sensitive detector are sent to the computer  110  by the position sensitive detector  90  as the vertical scanner  30  or the horizontal scanner  40  move the incident laser beam  20  across the surface of the substrate  60 . Typically, displacement measurements are recorded for locations which constitute an orthogonal inspection grid across the surface of the substrate. The scanning can be done by quick jumps to each desired point on the inspection grid, or by a continuous motion, or by a series of continuous motions interspersed with quick jumps. Scanning is generally quicker than alternate methods, such as using microposition controllers that could alternately be used to move the substrate under the incident laser beam  20 , or in combination with a scanning method. Also, with scanning, the effects of any low frequency vibrations of the substrate  60  are significantly reduced. In some embodiments the order and timing of the scan positions are varied to reduce the sensitivity of the system to different substrate  60  vibration mode frequencies. 
   A reflected laser beam  70 , such as the reflected ray  72 , may strike the position sensitive detector  90  off center for several reasons. One reason is that the substrate  60  may have become warped by the process of depositing and modifying thin films on the substrate  60 . Another reason is that the incident laser beam  20  may strike a dislocation in the thin films on the surface of the substrate  60 . Dislocations occur in the thin films as they are deposited to form the components of integrated circuits, particularly at the edges of the individual components and their interconnecting elements in each integrated circuit. A third reason that a reflected laser beam  70 , such as the reflected ray  72 , may strike the position sensitive detector  90  off center is that the reflected laser beam  70  may strike a local defect in the substrate  60  material or a thin film deposit on the substrate  60 . 
   It is of great value to distinguish anomalies attributable to the substrate  60  warping from anomalies attributable to discontinuities or local defects. If the substrate  60  is excessively warped, stresses are induced in the thin films, and those stresses may cause operational failures in a large fraction of the integrated circuits. Knowing the extent of such warping permits the fabricator to decide whether to scrap an entire substrate while it is still in process rather than invest further resources to complete its fabrication. 
   Integrated circuits are typically fabricated in an orthogonal layout with uniform spacing in both dimensions across the substrate. The amount of the spacing in one dimension is typically different than the spacing in the other dimension, but the spacing is uniform in each dimension. This uniform orthogonal spacing forms a repeat pattern in two axes across the surface of the substrate  60 . In a most preferred embodiment, the scan trace created by the vertical scanner  30  and the horizontal scanner  40  is programmable and the trace is adjusted to form a periodic orthogonal inspection grid on the substrate  60  such that the inspection grid is generally congruent with the layout of the integrated circuits. 
   Also, in a most preferred embodiment, the inspection grid spacing in each dimension is a submultiple of the repeat dimension of the repeat pattern in that dimension. For example, if the integrated circuits are spaced at six millimeter intervals in one dimension across the substrate, and if the inspection grid is congruent with the layout of the integrated circuits, preferred spacing of inspection points on the inspection grid would include spacings in that dimension of one, or one and one half, or two, or three millimeters. 
   The benefit of having an inspection grid spacing in each dimension that is a submultiple of the repeat dimension of the repeat pattern in that dimension is that submultiple spacing permits identification of anomalous displacement measurements that are attributable to dislocations in the thin films on the surface of the substrate  60 . These anomalies are referred to as pattern induced systematic anomalies. If integrated circuits are spaced at six millimeter intervals in one dimension, and an inspection point on the grid falls on a dislocation in the thin film that represents the edges of a particular individual component of the circuit, then by using an inspection grid spacing that is a submultiple of six millimeters, an inspection point is established six millimeters away and the incident laser beam  20  will hit the same edge of the same component on both integrated circuits. Except for any local surface differences between the two points, the displacement measurement will be the same for both locations. As subsequently described, this permits detection of the pattern and cancellation of its effects when mapping the surface of the substrate  60 . 
   In a preferred embodiment the computer  110  is programmed to compile a database of displacement measurements, examine the database for effects of a pattern induced systematic anomalies in the displacement measurements, and correct examined displacement measurements to produce an adjusted database. The most preferred method of correcting examined displacement measurement is to calculate the tilt represented by the displacement measurements, and for each point subtract the tilt from the equivalent point on a neighboring integrated circuit to arrive at a corrected tilt for that point. If the position sensitive detector  90  is a two dimensional detector, the tilt subtraction is generally a vector subtraction. 
   The corrected tilt values are preferably provided as input to software running on the computer  110  to calculate curvature of the substrate  60  at selected points. Such software may employ simple geometric algorithms to compute the radius of curvature of the substrate  60 , or more sophisticated algorithms to deduce a full surface height map of the substrate  60 , such as described in “A Line-integration Based Method for Depth Recovery from Surface Normals,” Zhongquan Wu and Linzao Li,  Computer Vision, Graphics, and Image Processing, volume  43 (1988), the disclosure of which is incorporated herein in its entirety by reference. The computer  110  typically stores this information for selected successive integrated circuit fabrication steps. Changes in substrate curvature between such steps are provided as input either to Stoney&#39;s equation or to more sophisticated two dimensional algorithms based on finite element analysis, which is used by the computer  110  to compute the resultant stress induced in the thin films by the intervening processes that were performed on the substrate. 
   In an alternate embodiment, the scan patterns are programmed so that the incident beam  20  only hits points on the substrate  60  where there will be no pattern dislocations in the thin films on the surface of the substrate  60 , so that distortions in the displacement measurements are minimal. In this embodiment, corrections for pattern induced anomalies is typically not necessary. It is also possible to vary the order and timing of the scan positions in order to reduce sensitivity to different substrate  60  vibration mode frequencies. 
   In preferred embodiments the computer  110  controls the operation of the laser  10 . As shown in  FIG. 4 , this is preferably achieved by using a laser interface cable  120  connected between the computer  110  and the laser electronics unit  130 , and using a laser controller cable  140  connected between the laser electronics unit  130  and the laser  110 . In many embodiments, electronic signals sent over the laser interface cable  120  are used by the computer  110  to instruct the laser electronics unit  130  to fire the laser  10 . When so instructed, electronic signals sent over the laser controller cable  140  to the laser  10  are preferably used to actually fire the laser  10 . In some embodiments, electronic signals sent over the laser interface cable  120  by the laser electronics unit  130  are used to convey timing information on the firing of the laser  10  to the computer  110 . In embodiments which employ a modulator  15 , the computer  110  preferably also controls the operation of the modulator  15 , and that control is most preferably accomplish by the laser electronics unit  130 . 
   In preferred embodiments the computer  110  also controls the operation of the vertical scanner  30  and the horizontal scanner  40 . As shown in  FIG. 4 , this is preferably achieved by using a scanner interface cable connected between the computer  110  and a scanner controller  160 . The scanner controller  160  drives the configuration of the vertical scanner  30  and the horizontal scanner  40  by electrical or mechanical mechanisms which are not illustrated. 
   In preferred embodiments both a vertical scanner  30  and a horizontal scanner  40  are employed to move the incident laser beam  20  across the substrate  60  in two dimensions. However, in some embodiments it may be sufficient to employ only a vertical scanner  30  or only a horizontal scanner  40 , which creates a one dimensional linear inspection grid. 
   In some embodiments the vertical scanner  30  and horizontal scanner  40  are moved by galvo driven mirrors. In some embodiments the vertical scanner  30  and horizontal scanner  40  employ acousto optic modulators to deflect the incident laser beam  20  to its intended inspection locations on the substrate  60 . In some embodiments a combination of galvo driven mirrors and acousto optic modulators are employed with the vertical scanner  30  and horizontal scanner  40 . 
   In the most preferred embodiments the acquisition of the displacement information by the computer  110  from the position sensitive detector  90  is synchronized with the operation of laser  10 , the vertical scanner  30 , and the horizontal scanner  40 . In some embodiments the vertical scanner  30  and the horizontal scanner  40  may continuously sweep the surface of the substrate  60 , and computer  110  may integrate the displacement measurement information over time to improve accuracy. 
   In some embodiments the substrate  60  is moved laterally under the incident laser beam  20 . Such lateral movement is typically accomplished by micro positioning devices (not shown). Such lateral movement may be employed instead of using a vertical scanner  30  or instead of using a horizontal scanner  40 , or instead of using either a vertical scanner  30  or a horizontal scanner  40 . In some embodiments, lateral movement of the substrate  60  is combined with the use of a vertical scanner  30  or the use of a horizontal scanner  40 . In the most preferred embodiments, one region of the substrate  60  is scanned both horizontally and vertically, and then the substrate  60  is moved so that another region of the substrate  60  can be scanned both horizontally and vertically. 
   In some embodiments an imaging detector, such as a charge coupled device, is used instead of a position sensitive detector  90 . In these embodiments the incident laser beam  20  is generally chopped by a beam modulator  15  in order to form a set of discrete spots on the imaging detector. In some embodiments the beam modulator  15  is an acousto optic device, in other embodiments it is an electro optic device. 
   In some embodiments the optics are arranged so that there is only one lens, and the incident laser beam  20  and the reflected laser beam  70  are substantially perpendicular to the surface of the substrate  60 , and a beam splitter is used to separate the incident laser beam  20  from the reflected laser beam  70 . 
   In some embodiments the incident laser beam  20  comprises at least two wavelengths of light. In some embodiments transmission of the incident laser beam  20  is multiplexed between two wavelengths of light. The use of more than one wavelength of light makes detection of the location of the surface of the substrate  60  easier in situations where the surface contains thin films that are transparent to a certain band of wavelengths, and the reflectivity at one of the wavelengths is relatively low. 
   In yet another embodiment, optics, such as the lenses  50  and  80 , are arranged so that there is only one lens. In this embodiment, the incident beam  20  and the reflected beam  70  are substantially perpendicular to the substrate  60 . A beam splitter is preferably used in this embodiment to separate the incident beam  20  and the reflected beam  70 . 
   The foregoing description of preferred 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 the best 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.