Abstract:
A method for controlling a polar coordinate stage moves an object relative to an imaging system. While moving the object, the image of the object is rotated to compensate for rotation of the object. Accordingly, the orientations of features in the image are preserved, and removal of apparent rotation in the image reduces confusion an operator experiences while directing movement of the object. The angular velocity of the motion of the object is controlled so that image shift speed is independent of the radial position of the point being viewed. Use of a polar stage, reduces the required foot print for a stage and facilitates prealignment. In particular, an edge detector measures the position of the edge of the object while the polar coordinate stage rotates the object. A prealignment process determines the position and orientation of the object from the measured edge positions. A further alignment process uses automated pattern recognition which more easily identifies features on the object when the image is rotated so that the orientations of the feature are approximately known.

Description:
BACKGROUND 
     1. Field of the Invention 
     This invention relates to measurement and inspection systems that use polar coordinate stages to position samples. 
     2. Description of Related Art 
     Many measurement and inspection systems mount samples such as semiconductor wafers on X,Y stages. An X,Y stage can move a sample in two independent orthogonal directions X and Y to select an area of the sample for viewing, imaging, or measurement. For example, an X,Y stage can move a wafer to select and position an area of the wafer in the field of view of an imaging system. The travel distances of the X,Y stage in the X and Y directions determine the size of the largest sample that can be inspected from edge to edge, and large samples require large travel distances. Accordingly, inspection systems have become larger to accommodate larger samples, for example, larger diameter semiconductor wafers. 
     The space required to accommodate the range of motion of an X,Y stage has a width that is equal to or greater than the width of the sample plus the travel distance in the X direction and a length that is equal to or greater than the length of the sample plus the travel distance in the Y direction. FIG. 1 illustrates a system  100  that uses an X,Y stage to position a circular sample  110 . System  100  includes an imaging and/or measurement system (not shown) that can be, for example, a video camera, a microscope, an interferometer, a reflectometer, an ellipsometer, an FTIR spectrometer, or any type of spectrophotometer. Such systems typically have a field of view  130  that is much smaller than sample  110 . To view the left edge of sample  110 , the X,Y stage moves sample  110  to a position  112  where the left edge of sample  110  is in field of view  130 . Position  112  is offset to the right from the central position of sample  110  by the radius r of sample  110 . A position  116  for viewing the right edge of sample  110  is offset a distance r to the left along the X axis from the central position. Accordingly, the X,Y stage must have a travel distance of 2r along the X axis for edge-to-edge inspection of sample  110 . Similarly, the X,Y stage must have a travel distance of 2r along the Y axis between positions  114  and  118 , and a minimum area  120  required for an X,Y stage capable of positioning sample  110  for edge-to-edge viewing is about 16*r 2 . 
     Many applications require the sample to be accurately positioned and oriented or at least require accurate information regarding the position and orientation of the sample relative to the X,Y stage. This requirement is common in automated semiconductor manufacturing where the samples are generally round semiconductor wafers. A wafer&#39;s position can be accurately determined by rotating the wafer about a rotation axis and monitoring the variation in the perimeter location of the wafer as a function of the rotation. An analysis of the measured perimeter variations can accurately determine the offset from the rotation axis to the center of the wafer. Additionally, the process can identify the orientation of the wafer because most semiconductor wafers have an orientation indicator such as a notch or a flat on its perimeter. An edge detector detects when the flat or notch in the wafer&#39;s perimeter rotates past. Examples of such position detector systems, which are often referred to as prealigners, are described in U.S. Pat. No. 4,457,664 of Judell et al., U.S. Pat. No. 5,308,222 of Bacchi et al., U.S. Pat. No. 5,511,934 of Bacchi et al., and U.S. Pat. No. 5,513,948 of Bacchi et al. Prealignment for an X,Y stage requires addition of structure such as a separate prealignment station, from which the wafer is transferred to the X,Y stage after prealignment, or a rotatable sub-stage on the X,Y stage for rotating the wafer. 
     FIG. 2 illustrates a system  200  using a polar coordinate stage  220  to position sample  110 . Polar coordinate stage  220  has a rotatable platform mounted on a linear drive mechanism. The linear drive mechanism moves the platform and a sample along a coordinate axis R, and the platform rotates the sample about the rotation axis of the platform. Polar coordinate stage  220  requires significantly less area when positioning sample  110  for edge-to-edge inspection. In particular, a travel distance r (the radius of the sample) along axis R out to a position  212  is sufficient to center in field of view  130  any radial coordinate ρ in the range from 0 to r. Rotation of sample  110  then selects an angular coordinate θ so that any point on sample  110  can be positioned in field of view  130 . Since polar coordinate stage  220  only requires one-dimensional linear motion and half the travel distance of an X,Y stage, the polar coordinate stage takes much less area than an X,Y stage requires. In particular, a polar coordinate stage needs an area of about 6*r 2 , which is less than 40% of the area that an X,Y stage requires. 
     A disadvantage of a polar stage is the portion of sample  100  in field of view  130  generally appears to rotate when the stage rotates sample  100  to move from one inspection location to another. Thus, different areas appear to have different orientations when an operator or machine vision software views the sample through an imaging system. Additionally, the speed of movement generally varies from one location to another for any constant stage rotation speed. In some measurement systems, an operator observes an image of a portion of the sample being measured or inspected and controls movement of the sample to select which areas are measured or inspected. With a polar stage, image rotation and variable image motion can easily confuse or disorient the operator when the operator is continuously viewing or inspecting sample  110  and moving the sample from one position to another. Accordingly, systems and methods are sought that provide the area savings of a polar coordinate stage but avoid the confusion of image rotation and variable speeds of motion. 
     SUMMARY 
     In accordance with an aspect of the invention, a system including a polar coordinate stage and an imaging system rotates an image to continuously compensate for image rotation that results when the polar coordinate stage moves from one part of a sample to another. In one embodiment, a control system accepts from a control such as a joystick, a mouse, or an external computer, control commands which define the desired direction and speed of an image shift. The control system determines the required motion of the polar coordinate stage and the required image correction to achieve the desired image shift. The control system generates the signal required to conform to the control commands and applies the required signals to the ρ and θ drives in the polar coordinate stage. Image correction is performed by mechanically varying the imaging system or by processing image signals to rotate the image being viewed. 
     In one embodiment of the invention, the imaging system includes active opto-mechanical image correction. For example, when the imaging system includes an optical microscope, an optical element such as a dove prism rotates an image by an amount that depends on the variable property of the optical element. When the imaging system includes a scanning beam microscope, such as a scanning electron beam microscope, the active image rotation unit rotates the scan direction to rotate the image. The control system calculates and applies the required signals to adjust the active image correction device and achieve the necessary image correction. For example, the control system can rotate a dove prism or a beam deflector at the appropriate rate and direction to maintain the image orientation while the stage moves. Alternatively, the imaging system provides a first image signal representing an image that rotates as the stage rotates the sample, and the control system electronically processes the first image signal to generate a second image signal that maintains the desired orientation while the stage rotates. 
     In accordance with another embodiment of the invention, a measurement system includes a sample measurement subsystem, an imaging system, a polar coordinate stage, and a sample prealigner. The prealigner includes an edge detector that during a prealignment process measures the location of the edge of a sample while the polar coordinate stage rotates the sample. Accordingly, prealignment can be completed without additional structure for rotating the sample. After prealignment, an alignment (or deskewing) process uses pattern recognition to precisely locate features on the sample and from the locations of the features, provides a precise indication of the location of the sample. During alignment, the polar coordinate stage moves the sample so that a feature is in the field of view of the imaging system. When moving the feature into the field of view, the image of the sample is rotated, so that the feature has a known orientation. Since the orientation of the feature is approximately known, pattern recognition software can more easily and more quickly identify the location of the feature within the field of view. Thus, the alignment process is relatively fast. Once the sample is aligned, the stage moves the sample to points where the measurement subsystem measures the sample. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1 and 2 respectively illustrate the areas required for Cartesian and polar coordinate stages that have the same coverage capability. 
     FIG. 3 is block diagram of an embodiment of the invention using an optical microscope and optical image rotation. 
     FIG. 4 is block diagram of an embodiment of the invention using software to rotate an image. 
     FIG. 5 is a plot of edge position measurements used in a prealignment process for the system of FIG. 3 or  4 . 
     FIG. 6 is a flow diagram of a system control program for the system of FIG. 3 or  4 . 
     FIG. 7 illustrates the relationship between the image axes and stage axes. 
     Use of the same reference symbols in different figures indicates similar or identical items. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In accordance with an aspect of the invention, a system for measuring, viewing, or inspecting a sample uses a polar stage on which the sample is mounted. A control system which receives operator commands for linear movement of the sample, generates signals to the polar coordinate stage as required to achieve the linear movement, and rotates the image during motion to preserve a fixed orientation of the sample as viewed by the operator. Accordingly, an operator can easily and intuitively control the direction and velocity of the sample&#39;s motion. 
     In accordance with another aspect of the invention, an edge detector detects the location of the edge of a circular sample such as a semiconductor wafer while the polar stage rotates the sample. Processing of the edge measurements allows a precise determination of the position of center of the wafer and identification of an orientation indicator such as a flat or a notch on the edge of the wafer. Accordingly, the stage does not require additional degrees of freedom or additional structures for prealignment of the wafer. 
     FIG. 3 illustrates a measurement system  300  in accordance with an embodiment of the invention. System  300  includes a polar coordinate stage  320 , an imaging system  330 , an operator interface  340 , a control system  350 , and an edge detector  360 . Polar coordinate stage  320  is a standard polar coordinate stage such as commercially available from a variety of sources and includes a linear drive that moves a rotatable platform on which sample  310  is mounted. Polar coordinate stage  320  can rotate sample  310  by 360° about a rotation axis of the platform. A rotary encoder monitors the angular orientation θ of the platform relative to a linear drive direction, which is the direction along which the linear drive moves the platform as the linear drive setting ρ changes. The linear drive direction is also referred to herein as the R coordinate axis. A linear encoder monitors the linear position of the platform along the R coordinate axis. The maximum linear travel of the platform along the R coordinate axis determines the radius of the largest sample which imaging system  330  can view completely, assuming that imaging system  330  is stationary. 
     Imaging system  330  is for viewing or inspecting regions of sample  310 . In system  300 , imaging system  300  is an optical microscope that includes a lamp  332 , a beam splitter  333 , lenses  334  and  335 , and a camera  338 . In operation, beam splitter  333  reflects light from lamp  332  onto an object area on sample  310 , and objective lens  334  produces a magnified reflected light image of the object area. Lens  335  projects the image into camera  338 , and camera  338  generates a signal representing the image that a monitor  348  displays. Lenses  334  and  335  are merely illustrative of optical elements. Additional optical elements are typically required to achieve the desired field of view and magnification of a suitable imaging system  330 . In one embodiment, imaging system  330  includes a confocal microscope. 
     A measurement subsystem  337  can also be associated with imaging system  330  to measure particular properties, such as reflectance, at a point in or near the region of sample  310  being viewed. For example, measurement subsystem  337  may include measuring equipment that measures reflectance at one or more points in the field of view. In an optical system, measurement subsystem  337  may include for example, an interferometer, a reflectometer, an ellipsometer, an FTIR spectrometer, or any type of spectrophotometer. Subsystem  337  can measure sample  310  through other elements of imaging system  330  or operate independently to measure a point or points in or near the field of view of imaging system  330 . 
     Imaging system  330  further includes image rotation optics  336  which provide an adjustable rotation of the image to cancel image rotation that stage  320  causes when moving sample  310 . In an exemplary embodiment of the invention, image rotation optics  336  includes a motor driven dove prism. Dove prisms are well known optical elements that provide image rotation about an optic axis, in this case the optical axis of imaging system  330 . Control system  350  generates a signal that rotates the dove prism at a rate that compensates for rotation of sample  310  and prevents the image formed in camera  338  from rotating. 
     In an alternative embodiment, imaging system  330  includes a scanning beam microscope such as an electron beam microscope or an ion beam microscope that scans a region of sample  310  and forms a video image. The video image conventionally has horizontal raster lines which correspond to the scanning direction of the scanned beam. In such an embodiment, image rotation unit  336  includes a beam deflection system that can rotate the direction of scanning. Rotating the direction of scanning direction results in a rotation of the image on monitor  348 . 
     Operator interface  340  is for observing the image of an object area of sample  310  and controlling movement of the field of view of imaging system  330  across sample  310 . Operator interface  340  includes monitor  348  and operator control  342 . Monitor  348  is a conventional video monitor capable of displaying an image represented by a signal from video camera  338 . In particular, monitor  348  displays the image of the object area of sample  310 , and an operator uses operator control  342  to change the object area in the field of view of imaging system  330 . Operator control  342  is for inputting movement commands and directing the motion of the field of view across sample  310 . In an exemplary embodiment of the invention, operator control  342  is a joystick but many alternative operator controls are suitable. For example, a region of monitor  348  can display control buttons that are software operated through the actions of a touch sensitive screen, a mouse, a track ball, a touch pad, or another pointing device. In the exemplary embodiment, an operator, observing the image from camera  338  on monitor  348 , moves the joystick in a direction which corresponds to the direction in which the field of view should move relative to the displayed image. The degree of joystick movement determines the speed of image motion. 
     Control system  350  is a computer system that receives control signals from operator interface  340  and generates control signals to stage  320  and imaging system  330 . Control system  350  includes a command module  352  that interprets the signals from operator control  342  and generates signals for controlling stage  320  and imaging system  330 . In particular, command module  352  includes software that control system  350  executes to monitor and control ρ and θ settings of stage  320  and control the angle through which image rotation unit  336  rotates the image. As stage  320  moves sample  310 , command module  352  sends a command via image rotation control unit  354  to image rotation unit  336  which responds by rotating the image. The image rotation is in a direction opposite the rotation of sample  310  so that the orientations of features appearing in the image remain fixed on monitor  348 . For example, when the operator directs movement of the image along a feature that initially appears horizontal on monitor  348 , control system  350  generates and applies a control signal to image rotation unit  336  to compensate for stage  320  rotating sample  310 , and the feature in the image remains horizontal as the image moves. In the exemplary embodiment, image rotation control  354  includes a hardware interface conveying information to and from image rotation unit  336 . 
     Control system  350  also determines and applies signals to an angle control unit  356  and a radius control unit  358  so that stage  320  moves sample  310  at the desired speed in the desired direction relative to the displayed image. In the exemplary embodiment, control units  356  and  358  combined include a hardware interface conveying information to and from stage  320 . Known computer controlled polar stages and their interfaces are suitable for system  300 . Control system  350  further receives signals from edge detector  360  for a prealignment process described below. The prealignment process provides a precise indication of the orientation and position of sample  310 . 
     FIG. 4 is block diagram of an inspection or measurement system  400  that uses software for image rotation. System  400  includes a stage  320 , an operator interface  340 , an edge detector  360 , and a measurement subsystem  337  which are the same as those described above in regard to system  300  of FIG.  3 . System  400  also includes an imaging system  430  and a control system  450 . In system  400 , imaging system  430  does not include an optical or mechanical system for rotating the image. Instead, control system  450  includes an image rotation unit  454 . Image rotation unit  454  includes an acquisition board that receives a video image signal from camera  338  and a module from rotating the video image. Typically, software performs the image rotation, but alternatively, hardware could be designed that electronically rotates the image. When command module  352  directs stage  320  to rotate sample  310 , the image from camera  338  is of a rotating (and moving) portion of sample  310 . Image rotation unit  454  processes the input video signal to compensate for the rotation and generates an output video signal representing a moving image which preserves the orientations of features on sample  310 . Control system  450  then provides a video signal representing the corrected image to video monitor  348 . 
     In one exemplary embodiment of system  400 , stage  320  is a polar stage available from Kensington Laboratories and is used to mount semiconductor wafers up to 200 mm in diameter. Additionally, a z coordinate stage can be added to or integrated into stage  320  for focusing for imaging system  430  and/or measurement system  337 . For example, imaging system  330  can attach to the z coordinate stage for focusing on a wafer on the polar coordinate stage. Imaging system  430  includes an optical microscope that provides a field of view at sample  310  which is about 1.3 mm×1 mm. Imaging system  430  also directs light from a small spot (about 15 microns in diameter) at the center of the field of view to a spectrometer which collects data on the reflectance. This data can be used for determining the film thickness. A co-filed provisional U.S. patent App. entitled “Compact Optical Reflectometer System”, of R. Yarussi and Blaine R. Spady, Ser. No. 60/092,384, describes some suitable measuring and imaging systems and is hereby incorporated by reference in its entirety. 
     Control system  450  is a computer such as a 400 MHz Pentium II-based personal computer having a video capture board for connection to video camera  338  and an interface for connection to stage  320 . Video capture boards capable of performing real time image rotation are commercially available from a variety of sources including, for example, Visicom, Inc. The interface board required for connecting control system  450  to stage  320  depends on the stage manufacturer. In this embodiment, operator control  342  is implemented in software as controls appearing on monitor  348 . 
     Before an operator uses system  300  or  400  to measure or inspect sample  310 , prealignment and alignment processes accurately determine the position and orientation of sample  310 . Typically when a sample such as a wafer is placed onto stage  320 , the position of the center of sample  310  is known only to within one or two millimeters, and the angular orientation of the sample  310  may be completely unknown. In accordance with an aspect of the invention, a prealignment procedure uses edge detector  360  and stage  320  to determine the position and orientation of sample  310 . For the prealignment procedure, a light source (not shown) below sample  310  illuminates sample  310 , and sample  310  casts a shadow onto edge detector  360 . Edge detector  360  includes a linear detector array located above sample  310  and precisely identifies the edge location of the shadow of sample  310  while stage  320  rotates sample  310  through 360°. If sample  310  is nearly circular but not perfectly centered on the stage, the position of the shadow on detector  360  moves slightly as stage  320  rotates sample  310 . 
     FIG. 5 shows a typical graph of the angular position of stage  320  versus the position of the shadow of sample  310  when sample  310  is a circular wafer having a notch in its perimeter. The position of the shadow of sample  310  generally follows a sinusoidal curve except where the notch causes a spike  510  in the sinusoid. The angular orientation of sample  310  is found from the position of spike  510 . The offset of the center of sample  310  from the rotation axis of stage  320  is a vector having a direction identified from maxima/minima of the sinusoidal curve and a magnitude of half the amplitude of the sinusoidal curve. Known statistical analysis techniques can be applied to the shadow position measurements (neglecting the spike) to determine the offset. For the exemplary embodiment of system  400 , the position of the wafer is then found within approximately 0.2 mm. 
     This is not accurate enough for many applications. The next level of alignment is a deskew procedure. This procedure can be done with a video camera that looks at a field of view on the order of 1 mm×1 mm and identifies a feature such as an alignment mark in the field of view of sample  310 . Imaging systems  330  (FIG. 3) and  430  (FIG. 4) fit these requirements, and can be used for the alignment process. Since the prealignment procedure aligns sample  310  to approximately 0.2 mm, a 1 mm×1 mm field of view centered on the expected location of the desired feature will include the feature. Pattern recognition software executed in the control system can then be used to find the position of the feature to within a few microns. Repeating the alignment process with a feature in another location on sample  310  can accurately find the position and orientation of sample  310 . If stage  320  is accurate enough, any point on sample  310  can be found within a few microns simply by controlling the settings of stage  320 . If stage  320  is not sufficiently accurate, the pattern recognition is repeated at subsequent measurement points. Now that the wafer has been accurately located, optical measurements can be made. 
     FIG. 6 shows a flow diagram illustrating a process  600  for control of the polar coordinate stage  320  and image rotation unit  454  and determination of the position of sample  310  on stage  320  of FIG. 4, during measurement or inspection of sample  310 . An initial block  610  initiates the system control program in control system  450 . An initial inquiry  620  of process  600  determines whether sample  310  is present on stage  320 . An object-present sensor or the operator responds to inquiry  620 . If no sample is present, process  600  ends in step  625  by reporting an error (no sample present). If sample  310  is present, block  630  implements the prealignment/alignment process described above. In particular, stage  320  rotates sample  310 , edge detector  360  measures edge positions, and control system  450  analyzes the edge position measurements to identify an offset between the rotation axis of stage  320  and the center of sample  310 . If necessary, sample  310  is then more precisely aligned or located using a deskewing procedure. Prealignment/alignment step  630  can be omitted when precise alignment of sample  310  is not required, for example, when sample  310  is simply inspected visually. 
     Once sample  310  is present and properly aligned, process  600  moves sample  310  according to the commands from an operator. In step  640 , control system  400  receives Cartesian input commands from the operator. The input commands indicate a desired movement direction and speed relative to the image on monitor  348 . Step  650  converts the Cartesian input commands to polar coordinate output commands for stage  320 , and block  660  applies the appropriate signals to stage  320  to move sample  310 . Step  670  is simultaneous with step  660  and rotates the image to cancel the rotation of sample  310  in step  660 . 
     To illustrate conversion and image rotation steps  660  and  670 , FIG. 7 shows the relationship between the X and Y coordinate axes and the R coordinate axis of stage  320 . The X and Y axes are fixed on sample  310  and centered on the rotation axis  710  of the platform on which sample  310  is mounted. As noted above, rotation axis  710  is typically offset from the center of sample  310  by an amount determined during prealignment and/or alignment. Rotation axis  710  of stage  320  passes through the R coordinate axis. The R coordinate axis corresponds to the direction of linear motion of stage  320  and has an origin that remains in the center of a field of view  740  of the imaging system. A view point  720  on sample  310 , which is currently at the center of field of view  740 , has polar coordinates ρ and θ relative to rotation axis  710 . Coordinate ρ is the distance that stage  320  moved sample  310 . Coordinate θ is the angle through which stage  320  rotated sample  310 . 
     Image rotation  670  preserves the orientation of the X and Y axes as viewed on monitor  348 . For example, the X axis if initially horizontal remains horizontal on monitor  348  regardless of how stage  320  rotates sample  310 . Accordingly, if the X axis is initially along the R axis, step  670  rotates the image by −θ, where θ is the polar coordinate of view point  720 . 
     In general, the Cartesian input commands indicate a vector V having X and Y components Vx and Vy, and the magnitude |V| of the vector indicates the speed of movement of the view point in the image. In one embodiment of the invention, step  650  continuously converts velocity components Vx and Vy to a radial velocity Vr and an angular velocity ω. Radial velocity Vr controls the velocity at which stage  320  moves sample  310  in along the R axis, and angular velocity ω determines the angular velocity of rotation of sample  310 . The velocity components can be determined from vector V and the coordinates of view point  720  using equations 1. 
     
       
         Vr=Vx*cos θ+Vy*sin θ  Equations 1 
       
     
     
       
         ω=(Vx*sin θ−Vy*cos θ)/ρ 
       
     
     Alternatively, when stage  320  uses coordinate settings rather than velocity settings, the input commands are sampled at a fixed frequency so that the components Vx and Vy indicate small displacements ΔX and ΔY which are the product of the velocity components and the time between samples. Displacements ΔX and ΔY shift a point  730  to the center of the field of view  740 . In this case, step  650  converts the displacements ΔX and ΔY to polar displacements Δθ and Δρ. The polar displacements Δθ and Δρ have magnitudes that depend on displacements ΔX and ΔY and the coordinates (X,Y) or (θ,ρ) of current view point  720 . Such conversions involve well known geometric techniques. It is desirable that stage  320  move sample  310  uniformly so that the displacements ΔX and ΔY require the full time between consecutive samplings of the input commands. Accordingly, to achieve this, the stage velocities need to vary according to the magnitude |V|, and the angular velocity needs to vary with radius. However, discontinuous shifts of sample  310  are imperceptible by the operator if the sampling period is sufficiently short, for example, if the sampling and shifting rate is higher than the frame rate of monitor  348 . 
     A step  680  gets and converts the stage position, and a step  685  can then store or display the position and the rotated image for inspection process  600 . After display of the inspection point, step  690  determines whether the inspection is complete. If not, process  600  branches back to step  640  to accept further Cartesian movement commands for movement to another inspection point. If the inspection is complete, process  600  ends in step  695 . 
     Although the invention has been described with reference to particular embodiments, the description is only an example of the invention&#39;s application and should not be taken as a limitation. In particular, even though much of the preceding was aimed at systems using imaging systems that are optical microscopes, alternative embodiments of this invention include other imaging systems such as electron-beam or ion-beam microscopes. Various other adaptations and combinations of features of the embodiments disclosed are within the scope of the invention as defined by the following claims.