Apparatus and method for examining a disk-shaped sample on an X-Y-theta stage

An apparatus and method for examining features of a planar, disk-shaped samples on a stage that holdings the sample and has an X-drive, a Y-drive and a θ-drive for rotating the stage about a center of rotation defined in the stage coordinates. The sample is placed on the stage such that the center of the sample is substantially aligned with the center of rotation and a measurement assembly is located above the sample to examine the features optically. A scheduling module coordinates the X-drive, the Y-drive and the θ-drive with the measurement assembly such that the sample is examined in an even number n of angular sectors defined by a sector angle Θ that is the same for each sector. Specifically, the sector angle Θ is defined in terms of n as follows:where n=4m and m is an integer, such that a multiple of sector angle Θ always includes angles 90° and 180°.

FIELD OF THE INVENTION

This invention relates generally to an apparatus for optically examining a disk-shaped sample placed on a stage capable of linear motion along the X and Y directions and rotation by an angle θ about a center of rotation.

BACKGROUND ART

The field of test and measurement spans a wide range of apparatus and techniques that include stages designed for accurate positioning and moving of samples under test. More specifically, the field of optical metrology requires stages that are capable of moving samples in a way that is accurate, reproducible and allows the inspection of the entire surface area of the sample. In other words, the stage needs to allow the optical inspection mechanism to access with its probe beam any surface portion of the sample. This is particularly important in samples that are disk-shaped, such as semiconductor wafers, whose features are being examined or inspected prior to dicing or cutting the wafer into individual circuit dies.

The prior art teaches a number of stages for retaining and moving samples, such as wafers for optical inspection. For example, U.S. Pat. No. 6,320,609 teaches an R-θ stage and assumes that the center of rotation of the polar coordinate stage can be made coincident with the center of measurement beam spot (or the center of the field of view of the imaging system). In practice, there is always some offset between these two centers. This means that there is an area, i.e., a blind spot that cannot be placed under the beam spot by an R-θ stage. In addition, when grating structures on a wafer are measured at oblique incidence of the test beam, the grating orientation relative to the direction of incidence varies from location to location, because the wafer will typically rotate as it moves. This is a serious problem for optical scatterometry measurements.

To overcome this problem, U.S. Pat. No. 6,882,413 teaches a very complicated optical setup. The setup is configured to move the optical assembly in a manner similar to the way in which rotation is compensated. Unfortunately, due to the small amounts of rotation, or even continuous rotation, the apparatus taught in this reference suffers from loss of angular resolution. Furthermore, misalignments in wafer positioning cause a blind spot that cannot be examined. A similar problem affects a system for measuring periodic structures, as disclosed in U.S. Pat. No. 6,721,052 by Zhao et al. Here the inventors are studying a periodic structure by illuminating it by polychromatic electromagnetic radiation and collecting radiation from the structure in two different polarizations. To reduce the footprint of the system, the measurement instrument and the wafer bearing the periodic structure are both moved. For example, they both undergo translational and rotational motion in such a way that the illumination beam from the apparatus scans a spiral path on the wafer. Thus, the system incurs the problems associated with continuous rotation as discussed above.

In fact, none of the above approaches offer a simple and effective apparatus and method for examining disk-shaped samples such as semiconductor wafers with a small footprint stage that is not susceptible to blind spots and issues associated with low angular resolution.

OBJECTS AND ADVANTAGES

In view of the above prior art limitations, it is an object of the invention to provide an apparatus and method for examining planar, disk-shaped samples in a small footprint apparatus that is insensitive to misalignments and problems associated with low angular resolution. In particular, the apparatus is intended to be able to operate within the footprint of the sample only while still being able to efficiently examine the entire surface of the sample.

It is a further object of the invention to provide an apparatus that can examine samples such as semiconductor wafers in a configuration that allows the measurement assembly to travel in a very flat and precise manner.

These and other objects and advantages of the invention will become apparent from the ensuing description.

SUMMARY OF THE INVENTION

The objects and advantages of the invention are addressed by an apparatus for examining the features of a disk-shaped sample. The sample can be any generally planar and disk-shaped object bearing features, especially microscopic ones, and in some particular cases it can be a semiconductor wafer. The apparatus has a stage for holding or supporting the sample. An X-drive and a Y-drive are provided for moving the stage along an Xsdirection and a Ysdirection defined in the stage coordinates, respectively. The apparatus also has a θ-drive for rotating the stage about a center of rotation defined in the stage coordinates. The sample is placed on the stage such that the center of the sample is substantially aligned with the center of rotation. If necessary, pre-alignment procedures are used to ensure proper placement of the sample on the stage.

The apparatus has a measurement assembly located above the sample. The measurement assembly is usually designed to examine the features of the sample optically. Such measurement is commonly performed by illuminating the sample with test or probe radiation, and studying a scattered radiation returning from the sample. In accordance with the invention, the apparatus is equipped with a scheduling module for coordinating the X-drive, the Y-drive and the θ-drive with the measurement assembly such that the sample is examined in an even number n of angular sectors. The angular sectors are defined by a sector angle Θ that is the same for each sector. Specifically, the sector angle Θ is defined in terms of n as follows:

Θ=360⁢°n;
where n=4m and m is an integer, such that multiple rotations of sector angle Θ always includes angles 90° and 180°.

In a preferred embodiment of the invention, the apparatus is also equipped with an image rotation module for rotating an image of a selected asymmetric reference feature identified on the sample. This image can be reconstructed from the scattered radiation in the field of view of the measurement assembly or it can be obtained independently with separate elements. A pattern matching module uses the image of the reference feature to perform a matching of the image of the reference feature with a stored image of the reference feature at the various possible values rotation angle θ. Preferably, the reference feature exhibits a sufficient level of asymmetry to distinguish a number of feature orientations at least equal to the number n.

The pattern matching performed with the aid of the image rotation module and the pattern matching module finds many uses in the apparatus of invention. In one embodiment, the apparatus has a drive feedback that is in communication with the pattern matching module. Based on matching the actual image with the stored reference feature the drive feedback controls at least one of the drives, i.e., X-drive, Y-drive and θ-drive, but preferably at least the θ-drive. In another embodiment, the apparatus has an angular sector identification module that is connected to the pattern matching module. The identification module identifies the angular sectors based on the pattern matching. In still another embodiment, the apparatus has a sample offset correction module that is in communication with the pattern matching module and performs offset correction of the sample based on the pattern matching. In any of these applications of the pattern matching module, as well as any other applications thereof, it is advantageous to ensure a corresponding level of asymmetry of the stored reference feature to ensure that the requisite angular resolution is obtained.

In addition to determining rotation angle θ, it is sometimes important to properly orient the measurement assembly with respect to the features that are to be examined in any given sector. A calibration pattern consisting of features to be examined can be used for this purpose. Specifically, the measurement assembly can be oriented with respect to the calibration pattern prior to commencing examination.

In some applications, the θ-drive has a stepper for rotating the sample by angular steps that are equal to the sector angle Θ. Any suitable mechanism can be used to enforce such step-wise rotation, including hard stops. In any of these applications the measurement assembly itself can be stationary or mobile. In the event it is mobile, it may also have hard stops for controlling its angular movement by increments equal to the sector angle Θ.

The invention further encompasses a method for examining features of a sample, e.g., a semiconductor wafer that is placed on a stage with three degrees of freedom of motion. Specifically, the method is practiced on a stage with an Xsdirection of motion, a Ysdirection of motion and a θ rotation.

The sample is placed on the stage such that its center is aligned with the center of rotation. The sample is also partitioned or subdivided into an even number n of angular sectors defined by sector angle Θ. Preferably, the sector angle Θ is selected from the following three angles: 90°, 45° and 22.5°. Note that smaller sector angles Θ are also possible, but may not be preferred due to the increased angular resolution required when implementing pattern matching for feedback and control of the drives, sector identification or correction of sample offset.

The method of invention can be practiced in many variants. In some a specific schedule of examination of the angular sectors can be implemented. For example, the schedule may dictate consecutive examination of diametrically opposite pairs of angular sectors.

A detailed description of the preferred embodiments of the invention is presented below in reference to the appended drawing figures.

DETAILED DESCRIPTION

The present invention and its principles will be best understood by first reviewing the limitations of a prior art R-θ stage10as shown inFIG. 1. Stage10is designed for optically examining a disk-shaped object12, e.g., a semiconductor wafer. During examination wafer12rotates over angle θ and is illuminated by a measurement beam spot14from an optical examination device (not shown). The examination device moves linearly and parallel to a Y-axis, as indicated by arrow A, and therefore measurement spot14moves linearly along arrow A as well. A camera (not shown) is used to track a selected pattern15on wafer12as wafer12rotates in order to keep track of which point on wafer12is actually being examined by spot14.

As pointed out above, the center of wafer12and the center of rotation of stage10are typically offset. This occurs even when eccentric placement of wafer12on stage10is corrected by prior art methods such as pre-alignment by tracking a notch17along a rim16of wafer12. As a result, a blind spot18is created that cannot be examined by beam spot14. Furthermore, as wafer12rotates continuously or in small increments of angle θ, pattern15rotates continuously or in small increments as well. To compensate, the image needs to be rotated either digitally or by rotating the camera. In performing the rotation a Dove prism can be used to rotate the image without distortion. It should be noted, that when the image is rotated by rotating the camera, then the entire prior art mechanism becomes unduly complicated.

An exemplary apparatus20that overcomes the prior art limitations in accordance with the invention is illustrated inFIG. 2. Apparatus20is designed for examining features24of a sample22that is typically disk-shaped and planar, such as a semiconductor wafer. Features24are usually small or microscopic, and may include lithographically produced patterns corresponding to circuits, connections and circuit elements. Only a few select features24in a greatly magnified format are indicated on a surface26of wafer22for drawing clarity.

Apparatus20has a stage28for holding or supporting wafer22. Stage28is a compound stage composed of two individual stages: a theta stage28A and a linear X-Y stage28B. Theta stage28A has a support arrangement in the form of support pins30for holding wafer22. Theta stage28A sits on top of linear X-Y stage28B. An X-drive32and a Y-drive34are provided for moving linear X-Y stage28B along directions defined along a stage Xsaxis and a stage Ysaxis, respectively. Theta stage28A has a rotary drive or θ-drive36for rotating theta stage28A by angle θ about a center of rotation Cr.

Theta stage28A is mounted on linear stage28A such that center of rotation Crlies on Ysaxis of linear X-Y stage28B. In fact, center of rotation Cris a distance R/2 from a center Csof X-Y linear stage28B along Ysaxis, where R is the radius of wafer22. The full travel range of linear X-Y stage28B along Ysaxis is R or ±R/2 centered on Cs. The full travel range along Xsaxis is 2Rsin(180°/n) or ±Rsin(180°/n) centered on Cs, where n is the number of sectors as defined below in reference toFIG. 2.

Wafer22is placed on stage28, and more precisely on support pins30of theta stage28A. The placement is such that a wafer center Cwis substantially aligned with center of rotation Crand center Cs, of an angular sector S1is aligned with center Cs. Any known technique can be used to ensure sufficient alignment. In the present embodiment, a pre-aligner38and a notch40along a rim42of wafer22are used to ensure substantial alignment between center of rotation Crand wafer center Cw.

Apparatus20has a measurement assembly44that is located above wafer22. Assembly44has an illumination source46and optics48for producing a measurement beam50of probe radiation52to examine features24on wafer22optically. Wafer22is illuminated with probe radiation52focused to a measurement beam spot54on surface26of wafer22. Assembly44has a detector56with optics58for collecting and studying a scattered portion60of probe radiation52. Any suitable scatterometry or other optical examination method can be employed in studying scattered portion60.

The above-described mounting relationship of stages28A and28B ensures that center Cwof wafer22is offset from center CS1of sector S1by R/2 and coincident with center Crof rotation of theta stage28A. Thus, when measurement assembly44is configured such that beam spot54is centered on center CS1of sector S1, then entire sector S1delimited by the dashed and dotted lines can be examined with beam spot54by moving linear X-Y stage28B only. In other words, each location on wafer22in sector S1can be arrived at by a displacement along Xsand a displacement along Ys, as indicated by the two dashed arrows dx, dy. In other words, beam spot54can be trained on all points within sector S1without moving measurement assembly44or rotating wafer22.

In accordance with the invention, this configuration of stages28A,28B of apparatus20is utilized by a scheduling module64for coordinating X-drive32, Y-drive34and θ-drive36with measurement assembly44such that wafer22is examined in an even number n of angular sectors. The angular sectors are defined by a sector angle Θ that is the same for each sector. Specifically, the sector angle Θ is defined in terms of n as follows:

Θ=360⁢°n;
where n=4m and m is an integer, such that multiple rotations of sector angle Θ always include angles 90° and 180°. In the present embodiment m=1 and thus n=4 and sector angle Θ is equal to 90°.

FIG. 3Ais a top plan view of surface26of wafer22subdivided into four (n=4, m=1) angular sectors S1, S2, S3and S4while in its canonical (θ=0) orientation corresponding to the orientation ofFIG. 2. In this position sector S1of wafer22is accessible for examination by measurement beam spot54originally centered on center CS1of sector S1of wafer22. Any location in sector S1can be accessed without rotating theta stage28A, but only displacing linear X-Y stage28B along Xsand Ysby displacements dxand dy.

Prior to examination of features24, the operator performs a pre-alignment procedure with pre-aligner38using notch40for reference. once pre-alignment is complete, scheduling module64coordinates X-drive32, Y-drive34and θ-drive36with measurement assembly44such that wafer22is examined in succession in the four angular sectors S1, S2, S3and S4. Measurement assembly44is positioned such that when X- and Y-drives32,34keep X-Y linear stage28B in the middle of the corresponding Xsand Ystravel ranges R, 2Rsin(180°/n), measurement beam spot54is incident on center CS1of sector S1of wafer22at an offset R/2 from wafer center Cw. When examining successive sectors S2, S3, S4measurement beam spot54is also initially centered on the centers of these sectors.

To commence examination in sector S1, scheduling module64instructs θ-drive36to rotate wafer22into the canonical position (θ=0). In this position notch40is aligned along Ywaxis, which in turn is parallel with Ysaxis of stage28(seeFIG. 2). Once wafer22is in the canonical position, module64instructs X- and Y-drives32,34to move linear X-Y stage in any suitable manner to cover Xsand Ystravel ranges R, 2Rsin(180°/n). Since n=4 and m=1, the Xstravel range is equal to 2Rsin(45°) or 1.41R. The movement is performed in a pattern selected such that spot54illuminates all features24that need to be optically examined by measurement assembly44. If the entire sector S1is to be examined, a raster pattern can be used.

To examine the next sector, module64instructs θ-drive36to rotate wafer22by one sector angle Θ, as shown inFIG. 3B. In this case wafer22is rotated to the left. Of course, wafer22could also be rotated to the right and/or be rotated by a multiple of sector angle Θ, e.g., by two sector angles2Θ. In any event, when wafer22is rotated, the coordinates on wafer22have to be recalculated, since they are no longer coincident with Xsand Ys. For example, one can continue to use wafer coordinates centered at wafer center Cwwith the polar axis pointed to notch40. Starting from the canonical position (when θ=0 and notch40is on the positive Ysaxis of X-Y linear stage28B) the coordinate transformation for any point xs, ysin stage28B coordinates Xs, Yscan be described in terms of center offset R/2 and rotation angle θ as:

Now, when examining sector S4as shown inFIG. 3B, the travel ranges for linear X-Y stage28B have to be adjusted. In particular, although Xsremains the same, travel range Ysdecreases to 0.292R (or R-Rsin(45)). In addition, the coordinate transformation of equation 1 has to be used to correlate the position xw, ywof beam spot54on wafer22to stage coordinates xs, ys.

After sector S4has been studied, module64instructs θ-drive36to rotate theta stage28A by another sector angle Θ to the left. This results in wafer22being positioned for examination of sector S2as shown inFIG. 3C. Then, once again, equation 1 is used to correlate the position xw, ywof beam spot54on wafer22to stage coordinates xs, ysduring examination. Finally, module64instructs θ-drive36to rotate theta stage28A by still another sector angle Θ to the left to examine sector S4.

The method of invention permits one to subdivide surface26of wafer22into more than four sectors. For example,FIG. 4is a top plan view in which wafer22is subdivided into eight (n=8, m=2) angular sectors S1, S2, . . . S8. Wafer22is shown in its canonical (θ=0) orientation with notch40on positive axis Ys. The travel range Xsis 2Rsin(22.5°) or 0.77R and sector angle Θ=45°. In order to examine successive sectors, module64instructs θ-drive36to rotate theta stage28A by sector angle Θ=45°. The travel ranges for each sector are summarized in table 1 below.

Equation 1 is used to transform from stage coordinates xs, ysto wafer coordinates xw, ywas before. Finally, theta stage28A is rotated by −θ and linear X-Y stage is translated by (−xs, −ys) to move point (xw, yw) to beam spot54. It should be noted that scheduling module64can enforce any examination order, including examining diametrically opposite pairs of angular sectors in succession.

To monitor which angular sector is actually being examined stage28, and in particular θ-drive36of theta stage28A is set up to communicates with scheduling module64. Specifically, after pre-alignment and initial calibration in the canonical position (θ=0), theta stage28A keeps track of rotation angle θ executed by θ-drive36and communicates it to module64. Since a small number of θ values are used in the examination process (e.g., 4, 8, 16 etc.), obtaining angle θ from theta stage28A directly without additional monitoring equipment is a particularly simple and reliable way of monitoring rotation angle θ. The value of θ thus obtained can be used not only for monitoring the rotation of wafer22for purposes of measurement or scheduling, but also for displaying any images of wafer22to an operator or user.

FIG. 5shows a partial three-dimensional view of another apparatus100for examining a planar disk-shaped sample102. In this embodiment apparatus100is equipped with a viewing mechanism104for imaging surface106of sample102with features110that are to be examined. As in the prior example, features110are microscopic, and shown in a magnified view in few areas only for reasons of drawing clarity. A measurement assembly112is provided for examining features110.

Apparatus100has an image rotation module108for rotating an image of certain pre-selected reference features125captured by mechanism104. It should be noted that reference features125are typically not selected from among features110which are to be examined. Rotation module108relies on pattern matching of rotated reference features125to determine the rotation angle θ. This embodiment is preferred when measurement assembly112requires precise calibration for proper measurement and analysis.

Viewing mechanism104may use scattered portion128of probe radiation120in the field of view of assembly112to reconstruct and image reference features125. Alternatively, it can obtain the image independently with separate elements such as additional illumination sources (not shown). In fact, a very wide range of imaging approaches can be employed by mechanism104, including integrating it with detector124, as will be appreciated by those skilled in the art.

Image rotation module108is in communication with viewing mechanism104so that it can receive the image of reference features125. Module108performs a rotation of the image of reference features125. In conjunction with a pattern matching module126, module108is configured to rotate the image of features125by multiple of 180°/n until it matches a certain pattern associated with a given orientation of sample102. For this purpose, features125in particular orientation or orientations may be selected and stored as a stored pattern127. In selecting features125for pattern127, it is important to make a choice that will permit reliable rotation control. However, since sector angles Θ are generally large even when there are many sectors, e.g., n=16 or more, the rotations performed by θ-drive are still significant enough that matching the rotated image of features125to stored pattern127is in general not a challenging problem. Thus, direct communication with θ-drive may not be necessary in most applications.

Another exemplary image rotation module108′ that can be used by the apparatus of invention is illustrated in the three-dimensional view ofFIG. 6A. Unlike module108, which performs the rotation digitally, rotation module108′ performs the rotation mechanically using a Dove prism134. To accomplish this, module108is positioned before viewing mechanism104.

Module108′ has a set of lenses136,138positioned before and after Dove prism134for focusing the light coming from feature125and guiding it through prism134to follow a total internal reflection path140, as shown. In this arrangement, a mechanical rotation of prism134about the optical axis by a mechanical rotation angle φ, results in an optical rotation by twice mechanical rotation angle φ, or simply 2φ. The orientation of feature125before and after undergoing the rotation is indicated by arrows I and I′, respectively. An image of feature125rotated by 2φ is shown on display132.

A person skilled in the art will appreciate that mechanical rotation module108′ admits of a large number of alternative mechanical rotation mechanisms. Also, any number of alternative modules that use no moving parts can be used. For example, rotation modules that employ acousto-optic beam deflectors and polygon mirrors emulating the operation of a Dove prism can be used to avoid the need for mechanical drives and/or moving parts.

Irrespective of the type of image rotation module and/or technique, it is important for pattern matching to select one asymmetric reference feature125and store it in the form of stored pattern127of images at various image rotations as shown inFIG. 6B. Reference feature125can be any asymmetric object on the surface of the die, including a fiducial, indicia or any type of markings or points on surface106of sample102. Feature125should be selected bearing in mind that pattern127should exhibit a certain level of asymmetry and in particular, a sufficient level of asymmetry to distinguish a number of pattern orientations at least equal to the number n of sectors (since the number of sectors is the number of discrete values that θ is allowed to assume). The images of feature125at different orientations corresponding to the different permissible values of θ based on the number of angular sectors are thus stored as pattern127and then matched to determine actual rotation angle θ.

There are several ways of doing pattern matching and thus finding rotation angle θ. When sample102is rotated by an unknown angle θ but with a step of 180°/n, an image is obtained by mechanism104. This is the current image and we define it as image B. To find rotation angle θ one can then perform any of the following steps:1. Store an image A of reference feature125as a standard image that is collected when θ=0. Rotate image B with a step 180°/n using image rotation module108and find the best match to image A using pattern matching module126. If the best match yields a rotation angle of alpha, it means the sample is rotated by alpha.2. Use pattern recognition software that matches a rotated image. When using such software, one just needs to try several or all the possible rotation angles θ in pattern matching module126and the best match will correspond to the correct rotation angle θ.3. Store a set of images A0, A1, A2, A3, . . . , A2n−1, of reference feature125collected at angles 0°, 180°/n, 2*180°/n, 3*180°/n, . . . , (2n−1)*180°/n. This is done at training time. During the measurement, the current image B will be compared with all stored images Ai, and the best match gives the correct rotation angle θ.4. Couple θ stage with pattern matching module126as is well-known to a person skilled in the art and monitor rotation angle θ.

The pattern matching performed with the aid of image rotation module108or108′ and pattern matching module126has many uses. In one embodiment, apparatus100has a drive feedback that is in communication with pattern matching module126. Based on matching stored pattern127, and in particular matching its rotated version with the pattern contained in the image of feature125observed by viewing mechanism104, the drive feedback controls at least one of the drives, i.e., X-drive, Y-drive and θ-drive, but preferably at least the θ-drive. In another embodiment, apparatus100has an angular sector identification module that is connected to pattern matching module126. The identification module identifies the angular sectors based on pattern matching. In this case, the pattern matching module can try several or all possible rotations with 180°/n steps, as described above, and the best pattern match yields the correct rotation angle θ.

In still another embodiment, apparatus100has a sample offset correction module that is in communication with pattern matching module126and performs offset correction of sample102based on the pattern matching. In any of these applications of pattern matching module126, as well as any other applications thereof, it is advantageous to ensure a corresponding level of asymmetry of the stored pattern to ensure that the requisite angular resolution is obtained. Once the correct rotation angle θ is found, the captured image can be rotated for display to an operator.

In addition to determining rotation angle θ, it is sometimes also important to properly orient measurement assembly112with respect to features110that are to be examined in any given sector. This is important when features110have a preferred orientation, i.e., when they are not isotropic. Examples of such features110include gratings and other non-isotropic structures encountered in the manufacture of electronic and photonic circuits. In the embodiment shown, features110are gratings and they need to be illuminated by measurement beam118from a certain angle of incidence.

To properly orient measurement assembly112a calibration pattern130consisting of features110is used.FIGS. 6C-Dare diagrams illustrating two possible calibration patterns130A,130B for calibrating measurement assembly112. In fact, each sector contains a copy of calibration pattern130, such that calibration of assembly112can be performed independently for measurements that are undertaken in each sector.

Patterns130A,130B are composed of features110present on sample102for calibrating the response of features110with respect to the angle of incidence of probe radiation120. Pattern130A illustrated inFIG. 6Bis designed for embodiments where sample102is segmented into four sectors and sector angle Θ=90°.FIG. 6Cillustrates a pattern130B that is optimized for embodiments where sample102is segmented into eight sectors and sector angle Θ=45°. Each pattern130is guaranteed to provide a calibration portion of features110that are parallel (or perpendicular) to actual features110that are to be examined in the sector.

Although measurement assembly112can be stationary, in order to take best advantage of calibration pattern130and achieve proper orientation rapidly, it is preferred that measurement assembly112be mobile.

In an alternative embodiment, image rotation module108of apparatus100can be in communication with the θ-drive of the theta stage (not shown). Thus, module108can obtain rotation angle θ from θ-drive directly rather than obtaining it from pattern matching or other process. Given that the number of possible rotation angles θ is small due to small number n, this is an effective way of obtaining rotation angle θ. Furthermore, rotation angle θ obtained from θ-drive can be used for rotating the image of sample102. Thus, for example, in cases where apparatus100is provided with a display module132, rotation angle θ obtained directly from θ-drive can be used to rotate the image. This image can be used for display to an operator or user, or else for monitoring the rotation of sample102.

Rotation angle θ obtained from θ-drive can also be used as a cross-check to confirm rotation angle θ obtained in pattern matching. Such cross-check becomes more important with a larger number n, i.e., as the number of angular sectors into which sample102is subdivided increases.

FIG. 7illustrates yet another segmentation of sample102into n=16 sectors S1, S2, . . . , S16. For clarity, sectors S1, S2, . . . , S16are merely designated by their sector number. In this embodiment the travel range Xsis equal to 2Rsin(11.25°) or 0.39R and sector angle Θ is 22.5°. In fact, for most applications sector angle Θ is preferably equal to 90°, 45° or 22.5°. Smaller sector angles Θ are also possible, but may not be preferred due to the increased complexity when implementing pattern matching for feedback and control of the drives, sector identification or correction of sample offset.

In any of the above embodiments, or still other embodiments with larger number of sectors, disk-shaped samples can be examined in a small footprint apparatus that is insensitive to misalignments. In particular, the apparatus operates within the footprint of the sample only while still being able to efficiently examine the entire surface of the sample. Further, the apparatus of invention can examine samples such as semiconductor wafers in a configuration that allows the measurement assembly to travel in a very flat and precise manner, whenever such travel is required. In addition, moving reliably between positions in various sectors is very precise and not as costly as in the prior art systems.

In some applications, the θ-drive has a stepper motor for rotating the sample by angular steps that are equal to the sector angle Θ. In one case an encoder with angular feedback can be used to control the position of the angular stepper motor. In another case, a simple DC motor can be used to drive the rotating table, and then any suitable mechanism can be used to enforce such step-wise rotation, including hard stops. The hard stops are used for controlling its angular movement by increments equal to sector angle Θ. In any of these applications the measurement assembly itself can be stationary or mobile.

As shown in the top plan view ofFIG. 8, a rotary stage200can use stepper or servo motors with rotary encoder feedback of the angular position at any given rotation angle θ. There is a closed loop control function used to position to any given θ.

Alternatively, if the θ positions to be measure are known, the rotary table can have adjustable tabs202,204,206,208at each position and then a hard stop210can be actuated to flip in and out of the tab path. A thru beam sensor212can be used to keep track of which pre-determined angular position rotary table200is in (in the present case the four permitted angular positions are 0°, 90°, 180° and 270°). In this embodiment a simple motor can be used without encoder feedback and exact position of the rotary table will be known because the actual hard stop and tab will be in contact.

The invention further encompasses a method for examining features of a sample, e.g., a semiconductor wafer that is placed on a stage with three degrees of freedom of motion. Specifically, the method is practiced on a stage with an Xsdirection of motion, a Ysdirection of motion and a θ rotation.

The method of invention can be practiced in many variants. In some a specific schedule of examination of the angular sectors can be implemented. For example, the schedule may dictate consecutive examination of diametrically opposite pairs of angular sectors.

Many other embodiments of the apparatus and method are possible. Therefore, the scope of the invention should be judged by the appended claims and their legal equivalents.