Abstract:
An ellipsometric apparatus provides two impinging focused probe beams directed to reflect off the sample along two mutually distinct and preferably substantially perpendicular directions. A rotating stage rotates sections of the wafer into the travel area defined by two linear axes of two perpendicularly oriented linear stages. As a result, an entire wafer is accessed for measurement with the linear stages having a travel range of only half the wafer diameter. The reduced linear travel results in a small travel envelope occupied by the wafer and, consequently, a small footprint of the apparatus. The use of two perpendicularly directed probe beams permits measurement of periodic structures along a preferred direction while permitting the use of a reduced motion stage.

Description:
CLAIM OF PRIORITY 
   The present application is a continuation of U.S. patent application Ser. No. 10/893,449, filed Jul. 16, 2004, now U.S. Pat. No. 6,985,228 which is a continuation of U.S. patent application Ser. No. 10/042,592, filed Jan. 9, 2002, now U.S. Pat. No. 6,798,512 which claims priority to U.S. Provisional Patent Applications Ser. No. 60/311,035, filed Aug. 9, 2001, and Ser. No. 60/336,437, filed, Nov. 1, 2001, each of which is hereby incorporated herein by reference. 

   TECHNICAL FIELD OF THE INVENTION 
   The present invention relates to ellipsometric systems for measuring feature configurations on wafers. Particularly, the present invention relates to a compact ellipsometer having multiple measurement beams. The measurement beams operate alternately and in conjunction with a rotating stage such that linear stages for positioning the wafer may be operated within reduced travel ranges. 
   BACKGROUND 
   Semiconductors are typically fabricated by depositing and etching a number of layers that are shaped and configured on the upper or top surface of a wafer. Controlling those fabrication steps and testing the wafer early during production helps to keep production costs low. An increasingly important technique for a non-destructive measurement of semiconductors is ellipsometry. In ellipsometry, a specifically configured probe light beam is directed to reflect off the wafer. The change in polarization state of the beam induced by the interaction with the wafer is monitored to provide information about the wafer. 
   Ellipsometers have been used extensively to monitor thin film parameters such as thickness, index of refraction and extinction coefficient. More recently, ellipsometers have been used to monitor the properties (critical dimensions) of small, repeating, periodic structures on wafers. These periodic structures are similar to a grating and the measured data can be subjected to a scatterometry analysis to derive information about the structure. Information of interest includes, but is not limited to, line width and spacing as well as sidewall profile. 
   Such periodic structures have distinct orientations. It has been found that the most useful information about such structures can be obtained if the probe beam of the ellipsometer is directed substantially perpendicular to the line structure. 
   As seen in  FIG. 1 , a typical wafer W will have multiple such periodic structures PS formed thereon. In some cases, all of the periodic structures will be oriented in the same direction (i.e. all lines parallel). In other cases, some of the structures will have lines running perpendicular to other structures. 
   A conventional, ellipsometer is typically provided with a stage for moving the wafer through full linear motions FX, FY as well as rotation about the central axis so that the probe beam PB can be directed to each of the periodic structures PS in the appropriate direction (usually perpendicular to the line structure). The linear motions FX, FY are about equal to the wafer diameter WD. The wafer W moved during the measurement consequently occupies a travel envelope LE that extends in the directions of each of the linear axes about twice the wafer diameter. The travel envelope LE determines the minimal footprint of an ellipsometer apparatus. 
   Recently, there has been a push to substantially reduce the size of ellipsometer apparatus. This effort is particularly directed to allowing an ellipsometer to be incorporated directly into a semiconductor processing tool. To achieve the desired miniaturization, stage system have been developed which reduce the total range of motion of the wafer, thereby reducing the travel envelope and consequently the footprint of the system (e.g. stages that implement a cylindrical coordinate system consisting of a linear and a rotational stage). The use of these stages has not significantly impeded the measurement of thin film parameters since such measurements are not effected by the direction in which the probe beam strikes the sample. However, such reduced motion stage systems have caused a problem with measuring periodic structures where the impinging direction of the probe beam PB has to correspond to a measurement relevant orientation of the periodic structure PS. 
   As shown in  FIGS. 2A and 2B  the wafer can be manipulated with the X- and Y-stages such that only one of the four quadrants of the sample is located at the intersection between the sample and the probing beam. To perform measurements within the other three quadrants of the sample, the rotating stage needs to move by multiples of 90°. To achieve perpendicular orientation of the periodic sample structures in these cases, a second probing beam perpendicular to the first one is necessary. 
   This difficulty can best be seen in  FIGS. 2A and 2B . In  FIG. 2A , the probe beam PB is shown striking periodic structure PSI perpendicular to the line structure. When the operator wishes to measure periodic structure PS 2 , the rotating stage is used to bring the sector of the wafer where that structure is located within the region which can be reached by the probe beam. As noted above, the stage travel in the X and Y directions is not sufficient to bring the structure PS 2  under the probe beam without a rotation. Unfortunately, and as seen in  FIG. 2B , the result of this rotation is to orient the periodic structure PS 2  so that the probe beam impinges thereon in a direction parallel to the lines. As noted above, it has been found that most relevant information can be obtained when the beam strikes the structure perpendicular to the line structure. 
   Accordingly, it would be desirable to develop an ellipsometer system, which can utilize a reduced motion stage but also provides for optimal measurement of both thin film parameters and periodic structures. 
   BRIEF SUMMARY 
   In the present invention, a probe beam is selectively directed along two or more beam paths to provide two or more focused beams impinging in various impinging directions on a tested wafer. Two perpendicularly operating linear stages provide a travel area that is only a fraction of the wafer size. Combined with the linear stages is a rotating stage positioned with its axis of rotation perpendicular to the movement plane defined by the linear stages. The movement plane is preferably parallel to the top of the fixed wafer. The rotating stage rotates the fixed wafer within a rotation range such that a number of sectors of the wafer top are brought within range of the respective focal spot during consecutive sector measurement steps. During a sector measurement step, only the linear stages are operated to move the wafer along the focal spot. 
   The focused beams are positioned in a number and in an angular orientation to each other that corresponds to the number of angular orientations of patterns within the measurement sectors. In the preferred embodiment, where wafer patterns have one measurement relevant orientation, two focused beams are provided in perpendicular impinging directions relative to each other such that the pattern can be measured in all four quadrants while still maintaining a perpendicular orientation of the pattern relative to the plane of the probing beam. Since two focused beams are utilized, the rotating stage operates only to adjust the wafers global orientation prior to the sector measurement steps and to rotate the predetermined sectors within the travel area so that is accessible by the focal spots. In the preferred embodiment, four sectors are defined for measuring a wafer in four consecutive sector measurement steps. The linear ranges of the linear stages are about half the wafer diameter, which significantly reduces the travel envelope and consequently the footprint of the apparatus. 
   The goal of the subject invention could be achieved using two completely separate ellipsometers mounted on the same support system. In other words, two light sources, two sets of focusing and collecting optics, two sets of polarizers and analyzers and two separate detectors could be used. However, in the preferred embodiment, only a single light source and a single detector are used. This approach not only conserves space, but also reduces complexity as only one light source and one detector needs to be adjusted and characterized for the measurement. 
   Preferably, a broad band light source is used to generate a polychromatic probe beam. At some point before striking the sample, optics are provided for either splitting the beam along two paths or selectively directing the beam along a first or a second beam path. After reflection off the sample, optics are provided before the detector to either recombine the previously split beam portions or selectively combine the two beam paths into a single path. In the preferred embodiment, movable mirrors are used to create two beam paths rather than splitting the beam to maximize the light energy being used for the measurement. 
   The subject invention is not limited to any particular ellipsometer configuration. Those skilled in the art will be aware of many variants such as rotating polarizer (analyzer) or rotating compensator systems. The elements necessary to create the polarization state of the incoming probe beam and analyze the polarization state of the reflected beam can be located in the common path regions or in the separate path regions. If located in the separate path regions, two sets of optical elements are needed as shown in the preferred embodiment illustrated herein. 
   In the illustrated embodiment, a broadband rotating compensator (waveplate retarder) system is shown. Such a system is disclosed in U.S. Pat. No. 5,973,787. A suitable rotating analyzer system is shown in U.S. Pat. No. 5,608,526. See also, U.S. Pat. No. 6,278,519. All of the above patents are incorporated herein by reference. 
   In the illustrated embodiment, a pair of movable mirrors is provided for controlling the beam propagation. In a first position, the mirrors are located out of the beam path and allow the beam to travel along a first beam path. In a second position, both mirrors are located within the beam path and cause the beam to travel along the second beam path. The moveable mirrors are specifically configured to provide high position accuracy and repeatability for a large number of switching cycles. 
   In the preferred embodiment, stepper motors that have a hollow shaft are utilized to rotate and control the waveplates. The stepper motors are placed such that the beam paths run through the hollow shaft of the stepper motors&#39; axes of revolution. The hollow shaft assembly reduces significantly the space otherwise occupied by the mechanism for driving the waveplates contributing to a reduced footprint of the optical assembly. As a result, the optical assembly fits into the apparatus despite the increased number of individual optical components. 
   The combination of single light source and single detector in combination with reduced travel range, multiple alternate focused beams, hollow shaft waveplate assembly and moving mirrors provides for an ellipsometry apparatus that has a footprint smaller than the travel envelope LE for a given wafer size. 
   As noted above, the subject invention allows periodic structures to be measured from a preferred direction while using a reduced motion stage. It should also be noted that this system allows any measurement area to be measured from any direction. While it is generally true that maximum information may be obtained when measuring a periodic structure when the beam is directed perpendicular to the line structure, additional information may be obtained from measurements where the beam is also directed parallel to the line structure or even at a 45 degree angle with respect thereto. In cases where the measured structure is rather simple, this information alone might even be sufficient. The subject invention allows measurements from any desired direction. Such measurements could be used individually or combined in a regression analysis to more fully characterize the structure. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  shows a simplified scheme of a prior art positioning system of an ellipsometer system where two linear stages move a wafer in ranges about equal the wafer diameter. 
       FIGS. 2A and 2B  show simplified schemes of a prior art positioning system of an ellipsometer system where two linear stages move a wafer in ranges about half the wafer diameter. In such prior art positioning system the probe beam is limited to direction irrelevant reflectance measurements. 
       FIG. 3  shows a first perspective view of an exemplary apparatus according to the preferred embodiment of the present invention. 
       FIG. 4A  shows a top view of the device of  FIG. 3  as indicated in  FIG. 3  by the arrow  17 . The device is shown with a light beam propagating along a first beam path. 
       FIG. 4B  shows schematic and simplified the main contents of  FIG. 4A  for the purpose of general understanding. 
       FIG. 5A  shows a top view of the device of  FIG. 3  as indicated in  FIG. 3  by the arrow  17 . The device is shown with the light beam propagating along a second beam path. 
       FIG. 5B  shows schematic and simplified the main contents of  FIG. 5A  for the purpose of general understanding. 
       FIG. 6  shows a second perspective view of the device of  FIG. 3  with the light beam propagating along the first beam path. 
       FIG. 7  shows the second perspective view of the device of  FIG. 3  with the light beam propagating along the second beam path. 
       FIG. 8  shows the second perspective view with the device of  FIG. 3  having a top portion removed. A travel envelope occupied by a work piece during the operational use of the device is illustrated. Also shown is a travel area provided by the travel ranges of the linear stages. 
       FIG. 9  shows the second perspective view with the device of  FIG. 3  having the top portion removed. A first focused beam is illustrated impinging a first sensitive measurement area in a first impinging direction. 
       FIG. 10  shows the second perspective view with the device of  FIG. 3  having the top portion removed. A second focused beam is illustrated impinging a second sensitive measurement area in a second impinging direction. 
       FIG. 11  is a perspective view of a hollow shaft stepper motor used to mount a rotating waveplate. 
   

   DETAILED DESCRIPTION 
   According to  FIG. 3 , an ellipsometric apparatus  1  of the preferred embodiment is configured in order to make directional reflectance (in this embodiment, ellipsometric) measurements on the top  9  (see  FIGS. 4A ,  5 A,  6 – 10 ) of a wafer  2 . The ellipsometric apparatus includes a base  5  and a top portion  8 , which carries an optical assembly  12 . The optical assembly  12  includes two rotating waveplate assemblies  10 ,  14 . 
   Directional reflectance measurements are measurements for which a certain impinging direction of a focused beam relative to structure  6 ,  11  (see  FIGS. 4A–10 ) needs to be maintained. The structures  6 ,  11  may be patterns as they are well known to those skilled in the art. The structures  6 ,  11  typically have a plurality of parallel lines in a grating-like configuration. The structures  6 ,  11  may also be layer configurations and other features on the wafer top  9  with properties measurable by the known techniques of ellipsometry. For more detailed information on related ellipsometry techniques, see the patents cited above. 
   It is noted that for the purpose of general understanding, the elements illustrated in the figures are schematically shown without any claim for accuracy. Moreover, for the purpose of clarity, propagating beams are shown as solids. 
   The ellipsometric apparatus  1  has a base  5  on which a first linear stage  4  is assembled. On top of the first stage  4  is mounted a second linear stage  3 , which itself carries a rotating stage  7 . The first stage  4  may have a first travel range TX (see  FIG. 8 ) along a first linear axis Al and the second stage  3  may have a second travel range TY (see  FIG. 8 ) along a second linear axis A 2  perpendicular to axis A 1 . First stage  4  and second stage  3  are positioned and computer controlled operated such that the rotating stage  7  may be moved within the apparatus as shown by a travel area TA (see  FIGS. 8–10 ). 
   The travel area TA is defined by the ranges TX, TY. The travel area TA is the area accessibly by focal spots  41 ,  81  (see  FIGS. 4A–7 ,  9 ,  10 ) by only operating the linear stages  3 ,  4 . The travel area TA is a fictive and global entity introduced for the purpose general understanding. It can be imagined as being drawn by the focal spots  41 ,  81  on an fictive element directly attached to the linear stage  3  while the linear stages  3 ,  4  move within their travel ranges TX, TY. 
   The rotating stage  7  has a rotating range preferably of 360 degrees around an axis of revolution AR (see  FIG. 8 ). The rotating stage  7  is fixed on the linear stage  3  such that any area of the concentrically supported wafer  2  may be brought within the travel area TA by rotating the wafer  2  via the rotating stage  7 . 
   In the preferred embodiment, the wafer  2  has a wafer diameter WD (see  FIG. 8 ) that is about twice the amount of the ranges TX, TY. The first range TX defines together with the diameter WD a first envelope extension EX (see  FIG. 8 ). The second range TY defines together with the diameter W) a second envelope extension EY (see  FIG. 8 ). Extensions EX, EY define a travel envelope  90  (see  FIG. 8 ). 
   Reflectance measurements are performed on essentially the whole wafer top  9  in a number of consecutive measurement steps where individual wafer sectors  91 – 94  (see  FIGS. 9 ,  10 ) are brought within the travel area TA. Following the step of positioning one of the sectors  91 – 94  within the travel area TA, the wafer  2  is linearly moved by the stages  3 ,  4  to bring predetermined test areas within the focal spots  41 ,  81 . The focal spots are fixed and defined by the optical assembly  12 . A predetermined test area may be part of the structures  6 ,  11 , which have different measurement relevant orientation on the wafer top  9 . Structures  6 ,  11  are solely shown for the purpose of general understanding to exemplarily represent the dense arrayed patterns that are measured by the apparatus  1 . The structures  6 ,  11  are direction sensitive measurement areas, which require a predetermined impinging direction in order to accomplish a reflectance measurement in accordance with known techniques of ellipsometry. A multitude of such direction sensitive measurement areas may be present on a wafer top  9  with varying angular fabrication orientation. 
   The apparatus top  8  is dimensioned in correspondence with the base  5  and may extend beyond the footprint of the base  5  for the purpose of providing secured access to supply and communication cables (not shown) as is clear to one skilled in the art. In the preferred embodiment, a common light source and a single final detector are utilized in order for the optical assembly  12  to fit within the apparatus top  8  together with eventual other functional elements like, for example a well known focusing unit and/or calibration unit (not shown). The apparatus top  8  is positioned in a gap height HG above the rotating stage  7  such that the rotating stage  7  is externally accessible for placing the wafer  2  on it. 
   In the preferred embodiment, the optical assembly  12  is configured to provide a first focused beam  40  (see  FIGS. 3 ,  4 A,  4 B,  6 ,  9 ) and alternately a second focused beam  80  (see  FIGS. 5A ,  5 B,  7 ,  10 ) from an initial light beam  30  (see  FIGS. 3–7 ). The first focused beam  40  provides a first focal spot  41  in a first impinging direction (see  FIG. 9 ). The second focused beam  80  provides a second focal spot  81  in a second impinging direction (see  FIG. 10 ). Impinging directions are the directions of center axes of the focused beams  40 ,  80 . 
   A first moveable mirror  28  (see  FIGS. 4A–7 ) alternately switches the initial light beam  30  between a first beam path and second beam path. According to  FIG. 4A ,  4 B, the first beam path includes the initial section  31  up to a first lens unit  37  from which the first focused beam  40  propagates towards the first focal spot  41 . The first beam path further includes a first inclining path segment  46  from a parabolic mirror  45  to the mirror  33  and a path end segment  50  from the mirror  33  up to a parabolic mirror  56 . 
   According to  FIGS. 5A and 5B , the second beam path includes the initial section  71  from a first moveable mirror  28  being in an in-position such that it redirects the initial light beam  30  up to a second lens unit  77  from which the second focused beam  80  propagates towards the second focal spot  81 . The second beam path further includes a second inclining path segment  86  from a parabolic mirror  85  to the mirror  73  and a path end segment  70  from the mirror  73  up to a second moveable mirror  54 . The second moveable mirror  54  is also in an in-position where it redirects the incoming light beam towards the parabolic mirror  56 . Light beams propagating along first or second beam path are both directed towards final optical elements, which prepare a terminating beam  60  to terminate on a single detector  61 . 
   The final optical elements include the parabolic mirror  56  together with mirrors  57 , a pin hole  58  and a holographic grating  59 , which prepare in a well known fashion the terminating beam  60  for impinging and terminating in the final detector  61 . In the preferred embodiment, the parabolic mirror  56  has a focus angle of 20° and a focal length of 100 mm, the holographic grating  59  is from Jobin Yvon, part number 543.02.190 and the final detector  61  is a CCD detector having 512 pixels, each corresponding to a different narrow wavelength bandwidth. The pin hole  58  includes a selectable element for changing the size of the pin hole so that the measurement spot size can be varied. 
   The scope of the invention includes embodiments in which other optical features well known for alternately redirecting an incoming light beam are used instead of the moveable mirrors  28 ,  54 . The scope of the invention is also not limited by a specific mode by which the moveable mirrors  28 ,  54  alternately redirect the incoming beams. For example, a configuration may be selected in which one of the moveable mirrors  28 ,  54  is in an in-position while the other one is in an out-position where it does not interfere with a light beam. The scope of the invention is also not limited by a particular shape of geometry of the beam paths or by any particular number and/or configuration of additional optical elements like, for example, mirrors  34 ,  74 . 
   According to  FIG. 4A , a first structure  6  has a measurement relevant orientation on the wafer top  9  requiring a first impinging direction provided by the first focused beam  40 . In order to perform the reflectance measurement in accordance with the known techniques of ellipsometry, the first focused beam  40  impinges the structure  6  within the first focal spot  41  along the first impinging direction and initiates a first reflected beam  42  whose polarization state has been changed away from the focal spot  41 . The first focused beam  40  is provided by a first lens unit  37  and a first polarizer  35 , which focus and polarize the initial light beam  30  propagating along the first beam path. In the preferred embodiment, the first lens unit  37  is a triplet lens with f=69 mm and the polarizer  35  is a Rochon prism. A mirror  32  is spatially oriented to redirect the propagating beam from a horizontal beam plane towards the angulated oriented and lower positioned polarizer  35  and lens unit  37 . The beam plane is sufficiently high above the top  8  to give room for mechanical features used, for example, for positioning and fixating the optical elements on the top  8 . 
   The initial light beam  30  is provided by a light source, which may include but is not limited to a white light source  20 , a lens  21 , a UV light source  22 , an ellipsoid mirror  23 , a source pinhole  24  and a parabolic mirror  25 . In the preferred embodiment, the white light source  20  is a tungsten light bulb, the UV light source  22  is a D2 UV-lamp, the ellipsoid mirror  23  has f=80 mm, and the parabolic mirror  25  has focus angle of 20° and f=50 mm. 
   To obtain an ellipsometric measurement with high accuracy, polarization detection needs to be performed on the reflected beams  42 ,  82  (see also  FIGS. 7 ,  10 ) with only a minimum number of additional reflections induced on the reflected beams  42 ,  82 . Since the reflected beams  42 ,  82  propagate conically away from the focal spots  41 ,  81 , at least one optical element is used to collimate the reflected beams  42 ,  82 . The collimating optic can be a lens, or as shown in the illustrated embodiment, parabolic mirrors  45 ,  85 , which have in the preferred embodiment has a focus angle of 45° and f=59.51 mm. 
   In the present invention, polarization detection is performed by rotating the waveplate (under computer control) around an axis of revolution, which is parallel to the propagation direction of the beam passing through the waveplate with the beam centered on the rotation symmetry axis. In the present invention, the polarization detection is at a high level of accuracy by introducing two alternately operating rotating waveplate assemblies  10 ,  14  such that each of the two reflected beams  42  is passed through one of the two rotating waveplates with only a single prior reflection induced by the parabolic mirrors  45 ,  85 . The waveplate assemblies  10 ,  14  include hollow shaft stepper motors with their rotor axes being collinear with a center axes of the reflected beams  42 ,  82 , which propagate parallel along the inclining path segments  46 ,  86 . The highly compact size of the waveplate assemblies  10 ,  14  accomplished by the use of hollow shaft stepper motors contributes significantly to the reduced space consumption of the optical assembly  12  and its successful integration into the apparatus  1  despite the increased number of optical components compared to that of a conventional apparatus having only a single focused beam. 
   Referring back to  FIG. 4A ,  4 B, a polarizer  47  is placed along each path segment after the first waveplate assembly  10 . In the preferred embodiment, the polarizer  47  is a Rochon prism. The spatially oriented mirror  33  reflects the incoming beam and directs it along the first beam path within the horizontal beam plane. 
   According to  FIG. 5A , a second structure  11  has an orientation on the wafer top  9  requiring a second impinging direction provided by the second focused beam  80 . This orientation may be the result of the fact that the wafer is provided with structures having different measurement orientations in a single quadrant or the same measurement orientation in a neighboring quadrant. In the latter case, a structure with the same orientation on a neighboring quadrant of the wafer would need to be measured with the second SE path, since it would require a 90 degree stage rotation to get it into the area where it can be reached by the beam. 
   In order to perform the reflectance measurement, the second focused beam  80  impinges the second structure  11  within the second focal spot  81  along the second impinging direction and initiates a second reflected beam  82  that carries reflectance information away from the focal spot  81 . The second focused beam  80  is provided by a second lens unit  77  and a second polarizer  75  that focus and polarize the initial light beam  30  propagating along the path section  71 . In the preferred embodiment, the second lens unit  77  is similar to the first lens unit  37  and the second polarizer  75  is the similar to the first polarizer  35 . A mirror  72  is spatially oriented to redirect the propagating beam from a horizontal beam plane towards the angulated oriented and lower positioned polarizer  75  and lens unit  77 . 
   The second reflected beam  82  is reflected by the parabolic mirror  85  from which it propagates parallel along the second inclining path segment  86 . A polarizer  87  is placed after the second waveplate assembly  14  along the path segment  86 . In the preferred embodiment, polarizers  87 ,  47  are similar. A mirror  73  is spatially oriented to direct the incoming beam along the second beam path within the horizontal beam plane. 
   The second moveable mirror  54  is in an in-position where it interferes with the beam traveling along the end section  70 . The second moveable mirror  54  reflects the beam again towards the parabolic mirror  56  and continues as described under  FIG. 4A . 
   The perspective views of  FIG. 6  and  FIG. 7  illustrate the extent to which the compactness of the waveplate assemblies  10 ,  14  contribute to the small scale of the optical assembly  12  especially in the proximity of the focal spots  41 ,  81 . For the purpose of clarity,  FIG. 6  shows the first beam path and  FIG. 7  shows the second beam path.  FIG. 6  illustrates the limited space available for the waveplate assembly  14  between the path segment  46  and the polarizer  87 .  FIG. 7  illustrates the limited space available for the waveplate assembly  10  between the path segment  86  and the polarizer  47 . 
   Referring to  FIG. 8 , the travel envelope  90  primarily defines the footprint of the apparatus  1  and consequently the available space on the apparatus top  8  as already explained in the above. Where the apparatus  1  is configured for measuring a wafer  2  having a diameter WD of about 300 mm, the envelope extensions EX, EY are according to the above description about 450 mm in each direction. The travel envelope  90  preferably remains within the overall boundaries of the apparatus  1  thus primarily defining a width FY (see  FIG. 3 ) and a depth FX (see  FIG. 3 ) of the apparatus  1 . However to allow this instrument to be used in applications where smaller samples (e.g. 200 mm wafers) are used, while still requiring a minimal footprint, this instrument was designed minimizing the size of the optics plate  8  ( FIG. 7 ) and allowing a 300 mm wafer to extend outside of the enclosure  5  ( FIG. 7 ) in certain situations. 
   Other factors like, for example, structural requirements, additional space for cabling and other well known components of an ellipsometric apparatus are secondarily defining the footprint of the apparatus  1 . Other well known components may be part of the base  5  and/or the top  8 . Such components may be required, for example, for controlling, processing, calibrating and/or focusing during the operation of the apparatus  1 . Some of these components, like for example, a focusing unit and or a calibration unit may be placed on the top  8 , which may additionally reduce the space available for the optical assembly  12 . 
   The sectors  91 – 94  ( FIG. 10 ) are predetermined and fictive areas on the wafer  2 , which can be accessed for measurement without rotating the wafer  2 . In the preferred embodiment and in accordance with the preferred travel ranges TX, TY, the sectors  91 – 94  are about one quarter of the wafer  2 . Hence, the wafer  2  has to be rotated four times in a case where all four sectors  91 – 94  are accessed for reflectance measurements. 
   Prior to performing a measurement, the wafer  2  is loaded on the rotating stage  7  by a wafer-loading tool like, for example, a robotic arm that holds the wafer  2  on its bottom surface via a vacuum fixture and releases the wafer  2  on the rotating stage  7 . The gap height HG is selected to provide sufficient space for loading and unloading of the wafer  2  even when a pin lifter assembly is used to raise the wafer to allow the robot arm to slip underneath and pick up the wafer. Due to eventual loading inaccuracies or other limitations in the loading cycle, the wafer  2  is typically globally reoriented such that the orientation of the structures  6 ,  11  corresponds to impinging directions. The rotating stage  7  may be configured to perform such initial global orienting. In this regard, a flat or notch finding procedure may be performed followed by a mask alignment procedure using a pattern recognition system. 
   The optical geometry relevant in that context includes an angle of incidence IA and a focusing angle FA (see  FIGS. 9 ,  10 ) of the focused beams  42 ,  82 . In the preferred embodiment, the angle of incidence IA (in the Figures, defined with respect to the surface of the wafer) is about 25 degrees. The focusing angle FA or cone is between 1–6 degrees. The reflected beams  42 ,  82  have a corresponding reflecting angle RA and a spreading angle SA. 
   As illustrated in  FIGS. 6 ,  7 , the sizes and positions of the lens units  37 ,  77  and the parabolic mirrors  45 ,  85  are influenced by the angles IA, FA, RA, SA in combination with the gap height HG. Sizes and positions of the lens units  37 ,  77  and the parabolic mirrors  45 ,  85  again define the available space for dimensioning and positioning the waveplate assemblies  10 ,  14 . The use of hollow shaft stepper motors significantly assists in down scaling the waveplate assemblies  10 ,  14  so that they are positioned along the inclining path segments  46 ,  86  without interfering with the lens units  37 ,  77 . 
   The waveplates are mounted in the hollow portions of the rotor shafts, and are concentrically fabricated relative to the rotor axes. The absence of separate waveplate bearings and a mechanical transmission system greatly simplifies the design and provides at the same time for a more accurate rotation control of the waveplates. Since the rotor bearing of the stepper motor is also the bearing for the waveplates, specific bearing tolerances and tolerances for concentricity of the hollow portion of the rotor shaft are defined to meet the precision demands of optical assembly  12 . In the preferred embodiment, linear actuator stepper motors from Eastern Air Devices were used. 
     FIG. 11  illustrates the optical mount  102  for the waveplate. Mount  102  supports stepper motor  106  having a rotating hollow shaft  108  therein. The waveplate is mounted to the end of the shaft  108  and is carried thereby. Reflected probe beam light ( 42 ,  82 ) passes through the hollow shaft and waveplate and thereafter passes through the analyzer (polarizer)  47 . The output of a home sensor  110  provides feedback for the position of the hollow shaft. 
   Another factor for keeping the size of the optical assembly  12  to a minimum is to utilize a common light source and a single final detector  61  as described above. In order to direct the initial light beam  30  and the reflected beams carrying the reflectance information, the moveable mirrors  28 ,  54  have to be switched into their in-position with highest accuracy over a high number of switching cycles. The movement and positioning of the moveable mirrors  28 ,  54  are provided by actuator units  29 ,  53 . In the preferred embodiment, the actuator units  29 ,  53  linearly move the mirrors  28 ,  54 . The actuator units  29 ,  53  are computer controlled and pneumatically operated. They provide custom designed hardened steel guides to achieve position precision over a large number of switching cycles. The precision requirements for the mirrors  28 ,  54  in their in-positions is 0.005° angular tolerance for 1 million switching cycles. 
   A measurement of a wafer  2  within the inventive apparatus may be performed by the following steps. In a first step, the wafer  2  is loaded, fixated and eventual globally reoriented. In a second step, the first sector  91  is rotated and brought within the test area TA. Then, one of the first or second focused beams ( 40 ,  80 ) is activated to perform the desired measurements. Conceivably, all of the areas of interest within one sector could be measured by only one of the two beams. However, depending on the orientation of the structures and the type of measurement sought, it may be either necessary or desirable to use both beam (at different times) to measure all the features of interest in a given sector. For example, to gain further information about a periodic structure, two measurements might be made, one with beam  40  perpendicular to the periodic structure and a second with beam  80  parallel to the periodic structure. 
   Once the measurements of the first sector  91  are completed, the rotating stage  7  rotates the second sector  92  within the travel area TA so that measurements in this sector can be obtained. After all predetermined sectors  91 – 94  have been measured, the wafer  2  is unloaded from the apparatus  1 . 
   The output from the detector  61  is supplied to a processor for analysis. The type of analysis performed is based on the type of measurement as is well known to those skilled in the art. For example, thin film parameters of a multi-layer structure can be characterized from multi-wavelength reflectometric or ellipsometric data using a theoretical model and the Fresnel equations. Information about small periodic structures (critical dimensions) can be derived using a diffraction model including, for example, rigorous coupled wave theory. (See U.S. Pat. Nos. 5,867,276 and 5,963,329, both incorporated herein by reference.) 
   The scope of the present invention includes embodiments, where the apparatus  1  is configured to make reflectance measurements on work pieces having features suitable to be measured with ellipsometric techniques as described in the above. Further more, the invention includes embodiments, where the work piece is non circular, and at least one of the ranges TX, TY is less than a parallel width of the fixed work piece. 
   In the preferred embodiment, two separate beam paths are provided within a single photodetector system as exemplarily illustrated by the final optical elements  56 ,  57 ,  58 ,  59  and the final detector  61 . Nevertheless, the scope of the present invention includes embodiments in which two physically separate photodetector systems may be provided. In such embodiment, each of the two separate photodetector systems receives one of the two reflected beams  50 ,  70  thus further reducing the number of optical elements in the path of the reflected beams  50 ,  70 . No moveable mirror  54  is present is such embodiments. Also, the scope of the present invention includes embodiments, where two separate light sources as exemplarily illustrated by the elements  20 – 25  are provided. In such embodiments, no moveable mirror  28  is present. 
   It should be recognized that a number of variations of the above-identified embodiments will be obvious to one of ordinary skill in the art in view of the foregoing description. Accordingly, the invention is not to be limited by those specific embodiments and methods of the present invention shown and described herein. Rather, the scope of the invention is to be defined by the following claims and their equivalents.