Patent Publication Number: US-2013241587-A1

Title: Wafer stage

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part of U.S. patent application Ser. No. 12/483,432, filed Jun. 12, 2009, which is incorporated by reference as if fully set forth. 
    
    
     FIELD OF INVENTION 
     The invention broadly relates to a wafer stage and to a method of supporting a wafer for inspection. 
     BACKGROUND 
     Integrated circuits (IC) are fabricated on semiconductor wafers. Each die on the wafer is tested and validated prior to dicing and packaging. A typical production test involves electrical testing using a wafer prober docked to an electronic test system (tester). 
     A probe card comprises a set of contacts or probes on a printed circuit board and is an interface between an electronic test system and a semiconductor wafer. In a wafer prober, the probe card is inserted and held in place. During testing, the wafer is loaded into the wafer prober, vacuum mounted on a wafer chuck and manipulated so that there can be a precise electrical contact between the probe card and the wafer. After a die has been electrically tested the wafer prober moves the next die on the wafer to the probe card and the next test can start. 
     A wafer test can separate the electrically functional dies from the non-functional. From the failed test patterns, it is possible to identify the functional blocks on the die that fails, but localization of the defect may not be possible. In order to find the cause of the failure and to increase wafer yield, further testing using defect isolation tools and techniques is required. 
     Defects can be classified as static or dynamic. In static defects, the die can easily be biased into a state where the defect can be measured i.e. short and open circuits, output stuck high or low. Dynamic defects cause otherwise functional dies to fail only at a particular frequency or temperature threshold or sequence of test vectors and loops. Dynamic defects require a tester to recreate. This requires the tester to be docked to a defect isolation tool with wafer probing capability. 
     In such a defect isolation tool, a scope transport can be located at the back side (i.e.: substrate side) of the wafer and is used to move a microscope to a location of interest on a die under test. Microscopes are used for imaging, and/or delivery of optical stimulus in order to locate defects through the back side of a die. 
     A wafer stage is used to hold the wafer in place during electrical testing by the wafer prober and image capturing by the microscope. 
       FIG. 1  is a top plan view of a typical semiconductor wafer stage  100 , comprising a platform  102  with a cavity  106  and a supporting rim  104 . The rim  104  is disposed along the circumference of the cavity  106 . A wafer (not shown) can be placed within the cavity  106  and is supported along its circumference by the rim  104 . However, the force that a wafer probe exerts onto the wafer can cause the wafer to deform and bend downwards, particularly around the centre where there is a lack of structural support from the rim  104 . The deformation can hinder testing by preventing a good electrical contact from forming between the contacts of a probe card and the wafer. 
       FIG. 2  is a top plan view of another typical wafer stage  200 , comprising a platform  202  with a cavity  206 , a supporting rim  204  and a network of a plurality of fixed support bars  208 . The rim  204  is disposed along the circumference of the cavity  206 . The wafer is placed within the cavity  206 , above the plurality of fixed support bars  208 , and is supported by the fixed support bars  208  and along its circumference by the rim  204 . Compared to the wafer stage  100 , wafer stage  200  can minimize deformation of the wafer during electrical testing by a wafer probe as it has additional support structures. However, the presence of the support bars  208  on the back side of the wafer means that a location of interest at the back side of the wafer may be obstructed. 
       FIG. 3  is a top plan view of another typical wafer stage  300 , comprising a platform  302 , a supporting rim  304  and a transparent plane parallel plate  306 . The transparent plate  306  can be made of a material like glass and can be disposed within the supporting rim  304 . The wafer is placed on the plate  306  and is supported by the supporting rim  304  and surface  306 . Compared to the wafer stage  200 , wafer stage  300  further reduces deformation of the wafer during electrical testing by a wafer probe as the entire back side of the wafer is supported by the plate  306 . In addition, the entire back side of the wafer can be observed without obstruction from the plurality of support bars (compare  FIG. 2 ). However, the presence of the transparent plane parallel plate  306  induces optical aberrations and results in microscope images that are aberration limited. 
     A need therefore exists to provide a wafer stage that seeks to address at least one of the abovementioned problems. 
     SUMMARY 
     In accordance with a first aspect of the present invention there is provided a wafer stage, comprising a platform for supporting a wafer such that a backside of the wafer is suspended above a cavity of the platform; and a support structure disposed substantially within the cavity for supporting a portion of the wafer; wherein the wafer stage is adapted for relative movement of the platform with respect to the support structure for alignment of the wafer with respect to a probe. 
     The support structure may comprise a support bar; and a support element projecting from a top surface of the support bar for supporting the portion of the wafer. 
     The support element may be hollow and may be disposed around an aperture formed in the support bar enabling an optical inspection of the backside of the wafer through the hollow support element and the aperture. 
     The support bar may be coupled to an anchor structure for inhibiting movement of the support bar in a plane parallel to the platform. 
     The anchor structure may be adapted for allowing movement of the support structure in a direction perpendicular to said plane parallel to the platform. 
     The support bar may be received in two slots formed in the platform and aligned across the cavity. 
     The hollow support may be coated with or formed from static dissipating low-friction material. 
     The support bar may be coated with or formed from the static dissipating low-friction material. 
     The static dissipating low-friction material may comprise PEEK Bearing Grade or Static-Dissipative Acetal Copolymer. 
     In accordance with a second aspect of the present invention there is provided a method of supporting a wafer for inspection, the method comprising the steps of supporting the wafer such that a backside of the wafer is suspended above a cavity of a platform; providing a support structure disposed substantially within the cavity for supporting a portion of the wafer; and effecting relative movement of the platform with respect to the support structure for alignment of the wafer with respect to a probe. 
     The support structure may comprise a support bar and a hollow support element disposed around an aperture formed in the support bar and projecting from a top surface of the support bar, and the method may further comprise performing an optical inspection of the backside of the wafer through the hollow support element and the aperture. 
     In accordance with a third aspect of the present invention there is provided a wafer analysis apparatus, comprising: a support element having an opening formed therein, the support element for supporting a portion of a wafer such that a back side of the wafer lies across the opening within the support element; and a microscope comprising a solid immersion lens having a diameter of around 2.5 mm or more, wherein either the support element, the microscope or both are adapted for relative movement with each other to align the solid immersion lens to predetermined locations on the back side of the wafer within the opening of the support element, so as to allow the solid immersion lens to press against the back side of the wafer. 
     The wafer analysis apparatus may further comprise a support structure within which the opening of the support element is formed and wherein the support element is provided on the platform, wherein the support element is in the form of a support ring; and a vacuum suction means incorporated in the support element, wherein the top surface of the support element has one or more cavities and wherein the vacuum suction means enables at least partial air evacuation of the one or more cavities of the support substrate that are in contact with the portion of the wafer. 
     The wafer analysis apparatus may further comprise a microscope; and an actuating mechanism coupled to the microscope for tilting the microscope within one or more of first and second planes that are all perpendicular to the support element, wherein the first plane is perpendicular to the second plane. 
     The wafer analysis apparatus may further comprise: a support element having an opening formed therein, the support element for supporting a portion of a wafer such that a back side of the wafer lies across the opening within the support element; a microscope; and an actuating mechanism coupled to the microscope for tilting the microscope within one or more of first and second planes that each form an angle with the support element, wherein the first plane is perpendicular to the second plane. 
     The actuating mechanism may be configured to tilt over a solid angle of 0.003 steradian. 
     The tilting of the microscope within one or more of the first planes may be provided by the actuating mechanism comprising a guide rail; and a roller arrangement coupled to the microscope, the roller arrangement being movably engaged with the guide rail. 
     The tilting of the microscope within one or more of the second planes may be provided by the actuating mechanism comprising a guide rail; and a roller arrangement coupled to the microscope, the roller arrangement being movably engaged with the guide rail. 
     The support element may be in the form of a support ring, wherein the wafer analysis apparatus further comprises a vacuum suction means incorporated in the support element, wherein the top surface of the support element has one or more cavities and wherein the vacuum suction means enables at least partial air evacuation of the one or more cavities of the support substrate that are in contact with the portion of the wafer. 
     In accordance with a fourth aspect of the present invention there is provided a method of sequentially examining wafers by a wafer analysis apparatus, the method comprising: providing the wafer analysis apparatus with a wafer from a sequence of wafers for examination; comparing an image of a die portion of the wafer against a stored corresponding die portion of a reference image upon which the wafer analysis apparatus is calibrated to perform wafer testing; and adjusting the position of the wafer until a substantial alignment exists between the die portion of the wafer and the stored corresponding die portion of the reference image. 
     The method may further comprise positioning a probe card of the wafer analysis apparatus to contact a portion of the front side of the wafer corresponding to the location of the support element; and positioning a solid immersion lens of the wafer analysis apparatus to press against the back side of the wafer within an opening of the support element. 
     The method may further comprise moving the wafer away from the support element to allow for analysis of a next wafer of the sequence of wafers. 
     The support element may have a vacuum suction means incorporated therein, wherein the top surface of the support element has one or more cavities and wherein the vacuum suction means enables at least partial air evacuation of the one or more cavities of the support substrate that are in contact with the portion of the wafer. 
     In accordance with a fifth aspect of the present invention there is provided a wafer analysis apparatus for sequentially examining wafers, the wafer analysis apparatus comprising: an imager for capturing an image of the wafer; a processor; and a memory for storing a reference image upon which the wafer analysis apparatus is calibrated to perform wafer testing and for storing computer program code, the computer program code configured to, with the processor, cause the wafer analysis apparatus to perform: comparing a die portion of the image of the wafer against a stored corresponding die portion of the reference image; and sending an alert when the position of the wafer is adjusted until a substantial alignment exists between the die portion of the wafer and the stored corresponding die portion of the reference image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Example embodiments of the invention will be better understood and readily apparent to one of ordinary skill in the art from the following written description, by way of example only, and in conjunction with the drawings, in which: 
         FIG. 1  is a top plan view of a typical wafer stage. 
         FIG. 2  is a top plan view of another typical wafer stage. 
         FIG. 3  is a top plan view of another typical wafer stage. 
         FIG. 4  is a schematic cross sectional view of a wafer stage according to an embodiment of the present invention. 
         FIG. 5   a  is a perspective view of part of a wafer stage according to an embodiment of the present invention. 
         FIG. 5   b  is a perspective cross sectional view of the wafer stage of  FIG. 5   a.    
         FIG. 6  is a cross sectional view of a detail of the wafer stage according to an example embodiment. 
         FIG. 7  is a cross sectional view of a detail of a wafer stage according to an alternate embodiment of the present invention. 
         FIG. 8  is a perspective view of a support ring with vacuum chuck for use with a wafer stage according to an example embodiment. 
         FIG. 9  shows a detail of the wafer stage according to an example embodiment. 
         FIG. 10  shows a flow-chart illustrating a method of supporting a wafer for inspection according to an example embodiment. 
         FIGS. 11 and 12  each shows a top plan view of a wafer stage according to an example embodiment. 
         FIG. 13A  shows a cut away isometric view of an actuating mechanism according to an example embodiment. 
         FIG. 13B  shows a top view of the actuating mechanism of  FIG. 13A . 
         FIG. 14  shows a flow chart illustrating a method of sequentially examining wafers by a wafer analysis apparatus according to an example embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The example embodiments described provide a wafer stage comprising a platform for supporting a wafer such that the wafer is suspended above a cavity of the platform and a support structure disposed substantially underneath the cavity. The wafer stage is adapted for relative movement with respect to the platform and for supporting a portion of the wafer. 
       FIG. 4  is a schematic cross sectional view of a wafer stage  400  according to an embodiment of the present invention. The wafer stage  400  advantageously allows optical access to the back side of the wafer  413  and comprises a base plate  401 , the base plate  401  supporting a wafer transport  402 , a bridge structure  403  and an anchor element  404 . A wafer probe card  405  is secured to the bridge structure  403  and a tester interface  406  is connected to the probe card  405  from above. An electronic test system (tester) test head  407  is aligned to guide pins  408 , and can be lowered onto and locked to the tester interface  406  for interconnection. The wafer transport  402  allows X, Y, Z and Theta movements and supports a wafer holder  409 . On the wafer holder  409 , the wafer  413  is suspended across a cavity  415 . The wafer transport  402  moves to align a selected die on the wafer  413  for electrical contact with the wafer probe card  405 . A support bar  410  is disposed within a slot formed in the wafer holder  409  and is constrained to move substantially only in the Z direction by the anchor element  404  with a vertically positioned linear guide  411 . 
     An opening  416  on the support bar  410  is centered about the wafer probe card  405 . A support ring  412  is disposed around the circumference of the opening  416  and projects from the top surface of the support bar  410  and abuts the back side of the wafer  413 . The top surface of the support ring  412  is flush with the top surface of the wafer holder  409 . The constraint on the support bar  410  due to the anchor  404  advantageously allows the support ring  412  to remain stationary in the X and Y direction and thus to the wafer probe card  405 . 
     In operation, the wafer  413  is moved by the wafer transport  402  in the X, Y or Theta direction with respect to the wafer probe card  405  and support ring  412 . After alignment, the wafer transport  402  moves in the Z direction to contact the selected die on the wafer  413  to the probe pins of the wafer probe card  405 , and the support bar  410  and support ring  412  move together with the wafer holder  409 . At the contact position, the support ring  412  advantageously prevents the wafer  413  from bending due to the force exerted by the probe pins. An optical lens  414  that is mounted on a scope transport (not shown) can now be moved into position to image the back side of the wafer  413  through the opening  416  on the support bar  410  and through the support ring  412 . 
       FIGS. 5   a  and  5   b  are a perspective view and a perspective cross sectional view respectively of a detail of an embodiment of a semiconductor wafer stage  500 , comprising a wafer holder  509  with a cavity  515 , a wafer ring  518  mounted at a circumference of the cavity  515  for receiving and supporting a wafer (not shown), and a support bar  510 . Slots  517   a  and  b  are provided within the wafer holder  509  and aligned across the cavity  515 . In  FIGS. 5   a  and  5   b , the wafer probe card, bridge structure, tester interface and electronic test system (tester) test head (compare  FIG. 4 ) have been omitted for better clarity. 
     The wafer holder  509  is supported by a wafer transport  502  with X, Y, Z and Theta movement. Wafer rings  518  of different inner diameter according to the size of a wafer undergoing testing can be selectively mounted to the wafer holder  509 , using latches e.g.  509   a  in the example embodiment. In this embodiment, the wafer ring  518  supports the wafer at three points, more particular at prongs or protrusions  518   a, b , and  c.    
     The support bar  510  is disposed within the wafer holder  509  in the slot  517 ; and has one end attached to a linear guide  511 . A hollow support element here in the form of a support ring  512  is provided on the support bar  510 . The support ring  512  is formed on the top surface of the support bar  510 , projecting substantially upwards, and approximately at the support bar&#39;s  510  mid-point. The support bar  510  comprises an aperture  516  aligned with the support ring  512  enabling optical inspection of the backside of a wafer (not shown) through the support ring  512  and the aperture  516 . The support ring  512  in this example embodiment is flush with the top surface of the wafer ring  518  and prongs  518   a, b, c.    
     The support ring  512  is advantageously coated with or formed from a static dissipating low-friction material such as PEEK Bearing Grade, or Static-Dissipative Acetal Copolymer, or other similar engineering plastic materials. PEEK Bearing Grade reduces friction between the support ring  512  and the back side of the wafer (not shown). Similarly, the support bar  510  or the slot  517  may be coated with or formed from a static dissipating low-friction material such as PEEK Bearing Grade to reduce friction between the support bar  510  and the wafer holder  509 . As will be appreciated by a person skilled art, the support bar  510  thus advantageously remains stationary while the wafer transport  502  moves in the X and Y directions during alignment of a selected die with a probe card (not shown). The thickness of the slot is preferably only slightly larger than the thickness of the support bar  510  to facilitate X and Y movement of the wafer holder  509  while preferably substantially minimizing any Z movement or play. The support bar  510  is coupled to an anchor element  520  mounted on a base  501  via the linear guide  511 . The anchor element  520  holds the support bar  510  stationary, in the X and Y directions, with respect to the probe pins of the wafer probe card (not shown), while allowing the wafer transport  502  and wafer holder  509  to position the selected die in the X or Y direction for alignment with the probe pins. On the other hand, the linear guide  511  allows the support bar  510  to move together with the wafer transport  502 , wafer holder  509  and wafer (not shown) in the Z direction to make contact with the probe pins. 
     The support ring  512  is in contact with and supporting the wafer during probing. This advantageously allows the support ring  512  to continuously support the selected die on the wafer undergoing testing so that bending induced by the probe card is minimized. This can ensure a good electrical contact between the probe pins and the die. 
     In the example embodiment, the X movement is implemented by way of a pair of linear guides  502   a  and a linear servomotor  502   b . The Y movement is implemented by way of a pair of linear guides  502   c  and a pair of linear servomotors  502   d . The Z movement is implemented by way of four linear guides  502   e  and four voice coils  502   f . The Theta movement is implemented by way of a pair of curved guides (not shown) and a linear servomotor  502   g  coupled to the wafer holder  509 . The movement axes are stacked in the order X, Y, Theta and Z. 
     The wafer transport  502 , wafer holder  509 , support ring  512  and support bar  510  are fabricated from aluminum in the example embodiment. However, it will be appreciated that other materials may be used in different embodiments. 
       FIG. 6  is a cross sectional view of a details of a wafer stage according to an example embodiment showing the wafer prober interface  621  and probe card  605 . The wafer prober interface  621  is supported by a bridge structure (or headplate)  603 . A wafer  613  is placed across the cavity  615  and supported by the wafer ring  618  and the support ring  612 . During testing of a plurality of dies on the wafer  613 , the wafer transport (not shown) sequentially positions each die on the wafer  613  to be tested underneath the probe card  605 . As described above, the support ring  612  remains stationary in the X and Y directions with respect to the probe card  605 . This advantageously allows the support ring  612  to continuously support a perimeter around the die being probed by the probe card  605 . A microscope lens  614  is disposed below the wafer  613  and the support ring  612 , to facilitate observation of the back-side of the corresponding die undergoing electrical testing. The microscope lens  614  is mounted on a scope transport  622 , which is independent of the wafer transport, and can move to and focus on a region of interest on the die. 
       FIG. 7  shows a cross sectional view of a wafer stage according to an alternate embodiment of the present invention, comprising a vacuum suction means incorporated in a support ring  712 , such that the support ring  712  facilitates in immobilizing a wafer  713 . The vacuum suction means advantageously provides a suction force to prevent lift-off of the wafer  713 . For example, when a solid immersion lens (SIL)  723  is used for optical imaging, it has to be pressed against the back side of the wafer  713  to eliminate the air gap between the surface of the SIL  723  and the wafer  713 . In this instance, the suction force to counter the force exerted by the SIL  723  prevents the wafer  713  from lifting-off. In this embodiment, the vacuum suction means is in the form of a plurality of vacuum tubing e.g.  724  disposed within the support bar  710  and coupled to internal vacuum conduits in the support ring  712 . 
       FIG. 8  is a perspective view of a support ring  812  for a wafer stage according to an example embodiment, comprising a rim  825  disposed on the support ring&#39;s  812  top surface. The support ring  812  and rim  825  comprises four cavities  826   a / 826   b / 826   c / 826   d  to enable air to be sucked out by one or more vacuum pumps (not shown) via the vacuum tubing (compare e.g.  724   FIG. 7 ). During probing of dies at the edge of a wafer, the whole rim  825  may not be in contact with the back side of the wafer. By selectively evacuating the cavities which remain in contact, the support ring is still advantageously able to immobilize the wafer. With reference to  FIG. 9 , the wafer  900  is supported at three points  902   a, b , and  c  by the wafer ring  902 , and held in place using stoppers  901   a, b , and  c  in this example implementation. The support ring  904  has a minimum clear aperture to accommodate the microscope objective (not shown). When probing edge dies e.g.  906 , part of the support ring  904  is not under the wafer  900 . By selectively evacuating the cavities that remain in contact, the support ring  904  is still advantageously able to immobilize the wafer. With this arrangement, while the some dies near the three support points  902   a, b , and  c  may still not be probed, this number is advantageously minimal compared to a scenario where the entire circumference of the wafer is supported. 
       FIG. 10  shows a flow chart  1000  illustrating a method of supporting a wafer for inspection according to an example embodiment. At step  1002 , the wafer is supported such that a backside of the wafer is suspended above a cavity of a platform. At step  1004 , a support structure disposed substantially within the cavity is provided for supporting a portion of the wafer. At step  1006 , relative movement of the platform with respect to the support structure is effected for alignment of the wafer with respect to a probe. 
     Embodiments of the present invention can advantageously provide continuous structural support around an area of a wafer undergoing testing so that deformation of the wafer may be minimized. This can ensure good electrical contact between the probe card and the die under test. Further, the absence of fixed support structures on the back side of the wafer may advantageously allow substantially all areas on the back side of the wafer to be observed without or with reduced obstructions and may also allow a microscope to move without or with reduced impediment in an X or Y direction during testing. In addition, optical aberrations may be minimized because no intermediate transparent element is present between wafer and the lens of the microscope. 
       FIG. 11  shows a top plan view of a wafer stage  1100  according to an example embodiment. The wafer stage  1100  includes a support structure  1102  within which an opening  1106  of a support element  1104  is formed. The support element  1104  supports a portion of a wafer (not shown) such that a back side of the wafer lies across the opening within the support element  1104 . In  FIG. 11 , the portion of the wafer that is supported by the support element  1104  is within the wafer  1108  circumference. In the embodiment shown in  FIG. 11 , the support element  1104  is in the form of a support ring, although other shapes are possible. 
       FIG. 12  shows a top plan view of the wafer stage of  FIG. 11  with a platform  1209  that is positioned over the support structure  1102 . The platform  1209  has a cavity. A wafer ring  1218  is mounted at a circumference of the cavity for receiving and supporting a wafer (not shown) along the wafer rim. 
     A vacuum suction means may be incorporated in the support element  1104 . The top surface of the support element  1104  may have one or more cavities through which the vacuum suction means facilitates at least partial air evacuation of the one or more cavities of the support element that are in contact with the portion of the wafer. As a result, the wafer stage  1100  is capable of allowing a wafer to be direct-docked, hard-docked or soft-docked to a tester while undergoing analysis by a microscope (not shown) through the back side of the wafer concurrently. In an implementation, a lens may be able to simultaneously land on the back side of the semiconductor die being examined while it is being tested by a tester through a probe card on the topside. 
     However, landing of the lens of the microscope on the back side of the wafer may cause physical deformity on the wafer due to the pressure the lens of the microscope placed on the back side of the wafer. Therefore, in order to prevent this disadvantage, in an embodiment of the present invention, the microscope has a solid immersion lens having a diameter of approximately 2.5 mm or more. Advantageously, these microscopes result in better landing because the pressure, caused by the solid immersion lens pressing on the back side of the wafer, is lessened. 
     Further, in an embodiment, the support element  1104 , the microscope or both may be adapted for relative movement with each other to align the solid immersion lens to predetermined locations on the back side of the wafer within the opening  1106  of the support element  1104 , so as to allow the solid immersion lens of the microscope to press against the back side of the wafer. 
     In an embodiment, the microscope may be coupled to an actuating mechanism which allows tilting of the microscope. The actuating mechanism is described in further detail with respect to  FIGS. 13A and 13B . 
       FIG. 13A  shows a cut away isometric view of an actuating mechanism  1300 , in accordance with one embodiment, in which some of the parts are cut away to show the essential components.  FIG. 13B  shows a top view of the actuating mechanism  1300 . 
     The actuating mechanism  1300  is coupled to a microscope  1302  for tilting the microscope  1302  within one or more of first and second planes  1350 ,  1352  that each form an angle with the support element (denoted as reference numeral  1104  in  FIGS. 11 and 12 ), wherein the first plane  1350  is perpendicular to the second plane  1352 . The actuating mechanism  1300  may be configured to allow the microscope  1302  to tilt over a solid angle of 0.003 steradian. Tilting of the microscope  1302  within one or more of first planes  1350  is provided by a first roller arrangement  1304  movably engaged with a first pair of guide rails  1310 . Similarly, tilting of the microscope  1302  within one or more of second planes  1352  is provided by a second roller arrangement  1312  movably engaged with a second set of guide rails  1318 . Further detail on the mechanical arrangements that allow the microscope  1302  to tilt is provided below. 
     As shown in  FIGS. 13A and 13B , the microscope  1302  is coupled to the first roller arrangement  1304 . The first roller arrangement  1304  may include a base plate  1306  upon which the microscope  1302  is mounted and rollers  1308  attached on the base plate  1306 . The first roller arrangement  1304  is movably engaged with a first pair of guide rails  1310  via the rollers  1308 . The movement of the first roller arrangement  1304  along the first pair of guide rails  1310  allows tilting of the microscope  1302  within the one or more of the first planes  1350 . 
     Each guide rail  1310  of the first pair of guide rails  1310  is mounted on a respective wall  1348 ,  1346  of a rectangular hollow box shaped structure  1314 . These walls  1348 ,  1346  are disposed opposite to each other, with inner surfaces on which each of the guide rail  1310  is mounted facing one another. For the sake of simplicity, only the wall  1348  is shown in  FIG. 13A . The walls  1348 ,  1346  are disposed opposite to each other by being coupled to a further wall  1344  of the rectangular hollow box shaped structure  1314 . 
     The second roller arrangement  1312  may include the rectangular hollow box shaped structure  1314  and rollers  1316  (partially hidden in both  FIGS. 13A and 13B ) mounted on an outer surface of the further wall  1344 . The first pair of guide rails  1310 , with which the first roller arrangement  1304  is movably engaged, is thus mounted on the second roller arrangement  1312 . 
     The second roller arrangement  1312  is movably engaged with the second set of guide rails  1318  via the rollers  1316 . The second set of guide rails  1318  may be mounted on a fixed plate  1320  which faces the further wall  1344  of the rectangular hollow box shaped structure  1314 . Given that the microscope  1302  is coupled to the second roller arrangement  1312  through the rollers  1308  of the first roller arrangement  1304 , the movement of the second roller arrangement  1312  along the second set of guide rails  1318  allows tilting of the microscope  1302  within the one or more of the second planes  1352 . 
     The embodiment shown in  FIGS. 13A and 13B  use two guide rails  1310 ,  1318  to allow tilting of the microscope  1302  within one or more of first and second planes  1350 ,  1352 . However, it will be appreciated that in another embodiment (not shown), only one guide rail is required to allow tilting of the microscope  1302  within one or more of first and second planes  1350 ,  1352 . 
     Actuators  1322 ,  1324  may be used to control the rollers  1308 ,  1316  to tilt the microscope  1302  within the one or more of first and second planes  1350 ,  1352 . The actuators  1322 ,  1324  may be controlled by a computer which forms part of the wafer probing system. 
     It was mentioned above that a vacuum suction means may be incorporated in the support element  1104 . Advantageously, the vacuum suction means provide a suction force to prevent lift off of the wafer when the solid immersion lens (SIL) of the microscope presses against the back side of the wafer. For example, when the SIL is used for optical imaging, it has to be pressed against the back side of the wafer to eliminate the air gap between the surface of the SIL and the wafer. In this instance, a suction force is provided by the vacuum suction means to counter the force exerted by the SIL and prevent the wafer from lifting-off. 
       FIG. 14  shows a flow chart  1400  illustrating a method of sequentially examining wafers by a wafer analysis apparatus. Such wafers may, for example, belong to a sequence of identically manufactured wafers, with each wafer being loaded into the wafer analysis apparatus. It will be appreciated that the method described below may be implemented via software or computer programs executed by a processor in a computer. 
     At step  1402 , the wafer analysis apparatus is provided with a wafer from a sequence of wafers for examination, wherein the support element supports a portion of the wafer. At step  1404 , an image of a die portion of the wafer is compared against a stored corresponding die portion of a reference image upon which the wafer analysis apparatus is calibrated to perform wafer testing. At step  1406 , the position of the wafer is adjusted until a substantial alignment exists between the image of the die portion of the wafer and the stored corresponding die portion of the reference image. A mathematical  2 D cross-correlation method may, for example, be used to perform the alignment between the image of the die portion of the wafer and the stored corresponding die portion of the reference image. Further, in one embodiment, the wafer is provided on a platform of the wafer analysis apparatus, so that it is movement of the platform of the wafer analysis apparatus that provides the required adjustment to establish the alignment. 
     Following which, a probe card of the wafer analysis apparatus may be positioned to contact a position of the front side of the wafer corresponding to the location of the support element and a solid immersion lens of the wafer analysis apparatus may be positioned to press against the back side of the wafer within an opening of the support element. The above method may then be repeated for subsequent wafers. 
     In a conventional wafer analysis apparatus, a user has to manually adjust the wafer analysis apparatus when a new wafer is to be examined. Since the accuracy of the positioning is required to be within 3-5 microns for examination of the dies, manual adjustment of the conventional wafer analysis apparatus may be rather time consuming. Thus a method, in accordance with the embodiment shown in  FIG. 14  provides a more efficient way to perform sequential testing of wafers. 
     In the method shown in  FIG. 14 , an image of a reference die is taken and stored in the wafer analysis apparatus. Features such as pads, tracks within the wafer are mapped out in the wafer analysis apparatus, so that these features (in addition to die portions) can be aligned to corresponding features of an image of the wafer being tested, as means to correctly position the wafer being tested. To automatically navigate the wafer analysis apparatus such that the die is in position for examination, the wafer analysis apparatus will collect information on the position of the die to be examined. This may be performed by taking an image of the die to be examined. The wafer analysis apparatus then compares the image of the die to the referenced image previously stored. Since normally all the dies in a wafer are replicates and the spacing between the dies in the sequence is regular, by comparing the images, the wafer analysis apparatus would be able to calculate and/or determine an approximate X-Y position to move the wafer, via the support element. The steps of taking an image of the die, comparing with the referenced image, determining the approximate X-Y position and moving the wafer analysis apparatus may be repeated for fine position adjustments. When the comparison of the images shows that the image of the die to be examined is within a predetermined threshold from the reference image, the wafer analysis apparatus determines that the die is in position for examination. Subsequently, the wafer analysis apparatus lands the probe card and the microscope on the wafer for examination. Since the vertical movement of the wafer stage in the Z direction is the same for all the dies in the sequence, landing of the probe card and the microscope may be performed automatically. In an embodiment, an option to manually control the final landing of the last 10 microns of the vertical movement in the Z direction may be provided as a safety precaution measure. 
     The method of  FIG. 14  may be performed in a wafer analysis apparatus that comprises: an imager, a processor and a memory. The imager, which may be a camera, captures an image of the wafer. The memory is for storing a reference image upon which the wafer analysis apparatus is calibrated to perform wafer testing. The memory also stores computer program code, the computer program code configured to, with the processor, cause the wafer analysis apparatus to compare a die portion of the image of the wafer against a stored corresponding die portion of the reference image and sending an alert when the position of the wafer is adjusted until a substantial alignment exists between the die portion of the wafer and the stored corresponding die portion of the reference image. In one embodiment, the wafer analysis apparatus includes a platform (see reference numeral  1209  of  FIG. 12 ) to hold the wafer, so that the platform is moved to establish the alignment between the wafer image and the reference image. 
     After the alert is sent by the processor, both a probe card and a microscope (along with its solid immersion lens) of the wafer analysis apparatus can be operated to land on the wafer to examiner the wafer. 
     It will be appreciated by a person skilled in the art that numerous variations and/or modifications may be made to the present invention as shown in the embodiments without departing from a spirit or scope of the invention as broadly described. The embodiments are, therefore, to be considered in all respects to be illustrative and not restrictive.