Abstract:
A method of performing alignment of an array of probe tips of a probe card to corresponding contact pads for wafer probing applications by performing the steps of: obtaining a backside image of the wafer; overlaying a mapping of the contact pads over the backside image; selecting contact pads as landing points; obtaining an image of the probe tips array; comparing the landing points to corresponding positions of probe tips; and, if the positions of probe tips are not aligned with the landing point, rotating the probe card to align the positions of probe tips to the landing points.

Description:
RELATED APPLICATIONS 
     This application claims priority benefit from U.S. Provisional Patent Application Ser. No. 61/114,430, filed Nov. 13, 2008. 
    
    
     BACKGROUND 
     1. Field 
     This invention is in the field of semiconductor wafer probing and, more specifically, relates to alignment of a probe tip to a device under test. 
     2. Related Art 
     Semiconductor wafers generally undergo probing using a probe card which makes contact to conductive pads on top of the IC. As technology advances, probing of wafers becomes more important in order to ensure proper designs and acceptable yield. However, as technology advances, the number of pads increases while the size of individual pads and the pitch between pads decrease. This makes it much harder to ensure that all contacts of the probe card make proper contact to the corresponding pads. 
     Device probe pads are the topmost layer of a chip. Prior art techniques for PTPA (Probe To Pad Alignment) require an unobstructed view of such pads from the backside of the wafer, and an infrared camera to see them through the silicon substrate. However, modern fabrication techniques for semiconductors place many layers of interconnecting metal (as many as 10 or more) between the topmost and bottommost layers in a device. These metal layers block all visibility of probe pads from below and thus render such existing techniques inadequate. 
       FIGS. 1-3  illustrate prior art arrangements for probe pins alignment. In  FIG. 1 , wafer  100  is held by a chuck  105 , which is attached to an x-y stage  110 . A camera  115  is also attached to the x-y stage  110 . To perform the alignment, the stage is scanned so that the camera  115  can take pictures of the probe tips  120  of probe card  125 . The image is used to form a mapping of the tips&#39; locations. In  FIG. 2 , an infrared camera  215  is used to image both the pads  202  on the wafer  200  and the probe tips  220  of probe card  225 . As shown in  FIG. 2 , in position A, camera  215  is focused on the plane where the pads are, so that it images the pads  202  through the silicon wafer (hence the use of infrared, to which silicon is transparent). At position B, camera  215  is focused on the plane where pins  220  are, so that it images the pins  220 , also through the silicon wafer. When no or very few metallic lines are fabricated on the wafer, the IR camera can image both the pads and the probe tip through the silicon. This is shown in  FIG. 3 , wherein the pads are shown as squares and the probe tips are shown as round. However, when several layers of metal lines are fabricated on the wafer, the lines block the view of the camera and prevents imaging the pads and/or the probe tips, so that proper alignment cannot be verified using this technique. 
     Therefore, new apparatus and methods are needed in order to ensure proper alignment of probe tips to contact pads, even when several layers of metal lines are fabricated on the wafer. 
     SUMMARY 
     The following summary is included in order to provide a basic understanding of some aspects and features of the invention. This summary is not an extensive overview of the invention and as such it is not intended to particularly identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts of the invention in a simplified form as a prelude to the more detailed description that is presented below. 
     Embodiments of this invention solve the problem of proper alignment of probe tips to contact pads when metal lines obstruct the view through the silicon. Aligning probe card pins to wafer device pads using upward-looking camera cannot be performed when the view of pins and pads is obscured due to probe card technology and multiple metal layer semiconductor fabrication technology. Accordingly, various embodiments are described which use a combination of views of the probe pin array, device images, and/or device CAD (Computer-Aided Design) map overlaid, so as to achieve alignment in X, Y, and Theta (rotation). Certain embodiments are facilitated by the upwards-looking infrared camera subsystem of a standard emission or laser probing microscope. 
     According to one embodiment, the device&#39;s CAD information is overlaid on a captured image of the actual device to provide the necessary probe pad location information. Therefore, the conventional view of the top layer through the backside of the device is not require. This “virtual device” information is then compared spatially with probe card pin array information taken by the upwards looking camera when the wafer is not present. Adjustments can then be made to the relative positions of wafer and probe card until the two are brought into alignment. 
     According to another embodiment, a high-resolution camera is used to take an image of the actual top side of a die on the wafer to be aligned, thereby showing the locations of the contact pads. The camera may be mounted in any location which affords an unobstructed view of the wafer top surface. In fact, it is not necessary that the camera be mounted to the probing system because the image may ultimately be manipulated by the probing system to match the scale and position of backside images generated by the IR microscope system in the tool. Once the image is captured, it may be manipulated digitally to scale, rotate, clip, and otherwise match it to the backside image of the die as captured by the upward looking IR camera. The image is then rendered translucent and merged to the backside image so as to overlay the two images. That is, the top side image is matched to edge features which are visible both from the top side and back side of the die and a composite “virtual transparent” die image is created. 
     The “transparent” feature refers to the fact that a normal image of the device backside, as captured by an IR microscope, cannot see through the device to its top side because intervening metal layers in the semiconductor device obscure the image. By merging the front side and back side images into a single image, we create an image similar to what would be seen if the device were transparent. Because this image is based on actual images of the die to be aligned it can be used in the wafer prober Probe-to-Pad alignment process. 
     According to yet another embodiment, a full composite image of backside, CAD, and topside is created to improve the accuracy and effectiveness of this method. However, when a CAD design is not available, the method can be performed without overlay of the CAD design. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, exemplify the embodiments of the present invention and, together with the description, serve to explain and illustrate principles of the invention. The drawings are intended to illustrate major features of the exemplary embodiments in a diagrammatic manner. The drawings are not intended to depict every feature of actual embodiments nor relative dimensions of the depicted elements, and are not drawn to scale. 
         FIGS. 1-3  illustrate prior art arrangements for probe pins alignment. 
         FIG. 4  illustrates a general schematic of a probing microscope which can be used for implementing embodiments of the invention. 
         FIG. 5  is a general schematic depicting major components of a probing microscope which may be used for implementing embodiments of the invention. 
         FIG. 6  shows a modern device as viewed with an infrared microscope. 
         FIG. 7  shows the infra-red image of device with corresponding CAD design data taken from a CAD database ( FIG. 5 ) and overlaid and registered to the infra-red image using physical device positions. 
         FIG. 8  shows desired probe positions identified and highlighted for comparison to the probe tip array. 
         FIG. 9  shows an image of the probe tip array as viewed with an infrared microscope. 
         FIG. 10  shows the probe tip array position compared to the probe pad locations on the device after alignment is completed. 
         FIG. 11  illustrates a flow chart of a process according to an embodiment of the invention for aligning probe card pins to contact pads of die on a wafer. 
         FIG. 12  illustrates an embodiment that can be used in such circumstances to reach alignment without the need for a special camera and optical setup to simultaneously view wafer dies and probe cards. 
         FIG. 13  is an example of a front image rendered translucent and merged to the backside image of the wafer. 
         FIG. 14  illustrates a process for aligning the probe card pins to the contact pads when no CAD design data is available. 
         FIG. 15  illustrates yet another embodiment according to which, when CAD design data is available, it is used as verification for the accuracy in selecting the contact pads for alignment. 
         FIG. 16  illustrates an example of an image showing selected contact pads (visible from the topside image and confirmed by the CAD data) are marked for alignment. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the invention provide apparatus and method for merging wafer die images from multiple sources to create a pseudo-transparent wafer die image to facilitate alignment of wafer dies to wafer probe cards. This inventive technique is applicable to all wafer types (including wafers with multiple metal layers which obscure through-wafer IR microscopy) and probe card types (including vertical, “cobra” type probe cards and other probe card types with no center viewing port). 
     While the invention may be implemented using a verity of arrangements, it is especially beneficial for use in probing microscopes, such as, e.g., emission microscopes and laser voltage probers such as the Meridian™ WaferScan available from DCG Systems of Fremont. Therefore, various embodiments of the invention will be described as implemented in such systems. 
     A general schematic of a probing microscope which can be used for implementing embodiments of the invention is depicted in  FIG. 4 . The system illustrated in Figure is particularly suitable for timing, emission, failure, and other testing of dies on wafers, especially from the backside through the substrate. The system is shown as operating in conjunction with a commercially available automated testing equipment  405  (ATE). The ATE  405  generally comprises a controller, such as a preprogrammed computer  481 , and a test head  424  which comprises an probing card  425  used to deliver signals (vectors) generated by the controller  481  to the device under test (DUT—in this context, the selected die on the wafer)  410  in a manner well known in the art. Specifically, the ATE  405  is used to generate signals that stimulate the DUT  410  to perform various tasks, as designed by the chip designer to check and/or debug the chip. 
     In the embodiment depicted in  FIG. 4 , the ATE test head  424  is placed on top of a vibration isolated test bench  415 , while the chamber  400  that houses the entire optics, imaging and sensing of the microscope system, is situated below. This provides a tremendous advantage as it allows the system to be used with any type and size of ATE without interference with, or making modification to any of the elements inside chamber  400 . Rather, the ATE is used to place the DUT from above, so that it is visible to the optics via opening  485 . A stage inside the chamber  400  enables placing of the collecting optics at any locations within the opening  485 . 
       FIG. 5  is a general schematic depicting major components of a probing microscope which may be used for implementing embodiments of the invention. In  FIG. 5 , dashed arrows represent optical path, while solid arrows represent electronic signal path. The optical paths represented by dashed lines are generally made using fiber optic cables. Probing system  500  comprises a laser light source, e.g., a CW or mode-locked laser source MLL  510 , an optical bench  512 , and data acquisition and analysis apparatus  514 . The optical bench  512  includes provisions for mounting the wafer  560  and includes beam optics  525 . The beam optics may include various elements to shape the beam, generally shown as beam manipulation optics, BMO  535 , and elements for pointing and/or scanning the beam over the DUT, such as a laser scanning microscope, LSM  530 . A computer  540  or other device, e.g., ATE, may be used to provide power and/or signals,  542 , to the DUT  560  via the probing card, and may provide trigger and clock signals,  544 , to the mode-locked laser source  510  and/or the analysis apparatus  514 . The analysis apparatus,  514 , includes workstation  570 , which controls processes and displays data from the signal acquisition board  550  and the optical bench  512 . 
     In operation, computer  540 , which may be a conventional ATE, generates test vectors that are electrically fed to the DUT  560 . When emission testing is performed, the optics collects the faint light that is emitted from active devices on the DUT, and directs the collected light to the photodetector  536 . The photodetector, e.g., avalanche photodiode (APD) converts the collected light into an electrical signal that is sent to the signal acquisition board. The signal can be then analyzed using the computer  570 . 
     On the other hand, when laser probing is performed, the ATE also sends sync signal  544  to the mode-locked laser source  510 , which emits a laser beam. The beam optics  525  is then used to point the beam to illuminates various positions on the DUT. The beam reflects from the DUT, but the reflection is perturbed by the DUT&#39;s response to the test vectors  542 . This perturbed reflection is detected by photodetector  536 , which converts it into an analog signal. The analog signal is acquired by the signal acquisition board  550  and is fed to computer  570 , where it is displayed as a waveform corresponding to the perturbed reflection from the DUT. By correlating the timeline of the waveform to that of the ATE, the response of the DUT can be analyzed. 
     As can be understood, in order to make the systems of  FIGS. 4 and 5  operable, the electrical vector signals need to be properly communicated to the DUT. For this purpose, the pins on the probe adapter must be precisely placed on the pads of the DUT. Improper alignment may cause incorrect signals or no signals being communicated to the DUT, and may damage the DUT or the pins. 
     When using optical probers such as those illustrated in  FIGS. 4 and 5 , one can use the laser of the prober to take an infrared image of the DUT, e.g., by scanning the laser using the LSM.  FIG. 6  shows a modern device as viewed with an infrared microscope. As can be seen, the inner metal layers completely obscure any view of probe pads or probe tips above the device. Therefore, existing methods of alignment, which rely upon a view of both probe pads and probe tips cannot be used on this device. 
       FIG. 7  shows the backside infrared image of device with corresponding CAD design data taken from a CAD database ( FIG. 5 ) and overlaid and registered to the infrared image using physical device positions. In  FIG. 7 , the circles indicate the positions of the top layer probe pads. The combination of the device image taken by the upward-looking camera and its overlaid CAD design form a “virtual device” which can be compared with the physical probe tip array. This overcomes the problem of attempting to take an image of the pads through the obscuring metal layers of the DUT. 
       FIG. 8  shows probe positions identified and highlighted for comparison to the probe tip array. That is, using the virtual device image of  FIG. 7 , several probing points are marked for the next step of registration. This can be done by simply having the operator “point and click” to desired probing point using a mouse or other pointing device coupled to computer  570 . Notably, not all of the probing points need to be highlighted, but rather only sufficient probing points to enable recognizing registration errors in x-y and theta (rotation). 
       FIG. 9  shows an image of the probe tip array as viewed with an infrared microscope. To obtain this image, the wafer has been moved to a holding position so that an unobstructed view of the probe tip array is available. Then, the infrared camera is used to obtain an image of the probe tip array. Wafer probe positions previously identified are shown overlaid on the image for position comparison. The probe card is then moved in theta (rotation) until the probe card angle matches the wafer angle. The wafer is then moved in X and Y until its position matches the probe array position.  FIG. 10  shows the probe tip array position compared to the probe pad locations on the device after alignment is completed. 
       FIG. 11  illustrates a flow chart of a process according to an embodiment of the invention for aligning probe card pins to contact pads of die on a wafer. At step  1100  the wafer is loaded to the system, wherein the system has an infrared imager with a view of the backside of the wafer. In this context, backside means the surface opposite to the surface having the contact pads. As explained with respect to  FIGS. 4 and 5 , the laser and infrared imaging optics are positioned to observe the backside of the wafer since that is the surface where photon emission and laser probing can be performed. Once the wafer is loaded, the infrared imager is used to scan the selected die on the wafer to obtain an image from the lower surface of the wafer. 
     At step  1110  the CAD design data of the wafer is loaded from a CAD database (see  FIG. 5 ) and the data is aligned and registered to the laser image of the die. That is, various visible elements in the laser image are registered to their CAD design data. An example is shown in  FIG. 7 , wherein device elements are shown as rectangles. The CAD data also includes the contact pads, and these are shown in  FIG. 7  as circles. Since the CAD data is registered to actual device elements on the die, the location of the contact pads indicated by the CAD data should be registered to the actual location of the contact pads, although the contact pads are obstructed and therefore not visible in the infrared image. 
     In step  1115 , selected contact pads are marked for matching to the probe card&#39;s pins. The pads can be selected by simply having an operator use a pointing device, such as a mouse, clicking on the desired contact pads, thereby marking these pads for the computer processor, which generates the graphic marks as illustrated in  FIG. 8 . 
     In step  1120  the wafer is removed and is parked, e.g., on parking pins  426 , such that the upward-looking imager has clear line of sight to the probe card. At this point, the image of the wafer can be removed from the screen, leaving only the markings for the contact pads. At step  1125  the upward-looking imager is used to image the probe card. The image of the probe card is then overlaid over the markings of the contact pads and at step  1130  the image is examined to see whether the pins of the probe card are rotationally aligned with the markings—see  FIG. 9 . If they are aligned, the process proceeds to step  1135 , wherein the wafer is loaded and is moved in x-y if needed. On the other hand, if at step  1130  the pins are not aligned, the process proceeds to step  1140 , wherein the probe board is rotated until the results of the inspection in step  1130  results in alignment. 
     There are occasions when wafer CAD design information is unavailable.  FIG. 12  illustrates an embodiment that can be used in such circumstances to reach alignment without the need for a special camera and optical setup to simultaneously view wafer dies and probe cards. The system shown in the embodiment of  FIG. 12  is similar to that shown in  FIG. 4 , so similar elements are indicated with similar reference, except that they are in the 12xx series rather than 4xx series. In the embodiment of  FIG. 12 , a high resolution camera is used to take a picture of the top side of the wafer, i.e., the contact pads side. Although not strictly required, a telecentric imaging camera is particularly beneficial for this embodiment, so as to assure a properly scaled view of the entire die, which does not have any distortion of magnification of the image radially from the center of the image. Also, the zoom and focus can be adjusted so that a whole die field of view can be achieved. 
     Then, the upward-looking infrared camera of the probing microscope is used to take an image of the underside of the die, similar to the image taken in the previous embodiment. The top-side digital image is then manipulated digitally to scale, rotate, clip, and otherwise match it to the backside image of the die as captured by the microscope&#39;s upward looking IR camera. The image is then rendered translucent and merged to the backside image. As shown in  FIG. 13 , the top image  1300  shows the pads  1310  and some other features  1305 . The backside image  1340  shows some features that are not visible in the topside image, but also some features  1315  which correspond to the features  1305  which are visible in the topside image. Using these features, the topside image is scaled and manipulated to fit the size and angular position of the backside image. The topside image is then flipped and “pinned” to the backside image using the features that are visible in both topside and backside images, so as to provide a composite image  1360 . That is, the top side image is matched to edge features which are visible both from the top side and back side of the die and a composite “virtual transparent” die image is created. 
     The “transparent” feature refers to the fact that a normal imaging of the device backside, as captured by an IR microscope, cannot see through the device to its top side because intervening metal layers in the semiconductor device obscure the image. By merging the front side and back side images into a single image, an image is created that is similar to what would be seen if the device were transparent. Because this image is based on actual images of the die to be aligned, it can now be used in the wafer prober Probe-to-Pad alignment process in the same fashion as has been described in the previous embodiment using CAD design data. Therefore, no device CAD information is required to use this technique. 
       FIG. 14  illustrates a process for aligning the probe card pins to the contact pads when no CAD design data is available. At step  1400  the wafer is loaded onto the system. At step  1405  the backside of a selected die on the wafer is imaged using the system&#39;s laser and imaging optics. At step  1410  the topside of the same die is imaged using a high resolution camera. Note that the order of steps  1405  and  1410  can be reversed. At step  1415  the topside digital image is digitally manipulated to correspond to the size and be aligned with the backside laser image, and is flipped and merged with the backside laser image to generate a virtual transparent image of the die. At step  1420  selected contact pads (visible from the topside image) are marked for alignment. At step  1425  the wafer is parked, e.g., on parking pins  1226 , and at step  1430  the laser imaging system images the tips of the probe card. At step  1435  it is checked whether the pins are aligned with the marks of the selected contact pads. If so, the wafer is reloaded at step  1440 . On the other hand, if the card is not properly aligned, at step  1445  the probe card is rotated and alignment is checked until the card is aligned. 
       FIG. 15  illustrates yet another embodiment according to which, when CAD design data is available, it is used as verification for the accuracy in selecting the contact pads for alignment. In step  1500  the wafer is loaded and in step  1505  the backside of a selected die is imaged using the system&#39;s laser and imaging optics. At step  1510  the topside of the same die is imaged using a high resolution camera. Note that the order of steps  1505  and  1510  can be reversed. At step  1515  the topside digital image is digitally manipulated to correspond to the size and be aligned with the backside laser image, and is flipped and merged with the backside laser image to generate a virtual transparent image of the die. At step  1520  the CAD design data is overlaid over the virtual transparent image to confirm the alignment of the top image to the backside image. If there are discrepancies, the topside digital image is manipulated to conform to the CAD design data. At step  1525 , selected contact pads (visible from the topside image and confirmed by the CAD data) are marked for alignment. A sample of such an image is shown in  FIG. 16 . At step  1530  the wafer is parked and at step  1535  the laser imaging system images the tips of the probe card. At step  1540  it is checked whether the pins are aligned with the marks of the selected contact pads. If so, the wafer is reloaded at step  1545 . On the other hand, if the card is not properly aligned, at step  1550  the probe card is rotated and alignment is checked until the card is aligned. 
     It should be understood that processes and techniques described herein are not inherently related to any particular apparatus and may be implemented by any suitable combination of components. Further, various types of general purpose devices may be used in accordance with the teachings described herein. It may also prove advantageous to construct specialized apparatus to perform the method steps described herein. The present invention has been described in relation to particular examples, which are intended in all respects to be illustrative rather than restrictive. Those skilled in the art will appreciate that many different combinations of functional elements will be suitable for practicing the present invention. Moreover, other implementations of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. Various aspects and/or components of the described embodiments may be used singly or in any combination in the relevant arts. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.