Abstract:
A testing system operable to accurately position a plurality of contact electrodes relative to a plurality of electrical contacts is disclosed. For one embodiment, the testing system comprises a first imaging system coupled to a wafer chuck. The wafer chuck is used to place the electrical contacts of a wafer in contact with the plurality of electrodes. To facilitate accurate positioning between the wafer electrical contacts and the contact electrodes, the first imaging system is configured to locate the plurality of contact electrodes. The testing system also comprises a second imaging system configured to locate the wafer electrical contacts. An image generator coupled to the first imaging system generate an alignment image on a focal point of the first imaging system. The testing system calibrates the first imaging system to the second imaging system using the alignment image.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an apparatus and method for projecting an alignment image. More particularly, the present invention relates to an apparatus and method that generates a projected reticle image to facilitate the calibration between a moveable direct probe sensor camera and a fixed camera. 
     BACKGROUND OF THE INVENTION 
     Improvements in manufacturing processes has led to an increase in the density and complexity of semiconductor devices placed on a single silicon wafer. The increased density of semiconductor devices, however, has reduced the accuracy of wafer sorts. Wafer sort, or wafer probe, describes the,process of using probe cards to identify semiconductor devices at the wafer stage of manufacture that have inter-connectivity or electrical malfunctions prior to the individual packaging of the semiconductor devices. In particular, a probe card includes a collection of electrical contacts, pins, or probes that are positioned to make contact with the bonding pads of the semiconductor device under test (“DUT”). Subsequently, Automatic Test Equipment (“ATE”) electrically connected to the probe cards, generates electrical tests to examine the inter-connectivity or electrical operation of the DUT. 
     As the density of semiconductor devices increase, the dimensions of the probe card have dramatically shrunk to ensure proper probe-to-pad alignment. Probe-to-pad alignment describes accurately positioning the bonding pads of a semiconductor device located on a wafer in such a way that the bonding pads of the device make good electrical contact with the probe tips of the probe card. The modified probe card dimensions, however, create numerous problems during probe-to-pad alignment. To ensure accurate probe-to-pad alignment numerous methods have been developed in the prior art. 
     One method of a prior art probe-to-pad alignment uses a dummy wafer in conjunction with an auto-align fixed camera. The fixed camera is a downward looking camera with a fixed position and a known field of view. Using the fixed downward looking camera to view the bonding pads and other features on a wafer, the location of the bond pads on the DUT are determined in horizontal dimensions ‘x’ and ‘y.’ The ‘z,’ or vertical location, of the wafer surface, or equivalently, of the bond pads, is determined using a separate system. Next, a dummy wafer with a soft markable surface, such as an aluminum layer, is probed. The probing causes the probe tips to leave indentations on the dummy wafer. Based on the location of the probe indentations the fixed camera determines the ‘x-y’ coordinates of the probe tips relative to the dummy wafer. Using the derived ‘x-y’ coordinates of the probe tips, the prober positions the bond pads of a DUT in contact with the probe tips. Thus, probe-to-pad alignment is achieved. The method of using dummy wafers for probe-to-pad alignment, however, has numerous drawbacks. In particular, this method results in wasted wafers, possible damage of probe tips, reliance on an alternate system to measure ‘z’ coordinates, and reliance on probe indentations to interpret actual probe tip position. 
     To counteract the reliance on dummy wafers, prior art probers developed a direct probe sensor (“DPS”) camera. In the prior art, the DPS camera is used in conjunction with the fixed camera to align probe tips and bond pads. In particular, the DPS camera is an upward looking camera that records the x, y, and z coordinates of the probe tips of a probe card. As previously described, the fixed camera is a down ward looking camera that determines the x,y, and z coordinates of the bond pads of a DUT located on a wafer. Based on the x, y, and z coordinates of the probe tips and the bond pads, the prober positions the wafer to align the probe tips of the probe card with the bond pads of the DUT. 
     FIG. 1 illustrates a prior art prober using a DPS camera. In particular, system  100  includes a probe card  160  with probe tips  165 . System  100  also includes lens system  120 , physical reticle  140 , and DPS  110 —a charge coupled device (“CCD”) that records images on pixel grid  115 . System  100  records the location of probe tips  165  via, lens system  120 . System  100  also includes wafer chuck  170 . Wafer chuck  170  is coupled to lens system  120 . System  100  moves wafer chuck  170  in the x, y, and z coordinates to place a wafer (not shown) in contact with probe tips  165 . System  100  also moves wafer chuck  170  in the x, y, and z coordinates to record the location of probe tips  165 . 
     Prior to recording the probe tip locations, the x, y, and z coordinates of the field of view of DPS  110  is calibrated with a fixed camera (not shown). As previously described, the fixed camera is a downward looking camera with a fixed position and a known field of view. The calibration between DPS  110  and the fixed camera is performed via physical reticle  140 . In the prior art, physical reticle  140  is a thin plate of glass with cross-hair pattern  150  located in the center of the glass plate. During calibration, physical reticle  140  is placed at the focal point of DPS  110 —denoted as focal  180 . Using the image generated by cross-hair pattern  150 , DPS  110  generates a pixel representation of cross-hair pattern  150  on pixel grid  115 . The pixel representation is relayed to a prober (not shown). Subsequently, housing  170  moves physical reticle  140  under the fixed camera and the fixed camera&#39;s field of vision relative to cross-hair pattern  150  is determined and relayed to the prober. 
     The prober correlates the pixel representation of cross-hair pattern  150  generated by DPS  110  to the known location and field of view of the fixed camera. Thus, the position of a probe tip viewed by DPS  110  is accurately determined because both cameras, DPS  110  and the fixed camera, are calibrated to each other by focusing on the same intermediate target—cross-hair  150 . Using physical reticle  140  for alignment between DPS  110  and the fixed camera, however, create numerous disadvantages. 
     One disadvantage of using a physical reticle results from the design characteristics of the physical reticle. In particular, as previously described, physical reticle  140  is designed using a glass plate. The glass plate, however, creates an image offset because there is an optical path difference between glass and the air surrounding physical reticle  140 . The image offset results in a shifted cross-hair  150 , which in turn results in a calibration offset in the “z” direction. 
     Another disadvantage of using a physical reticle results from the requirement of operator intervention of the physical reticle. In particular, physical reticle  140  is removed during non-calibration (i.e. normal testing) use. Thus, full automation is prevented. 
     Yet another disadvantage of using a physical reticle results from the close proximity of the physical reticle to the probe tips. In particular, during the calibration of DPS  110 , the physical reticle  110  may cause damage to the probe tips through accidental contact. 
     SUMMARY OF THE INVENTION 
     A testing system operable to accurately position a plurality of contact electrodes relative to a plurality of electrical contacts is disclosed. For one embodiment, the testing system comprises a first imaging system coupled to a wafer chuck. The wafer chuck is used to place the electrical contacts of a wafer in contact with the plurality of electrodes. To facilitate accurate positioning between the wafer electrical contacts and the contact electrodes, the first imaging system is configured to locate the plurality of contact electrodes. The testing system also comprises a second imaging system configured to locate the wafer electrical contacts. To calibrate the objects viewed by the first imaging system and the second imaging system, an image generator coupled to at least one of the imaging systems generates an alignment image along the optical path of the imaging system. The testing system calibrates positioning and imaging information between the first imaging system and the second imaging system using the alignment image. 
     According to another embodiment, an imaging system operable to generate an alignment image is disclosed. The imaging system comprises an image generator configured to generate the alignment image. The imaging system also comprises an objective coupled to the image generator that has an optical path including an objective lens, a rear image forming lens, and a beam-splitter coupled between the objective lens and the rear image forming lens. The beam-splitter is configured to inject the alignment image into the optical path of the imaging system. For one embodiment, the imaging system generates the alignment image on the focal point of the imaging system via a charge coupled device. Specifically, a reflective charge coupled device is coupled to the objective. The reflective charge coupled device is configured to reflect the alignment image onto the focal point of the imaging system. 
     For yet another embodiment, the alignment image projected on the charge coupled device and the reflected alignment image are optically conjugate points. Thus, a second imaging system viewing the projected alignment image of a first imaging system results in both imaging system viewing the identical image at the same point in space. 
     Other features and advantages of the present invention will be apparent from the accompanying drawings and from the detailed description that follows. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The features and advantages of the present invention are illustrated by way of example and not limitation in the figures of the accompanying drawings in which like references indicate similar elements and in which: 
     FIG. 1 illustrates a prior art direct probe sensor camera; 
     FIG. 2 illustrates one embodiment of an automatic test equipment; 
     FIG. 3 illustrates one embodiment of a direct probe sensor camera generating a calibration image; 
     FIG. 4 illustrates one embodiment of an objective included in the direct probe sensor camera of FIG. 3; 
     FIG. 5 a  illustrates one embodiment of an image generator included in the direct probe sensor camera of FIG. 3; and 
     FIG. 5 b  illustrates one embodiment of a charge coupled device included in the direct probe sensor camera of FIG.  3 . 
    
    
     DETAILED DESCRIPTION 
     An automatic test equipment that generates an image to calibrate a direct probe sensor camera and a wafer sort camera is disclosed. For one embodiment, the image is generated within the direct probe sensor (“DPS”) camera. The generated image is located at the focal point of the DPS camera. In the present embodiment, both the DPS camera and the wafer sort camera include a charge coupled device (“CCD”) to record viewed objects. Accordingly, the generated image is located at both the focal point of the DPS camera and on the CCD of the DPS camera. During calibration, the DPS camera records the pixel location of the image on the CCD of the DPS. Alternatively, during calibration, the DPS camera records the pixel location of the image oh the CCD of the DPS. The DPS camera transfers the pixel representation to a prober. Subsequently, the prober moves the image over to the wafer sort camera. The wafer sort camera focuses on the image and generates a pixel representation of the image. Alternatively, the wafer sort camera focuses on the image and records the pixel location of the image. The pixel image recorded by the wafer sort camera is also transferred to the prober. Accordingly, for each pixel, the prober correlates the pixel image recorded by DPS camera to the pixel image recorded by the wafer sort camera, thus calibrating the two camera system. The calibration allows the prober to position a first object viewed by the DPS camera relative to a second object viewed by the wafer sort camera. 
     For one embodiment, the prober uses the DPS camera to view probe pins of a probe card. The prober also uses the wafer sort camera to view bond pads. Accordingly, the calibration allows the prober to accurately place the probe pins in contact with the bond pads. For another embodiment, the wafer sort camera is replace by a wafer alignment camera. 
     For an alternative embodiment, the calibration between the DPS camera and the wafer sort camera is implemented without the generated image. Instead, the entire CCD of the DPS is illuminated. After the illumination of the DPS CCD, the prober moves the DPS camera below the wafer sort camera. Subsequently, the wafer sort camera records the position of the pixels of the CCD included in the DPS camera. The prober correlates the pixels recorded by the wafer sort camera to the actual pixels of the DPS camera, thus calibrating the two camera system. 
     FIG. 2 illustrates an embodiment of an automatic test equipment (“ATE”) implemented by the present invention. In particular, system  200  comprises a wafer chuck ( 202 ) coupled to an orientation mechanism ( 204 ) in a manner which allows wafer chuck  202  to be moved in the X, Y, Z, and theta directions  290 . Wafer chuck  202  accepts the attachment of a wafer ( 222 ). System  200  also includes a probe card holder ( 240 ) which accepts a probe card ( 230 ). For one embodiment, probe card  230  may be any of the different varieties of probe cards, including for example membrane probe cards. For an alternative embodiment, probe card holder  240  may be configured to provide movement of probe card  230  in any of the X, Y, Z, or theta directions  290 . As illustrated in FIG. 2, probe card  230  includes a number of conducting contact electrodes. The contact electrodes may in one embodiment include metallic pins ( 232 ). Provided the probe card and the wafer are properly aligned by system  200 , pins  232  make contact with pads  224  of wafer  222 , thus allowing system  200  to test the inter-connectivity and electrical operation of devices located on wafer  222 . For one embodiment, pads  224  comprise any contact electrode surface including, but not limited to, a flat surface, a solder bump, pins, or posts. 
     Pads  224  and pins  232  are placed in contact via direct probe sensor (“DPS”) camera  206 - 210  and fixed camera  220 , alternatively referred to as a wafer alignment camera. In particular, DPS camera  206 - 210 , is configured to view pins  232  on probe card  230 . Fixed camera  220  is coupled to a fixed reference point, base  250 , and is configured to view pads  224  on wafer  222 . For one embodiment, system  200  uses the location of pins  232  recorded by DPS camera  206 - 210  in conjunction with the current pad  224  location viewed by fixed camera  220  to incrementally move wafer chuck  202  until pads  224  come in contact with probe pins  232 . For alternative embodiments, fixed camera  220 , may contain both coaxial and oblique illumination sources. For another embodiment, probe card holder  240  is coupled to base  250 . For yet another embodiment, system  200  includes a computer system (not shown) with a central processing unit and memory. Based on the DPS camera  206 - 210  and fixed camera  220  data, computer system applies control signals to orientation mechanism ( 204 ), thus moving wafer chuck  202  until pads  224  come in contact with probe pins  232 . The computer system is also used to calibrate DPS camera  206 - 210  and fixed camera  220 . 
     As illustrated in FIG. 2, DPS camera  206 - 210  and fixed camera  220  comprise two physically disjointed camera systems. Specifically, the camera systems do not share the same objective or lenses. Thus, calibration between the two camera systems is necessary to ensure the accurate positioning of wafer chuck  202  relative to pins  232 . 
     For one embodiment, the calibration between the two systems is performed by an image generated by DPS camera  206 - 210 . In particular, both DPS camera  206 - 210  and fixed camera  220  simultaneously focus on the generated image. Subsequently, system  200  correlates the image and positioning information determined by DPS camera  206 - 210  with the image and positioning information determined by fixed camera  220 , thus calibrating the two cameras. 
     For an alternative embodiment, DPS camera  206 - 210  focuses on the generated image, hereinafter referred to as a calibration image or alternatively as an alignment image. Subsequently, orientation mechanism  204  moves the generated image below fixed camera  220  so that fixed camera  220  can focus on the generated image. Based on the movement of orientation mechanism  204  and the images record by both cameras, system  200  determines the relative position between the two camera&#39;s focal points. Thus, calibrating DPS camera  206 - 210  to fixed camera  220 . For an alternative system, based on the movement of orientation mechanism  204  and the images recorded by both cameras, a computer system coupled to system  200  determines the relative position between the two camera&#39;s focal points. 
     FIG. 3 illustrates one embodiment of a DPS camera generating a calibration image. In particular, system  300  includes an objective ( 330 ) coupled to both an image generator ( 320 ) and a CCD ( 310 ). For one embodiment, system  300  is a video microscope with a fixed field of view. For an alternative embodiment, system  300  generates a calibration image ( 340 ) at the focal point ( 350 ) of the video microscope. 
     For one embodiment, system  300  is included in system  200 . Accordingly, section  206  of DPS camera  206 - 210  corresponds to objective  330 . Similarly, sections  208  and  210  of DPS camera  206 - 210  correspond to image generator  320  and CCD  310 , respectively. 
     As illustrated in FIG. 3, image  340  is cross-hair pattern located directly above objective  330 . Accordingly, CCD  310  generates a pixel representation of the cross-hair pattern. For one embodiment, the image recorded by CCD  310  is correlated to a fixed camera recording of image  340 , thus resulting in the calibration of system  300  and the fixed camera. For another embodiment, system  300  generates a calibration image by illuminating either all or a subset of all the pixels included in CCD  310 . The illuminated pixels are subsequently recorded by a fixed camera. Accordingly, each pixel detected by the fixed camera is correlated to each pixel recorded by CCD  310 , thus calibrating the fixed camera and system  300 . 
     FIG. 4 illustrates one embodiment of an objective included in the DPS camera of FIG.  3 . In particular, objective  400  includes a rear image forming lens ( 420 ), a beam-splitter ( 430 ), and an objective lens ( 440 ). Objective  400  also includes three illumination paths ( 410   a-c ). Illumination path  410   b  and  410   c  are the normal optical path through which objective  400  views images. 
     For one embodiment, beam-splitter  430  is a partially reflecting mirror with an anti-reflective coat on side ‘A’ and a plane of glass coated for 4-6% refection on side ‘B.’ For alternative embodiments, the reflective qualities of the glass coat is varied based on the light generated from path  410   b . The dual qualities of beam-splitter  430  allow the beam splitter to either deflect light from path  410   a  to  410   b  or to effectively transmit light in a bidirectional fashion between path  410   c  and path  410   b . It will be appreciated by one skilled in the art, that the reflective qualities of beam-splitter  430 , the displacement of the lenses ( 420  and  440 ), and the magnification strength of the lenses ( 420  and  440 ) may be varied depending on the focal point and illumination characteristics of the video microscope that houses objective  400 . 
     For one embodiment, objective  400  is used in DPS camera  206 - 210  of system  200 . Accordingly, objective  400  is coupled to image generator  208  CCD  210  at nodes  411   a  and  411   b , respectively. System  200  controls the light source generated along illumination paths  410   a-c  to perform two functions, probe-to-pad alignment and calibration. In particular, during probe-to-pad alignment, system  200  turns image generator  208  off. Thus, only ambient light source information (including images of probe pins  232 ) is transmitted from path  410   c  to path  410   b . Subsequently, the ambient light source information is recorded by CCD  210 . In particular, it will be appreciated by one skilled in the art that the arrangement of system  200  does not interfere with the use or placement of other illumination sources, such as coaxial or oblique illumination, that are normally associated with normal image generation in optical systems. 
     To perform the calibration function, system  200  turns image generator  208  on, thus generating a light source that includes a calibration image along path  410   a . Beam-splitter  430  deflects the light source transmitted on path  410   a  and injects the calibration image into the normal path of light in objective  400 , path  410   b . In particular, beam-splitter  430  and lens  420  create an image along path  410   b  that mimics an actual image placed at the focal point ( 450 ) of objective  400 . CCD  210  records the calibration image transmitted along path  410   a  and  410   b.    
     For one embodiment, CCD  210  is a reflective CCD. Accordingly, the light source transmitted along path  410   b  is reflected through lens  420 , through beam-splitter  430 , and lens  440  onto focal point  450 . As previously described, the light source transmitted along path  410   b  includes a calibration image. Thus, a virtual calibration image is generated at focal point  450 . In the present embodiment, system  200  uses the virtual calibration image to calibrate DPS camera  206 - 210  with fixed camera  220 . In particular, system  200  correlates the pixel image recorded by CCD  210  to a recording of the virtual pixel image generated by fixed camera  220 , thus determining the orientation and focal point of CCD  210  relative to fixed camera  220 . System  200  also uses the predetermined location of both the virtual calibration image and the fixed camera  220  to correlate the field of view between DPS camera  206 - 210  and fixed camera  220 . Additionally, system  200  uses the predetermined location of both the virtual calibration image and fixed camera  220  to clibrate the initial X, Y, and Z coordinates of DPS camera  206 - 210  relative to fixed camera  220 . Based on the afore-mentioned calibration, system  200  ensures proper probe-to-pad alignment. 
     FIG. 5 a  illustrates one embodiment of an image generator included in the direct probe sensor camera of FIG.  3 . In particular, image generator  500  includes an illumination source ( 510 ), a reticle ( 520 ) and a reticle lens ( 530 ). Reticle  520  is a flat circular glass plate with a metal deposit applied to the surface of the glass plate. For one embodiment, with the exception of the surface area delineated by cross-hair pattern  525 , the metal deposit is uniformly applied to the entire glass surface. The space in the metal deposit allows the light from illumination source  510  to generate a cross-hair light pattern (i.e. a calibration image) that is focused through reticle lens  530 . For alternative embodiments, the metal deposit on reticle  520  is varied to generate different calibration images. It will be appreciated by one skilled in the art, that the brightness of illumination source  510 , the characteristics of reticle  520  (including but not limited to thickness and impurity content), and the magnification strength of lens  530  may be varied depending on the desired dimensions and brightness of the calibration image. 
     For one embodiment, image generator  500  is used in conjunction with objective  400  and a reflective CCD. In particular, image generator  500  is coupled to node  411   a  and the reflective CCD is coupled to node  411   b . Accordingly, the calibration image generated by image generator  500  is transmitted along illumination path  410   a  as a light source. Beam-splitter  430  deflects the light source transmitted on path  410   a  and injects the calibration image into the normal path of light in objective  400 , path  410   b . In particular, beam-splitter  430  and lens  420  create an image along path  410   b  that mimics an actual calibration image placed at focal point  450 . The reflective CCD records the calibration image. The reflective CCD also reflects the light source transmitted along path  410   b  back through lens  420 , beam-splitter  430 , and lens  440  onto focal point  450  as a virtual calibration image. As previously described, the virtual calibration image is used to calibrate a DPS camera housing objective  400  to a fixed camera. 
     For an alternative embodiment, reticle  520  is removed from system  500 . Accordingly, the virtual calibration image is either all or a subset of all the pixels illuminated in the reflective CCD. The illuminated pixels are subsequently recorded by a fixed camera. Thus, each pixel detected by the fixed camera is correlated to each pixel recorded by a DPS camera that houses objective  400 . The correlation results in the calibration of the fixed camera and the DPS camera that houses objective  400 . 
     FIG. 5 b  illustrates one embodiment of a charge coupled device included in the direct probe sensor camera of FIG.  3 . In particular, CCD  540  includes an array of light sensitive transistor diodes ( 560 ), also referred to as cells, that are deposited on a wafer ( 570 ). Each cell is addressable through a control circuitry ( 580 ) that supplies power to CCD  540 . For one embodiment, control circuitry  580  activates all the cells in CCD  540  for a twenty mill-second period. During the twenty milliseconds, each cell accumulates charge depending on the amount and intensity of photons striking the particular cell. For one embodiment, control circuitry  580  generate a pixel representation of the light source striking CCD  540  based on the cells with accumulated charge. For alternative embodiments, control circuitry  580  activates all the cells in CCD  540  for different time periods depending on the photon absorption qualities of the specific CCD. 
     In the present embodiment, CCD  540  is used in conjunction with objective  400  and image generator  500 . In particular, image generator  500  is coupled to node  411   a  and CCD  540  is coupled to node  411   b . Accordingly, the calibration image generated by image generator  500  is transmitted along illumination path  410   a  as a light source. Beam-splitter  430  deflects the light source transmitted on path  410   a  and injects the calibration image into the normal path of light in objective  400 , path  410   b . In particular, beam-splitter  430  and lens  420  create an image along path  410   b  that mimics an actual calibration image placed at focal point  450 . 
     FIG. 5 b  illustrates the charge accumulation of CCD  540  as photons from the light source along path  410   b  strike the surface of CCD  540 . In particular, the cells delineated by cross-hair  550  are struck by the light source created by image generator  500 . CCD  540  records the cells with accumulated charge via control circuitry  580 , thus generating a pixel representation of the light source striking CCD  540 . 
     Following the previous example, for an alternative embodiment, CCD  540  is a reflective CCD. Accordingly, the cells struck by the light source transmitted along path  410   b  reflects the light source back through lens  420 , beam-splitter  430 , and lens  440  onto focal point  450  as a virtual calibration image. As previously described, the virtual calibration image is used to calibrate a DPS camera housing objective  400  to a fixed camera. For one embodiment, the cells of CCD  540  reflect ten to thirty percent of the photons absorbed by the illuminated cells. For alternative embodiments, the reflective qualities of beam-splitter  430 , the displacement of the lenses ( 420  and  440 ), and the magnification strength of the lenses ( 420  and  440 ) may be varied depending on the reflective characteristics of CCD  540 . 
     Thus, an apparatus and method for projecting an alignment image have been provided. Although the present invention has been described with reference to specific exemplary embodiments, it will be evident that various modifications and changes may be made to these embodiments without departing from the broader spirit and scope of the invention as set forth in the claims. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense.