Abstract:
A probe card for production testing of semiconductor imaging die includes a stiffener supported on a bottom side of the probe card. The top of the stiffener is substantially flush with a top surface of the probe card. A light passage through the stiffener features non-reflective surfaces. Surfaces surrounding the light passage are arranged to avoid casting any shadows on the imaging die being tested. The arrangement provides a low profile probe card. A source of light used to illuminate the imaging die through the light passage can be placed close to the imaging device under test, providing few false negatives and more consistent results from wafer to wafer.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The invention relates to testing of integrated circuits using probe cards, and more particularly to bottom side stiffeners for probe cards used in production testing of wafers containing imaging die. 
   2. Description of the Related Art 
   Semiconductor integrated circuits (ICs) are produced extensively and are used in a vast number of products. As semiconductor ICs become increasingly smaller and production levels increase, testing of the ICs becomes more difficult, and more critical. 
   ICs are mass-produced on round, thin sheets of semiconducting material referred to as wafers. Wafers most commonly are made of silicon. Anywhere from 50 to 200 or more identical circuits, or die, can be produced in a single wafer. Each circuit is provided with contacts, such as peripheral pads, to connect the circuit die with other components or circuits. Testing devices need to make connections with the circuits of each individual die by engaging the contacts. Rather than testing each die on a wafer one at a time, the testing devices typically test several die at once. All of the die are tested by moving the wafer under test relative to the fixed probe card of the testing device in a sequential testing pattern. On a wafer with 200 circuit die, for example, a testing device that tests four die at once will use a pattern of at least 50 moves to cover the entire wafer. Thus, the connections made by the probes on a probe card of a testing device with the die contacts must be made repeatedly and reliably for each of hundreds of wafers in a production batch, for example. 
   In addition to making reliable connections with each die, active testing of imaging die also requires illuminating each die using a light source. In order to achieve consistent active testing from die to die and wafer to wafer, the illumination must be constant and evenly distributed onto each of the several die being tested. In the example above, for instance, the illumination provided to each of the four die being tested simultaneously must be as consistent as possible. In the past, stiffeners provided on the top side of probe cards for imaging die have contributed to inconsistent lighting during testing, which has lead to inconsistencies and false-negative results. 
   Thus, there is a need and desire for a probe card for imaging die provided with a stiffener which allows consistent lighting of the imaging die during active production testing. 
   BRIEF SUMMARY OF THE INVENTION 
   An exemplary embodiment of the invention provides a probe card for production testing of semiconductor imaging die. The probe card includes a stiffener supported on a bottom side of the probe card, nearest the imaging die being tested. The top of the stiffener is substantially flush with a top surface of the probe card. A light passage through the stiffener is provided with non-reflective surfaces. Surfaces surrounding the light passage are arranged to minimize the casting of any shadows on the imaging die being tested. The arrangement provides a low profile probe card. A light source shining through the passage in the stiffener can be placed close to the imaging die for use during active testing, providing few false negatives and more consistent results from wafer to wafer. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above-described features and advantages of the invention will be more clearly understood from the following detailed description, which is provided with reference to the accompanying drawings in which: 
       FIG. 1  is a bottom side plan view of a portion of a probe card according to an exemplary embodiment of the invention; 
       FIG. 2  is a cross-section of a probe card taken along the line II—II of  FIG. 3  according to an exemplary embodiment of the invention; 
       FIG. 3  is a top plan view of a probe card according to an exemplary embodiment of the invention; 
       FIG. 4  is a bottom plan view of a probe card according to an exemplary embodiment of the invention; 
       FIG. 5  is an exploded view of a stiffener being assembled to the bottom side of a probe card according to an exemplary embodiment of the invention; 
       FIG. 6  is a perspective view of the top side of a probe card according to an exemplary embodiment of the invention; 
       FIG. 7  is a cut-away plan view of a probe card being used to test an imaging die wafer according to an exemplary embodiment of the invention; 
       FIG. 8  illustrates a wafer tester according to an exemplary embodiment of the invention; and 
       FIG. 9  is an enlarged view of section IX of the wafer tester illustrated in  FIG. 8 . 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Referring to  FIGS. 1–9 , a probe card  2  according to an exemplary embodiment of the invention includes a printed circuit board  4  having a central opening framed by a stiffener  6 . A plurality of probes  8  are supported by the stiffener  6 . The probes  8  are arranged to contact peripheral pads formed on imaging die in a silicon wafer, as described further below. The probes are secured to the stiffener  6  by a layer of adhesive, such as epoxy  7 . Individual probes  8  are connected by leads  10  to circuitry provided on the printed circuit board  4 . Portions of the probes and their connections to leads  10  are encased in epoxy  7 , as indicated for representative probes by broken lines  9 . Some of the circuitry, which is not shown in detail in the Figures, includes discrete components such as capacitors  12 . 
   Referring to  FIG. 2 , the probe card  2  is shown in cross-section. The printed circuit board  4  includes a top side  14  and a bottom side  16 . The stiffener  6  is disposed substantially flush with the surface of the top side  14  of printed circuit board  4 . The stiffener  6  includes a metal frame  32 , an axially-extending portion of which fits into a recess formed in the bottom side  16  of printed circuit board  4 . The stiffener also includes a ceramic ring  34 . The ceramic ring  34  fits into a recess formed in the bottom side of frame  32 . The interfitting of the printed circuit board  4 , the frame  32 , and the ceramic ring  34  provides the probe card with a narrow profile. 
   Referring to  FIG. 3 , the top side  14  of printed circuit board  4  is shown in plan view. The top side  14  of printed circuit board  4  includes conductive traces  18  shown schematically around an opening  20  ( FIG. 5 ) into which stiffener  6  is installed. The traces  18  are used to interconnect probe card circuit components (not shown). The top side  14  of the printed circuit board  4  includes wire openings and other traces not shown in the Figures for the sake of clarity. In particular, a plurality of connector pads is disposed in outer circuit board ring  22  for interlinking with a computerized (micro-processor based) IC tester  26  (illustrated in  FIG. 8 ). Surface  28  of stiffener  6  is treated to reduce light reflection and scattering. According to an exemplary embodiment, surface  28  is formed of an epoxy that is black in color. The epoxy surface is roughened to give it a coarse texture, for example, to provide a non-reflective surface and minimize light reflection. 
     FIG. 4  illustrates the bottom side  16  of the printed circuit board  4  and the bottom of stiffener  6  shown in plan view. As in  FIG. 3 , various interconnects, components, and traces are not shown for the sake of clarity. The bottom side of frame  32  supports ceramic ring  34 , upon which probes  8  ( FIG. 1 ) are supported. Probes  8  are not shown in  FIG. 4  for clarity of illustration purposes. 
   Referring to  FIG. 5 , a step in the assembly of the stiffener  6  to the printed circuit board  4  is illustrated according to an exemplary embodiment of the invention. The stiffener  6  is installed into the opening  20  from the back side  16  of printed circuit board  4 . A relief  30  is formed in printed circuit board  4  to receive the stiffener  6 . Stiffener  6  includes the metal frame  32  supporting the ceramic ring  34 . The ring  34  can be formed of any material that is an electrical insulator capable of withstanding temperatures up to as high as about 200° C. (˜392° F.). In an exemplary embodiment, the ring  34  is formed of layers of ceramic material. Advantageously, the ceramic material is sturdy and has a coefficient of expansion similar to that of the material (silicon) used to form the printed circuit board  4 . 
   An adhesive layer, such as epoxy  36 , holds probes  8  ( FIGS. 1 and 8 ) in place on ceramic ring  34 . Epoxy  36  is sturdy, electrically non-conductive, and able to withstand high temperatures. In addition, epoxy  36  adheres well to both the ceramic ring  34  and the probes  8 , in order to secure the probes in position. 
     FIG. 6  illustrates the top side  14  of the probe card  2 , with the stiffener  6  assembled to the printed circuit board  4  as discussed above in connection with  FIG. 5 . A top surface  38  of stiffener  6  is substantially flush with the surface of top side  14  of circuit board  4 . The stiffener  6  is attached to the probe card  4  at attachment points  40 . 
   Referring to  FIG. 7 , a method of testing a wafer  42  using probe card  2  is now explained. The probe card  2  is shown with a portion cut away to reveal the wafer  42 . The wafer  42  contains a plurality of imaging die  44 . The wafer  42  is positioned under the probe card  2 , and a passage  46  through the stiffener is aligned over four of the imaging die  44 . The wafer  42  under test is advanced with respect to the probe card  2  such that the probes  8  (shown in  FIGS. 1 ,  8 , and  9 ) contact peripheral pads on each of the four imaging die  44 . With the probe card  2  in place, four imaging die  44  can be tested simultaneously. Tests take place under light and dark conditions, as described further below. The passage  46  and its surrounding surfaces are arranged such that shadowing is minimized on the four imaging die  44  (e.g., by light projected through the passage  46  during active testing). Once a set of four die have been tested and the results recorded, the wafer  42  is moved relative to the probe card  2  so that the passage  46  is aligned with another set of four imaging die  44 . The process is repeated until each of the die  44  on the wafer  42  is tested. 
     FIGS. 8 and 9  illustrate an integrated circuit tester  26  interconnected to the probe card  2 . The wafer  42  is placed on a prober chuck  48 . A prober chuck  48  is arranged to advance the wafer  42  toward the probe card  2  on probe card support  50  to bring probes  8  into contact with peripheral pads on the four imaging die  44 . A light source (not shown) is provided in probe card support  50  for shining light through passage  46  in stiffener  6  onto the imaging die  44  to be used in active tests requiring illumination. Advantageously, the light source can be placed close to the top side  14  of the printed circuit board  2 , and closer to the imaging die  44  than in other probe card arrangements, because the top of the stiffener  6  is substantially flush with the top side  14  of the printed circuit board  2 . Placing the source of light closer to the imaging die under test provides few false negatives and more consistent results from wafer to wafer. The light source may be fitted with a diffuser to enhance a more even distribution of light illuminating the imaging die. The stiffener  6  is arranged to minimize or eliminate shadows cast by the light onto the imaging die. 
   Various applications of the devices and methods of the invention will become apparent to those of skill in the art as a result of this disclosure. Although certain advantages and embodiments have been described above, those skilled in the art will recognize that substitutions, additions, deletions, modifications and/or other changes may be made without departing from the spirit or scope of the invention. Accordingly, the invention is not limited by the foregoing description but is only limited by the scope of the appended claims.