Abstract:
A probe head for testing semiconductor wafers has a probe contactor substrate have a first side and a second side. A plurality of probe contactor tips are coupled to the first side and the plurality of tips lie in a first plane. A plurality of mounting structures are coupled to the second side with each of the mounting structures each having a top surface lying in a second plane, wherein the first plane is substantially parallel to the second plane.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/346,954, filed Feb. 3, 2006, titled “PROBE HEAD WITH MACHINED MOUNTING PADS AND METHOD OF FORMING SAME,” the disclosure of which is incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    Embodiments of the present invention are directed to a probe head assembly used in semiconductor testing apparatus, and more particularly to a probe contactor substrate having probe contactors electroplated on a first side and machined mounting pads electroplated on a second side. 
         [0004]    2. Description of Related Art 
         [0005]    A modern probe card assembly used to test wafers of semiconductor chips generally consists of a Printed Circuit Board (PCB) (also referred to as a printed wiring board or probe card wiring board), a probe contactor substrate having probes for contacting the wafer (sometimes referred as a probe head), and an interposer connecting the PCB to the probe contactor substrate. 
         [0006]    Probes (also referred to as probe contactors) are generally compliant mechanisms including at least one spring which have some limited range of compliance in a vertical direction (the “z” direction). Some probes have minimal or no compliance. When in use, a wafer under test is urged upward to contact the tips of the probes. In practice, there is some manufacturing process-related z error (non-planarity of the probe tips) caused by film stresses, etch control, assembly control, etc. as well as systemic z errors caused by a warping or curving in the surface of the probe contactor substrate. If the probe contactor substrate is curved or warped, so will be the imaginary surface that goes through the tips (assuming that the probes are of uniform height). Thus some probe tips will contact the wafer first (called the first touch z height) and some probe tips will contact the wafer last (last touch z height). Because probes generally have a limited range of compliance (as small as 50 μm or less for many compliant microfabricated technologies and effectively 0 for non-compliant technologies), it is desirable to minimize both the process-related and systemic errors in tip z height. Some errors are most directly related to the fabrication of the probes on the probe contactor substrate rather than the probe card assembly design. However, some errors are usually directly related to the probe card assembly and the way the probe contactor substrate (or substrates) are mounted to the rest of the probe card assembly. The minimization of these latter errors is the subject of the present invention. 
         [0007]    In older probe card applications, a prober has a surface which has been planarized to that of the chuck that carries the wafer under test. The probe card PCB is generally mounted to this planarized surface of the prober. Thus, such probe card assemblies require well controlled parallelism between the plane of the probe tips (the best-fit plane that minimizes the overall root-mean-square z error between the probe tips and the plane) and the plane of the PCB (the PCB can be thought of as the “reference plane”). If the probe tips are co-planar with the PCB, then they are also co-planar with the chuck, and thus with the wafer under test. Such a design will lead to a more uniform contact of the probes to the wafer under test (less of a distance between first touch z distance and last touch z distance). In newer probe cards, the probe tips are referenced to mounting points on the probe card which are typically kinematic mounts of some type (used here to describe a mount that provides accurate and repeatable mechanical docking of the probe card into the test equipment and provides constraint in at least the two degrees of freedom necessary to achieve parallelism to the plane of the wafer chuck). In either case, it is necessary to align the tips of the probe contactors such that they are parallel to a reference plane which is itself parallel to the plane of the wafer chuck. Furthermore, it is desirable to mount the probe head to the probe card assembly in a fixed manner without the need for shimming or dynamic adjustment of the planarity of the probe card substrate once it has been mounted. 
         [0008]    A common problem with mounting the probe head to the probe card assembly, is that the probe contactor substrate  100  to which the probe contactors  110  are attached is generally non-uniform and has thickness variations across its surface, as shown in  FIG. 1 . (While not shown in  FIG. 1 , the front side of the probe contactor substrate may also have variations in the in the thickness as well, however, the method of forming the probe contactors on the substrate accounts for this and allows the tips of the probe contactor to be coplanar.) The probe contactor substrate  100  is generally mounted to the probe card assembly  200  in such a manner that the planarity is set by the location of discrete points along the back surface (the surface opposite of the probe contactors) of the probe contactor substrate  100  and thus, due to the non-uniform thickness of the probe contactor substrate, the plane  210  of the probe contactor tips may not be co-planar with the plane  220  of the reference plane as shown in  FIG. 2 . It is a further problem that due to manufacturing tolerances or errors, the overall heights of the probe contactors can vary linearly across the substrate such that when referenced from the back of a planar substrate, the probe tips lie in a plane that is tilted. 
         [0009]    It is a further problem that in applications requiring more than one probe contactor substrate to function in unison, the overall thickness or z distance between the back of each substrate to the tips of the substrates should be identical to eliminate the need to individually compensate the mounting means of the substrates for variation in thickness. 
         [0010]    Thus, what is needed is a probe head which has mounting structures (for mounting to the probe card assembly) that are planar to each other and co-planar with the plane of the tips of the probe contactors and are a known predetermined distance from the plane of the tips. 
       BRIEF SUMMARY OF THE INVENTION 
       [0011]    In the present invention, rather than adjust the orientation of the substrate to the reference plane after mounting or during the process of mounting, mounting structures are provided on a back side of the substrate which are a precise z distance from the probe contactor tips and the plane of the mounting structures are parallel to the plane of the probe contactor tips. These machined mounting structures are meant to alleviate the concerns caused by the non-uniformity of the probe contactor substrate or in tilt of the plane of the probe contactor tips relative to the substrate in that they the mounting points are machined to be planar to one another and parallel to the plane of the tips of the probe contactors. Thus, the probe head may be fixedly mounted to the probe card assembly, using these mounting structures, without the need for shimming or adjustment of planarity during the mounting process. Moreover, these machined mounting structures may be electroplated on to the back of the probe contactor substrate using the same processes utilized when the probe contactors are electroplated to the probe contactor substrate, thus saving time and expense. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  illustrates a probe head as is known in the art. 
           [0013]      FIG. 2  illustrates a probe card assembly as is known in the art. 
           [0014]      FIG. 3  illustrates a probe head according to an embodiment of the present invention. 
           [0015]      FIG. 4   a  illustrates a mounting technique according to an embodiment of the present invention. 
           [0016]      FIG. 4   b  illustrates another mounting technique according to an embodiment of the present invention. 
           [0017]      FIG. 5  illustrates another mounting technique according to an embodiment of the present invention. 
           [0018]      FIGS. 6-8  illustrate steps for forming mounting pads on a probe contactor substrate according to an embodiment of the present invention. 
           [0019]      FIG. 9  illustrates a probe card assembly according to an embodiment of the present invention. 
           [0020]      FIG. 10  illustrates a probe card assembly incorporating a sub-mount according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0021]      FIG. 3  illustrates a probe head according to an embodiment of the present invention. A probe contactor substrate  300  has a plurality of probe contactors  310  coupled to a first side  300   a  of the probe contactor substrate  300 . On a second side  300   b,  opposite the side with the probe contactors  310 , a plurality of mounting structures  320  are coupled to the probe contactor substrate  300 . According to an embodiment of the present invention, the top surfaces of the mounting structures  320  lie in a plane  340  wherein the top surfaces of the mounting structures are co-planar (although in  FIG. 3  only two mounting structures are shown, there will generally be at least three mounting structures to define a plane in three-dimensional space), and wherein the plane  340  of the mounting structures is parallel to the plane  330  of the tips of the probe contactors  310 . While the overall planarity target of the probe card may be customer specified, ideally the plane  340  of the mounting structures should be within ±10 μm total error from parallelism relative to the plane  330  of the tips of the probe contactors. 
         [0022]    The mounting structures  320  may be of any shape or size. They may be round, square, rectangular, “doughnut” or any other convenient shape for mounting to a mechanical structure. The mounting structures may include additional alignment features for locating the substrate in x and y directions. The mounting structures may be made of any rigid and thermally stable material that will conveniently serve the mounting function (for example any metal, epoxy, even glass or ceramic). The mounting structures of course are planarized and the material is selected so as to accommodate a planarization machining step such as grinding or cutting, including milling and fly-cutting for example with a single point diamond fly-cutter. The preferred material is Ni or a Ni alloy. The mounting structures may include features for the convenient and effective use of adhesive mounts. Such features include wells or pockets to capture and wick adhesive at an appropriate adhesive bond line thickness when the mounting structure surface is firmly seated against a corresponding surface. 
         [0023]    As shown in  FIG. 9 , the rest of the probe card assembly may then be coupled to the probe head at the mounting structures  320 . In an embodiment of the present invention, the rest of the probe card assembly may include a printed circuit board  1010  (also known as a wiring board), a stiffener ring  1020  (or other similar structure), substrate supports  1030 , and an interposer  1040 . In this embodiment, the substrate supports  1030  may be coupled to the mounting structures  320 . The resulting probe card assembly exhibits excellent parallelism between the probe card reference plane  220  and the probe tip best fit plane  330 . It should be noted that the substrate supports may be of any size or structure, including round, square, triangular, elongated, or thick. 
         [0024]    In another embodiment of the present invention, the rest of the probe card assembly may incorporate a sub-mount  1100 , as shown in  FIG. 10 . In this embodiment the sub-mount  1100  may have support surfaces  1120  to which the mounting structures  320  are coupled. As with substrate supports, the support surfaces may be of any size or shape so long as they adequately connect the mounting structures to the wiring board (as with substrate support) or to the sub-mount (as with the support surfaces). Embodiments of probe cards that utilize multiple “tiles” of probe contactor substrates often utilize this sub-mount assembly method. If multiple probe contactor substrates are used in a probe card, the overall planarity of the tips of the probe contactors may be affected by a difference in the thickness between the probe card substrates as well as the other factors already discussed. Thus, to lessen the error caused by differences in thickness of probe contactor substrates and other factors, ideally the z distance between the top exposed surface of a mounting structure  320  and the tip plane  330  of any given “tile” will be within ±5 μm of the z distance of a mounting structure and the tip plane of any other given “tile.” Also ideally, in a multiple-substrate probe card, the plane  340  of the mounting structures of a given tile is within ±5 μm from the plane  330  of the tips of the probe contactors. This will ensure that the overall planarity target of the probe card is better than ±10 μm from the ideal planarity. 
         [0025]    The mounting structures  320  may be attached to the substrate supports  1030  or the support surfaces  1120  (or any other structure that is utilized in mounting the probe substrate to the probe card assembly) using any appropriate technique that provides a precise and well controlled attachment at the mounting structures  320 . Such techniques include adhesive bonding, solder bonding, or mechanical attachment using clamps, screws, clips, and the like. If adhesive or solder is used, features may be incorporated into the mounting structures to provide bond line thickness control such as adhesive wells providing an area that is a few microns to a few tens of microns distant from the surface so that when the structure is placed in close contact to a mounting surface, a well controlled adhesive or solder bond line is maintained. 
         [0026]      FIG. 4   a  illustrates an adhesive (or solder) mounted arrangement where the mounting structure  320  is configured to have wells  550  to allow for adhesive  500  (or solder) over-fill while providing metal-to-metal contact for precise z axis placement independent of bond-line thickness. Any adhesive may be used but high temperature epoxies are preferred (those with a glass transition temperature (Tg) greater than 150° C.).  FIG. 4   b  illustrates another mounting technique similar to that illustrated in  FIG. 4   a,  however, there is only one “well” in  FIG. 4   b  and is included to illustrate the bond line thickness at reference numeral  555 . Different adhesives or solders may have different bond line thicknesses at which they are most efficiently effective. If solder is used, a low creep, high temperature material such as gold-tin eutectic is preferred.  FIG. 5  illustrates a mechanical clamp arrangement that provides direct metal-to-metal contact between the mounting structure and the corresponding support structure. The clamp shown employs a screw  600  (or other type of fastener), a clamp element  610 , and a nut  630 , which tightens the screw  600  to the clamp element  610 . Similarly, the sub-mount  1100  may be clamped directly to a threaded insert which is coupled to the probe contactor substrate  300 , or by a screw fastener going directly through a through-hole in the probe contactor substrate  300 . 
         [0027]    The process of forming the mounting structures  320  on the probe contactor substrate  300  can be efficiently and precisely done during the same process in which the probe contactors  310  are formed on the probe contactor substrate. In one embodiment, processes similar to those described in U.S. Pat. No. 5,190,637, titled “Formation of microstructures by multiple level deep X-ray lithography with sacrificial metal layers,” assigned to the Wisconsin Alumni Research Foundation, and/or any of U.S. patent application Ser. Nos. 11/019,912, 11/102,982, and 11/194,801, all of which are assigned to the present applicant, Touchdown Technologies, Inc. and all of which are incorporated by reference herein, may be used. The following is an example of one way in which a probe head may be formed with mounting structures  320  on the probe contactor substrate, although other methods are possible and intended to be covered by this application. 
         [0028]      FIG. 6  depicts a probe contactor substrate  300  upon which probe contactors  310  are lithographically electroplated using the methods described in the patent and patent applications listed above. Reference numeral  660  refers to sacrificial metal which is plated around the probe contactors during their formation as described in the patent and patent applications above. After forming the probe contactors  310  and the sacrificial metal  660  on the probe contactor substrate  300 , the bottom of the probe contactor substrate (the side without probe contactors) is mounted to a machine chuck. The assembly then passes a cutter or a grinder parallel to the chuck surface and over the probe contactors to cut them to a planar surface (at a plane parallel to the chuck) at a known distance from the chuck. The cutting process may be followed by polishing process as needed to refine the surfaces of the tips of the probe contactors  310 . 
         [0029]    After planarization of the probe contactors  310  and sacrificial metal  660 , a plating seed layer is electroplated on the back side of the probe contactor substrate  300 . The seed layer is deposited by PVD (physical vapor deposition), electroless plating, screen printing or any other suitable means of obtaining an adherent conductive layer suitable as a plating seed. A photoresist compound is applied to the back side of the probe contactor substrate  300  and is exposed in such a manner so as to define an area and volume for plating of the mounting structures  320 . The mounting structures  320  will preferably be plated to a height that is larger than the intended height of the finished mounting structures  320 . The rest of the photoresist is then dissolved and the back side of the probe contactor substrate  300  (including the mounting structures  320 ) is optionally plated with a sacrificial metal  680 . Preferably, the sacrificial metal  680  which is plated around the mounting structures  320  is dissolved by the same solution or medium as the sacrificial metal  660  which surrounds the probe contactors  310 . The probe head is then placed in the machine chuck again, this time with the probe contactors  310  and sacrificial metal  660  placed against the chuck&#39;s surface. The assembly then passes a cutter or grinder parallel to the planarized surface of the probe contactors/sacrificial substrate, and the mounting structures  320  and sacrificial metal  680  is planarized such that they are parallel to, and a known distance from, the plane of the probe contactors  310 . The result is illustrated in  FIG. 7 . The final step is the etching away of the sacrificial metals  660 ,  680  to expose the probe contactors  310  and the mounting structures  320  as shown in  FIG. 8 . 
         [0030]    By using this process, the mounting structures  320  may be formed with lithographic precision and using the same process used to create the probe contactors, thereby doing so efficiently. 
         [0031]    While the description above refers to particular embodiments of the present invention, it will be understood that many alternatives, modifications and variations may be made without departing from the spirit thereof. The accompanying claims are intended to embrace such alternatives, modifications and variations as would fall within the true scope and spirit of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims, rather than the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are therefore intended to be embraced therein.