Abstract:
A probe tip structure that decreases the accumulation rate of Sn particles to the probe tip and enables considerably more efficient and complete laser cleaning is disclosed. In an embodiment, the probe structure includes an array of probe tips, each probe tip having an inner core; an interfacial layer bonded to the inner core; and an outer layer bonded to the interfacial layer, wherein the outer layer is resistant to adherence of the solder of the ball grid array package.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    Embodiments of the invention relate generally to integrated circuit (IC) devices. More particularly, the subject matter disclosed herein relates to a probe tip structure for testing an array of solder bumps of an integrated circuit and for testing an integrated circuit. 
         [0002]    Testing products with very large arrays of solder bumps require probes that are durable, accurate and reliable. Achieving good (low resistance, repeatable) electrical probe contact with a solder bump requires some degree of penetration of the bump by the probe tip in order to bypass any surface oxide. As shown in  FIG. 1 , the present state of art is to force rigid cylindrical probe tips  10  deposited on a ceramic or organic substrate (not shown) into the surface of tin (Sn) solder bumps  15 . The cylinders are generally formed from copper (Cu) electroplating through a resist mask, after which the resist is stripped, and nickel (Ni) and gold (Au) layers are plated over the Cu. 
         [0003]    An array of probe tips cannot be made to be perfectly planar and parallel to the solder bumps on the device to be tested. One common way to overcome this challenge is to have some compliance built into the design of the probe. Examples of compliant probes are cantilever, cobra, buckling beam as well as some membrane or elastomeric probes. However, compliant probes take up too much space due to their spring structure. Additionally, compliant probes are more expensive and complex, and also have limited current carrying capability. 
         [0004]    Rigid probes such as thin film interposer (TFI) and probe on substrate (POS) were developed to enable testing of high power products that have a large number of closely spaced solder balls. Rigid probes overcome planarity variations by using a high force to deform the highest solder bumps enough so all bumps make electrical contact. The amount of force required can be controlled by making the probe tips smaller in diameter, minimizing the hardness of the solder bumps and improving control of solder bump and probe tip planarity. In other words, a rigid probe structure relies on compliance of the solder bumps themselves. 
         [0005]    Referring back to  FIG. 1 , a problem with probe tips  10  of the present state of art is that after testing several wafers, the contact resistance begins to rise, eventually obscuring some of the desired test results. Contact resistance rises partially due to the accumulation and oxidation of Sn particles  20  which cling to probe tips  10  and decrease the surface of the probe tips  10  that contacts the solder. To restore probe tip  10  to its original low contact resistance value, probe tip  10  is subjected to a chemical cleaning process to remove Sn particles  20 . Unfortunately, this procedure is lengthy and can damage probe tip  10  or other parts of the assembly such as the substrate. The process also uses dangerous chemicals. 
         [0006]    In order to increase efficiency and safety, a laser cleaning process was introduced to ablate Sn particles  20  from probe tips  10 . This process is relatively fast and does not cause damage to probe tip  10 . However, the results of the laser cleaning process are not as good as the chemical clean. Consequently, laser cleaned probes need to be cleaned more often than chemically cleaned probes. During the laser clean, the temperature of the probe tip  10  can exceed the melting point of Sn, such that the Au and Sn form alloys at relatively low temperatures. While the Sn melting temperature is much lower than that of Au or Cu, by the time enough energy is applied to evaporate the Sn particles  20 , there is sufficient energy to alloy the Sn and the Au or Cu. In some cases, these alloys can cause the Sn particles  20  to become more firmly attached to the Au or Cu. These residual Sn particles  20  accrue additional particles more easily than a clean surface and shorten the time between cleanings. 
       SUMMARY OF THE INVENTION 
       [0007]    Various embodiments include a probe tip structure that decreases the accumulation rate of Sn particles to the probe tip structure and enable considerably more efficient and complete laser cleaning. 
         [0008]    In one embodiment, a probe tip structure for a test application of solder on a ball grid array package is provided. The probe tip structure comprises an array of probe tips, each probe tip having an inner core; an interfacial layer bonded to the inner core; and an outer layer bonded to the interfacial layer, wherein the outer layer is resistant to adherence of the solder of a ball grid array package. 
         [0009]    In another embodiment, a solder bump array probe is provided. The solder bump array probe comprises a substrate; an array of probe tips directly on the substrate, the probe tips having an inner core; an interfacial layer bonded to the inner core; and an outer layer bonded to the interfacial layer, wherein the outer layer is resistant to adherence of a solder; wherein the inner core of the probe tips comprises a layer of high conductivity metal; and wherein the outer layer comprises a material with a higher melting point than that of the inner core. 
         [0010]    Another embodiment of the invention provides for a rigid probe structure. The rigid probe structure comprises an array of probe tips disposed on a rigid substrate, each probe tip having an inner core; an interfacial layer bonded to the inner core; and an outer layer bonded to the interfacial layer, wherein the outer layer is resistant to adherence of solder of a ball grid array package; and wherein the substrate is mounted to a turn table; and wherein the turn table is inclined at an angle to an incident laser and the turn table is rotatable about a perpendicular axis of the substrate. 
     
    
     
       BRIEF DESCRIPTION OF SEVERAL VIEWS OF THE DRAWINGS 
         [0011]    These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
           [0012]      FIG. 1  shows a cross-sectional depiction of the prior art. 
           [0013]      FIG. 2  shows a cross-sectional depiction of an inner core in cylindrical form in accordance with embodiments of the invention. 
           [0014]      FIG. 3  shows a cross-sectional depiction of an inner core having tapered side walls after undergoing an etching process. 
           [0015]      FIG. 4  shows a cross-sectional depiction of an inner core having side walls with a substantially cylindrical bottom and substantially hemispherically shaped top. 
           [0016]      FIG. 5  shows a cross-sectional depiction of an inner core having side walls with a substantially cylindrical bottom and a tapered top. 
           [0017]      FIG. 6  shows a cross-sectional depiction of an inner core with bell-curve shaped side walls. 
           [0018]      FIG. 7  shows a cross-sectional depiction of an inner core after deposition of an interfacial layer. 
           [0019]      FIG. 8  shows a cross-sectional depiction of an inner core and interfacial layer after deposition of an outer layer. 
           [0020]      FIG. 9  shows a cross-sectional depiction of another embodiment of the invention with an inner core, first interfacial layer, second interfacial layer, and outer layer. 
           [0021]      FIG. 10  shows a cross-sectional depiction of a portion of an embodiment of the invention mounted on a turn table exposed to a laser. 
           [0022]      FIG. 11  shows a perspective view of a probe tip structure having an array of probe tips according to embodiments of the invention 
       
    
    
       [0023]    It is noted that the drawings of the invention are not necessarily to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0024]    As indicated above, the subject matter disclosed herein relates to integrated circuit (IC) devices. More particularly, the subject matter disclosed herein relates to a probe tip structure for testing an array of solder bumps of an integrated circuit and for testing an integrated circuit. 
         [0025]    In the following description, reference is made to the accompanying drawings that form a part thereof, and in which is shown by way of illustration specific exemplary embodiments in which the present teachings may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the present teachings and it is to be understood that other embodiments may be utilized and that changes may be made without departing from the scope of the present teachings. The following description is, therefore, merely exemplary. 
         [0026]    The rigid probe tip structure disclosed herein decreases the accumulation rate of solder particles on the probe tip. Conventional cylindrical probe tips have sharp corners which are the primary sites for solder particle retention after repeatedly inserting and withdrawing the probe tips from solder bumps. Etching removes the sharp corners of conventional probe tips, and the resulting shape resists the adherence of solder particles. Thus, the surface area that may contact the solder bump may increase with the decrease of solder particle adherence. This allows the passage of more time between probe cleanings, and improves the overall life and efficiency of the probe tip. 
         [0027]      FIG. 8  shows one embodiment of the invention wherein a rigid probe tip  100  has tapered side walls  101 . As will be described in detail, probe tip  100  may include an inner core  112 , an interfacial layer  114 , and an outer layer  118 . It is to be understood that  FIGS. 2-9  show a single probe tip for ease of understanding. However, the probe tip structure may comprise an array of probe tips. 
         [0028]    Referring now to  FIGS. 2-7 , various embodiments of inner core  112  according to embodiments of the invention will now be described.  FIG. 2  shows a cross-sectional view of one embodiment of inner core  112 . Inner core  112  may comprise a layer of high conductivity metal, for example, Cu. Inner core  112  may be created using well known photolithography processes. For example, a photo resist may be applied to the substrate, and then exposed and etched to create holes into which copper is deposited, the excess copper and resist is subsequently stripped off leaving the substantially cylindrical copper posts of  FIG. 2 . The substantially cylindrical posts of  FIG. 2  may then be further modified with additional etching and perhaps mask steps to produce the embodiments shown in  FIGS. 3 ,  4 ,  5  and  6 . “Etching” generally refers to the removal of material from a substrate (or structures formed on the substrate), and is often performed with a mask in place so that material may be selectively removed from certain areas of the substrate, while leaving the material unaffected in other areas of the substrate. Etching may include any now known or later developed techniques appropriate for the material to be etched including but not limited to, for example: isotropic etching, anisotropic etching, plasma etching, sputter etching, ion beam etching, reactive-ion beam etching and reactive-ion etching (RIE). Etching may also be accomplished with mechanical abrasion of the probe tip for example with abrasive loaded elastomers. 
         [0029]    Etching inner core  112  as described above removes the upper corners of the cylinder where solder particles are most likely retained. Inner core  112  may be etched such that inner core  112  may be shaped in any of the embodiments shown in  FIGS. 3-6 .  FIG. 3  shows inner core  112  with tapered side walls  101 .  FIG. 4  shows inner core  412  with side walls  401  that have a substantially cylindrical bottom  401   a  with a substantially hemispherically shaped top  401   b .  FIG. 5  shows inner core  512  with a shape that has a substantially cylindrical bottom  501   a  with a tapered top  501   b , such that a portion of probe tip  500  has substantially vertical side walls  501 . 
         [0030]      FIG. 6  shows a probe tip  600  that is substantially bell curve shaped. In this embodiment, inner core  612  has undergone a heavier etch such that more of inner core  612  is removed, providing a probe tip  600  that is thinner than the other embodiments shown in  FIGS. 3-5 .  FIG. 6  also shows probe tip  600  wherein side walls  601  meet the bottom of probe tip  600  at a slant  625  such that the lower corners of the Cu cylinder are also removed. Slant  625  prevents solder particles  920  (shown in  FIG. 10 ) from becoming lodged in the lower corners. It is understood that the lower corners of the Cu cylinder of the embodiments of  FIGS. 3-5  may also be removed during the etching process such that each of the embodiments have slants  625 . 
         [0031]    The shape of the probe tips can be optimized based on the type of product tested. For example, a large high power chip with a solder bump of low elasticity may have a probe tip with more vertical sides, such as probe tip as shown in  FIG. 5 , to maintain contact with the solder bump during small thermally induced deformations. A chip with lower power, or more elastic solder bumps may be tested with probes that have more sloping sides so the probes do not need to be cleaned as often, such as probe tip as shown in  FIGS. 3 ,  4 , and  6 . It is also understood that embodiments of the present invention are not limited to the geometries shown in  FIGS. 3-6  and may be of other shapes that accomplish the same advantages. 
         [0032]    Referring now to  FIGS. 5-10 , subsequent processing of a probe tip will now be described. While  FIGS. 7-10  refer to only inner core  112  as illustrated in  FIG. 3 , it is understood that the teachings are applicable to any embodiment of inner core from  FIGS. 2-6 . As shown in  FIG. 7 , once inner core  112  has been etched to a desired shape, interfacial layer  114  may be deposited over inner core  112  such that it is bonded mechanically and electrically to inner core  112 . Interfacial layer  114  may include, but is not limited to, nickel (Ni), titanium (Ti), and/or tantalum (Ta). As described herein, “deposition” may include any now known or later developed techniques appropriate for the material to be deposited including but are not limited to, for example: chemical vapor deposition (CVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), semi-atmosphere CVD (SACVD) and high density plasma CVD (HDCVD), rapid thermal CVD (RTCVD), ultra-high vacuum CVD (UHCVD), limited reaction processing CVD (LRPCVD), metalorganic CVD (MOCVD), sputtering deposition, ion beam deposition, electron beam deposition, laser assisted deposition, thermal oxidation, thermal nitridation, spin-on methods, physical vapor deposition (PVD), atomic layer deposition (ALD), chemical oxidation, molecular beam epitaxy (MBE), electro-plating and electroless-plating, and evaporation. 
         [0033]    As shown in  FIG. 8 , outer layer  118  may be deposited on interfacial layer  114  such that it is bonded mechanically and electrically to interfacial layer  114 . Outer layer  118  may be deposited using any appropriate deposition techniques as listed herein. Outer layer  118  may include, but is not limited to, molybdenum (Mo), titanium nitride (TiN), and/or tantalum nitride (TaN). For example, inner core  112  may include Cu, interfacial layer  114  may include Ni, and outer layer  118  may include Mo. In another example, inner core  112  may include Cu, interfacial layer  114  may include Ta, and outer layer  118  may include TaN. 
         [0034]    With regard to the deposition materials chosen for the present application, outer layer  118  has a melting point much higher than that of Sn. If the melting point of outer layer  118  is relatively close to that of Sn, then there is a possibility that part of probe tip  100  will be evaporated if enough energy is provided on the tip to evaporate the Sn. Therefore, outer layer  118  may have a melting point six to ten times higher than that of Sn so that outer layer  118  will not be damaged by the laser energy sufficient to vaporize Sn. Additionally, outer layer  118  has a lower thermal conductivity which causes the heat generated by the laser to be contained more closely to the surface where solder particles  920  (shown in  FIG. 10 ) are located. This prevents alloying of the Sn particles to the layers  112 ,  114 ,  118 . 
         [0035]    Interfacial layer  114  and outer layer  118  may be deposited such that the coating is thick enough (100 to 30,000 Angstroms) that it achieves good coverage over inner core  112 , and so that the heat penetration from the laser does not diffuse into inner core  112 . Additionally, etching inner core  112  before depositing interfacial layer  114  and outer layer  118  allows probe tip  100  to more readily accept deposited materials thereon. For example, with respect to cylindrical probes, it is more difficult to coat the vertical side walls with other materials by some types of deposition. 
         [0036]    Interfacial layer  114  and outer layer  118  need to bond to inner core  112 . Therefore, there may be some alloying between probe tip layers  112 ,  114 ,  118  themselves. For example, in the case where outer layer  118  includes TaN, TaN does not bond very well to an inner layer  112  of Cu, thus an interfacial layer  114  of Ta may be used. Ta bonds well to both Cu and TaN. This same principle applies in the case where interfacial layer  114  includes Ni and outer layer  118  includes Mo. Thus, the constitution of interfacial layer  114  may be selected by both the materials used for inner core  112  and outer layer  118 . 
         [0037]    Referring now to  FIG. 9 , another embodiment of the invention is shown wherein interfacial layer  114  of  FIG. 8  further comprises a first interfacial layer  815  and a second interfacial layer  817 . An outer layer  818  is positioned over second interfacial layer  817 . In this embodiment, an inner core  812  (similar to inner core  112 ) may include Cu, first interfacial layer  815  may include Ni, second interfacial layer  817  may include Ti, and outer layer  818  may include TiN. In this embodiment, first interfacial layer  815  acts as a barrier between inner core  812  and second interfacial layer  817  because Ti diffuses easily with Cu. Again, it is to be understood that first and second interfacial layer  815 ,  817  are not limited to include only Ti and TiN, but instead are determined by both the materials used for inner core  812  and outer layer  818 . For example, other materials may include, but are not limited to, Rhodium (Rh), Rhenium (Re), Rhenium Diboride (ReB 2 ), and Iridium (Ir). 
         [0038]      FIG. 10  shows a portion of an embodiment of the invention wherein the probe tip structure  980  has an array of probe tips  900  having solder particles  920  are subjected to laser cleaning. The array of probe tips  900  may be deposited on a rigid ceramic or organic substrate  930 . The embodiment of  FIG. 10  is shown with the array of probe tips  900  having tapered side walls  901 , however, it is understood that the following discussion also applies to any of the other embodiments and geometries as shown in  FIGS. 3-6 . As probe tip structure  980  is exposed to a laser (shown by the dotted arrows), tapered side walls  901  of probe tips  900  will absorb more laser energy than probes with more vertical sides; however some of the laser energy will be reflected from the tapered sides. 
         [0039]    The amount of laser energy absorbed by a surface will depend on several factors including the angle of the surface with respect to the laser energy. A vertical laser will heat up horizontal surfaces much more than vertical surfaces. Laser cleaning will be more effective on probes with sloping sides. The sides of probes can be more effectively cleaned by temporarily mounting the substrate  930  to a turntable  940  during laser cleaning. Turntable may be inclined at an angle to the laser power and rotated during cleaning, so that all surfaces will receive enough energy to clean contamination off the surfaces. For example, turntable  940  may be inclined at an angle of 25-75 degrees. The turn table  940  may also rotate about an axis that is perpendicular to substrate  930 , as indicated by arrows  945 , so that all sides of the probes may get cleaned. 
         [0040]    Referring now to  FIG. 11 , the probe tip structure  980  having an array of probe tips  900  is shown for testing an array of solder bumps (not shown) wherein each probe tip  900  tests a solder bump. Generally, an array of probes may require between 2,000 and 20,000 probes. However, in other aspect, an array may require over 150,000 probes. Each probe tip  900  may include any of the probe tips described above. 
         [0041]    Spatially relative terms, such as “inner,” “outer,” and the like, may be used herein for each of description to describe one element or features relationship to another element(s) or feature(s) as illustrated in the figures. Spatially relative terms may be intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. 
         [0042]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand embodiments disclosed herein.