Patent Publication Number: US-11029334-B2

Title: Low force wafer test probe

Description:
DOMESTIC PRIORITY 
     This application is a continuation of U.S. application Ser. No. 15/208,185, titled “Low Force Wafer Test Probe” filed Jul. 12, 2016, the contents of which are incorporated by reference herein in its entirety. 
    
    
     BACKGROUND 
     The present invention generally relates to testing wafers on which electronic circuits are formed, and more particularly, to a test configuration for testing an electronic circuit. 
     An important facet of the semiconductor industry resides in being able to provide satisfactorily functioning semiconductor devices. In particular, such semiconductor devices may comprise wafers which are divided into areas which form chips, the shapes and dimensions of which are as close to identical as possible, so as to impart consistent uniform electrical properties thereto. 
     Generally, semiconductor devices on chips are ordinarily connected to each other with thin strips of metal, referred to in the art as interconnection metallurgy, which in turn contact the wafer surface through a series of pads or bumps. Other connector pad configurations may include an array of electrical contacts or bumps which are distributed over an area; for instance, the widely employed C4 bumps (controlled collapse chip connects). Such bumps or electrical contacts extend above the integrated circuits and have a generally spherical or round cross-sectional configuration. 
     Although wafers are formed as uniformly as possible through current manufacturing techniques, it is not always feasible that every chip produced is perfect. In order to identify defective chips, electrical tests are performed to facilitate the sorting out of good chips and eliminating defective chips prior to the next step of manufacture. 
     Ordinarily, active testing of the wafers is performed by a test facility in which the pads or areas on wafers possessing arrays of bumps, such as of C4 bumps, are contacted by an assembly incorporating test probes. In order to successfully probe the integrity of the pads or bumps, it is desirable that an oxide layer, which inevitably forms on the surface of the C4 bumps, be ruptured and penetrated to ensure good electrical contact with the probe while employing only a minimal force to inhibit damaging the pads or bumps. 
     SUMMARY 
     In one aspect, the present invention provides a probe including a pedestal and at least one feature extending from the pedestal to engage a surface of a corresponding contact at a position offset from a central longitudinal axis of the contact. 
     In one aspect, an embodiment of the present invention provides a probe including a pedestal and at least one feature extending from the pedestal to engage a surface of a corresponding contact. The at least one feature applies a lateral force to the contact to urge the contact into a desired position relative to the probe when the contact is misaligned relative to the probe. 
     In another aspect, an embodiment of the present invention provides a probe including a pedestal and at least one feature extending from the pedestal to engage a surface of a corresponding contact. The at least one feature is configured to shear an outer surface of the contact. 
     In one aspect, an embodiment of the present invention provides a probe including a pedestal and a cavity extending inwardly from a first end of the pedestal. At least one feature is formed within the cavity to engage a surface of a corresponding contact. A radial diameter of the at least one feature is less than ⅕ of a diameter of the contact to minimize the contact force applied by the at least one feature to the contact. 
     In another aspect, an embodiment of the present invention provides a probe including a pedestal and a cavity extending inwardly from a first end of the pedestal. At least one feature formed within the cavity is configured to engage a surface of a corresponding contact. A configuration of the at least one feature is selected such that a first test and a second test are performed on the contact without a reflow operation. 
     In one aspect, an embodiment of the present invention provides a probe includes a pedestal and at least one feature extending from the pedestal to engage a surface of a corresponding contact. The probe being configured such that when the probe is engaged with the contact, a center of the contact remains undeformed. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       In order that the advantages of the invention will be readily understood, a more particular description of the invention briefly described above will be rendered by reference to specific embodiments that are illustrated in the appended drawings. Understanding that these drawings depict only typical embodiments of the invention and are not therefore to be considered to be limiting of its scope, the invention will be described and explained with additional specificity and detail through the use of the accompanying drawings. 
         FIG. 1  is side view of a system including an array of probes and a wafer in an inactive position according to an embodiment; 
         FIG. 2  is side view of a system including an array of probes and a wafer in an active position according to an embodiment; 
         FIG. 3  is a perspective view of a probe according to an embodiment; and 
         FIG. 4  is a perspective view of the probe of  FIG. 3  arranged in engagement with a C4 bump according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     Citation of “a specific embodiment” or a similar expression in the specification means that specific features, structures, or characteristics described in the specific embodiments are included in at least one specific embodiment of the present invention. Hence, the wording “in a specific embodiment” or a similar expression in this specification does not necessarily refer to the same specific embodiment. 
     Hereinafter, the present invention and various embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Nevertheless, it should be understood that the present invention could be modified by those skilled in the art in accordance with the following description to achieve the excellent results of the present invention. Therefore, the following description shall be considered as a pervasive and explanatory disclosure related to the present invention for those skilled in the art, not intended to limit the claims of the present invention. 
     Referring now to  FIG. 1 , an example of an array  30  of test probes  32  used to test the electrical conductivity of an electrical semiconductor wafer  20  is illustrated. The semiconductor wafer  20  includes a plurality of conductive bumps  22 , also referred to as controlled collapse chip connects (“C4 bumps”), which have a generally spherical or curved top shape. The semiconductor wafer  20  may include a base layer  24  formed from a substrate, such as a silicon material for example, having a specific structure or openings. The base layer  24  may include a single or multiple layers of material. The wafer  20  as supported in a test fixture may or may not have the capability of moving in one or more directions prior to electrical contact for indexing the position of the wafer  20  and the circuit on the wafer  20  to be tested. 
     As shown, the probes  32  of the array  30  are mounted to a structure  34  in a configuration such that each probe  32  is substantially aligned with one of the C4 bumps  22  on a semiconductor wafer  20 . Each of the probes  32  has a longitudinal axis X which passes through the center of the probe  32  such that a height of each probe  32  may be measured along the longitudinal axis. When the probe  32  is in contact with C4 bumps  22 , as shown in  FIG. 2 , the distance between the structure  34  and the C4 bumps  22  is less than the length of the probes  32 , to ensure that a pressure or force is applied to each of the C4 bumps  22 . The pressure or force provides penetration of the C4 bumps  22  resulting in piercing and exposing a new clean contact surface free of oxide below the former surface  26  of the C4 bump  22 . 
     Referring now to  FIGS. 3 and 4 , a perspective view of a probe  32  configured for use in the array  30  and configured to engage a corresponding contact on the wafer  20  is illustrated. The probe  32  is formed from a substantially rigid material, such as copper for example. In the illustrated, non-limiting embodiment, the probe  32  includes a generally cylindrical pedestal  40  having a cavity  42  formed therein. The cavity  42  may be formed through any suitable process, such as by removing material via photo etching or micro-machining, or alternatively, may be formed through an additive manufacturing process such as electroplating for example. 
     The cavity  42  extends inwardly from a first planar surface at the first end  36  of the probe  32  positioned generally adjacent and parallel to the upper surface  28  of the wafer  20 . The diameter of the cavity  42  may vary based on the size of a corresponding C4 bump  22  that the probe  32  is configured to contact; however, the diameter of the cavity  42  is generally less than the diameter of the C4 bump  22  such that during a test operation, only a limited portion of the C4 bump  22  is received within the cavity  42 . 
     Formed within the cavity  42  are one or more features  46  configured to contact the exterior surface  26  of the C4 bump  22 . In an embodiment, the features  46  include blades that extend generally inwardly from the periphery of the cavity  42  towards a central axis thereof. The illustrated, non-limiting embodiment includes three blades  46  spaced substantially equidistantly about the periphery of the sidewall  48  of the cavity  42 . However, embodiments having any number of features  46  located at any position about the cavity  42  are contemplated herein. The support of these features  46  by an outer ring or pedestal  40  increases the strength of the probe elements  32  and reduces the stress of the probe  32  generated during contact with a corresponding C4 bump. This described configuration allows for the use of smaller features, thereby achieving a lower force performance than required by a probe having a free standing pin structure. In addition, these smaller, sharper, strategically placed features  46  impart higher localized stress on the C4 bumps for purpose of piercing the oxide layer, while minimizing global deformation. 
     The portion of each blade  46  adjacent the first end  36  of the probe  32 , extends generally vertically, parallel to the exterior surface  38  of the probe  32  and the central longitudinal axis of the probe  32  and the C4 bump. As the blade  46  approaches the interior end  50  of the cavity  42 , the blades may have a contour, such as a radius for example, generally complementary to the contour of the cavity  42 . With such an arrangement, the features  46  are generally configured to contact a portion of the C4 bump  22  offset from the central longitudinal axis Y thereof. As a result, the center of the C4 bump  22  remains undisturbed through the testing, thereby eliminating the need for a reflow operation. In addition, the orientation and contour of the features  46  may be selected to apply a lateral force on the C4 bump  22  to urge the corresponding C4 bump  22  into a desired position within the cavity  42  in the event that central axis of the probe  32  and central axis of the C4 bump  22  are misaligned. By including a plurality of features  46  positioned about the periphery of the cavity  42 , the features  46  ensure proper alignment between the probe and C4 bump  22  regardless of the direction of offset of the C4 bump  22  relative to the probe  32 . 
     The portion of each feature  46  configured to contact a C4 bump  22  is generally small in relation to not only the surface area of the C4 bump  22  but also to the diameter of the probe  32 . For example, a total surface area of the plurality of features configured to engage the contact is generally less than 10%, such as between about 5% and 10% of a total surface area of the contact. In an embodiment, the radial thickness of each feature is between 1/20 and ⅕ of the outer diameter of the C4 bump  22 . In an embodiment, the portion of each feature  46  configured to contact the C4 bump  22  is designed to shear the surface layer  26  of the C4 bump  22 , as opposed to compressing the surface  26  as occurs with conventional probes. This shearing that occurs maximizes the displacement of the oxide layer and provides optimized electrical contact between the probe  32  and an unoxidized solder layer. 
     By limiting the area of contact between the features  46  and the C4 bump  22 , the stress at the interface between the probe  32  and a corresponding C4 bump  22  may be reduced or even minimized. In addition, the amount of solder that is deformed during a test is also reduced. As a result, multiple tests may be performed on a C4 bump  22  before the solder of the C4 bump  22  requires reforming. For example, a first test using a first probe having one or more features  46  as described herein may be performed on a C4 bump  22 . A second test may then be performed using a second probe having features  46  positioned at different areas of the C4 bump  22  than the first probe. The first probe and the second probe may have different configurations, or alternatively may be substantially identical, but oriented at an angle relative to one another. 
     It should be understood that the probe illustrated and described herein is intended as an example only and that other probe configurations are also within the scope of the disclosure. 
     The foregoing detailed description of the embodiments is used to further clearly describe the features and spirit of the present invention. The foregoing description for each embodiment is not intended to limit the scope of the present invention. All kinds of modifications made to the foregoing embodiments and equivalent arrangements should fall within the protected scope of the present invention. Hence, the scope of the present invention should be explained most widely according to the claims described thereafter in connection with the detailed description, and should cover all the possibly equivalent variations and equivalent arrangements.