Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2001-02-20
2003-01-14
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S754090
Reexamination Certificate
active
06507207
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to semiconductor wafer probing apparatus, and more particularly, to an improved contact probe pin device including an elongated conductor component having a coating of a substantially less conductive metal with high elastic properties.
2. Description of the Prior Art
Discrete semiconductor devices are typically formed on a semiconductor substrate employing precise photolithographic techniques. A plurality of such devices are usually formed on a single wafer and the wafer is subsequently sliced into separate individual die upon completing the fabrication process. However, prior to slicing, the partially finished devices are often tested using a wafer probe apparatus to electrically engage bond pads on the dice to evaluate the electrical characteristics thereof.
Conventional probe technologies have historically included two basic types of probe apparatus, i.e., those using cantilever type bond pad engaging probe pins and those using vertically aligned die pad engaging probe pins. Cantilever probes are comprised of a plurality of substantially horizontally disposed cantilevered probe pins extending radially inward from a probe card and then usually downwardly to form die pad engaging tips. Contact pressure is determined by the amount shear strain or bending displacement of the probe tip and the pin's stress-strain profile. Since the stress-strain profile and hence the contact force, decreases with increasing cantilever length, poor or incomplete penetration of a device's inner bond pad's passivation layer may result when longer cantilever probe pins are used. Accordingly, cantilever type probes are typically design restricted to devices having peripheral device bond pads.
Vertical probe pin cards include generally vertically aligned, straight or near straight pins arranged in an ordered, addressable array. The array coincides with the bond pad positions of the device under test. Vertical probe pin densities are normally higher than cantilever-type probes, and multiple devices may be tested by a single probe card (i.e., grid array probing).
The contact force of vertical-type probe pins is determined by either the compressive strain of the pin, or by resilient material against which the end of the probe pin may act. One problem with this type of probe is that, depending on the pin configuration, the contact force of the probe pin tip may increase with increasing vertical displacement of the probe. Another problem is that in order to ensure uniform probe tip contact force across all bond pads, the probe tips must be carefully aligned in the Z-plane; i.e., have a high probe tip planarity.
Still another problem with vertical probe tip assemblies is the criticality of precise vertical control of the probe assembly. The vertical position of the entire probe assembly must be precisely controlled to ensure that the probe pin contact pressure is uniform and within a predetermined contact force across all of the bond pads on the semiconductor device. Whereas the extent of the probe assembly's vertical travel must be sufficient to ensure a contact force sufficiently high to pierce the passivation layer of all the bond pads, it must not extend so far as to exert excessive contact pressure which might damage the probe, the bond pad, or nearby wire connections. This vertical displacement criticality arises in part as a result of the compressive strain spring constant of vertical-type probe pins. Since contact force is proportional to probe tip vertical displacement, precise control of the vertical travel of the probe assembly is necessary to ensure uniform contact force and penetration of the passivation layer at each bond pad.
The ability of vertical-pin probe tips to penetrate the passivation layer of the bond pad is aggravated in some probe devices in that the vertical displacement of the probe tip does not create an X or Y force component to allow the probe to “scrub” or “shear” through the passivation layer. Consequently, vertical pin probe tips crush the passivation in order to establish electrical contact between the probe tip and the bond pad. As a result, relatively higher contact forces are needed to pierce the passivation layer. This in turn results in lower device yields due to an increased likelihood of damage to the bond pads and connecting wires, and increased particulate contamination.
A further problem with vertical pin probes arises as the bond pad density increases with increasing integration density of a semiconductor device. The resulting increase in the number of probe pins needed to test the semiconductor device exerts a correspondingly increased cumulative force in the semiconductor device and the probe card. For example, in the case of a probe card having 500 probe pins, a force of 5 grams exerted by each probe pin results in a cumulative force of about 2.5 kilograms exerted on the device under test and on the entire probe card. As the number of probe pins increases even further, the vertical load on the probe card also increases resulting in possible deformation of the probe assembly, which in turn degrades probe tip planarity.
Still another problem with vertical pin probes is that because the wire used to make the probe pins is very small in diameter, there is a likelihood that as axial load is applied to the pin, it may experience deformation exceeding its intended delection and engage other pins. It is possible that the deflection may exceed the pin's elastic limit and cause it to become permanently deformed. On the other hand, pins made of such wire may experience situations in which the wire stiffness is not sufficient to cause penetration of the passivation layer or otherwise not make good ohmic contact with the bond pad. Accordingly, material selection for the conductive wire plays a substantial part in the design of the probe pin; that is, not only must the material have high conductivity, it must have good resilience properties. This usually affects the cost of the material selected.
Various attempts to address the aforementioned problems have been tried, including the designs disclosed in my prior U.S. Pat. No. 5,952,843 entitled “Variable Contact Pressure Probe”, the disclosure of which is expressly incorporated herein by reference. However, there still exists, a need for an improved pin design for use in wafer probe assemblies having high pin density. There's also a need for a probe design wherein the cumulative contact force of multiple probes within a wafer probe assembly may be controlled even as probe pin count/density increases. Such pin design usually must be small in transverse dimension in order to enable high pin density, be stiff enough to apply appropriate contact force, and be resilient enough to return to its rest configuration when withdrawn from contact with the semiconductor devices. There is also a need for a pin design allowing wire of high conductivity but limited stiffness to be used.
SUMMARY OF THE INVENTION
It is therefore an important objective of the present invention to provide a pin design having means for enhancing the resiliency characteristic of the conduction wire used to form a probe pin.
Another objective of the present invention is to provide a probe pin design in which the elastic or resilient properties of the wire material used in the pin plays a less critical role than was heretofore the case.
Still another objective is to provide a probe pin design in which the flex characteristics can be controlled substantially independent of the flex characteristics of the conductor material.
Briefly, a probe pin in accordance with the present invention includes an elongated metal conductor having a connector (or proximal) end, a center or medial section coated with an elastic material, and a contact pad engaging tip end. More specifically, at least a portion of the center section of the probe pin is coated with a poorly conductive, but highly elastic metal so as to enhance the flex characteristics of the probe
Hamrick Claude A. S.
Oppenheimer Wolff & Donnelly LLP
Sherry Michael
Tang Minh N.
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