Probe head structure for probe test cards

A probe head assembly for testing a device under test includes a plurality of test probes and a probe head structure. The probe head structure includes a guide plate and a template and supports a plurality of test probes that each includes a tip portion with a tip end for making electrical contact with a device under test, a curved compliant body portion and a tail portion with a tail end for making electrical contact with the space transformer. Embodiments of the invention include offsetting the position of the tail portions of the test probes with respect to the tip portions of the test probes so that the tip portions of the test probes are biased within the apertures of the guide plate, using hard stop features to help maintain the position of the test probes with respect to the guide plate and probe ramp features to improve scrubbing behavior.

FIELD OF THE INVENTION

This invention relates generally to integrated circuit testing using probe test cards.

BACKGROUND

The approaches described in this section are approaches that could be pursued, but not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated, the approaches described in this section may not be prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.

In semiconductor integrated circuit manufacturing, it is conventional to test integrated circuits (“IC's”) during manufacturing and prior to shipment to ensure proper operation. Wafer testing is a well-known testing technique commonly used in production testing of wafer-mounted semiconductor IC's, wherein a temporary electrical connection is established between automatic test equipment (ATE) and each IC formed on the wafer to demonstrate proper performance of the IC's. Components that may be used in wafer testing include an ATE test board, which is a multilayer printed circuit board that is connected to the ATE, and that transfers the test signals between the ATE and a probe card assembly. Conventional probe card assemblies include a printed circuit board, a probe head assembly having a plurality of flexible test probes attached thereto, and an interposer that electrically connects the test probes to the printed circuit board. The test probes are conventionally mounted to electrically conductive, typically metallic, bonding pads on a substrate using solder attach, wire bonding or wedge bonding techniques.

In operation, a device under test is moved into position so that the test probes make contact with corresponding contact points on the device under test. When contact is made, the test probes flex, which causes the tips of the test probes to move laterally on and “scrub” the contact points on the device under test. This scrubbing action is desirable because it removes any oxides or other material that may be present on the contact points, providing better electrical contact.

One of the challenges with probe card assemblies is how to reduce the amount of “overdrive” that is required to ensure that all test probes contact a device under test. In the context of probe test cards, the term “overdrive” generally refers to the distance traveled after the first test probe has made contact with a device under test. In most probe card assemblies, since the tips of the test probes are not co-planar, once the first test probe has made contact with the device under test, additional travel is required to ensure that all test probes make contact with the device under test. In applications with poor planarity, the amount of required overdrive can be substantial. For example, in situations where the tip-to-tip planarity is in the range of about 30 to 50 microns, approximately 100 microns of overdrive may be required to ensure that all test probes contact the device under test. Large amounts of overdrive are undesirable because it can shorten the life of test probes, typically measured in the number of “touchdowns”, damage test probes, and/or cause shorts between test probes. In view of the foregoing, a probe head assembly that does not suffer from limitations of prior probe head assemblies is highly desirable.

DETAILED DESCRIPTION

II. Probe Head Structure

III. Test Probe Offset

IV. Hard Stop Features

V. Probe Ramp Features

A probe head assembly for testing a device under test includes a plurality of test probes and a probe head structure. One or more test probes from the plurality of test probes includes a tip portion with a tip end for making electrical contact with the device under test, a curved compliant body portion and a tail portion with a tail end for making electrical contact with another element in the probe head assembly, for example a space transformer.

The probe head structure includes a guide plate having a plurality of apertures formed therein and a template having a plurality of apertures formed therein. The tip portions of the one or more test probes are disposed through the plurality of apertures in the guide plate, the tail portions of the one or more test probes are disposed through the plurality of apertures in the template and the curved compliant body portions of the one or more test probes are disposed between the guide plate and the template. The template and guide plate position and align the one or more test probes to match a pattern of desired test points on the device under test.

Embodiments of the invention include configuring the shape of the curved compliant body portions of the one or more test probes so that for each test probe, the tail portion is offset with respect to the tip portion so that the tip portion is biased within one of the apertures of the guide plate. The biasing of the test probes in this manner improves the allowable movement and corresponding scrubbing provided by the tip ends against the device under test. According to one embodiment of the invention, the offset is configured to provide little or no lateral force on the tail portions of the test probes in the template. This helps the tail ends maintain contact with corresponding contacts on the space transformer while reducing the tendency of the tail ends to scrub on the space transformer (or other element). Embodiments of the invention also include using hard stop features to help maintain the position of the test probes with respect to the guide plate and probe ramp features to improve scrubbing behavior. The probe head assembly allows probe head structures with easier assembly, reduced test probe pitch, increased flexibility in test probe design, improved test probe tip-to-tip planarity, improved control over tail ends on the space transformer or other element during contact with a device under test and improved scrubbing behavior at the test probe tips.

II. Probe Head Structure

FIG. 1depicts an example probe card assembly100for testing a device under test (DUT). Probe card assembly100includes a printed circuit board (PCB)102, a space transformer104and a probe head structure106. The PCB102provides electrical connections to test equipment and space transformer104provide electrical connections between the PCB102and the probe head structure106, which is typically smaller than the space transformer104. Space transformer104may be made as a single layer of material or from multiple layers. For example, space transformer104may be a multi-layer ceramic (MLC). As another example, space transformer104may be a Multi-Layer Silicon (MLS) space transformer made using silicon wafer fabrication techniques. An MLS space transformer may provide finer contact pitch, compared to an MLC space transformer. PCB102and space transformer104are not required for the invention, but are depicted inFIG. 1to show how probe head structure106interacts with other example elements.

Probe head structure106supports a plurality of test probes108that make contact with a device under test. Probe head structure106includes a template110and a guide plate112positioned via spacer elements114. Test probes108extend through apertures in guide plate112and through apertures in template110and make contact with contact points on space transformer104. Examples of contact points include, without limitation, pads and stud bumps. Template110and guide plate112position and align test probes108to match a pattern of desired test points on a device under test. The use of a single guide plate112with template110provides easier assembly than designs that use two guide plates and reduces the likelihood of damaging test probes during assembly. Using a single guide plate112with template110also allows a greater variety of test probes to be used and provides good planarity between test probes. Spacer elements114provide a desired spacing between the template110and the guide plate112and may also be used to attach the probe head structure106to the probe card assembly100. Template110may be attached directly to space transformer104, e.g., by bonding, or may be spaced apart from space transformer104as depicted inFIG. 1, depending upon a particular implementation. Other structures, e.g., a fastener structure, may be used to hold probe head structure106in position with respect to space transformer104that are not depicted inFIG. 1for purposes of explanation.

Template110, guide plate112and spacer elements114may have a variety of shapes and dimensions, depending upon a particular implementation. For example, template110, guide plate112and spacer elements114may be rectangular or circular in shape, or may have irregular shapes. An example range of thickness for template110is about 50 microns to about 125 microns. An example range of thickness for guide plate112is about 250 microns to about 675 microns. According to one embodiment of the invention, the thickness of template110is about 75 microns and the thickness of guide plate112is about 500 microns. Guide plate112may be implemented by a single layer of material or multiple layers of material, e.g., multiple layers of silicon, and may include multiple layers of the same material or multiple layers of different materials, depending upon a particular implementation.

Template110, guide plate112and spacer elements114may be made from a variety of materials. Example materials for guide plate112include, without limitation, silicon, silicon nitride, plastic and quartz. An example material for template110is a polyimide. Space transformer104may be made of silicon to provide accurate positioning of contact pads that contact the tail ends of the test probes. Example processes for making guide plate112from silicon include, without limitation, Micro-Electro-Mechanical Systems (MEMS) and Deep Reactive Ion Etching (DRIE) micromachining processes. One benefit provided by the MEMS and DRIE processes is that they allow rectangular apertures or slots to be formed in guide plate112, which are more compatible with rectangular-shaped test probes, e.g., when the test probes are made using semiconductor fabrication techniques. Rectangular apertures or slots also provide better directional control over the deflection and bending of test probes. For silicon nitride, a laser fabrication process may be used. Other materials may be used, depending upon the requirements of a particular implementation.

According to one embodiment of the invention, template110and/or guide plate112are made from a rigid material to provide adequate alignment and thermal stability of the test probes, to ensure proper contact with a device under test. One or more portions or the entirety of template110and/or guide plate112may be coated, for example, with a non-conductive material. Example non-conductive materials include, without limitation, insulating coating materials such as silicon dioxide (SiO2), rubber and other non-conductive materials. The use of non-conductive material in the apertures of guide plate112prevents shorts between test probes if the guide plate material is not sufficiently insulating. Example materials for spacer elements114include, without limitation, metals, such as steel, or other rigid materials that have good flatness and provide stability for template110and guide plate112.

Test probes108may be fabricated using a variety of techniques, depending upon a particular implementation. For example, test probes108may be stamped, electro-formed, or fabricated using semiconductor fabrication techniques. Electrolithographically-produced test probes may have fine features that are difficult to create using stamping techniques. Test probes108may be any type of test probe, such as cantilever test probes or vertical test probes. Test probes108may be made from a wide variety of materials, depending upon a particular implementation, and the invention is not limited to test probes made of particular materials. Example materials include, without limitation, nickel alloys, copper alloys, aluminum, copper or any other metals or alloys. Test probes108may also have a wide variety of shapes, depending upon a particular implementation. For example, test probes108may be round or rectangular and may be straight, bent or curved. Test probes108made from wires are typically round, while test probes108made using semiconductor fabrication techniques are typically rectangular. Test probes108may be partially or fully coated to change their physical or conductive characteristics. Test probes108may also be fabricated with features, e.g., notches, ridges, lips, protrusions, etc., that automatically position the test probes108within the template110and guide plate112.

According to one embodiment of the invention, test probes108are pre-buckled so that they deflect in generally a specified direction when test probes108make contact with a device under test. This reduces the likelihood that test probes108will deflect and/or bend in different directions and contact each other causing shorts when moved into contact with a device under test. It also increases the predictability of positioning of test probe tips on a device under test.

III. Test Probe Offset

FIG. 2Ais a block diagram that depicts an expanded portion of the probe card assembly100depicted inFIG. 1. InFIG. 2A, only portions of template110and guide plate112and a single test probe200are depicted. Test probe200includes a tip portion202with a tip end204, a curved compliant body portion206and a tail portion208with a tail end210. Tip portion202extends through an aperture in guide plate112. Tail portion208extends through an aperture in template110.

Curved compliant body portion206is disposed between template110and guide plate112and provides a downward spring force on test probe200. Curved compliant body portion206may be the same size as the other portions of test probe200, or may be a different size, depending upon a particular implementation. For example, curved compliant body portion206may have a smaller or larger cross sectional area than the remaining portions of test probe200to provide a desired amount of spring force. The cross sectional area of curved compliant body portion206may vary along the length of curved compliant body portion206. Curved compliant body portion206may also have different shapes, depending upon a particular implementation. Although depicted in the figures as having a single, continuous curve, curved compliant body portion206may have different shapes, such as S-shaped or a compound curve.

Curved compliant body portion206may also have a wide variety of dimensions, depending upon a particular implementation. For example, where test probe200has a total length of about 2000 microns to about 3000 microns, an example length212of curved compliant body portion206is about 1000 microns to about 2000 microns. An example width214, as measured from a longitudinal axis216of tip portion202, is about 200 microns to about 500 microns. According to one embodiment of the invention, curved compliant body portion206has a length212of about 1500 microns and a width214of about 300 microns.

As depicted inFIG. 2A, tail portion208is offset from tip portion202. More specifically, the longitudinal axis218of tail portion208is offset from the longitudinal axis216of tip portion202by an offset220. The amount of offset220may vary depending upon a particular implementation. One example range of values for offset220is about 50 microns to about 150 microns. According to one embodiment of the invention, the value of offset220is about 100 microns.

Offsetting tail portion208with respect to tip portion202biases test probe200within an aperture in guide plate112, which maximizes the allowable movement and provides improved control over the scrubbing provided by tip end204against a device under test.FIG. 2Bdepicts a close-up view of test probe200depicted inFIG. 2A. InFIG. 2B, test probe200is shown biased against the walls of aperture222. Specifically, the spring force provided by curved compliant body portion206and the offset of the tail portion208with respect to the tip portion202causes test probe200to contact guide plate112at a first location224aadjacent a top surface112aof guide plate112. Test probe200also contacts guide plate112at a second location224badjacent a bottom surface112bof guide plate112. According to one embodiment of the invention, second location224bis generally opposite first location224a.

Offsetting tail portion208with respect to tip portion202also biases test probe200within an aperture of template110, which reduces movement of the tail portion208and allows tail end210to maintain contact with a corresponding contact on space transformer104when the test probe200contacts a device under test. According to one embodiment of the invention, curved compliant body portion206is configured so that there is little or no lateral force on tail portion208. This reduces the likelihood of damage to tail portion208. InFIG. 2B, tip end204is depicted as being pyramidal-shaped for explanation purposes only and tip end204may be a wide variety of shapes and sizes, depending upon a particular implementation, and the invention is not limited to test probes having a particular probe tip shape.

IV. Hard Stop Features

As previous described herein, curved compliant body portion206applies a spring force to probe200, causing tip portion202to be pushed downward into guide plate112. According to one embodiment of the invention, one or more hard stop features are provided on test probes to help maintain the position of the test probes with respect to the guide plate. The hard stop features provide improved tip-to-tip planarity by ensuring the tips of the test probes are in close vertical proximity. For example, using the probe head structure described herein, tip-to-tip planarity in the range of a few microns has been achieved. This significantly reduces the amount of overdrive required to ensure that all test probe tips make contact with the device under test.

FIG. 2Cdepicts a test probe200that includes a hard stop feature226attached to tip portion202of test probe200adjacent guide plate112. Curved compliant body portion206, only a portion of which is depicted inFIG. 2C, exerts a downward force on test probe200. As depicted inFIG. 2C, hard stop feature226limits the amount of downward travel of test probe200when hard stop feature makes contact with upper surface112aof guide plate112.FIG. 2Ddepicts test probe200when in contact with a device under test. In this figure, the contact with the device under test has moved test probe200upward so that hard stop feature226is not in contact with upper surface112aof guide plate112. This also compresses curved compliant body portion206. When test probe200no longer contacts the device under test, then the spring force provided by curved compliant body portion206forces test probe200downward until hard stop feature226contacts the upper surface112aof guide plate112, as depicted inFIG. 2C.

Hard stop feature226may be formed as part of test probe200. For example, hard stop feature226may be formed as part of test probe200when test probe200is created, for example using a wide variety of lithography techniques. Thus, test probe200and hard stop feature226may be formed together as a single element. Alternatively, hard stop feature226may be formed separately and bonded to test probe200. Hard stop feature226may have a wide variety of shapes, for example, round, rectangular, or odd shaped, and is not limited to the example shape depicted in the figures. An example width228of hard stop feature226is about 5 microns to about 25 microns, but the width228may vary considerably depending upon a particular implementation. Since the width228of hard stop feature226needs to be sufficient to prevent further downward movement of test probe200, the width may vary, for example, based upon a variety of factors including, without limitation, the cross sectional area of tip portion202within guide plate112and the cross sectional area of aperture222.

A single hard stop feature226is depicted in the figures for purposes of explanation, but multiple hard stop features may be used. Multiple hard stop features may be disposed on a single side of test probe200above the guide plate112. Alternatively, hard stop features may be disposed on both sides of test probe200above guide plate112. Hard stop feature226may be made from a variety of materials and may be the same material as test probe200, or a different material, depending upon a particular implementation. Example materials include, without limitation, nickel alloys, copper alloys, aluminum, copper or any other metals or alloys. Hard stop feature226may also be made from a non-conducting material that does not significantly interfere with the conductivity of test probe200.

V. Probe Ramp Features

According to one embodiment of the invention, probe ramp features are used to improve scrubbing characteristics. As used herein, a “probe ramp feature” refers to a portion of a test probe where the cross sectional area of the test probe varies along the length of test probe. For example,FIG. 2Edepicts the use of a probe ramp feature230on the tip portion202of test probe200that is between guide plate112and tip end204. In this example, the tip portion202of test probe200below guide plate112has a cross sectional area that increases along the length of the tip portion202and then decreases adjacent tip end204. When the test probe200makes contact with a device under test, probe ramp feature230makes contact with guide plate112, which provides a controlled scrub.FIG. 2Fdepicts test probe200after making contact with a device under test and, because of the contact, test probe200has moved upward in aperture222, but not yet made contact with probe ramp feature230.FIG. 2Gdepicts test probe200making contact with the tapered portion of probe ramp feature230, which causes test probe200to move to the left and scrub the device under test, as indicated by reference numeral232. Depending upon the amount of overdrive, the flat portion of ramp230may make contact with guide plate112.

Compared to test probes without a ramp feature230, test probes with a probe ramp feature230exhibit a more controlled and consistent scrub, with a reduced likelihood of test probe-to-test probe shorting and a reduced likelihood of damage to test probes. The use of a probe ramp feature230also provides a more consistent contact resistance (CRes) reading. An example test probe force provided by the use of a probe ramp feature230as described herein is from about 0.02 grams/micron to about 0.10 grams per micron. According to one embodiment of the invention, a test probe force of about 0.04 grams/micron to about 0.08 grams per micron is achieved.

Probe ramp feature230may have a variety of shapes and sizes, depending upon a particular implementation, and the shape and size of probe ramp feature230is not limited to the examples depicted in the figures and described herein. For example,FIG. 3Adepicts a tip portion300of a test probe that includes a probe ramp feature302. In this example, probe ramp feature302includes a more rounded top portion304than ramp feature230. As another example,FIG. 3Bdepicts that ramp feature302includes both a more rounded top portion304and a more rounded bottom portion306.FIG. 3Cdepicts example dimensions for probe ramp feature302. In this example, probe ramp feature302has a length308of about 25 microns to about 50 microns and a width310of about 2 microns to about 10 microns. According to one embodiment of the invention, probe ramp feature302has a length308of about 45 microns and a width310of about 6 microns. Using a wider probe ramp feature increases the amount of lateral movement at the test probe tip and therefore increases the amount of scrub. Increasing the steepness or slope of a probe ramp feature increases the rate at which the test probe tip moves. For example, for a given speed at which a device under test is moved towards a probe head assembly, or vice versa, a steeper probe ramp feature will cause a test probe tip to scrub faster, while a more gradual probe ramp feature will cause the test probe tip to scrub slower.

The use of test probe tail offsets, hard stop features and probe ramp features may be used alone or in any combination, depending upon a particular implementation, and the invention is not limited to any particular combination of these features. For example, some implementations may include only test probe tail offsets, hard stop features, or probe ramp features. Other implementations may include various combinations of test probe tail offsets, hard stop features and probe ramp features.

In the foregoing specification, embodiments of the invention have been described with reference to numerous specific details that may vary from implementation to implementation. Thus, the sole and exclusive indicator of what is, and is intended by the applicants to be the invention is the set of claims that issue from this application, in the specific form in which such claims issue, including any subsequent correction. Hence, no limitation, element, property, feature, advantage or attribute that is not expressly recited in a claim should limit the scope of such claim in any way. The specification and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.