Small contactor for test probes, chip packaging and the like

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This invention relates generally to semiconductor integrated circuits and more specifically to structures for making reliable electrical contact thereto.

The use of small scale packaging, such as Ball Grid Array technology, has created additional challenges in making electrical contact to integrated circuit chips. One problem is making contact to semiconductor chips for testing. Each chip has many test points that must be contacted. The test points are very close together, making it difficult to make good electrical contact to all of the test points at once. The problem exists whether the chip is tested as a packaged part or while still on a semiconductor wafer.

Making contact to a packaged semiconductor part for testing is generally accomplished by pressing the part into a contactor. A contactor is a socket with numerous springy contact elements. One end of each contact element is connected to an automatic test system. The other end of the contact element makes contact with a test point on the integrated circuit chip. For a ball grid array, each solder ball is generally used as a test point.

There is generally one contact element for each test point on the package of the integrated circuit chip. When the test points are very close together, it becomes difficult to provide contact elements that have the required amount of compliance and generate sufficient spring force to make good electrical connection. There are few commercially available contactors suitable for use with ball grid arrays, particularly ball grid arrays where the balls are on a very small pitch, such as 1.27 mm or smaller.

One commercially available contactor has S-shaped contact elements. The looped parts of the "S" engage an elastomeric material to form two springy elements. One side of the S is pressed against the test point and the other is pressed against a pad that is connected to the test system. Such a contactor is difficult and expensive to make. In addition, each time a part is pressed into the contactor, the S shaped contact element scrapes across the pad. This scraping action can cause excessive wear on the pad, limiting the useful life of the contactor. In addition, the ends of the S shaped contact elements act like coupled parallel plate capacitors, which limits electrical performance.

In general, a limiting factor in making contactors for small BGA devices is that the physical space available for the contact elements is very limited. The beams which form the springy portions of the contact elements must therefore be relatively short. Short beams are not as compliant as longer beams and the distance the beams can deflect when making electrical contact, or the "stroke" of the contact element, is therefore limited.

A problem with having a limited stroke is that it significantly increases the accuracy with which the test point on the chip must be positioned relative to the contactor for the contactor to work. This is referred to as the planarity of the contact points. When pressing the chip into the contactor, the test point on the chip should press down on the contact element so that the springy portion of the contact element is deflected enough to generate sufficient spring force to make good electrical contact. For example, the contactor might be designed so that the test point on the chip is intended to deflect the contact element by an amount equal to one half of its stroke. In this case, one half of the stroke is an upper limit on the amount of inaccuracy that is possible in positioning the test point. If the test point is too high by that amount, it will not make contact with the contactor at all. Thus, the greater the stroke of the contact element, the more tolerance there is for inaccurate positioning of the test points on the chip during testing, i.e. lack of planarity of the contact points. Greater tolerance in positioning the test points will translate into a greater likelihood likely that good electrical contact will be made to the test points.

This problem is particularly severe for ball grid array type packages. A ball grid array package might have hundreds of test points. All test points in the array must be contacted simultaneously. Thus, not only must the BGA package be positioned accurately relative to the contact elements, the test points on the BGA package must be positioned accurately relative to the package. For a BGA, this is difficult because the test points are solder balls. The solder balls are generally formed and then placed on pads on the package. There is significant variability in the processes of forming the balls and attaching them to the pads. For example, thermal "warp" of the chip can lead to significant variability. As a result that there can be significant variation in the height of the solder balls. Given the small area to make contact elements for a BGA package, the variation in heights of solder balls is likely to exceed the tolerance because of limited stroke of the contact elements.

Thus, it would be highly desirable to have contact elements, which fit in a limited area, but still have good stroke.

A similar problem exists with making contact to the chips before they are packaged. Many integrated circuit chips are tested while they are still part of the wafer on which they are fabricated. The test points are pads on the surface of the integrated circuit chip. These test points are even smaller and closer together than the test points on the packaged part.

Contact is generally made to the chip for wafer level testing using a "probe card." Traditional probe cards use long tungsten needles for contact elements. Such probe cards are expensive to make and are also easily damaged. In addition, a wafer contains many chips. For economical processing, many chips are tested simultaneously. The probe card must therefore have contact elements which are aligned with the pads on each of several chips. However, there is not sufficient space to position long needles such that they align with pads on adjacent chips on the die. Therefore, when a probe card is set up to probe multiple chips simultaneously, the contact elements on the probe card are positioned to make contact with chips that are widely spaced on the wafer. This arrangement of contact elements reduces the efficiency of the testing process.

Some probe cards have recently been made by attaching small, S-shaped perpendicular to the surface of a ceramic substrate. Tips might be attached to the S-shaped wires with points for better electrical contact. However, such probe cards are complex and expensive to manufacture. They are also very delicate and easily damaged.

It would be highly desirable to have a probe card that is made with very short contact elements that can be aligned with contact pads on adjacent chips on the wafers. It would also be desirable to have such a probe card that is less expensive to manufacture and less susceptible to damage.

It has been suggested to make probe cards with micromachined beams that miniaturize the geometry and mimic the tungsten needles. Micromachining involves processing techniques similar to those used to make semiconductor chips. Layers of material are deposited on a substrate. Masks are used to form protective coatings in particular patterns on the layers. When the layers are exposed to various etching solutions, structures are left that have the required shape. Micromaching has been used for making various structures, such as relays.

However, micromachined beams suffer from various drawbacks. One problem is that it is difficult to generate the required spring force to make good electrical contact. Often, oxide layers form over metal structures, such as would be used to form most test points. There must be sufficient contact force to ensure that the oxide layer is either broken up or pierced by the contact element. A second problem is that the micromachined beams are often so small that, if electric charge builds up on the beam, electrostatic force can deflect the beam. If the beam is deflected, it does not make good electrical contact with the integrated circuit chip.

It has been suggested that probe cards be made with micromachined contacts on a flexible membrane, instead of a rigid substrate. Extra stroke and more contact force is provided by the membrane. However, it would be desirable to have a more effective probe card that does not require additional mechanical structure to flex the probe card. Further, using a flexible substrate, while it might be possible in some probing applications, is impractical for other applications in which small, clustered contact elements are required.

Another problem that has come about because of the high levels of integration of semiconductor devices into small packages is thermal stress. Thermal stress occurs when two items, made of materials with different coefficients of thermal expansion, are rigidly attached or when two items--of similar or dissimilar material--experience a large temperature gradient. If an integrated circuit chip, which is generally made of silicon, were soldered directly to the printed circuit board, differences in the coefficient of thermal expansion between the silicon in the chip and the printed circuit board would stress the solder bonds and might cause the bonds to break. The problem is particularly acute for highly integrated parts because such parts can generate a lot of heat in operation and also because a limited amount of solder can be used for attaching the numerous points of the packaged part to the printed circuit board. Even a silicon chip mounted to a ceramic substrate, if the chip dissipates high power, can experience problems with conventional attachment techniques.

To avoid problems associated with thermal stress, BGA packages are generally made by attaching the integrated circuit chip to a small printed circuit board that is separated from the chip by a compliant structure, such as an elastomeric pad. The solder balls are formed on one side of the small printed circuit board. Wire bonds run from the chip to the printed circuit board. Traces in the printed circuit board connect the wire bonds to the solder balls. The entire structure is encapsulated, leaving just the solder balls exposed.

This type of packaging adjusts for thermal stress that might occur when the BGA is mounted to a large printed circuit board. The printed circuit board inside the BGA package has a coefficient of thermal expansion that matches the printed circuit board to which the BGA package is attached, thereby reducing the thermal stress on the solder joints. There is still a mismatch of thermal expansion between the chip and the small circuit board inside the BGA package. However, the wire bonds between the chip and printed circuit board are flexible enough to adjust for forces caused by the mismatch.

While such packages satisfy the requirement for mounting semiconductor chips, they are relatively complicated to manufacture. It would be highly desirable to be able to package integrated circuit chips without the need for a small printed circuit board inside the package.

In addition, small scale packaging is also generally associated with integrated circuits that operate at high frequencies. It would be desirable to have contact elements suitable for testing at very high frequency.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of the invention to provide a contact structure that can easily be made for use with semiconductor devices.

It is also an object to provide a contact that provides compliance in multiple directions.

It is also an object to provide a contact that provides compliance and a mounting point for a solder ball to thereby allow solder balls of a BGA package to be mounted without the use of an intervening circuit board.

The foregoing and other objects are achieved with a contact having a contact point mounted on a compliant member. The compliant member is bent to provide compliance in multiple directions.

In a preferred embodiment, the number of bends in the compliant member equals or exceeds the number of points at which the compliant member is affixed to a substrate.

In a preferred embodiment, the contact members have a very small scale. They are formed using microelectrical mechanical fabrication techniques and are formed on silicon substrates.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the following more detailed description and accompanying drawings in which

FIG. 1A is an isometric view of an array of contact elements according to the invention;

FIG. 1B is an enlarged view of a portion of FIG. 1A showing greater detail of one contact element;

FIG. 1C shows further detail of the compliant member of a contact element of FIG. 1A;

FIG. 2 shows a step in the fabrication of the array of contact elements of FIG. 1A;

FIG. 3 shows a step in the fabrication of an alternative embodiment of the array of contact elements made according to the invention;

FIG. 4A shows a contact element according to the invention configured to provide a contact pad;

FIG. 4B shows a contact element according to the invention configured to provide a probe tip;

FIG. 5A shows a contact element according to the invention configured with a contact pad holding a solder ball;

FIG. 5B shows an alternative embodiment of a contact element according to the invention;

FIG. 6 is an isometric view of the bonding pads on the surface of an integrated circuit chip;

FIG. 7 shows an alternative embodiment of a contact element according to the invention which is adapted for making a probe for probing bonding pads as shown in FIG. 6;

FIG. 8A is an isometric view of a probe for probing bonding pads as shown in FIG. 6;

FIG. 8B is an enlarged view of a portion of the probe in FIG. 8A;

FIG. 9 is an alternative implementation of the contact element in FIG. 7; and

FIG. 10 is a sketch illustrating a ball grid array package formed according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Copending U.S. patent application by Slocum et al. entitled ROBUST, SMALL ELECTRICAL CONTACTOR, filed simultaneously with this application and hereby incorporated by reference, describes a contact structure which provides reliable and repeatable contact. That contact structure is designed to withstand forces parallel to the substrate which carries an array of contact elements. Because the contact structure is resistant to forces parallel to the substrate, it is not well suited to provide compliance when forces are applied parallel to the substrate.

FIG. 1A shows an array 100 of contact elements 110 fabricated on a substrate 101. Though specific size of the contact elements 110 or the array 100 are not critical to the invention, the invention will be most useful in conjunction with large arrays of relatively small contact elements. Such arrays are needed in many facets of semiconductor fabrication or utilization. It is contemplated that contact elements 110 will be less than 2 mm in any direction. However, the invention will be most useful for making contact to arrays of points that are very close together. For example, ball grid array packages have solder balls that are on a pitch of 1.27 mm and smaller. Thus, in a preferred embodiment contact elements 110 will be 1.3 mm or less in any dimension. Very significantly, the contact elements of the invention are intended to provide the spring force, compliance and robustness for making contact on a very small pitch. Thus, in some embodiments, contact elements 110 will have a maximum dimension of 0.5 mm or less, allowing an array 100 to be constructed with contact elements on a pitch of 0.5 mm.

Various items might be used as a substrate. The choice of substrate material will depend in large measure on the intended use of the array 100 of contact elements. As will be described below, silicon, such as is conventionally used in the fabrication of integrated circuits, and epoxy based material, such as is conventionally used to make printed circuit boards, are two desirable substrate materials.

In a preferred embodiment, substrate 101 will have electrically conducting traces (not shown) running through it. Each contact element 110 is connected to a trace, thereby allowing electrical signals to be routed to and from the contact elements 110.

The contact elements 110 are made of one or more conducting members, generally parallel with substrate 101. The conducting members are made from a springy material with bends in them.

As described in greater detail below, conducting members 110 could be a metal, such as aluminum, nickel or chrome. Alternatively, they could be made from a semiconductor, such as silicon, germanium or gallium arsenide, either doped or undoped. Alternatively, contact elements 110 could be made from a ceramic, such as silicon nitride or silicon carbide, or a silicon oxide. If an insulative material is used as the springy material, a conductive material will be overlayed on the insulative material. For example, a highly conductive material without adequate springiness, such as gold, might be deposited over a silicon nitride beam.

The bends give the contact elements 110 compliance in multiple directions. As shown in FIG. 1C, the bends give contact element 110 four approximately equal lobes. The lobes give contact element 110 a clover-shape. Contact element 110 is bent into two arm portions 106A and 106B and a tip portion 104 (FIG. 1B). Arm portions 106A and 106B give contact element 110 bending and torsional compliance when subjected to forces at tip portion 104.

As a result clover-shaped contact element 110 as shown in FIG. 1A has compliance with three degrees of freedom. Tip portion 104 can move in a direction perpendicular to the surface of substrate 101. It can also move in the two directions that define a plane parallel with the surface of substrate 101.

Additionally, the clover-shaped structure combines bending and torsional stress in a small area, thereby maximizing the compliance potential of the material. This shape allows the contacts to be miniaturized, while still providing the required contact force and compliance.

In a preferred embodiment, conducting element 110 has a thickness of approximately 0.01 mm to 0.05 mm. The overall length of conducting element is approximately 0.15 mm to 1 mm. The bend radius of each of the lobes is 0.01 mm to 0.1 mm.

As seen in greater detail in FIG. 1B, contact element 110 is mounted above the surface of substrate 101. Contact elements 110 are mounted to posts 102A and 102B, that hold contact element 110 above substrate 101. Posts 102A and 102B project, in the illustrated embodiment, above the surface of substrate 101 by about 0.1 mm. This dimension defines the stroke of the connector and will thus be set to equal or exceed the desired amount of deflection at the tip 104.

As shown in FIG. 1B, contact element 110 has two ends 103A and 103B that are mounted to separate posts 102A and 102B. This configuration is desirable for making a probe for "fly-by" testing, as will be described below. Briefly, fly-by testing occurs when a signal is applied to a test point over a separate path than is used to measure signals at the same point. "Fly-by" testing is used with high speed signals in which transmission line effects might cause interference between signals being applied to and measured at a test point. Where "fly-by" type contact is not required, a single post might be used.

Preferably, posts 102A and 102B are electrically conducting. Posts 102A and 102B connect contact element 110 to the conductive traces within substrate 101. If posts 102A and 102B are made of a non-conducting material, conducting pathways can be included through them, such as vias used in circuit board or semiconductor fabrication, to make electrical connection between contact element 110 and the traces within substrate 101.

FIG. 1C shows contact element 110 in greater detail. FIG. 2 shows the posts 102A and 102B on substrate 101 without showing contact element 110. In the embodiment of FIG. 2, posts 102A and 102 are electrically conducting and are physically separate. In the embodiment of FIG. 2, the surface of substrate 101 is an insulator, which may overlay a ground plane within substrate 101. If contact element 110 is formed from an insulator with a conductive trace on one surface, then the surface of substrate 101 can be a conductive ground plane.

FIG. 3 shows an alternative implementation of substrate 101, which forms a base for mounting array 100. In FIG. 3, substrate 101 is formed with a well 111 associated with each contact element 110. As described above, tips 104 of contact elements 110 bend toward substrate 101 when making electrical contact with another structure, such as an integrated circuit chip or printed circuit board. In the embodiment of FIG. 1A, posts 102A and 102B have a height sufficient to provide the required contact stroke. In the embodiment of FIG. 3, posts 102A and 102B do not need to be so high. When a contact element 110 is making electrical contact, tip 104 will bend into recess 111, thereby providing the required stroke. The embodiment of FIG. 3 is useful for manufacturing processes in which it is easier to form a recess than to construct a tall post.

FIG. 4A and 4B illustrate important advantages of a contact element according to the invention. A contact element 402 is shown with the same "clover-leaf" structure of contact element 110. However, base 401 of contact element 402 is solid instead of split as shown in FIG. 1A. Additionally, a post 403 has been fabricated at the tip of contact element 402. Post 403 might, for example, be etched or otherwise fabricated with a point to facilitate good electrical connection with a metal surface on which an oxide layer has formed. Post 403 aids in breaking the oxide layer such that good electrical contact is made to the underlying metal conductor.

The embodiment of FIG. 4B can also be used as the basis form making a compliant pad structure 400. In the embodiment of FIG. 4A, post 403 serves as an attachment point for a pad 410. While the embodiment of FIG. 4B is suited for using the array of contact elements as a probe to make electrical contact to numerous points on an integrated circuit chip, the embodiment of FIG. 4A is useful for making compliant pads on an integrated circuit chip. Pads are often included on the exterior surfaces of packaged integrated circuit chips, particularly BGA type packages. The pads serve as an attachment point of the chip to a printed circuit board or other substrate, such as might be found in a multichip module (MCM). An important advantage of making compliant pads on an integrated circuit chip is that problems associated with thermal stress are greatly reduced.

In FIG. 4A, pad 410 is attached at one end to post 403. Post 403 lifts pad 410 above contact element 402, such that contact element 402 will still provide compliance both parallel and perpendicular to substrate (such as 101 in FIG. 1A) to which the array is mounted.

Pad 410 may optionally be formed with a ring 411. Ring 411 serves to retain a solder ball on pad 410, thereby aiding in miniaturization of the BGA technology. In making BGA type devices, a solder ball is placed on a pad.

FIGS. 5A and 5B show an alternative embodiment of a contact element 500. Contact element 500 has two spiral shaped beams 502A and 502B. One end of each beam 502A and 502B is attached to post 501A or 501B, respectively. Posts 501A and 501B elevate beans 502A and 502B above the surface of a substrate (such as substrate 101 in FIG. 1A). The other end of each beam is attached to tip 503. Tip 503 is thicker than beams 502A and 502B such that the upper surface of tip 503 is above the beams 502A and 502B. In the case of a probe card, tip 503 can be shaped into a point appropriate for piercing an oxide layer on a pad it contacts.

Beams 502A and 503B can deflect down towards a substrate. Alternatively, they can flex at their bends to allow compliance of tip 503 in a plane parallel to the surface of a substrate (such as substrate 101 in FIG. 1A).

Contact clement 500, in the configuration of FIG. 5B, could be used to probe pads, such as might be found on an integrated circuit chip. For probing, tip 503 might be etched to be pointed. A pointed tip improves electrical contact by piercing an oxide layer which might build up on a metal pad to which contact element 500 us to make electrical contact.

Alternatively, a larger pad 510 might be attached to tip 503. Pad 510 is elevated above beams 502A and 502B, thereby allowing tip 503 to have its full range of compliant motions. Pad 510, because it is attached to tip 503 also has a full range of compliant motion perpendicular and in a plane parallel to a substrate (such as 101 in FIG. 1A). Pad 510 might thus be used as an attachment point for a solder ball in a BGA package that is resistant to thermal stress. Pad 510 has an optional ring 511 around its periphery to act as a lip to help restrain solder ball 551.

FIG. 10 illustrates a semiconductor device with a ball grid array package according to the invention. FIG. 10 is drawn with a scale that exaggerates the contact elements. In FIG. 10, a substrate 101 is shown in cross section. In this embodiment, the substrate 101 contains the electronic circuitry of the semiconductor chip. On one surface of the chip, an array of contact elements is formed. In FIG. 10, five such contact elements are shown in a row. In a practical application, there would likely be many more such contact elements, but only five are shown for clarity.

Each contact element 402 is mounted on a post 401 to a surface of the chip that acts as substrate 101. Pads 410 are elevated above contact elements 402 on posts 403. Ring 411 retains a solder ball 551. The entire chip is encased in a housing 1010, which is optional.

In use, the solder balls 551 would be placed against a printed circuit board and heated until they reflow, thereby securing pads 510 to the printed circuit board. Contact element 500 would then provide compliance between the chip and the printed circuit board, making the structure immune to thermal stress.

Other shaped contact elements might be preferable for other configurations. FIG. 6 shows the surface of an integrated circuit chip 600. Integrated circuit chip 600 has numerous pads 601 on it. Pads 601 are laid out in rows along the periphery of chip 600 in what is commonly referred to as a peripheral pad layout. Pads 601 connect to circuit elements within chip 600. When packaging chip 600, wire bonds are made from pads 601 to contact points extending from the package.

Having pads 601 near the periphery of chip 600 is important to allow wire bonding. However, placing all the pads along th