This invention relates to a compliant probe apparatus. In particular, this invention relates to the burn-in and testing of microelectronic devices, and specifically to contact assemblies used for connecting electrical signals to integrated circuits during burning-in and testing of individual chips and of full wafers.
Microelectronic devices are subjected to a series of test procedures during the manufacturing process in order to verify functionality and reliability. Prior art testing procedures conventionally include wafer probe testing, in which microelectronic device chips are tested to determine operation of each chip before it is diced from the wafer and packaged. Prior art probe cards are built of long cantilever wires that are used to test one or several chips at a time at the wafer level.
Typically, not all chips on a wafer are found to be operable in the wafer probe test, resulting in a yield of less than 100% good devices. The wafer is diced into individual chips and the good chips are assembled into packages. The packaged devices are dynamically burned-in by loading into sockets on burn-in boards and electrically operating at a temperature of from 125xc2x0 C. to 150xc2x0 C. for a burn-in period of 8 to 72 hours in order to induce any defective devices to fail. Burn-in accelerates failure mechanisms that cause infant mortality or early failure of the devices, and allows these defective devices to be screened out by a functional electrical test before they are used commercially.
A full functional test is done on packaged devices, which are operated at various speeds in order to categorize each by maximum speed of operation. Testing discrete packaged devices also permits elimination of any devices that failed during the burn-in process. Burn-in and test of packaged devices is accomplished by means of sockets specially suited to the burn-in reconditions and to high speed testing respectively. As a result, conventional manufacturing processes are expensive and time consuming because of repeated handling and testing of individual discrete devices through a lengthy set of steps that adds weeks to the total manufacturing time for the device.
A considerable advantage in cost and in process time can be obtained by burn-in and test of the wafer before it is diced into discrete devices. Additional savings can be obtained by fabricating chip size packages on each device on a wafer before the wafer is diced into discrete devices. A considerable effort has been expended by the semiconductor industry to develop effective methods for wafer level packaging, burn-in and test in order to gain benefits of a greatly simplified and shortened process for manufacturing microelectronic devices. In order to reap these benefits, it is necessary to provide means to burn-in and speed test chips before they are diced from the wafer into individual discrete devices.
Conventional cantilever wire probes, however, are not suited to burn-in and speed testing of devices on the wafer. Cantilever wire probes are too long and bulky to allow simultaneous contact to all of the devices on a wafer, as required for simultaneous burn-in of all of the devices on the wafer. In addition, long cantilever wire probes are not suitable for functional testing of high-speed devices, among other things, because of a high self and mutual inductance of the long, parallel wires comprising the probes.
A small, high-performance probe that can be made at low cost is required for practical application of wafer burn-in and test procedures. To be useful for wafer burn-in and test, the desired probes must reliably contact all of the pads on the devices under test while they are on the undiced wafer. Probes for contacting the wafer must also provide electrical contact to pads on devices even, and especially, where the pads vary in height on the surface of the wafer. In addition, the probes must break through any oxide layers on the surface of the contact pads in order to make a reliable electrical contact to each pad. Many approaches have been tried to provide a cost-effective and reliable means to probe wafers for burn-in and test, without complete success.
The prior art reveals a number of attempts that have been tried to provide small, vertically compliant probes for reliably contacting the pads on devices on a wafer. According to the invention represented by U.S. Pat. No. 4,189,825, a cantilever probe is provided for testing integrated circuit devices. In FIG. 1, cantilever 22 supports sharp tips 26 above aluminum contact pads 24 on a chip 23. A compliant member 25 is urged downward to move tips 26 into contact with pads 24. An aluminum oxide layer on pad 24 is broken by sharp tip 26 in order to make electrical contact between tip 24 and the aluminum metal of pad 24. The rigidity of small cantilever beams, however is generally insufficient to apply the force to a tip that is necessary to cause it to break through an aluminum oxide layer on a contact pad, without an external means of applying force to the cantilever. Cantilever beams of glass, silicon, ceramic material, and tungsten have also been tried in various configurations, without success in providing burn-in probes of sufficient force and flexibility.
A flexible membrane probe is described in Flexible Contact Probe, IBM Technical Disclosure Bulletin, October 1972, page 1513 as shown in FIG. 2A. A flexible dielectric film 32 includes terminals 33 that are suited to making electrical contact with pads on integrated circuits. Terminals 33 are connected to test electronics by means of flexible wires 34 attached to contact pads 35 on terminals 33. Probes fabricated on a flexible polyimide sheet were described in the Proceedings of the IEEE International Test Conference (1988) by Leslie et al. The flexible sheet allows a limited amount of vertical motion to accommodate variations in height of bond pads on integrated circuits on a wafer under test. Membrane probes such as that described by Leslie et al provide connections to integrated circuit chips for high performance testing. However, dimensional stability of the membrane is not sufficient to allow contacts to pads on a full wafer during a burn-in temperature cycle.
Fabrication of the contacts on a thin silicon dioxide membrane as described in U.S. Pat. No. 5,225,771 is shown in FIG. 2B. A silicon dioxide membrane 40 has better dimensional stability than polyimide, thereby somewhat ameliorating the dimensional stability problem of mating to contact pads on a wafer under burn-in test. Probe tips 41 are connected by vias 44 through membrane 40 to circuit traces 45 that are linked to an additional layer of circuitry 42 above a dielectric film 43. However, limited vertical compliance of the test probes on silicon dioxide membrane 40 renders use of such probe arrays unreliable for use in burn-in of devices on a semiconductor wafer.
Fabrication of an array of burn-in probes on a semiconductor wafer is described in U.S. Pat. No. 4,585,991, especially as illustrated in FIGS. 3A and 3B showing a top plan view and a sectional view respectively. Probe 51 is a pyramid attached to semiconductor wafer substrate 52 by arms 54. Material 53 is removed from the semiconductor wafer 52 in order to mechanically isolate the probe 51. A probe as in FIG. 3A provides a limited vertical movement but it does not allow space on the substrate for wiring needed to connect an array of probes to test electronics required for dynamic burn-in.
Another marginally successful approach to providing flexible probes to device contact pads involves the use of flexible wires or posts to connect the test circuitry to the pads. A flexible probe is described in U.S. Pat. No. 5,977,787 as shown in FIG. 4A. There, probe 60 is a buckling beam probe, earlier generally described in U.S. Pat. No. 3,806,801. The buckling beam probe 60 is adapted for use in burn-in of devices on a wafer. The buckling beam probe 60 is held by guides 61 and 62 that have a coefficient of expansion similar to that of the wafer being tested. The probe tip 63 is offset by a small distance 64 to provide a definite modality of deflection for the buckling beam probe 60. Although buckling beams are well suited to testing individual integrated circuit chips, they are too expensive to be used for wafer burn-in where thousands of contacts are required. Further, electrical performance of buckling beam probes is limited because of the length required for adequate flexure of the beam.
Another approach using flexible posts as disclosed in U.S. Pat. No. 5,513,430 is shown in FIG. 4B. FIG. 4b shows flexible probes in the form of posts 66 that are able to bend in response to force on probe tip 67. Posts 66 are formed at an angle to a substrate 69 in order to allow them to flex vertically in response to a force on tip 67 from mating contact pads. Posts 66 have a taper 65 from the base terminal 68 to tip 67 in order to facilitate flexure.
Yet another approach using flexible wires and posts as disclosed in U.S. Pat. No. 5,878,486 is shown in FIG. 4C. The probe shown in FIG. 4C comprises a probe tip 72 on a spring wire 71 that is bent to a specific shape in order to facilitate flexure. Wire 71 is joined to substrate 74 by a conventional wire bond 73. Probes of the type shown in FIG. 4C require a long spring length to achieve the contact force and compliancy needed for wafer burn-in. Additionally, such probes that use individual wires are too expensive for use in wafer burn-in where many thousands of probes are required for each wafer.
Further approaches to providing flexible probes involve the use of compliant layers interposed between a test head and a device being tested, such that terminals on the test head are electrically connected to mating contact pads on the device. The electrical connector described in U.S. Pat. No. 3,795,037 utilizes flexible conductors embedded in an elastomer material to make connections between mating pairs of conductive lands that are pressed into contact with the top and bottom surfaces of the electrical connector. Many variations of flexible conductors are known including slanted wires, conductive filled polymers, plated posts and other conductive means in elastomeric material in order to form compliant interposer layers.
The approaches listed above, however, and other attempts have been unsuccessful in providing a high performance probe that allows economical burn-in and speed test of microelectronic devices on a wafer before the wafer is diced into discrete chips.
In accordance with the present invention, a small compliant probe is disclosed that includes a conductive tip, which is positioned on a supporting surface in a manner that allows a tip on the probe to move flexibly with respect to the supporting surface. In a preferred embodiment, the probe tip moves vertically in response to the force of a mating contact pad as it is biased against the tip. Mechanical compliance of the probe of the present invention allows electrical contact to be made reliably between the probe and a corresponding contact pad on a microelectronic device, where the mechanical compliance accommodates variations in height of the contact pad.
It is an object of the present invention to provide a method and means for making electrical connection to contact pads on microelectronic devices on an undiced wafer in order to burn-in the devices before they are diced into separate chips. Compliant probes according to the invention allow reliable electrical connections to be made simultaneously to all of the contact pads arrayed on the surface of a wafer so that microelectronic devices on the wafer can be burned-in economically.
Another object of the present invention is to provide a fixture for burn-in of microelectronic devices on undiced wafers. The fixture electrically connects contact pads on each device to drive circuitry that supplies electrical signals to the device as required during dynamic burn-in at high temperature. Electrical signals and power are supplied to all of the chips on a wafer simultaneously. Mechanical compliance of probes in the fixture accommodates variations in height of the contact pads and in the probe tips such that each probe tip remains in contact with its mating contact pad throughout the temperature cycle of the burn-in process.
Yet another object of the preferred embodiment of the present invention is to provide an electrical probe card that allows high speed testing of unpackaged microelectronic devices. Small, compliant probes as taught herein are used to make temporary connections to corresponding pads on a device in order to apply electrical test signals to that device and to measure electrical signals from that device. The small size of the compliant probe allows high speed electrical signals to be passed to and from the device without losses due to excessive inductance or capacitance associated with wire probes as used in the prior art.
A further object of the present invention is to provide a means for burning-in, testing and operating microelectronic devices where electrical contacts on the device are disposed in an area array over a surface of the device. Small, compliant probes as taught in this disclosure are used to make reliable electrical connections to contacts on the device, where the contacts are arranged in an area array. Mechanical compliance allows the tip of each probe to maintain electrical contact with a mating contact on the device notwithstanding variations in the height of contacts on the device both at room temperature and at the operating temperature range of the device.
Another object of a preferred embodiment of the present invention is to provide a small socket for connecting integrated circuit chips to electrical circuits for purposes of burn-in, test and operation of the chip. The small size of each probe contact in the socket allows high-speed operation of a chip mounted in the socket. Mechanical compliance of the probes as taught herein enables reliable electrical connections to be made to a rigid chip with minimal or no packaging. Compliant probes according to the present invention allow construction of small, economical sockets for chip scale packages and for flip-chips.
The probe disclosed herein is significantly improved over conventional cantilever probes in that it provides a greater range of compliant motion of the probe tip for any given probe force and probe size. A conventional cantilever probe is limited in the range of motion it provides in response to a given force before the elastic limit of the probe material is reached. The maximum mechanical stress in cantilever probes is concentrated on the surface of the cantilever material at the point of flexure. The present invention provides a greater range of motion for a given spring material and probe force, before reaching the elastic limit of that material.
The invention increases manufacturing efficiency for microelectronic devices by reliably providing test and burn-in functions at the wafer level, while at the same time reducing the size of the test fixture. The mechanically compliant probe of the present invention provides a large range of motion relative to the size of the probe. This range of motion is important in making connections to a device with contact pads that are not substantially in the same plane. The compliant probe tip of the present invention moves flexibly to accommodate differences in the height of mating contact pads while maintaining sufficient force of the probe tip on the contact pad to assure reliable electrical contact there between.
These as well as other objects of the invention are met by providing a mechanically compliant electrical probe. In a preferred embodiment, a probe tip is disposed on an elongated thin strip of material that is supported at both ends and wherein the tip is positioned at a predetermined distance from a center line connecting the centers of the supports at each end of the strip. The probe tip thus supported moves compliantly in a vertical direction by torsional and bending flexure of the thin strip of material.