Abstract:
An apparatus to heat and test a semiconductor wafer includes a probe card and tests a plurality of die simultaneously at the wafer level. In the present invention, the apparatus heats the wafer to sufficient temperatures to perform burn-in and a speed test. A method of testing the semiconductor allows certain die to be repaired that would otherwise be scrapped in a conventional process where bun-in and other tests are performed on packaged die. The method also eliminates steps associated with handling individually packaged parts, reduces burn-in space and consolidates certain test steps.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to semiconductor testing devices and to testing and/or burn-in methods for semiconductor devices, and particularly to the testing of high density integrated circuits such as DRAMs, SRAMs and embedded devices including “system on a chip” and graphic accelerators with embedded DRAM. 
     2. Description of the Related Art Integrated circuit devices, including DRAM and SRAM memory devices, are typically tested several times at the wafer level and again after dicing and packaging to ensure device quality and reliability. In a conventional testing process, individual devices on the wafer are tested to distinguish good devices from defective ones. The defects found at this stage of testing typically originated during device fabrication. If redundant circuits are present on the devices, a test/sort process is performed to detect reparable devices, which are typically laser repaired to activate the redundant circuitry. The good devices are sorted from the irreparable defective devices. A wafer separation unit, such as a scribe and break mechanism, separates the wafer into individual devices or die, and the good devices are individually packaged. After packaging, a pre-burn-in test (or open/short test) is typically performed to screen out packaging assembly defects, for example shorts or the existence of leakage current that will further degrade during burn-in. Burn-in is then performed in the form of a reliability stress test designed to accelerate failure mechanisms by operating at elevated temperatures. Following burn-in, a post burn-in test is performed to screen out the burn-in stress test defects. Those devices lost during burn-in are devices that likely would fail before the end of their specified life during normal operation. Defects are identified in these parts because the failure modes of the parts are accelerated by the burn-in process. The devices are then unloaded from the burn-in apparatus and a speed test at high temperature is performed. The packaged devices are then marked, and final tests are carried out to ensure that they will operate reliably at room temperature. Consequently, four or five different tests and anywhere from four to seventy-six hours of burn-in are required to ensure the quality and reliability of any given device. 
     High-density memory integrated circuits, e.g. 64 megabit or larger memory chips, require even longer test times than prior generation memories, since more time is needed to test these larger memory chips due to the longer execution test pattern. In addition, to achieve an acceptable level of yield from high density memories, extra die repair tests and processes are typically needed. At the end of these test and repair cycles, often 60-80% of the finally yielded good parts have undergone repairs. This more significant level of testing required by higher density memories can be costly and time-consuming. Conventional wafer probe technology may only test a single die at a time, or at most, up to 32 die simultaneously, and simultaneous testing of a greater number of die is constrained, in part, by the physical limitation of probe tip design. 
     Conventional wafer probing systems have relatively long probe tips which cause impedance mismatch problems when tests are carried out on the high density memory units. As the density of a device increases, high speed testing becomes necessary, and the tester must have good high frequency characteristics. Accordingly, probe lengths should be reduced as much as possible to allow testing at high speed or at high temperatures using a high frequency test signal. Due to the above limitations, post burn-in testing of high speed devices is typically performed after packaging. Such discrete component testing requires a large quantity of expensive burn-in systems, burn-in boards with expensive sockets that can accept packaged units, and additional labor associated with the loading and unloading of individual devices to and from the burn-in board. Further, these conventional processes typically scrap an additional one to three percent of the devices after post burn-in that would otherwise have been repairable through activation of redundant circuitry had they not been packaged before the burn-in test. The total cost per yielded packaged device is therefore unnecessarily increased by the equipment cost of the discrete component testing, the loss of devices that would be repairable had their defects been detected at the wafer level and the requirement of additional test cycles for the packaged parts. Accordingly, there is a need for a wafer level defect and reliability testing apparatus that can also perform many of the tests that currently take place after packaging. For example, it would be advantageous to perform the burn-in and high-temperature/high speed testing on all devices simultaneously while they are still in wafer form. 
     SUMMARY OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a probe card with a short probe length to enable high temperature, high speed testing of high density devices while reducing the impedance mismatching problems typically associated with such testing. The invention may also relate to a probing and heating apparatus that uses the probe card to enable substrate level testing at elevated temperatures and methods for using the apparatus to test and burn-in devices at the substrate, e.g. wafer, level. 
     A probe card for electrically interfacing a plurality of devices on a substrate to be tested to a testing unit includes a plurality of probe tips disposed on a surface of the probe card facing the substrate and arranged in a manner corresponding to a plurality of contact pads on the devices of the substrate. The probe card also includes a plurality of signal contacts for conducting signals to and from the testing unit, each signal contact electrically connected to a probe tip. Use of preferred embodiments of the present invention may provide reduced impedance mismatch during high speed testing of high density semiconductor devices. 
     The probe card described is included in a substrate probing and heating apparatus for testing the electrical characteristics of a plurality devices on a substrate at a plurality of temperatures. The probing and heating apparatus also includes a support unit to align and hold the substrate and the probe card to one another. The support unit includes two support members that may be disengagably coupled to one another. A substrate is removably mounted on a planar surface of the first support member, the substrate having device contact pads facing away from the planar surface. The first support member also includes a heater. The probe card is removably mounted on a second support member. The apparatus also includes a means for disengagably coupling the first member and the second member to one another in a fixed position with the substrate and probe card therebetween to achieve electrical contact between each probe tip of the probe card and the corresponding contact pad of the substrate. Use of preferred embodiments of the present invention may allow testing of high density devices at high speeds in substrate form thereby eliminating a number of steps of the conventional process required when the substrate is separated into individual packaged units before performing certain tests such as burn-in. 
     A method for handling and testing a plurality of devices on a substrate at a plurality of temperatures includes the steps of providing a substrate having a plurality of devices to be tested through a plurality of contact pads, providing a probing and heating apparatus as described above, a defect testing system and a heater driver/control unit. The substrate is mounted on the first member of the support unit and the two support members are coupled to one another. The substrate is then heated to a pre-determined temperature and the electrical characteristics of the devices are tested with the tester while the devices are maintained at the pre-determined temperature. In other embodiments additional steps are performed, such as providing a laser circuit repair unit and repairing defective devices detected during testing, distinguishing the good devices from the defective devices, providing a substrate separation unit to separate the substrate into individual devices, separating the substrate into individual devices using the substrate separation unit and sorting the good devices for packaging and final test. 
     Embodiments of the present invention are of particular use when the tested substrate is a semiconductor wafer containing high density memory circuits. 
     The invention will be best understood from the following description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 a  is an exploded lateral sectional view of an embodiment of the substrate probing and heating apparatus. 
     FIG. 1 b  is alateral sectional view of the embodiment of FIG. 1 a  assembled. 
     FIG. 2 a  is a top view of a detail of an embodiment of a probe card. 
     FIG. 2 b  is a bottom view of the embodiment of FIG. 2 a.    
     FIG. 3 is a lateral sectional view of a detail of an embodiment of the probe card of FIG. 2 a  through section line  3 — 3  aligned above a lateral sectional view of a substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention relates to a probe card with a short probe length to enable high temperature, high speed testing of high density devices while reducing the impedance mismatching problems typically associated with such testing. Other aspects of the invention relate to a probing and heating apparatus that uses a probe card in accordance with the invention to enable substrate level testing at elevated temperatures. Still other aspects of the invention relate to methods for using the apparatus to handle, test and burn-in devices on a substrate. 
     As shown in FIGS. 1 a  and  1   b  (collectively, FIG.  1 ), an embodiment of a substrate heating and testing apparatus  10  may include a support unit that includes a first support member  11 , a second support member  12  and a probe card  13 . A substrate  14 , for example a semiconductor wafer, can be removably mounted on the first support member on a planar surface  15 . The substrate has a plurality of devices (not shown) having a plurality of contact pads (not shown) that face away from the planar surface  15 . The first support member also includes a heater  16 . The probe card  13  is removably mounted on the second support member  12 . The probe card  13  has a plurality of probe tips  34  (FIG. 3) that are disposed on the surface of the probe card that faces the substrate  14  (FIG.  1 ). Each probe tip is electrically connected to a signal lead for conducting signals between an external tester and the probe tip. The support unit also includes a means for disengagably coupling the first member and the second member to one another in a fixed position to achieve alignment and electrical contact between the probe tips of the probe card  13  and the contact pads of the substrate  14 . In some embodiments, the coupling means is a mechanical clasp that fixes the two members to one another. In the embodiment of FIG. 1, the mechanical clasp includes two L-shaped hinges  17 , each coupled to a support member. Each hinge has two arms, a first arm  18  coupled to the second support member and a second arm  19  pivotally associated with the first arm  18 . The second arm may pivot about the first so that it comes to rest at an open position having an approximate 180° angle with the first arm as shown in the phantom drawing of FIG.  1 . When both second arms  19 ,  19  are in this open position, the support members may be brought together and coupled to one another as shown in FIG. 1 b . The second arm may then be pivoted about the first to lock into an approximate 90° angle, thereby coupling the two support members to one another. Although FIG. 1 shows both hinges coupled to the second support member, other coupling configurations will be apparent. 
     Preferably, the substrate heating and probing apparatus  10  simultaneously tests a plurality of devices on the substrate over a range of pre-selected test conditions, at least one at an elevated temperature. Most preferably, the apparatus  10  tests all devices on the substrate simultaneously. 
     The embodiment shown in FIG. 1 includes a vacuum hold-down device to removably mount the substrate  14  to the planar surface  15  and substantially flatten it when sufficient vacuum is applied. In the embodiment of FIG. 1, the vacuum hold-down device includes a region of the first support member surrounding a vacuum cavity  20 . The cavity  20  is in fluid communication with the external environment through an array of first orifices  21 , each having an opening in the planar surface  15  of the first member  11 . In the preferred embodiment of FIG. 1, there are a plurality of first orifice elements although other configurations are possible. The cavity  20  is also in fluid communication with the external environment through a second orifice  22 , having an opening at a surface of the first member not in contact with the substrate. When an external vacuum source (not shown) is applied to the external opening of the second orifice, pressure in the cavity  20  is reduced below ambient, drawing a vacuum at the external opening of the first orifice  21 . The substrate  14  is thereby drawn to the planar surface  15  of the first member  11 . Such a vacuum at the planar surface  15  allows the substrate  14  placed thereon to be held substantially flat against the surface  15 . In the embodiment of FIG. 1, the second orifice  22  is sealably associated with an end  23  of a vacuum tube  24  housed in the second support member  12  when both members are engaged. An external vacuum source may be applied to the opposite end of the vacuum tube  24 . The tube  24  may optionally include a vacuum control valve  26  that is sealed to hold the wafer in place over the course of testing. 
     The embodiment of FIG. 1 further includes a temperature sensor  27  in the proximity of the planar surface  15 . The sensor is operationally connectable to an external heater driver and control unit (not shown) through a conductive path  28 . In a preferred embodiment, the sensor  27  and the external heater driver control unit operate within a feedback and control system, the sensor sensing a first temperature and supplying a first temperature signal to the heater driver and control unit, the control unit comparing the first temperature signal to a stored temperature signal and adjusting the heater driver unit to reduce any difference between the signals. Accordingly, the desired substrate temperature can be maintained at a temperature substantially the same as the stored (target test) temperature. 
     The embodiment of FIG. 1 further includes a buffer to cushion the force of the probe card against the substrate contacts and to apply pressure to maintain electrical contact. In this embodiment of the buffer, the first support member  11  is divided into two plates  29 ,  30  in a plane parallel to the planar surface  15 . One plate slidably engages the other plate about the periphery of the plates. In the embodiment of FIG. 1, the top plate  29 , which includes the vacuum cavity  20 , the first orifice  21 , the second orifice  22  and the heater  16 , slidably inserts into a recess  31  of the bottom plate  30 . Other similarly functioning configurations will be appreciated by those of ordinary skill in the art. Sliding the two plates in opposite directions creates a gap  32  therebetween. In the embodiment of FIG. 1, the top plate  29  slides in an upward direction and the bottom plate  30  slides in a downward direction to create the gap  32 . In preferred embodiments, an elastic member  33  is disposed within the gap between the two plates  29 ,  30 . The elastic member  33  is biased to sustain the gap  32  and hold the contact pads (not shown) of the substrate  14  against the probe tips (not shown) of the probe card  13  to enable electrical contact therebetween. In the embodiment of FIG. 1, the elastic member  33  is a spring, biased upwardly. In certain embodiments, a plurality of elastic elements are disposed within the gap, for example, the plurality of springs shown in FIG.  1 . The relative resilience of the elastic member also inhibits excess pressure on the substrate contacts and the probe tips of the probe card when the two support members are engaged. 
     Details of an embodiment of a probe card  13  for electrically interfacing a plurality of contact pads on a substrate to a testing unit are shown in FIGS. 2 a ,  2   b  and  3 . FIG. 3 is a lateral section of a detail of an embodiment of the probe card of FIG. 2 a  through section line  3 — 3 , shown aligned above a section of a substrate  14  showing a plurality of probe tips  34  on a surface  35  of the probe card that faces the substrate  14 . The probe tips  34  are arranged in a manner corresponding to a plurality of contact pads  36  on the devices of the substrate  14  so that when the probe tip surface  35  of the probe card  13  faces the contact pad surface  37  of the substrate  14  in proper alignment, each probe tip will electrically contact a contact pad. FIG. 3 also shows a plurality of signal contacts  38  on the probe card  13  for conducting signals to and from a testing unit (not shown). Each signal contact is electrically connected to a probe tip. In a preferred embodiment, as shown in FIG. 3, each probe tip  34  extends beyond the surface  35  of the probe card and the probe conductive path  39  between a given signal contact and its corresponding probe tip is shortened to the greatest extent possible to reduce the impedance mismatch problems previously discussed that are exacerbated as the probe wire lengthens. In a preferred embodiment, each signal contact  38  is electrically associated with a ground plane  40  to minimize interference among individual signals. 
     FIG. 2 a  is a top elevation of the embodiment of the probe card just described in FIG. 3, showing a plurality of signal contacts  38 . FIG. 2 b  is a bottom view of the embodiment of the probe card of FIG. 3, showing a plurality of probe tips  34 . Some of the more numerous signal contacts  38  may be connected to more than one probe tip  34 . This may allow different tests to be performed without repositioning the probe card or may allow the probe card to be used with multiple testers. It should be appreciated that different circuits, for example having different loads or impedance matching characteristics, might be included in the different connection paths provided by the embodiments of FIGS. 2 a  and  2   b . The probe card  13  and certain of its elements, for instance the signal contacts  38  and any circuits can be manufactured using standard printed circuit board (“PCB”) technology. In one embodiment, the signal contacts  38  and circuits (not shown) are made from boron tungsten. In another embodiment, they are made from copper. Other conductive materials having good electrical properties might also be used. The probe tips  34  can be manufactured using standard fabrication technology. Preferred embodiments of the probe tips  32  include tungsten, although other conductive materials with good durability properties may be used. 
     PCB technology is desirable for the board due to the low cost and ready availability of the technology. Different length connection paths can be provided to achieve impedance matching to sets of contact points for different tests. Providing such connection paths or providing different circuits for testing is greatly facilitated using the multi-layer fabrication provided by printed circuit board technology. 
     Returning to the embodiment of FIG. 1 a , the plural signal contacts (not shown) associated with the upper surface  41  of the probe card  13 , and the circuits of certain embodiments, enable direct electrical coupling to a load board of a tester (not shown) or burn-in board. In a preferred embodiment, an adapter  42  electrically interfaces between the signal contacts of the probe card and an external device, for example, the load board  43  of a tester or a burn-in board. These adapters, which can be variously configured, further facilitate use of standard load board and burn-in board equipment to interface with the probe card. 
     In a preferred embodiment, the probe card  13  includes an alignment mechanism to removably mount the probe card  13  to the second support member  12 , providing alignment for electrical contact between each probe tip of the probe card and the corresponding contact pad on the substrate when the two support members are coupled to one another. In some embodiments, the alignment mechanism comprises a plurality of receiving cavities in the probe tip surface  35  of the probe card  13 , and a plurality of protrusions from the surface of the second support member where the probe card is mounted, each protrusion sidably insertable into a corresponding receiving cavity in the probe card when the probe card is mounted on the second support. Most preferably, the alignment mechanisms are spaced about the periphery of the support member  12  so as not to interfere with the central area that provides testing access to the wafer under test. These alignment mechanisms assure proper alignment between the probe tips of the probe card and the corresponding contact pads of the substrate when the two support members are coupled to one another. In the embodiment of FIG. 1, the receiving cavities in the probe card are alignment holes  44  and the protrusions are cylindrical pillars  45  projecting upward from the surface of the second support member. Each of the pillars  45  slidably inserts into the corresponding alignment hole  44  of the probe card when the probe card is properly mated to the second support member. Various shapes and layouts of cavities and protrusions can be used that assure a desired unique orientation of the probe card on the second support. 
     The substrate probing and heating apparatus is particularly suitable for testing integrated circuits on a semiconductor wafer, and especially high density devices such as SRAM and DRAM memories. Other particularly well-suited applications include testing embedded devices, system on a chip (SOC) devices and other special circuits. 
     An embodiment of a method for handling and testing a plurality of devices on a substrate at a plurality of temperatures includes the steps of first providing a substrate having a plurality of devices to be tested through a plurality of contact pads, a probing and heating apparatus and a probe card, each as described herein, a device testing system operationally associated and electrically connected to each signal point on the probe card and a heater driver and control unit operationally associated with the heater in the probing and heating apparatus. The substrate is mounted on the first support so that the contact pads of the substrate face the probe tips of the probe card, and the two supports are coupled to one another so that the probe tips electrically contact the contact pads. Next, the substrate is heated to a pre-determined testing temperature, and then the electrical characteristics of the devices are tested with the tester while the devices are maintained at the pre-determined temperature. Thus burn-in testing can be performed at the substrate level, for example wafer level, without the need for first packaging the parts. Further, the testing can be done at high speed, because unwanted impedance mismatch effects have been minimized by the probe card design and construction. 
     In a preferred embodiment in which the devices are reparable devices with redundant circuitry, following the testing step, the method further includes the steps of providing a laser circuit repair unit, disengaging the two support members from one another and repairing defective devices detected during testing using the laser circuit repair unit to activate the redundant circuitry of the device. Although wafers tested as above can be shipped as final products or applications, in many application packaged parts are desired. For such applications, following the step of repairing the defective devices the method further comprises the steps of providing a substrate separation unit to separate the substrate into individual devices, distinguishing the good devices from the defective devices, separating the substrate into individual devices using the substrate separation unit, and sorting the good devices. These good devices may then be packaged, marked and finally tested as discrete units. 
     For comparison, a conventional testing method may include: (1) sorting at room temperature, currently up to thirty-two devices at a time, (2) redundancy testing, (3) sorting the repaired devices, ( 4 ) packaging the devices, (5) pre burn-in testing (open/short test), (6) loading parts for burning-in testing, ( 7 ) burn-in, (8) unloading parts after burn-in testing, (9) post burn-in testing (speed testing at high temperatures), (10) marking the packaged devices and (11) final testing at room temperature. In processes embodying aspects of in the present invention, a number of steps typically performed are eliminated, thus greatly reducing testing time and cost. For instance, the steps of initial sort at room temperature and post burn-in can be merged into one test. Additionally, it is possible that one or more other tests might be avoided, including the redundancy test, loading and unloading of packaged parts from the burn-in board, and pre-burn-in in test. In addition, a number of devices that are typically scrapped in the conventional process can be laser repaired because defects are identified before packaging. Reduction of unnecessary packaging of initially defective, but repairable, die also saves costs. 
     Although the present invention has been described in detail with reference only to the presently-preferred embodiments, those of ordinary skill will appreciate that various modifications can be made without departing from the invention. Accordingly, the invention is defined by the following claims.