Abstract:
A probe assembly suitable for making test measurements using test signals having high currents. The disclosed probe assembly provides for a test signal exhibiting relatively low inductance when compared to existing probe assemblies by preferably reducing the electrical path distance between the test instrumentation and the electrical device being tested.

Description:
This application claims the benefit of Provisional U.S. Patent Application Ser. No. 60/429,082 filed Nov. 25, 2002. 

   BACKGROUND OF THE INVENTION 
   The present invention relates to probe stations, commonly known as package or wafer probers, used manually, semi-automatically, or fully automatically to test electrical devices such as semiconductor wafers. 
   Existing probe stations are capable of performing both low-current and high frequency measurements in an electronically quiet environment. The environment may be provided by, for example, incorporating one or more guard and electromagnetic interference (EMI) shield structures within an environmental enclosure. Guard and EMI shield structures are well known and discussed extensively in technical literature. See, for example, an article by William Knauer entitled “Fixturing for Low Current/Low Voltage Parametric Testing” appearing in  Evaluation Engineering , November, 1990, pages 150-153. Examples of existing probe stations that provide such guard and EMI shield structures can be found in commonly owned U.S. Pat. Nos. 5,434,512; and 5,266,889 which are hereby incorporated by reference. 
   Probe stations deliver a test signal to an electrical device, such as a semiconductor wafer, whose characteristics are to be measured. Test conditions are desirably controlled and substantially free of electromagnetic interference, though not necessarily, that may emanate from test instrumentation or other nearby electrical equipment, or that may result from spurious air currents or the like. To provide a controlled and substantially noise-free test environment, existing probe stations that incorporate guard structures will usually at least partially surround the test signal path with a guard signal that closely approximates the test signal, thus inhibiting electromagnetic current leakage from the test signal path to its immediately surrounding environment. Similarly, EMI shield structures may provide a shield signal to the environmental enclosure surrounding much of the perimeter of the probing environment. The environmental enclosure may typically be connected to shield, earth ground, instrumentation ground, or some other desired potential. 
   To provide test, guard, and shield signals to the probe station, existing probe stations often include a multistage chuck upon which the electrical device rests while being tested. The top stage of the chuck, which supports the electrical device, typically comprises a solid, electrically conductive metal plate through which the test signal may be routed. A middle stage and a bottom stage of the chuck similarly comprise solid electrically conductive plates through which a guard signal and a shield signal may be routed, respectively. In this fashion, an electrical device resting on such a multistage chuck may be both guarded and shielded from below. Similarly, single stage and dual stage chucks, and chucks with substantial openings centrally defined therein are likewise frequently employed. 
   Further reduction in interference can be obtained by locating a suspended conductive plate over the electrical device which is typically electrically insulated from the test signal path and connected to the guard signal. The suspended plate defines a central opening so that the probe assembly may make electrical contact with the electrical device. In this fashion, the electrical device can be guarded from both below and above by signals closely approximating that delivered to the electrical device. 
   Though such a probe station is effective in performing low-current testing and high frequency testing of electrical devices, the aforementioned existing probe stations unfortunately often exhibit significant inductance to high current measurements, and particularly when testing using pulsed signals. The high inductance tends to resist fast changes in the current levels, and results in higher than desirable voltage and current levels. 
   What is desired, therefore, is a probe station that is suitable for performing high current and/or pulsed tests. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  shows a schematic of an existing probe station having guard and electromagnetic shield structures. 
       FIG. 2  illustrates a general schematic of  FIG. 1 . 
       FIG. 3  shows schematic of a modified probe station exhibiting reduced inductance. 
       FIG. 4  illustrates a general schematic of  FIG. 3 . 
       FIG. 5  shows schematic of another modified probe station exhibiting reduced inductance. 
       FIG. 6  shows schematic of yet another modified probe station exhibiting reduced inductance. 
       FIG. 7  shows schematic of a further modified probe station exhibiting reduced inductance. 
       FIG. 8  shows schematic of a modified probe station exhibiting reduced inductance. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     FIG. 1  shows a general schematic diagram of an existing probe station  10  having guard and electromagnetic shield structures. A test signal is provided through a test path  12  to a probe  14  having probe needles  16  that contact an electrical device  18  resting upon a chuck  20 . The probe needles may alternatively be any type of contacts, such as for example, probe cards, probes on movable positioners, optical signals, and membrane probes. The chuck  20  receives a guard signal through a first transmission line  22  while a suspended guard member  24  receives a guard signal through a second transmission line  26 . The first transmission line  22  likewise includes a test signal path to the chuck  20 . The first transmission line  22 , the test path  12 , the probe  14 , the needles  16 , the device  18 , and the chuck  20  together form a large loop, as shown in  FIG. 1 , to a common signal source at the test instrumentation. Normally within the probe station the transmission line  22  is within a service loop that is several feet long to accommodate movement of the chuck  20 . 
   The present inventors came to the realization that when using high current or pulsed tests, the large test loop that originates from the test equipment and passes through the chuck creates undesirable inductance. The inductance resulting from this large loop often interferes with test measurements, and in particular high current and/or pulsed signals. In addition, the transmission line  22  is normally a small conductor which is not especially suitable for carrying high currents.  FIG. 2  illustrates more schematically the resulting test loop for purposes of clarity. 
   The present inventors further determined that reducing or otherwise modifying this previously unrecognized source of inductance for high current and/or pulsed signals, namely, the inductive test loop could improve such measurements. The modification may include modifying or otherwise providing another test signal path from the chuck  20  to the test instrumentation.  FIG. 3  shows one embodiment of a probe station  10  with a test loop having a decreased length. Rather than routing the test signal from the chuck  20  through transmission line  22 , a transmission line  28  may interconnect the chuck  20  with the suspended guard member  24 , which is then electrically connected to the test instrumentation by another transmission line  29 . The suspended guard member  24  typically has its guard potential removed when performing this test. Accordingly, the suspended guard member  24  is being used in a non-traditional manner, namely, not interconnected to a guard potential. The interconnection of the transmission line  28  at the chuck  20  may be one of the layers of the chuck  20  such as the top layer  20 A of the chuck  20  that defines the surface  20 B that supports an electrical device being probed. The at least partially encircling conductive member  33 , normally connected to guard potential, may have a height greater than the top surface of the chuck, even with the top surface of the chuck, or below the top surface of the chuck. Preferably, there is an air gap between the conductive member  33  and the chuck  20 . The air gap may be partially filled, substantially filled, or completely filled with dielectric material. The signal path to or from the top surface of the chuck may be provided through an opening in the conductive member  33 . Electrically connecting the chuck  20  to the suspended guard member  24  by the transmission line  28 , and to the test instrumentation by transmission line  29 , results in a smaller loop path than that provided by previously existing probe stations, as shown schematically in  FIG. 4 . By reducing the length of the test path loop, electrical performance is improved, particularly when testing an electrical device using high-current and/or pulsed signals. 
   It is to be understood that the suspended plate may be suspended from above, typically using insulators, or supported by supports from within the probe station, or supported by the chuck or chuck assembly. Normally the suspended plate does not move together with the chuck  20 , but is rather maintained in a fixed spatial relationship with respect to the probe station  10 . Also, it is to be understood that the suspended plate may be any conductive member within the probe station that has the characteristic that it does not move together with the chuck  20 , but is rather maintained in a fixed spatial relationship with respect to the probe station  10 . Alternatively, the suspended member may be any conductive member within the probe station that is free from being electrically connected to a guard and/or shield potential when used in the aforementioned configuration. 
   The interconnections from the chuck  20  to the suspended guard  24  is preferably totally within the environmental enclosure. A further explanation of the environmental enclosure is disclosed in U.S. Pat. No. 5,457,398, incorporated by reference herein. Interconnection within the environmental enclosure potentially reduces the length of the conductive path to less than it would have been had the interconnection been, at least in part, exterior to the environmental enclosure, or otherwise the test path passing from within the environmental enclosure to outside the environmental enclosure to within the environmental enclosure. 
   The transmission lines  28  and  29 , shown schematically in  FIGS. 2-4  may be embodied in many different structures. For example, the transmission lines  28  and  29  may be a traditional transmission line, such as a wire, coaxial cable, triaxial cable, and one or more conductive tabs. Alternatively, as depicted in  FIG. 5 , the transmission line  28  may comprise a conductive shell or bowl  50  that contacts the test path of the chuck  20  (e.g., top layer) at its lower end and the suspended plate  24  at its upper end. The shell  50  preferably encircles a major portion of the chuck  20  and more preferably substantially all of the chuck  20 . In addition, the shell  50  while preferably forming a substantially closed loop may have a size less than, at least in part, the exterior periphery defined by the chuck  20 . Also, preferably the conductive shell  50  includes a flexible upper portion in contact with the suspended member so that upon pressing engagement a good conductive interconnection is made even while the conductive shell  50  moves horizontally relative to the suspended plate  24 . Moreover, the shell  50  may be detachably engageable with the suspended member by changing its height, such as for example, using “flip-up” fingers. In addition, a flexible upper portion also permits a greater range of movement of the chuck in the z-axis direction. In addition, the shell may be solid, flexible, and/or perforated with openings as desired. The openings, in particular, may be useful for permitting air flow around the device under test. 
   Referring to  FIG. 6 , the reduced inductance test path may be included within the structure that includes an enclosure  37  that surrounds the chuck therein. During testing of the device under test the enclosure  37  moves together with the chuck  20 . The interconnection  28  to the suspended member may be by a cable or otherwise from a location within the chamber or otherwise connected to the chuck therein. 
   Referring to  FIG. 7 , a dual probe assembly may be used to provide a test signal path. A first probe  70  may provide a test signal to the device under test. The test signal then passes through the device under test and to the chuck  20 . The chuck  20  is electrically interconnected to the suspended plate  24 . A second probe  72  may receive the test signal from the suspended plate  24 . Alternatively, the second probe  72  may be directly interconnected to the chuck  20  to receive the test signal. 
   Referring to  FIG. 8 , a single probe assembly  80  may be used to provide and sense a test signal path. The probe  80  may provide a test signal to the device under test through a first probe tip  82 . The test signal then passes through the device under test and to the chuck  20 . The chuck  20  is electrically interconnected to the suspended plate  24 . The single probe assembly  80  may receive the test signal from the suspended plate  24  through a second probe tip  84 . Alternatively, the second tip of the probe assembly  80  may be direct interconnection to the chuck  20  to receive the test signal. In this manner a single probe assembly may both provide the test signal and sense the test signal. Also, it is preferred that the interconnected from the probe assembly  80  to the test instrumentation is a single cable assembly, more preferably a twisted pair of wires, to minimize inductance. The twisted pair of wires preferably extends at least 50% of the distance between the probe and the test instrumentation. 
   The terms and expressions employed in the foregoing specification are used therein as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding equivalents of the features shown and described or portions thereof, it being recognized that the scope of the invention is defined and limited only by the claims that follow.