Abstract:
A DC-AC probe card for testing a DUT includes: a plurality of probe needles, each probe needle having a distal end for contacting said DUT; and a plurality of connection pathways operable to connect test instrumentation to the probe needles, wherein each connection pathway provides both a desired characteristic impedance for AC measurements and a guarded pathway for DC measurements between respective test instrument connections and probe needles.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    The present invention relates to electrical measurements and, in particular, to probe cards. 
         [0002]    In testing small electrical devices (e.g., integrated circuits and circuit boards), it is common to use probe cards that provide an interface between test instruments and the device under test (DUT). The probe card provides the connections between probe needles and the test instruments/cables. In turn, the probe needles make electrical connections with the DUT during testing. By substituting probe cards with differing probe layouts, it is possible to change where on a DUT tests are made. A common configuration for probe cards is have the probe needles arranged radially about the DUT with connection pathways radiating outward from the probe needles. The opposite ends of the connection pathways then provide connection points for the test instruments/cables. Typically, the connection pathways are either integral with or mounted on a supporting structure along with the probe needles (e.g., a printed circuit board). 
         [0003]    Besides the touch down locations of the needles, it is often important to consider the nature of the required connections to the DUT. For example, in the case of very low current DC measurements, minimizing extraneous leakage current is often important, while in AC measurements, losses and reflections due to impedance mismatches are often important. 
         [0004]    These considerations often result in having to change probe cards depending on the type of measurement to be made, even if the probe needles themselves are in the correct location, the connection pathways between the instrument connections and the probe needles may not be suitable for the measurement. 
         [0005]    Referring to  FIG. 14 , a probe card  1  suitable for low current testing connects the source measure unit  2  (SMU  2 ) to the probe needle  3  and the SMU  4  to the probe needle  5 . A SMU can source a voltage/current and measure a current/voltage, respectively. The probe needles  3 ,  5  provide test connections to the DUT. As is typical in low current DC measurements, guard conductors  6 ,  7  containing a “guard” voltage are provided next to the signal conductors  8 ,  9 , respectively. The guard voltage is typically supplied by the respective SMU  2 ,  4 . This voltage is a buffered version of the signal on the respective signal conductor. Because the guard voltage and the signal voltage are equal, the signal conductors  8 ,  9  do not “see” any potential difference to cause leakage from the signal conductors  8 ,  9 . It is said that this “guards” the signal conductors from leakage. Therefore, the guard conductors  6 ,  7  are often simply called guards. They are often coaxial with the signal conductors, but other configurations are also used, for example, such as being a planar conductor near to the signal conductor while having a width substantially greater than that of the signal conductor or other strip line configurations suitable for printed circuit board implementation. The ground connection  10  between the SMUs  2 ,  4  is shown off the probe card  1 , but may also be on the probe card  1 . 
         [0006]    Referring to  FIG. 15 , the probe card  1  can be used to connect AC test instrumentation  11 ,  12  to the DUT. However, this configuration will often not be satisfactory for AC measurements as the guard conductors  6 ,  7  are “floating” at the AC voltage. From a transmission line perspective, the guard conductors  6 ,  7  are transmission line stubs as they near the DUT. The return path for the AC signals is through the ground connection  10 , not through the ends of the guard conductors  6 ,  7 . The configuration is unlikely to exhibit the measurement systems&#39; characteristic impedance, instead causing losses and reflections of the AC signal. If the AC signal is such that this transmission line effect is significant (e.g., radio frequencies), then a different probe card configuration will be needed for AC measurements. 
       SUMMARY OF THE INVENTION 
       [0007]    A DC-AC probe card for testing a DUT includes: a plurality of probe needles, each probe needle having a distal end for contacting said DUT; and a plurality of connection pathways operable to connect test instrumentation to the probe needles, wherein each connection pathway provides both a desired characteristic impedance for AC measurements and a guarded pathway for DC measurements between respective test instrument connections and probe needles. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is schematic diagram of an example of a DC-AC probe card according to an aspect of the invention; 
           [0009]      FIG. 2  is a schematic diagram of an example of a DC-AC probe card according to another aspect of the invention in an AC configuration; 
           [0010]      FIG. 3  is a schematic diagram of the DC-AC probe card of  FIG. 2  in a DC configuration; 
           [0011]      FIG. 4  is a schematic diagram of an example of a DC-AC probe card according to still another aspect of the invention; 
           [0012]      FIG. 5  is a schematic diagram of the DC-AC probe card of  FIG. 4  in a DC configuration 
           [0013]      FIG. 6  is a schematic diagram of an example of a DC-AC probe card according to an additional aspect of the invention; 
           [0014]      FIG. 7  is a schematic diagram of the DC-AC probe card of  FIG. 6  in a DC configuration 
           [0015]      FIG. 8  is a schematic diagram of an example of a DC-AC probe card according to another additional aspect of the invention; 
           [0016]      FIG. 9  is a schematic diagram of the DC-AC probe card of  FIG. 8  in a DC configuration; 
           [0017]      FIG. 10  is a schematic diagram of an example of a DC-AC probe card according to still another additional aspect of the invention; 
           [0018]      FIG. 11  is a schematic diagram of the DC-AC probe card of  FIG. 10  in a DC configuration; 
           [0019]      FIG. 12  is a schematic diagram of an example of a DC-AC probe card according to a further aspect of the invention; 
           [0020]      FIG. 13  is a schematic diagram of the DC-AC probe card of  FIG. 12  in a DC configuration; 
           [0021]      FIG. 14  is a schematic diagram of a prior art example of a probe card for DC testing; and 
           [0022]      FIG. 15  is a schematic diagram of the prior art probe card of  FIG. 8  in an AC testing configuration. 
       
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0023]    Referring to  FIG. 1 , a DC-AC probe card  20  connects an instrumentation connection  22  to a connection pathway  24 . The connection pathway  24  connects in turn to a probe needle  26 , which during operation of the probe card  20  connects to a device under test (DUT). 
         [0024]    Similarly, an instrumentation connection  28  connects to a connection pathway  30 . The connection pathway  30  connects in turn to a probe needle  32  which during operation of the probe card  20  connects to the DUT. 
         [0025]    The DUT is not part of the probe card  20  but will typically be located where the probe needles can touch down on the DUT to make connections thereto. This is the case for all of the examples set forth herein. 
         [0026]    Further, when reference is made to proximal and distal herein, proximal is electrically towards test instrumentation and distal is electrically towards the DUT. For example, the distal end of a probe needle is used to make contact with the DUT. 
         [0027]    For ease of understanding, the examples herein only employ two connections to the DUT, actual probe cards will typically be capable of many more connections. 
         [0028]    The term “instrumentation connection” as used herein encompasses both some form of AC or DC test instrumentation and associated cabling between the test instrumentation and the probe card. It is expected that the probe cards described herein will typically include electrical connectors between the probe card and the instrumentation connection. These provide convenient connection/disconnection of the probe card from the instrumentation connections. For ease of understanding, these electrical connectors are not shown. The frequency of the AC signals herein may be, for example, between 1 KHz and 1 GHz. 
         [0029]    In the case of DC measurements, the connection pathways  24 ,  30  each provide a guarded pathway between the respective instrumentation connections  22 ,  28  and the respective probe needles  26 ,  32 . In the case of AC measurements, the same connection pathways  24 ,  30  provide a connection having a desired characteristic impedance (e.g., 50 or 75 ohms). 
         [0030]    Referring to  FIG. 2 , a DC-AC probe card  32  includes a first conductor pair  34  and a second conductor pair  36 . The first conductor pair  34  includes a first conductor  38  and a second conductor  40 . The second conductor pair  36  includes a third conductor  42  and a fourth conductor  44 . A first probe needle  46  and a second probe needle  48  are located in positions suitable for contacting a DUT. 
         [0031]    The distal end of the first conductor  38  is connected to the first probe needle  46 . The distal end of the third conductor  42  is connected to the probe needle  48 . 
         [0032]    The distal ends of the second and fourth conductors are connected to the commonly connected capacitors  50 ,  52 . In this example, the commonly connected capacitors  50 ,  52  constitute a coupling circuit as more fully described below. 
         [0033]    In operation, the proximal ends of the first conductor  38  and the second conductor  40  are connected to AC test instrumentation  54  and the proximal ends of the third conductor  42  and the second conductor  44  are connected to the AC test instrumentation  56 . In this example, for ease of understanding, the AC test instrumentation  54  is an AC voltage source with a 50 ohm impedance and the AC test instrumentation  56  is 50 ohm load. In general, the AC test instrumentation may be, for example, signal generators, pulse generators, oscilloscopes, AC voltage and current meters, or AC power meters. It is expected that the AC test instrumentation will have a characteristic impedance (e.g., 50 ohms). As a result, it is desirable that the probe card  32  also have the same characteristic impedance. Having this same desired characteristic impedance minimizes losses and detrimental reflections in the overall measurement system. 
         [0034]    The conductor pairs  34 ,  36  are each shown as coaxial cables. Such cables are available with various characteristic impedances, 50 ohm and 75 ohm being common. It is also common to use strip line structures on printed circuit boards to provide a conductor pair with a desired characteristic impedance. 
         [0035]    The coupling circuit comprised of the commonly connected capacitors  50 ,  52  is used to provide the desired characteristic impedance up to as close to the probe needles  46 ,  48  as possible. For example, the capacitors  50 ,  52  each have a capacitance that will pass AC signals being used for testing between the distal ends of the conductors  40 ,  44 . Absent this coupling, the distal ends of the conductors  40 ,  44  would be floating transmission line stubs of uncertain impedance likely to cause losses and reflections that degrade measurements of the DUT. 
         [0036]    Referring to  FIG. 3 , the DC-AC probe card  32  is shown instead connected to DC test instrumentation. The proximal end of the conductor pair  34  is connected to DC test instrumentation  58  and the proximal end of the conductor pair  36  is connected to the DC test instrumentation  60 . 
         [0037]    In this example, the DC test instrumentation  58 ,  60  are each a SMU. A SMU can source a DC voltage/current and measure a DC current/voltage, respectively. SMU are often used to measure low currents (e.g., microamperes, femtoamperes). At low currents, leakage current unrelated to the DUT may be particularly detrimental. SMUs provide guard voltages to help minimize leakage currents. The guard voltage is a buffered version of the actual signal voltage. 
         [0038]    The guard voltage of the SMU  58  is connected to the proximal end of the second conductor  40 . The actual signal of the SMU  58  is connected to the proximal end of the first conductor  38 . 
         [0039]    Similarly, the guard voltage of the SMU  60  is connected to the proximal end of the fourth conductor  44 . The actual signal of the SMU  60  is connected to the proximal end of the third conductor  42 . 
         [0040]    In this configuration, the voltage on the second conductor  40  guards the first conductor  38  and the voltage on the fourth conductor  44  guards the third conductor  42 . 
         [0041]    Because the signals are DC, the coupling circuit comprised of the commonly connected capacitors  50 ,  52  now decouples the distal ends of the conductors  40 ,  44  instead of coupling them as in  FIG. 2 . 
         [0042]    In the case of AC test instrumentation, the coupling circuit couples the distal ends of the second conductor  40  and the fourth conductor  44 . In the case of DC test instrumentation, the coupling circuit decouples the distal ends of the conductors  40 ,  44 . 
         [0043]    The capacitors  50 ,  52  allow the conductors  40 ,  44  to carry AC ground current for higher frequencies where the capacitors  50 ,  52  are able to effectively short the distal ends together. Frequencies that are lower than where the guards are effectively shorted together must return their ground current though the actual ground path  62 . Increasing the capacitance of the capacitors  50 ,  52  will lower the frequency at which the guards are effectively shorted together. However, increasing these capacitances makes it harder for the SMUs to drive the guards during DC testing. 
         [0044]    The loop area of the ground pathway also affects how wide of a frequency band for which the pathway will not effectively function. Decreasing the loop area of the ground pathway reduces the inductance of the loop area. The ground current can then flow through the ground path for lower frequencies without further increasing the capacitance of the capacitors  50 ,  52 . 
         [0045]    Referring to  FIG. 4 , a DC-AC probe card  32 ′ similar to the previous example of  FIG. 2  adds another parallel ground path  64  that is on the DC-AC probe card  32 ′. The ground path  64 , being closer than the ground path  62  to the conductors  40 ,  44 , results in a lower inductance. This lower impedance further lowers the frequency at which the conductors  40 ,  44  are effectively shorted together (or, if desired, the values of the capacitors  50 ,  52  may be also lowered to maintain the same frequency performance). 
         [0046]    Referring to  FIG. 5 , the DC-AC probe card  32 ′ is shown in a DC measurement configuration similar to  FIG. 3 . 
         [0047]    Referring to  FIG. 6 , the DC-AC probe card  32 ″ makes use of triaxial structures  35 ,  37  to further control the inductance. The outer shield conductors  64 ′,  64 ″ now provide the ground path  64  of  FIG. 4 . The loop area and thus the inductance is further reduced. This further lowers the frequency at which the conductors  40 ,  44  are shorted together. 
         [0048]    Referring to  FIG. 7 , the DC-AC probe card  32 ″ is shown in a DC measurement configuration similar to  FIG. 5 . 
         [0049]    Referring to  FIG. 8 , the DC-AC probe card  32 ′″ is similar to the example of  FIG. 6  except instead of a triaxial structure, coaxial conductors  66 ,  68  are arranged around respective portions of the ground path  64 . The proximal and distal ends of the coaxial conductors  66 ,  68  are connected to respective portions of the conductors  40 ,  44 . This couples the upper and lower coaxial structures together. 
         [0050]    Referring to  FIG. 9 , the DC-AC probe card  32 ′″ is shown in a DC measurement configuration similar to  FIG. 5 . 
         [0051]    It should be noted that presence of capacitance and inductance will lead to possible issues with resonance frequencies. This can be managed by the addition of resistance to the loops. 
         [0052]    Referring to  FIG. 10 , the DC-AC probe card  32 ″″ is similar to the example of  FIG. 8  with the resistors  70 ,  72  substituted for direct connections and the coaxial conductors  66 ′,  68 ′ having the shield and center connections interchanged from the coaxial conductors  66 ,  68 . This reduces the Q of the resonance of the pathway inductance and capacitance. 
         [0053]    Referring to  FIG. 11 , the DC-AC probe card  32 ″″ is shown in a DC measurement configuration similar to  FIG. 9 . 
         [0054]    Referring to  FIG. 12 , a DC-AC probe card  74  provides both DC guarding and an AC path that has the desired characteristic impedance. 
         [0055]    The probe card  74  has a first transmission line  76  that includes a first conductor  78  and a second conductor  80 . The distal end of the first conductor  78  is connected to a first probe needle  82 . During operation, the proximal ends of the first and second conductors  78 ,  80  are connected to the first test instrumentation  84 . 
         [0056]    A second transmission line  86  includes a third conductor  88  and a fourth conductor  90 . The distal end of the fourth conductor  90  is connected to the distal end of the second conductor  80 . During operation, the proximal ends of the third and fourth conductors  88 ,  90  are connected to the first test instrumentation  84 . During AC testing, the proximal ends of the second and fourth conductors  80 ,  90  are connected together as shown. 
         [0057]    Similarly, the probe card  74  has a third transmission line  92  that includes a fifth conductor  94  and a sixth conductor  96 . The distal end of the fifth conductor  94  is connected to a second probe needle  98 . During operation, the proximal ends of the fifth and sixth conductors  94 ,  96  are connected to the second test instrumentation  100 . 
         [0058]    A fourth transmission line  102  includes a seventh conductor  104  and a eighth conductor  106 . The distal end of the eighth conductor  106  is connected to the distal end of the sixth conductor  96 . The distal ends of the third and seventh conductors  88 ,  104  are connected. During operation, the proximal ends of the seventh and eighth conductors  104 ,  106  are connected to the second test instrumentation  100 . During AC testing, the proximal ends of the sixth and eighth conductors  96 ,  106  are connected together. 
         [0059]    The transmission lines  76 ,  86  are of equal electrical length, as are the transmission lines  92 ,  102 . 
         [0060]    In this example, the AC signals follow the same path independent of frequency, eliminating the resonance issues of the examples above. 
         [0061]    Referring to  FIG. 13 , the DC-AC probe card  74  is shown instead connected to DC test instrumentation in the test instrumentation  84 ,  100 . The proximal end of the transmission line  76  is connected between the signal and guard of the SMU in the test instrumentation  84  and the proximal end of the transmission line  92  is connected between the signal and the guard of the SMU in the test instrumentation  100 . The proximal end of the conductor  88  is connected to the ground of the SMU in the test instrumentation  84  and the proximal end of the conductor  108  is connected to the ground of the SMU in the test instrumentation  84 . 
         [0062]    The transmission lines  76 ,  86 ,  92 ,  102  are shown as coaxial cables, but other transmission line structures may be used as mentioned above. 
         [0063]    It should be evident that this disclosure is by way of example and that various changes may be made by adding, modifying or eliminating details without departing from the fair scope of the teaching contained in this disclosure. The invention is therefore not limited to particular details of this disclosure except to the extent that the following claims are necessarily so limited.