Abstract:
To reduce noise in measurements obtained by probing a device supported on surface of a thermal chuck in a probe station, a conductive member is arranged to intercept current coupling the thermal unit of the chuck to the surface supporting the device. The conductive member is capacitively coupled to the thermal unit but free of direct electrical connection thereto.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]    This is a continuation of application Ser. No. 09/345,571, filed Jun. 30, 1999. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    The present invention is directed to probe stations suitable for making low current and low voltage measurements and, more particularly, to a system for reducing noise due to capacitive currents resulting from the operation of a thermal chuck for a probe station.  
           [0003]    Integrated circuit devices are typically manufactured in and on a wafer of semiconductor material using well-known techniques. Prior to cutting the individual integrated circuit devices from a wafer, tests are run on individual devices to determine if the devices operate properly. The wafer is supported on a chuck inside an environmental enclosure in a probe station. Probes are brought into contact with test points or pads on the integrated circuit devices and a series of measurements are preformed. Schwindt et al., U.S. Pat. No. 5,663,653, disclose an example of a probe station in which the present invention might be used and the patent is incorporated herein by reference.  
           [0004]    Many integrated circuit devices are designed to operate at temperatures other than room temperature. To accommodate device testing at temperatures other than the ambient temperature, a thermal chuck may be employed. One design of a thermal chuck comprises a multilayered chuck for securing a wafer having a thermal driver to modify the temperature of the chuck. A thermal chuck of this design is disclosed by Schwindt in U.S. Pat. No. 5,610,529 which is incorporated herein by reference.  
           [0005]    The thermal driver may provide for either heating, cooling, or heating and cooling of the chuck. To modify the temperature of the chuck, the thermal driver may comprise one or more thermal units including a thermal device and a plurality of power conductors connecting the thermal device to a power source. Thermal devices, typically electric resistance heaters or thermoelectric heat pumps, are provided to heat the chuck to temperatures above the ambient temperature. The thermoelectric heat pump, also known as a Peltier device, is reversible and can be used for cooling as well as heating the chuck. The thermoelectric heat pump comprises a number of thermocouples sandwiched between two electrically insulating, thermally conductive plates. When DC power is supplied to the thermocouples, the Peltier effect causes heat to be transferred from one plate to the other. The direction of heat flow is reversible by reversing the direction of current flow in the thermocouples. Exposing the chuck to the warmer plate or the cooler plate of the thermoelectric heat pump will, respectively, either heat or cool the chuck. For testing at temperatures below ambient, the thermal chuck may also include passages for circulating coolant to cool the chuck directly or remove excess heat from the thermoelectric heat pump.  
           [0006]    When making the low voltage and low current measurements common to testing integrated circuit devices, even very low levels of electrical noise are unsatisfactory. Thermal chucks include several sources of noise and unacceptably high levels of noise are a common problem when using a thermal chuck. One known source of noise is the result of expansion or contraction of the components of the thermal chuck due to changing temperature. Expansion or contraction changes the spacing between conductive components resulting in the generation of capacitive currents which can reach the conductive surface of the chuck. Expansion or contraction due to temperature change can also cause relative transverse movement between the multiple material layers of the chuck. Relative movement between contacting layers of insulating and conductive materials can generate triboelectric current. In a probe station chuck, the triboelectric current can appear as noise in the test measurements. Triboelectric currents can be reduced by a chuck design which prevents movement between contacting layers of insulating and conducting materials.  
           [0007]    The operation of the thermal units by the thermal driver controller is another potential source of noise when using a thermal chuck. To change or maintain the temperature of the thermal chuck, the thermal driver controller fluctuates the electrical power to the thermal units in response to a temperature control system. As a result of the voltage drop within the conductors of the thermal units, physically adjacent portions of the electrical conductors leading to and from, and internal to the thermal devices, will be at different potentials. As the power fluctuates, the difference in voltage between the power conductors changes with time. This results in a displacement of charges in the dielectric material surrounding the conductors which manifests itself as a displacement or capacitive current coupled to the conductive top surface of the chuck. This capacitive current appears as noise in the test measurements.  
           [0008]    The currently accepted technique to reduce the effects of capacitive currents involves shielding the chuck from external electromagnetic sources. However, the shielding layers of conductive material in the chuck have proven unsuccessful in eliminating the noise from the thermal driver. To reduce noise due to capacitive currents originating in the thermal chuck, users of probe stations often shut off the thermal units and wait for the current to dissipate. However, the RC time constant involved can be greater than five seconds. Waiting a period of five time constants (e.g. 25 seconds) for the observed noise to dissipate to an acceptable level before making a measurement substantially effects the productivity of the probe station.  
           [0009]    What is desired, therefore, is a system for reducing the electrical noise generated by the operation of the thermal unit of a probe station&#39;s thermal chuck. Reducing noise generated by the thermal chuck reduces the time for the noise to dissipate to acceptable levels improving the productivity of the probe station.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    [0010]FIG. 1 is a cross-section of a probe station incorporating a thermal chuck.  
         [0011]    [0011]FIG. 2 is a cross section of an exemplary thermal chuck constructed in accordance with the present invention.  
         [0012]    [0012]FIG. 3 is an exemplary schematic diagram of a thermal unit and shielding in accordance with a first aspect of a preferred embodiment of the present invention.  
         [0013]    [0013]FIG. 4 is an exemplary schematic diagram of a thermal unit and shielding in accordance with a second aspect of a preferred embodiment of the present invention.  
         [0014]    [0014]FIG. 5 is an exemplary schematic diagram of a thermal unit and shielding in accordance with a third aspect of a preferred embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0015]    As illustrated in FIG. 1, a probe station generally includes an environmental enclosure  2  in which is located a chuck  4  and one or more probes  6 . The environmental enclosure  2  is typically constructed of a conductive material and grounded  7  so that the chamber, interior to the enclosure  2 , is shielded from electromagnetic fields emanating from outside of the enclosure  2 . The chuck  4  typically comprises multiple layers of conductive and dielectric materials that are connected to the various conductors of a coaxial or triaxial cable  8 . The chuck  4  includes a securement technique for securing a device under test  10 , generally a wafer of semiconductor material, to the upper surface  12  of the chuck  4 . The upper surface  12  of the chuck  4  is typically conductive. One technique for securing a device under test  10  relies on a vacuum source (not shown) located outside of the environmental enclosure. The vacuum source communicates through appropriate control valves and piping with apertures (not shown) in the upper surface  12  of the chuck  4 . When the device under test  10  is placed on the chuck  4  the device blocks apertures leading to the vacuum source. Air pressure holds the device under test  10  against the chuck&#39;s upper surface  12 . One or more probes  6  can be positioned over the device under test  10  and brought into contact with test pads on the circuit to be tested. Instrumentation connected to the probes  6  measures selected operating parameters of the circuit at the test pads.  
         [0016]    A thermal chuck  14 , bracketed, may be used to test the operation of devices at temperatures other than the ambient temperature of the environmental enclosure  2 . Referring to FIG. 2, the thermal chuck  14 , indicated with a bracket, may include a thermal driver  16  having facilities for modifying the temperature of a chuck  4 , indicated with a bracket, supported on the top of the thermal driver  16 . The thermal driver  16  may be arranged to provide for either heating, cooling, or heating and cooling of the chuck  4 . The thermal driver  16  comprises one or more electrically powered thermal units  20  each of which includes one or more thermal devices  22  and a plurality of insulated power conductors  24  connecting the thermal devices  22  to a thermal driver controller  18 . Typically, the thermal devices  22  are resistance heaters or thermoelectric heat pumps. Resistance heaters and thermoelectric heat pumps can increase the temperature of the chuck  4 . The thermoelectric heat pump can also be used to cool the chuck  4 . The thermoelectric heat pump, also known as a Peltier device, comprises a plurality of electrically connected thermocouples of p-type and n-type semiconductor materials sandwiched between two plates of an electrically insulating, thermally conducting material. When DC power is supplied to the thermocouples, heat is transferred from one plate to the other as a result of the Peltier effect. The direction of heat flow is reversible by reversing the direction of current flow in the semiconductors. Exposing the chuck  4  to the warmer plate or the cooler plate of the thermoelectric heat pump will, respectively, heat or cool the chuck  4 .  
         [0017]    The thermal driver  16  may also include passages  26  for circulating coolant supplied by a coolant source (not shown) typically located outside of the environmental enclosure  2 . For testing at temperatures below the ambient temperature, the chuck  4  may be cooled directly by the coolant. If a thermoelectric heat pump is used to cool the chuck, circulating coolant may be necessary to remove heat transferred to the thermal driver  16  by the heat pump.  
         [0018]    Electric power for the thermal units  20  is supplied by the thermal driver controller  18  located outside of the environmental enclosure  2 . Insulated power conductors  24  transfer the electrical power to the thermal devices  22  in the thermal chuck  14 . In response to a temperature sensing system, the thermal driver controller  18  fluctuates the electrical power to the thermal unit  20  to vary its thermal output to either reduce or increase the rate of addition or removal of thermal energy to or from the chuck  4 . As a result of the voltage drop in the thermal unit  20 , adjacent portions of the insulated power conductors  24  and the conductors inside the thermal devices  22  are at differing potentials. This causes a displacement of charge in the dielectric material surrounding the conductors. As the thermal driver controller  18  fluctuates the power to the thermal unit  20  the difference in voltage between adjacent conductors also varies with time. The present inventors came to the realization that this displacement of charge varying with time causes a displacement or capacitive current which is coupled to the conductive upper surface  12  of the chuck  4 . The present inventors further realized that this capacitive current manifests itself as noise in the test measurements.  
         [0019]    The present inventors came to the realization that the aforementioned capacitive currents are a significant source of noise when making measurements in the femtoamp range with state of the art probe stations. The present inventors further realized that conductive shielding of the thermal unit  20  that is capacitively coupled to the conductors of the thermal unit  20  can intercept a substantial amount, and preferably substantially all, of the capacitive currents resulting from the operation of the thermal unit  20  and provide a conductive path to return any current induced in the conductive shielding to the thermal driver controller  18  and to ground. This is in contrast to the presently accepted techniques of adding more shielding to the chuck itself. Referring also to FIG. 3, a conductive thermal device shell  28  substantially encloses the thermal devices  22  and the power conductors  24  at their connection to the thermal devices  22 . Variation in charge displacement resulting from the operation of the electric circuit of the thermal device  22  results in a displacement current in the conductive thermal device shell  28 . In other words, the thermal device shell  28  is capacitively coupled through “virtual” coupling capacitors  30  to the electric circuit of the thermal device  22  and intercepts capacitive currents that would otherwise find their way to the upper surface  12  of the chuck  4 . Although apertures may be required in the thermal device shell  28  they should be minimized in relation to the total surface area of the thermal device shell  28 . The more completely the thermal device shell  28  spatially encloses the thermal device  22  the more completely it will intercept capacitive currents emanating from the thermal device  22 . The thermal device shell  28  is conductively connected to the thermal driver controller  18  through the conductive shield of the cable  32 . The conductive connection of the thermal device shell  28  to the thermal driver controller  18  provides a path for any current in the thermal device shell  28  to exit the environmental enclosure  2  to the thermal driver controller  18 . The driver controller  18  is connected to ground  7  extending the conductive return path for capacitive currents to ground  7 .  
         [0020]    The present inventors also came to the stark realization that by enclosing the thermal devices  22  with a conductive shell  28  the RC time constant of the thermal chuck is dramatically reduced. The thermal devices  22  do not need to be turned off in order for the noise to be sufficiently reduced. The present inventors determined that this reduction in RC time constant is due to a reduction in the stored capacitive charge in the dielectric material within the chuck, referred to as absorption capacitance. The absorption capacitance of a material includes a series resistance so, in effect, it has a memory of previous charges and is slow to dissipate. This absorption capacitance was not previously considered in the design of thermal chucks. There was little, if any, motivation to enclose the thermal devices  22  in a conductive enclosure, as it was believed that noise from the thermal devices  22  could be removed by layers of shielding in the chuck  4 . The layers of the chuck  4  include, however, dielectric material which the inventor realized is, in fact, a source of the long RC time constant.  
         [0021]    The cable  32  includes the power conductors  24  connecting the thermal driver controller  18  to the thermal devices  22 . The shield of the cable  32  ideally extends through the wall of the environmental enclosure  2  and encompasses the power conductors  24  at their entrance into the thermal device shell  28 . The shield of the cable  32  is capacitively coupled to the power conductors  24  and will intercept and return to the thermal driver controller  18  currents emanating from the capacitive effects of power fluctuation in the power conductors  24 . The thermal driver controller  18  is grounded at ground connection  21 . The more complete the enclosure of all conductors in the thermal unit  20  by the conductive shielding, the more complete will be the protection of the test measurement from noise generated by the operation of the thermal unit  20 .  
         [0022]    The walls of the environmental enclosure  2  are typically conductive material. The conductive material shields the chamber inside the environmental enclosure  2  from electromagnetic (EM) fields originating outside of the enclosure  2  which would otherwise result in noise within the probe  6 . The environmental enclosure  2  is grounded to return to ground the currents generated in the conductive wall by the EM fields. In a preferred embodiment of the present invention, the conductive wall of the environmental enclosure is extended to substantially surround parts of the thermal units. The extension of the wall of the enclosure provides a conductive shield capacitively coupled to the thermal units which can return capacitive currents to the enclosure ground.  
         [0023]    Referring to FIG. 3, in a first aspect of this preferred embodiment the wall of the environmental enclosure  2  is extended coaxially with yet another shield layer  34  of the cable  32  to a point of close physical proximity to the thermal device shell  28  yet being free from direct electrical connection to the shield of the cable  32 , the thermal driver controller  18 , and the thermal device shell  28 . The wall of the environmental enclosure  2  is extended proximate to the thermal device shell  28  by connecting the outer shield layer  34  of the cable  32  to the wall of the environmental enclosure  2 . The cable  32  includes the power conductors  24  connecting the thermal driver controller  18  to the thermal devices  22 . Capacitive currents emanating from the power conductors  24  are intercepted by the shield of cable  32  and returned to the thermal driver controller  18  and the thermal driver controller ground  21 . The extension of the wall of the environmental enclosure  2  through the outer shield  34  of the power cable  32  is capacitively coupled to the shield of the cable  32  by a “virtual” capacitor  36  and intercepts capacitive currents leaking from within the cable  32  which might otherwise couple to the chuck  4 . Any current in the extension of the environmental enclosure  2  is returned to ground  7  outside of the environmental enclosure  2  if switch  23  is closed. If the switch  23  is open, capacitive currents are returned to the ground  25  of an instrument  27  which is connected by leads  29  to probes inside the chamber.  
         [0024]    Referring to FIG. 4, in a second aspect of this preferred embodiment the wall of the environmental enclosure  40  is extended to substantially surround the thermal devices  42 , the thermal device shell  44  and the power cable  46  connecting the thermal devices  42  to the thermal driver controller  50 . Heat is transferred to and from the chuck  56  through the thermal device shell  44  and the wall of the environmental enclosure  40 . The thermal devices  42  are capacitively coupled to the thermal shell  44  by virtual capacitors  48 . The thermal device shell  44  and the shield of the power cable  46  are, in turn, capacitively coupled to the wall of the environmental enclosure  40  by virtual coupling capacitors  52 . Capacitive currents in the thermal device shell  44  or the shield of the cable  46  are returned to the thermal driver controller  50  through the conductive shield layer of the cable  46 . The thermal driver controller  50  is connected to the thermal devices  42  by power conductors  43  and to ground at ground  51 . Capacitive currents leaking from the thermal device shell  44  or the power cable  46  will be intercepted by the wall of the enclosure  40  and returned to the enclosure ground  54  when the switch  53  is closed. When the switch  53  is open, capacitive currents in the wall of the environmental enclosure  40  are returned to the ground  55  of instrument  57 . The instrument  57  is connected to the probes  6  inside the environmental enclosure by instrument leads  47 .  
         [0025]    Referring to FIG. 5, in a third aspect of this preferred embodiment the wall of the environmental enclosure  60  is extended to surround the thermal devices  64  and the power conductors  62  connecting the thermal devices  64  to the thermal driver controller  63 . The thermal driver controller is grounded at ground  74 . In this aspect of the invention, the thermal devices  64  and the power conductors  62  are capacitively coupled to the wall of the environmental enclosure  60  through the virtual coupling capacitors  66 . Capacitive currents generated in the thermal devices  64  or power cables  62  are intercepted by the shield formed by the conductive wall of the enclosure  60  and returned to the enclosure ground  68  when the switch  69  is closed. If the switch  69  is open the walls of the enclosure  60  are grounded through the instrument  73  to the instrument ground  71 . Heat is transferred to and from the chuck  70  through the wall of the environmental enclosure  60 .  
         [0026]    The terms and expressions that have been employed in the foregoing specification are used 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.