Abstract:
A probe measurement system for measuring the electrical characteristics of integrated circuits or other microelectronic devices at high frequencies.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of U.S. application Ser. No. 10/445,099 filed May 23, 2003, which claims the benefit of U.S. patent application Ser. No. 60/383,017 filed May 23, 2002. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     The present invention relates to probe measurement systems for measuring the electrical characteristics of integrated circuits or other microelectronic devices at high frequencies.  
         [0003]     There are many types of probing assemblies that have been developed for the measurement of integrated circuits and other forms of microelectronic devices. One representative type of assembly uses a circuit card on which the upper side are formed elongate conductive traces that serve as signal and ground lines. A central opening is formed in the card, and a needle-like probe tip is attached to the end of each signal trace adjacent the opening so that a radially extending array of downwardly converging needle-like tips is presented by the assembly for selective connection with the closely spaced pads of the microelectronic device being tested. A probe assembly of this type is shown, for example, in Harmon U.S. Pat. No. 3,445,770. This type of probing assembly, however, is unsuitable for use at higher frequencies, including microwave frequencies in the gigahertz range, because at such frequencies the needle-like tips act as inductive elements and because there are no adjoining elements present to suitably counteract this inductance with a capacitive effect in a manner that would create a broadband characteristic of more or less resistive effect. Accordingly, a probing assembly of the type just described is unsuitable for use at microwave frequencies due to the high levels of signal reflection and substantial inductive losses that occur at the needle-like probe tips.  
         [0004]     In order to obtain device measurements at somewhat higher frequencies than are possible with the basic probe card system described above, various related probing systems have been developed. Such systems are shown, for example, in Evans U.S. Pat. No. 3,849,728; Kikuchi Japanese Publication No. 1-209,380; Sang et al. U.S. Pat. No. 4,749,942; Lao et al. U.S. Pat. No. 4,593,243; and Shahriary U.S. Pat. No. 4,727,319. Yet another related system is shown in Kawanabe Japanese Publication No. 60-223,138 which describes a probe assembly having needle-like tips where the tips extend from a coaxial cable-like structure instead of a probe card. A common feature of each of these systems is that the length of the isolated portion of each needle-like probe tip is limited to the region immediately surrounding the device-under-test in order to minimize the region of discontinuity and the amount of inductive loss. However, this approach has resulted in only limited improvement in higher frequency performance due to various practical limitations in the construction of these types of probes. In Lao et al., for example, the length of each needle-like tip is minimized by using a wide conductive blade to span the distance between each tip and the supporting probe card, and these blades, in turn, are designed to be arranged relative to each other so as to form transmission line structures of stripline type. As a practical matter, however, it is difficult to join the thin vertical edge of each blade to the corresponding trace on the card while maintaining precisely the appropriate amount of face-to-face spacing between the blades and precisely the correct pitch between the ends of the needle-like probe tips.  
         [0005]     One type of probing assembly that is capable of providing a controlled-impedance low-loss path between its input terminal and the probe tips is shown in Lockwood et al. U.S. Pat. No. 4,697,143. In Lockwood et al., a ground-signal-ground arrangement of strip-like conductive traces is formed on the underside of an alumina substrate so as to form a coplanar transmission line on the substrate. At one end, each associated pair of ground traces and the corresponding interposed signal trace are connected to the outer conductor and the center conductor, respectively, of a coaxial cable connector. At the other end of these traces, areas of wear-resistant conductive material are provided in order to reliably establish electrical connection with the respective pads of the device to be tested. Layers of ferrite-containing microwave absorbing material are mounted about the substrate to absorb spurious microwave energy over a major portion of the length of each ground-signal-ground trace pattern. In accordance with this type of construction, a controlled high-frequency impedance (e.g., 50 ohms) can be presented at the probe tips to the device under test, and broadband signals that are within the range, for example, of DC to 18 gigahertz can travel with little loss from one end of the probe assembly to another along the coplanar transmission line formed by each ground-signal-ground trace pattern. The probing assembly shown in Lockwood et al. fails to provide satisfactory electrical performance at higher microwave frequencies and there is a need in microwave probing technology for compliance to adjust for uneven probing pads.  
         [0006]     To achieve improved spatial conformance between the tip conductors of a probe and an array of non-planar device pads or surfaces, several high-frequency probing assemblies have been developed. Such assemblies are described, for example, in Drake et al. U.S. Pat. No. 4,894,612; Coberly et al. U.S. Pat. No. 4,116,523; and Boll et al. U.S. Pat. No. 4,871,964. The Drake et al. probing assembly includes a substrate on the underside of which are formed a plurality of conductive traces which collectively form a coplanar transmission line. However, in one embodiment shown in Drake et al., the tip end of the substrate is notched so that each trace extends to the end of a separate tooth and the substrate is made of moderately flexible nonceramic material. The moderately flexible substrate permits, at least to a limited extent, independent flexure of each tooth relative to the other teeth so as to enable spatial conformance of the trace ends to slightly non-planar contact surfaces on a device-under-test. However, the Drake et al. probing assembly has insufficient performance at high frequencies.  
         [0007]     With respect to the probing assembly shown in Boll et al., as cited above, the ground conductors comprise a pair of leaf-spring members the rear portions of which are received into diametrically opposite slots formed on the end of a miniature coaxial cable for electrical connection with the cylindrical outer conductor of that cable. The center conductor of the cable is extended beyond the end of the cable (i.e., as defined by the ends of the outer conductor and the inner dielectric) and is gradually tapered to form a pin-like member having a rounded point. In accordance with this construction, the pin-like extension of the center conductor is disposed in spaced apart generally centered position between the respective forward portions of the leaf-spring members and thereby forms, in combination with these leaf-spring members, a rough approximation to a ground-signal-ground coplanar transmission line structure. The advantage of this particular construction is that the pin-like extension of the cable&#39;s center conductor and the respective forward portions of the leaf-spring members are each movable independently of each other so that the ends of these respective members are able to establish spatially conforming contact with any non-planar contact areas on a device being tested. On the other hand, the transverse-spacing between the pin-like member and the respective leaf-spring members will vary depending on how forcefully the ends of these members are urged against the contact pads of the device-under-test. In other words, the transmission characteristic of this probing structure, which is dependent on the spacing between the respective tip members, will vary in an ill-defined manner during each probing cycle, especially at high microwave frequencies.  
         [0008]     Burr et al., U.S. Pat. No. 5,565,788, disclose a microwave probe that includes a supporting section of a coaxial cable including an inner conductor coaxially surrounded by an outer conductor. A tip section of the microwave probe includes a central signal conductor and one or more ground conductors generally arranged normally in parallel relationship to each other along a common plane with the central signal conductor so as to form a controlled impedance structure. The signal conductor is electrically connected to the inner conductor and the ground conductors are electrically connected to the outer conductor, as shown in  FIG. 1 . A shield member is interconnected to the ground conductors and covers at least a portion of the signal conductor on the bottom side of the tip section. The shield member is tapered toward the tips with an opening for the tips of the conductive fingers. The signal conductor and the ground conductors each have an end portion extending beyond the shield member and the end portions are able to resiliently flex, despite the presence of the shielding member, relative to each other and away from their common plane so as to permit probing devices having non-planar surfaces.  
         [0009]     In another embodiment, Burr et al. disclose a microwave probe that includes a supporting section of a coaxial cable including an inner conductor coaxially surrounded by an outer conductor, as shown in  FIGS. 2A, 2B , and  2 C. A tip section of the microwave probe includes a signal line extending along the top side of a dielectric substrate connecting a probe finger with the inner conductor. A metallic shield may be affixed to the underside of the dielectric substrate and is electrically coupled to the outer metallic conductor. Ground-connected fingers are placed adjacent the signal line conductors and are connected to the metallic shield by way of vias through the dielectric substrate. The signal conductor is electrically connected to the inner conductor and the ground plane is electrically connected to the outer conductor. The signal conductor and the ground conductor fingers (connected to the shield via vias) each have an end portion extending beyond the shield member and the end portions are able to resiliently flex, despite the presence of the shielding member, relative to each other and away from their common plane so as to permit probing devices having non-planar surfaces. While the structures disclosed by Burr et al. are intended to provide uniform results of a wide frequency range, they unfortunately tend to have non-uniform response characteristics at high microwave frequencies. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  illustrates an existing probe.  
         [0011]      FIGS. 2A-2C  illustrate an existing probe.  
         [0012]      FIG. 3  illustrates a modified probe.  
         [0013]      FIG. 4  illustrates a side view of a portion of the probe of  FIG. 3 .  
         [0014]      FIG. 5  illustrate a bottom view of a portion of the probe of  FIG. 3 .  
         [0015]      FIG. 6  illustrates a force versus vertical probe deformation graph.  
         [0016]      FIG. 7  illustrates probe pre-loading.  
         [0017]      FIG. 8  illustrates a force versus vertical probe deformation graph for probe pre-loading.  
         [0018]      FIG. 9  illustrates a probe contact.  
         [0019]      FIG. 10  illustrates a modified probe contact.  
         [0020]      FIG. 11  illustrates contact resistance.  
         [0021]      FIG. 12  illustrates contact resistance.  
         [0022]      FIG. 13  illustrates a membrane type probe tip. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0023]     The present inventors considered the co-planar fingered probing devices disclosed by Burr et al., including the co-planar finger configuration and the microstrip configuration with fingers extending therefrom. In both cases, electromagnetic fields are created during probing between the fingers. These electromagnetic fields encircle each of the fingers, electrically couple the signal finger to the ground fingers, and electrically couple the ground fingers one another. While the probing device is being used for probing, the resulting electromagnetic fields surrounding the fingers interact with the wafer environment. While probing in different regions of the wafer, the interaction between the electromagnetic fields around the fingers and the wafer change, typically in an unknown manner. With a significant unknown change in the interaction it is difficult, if not impossible, to accurately calibrate out the environmental conditions while probing a device under test.  
         [0024]     When multiple probes are being simultaneously used for probing the same area of the wafer, the probe tips come into close proximity with one another and result in additional coupling between the probes, normally referred to as cross-talk. In addition, the region between the support for the fingers, such as a dielectric substrate, and the extended portion of the fingers results in a significant capacitance, which impedes high frequency measurements.  
         [0025]     The present inventors were surprised to determine that the microstrip structure disclosed by Burr et al. further does not calibrate well on calibration test substrates at very high frequencies, such as in excess of 70 GHz. This calibration is independent of potential interaction with a wafer at a later time during actual probing of a device under test. After examination of this unexpected non-calibration effect the present inventors speculate that an energy is created in an “undesired mode”, other than the dominant field modes, at such extreme frequencies. This “undesired mode” results in unexpected current leakages from the signal path thus degrading the signal integrity. The present inventors further speculate that this “undesired mode” involves resonating energy in the ground plane as a result of discontinuities in the ground path, including for example, the connection between the ground plane and the external portion of the cable, and the inductance in the ground plane. This ground plane resonant energy results in unpredictable changing of the energy in the signal path to the device under test, thus degrading performance. This degradation wasn&#39;t apparent at lower operating frequencies, so accordingly, there was no motivation to modify existing probe designs in order to eliminate or otherwise reduce its effects.  
         [0026]     Referring to  FIG. 3 , a semi-rigid coaxial cable  40  is electrically connected at its rearward end to a connector (not shown). The coaxial cable  40  normally includes an inner conductor  41 , a dielectric material  42 , and an outer conductor  43 . The coaxial cable  40  may likewise include other layers of materials, as desired. The forward end of the cable  40  preferably remains freely suspended and, in this condition, serves as a movable support for the probing end of the probe.  
         [0027]     A microstrip style probe tip  80  includes a dielectric substrate  88  that is affixed to the end of the coaxial cable  40 . The underside of the cable  40  is cut away to form a shelf  85 , and the dielectric substrate  88  is affixed to the shelf  85 . Alternatively, the dielectric substrate  88  may be supported by an upwardly facing shelf cut away from the cable or the end of the cable without a shelf. Referring also to  FIG. 4 , a conductive shield  90 , which is preferably planar in nature, is affixed to the bottom of the substrate  88 . The conductive shield  90 , may be for example, a thin conductive material (or otherwise) that is affixed to the substrate  88 . By using a generally planar conductive material having a low profile the shield  90  is less likely to interfere with the ability to effectively probe a device under test by accidently contacting the device under test. The conductive shield  90  is electrically coupled to the outer conductor  43  to form a ground plane. The outer conductor  43  is typically connected to ground, though the outer conductor  43  may be provided with any suitable voltage potential (either DC or AC). The conductive shield  90  preferably covers all of the lower surface of the substrate  88 . Alternatively, the conductive shield  90  may cover greater than 50%, 60%, 70%, 80%, 90%, and/or the region directly under a majority (or more) of the length of a conductive signal trace on the opposing side of the substrate  88 .  
         [0028]     One or more conductive signal traces  92  are supported by the upper surface of the substrate  88 . The conductive traces  92 , may be for example, deposited using any technique or otherwise supported by the upper surface of the substrate. The conductive trace(s)  92  is electrically interconnected to the inner conductor  41  of the coaxial cable  40 . The inner conductor  41  of the coaxial cable  40  and the conductive trace(s)  92  normally carries the signal to and from the device under test. The conductive trace(s)  92  together with the shield layer  90  separated by a dielectric material  88  form one type of a microstrip transmission structure. Other layers above, below, and/or between the shield  90  and the conductive trace  92  may be included, if desired.  
         [0029]     To reduce the effects of the aforementioned unexpected high frequency signal degradation, the present inventors determined that the signal path may include a conductive via  94  passing through the substrate  88 . The conductive via  94  provides a manner of transferring the signal path from the upper surface of the substrate to the lower surface of the substrate. The conductive via  94  avoids the need for using a conductive finger extending out from the end of the substrate  88  that would otherwise result in a significant capacitance between the extended finger and the end of the substrate  88 . The conductive via  94  provides a path from one side of the substrate  88  to the other side of the substrate  88  in a manner free from an air gap between the conductive via  94  and the substrate  84  for at least a majority of the thickness of the substrate  88 . In addition, the shield layer  90  preferably extends beyond the via  94  to provide additional shielding.  
         [0030]     Referring also to  FIG. 5 , the lower surface of the substrate  88  illustrates a contact bump  100  electrically connected to the via  94  and the trace  92  extending below the lower surface of the substrate  88  and the shield  90  which may be used to make contact with the device under test during probing. The conductive shield  90  may include an “patterned” section around the contact “bump”  100  so that the shield and the signal path are free from being electrically interconnected (e.g., the shield layer  90  may be greater than 50%, 75%, or laterally surrounding all of the contact at some point). It is to be understood that the contact may take any suitable form, such as a bump, a patterned structure, or an elongate conductor. The conductive shield  90  may laterally encircle the conductive bump which increases the resistance to external electromagnetic fields. Also, the conductive shield  90  extending beyond the conductive bump  100  reduces the crosstalk from other probes. For some probing applications, one or more shield contacts  102  may be provided, if desired. The shield layer and the conductive trace are normally constructed to provide a microstrip transmission line controlled impedance structure.  
         [0031]     To further increase the performance at high frequencies the present inventors considered the effects of the substrate material. In many cases the dielectric constant of the dielectric substrate material is high, such as Al 2 O 3  which has a 9.9 dielectric constant. Materials with a high dielectric constant have a tendency to concentrate the electromagnetic fields therein, thus decreasing the electromagnetic fields susceptible to influence by other devices. In addition, the thickness of the substrate is typically 250-500 microns to provide mechanical stability. Thus the fields tend to concentrate within the substrate.  
         [0032]     Referring to  FIG. 13 , while considering such substrates the present inventors came to the realization that the flexible membrane substrate may be substituted for the more rigid substrate  88 . An example of membrane material is described in U.S. Pat. No. 5,914,613. In general, membrane based probes are characterized by a flexible (or semi-flexible) substrate with traces supported thereon together with contacting portions being supported thereon. The contacting portions come into contact with the device under test and the traces are normally on the opposing side of the membrane connected to the contacting portions using vias. In many cases, the membrane technology may be significantly thinner than ceramic based substrates, (see e.g., substrate  88 ) such as 40, 30, 20, 10, 5, or 3 microns or less. Normally the dielectric constant of the membrane material is 7 or less, sometimes less than 6, 5, or 4 depending on the particular material used. While normally using a lower dielectric constant substrate is unsuitable, using a significantly thinner substrate together with a lower dielectric constant substrate raises the theoretical frequency range of effective signal transmission to 100&#39;s of GHz. The significantly thinner substrate material permits positioning the lower shield material significantly closer to the signal traces than the relatively thick ceramic substrate, and therefore tends to more tightly confine the electromagnetic fields there between.  
         [0033]     When the membrane based probe comes into contact with a device under test, as in most probes, it tends to skate across the pad as additional pressure is exerted. This skating is the result of the angled probe and/or co-axial cable flexing while under increasing pressure against the test pad. A limited amount of skating is useful to “scrub” away oxide layers, or otherwise, that may be built up on the contact pad, which results at least in part from a suitable amount of pressure and/or skating. In many cases the test pad is typically relatively small and excessive skating from slightly too much pressure being applied results in the probe simply skating off the test pad. In addition, if excessive pressure is exerted damage to the probe and/or contact pad may result. Accordingly, there is an acceptable range of pressure and skating that should be maintained.  
         [0034]     Referring to  FIG. 6 , for purposes of illustration the force applied by the probe versus vertical deformation of the probe as a result of the force being applied is shown. Line  400  is for a low stiffness probe and line  402  is for a high stiffness probe. Vertical line  404  illustrates the maximum skating distance before the probe is likely off the contact pad, and accordingly the greatest distance of over travel of the probe after contact with the contact pad. Vertical line  406  illustrates the minimum generally acceptable skating distance of the probe to ensure sufficient scrubbing distance of the oxide layer or otherwise that may be built up on the contact pad, and accordingly the minimum generally acceptable distance of over travel of the probe after contact with the contact pad. Typically the range of useful over-travel is approximately 50 to 200 microns. Horizontal line  408  illustrates the maximum acceptable force that the probe may apply so that damage to the probe and/or contact pad is minimized. Horizontal line  410  illustrates the minimum acceptable force that the probe should apply so that sufficient pressure is exerted to break through the oxide layer or otherwise that may be built up on the contact pad.  
         [0035]     It may be observed that there is a rectangular region (in this example) within which acceptable over-travel and force is applied by the probe to the contact pad. For the low stiffness probe  400  a range of  420  is shown where acceptable probing takes place. It may be observed that this distance uses less than the maximum range between vertical lines  404  and  406 , and thus the over-travel must be carefully controlled by the operator. For the high stiffness probe  402  a range of  422  is shown where acceptable probing takes place. It may be observed that this distance uses less than the maximum range between vertical lines  404  and  406 , and thus the over-travel must be carefully controlled by the operator. Accordingly, the stiffness of the probe needs to be carefully controlled, which is difficult, in order to establish an acceptable operating region. Further, it is noted that there is some relationship between skate to over-travel which may be controlled also. To little skate is problematic because some scrubbing action improved contact resistance and lateral motion provides visual confirmation (through the microscope) that contact has been made. To much skate is problematic because then the probe tip slides across and off the pad before getting enough force for good contact resistance. The pre-load provides the opportunity to tune that ratio by varying the curvatures of the probe and the pre-load location.  
         [0036]     After consideration of the limitations seemingly inherent with the stiffness of the probe, the present inventors came to the realization that by using a relatively flexible probe together with pre-loading a portion of the force to be applied by that probe, a modified force-distance profile may be obtained that is more readily within the acceptable region. The modified force-distance profile may include more of the acceptable probing range, namely a wider probing range within the rectangular region, than otherwise achieved if the probe were not otherwise modified. Referring to  FIG. 7 , this pre-loading may be achieved by using a string  440  or other support member to upwardly flex the probe. If the low stiffness probe  400  is used, then a modified force profile  444  (see  FIG. 8 ) may be obtained. It is noted that the lower curved portion  446  is as a result of the pre-loading of the probe. The upper portion  448  is a result of the probe itself and generally has the same force slope as the probe without pre-loading. It may be observed that in this manner a probe profile that has a relatively low slope that is suitable to extend across more of the useful probing range while maintaining reasonable forces may be used. The pre-loading raises the initial force to a range near the minimum generally acceptable force. The height of the profile  444  may be modified by adjusting the pre-loading. Also, the slope of the profile  444  may be lessened by selecting a more flexible probe or otherwise modifying the orientation of the probe in relation to the contact pad. This pre-load system, while especially useful for membrane type probes, is likewise useful with other probing technologies.  
         [0037]     When making probing measurements the contact resistance between the probe and the device under test is an important consideration. The tip of the probe may be designed in such a manner as to achieve a low contact resistance while permitting effective viewing of the area to be probed with an associated microscope. The probe tip  438  (see  FIG. 9 ) is typically constructed in such a manner that the resulting structure has a pair of opposing inclined surfaces  450  and  452 . The tip  454  of the probe is preferably extended from the inclined surfaces  450  and  452 . The construction of the probe tip may be done using a sacrificial substrate into which is created a depression. Into this depression is located conductive material, traces are located thereon if desired, and flexible dielectric material is located on or under the traces. See U.S. Pat. No. 5,914,613, incorporated herein by reference, together with all references cited herein. Thereafter the sacrificial substrate is removed leaving the probe tip, traces, and membrane material. The probe tip  438  is acceptable, however, it is difficult to see the region proximate the tip  438  when contacting the device under test because of the inclined surfaces  450  and  452 .  
         [0038]     To improve the visibility of the tip  438  during probe it has been determined that the probe  454  may be ground back or otherwise a portion of the probe removed, as illustrated in  FIG. 10 . By removal of a portion of the probe a greater visibility may be achieved during probing of the device under test as illustrated in  FIG. 10 . It is also to be understood that the probe may be constructed in a manner such that a portion of the probe does not need to be removed. The tip portion  454  is preferably approximately 12 μm×12 μm, with about 2-3 mills of vertical over-travel resulting in about 1 mil of longitudinal tip scrub. The probe may likewise retain a lip  460  that provides additional structural support for the tip  454 . The backside  462  of the probe may even be undercut with respect to the plane of the base  464  of the probe. Alternatively, the backside  462  of the probe may be within 30 degrees of vertical (undercut or not) with respect to the plane of the base  464  of the probe.  
         [0039]     The contact resistance resulting from the described structure turns out to be exceedingly low, especially in comparison to other types of probing systems like Tungsten Probes. Referring to  FIG. 1 , the contact resistance on un-patterned aluminum is less then 30 mΩ over 5000 contact cycles, which is considerably lower than conventional tungsten probes where the contact resistance is approximately 130 mΩ. Referring to  FIG. 12 , with the probe being held in contact with the aluminum pads the contact resistance is shown as a function of time. As illustrated in  FIG. 12 , only 10 mΩ of variation was observed during a 5-hour interval. In a similar test, conventional tungsten probes show significant changes over the same period, typically the contact resistance varies from 35 mΩ to 115 mΩ.  
         [0040]     Another consideration in the design of the probe is the characteristics of the different transmission structures. The coaxial cables provide good high frequency transmission characteristics. Within the membrane structure, connected to the coaxial cables, the micro-strip structure provides good high frequency characteristics. Connected to the membrane structure includes a set of contacts, such as a signal contact and a pair of ground contacts. The contacts provide a co-planar transmission structure which has less bandwidth capability than the coaxial cable and the micro-strip structure. To achieve acceptable bandwidth the present inventors have determined that the contacts should be no longer than 150 microns. More preferably the contacts should be no longer (e.g., height from planar surface) than 100 microns, or no longer than 75 microns, or no longer than 55 microns.