Abstract:
A system for low-current testing of a test device includes a probing device for probing a probing site on the test device. The probing device includes a dielectric substrate having first and second sides, an elongate conductive path on the first side of the substrate, an elongate probing element connected to the elongate conductive path so as to extend in a cantilevered manner beyond the substrate, and a conductive area on the second side of the substrate. The probe housing is matingly detachably engageable with the probing device.

Description:
This application is a continuation of patent application Ser. No. 09/815,182, filed on Mar. 22, 2001, now U.S. Pat. No. 6,384,615, which is a Division of patent application Ser. No. 08/864,287, filed on May 28, 1997, now U.S. Pat. No. 6,232,789. 
    
    
     The present invention relates to a probe holder suitable for low current measurements. 
     Typically, in the construction of a probe card, a dielectric board is used as a base. A plurality of probing devices are mounted in a radial arrangement about an opening in the board so that the probing elements of these devices, which may, for example comprise slender conductive needles, terminate below the opening in a pattern suitable for probing the contact sites of the test device. The probing devices are individually connected to the respective channels of a test instrument by a plurality of interconnecting lines, where the portion of each line that extends between the corresponding probing device and the outer edge of the dielectric board may comprise an interconnecting cable or a conductive trace pattern formed directly on the board. In one conventional type of setup where the test devices are integrated circuits formed on a semiconductor wafer, the probe card is mounted by a supporting rig or test head above the wafer, and a support beneath the wafer moves the wafer so that each device thereon is consecutively brought into contact with the needles or probing elements of the probe card. 
     With particular regard to probe cards that are specially adapted for use in measuring ultra-low current (down to the femtoamp region or lower), probe card designers have been concerned with developing techniques for eliminating or at least reducing the effects of leakage currents, which are unwanted currents that can flow into a particular cable or channel from surrounding cables or channels so as to distort the current measured in that particular cable or channel. For a given potential difference between two spaced apart conductors, the amount of leakage current that will flow between them will vary depending upon the volume resistivity of the insulating material that separates the conductors, that is, if a relatively lower-resistance insulator is used, this will result in a relatively higher leakage current. Thus, a designer of low-current probe cards will normally avoid the use of rubber-insulated single-core wires on a glass-epoxy board since rubber and glass-epoxy materials are known to be relatively low-resistance insulators through which relatively large leakage currents can flow. 
     One technique that has been used for suppressing interchannel leakage currents is surrounding the inner core of each lead-in wire with a cylindrical “guard” conductor, where the “guard” conductor is maintained at the same potential as the inner core by a feedback circuit in the output channel of the test instrument. Because the voltage potentials of the outer guard conductor and the inner conductive core are made to substantially track each other, negligible leakage current will flow across the inner dielectric that separates these conductors regardless of whether the inner dielectric is made of a low- or high-resistivity material. Although leakage current can still flow between the guard conductors of the respective cables, this is typically not a problem because these guard conductors, unlike the inner conductive cores, are at low impedance. By using this guarding technique, significant improvement may be realized in the low-level current measuring capability of certain probe card designs. 
     To further improve low-current measurement capability, probe cards have been constructed so as to minimize leakage current between the individual probing devices which mount the probing needles or other elements. With respect to these devices, higher-resistance insulating materials have been substituted for lower resistance materials and additional conductive surfaces have been arranged about each device in order to perform a guarding function in relation thereto. In-one type of assembly, for example, each probing device is constructed using a thin blade of ceramic material, which is a material known to have a relatively high volume resistivity. An elongate conductive trace is provided on one side of the blade to form the signal line and a backplane conductive surface is provided on the other side of the blade for guarding purposes. The probing element of this device is formed by a slender conductive needle, such as of tungsten, which extends in a cantilevered manner away from the signal trace. Such devices are commercially available, for example, from Cerprobe Corporation based in Tempe, Ariz. During assembly of the probe card, the ceramic blades are edge-mounted in a radial arrangement about the opening in the card so that the needles terminate within the opening in a pattern suitable for probing the test device. The conductive backplane on each blade is connected to the guard conductor of the corresponding cable and also the corresponding conductive pad or “land” adjacent the opening in the card. In this manner each conductive path is guarded by the backplane conductor on the opposite side of the blade and by the conductive land beneath it. 
     It has been found, however, that even with the use of guarded cables and ceramic probing devices of the type just described, the level of undesired background current is still not sufficiently reduced as to match the capabilities of the latest generation of commercially available test instruments, which instruments are able to monitor currents down to one femtoamp or less. Thus, it was evident that other changes in probe card design were needed in order to keep up with the technology of the latest test instrument design. 
     However, in the design of such probe cards the ceramic blades are permanently mounted to the probe card and thus when damaged the entire probe card may need to be replaced or the damaged ceramic blade somehow repaired at substantial expense and effort. Referring to FIG. 1, in order to provide probe tips that are more easily replaced, a probe housing  10  with a replaceable probe tip  12  was designed. A pair of triaxial cables (not shown), each of which includes a shield, a guard, and a signal conductor, extend from measurement equipment (not shown) to a location within a chamber (not shown) that encloses the probe tip  12 , the probe housing  10 , and the test device. Each triaxial cable is connected to a respective coaxial cable  14  and  16  that includes a guard and a signal conductor. The shield conductor of each of the triaxial cables may be connected to the chamber, if desired. The chamber environment is shielded so it is unnecessary to include the shield conductors all the way to the probe housing  10 . In addition, the probe housing  10  includes relatively small connectors which are much more suitable for connection to relatively small coaxial cables  14  and  16 , as opposed to relatively large triaxial cables. The probe housing  10  includes a pair of connectors  18  and  20 , each of which provides a connection to a respective one of the coaxial cables  14  and  16 . The guard of each of the coaxial cables  14  and  16  is electrically connected to the conductive exterior of the probe housing  10 , which reduces the capacitance and leakage currents to the probe tip  12 . 
     Ideally in a two lead coaxial cable system a “true Kelvin” connection is constructed, although not shown in FIG.  1 . This involves using what is generally referred to as a force signal and a sense signal. The signal conductor from one of the coaxial cables is considered the force conductor, while the signal conductor from the other coaxial cable is considered the sense conductor. The force conductor is brought into contact with a test pad on the wafer. The force conductor is a low impedance connection, so a current is forced through the force conductor for testing purposes. The sense conductor is a high impedance connection and is also brought into contact with the same test pad on the wafer, preferably in close proximity to the sense conductor, in order to sense the voltage. As such the current versus voltage characteristics of the test device can be obtained using the force and sense conductors. 
     To calibrate the “true Kelvin” connection, first an open circuit test is performed to measure the capacitance without the test pad capacitance. This is performed by picking up the probe and shorting the probe tips of the sense and force conductors together with both suspended in air. The open circuit test is difficult to perform. Second, a short circuit test is performed to measure the capacitance when the force and sense conductor tips are on the test pad. From the open circuit test and the short circuit test the cable impedance is obtained and thereafter used for offsetting during subsequent measurements. Unfortunately, calibration of a “true Kelvin” connection is difficult and time consuming to perform. Additionally, the current flowing through the force conductor is generally known but the resistance drop along the length of force conductor results in the exact voltage at its end to be unknown, therefore the measurement can be inaccurate. Further, the test pads on the test device are normally small, in order to minimize cost, which makes it difficult to position two needles on the test pad. Furthermore, using two needles requires additional space for the needles and supporting structure that may not be available when a large number of probe needles are simultaneously necessary to test a small area of the test device, such as a silicon wafer. 
     Referring again to FIG. 1, to permit the use of a single probe tip, which permits more tests to be simultaneously performed in a confined area, the force conductor  22  and the sense conductor  24  are electrically connected together with a combined conductor  26  within the probe housing  10 . Coaxial cable  14  would be the force connection while cable  16  would be the sense connection. The guard conductor of the force cable  14  and the guard conductor of the sense cable  16  are electrically connected to the conductive probe housing  10 . The combined force and sense conductor  26  is electrically connected to a probe connector  28  at one end of the probe housing  10 . A rigid coaxial probe tip cable  30  is detachably connected to the probe connector  28 . The rigid coaxial probe tip cable  30  includes both a copper guard conductor  32  plated with gold and a central signal conductor  34  made of tungsten. The guard conductor  32  of the rigid coaxial probe tip cable  30  is electrically connected to the probe housing  10 , which is in turn connected to the guard conductors of the coaxial cables  14  and  16 . The length of the signal path extending from the point that the force conductor  22  and sense conductor  24  are connected together carries current during measurements which results in a voltage drop from any internal resistance in that portion of the signal path. The assumption is that for low current applications, the voltage drop due to the resistance is small because the junction is close to the probe tip  12  and the conductor has low resistance. However, the rigid coaxial probe tip cable  30  is difficult to replace if defective or damaged during use. The test device shown in FIG. 1, provides reasonably accurate low current measurements. Unfortunately, it was observed that the device shown in FIG. 1, when used over a wide range of temperatures, such as −65 degrees celsius to 300 degree celsius, results in unacceptable levels of noise. For example, in one commercial embodiment, noise in the range of +−100 femtoamps was observed over only a temperature range from room temperature to 150 degrees celsius. As previously mentioned, modern measurement instruments are capable of measuring much lower current levels and thus such noise levels obscures low current measurement levels under 10 femtoamps. 
     What is desired, therefore, is a low current measurement device that has substantially lower noise levels. In addition, such a measurement device should be provide for easy replacement of probe tips. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view of a conventional probe housing and cable type probing device. 
     FIG. 2 is a pictorial view of an exemplary embodiment of a probe housing with a probe connector, and probing device of the present invention. 
     FIG. 3 is a sectional view of the probe housing and probing device engaged within the probe connector of FIG.  1 . 
     FIG. 4 is a sectional view of the probe connector and probing device of FIG.  1 . 
     FIG. 5 is an exemplary low-noise cable. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     After observing excessive noise levels over wide temperature ranges resulting from the device shown in FIG. 1, the present inventor came to the startling realization that a Teflon insulator  36  between the signal conductor  34  and the guard conductor  32  of the coaxial probe tip cable  30  expands and contracts when the temperature is increased and decreased, respectfully. The expansion and contraction of the Teflon insulator  36  results in relative movement and friction with respect to the signal conductor  34  and the guard conductor  32 . The relative movement generates electrical charges between both the guard conductor  32  and the signal conductor  34  in contact with the Teflon insulator  36  due to friction. 
     Free electrons rub off the signal conductor  32  and the guard conductor  34  which creates a charge imbalance resulting in a current flow. 
     Triboelectric currents are traditionally considered to arise in relation to the physical bending of materials, such as coaxial cables. The time during which the coaxial cable undergoes physical bending is relatively long, e.g. low frequency, and does not significantly contribute to the triboelectric current generation. The present inventor believes that instead of the physical bending resulting in the major source of the generation of triboelectric currents, it is in fact a much more subtle phenomena involving the slight contraction and expansion of the Teflon insulator that creates the frictional movement previously described, and hence the undesired triboelectric current generation. 
     With the identification of the source of the noise, which had not been previously identified by previous probe card designers, the present inventor developed ingenious solutions to the identified problem. 
     In order to reduce the triboelectric current generation, the cable  30  is replaced with “low-noise” cable. “Low-noise” cable greatly reduces triboelectric currents by typically using an inner insulator of polyethylene coated with graphite. The graphite provides lubrication and a conduction equipotential cylinder to equalize charges and minimize charges generated by frictional effects of cable movement. 
     It will be noted that the inventor does not claim to have discovered a new solution to the problem of the triboelectric effect as embodied by the use of “low-noise” cable. A relatively straightforward solution to this problem can be found in the field of cable technology wherein it is known how to construct a “low-noise” cable by using an additional layer of material between the outer conductor and the inner insulator, which material is of suitable composition for suppressing the triboelectric effect. This layer, in particular, includes a nonmetallic portion that is physically compatible with the inner insulator so as to be prevented from rubbing excessively against this dielectric and, on the other hand, includes a portion that is sufficiently conductive that it will immediately dissipate any charge imbalance that may be created by free electrons that have rubbed off the conductor. It is not claimed by the inventor that this particular solution to the triboelectric effect problem is his invention. Rather it is the recognition that this specific problem is a major source of performance degradation in the field of low-current probe station design, and in particular degradation when testing over a range of temperatures, that the inventor regards as his discovery. 
     As previously described, one embodiment of a probe holder design of the present invention includes the replacement of cable  30  with a “low-current” cable that includes conductive and dielectric layers in a coaxial arrangement with each other and further includes at least one layer of material within each cable adapted for suppressing the triboelectric effect so as to minimize any undesirable currents that would otherwise be generated internally in each cable due to this effect. This layer of material on the probe holder enables the probe station to be used for the measurement of ultra-low currents even over a range of temperatures. 
     In the field of radio frequency (rf) cable technology, cables that include a layers material of the type just described are generally referred to as “low-noise” cables. Commercial sources for this type of cable include Belden Wire and Cable Company based in Richmond, Ind., Suhner HR-Kabel based in Herisau, Switzerland, and Times Microwave Systems based in Wallingford, Conn. 
     While the replacement of the cable  30  with “low-noise” cable significantly enhances the low-noise characteristics of the device  10 , the cable is expensive to obtain in small quantities, awkward to replace in a confined environment if damaged, and the precise bending of the cable to the test pad together with the precise location of the probe tip is difficult to control with a cable type connector. Even more importantly, the Teflon insulator material in coaxial cables are susceptible to “cold flow” out of the end of the cable when subjected to significant temperatures. The resulting cable insulator will be thinner in portions thereby changing the characteristics of the cable over time. 
     Referring to FIG. 2, in order to overcome the limitations of cable type connectors, an alternative embodiment includes a conductive probe housing  50 , similar in structure to the probe housing  10  shown in FIG. 1, with an elongate probe connector  52 . The probe connector  52  is conductive and preferably has a rectangular cross section. An insert  54  is sized to fit within the probe connector  52 . The insert  52  includes a ceramic insulator  56  and a conductive bent connector  58  attached to one side of the insulator  56 . The insulator  56  is in face-to-face abutment with the interior upright surface  59  of the probe connector  52 . A “blade” type probe  60 , as described in the background, is matingly detachably engageable within the probe connector  52 . 
     Referring also to FIG. 3, the blade  60  preferably includes a dielectric substrate  62  formed of a ceramic or a comparable high-resistance insulating material. The blade  60  has a pair of broad parallel sides or faces interconnected by a thin edge. Formed on one side of the blade  60  is an elongate conductive path  64 , while the other side includes a backplane conductive surface  66 . A needle  68  is supported by the dielectric substrate  62  and electrically connected to the elongate conductive path  64 . In the particular embodiment shown, the blade  60  is generally L-shaped in profile and is edge-mounted within the probe connector  52  so that the short arm of the L-shaped blade  60  extends downwardly making contact with the test device. As previously indicated, blades  60  having a construction of the type just described are commercially available from Cerprobe Corporation of Tempe, Ariz. 
     Referring also to FIG. 4, when the blade  60  is slidably engaged within the probe connector  52 , the backplane conductive surface  66  is in face-to-face contact with the inner upright surface  59  of the probe connector  52 . Accordingly, a ground signal path is provided from the guard conductors of the force and sense cables  14  and  16 , though the probe housing  50  and probe connector  52  to the backplane conductive area  66  of the blade  60 . This provides a ground path to a location near the end of the needle  68 . In addition, a conductive path is provided from force and sense conductors  72  and  74  connected to cables  14  and  16 , through a combined conductor  70  to the bent connector  58 . It is to be understood that the combined connector  70  may be any suitable type of coupler that electrically connects the force and sense cables to the conductive path  64  on the blade  60 . Likewise it is to be understood that the electrical connection between the backplane  66  on the blade  60  and the cables  14  and  16  may be any suitable type of coupler. The bent connector  58  is resiliently deformable as the blade  60  is inserted into the probe connector  52  and exerts pressure between the backplane conductive surface  66  and the upright surface  59  so a low loss connection is maintained. Also the pressure maintains the position of the blade  60  during use. Simultaneously, the bent connector  58  exerts pressure between the conductive path  64  and the bent connector  58  to provide a low loss connection. A signal path is thus provided through the needle  68 , the conductive path  64 , the bent connector  58 , and the combined conductor  70  to the force conductor  72  and sense conductor  74 . 
     The probe embodiment shown in FIGS. 2-4 does not include structure that is subject to the generation of triboelectric currents and further is free from materials that deform over a normal range of temperatures used during probing. In addition, the blades  60  are readily replaceable if damaged during use. 
     While the preferred embodiment of the present invention embodies a set of two cables  14  and  16 , with one including the sense conductor and the other including the force conductor, it is to be understood that the present invention also encompasses a probe holder with a single coaxial, or triaxial, cable. 
     The terms and expressions which have been 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 which follow.