Abstract:
An apparatus, system, and method are provided for testing an integrated circuit with a probe card having optical fibers. The optical fibers of the probe card are fixed in alignment with test structures in the integrated circuit, and each optical fiber is coupled to an avalanche photo-diode for measuring photoemissions from the test structures. The photoemissions can be analyzed to verify correct circuit behavior. The optical fibers can be alternatives or complements to electrically conductive probes of the probe card.

Description:
RELATED PATENT APPLICATIONS 
     This patent application is a continuation-in-part of the patent application entitled, “SINGLE POINT HIGH RESOLUTION TIME RESOLVED PHOTOEMISSION MICROSCOPY SYSTEM AND METHOD” by Bruce et al., filed on Dec. 4, 1999, and having application Ser. No., 09/205,589, and is related to the co-pending patent application entitled, “QUADRANT AVALANCHE PHOTODIODE TIME-RESOLVED DETECTION”, by Bruce et al., filed on Sep. 30, 1999, and having application Ser. No. 09/409,088, now U.S. Pat. No. 6,483,327 issued Nov. 19, 2002, the contents of which are hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to probe cards for testing integrated circuits, and more particularly to probe cards having optical fibers. 
     BACKGROUND OF THE INVENTION 
     Probe card assemblies generally include a probe card which is a printed circuit board having electrically conductive traces for carrying signals from contact pads formed on the integrated circuit. The probe card is interfaced with a programmed computer that generates test signals and senses responses from the integrated circuit. Electrically conductive probes are connected to the traces in the probe card and are arranged to be placed in electrical contact with the contact pads on the integrated circuit. Some assemblies also include an additional layer to transition from the low density probe card to a higher density of probes. 
     Problems recognized in the design of probe card assemblies include the uneveness of the contact pads, fatigue of probes, and misalignment of the probes caused by repetitive use. As integrated circuits become denser, the separation of contact pads is reduced and the challenges relating to the physical characteristics of the probe are increased. For example, as the probes become smaller, materials having higher conductivity must be used to compensate for the loss in probe size. 
     The increasing clock rate of integrated circuits is also creating design challenges for probe card assemblies. In particular, present circuits often run faster than 200 MHz, while probe card assemblies are designed to run much slower. An apparatus that addresses these problems is therefore desirable. 
     SUMMARY OF THE INVENTION 
     In various embodiments, the invention is a probe card having one or more optical fibers. Time-correlated single-photon-counting (TCSPC) techniques are used in one embodiment to detect a state change in a test structure of a circuit under test. Using optical fibers instead of traditional electrically conductive probes supports capture of device state changes that would be undetectable with traditional probes. In addition, the ability to detect a signal with an optical fiber is not dependent on establishing a good electrical contact between the fiber and a contact pad. Thus, positioning an optical fiber in order to detect a desired signal is not dependent on application of a suitable force to the fiber. 
    
    
     The above summary is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description which follow more particularly exemplify these embodiments. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following detailed description of the preferred embodiments can best be understood when read in conjunction with the following drawings, in which: 
     FIG. 1 illustrates a probe card constructed in accordance with one embodiment of the invention; 
     FIG. 2 illustrates a probe card constructed in accordance with another embodiment of the invention; 
     FIG. 3 is a block diagram of a system for time resolved photoemission detection using a probe card having optical fibers; and 
     FIG. 4 is a graph of an example histogram of data compiled while detecting photoemissions from a test structure using time-correlated single-photon-counting techniques. 
    
    
     While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     The present invention is believed to be applicable to front-side or backside analysis of a variety of circuits implemented on semiconductor wafers. While the present invention is not so limited, an appreciation of various aspects of the invention is best gained through a discussion of the example embodiments set forth below in which the front-side of a wafer is probed. 
     FIG. 1 illustrates a probe card constructed in accordance with one embodiment of the invention. Probe card  10  includes a plurality of probes  12 , along with a plurality of optical fibers  14 . Probes  14  and fibers  14  are held in fixed locations in probe card  10  by substrate  16 . 
     Probes  12  are generally aligned with contact pads  20  of wafer  22 . In one embodiment, the probes are exactly aligned with the contact pads and no additional apparatus is required during testing to properly align the probes with the contacts. In another embodiment, the probes in the substrate may be generally aligned and an alignment guide (not shown) is used to exactly align the probes with the contact pads during testing. Example probe and substrate structures can be found in U.S. Pat. Nos. 5,751,157 and 5,828,226 to Kister and Higgens et al., respectively. 
     Probes  12  are connected to electrical traces  24  in substrate  16 . Test equipment (not shown) can be coupled to external contacts (not shown) that are connected to traces  24 , as known in the art. 
     Probe card  10  also includes optical fibers  14  that are available for testing circuitry on wafer  22 . Optical fibers  14  are used to collect photoemissions from test structures (not shown) formed within wafer  22 . For example, it is known that CMOS transistors emit photons via hot carriers when driven into saturation. During a CMOS logic state change, the photon emission, primarily from n-MOS transistors, has been shown to occur transiently on picosecond time scales. This is particularly useful for analysis of ring oscillator test structures. As explained further below, integrated circuit structures can be tested using the photoemissions. 
     For each test structure to be tested within wafer  22 , a separate optical fiber is disposed in a corresponding position within probe card  10 . As with probes  12 , a guide member (not shown) may be provided for exact alignment of the optical fibers with the test structures. 
     In one embodiment, fiber guides  26  are disposed at desired locations of probe card  10 . The fiber guides provide accurate placement of the fibers in substrate  16  relative to the test structures and also provide lateral support for the fibers between substrate  16  and wafer  22 . Hollow metal tubes or glass capillaries can be used to implement the fiber guides. Those skilled in the art will recognize other suitable materials. It is preferable that fiber guides  26  have blunt edges on the ends nearest the wafer in order to minimize damage if inadvertent contact is made. As an alternative to forming the fiber guides as within substrate  16 , tubes could be wrapped around the substrate or probe pins and clamped or glued thereon. 
     It will be appreciated that the length of optical fibers  14  (extending from substrate  16  toward the wafer) will depend on the distance that separates the wafer and probe card when probes  12  are in electrical contact with contact pads  20 , as shown in FIG.  1 . It will be appreciated that the fibers need not directly contact the wafer, but can be disposed a small distance above the wafer and still collect photoemissions from the test structures. The length of fiber guides  26  extending from substrate  16  toward wafer  22  should be less than the length of the fibers in order to prevent contact between the fiber guides and the wafer. 
     As shown in FIG. 1, placement of optical fibers  14  within substrate  16  can be separate from probes  12  or interleaved therewith, depending on the relative placement of the test structures and contact pads of the wafer. 
     FIG. 2 illustrates a probe card constructed in accordance with another embodiment of the invention. Probe card  50  includes a plurality of optical fibers with no electrically conductive probes. With no electrically conductive probes and conductive traces, probe card  50  can be built to support the optical fibers without having to accommodate the electrically conductive structures. Thus, alternative structures and materials may be suitable for probe card  50  as compared to probe card  10 . In another embodiment, an optical fiber could be attached to a thin metal wire, for example glued to a probe pin. It will be appreciated that the optical fibers need not pass through the probe card and could instead be attached to the surface of the probe card. Those skilled in the art of optical fiber connectors and carrier assemblies for optical fibers will recognize additional possibilities. Such carriers and connectors may also be used in combination with the embodiment of FIG.  1 . 
     FIG. 3 is a block diagram of a system for time resolved photoemission detection using a probe card  210  having optical fibers. Each of optical fibers  214  is coupled to a respective avalanche photodiode (APD) detector  212  to detect photoemissions. The fiber optic cable  214  is a 0.002″ glass fiber. Commercially available products are suitable, such as the model SPCM-QC4 fiber optic cable with an FC connector at one end and bare pig-tailed at the probe card end and an opaque PVC sheathing from EG&amp;G Optoelectronics. The optical fibers need not provide transmission of coherent light. However such a cable could be used. The fibers  214  must be shielded with material that does not permit penetration of external light to the light transmitting fiber. In addition, a dark enclosure is desirable for the probe card and wafer under test. 
     APD detector  212  of the example embodiment provides high efficiency single-photon detection, for example the single photon counting module SPCM-AQ-151-FC from EG&amp;G CANADA having a specified timing resolution of 300 ps. It will be appreciated that faster detectors can be used to analyze today&#39;s circuits in which devices, such as inverters, change state faster than 100 ps. In addition, as technology progresses and state changes occur at faster rates, even faster detectors can be used. Examples of other detectors that can be used include a photomultiplier such as the model 8852 from Burle Industries or the R3809U micro-channel plate photomultiplier tube (MCP-PMT) from Hamamatsu. Prototype detectors have been demonstrated having a timing resolution in the range of 20 ps. In another embodiment, a quadrant APD detector such as the C30927E series of quadrant APDs from the EG&amp;G company, can be used to spatially resolve photoemissions as well as time-resolve the photoemissions. 
     Test controller  228  is coupled to wafer  202  for providing test signals as input to the circuit being tested. The test controller includes conventional hardware and software for configuring and loading test vectors circuit being tested. 
     A time-to-amplitude converter  222  is coupled to test controller  228  and to APD detector  212 . The converter  222  generates an output pulse whose amplitude is directly proportional to the time between the start and stop pulses from the detector  212  and the test controller, respectively. The APD detector  212  generates a start pulse each time a photon is detected, and test controller  228  provides a clock signal which the time-to-amplitude converter  222  uses as a stop pulse. 
     The output pulse from the converter  222  is digitized by a conventional analog-to-digital converter  224 , and the digitized pulse is provided as input data to a computer  226  via a multi-channel analyzer (MCA) card. The input data represents the pulse height of the pulse output from the converter  222 . The computer/MCA analyzes the digitized pulse height and increases the count of a histogram data point, where the data point is selected based on the pulse height. For example, the pulse height represents a time interval in which the photoemission was detected. Thus, the count for that time interval is incremented upon detection of a photoemission. 
     The time-to-amplitude converter is, for example, a model TC862 from Oxford. Other suitable converters include the Canberra model 2145. In an alternative embodiment, a time-to-digital converter could be used in place of the time-to-amplitude converter  222  and ADC  224 . An example time-to-digital converter is the model 1875 TDC that is available from Lecroy. 
     For each test structure of wafer  202  to be monitored, a separate parallel arrangement including an APD detector, a time-to-amplitude converter, and an ADC is required. Histograms of photoemissions from the respective test structures can be created based on the temporal relationships between output pulses of the respective photo-diodes and a clock signal that is output by test controller  228 . Time-correlated single-photon-counting (TCSPC) techniques are used in the example embodiment. 
     FIG. 4 is a graph of an example histogram of data compiled while detecting photoemissions from a test structure using time-correlated single-photon-counting techniques. The example histogram represents data compiled from a circuit that outputs a clock pulse every 2 ns, wherein each data point in the graph represents a count of photoemission events detected in a 12.5 ps interval. 
     The 5 ns interval between the peaks in the histogram illustrates the time interval between transitions of two inverters at an end of a ring oscillator. Given that the cycle time for an inverter is 10 ns in the example system, and photons are emitted only once during a cycle (e.g., going from a high logic level to a low logic level, but not in the transition from a low logic level to a high logic level), photons will be emitted every 5 ns by the two adjacent inverters. 
     Once photoemission data is collected, such as the example data of FIG. 4, timing specifications for the circuit can be analyzed and compared to the photoemission data to determine whether the circuit is functioning as expected. 
     Usage of conventional equipment in the example embodiments set forth above results in a system that is cost effective, yet highly functional. 
     As noted above, the present invention is applicable to analysis of a number of different semiconductor structures and arrangements. Furthermore, the invention may be implemented in various forms using equipment that is comparable to that identified herein. Accordingly, the present invention should not be considered limited to the particular examples described above, but rather should be understood to cover all aspects of the invention as fairly set out in the attached claims. Various modifications, equivalent structures, as well as numerous structures to which the present invention may be applicable will be readily apparent to those of skill in the art upon review of the present specification. The claims are intended to cover such modifications and devices.