Abstract:
Method and apparatus for optically testing (e.g., using a laser beam) an operating integrated circuit (device under test—DUT) that actively control the operating temperature of the DUT. This is chiefly useful with flip-chip packaged ICs. The temperature of the DUT varies with its operating power consumption, and this fluctuation in temperature adversely affects the results obtained during optical probing or other optical testing. Furthermore, the DUT may be damaged if its temperature exceeds design limits. The temperature of the DUT is controlled by thermally contacting the exposed backside surface of the DUT die to a diamond film heat conductor, an associated heat sink structure, and at least one thermoelectric device. The thermoelectric device is controlled by a temperature sensor proximal to the DUT. By controlling the amount and direction of the electrical current supplied to the thermoelectric device in response to the sensed temperature, the temperature of the DUT is maintained.

Description:
CROSS REFERENCE TO RELATED APPLICATION  
       [0001]     This application claims priority to and is a divisional application of U.S. application Ser. No. 10/264,716, filed Oct. 3, 2002 and entitled “OPTICAL TESTING OF INTEGRATED CIRCUITS WITH TEMPERATURE CONTROL,” which is hereby incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0002]     The present invention relates to testing, and more specifically, to optical probing of integrated circuits.  
       BACKGROUND  
       [0003]     Many advancements have been made in the use of optical probing techniques used for testing integrated circuits; see, e.g., Paniccia, U.S. Pat. No. 5,895,972, Apr. 20, 1999 (“Paniccia”), incorporated by reference herein in its entirety. As described in Paniccia, flip-chips (integrated circuits which are mounted face down in the package) can be optically tested using laser probing. This is because the backside surface of the die of the integrated circuit (“IC”) device under test (“DUT”), generally having a silicon substrate, is accessible for the purposes of laser or optical probing and/or for detecting photon emissions emitted from an active DUT. (This of course requires that the package be opened up.) Thus optical probing involves detecting optically electrical activity in the DUT while it is operating.  
         [0004]     Although such optical probing is not limited to flip-chips, the backside of the die of the DUT must be visible to the optical probe so that waveform measurements can be made. During the testing or debugging of a DUT, it is generally desirable to operate the DUT at its full operating capacity and speed. Since the electric power consumption in, e.g., a microprocessor, is typically high, it is known in the art to exhaust heat created by the operation of the DUT to maintain an acceptable operating temperature; otherwise the DUT may be damaged.  
         [0005]      FIG. 1A  illustrates in a side view the approach of Paniccia (taken from his  FIG. 5A ) to dissipating heat from a flip-chip mounted integrated circuit die DUT  102 . DUT  102  is conventionally mounted, via a plurality of ball bonds designated collectively as  107 , to its package  110  having pins designated collectively as  115  used for coupling signals to/from and power to DUT  102 . Pins  115  are electronically connected to contacts on a conventional test head  101  that also mechanically supports DUT  102  and the associated heat dissipating structures. Test head  101  also supplies the signals to and receives signals from the pins  115  and supplies power thereto. An infrared transmissive heat conductor film  120  is disposed over the backside surface  105  of DUT  102 . A heat sink  150  (i.e., a structure capable of absorbing dissipating heat, being typically of large thermal mass and made of, e.g., copper) is in thermal contact with the outer edges of the heat conductor&#39;s top surface. Heat conductor film  120  is of a material that is transmissive of infrared wavelength light, e.g., diamond. (The remainder of the test apparatus is not shown here.)  FIG. 1 B  illustrates a plan view of the  FIG. 1A  apparatus.  
       SUMMARY OF THE INVENTION  
       [0006]     Although U.S. Pat. No. 5,895,972 describes the removal of heat from an active integrated circuit, we have recognized the additional advantage of actively controlling temperature of the DUT during optical testing. In other words, if during optical probing or other optical testing of the active DUT the electric power consumption and hence heat output of the DUT fluctuates, we have recognized there will be a corresponding fluctuation in the temperature of the DUT. This fluctuation in temperature will adversely affect the test results obtained by interfering with the propagation of the probing beam and/or the detected light. Passive observation of photons emitted from transistors that are switching is also known.  
         [0007]     This fluctuation in temperature will also adversely affect the test results due to the fluctuating operating conditions of the IC die. The goal is to collect the data from the die while maintaining the die at a specific temperature, with only small temperature variations.  
         [0008]     Therefore, recognizing that the DUT temperature should be kept constant, we determined there is needed a method and an apparatus for controlling the temperature of an integrated circuit DUT during its active operation while simultaneously allowing the DUT to be probed, or otherwise tested in accordance with an optical-based testing technique.  
         [0009]     Therefore, we describe here an apparatus and method for optical based testing through or at the backside surface of an active IC semiconductor substrate (die) that permits active temperature control of the DUT.  
         [0010]     In one embodiment, the present apparatus is used with a an integrated circuit DUT having an optically transmissive heat conductor in thermal contact with, e.g., the backside surface of the DUT. The optically transmissive heat conductor is in thermal contact with an additional heat conductor structure, which in turn is in thermal contact with a thermoelectric device. The thermoelectric device, capable of heating or cooling, advantageously controls the temperature of the active IC DUT during optical probing and/or testing. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention is illustrated by way of example and is not limited by the figures of the accompanying drawings, in which like references indicate similar elements, and in which:  
         [0012]      FIG. 1A  illustrates a prior art heat removal apparatus.  
         [0013]      FIG. 1B  illustrates a plan view of the  FIG. 1A  apparatus.  
         [0014]      FIG. 2  illustrates a temperature control apparatus in accordance with the present invention.  
         [0015]      FIG. 3  illustrates a plan view of the apparatus of  FIG. 2 .  
         [0016]      FIG. 4  illustrates an optical probing system using the  FIG. 2  apparatus. 
     
    
     DETAILED DESCRIPTION  
       [0017]     An apparatus and method with temperature control for optically probing and/or testing an active (operating) semiconductor device (DUT) is described here. In the following description, numerous specific details are set forth such as material types, etc., in order to provide a thorough understanding. However, it will be apparent to one of skill in the art that the invention may be practiced without these specific details. In other instances, well known elements and techniques have not been shown in particular detail in order to avoid unnecessarily obscuring the present invention. This discussion is mainly of controlling the temperature of flip-chip semiconductor devices (e.g., ICs) during optical (laser or other) probing and/or testing. It will be recognized, however, that such is for descriptive purposes only and that the present apparatus and methods are applicable to ICs having other types of packaging.  
         [0018]     As explained above, optical-based testing and debugging techniques, such as laser based probing and photon emission detection, are particularly useful for evaluating IC parameters and defects through the backside surface of the IC substrate (die). Since the IC substrate is typically of crystalline silicon which has a band gap energy of 1.1 eV, the energy of a photon used to probe the substrate should have an energy that is less than or equal to 1.1 eV (since the substrate will merely be partially transmissive to such photons). Thus for optical probing the photons propagate to and from the active (transistor) regions located on the principal (front-side) region of the substrate.  
         [0019]      FIG. 2  illustrates a side view (not to scale) of the present temperature control apparatus  100  in one embodiment. In  FIG. 2 , a flip-chip mounted integrated circuit die DUT  102  is mounted to its conventional flip-chip IC package  110  and conventionally electrically powered from and receives and transmits signals through its pins  115 . The portion of the IC package uppermost in the figure has earlier been cut away to expose the die&#39;s backside. Any backside heat sink inside the package is also removed. An, e.g., infrared transmissive heat conductor film  120  is provided in thermal contact with the backside surface of die DUT  102 . A heat spreader structure  130  is in thermal contact with the top surface outer edges of the film  120 . Thermoelectric devices (“TED”)  140   a,    140   d  are in thermal contact with the top surface of the heat sink structure  130 . An additional heat sink structure  150   a,    150   d  is in thermal contact with respectively the top surface of each TED  140   a,    140   d  in one embodiment. It is to be understood that the heat spreader and heat sink may be integrated or combined.  
         [0020]     Heat conductor film  120  is of a material that is both a conductor of heat and transmissive to, e.g., infrared light. In one embodiment, heat conductor film  120  is made of synthetically grown, optically clear diamond since diamond is both an excellent conductor of heat and is transmissive to infrared light. Since diamond has a thermal conductivity that is approximately twelve times larger than that of silicon, it provides an excellent means for conducting heat away from die DUT  102 . The diamond film  120  is placed in intimate contact with the IC, and clamped thereto by the heat spreader  130 . The diamond film is a part purchased from Norton Diamond Films or Harris Diamond, and is, e.g., 300 microns thick; its other dimensions depend on those of the underlying DUT die; exemplary dimensions are 22 mm×0.22 mm. Sufficient pressure is exerted on the die by film  120  to obtain good thermal contact.  
         [0021]     In one embodiment heat spreader structure  130  is a machined, oxygen free copper plate concentric around a central window to allow passage of the probe beam. A suitable copper is UNS C14200. An alternative material is, e.g., aluminum. In one embodiment, additional heat sink structures  150   a,    150   d  are provided. Each is an additional machined, oxygen free copper plate having a large thermal mass and a large heat transfer area and defining internal channels  155   a,    155   b,  for carrying coolant (e.g., air or water). Such use of internal coolant channels in a heat sink structure is conventional. Cooling fins may also be provided for heat sink structures  150   a,    150   d  either with or without internal channels  155   a,    155   b.  Heat spreader structure  130  is shaped to allow maximum surface contact with heat conductor film  120 , and is a standard part purchased from Melcor. It defines internal cooling channels to carry coolant and is connected to TEDs with screws.  
         [0022]     TEDs  140   a,    140   d  are in thermal contact, being clamped with one screw respectively to heat sinks  150   a,    150   d  and heat sink structure  130 . Each TED  140   a,    140   d  is of the type that, e.g., operates on the Peltier effect, capable of heating or cooling depending on the voltage bias applied to it. In one embodiment, each TED  140  is a standard part available also from Melcor, of Trenton, N.J.  
         [0023]      FIG. 3  illustrates a plan view of the temperature control apparatus shown in  FIG. 2 . In the embodiment shown, four TEDs, TEDs  140   a,    140   b,    140   c,    140   d,  are in thermal contact with heat sink structure  130  and with respectively associated heat sinks  150   a,  . . . ,  150   d,  however, this configuration is not limiting. Thus in this embodiment each TED  140  has its associated separate heat sink.  
         [0024]     As previously stated, it is advantageous to control the temperature of the active (operating) integrated circuit because we have found that fluctuations in temperature adversely affect the results obtained during the optical testing. While being tested, if DUT  102  is operated at full electric power, TEDs  140  cool DUT  102  to maintain DUT  102  at a predetermined temperature in a range of, e.g., 0 C-150 C. If electric power to DUT  102  is reduced, the power to the TEDs  140  is adjusted so that DUT  102  is cooled less, again, so that DUT  102  is advantageously maintained at the predetermined temperature. If the power to DUT  102  is reduced to the point where the predetermined temperature is not maintained by the internal exhaust heating of the operating DUT  102 , the electrical current bias to TED  140  is reversed to provide heating to the operating DUT  102  by TEDs  140 . Each TED, e.g.,  140   c  is powered via leads  185   a,    185   b  by a suitably controlled power supply  180 . In one embodiment, the TED power supply  180  is controlled by a standard TED controller  170  from Melcor coupled to a standard temperature sensor  160  from Omega Engineering, Inc. of Stanford, Conn. located on heat sink structure  130  as shown in  FIG. 3 , connected thereto by epoxy. In an alternate embodiment (not shown), temperature sensor  160  is located on DUT  102  by use of a thermal sensor located on the die so that the signal must be transmitted out of the die through a pin (terminal). Also shown are the coolant connections, e.g.,  187 , to the heat sinks.  
         [0025]     In another embodiment (also shown in  FIG. 3 ), TED power supply  180  is instead controlled using a feedback loop  175  coupling TED power supply controller  170  and the conventional DUT power supply  195  to DUT  102  (the electrical coupling of power supply  195  to DUT  102  is not shown). In this embodiment, rather than controlling TEDs  140  in response to changes in sensed temperature, TEDs  140  are controlled based upon changes in the electric power (e.g., current) drawn by DUT  102 . In such an embodiment, if power consumption is high, then TEDs  140  will operate to cool the operating DUT  102 . Alternatively, if DUT  102  power draw is low, the electrical current bias of TEDs  140  is reversed to provide heating to DUT  102 . The TEDs are coupled in parallel to their power supply  180 .  
         [0026]      FIG. 4  illustrates in somewhat simplified form one embodiment of the conventional optical probing system (optical probing microscope) used in conjunction with the active DUT temperature control apparatus  100  as described here. DUT  102  is coupled to test apparatus  190 . Test apparatus  190  conventionally operates (exercises) the DUT  102  by applying thereto test signal patterns. Test apparatus  190  also supplies power to the DUT  102  via conventional DUT power supply  195 , which, for simplicity, is not shown in  FIG. 4 . It is understood that DUT  102  has a plurality of signal and power pins  115  connected via its package  110  to test apparatus  190 . Although not illustrated, it is further understood that in the present system test apparatus  190 , and the elements of temperature control apparatus  100  as previously described, are configured and assembled to allow the DUT to fit within the overall constraints of a typical commercially available optical probing microscope such as the Schlumberger IDS™ 2500, without interfering with the primary objective of obtaining optical signals from the DUT.  
         [0027]     As described here, and illustrated in  FIG. 4 , the DUT is in one embodiment conventionally probed optically with laser beam  210  generated by laser source  200 . A suitable laser is a YAG 1064 mode-locked laser. For detail on the laser and the microscope, See U.S. patent application Ser. No. 09/500,757 filed Feb. 8, 2000, K. Wilsher, W. K. Lo, incorporated herein by reference in its entirety. Laser beam  210  is focused onto a location (e.g., one transistor) on the DUT through objective lens  230 . After the DUT is probed by laser beam  210 , the resultant photons reflected from a particular transistor of the DUT, pass back up through the lens  230  and are detected by the photon detector  240  and subsequently processed conventionally. Conventional reference arm  230  is for calibration purposes.  
         [0028]     Thus, what has been described is an apparatus and method for optically probing or otherwise testing an active IC while controlling the temperature of the IC device under test. In the foregoing detailed description, the methods and apparatus of the present invention have been described with reference to specific exemplary embodiments thereof. It will, however, be evident that various modifications and changes may be made thereto without departing from the broader spirit and scope of the present invention. The present specification and figures are accordingly to be regarded as illustrative rather than restrictive.  
         [0029]     The invention is not limited to the embodiments described. Those of skill in the art will recognize that these and other modifications can be made within the spirit and scope of the invention as defined in the claims.