Abstract:
A combination cooling plate and micro-spray cooling system beneficial for use in testers of electrically stimulated integrated circuit chips is disclosed. The system includes a transparent heat spreader and micro-spray heads disposed about the heat spreader. The spray heads spray cooling liquid onto a periphery of said heat spreader so as to remove heat from the chip. Alternatively, and micro-spray heads are provided inside the cooling plate holder so as to spray cooling liquid inside the interior of the holder so that the holder is cooled. The holder is in physical contact with the heat spreader, so that as the holder is cooled by the spray, heat is removed from the heat spreader, and thereby from the chip.

Description:
REFERENCE TO RELATED APPLICATION  
       [0001]    This application is a continuation in part of application Ser. No. 10/222,107, which is incorporated herein by reference in its entirety. 
     
    
     
       BACKGROUND OF THE INVENTION  
         [0002]    1. Field of the Invention  
           [0003]    The present invention relates to a system and a method for thermal management of an electrically stimulated semiconductor integrated circuit undergoing probing, diagnostics, or failure analysis.  
           [0004]    2. Description of the Related Art  
           [0005]    Integrated circuits (ICs) are being used in increasing numbers of consumer devices, apart from the well-known personal computer itself. Examples include automobiles, communication devices, and smart homes (dishwashers, furnaces, refrigerators, etc.). This widespread adoption has also resulted in ever larger numbers of ICs being manufactured each year. With increased IC production comes the possibility of increased IC failure, as well as the need for fast and accurate chip probing, debug, and failure analysis technologies. The primary purpose of today&#39;s probing, debug, and failure analysis systems is to characterize the gate-level performance of the chip under evaluation, and to identify the location and cause of any operational faults.  
           [0006]    In the past, mechanical probes were used to quantify the electrical switching activity. Due to the extremely high circuit densities, speeds, and complexities of today&#39;s chips, including the use of flip-chip technology, it is now physically impossible to probe the chips mechanically without destructively disassembling them. Thus, it is now necessary to use non-invasive probing techniques for chip diagnostics. Such techniques involve, for example, laser-based approaches to measure the electric fields in silicon, or optically-based techniques that detect weak light pulses that are emitted from switching devices, e.g., field-effect transistors (FETs), during switching. Examples of typical microscopes for such investigations are described in, for example, U.S. Pat. Nos. 4,680,635; 4,811,090; 5,475,316; 5,940,545 and Analysis of Product Hot Electron Problems by Gated Emission Microscope, Khurana et al., IEEE/IRPS (1986), which are incorporated herein by reference.  
           [0007]    During chip testing, the chip is typically exercised at relatively high speeds by a tester or other stimulating circuit. Such activity results in considerable heat generation. When the device is encapsulated and is operated in its normal environment, various mechanisms are provided to assist in heat dissipation. For example, metallic fins are often attached to the IC, and cooling fans are provided to enhance air flow over the IC. However, when the device is under test, the device is not encapsulated and, typically, its substrate is thinned down for testing purposes. Consequently, no means for heat dissipation are available and the device under test (DUT) may operate under excessive heat so as to distort the tests, and may ultimately fail prematurely. Therefore, there is a need for effective thermal management of the DUT.  
           [0008]    One prior art system used to cool the DUT is depicted in FIG. 1 a . The cooling device  100  consists of a cooling plate  110  having a window  135  to enable optical probing of the DUT. The window  135  may be a simple cut out, or may be made of thermally conductive transparent material, such as synthetic diamond. The use of synthetic diamond to enhance cooling is described in, for example, U.S. Pat. No. 5,070,040, which is incorporated herein by reference. Such a solid transparent window is often referred to as a transparent heat spreader. Conduits  120  are affixed to the cooling plate  110  for circulation of cooling liquid. Alternatively, the conduits may be formed as an integral part of the plate, see, e.g., U.S. Pat. No. 6,140,141.  
           [0009]    [0009]FIG. 1 a  depicts in broken line a microscope objective  105  used for the optical inspection, and situated in alignment with the window  135 . During testing, the cooling plate is placed on the exposed surface of the DUT  160 , with the window  135  placed over the location of interest. When the cooling plate  110  is used with a transparent heat spreader  135 , an oil layer, or other high index of refraction fluid, is sometimes provided between the transparent heat spreader  135  and the DUT  160  in order to improve the optical coupling from the DUT  160  to the transparent heat spreader  135 . Heat from the device is conducted by the cooling plate to the conduits and the cooling liquid. The cooling liquid is then made to circulate through a liquid temperature conditioning system, such as a chiller, thereby removing the heat from the device. Typically, however, the DUT includes auxiliary devices  165 , which limit the available motion of the cooling plate, thereby limiting the area available for probing. To overcome this, custom plates are made for specific devices, leading to increased cost and complexity of operation of the tester.  
           [0010]    Another problem with the conventional cooling plate is insufficient and non-uniform heat removal from the DUT. FIGS. 1 b  and  1   c  schematically show a conventional cooling plate with a transparent heat spreader of a somewhat modified design from that of FIG. 1 a . FIG. 1 b  is a top view, while FIG. 1 c  is a partial cross section along lines A-A in FIG. 1 b . A transparent heat spreader  110 ′ is soldered to a frame  130  using, for example, an indium solder at interface  1115 . A DUT (not shown) is observable through transparent heat spreader  110 ′, and oil or other fluid may be provided between the DUT and the heat spreader. The frame  130  is attached to, or is formed as an integral part of, an inner metallic heat sink  140  which, in turn, is attached to an outer metallic heat sink  150 . Conventionally, the inner metallic heat sink  140  is attached to the outer metallic heat sink  150  using screws and having no heat conducting material there-between. Chilled air is pumped through inlet  170  to circulate through the outer metallic heat sink  150 , and is exhausted through outlet  175 .  
           [0011]    As can be understood, heat is transported from the DUT to the transparent heat spreader  110 ′, to the frame  130 , to the inner metallic heat sink  140 , to the outer metallic heat sink  150 , and to the chilled fluid. However, the interfaces between the various elements act to resist heat conduction, thereby reducing the efficiency of heat removal from the DUT. Additionally, the temperature gradient across the various elements encourages heat gain from the ambient. In fact, studies have shown that heat gain from the ambient can be greater than the heat removal from the DUT. The thermal resistance present in the heat conduction path, along with the significant heat gained from the ambient, combine to dramatically increase the difficulty in lowering the temperature of the transparent heat spreader and, thereby, lowering the temperature of the DUT.  
           [0012]    Of particular interest to the present inventors is the temperature at the periphery of the transparent heat spreader (locations of  1 - 8  in FIG. 1 b ). That is, the inventors speculated that a system having efficient heat transport will lower the temperature at the periphery of the transparent heat spreader, and thereby the temperature of the heat spreader and the DUT. To investigate that, a temperature distribution of an industry standard semiconductor thermal test chip, cooled by transparent heat spreader as exemplified in FIG. 1 b , was simulated using a Finite Element model. The model simulated the temperature distribution in the transparent heat spreader, as well as the heat conduction from the transparent heat spreader, across the indium solder, to the periphery of the inner metallic heat sink. Using a one dimensional heat conduction analysis to calculate the temperature rise from the surface of the transparent heat spreader to the chip, the chip&#39;s temperature distribution, and its maximum temperature, were determined. The accuracy of the prediction of the Finite Element model is directly tied to the accuracy of the imposed boundary conditions. In this case, a key boundary condition is the temperature at the inner periphery of the inner metallic heat sink, i.e., at the indium solder contact area. As can be understood, the temperature at this periphery is dependent upon the heat removal efficiency of the entire assembly.  
           [0013]    The impact of the boundary condition on the DUT&#39;s maximum temperature can be determined from FIG. 2 (determined with the Finite Element model). FIG. 2 is a plot of the transparent window&#39;s maximum temperature as a function of the boundary condition for various heat loads, wherein this maximum temperature drives the maximum DUT temperature. As is evident from FIG. 2, the lower the temperature at the periphery of the heat sink, the lower the maximum temperature of the transparent window, which will result in a lower maximum DUT temperature. That is, the inventors speculated that a system that can efficiently lower the temperature at the boundary of the transparent window, will also efficiently remove heat from the DUT.  
           [0014]    To verify the accuracy of the model, extensive experiments were conducted with a test chip, using a cooling plate assembly similar to that of FIG. 1 b . The DUT was powered to various heat fluxes, while being cooled by the cooling plate in a conventional manner. The cooling plate was instrumented for temperature measurements at the locations marked  1 - 18  in FIG. 1 b , and the results for a chip powered at 20 W/cm 2  are shown in FIG. 3 (data are provided in degrees Centigrade). The results at the periphery of the transparent heat spreader were used as the boundary condition for further Finite Element model analysis. The model was executed and a prediction was made of the temperature distribution in the transparent window, as well as its maximum temperature. The maximum DUT temperature was then calculated as a function of the oil layer thickness and plotted in FIG. 4. For a measured oil layer thickness of 80 μm, the results of FIG. 4 correspond with the experimentally measured temperatures of the test DUT, thereby verifying the accuracy of the model.  
           [0015]    As can be readily understood from the above discussion there is a need for an innovative, inexpensive, flexible, and thermally effective thermal management solution for chip testers or probers.  
         SUMMARY OF THE INVENTION  
         [0016]    The results of the investigation detailed above highlight the importance of lowering the temperature at the periphery of the transparent window to the maximum extent possible. The present invention provides effective solutions for heat removal from the periphery of the transparent window, thereby providing a mechanism for removing heat from a DUT and allowing for inspection of the device under electrical stimulation. Therefore, the system is particularly adaptable for use with optical microscopes used for probing, diagnostics and failure analysis of the DUT.  
           [0017]    In one aspect of the invention, a thermal management system is provided which utilizes a heat spreader for removing heat from the DUT and an atomized liquid spray system for removing heat from the heat spreader.  
           [0018]    In another aspect of the invention an objective lens housing and a transparent cooling plate are placed inside a spray chamber. A spray cooling arrangement is provided to spray coolant onto the cooling plate. The spray chamber is sealed to a plate upon which the DUT is situated. The pressure inside the chamber may be controlled to obtain the proper evaporation of the sprayed coolant. Pressure transducers and temperature sensors may be installed on the pressure chamber to monitor the operation of the thermal management system.  
           [0019]    In another aspect of the invention, the spray cooling is accomplished using several banks of atomizers to cool the periphery of the transparent heat spreader. According to one implementation, all of the atomizers are commonly connected to one liquid supply. On the other hand, according to other implementations, liquid delivery to each, or to groups, of atomizers may be controlled separately so as to vary the pressure, the timing, and/or the type of liquid delivered to various atomizers.  
           [0020]    In yet another aspect of the invention, a cooling plate is soldered onto a holder. The holder is used to press the cooling plate against the DUT. Several atomizers are provided for spraying cooling fluid onto the periphery of the cooling plate. The holder may additionally serve to prevent the sprayed fluid from reaching the central part of the cooling plate, so as not to obscure the optical path.  
           [0021]    In a further aspect of the invention, a cooling plate is soldered onto a holder. The holder is used to press the cooling plate against the DUT. The holder is provided with a hollow cavity, inside which the atomizers are a fixed. The atomizers spray cooling liquid onto the upper interior part of the holder, and the sprayed liquid is then evacuated via the hollow cavity inside the holder. In this manner, no liquid reaches the cooling plate; rather, heat is removed from the cooling plate via the cooled holder.  
           [0022]    According to a further aspect of the invention, a DUT is affixed onto a pc board and the cooling plate is provided over the DUT. A metallic clamp holds the cooling plate and the DUT onto the pc board. An indium gasket may be provided between the metallic clamp and the cooling plate. A hollow holder is then pressed against the metallic clamp. Another indium gasket may be placed between the metallic gasket and the holder. The holder is provided with hollow injection chamber, inside which the atomizers are affixed, and hollow return chamber for collecting sprayed liquid. The atomizers spray cooling liquid onto the upper interior part of the holder, and the sprayed liquid is then evacuated via the hollow return chamber inside the holder. In this manner, no liquid reaches the cooling plate; rather, heat is removed from the cooling plate via the cooled holder.  
           [0023]    According to yet another aspect of the invention, a transparent heat spreader is movably attached to the objective assembly via a holder. The holder may slide freely, be spring loaded, or flexibly mounted to the objective assembly. This arrangement is provided so that once the heat spreader is placed against the DUT, the objective assembly may be moved further in order to reach appropriate focus point. Coolant is delivered to the spray heads, which deliver coolant spray onto the heat spreader or, optionally, also onto the DUT itself.  
           [0024]    According to yet another aspect of the invention, a transparent heat spreader is provided having both a cooling channels and a cooling spray.  
           [0025]    In a further aspect of the invention, control instrumentation is provided for accurate operation of the thermal management system. The DUT temperature can be controlled via coolant temperature, coolant flow rate (directly tied to coolant delivery pressure), and coolant boiling point (a function of spray chamber pressure and vapor temperature. Note that at its saturation temperature, the temperature of the saturation liquid is the same as its vapor (non-superheated)). A temperature sensor close to the coolant delivery point monitors the coolant delivery temperature, which is fed back to the thermal management system&#39;s controller. The controller controls a liquid temperature conditioning system, which may be a chiller or other device to control the coolant&#39;s temperature to a pre-determined value. Such systems are well known to those skilled in the art.  
           [0026]    Spray chamber pressure is measured with a pressure transducer in communication with the spray chamber. Vapor temperature (measured with a temperature sensor in communication with the spray chamber) and spray chamber pressure determine the coolant&#39;s boiling point, which in turn influences the manner in which the DUT temperature is controlled (via the transparent cooling plate). The spray chamber pressure can be manipulated to influence the coolant&#39;s boiling point. The spray chamber pressure may be affected, for example, by a solenoid valve in communication with the spray chamber, by adjusting the return pump&#39;s speed, or by manipulating the pressure inside the liquid temperature conditioning system&#39;s reservoir. A mechanical pressure relief valve provides a safety release in the event that the solenoid valve fails.  
           [0027]    One or more of the afore-mentioned approaches, individually or in combination, may be used to control the coolant flow rate and/or the coolant&#39;s boiling point. The ultimate goal is to use the instrumentation to control the DUT to a predetermined temperature. The temperature of the DUT may be measured by mechanical contact with a thermocouple or other sensor, by non-contact means such as a thermal imaging camera, or by any other means suitably accurate for the intended temperature stability. Any means for measuring the DUT temperature may be employed in the control of the DUT temperature. The specific examples given here are meant for illustrative purposes only and are not meant to limit this invention in any way.  
           [0028]    A computer or other electronic or mechanical control system may be used to monitor DUT temperature and provide the necessary adjustment of spray. For example, if the DUT temperature rises, the computer could increase the flow rate, decrease the fluid temperature, or both.  
           [0029]    The terms “transparent window” and “transparent heat spreader” are used herein somewhat interchangeably. As can be appreciated, the element functions as a window for the optical system, and as a heat spreader for the thermal management system. Also, the term “transparent” is used herein loosely. That is, as can be appreciated, a window may be transparent to a system operating a one wavelength, while opaque for system operating at another wavelength. For example, a “transparent” window for a system operating at the IR range, may be opaque at the visible range. Therefore, when used herein, the term “transparent” means transparent for the wavelength of interest.  
           [0030]    The invention further comprises a method for controlling the temperature of an integrated circuit (IC) undergoing diagnostics, the method comprising: attaching the IC to a socket; providing a transparent heat spreader over the IC; and injecting coolant onto the transparent heat spreader from at least one spray head. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0031]    The invention is described herein with reference to particular embodiments thereof, which are exemplified in the drawings. It should be understood, however, that the various embodiments depicted in the drawings are only exemplary and may not limit the invention as defined in the appended claims.  
         [0032]    [0032]FIGS. 1 a - 1   c  depict cooling plate systems according to the prior art.  
         [0033]    [0033]FIG. 2 is a graph of maximum temperature of the transparent heat spreader as a function of its boundary conditions, obtained from a Finite Element model.  
         [0034]    [0034]FIG. 3 depicts experimentally measured temperatures measured across a conventional transparent heat spreader assembly for 20 W/cm 2  applied to the DUT.  
         [0035]    [0035]FIG. 4 is a graph of maximum die temperature as a function of oil film thickness at 20 W/cm 2  applied to the DUT.  
         [0036]    [0036]FIG. 5 is an exploded view of one embodiment of the inventive cooling system of the present invention.  
         [0037]    [0037]FIG. 6 is a cross section schematic of an embodiment of the inventive cooling system of the present invention.  
         [0038]    [0038]FIG. 7 depicts a cross section schematic of an embodiment of the inventive cooling system.  
         [0039]    [0039]FIG. 8 is a schematic of another embodiment of the inventive cooling system.  
         [0040]    [0040]FIGS. 9 a  and  9   b  are cross section schematics of DUT with a cooling plate holder arrangement according to two alternative embodiments of the invention.  
         [0041]    [0041]FIG. 10 is a cross section schematic of a DUT with a cooling plate holder arrangement according to an embodiment of the invention.  
         [0042]    [0042]FIG. 11 is a cross section schematic of a DUT with a cooling plate holder arrangement according to an embodiment of the invention.  
         [0043]    [0043]FIG. 12 is a schematic of another embodiment of the inventive cooling system.  
         [0044]    [0044]FIG. 13 is a schematic of another embodiment of the inventive cooling system.  
         [0045]    [0045]FIG. 14 is a schematic of another embodiment of the inventive cooling system.  
         [0046]    [0046]FIG. 15 is a schematic of a further embodiment of the inventive cooling system. 
     
    
     DETAILED DESCRIPTION  
       [0047]    Various embodiments and implementations of the present invention can be used in conjunction with various IC testers and probers, so as to provide cooling of an IC that is electrically stimulated. For example, various embodiments of the invention can be easily implemented with IDS-PICA available from NPTest of San Jose, Calif., or with Gemini MS available from IMS of Beaverton, Oreg. In one general aspect, a transparent heat spreader and an atomized liquid spray is provided about a probe head so as to cool the DUT as the probe head collects data. Any probe head may be used, for example, the probe head may be in the form of an optical photon-counting time-resolved receiver, optical emission microscope, or laser-based probing tool. In order to provide a more detailed explanation of various aspects and features of the invention, the invention will be described with reference to more specific IC probers, i.e., optical photon-counting time-resolved emission probers. However, it should be appreciated that such detailed description is provided only as an example and not by way of limitation.  
         [0048]    [0048]FIG. 5 depicts an exploded view of one embodiment of the inventive cooling system. The cooling system depicted in FIG. 5 may be used with any type of microscope used for inspection and/or testing of ICs. For clarity, FIG. 5 shows only the objective lens portion optical inspection/probing system, and the parts relating to its cooling system. As shown in FIG. 5, a retention frame  570  holds the DUT  560  onto seal plate  580 . The seal plate is mounted to a load board, which in turn is connected to a conventional test head (not shown) of a conventional automated testing equipment (ATE). The ATE sends stimulating signals to the DUT  560 , to simulate operating conditions of the DUT  560 . This is done conventionally using the load board with an appropriate socket for the DUT.  
         [0049]    An objective housing  505  houses the objective lens of the testing system. The housing  505  and objective lens generally form an optical receiver of the system, i.e., the probe head. A transparent heat spreader  510  is mounted onto a holder  550 , which generally may be a metallic holder. The heat spreader is made of a material that is permeable to the wavelength of radiation that is monitored by the particular tester used. For infrared, visible and UV radiation, the heat spreader may be made of ceramic, such as, for example, aluminum oxide, silicon oxide or mixture thereof. The heat spreader may also be made of a monocrystalline material such as, for example, sapphire. In one embodiment, the transparent heat spreader  510  is soldered, using, e.g., indium solder, to the holder  550 .  
         [0050]    Atomizers  515  are arranged so as to spray cooling fluid onto the periphery of the heat spreader  510 . The atomizers are shown here only schematically with respect to their size, number and arrangement. For example, while only two banks of atomizers are shown on two opposite sides of the heat spreader  510 , four banks may be provided so as to cool the entire periphery of the heat spreader  510 . Also, in the context of this disclosure, the term “fluid” is used herein to signify both liquid and gaseous forms of cooling media.  
         [0051]    The entire assembly is situated inside spray chamber  525 , having a seal  530  affixed to its upper surface. The spray chamber  525  is affixed to a translation stage, e.g., an x-y-z stage (not shown). When performing testing in an embodiment employing the sliding seal, the spray chamber  525  is brought in contact with the sealing plate  580 , so that sliding seal  530  creates a seal with the sealing plate  580 . The seal may be hermetic, but a hermetic seal is not required. In this manner, the spray chamber  525  may be moved about so as to bring the objective lens into registration with the particular area of the DUT sought to be imaged, without breaking the seal with the sealing plate  580 . Alternatively, the objective housing  505  may be movable with respect to the heat spreader  510 , so that once the heat spreader  510  is applied against the DUT it need not be moved for testing different locations on the DUT.  
         [0052]    In another embodiment, the housing  525  is connected to the sealing plate  580  through a flexible bellows (not shown). The bellows material should be compatible with the coolant temperature and chemical properties. Some potential materials include folded thin-walled steel and rubber.  
         [0053]    During testing, fluid is supplied to the atomizers  515  via coolant supply manifold  555 . The boiling point of the coolant can be controlled by controlling the pressure inside the spray chamber  525  using solenoid  520 , or otherwise. In one implementation of the invention, the pressure inside the spray chamber  525  is measured using pressure transducer  552  and of that of the coolant supply is measured using pressure transducer  540 , while the temperature of the cooling fluid is measured with temperature sensor  541  and of the vapor in the spray chamber using temperature sensor  545 . As a safety measure, a mechanical pressure relief valve  535  is optionally provided. For fixed or varying coolant temperature and spray chamber pressure, the measured coolant delivery pressure is fed back to the controller to ensure adequate coolant delivery pressure for a required DUT temperature. The flow rate, and thus the cooling rate, can be controlled by the coolant delivery pressure.  
         [0054]    [0054]FIG. 6 is a cross-sectional schematic of the spray cooling system according to an embodiment of the present invention. Specifically, DUT  660  is held in place by the DUT retention frame  672 , which is attached to the DUT socket (not shown). The seal plate  670  is attached to the DUT load board (not shown), which is connected to a test adapter in a conventional manner. In this embodiment, spray chamber  625  is pressed against the seal plate  670  so as to form a seal using a sliding seal  630 . Objective housing  605  is positioned below a transparent heat spreader  617 , which is fitted with spray heads  615 . Pump  680  is used to return fluid to the fluid temperature conditioning system, such as a chiller  650 , and can also be used to control the pressure inside the chamber interior  635 , typically at about 1 atm. It should be understood that the desired spray chamber pressure could be calculated according to the characteristics of the cooling fluid used and the boiling point desired (in a given embodiment).  
         [0055]    Pump  665  is used to pump coolant through supply piping  695  to be injected onto the transparent heat spreader via spray heads, or atomizer banks,  615 . In one embodiment of the invention, coolant is sprayed onto the heat spreader  617  in a liquid form, whereupon it is heated to its boiling point and then evaporates and vapor forms in the interior  635 . The vapor may then condense on the chamber  625  walls, and is drained through channels  655 , back into the pump  680 . The vapor may also be directly fed into the chiller  650 , although the load on the chiller will be increased. In another embodiment, the coolant simply absorbs the heat from the heat spreader  617  without evaporating, whereupon the unevaporated liquid is returned to the fluid temperature conditioning system. While two thermal management scenarios have been presented, those skilled in the art can appreciate the fact that the relative cooling strengths of the fluid heat absorption and the evaporation may be adjusted, for example, by choosing different fluids, nozzle design and number, fluid flow rate, fluid temperature, and chamber pressure as described above.  
         [0056]    The fluid may then be circulated through the fluid temperature conditioning system  650  before being sprayed again onto the heat spreader  617 . The coolant used in this particular embodiment is of high vapor pressure, e.g., hydrofluoroethers or perfluorocarbons. Consequently, such fluids evaporate readily when exposed to atmospheric condition. Therefore, as shown in this embodiment, the entire cooling system forms a closed loop system. The closed system may be vented through the solenoid valve  685 , which may also be operated in conjunction with a vapor recovery system such as a reflux condenser to mitigate additional vapor loss. For this purpose, the fluid temperature conditioning system  650  comprises a sealed chiller reservoir  690 , capable of operating at both high and low pressures, i.e., 10 psi above atmospheric pressure or a full vacuum of −1 atm. The reservoir  690  may also include a fluid agitation system (not shown) to enhance heat transfer from the coolant to the chiller coils (not shown). In this example, the chiller  650  and reservoir  690  are capable of operating at low temperatures of down to, for example, −80° C.  
         [0057]    Using this system, the temperature of the DUT can be varied so as to be tested under various operating conditions. For example, the operator may input a certain operating temperature for testing the DUT. In one embodiment, the actual temperature of the DUT can be detected by the ATE (not shown) in a manner known to those skilled in the art. For example, a temperature diode may be embedded in the DUT, and its signal sent to the ATE. This is conventionally done for safety reasons such as, for example, to shut down the system if the DUT gets too hot. However, according to this embodiment of the invention, the temperature of the DUT is sent from the ATE to the controller  600 . Using the actual DUT temperature, the controller  600  adjusts the cooling rate so as to operate the DUT at the temperature selected by the operator. To control the cooling rate, the controller  600  may adjust, for example, the flow rate of coolant, the temperature of the coolant, or change the pressure in the chamber so as to change the boiling point of the cooling liquid.  
         [0058]    As shown in FIGS. 5 and 6, and as alluded to above, various sensors and instrumentation may be used to control the operation of the inventive cooling system. A pressure transducer  620  measures the coolant delivery pressure so as to control the pump  665  speed. Additionally, a pressure transducer  622  measures the pressure inside the spray chamber so as to control a solenoid valve  685  to obtain the appropriate coolant boiling point inside the spray chamber. Optionally, another transducer  694  is provided to monitor the pressure of the returned coolant. Temperature sensor  640  is used to measure the coolant temperature close to the point of delivery, while the vapor temperature in the spray chamber is measured with temperature sensor  645 . Notably, from the spray chamber pressure and the vapor temperature (or coolant at its saturation temperature), it is possible to determine the thermodynamic state of the coolant delivered to the stimulated DUT. A mechanical pressure relief valve  626  provides a safety release in the event that the solenoid valve  685  fails.  
         [0059]    In the embodiments of FIGS. 5 and 6, the effect of the atomized coolant on imaging needs to be minimized. One way to do this is by using the heat spreader carrier,  550 ,  618 , operating as a shield, so as to prevent the mist from entering the optical axis of the imaging system. In this manner, the sprayed fluid impinges only on the periphery of the heat spreader, so that the field of view of the objective is not obstructed by fluid. However, as noted above, effective control of the temperature at the periphery of the heat spreader results in effective control of the DUT&#39;s temperature. Therefore, the embodiments described herein are very effective in controlling the DUT&#39;s temperature.  
         [0060]    As noted above, oil, or other high index of refraction liquid, may be provided between the heat spreader and the DUT. Additionally, in various embodiments of the invention, oil, or other high index of refraction liquid, is placed between the heat spreader and the objective lens. In other embodiments of the invention, an optional solid immersion lens (SIL)  619  is provided in contact with the heat spreader so as to efficiently collect light from the DUT and direct it to the objective lens. The SIL enables transmission of optical energy between the heat spreader and the objective lens and may be used alone, or in combination with a high index of refraction liquid.  
         [0061]    Solid immersion lenses (SIL) are well known in the art and are described in, for example, U.S. Pat. Nos. 5,004,307, 5,208,648, and 5,282,088, which are incorporated herein by reference. FIG. 6 depicts in broken line an embodiment of the cooling system of the invention used in conjunction with an optional SIL. In this embodiment, a SIL  619  is affixed to the tip of the objective housing  605 . In operation, the SIL  619  is “coupled” to the heat spreader  617 , so as to allow communication of evanescent wave energy. In other words, the SIL is coupled to the heat spreader  617  so that it captures rays propagating in the heat spreader  617  at angles higher than the critical angle (the critical angle is that at which total internal reflection occurs). As is known in the art, the coupling can be achieved by, for example, physical contact with the imaged object, very close placement (up to about 20-200 micrometers) from the object, or the use of index matching material or fluid.  
         [0062]    In the embodiment of FIGS. 5 and 6, two banks of atomizers are shown. It should be appreciated, however, that the number of atomizers and the number of banks of atomizers are provided only as examples, and other numbers and arrangements may be used. For example, the atomizers may be placed in a circular arrangement about the objective housing, rather than in linear banks. Furthermore, various injectors may be operated at different spray rates or be provided with different cooling fluid, or same cooling fluid, but at different temperature. Optionally, different spray heads may be adjusted to provide spray at different angles.  
         [0063]    An exemplary arrangement of mounting the DUT to a conventional tester head adapter is depicted in FIG. 7. The DUT  760  is mounted via a socket (not shown) onto the DUT board  790  and held in place by retention frame  770 . An o-ring seal  710  may be provided between the DUT retention frame  770  and the DUT carrier  765 . In addition, an o-ring seal  720  may be provided between the seal plate  780  and the load board  790 .  
         [0064]    A further embodiment of the inventive spray cooling system is depicted in FIG. 8. In this embodiment the transparent heat spreader  864  is pressed against the DUT  860 , which, in turn, is attached to the DUT carrier  866  and socket  862 . The heat spreader  864  is held in place by the retention frame  861 , rather than by the optics carrier. Atomizer banks  815  are provided about the periphery of the heat spreader  864 . In this particular example, an optional seal  867  is provided between seal plate  870 , socket  862 , and DUT board  863 . This ensures that no vapor will escape in the space between these parts.  
         [0065]    The spray chamber  825  is held against the seal plate  870  so that seal  830  makes a seal with the seal plate  870 . The condition inside chamber interior  835  is monitored using pressure transducer  822  and temperature sensor  845 . The pressure inside chamber interior  835  is controlled using solenoid valve  885 . Additionally, a mechanical pressure relief valve  826  is provided for safety.  
         [0066]    Cooling fluid is provided to the atomizer banks  815  using supply pump  865 . The pressure of the delivered fluid is measured by pressure transducer  820 , and the temperature is measured by temperature sensor  840 . After being sprayed, the fluid is collected and is pumped back to chiller reservoir  890  using return pump  880 . The fluid level inside the chiller is monitored by level sensor  896 , which can also be used as an added variable for thermal management control, while the pressure inside the chiller is monitored by pressure transducer  891 . A mechanical pressure relief valve  892  is provided for safety. The temperature of the fluid inside the chiller is controlled using chiller coils  893  and heater  894 . As shown, all the sensors, actuators, and pumps are connected to computer/controller  800 .  
         [0067]    As is known, in order to inspect the DUT, it is customary to thin the DUT. Consequently, when devices generate heat, the heat does not spread well over the entire DUT and a localized hot spot is created. In this embodiment, a transparent heat spreader  864  is provided in contact with the DUT  860 , so as to enhance spreading of heat from localized hot spots. As in the above embodiments, oil or other index matching fluid may be used between the heat spreader  864  and the DUT  860 . The spray is then applied onto the periphery of the heat spreader  864 , which may be made from, for example, silicon, sapphire, diamond, etc.  
         [0068]    [0068]FIGS. 9 a  and  9   b  are cross-section schematics of DUT holder arrangements with heat spreader retention frame, which can be used in the embodiment of FIG. 8. The DUT  960  is held against socket  965  by the transparent heat spreader  974 , which is held in place by retention frame  970 . An o-ring  920  is provided between the DUT carrier  962  and socket  965 . The transparent cooling plate  974  may be sealed to the retention frame  970  by, for example, indium or epoxy bonding, silicon sealant, and the likes. For enhanced heat conduction, indium solder may be used. The retention frame  970  may be screwed or otherwise attached to the test adapter (not shown). Fluid inlets  915  and outlets  917  are provided in the retention frame  970 . As can be appreciated from FIGS. 9 a  and  9   b , the retention frame  970 , heat spreader  974 , and socket  965  define a void  990 . Coolant is circulated in void  990  using inlets  915  and outlets  917 , as shown in FIG. 9 a . On the other hand, in FIG. 9 b  the fluid is sprayed into void  990  using spray heads  916 , and is evacuated using outlets  917 . Notably, in either example coolant fluid is not circulated in the optical path of the objective, but rather cools only the periphery of the heat spreader  974  and DUT  960 . If collection of spent coolant is not important, such as, for example, when the coolant is air or liquid nitrogen, outlet  917  can simply be a non-hermetic setting between the frame  970  and socket  965 . In such a case, spent coolant will simply evaporate via the non-hermetic setting into the atmosphere.  
         [0069]    [0069]FIG. 10 is a cross section schematic of a transparent heat spreader arrangement held against a DUT. This illustrative example may be used with the embodiment of FIGS. 5 and 6. That is, in this embodiment, the DUT is attached to the DUT socket, while the transparent heat spreader is pressed against the DUT. As shown, DUT  1060  is attached to the DUT carrier  1065 , which may or may not include periphery devices  1020  (e.g., capacitors). Prior to testing, a transparent heat spreader  1074  is pressed against the DUT  1060 , with or without an index matching fluid in between. The heat spreader  1074  is attached to its holder  1070  by, for example, indium soldering  1012 . Atomizers  1015  are attached to the holder  1070  so that they spray on the periphery of the heat spreader  1074 . As shown, the holder  1070  may act as a “shield” so as to prevent the spray from reaching the central area of the heat spreader  1074  and obstruct the optical path.  
         [0070]    [0070]FIG. 11 is a cross section schematic of a transparent heat spreader arrangement held against a DUT. This illustrative example may be used with the embodiment of FIGS. 5 and 6. However, this embodiment may also be used in situation where no evaporation chamber, such as that shown in FIGS. 5 and 6, is provided. Notably, in this embodiment sprayed fluid is contained within the heat spreader holder and is circulated to the chiller for temperature control. As shown, DUT  1160  is attached to the DUT carrier  1165 , which may or may not include periphery devices  1120  (e.g., capacitors). Prior to testing, a transparent heat spreader  1174  is pressed against the DUT  1160 , with or without an index matching fluid in between. The heat spreader  1174  is attached to a holder  1170  by, for example, indium soldering  1112 . The holder  1170  is made of a solid part  1180 , which may be made of solid metal, and a hollow part  1175 . Atomizers  1115  are situated inside the hollow part of the holder  1170 , as shown. The atomizers spray the solid part  1180  of the holder  1170 . The fluid then flows down the hollow part  1175  to be collected and/or circulated into the chiller. In this arrangement, heat from the DUT is conducted to the heat spreader  1174 , then to the solid part  1180 , and is removed therefrom by the spray from the atomizers  1115 . Since in this case the fluid path is “closed-loop,” no sealing plate or the like need to be provided and the arrangement may easily be used in various testing situations.  
         [0071]    Another embodiment of the invention is depicted in FIG. 12 in an exploded view. In this embodiment, the DUT and heat spreader can be fixed onto a PC board and be configured onto the test bench of an ATE or other testing equipment. The cooling to the heat spreader is provided separately and removably from the DUT-PC board arrangement. This embodiment is particularly advantageous since the PC Board, DUT and heat spreader form a separate and independent arrangement from the rest of the testing and cooling system, and may be conveniently assembled on a work bench and then moved and configured onto the testing system.  
         [0072]    The PC board assembly comprises a conventional PC Board  1200 , upon which a socket  1265  and a DUT package  1285  are affixed. The DUT  1260  is secured onto the socket  1265  in a conventional manner. A transparent heat spreader  1274  is then provided over the DUT  1260 , with or without an index matching fluid. The heat spreader  1274  is pressed against the DUT  1260  using a copper clamp  1230  and an indium gasket  1222 . The entire assembly is then secured by steel clamp  1240  via bolts  1250 , which is aligned using guide pins  1255 .  
         [0073]    The spray cooling assembly comprises a holder  1270  having two hollow chambers therein: injection chamber  1245  and exhaust chamber  1275 . The cooling spray assembly is pressed against the copper DUT clamp  1230 , with an indium gasket  1212  provided in-between. The indium gasket may be a separate and replaceable part, or it may be soldered to the holder  1270 . Cooling liquid is pumped into injection chamber  1245 , and is being sprayed onto the top part of the holder  1270  by injectors  1215 . The sprayed liquid then flows down the exhaust chamber and is pumped into the chiller for temperature control.  
         [0074]    In the embodiments of FIGS. 6 and 8, it is shown that the cooling fluid is collected and is returned to the chiller system. However, this is not always necessary. For example, the cooling fluid may be chilled air. In such a case, the air may be chilled and is delivered under pressure to the spray heads, but it need not be collected after spraying. So, for example, pumps  680  and  880  may be eliminated. On the other hand, liquid nitrogen is generally readily available in institutions working on semiconductor chips. Therefore, liquid nitrogen can be used in embodiments of the invention.  
         [0075]    [0075]FIG. 13 depicts an embodiment wherein liquid nitrogen is used as the coolant fluid. That is, most research and production facilities handling semiconductors have liquid nitrogen readily available “on tap.” That is, facilities  1380  are provided to supply pressurized liquid nitrogen through an outlet valve connection  1382 . Consequently, when using such facility for the supply of coolant, there is no need for the pumps and heat exchangers depicted in the embodiments of FIGS. 6 and 8. Rather, the liquid nitrogen can be delivered directly to the spray heads  1315 .  
         [0076]    Once the liquid nitrogen is sprayed upon the heat spreader  1317 , it may or may not be collected. That is, one may use a cooling chamber  1325  with a sealing ring  1330  and plate  1370  in order to construct an enclosed environment that enables collection of the “spent” nitrogen via outlet  1355 . On the other hand, the spent nitrogen may be discarded to the atmosphere. In such a case, there is no need to construct a cooling chamber and the spray heads  1315  and cooling plate  1317  may be open to the environment.  
         [0077]    As in the embodiments of FIGS. 5 and 6, various sensors and instrumentation may be used to control the operation of the inventive cooling system. A pressure transducer  1320  measures the coolant delivery pressure so as to control the valve  1382 . Additionally, a pressure transducer  1322  measures the pressure inside the spray chamber so as to control a solenoid valve  1385 . Temperature sensor  1340  is used to measure the coolant temperature close to the point of delivery, while the vapor temperature in the spray chamber is measured with temperature sensor  1345 . A mechanical pressure relief valve  1326  provides a safety release in the event that the solenoid valve  1385  fails. As can be readily understood, some or all of these various sensors and transducers may be changed or omitted if the system is designed so that spent nitrogen is released to the atmosphere.  
         [0078]    [0078]FIG. 14 depicts another embodiment of the inventive system, wherein the transparent heat spreader  1474  is movably attached to the objective assembly  1405  via holder  1470 . In this embodiment, holder  1470  may slide freely in housing  1475 , or it may be spring loaded or flexibly mounted inside holders  1475 . This arrangement is provided so that once the heat spreader  1474  is placed against the DUT  1460  (mounted onto socket  1485 ), the objective assembly  1405  may be moved further in order to reach appropriate focus point. When the holder  1470  is spring loaded, the transparent heat spreader  1474  can be physically pressed against the DUT  1460  so as to prevent or reduce any mechanical movement and vibrations between the transparent heat spreader  1474  and the DUT  1460 . Coolant is delivered to the spray heads  1415  via pipes or hoses  1480 . The spray heads  1415  deliver coolant spray onto the heat spreader  1474 . Optionally, the spray heads  1415  may also deliver coolant spray onto the DUT  1460  itself. Optionally, the holder  1470  may be formed as a cylinder, movably attached to a cylindrical housing  1475 . Such an arrangement is advdentageous as the cylindrical holder  1470  would prevent any coolant from reaching the optical path of the objective. As also exemplified in FIG. 14, the arrangement of this embodiment may be used in conjunction with another transparent heat spreader  1410 , such as, for example, the conventional heat spreader  110  shown in FIG. 1 a . In such a case, the heat spreader  1474  would be pressed against the heat spreader  1460 . Further optionally, cooling channels may be provided in or on the heat spreader  1410 , such as, for example, the conventional cooling channels  120  shown in FIG. 1 a.    
         [0079]    [0079]FIG. 15 is a schematic of a further embodiment of the inventive cooling system, wherein a conventional cooling plate  1540  is used. The conventional cooling plate includes a transparent heat spreader  1510 ′, cooling fluid input/output hookups  1570 , and cooling fluid channel  1530 . The heat spreader  1510 ′ may be connected to the cooling plate via an indium solder  1525 . However, according to this embodiment, in order to enhance heat removal from the periphery of the heat spreader, spray heads  1515  are positioned to provide cooling spray onto the cooling plate.  
         [0080]    While the invention has been described with reference to particular embodiments thereof, it is not limited to those embodiments. Specifically, various variations and modifications may be implemented by those of ordinary skill in the art without departing from the invention&#39;s spirit and scope, as defined by the appended claims. Additionally, all of the above-cited prior art references are incorporated herein by reference.