Abstract:
A semiconductor-wafer chuck for heating and cooling a device-under-test includes a heat-spreader plate with a clamping surface for a semiconductor wafer. A heater is disposed within the heat-spreader plate and provides for temperature elevations. A chiller heat-exchanger independent of the heat-spreader plate provides for heat removal. A motion control system is used to move the chiller heat-exchanger in relation to the heat-spreader plate, and thus provide for an adjustment of the thermal resistance and thermal coupling between the two. The heater typically comprises electric heating elements with a variable power input, and the chiller heat-exchanger is moved sufficiently far away to prevent boiling and evaporation of a coolant disposed inside when the heater is switched on. A device-under-test temperature controller has outputs connected to the heater, the chiller and the position control system, and an input for sensing the temperature of a device-under-test clamped to the heat spreader plate. It can then optimally and flexibly control the device-under-test temperature by controlling the heater power, chiller fluid temperature and/or by moving the chiller heat-exchanger in relation to the heat spreader plate.

Description:
1. FIELD OF THE INVENTION  
         [0001]    The present invention relates to methods and devices for cycling the temperature of a device-under-test, and more particularly to chuck systems for semiconductor wafers that provide for rapidly obtained set-point temperatures over a wide control range.  
         2. DESCRIPTION OF THE PRIOR ART  
         [0002]    Thermal testing systems used in the semiconductor industry have advanced to the point that wide temperature variations for device testing can be induced in semiconductor wafers. For example, Temptronic Corporation (Sharon, Mass.) markets a thermal test system called THERMOCHUCK®. This thermal inducing vacuum platform allows for wafer probing, testing, and failure analysis at precise, controlled temperatures. Wafers as big as 300-mm in diameter can be accommodated and temperature controlled with a range of −65° C. to +400° C.  
           [0003]    A modern wafer probing system is described by Warren Harwood, et al., in U.S. Pat. No. 6,313,649 B2, issued Nov. 6, 2001, and titled WAFER PROBE STATION HAVING ENVIRONMENT CONTROL ENCLOSURE. A positioning mechanism is included to facilitate microscopic probing.  
           [0004]    Operating temperatures over +200° C. and certainly those as high as +400° C. resulted in a prior art requirement to valve cooling air and liquid coolant between high temperature and low temperature evaporators. One such arrangement is described by George Eager, et al., in U.S. Pat. No. 4,784,213, issued Nov. 15, 1988, and titled MIXING VALVE AIR SOURCE.  
           [0005]    Typical device-under-test chucks used for probing semiconductor wafers have a flat plate with holes in it so the semiconductor wafer can be drawn tightly down with a vacuum. For example, see U.S. Pat. No. 6,073,681, issued to Paul A. Getchel, et al., on Jun. 13, 2000, for a WORKPIECE CHUCK. The flat plate usually has an electric heater and a chiller heat-exchanger for heating and cooling the device-under-test. A fluorocarbon liquid is pumped from an external chiller through the chiller heat-exchanger to bring the temperature down below −65° C. The electric heating elements can raise the device-under-test temperature as high as +400° C. Thermocouples are used to measure the chuck temperature and provide feedback to a closed-loop control system with a temperature setpoint manipulated by a user.  
           [0006]    William Wheeler describes a hot/cold chuck in U.S. Pat. No. 4,609,037, issued Sep. 2, 1986. An electric heater is used in a top plate and a coolant circulating plate below it is brought in contact during the cooling phase. A power and control system for such a device-under-test chuck is described in U.S. Pat. No. 6,091,060, issued Jul. 18, 2000, to Getchel, et al.  
           [0007]    Unfortunately, the fluorocarbon liquid pumped from the external chiller through the chiller heat-exchanger is subject to boiling and evaporation loss when the electric heaters are used. Such fluorocarbon liquids are very expensive, and even a teaspoonful loss every temperature cycle can add up to thousands of dollars of expense over a relatively short time.  
         SUMMARY OF THE INVENTION  
         [0008]    It is therefore an object of the present invention to provide a method for rapidly heating and cooling a device-under-test.  
           [0009]    It is another object of the present invention to provide a vacuum chuck system that is simple and inexpensive to manufacture and operate.  
           [0010]    Briefly, a semiconductor-wafer chuck embodiment of the present invention provides for heating and cooling of a device-under-test. It includes a heat-spreader plate with a clamping surface for a semiconductor wafer. A heater is disposed within the heat-spreader plate and provides for temperature elevations. A chiller heat-exchanger independent of the heat-spreader plate provides for heat removal. A position control system is used to move the chiller heat-exchanger in relation to the heat-spreader plate, and thus provide for an adjustment of the thermal resistance and thermal coupling between the two. The heater typically comprises electric heating elements with a controlled power input including full on and off, and the chiller heat-exchanger is moved sufficiently far enough away to prevent boiling and evaporation of a coolant disposed inside when the heater is switched on. A device-under-test-temperature controller has outputs connected to the heater and the position control system, and an input for sensing the temperature of a device-under-test clamped to the heat-spreader plate. It then can control the device-under-test temperature by controlling the heater power, and/or by moving the chiller heat-exchanger in relation to the heat-spreader plate.  
           [0011]    An advantage of the present invention is that a method is provided for rapid heating and cooling of devices-under-test.  
           [0012]    Another advantage of the present invention is that a hot/cold vacuum chuck system is provided that does not boil off and evaporate coolant, and therefore is inexpensive to operate.  
           [0013]    A further advantage of the present invention is that a hot/cold chuck system is provided that avoids the use of complex valving systems for coolant circulation and control, and therefore is less expensive to manufacture.  
           [0014]    Another advantage of the present invention is that a hot/cold chuck system is provided that does not depend on valves to route coolant and cool-down air.  
           [0015]    A still further advantage of the present invention is that a hot/cold chuck system is provided that does not need to expel vapor, fumes or gases too hot for plastic pipes and pieces to be used.  
           [0016]    These and other objects and advantages of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiments which are illustrated in the various drawing figures.  
       
    
    
     IN THE DRAWINGS  
       [0017]    [0017]FIG. 1 is a schematic diagram of a device-under-test heating-and-cooling embodiment of the present invention;  
         [0018]    [0018]FIG. 2 is a block diagram of a wafer-probing system embodiment of the present invention and includes a hot/cold chuck based on the elements of FIG. 1;  
         [0019]    [0019]FIGS. 3A and 3B are cross-sectional diagrams of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2, FIG. 3A shows the cooling heat-exchanger close to the top of its travel, and FIG. 3B shows it close to its bottom travel limit;  
         [0020]    [0020]FIG. 4 is a perspective view diagram of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2 mounted on an X-Y-Z positioning platform to facilitate semiconductor wafer probing;  
         [0021]    [0021]FIG. 5 is a cross-sectional close-up diagram of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2, and showing some details of the quartz ring supports;  
         [0022]    [0022]FIG. 6 is a chart showing a cool-down test of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2; and  
         [0023]    [0023]FIG. 7 is a chart showing a heat-up test of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2.  
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0024]    [0024]FIG. 1 illustrates a device-under-test chuck heating-and-cooling method embodiment of the present invention, and is referred to herein by the general reference numeral  100 . Embodiments other than this one are more preferred in many applications. However, this embodiment provides a good vehicle here to discuss the principle critical components and methods used in all embodiments.  
         [0025]    The method  100  provides a heat-spreader plate  102  with a clamping surface  104  for a device-under-test  106 . The device-under-test  106  is typically a semiconductor wafer device-under-test that is heated and cooled to various setpoint temperatures for probing and failure analysis. The method  100  includes allowing the rapid heating of the heat-spreader plate  102  by increasing a variable thermal resistance, represented by schematic symbol  108 , to a chiller heat-exchanger  110 . The heat-spreader plate  102 , and therefore the device-under-test  106 , are cooled by decreasing the thermal resistance  108  and thus increasing the thermal coupling to the chiller heat-exchanger  110 .  
         [0026]    The thermal resistance  108  is not a physical part, it represents the effect of moving the chiller heat-exchanger  110  relative to the heat-spreader plate  102 .  
         [0027]    The heat-spreader plate  102  unavoidably has a thermal mass that can slow down temperature ramping. However, in order to spread heat well, it must be constructed of metal and metal will have a significant thermal mass. What is important is the ratio of the thermal masses of the heat spreader and the chiller heat-exchanger. When the chiller heat-exchanger has a large thermal mass relative to the spreader, the temperature increase it experiences when brought into to contact with a hotter spreader plate is reduced, easing fluid overheating problems.  
         [0028]    One way to increase the thermal resistance  108  is accomplished by increasing a separation distance between the heat-spreader plate  102  and the chiller heat-exchanger  110 . This would lengthen the path heat would have to travel over the thermally inefficient air gap. Alternatively, the step of heating comprises increasing the thermal resistance by introducing a lesser thermally conductive intervening medium between the heat-spreader plate and the chiller heat-exchanger, e.g., a vacuum.  
         [0029]    The step of cooling comprises decreasing the thermal resistance by decreasing a separation distance between the heat-spreader plate  102  and the chiller heat-exchanger  110 . Alternatively, the step of cooling comprises decreasing the thermal resistance by introducing a more thermally conductive intervening medium between the heat-spreader plate and the chiller heat-exchanger, e.g., a dense gas or liquid.  
         [0030]    A positioning motor  112  with a leadscrew or jackscrew  114  can be used to position the chiller heat-exchanger  110  closer to or farther from the heat-spreader plate  102 . At the minimum thermal resistance  108 , the heat-spreader plate  102  may be in full face contact with the chiller heat-exchanger  110 . A useful maximum separation was discovered to be only a scant 0.30 inches. A positioning controller  116  can be used to control the effective thermal resistance  108 . A setpoint temperature (S)  118  is compared to a device-under-test temperature (T)  119  and the difference causes control signals to be developed for an electric heater  120  via heater controller  116  and an external chiller  124 . An electric power source  126  supplies operating current to the heater  120 . The heater is operated after the heat-spreader plate  102  and chiller heat-exchanger  110  are separated, and then the external chiller  124  is idled. A typical idle temperature for the chiller heat-exchanger is 0° C., and this helps to heat shield any control electronics disposed below and inside an environmental chamber  128 . A dry atmosphere  130  is disposed and maintained inside the environmental chamber  128  to prevent and control frosting.  
         [0031]    In general, the thermal resistance  108  to the chiller heat-exchanger is preferably sufficient to prevent boiling off a coolant fluid circulating within the chiller heat-exchanger  110  when the heater  120  is operating. The chiller heat-exchanger  110  and external chiller  124  typically circulate a fluid comprising a fluorocarbon, e.g., as marketed by 3M Company.  
         [0032]    The temperature control system  116  is a supervisory controller, most likely implemented as a program running on a small single board computer. It may receive instructions from a main probing system-computing controller or directly from a built-in control panel. It issues setpoint values to two temperature controllers, typically via RS-232 interfaces. The controllers control the chiller fluid temperature and the heat-spreader temperature. The temperature controllers may be built into the chiller and heater power supply, as hinted in FIG. 2. The supervisory controller also controls the heat-exchanger positioning. This may be via a motor servo loop, open control system, or perhaps by a less complex control strategy. The movement primarily controls position and does not necessarily directly control temperature. Heater power is generally turned off during cooling.  
         [0033]    [0033]FIG. 2 illustrates a wafer-probing system embodiment of the present invention, and such is referred to herein by the general reference numeral  200 . The wafer-probing system  200  includes a hot/cold chuck  202  mounted on a motion stage  204  inside an enclosure  206 . An air drier  208  supplies dry air that will not form frost on the components inside enclosure  206 . A probe  210  provides for semiconductor wafer testing on the chuck  202 . A chuck heater power supply and temperature controller  212  operate on heating cycles, e.g., to +400° C. A recirculating fluid chiller and temperature controller  214  chill a movable cooling heat-exchanger  215  during cooling cycles, e.g., to as low as −80° C. An electronic test instrument  216  may be electrically connected to the probe  210  and the chuck  219  to measure the electrical parameters of the DUT (Device Under Test)  221 .  
         [0034]    In particular, the thermal system supervisory controller  217  can operate a cooling-heat-exchanger-positioning motor  220  to increase or decrease the effective thermal coupling between the cooling heat-exchanger  215  and the fixed top portion of chuck  202 . The probing system computing controller  218  provides direction to and receives data from the instrumentation  216 . The thermal systems supervisory controller  217  could be integral to the computing controller  218 , but is equally likely to be built into a separate box with a human interface, or a separate box which receives control instructions from the computing controller  218 .  
         [0035]    [0035]FIGS. 3A and 3B represent a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2, and are referred to herein by the general reference numeral  300 . One or more additional layers  304  may cover the heat spreader plate  302  to enhance electrical measurement capabilities by reducing noise and leakage currents. Typically these layers are alternately thin insulator and conducting sheets, which may variously be fabricated as solid plates, metallic foils, and/or deposited films.  
         [0036]    The hot/cold vacuum chuck  300  primarily heats or cools the semiconductor wafer  306  to various target temperatures so probing tests and failure analysis can conducted. FIG. 3A shows how during cooling of the semiconductor wafer  306  a cooling heat-exchanger  308  is lifted by a set of jackscrews  310  and  312  to be in close proximity or contact with the heat spreader  302 . An electric heater element  314  is turned off during cooling. A set of motors, or a motor and belt, can be used to run the jackscrews  310  and  312  up and down as needed. Alternatively, a manually driven thumbscrew can be manipulated for the same purpose.  
         [0037]    [0037]FIG. 3B shows how during heating of the semiconductor wafer  306  the cooling heat-exchanger  308  is dropped down away from the heat spreader  302  by the jackscrews  310  and  312 . The electric heater element  314  is turned on during heating. The separation distance between the heat spreader  302  and the cooling heat-exchanger  308  removes a major part of the heat load from the cooling system.  
         [0038]    [0038]FIG. 4 is a perspective view diagram of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2, and is referred to herein by the general reference numeral  400 . A semiconductor wafer  402  being tested is placed on the vacuum-clamping surface  404  of a heat spreader  406 . The electrical heater is built into the heat spreader  406  that has a fixed position. A moveable cooling plate  408  can be moved up and down by the motion control motor. A base plate  414  supports the above components and is pierced by coolant supply and return piping  416 . A positioning stage  418  is mounted on a base foundation  420  and can adjust the X-Y-Z and angular (Ø) position of the semiconductor wafer  402  during probing.  
         [0039]    [0039]FIG. 5 represents a hot/cold vacuum chuck  500  in a preferred embodiment of the present invention. A sandwich of plates  502  and  504  are clamped to the top of a heat-spreader plate  506 . In this embodiment the lower plate  504  may be an insulator, and the upper plate  502  may be a conductor.  
         [0040]    A cooling heat-exchanger  508  with coolant chambers  510  is raised and lowered on a jackscrew  512  driven by a positioning motor, e.g., via lift mechanism  514 . Such motion will adjust the effective thermal coupling and thermal resistance between the heat-spreader plate  506  and the cooling heat-exchanger  508 . A clamping ring  516 , a spring  518 , and a fastener  520  clamp the edge of an annular, quartz support ring  522  to mount the heat spreader and top plate assembly to the base  524 . The support ring  522  has the shape of a straight, parallel section of a hollow right cylinder. For example, it could be cut from a length of large-diameter glass tubing.  
         [0041]    The operating range of the hot/cold vacuum chuck  500  can span −80° C. to +400° C., and so the expansion and contraction of these pieces can be substantial. The quartz support ring  522  tolerates such extreme heating and cooling very well, and provides a solid support from a base plate  524 . A protective shield  526  surrounds the quartz support ring  522  all around its circular perimeter.  
         [0042]    The annular, quartz support ring  522  is a critical component in many embodiments of the present invention. It places a support member with a crucial low-coefficient of thermal expansion at a place that principally defines the plane of the top surface of the work area.  
         [0043]    [0043]FIG. 6 is a chart  600  showing a cool-down test of a hot/cold vacuum chuck embodiment of the present invention like that shown in FIG. 2. Three thermocouples were attached to various points on the chuck: a first on a heat spreader (Ts), a second to the top surface of the chuck near the edge (Te), and the third to the top surface of the chuck near the center (Tc). A fourth thermocouple was attached to a chiller heat-exchanger. These respectively produced temperature curves  601 - 604 . At time zero, e.g., 0.00 minutes, the device-under-test was stabilized at over 200° C. and the cooling heat-exchanger was idling at 0° C. In the first minute, the heater was turned off, the chiller reactivated, and the cooling plate moved in to thermally couple with the heat-spreader and device-under-test. This caused a small bump in curve  604 , but not so high as to evaporate the coolant or cause it to decompose into potentially non-benign constituents. The curves  601 - 603  drop precipitously, and demonstrate good performance. The surface of the spreader plate was stabilized at less than −60° C. in less than forty minutes. Faster speeds are possible.  
         [0044]    [0044]FIG. 7 is a chart  700  showing an actual heat-up test of the hot/cold vacuum chuck mentioned in connection with FIG. 6. which starts from an extremely cold temperature. The thermocouples attached to various points respectively produced temperature curves  701 - 704 . The heater was inadvertently shut off in the 8-9 minute period. The graph is nevertheless informative.  
         [0045]    At time zero, e.g., 0.00 minutes, the device-under-test was stabilized at under −60° C. and the cooling heat-exchanger was running at maximum. In the first minute, the heater was turned on and the chiller set to 0° C., but the cooling plate remained in contact with the heat spreader. At 7 minutes the cooling heat-exchanger was positioned far away from the heat spreader. This allowed the temperatures to rapidly separate, e.g., as seen in the diversion of curves  701 - 703  from curve  704 . The curves  701 - 703  plateau above +200° C. in under fifteen minutes.  
         [0046]    A preferred system embodiment of the present invention uses two temperature controllers, and one chiller heat-exchanger positioner. One temperature controller controls the electric heater plate, and the other controls the chiller fluid temperature, for example, controllers  212  and  214 , A third controller controls the positioning motor  220  (FIG. 2). These three controllers and positioners are, in turn, connected to a master controller, e.g., the thermal systems supervisory controller  217  (FIG. 2). Alternately, such supervisory controller could be realized in software within the probing system computing controller  218 .  
         [0047]    Lesser-preferred embodiments of the present invention allow the heating and cooling systems to battle one another. For instance, where the heater is left on and the chiller heat-exchanger position is moved in and out to hold a desired device-under-test temperature. Typically this method would be inefficient, but may have other advantages such as faster response time or enhanced temperature accuracy.  
         [0048]    Therefore, a preferred operating-method embodiment of the present invention begins by heating a device-under-test chuck from near room temperature. To do this without causing a battle with the cooling system, the chiller&#39;s heat-exchanger is lowered away to open up a large insulating gap. The chiller-fluid temperature controller is reset to a moderate temperature setpoint, e.g., 0-25° C. The electric-heat controller is used to proportionally control heater-power to maintain the desired hot temperature setpoint.  
         [0049]    The device-under-test is cycled cold by idling electric-heat controller, i.e., to essentially turn off the heater filaments. The fluid temperature of the chiller system is brought near to the desired cold temperature by issuing a setpoint-value to the chiller-fluid controller. Then the chiller&#39;s heat-exchanger is moved close enough to the heater plate to instigate rapid cooling, but not close enough to overheat the chiller fluid or induce plate warping. In less extreme temperature ramping, such chiller fluid boiling and plate warping will not be an issue. So when it is “safe”, the chiller heat-exchanger can be raised to actually contact the heater plate. The chiller-fluid controller then operates to further reduce the device-under-test chuck temperature to the cold setpoint-value.  
         [0050]    The device-under-test chuck temperature is brought up from cold temperatures by first sending the chiller chiller-fluid controller a setpoint-value near room temperature, e.g., 0° to 25° C. The desired hot setpoint-value is sent to the electric-heat controller, and heating commences. The chiller heat-exchanger contact with the heater plate is preferably maintained until the chiller fluid temperature comes up to the desired fluid idle temperature. The chiller heat-exchanger is then moved away to its maximum separation position. Such frees the electric-heat controller to more rapidly drive chuck temperature up to the hot setpoint-value.  
         [0051]    In many of the lift and pulley mechanisms illustrated, the center through-hole of a wheel is threaded to mate with a jackscrew that passes through it and is fixedly attached to the chiller heat exchanger. Each wheel is captured between the base plate (e.g.,  324 ) and a support bracket (e.g.  318 ). When the wheel is turned, the jackscrew and the attached chiller heat-exchanger move up and down. Three sets of jackscrews and wheels are normally used to define and retain chiller heat-exchanger and spreader surfaces in parallel planes. The threaded jackscrew drive wheels are simultaneously driven by a common belt or chain and motor, e.g., as can be partially seen in FIG. 4.  
         [0052]    Although the present invention has been described in terms of the presently preferred embodiments, it is to be understood that the disclosure is not to be interpreted as limiting. Various alterations and modifications will no doubt become apparent to those skilled in the art after having read the above disclosure. Accordingly, it is intended that the appended claims be interpreted as covering all alterations and modifications as fall within the “true” spirit and scope of the invention.