Patent Publication Number: US-2005121186-A1

Title: Apparatus and method for reducing electrical noise in a thermally controlled chuck

Description:
RELATED APPLICATION  
      This application is based on U.S. Provisional Patent Application Ser. No. 60/526,030, filed on Nov. 26, 2003, the contents of which are incorporated herein in their entirety by reference. 
    
    
     BACKGROUND OF THE INVENTION  
      In the semiconductor integrated circuit industry, the cost of individual integrated circuit chip die is continuing to decrease in comparison to IC package costs. Consequently, it is becoming more important to perform many IC process steps while the die are still in the wafer, rather than after the relatively expensive packaging steps have been performed.  
      Typically, in IC processing, semiconductor wafers are subjected to a series of test and evaluation steps. For each step, the wafer is held in a stationary position at a process station where the process is performed. For example, circuit probe testing is increasingly performed over a wide temperature range to temperature screen the ICs before assembly into a package. The wafer is typically held stationary relative to a vacuum support surface of a prober machine which electrically tests the circuits on the wafer. The prober includes a group of electrical probes which, in conjunction with a tester, apply predetermined electrical excitations to various predetermined portions of the circuits on the wafer and sense the circuits&#39; responses to the excitations.  
      In a typical prober system, the wafer is mounted on the top surface of a wafer chuck, which is held at its bottom surface to a support structure of the prober. A vacuum system is typically connected to the chuck. A series of channels or void regions in communication with the top surface of the chuck conduct the vacuum to the wafer to hold it in place on the top surface of the chuck. The prober support structure for the chuck is then used to locate the wafer under the probes as required to perform the electrical testing on the wafer circuits.  
      The chuck can also include a temperature control system which raises and lowers the temperature of the chuck surface and the wafer as required to perform the desired temperature screening of the wafer. It is important to the accuracy of such testing that the temperature of the wafer and, therefore, the temperature of the chuck surface, be controlled as accurately and precisely as possible.  
      Various approaches to controlling the wafer temperature have been employed. For example, the chuck can include a circulation system through which a temperature control fluid is circulated. The temperature of the fluid is controlled such that the temperature of the chuck and, therefore, the temperature of the wafer, are controlled. Such temperature-controlled chucks can also include heaters such as electrical resistive heaters within the chuck. By applying power to the resistive heating element, heat is introduced into the chuck.  
      In some applications, such as where a wafer is being tested on a circuit prober, the presence of electrical noise in the system can degrade the performance accuracy of the testing. Therefore, it is important to minimize the electrical noise introduced into the system. One source of electrical noise is the temperature control fluid that circulates through the chuck. The motion and other behavior of the fluid in the chuck or in the lines carrying the fluid to and from the chuck can create electrical noise by triboelectric effects whereby electrical charge is produced by friction between two objects.  
     SUMMARY OF THE INVENTION  
      It is a feature of the invention to provide a temperature control system for controlling temperature in a workpiece or wafer chuck in which the electrical noise effects introduced by the fluid are reduced.  
      In one aspect, the invention is directed to a temperature control system and a method for controlling temperature in a workpiece chuck. The system includes a temperature control device for controlling temperature of a temperature control fluid and a fluid inlet and a fluid outlet, the temperature control fluid being carried to and from the chuck via the fluid inlet and the fluid outlet. A controller controls the temperature control device, such as by controlling a flow rate of the temperature control fluid through the chuck, such that the flow rate of the temperature control fluid through the chuck when the temperature of the chuck is in transition is higher than the flow rate of the temperature control fluid through the chuck when the temperature of the chuck is being maintained at a set point temperature and/or by controlling the temperature control device such that, when a temperature of the chuck is above a predetermined temperature, e.g., the boiling temperature of the temperature control fluid, the temperature control fluid is prevented from entering the chuck.  
      In one embodiment, a flow controlling device is controllable to change the flow rate of the temperature control fluid.  
      In one embodiment, the system includes a first fluid carrying path connected to one of the inlet and the outlet and a second fluid carrying path connected to the other of the inlet and the outlet. The flow controlling device can be connected in one of the first and second fluid carrying paths. The flow controlling device can include a fixed-orifice flow reducing device in one of the first and second fluid carrying paths and a controllable valve in parallel with the fixed-orifice flow reducing device. The controller can control the controllable valve to be open when temperature in the chuck is in transition and closed when the temperature in the chuck is to be maintained at a desired set point.  
      Alternatively, the flow controlling device can include a first controllable valve connected in one of the first and second fluid carrying paths and a second controllable valve connected in the one of the first and second fluid carrying paths in parallel with the first controllable valve. The controller can control the first and second controllable valves such that one of the first and second controllable valves is open when temperature in the chuck is in transition and closed when the temperature in the chuck is to be maintained at a desired set point.  
      In one embodiment, a capillary tube can be connected between the first and second fluid carrying paths to balance pressures in the first and second fluid carrying paths.  
      In one embodiment, a first valve is connected in the first fluid carrying path, and a second valve is connected in the second fluid carrying path. The first and second valves are closed when the temperature of the chuck is above a predetermined temperature, e.g., the boiling temperature of the temperature control fluid, such that the temperature control fluid is prevented from flowing into the chuck.  
      The various techniques of the invention result in controlling flow and other behavior of the temperature control fluid used to control temperature in a chuck. The result is that electrical noise introduced by the fluid is reduced, and, accordingly, wafer processing such as prober testing over temperature, can be carried out more accurately. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.  
       FIG. 1  contains a top-level schematic block diagram of a temperature control system and workpiece chuck connected to control temperature of a workpiece such as a semiconductor wafer mounted on the workpiece chuck.  
       FIG. 2  is a schematic diagram of the temperature control system used to control temperature in a workpiece chuck, in accordance with an embodiment of the invention.  
       FIG. 3  is a schematic diagram of the temperature control system used to control temperature in a workpiece chuck, in accordance with another embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION  
       FIG. 1  contains a top-level schematic block diagram of a temperature control system and workpiece chuck connected to control temperature of a workpiece such as a semiconductor wafer mounted on the workpiece chuck. The system  9  includes a temperature control system such as a chiller  11 , which controls the temperature of a temperature control fluid. The chiller  11  can be of the type manufactured and sold by Temptronic Corporation of Sharon, Mass., the assignee of the present application. Specifically, the chiller can be a Model Number TP03500A Atlas Chiller, or similar system, which is modified to include features of the invention used to reduce electrical noise as described herein.  
      The temperature control fluid is routed to and from a test system, such as, for example, a wafer prober  13 , along a hose or pipe  15 . The hose or pipe  15  can be of the type described in U.S. Pat. No. 6,070,413, assigned to Temptronic Corporation of Sharon, Mass., and incorporated herein in its entirety by reference. The hose or pipe  15  actually includes at least two fluid carrying hoses or pipes, one of which carries the temperature control fluid from the chiller  11  to the prober  13  and the other of which carries the temperature control fluid from the prober  13  back to the chiller  11 . The workpiece or wafer  21  being tested by the prober  13  is mounted on a chuck  19 . The chuck  19  and/or the approach by which temperature of the chuck  19  and wafer  21  is controlled can include any features disclosed in any of U.S. Pat. Nos. 4,734,872; 6,073,681; 6,540,014; 6,415,858; 6,505,478; 6,802,368; 6,091,060; 6,019,164; 6,328,096; 6,375,176; 6,700,099; 6,552,561; 6,744,270; 6,745,575, all of which are assigned to Temptronic Corporation of Sharon, Mass. and all of which are incorporated herein in their entirety by reference.  
       FIG. 2  is a schematic diagram of the temperature control system used to control temperature in a workpiece chuck, in accordance with an embodiment of the invention. Referring to  FIG. 2 , the temperature control system or chiller  11  is connected to the prober  13  via the hose or pipe  15 , which carries the temperature control fluid back and forth to the chuck  19  in the prober  13 . The chiller  11  includes a heat exchanger  23  used in controlling the temperature of the temperature control fluid. A pump  25  circulates the fluid through the system. The fluid is carried from the heat exchanger  23  to a fluid outlet of the chiller  11  on a first fluid carrying path  27 . The fluid circulates through the chuck  19  and returns to the chiller  11  via the hose or pipe  15 . The fluid is pumped along a second fluid carrying path  29  from a fluid inlet of the chiller  11  back to the heat exchanger  23 . The chiller  11  includes a controller  7 , which controls the functions of the chiller  11  to implement generation and delivery of the temperature control fluid, including, for example, the opening and closing of valves, and the timing of the opening and closing of the valves.  
      In accordance with the invention, a first valve  14  is connected in the first fluid carrying path  27 , and a second valve  16  is connected in the second fluid carrying path  29 . A fixed-orifice flow reducer  12  is connected in the first fluid carrying path  27 , and a third valve  10  is connected in the first fluid carrying path  27  in parallel with the fixed-orifice flow reducer  12 . A capillary tube  18  is connected between the first and second fluid carrying paths. As shown in  FIG. 2 , the capillary tube  18  is located between the chuck  19  and the valves  10 ,  12 ,  14 ,  16 .  
      One or more approaches in accordance with the invention are utilized in the work chuck thermal control system of the invention to reduce the triboelectric electrical noise in the chuck. The use of these approaches is controlled by the chiller control system and is synchronized to the appropriate thermal control range where their use is most beneficial. These approaches all reduce electrical noise independently in different modes of system operation.  
      The first approach significantly reduces the flow of coolant through the chuck  19  from the flow rate used to temperature transition the chuck  19 . It includes the valve  10  and the flow reducer  12 . This reduction in coolant flow results in reduced movement (vibration) of the chuck and other chuck components, which in turn results in reduced electrical noise level on the chuck. In one embodiment, the reduction in flow is achieved by switching the coolant flow from a high flow line to one with a flow restriction. In one embodiment, the restriction is an orifice that has significantly lower flow area than the high flow line.  
      In the thermal chuck of the invention, rapid temperature transitions during testing are highly desirable. One way to achieve rapid temperature transition is to provide high rate of flow of the temperature control fluid. However, high flow rate induces high electrical noise due to vibrations in the chuck and the triboelectric effect. In accordance with the invention, the flow reducer  12  and valves  10 ,  14  and  16  are used to achieve a high fluid flow rate during temperature transitions and a relatively low flow rate while maintaining the temperature of the chuck and wafer at a desired set point temperature. During temperature transitions, valves  10 ,  14  and  16  are opened, allowing fluid to flow at its maximum rate through the valves  10 ,  14  and  16 , as well as the flow reducer  12 . When the temperature of the chuck  19  reaches the desired set point, the valve  10  is closed, and the valves  14  and  16  remain open. As a result, the total fluid flow is reduced to that allowed by the fixed-orifice flow reducer  12 . In one embodiment, the flow rate at constant temperature is about half of the flow rate during transitions. The lower flow rate while temperature is held constant results in much lower electrical noise, which provides increased accuracy in prober testing of the wafer at the desired set point temperature.  
      The second approach also involves the use of valves  14  and  16 . This approach prevents coolant fluid from entering the chuck after the chuck temperature is above the boiling point of the cooling fluid. If the coolant fluid is allowed to enter a work chuck that is at a temperature above the boiling point of the coolant fluid, the coolant vaporizes violently and the resulting pressure pulsation induces vibrations in the chuck causing electrical noise. The coolant fluid can enter the work chuck by periodically flowing from a coolant fluid storage reservoir through the work chuck supply and/or return line. The flow is typically induced by the pressure created by the coolant fluid boiling itself or by fluid buoyancy differences throughout the system. The capillary tube  18  placed between the chuck inlet and outlet also balances the pressure between these lines preventing the coolant fluid in one line from being closer to the chuck than the fluid in the other line. This reduces the possibility that fluid will reenter the high-temperature chuck and flash boil to create noise-inducing motion.  
       FIG. 3  is a schematic diagram of the temperature control system used to control temperature in a workpiece chuck, in accordance with another embodiment of the invention. In this embodiment, the valve  10  and flow reducer  12  of  FIG. 2  are replaced with a pair of valves  45  and  47  connected in parallel in the first fluid carrying path  27 . In this embodiment, during temperature transitions, both valves  45  and  47  are opened to allow for maximum rate of flow of the temperature control fluid. When the chuck reaches a set point temperature, one of the valves, e.g., valve  45 , is closed, and the other valve, e.g., valve  47 , remains open. As a result, the temperature control fluid is allowed to flow through only the open valve  47  at a reduced flow rate, resulting in a reduction in electrical noise caused by fluid at a comparatively high flow rate. It should be noted that in the embodiment of  FIG. 3 , the valve  14  is optional. The valve  14  can be omitted, because valves  45  and  47  can be used to meet the purpose of valve  14 .  
      In accordance with the invention, when a temperature-controlled workpiece or wafer chuck is transitioned in temperature, high transition rate is achieved by using a high flow rate of temperature control fluid through the chuck. When the chuck temperature reaches a desired set point temperature, the chuck temperature is maintained at the set point. Under this steady-state condition, the flow rate of the temperature control fluid is reduced, such that electrical noise that may be introduced by the moving fluid is reduced. In embodiments of the invention described herein, this dual-flow-rate approach is achieved by using a flow control device connected in one or more of the lines carrying the temperature control fluid to and from the chuck. This flow control device can be a pair of valves or a valve and a fixed-orifice flow restrictor connected in parallel. At least one on the valves can be controlled such that it is open during temperature transitions to achieve high-rate transition and closed during steady-state temperature maintenance such that electrical noise is reduced. In another embodiment of the invention, the dual-flow-rate approach is achieved by using a variable-speed pump as the pump  25 . The pump can be set to a relatively high speed during temperature transitions such that the fluid flow rate is relatively high.  
      At steady-state temperature set point maintenance, the pump can operate at a reduced speed such that the fluid flow rate is reduced, resulting in reduced electrical noise in the chuck during testing.  
      It should be noted that throughout the foregoing description and the following claims, the application refers to the temperature control fluid. The temperature control fluid can be in a gaseous state or a liquid state. As referred to herein, the fluid is assumed to be in a liquid state, however, it is known that the fluid can also be in a gaseous state in the system, such as where the liquid state of the fluid boils in the chuck to enter its gaseous state. Unless otherwise specified herein, when the temperature control fluid is referred to, such as where the fluid is prevented from entering the chuck in certain embodiments of the invention, it is the liquid state of the fluid that is being referred to. For example, where the fluid, i.e., liquid state of the fluid, is prevented from entering the chuck, it is understood that the gaseous state of the fluid may be present in the chuck.  
      While this invention has been particularly shown and described with references to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined in the following claims.