Abstract:
An apparatus for polishing wafers includes a polishing table with a heating device. A conduit connects a tank holding a liquid polishing agent to a distributor for feeding the liquid polishing agent to the polishing table. A heat exchanger is disposed along the conduit between the tank and the distributor for heating the liquid polishing agent. The heat exchanger is independent of said heating device. A method for heating a polishing agent is also provided.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of copending International Application PCT/DE98/02492, filed Aug. 24, 1998, which designated the United States. 
    
    
     BACKGROUND OF THE INVENTION 
     FIELD OF THE INVENTION 
     The invention relates to an apparatus and to a method for heating a liquid or viscous medium, in particular a polishing agent for a chemical mechanical polishing. Furthermore, the invention relates to an apparatus for polishing, in particular for a chemical mechanical polishing of wafers. 
     Depending on the strength of the chemical component of the polishing process, the temperature during chemical mechanical polishing (CMP) has a decisive influence on the process result. In this case, the process temperature, that is to say the temperature on the side of the wafer to be polished during the polishing operation, is influenced essentially by three thermal components: 1) the frictional heat occurring during the polishing process; 2) the heating of the polishing table; and 3) the temperature of the polishing agent (slurry). 
     The frictional heat produced during the polishing process can be influenced only in a limited manner, since the polishing pressure and the rotational speeds of the table and the carrier are generally subject to other process preconditions. Heating for the polishing table is usually provided in the apparatuses for polishing. 
     A specific and defined setting of the temperature of the polishing agent has so far not been taken into consideration. If the temperature of the polishing agent is controlled at all, this is done in such a way that the conduit for the polishing agent is guided through the heating device which is responsible for the temperature control of the polishing table. Such heating has, however, the disadvantage that it is not possible for the polishing agent and the polishing table to be heated independently. 
     In a further known apparatus, a heating coil is wound around the conduit for the polishing agent. However, this has the disadvantage that by comparison with the desired temperature of the polishing agent the heating coil operates at very high temperatures in order to be able to react quickly to temperature fluctuations. However, this can lead locally to very high temperatures in the polishing agent and thus to a degradation of the polishing agent. 
     SUMMARY OF THE INVENTION 
     It is accordingly an object of the invention to provide a method and an apparatus for heating a liquid or viscous medium which overcome the above-mentioned disadvantages of the heretofore-known methods and apparatuses of this general type and which make it possible to set the temperature of the polishing agent in a defined manner independently of other process parameters. 
     It is furthermore an object of the invention to provide an apparatus for polishing wafers which allows to polish the wafers simply and satisfactorily while avoiding the abovementioned disadvantages. 
     The object of the invention is achieved by an apparatus for heating a liquid or viscous medium, in particular a polishing agent for chemical mechanical polishing, having a conduit for the medium to be heated and a heating device for the medium, the heating device being constructed as a heat exchanger which is disposed at the conduit for the medium to be heated. 
     With the heating apparatus according to the invention, it is possible to provide the polishing agent always at a precisely defined and constant temperature for the polishing process, virtually independently of the fact that the polishing agent is not continuously removed because of the polishing cycle which has a loading phase, a polishing phase and an unloading phase. 
     In accordance with a preferred embodiment, a heating agent, preferably a glycol-water mixture or deionized water, can flow through the heat exchanger. The selection of the heating agent is however not limited to these two variants. The temperature of the heating agent is advantageously set with a heating controller to a value between 30 and 90° C., preferably between 45 and 70° C., and most preferably between 55 and 60° C. The heating controller is advantageously a thermostat, and has a power of approximately 3 kW, for example. The transportation of the heating agent from a tank provided therefor into the heat exchanger can be supported, for example, by a pump which can have a performance of approximately 24 l/min. If the heating controller for the heating agent is disposed in the heating agent tank, then this configuration has the advantage that the heating agent temperature in the heat exchanger corresponds virtually to the heating agent temperature in the heating agent tank, wherein it is possible to provide the heating agent tank separated and away from the heat exchanger. However, the heating controller can also be provided directly in the heat exchanger. 
     In accordance with another feature of the invention, the conduit for the medium (polishing agent) to be heated can have a conduit region which is connected to a tank for the medium to be heated, and a conduit region which is connected to a distributor. The latter conduit region is advantageously short by comparison with the first-named conduit region. That is to say, the heat exchanger of the heating apparatus according to the invention is provided in the immediate vicinity of the distributor. The short length of the conduit ensures that the medium cannot cool down much after exiting from the heat exchanger and until entering the distributor. The defined adjustability of the temperature of the medium—for example the polishing agent—is therefore further increased. 
     In accordance with a further feature of the invention, the heat exchanger has a conduit section which is flushed on its outside by a heating agent. The conduit section is advantageously connected to the two conduit regions of the conduit for the medium (polishing agent) to be heated. The conduit section can, for example, be a spiral hose, but other configurations are also possible. 
     In accordance with another feature of the invention, the heat exchanger can have a cover with inlet openings on the incoming side. For example, two inlet openings for the heating agent conduit and the corresponding conduit region of the conduit for the medium (polishing agent) to be heated are provided. Furthermore, the heat exchanger can have a cover with outlet openings on the outgoing side. Again, two outlet openings can be provided for the conduits mentioned with regard to the incoming openings. Finally, the heat exchanger can have a medium pipe and at least one limiting rod. The limiting rod serves the purpose of holding the conduit section inside the heat exchanger in a precisely defined position, so that the conduit section is flushed on its outside simultaneously from all sides with heating agent, and the medium (polishing agent) to be heated, which is provided in it, is heated to a uniform, defined temperature. 
     The cover can advantageously have an outside diameter of approximately 126 mm and a maximum thickness in the region of the openings of 15 mm. An exemplary embodiment of the heat exchanger has a length of 510 to 540 mm. 
     According to another feature of the invention, the flow rate of the medium to be heated can be in the range of 100 to 1000 ml/min in the individual conduit regions and/or in the conduit section of the heat exchanger. Preferred flow rates are, for example, 150 ml/min, 200 ml/min and 250 ml/min. 
     The individual elements of the heat exchanger can advantageously be formed from a plastic, preferably from a polyurethane-based plastic or a PVA (polyvinyl alcohol) based plastic. The invention is not, however, limited to the use of these materials. Rather, any material can be used which has a suitable thermal conductivity, chemical stability with respect to the medium to be heated, and thermal stability. In particular, it is possible to use those materials which are, furthermore, compatible with the purity requirements and contamination requirements of the semiconductor industry. 
     According to the invention, the inside diameter of the conduit regions and/or of the conduit section can be 5 to 8 mm, preferably approximately 6.4 mm. 
     In accordance with another feature of the invention, the conduit region, leading towards the heat exchanger and/or away from it, of the conduit for the medium to be heated can additionally be thermally insulated. In particular, the additional insulation of the conduit region leading from the heat exchanger to the distributor further enhances the precise adjustability of the temperature of the medium. 
     As a result of the heating device according to the invention as described above, the temperature of the medium to be heated—for example, a polishing agent—can be set independently of other process parameters and in a precisely defined fashion. The reason for this, inter alia, is that the heat exchanger is disposed in the immediate vicinity of the distributor. A cooling of the medium after exiting from the heat exchanger is therefore prevented. An additional minimizing of the heat loss can be achieved by the additional thermal insulation of the conduits. Furthermore, the quantity of the polishing agent and the period over which the polishing agent has a raised temperature can be minimized with the positioning of the heat exchanger according to the invention. A degradation of the polishing agent is thereby avoided. 
     The required dimensioning of the heat exchanger can be determined by the formulae given below. The required heating power is calculated using the formula 
     
       
         Q=c p ρV(T out −T in )+power loss  (1) 
       
     
     The temperature downstream of the conduit section 1 in the heat exchanger is yielded from                T        (   l   )       =       T   Bad     -       (       T   Bad     -     T   in       )                 -     1   B                     l                     where               (   2   )               B   =       1     2                 π                c   p        ρ     λ        ln                     r   a       r   i            V   .               (   3   )                                
     In this case, T Bad  indicates the temperature of the heating agent, T in  indicates the input temperature of the medium to be heated at the heat exchanger, T out  indicates the output temperature of the heated medium upon exiting from the heat exchanger, c p  signifies the thermal capacitance of the medium to be heated, ρ signifies the density of the medium to be heated, λ signifies the thermal conductivity of the conduit material of the conduit section, r a  signifies the outside radius of the conduit section, r i  signifies the inside radius of the conduit section, and V signifies the flow rate of the medium to be heated. 
     The required length of the conduit section located in the heat exchanger at a desired output temperature of the medium T out  at the output of the heat exchanger is yielded after appropriate transformation of equation (2) in terms of the length 1 as              l   =     B                 ln                       T   Bad     -     T   in           T   Bad     -     T   out                   (   4   )                                
     With the objects of the invention in view there is also provided, an apparatus for polishing wafers, including: 
     a polishing table for placing a wafer thereon, the polishing table having a heating device; 
     a tank for holding a liquid polishing agent; 
     a distributor; 
     a conduit connecting the tank to the distributor for feeding the liquid polishing agent to the polishing table; and 
     a heat exchanger disposed along the conduit between the tank and the distributor for heating the liquid polishing agent, the heat exchanger being independent of the heating device. 
     In other words, in accordance with the invention, an apparatus is provided for polishing wafers, in particular for a chemical mechanical polishing of wafers, having a polishing table, wherein a polishing agent is applied to the polishing table via a distributor. The polishing agent has a temperature adjusted in a defined fashion at the distributor. A heating apparatus according to the invention as described above is used for the purpose of setting the polishing agent temperature. 
     It is possible for temperature fluctuations in the polishing agent to be minimized by such a device even in the case of non-continuous tapping or extraction. It is thereby possible to carry out polishing processes in an optimum fashion. Since important aspects of the polishing apparatus according to the invention result from the heating apparatus according to the invention, reference is made to the advantages, results, effects and functions described in context with the heating apparatus. 
     In accordance with an advantageous feature of the invention, the heating apparatus, and in particular the heat exchanger, can be provided in the vicinity of the distributor. As a result, in addition to the advantages mentioned with regard to the heating apparatus, it is possible, in particular, to reduce the heat losses inside the conduit region leading from the heat exchanger to the distributor. 
     In accordance with another feature of the invention, the temperature of the polishing agent at the distributor is in the region of between 20 and 80° C. 
     According to the invention, the polishing table can be heated via a heating device independent of the heating apparatus of the invention. As a result, the process temperature can be set even more precisely during polishing of the wafer. 
     With the objects of the invention in view there is also provided, in accordance with the invention, a method for heating a polishing agent for a chemical mechanical polishing. The method includes the steps of: 
     heating a heating agent using a heating controller; 
     introducing the heated heating agent into a heat exchanger; 
     guiding a polishing medium to be heated through the heated heating agent in the heat exchanger; and 
     outputting the heated polishing medium from the heat exchanger to a distributor. 
     In other words, a method is provided for heating a liquid or viscous medium, in particular a polishing agent for a chemical mechanical polishing, in particular by using a heating apparatus according to the invention in an apparatus for polishing wafers according to the invention. As described above, the method is characterized by the following steps: 1) introducing a heated heating agent into a heat exchanger via a heating controller; 2) feeding the medium to be heated through the heated heating agent in the heat exchanger; and 3) outputting the heated medium from the heat exchanger to a distributor in the vicinity of the distributor. 
     The advantages, results, effects and functions described with reference to the heating apparatus and polishing apparatus according to the invention are thereby achieved, and thus reference is made to the above description relating to these aspects of the invention. 
     In accordance with another mode of the invention, the temperature of the heating agent can be set to a temperature in the range of 30 to 90° C., preferably 45 to 70° C., and most preferably 55 to 60° C. 
     In accordance with yet another mode of the invention, the medium to be heated can be conducted through the heat exchanger at a flow rate of 100 to 1000 ml/min. 
     Finally, when being output from the heat exchanger to the distributor the medium to be heated can advantageously have a temperature between 20 and 80° C. 
     Other features which are considered as characteristic for the invention are set forth in the appended claims. 
     Although the invention is illustrated and described herein as embodied in an apparatus and method for heating a liquid or viscous polishing agent, and a device for polishing wafers, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims. 
    
    
     The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of the structure of an exemplary embodiment of the heating apparatus according to the invention; 
     FIG. 2 is a diagrammatic cross-sectional view of the heat exchanger of the heating apparatus according to the invention; and 
     FIG. 3 is a detailed cross-sectional view of the heat exchanger shown in FIG.  2 . 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to the figures of the drawings in detail and first, particularly, to FIG. 1 thereof, there is shown a heating apparatus  10  for heating a polishing agent for a chemical mechanical polishing. The heating apparatus  10  generally includes a heating agent tank  11  in which a heating agent  16  can be heated via a heating controller  12 . The heating agent tank  11  is connected to a heat exchanger  30  on the incoming side thereof via a conduit  13  in which a pump  15  is additionally provided. Provided on the outgoing side of the heat exchanger  30  is a further conduit  14  for the heating agent  16 , via which the heating agent  16  is returned to the heating agent tank for the purpose of a renewed heating after leaving the heat exchanger  30 . 
     The heat exchanger  30  further has a conduit section  40  which is flushed on its outer side by the heating agent in the heat exchanger  30  and which is connected on the incoming side to a conduit region  20   a , and on the outgoing side to a conduit region  20   b  of a conduit  20 . The polishing agent to be heated is led by the conduit  20  from a polishing agent tank  53  through the heat exchanger  30  to a distributor  25  in a schematically shown polishing device  50 , and is heated in the heat exchanger  30  in the process. The schematically shown polishing device  50  has a polishing table  54  for placing a wafer  52  on it and also has a heating device  51 , which can be operated independently from the heat exchanger  30 . 
     The basic configuration of the heat exchanger  30  can be seen in FIG.  2 . The heat exchanger  30  has a cover  31  with two inlet openings  32 ,  33  for the conduits  13  and  20   a . Likewise provided on the outgoing side is a cover  34  with two outlet openings  35 ,  36  for the conduits  14  and  20   b . A medium pipe  37  is provided between the covers. The covers  31 ,  34  are welded to the medium pipe  37  and thereby form a closed container for holding the heating agent  16 . 
     As follows further from FIG. 3, the spirally constructed conduit section  40  is provided inside the medium pipe  37  of the heat exchanger  30 . The conduit section  40  is held in a defined position inside the medium pipe  37  via three limiting rods  38 . The result of this is to ensure that the conduit section  40  is continuously and uniformly flushed on its outside by the heating agent  16  at all points. 
     The conduit section  40  is connected respectively to the conduit regions  20   a  and  20   b  of the conduit  20  via a connecting element  41  and a connecting flange  42 . The connection of the heating agent conduits  13  and  14  to the heat exchanger  30  is performed via an inlet stub  43  and an outlet stub  44 . 
     The mode of operation of the heating apparatus  10  will now be described below. 
     The heating apparatus  10  is used, for example, when a wafer is to be polished in a polishing apparatus by a polishing agent. The polishing plate used for this purpose can be heated to a defined setting of the process temperature. The polishing agent to be used is fed into the polishing apparatus via a distributor  25 . The advantageous heating of the polishing agent, through the use of which the polishing process can be further optimized, is made possible by the heating apparatus  10 . 
     An essential element of the heating apparatus  10  is the heat exchanger  30 . The first step is to introduce into the heat exchanger  30  a heating agent  16  which is heated outside the heat exchanger  30  in a heating agent tank  11  to a temperature of 30 to 90°, preferably to a temperature of 55 to 60°. A water-glycol mixture is used in the present case as heating agent. The heating is performed by a heating controller  12  which is constructed in the exemplary embodiment in the form of a thermostat with a power of 3 kW. The heating agent thus heated is fed into the heat exchanger  30  via the conduit  13  and the inlet stub  43 . In order to permit a permanent circulation of the heating agent  16  through the heat exchanger  30 , as a result of which it is possible to set a constant heating agent temperature in the heat exchanger, the heat exchanger is connected on the outgoing side via the outlet stub  44  to a heating agent conduit  14  which conducts the emerging heating agent for the purpose of a renewed heating back into the heating agent tank  11 . The continuous circulation of the heating agent is achieved by a pump  15  which in the present case has a conveying capacity of 24 l/min. 
     In order to heat the polishing agent, the latter is firstly fed into the conduit region  20   a  of the conduit  20  from the polishing agent tank  53 . The conduit region  20   a  is connected to the conduit section  40  via the connecting element  41  and the connecting flange  42 . This section is located with a spiral construction inside the medium pipe  37  of the heat exchanger  30 , and is uniformly and from all sides flushed on the outside by the heated heating agent. The polishing agent is heated to the desired temperature while spirally running through the conduit section  40 . 
     On the outgoing side, the conduit section  40  is connected to the conduit region  20   b  via the connecting element  41  and the connecting flange  42 , and to the distributor  25  via the conduit region. 
     The polishing agent temperature reached at the output of the heat exchanger  30  is therefore dependent on the temperature set in the water-glycol circuit, and on the flow rate set for the polishing agent. The rate is advantageously 150 ml/min, 200 ml/min or 250 ml/min. The polishing agent immediately downstream of the heat exchanger  30  can assume at most the temperature of the water-glycol mixture. 
     The polishing agent in the conduit region  20   b  cools down slightly after exiting from the heat exchanger  30  and before entering the distributor  25 . Such a cooling can, however, be reduced by keeping the length of the conduit region  20   b  as short as possible. The resulting configuration or placement of the heat exchanger in the immediate vicinity surrounding the distributor  25  greatly reduces the possibility of a cooling of the polishing agent. In addition, the conduit region  20   b  can further be thermally insulated, as a result of which the cooling is reduced even more. 
     After entering the polishing apparatus through the distributor  25 , the heated polishing agent can be used to polish the wafers. 
     The following effects must be taken into account with regard to the heating apparatus  10  when setting the suitable polishing agent temperature for the polishing process. The polishing agent cools slightly on the way from the heat exchanger  30  to the distributor  25 . This has the effect that the polishing agent temperature at the distributor  25  is lower than immediately downstream of the heat exchanger  30 . 
     Consequently, when the polishing agent exits from the heat exchanger  30  its temperature must be set such that it is somewhat higher than the desired temperature of the polishing agent. 
     In addition, there is the effect that when no polishing agent is removed the polishing agent present in the conduit region  20   b  downstream of the heat exchanger  30  cools down markedly. When a tapping or removal of the polishing agent begins, the cooled polishing agent first has to be pumped out of the conduit region  20   b , until the desired temperature is established after some time. Since the conduit region  20   b  is, however, selected to be short and is in addition to that advantageously thermally insulated, the losses of nonusable polishing agent are small. Moreover, the desired temperatures are already established shortly after starting to remove or tap the polishing agent, such that no long time delays occur during the polishing process. 
     A concrete example of a dimensioning of the heating apparatus  10  undertaken in accordance with the formulas (1) to (4) will now be described below. 
     The temperature of the heating agent T Bad  was set to a value of 55° C. The incoming temperature of the medium to be heated at the heat exchanger  30 —that is to say in the conduit region  20   a −T in  was 20° C. The outgoing temperature of the heated medium upon exiting from the heat exchanger  30  into the conduit region  20   b  T out  was 50° C. The thermal capacitance of water at 20° C., that is to say c p  is equal to 4180 J/kgK, was used as thermal capacitance. Likewise, the density ρ of water was set to 1000 kg/M 3 . The thermal conductivity λ had a value of 0.19 W/mK, a PVA (polyvinyl alcohol) plastic pipe being used as conduit section. The conduit section had an outside diameter r a  of 8 mm and an inside radius r i  of 6.4 mm. The flow rate ΔV/Δt of the polishing agent was 250 ml/min. 
     On the basis of these values, the formulas (1) to (4) were used to calculate a heating power of approximately 520 W and a required length 1 of the conduit section of 6.56 m. 
     Finally, it was investigated in a series of experiments how far the polishing agent temperatures differ upon exiting from the heat exchanger  30  from the temperatures of the heating agent  16  flushing the conduit section  40 . The experiments were carried out for various heating agent temperatures and flow rates. The results are shown in the following Table 1. 
     
       
         
               
               
             
               
               
               
               
             
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                   
                 Polishing agent temperature at a flow 
               
               
                 Heating agent 
                 rate of 
               
             
          
           
               
                 temperature (° C.) 
                 150 ml/min 
                 200 ml/min 
                 250 ml/min 
               
               
                   
               
             
          
           
               
                 30 
                 28.0 
                 28.1 
                 27.8 
               
               
                 35 
                 32.0 
                 31.8 
                 31.7 
               
               
                 40 
                 36.0 
                 35.7 
                 35.3 
               
               
                 45 
                 39.5 
                 39.5 
                 39.2 
               
               
                 50 
                 43.4 
                 43.2 
                 43.0 
               
               
                 55 
                 47.0 
                 46.8 
                 46.6 
               
               
                   
               
             
          
         
       
     
     As may be seen from the results of Table 1, the polishing agent temperatures have a value which is respectively somewhat lower than the various heating agent temperatures. However, the value is approximately constant in each case for the various flow rates. Consequently, the desired polishing agent temperature value can be precisely set by a correspondingly higher setting of the heating agent temperature value.