Abstract:
The susceptor ( 10 ) of a plasma treating device, or the electrostatic chuck ( 12 ) of a substrate table is formed by ceramic thermal spray method. A ceramic spray layer ( 12 A) is pore-sealed by methacrylic resin ( 12 D). Resin raw material mainly containing methyl methacrylate is applied to and impregnated into the ceramic spray layer and then is cured to thereby fill pores between ceramic particles in the ceramic spray layer with methacrylic resin. Methacrylic resin raw material solution, which does not produce pores at curing, can complete perfect pore sealing.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to an improvement of a substrate table for mounting thereon a substrate such as a semiconductor wafer in a chamber of a plasma processing apparatus; and, more particularly, to a technique for forming an electrostatic chuck on a substrate table by employing a ceramic thermal spray method and for sealing pores of the electrostatic chuck.  
         BACKGROUND OF THE INVENTION  
         [0002]    A plasma processing apparatus performs a processing such as a film formation on a substrate, e.g., a semiconductor wafer, which is mounted on a substrate table called a susceptor disposed in a process chamber, by using a plasma generated from a process gas introduced into the process chamber. A susceptor employed in the plasma processing apparatus includes a main body  1 , an electrostatic chuck  2  disposed on the main body  1  and a focus ring  3  disposed along an outer periphery of the main body  1  in such a manner that it surrounds the electrostatic chucks  2 , as shown in FIG. 8A for example. A wafer W is adsorptively fixed on the susceptor by the electrostatic chuck  2 . The process chamber is maintained at a predetermined vacuum level and a predetermined high frequency power is applied from a high frequency power supply  4  connected to the main body  1  via a matching unit  4 A. The plasma of the process gas generated between an upper electrode (not shown) and the susceptor is converged onto the wafer W by the focus ring  3 . Further, formed within the main body  1  is a coolant passageway  1 A through which a coolant circulates to cool the main body  1 , which in turn cools the wafer W whose temperature rises during the plasma processing, so that the wafer W is maintained at a constant process temperature. Further prepared within the main body  1  is a gas channel  1 B of a thermally conductive gas (e.g., He gas), which has openings at plural spots on a top surface of the main body  1 . Formed in the electrostatic chuck  2  are through holes  2 A corresponding to the gas channel  1 B. The He gas is supplied between the electrostatic chuck  2  and the wafer W via the gas channel  1 B and the through holes  2 A to serve as a heat transfer medium in a narrow elongated gap existing between the electrostatic chuck  2  and the wafer W. As a result, a heat flow from the wafer W to the electrostatic chuck  2  and, further, to the main body  1  is facilitated, thereby efficiently cooling the wafer W. The electrostatic chuck  2  is of a plate shape obtained by sintering a ceramic such as alumina. Embedded in the electrostatic chuck  2  is an electrode plate  2 B connected to a DC power supply  5 . The electrostatic chuck  2  adsorbs the wafer W by an electrostatic force generated by a high voltage applied thereto from the DC power supply  5 .  
           [0003]    Since it is hard to produce a large sized thin plate by ceramic sintering, a manufacture of the electrostatic chuck  2  of a reasonable size for a large wafer W is also difficult. Therefore, recently, electrostatic chucks are fabricated by employing a ceramic thermal spray technique (see, for example, Japanese Patent No. 2971369). An electrostatic chuck obtained by ceramic thermal spray is hygroscopic due to pores existing between ceramic particles. Therefore, a pore sealing process is executed on the electrostatic chuck by using a silicone resin. The pore sealing process involves the steps of impregnating a silicone resin raw material solution, which is obtained by dissolving methyl sillyl triisocynate in ethyl acetate, into a thermally sprayed alumina layer of the electrostatic chuck; and then heating the electrostatic chuck in the atmosphere at a temperature of about 70° C. for about 8 hours, so that the methyl sillyl triisocynate is polymerized and cured to become the silicone resin. By repeating the impregnation step and the curing step plural times, the pore sealing process is completed.  
           [0004]    However, in case of performing a plasma processing on the wafer W by applying a high frequency power at a high vacuum region (e.g., 100 mTorr) through the use of the ceramic sprayed electrostatic chuck on which the pore sealing process using the silicon resin is executed, there occurs a phenomenon that a surface temperature of the wafer is gradually reduced during plasma processing as the application time of the high frequency power increases, as illustrated by a graph {circumflex over (2)} in FIG. 7.  
         SUMMARY OF THE INVENTION  
         [0005]    It is, therefore, an object of the present invention to provide a substrate table capable of processing a substrate at a stable temperature and a method for the manufacture thereof.  
           [0006]    The inventors of the present invention investigated causes leading to a decrease in temperature of a wafer during a substrate processing and reached a conclusion as follows. If a silicone resin raw material solution is polymerized and cured after being impregnated into a thermally sprayed alumina layer, surfaces of alumina grains are coated with a silicone resin, leading to a formation of a silicon resin layer. At this time, ethyl acetate, which is an organic solvent for dilution, evaporates in the thermally sprayed alumina layer. As a result, there are left traces of evaporation in the form of pores  2 C between the alumina grains as conceptually illustrated in FIG. 8B. Further, as shown in FIG. 9A, there is formed a film  2 D of silicone resin on an adsorption surface of the electrostatic chuck  2 .  
           [0007]    At a time of removing the wafer W from a susceptor after processing the wafer, a part  2 E of the silicone resin film  2 D on the adsorption surface of the electrostatic chuck may be peeled off therefrom because the part  2 E is stuck to the wafer W, as shown in FIG. 9B. In such a case, pores  2 C between the alumina grains are disposed open through the adsorption surface of the electrostatic chuck  2 . The inventors of the present invention concluded that a surface temperature of an object to be processed is decreased with an aging characteristic due to these pores  2 C being disposed open on the surface of the electrostatic chuck  2 . In FIGS. 9A and 9B, the silicone resin is schematically shown as block dots to express a state where ceramic particles are coated with the silicone resin.  
           [0008]    The present invention is conceived from the above conclusion. In accordance with the present invention, there is provided a substrate table including: a table main body; and an electrostatic chuck layer formed on the table main body and made of a thermally sprayed ceramic layer incorporating therein an electrode layer, wherein the thermally sprayed ceramic layer is pore-sealed by methacrylic resin.  
           [0009]    Since the methacrylic resin does not produce pores while being cured, an adverse effect of pores in the electrostatic chuck on a substrate temperature control can be avoided.  
           [0010]    Preferably, the methacrylic resin is obtained by curing a resin raw material solution containing methyl methacrylate as a main component thereof. The resin raw material further containing a stabilizer, an additive, and the like in addition to the main component of methyl methacrylate is used.  
           [0011]    The thermally sprayed ceramic layer is made of at least one of aluminum oxide, aluminum nitride, silicon nitride, and titanium oxide.  
           [0012]    The substrate table in accordance with the present invention can be appropriately applied to various plasma processing apparatus.  
           [0013]    In accordance with the present invention, there is further provided a method for manufacturing a substrate table, including the steps of: preparing a table main body; forming on the table main body an electrostatic chuck layer made of a ceramic layer incorporating therein an electrode layer, wherein the step of forming the electrostatic chuck has the steps of forming the ceramic layer by thermally spraying a ceramic material; and sealing pores in the ceramic layer by using methacrylic resin.  
           [0014]    Preferably, the pore sealing step has the steps of: impregnating a resin raw material solution containing methyl methacrylate as a main component thereof into the ceramic layer; and curing the resin raw material.  
           [0015]    Further, at the step of forming the ceramic layer by thermal spraying, the spraying is preferably conducted while the table main body is heated.  
           [0016]    If a gas channel for supplying a gas to a substrate is formed in the table main body, it is preferable that, at the step of forming the ceramic layer by thermal spraying, the spraying is performed while ejecting a pressured gas from the gas channel. By this, a gas channel communicating with the gas channel in the table main body can be formed in the thermally sprayed ceramic layer without recourse to an additional machine work. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    [0017]FIG. 1 shows a substrate table in accordance with an embodiment of the present invention, wherein FIG. 1A is a cross sectional view thereof and FIG. 1B is a conceptual view of an electrostatic chuck layer shown in FIG. 1A;  
         [0018]    [0018]FIG. 2 describes a manufacturing process of the substrate table shown in FIG. 1, wherein there is illustrated a cross sectional view of a state in which a thermally sprayed alumina layer constituting an electrostatic chuck layer is formed on a top surface of a main body;  
         [0019]    [0019]FIG. 3 explains the manufacturing process of the substrate table shown in FIG. 1, wherein a cross sectional view of a state in which an electrode layer constituting the electrostatic chuck layer is formed is illustrated;  
         [0020]    [0020]FIG. 4 illustrates the manufacturing process of the substrate table shown in FIG. 1, wherein a cross sectional view of a state in which the electrostatic chuck layer is formed is illustrated;  
         [0021]    [0021]FIG. 5 describes the manufacturing process of the substrate table shown in FIG. 1, wherein a cross sectional view of a state in which a thermally sprayed alumina layer is formed on an outer peripheral surface of the main body is illustrated;  
         [0022]    [0022]FIG. 6 demonstrates a manufacturing process of the substrate table shown in FIG. 1, wherein a cross sectional view of a state after polishing the thermally sprayed alumina layer is illustrated;  
         [0023]    [0023]FIG. 7 describes relationships between an application time of a high frequency power and a wafer temperature for the cases where plasma process is performed by using the substrate table shown in FIG. 1 and a conventional substrate table, respectively;  
         [0024]    [0024]FIG. 8 is a counterpart view of FIG. 1, illustrating the conventional substrate table, wherein FIG. 8A is a cross sectional view thereof and FIG. 8B is a conceptual view of a thermally sprayed alumina layer of an electrostatic chuck in FIG. 8A;  
         [0025]    [0025]FIG. 9 is a conceptual cross sectional view of a part of the thermally sprayed alumina layer of the electrostatic chuck shown in FIG. 8, wherein FIG. 9A is a cross sectional view thereof and FIG. 9B is a cross sectional view showing a case where a portion of the thermally sprayed alumina layer of the electrostatic chuck in FIG. 9A is being peeled off; and  
         [0026]    [0026]FIG. 10 is a schematic cross sectional view of an exemplary plasma processing apparatus incorporating therein the substrate table shown in FIG. 1. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0027]    As shown in FIG. 1A, a susceptor  10 , i.e., a substrate table, includes an aluminum main body  11  having an outer peripheral portion formed lower than a central portion thereof at a top surface thereof; an electrostatic chuck layer  12  formed on the top surface of the main body  11  by thermal spraying; and a focus ring  13  formed to surround the electrostatic chuck layer  12 . A peripheral surface of the main body  11  is coated with a thermally sprayed alumina layer  14  formed by alumina thermal spraying. A thermally sprayed alumina layer constituting the electrostatic chuck  12  is formed as a single body with the thermally sprayed alumina layer  14 .  
         [0028]    The electrostatic chuck layer  12  has a thermally sprayed alumina layer  12 A and an electrode layer  12 B made of tungsten and installed within the thermally sprayed alumina layer  12 A. A total thickness of the electrostatic chuck layer  12  is about 600 μm. The electrode layer  12 B is formed by thermally spraying tungsten and its thickness is about 50 μm.  
         [0029]    Further, a ceramic material constituting the thermally sprayed layers  12 A and  14  is not limited to alumina. Any one of alumina (aluminum oxide), aluminum nitride, silicon nitride and titanium oxide or a mixture of at least two of them can be employed. The thermally sprayed alumina layer  12 A is pore-sealed by a methacrylic resin as will be described later in detail.  
         [0030]    Formed within the main body  11  are a coolant passageway  11 A and a gas channel  11 B. Provided in the electrostatic chuck layer  12  are through holes  12 C corresponding to the gas channel  11 B. A thermally conductive gas such as He gas is supplied between a wafer W and the electrostatic chuck layer  12  from the through holes  12 C, thereby increasing a thermal conductivity therebetween. Accordingly, the wafer W can be efficiently cooled by the main body  11 . As in the prior art, a high frequency power supply  15  is connected to the main body  11  via a matching unit  15 A, while a DC power supply  16  is connected to the electrode layer  12 B of the electrostatic chuck layer  12 .  
         [0031]    A method for manufacturing the susceptor  10  will now be described with reference to FIGS.  2  to  6 . First, a main body  11  having the coolant passageway  11 A and the gas channel  11 B formed therein is prepared. A lower peripheral portion of top surface of the main body  11  is masked. Then, the main body  11  is heated up to about 150° C., and a pressurized air having a gauge pressure of about 98 KPa is provided into the gas channel  11 B of the main body  11  to be ejected through openings thereof. In this state, alumina is sprayed onto the main body  11  to thereby form the thermally sprayed alumina layer  12 A of 450 μm as shown in FIG. 2. By ejecting the pressurized air, through holes  12 C corresponding to the gas channel  11 B can be formed in the thermally sprayed alumina layer  12 A concurrently with the alumina thermal spraying. Then, the thermally sprayed alumina layer  12 A is polished until its thickness becomes about 300 μm.  
         [0032]    Subsequently, an electrode layer  12 B is formed. Specifically, a top surface of the thermally sprayed alumina layer  12 A excluding a region thereof on which the electrode layer  12 B is to be formed is masked. Then, tungsten is thermally sprayed on the thermally sprayed alumina layer  12 A while supplying a pressurized air into the gas channel  11 B at room temperature, thereby forming the electrode layer  12 B of 50 μm as shown in FIG. 3. The through holes  12 C are also formed in the electrode layer  12 B concurrently with the tungsten thermal spraying. Afterward, neighboring portions around the through holes  12 C are blast-processed by using an abrasive of grade  60  to prevent the through holes  12 C from being clogged.  
         [0033]    Next, the main body  11  is heated up to 150° C. In this state, alumina is thermally sprayed onto the electrode layer  12 B while ejecting a pressurized air through the openings of the gas channel  11 B, thereby forming another thermally sprayed alumina layer  12 A of 400 μm on the electrode layer  12 B as shown in FIG. 4. The through holes  12 C are also formed in the thermally sprayed alumina layer  12 B concurrently with the alumina thermal spraying.  
         [0034]    By the thermal spraying processes described above, the electrostatic chuck layer  12  having the electrode layer  12 B embedded in the thermally sprayed alumina layer  12 A is formed as a single body with the main body  11 , as illustrated in FIG. 4.  
         [0035]    Thereafter, a pore sealing process of the thermally sprayed alumina layer  12 A is executed. First, a methacrylic resin raw material solution containing a liquid of methyl methacrylate as a main component thereof is coated on the electrostatic chuck layer  12  by using a roller. As a result, the methacrylic resin raw material solution permeates into pores in the thermally sprayed alumina layer  12 A of the electrostatic chuck layer  12 .  
         [0036]    Subsequently, the main body  11  on which the electrostatic chuck layer  12  is formed is loaded into a vacuum vessel and a deaeration process is performed at a vacuum level of 0.1 Torr. During the vacuum deaeration process, methacrylic resin is formed in the thermally sprayed alumina layer  12 A via a copolymerization by a polymerization catalyzer contained in the methacrylic resin raw material solution. Consequently, the pores in the thermally sprayed alumina layer  12 A are filled with the methacrylic resin as shown in FIG. 1B.  
         [0037]    Since no volatile compound is contained in the methacrylic resin raw material solution, it is possible to completely fill the pores between alumina grains with the methacrylic resin (hatched portion), as conceptually illustrated in FIG. 1B. Accordingly, there occurs no pore formation due to an evaporation of an organic solvent, unlike in a pore sealing process using silicone resin.  
         [0038]    Further, the curing of the methacrylic resin raw material solution can also be achieved by heating and sintering the main body  11  at a temperature ranging from 60 to 70° C. for 5 to 8 hours.  
         [0039]    Though the pore sealing process is performed only on the thermally sprayed alumina layer  12 A disposed on the electrode layer  12 B in the above embodiment, the pore sealing process may also be executed on the thermally sprayed alumina layer  12 A located below the electrode layer  12 B before the formation of the electrode layer  12 B. In such a case, a formation of pores in the electrostatic chuck layer  12  can be prevented more effectively.  
         [0040]    After performing the pore sealing process by using the methalcrylic resin, the electrostatic chuck layer  12  excluding an outer peripheral portion thereof is masked and, at the same time, the mask material of the outer peripheral portion of the top surface of the main body  11  is removed. Then, alumina is thermally sprayed on the outer peripheral surface of the main body  11  at room temperature, thereby forming a thermally sprayed alumina layer  14  of 750 μm as shown in FIG. 5. Thus, the thermally sprayed alumina layers  12 A and  14  are integrated as one body, as shown in FIG. 5.  
         [0041]    Next, a thermally sprayed alumina layer  14 A around an outer peripheral surface of the electrostatic chuck layer  12  is coated and impregnated with the methacrylic resin raw material solution, while another thermally sprayed alumina layer  14 B is coated and impregnated with a silicone resin raw material solution. Then, the resin raw material solutions are cured. Further, the thermally sprayed alumina layer  14 B may also be coated and impregnated with the methacrylic resin raw material solution.  
         [0042]    Thereafter, the masking is removed from the surface of the electrostatic chuck layer  12 , and the electrostatic chuck layer  12  and the thermally sprayed alumina layer  14  are ground through the use of a grinder, thereby flattening surfaces of the electrostatic chuck layer  12  and the thermally sprayed alumina layer  14  and a neighboring circumferential surface of the main body  11  as shown in FIG. 6. At this time, the surface of the electrostatic chuck layer  12 , i.e., a surface that makes a contact with the wafer is set to have a flatness of Ra=0.2 to 0.3. It is preferable to set a thickness of the thermally sprayed alumina layer deposited on the electrode layer  12 B to be equal to or smaller than 250 μm.  
         [0043]    Thereafter, a red check was conducted on the susceptor  10  fabricated by the sequence of processes described above to investigate a filling state of the thermally sprayed alumina layer  12 A by the methacrylic resin  12 D.  
         [0044]    The red check which is equivalent to Dye Penetrant Testing is carried out by coating a red dye and spraying a white developing solution after wiping the red dye on a surface. If there exists a defect on the surface of a test object (i.e., if there exists a pore which is not sealed in this case), the red dye left in such a defect floats on a dried white developing solution layer, so that the defect can be identified with the eye. Moreover, it is also possible to check whether pores are being sealed or not by shades of color of the coated red dye, instead of using the white developing solution.  
         [0045]    The test was executed after conducting the pore sealing process three times by using the methacrylic resin raw material solution. The result of the red check performed on the electrostatic chuck layer  12  showed no unsealed pores. Further, the red check was performed on a surface of the electrostatic chuck layer  12  whose surface was cut down to a depth of 75 μm. However, there was found no unsealed pore. Furthermore, the surface of the electrostatic chuck layer was cut down to 100 μm, 150 μm, 200 μm and 250 μm step by step and the red check was conducted at each step, but there was found no unsealed pore at each step. Such a red check revealed that the sealing of pores in the electrostatic chuck layer  12  was perfectly conducted at least down to a depth 250 μm by the methacrylic resin  12 D.  
         [0046]    Further, as comparative examples, there were prepared electrostatic chuck layers on which the pore sealing process was executed 5, 10, 15 and 20 times by using the silicone resin and the red check was conducted for each of the electrostatic chuck layers. The result showed the coloration in all the electrostatic chuck layers, though the color becomes lighter as the number of sealing processes increases.  
         [0047]    Further, each electrostatic chuck layer was cut after completing the pore sealing process in order to investigate a permeation state of the red dye. The result also showed that coloration of a cross section of each electrostatic chuck layer becomes lighter as the sealing process is repeated.  
         [0048]    Accordingly, it was proven that pores remain in the electrostatic chuck layers in case a pore sealing process is conducted by using the silicone resin.  
         [0049]    Subsequently, a plasma processing was conducted on a wafer W by using the susceptor  10  manufactured as described above and, then, a high frequency power application time and a surface temperature of the wafer W were obtained. As a result, it was found that the surface temperature of the wafer W was maintained constant without being reduced even though the high frequency power applying time increases, as shown in FIG. 7. That is, if the electrostatic chuck layer  12  is sealed by the methacrylic resin, there remains no pore between alumina grains, unlike in the conventional cases. Accordingly, a temperature decrease in the wafer due to the lapse of time can be prevented in conducting the plasma processing at a high vacuum range.  
         [0050]    Particularly, even in a case where a feed pressure of a thermally conductive gas is changed from 10 to 40 Torr to a low pressure of 5 to 10 Torr in a control process for changing the temperature of the wafer W from 100° C. to 120° C., the thermally conductive gas of the low pressure can reach the backside of the wafer W without permeating between alumina grains in the thermally sprayed alumina layer  12 A. Therefore, the temperature of the wafer W can be controlled much more accurately.  
         [0051]    In accordance with the embodiment of the present invention, a decrease in the surface temperature of the wafer W can be prevented and a plasma processing can be stably performed at a predetermined temperature even though the processing of wafer W processing is conducted at a high vacuum range for a long time.  
         [0052]    Further, there was a limit in a service temperature of a conventional susceptor since there was a risk in that a thermally sprayed ceramic layer constituting an electrostatic chuck layer could be cracked due to a difference between a thermal expansion coefficient of the aluminum material and that of the thermally sprayed ceramic layer. In the embodiment, however, a ceramic is thermally sprayed on the aluminum material which is thermally expanded by heating the main body  11 . Therefore, a thermal stress between the aluminum material and the thermally sprayed ceramic layer  12 A can be reduced when the temperature of the susceptor increases, and thus a heat resisting temperature of the susceptor  10  can be raised.  
         [0053]    The susceptor  10 , i.e., the substrate table, provided by the present invention can be widely applied to various plasma processing apparatuses such as a capacitive coupling type plasma processing apparatus, an induction coupling type plasma processing apparatus, a microwave plasma processing apparatus, and the like. FIG. 10 depicts an overall structure of a microwave plasma processing apparatus to which the susceptor described above is applicable.  
         [0054]    Installed within a processing vessel  20  made of a conductive material such as aluminum is a susceptor  10  which can be moved up and down by an elevator unit  21  such as an air cylinder. The susceptor  10  has the same configuration as the one described in FIG. 1. An electrostatic chuck layer  12  is formed by the method described earlier with reference to FIGS.  3  to  6 . A properly temperature-adjusted coolant is introduced into a coolant passageway  11 A prepared in a main body  11  through an inlet tube  22  and then is discharged through an outlet tube  23  after being used for a heat exchange. A high DC voltage is applied to a tungsten electrode layer  12 B within the electrostatic chuck layer  12  from a DC power supply  16  via a filter  16 A and a lead line  16 B, whereby the electrostatic chuck layer adsorbs the wafer W.  
         [0055]    Disposed around the wafer W adsorptively held on the electrostatic chuck layer  12  is an annular focus ring  13 , which is formed of an insulating or a conductive material depending on a process performed by a plasma processing apparatus. The focus ring  13  serves to confine or expand reactive ions (plasma). Further, installed between the susceptor  10  and the processing vessel  20  is an exhaust ring  24  provided with exhaust openings not shown in the drawing. By the exhaust ring  24 , an exhaust flow from a processing space can be controlled and, further, plasma can be appropriately confined between the susceptor  10  serving as a lower electrode and an upper electrode  25  disposed above the susceptor  10 .  
         [0056]    The susceptor is moved up and down depending on a process to be performed, and the distance between the upper electrode  25  and the susceptor  10  is adjusted. A high frequency power of 2 to 13.56 MHz is applied to the main body  11  of the susceptor  10  from a high frequency power supply  15  via an impedance matching unit  15 A. A high frequency power of 13.56 to 100 MHz is applied to the upper electrode  25  from a high frequency power supply  27  via an impedance matching unit  26 .  
         [0057]    A process gas is supplied to the upper electrode  25  from a process gas supply unit  29  via a process gas supply tube  28  and a mass flow controller  30 . The process gas is converted into plasma by the high frequency powers, so that a predetermined processing can be carried out on the wafer W by the plasma. A loadlock chamber  32  is connected to a side wall of the processing vessel  11  via a gate valve  31 . A transfer arm  33  mounted in the loadlock chamber  32  transfers the wafer W between the loadlock chamber  32  and the processing vessel.