Abstract:
An electrostatic chuck comprises a sample stage  9  provided with electrodes  1, 2  and  3  for applying n-phase alternating voltage wherein n is equal to or greater than 2, and a circuit for applying the n-phase alternating voltage. The present electrostatic chuck realizes smooth and speedy attraction/separation of the substrate without the need for a charge elimination process, and the present chuck solves the problem of substrate vibration.

Description:
FIELD OF THE INVENTION 
     The present invention relates to an electrostatic chuck for attracting and holding substrates such as semiconductor wafers, and especially relates to an electrostatic chuck that realizes quick attraction and separation of the substrate (wafer) without causing wafer vibration. 
     DESCRIPTION OF THE RELATED ART 
     Heretofore, an apparatus that attracts and holds a wafer using electrostatic force, so-called an electrostatic chuck, is utilized to hold substrates such as semiconductor wafers in a semiconductor fabrication system and the like. There are two types of electrostatic chucks. One is a so-called monopole-type electrostatic chuck comprising a single electrode, wherein a certain amount of voltage as opposed to ground potential is applied to the electrode while the substrate is grounded through plasma etc., thus utilizing the electrostatic force generated between the wafer and the electrode. Another is a so-called dipole-type electrostatic chuck comprising either a pair of or plural pairs of electrodes, wherein voltage is applied between the electrodes by which reverse polarity charge is induced in the substrate, thus utilizing the electrostatic force generated between the substrate and the electrodes. 
     The voltage applied to the above-mentioned single electrode or a pair of (pairs of) electrodes can either be DC voltage or single-phase alternating voltage. 
     The prior art technology related to these types of electrostatic chucks are disclosed for example in Japanese Patent Laid-Open Provisional Publication Nos. 7-273177 (273177/95) 9-213780 (213780/97) and 6-244271 (244271/94). 
     However, the conventional electrostatic chucks have the following problems. First, according to the system where DC voltage is applied to the electrode(s), electric charge is accumulated in the back surface of the substrate while the wafer is being attracted in both the monopole type and dipole type apparatuses, and the accumulated charge makes the separation of the substrate difficult. Especially in a plasma processing unit and the like where the substrate is in contact with plasma, the substrate becomes negatively charged as opposed to the ground potential through plasma, so the amount of accumulated charge is varied according to the state of plasma, and thus the process of charge being accumulated in the back surface of the substrate becomes more complex. For example, Japanese Patent Laid-Open Provisional Publication Nos. 7-273177 and 9-213780 disclose a method of removing the charge being accumulated in the substrate by applying a reverse voltage so-called a charge eliminator thereto. However, this charge elimination process required in the conventional techniques deteriorates the throughput of the apparatus. 
     In comparison thereto, Japanese Patent Laid-Open Provisional Publication No. 6-244271 discloses a method of applying a single-phase alternating voltage according to which no electric charge is accumulated in the back surface of the substrate, so there is no need to perform the charge elimination process when separating the substrate. However, this prior art method has a drawback in that attraction force becomes zero when the voltage reaches zero, and as a result, the wafer starts to vibrate. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide an electrostatic chuck that does not require a charge elimination process when separating the substrate, which has been required in the conventional art where DC voltage is applied, and a method of treating a substrate using this electrostatic chuck. The present electrostatic chuck solves the problem of substrate vibration, which had been observed in the conventional apparatuses utilizing single-phase alternating voltage. 
     In order to solve the problems of the prior art, the present invention provides an electrostatic chuck characterized in applying an n-phase alternating voltage, wherein n is a number equal to or greater than 2. FIG. 1 illustrates a basic structure of the present invention, in which three-phase alternating voltage is applied (n=3). A substrate  5  is mounted on a sample stage  9  comprising an R-phase electrode  1 , an S-phase electrode  2 , a T-phase electrode  3  and an insulator  4 , having a three-phase alternator  6  connected to the electrodes  1 ,  2  and  3  via switch  11 . By switching the switch  11  on and off, the wafer  5  can be attracted to or separated from the stage. 
     The time variation of voltage applied between the substrate  5  and the R-phase electrode  1 , S-phase electrode  2  and T-phase electrode  3  is shown in FIG.  3 . Moreover, the time variation of attraction force provided between the wafer  5  and sample stage  9  is illustrated in FIG.  4 . As can be seen from FIG. 4, the attraction force provided between the wafer  5  and sample stage  9  is not varied by time, and as a result, the wafer will not vibrate. 
     Moreover, since no DC component is included in the voltage applied to electrodes  1 ,  2  and  3 , no charge is accumulated in the back surface of the wafer, which had caused problems when DC voltage was applied to the electrodes. Furthermore, even in a plasma processing equipment and the like where the substrate comes into contact with plasma and the substrate takes a negative potential as opposed to ground potential, by insulating the primary side and the secondary side of the three-phase alternator  6  and by maintaining the secondary side providing three-phase alternating voltage to the electrodes  1 ,  2  and  3  to a floating potential as opposed to ground potential, no DC voltage is generated between the wafer  5  and electrodes  1 ,  2  and  3 , and thus no charge accumulates in the back surface of the substrate. According to the present invention, prompt attraction and separation of the wafer is realized simply by switching the switch  11  on and off, without having to perform a charge elimination process. 
     In other words, by utilizing three-phase alternating voltage, the present invention provides an electrostatic chuck that realizes smooth and secure attraction and separation of the substrate, that does not require a charge elimination process and that solves the problem of substrate vibration. 
     Such advantages of the present invention can also be enjoyed by applying an n-phase alternating voltage wherein n equals 4 or more. FIG. 2 illustrates the basic structure of the present invention in which a four-phase alternating voltage is applied (n=4). 
     Furthermore, the above advantages can be enjoyed by applying a two-phase alternating voltage (n=2). FIG. 15 illustrates the basic structure of the present invention in which a two-phase alternating voltage is applied (n=2). FIG. 16 illustrates the time variation of the R-phase voltage and S-phase voltage of a two-phase alternator  19 . The phases of the R-phase voltage and S-phase voltage are varied by 90 degrees so that the attraction force applied to the substrate  5  and the sample stage  9  does not fall to zero, and as a result, the substrate will not vibrate. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 illustrates the basic structure of the electrostatic chuck according to the present invention in which a three-phase alternating voltage is applied (n=3); 
     FIG. 2 illustrates the basic structure of the electrostatic chuck according to the present invention in which a four-phase alternating voltage is applied (n=4); 
     FIG. 3 illustrates the time variation of voltage applied between electrodes  1 ,  2  and  3  and wafer  5  when three-phase alternating voltage is applied to the electrostatic chuck of the present invention (n=3); 
     FIG. 4 illustrates the time variation of attraction force generated between the wafer  5  and sample stage  9  when three-phase alternating voltage is applied to the electrostatic chuck of the present invention (n=3); 
     FIG. 5 illustrates one embodiment of the present electrostatic chuck in which three-phase alternating voltage is applied (n=3); 
     FIG. 6 illustrates another embodiment of the present electrostatic chuck in which three-phase alternating voltage is applied (n=3). 
     FIG. 7 is a plan view illustrating one embodiment of the sample stage  9  of the electrostatic chuck according to the present invention in which three-phase alternating voltage is applied (n=3); 
     FIG. 8 is a plan view illustrating another embodiment of the sample stage  9  of the electrostatic chuck according to the present invention in which three-phase alternating voltage is applied (n=3); 
     FIG. 9 is a plan view illustrating yet another embodiment of the sample stage  9  of the electrostatic chuck according to the present invention in which three-phase alternating voltage is applied (n=3); 
     FIG. 10 is a plan view showing yet another embodiment of the sample stage  9  of the electrostatic chuck according to the present invention in which three-phase alternating voltage is applied (n=3); 
     FIG. 11 illustrates one embodiment of a processing equipment for a substrate utilizing the electric chuck of the present invention; 
     FIG. 12 illustrates another embodiment of a processing equipment for a substrate utilizing the electric chuck of the present invention; 
     FIG. 13 is a flowchart showing one embodiment of the method for processing a substrate using the electric chuck according to the present invention; 
     FIG. 14 is a flowchart showing another embodiment of the method for processing a substrate using the electric chuck according to the present invention; 
     FIG. 15 illustrates the basic structure of the electrostatic chuck according to the present invention in which two-phase alternating voltage is applied (n=2); and 
     FIG. 16 illustrates the time variation of R-phase voltage and S-phase voltage of the two-phase alternator when applying two-phase alternating voltage to the electrostatic chuck of the present invention (n=2). 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     One preferred embodiment of the electrostatic chuck according to the present invention is illustrated in FIG. 5, in which a three-phase alternating voltage is applied to the chuck (n=3). The substrate  5  is mounted on a sample stage  9  composed of an R-phase electrode  1 , an S-phase electrode  2 , a T-phase electrode  3  and an insulating body  4 . FIG. 5 illustrates the case where the electrodes  1 ,  2  and  3  are covered by the insulating body  4  on the mounting surface of wafer  5 , but if the substrate  5  is a liquid crystal substrate and the like whose back surface is covered with an insulator, the electrodes  1 ,  2  and  3  can be exposed on the mounting surface. The voltage applied to the electrodes  1 ,  2  and  3  and the frequency thereof are appropriately selected according to the material of the insulator, the material of the wafer, and so on. In the example of FIG. 5, the effective value of the voltage applied to electrodes  1 ,  2  and  3  is 400 V, and the frequency is either 50 Hz or 60 Hz. A commercial three-phase alternator  13  is connected to the R-phase electrode  1 , S-phase electrode  2  and T-phase electrode  3  via a transformer  10 , a switch  11  and an overcurrent protector  12 . In the example of FIG. 5, a delta-Y connection transformer  10  is utilized, but a delta—delta connection, a Y-delta connection, or a Y—Y connection transformer can also be used. The winding ratio of the transformer  10  is set to 1:2 so that the effective value of the secondary side phase voltage becomes 400 V. If the voltage applied to the electrodes is varied or if the connection of the transformer  10  differs, the winding ratio must be changed so as to correspond thereto. The frequency of the alternating voltage being applied is either 50 Hz or 60 Hz, which are provided as commercial power. The primary side of the transformer  10  is insulated from the secondary side thereof, and even if the substrate  5  within a plasma processing equipment and the like takes a negative potential in comparison to the ground potential through plasma, no DC voltage is generated between the wafer  5  and the electrodes  1 ,  2  and  3 , and thus no charge is accumulated in the wafer. By switching the switch  11  on and off, the wafer can be attracted to and separated from the sample stage. 
     FIG. 6 is a drawing showing another preferred embodiment of the electrostatic chuck according to the present invention in which three-phase alternating voltage is applied (n=3). The wafer is attracted to and separated from the stage according to the on/off of the output command  15  to an inverter  14 . By varying the value of the voltage command  16  and the frequency command  17  transmitted from a controller  18  to the inverter  14 , the three-phase alternating voltage and the frequency applied between the electrodes  1 ,  2  and  3  and the wafer  5  can be changed easily. 
     FIG. 7 is a plan view illustrating one embodiment of the sample stage  9  of the present electrostatic chuck in which a three-phase alternating voltage is applied (n=3). The ratio of areas of R-phase electrode  1 , S-phase electrode  2  and T-phase electrode  3  is set to 1. 
     FIGS. 8,  9  and  10  are plan views illustrating other embodiments of the sample stage  9  of the electrostatic chuck according to the present invention in which three-phase alternating voltage is applied (n=3). The ratio of areas of R-phase electrode  1 , S-phase electrode  2  and T-phase electrode  3  is set to 1. 
     FIGS. 11 and 13 illustrate one embodiment of the method for treating a substrate utilizing the electrostatic chuck according to the present invention. FIG. 11 is a drawing showing the substrate processing equipment according to the present embodiment. In FIG. 11, the sample stage  9  is disposed within a vacuum chamber  20 , and the substrate  5  is mounted on the sample stage  9 . Process gas  21  and microwave power  24  are introduced to the vacuum chamber so as to create plasma  23 . FIG. 13 is a flowchart showing the method of processing the substrate according to the present embodiment. In step  41 , the substrate  5  is mounted on the sample stage  9 , and thereafter in step  42 , the substrate  5  is attracted to the sample stage  9 . After the substrate is treated in step  44 , the procedure advances to step  46  where the substrate  5  is separated from the stage  9 , and in step  47  the substrate  5  is taken out. 
     The electrostatic chuck according to the present invention can be applied to other substrate processing methods. FIGS.  12  and  14  illustrate another embodiment of a substrate processing method utilizing the electrostatic chuck of the present invention. FIG. 12 illustrates a substrate processing equipment according to the present invention. A sample stage  9  is disposed within a vacuum chamber  20 , and a substrate  5  is mounted on the sample stage  9 . Process gas  21  is introduced to the vacuum chamber. FIG. 14 is a flowchart showing the method of processing the substrate using the equipment illustrated in FIG.  12 . In step  51 , the substrate  5  is mounted on the sample stage  9 , and thereafter in step  52 , the substrate  5  is attracted to the sample stage  9 . After the substrate  5  is treated in step  54 , the procedure advances to step  56  where the substrate  5  is separated from the stage  9 , and in step  57  the substrate  5  is taken out. 
     The electrostatic chuck according to the present invention is characterized in that an n-phase alternating voltage is applied to the equipment, wherein n is equal to or greater than 2. Unlike the conventional electrostatic chuck where direct voltage is applied, the present chuck is free from accumulated charges in the back surface of the wafer, and unlike the conventional chuck where single-phase alternating voltage is applied, the present equipment realizes an attraction force that does not fall to zero. Thus, the present invention provides an electrostatic chuck that does not require a charge elimination process, that realizes quick attraction and separation of the wafer and that eliminates wafer vibration.