Patent Publication Number: US-2021175054-A1

Title: Electrostatic chuck and substrate fixing device

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Note that the present application is based on Japanese Patent Application No. 2019-222739 filed on Dec. 10, 2019, which is incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to an electrostatic chuck, and a substrate fixing device. 
     BACKGROUND ART 
     In the related art, a film formation apparatus and a plasma etching apparatus that are used when manufacturing a semiconductor device each have a stage for holding accurately a wafer in a vacuum treatment chamber. As the stage, for example, a substrate fixing device configured to suck and hold a wafer by an electrostatic chuck mounted on a base plate is suggested. 
     A substrate fixing device having a structure where a gas supply unit for cooling a wafer is provided may be exemplified. The gas supply unit supplies a gas to a surface of the electrostatic chuck through a gas flow path in the base plate and gas holes formed in the electrostatic chuck, for example (for example, refer to PTL 1). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] JP-A-H07-45693 
     An electric discharge may occur around the gas holes of the electrostatic chuck. When the electric discharge occurs, there is a risk of burning or melting a rear surface of a suction target. 
     SUMMARY OF INVENTION 
     Aspect of non-limiting embodiments of the present disclosure is to provide an electrostatic chuck capable of suppressing occurrence of an electric discharge around a gas hole. 
     An electrostatic chuck according to non-limiting embodiment of the present disclosure is an electrostatic chuck configured to suck and hold a suction target, comprising: 
     a base body on which the suction target is placed, the base body having a gas hole for supplying a gas to the suction target; and 
     a plurality of electrostatic electrodes embedded in the base body, the electrostatic electrodes comprising a positive electrode and a negative electrode, 
     wherein as seen from above, the positive electrode and the negative electrode are arranged to face each other with a first gap around the gas hole, the positive electrode and the negative electrode are arranged to face each other with a first path on the positive electrode-side and a second path on the negative electrode-side, the first path and the second path formed with the gas hole being interposed therebetween, and the positive electrode and the negative electrode are arranged to face each other with a second gap around the gas hole, 
     the first path and the second path extend along an outer periphery of the gas hole with the gas hole being interposed therebetween, converge to be the first gap at a first end, and converge to be the second gap at a second end, 
     wherein as seen from above, at a place at which the first path and the second path converge to be the first gap, a first corner portion formed by the positive electrode is rounded and a second corner portion formed by the negative electrode is rounded, and at a place at which the first path and the second path converge to be the second gap, a third corner portion formed by the positive electrode is rounded and a forth corner portion formed by the negative electrode is rounded, and 
     wherein as seen from above, a first distance between the gas hole and the positive electrode is constant in the first path except the rounded first and third corner portions, a second distance between the gas hole and the negative electrode is constant in the second path except the rounded second and fourth corner portions, and the first distance and the second distance are the same. 
     According to the disclosed technology, it is possible to provide the electrostatic chuck capable of suppressing occurrence of an electric discharge around the gas hole. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a simplified sectional view for depicting a substrate fixing device in accordance with a first embodiment. 
         FIG. 2  is a partially enlarged plan view of an A part in  FIG. 1 . 
         FIG. 3  illustrates distances between a positive electrode and a negative electrode of an electrostatic chuck and a gas hole in Comparative Example. 
         FIG. 4  illustrates a relation between a distance from a positive electrode or a negative electrode and a voltage. 
         FIG. 5  is a simplified sectional view for depicting a substrate fixing device in accordance with a second embodiment. 
         FIG. 6  is a partially enlarged sectional view of a B part in  FIG. 5 . 
         FIG. 7  is a partially enlarged sectional view of the B part in  FIG. 5 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinbelow, embodiments of the present invention will be described with reference to the drawings. In the respective drawings, the same constitutional parts are denoted with the same reference signs, and overlapping descriptions may be omitted. 
     First Embodiment 
       FIG. 1  is a simplified sectional view for depicting a substrate fixing device in accordance with a first embodiment. Referring to  FIG. 1 , a substrate fixing device  1  includes, as main constitutional elements, a base plate  10 , an adhesion layer  20 , and an electrostatic chuck  30 . The substrate fixing device  1  is a device configured to suck and hold a substrate (a wafer and the like), which is a suction target, by the electrostatic chuck  30  mounted on one surface of the base plate  10 . 
     The base plate  10  is a member for mounting the electrostatic chuck  30 . A thickness of the base plate  10  is, for example, about 20 mm to 40 mm. The base plate  10  is formed of, for example, aluminum, and can be used as an electrode for controlling plasma. By supplying predetermined high-frequency electric power to the base plate  10 , it is possible to control energy for causing ions in a generated plasma state to collide with the substrate sucked on the electrostatic chuck  30 , and to effectively perform an etching treatment. 
     In the base plate  10 , a gas supply unit  11  is provided. The gas supply unit  11  has a gas flow path  111 , a gas injection part  112 , and gas discharge parts  113 . 
     The gas flow path  111  is an annular hole formed in the base plate  10 , for example. The gas injection part  112  is a hole having one end communicating with the gas flow path  111  and the other end exposed to an outside from a lower surface  10   b  of the base plate  10 , and is provided to introduce an inert gas (for example, He, Ar and the like) for cooling the substrate sucked on the electrostatic chuck  30 , from an outside of the substrate fixing device  1 . The gas discharge part  113  is a hole having one end communicating with the gas flow path  111  and the other end exposed to an outside from an upper surface  10   a  of the base plate  10  and penetrating the adhesion layer  20 , and is provided to discharge the inert gas introduced into the gas flow path  111 . As seen from above, the gas discharge parts  113  are scattered on the upper surface  10   a  of the base plate  10 . The number of the gas discharge parts  113  can be determined as appropriate, as necessary, and is, for example, about several tens to several hundreds. 
     In the meantime, the description “as seen from above” means that a target is seen in a normal direction of the upper surface  10   a  of the base plate  10 , and a planar shape indicates a shape seen in the normal direction of the upper surface  10   a  of the base plate  10 . 
     In the base plate  10 , a cooling mechanism  15  may also be provided. The cooling mechanism  15  has a coolant flow path  151 , a coolant introduction part  152 , and a coolant discharge part  153 . The coolant flow path  151  is an annular hole formed in the base plate  10 , for example. The coolant introduction part  152  is a hole having one end communicating with the coolant flow path  151  and the other end exposed to the outside from the lower surface  10   b  of the base plate  10 , and is provided to introduce a coolant (for example, cooling water, GALDEN and the like) from the outside of the substrate fixing device  1  into the coolant flow path  151 . The coolant discharge part  153  is a hole having one end communicating with the coolant flow path  151  and the other end exposed to the outside from the lower surface  10   b  of the base plate  10 , and is provided to discharge the coolant introduced into the coolant flow path  151 . 
     The cooling mechanism  15  is connected to a coolant control device (not shown) provided outside of the substrate fixing device  1 . The coolant control device (not shown) is configured to introduce the coolant from the coolant introduction part  152  into the coolant flow path  151 , and to discharge the coolant from the coolant discharge part  153 . The coolant is caused to circulate in the cooling mechanism  15  to cool the base plate  10 , so that it is possible to cool the wafer sucked on the electrostatic chuck  30 . 
     The electrostatic chuck  30  is a part for sucking and holding the wafer that is a suction target. A planar shape of the electrostatic chuck  30  is, for example, circular. A diameter of the wafer that is a suction target of the electrostatic chuck  30  is, for example, 8 inches, 12 inches, or 18 inches. 
     The electrostatic chuck  30  is provided on the upper surface  10   a  of the base plate  10  with the adhesion layer  20  being interposed therebetween. The adhesion layer  20  is, for example, a silicon-based adhesive. A thickness of the adhesion layer  20  is, for example, about 0.1 mm to 1.0 mm. The adhesion layer  20  bonds the base plate  10  and the electrostatic chuck  30  each other, and has an effect of reducing stress that is generated due to a difference in thermal expansion coefficient between the ceramic electrostatic chuck  30  and the aluminum base plate  10 . 
     The electrostatic chuck  30  has a base body  31 , positive electrodes  32 P and negative electrodes  32 N. An upper surface of the base body  31  is a placement surface  31   a  for a suction target. The electrostatic chuck  30  is, for example, a Johnson-Rahbek type electrostatic chuck. However, the electrostatic chuck  30  may also be a Coulomb force type electrostatic chuck. 
     The base body  31  is a dielectric body. As the base body  31 , for example, a ceramic such as aluminum oxide (Al 2 O 3 ) and aluminum nitride (AlN) is used. A thickness of the base body  31  is, for example, about 1 mm to 5 mm, and a specific permittivity (1 kHz) of the base body  31  is, for example, about 9 to 10. 
     The positive electrodes  32 P and the negative electrodes  32 N are bipolar electrostatic electrodes formed of thin films, and are embedded in the base body  31 . The positive electrodes  32 P and the negative electrodes  32 N are formed in a comb teeth-shaped electrode pattern, for example, and teeth of each electrode are alternately arranged at predetermined intervals. The positive electrodes  32 P and the negative electrodes  32 N are connected to a power supply provided outside of the substrate fixing device  1 , and generate a suction force by static electricity between the electrodes and the wafer when a predetermined voltage is applied from the power supply. Thereby, the wafer can be sucked and held on the placement surface  31   a  of the base body  31  of the electrostatic chuck  30 . When a higher voltage is applied between the positive electrode  32 P and the negative electrode  32 N, a suction holding force becomes stronger. As materials of the positive electrode  32 P and the negative electrode  32 N, tungsten, molybdenum and the like are used, for example. 
     In the base body  31 , a heating element (heater) that generates heat when a voltage is applied from the outside of the substrate fixing device  1  and heats the placement surface  31   a  of the base body  31  to a predetermined temperature may also be provided. 
     Gas holes  311  formed to penetrate the base body  31  and to expose the other ends of the gas discharge parts  113  are provided in positions corresponding to the gas discharge parts  113  of the base body  31 . A planar shape of the gas hole  311  is, for example, a circular shape having an inner diameter of about 0.1 mm to 1 mm. The gas holes  311  can be formed by drilling or laser processing, for example. The inert gas is supplied to a rear surface of the suction target sucked on the electrostatic chuck  30  through the gas holes  311 , so that the suction target is cooled. 
       FIG. 2  is a partially enlarged plan view of an A part in  FIG. 1 , depicting distances between the positive electrode  32 P and the negative electrode  32 N and the gas hole  311  around the gas hole  311 . In  FIG. 2 , the base body  31  is not shown. In  FIG. 2 , a thickness direction of the electrostatic chuck  30  is denoted as a Z direction, a direction in which a gap separating the positive electrode  32 P and the negative electrode  32 N in a plane perpendicular to the Z direction extends is denoted as a Y direction, and a direction orthogonal to the Y direction in the plane perpendicular to the Z direction is denoted as an X direction (width direction). 
     As shown in  FIG. 2 , the positive electrode  32 P and the negative electrode  32 N are patterned at predetermined distances from the gas hole  311  so as not to contact the gas hole  311 , around the gas hole  311 . 
     Specifically, as seen from above, the positive electrode  32 P and the negative electrode  32 N are arranged to face each other with a first gap  321  on a −(negative)Y-side of the gas hole  311  around the gas hole  311 . The first gap  321  is divided into a first path  322  on the positive electrode  32 P-side and a second path  323  on the negative electrode  32 N-side at the gas hole  311 . The positive electrode  32 P and the negative electrode  32 N are arranged to face each other with the first path  322  and the second path  323  between which the gas hole  311  is interposed. The first path  322  and the second path  323  extend along an outer periphery of the gas hole  311  with the gas hole  311  being interposed therebetween, and converge to be one second gap  324  on a +(positive)Y-side of the gas hole  311 . The positive electrode  32 P and the negative electrode  32 N are arranged to face each other with the second gap  324  on the −(negative)Y-side of the gas hole  311  around the gas hole  311 . The first path  322  and the second path  323  extend along an outer periphery of the gas hole  311 , and converge to be the first gap  321  at a first end on the −Y-side of the gas hole  311  and converge to be the second gap  324  at a second end on the +Y-side of the gas hole  311 . 
     As used herein, the gap means that the targets are arranged spaced, and does not mean that there is a space in the gap (no material exists in the gap). In the first gap  321 , the first path  322 , the second path  323  and the second gap  324 , the base body  31  is arranged. 
     As seen from above, at a place at which the first gap  321  is divided into the first path  322  and the second path  323  (in other words, at a place at which the first path  322  and the second path  323  are converged to be the first gap  321 ) and at a place at which the first path  322  and the second path  323  are converged to be the second gap  324 , corner portions formed by the positive electrode  32 P and corner portions formed by the negative electrode  32 N are not sharpened but rounded (four places in circles of broken lines). The rounded corner portion preferably has R=0.1 μm or larger. 
     As seen from above, a first distance a between the gas hole  311  and the positive electrode  32 P is constant in the first path  322  except the rounded corner portions. Also, a second distance b between the gas hole  311  and the negative electrode  32 N is constant in the second path  323  except the rounded corner portions. The first distance a and the second distance b are the same. Here, the distance between the gas hole  311  and the positive electrode  32 P or the negative electrode  32 N is a distance in a normal direction of a tangent line at each point of an inner wall  311 W of the gas hole  311 , as seen from above. 
     Also, a maximum distance in the X direction between the positive electrode  32 P and the negative electrode  32 N around the gas hole  311  is a distance c, as seen from above. 
     A third distance d in a width direction (X direction) of the first gap  321  between the positive electrode  32 P and the negative electrode  32 N is preferably the same as a fourth distance e in the width direction (X direction) of the second gap  324  between the positive electrode  32 P and the negative electrode  32 N. More preferably, the first distance a, the second distance b, the third distance d and the fourth distance e are all the same. The first distance a, the second distance b, the third distance d, and the fourth distance e are arbitrarily set within a range of about 0.1 mm to 10 mm, for example. 
     Here, effects achieved by the electrostatic chuck  30  are described with reference to Comparative Example. 
       FIG. 3  illustrates distances between a positive electrode and a negative electrode of an electrostatic chuck and a gas hole in Comparative Example, and is a partially enlarged plan view corresponding to  FIG. 2 . In  FIG. 3 , the base body  31  is not shown. 
     An electrostatic chuck  30 X in accordance with Comparative Example shown in  FIG. 3  is different from the electrostatic chuck  30  shown in  FIG. 2 , in that an end portion of the negative electrode  32 N facing the positive electrode  32 P with the gas hole  311  being interposed therebetween is linear in the Y direction, as seen from above. Also, as seen from above, corner portions formed by the positive electrode  32 P are not rounded but sharpened (two places in circles of broken lines), which is different from the electrostatic chuck  30  shown in  FIG. 2 . 
     In the electrostatic chuck  30 X, the first distance a between the gas hole  311  and the positive electrode  32 P is constant on a −X-side from a center of the gas hole  311 . However, since the negative electrode  32 N has a shape as described above, the second distance b between the gas hole  311  and the negative electrode  32 N is not constant on a +X-side from the center of the gas hole  311 . Therefore, a relation of the first distance a and the second distance b (the first distance a is equal to the second distance b) is not made. 
     For example, in a case where the first distance a and the second distance b are not the same (it is assumed that a&lt;b) at the position shown in  FIG. 3 , it is assumed that a DC voltage of +10 kV is applied to the positive electrode  32 P and a DC voltage of −10 kV is applied to the negative electrode  32 N. In this case, as shown in  FIG. 4 , a voltage Va at the gas hole  311  located at a position of the distance a from the positive electrode  32 P is higher than a voltage Vb at the gas hole  311  located at a position of the distance b from the negative electrode  32 N. In this case, the voltage is biased to a plus side, so that positive charges are likely to be accumulated. When the positive charges are accumulated to some extent or more, an electric discharge occurs. The electric discharge may also be referred to a dielectric barrier electric discharge. In the meantime, even when the voltage is biased to a minus side, the charges are unevenly distributed, so that the electric discharge occurs. 
     Even if the first distance a and the second distance b are made to be the same at the position shown in  FIG. 3 , no countermeasure against the electric discharge is provided. This is because the second distance b between the gas hole  311  and the negative electrode  32 N is not constant on the +X-side from the center of the gas hole  311 , as described above. That is, in the shape of  FIG. 3 , since a part where the first distance a is not the same as the second distance b always exists around the gas hole  311 , the electric discharge as described above occurs. When the electric discharge as described above occurs, there is a risk of burning or melting a rear surface of the substrate that is a suction target. For this reason, it is required to suppress the electric discharge as described above. 
     Therefore, in the electrostatic chuck  30 , the corner portions formed by the positive electrode  32 P and the negative electrode  32 N are rounded, and the first distance a and the second distance b are made to be the same, as seen from above, except the rounded corner portions. That is, in the electrostatic chuck  30 , since the first distance a is the same as the second distance b over the substantially entire region on the outer periphery-side of the gas hole  311 , the voltage Va and the voltage Vb described with respect to  FIG. 4  are the same, so that charges are not unevenly distributed. 
     As a result, it is possible to suppress occurrence of the electric discharge. 
     Also, the electric discharge is more likely to occur at an acuter portion. However, in the electrostatic chuck  30 , the corner portions formed by the positive electrode  32 P and the negative electrode  32 N are rounded, so that it is also possible suppress occurrence of the electric discharge. The rounded corner portion having R of 0.1 μm or greater can contribute to suppression of occurrence of the electric discharge. 
     Also, it is preferably that the third distance d is the same as the fourth distance e. Thereby, the charges are not unevenly distributed even in a region slightly distant from the gas hole  311 , so that it is possible to further suppress occurrence of the electric discharge. 
     Also, it is more preferably that the first distance a, the second distance b, the third distance d and the fourth distance e are all the same. The third distance d and the fourth distance e are made to be the same as the first distance a and the second distance b, so that the distances from each of the positive electrode  32 P and the negative electrode  32 N are the same and charge amounts in each position are the same. For this reason, for example, when stopping the applying of the voltage to the positive electrode  32 P and the negative electrode  32 N and demounting the wafer from the electrostatic chuck, the loss of charges is the same, so that it is possible to suppress the wafer from being positionally misaligned. 
     Second Embodiment 
     In a second embodiment, an example where a porous body is arranged in the gas hole is described. In the second embodiment, the descriptions of the same constitutional parts as the above-described embodiment may be omitted. 
       FIG. 5  is a simplified sectional view for depicting a substrate fixing device in accordance with a second embodiment.  FIG. 6  is a partially enlarged sectional view of a B part in  FIG. 5 . Referring to  FIGS. 5 and 6 , a substrate fixing device  2  is different from the electrostatic chuck  30  (refer to  FIG. 1  and the like), in that a porous body  60  is arranged in the gas hole  311 . 
     The porous body  60  includes a plurality of spherical oxide ceramic particles  601 , and a mixed oxide  602  that binds and integrates the plurality of spherical oxide ceramic particles  601 . 
     A diameter of the spherical oxide ceramic particle  601  is, for example, within a range of 30 μm to 1000 μm. As a favorable example of the spherical oxide ceramic particle  601 , a spherical aluminum oxide particle may be exemplified. Also, the spherical oxide ceramic particles  601  are preferably contained in a weight ratio of 80 wt % or more (and 97 wt % or less) in the porous body  60 . 
     The mixed oxide  602  adheres to some of outer surfaces (spherical surfaces) of the plurality of spherical oxide ceramic particles  601  and supports the same. The mixed oxide  602  is formed of oxides of two or more elements selected from silicon (Si), magnesium (Mg), calcium (Ca), aluminum (Al) and yttrium (Yt), for example. 
     In the porous body  60 , pores P are formed. The pores P communicate with the outside so as to cause the gas to pass through from a lower side toward an upper side of the porous body  60 . A porosity of the pores P formed in the porous body  60  is preferably within a range of 20% to 50% of an entire volume of the porous body  60 . To inner surfaces of the pores P, some of the outer surfaces of the spherical oxide ceramic particles  601  and the mixed oxide  602  are exposed. 
     When the base body  31  is formed of aluminum oxide, the base body  31  preferably contains, as other components, oxides of two or more elements selected from silicon, magnesium, calcium and yttrium. A composition ratio of oxides of two or more elements selected from silicon, magnesium, calcium and yttrium in the base body  31  is preferably set to be the same as a composition ratio of oxides of two or more elements selected from silicon, magnesium, calcium and yttrium in the mixed oxide  602  of the porous body  60 . 
     In this way, the composition ratios of oxides are made to be the same between the base body  31  and the mixed oxide  602  of the porous body  60 , so that mutual material transfer does not occur when sintering the porous body  60 . Therefore, it is possible to secure flatness of an interface between the base body  31  and the porous body  60 . 
     The porous body  60  can be formed by charging paste, which is a precursor of the porous body  60 , in the gas hole  311  by using a squeegee or the like and sintering the same. When a part of the porous body  60  protrudes from a lower surface-side of the base body  31 , grinding or the like is preferably performed so that an end face of the porous body  60  is substantially flush with the lower surface of the base body  31 . 
     The paste that is a precursor of the porous body  60  contains, for example, spherical aluminum oxide particles in a predetermined weight ratio. The rest of the paste contains oxides of two or more elements selected from silicon, magnesium, calcium, aluminum and yttrium, for example, and further contains an organic binder and a solvent. As the organic binder, for example, polyvinyl butyral may be used. As the solvent, for example, alcohol may be used. 
     As described above, the porous body  60  may be arranged in the gas hole  311 . Since the porous body  60  is also a dielectric body, charges are accumulated. Since the porous body  60  has the pores P, the electric discharge may occur at a short distance. In the process of arranging the porous body  60  in the gas hole  311 , it is difficult to control the size of the pores P and the size of the mixed oxide  602 . Therefore, as shown in  FIG. 7 , distances (for example, F1, F2, F3) between the adjacent the spherical oxide ceramic particles  601  are not constant, and thus discharge may occur at the shorter distance (for example, F2) in these distances. 
     Therefore, similar to the first embodiment, the corner potions formed by the positive electrode  32 P and the negative electrode  32 N are rounded, and the first distance a and the second distance b are made to be the same, as seen from above, except the rounded corner portions. Thereby, charges are not unevenly distributed, so that occurrence of the electric discharge can be suppressed. 
     Also, the corner potions formed by the positive electrode  32 P and the negative electrode  32 N are preferably rounded, the third distance d is more preferably the same as the fourth distance e, and further preferably, the first distance a, the second distance b, the third distance d and the fourth distance e are all the same. 
     Although the preferred embodiments have been described in detail, the present invention is not limited to the above embodiments and the above embodiments can be diversely modified and replaced without departing from the scope of the claims. 
     For example, as the suction target of the substrate fixing device of the present invention, a glass substrate and the like that are used in a manufacturing process of a liquid crystal panel and the like may be exemplified, in addition to the semiconductor wafer (silicon wafer, and the like).