Patent Publication Number: US-2023142870-A1

Title: Electrostatic chuck

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims the benefit of priority from prior Japanese patent application No. 2021-183550 filed on Nov. 10, 2021, the entire contents of which are incorporated herein by reference. 
     TECHNICAL FIELD 
     The present invention relates to an electrostatic chuck. 
     BACKGROUND ART 
     For example, when manufacturing a semiconductor component and the like, an  15  electrostatic chuck (ESC) configured to adsorb and hold a wafer may be used. The electrostatic chuck has a structure in which a ceramic plate having an electrode embedded therein is bonded to a metallic base plate by a resin layer. The electrostatic chuck is configured to adsorb the wafer to the ceramic plate by using an electrostatic force when a voltage is applied to the electrode embedded in the ceramic plate. 
     As an adhesive forming the resin layer, for example, a flexible silicone resin-based adhesive is used so as to favorably maintain thermal conductivity and to absorb a difference in thermal expansion between the base plate and the ceramic plate. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: JP2020-23088A 
     SUMMARY OF INVENTION 
     In the meantime, the manufacturing of a semiconductor component using an electrostatic chuck may be performed at a low temperature of, for example, −60° C. or lower. When the electrostatic chuck is used at the low temperature of −60° C. or lower, the flexibility of the resin layer between the base plate and the ceramic plate in the electrostatic chuck may decrease. This is because a storage elastic modulus, which is a physical property value representing hardness of the adhesive forming the resin layer, rapidly increases near −60° C. and the adhesive is cured. When the flexibility of the resin layer decreases at the low temperature, the stress resulting from a difference in thermal expansion between the base plate and the ceramic plate is not sufficiently relaxed by the resin layer, and as a result, the resin layer may be destroyed. The destruction of the resin layer locally reduces a heat transfer characteristic between the base plate and the ceramic plate via the resin layer, and as a result, reduces temperature uniformity on a surface of the ceramic plate as an adsorption surface. 
     An object of the present disclosure is to provide an electrostatic chuck capable of suppressing destruction of a resin layer at low temperatures. 
     According to one aspect of the present disclosure, an electrostatic chuck includes a base plate, a ceramic plate, and a resin layer. The ceramic plate is fixed to the base plate, and is configured to adsorb a target object by an electrostatic force generated by energization to an embedded electrode. The resin layer is a resin layer for bonding the base plate and the ceramic plate, and is formed of one or more laminated adhesives, and at least one of the one or more adhesives has a temperature of −70° C. or lower corresponding to an extreme value of a loss tangent in a temperature range of −150° C. to 250° C. 
     According to one aspect of the electrostatic chuck of the present disclosure, an effect capable of suppressing destruction of the resin layer at low temperatures is exhibited. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a perspective view showing a configuration of an electrostatic chuck according to an embodiment. 
         FIG.  2    is a schematic view showing a cross section of the electrostatic chuck according to the embodiment. 
         FIG.  3    is a graph showing a measurement result of a loss tangent of an adhesive. 
         FIG.  4    illustrates an example of a relationship among a thickness of an adhesive forming a resin layer, delamination and temperature uniformity on an adsorption surface. 
         FIG.  5    is a schematic view showing a cross section of an electrostatic chuck according to a first modified embodiment of the embodiment. 
         FIG.  6    is a schematic view showing a cross section of an electrostatic chuck according to a second modified embodiment of the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, embodiments of the electrostatic chuck of the present disclosure will be described in detail with reference to the drawings. Note that, the disclosed technology is not limited to the embodiments. 
     Embodiment 
       FIG.  1    is a perspective view showing a configuration of an electrostatic chuck  100  according to an embodiment. The electrostatic chuck  100  shown in  FIG.  1    includes a base plate  110  and a ceramic plate  120 . 
     The base plate  110  is a circular member made of metal such as aluminum, for example. The base plate  110  is a base member of the electrostatic chuck  100 . A refrigerant passage through which a refrigerant such as cooling water passes is formed in the base plate  110 , so that temperatures of the ceramic plate  120  and a wafer or the like adsorbed to the ceramic plate  120  are adjusted. 
     The ceramic plate  120  is a circular member made of insulating ceramic. A diameter of the ceramic plate  120  is smaller than a diameter of the base plate  110 , and the ceramic plate  120  is fixed to a center of the base plate  110 . That is, one surface of the ceramic plate  120  serves as an adhesive surface that is bonded to the base plate  110 , and the adhesive surface is bonded to the base plate  110  by the resin layer, so that the ceramic plate  120  is fixed. A surface on an opposite side to the adhesive surface of the ceramic plate  120  is, for example, an adsorption surface for adsorbing a target object such as a wafer to be adsorbed. 
     A conductive electrode is arranged in the ceramic plate  120 , an electrostatic force is generated by energization to the electrode, and the target object is adsorbed to the adsorption surface of the ceramic plate  120  by the electrostatic force. 
     In addition, a heater electrode is arranged in the ceramic plate  120 , and the heater electrode generates heat when energized, thereby adjusting temperatures of the ceramic plate  120  and the target object such as a wafer to be adsorbed to the ceramic plate  120 . 
       FIG.  2    is a schematic view showing a cross section of the electrostatic chuck  100  according to the embodiment. In  FIG.  2   , a cross section taken along a line II-II in  FIG.  1    is shown. As shown in  FIG.  2   , the electrostatic chuck  100  is configured by bonding the ceramic plate  120  to the base plate  110  via the resin layer  130 . 
     The base plate  110  is, for example, a metallic member having a thickness of about 20 to 50 mm in which a refrigerant passage  111  becoming a passage of refrigerant such as cooling water and cooling gas is formed. The refrigerant passes through the refrigerant passage  111 , so that the ceramic plate  120  and the wafer adsorbed to the ceramic plate  120  are cooled. The ceramic plate  120  is cooled, so that the target object such as a wafer adsorbed to the ceramic plate  120  is cooled. 
     The ceramic plate  120  is, for example, a plate made of ceramic, having a thickness of about 1 mm to 10 mm and including an electrode  121  and a heater electrode  122  therein. The ceramic is obtained by, for example, firing a green sheet fabricated using aluminum oxide. A lower surface of the ceramic plate  120  is an adhesive surface to be bonded to the base plate  110 , and is bonded to an upper surface of the base plate  110  by the resin layer  130 . 
     When a voltage is applied to the electrode  121  of the ceramic plate  120 , the ceramic plate  120  adsorbs, for example, a target object such as a wafer by an electrostatic force. That is, in  FIG.  2   , an upper surface of the ceramic plate  120  serves as an adsorption surface, and when a voltage is applied to the electrode  121 , the target object is adsorbed to the adsorption surface. 
     In addition, when a voltage is applied to the heater electrode  122  of the ceramic plate  120 , the heater electrode  122  generates heat, so that the ceramic plate  120  is heated and the target object adsorbed to the ceramic plate  120  is heated. The heating by the heater electrode  122  and the cooling by the base plate  110  adjust the temperature of the ceramic plate  120  and adjust the temperature of the target object adsorbed to the ceramic plate  120  to a desired temperature. 
     As the electrode  121  and the heater electrode  122 , for example, a conductor such as tungsten can be used. 
     The resin layer  130  is, for example, a layer formed of a silicone resin-based adhesive and having a thickness of about 0.05 mm to 3.0 mm, and bonds the lower surface of the ceramic plate  120  to the upper surface of the base plate  110 . In the adhesive forming the resin layer  130 , a temperature of corresponding to an extreme value of a loss tangent is −70° C. or lower.  FIG.  3    is a graph showing a measurement result of the loss tangent of the adhesive forming the resin layer  130 .  FIG.  3    shows a measurement result obtained by dynamic viscoelasticity measurement (DMA) for each test piece of an adhesive A and an adhesive B used as the adhesive forming the resin layer  130 . 
     The measurement conditions of the measurement result shown in  FIG.  3    are as follows. 
     Measurement device: DMA6100 (made by Hitachi High-Tech Corporation) 
     Measurement temperature range: −150° C. to 250° C. 
     Temperature increase rate: 5° C./min 
     Measuring Mode: Tensile 
     Measurement frequency: 1 Hz 
     Shape of test piece: Rectangular shape 
     Dimensions of test piece: length 15 mm x width 15 mm x thickness 0.1 to 1 mm 
     Distortion amplitude: 10 μm 
     In the DMA, the complex elastic modulus (G*), the storage elastic modulus (G′), and the loss elastic modulus (G″) expressed by the following formula (1) were measured. 
         G*=G′+G″i   (1)
 
     Here, the storage elastic modulus (G′) is a value representing hardness of the viscoelastic body, and the loss elastic modulus (G′) is a value representing the viscosity of the viscoelastic body. 
     In addition, in the DMA, the loss tangent tan δ represented by the following formula (2) was calculated from the storage elastic modulus (G′) and the loss elastic modulus (G″). The loss tangent tan δ is a value representing a degree of contribution of viscosity to the viscoelastic body. The temperature corresponding to the extreme value of the loss tangent tan δ is also called the glass transition temperature (Tg). 
       tan δ= G″/G′   (2)
 
     As shown in  FIG.  3   , the measurement data obtained from the test piece shows that, for the adhesive A, the temperature corresponding to the extreme value of the loss tangent tan δ is −102.5° C., and for the adhesive B, the temperature corresponding to the extreme value of the loss tangent tan δ is −118.5° C. That is, it can be seen that the adhesive (adhesive A or adhesive B) forming the resin layer  130  has a temperature corresponding to the extreme value of the loss tangent, i.e., a glass transition temperature (Tg) of −70° C. or lower. 
     When the glass transition temperature (Tg) of the resin layer  130  is higher than −70° C., the flexibility of the resin layer  130  at a low temperature of −60° C. or lower, for example, may decrease. This is because the storage elastic modulus (G′) of the adhesive forming the resin layer  130  rapidly increases near −60° C. and the adhesive is cured. When the flexibility of the resin layer  130  decreases at the low temperature, the stress resulting from a difference in thermal expansion between the base plate  110  and the ceramic plate  120  is not sufficiently relaxed by the resin layer  130 , and as a result, the resin layer  130  may be destroyed. The destruction of the resin layer  130  locally lowers the heat transfer characteristic between the base plate  110  and the ceramic plate  120  via the resin layer  130 , and as a result, reduces temperature uniformity on the surface of the ceramic plate  120  as an adsorption surface. 
     In contrast, the resin layer  130  is formed by an adhesive having a glass transition temperature (Tg) of −70° C. or lower in the temperature range of −150° C. to 250° C., so that even when the electrostatic chuck  100  is used at the low temperature of −60° C. or lower, for example, the resin layer  130  maintains favorable flexibility. That is, since the glass transition temperature (Tg) of the resin layer  130  is −70° C. or lower, the storage modulus (G′) of the resin layer  130  at the low temperature of −60° C. or lower is maintained at the same level as the value at a room temperature and the curing of the resin layer  130  is suppressed. For this reason, even when the electrostatic chuck  100  is used at the low temperature, the resin layer  130  deforms, and therefore, can sufficiently relax the stress resulting from the difference in thermal expansion between the base plate  110  and the ceramic plate  120 . Thereby, the destruction of the resin layer  130  at the low temperature can be suppressed, and as a result, the temperature difference on the adsorption surface of the ceramic plate  120  becomes small, and the electrostatic chuck  100  can obtain the sufficiently high temperature uniformity. The glass transition temperature (Tg) of the adhesive forming the resin layer  130  is more preferably −100° C. or lower. 
     Further, the adhesive forming the resin layer  130  has a thermal conductivity of 0.5 W/mK or more at −60° C. For example, the adhesive A shown in  FIG.  3    has a thermal conductivity of 2.16 W/mK at −60° C., and the adhesive B has a thermal conductivity of 1.12 W/mK at −60° C. Note that, the thermal conductivity is calculated based on the thermal diffusivity, the specific heat capacity, and the density. The thermal diffusivity can be measured by, for example, a laser flash method, the specific heat capacity can be measured by, for example, an adiabatic continuous method, and the density can be measured by, for example, a weighing in liquid method. 
     If the thermal conductivity at −60° C. of the adhesive forming the resin layer  130  is less than 0.5 W/mK, the thermal conductivity of the resin layer  130  at the low temperature of −60° C. or lower, for example, is lowered. In contrast, the resin layer  130  is formed by an adhesive having a thermal conductivity of 0.5 W/mK or more at −60° C., so that a decrease in thermal conductivity of the resin layer  130  at the low temperature can be suppressed. The thermal conductivity at −60° C. of the adhesive forming the resin layer  130  is preferably 1 W/mK or more. 
     On the other hand, when the thickness of the resin layer  130  is thin, the resin layer  130  cannot sufficiently relax stress caused by bonding of different materials, which leads to destruction of the resin layer  130 . Therefore, the inventors of the present application conducted an experiment to investigate a relationship among the thickness of the adhesive forming the resin layer  130 , delamination between layers (delamination), and temperature uniformity on the adsorption surface. A result of this experiment is shown in  FIG.  4   .  FIG.  4    illustrates an example of the relationship among the thickness of the adhesive forming the resin layer  130 , delamination and temperature uniformity on the adsorption surface. Note that, delamination between layers (delamination) refers to delamination between the resin layer  130  and the ceramic plate  120 , for example. 
     In the experiment shown in  FIG.  4   , the thickness of the adhesive forming the resin layer  130  was set to 4 types, a thermal cycle test in a predetermined temperature range was conducted 1000 cycles, and the electrostatic chuck  100  after the thermal cycle test was evaluated as to the presence or absence of occurrence of delamination and the temperature difference on the adsorption surface. In this experiment, the thickness of the adhesive forming the resin layer  130  was set to four types of 0.3 mm, 0.25 mm, 0.2 mm, and 0.15 mm. In addition, in this experiment, the temperature range in the thermal cycle test was set to −40° C. to 60° C. 
     As shown in  FIG.  4   , when the thickness of the adhesive was 0.25 mm or more, delamination between the resin layer  130  and the ceramic plate  120  did not occur. In addition, when the thickness of the adhesive was 0.25 mm or more, the temperature difference on the adsorption surface of the ceramic plate  120  was smaller, as compared with a case where the thickness of the adhesive was less than 0.25 mm. That is, it can be seen from the result of  FIG.  4    that, when the thickness of the adhesive forming the resin layer  130  is 0.25 mm or more, the occurrence of delamination can be suppressed and high temperature uniformity on the adsorption surface can be maintained. Therefore, it is preferable that the thickness of the adhesive forming the resin layer  130  is 0.25 mm or more. 
     Modified Embodiments 
     Note that, in the above embodiment, the case where the resin layer  130  bonding the base plate  110  and the ceramic plate  120  is formed by the adhesive of one layer has been shown as an example. However, the bonding layer may be formed by laminating a plurality of adhesives. That is, the bonding layer may be formed of one or more laminated adhesives. Specifically, for example, as shown in  FIG.  5   , in an electrostatic chuck  100 A according to a first modified embodiment, a resin layer  130 A may be formed by laminating a first adhesive  131  and a second adhesive  132 .  FIG.  5    is a schematic view showing a cross section of the electrostatic chuck  100 A according to the first modified embodiment. The first adhesive  131  is applied to the base plate  110 . The second adhesive  132  is applied between the first adhesive  131  and the ceramic plate  120 . The first adhesive  131  and the second adhesive  132  each have a temperature corresponding to the extreme value of the loss tangent, i.e., a glass transition temperature (Tg) of −70° C. or lower in the temperature range of −150° C. to 250° C. The glass transition temperature (Tg) of the first adhesive  131  and the glass transition temperature (Tg) of the second adhesive  132  may be the same or different. In this way, by applying the first adhesive  131  and the second adhesive  132  to form the resin layer  130 A, deformation of the resin layer  130 A at a low temperature is promoted and displacement in a horizontal direction is absorbed, so that the destruction of the resin layer  130  at the low temperature can be further suppressed. Note that, the resin layer  130  may be formed by laminating the sheet-like first adhesive  131  and the sheet-like second adhesive  132 . 
     In addition, when the bonding layer is formed of one or more laminated adhesives, at least one of the one or more adhesives may have a temperature of −70° C. or lower corresponding to the extreme value of the loss tangent in the temperature range of −150° C. to 250° C. For example, for only one of the one or more adhesives forming the bonding layer, the temperature corresponding to the extreme value of the loss tangent in the temperature range of −150° C. to 250° C. may be −70° C. or lower. A modified example in this case is shown in  FIG.  6   .  FIG.  6    is a schematic view showing a cross section of an electrostatic chuck  100 B according to a second modified embodiment. As shown in  FIG.  6   , in the electrostatic chuck  100 B according to the second modified embodiment, a resin layer  130 B is formed by laminating a sheet-like first adhesive  133  and a sheet-like second adhesive  132 . The first adhesive  133  is laminated on the base plate  110 . The second adhesive  132  is laminated between the first adhesive  133  and the ceramic plate  120 . The second adhesive  132  of the first adhesive  133  and the second adhesive  132  has a temperature of −70° C. or lower corresponding to the extreme value of the loss tangent in the temperature range of −150° C. to 250° C. On the other hand, in the case of the first adhesive  133 , the temperature corresponding to the extreme value of the loss tangent is higher than 70° C. In this case, the viscosity before thermosetting and the storage elastic modulus after thermosetting of the first adhesive  133  may be greater than those of the second adhesive  132 . Thereby, since the hardness of the first adhesive  133  is maintained as appropriate, the thickness of the resin layer  130 B can be appropriately adjusted by the thickness of the first adhesive  133 . In the example of  FIG.  6   , the thickness of the first adhesive  133  is greater than the thickness of the second adhesive  132 . For example, the thickness of the first adhesive  133  is about 0 to 1 mm, and the thickness of the second adhesive  132  is about 0.05 to 0.5 mm. 
     Note that, in the examples of  FIG.  5    and  FIG.  6   , the resin layers  130 A and  130 B were formed by laminating adhesives of two layers. However, the resin layer may be formed by laminating adhesives of three or more layers. 
     Other Modified Embodiments 
     Note that, in the electrostatic chuck  100  described above, the thermal conductivity of the resin layer  130  may be improved by containing a filler such as aluminum oxide, silicon carbide or zinc oxide in the resin layer  130 . In addition, from a viewpoint of improving the adhesiveness between the base plate  110  and the ceramic plate  120  by the resin layer  130 , a silane coupling layer may be provided on the upper surface and the lower surface of the resin layer  130 , as required. 
     As described above, the electrostatic chuck (for example, electrostatic chuck  100 ) according to the embodiment includes a base plate (for example, base plate  110 ), a ceramic plate (for example, ceramic plate  120 ), and a resin layer (for example, a resin layer  130 ;  130 A;  130 B). The ceramic plate is fixed to the base plate, and is configured to adsorb a target object by an electrostatic force generated by energization to an embedded electrode. The resin layer is a resin layer for bonding the base plate and the ceramic plate, and is formed of one or more laminated adhesives, and at least one of the one or more adhesives has a temperature of −70° C. or lower corresponding to an extreme value of a loss tangent in a temperature range −150° C. to 250° C. Thereby, according to the electrostatic chuck of the embodiment, it is possible to suppress destruction of the resin layer at a low temperature, and as a result, to obtain sufficiently high temperature uniformity. 
     In addition, at least one of the one or more adhesives may have a thermal conductivity of 0.5 W/mK or more at −60° C. Thereby, according to the electrostatic chuck of the embodiment, it is possible to suppress decrease in thermal conductivity of the resin layer at the low temperature. 
     Further, at least one of the one or more adhesives may contain a filler. Thereby, according to the electrostatic chuck of the embodiment, it is possible to improve the thermal conductivity of the resin layer. 
     Further, the resin layer may include a first adhesive (for example, first adhesive  131 ;  133 ) applied to the base plate, and a second adhesive (for example, second adhesive  132 ) applied between the first adhesive and the ceramic plate. At least one of the first adhesive and the second adhesive may have a temperature of −70° C. or lower corresponding to the extreme value of the loss tangent in the temperature range of −150° C. to 250° C. Thereby, according to the electrostatic chuck of the embodiment, deformation of the resin layer at the low temperature is promoted and displacement in a horizontal direction is absorbed, so that the destruction of the resin layer at the low temperature can be further suppressed. 
     Further, one of the first adhesive and the second adhesive may have the temperature of −70° C. or lower corresponding to the extreme value of the loss tangent in the temperature range of −150° C. to 250° C. In addition, the viscosity before thermosetting and the storage elastic modulus after thermosetting of the other of the first adhesive and the second adhesive may be greater than those of one of the first adhesive and the second adhesive. Thereby, according to the electrostatic chuck of the embodiment, it is possible to appropriately adjust the thickness of the resin layer. 
     Further, the thickness of the other of the first adhesive and the second adhesive may be greater than the thickness of one of the first adhesive and the second adhesive. Thereby, according to the electrostatic chuck of the embodiment, it is possible to appropriately adjust the thickness of the resin layer.