Electrostatic chuck

Abstract

To provide an electrostatic chuck that can optimize a value of Young's modulus of a bonding layer and can reduce the thermal stress applied to a dielectric substrate.SOLUTION: An electrostatic chuck 10 includes: a dielectric substrate 100 in which through-holes 140 are formed; a base plate 200 formed of a metal material; and a bonding layer 300 that bonds the dielectric substrate 100 and the base plate 200. If E (MPa) refers to Young's modulus of the bonding layer 300 and X (mm) refers to a distance between a central axis AX0 of the dielectric substrate 100 and a central axis AX1 of a through-hole 140, then E≤0.2063×X2-59.3887×X+4278.8065 is satisfied for the through-hole 140 positioned farthest from the central axis AX0.SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrostatic chuck. [Background technology]

[0002] For example, in semiconductor manufacturing equipment such as an etching device, an electrostatic chuck is provided as a device for attracting and holding a substrate such as a silicon wafer to be processed. The electrostatic chuck includes a dielectric substrate provided with an attracting electrode and a base plate for supporting the dielectric substrate, which are joined together. When a voltage is applied to the attracting electrode, an electrostatic force is generated, and the substrate placed on the dielectric substrate is attracted and held.

[0003] During processing, the temperature of the substrate rises due to exposure to plasma, and the temperature of the dielectric substrate also rises. Meanwhile, because a low-temperature coolant is supplied to the base plate, the temperature of the base plate may drop to -60°C or even lower. The dielectric substrate is subjected to a large thermal stress due to temperature changes in each part associated with substrate processing and the temperature difference between the dielectric substrate and the base plate.

[0004] In order to prevent the dielectric substrate from being damaged by thermal stress, it is necessary to select a material with appropriate physical properties as the material for the bonding layer connecting the dielectric substrate and the base plate. For example, Patent Document 1 below proposes that the storage modulus of the bonding layer (adhesive member) at -60°C be 100 MPa or less. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] JP 2020-23088 A Summary of the Invention [Problem to be solved by the invention]

[0006] If a material with as small a Young's modulus as possible is used as the material for the bonding layer, the thermal stress applied to the dielectric substrate can be kept small. However, in consideration of the heat transfer performance required for the bonding layer, the Young's modulus of the bonding layer cannot be made as small as possible. The material for the bonding layer must be appropriately selected in consideration of the required heat transfer performance, etc., under the condition that the Young's modulus is equal to or less than a predetermined upper limit.

[0007] Meanwhile, the dielectric substrate is formed with a through hole for the purpose of supplying a cooling gas or the like. According to the experiments conducted by the present inventors, it has been found that the thermal stress applied to the dielectric substrate is particularly large in the through hole portion, and the magnitude of the stress varies depending on the position of the through hole. Specifically, when the distance between the central axis of the dielectric substrate and the central axis of the through hole is X, the larger the value of X, the larger the thermal stress applied to the through hole portion. In other words, the larger the value of X, the narrower the upper limit of the allowable range for the Young's modulus of the bonding layer becomes.

[0008] Therefore, in order to suppress the thermal stress applied to the dielectric substrate while satisfying the required specifications such as heat transfer performance, it is necessary to appropriately set each parameter while considering the correlation between "the distance between the central axis of the dielectric substrate and the central axis of the through hole" and "the upper limit value of the Young's modulus of the bonding layer." However, no specific study has been conducted so far on how to consider these correlations.

[0009] The present invention has been made in consideration of the above problems, and an object of the present invention is to provide an electrostatic chuck that has an appropriate Young's modulus value for a bonding layer and can reduce thermal stress applied to a dielectric substrate. [Means for solving the problem]

[0010] In order to solve the above problems, an electrostatic chuck according to the present invention includes a dielectric substrate having a through hole formed therein, a base plate made of a metal material, and a bonding layer bonding the dielectric substrate and the base plate together. In this electrostatic chuck, when the Young's modulus of the bonding layer is E (MPa) and the distance between the central axis of the dielectric substrate and the central axis of the through hole is X (mm), the Young's modulus of the bonding layer satisfies E≦0.2063×X 2 -59.3887×X+4278.8065 holds.

[0011] In many cases, the through hole located at the farthest position from the central axis of the dielectric substrate is the part of the dielectric substrate to which the greatest thermal stress is applied. 2 It has been found that if the position (X) of the through hole and the Young's modulus (E) of the bonding layer are set so as to satisfy the condition of -59.3887 x X + 4278.8065, the thermal stress in the through hole portion of the dielectric substrate can be sufficiently reduced. By configuring the electrostatic chuck as described above, it is possible to reduce the thermal stress applied to the dielectric substrate while keeping the Young's modulus value of the bonding layer appropriate.

[0012] The above formula for the Young's modulus of the bonding layer may be valid not only for the through hole located farthest from the central axis of the dielectric substrate, but also for all through holes formed in the dielectric substrate. Effect of the Invention

[0013] According to the present invention, it is possible to provide an electrostatic chuck in which the Young's modulus of the bonding layer is set to an appropriate value and the thermal stress applied to the dielectric substrate can be reduced. [Brief description of the drawings]

[0014] [Figure 1] 1 is a cross-sectional view illustrating a schematic configuration of an electrostatic chuck according to an embodiment of the present invention. [Diagram 2] 2 is a diagram showing a configuration of a dielectric substrate provided in the electrostatic chuck of FIG. 1. [Diagram 3] FIG. 2 is a diagram showing the actual shape of an electrostatic chuck according to the present embodiment. [Figure 4] FIG. 13 is a diagram showing the relationship between the Young's modulus of the bonding layer and the maximum stress generated in the dielectric substrate. [Diagram 5] FIG. 13 is a diagram showing the relationship between the position of a through hole and the allowable Young's modulus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0015] Hereinafter, the present embodiment will be described with reference to the accompanying drawings. In order to facilitate understanding of the description, the same components in each drawing are denoted by the same reference numerals as much as possible, and duplicated description will be omitted.

[0016] The electrostatic chuck 10 according to this embodiment is configured to electrostatically attract and hold a substrate W to be processed inside a semiconductor manufacturing apparatus (not shown), such as an etching apparatus. The substrate W is, for example, a silicon wafer. The electrostatic chuck 10 may be used in an apparatus other than a semiconductor manufacturing apparatus.

[0017] 1 shows, in a schematic cross-sectional view, the configuration of an electrostatic chuck 10 in a state in which the electrostatic chuck 10 attracts and holds a substrate W. The electrostatic chuck 10 includes a dielectric substrate 100, a base plate 200, and a bonding layer 300.

[0018] The dielectric substrate 100 is a substantially disk-shaped member made of a sintered ceramic body. The dielectric substrate 100 contains, for example, high-purity aluminum oxide (Al2O3), but may contain other materials. The purity, type, and additives of the ceramics in the dielectric substrate 100 can be appropriately set in consideration of the plasma resistance and other properties required of the dielectric substrate 100 in semiconductor manufacturing equipment.

[0019] The upper surface 110 of the dielectric substrate 100 in FIG. 1 is a "mounting surface" on which the substrate W is placed. The lower surface 120 of the dielectric substrate 100 in FIG. 1 is a "bonded surface" that is bonded to the base plate 200 via a bonding layer 300 described later. The viewpoint when the electrostatic chuck 10 is viewed from the surface 110 side along a direction perpendicular to the surface 110 is hereinafter also referred to as "top view". The thickness of the dielectric substrate 100, i.e., the distance between the surface 110 and the surface 120, is 0.9 mm in this embodiment, but may be a different thickness.

[0020] An adsorption electrode 130 is embedded inside the dielectric substrate 100. The adsorption electrode 130 is a thin, flat layer made of a metal material such as tungsten, and is disposed so as to be parallel to the surface 110. The material of the adsorption electrode 130 may be molybdenum, platinum, palladium, or the like, in addition to tungsten. When a voltage is applied to the adsorption electrode 130 from the outside via the power supply path 13, an electrostatic force is generated between the surface 110 and the substrate W, thereby adsorbing and holding the substrate W. Two adsorption electrodes 130 may be provided as so-called "bipolar" electrodes as in this embodiment, or only one may be provided as a so-called "monopolar" electrode.

[0021] In Fig. 1, the entire power supply line 13 is depicted in a simplified manner. A portion of the power supply line 13 inside the dielectric substrate 100 is configured as, for example, a long and narrow via (hole) filled with a conductor, and an electrode terminal (not shown) is provided at the lower end thereof. A portion of the power supply line 13 penetrating a base plate 200 described below is a conductive metal member (for example, a bus bar) having one end connected to the electrode terminal. A through hole (not shown) for inserting the power supply line 13 is formed in the base plate 200. For example, a cylindrical insulating member may be provided between the inner surface of the through hole and the power supply line 13.

[0022] 1, a space SP is formed between the dielectric substrate 100 and the substrate W. When a process such as etching is performed in the semiconductor manufacturing apparatus, helium gas for temperature adjustment is supplied from the outside to the space SP via a through hole 140 described below. By providing helium gas between the dielectric substrate 100 and the substrate W, the thermal resistance between them is adjusted, and the temperature of the substrate W is thereby maintained at an appropriate temperature. The temperature adjustment gas supplied to the space SP may be a type of gas other than helium.

[0023] 2 is a top view of the dielectric substrate 100. As shown in the figure, a seal ring 111 and dots 112 are provided on a surface 110 that is a mounting surface, and the above-mentioned space SP is formed around these.

[0024] The seal rings 111 are walls that divide the space SP, and a plurality of seal rings 111 are provided so as to be arranged concentrically when viewed from above. The upper end of each seal ring 111 forms part of the surface 110 and abuts against the substrate W. In this embodiment, a total of four seal rings 111 are provided, thereby dividing the space SP into four. With this configuration, it is possible to individually adjust the pressure of the helium gas in each space SP and make the surface temperature distribution of the substrate W during processing closer to uniform.

[0025] 1 and 2, the portion marked with the reference symbol "116" is the bottom surface of the space SP. Hereinafter, this portion will also be referred to as the "bottom surface 116." The seal ring 111, together with the dots 112 described below, is formed by digging down a part of the surface 110 to the position of the bottom surface 116.

[0026] The dots 112 are circular protrusions protruding from the bottom surface 116. As shown in Fig. 2, a plurality of dots 112 are provided and are distributed approximately evenly on the mounting surface of the dielectric substrate 100. The upper end of each dot 112 forms part of the surface 110 and abuts against the substrate W. By providing a plurality of such dots 112, bending of the substrate W is suppressed.

[0027] Grooves 113 are formed in the bottom surface 116 of the space SP. Grooves 113 are formed so as to recede further from the bottom surface 116 toward the surface 120. Grooves 113 are formed for the purpose of quickly diffusing the helium gas supplied from the through holes 140 into the space SP and making the pressure distribution in the space SP approximately uniform within a short period of time. Depending on the number and arrangement of the through holes 140, grooves 113 may not be necessary.

[0028] The dielectric substrate 100 is formed with through holes 140 extending vertically from the surface 120 toward the surface 110. As shown in Fig. 2, the end of the through hole 140 on the surface 110 side opens at the bottom surface of the groove 113. A plurality of through holes 140 are formed in the dielectric substrate 100, and these are lined up along the groove 113. In this embodiment, a plurality of through holes 140 are connected to each of the four divided spaces SP.

[0029] 2, for convenience of illustration, the diameter of the through hole 140 is drawn to be larger than the width of the groove 113, but as shown in Fig. 1, the actual diameter of the through hole 140 is smaller than the width of the groove 113. The width of the groove 113 may be locally larger at the position of the through hole 140 so that the through hole 140 fits inside the groove 113. The diameter of the through hole 140 at the end opposite to the bonding layer 300 (i.e., the end on the surface 110 side) is 0.2 mm or less in this embodiment.

[0030] As shown in FIG. 1, the portion of the through hole 140 on the surface 120 side is enlarged in diameter compared to the portion on the surface 110 side, and a vent plug 145 is disposed inside the portion. The vent plug 145 is a porous body made of, for example, alumina, and has air permeability as a whole. By disposing such a vent plug 145 inside the through hole 140, it is possible to suppress the occurrence of dielectric breakdown in the path through the through hole 140 while ensuring the flow of gas in the through hole 140. Note that, if dielectric breakdown can be sufficiently prevented, the through hole 140 may be configured without disposing the vent plug 145. In this case, the inner diameter of the through hole 140 may be uniform throughout the entire vertical direction.

[0031] In FIG. 2, the reference numeral "115" denotes a through hole through which a lift pin (not shown) provided in the semiconductor manufacturing apparatus is inserted. The through hole is also referred to as "through hole 115" below. Like the above-mentioned through hole 140, the through hole 115 is formed so as to extend vertically from the surface 120 toward the surface 110. A total of three through holes 115 are formed, and are arranged so as to be equidistant from each other by 120 degrees. The substrate W is attached to and detached from the surface 110 of the dielectric substrate 100 by the lift pin that moves up and down through the through hole 115.

[0032] The base plate 200 is a substantially disk-shaped member that supports the dielectric substrate 100. The base plate 200 is formed of a metal material such as aluminum. The upper surface 210 of the base plate 200 in FIG. 1 is a "bonded surface" that is bonded to the dielectric substrate 100 via a bonding layer 300.

[0033] 1, the base plate 200 has through holes 240 formed therein, which extend vertically from the surface 210 toward the surface 220 on the opposite side. The through holes 240 are formed at positions overlapping with the through holes 140 of the dielectric substrate 100 in a top view, and communicate with the through holes 140 via through holes 310 provided in the bonding layer 300. The through holes 240, together with the through holes 140 of the dielectric substrate 100, form part of a path for supplying helium gas toward the space SP.

[0034] As shown in FIG. 1, the portion of the through hole 240 on the surface 210 side is enlarged in diameter compared to the portion on the surface 220 side, and a vent plug 245 is disposed inside the portion. The vent plug 245 is a porous body made of, for example, alumina, and has air permeability as a whole. By disposing such a vent plug 245 inside the through hole 240, it is possible to suppress the occurrence of dielectric breakdown in the path through the through hole 240 while ensuring the flow of gas in the through hole 240. Note that, if dielectric breakdown can be sufficiently prevented, the through hole 240 may be configured not to have the vent plug 245 disposed therein. In this case, the inner diameter of the through hole 240 may be uniform throughout the entire vertical direction.

[0035] The through-holes 240 may be formed so as to extend linearly as a whole as in this embodiment, but may be formed so as to bend on the way to the surface 220. Also, the multiple through-holes 240 on the surface 210 side may be aggregated into a small number of flow paths inside the base plate 200, and the flow paths may be configured to extend to the surface 220 side.

[0036] A coolant flow path 250 for flowing a coolant is formed inside the base plate 200. When a process such as film formation is performed in the semiconductor manufacturing apparatus, a coolant is supplied to the coolant flow path 250 from the outside, thereby cooling the base plate 200. Heat generated in the substrate W during the process is transferred to the coolant via the helium gas in the space SP, the dielectric substrate 100, and the base plate 200, and is discharged to the outside together with the coolant.

[0037] In the base plate 200, through holes (not shown) for passing lift pins are formed at positions overlapping with the through holes 115 in a top view.

[0038] An insulating film may be formed on the surface of the base plate 200. The insulating film is preferably formed on the surface of the base plate 200, at least in a range including the entire face 210. As the insulating film, for example, an alumina film formed by thermal spraying can be used. By covering the surface of the base plate 200 with an insulating film, the dielectric strength of the base plate 200 can be increased.

[0039] In FIG. 1, the diameter of the surface 120 of the dielectric substrate 100 and the diameter of the surface 210 of the base plate 200 are illustrated as being equal to each other, but they are often different from each other. FIG. 3 shows the actual configuration of the electrostatic chuck 10. As shown in the figure, in the electrostatic chuck 10 of this embodiment, the diameter of the surface 120 of the dielectric substrate 100 is slightly larger than the diameter of the surface 210 of the base plate 200. Therefore, the outer peripheral portion of the dielectric substrate 100 protrudes outward from the base plate 200 and is not supported from the lower side. However, this configuration is merely an example, and the above-mentioned diameters may be the same as each other.

[0040] The bonding layer 300 is a layer provided between the dielectric substrate 100 and the base plate 200, and bonds them together. The bonding layer 300 is formed by curing an adhesive made of an insulating material. In this embodiment, a silicone adhesive is used as the adhesive. However, the bonding layer 300 may be formed by curing another type of adhesive. In either case, it is preferable to use a material with as high a thermal conductivity as possible as the material for the bonding layer 300 so that the thermal resistance between the dielectric substrate 100 and the base plate 200 is small. The bonding layer 300 may be configured such that a plurality of particulate fillers (fillers) are arranged inside the bonding layer 300 to increase the thermal conductivity. For example, particles mainly composed of alumina can be used as the filler.

[0041] The thickness of the bonding layer 300 can be appropriately set according to the heat transfer performance required for the bonding layer 300. In this embodiment, the thickness of the bonding layer 300 is 250 μm, but it may be a different thickness. In addition, the thickness of the bonding layer 300 may not be uniform as a whole, such as when an electrode terminal is embedded on the surface 120 side of the dielectric substrate 100 or when a groove is formed in a part of the surface 210 of the base plate 200. In this case, the "thickness of the bonding layer 300" refers to the thickness of the range excluding the part where the thickness is locally different from the others as described above.

[0042] The bonding layer 300 has a smaller Young's modulus (modulus of longitudinal elasticity) than the dielectric substrate 100 and the base plate 200. Even if a difference in thermal expansion occurs between the dielectric substrate 100 and the base plate 200, the bonding layer 300 deforms and absorbs the difference in thermal expansion, so that the thermal stress in the dielectric substrate 100 etc. can be kept small.

[0043] The Young's modulus of the bonding layer 300 changes depending on the temperature of the bonding layer 300. The Young's modulus of the bonding layer 300 when the temperature of the bonding layer 300 is −60° C. is expressed in units of “MPa” and is hereinafter referred to as “E”. For example, when the Young's modulus of the bonding layer 300 at −60° C. is 0.01 GPa, E=10. Note that the temperature of “−60° C.” in the above description is merely a convenient standard for identifying the physical property (Young's modulus) of the bonding layer 300, and does not limit the temperature of the coolant actually supplied to the coolant flow path 250.

[0044] In recent years, there is a tendency that, due to an increase in the energy incident on the substrate W during processing, a higher cooling performance than before is required for the base plate 200. For example, a coolant at a temperature of -60°C or lower may be supplied to the coolant flow passage 250 of the base plate 200. As the plasma output increases, the coolant temperature will become even lower, and it is possible that a coolant at about -100°C will be supplied in the future.

[0045] Large thermal stress is applied to the dielectric substrate 100 due to temperature changes in various parts accompanying the start of processing of the substrate W, the temperature difference between the dielectric substrate 100 and the base plate 200, etc. According to experiments conducted by the present inventors, it has been found that the thermal stress applied to the dielectric substrate 100 is particularly large in the through-hole portion (e.g., the outlet portion of the through-hole 140), and the magnitude of the thermal stress varies depending on the position of the through-hole.

[0046] For ease of explanation, the central axis of the dielectric substrate 100 will also be referred to as the "central axis AX0" hereinafter (see FIG. 1). The central axis AX0 passes through the center of the dielectric substrate 100, which is circular in top view, and is perpendicular to the surface 110. Note that, for example, an orientation flat or the like may be formed on a part of the outer periphery of the dielectric substrate 100, and the shape of the dielectric substrate 100 in top view may not be strictly circular. In this case, it is assumed that the orientation flat or the like is not formed, and the overall shape of the dielectric substrate 100 is considered to be circular, and the central axis AX0 is defined as the axis passing through the center of the circle.

[0047] Moreover, the central axis of through hole 140 will be referred to as "central axis AX1" hereinafter (see FIG. 1). As described above, through hole 140 is a hole formed to extend perpendicularly from surface 120 toward surface 110. Therefore, central axis AX1 is an axis perpendicular to surface 110, similar to central axis AX0. Central axis AX1 is defined individually for each of the multiple through holes 140.

[0048] The distance from the central axis AX0 of the dielectric substrate 100 to the central axis AX1 of the through hole 140, expressed in units of "mm", is hereinafter referred to as "X" or "distance X". The distance X is defined individually for each of the multiple through holes 140. According to experiments conducted by the present inventors, the greater the distance X from the central axis AX0 to the central axis AX1 of the through hole 140, the greater the magnitude of the thermal stress applied to the portion of the through hole 140 tends to be. In other words, in many cases, the portion of the dielectric substrate 100 that is subjected to the greatest thermal stress is the portion of the through hole 140 formed at the outermost periphery.

[0049] If a material with as small a Young's modulus as possible is used as the material for the bonding layer 300, the thermal stress applied to the dielectric substrate 100 can be kept small. However, in consideration of the heat transfer performance, etc. required for the bonding layer 300, the Young's modulus of the bonding layer 300 cannot be made as small as possible. The material for the bonding layer 300 needs to be appropriately selected in consideration of the required heat transfer performance, etc., under the condition that the Young's modulus is equal to or less than a predetermined upper limit.

[0050] The smaller the distance X from the central axis AX0 of the dielectric substrate 100 to the central axis AX1 of the through hole 140, the smaller the thermal stress applied to the through hole 140, and therefore the larger the upper limit of the permissible Young's modulus (E). In this way, the position (X) of the through hole 140 and the upper limit of the Young's modulus (E) of the bonding layer 300 are parameters that are correlated with each other. The present inventors have been able to obtain the following new findings regarding the above correlation by performing various experiments and analyses.

[0051] The four graphs shown in FIG. 4 show the relationship between the Young's modulus of the bonding layer 300 at −60° C. (horizontal axis) and the maximum stress generated in the dielectric substrate 100 (vertical axis).

[0052] For example, the graph with "X1" on the right side was obtained by performing an analysis on a dielectric substrate 100 in which the through hole 140 located at the position farthest from the central axis AX0 (i.e., the outermost circumference) is X=X1, changing the Young's modulus of the bonding layer 300 (specifically, Young's modulus at -60°C), and plotting the maximum value of the thermal stress (hereinafter also referred to as "maximum stress") generated in the dielectric substrate 100 at low temperatures.

[0053] The above "at low temperature" specifically refers to a time when the bonding layer 300 is cured when the temperature of the entire electrostatic chuck 10 is 40° C., and then the temperature of the entire electrostatic chuck 10 is lowered to -60° C. In both analyses, the "maximum value of stress generated in the dielectric substrate 100" was the value of stress generated in the portion near the end of the surface 110 side of the through hole 140 arranged on the outermost periphery (i.e., the through hole 140 where X=X1).

[0054] The graphs labeled "X2" through "X4" on the right side were also obtained based on the same analysis as above. X2 is a distance greater than X1, X3 is a distance greater than X2, and X4 is a distance greater than X3.

[0055] 4, it can be seen that the maximum stress generated in the dielectric substrate 100 at low temperatures increases as the Young's modulus of the bonding layer 300 increases. In addition, when comparing under conditions where the Young's modulus of the bonding layer 300 is the same, it can be seen that the maximum stress generated in the dielectric substrate 100 at low temperatures tends to increase as the distance X of the through holes 140 arranged at the outermost periphery increases.

[0056] However, the maximum stress when X=X4 is slightly smaller than the maximum stress when X=X3, which is the opposite of the above trend. This is because the position of the through hole 140 corresponding to X4 is on the outer periphery side of the surface 210 of the base plate 200, that is, a position where the lower side of the dielectric substrate 100 is not supported by the base plate 200. In reality, it is considered unlikely that the through hole 140 will be formed at such a position.

[0057] The "threshold" shown on the vertical axis of FIG. 4 is the upper limit of the range of maximum stress that will not cause damage to the dielectric substrate 100.

[0058] "E1" in FIG. 4 is the value of Young's modulus (E) at which the maximum stress value is the threshold value in the dielectric substrate 100 where the distance X of the through-hole 140 at the outermost periphery is X1. "E2" is the value of Young's modulus (E) at which the maximum stress value is the threshold value in the dielectric substrate 100 where the distance X of the through-hole 140 at the outermost periphery is X2. The same applies to "E3" and "E4". Each of E1 to E4 can be said to be the upper limit value of the allowable range of the Young's modulus (E) value of the bonding layer 300 so as not to cause damage to the dielectric substrate 100. Such an upper limit value of the Young's modulus is also referred to as "allowable Young's modulus" below. As is clear from FIG. 4, the allowable Young's modulus (E1 to E4) is a different value depending on the distance X (X1 to X4) of the through-hole 140 at the outermost periphery.

[0059] The relationship between the distance X (horizontal axis) of the through holes 140 at the outermost periphery and the allowable Young's modulus (vertical axis) is as shown in the graph in FIG. 5. As shown in the figure, it can be seen that there is a correlation between the two in the shape of a roughly double curve. As the distance X increases, the allowable Young's modulus gradually decreases. However, when the distance X becomes large to a certain extent, the allowable Young's modulus increases conversely as the distance X increases. This tendency to increase occurs because the outermost periphery of the dielectric substrate 100 is not supported from below by the base plate 200. The relationship between the two shown in FIG. 5 can be expressed by the following formula (1). Allowable Young's modulus = 0.2063 x X 2 -59.3887×X+4278.8065···(1)

[0060] Therefore, if the Young's modulus E of the bonding layer 300 and the distance X of the outermost through holes 140 satisfy the condition shown in the following formula (2), the value of the maximum stress generated in the dielectric substrate 100 will be below the "threshold" in FIG. 4, and damage to the dielectric substrate 100 will be reliably prevented. E≦0.2063×X 2 -59.3887×X+4278.8065···(2)

[0061] For example, if the outer diameter of the dielectric substrate 100 is different from that of this embodiment, the coefficients of the formulas (1) and (2) calculated by the above method will also have values ​​that are different from those in the above in the strict sense. However, since the "threshold value" shown in Fig. 4 is set with a certain margin in mind, even if the configuration of the electrostatic chuck 10 is somewhat different from that of this embodiment, it is possible to make a determination by using formula (2) as it is.

[0062] As described above, the present inventors have been able to obtain the above formula (2) as a condition for making the distance X of the through-holes 140 at the outermost periphery and the Young's modulus E of the bonding layer 300 appropriate through the analysis and the like recently performed. If the positions of the through-holes 140 and the Young's modulus E of the bonding layer 300 are selected so as to satisfy the condition of the above formula (2), the thermal stress of the dielectric substrate 100 during processing of the substrate W can be reduced to a level that does not cause damage. The specific values ​​of E and X may be appropriately set within a range that satisfies the required specifications such as the heat transfer performance required for the bonding layer 300 and satisfies the above condition (2).

[0063] The group of through holes 140 that satisfies the condition of the above formula (2) may include at least the through hole 140 located farthest from the central axis AX0 of the dielectric substrate 100, but may also include other through holes 140. It is preferable that all the through holes 140 formed in the dielectric substrate 100 satisfy the condition of formula (2). With this configuration, damage to the dielectric substrate 100 due to thermal stress can be more reliably prevented.

[0064] It has been found that the smaller the diameter of the end of the through hole 140 or the like on the opposite side to the bonding layer 300 (i.e., the end on the surface 110 side), the greater the thermal stress generated in the dielectric substrate 100. A through hole for supplying an inert gas, such as the through hole 140, is often a small hole with a diameter of 0.2 mm or less to prevent dielectric breakdown. It has been confirmed by analysis, etc. that if the thickness and Young's modulus of the bonding layer 300 are set to satisfy the condition of the above formula (2), even if the diameter of the through hole is 0.2 mm or less, the thermal stress can be suppressed to a level that does not cause damage to the dielectric substrate 100. It is to be noted that the above description does not in any way deny the diameter of the through hole being 0.2 mm or more.

[0065] Various types of adhesives can be used as the bonding layer 300 as long as they can satisfy the condition of the above formula (2). For example, in addition to the silicone adhesive of this embodiment, various adhesives such as epoxy, polyimide, acrylic, and modified silicone resin can be used to form the bonding layer 300. However, since the Young's modulus of the silicone adhesive after curing is relatively small, it is easy to select one that satisfies the condition of the above formula (2) from various types of silicone adhesives. For this reason, as in this embodiment, it is preferable that the bonding layer 300 is made by curing the silicone adhesive.

[0066] As the adhesive for forming the bonding layer 300, a commercially available existing adhesive may be used as it is, but an existing adhesive with an adjusted Young's modulus may be used so as to satisfy the condition of formula (2). Various known methods can be adopted as a method for adjusting the Young's modulus of the adhesive. For example, by adding a functional group or a filler to the adhesive and adjusting the type and amount of each, the Young's modulus in a low temperature range such as -60 degrees can be changed to a desired value. As an example, when the adhesive is a silicone resin, the Young's modulus can be adjusted, particularly in a low temperature range, by adjusting the amount of phenyl group added. The Young's modulus can also be reduced by reducing the amount of inorganic filler added.

[0067] As described above, the greater the distance X of the through hole 140, the greater the thermal stress applied to the portion of the through hole 140. In a dielectric substrate 100 in which at least one through hole 140 is formed in an outer peripheral portion where X≧75 mm, the thermal stress applied to the portion of the through hole 140 is often large enough to be not negligible. However, if a material for the bonding layer 300 is selected so as to satisfy the condition of the above formula (2), the thermal stress can be suppressed to a level that does not cause damage to the dielectric substrate 100.

[0068] In this embodiment, the through hole formed on the outermost side among the multiple through holes formed in the dielectric substrate 100 is the through hole 140 (not the through hole 115). When another through hole is formed at a position further outside the through hole 140, the Young's modulus (E) of the bonding layer 300 may be set so that the position (X) of the through hole satisfies the condition of formula (2).

[0069] The present embodiment has been described above with reference to specific examples. However, the present disclosure is not limited to these specific examples. Any design modifications made by a person skilled in the art to these specific examples are also included within the scope of the present disclosure as long as they have the features of the present disclosure. The elements of each of the above-mentioned specific examples and their arrangements, conditions, shapes, etc. are not limited to those exemplified and can be changed as appropriate. The combination of each of the elements of each of the above-mentioned specific examples can be changed as appropriate as long as no technical contradiction occurs. [Explanation of symbols]

[0070] 10: Electrostatic chuck 100: Dielectric substrate 140:Through hole 200: Base plate 300: Bonding layer AX0,AX1: Center axis

Claims

1. a dielectric substrate having a through hole; A base plate formed of a metal material; a bonding layer that bonds the dielectric substrate and the base plate, The Young's modulus of the bonding layer is E (MPa), When the distance between the central axis of the dielectric substrate and the central axis of the through hole is X (mm), For the through hole located at the farthest position from the central axis of the dielectric substrate, E≦0.2063×X ２ -59.3887×X+4278.8065 The electrostatic chuck is characterized in that:

2. 2. The electrostatic chuck according to claim 1, wherein the through hole has a diameter of 0.2 mm or less at an end portion opposite to the bonding layer.

3. 2. The electrostatic chuck according to claim 1, wherein the bonding layer is a cured silicone adhesive.

4. 2. The electrostatic chuck according to claim 1, wherein at least one of the through holes is formed at a position where X is greater than or equal to 75 mm.

5. a dielectric substrate having a through hole; A base plate formed of a metal material; a bonding layer that bonds the dielectric substrate and the base plate, The Young's modulus of the bonding layer is E (MPa), When the distance between the central axis of the dielectric substrate and the central axis of the through hole is X (mm), For all of the through holes, E≦0.2063×X ２ -59.3887×X+4278.8065 The electrostatic chuck is characterized in that:

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