Patent Publication Number: US-2022223454-A1

Title: Substrate fixing device

Description:
This application claims priority from Japanese Patent Applications No. 2021-004453, filed on Jan. 14, 2021, and No. 2021-104338, filed on Jun. 23, 2021, the entire contents of which are herein incorporated by reference. 
     BACKGROUND 
     Technical Field 
     The present disclosure relates to a substrate fixing device. 
     Background Art 
     In a substrate fixing device used for fixation of a substrate such as a wafer, an electrostatic adsorption member is typically adhesively bonded to a base plate by an adhesive layer (see e.g., WO2016/035878, JP-A-2014-207374, WO2009/107701, JP-A-2011-091297, JP-A-2020-023088. JP-A-2015-061913, JP-A-2019-220503, and JP-A-2007-299837). 
     The substrate fixing device according to the background art may not be able to obtain sufficient heat uniformity in an adsorption face of the electrostatic adsorption member. In other words, temperature of the adsorption face of the electrostatic adsorption member may vary. 
     SUMMARY 
     One of aspects of the present disclosure is to provide a substrate fixing device that can improve uniformity of temperature of an adsorption face of an electrostatic adsorption member. 
     A certain embodiment provides a substrate fixing device comprising: a base plate; an electrostatic adsorption member that adsorbs and holds a substrate; and a first adhesive layer that adhesively bonds the electrostatic adsorption member to the base plate. A storage modulus of the first adhesive layer is not less than 0.01 MPa and not more than 25 MPa within a temperature range of −110° C. to 250° C. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a sectional view showing a substrate fixing device according to a first embodiment; 
         FIG. 2  is a graph showing relationship between temperature and storage modulus in examples of an adhesive layer; 
         FIG. 3  is a graph showing the relationship between the temperature and the storage modulus in the examples of the adhesive layer and an example of an auxiliary adhesive layer; 
         FIG. 4  is a graph showing relationship between temperature and storage modulus in another example of the adhesive layer; 
         FIG. 5  is a graph showing results of reliability tests; 
         FIG. 6  is a sectional view of a substrate fixing device according to a second embodiment; and 
         FIG. 7  is a sectional view of a substrate fixing device according to a third embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Inventors of the present application have diligently performed investigation in order to look deep into the cause of the variation in the temperature of the adsorption face of the electrostatic adsorption member in the background-art substrate fixing device. As a result, it has been apparent that when the substrate fixing device is exposed to a temperature as low as about −60° C., a large difference may occur between an amount of thermal deformation of the electrostatic adsorption member and an amount of thermal deformation of a base plate to cause large stress to act on the adhesive layer, thereby resulting in occurrence of cohesive failure in the adhesive layer. Due to the occurrence of the cohesive failure, in-plane uniformity of thermal resistance of the adhesive layer is reduced to cause the variation in the temperature of the adsorption face of the electrostatic adsorption member. 
     The present disclosure has been accomplished based on such a finding to suppress the cohesive failure of the adhesive layer and improve uniformity of the temperature of the adsorption face of the electrostatic adsorption member even when the substrate fixing device is exposed to the low temperature. 
     Embodiments of the present disclosure will be specifically described below with reference to the accompanying drawings. Incidentally, in description of the present disclosure and the drawings, the same reference signs will be appended to constituent elements having substantially the same functional configurations, so that duplicate description thereof may be omitted. 
     First Embodiment 
     First, a first embodiment will be described.  FIG. 1  is a sectional view showing a substrate fixing device according to the first embodiment. 
     As shown in  FIG. 1 , the substrate fixing device  1  according to the first embodiment has a base plate  10 , an adhesive layer  20 , an auxiliary adhesive layer  21 , and an electrostatic adsorption member  50  as major constituent elements. 
     The base plate  10  is a member for mounting the electrostatic adsorption member  50  thereon. Thickness of the base plate  10  can be, for example, set in a range of about 20 mm to 50 mm. The base plate  10  is, for example, formed from aluminum, and can be also used as an electrode or the like for controlling plasma. By feeding predetermined high-frequency electric power to the base plate  10 , energy for making ions or the like in a generated plasma state collide against a substrate such as a wafer adsorbed on the electrostatic adsorption member  50  can be controlled to effectively perform an etching process. 
     A water channel  15  is provided inside the base plate  10 . The water channel  15  has one end where a cooling water introduction portion  15   a  is provided, and the other end where a cooling water discharge portion  15   b  is provided. The water channel  15  is connected to a cooling water control device (not shown) provided outside the substrate fixing device  1 . The cooling water control device (not shown) introduces cooling water into the water channel  15  from the cooling water introduction portion  15   a  and discharges the cooling water from the cooling water discharge portion  15   b . When the base plate  10  is cooled by the cooling water circulated in the water channel  15 , the substrate adsorbed on the electrostatic adsorption member  50  can be cooled. In addition to the water channel  15 , a gas channel or the like for introducing inert gas to cool the substrate adsorbed on the electrostatic adsorption member  50  may be provided in the base plate  10 . 
     The electrostatic adsorption member  50  having a heating portion  30  and an electrostatic chuck  40  adsorbs and holds the substrate such as the wafer, that is an object to be adsorbed. A primer layer  62  is applied to a face of the electrostatic adsorption member  50  on the base plate  10  side. The primer layer  62  may, for example, contain titanium. The primer layer  62  is an example of a second primer layer. 
     The heating portion  30  has an insulating layer  31 , and a heating element  32  that is built into the insulating layer  31 . The heating element  32  is covered with the insulating layer  31  to be protected from the outside. For example, a sintered body of tungsten or molybdenum can be used as the heating element  32 . A rolled alloy may be used as the heating element  32 . Incidentally, the heating element  32  does not necessarily have to be built into a thicknesswise central portion of the insulating layer  31 , and may be unevenly distributed more toward the base plate  10  side or the electrostatic chuck  40  side than the thicknesswise central portion of the insulating layer  31  according to requirement specifications. 
     For example, an epoxy resin, a bismaleimide-triazine resin, or the like, having high thermal conductivity and high heat resistance can be used as the insulating layer  31 . Thermal conductivity of the insulating layer  31  is preferably set to be not less than 3 W/(m·K). By including a filler of alumina, aluminum nitride, or the like, in the insulating layer  31 , the thermal conductivity of the insulating layer  31  can be improved. In addition, glass transition temperature (Tg) of the insulating layer  31  is preferably set to be not lower than 250° C. Moreover, thickness of the insulating layer  31  is preferably set in a range of about 100 μm to 150 μm, and a variation in the thickness of the insulating layer  31  is preferably set in a range of +10% or less. 
     Incidentally, at least one face (one or each of upper and lower faces) of the heating element  32  is preferably roughened in order to improve adhesiveness between the heating element  32  and the insulating layer  31  under high temperature. It is a matter of course that each of the upper and lower faces of the heating element  32  may be roughened. In this case, different roughening methods may be used for the upper face and the lower face of the heating element  32 . The roughening methods are not particularly limited, but examples of the roughing methods can include a method using etching, a method using a coupling agent-based surface modification technique, a method using dot processing by a UV-YAG laser having a wavelength of 355 nm or less, and the like. 
     The electrostatic chuck  40  adsorbs and holds the substrate such as the wafer, which is the object to be adsorbed. A diameter of the substrate, which is the object to be adsorbed by the electrostatic chuck  40 , can be, for example, set at about 8 inches, 12 inches, or 18 inches. 
     The electrostatic chuck  40  is provided on the heating portion  30 . The electrostatic chuck  40  has a base body  41 , and an electrostatic electrode  42 . The electrostatic chuck  40  is, for example, a Johnsen-Rahbek type electrostatic chuck. However, the electrostatic chuck  40  may be a Coulomb force type electrostatic chuck. 
     The base body  41  is a dielectric. For example, ceramics such as aluminum oxide (Al 2 O 3 ) and aluminum nitride (AlN) can be used as the base body  41 . Thickness of the base body  41  can be, for example, set in a range of about 1 mm to 10 mm, and a relative dielectric constant of the base body  41  can be, for example, set in a range of about 9 to 10 at a frequency of 1 kHz. The electrostatic chuck  40  and the insulating layer  31  of the heating portion  30  are directly bonded to each other. By directly bonding the heating portion  30  and the electrostatic chuck  40  to each other without any adhesive agent, heat-resistant temperature of the substrate fixing device  1  can be improved. Although heat-resistant temperature of a substrate fixing device in which the heating portion  30  and the electrostatic chuck  40  are bonded to each other by an adhesive agent is about 150° C., the heat-resistant temperature of the substrate fixing device  1  can be set at about 200° C. 
     The electrostatic electrode  42  that is a thin-film electrode is built into the base body  41 . The electrostatic electrode  42  is connected to a power supply provided outside the substrate fixing device  1 . When a predetermined voltage is applied to the electrostatic electrode  42 , adsorption force caused by static electricity is generated between the electrostatic electrode  42  and the substrate such as the wafer so that the substrate can be adsorbed and held on the electrostatic chuck  40 . As the voltage applied to the electrostatic electrode  42  is higher, the adsorption and holding force is stronger. The electrostatic electrode  42  may be a monopolar shape or a bipolar shape. For example, a sintered body of tungsten or molybdenum can be used as the electrostatic electrode  42 . 
     The adhesive layer  20  adhesively bonds the heating portion  30  to the base plate  10 . The adhesive layer  20  directly contacts the primer layer  62 . For example, a silicone-based adhesive agent can be used as the adhesive layer  20 . A filler of alumina, aluminum nitride, or the like may be included in the adhesive layer  20 . A storage modulus of the adhesive layer  20  is not less than 0.01 MPa and not more than 25 MPa within a temperature range R 1  of −110° C. to 250° C. Preferably, the storage modulus of the adhesive layer  20  is not less than 0.01 MPa and not more than 21 MPa within a temperature range R 2  of −100° C. to 140° C.  FIG. 2  shows relationship between temperature and storage modulus in each of examples of the adhesive layer  20 . The relationship shown in  FIG. 2  can be acquired by dynamic mechanical analysis (DMA).  FIG. 2  shows characteristics of a first example  20 A and a second example  20 B of the adhesive layer  20 . In each of the first example  20 A and the second example  20 B, the storage modulus decreases as the temperature increases, and the storage modulus is not more than 25 MPa at −110° C., and is not less than 0.01 MPa and not more than 25 MPa within the temperature range R 1 . Moreover, in each of the first example  20 A and the second example  20 B, the storage modulus is not less than 0.01 MPa and not more than 21 MPa within the temperature range R 2  of −100° C. to 140° C. In particular, in the first example  20 A, the storage modulus is 11 MPa at 100° C., and is not less than 1 MPa and not more than 11 MPa within the temperature range R 2 . The adhesive layer  20  is an example of a first adhesive layer. Moreover, in the second example  20 B, the storage modulus is not less than 0.5 MPa and not more than 21 MPa within the temperature range R 2 . 
     Thickness of the adhesive layer  20  is preferably not less than 0.05 mm and not more than 0.4 mm. This is because thermal resistance between the electrostatic chuck  40  and the base plate  10  is likely to be excessive if the thickness of the adhesive layer  20  is more than 0.4 mm. The thickness of the adhesive layer  20  is more preferably not less than 0.05 mm and not more than 0.3 mm, and further preferably not less than 0.05 mm and not more than 0.2 mm. Moreover, from the viewpoint of securing the adhesion force, the thickness of the adhesive layer  20  is preferably not less than 0.05 mm and not more than 0.1 mm. 
     Within the temperature ranges R 1  and R 2 , thermal conductivity of the adhesive layer  20  is preferably not less than 0.5 W/(m·K) and not more than 10 W/(m·K), and more preferably not less than 0.9 W/(m·K) and not more than 10 W/(m·K). This is because the thermal resistance between the electrostatic chuck  40  and the base plate  10  is likely to be excessive if the thermal conductivity of the adhesive layer  20  is less than 0.5 W/(m·K). Within the temperature ranges R 1  and R 2 , the thermal conductivity of the adhesive layer  20  is further preferably not less than 1.0 W/(m·K) and not more than 10 W(m·K), and further more preferably not less than 1.1 W/(m·K) and not more than 10 W(m·K). 
     The auxiliary adhesive layer  21  is thinner than the adhesive layer  20 . Thickness of the auxiliary adhesive layer  21  is, for example, not less than 0.05 mm and not more than 0.12 mm. Within the temperature ranges R 1  and R 2 , thermal conductivity of the auxiliary adhesive layer  21  is preferably higher than the thermal conductivity of the adhesive layer  20 . For example, within the temperature ranges R 1  and R 2 , the thermal conductivity of the auxiliary adhesive layer  21  is not less than 2.0 W/(m·K) and not more than 10 W(m·K). The auxiliary adhesive layer  21  is an example of a second adhesive layer. 
     In the substrate fixing device  1  according to the first embodiment, the storage modulus of the adhesive layer  20  is as low as 25 MPa or less within the temperature range R 1  of −110° C. to 250° C. Therefore, even when a large difference has occurred between an amount of thermal deformation of the electrostatic adsorption member  50  and an amount of thermal deformation of the base plate  10 , the adhesive layer  20  is easily deformed within the temperature range R 1  so that stress acting on the adhesive layer  20  is suppressed. Therefore, cohesive failure of the adhesive layer  20  is suppressed, so that excellent heat uniformity in an adsorption face of the electrostatic adsorption member  50  can be obtained. In other words, uniformity of temperature of the adsorption face of the electrostatic adsorption member  50  can be improved. 
     Within a temperature range S of −100° C. to 0° C., the storage modulus of the auxiliary adhesive layer  21  is preferably higher than the storage modulus of the adhesive layer  20 . This is to easily relax the stress acting on the adhesive layer  20 .  FIG. 3  shows the relationship between the temperature and the storage modulus in the examples (the first example  20 A and the second example  20 B) of the adhesive layer  20  and an example of the auxiliary adhesive layer  21 . The relationship shown in  FIG. 3  can be obtained by DMA. In each of the examples shown in  FIG. 3 , the storage modulus of the auxiliary adhesive layer  21  is higher than the storage modulus of the adhesive layer  20  within the temperature ranges R 1  and R 2 . That is, the storage modulus of the auxiliary adhesive layer  21  is higher than the storage modulus of the adhesive layer  20  within the temperature range S of −100° C. to 0° C. 
     The auxiliary adhesive layer  21  and the primer layer (undercoat layer)  62  may not be provided. 
     Incidentally, in manufacturing the substrate fixing device  1  according to the first embodiment, for example, an adhesive agent whose viscosity is 2 Pa·s to 40 Pa·s can be used as a raw material for the adhesive layer  20 . The adhesive agent may be provided by coating, or may use a semi-cured (B-stage) insulating resin film. 
     Moreover, in manufacturing the substrate fixing device  1  according to the first embodiment, for example, an adhesive agent whose viscosity is 40 Pa·s to 70 Pa·s can be used as a raw material for the auxiliary adhesive layer  21 . The adhesive agent may be provided by application, or may use a semi-cured (B-stage) insulating resin film. Grinding may be performed after the adhesive agent is applied and cured. 
     The substrate fixing device  1  according to the first embodiment can be, for example, manufactured as follows. First, the auxiliary adhesive layer  21  is formed on the base plate  10 . Surface grinding may be performed on the auxiliary adhesive layer  21  after the auxiliary adhesive layer  21  has been formed. In addition, the primer layer  62  is applied to a lower face of the electrostatic adsorption member  50  and dried by air. Then, an upper face of the auxiliary adhesive layer  21  and the lower face of the electrostatic adsorption member  50  to which the primer layer  62  has been applied are adhesively bonded to each other by the adhesive layer  20 . On this occasion, a spacer may be used to adjust a distance between the base plate  10  and the electrostatic adsorption member  50 . 
     The materials for the adhesive layer  20  and the auxiliary adhesive layer  21  are not limited. Examples of the materials for the adhesive layer  20  and the auxiliary adhesive layer  21  include a silicone resin, an epoxy resin, an acrylic resin, and a polyimide resin. Composite materials based on these resins may be used for the adhesive layer  20  and the auxiliary adhesive layer  21 . In addition, a filler may be included in each of the adhesive layer  20  and the auxiliary adhesive layer  21 . Examples of the filler include silica, alumina, aluminum nitride, and the like. 
     Next, a result of a reliability test concerning a substrate fixing device (device No. 1) manufactured in accordance with the first embodiment will be described in comparison with a result of a reliability test concerning a reference example (device No. 2). 
     The device No. 1 has a first example  20 A of an adhesive layer  20  and an auxiliary adhesive layer  21  having characteristics shown in  FIG. 3 . Thickness of the adhesive layer  20  was set at 0.2 mm, and thickness of the auxiliary adhesive layer  21  was set at 0.1 mm. 
     The device No. 2 has an adhesive layer  22  having a characteristic shown in  FIG. 4  in place of the first example  20 A of the adhesive layer  20 . The remaining configuration of the device No. 2 is similar to or the same as the configuration of the Device No. 1. Thickness of the adhesive layer  22  was set at 0.2 mm.  FIG. 4  is a graph showing relationship between temperature and storage modulus in another example (the adhesive layer  22 ) of the adhesive layer. The relationship shown in  FIG. 4  can be obtained by DMA. 
     Incidentally, the adhesive layers  20  and  22  include fillers, and a percentage of the filler content in the adhesive layer  20  (mass of the filler per unit mass of the adhesive layer) is lower than a percentage of the filler content in the adhesive layer  22 . 
     In the reliability test, repetition of a thermal load of lowering temperature to −45° C. and raising the temperature to 150° C. (test A) and repetition of a thermal load of lowering temperature to −45° C. and raising the temperature to 170° C. (test B) were performed. Then, a variation in temperature of an adsorption face was measured when a predetermined number of days had lapsed since the start of the test. The number of times of the repetition of the thermal load per day in the test A was set at 16, and the number of times of the repetition of the thermal load per day in the test B was set at 11. Results of the tests are shown in  FIG. 5 .  FIG. 5  is a graph showing the results of the reliability tests. In  FIG. 5 , a horizontal axis shows the number of elapsed days, and a vertical axis shows the variation in the temperature of the adsorption face. 
     In each of the test A and the test B, as shown in  FIG. 5 , the variation in the temperature of the adsorption face of the device No. 1 was much smaller than the variation in the temperature of the adsorption face of the device No. 2. This indicates that cohesive failure of the adhesive layer  20  is less likely to occur than cohesive failure of the adhesive layer  22 , and that uniformity of thermal resistance of the adhesive layer  20  is excellent over a long period of time. 
     Incidentally, the surface of the base plate  10  may be insulated by alumina spraying or the like. By the insulating treatment, discharge between the base plate  10  and an electrostatic electrode  42  can be suppressed more surely. 
     The insulating layer  31  may not be provided, but the heating element  32  may be built into the base body  41  alternatively. Moreover, the heating portion  30  may not be provided in the substrate fixing device  1 . In these cases, the adhesive layer  20  adhesively bonds the base body  41  to the base plate  10 . 
     Second Embodiment 
     Next, a second embodiment will be described. The second embodiment mainly differs from the first embodiment in a configuration of an electrostatic adsorption member.  FIG. 6  is a sectional view showing a substrate fixing device according to the second embodiment. 
     In the substrate fixing device  2  according to the second embodiment, as shown in  FIG. 6 , the electrostatic adsorption member  50  does not include a heating portion  30 , but a heating element  32  is built in an electrostatic chuck  40 . The remaining configuration is similar to or the same as that according to the first embodiment. 
     An effect similar to or the same as that according to the first embodiment can be obtained also by the second embodiment. 
     Third Embodiment 
     Next, a third embodiment will be described. The third embodiment mainly differs from the first embodiment in a configuration of a portion between a base plate  10  and an electrostatic adsorption member  50 .  FIG. 7  is a sectional view showing a substrate fixing device according to the third embodiment. 
     In the substrate fixing device  3  according to the third embodiment, as shown in  FIG. 7 , a primer layer  61  is applied to a face of the base plate  10  on the electrostatic adsorption member  50  side. The primer layer  61  may, for example, contain titanium. The substrate fixing device  3  does not include an auxiliary adhesive layer  21 , but an adhesive layer  20  directly contacts the primer layer  61 . The remaining configuration is similar to or the same as that according to the first embodiment. The primer layer  61  is an example of a first primer layer. 
     An effect similar to or the same as that according to the first embodiment can be obtained also by the third embodiment. In addition, since the auxiliary adhesive layer  21  is not provided, thermal resistance between the electrostatic adsorption member  50  and the base plate  10  can be more reduced. 
     The substrate fixing device  3  according to the third embodiment can be, for example, manufactured as follows. First, the primer layer  61  is applied to an upper face of the base plate  10  and dried. The primer layer  62  is applied to a lower face of the electrostatic adsorption member  50  and dried. The primer layers  61  and  62  may be dried by heating. Then, the upper face of the base plate  10  to which the primer layer  61  has been applied and the lower face of the electrostatic adsorption member  50  to which the primer layer  62  has been applied are adhesively bonded to each other by the adhesive layer  20 . On this occasion, a spacer may be used to adjust a distance between the base plate  10  and the electrostatic adsorption member  50 . 
     In the second embodiment, a primer layer  61  may be applied to the face of the base plate  10  on the electrostatic adsorption member  50  side as in the third embodiment, and the auxiliary adhesive layer  21  may not be provided, so that the adhesive layer  20  directly contacts the primer layer  61 . 
     Incidentally, in a thicknesswise direction (up/down direction) of the substrate fixing device  3 , the distance between the base plate  10  and the electrostatic adsorption member  50  is preferably not less than 0.025 mm and not more than 1.025 mm, and more preferably not less than 0.050 mm and not more than 1.000 mm. This is to achieve both excellent adhesiveness and heat dissipation. Here, the distance between the base plate  10  and the electrostatic adsorption member  50  is specifically defined by a distance between the upper face of the base plate  10  and the lower face of the electrostatic adsorption member  50 . 
     Although the preferred embodiments etc. have been described above in detail, the present disclosure is not limited to the aforementioned embodiments etc., and various modifications and substitutions can be added to the aforementioned embodiments etc. without departing from the scope described in Claims.