Patent Publication Number: US-2023136771-A1

Title: Substrate joining method, substrate joining system and method for controlling hydrophilic treatment device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a substrate joining method, a substrate joining system, and a method for controlling a hydrophilic treatment device. 
     BACKGROUND ART 
     In the prior art, hydrophilic joining of substrates is achieved by attaching the substrates together in the atmosphere with water molecules interposed, whereby even if a sufficient amount of OH groups are not produced on the joint surfaces of the original substrates, the water molecules that are interposed between the joint surfaces of substrates can turn into OH groups and then turn into a strong covalent bond through heating. However, the joining in the atmosphere causes large voids due to incorporation of the air. Therefore, it is necessary to employ a joining method of attaching the substrates together while deflecting the substrates in the center. However, this joining method has problems such as distortion in the substrates and deterioration in the accuracy of alignment between the substrates. Moreover, other problems include occurrence of microvoids through heating of the substrates because the substrates are joined with water molecules interposed on the interface of the substrates. Therefore, joining the substrates together in a vacuum makes it possible to prevent air from being incorporated between the substrates so as to reduce occurrence of voids. Furthermore, joining the substrates together in a vacuum results in joining the substrates while expelling water molecules that are present between the substrates, whereby no voids occur. As just stated, the method of joining the substrates together in a vacuum is an effective method of achieving excellent joining between the substrates. However, it is required to produce a sufficient amount of OH groups on the joint surfaces of the substrates in activating the joint surfaces of the substrates before joining the substrates together. Then, the process of activating the joint surfaces of the substrates by an RIE treatment and subsequent exposure to the atmosphere alone as in the prior art does not produce a sufficient amount of OH groups on the joint surfaces of the substrates, whereby the joined substrates will have an insufficient joint strength. With the view of the above, there is a demand fora method of activating the joint surfaces of substrates that makes it possible to produce a sufficient amount of OH groups that endure through the joining in a vacuum. 
     On the other hand, a substrate joining method for joining two substrates by a combination of reactive ion etching and radical irradiation on the joint surfaces of the two substrates to join has been proposed (for example, see Patent Literature 1). This substrate joining method is a method for joining two substrates together by producing hydroxyl groups (OH groups) on the joint surfaces of the two substrates and bringing into mutual contact and pressurizing the joint surfaces of the two substrates to form the hydrogen bond between the joint surfaces. Then, in this substrate joining method, the joint surfaces of the two substrates to join are exposed to oxygen plasma to perform reactive ion etching, and then, the joint surfaces of the two substrates are irradiated with nitrogen radicals. Subsequently, the joint surfaces of the two substrates are brought into mutual contact and pressurized to join the two substrates. 
     CITATION LIST 
     Patent Literature 
     Patent Literature 1: Unexamined Patent Application Kokai Publication No. 2005-79353 
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in the substrate joining method described in the Patent Literature 1, an insufficient amount of OH groups are produced on the joint surfaces of the two substrates and the joint strength of the two substrates joined may not be sufficient in some cases. Then, there is a demand for a substrate joining method that makes it possible to join two substrates together more solidly by producing a sufficient amount of OH groups on the joint surfaces of the two substrates. 
     The present disclosure is made with the view of the above reason and an objective of the present disclosure is to provide a substrate joining method, a substrate joining system, and a method for controlling a hydrophilic treatment device that make it possible to join two substrates solidly. 
     Solution to Problem 
     In order to achieve the above objective, the substrate joining method according to the present disclosure is a substrate joining method for joining two substrates, including: 
     a hydrophilic treatment step of hydrophilizing at least one of respective joint surfaces of the two substrates that are to be joined to each other, and 
     a joining step of joining the two substrates after the hydrophilic treatment step, 
     wherein the hydrophilic treatment step includes: 
     a first etching step of performing reactive ion etching using nitrogen gas on the joint surfaces of the substrates, and 
     a radical treatment step of performing a radical treatment to irradiate the joint surfaces of the substrates with nitrogen radicals after the first etching step. 
     The substrate joining system according to the present disclosure from another aspect is a substrate joining system for joining two substrates, comprising: 
     a hydrophilic treatment device configured to perform a hydrophilic treatment to hydrophilize at least one of respective joint surfaces of the two substrates that are to be joined to each other; 
     a substrate joining device configured to join the two substrates of which the joint surfaces are subjected to the hydrophilic treatment by the hydrophilic treatment device; and
         a controller configured to control the hydrophilic treatment device and the substrate joining device,   wherein the hydrophilic treatment device comprises:   a chamber;   a stage configured to support the substrates within the chamber;   a nitrogen gas supplier configured to supply nitrogen gas into the chamber;   a plasma generation source configured to generate plasma and supply radicals in the plasma to the joint surfaces of the substrates that are supported by the stage; and   a bias applier configured to apply a high frequency bias to the substrates that are supported by the stage, and   the controller is configured to   control the nitrogen gas supplier to introduce nitrogen gas into the chamber and then control the bias applier to apply a high frequency bias to the substrates to perform reactive ion etching using nitrogen gas on the joint surfaces of the substrates, and then control the plasma generation source and the bias applier to generate plasma with the nitrogen gas and stop the application of the high frequency bias to the substrates to perform a radical treatment to irradiate the joint surfaces of the substrates with nitrogen radicals.       

     The method for controlling a hydrophilic treatment device according to the present disclosure from another aspect is a method for controlling a hydrophilic treatment device that comprises a chamber, a stage configured to support substrates within the chamber, a nitrogen gas supplier configured to supply nitrogen gas into the chamber, a plasma generation source configured to generate plasma and supply radicals in the plasma to joint surfaces of the substrates that are supported by the stage, and a bias applier configured to apply a high frequency bias to the substrates that are supported by the stage, including:
         a step of introducing nitrogen gas into the chamber by means of the nitrogen gas supplier and then applying a high frequency bias to the substrates by means of the bias applier to perform reactive ion etching using nitrogen gas on the joint surfaces of the substrates, and   a step of generating plasma with nitrogen gas by means of the plasma generation source and stopping the application of the high frequency bias to the substrates by means of the bias applier to perform a radical treatment to irradiate the joint surfaces of the substrates with nitrogen radicals.       

     Advantageous Effects of Invention 
     According to the present disclosure, the hydrophilic treatment step includes a first etching step of performing reactive ion etching using nitrogen gas on the joint surfaces of the substrates and a radical treatment step of performing a radical treatment to irradiate the joint surfaces of the substrates with nitrogen radicals after the first etching step. Moreover, the controller according to the present disclosure controls the nitrogen gas supplier to introduce nitrogen gas into the chamber and then controls the bias applier to apply a high frequency bias to the substrates to perform reactive ion etching using nitrogen gas on the joint surfaces of the substrates, and then controls the plasma generation source and the bias applier to generate plasma with nitrogen gas and stop the application of the high frequency bias to the substrates to perform a radical treatment to irradiate the joint surfaces of the substrates with nitrogen radicals. In other words, the reactive ion etching using nitrogen gas serves for nitrogen ions to collide against the joint surfaces of the substrates with a relatively strong collision force to form a large number of sites for OH groups to adhere. Then, the subsequent highly reactive radical treatment with a relatively weak collision force of nitrogen radicals against the joint surfaces of the substrates serves to form sites for OH groups to adhere while suppressing release of the OH groups that have adhered to the sites. As a result, it is possible to efficiently augment adherence of OH groups to the joint surfaces of the substrates and produce a large number of OH groups on the joint surfaces of the substrates. Thus, when the joint surfaces of two substrates are brought into mutual contact to join the two substrates, it is possible to form a large number of hydrogen bonds between the joint surfaces for a large number of OH groups being produced and thus, as the hydrogen bond turns into the covalent bond through a subsequent heating step, improve the joint strength of the substrates that are joined to each other. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a general configuration diagram of the substrate joining system according to an embodiment of the present disclosure; 
         FIG.  2    is a general front view of the hydrophilic treatment device according to the embodiment; 
         FIG.  3    is a general front view of the substrate joining device according to the embodiment; 
         FIG.  4    is a block diagram that shows the configuration of the controller according to the embodiment; 
         FIG.  5    is a flowchart that shows the process flow of the substrate joining procedure that is executed by the substrate joining system according to the embodiment; 
         FIG.  6    is a flowchart that shows the process flow of the hydrophilic treatment that is executed by the substrate joining system according to the embodiment; 
         FIG.  7 A  is a time chart for explaining the substrate joining method according to Comparative Embodiments 1 to 4; 
         FIG.  7 B  is a time chart for explaining the substrate joining method according to Comparative Embodiments 5 to 7; 
         FIG.  7 C  is a time chart for explaining the substrate joining method according to the embodiment; 
         FIG.  7 D  is a time chart for explaining the substrate joining method according to the embodiment; 
         FIG.  7 E  is a time chart for explaining the substrate joining method according to Comparative Embodiment 8; 
         FIG.  8 A  is an illustration for explaining the method of measuring the joint strength (calculated in the surface energy) of the substrates by the blade insertion method; 
         FIG.  8 B  is an illustration for explaining the method of assessing the joint strength according to the embodiment; 
         FIG.  9 A  is a photograph of the appearance of Sample 1; 
         FIG.  9 B  is a photograph of the appearance of Sample 2; 
         FIG.  9 C  is a photograph of the appearance of Sample 3; 
         FIG.  9 D  is a photograph of the appearance of Sample 4; 
         FIG.  10 A  is a photograph of the appearance of Sample 5; 
         FIG.  10 B  is a photograph of the appearance of Sample 6; 
         FIG.  10 C  is a photograph of the appearance of Sample 7; 
         FIG.  10 D  is a photograph of the appearance of Sample 8; 
         FIG.  11 A  is a photograph of the appearance of Sample 9; 
         FIG.  11 B  is a photograph of the appearance of Sample 10; 
         FIG.  11 C  is a photograph of the appearance of Sample 11; 
         FIG.  11 D  is a photograph of the appearance of Sample 12; and 
         FIG.  12    is a general configuration diagram that shows part of the hydrophilic treatment device according a modified embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     The substrate joining system according to an embodiment of the present disclosure will be described below with reference to the drawings. 
     The substrate joining system according to this embodiment is a system for joining two substrates by performing a hydrophilic treatment on the joint surfaces of two substrates, bringing the substrates into mutual contact, and then pressurizing and heating the substrates in a chamber under reduced pressure. In the hydrophilic treatment, the joint surfaces of the two substrates are hydrophilized by performing, on the joint surface of each of the two substrates, reactive ion etching using nitrogen gas and a radical treatment to irradiate nitrogen radicals. 
     The substrate joining system, according to this embodiment, comprises, as shown in  FIG.  1   , an introduction port  961 , a retrieval port  962 , a first transportation device  930 , a cleaning device  940 , a contour alignment device  800 , a flipping device  950 , a hydrophilic treatment device  600 , a substrate joining device  100 , a second transportation device  920 , a controller  700 , and a load lock chamber  910 . Moreover, a water gas supplier  960  for supplying water gas into the load lock chamber  910  is connected to the load lock chamber  910 . The controller  700  controls the first transportation device  930 , the cleaning device  940 , the contour alignment device  800 , the flipping device  950 , the hydrophilic treatment device  600 , the substrate joining device  100 , the second transportation device  920 , and the water gas supplier  960 . A high efficiency particulate air (HEPA) filter (not shown) is provided in each of the first transportation device  930 , the cleaning device  940 , and the contour alignment device  800 . As a result, an atmospheric environment with a very little particles is established in these devices. On the other hand, a reduced pressure atmosphere is established in the flipping device  950 , the hydrophilic treatment device  600 , and the substrate joining device  100 . 
     The first transportation device  930  has an atmospheric transportation robot  931  that has an arm for grabbing substrates  301  and  302 . The second transportation device  920  also has a vacuum transportation robot  921  that has an arm for grabbing the substrates  301  and  302 . The cleaning device  940  cleans the transported substrates  301  and  302  while ejecting water toward the substrates  301  and  302 . The contour alignment device  800  has an edge recognition sensor and a substrate thickness measurer and while rotating a stage  803  on which the substrates  301  and  302  are placed, recognizes the edges of the substrates  301  and  302  by means of an edge recognition sensor  810  and measures the thicknesses of the substrates  301  and  302  by means of a substrate thickness measurer  802 . The flipping device  950  flips over and holds the transported substrate  302 . Then, the vacuum transportation robot  921  can grab the substrate  302  that is flipped over and held by the flipping device  950 . 
     The hydrophilic treatment device  600  performs a hydrophilic treatment to hydrophilize the joint surface of each of the substrates  301  and  302 . The hydrophilic treatment device  600  has, as shown in  FIG.  2   , a stage  610 , a chamber  612 , a trap plate  614 , a waveguide  615 , a magnetron  616 , and a high frequency power source  617 . Moreover, the hydrophilic treatment device  600  has a N 2  gas supplier (nitrogen gas supplier)  620 A and an O 2  gas supplier (oxygen gas supplier)  620 B. The N 2  gas supplier  620 A has a N 2  gas reservoir  621 A, a supply valve  622 A, and a supply pipe  623 A. The O 2  gas supplier  620 B has an O 2  gas reservoir  621 B, a supply valve  622 B, and a supply pipe  623 B. The substrates  301  and  302  are placed on the stage  610 . The chamber  612  is connected to the waveguide  615  via a glass window  613 . The chamber  612  is connected to a vacuum pump  201  via a discharge pipe  202 A and a discharge valve  203 A. When the vacuum pump  201  is operated with the discharge valve  203 A opened, the gas within the chamber  612  is discharged outside the chamber  612  through the discharge pipe  202 A and the gas pressure within the chamber  612  is reduced (depressurized). 
     Microwaves that are generated by the magnetron  616  are introduced into the chamber  612  through the waveguide  615 . As the magnetron  616 , for example, one that generates microwaves of a frequency of 2.45 GHz can be employed. Then, as the microwaves are introduced from the waveguide  615  with the N 2  gas introduced within the chamber  612 , the microwaves serve to form plasma PLM within the chamber  612  in the vicinity of the glass window  613 . The trap plate  614  traps ions that are contained in the plasma PLM and allows only radicals to flow down to the stage  610 . The magnetron  616 , the N 2  gas supplier  620 A, and the trap plate  614  constitute a plasma generation source that generates plasma PLM within the chamber  612  and supplies N 2  radicals in the plasma to the joint surfaces of the substrates  301  and  302  that are supported by the stage  610 . Here, the explanation is made with regard to the configuration that comprises the magnetron  616  and the waveguide  615  as the hydrophilic treatment device  600 . However, this is not restrictive. Instead, a configuration that comprises plate electrodes that are provide on the glass window  613  and a high frequency power source that is electrically connected to the plate electrodes may be contemplated. In such a case, as the high frequency power source, for example, one that applies a high frequency bias of 27 MHz can be employed. 
     The high frequency power source (the bias applier)  617  applies a high frequency bias to the substrates  301  and  302  that are supported by the stage  610 . As the high frequency power source  617 , for example, one that generates a high frequency bias of 13.56 MHz can be employed. As juts stated, with a high frequency bias being applied to the substrates  301  and  302  by the high frequency power source  617 , a sheath region in which ions that have kinetic energy repeatedly collide against the substrates  301  and  302  occurs in the vicinity of the joint surfaces of the substrates  301  and  302 . Then, the ions that are present in the sheath region and have kinetic energy etch the joint surfaces of the substrates  301  and  302 . 
     The water gas supplier  960  has a water gas generation device (not shown). This water gas generation device produces water gas by bubbling in the retained water a carrier gas such as argon (Ar), nitrogen (N 2 ), helium (He), and oxygen ( 02 ). The water gas generation device is connected to the load lock chamber  910  via a supply valve and a supply pipe. The flow rate of the water gas and the carrier gas that are introduced into the load lock chamber  910  is adjusted by controlling the degree of opening of the supply valve. Here, the water gas supplier  960  may be configured to accelerate and eject water (H 2 O) molecules or clusters toward the joint surfaces of the substrates  301  and  302 . Here, the water gas supplier  960  may comprise a particle-beam source that ejects accelerated water (H 2 O) molecules. In such a case, as the particle beam source, for example, a configuration in which an ultrasonic waves generation element is used to generate water gas may be contemplated. Alternatively, a configuration in which a mixed gas of a carrier gas that is generated by the above-described bubbling, ultrasonic vibration, or the like and water (H 2 O) is introduced into the above-described particle beam source to generate and eject a water particle beam to the joint surfaces of the substrates  301  and  302  may be contemplated. 
     The load lock chamber  910  is provided with a cooling device (not shown) for cooling the stage that supports the substrates  301  and  302 . Then, for example, for a setting of the humidity being 50% when the temperature is 25° C. within the load lock chamber  910 , the cooling device cools the stage to 18° C. so that the humidity in the vicinity of the substrates  301  and  302  that are placed on the stage becomes 80% or so. As a result, it is possible to reduce the amount of water gas that is supplied from the water gas supplier  960  into the load lock chamber  910 . 
     The substrate joining device  100  joins the substrates  301  and  302  together that are subjected to the hydrophilic treatment in the hydrophilic treatment device  600 . The substrate joining device  100  comprises, as shown in  FIG.  3   , a chamber  200 , a stage  401 , a head  402 , a stage driver  403 , a head driver  404 , substrate heaters  421  and  422 , and a misalignment amount measurer  500 . Here, the following explanation will be made on the assumption that the +Z direction is the vertical direction and the XY direction is the horizontal direction in  FIG.  3    as appropriate. The chamber  200  is connected to the vacuum pump  201  via a discharge pipe  202 B and a discharge valve  203 B. As the vacuum pump  201  is operated with the discharge valve  203 B opened, the gas within the chamber  200  is discharged outside the chamber  200  through the discharge pipe  202 B and the gas pressure within the chamber  200  is reduced (depressurized). Moreover, the gas pressure (the degree of vacuum) within the chamber  200  can be adjusted by changing the amount of open/close of the discharge valve  203 B and thus adjusting the amount of discharge. 
     The stage  401  and the head  402  are placed to face each other in the Z direction within the chamber  200 . The stage  401  supports the substrate  301  with its top surface, and the head  402  supports the substrate  302  with its bottom surface. Here, the top surface of the stage  401  and the bottom surface of the head  402  may be roughened in consideration of the case in which the contact surfaces of the substrates  301  and  302  with the stage  401  and the head  402  are mirror surfaces and difficult to be released from the stage  401  and the head  402 . The stage  401  and the  402  each have a holding mechanism (not shown) for holding the substrate  301  or  302 . The holding mechanism comprises an electrostatic chuck or a vacuum chuck. 
     The stage driver  403  can move the stage  401  in the XY direction and rotate the stage  401  about the Z axis. The head driver  404  moves the head  402  up and down in the vertical direction (seethe arrow AR 1  in  FIG.  3   ). The head driver  404  moves the head  402  downward to make the head  402  closer to the stage  401 . Moreover, the head driver  404  moves the head  402  upward to make the head  402  away from the stage  401 . Then, with the substrates  301  and  302  in the state of mutual contact, as the head driver  404  effects on the head  402  a drive force in the direction of moving toward the stage  401 , the substrate  302  is pressed against the substrate  301 . Moreover, the head driver  404  is provided with a pressure sensor  408  that measures the drive force in the direction of moving toward the stage  401  that is effected by the head driver  404  on the head  402 . The measurement of the pressure sensor  408  makes it possible to detect a pressure that acts on the joint surfaces of the substrates  301  and  302  when the substrate  302  is pressed against the substrate  301  by the head driver  404 . The pressure sensor  408  comprises, for example, a load cell. 
     The substrate heaters  421  and  422  comprise, for example, an electrothermal heater. The substrate heaters  421  and  422  transfer heat to the substrates  301  and  302  that are supported by the stage  401  and the head  402  to heat the substrates  301  and  302 . Moreover, the temperature of the substrates  301  and  302 , and their joint surfaces can be adjusted by adjusting the amount of heat generation of the substrate heaters  421  and  422 . 
     The misalignment amount measurer  500  recognizes the position of a positioning mark (an alignment mark) that is provided to each of the substrates  301  and  302  to measure the amount of horizontal misalignment of the substrate  301  with respect to the substrate  302 . The misalignment amount measurer  500  recognizes the alignment marks of the substrates  301  and  302  using, for example, light (for example, infrared light) that is transmitted through the substrates  301  and  302 . The stage driver  403  executes the operation of mutual alignment of the substrates  301  and  302  (the alignment operation) by moving in the horizontal direction and/or rotating the stage  401  based on the amount of misalignment that is measured by the misalignment amount measurer  500 . The measuring of the amount of misalignment by the misalignment amount measurer  500  and the alignment operation by the stage driver  403  are both executed under the control of the controller  700 . 
     The controller  700  has, as shown in  FIG.  4   , a micro processing unit (MPU)  701 , a main storage  702 , an auxiliary storage  703 , an interface  704 , and a bus  705  that connects the parts. The main storage  702  comprises a volatile memory and is used as the work area of the MPU  701 . The auxiliary storage  703  comprises a nonvolatile memory and stores programs that are executed by the MPU  701 . The interface  704  converts measurement signals that are entered from the pressure sensor  408 , the misalignment amount measurer  500 , and the like to measurement information and outputs the measurement information to the bus  705 . Moreover, the MPU  701  reads into the main storage  702  and executes the programs that are stored in the auxiliary storage  703 , thereby outputting control signals to the stage driver  403 , the head driver  404 , the substrate heaters  421  and  422 , the magnetron  616 , the high frequency power source  617 , the supply valves  622 A and  622 B, the vacuum transportation robot  921 , the atmospheric transportation robot  931 , the water gas supplier  960 , and the like via the interface  704 . 
     Here, in regard to the entire substrate joining system according to this embodiment, the operation flow from introduction of the substrates  301  and  302  into the substrate joining system to retrieval of the joined substrates  301  and  302  from the substrate joining system will be described. The substrates  301  and  302  are, first, placed at the introduction port  961 . The substrates  301  and  302  are, for example, any of glass substrates, oxide substrates (for example, silicon oxide (SiO 2 ) substrates or alumina substrates (Al 2 O 3 )), and nitride substrates (for example, silicon nitride (SiN), aluminum nitride (AlN)). 
     Next, the substrates  301  and  302  are transported from the introduction port  961  to the cleaning device  940  by the atmospheric transportation robot  931  of the first transportation device  930  and in the cleaning device  940 , cleaned to remove foreign substances that are present on the substrates  301  and  302 . Subsequently, the substrates  301  and  302  are transported from the cleaning device  940  to the contour alignment device  800  by the atmospheric transportation robot  931  and in the contour alignment device  800 , subject to alignment in their contour and measurement of the substrate thickness. Subsequently, the substrates  301  and  302  are transported to the load lock chamber  910  that is open to the atmosphere by the atmospheric transportation robot  931 . Then, the gas that is present within the load lock chamber  910  is discharged to make equal the degree of vacuum within the load lock chamber  910  and the degree of vacuum within the second transportation device  920 , and then the substrates  301  and  302  are transported to the hydrophilic treatment device  600  by the vacuum transportation robot  921  of the second transportation device  920 . 
     Subsequently, the substrates  301  and  302  are transported to the hydrophilic treatment device  600 , where the hydrophilic treatment is performed on the joint surface of each of the substrates  301  and  302 . The hydrophilically-treated substrates  301  and  302  are transported into the load lock chamber  910  again by the vacuum transportation robot  921 . Then, water gas is supplied from the water gas supplier  960  to the load lock chamber  910  and the joint surfaces of the substrates  301  and  302  are exposed to the water gas. As a result, a much larger amount of OH groups are produced on the joint surfaces of the substrates  301  and  302 . Next, the substrate  301  is transported to the substrate joining device  100  by the vacuum transportation robot  921 . On the other hand, the substrate  302  is transported to the flipping device  950  by the vacuum transportation robot  921  and in the flipping device  950 , flipped over. Subsequently, the substrate  302  is transported to the substrate joining device  100  by the vacuum transportation robot  921 . 
     Subsequently, the substrates  301  and  302  are joined to each other in the substrate joining device  100 . The joined substrates  301  and  302  are transported to the load lock chamber  910  again by the vacuum transportation robot  921 . Subsequently, as the load lock chamber,  910  is opened to the atmosphere, the substrates  301  and  302  that are joined to each other are transported from the load lock chamber  910  to the retrieval port  962  by the atmospheric transportation robot  931 . The operation flow of the entire substrate joining system, according to this embodiment is described above. 
     Next, the substrate joining procedure that is executed by the substrate joining system (the controller  700 ) according to this embodiment, will be described with reference to  FIG.  5   . Triggered by the controller  700  activating the program for executing the substrate joining procedure, this substrate joining procedure starts. Here,  FIG.  5    shows the process flow up to the substrates  301  and  302  being joined to each other after transported to the load lock chamber  910 . 
     First, the controller  700  controls the vacuum transportation robot  921  of the second transportation device  920  to transport the substrate  301  ( 302 ) to the hydrophilic treatment device  600  and in the hydrophilic treatment device  600 , the transported substrate  301  ( 302 ) is held on the stage  610  (Step S 1 ). 
     Next, the controller  700  controls the hydrophilic treatment device  600  to execute the hydrophilic treatment to hydrophilize the joint surface of the substrate  301  ( 302 )(the hydrophilic treatment step) (Step S 2 ). The hydrophilic treatment will be described in detail later. As a result, OH groups are produced on the joint surface of the substrate  301  ( 302 ). 
     Subsequently, the controller  700  controls the vacuum transportation robot  921  to transport the substrate  301  ( 302 ) from the hydrophilic treatment device  600  to the load lock chamber  910  and in the load lock chamber  910 , the transported substrate  301  ( 302 ) is held on the stage (Step S 3 ). 
     Subsequently, the controller  700  controls the water gas supplier  960  to introduce water gas into the load lock chamber  910  from the water gas generation device to expose the substrate  301  ( 302 ) to the water gas (H 2 O), thereby performing a water gas exposure treatment to supply water to the joint surfaces of the substrates  301  and  302  (the first water supply step)(Step S 4 ). As a result, moisture, which was short during the plasma treatment, is replenished and OH groups are additionally produced on the joint surface of the substrate  301  ( 302 ). 
     Next, the controller  700  controls the vacuum transportation robot  921  to transport the substrate  301  ( 302 ) from the load lock chamber  910  to the substrate joining device  100  and in the substrate joining device  100 , the transported substrate  301  ( 302 ) is held on the stage  401  (the head  402 )(Step S 5 ). Here, for the substrate  302  that is to be held on the head  402  of the substrate joining device  100 , the controller  700  first transports the substrate  302  from the load lock chamber  910  to the flipping device  950  and controls the flipping device  950  to flip over the substrate  302 . Subsequently, the controller  700  controls the vacuum transportation robot  921  to transport the substrate  302  that is flipped over by the flipping device  950  to the substrate joining device  100 . As a result, the substrate  301  is held on the stage  401 , and the substrate  302  is held on the head  402 . 
     Subsequently, the controller  700  controls the substrate joining device  100  to move the substrate  302  in the direction of the substrates  301  and  302  mutually approaching from a mutually distant state of the substrates  301  and  302  so as to bring the joint surfaces of the substrates  301  and  302  into mutual contact (Step S 6 ). Here, the substrate joining device  100  first moves the head  402  that supports the substrate  302  toward the stage  401  that supports the substrate  301  so that both substrates  301  and  302  get closer to each other. Next, with both substrates  301  and  302  being closer to each other, the substrate joining device  100  executes the alinement operation of both substrates  301  and  302  based on the amount of misalignment that is measured by the misalignment amount measurer  500 . Subsequently, the substrate joining device  100  moves the head  402  toward the stage  401  again to bring the two substrates  301  and  302  into contact. 
     Subsequently, the controller  700  controls the substrate joining device  100  to execute the joining process to join the substrates  301  and  302  in a vacuum (the joining step)(Step S 7 ). Here, the substrate joining device  100  pressurizes the two substrates  301  and  302  of which the joint surfaces are in mutual contact to join the two substrates  301  and  302 . At this point, the joint surfaces of the substrates  301  and  302  are each covered with OH groups or water molecules. As a result, with the joint surfaces of the substrates  301  and  302  being brought into mutual contact, the substrates  301  and  302  are temporarily joined together by the hydrogen bond between OH groups or water molecules. 
     Next, the controller  700  executes a heat treatment to heat the substrates  301  and  302  that are temporarily joined to each other (Step S 8 ). Here, the substrate joining device  100  keeps the substrates  301  and  302  heated, for example, to 120 to 200° C. for two to seven hours by means of the substrate heaters  421  and  422 . As a result, presumably, the majority of the water molecules and hydrogen that are produced while the OH groups that are present on the joint surfaces of the substrates  301  and  302  make the transition from the hydrogen bond to the covalent bond or the water molecules and hydrogen that remain on the joint surfaces of the substrates  301  and  302  in a vacuum escape outside the joint interface of the substrates  301  and  302  and a solid covalent bond is formed between the joint surfaces. At this point, presumably, in the course of the water molecules and hydrogen escaping from the joint interface of the temporarily-joined substrates  301  and  302 , the joint surfaces of the substrates  301  and  302  come into mutual contact even in parts where no contact was made during the temporary joining, whereby the joint interface is substantially extended, and the joint area is increased. 
     Here, the above-mentioned hydrophilic treatment that is executed by the hydrophilic treatment device  600  will be described in detail with reference to  FIG.  6   . First, the controller  700  controls the O 2  gas supplier  620 B to introduce O 2  gas into the chamber  612  (Step S 21 ). Specifically, the controller  700  opens the supply valve  622 B so as to introduce O 2  gas into the chamber  612  from the O 2  gas reservoir  621 B through the supply pipe  623 B. 
     Next, with supply of microwaves to the chamber  612  from the magnetron  616  being stopped, the controller  700  controls the high frequency power source  617  to apply a high frequency bias to the substrates  301  and  302  that are held on the stage  610 . As a result, an O 2  reactive ion etching (RIE) treatment to perform RIE using O 2  gas is performed on the joint surfaces of the substrates  301  and  302  (the second etching step) (Step S 22 ). 
     Subsequently, the controller  700  closes the supply valve  622 B to stop supply of O 2  gas into the chamber  612  from the O 2  gas reservoir  621 B, thereby discharging O 2  gas within the chamber  612 . Subsequently, the controller  700  controls the N 2  gas supplier  620 A to introduce N 2  gas into the chamber  612  (Step S 23 ). Specifically, the controller  700  opens the supply valve  622 A to introduce N 2  gas into the chamber  612  from the N 2  gas reservoir  621 A through the supply pipe  623 A. 
     Subsequently, with supply of microwaves to the chamber  612  from the magnetron  616  being still stopped, the controller  700  controls the high frequency power source  617  to apply a high frequency bias to the substrates  301  and  302 . As a result, a N 2  reactive ion etching (RIE) treatment to perform RIE using N 2  gas is performed on the joint surface of each of the substrates  301  and  302  (the first etching step)(Step S 24 ). 
     Next, the controller  700  controls the magnetron  616  to start supply of microwaves to the chamber  612  from the magnetron  616  to generate plasma with N 2  gas. At this point, the controller  700  controls the high frequency power source  617  to stop application of a high frequency bias to the substrates  301  and  302  by the high frequency power source  617 . In this way, a N 2  radical treatment to irradiate the joint surfaces of the substrates  301  and  302  with N 2  radicals (the radical treatment step) is performed (Step S 25 ). Subsequently, the above-described processing of the Step S 3  in  FIG.  5    is executed. 
     Here, in the hydrophilic treatment, according to this embodiment, the above-described processing of the Steps S 21  and S 22  may be omitted. In other words, in the hydrophilic treatment, only the processing of the Steps S 23  to S 25  may be performed. 
     Next, results of assessment of the joint strength of the two substrates  301  and  302  that are joined by the substrate joining system according to this embodiment will be described. Here, results of assessment of the joint strength of the two substrates  301  and  302  that are joined by the substrate joining method according to this embodiment and results of assessment of the joint strength of two substrates that are joined by the substrate joining methods according to Comparative Embodiments 1 to 8 that are described later are described. First, the substrate joining methods according to Comparative Embodiments 1 to 8 will be described. 
     In the substrate joining method, according to Comparative Embodiments 1 to 4, as shown in  FIG.  7 A , only the O 2  RIE treatment is executed on the joint surfaces of the substrates  301  and  302  in the hydrophilic treatment. Moreover, in the substrate joining method according to Comparative Embodiments 5 to 7, as shown in  FIG.  7 B , the O 2  RIE treatment is executed, and then the N 2  radical treatment is executed in the hydrophilic treatment. On the other hand, in the first substrate joining method according to this embodiment (which is hereafter referred to as “the first substrate joining method”), as shown in  FIG.  7 C , the N 2  RIE treatment is executed, and then the N 2  radical treatment is executed in the hydrophilic treatment. Moreover, in the second substrate joining method according to this embodiment (which is hereafter referred to as “the second substrate joining method”), as shown in  FIG.  7 D , the O 2  RIE treatment is executed, then the N 2  RIE treatment is executed, and subsequently, the N 2  radical treatment is executed in the hydrophilic treatment. Furthermore, in the substrate joining method according to Comparative Embodiment 8, as shown in  FIG.  7 E , the N 2  RIE treatment is executed, then the O 2  RIE treatment is executed, and subsequently, the N 2  radical treatment is executed in the hydrophilic treatment. 
     Next, results of assessment of the joint strength of the two substrates  301  and  302  that were joined to each other by the substrate joining methods according to Comparative Embodiments 1 to 8 and by the first substrate joining method and the second substrate joining method according to this embodiment will be described. Here, the explanation is made with regard to a case in which glass substrates were employed as the substrates  301  and  302 . The joint strength was assessed on 12 different Samples 1 to 12 that were obtained by employing different substrate joining methods and using different combinations of treatment times ΔT 1 , ΔT 2 , and ΔT 3  of the O 2  RIE treatment, the N 2  RIE treatment, and the N 2  radical treatment. Here, as the hydrophilic treatment device  600  to use for the O 2  RIE treatment, the N 2  RIE treatment, and the N 2  radical treatment of the substrates  301  and  302 , a configuration comprising, in the configuration shown in  FIG.  2   , plate electrodes that are provided on the glass window  613  and a high frequency power source that is electrically connected to the plate electrodes in place of the magnetron  616  and the waveguide  615  was used. Moreover, the bias power of a high frequency bias that is applied to the substrates  301  and  302  during the O 2  RIE treatment and the N 2  RIE treatment was set to 250 W in all cases. Moreover, the power that is supplied to the chamber  612  from the magnetron  616  during the N 2  radical treatment was set to 250 W in all cases. Moreover, the degree of vacuum within the chamber  200  of the substrate joining device  100  was set to 5.0×10 3  Pa for all samples. Moreover, for joining the substrates  301  and  302 , the substrates  301  and  302  were kept under a pressure of 10N for one minute in all sample cases. Furthermore, the heat treatment was conducted at a treatment temperature of 200° C. for a treatment time of two hours. Table 1 below collectively shows, for each of the 12 different Samples 1 to 12, the substrate joining method employed and the treatment times of the O 2  RIE treatment, the N 2  RIE treatment, and the N 2  radical treatment in the hydrophilic treatment of the substrate joining method. Here, in the column “treatment time” in Table 1, “O 2  RIE” presents the treatment time ΔT 1  of the O 2  RIE treatment, “N 2  RIE” presents the treatment time ΔT 3  of the N 2  RIE treatment, and “N 2  radical” presents the treatment time ΔT 2  of the N 2  radical treatment. Moreover, in the column “substrate joining method,” “Comparative Embodiment 1 (2 to 8)” indicates that the above-described substrate joining method according to Comparative Embodiment 1 (2 to 8) was employed, and “the first substrate joining method” indicates that the above-described first substrate joining method according to the embodiment was employed. Moreover, “the second substrate joining method” indicates that the above-described second substrate joining method, according to the embodiment was employed. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 treatment time (sec) 
                   
               
            
           
           
               
               
               
               
               
            
               
                   
                 O 2   
                 N 2   
                 N 2   
                   
               
               
                   
                 RIE 
                 RIE 
                 radical 
                 substrate joining method 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
            
               
                 Sample 1 
                 30 
                 — 
                 — 
                 Comparative Embodiment 1 
               
               
                 Sample 2 
                 60 
                 — 
                 — 
                 Comparative Embodiment 2 
               
               
                 Sample 3 
                 120 
                 — 
                 — 
                 Comparative Embodiment 3 
               
               
                 Sample 4 
                 180 
                 — 
                 — 
                 Comparative Embodiment 4 
               
               
                 Sample 5 
                 120 
                 — 
                 7.5 
                 Comparative Embodiment 5 
               
               
                 Sample 6 
                 120 
                 — 
                 15 
                 Comparative Embodiment 6 
               
               
                 Sample 7 
                 120 
                 — 
                 30 
                 Comparative Embodiment 7 
               
               
                 Sample 8 
                 — 
                 120 
                 15 
                 First substrate joining method 
               
               
                 Sample 9 
                 — 
                 60 
                 15 
                 First substrate joining method 
               
               
                 Sample 10 
                 30 
                 90 
                 15 
                 Second substrate joining method 
               
               
                 Sample 11 
                 60 
                 60 
                 15 
                 Second substrate joining method 
               
               
                 Sample 12 
                 60 
                 60 
                 15 
                 Comparative Embodiment 8 
               
               
                   
               
            
           
         
       
     
     Moreover, the joint strength of the substrates  301  and  302  of Samples 1 to 12 was assessed by measuring the joint strength (calculated in the surface energy) using the crack and opening method in which a blade is inserted. In the crack and opening method, first, as indicated by the arrow in  FIG.  8 A , the detachment length L between the substrates  301  and  302  when a blade BL such as a razor blade is inserted into a joint part from the rims of the two substrates  301  and  302  that were joined to each other is measured. As the blade BL, for example, a blade of 100 μm in thickness is used. Moreover, as shown in  FIG.  8 B , the detachment length L from the blade contact point when the blade BL was inserted at six points (Pos1, Pos2, Pos3, Pos3, Pos4, Pos5, Pos6) on the rims of the two substrates  301  and  302  that were joined to each other (see the arrows in  FIG.  8 B ) was measured. Then, the joint strength of the substrates  301  and  302  was assessed by obtaining the strength of the joint interface of the substrates  301  and  302  that is calculated in the surface energy per unit area from the detachment length L at each of the six points on the rims of the substrates  301  and  302 . Here, the relational expression (1) below was used to calculate the joint strength (calculated in the surface energy) Eb from the detachment length L. 
     
       
         
           
             
               
                 
                   [ 
                   
                     Math 
                     ⁢ 
                         
                     1 
                   
                   ] 
                 
               
               
                  
               
             
             
               
                 
                   Eb 
                   = 
                   
                     
                       3 
                       × 
                       Y 
                       × 
                       
                         Ts 
                         
                           ? 
                         
                       
                       × 
                       
                         Tb 
                         
                           ? 
                         
                       
                     
                     
                       32 
                       × 
                       
                         L 
                         
                           ? 
                         
                       
                     
                   
                 
               
               
                 
                   Expression 
                   ⁢ 
                       
                   
                     ( 
                     1 
                     ) 
                   
                 
               
             
           
         
       
       
         
           
             
               ? 
             
             indicates text missing or illegible when filed 
           
         
       
     
     in which Y is the Young&#39;s modulus, Ts is the thickness of the substrates  301  and  302 , and Tb is the thickness of the blade BL. In assessment of the joint strength of the substrates  301  and  302  of Samples 1 to 12, the Young&#39;s modulus Y was 6.5×10 10  [N/m 2 ], the thickness Ts of the substrates  301  and  302  was 0.0011 m (1.1 mm), and the thickness Tb of the blade BL was 0.0001 m (0.1 mm). From the calculation formula, the shorter the detachment length is, the higher the joint strength is. Table 2 shows the average values of the joint strengths (calculated in the surface energy) at the six points on the rims of the substrates  301  and  302 . Here, higher calculated joint strengths (calculated in the surface energy) indicate higher joint strengths of the substrates  301  and  302 . Those that were subjected to “bulk destruction” are indicated by “bulk destruction.” Generally, bulk destruction beyond 2 J/m 2  is preferable. Moreover, among those that were subjected to bulk destruction, presumably, the shorter the detachment length was, the higher the joint strength was. Therefore, the detachment lengths are listed as well. 
     Results of assessment of the joint strength of the substrates  301  and  302  of Samples 1 to 12 are shown in Table 2 below and  FIGS.  9 A to  11 D . Here, in Table 2, the column “Sample name” corresponds to each of Samples 1 to 12 in the above-described Table 1. The values in the column “detachment length” present the average values of detachment lengths at the six points (“Pos1” to “Pos6”) on the rims of the two substrates  301  and  302  shown in  FIG.  8 B . Moreover, the values in the column “joint strength (calculated in the surface energy)” present the average values of joint strengths (calculated in the surface energy) at the six points (“Pos1” to “Pos6”) on the rims of the two substrates  301  and  302  shown in  FIG.  8 B . Furthermore, in the column “presence/absence of bulk destruction,” “absence” indicates that no bulk destruction occurred and the numeric values indicate the number of points where bulk destruction occurred among the six points (“Pos1” to “Pos6”) on the rims of the two substrates  301  and  302 .  FIGS.  9 A to  11 D  are photographs that show the appearance of each of Samples 1 to 12. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                 detachment 
                 joint strength 
                 presence/ 
               
               
                   
                 length 
                 (calculated in surface 
                 absence of bulk 
               
               
                 Sample name 
                 (mm) 
                 energy (J/m 2 )) 
                 destruction 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Sample 1 
                 20.3 
                 0.48 
                 absence 
               
               
                 Sample 2 
                 19.3 
                 0.59 
                 absence 
               
               
                 Sample 3 
                 18.5 
                 0.69 
                 absence 
               
               
                 Sample 4 
                 18.8 
                 0.66 
                 absence 
               
               
                 Sample 5 
                 17.3 
                 0.90 
                 absence 
               
               
                 Sample 6 
                 12.6 
                 bulk destruction 
                 2 
               
               
                 Sample 7 
                 15.9 
                 1.26 
                 absence 
               
               
                 Sample 8 
                 6.2 
                 bulk destruction 
                 6 
               
               
                 Sample 9 
                 12.3 
                 bulk destruction 
                 3 
               
               
                 Sample 10 
                 7.0 
                 bulk destruction 
                 5 
               
               
                 Sample 11 
                 5.1 
                 bulk destruction 
                 6 
               
               
                 Sample 12 
                 14.3 
                 1.9  
                 absence 
               
               
                   
               
            
           
         
       
     
     Of Samples 1 to 4 for which the conditions were set in the method that employed the substrate joining method according to Comparative Embodiments 1 to 4 (only the O 2  RIE treatment is performed in the hydrophilic treatment), the joint strength (calculated in the surface energy) was 0.48 to 0.69 J/m 2  and any of Samples 1 to 4 reached the bulk destruction strength at none of the points. Moreover, as shown in  FIGS.  9 A to  9 D , almost no part where the joint surfaces of the substrates  301  and  302  were not joined together was observed in Sample 2 while some parts where the joint surfaces of the substrates  301  and  302  were not joined were observed in Samples 1, 3, and 4. Consequently, the wettability between the substrates  301  and  302  is the best when the treatment time of the O 2  RIE treatment was 120 sec and the joint strength was the highest when the treatment time of the O 2  RIE treatment was 60 sec. From these, presumably, the optimum value of the treatment time of the O 2  RIE treatment is a treatment time of the O 2  RIE treatment between 60 sec and 120 sec or so. Consequently, it is understood that with all efforts of setting conditions in the conventional hydrophilic treatment method according to Comparative Embodiments 1 to 4, a sufficient joint strength cannot be obtained in joining the substrates  301  and  302  together in a vacuum. 
     Moreover, the joint strength (calculated in the surface energy) of Samples 5 and 7 that were obtained by employing the above 120 sec as the treatment time of the O 2  RIE treatment and varying the treatment time of the N 2  radical treatment in the method that employed the substrate joining method according to Comparative Embodiments 5 to 7 (the O 2  RIE treatment and the N 2  radical treatment are performed in the hydrophilic treatment) was 0.90 to 1.26 J/m 2 . Moreover, Sample 6 reached the bulk destruction strength at two points on the rims of the two substrates  301  and  302 . As just stated, the joint strength of the substrates  301  and  302  was improved when the substrate joining method according to Comparative Embodiments 5 to 7 was employed compared with when the substrate joining method according to Comparative Embodiments 1 to 4 was employed. Moreover, presumably, the optimum value of the treatment time of the N 2  radical treatment is around 15 sec. Moreover, as shown in  FIGS.  10 A to  10 C , almost no part where the substrates  301  and  302  were not joined was observed in Samples 5 to 7. In regard to these results, in the substrate joining method according to Comparative Embodiments 5 to 7, presumably, performing the N 2  radical treatment in addition to the O 2  RIE treatment in the hydrophilic treatment resulted in increasing the amount of OH groups that are present on the joint surfaces of the substrates  301  and  302  and thus increasing the joint strength of the substrates  301  and  302 . From these results, in the hydrophilic treatment method according to Comparative Embodiments 5 to 7, presumably, the optimum value of the treatment time of the N 2  radical treatment is around 15 sec. 
     Sample 8 that was obtained by setting conditions in the method that employed the first substrate joining method according to this embodiment reached the bulk destruction strength at all six points on the rims of the two substrates  301  and  302  and the detachment length L was 6.2 mm. Moreover, Sample 9 also reached the bulk destruction strength at three points on the rims of the two substrates  301  and  302  and the detachment length L was 12.3 mm. As just stated, the joint strength of the substrates  301  and  302  was further improved when the first substrate joining method according to this embodiment was employed compared with when the substrate joining method according to Comparative Embodiments 5 to 7 was employed. Moreover, as shown in  FIGS.  10 D and  11 A , defective mutual joint parts of the substrates  301  and  302  occurred in Sample 8 while only voids due to particles occurred at several points in Sample 9. From these results, in the hydrophilic treatment method according to the first substrate joining method in which the processing is executed in the order of the N 2  RIE treatment→N 2  radical treatment, presumably, the optimum value of the treatment time of the N 2  RIE treatment is around 120 sec. In regard to this result, in the first substrate joining method according to this embodiment, presumably, performing the N 2  RIE treatment in place of the O 2  RIE treatment in the hydrophilic treatment resulted in further increasing the amount of OH groups that are present on the joint surfaces of the substrates  301  and  302  and thus further increasing the joint strength of the substrates  301  and  302 . 
     Furthermore, Sample 10 that was obtained by setting conditions in the method that employed the second substrate joining method according to this embodiment reached the bulk destruction strength at five points on the rims of the two substrates  301  and  302  and the detachment length L was 7.0 mm. Moreover, Sample 11 reached the bulk destruction strength at all six points on the rims of the two substrates  301  and  302  and the detachment length L was 5.1 mm. As just stated, given that the treatment time of the N 2  RIE treatment was equal, the joint strength of the substrates  301  and  302  was further improved when the second substrate joining method according to this embodiment was employed compared with when the first substrate joining method was employed. Moreover, as shown in  FIGS.  11 B and  11 C , except for parts due to voids, almost no part where the joint surfaces of the substrates  301  and  302  were not joined together was observed in Samples 10 and 11. Particularly, as shown in  FIG.  11 C , Sample 11 had no void and a high joint strength, whereby presumably, nearly the optimum conditions were set. In regard to these results, in the second substrate joining method according to this embodiment, presumably, performing the O 2  RIE treatment before the N 2  RIE treatment in the hydrophilic treatment resulted in further increasing the joint strength of the substrates  301  and  302  and increasing the wettability between the substrates  301  and  302  as well. 
     On the other hand, Sample 12 reached the bulk destruction strength at none of the six points on the rims of the substrates  301  and  302 . In other words, with the order of the N 2  RIE treatment and the O 2  RIE treatment being reversed from the second substrate joining method, the joint strength was lower than Samples 8 and 6 that were obtained separately by performing only the N 2  RIE treatment or only the O 2  RIE treatment. Moreover, both the wettability and the joint strength of the substrates  301  and  302  were lower compared with Sample 11. From these results, it is understood that the order of the O 2  RIE treatment→the N 2  RIE treatment is important in the hydrophilic treatment. 
     As stated above, it is understood that the joint strength of the substrates  301  and  302  improves when the first substrate joining method or the second substrate joining method according to this embodiment is employed compared with when the substrate joining methods according to Comparative Embodiments 1 to 7 are employed. Moreover, it is also understood that occurrence of parts where the joint surfaces of the substrates  301  and  302  are not joined together is suppressed. Moreover, from the above-described assessment results, it is understood that the O 2  RIE treatment serves to improve the mutual wettability of the substrates  301  and  302  and the N 2  RIE treatment serves to improve the mutual joint strength of the substrates  301  and  302 . In other words, the N 2  RIE treatment alone fails to lead to a sufficient mutual wettability of the substrates  301  and  302  while the O 2  RIE treatment alone fails to lead to a sufficient mutual joint strength of the substrates  301  and  302 . Then, in the processing in the order of the O 2  RIE treatment→the N 2  RIE treatment→the N 2  radical treatment, as the N 2  RIE treatment is performed after removing contaminants such as organic substances that are present on the joint surfaces of the substrates  301  and  302  in the O 2  RIE treatment, presumably, the effect of improving the mutual joint strength of the substrates  301  and  302  in the N 2  RIE treatment is obtained along with improvement in the mutual wettability of the substrates  301  and  302 . Then, presumably, the order of the N 2  RIE treatment→the O 2  RIE treatment does not yield these effects. 
     As described above, in the substrate joining system according to this embodiment, the N 2  RIE treatment is performed on the joint surface of each of the two substrates  301  and  302  and then the N 2  radical treatment is performed on the joint surface of each of the two substrates  301  and  302  in the hydrophilic treatment Specifically, the controller  700  of the substrate joining system controls the N 2  gas supplier  620 A to supply N 2  gas into the chamber  612 . Then, the controller  700  controls the high frequency power source  617  to apply a high frequency bias to the substrates  301  and  302  to perform the N 2  RIE treatment on the joint surfaces of the substrates  301  and  302 . Subsequently, the controller  700  controls the magnetron  616  and the high frequency power source  617  to generate plasma with N 2  gas while stopping application of the high frequency bias to the substrates  301  and  302  to perform the N 2  radical treatment on the joint surfaces of the substrates  301  and  302 . In other words, the N 2  RIE treatment serves for N ions to collide against the joint surfaces of the substrates  301  and  302  with a relatively strong collision force to form many sites for OH group to adhere. Then, the subsequent highly reactive N 2  radical treatment with a relatively weak collision force of N 2  radicals against the joint surfaces of the substrates  301  and  302  serves to form sites for OH groups to adhere while suppressing release of OH groups that have adhered to the sites. As a result, adherence of OH groups to the joint surfaces of the substrates  301  and  302  is efficiently augmented and it is possible to produce a large number of OH groups on the joint surfaces of the substrates  301  and  302 . Thus, when the joint surfaces of the substrates  301  and  302  are brought info mutual contact and the substrates  301  and  302  are joined, it is possible to form a large number of hydrogen bonds between the joint surfaces as a large number of OH groups are produced, whereby the hydrogen bond turns into the covalent bond through a subsequent heating step and the joint strength of the substrates  301  and  302  that are joined to each other is improved. 
     Moreover, the substrate joining system according to this embodiment joins the substrates  301  and  302  in a vacuum. As a result, there is no air that is interposed between the substrates  301  and  302  as in the case of, for example, joining the substrates  301  and  302  in the atmosphere. Thus, air gaps and extra water molecules on the joint interface of the substrates  301  and  302 , which are as a result of the substrates  301  and  302  being joined together with air incorporated in parts of the substrates  301  and  302 , are blown off and occurrence of microvoids is suppressed. 
     Furthermore, in the hydrophilic treatment according to this embodiment, the O 2  RIE treatment using O 2  gas is performed on the joint surface of each of the substrates  301  and  302  before the N 2  RIE treatment. Specifically, the controller  700  of the substrate joining system controls the O 2  gas supplier  620 B to supply O 2  gas into the chamber  612  and then controls the high frequency power source  617  to apply a high frequency bias to the substrates  301  and  302  to perform the O 2  RIE treatment on the joint surface of each of the substrates  301  and  302 . As a result, it is possible to improve the joint strength of the joined substrates  301  and  302 . The strength went up presumably because the O 2  RIE treatment served for the wettability to go up and to remove organic substances that have adhered to the joint surfaces of the substrates  301  and  302 , whereby the N 2  RIE treatment was performed in the absence of contaminants. Conversely, an attempt of the reversed processing (the N 2  RIE treatment→the O 2  RIE treatment) resulted in a deteriorated wettability and a lower strength than the separate processing. The effect of reaction due to the order of the O 2  RIE treatment→the N 2  RIE treatment is presumable. From the above results, it is understood that execution of the processing in the order of the O 2  RIE treatment→the N 2  RIE treatment→the N 2  radical treatment is effective in improving the mutual joint strength of the substrates  301  and  302 . 
     An embodiment of the present disclosure is described above. However, the present disclosure is not restricted to the configuration of the above-described embodiment. For example, a configuration in which the hydrophilic treatment device produces N 2  radicals by inductively coupled plasma (ICP) may be contemplated. For example, as shown in  FIG.  12   , a hydrophilic treatment device  2600  has the stage  610 , a chamber  2612 , a solenoid coil  2616 , and the high frequency power source  617 . Here, in  FIG.  12   , the similar configurations to those in the embodiment are referred to by the same reference numbers as in  FIG.  2   . The N 2  gas reservoir  621 A is connected to the chamber  2612  via the supply valve  622 A and the supply pipe  623 A. Moreover, the O 2  gas reservoir  621 B is also connected to the chamber  2612  via the supply valve  622 B and the supply pipe  623 B. Moreover, the chamber  2612  is connected to the vacuum pump  201  via the discharge pipe  202 A and the discharge valve  203 A. 
     The solenoid coil  2616  is supplied with a high frequency current of, for example, 27.12 MHz. Then, as the high frequency current flows through the solenoid coil  2616  with N 2  gas introduced within the chamber  2612 , high density plasma PLM is formed within the chamber  2612 . Here, ions in the plasma PLM are trapped by the magnetic field that is produced by the solenoid coil  2616  and only radicals in the plasma PLM flow down to the stage  610 . 
     Here, the hydrophilic treatment that is executed by the substrate joining system (the controller of the substrate joining system) according to this modified embodiment will be described in detail with reference to  FIG.  6   . First, the controller controls the O 2  gas supplier  620 B to introduce O 2  gas into the chamber  2612  (Step S 21 ). Next, with the high frequency current flowing through the solenoid coil  2616 , the controller applies a high frequency bias to the substrates  301  and  302  that are held on the stage  610  by means of the high frequency power source  617 . In this way, the O 2  RIE treatment is performed on the joint surfaces of the substrates  301  and  302  (Step S 22 ). 
     Subsequently, the controller discharges the O 2  gas within the chamber  2612  and then controls the N 2  gas reservoir  620 A to introduce N 2  gas into the chamber  2612  (Step S 23 ). Subsequently, with the high frequency current flowing through the solenoid coil  2616 , the controller controls the high frequency power source  617  to apply a high frequency bias to the substrates  301  and  302 . In this way, the N 2  RIE treatment is performed on the joint surfaces of the substrates  301  and  302  (Step S 24 ). 
     Next, with the high frequency current flowing through the solenoid coil  2616 , the hydrophilic treatment device  2600  controls the high frequency power source  617  to sop application of the high frequency bias to the substrates  301  and  302 . In this way, the N 2  radical treatment (the radical treatment step) is performed on the joint surfaces of the substrates  301  and  302  (Step S 25 ). Subsequently, the above-described processing of the Step S 3  in  FIG.  5    is executed. 
     This configuration makes it possible to form higher density plasma PLM in the chamber  2612  than the plasma PLM that can be produced by the hydrophilic treatment device  600  according to Embodiment 1. Thus, it is possible in the N 2  radical treatment to increase the supply amount of N 2  radicals per unit time that are supplied to the joint surfaces of the substrates  301  and  302  and thus increase the amount of N 2  radicals, which is more effective to improve the joint strength of the substrates  301  and  302 . 
     Moreover, the embodiment is described with regard to the case in which the hydrophilic treatment is performed on both of the two substrates  301  and  302 . However, this is not restrictive. For example, a configuration in which the hydrophilic treatment is performed on only one of the two substrates  301  and  302  may be contemplated. 
     Furthermore, the embodiment is described with regard to the case in which the substrates  301  and  302  are any of glass substrates, oxide substrates, and nitride substrates. However, there is no restriction to cases in which both of the substrates  301  and  302  are any of glass substrates, carbide substrates, ceramic substrates, oxide substrates, and nitride substrates and for example, one of the substrates  301  and  302  may be another kind of substrate such as a Si substrate and a sapphire substrate. 
     Furthermore, the embodiment is described with regard to the case in which with the entire joint surfaces of the substrates  301  and  302  being in contact with each other, the substrate joining device  100  pressurizes the substrates  301  and  302  and heats the substrates  301  and  302  by means of the substrate heaters  421  and  422 . However, this is not restrictive. For example, a configuration in which with the entire joint surfaces of the substrates  301  and  302  being in contact with each other, the substrate joining device  100  only pressurizes, does not heat, the substrates  301  and  302  may be contemplated. Alternatively, a configuration in which with the entire joint surfaces of the substrates  301  and  302  being in contact with each other, the substrate joining device  100  only heats, does not pressurize, the substrates  301  and  302  may be contemplated. 
     The embodiment is described with regard to the case in which the substrate joining device  100  pressurizes and heats the substrates  301  and  302 . This configuration is not restrictive. A configuration in which a device that is different from the substrate joining device  100  pressurizes/heats the substrates  301  and  302  may be contemplated. For example, a configuration in which the substrate joining device  100  executes up to temporary joining of the substrates  301  and  302  and subsequently, another heating device (not shown) executes the heat treatment may be contemplated. In such a case, the heat treatment is set for a condition of 180° C. for two hours or so. This results in improving the production efficiency. 
     The embodiment is described with regard to the configuration in which the substrates  301  and  302  are joined together in a vacuum. However, this is not restrictive. A configuration in which the substrates  301  and  302  are joined together under the atmospheric pressure or a configuration in which the substrates  301  and  302  are joined together in an atmosphere that is filled with any gas may be contemplated. 
     Moreover, the embodiment is described with regard to the case in which the cleaning of the substrates  301  and  302  is performed in the cleaning device  940 , then the hydrophilic treatment on the joint surfaces of the substrates  301  and  302  is executed in the hydrophilic treatment device  600 , and subsequently the joining process of the substrates  301  and  302  is executed in the substrate joining device  100 . However, the order of the cleaning, the hydrophilic treatment, and the joining process of the substrates  301  and  302  is not restricted to the above. For example, it may be possible to execute the hydrophilic treatment on the joint surfaces of the substrates  301  and  302 , perform the cleaning of the substrates  301  and  302 , and subsequently execute the joining process of the substrates  301  and  302 . 
     Furthermore, the embodiment is described with regard to the configuration in which the water supply process to expose the joint surfaces of the substrates  301  and  302  to water gas (H 2 O) is performed in the load lock chamber  910  without opening the load lock chamber  910  to the atmosphere. However, the place where the joint surfaces of the substrates  301  and  302  are exposed to water gas is not restricted to the load lock chamber  910 . For example, a configuration in which the joint surfaces of the substrates  301  and  302  are exposed to water gas in the hydrophilic treatment device  600  may be contemplated. In such a case, a configuration in which the above-described water gas generation device is connected to the chamber  612  of the hydrophilic treatment device  600  via a supply valve and a supply pipe may be contemplated. Moreover, a configuration in which the water supply process is performed within the chamber  200  of the substrate joining device  100  or within a chamber that is possessed by the second transportation device  920  may be contemplated. Alternatively, a substrate joining method by which the substrates  301  and  302  are transported to the load lock chamber  910  and then the load lock chamber  910  is opened to the atmosphere to supply moisture to the joint surfaces of the substrates  301  and  302  in the substrate joining method according to the embodiment may be contemplated. In such a case, the atmosphere that is present outside the load lock chamber  910  and has a given humidity is introduced into the load lock chamber  910 . Then, in order to prevent unfavorable contaminants (for example, carbon) in the atmosphere from adhering to the joint surfaces of the substrates  310  and  302  while introducing the atmosphere into the load lock chamber  910 , it is preferable that the load lock chamber  910  has a configuration in which the atmosphere is introduced into the load lock chamber  910  through a given filter. Then, after opening the load lock chamber  910  to the atmosphere, it is merely required to depressurize the load lock chamber again and subsequently transport the substrates  301  and  302  from the load lock chamber  910  to the substrate joining device  100 . With this configuration, there is no need of connecting the water gas supplier  960  to the load lock chamber  910  and thus the configuration of the substrate joining system can be simplified. 
     Alternatively, the substrate joining method may be configured to perform the water supply process to expose the joint surfaces of the substrates  301  and  302  to water gas in the load lock chamber  910  without opening the load lock chamber  910  to the atmosphere and then open the load lock chamber  910  to the atmosphere. 
     The embodiment is described with regard to the configuration in which the water supply process to supply water gas (H 2 O) to the joint surfaces of the substrates  301  and  302  is performed. However, this is not restrictive and for example, gas containing H and OH groups may be supplied to the joint surfaces of the substrates  301  and  302  in place of water gas. 
     The embodiment is described with regard to the case in which immediately after the O 2  RIE treatment is performed on the joint surfaces of the substrates  301  and  302 , O 2  gas is discharged from and N 2  gas is introduced into the chamber  2612  to perform the N 2  RIE treatment on the joint surfaces of the substrates  301  and  302 . However, this is not restrictive and for example, it may be possible to supply gas containing at least one of H 2 O, H and OH groups into the chamber  2612  after performing the O 2  RIE treatment and then perform the N 2  RIE treatment on the joint surfaces of the substrates  301  and  302 . Moreover, it may also be possible to perform the water supply step of supplying gas containing at least one of H 2 O, H, and OH groups to the joint surfaces of the substrates  301  and  302  (the second water supply step) after the step of performing the N 2  RIE treatment (the first etching step). Then, after this water supply step, the N 2  RIE treatment is performed on the joint surfaces of the substrates  301  and  302 . 
     During the N 2  RIE treatment and/or during the N 2  radical treatment, because of a high degree of vacuum within the chamber  2612  and an insufficient amount of water molecules present within the chamber  2612 , a sufficient amount of OH groups may not be produced on the joint surfaces of the substrates  301  and  302  in some cases. On the other hand, with this configuration, gas containing H 2 O or H, OH groups is supplied to the joint surfaces of the substrates  301  and  302  after the O 2  RIE and/or after the N 2  RIE treatment. As a result, it is possible to increase the amount of water molecules that are present in the vicinity of the joint surfaces of the substrates  301  and  302  and thus produce a sufficient amount of OH groups on the joint surfaces of the substrates  301  and  302 . Moreover, the N 2  RIE treatment and/or the N 2  radical treatment is performed on the joint surfaces of the substrates  301  and  302  with H 2 O adhering to the joint surfaces of the substrates  301  and  302 , whereby H 2 O adhering to the joint surfaces of the substrates  301  and  302  turns into plasma and active OH groups easily adhere to the joint surfaces of the substrates  301  and  302 . Alternatively, active H can produce OH groups on the surfaces of oxides that are formed on the joint surfaces of the substrates  301  and  302 . In this way, it is possible to increase the amount of OH groups that are produced on the joint surfaces of the substrates  301  and  302 . Moreover, preferably, it is possible to realize the subsequent plasma treatment without once discharging the gas containing H 2 O or H, OH groups by adding the water supply step of supplying gas containing H 2 O or H, OH groups after the O 2  RIE treatment and subsequent discharge of O 2  gas and introduction of N 2  gas. Moreover, preferably, it is possible to perform the radical treatment with a large amount of gas containing H 2 O or H, OH groups and produce a sufficient amount of OH groups by adding the water supply step of supplying gas containing H 2 O or H, OH groups after the N 2  RIE treatment and before the N 2  radical treatment so as to realize the subsequent plasma treatment without once discharging for exchanging the gas. Moreover, this may be performed both between the O 2  RIE treatment and the N 2  RIE treatment and between the N 2  RIE treatment and the N 2  radical treatment. 
     The embodiment is described with regard to the configuration in which a cooling device for cooling the stage that supports the substrates  301  and  302  is provided in the load lock chamber  910 . However, this is not restrictive and a configuration in which no cooling device is provided in the load lock chamber  910  may be contemplated. 
     The embodiment is described with regard to the case in which the heat treatment to heat the substrates  301  and  302  is performed in the substrate joining device  100 . However, the configuration in which the heat treatment is performed in the substrate joining device  100  is not restrictive. For example, a configuration in which the joining process (temporary joining) is performed in the substrate joining device  100 , and then, the heat treatment is performed on the substrates  301  and  302  in an annealing furnace (not shown) that is a separate body from the substrate joining device  100  may be contemplated. 
     The embodiment is described with regard to the case in which the substrates  301  and  302  that are glass substrates are joined together. This is not restrictive, and one of the substrates  301  and  302  may be a Si substrate. Moreover, the same effect as the embodiment is obtained even when one of the substrates  301  and  302  is a Si substrate, the other is a glass substrate, and the substrates  301  and  302  are anodically joined together. 
     Moreover, when one of the substrates  301  and  302  is a Si substrate, and the other is a glass substrate, it may be possible to perform the conventional hydrophilic treatment on the joint surface of the Si substrate (for example, the process according to Comparative Embodiments 1 to 4) and perform the hydrophilic treatment according to the embodiment only on the joint surface of the glass substrate. Furthermore, the substrates  301  and  302  are not restricted to glass substrates and may be Si substrates, substrates on which an oxide film is formed, substrates on which a nitride film is formed, carbide substrates, or ceramic substrates. Moreover, the substrates  301  and  302  may be a combination of two substrates that are selected from among a glass substrate, a Si substrate, a substrate on which an oxide film is formed, a substrate on which a nitride film is formed, a carbide substrate, and a ceramic substrate. 
     The foregoing describes some example embodiments for explanatory purposes. Although the foregoing discussion has presented specific embodiments, persons skilled in the art will recognize that changes may be made in form and detail without departing from the broader spirit and scope of the invention. Accordingly, the specification and drawings are to be regarded in an illustrative rather than a restrictive sense. This detailed description, therefore, is not to be taken in a limiting sense, and the scope of the invention is defined only by the included claims, along with the full range of equivalents to which such claims are entitled. 
     The present application is based on Japanese Patent Application No. 2016-217582, filed on Nov. 7, 2016. The specification, the scope of claims, and the drawings of the Japanese Patent Application No. 2016-217582 are entirely incorporated herein by reference. 
     INDUSTRIAL APPLICABILITY 
     The present disclosure is preferable for manufacturing; for example, complementary MOS (CMOS) image sensors and memories, arithmetic operation elements, and micro electromechanical systems (MEMS). 
     REFERENCE SIGNS LIST 
     
         
         
           
               100  Substrate joining device 
               200 ,  612 ,  2612  Chamber 
               201  Vacuum pump 
               202 A,  202 B Discharge pipe 
               203 A,  203 B Discharge valve 
               301 ,  302  Substrate 
               401 ,  610 ,  803  Stage 
               402  Head 
               403  Stage driver 
               404  Head driver 
               408  Pressure sensor 
               421 ,  422  Substrate heater 
               500  Misalignment amount measurer 
               600 ,  2600  Hydrophilic treatment device 
               613  Glass window 
               614  Trap plate 
               615  Waveguide 
               616  Magnetron 
               617  High frequency power source 
               620 A N 2  gas supplier 
               620 B O 2  gas supplier 
               621 A N 2  gas reservoir 
               621 B O 2  gas reservoir 
               622 A,  622 B Supply valve 
               623 A,  623 B Supply pipe 
               700  Controller 
               701  MPU 
               702  Main storage 
               703  Auxiliary storage 
               704  Interface 
               705  Bus 
               800  Contour alignment device 
               802  Substrate thickness measurer 
               810  Edge recognition sensor 
               910  Load lock chamber 
               920  Second transportation device 
               930  First transportation device 
               921  Vacuum transportation robot 
               931  Atmospheric transportation robot 
               940  Cleaning device 
               950  Flipping device 
               960  Water gas supplier 
               961  Introduction port 
               962  Retrieval port 
               2616  Solenoid coil