JPWO2019054368A1 - Substrate joining method and sealing structure - Google Patents

Abstract

In a sealing structure formed by joining two substrates, a diffusion coefficient at room temperature such as Ru, Ta, Mo, Hf, W as a base layer between the substrate and Au or Au alloy which is a bonding layer. Use a metal having a value of 1 × 10-60 (m2 / s) or less. Alternatively, a commonly used material such as Ti or Cr is used as the base layer, and a diffusion barrier material such as Pt, Co, Ru, Ta, TiN, TaN, WN, In2O3, or Ni is used between the base layer and the bonding layer. Place layers of.

Description

The present invention is required in the manufacturing process of a chip in which elements such as semiconductor elements, optical elements, MEMS, and gas cells are enclosed inside a substrate, which is produced by sealing two or more wafer-sized substrates. The present invention relates to a joining method for tight sealing.

As one of the packaging methods for integrated devices, as shown schematically in FIG. 1, two wafer-sized substrates are bonded to each other to be formed inside the recess 13 of the device forming substrate 11. The process of sealing the element with the sealing substrate 12, sealing the element in the bonded substrate, and then cutting out (dicing) to individualize the chip 14 to produce the chip 14 is a process that can be easily mass-produced. It has been done a lot in recent years.

As a specific joining technique, a technique of superimposing two substrates and applying high heat and high pressure to join them is generally used. However, in such a joining method, elements, substrates and the like are damaged by high heat and high pressure, so atomic diffusion bonding has been proposed as a method of bonding at low temperature and low pressure (Patent Document 1). In atomic diffusion bonding, as shown in FIG. 2, titanium (Ti), vanadium (V), chromium (Cr), zirconium (Zr), niobium (Nb), and molybdenum are formed on the bonding surface of the smooth substrate to be bonded. From underlayers such as (Mo), hafnium (Hf), titanium (Ta), tungsten (W) and single metals / alloys with gold (Au) or diffusion coefficient of 1 × ^10-45 (m ² / s) or more. Bonding layers are formed under vacuum and brought into contact with each other. Then, atomic diffusion occurs at the bonding interface and grain boundaries, and strong bonding is achieved even at room temperature.

However, in this method, when the thickness of the bonding film increases, it becomes difficult to achieve a bonding interface without defects desirable for airtight sealing, as described in Non-Patent Document 1. For defect-free bonding, the thickness of the bonding film is limited to about 50 nm or less as described in Non-Patent Document 2.

Japanese Patent No. 5569964 JP-A-2016-171393

Yuichi Kurashima, Atsuhiko Maeda, Ryo Takigawa, Hideki Takagi, "Room temperature wafer wafer bonding of metal films using flattening by thermal imprint process", MICROELECTRONIC 12th month, 13th edition, 13th edition, MICROELECTRONIC. 52-56 Eiji Higurashi, Ken Okumura, Yutaka Kunimune, Yuichi Suga, Kei Hagiwara, "Room Temperature Bonding of Wafers with Smart Auto Engine Electronics in Ambient Electronics Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine Engine , February 2017, p. 156-160 Masahiro Kitada, Noboru Shimizu, "Effects of temperature, chicness and atmosphere on mixing in Au-Ti bilayer thin films", Journal of Materials science, 28th edition, p. 5088-5091

The base layer and the bonding layer contain molecules such as hydrogen taken in from the atmosphere during film formation, and gas molecules adhere to the surface of the device during the manufacturing process. After sealing, these molecules are released into the recess (cavity) sealed as a gas by the temperature rise of the device or an electric action, which causes deterioration of the operating accuracy of the device and the product life. Therefore, in a general substrate sealing process, it is essential to perform an annealing treatment (baking) before sealing in order to degas the inclusions and adhered molecules.

Patent Document 2 discloses that in a manufacturing process of a MEMS oscillator, a device is manufactured in a recess of a device wafer, then high-temperature baking is performed, and then the sealed wafer is joined in a vacuum. However, since the step of forming the bonding layer (connection electrode) formed on the surface of the sealed wafer is performed after the baking step and the baking is not performed thereafter, degassing from this layer and these are performed. There is a problem that the sealing environment deteriorates due to degassing of the surface substance adhering in the process. Further, Patent Document 2 has a problem that 390 ° C. or higher is required for joining.

Further, depending on the type of the element, a getter material such as Ti may be arranged in advance in the recess in order to control the atmosphere inside the sealed cavity. In this case, the getter material is used. However, it is necessary to perform degassing treatment by annealing before joining.

However, when the annealing treatment is performed, metal diffusion occurs between the bonding layer and the underlying layer, and the atoms of the underlying layer, which are easily oxidized, diffuse to the surface to form an oxide film on the surface. This phenomenon is more prominent when the bonding layer is thin as described in Non-Patent Document 3. When the substrates are bonded in such a state, the formation of interatomic bonds at the bonding interface is inhibited by this oxide film, and as a result, there arises a problem that the bonding strength is reduced. In particular, in a very thin bonding layer used for atomic diffusion bonding, an oxide layer is formed on the surface even by annealing at a relatively low temperature for a short time, and the bonding is hindered.

In this way, degassing from the substrate and internal structure causes deterioration of the atmosphere inside the seal, so degassing treatment by annealing before joining is essential, and strong joining can be achieved even after this annealing treatment. A joining method is required.

Solution of the invention

In order to solve the above problems, in the present invention, as a base layer between a substrate and an Au alloy such as Au or Au-Ag or Au-Cu having a thickness of 50 nm or less, which is a bonding layer, Ru, Ta, Mo, W and the like are used. Use a metal having a diffusion coefficient of 1 × 10 ^-60 (m ² / s) or less at room temperature. Alternatively, a commonly used material such as Ti or Cr is used as the base layer, and platinum (Pt), cobalt (Co), ruthenium (Ru), tantalum (Ta), etc. are used between the base layer and the bonding layer. Layers of diffusion barrier material such as titanium nitride (TiN), tantalum nitride (TaN), tungsten nitride (WN), indium oxide (In ₂ O ₃ ), nickel (Ni) are arranged.

Then, as schematically shown in FIG. 3, if the baking is not performed, degassing occurs after sealing, and the atmosphere in the cavity deteriorates. On the other hand, after the base layer and the bonding layer are formed on the substrate, they are annealed in vacuum or in an inert gas to be degassed, and then bonded, so that the inside is released by gas even after sealing. It is possible to realize joining with less deterioration of the atmosphere. The annealing temperature should be at least 100 ° C., which is the boiling point of water, and more preferably 200 ° C. or higher. On the other hand, at a high temperature of more than 300 ° C., the annealing temperature is preferably 300 ° C. or lower because bonding becomes difficult due to an increase in surface roughness due to recrystallization of the metal and diffusion of the base film.

In order to reduce the influence of thermal residual stress after sealing, the bonding is performed by cooling to a suitable temperature and performing low temperature bonding. Silicon (Si), glass, crystal, sapphire, etc. are used as the substrate, but by using different materials for the element forming substrate and the sealing substrate, a semiconductor material such as Si, which is easy to be finely processed, is used for the element forming substrate. , It is possible to use transparent and light-transmitting sapphire for the sealing substrate. However, when such dissimilar substrates are bonded to each other, problems such as deformation of the bonded body occur due to the difference in the coefficient of thermal expansion between the two, so low temperature bonding is particularly preferable.

The method of joining two substrates of the present invention to form a sealed structure includes the following steps.
A step of preparing a first substrate having at least one recess and a flat surface around it and a second substrate having a flat surface.
A step of forming an underlayer on the flat surface of the first substrate and the flat surface of the second substrate.
A step of forming a metal bonding layer on the base layer of the first and second substrates, and
A step of degassing the first and second substrates on which the base layer and the metal bonding layer are formed by annealing.
The recesses of the first substrate are sealed by the second substrate by atomic diffusion bonding between the metal bonding layers of the first and second substrates after the degassing treatment by the annealing. Step.

For the base layer according to the bonding method of the present invention, a metal material having a diffusion coefficient of 1 × 10 ^-60 (m ² / s) or less at room temperature may be used, and in particular, Ta, Ru, Mo, Hf, and W. It may contain at least one of.

Alternatively, the bonding method of the present invention may further include a step of forming a diffusion barrier layer between the base layer and the metal bonding layer of the first and second substrates, and in particular, the base layer is Ti. Alternatively, the diffusion barrier layer containing Cr may be at least one of Pt, Co, Ru, Ta, TiN, TaN, WN, In ₂ O ₃ and Ni.

Further, the metal bonding layer according to the bonding method of the present invention may contain Au.

Further, at least one of the first and second substrates according to the joining method of the present invention may be a Si substrate.

The degassing treatment by annealing according to the bonding method of the present invention heats the first and second substrates at a temperature of 100 ° C. or higher, preferably 200 ° C. or higher, in dry air, a reduced pressure atmosphere or an inert atmosphere. You may.

Further, the step of atomic diffusion bonding of the first and second substrates by the bonding method of the present invention may be performed at a temperature lower than the temperature of the degassing treatment.

Further, the sealing structure according to the joining method of the present invention may include a gas cell of an atomic clock. In the gas cell according to the present invention, the first and second substrates are translucent, two or more recesses are formed in the first substrate, and a Cs atom supply source (cesium dispenser) is formed in one recess. ) And the getter material are arranged in advance, and the plurality of recesses are formed in an atmosphere containing neon (Ne), which is the atmosphere at the time of sealing, and Cs atoms generated from the Cs atom source, and are formed by sealing. It is used as an atomic clock by transmitting light through a cell in which the Cs atom supply source and the getter material are not arranged in advance and measuring the resonance frequency among the cells. The dispenser is not limited to the cesium dispenser, and may be a rubidium (Rb) dispenser or the like. Further, the atmospheric gas at the time of sealing is not limited to Ne gas, and may be another inert gas such as Ar, or a mixed gas thereof including Ne.

According to the joining method of the present invention, long-term stability of devices such as semiconductor devices, optical devices, MEMS, gas cells, etc., which are manufactured by sealing and joining the devices manufactured in the recesses of the device forming substrate with a sealing substrate. Sex can be realized.

Further, especially in the case of a gas cell of an atomic clock, since Cs dispersed in Ne sealed in the cell easily reacts with a degassing component, a Cs supply source and a gas getter material are arranged in a recess before sealing. Cs can be stably sealed in the cell by sealing after the degassing treatment by annealing.

FIG. 1 is a schematic explanatory view showing a process of manufacturing an element by joining wafer-sized substrates. FIG. 2 is a schematic explanatory view showing a joining process by a conventional method. FIG. 3 is a schematic explanatory view showing the effect of “burning”. FIG. 4 is a schematic explanatory view showing a joining process according to the present invention. FIG. 5 is an explanatory diagram showing the principle of a method for measuring joint strength (crack opening method). FIG. 6 is a table comparing the bonding strengths according to the examples and comparative examples of the present invention. FIG. 7 is a schematic view showing a manufacturing process of a gas cell. FIG. 8 is a schematic view showing a cross section of the gas cell. FIG. 9A is a photograph of the cell portion of the calcined gas cell observed from above with a microscope. (B) is a spectrum of a transition frequency peculiar to Cs in Ne gas measured using the cell of (A). (C) is a photograph of the cell portion of the gas cell that is not baked, observed from above with a microscope. FIG. 10 is a diagram showing the atomic distribution in the depth direction by XPS after the annealing treatment of the Au / Ti / Si laminated structure. FIG. 11 is a diagram showing the atomic distribution in the depth direction by XPS after the annealing treatment of the Au / Cr / Si laminated structure. FIG. 10 is a diagram showing the atomic distribution in the depth direction by XPS after the annealing treatment of the Au / Pt / Ti / Si laminated structure. FIG. 13 is a diagram showing the atomic distribution in the depth direction by XPS after the annealing treatment of the Au / Ta / Si laminated structure. FIG. 14 is a diagram showing the surface roughness of each laminated structure after sputtering, plasma treatment, and annealing treatment measured by AFM.

Embodiments of the present invention will be described in detail below with reference to the drawings.

1. 1. Substrate bonding Fig. 4 shows an outline of the substrate bonding method according to the present invention. As the first method, a metal having a diffusion coefficient of 1 × 10 ^-60 (m ² / s) or less is used as the base layer. Table 1 shows the self-diffusion coefficient of the main metals used in the semiconductor process at room temperature. ^{Examples of the} metal having a diffusion coefficient of 1 × 10 ^-60 (m ² / s) or less include Hf, Mo, Nb, Ta, and W. Here, Ta was formed as a base layer at 10 nm and Au was formed as a bonding layer on a 400 μm-thick Si substrate by a 12 nm sputtering method. Before joining these samples together, annealing treatment was performed at 200 ° C. for 10 minutes in a reduced pressure atmosphere of about 1 × 10 ^-3 (Pa). After that, the sample was cooled to 40 ° C., and a load of 123 kPa was applied to the sample to join the two substrates. Bond strength was measured in the Massara blade test. The Maszara blade test is a method of measuring the energy γ required to peel off the substrate by inserting the blade (here, the teeth of a safety razor) between the bonded substrates, as shown in the outline of the principle in FIG. is there. As a result, a large value of 5.0 (J / m ² ) was obtained in this example.

The second method of substrate bonding according to the present invention is a method of sandwiching a diffusion barrier layer using a material having a diffusion barrier effect between the base layer and the bonding layer. The embodiment will be described below. On a 400 μm-thick Si substrate, Ti was formed as a base layer at 5 nm, followed by Pt as a diffusion barrier layer at 10 nm, and Au was formed as a bonding layer on the Si substrate by 12 nm sputtering. Before joining the samples prepared in this way, annealing treatment was performed at 200 ° C. for 10 minutes in a reduced pressure atmosphere of about 1 × 10 ^-3 (Pa). The sample was then cooled to 40 ° C. and a load of 123 kPa was applied to the sample for bonding. The bonding strength measured in the Massara blade test was 3.2 (J / m ² ).

On a 400 μm-thick Si substrate, Ti was formed as a base layer at 5 nm, followed by Pt as a diffusion barrier layer at 10 nm, and Au was formed as a bonding layer on the Si substrate by 12 nm sputtering. Before joining the samples thus prepared, annealing treatment was performed at 250 ° C. for 10 minutes in a reduced pressure atmosphere of about 1 × 10 ^-3 (Pa). After that, the sample was cooled to around 40 ° C., and a load of 123 kPa was applied to the sample for joining. When the Massara blade test was performed, the fracture occurred from the Si substrate, not from the bonding interface.

Comparative Example 1

Next, as a comparative example, the results when a metal having a diffusion coefficient larger than 1 × ^10-60 (m ² / s) is used as the base layer will be described. A sample in which Cr was 5 nm as a base layer and an Au bonding layer was formed on the Si substrate with a thickness of 400 μm at 12 nm was prepared by stappering, and before joining these samples, 1 × 10 ^-3 (Pa) was prepared. Annealing treatment was performed at 200 ° C. for 10 minutes in a reduced pressure atmosphere. The sample was then cooled to 40 ° C. and a load of 123 kPa was applied to the sample for bonding. In this case, the bonding strength measured in the bonding Massara blade test was 0.04 (J / m ² ), which was a very small value (FIG. 6).

Comparative Example 2

A sample in which Ti was formed as a base layer on a 400 μm-thick Si substrate by 5 nm and an Au bonding layer was formed on it by 12 nm stappering was prepared, and before joining these samples, 1 × 10 ^-3 (Pa) was prepared. Annealing treatment was performed at 200 ° C. for 10 minutes in a reduced pressure atmosphere. After that, the sample was cooled to 40 ° C., and a load of 123 kPa was applied to the sample to perform bonding, but bonding was not performed (FIG. 6).

The bonding strength (J / m ² ) according to each layer structure and bonding conditions obtained in this manner is summarized in FIG. The column of "Joining (room temperature)" shows the results of conventional studies for comparison. "Base material fracture" means that the base metal was broken because the joint strength was much higher than the strength of the Si base material (2.5 J / m ² ), and "joining failure" means that the joint could not be joined. .. As shown in FIG. 6, good bonding strength was obtained by the method using the layer structure of Au / Ta / Si or Au / Pt / Ti / Si according to the present invention.
2. 2. Fabrication of gas cell according to the present invention

FIG. 7 shows an outline of a gas cell manufacturing process using the present invention. A transparent sapphire is used for the substrate 21 for forming the element, and two columnar recesses 23 having a diameter and a depth of 26 mm of 2 mm are formed from one side of the substrate with a center spacing of 4 mm, and the two recesses are surfaces. It is connected by a groove 25 having a line width of 0.5 mm and a depth of 0.1 mm provided on the side. This is not necessarily limited to two, and three or more may be formed and each may be connected by a groove. A flat sapphire substrate was also used for the upper sealing substrate 22.

Next, Ti was formed as a base layer at 5 nm on the flat outermost surface of the device forming substrate 21, Pt was formed at 10 nm as a diffusion barrier layer, and Au was formed on the flat outermost surface as a bonding layer by 12 nm sputtering to form a multilayer metal structure. 31 was prepared. On the other hand, on the sealing substrate 22, Ti was similarly formed at 5 nm as a base layer and then Pt at 10 nm as a diffusion barrier layer on the surfaces other than the portions corresponding to the recesses 23 and the grooves 25 of the element forming substrate. Au was formed as a bonding layer on the film by 12 nm sputtering, respectively, and a multilayer metal structure 32 was similarly produced.

Next, after irradiating each Au surface on both substrates with Ar plasma at 200 W for 30 seconds to remove organic substances adhering to the bonded surface, the cesium dispenser 41 and the getter material 42 are placed in one recess 23 of the element forming substrate. Was placed. Then, using an airtight sealing joining device, the inside of the joining device was evacuated to about ^10-4 Pa, and then both substrates were heated to 200 ° C. and baked for 10 minutes. Next, the Ne gas was introduced into the apparatus until air pressure in the 10 4 ^Pa, these recesses do hermetically joining the inside of the recess as Ne gas atmosphere was gas cell.

After bonding, the cesium dispenser was irradiated with a 5 W laser for 1 minute and heated to release cesium atoms into the gas cell. FIG. 9A shows a photograph of the cell prepared in this way and after releasing cesium, observed from above with a microscope. Gloss due to the deposition of Cs metal was observed on the side surface of the cylindrical recess. On the other hand, due to the presence of the groove 25 between the two recesses, Cs atoms are diffused even in the recesses where the cesium dispenser and the getter material are not arranged, and the recesses without the cesium dispenser and the getter material are used for time measurement. Use. That is, as shown in FIG. 8, by passing the light from the light source 44 through this cell and detecting it with the detector 46, as shown in FIG. 9B, the transition frequency peculiar to Cs in Ne gas is obtained. I measured it.

Comparative Example 3

Instead of the multilayer metal structure (Au / Pt / Ti) of the above example, Ti was formed by 5 nm and an Au bonding layer was formed on it by 12 nm sputtering without using Pt. Sealing and joining was attempted by the same process. As a result, joining was not possible.

Comparative Example 4

Using the same multilayer metal structure (Au / Pt / Ti) as in Example 3, sealing and bonding were performed in Ne gas at room temperature without performing a baking step. The other steps are the same as in the third embodiment. An attempt was made to emit Cs by irradiating the cesium dispenser with a laser under the same conditions as in Example 3, but as shown in FIG. 9 (C), metallic luster due to metallic cesium could not be confirmed on the side surface of the recess. It was. This is because water and other gases were generated inside the gas cell because the baking treatment was not performed before joining, and cesium reacted with water to form cesium hydroxide.
3. 3. Component diffusion analysis

In order to investigate the cause of the difference in the bonding strength due to the difference in the layer structure described above, the distribution in the depth direction of the component elements of the film after the annealing treatment (baking) was examined by XPS (X-ray Photoelectron Spectroscopy). The results are shown in FIGS. 10 to 13. In FIG. 10, a sample having a layered structure of Au (12 nm) / Ti (5 nm) / Si (400 μm) prepared by sputtering is ultrasonically cleaned, surface-cleaned with Ar plasma, and then 200 ° C. at 1 × 10 ^-3 (Pa). The distribution in the depth direction of the component elements after the annealing treatment for 10 minutes is shown. As can be seen from this result, Ti and O are detected on the surface, and it can be seen that TiOx is formed. Therefore, it can be inferred that the presence of this oxide film reduces the bonding strength.

FIG. 11 shows the depth distribution of the component elements of the similarly prepared sample having a layer structure of Au (12 nm) / Cr (5 nm) / Si (400 μm) after undergoing exactly the same treatment as the above sample. Similar to FIG. 10, Cr and O are detected on the surface, and it can be seen that CrOx is formed. Therefore, it can be inferred that the presence of this oxide film also reduces the bonding strength.

FIG. 12 shows the depth of the component elements of the similarly prepared sample having a layer structure of Au (12 nm) / Pt (10 nm) / Ti (5 nm) / Si (400 μm) after the same treatment as the above sample. Shows the directional distribution. Unlike FIG. 10, no Ti element was found on the surface, indicating that TiOx was not formed on the surface. Therefore, it can be inferred that the joint strength does not decrease. It is considered that this is because Pt acts as a heat diffusion barrier.

FIG. 13 shows the distribution of the component elements in the depth direction of the similarly prepared sample having a layer structure of Au (12 nm) / Ta (10 nm) / Si (400 μm) after undergoing exactly the same treatment as the above sample. It can be seen that no Ta element is found on the surface. Therefore, it can be inferred that the bonding strength does not decrease because no oxide is formed on the surface. It is considered that this is because the body diffusion coefficient of Ta is very small, so that the annealing treatment at such a temperature and time does not diffuse to the surface.

Factors other than surface oxides that affect the bonding strength include changes in surface smoothness due to annealing treatment. Therefore, in order to investigate the smoothness of each of the above samples after each treatment, the result of measuring the RMS (Root Mean Square) of the surface profile of AFM (Atomic Force Microscope) is shown in FIG. .. In the case of Au / Ti / Si and Au / Cr / Si having an oxide film formed on the surface, the RMS (roughness) of the surface is greatly increased after annealing, whereas the oxide film is formed on the surface. In the case of Au / Pt / Ti / Si and Au / Ta / Si that were not used, it can be seen that the increase in RMS was small even after annealing and was 0.6 nm or less. It is considered that one of the factors that the bonding strength does not decrease is that the deterioration of the surface smoothness due to this annealing treatment is small.

Although the above description has been made for Examples, it will be apparent to those skilled in the art that the present invention is not limited thereto and various modifications and modifications can be made within the scope of the spirit of the present invention and the appended claims. For example, in the above embodiment, the case where the surface of the element-forming substrate in which the element is formed in the recess and the surface of the sealing substrate for sealing the recess are the same has been described, but the layer configurations are different in both substrates. You may. For example, the device forming substrate may be formed by a method using a metal having a diffusion coefficient of 1 × 10 ^-60 (m ² / s) or less, and the sealing substrate may be formed by a method using a diffusion barrier layer, or vice versa. Good. Further, the base layer and the bonding layer of the present invention are formed on both sides of the third substrate, and the layers are arranged so as to be sandwiched between the element forming substrate and the sealing substrate, and the three substrates are bonded to form the element. It may be produced.

11, 21 Element forming board 12, 22 Encapsulation board 13, 23 Recess 14 Chip 25 Groove 26 Recess depth 31, 32 Multi-layer metal structure 40 Gas cell 41 Cesium dispenser 42 Getter material 44 Light source 46 Detector

Claims (20)

It is a method of forming a sealing structure by joining two substrates.
A step of preparing a first substrate having at least one recess and a flat surface around it, and a second substrate having a flat surface.
A step of forming a base layer on the flat surface of the first substrate and the flat surface of the second substrate.
A step of forming a metal bonding layer on the base layer of the first and second substrates, and
A step of degassing the first and second substrates on which the base layer and the metal bonding layer are formed by annealing.
The recesses of the first substrate are sealed by the second substrate by atomic diffusion bonding between the metal bonding layers of the first and second substrates after the degassing treatment by the annealing. Substrate joining method including steps. The method for joining a substrate according to claim 1, wherein the base layer is made of a metal material having a diffusion coefficient of 1 × 10 ^-60 (m ² / s) or less at room temperature. The method for joining a substrate according to claim 2, wherein the base layer contains at least one of Ta, Ru, Mo, Hf, and W. The method for joining a substrate according to claim 1, further comprising a step of forming a diffusion barrier layer between the base layer and the metal bonding layer of the first and second substrates. The fourth aspect of the present invention, wherein the base layer contains Ti or Cr, and the diffusion barrier layer contains at least one of Pt, Co, Ru, Ta, TiN, TaN, WN, In ₂ O ₃ and Ni. How to join the substrates. The method for joining a substrate according to any one of claims 1 to 5, wherein the metal bonding layer contains Au. The method for joining a substrate according to any one of claims 1 to 6, wherein the thickness of the metal bonding layer is 50 nm or less. The method for joining a substrate according to any one of claims 1 to 7, wherein the surface roughness of the metal bonding layer after the degassing treatment by annealing is 0.6 nm or less in terms of root mean square roughness. .. Any of claims 1 to 8, wherein the degassing treatment by annealing includes heating the first and second substrates at a temperature of 100 ° C. or higher and 300 ° C. or lower in a reduced pressure atmosphere or an inert atmosphere. The method for joining the substrates according to item 1. The substrate according to claim 9, wherein the degassing treatment by annealing includes heating the first and second substrates at a temperature of 200 ° C. or higher and 300 ° C. or lower in a reduced pressure atmosphere or an inert atmosphere. Joining method. The method for joining a substrate according to claim 9 or 10, wherein the step of atomic diffusion bonding of the first and second substrates is performed at a temperature lower than the temperature of the degassing treatment. It is a sealing structure including a gas cell formed by joining two substrates, and the sealing structure is
It has a first substrate having at least one recess and a flat surface around it, including a base layer formed on the flat surface and a metal bonding layer formed on the base layer, and a flat surface. A second substrate including a base layer formed on the flat surface and a metal bonding layer formed on the base layer is included.
The base layer, the metal bonding layer, and the recess formed on the first and second substrates, respectively, are annealed at a temperature of at least 100 ° C. or higher and 300 ° C. or lower, degassed, and then degassed. A sealing structure in which the metal bonding layers of the first and second substrates are bonded to each other. The first and second substrates are translucent, a plurality of the recesses are formed in the first substrate, and a metal atom source of Cs or Rb is arranged in advance in at least one of the recesses. , The plurality of recesses are formed as a plurality of cells by being sealed in the external atmosphere of the inert gas, and each cell has the inert gas atmosphere containing metal atoms generated from the metal atom supply source. The gas cell according to claim 12, wherein light is transmitted through a cell in which the metal atom supply source is not arranged in advance, the resonance frequency of the metal atom is measured, and the time is measured. The gas cell according to claim 13, wherein the inert gas is Ne or a rare gas other than Ne, or a mixed gas of Ne and a rare gas other than Ne. The sealing structure according to any one of claims 12 to 14, wherein the base layer is made of a metal material having a diffusion coefficient at room temperature of 1 × 10 ^-60 (m ² / s) or less. The sealing structure according to claim 15, wherein the base layer contains at least one of Ta, Ru, Mo, Hf, and W. The sealing structure according to any one of claims 12 to 14, wherein a diffusion barrier layer is formed between the base layer and the metal bonding layer of the first and second substrates. 17. The underlying layer comprises Ti or Cr, and the diffusion barrier layer comprises at least one of Pt, Co, Ru, Ta, TiN, TaN, WN, In ₂ O ₃ and Ni. Sealing structure. The sealing structure according to any one of claims 12 to 18, wherein the metal bonding layer contains Au. The sealing structure according to any one of claims 12 to 19, wherein the thickness of the metal bonding layer is 50 nm or less.