WO2019054368A1

WO2019054368A1 - Method for joining substrates, and sealing structure

Info

Publication number: WO2019054368A1
Application number: PCT/JP2018/033626
Authority: WO
Inventors: 貴司松前; 優一倉島; 高木　秀樹
Original assignee: 国立研究開発法人産業技術総合研究所
Priority date: 2017-09-15
Filing date: 2018-09-11
Publication date: 2019-03-21
Also published as: JPWO2019054368A1; JP7028473B2

Abstract

Provided is a sealing structure formed by joining two substrates, wherein a metal having a diffusion coefficient of 1 × 10-60 (m2/s) or less at normal temperature, such as Ru, Ta, Mo, Hf, or W, is used as an underlayer between the substrates and either Au or an Au alloy serving as a joining layer. Alternatively, a typically used material such as Ti or Cr is used as the underlayer, and a layer of a diffusion barrier material such as Pt, Co, Ru, Ta, TiN, TaN, WN, In2O3, or Ni is disposed between the underlayer and the joining layer.

Description

Bonding method of substrate and sealing structure

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

As one of the packaging methods for integrated devices, as shown schematically in FIG. 1, substrates of two wafer sizes were bonded to each other, and they were fabricated in the recess 13 of the element forming substrate 11. After the element is sealed with the sealing substrate 12 and the element is sealed in the bonded substrate, the process of fabricating the chip 14 into pieces by dicing (dicing) is a process that can be easily mass-produced. It has been carried out in recent years.

As a specific bonding technique, a technique in which two substrates are stacked and bonded by applying high heat and pressure is generally used. However, in such a bonding method, an element, a substrate and the like are damaged by high heat and high pressure, so atomic diffusion bonding is 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), molybdenum are formed on the bonding surface of a smooth substrate which is a bonding material, as shown in FIG. Underlayers such as (Mo), hafnium (Hf), tantalum (Ta), tungsten (W) and gold (Au) or a single metal or alloy having a diffusion coefficient of 1 × 10 ^-45 (m ² / s) or more The resulting bonding layer is formed under vacuum and brought into contact. Then, atomic diffusion occurs at the bonding interface and grain boundaries, and a strong bonding can be achieved even at room temperature.

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

Patent No. 5569964 gazette Unexamined-Japanese-Patent No. 2016-171393 specification

The above-described base layer and bonding layer contain molecules such as hydrogen taken in from the atmosphere at the time of film formation, and gas molecules are also attached to the surface of the element in the manufacturing process. These molecules are released into the inside of the recess (cavity) sealed as a gas as a result of temperature rise of the element or electric action after sealing, and cause deterioration of the operation accuracy and product life of the element. Therefore, in a general substrate sealing process, it is essential to perform annealing (baking out) before sealing in order to degas the molecules contained and attached.

Patent Document 2 discloses that, in a process of manufacturing a MEMS vibrator, high-temperature bake-out is performed after manufacturing an element in a recess of an element wafer, and then the sealed wafer is bonded in a vacuum. However, since the step of forming the bonding layer (connection electrode) formed on the surface of the sealing wafer is performed after the baking step, and the baking is not performed thereafter, degassing from this layer, and the like There is a problem that the sealing environment is degraded due to the degassing of the surface material attached in the process. Moreover, in patent document 2, there also exists a problem that 390 degreeC or more is required for joining.

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

However, when the annealing process is performed, metal diffusion occurs between the bonding layer and the base layer, atoms of the base layer which is easily oxidized diffuse to the surface, and an oxide film is formed on the surface. As described in Non-Patent Document 3, this phenomenon is more pronounced when the bonding layer is thin. When the substrates are bonded in such a state, formation of interatomic bonds at the bonding interface is inhibited by the oxide film, resulting in a problem that the bonding strength is reduced. In particular, in the case of 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 and for a short time, and the bonding is inhibited.

As described above, since degassing from the substrate and the internal structure causes deterioration of the atmosphere inside the seal, degassing treatment by annealing before bonding is essential, and a strong bonding can be achieved even after this annealing treatment. A bonding method is required.

Solution of the invention

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

Then, as schematically shown in FIG. 3, when the baking is not performed, degassing after sealing occurs, and the atmosphere in the cavity is degraded. On the other hand, after forming the base layer and the bonding layer on the substrate, annealing and degassing in vacuum or in an inert gas are performed, and thereafter bonding is performed, so that the inside due to gas release is obtained even after sealing. Bonding with less deterioration of the atmosphere can be realized. The annealing temperature needs to be at least 100 ° C., which is the boiling point of water, at least 200 ° C., more preferably. On the other hand, at a high temperature exceeding 300 ° C., bonding becomes difficult due to an increase in surface roughness due to recrystallization of metal and diffusion of an underlayer, so the annealing temperature is preferably 300 ° C. or less.

The bonding is cooled to a suitable temperature to perform low temperature bonding in order to reduce the influence of thermal residual stress after sealing. As the substrate, silicon (Si), glass, quartz, sapphire, or the like is used. By using different materials for the element formation substrate and the sealing substrate, the element formation substrate is a semiconductor material such as Si which is easily microfabricated. For the sealing substrate, it is possible to use sapphire or the like which is transparent and transmits light. However, when such dissimilar substrates are bonded to each other, a low temperature bonding is particularly preferable because a defect such as deformation of a bonded body occurs due to the difference in thermal expansion coefficient between the two.

The method of bonding two substrates of the present invention to form a sealed structure includes the following steps.
Providing a first substrate having at least one recess and a flat surface around it, and a second substrate having a flat surface,
Forming an underlayer on the planar surface of the first substrate and the planar surface of the second substrate;
Forming a metal bonding layer on the underlayers of the first and second substrates;
Degassing the first and second substrates on which the base layer and the metal bonding layer are formed by annealing;
The metal bonding layer of each of the first and second substrates after the degassing process by the annealing is atomic diffusion bonded to seal the recess of the first substrate with the second substrate. Step.

The base layer according to the bonding method of the present invention may use a metal material having a diffusion coefficient of 1 × 10 ⁻⁶⁰ (m ² / s) or less at normal temperature, and in particular, Ta, Ru, Mo, Hf, and W. At least one of the above.

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

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 bonding method of the present invention may be a Si substrate.

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

The step of atomic diffusion bonding the first and second substrates according to 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 bonding 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, and two or more recesses are formed in the first substrate, and a Cs atom source (cesium dispenser is provided in one recess) And the getter material are arranged in advance, and the insides of the plurality of recesses are made in an atmosphere containing neon (Ne) which is an atmosphere at the time of sealing and Cs atoms generated from the Cs atom source, and formed by sealing. Among the cells, the Cs atom source and the getter material are not arranged in advance, and light is transmitted through the cells to measure the resonance frequency, whereby the atomic clock is used. The dispenser is not limited to the cesium dispenser, and may be a rubidium (Rb) dispenser or the like. Further, the atmosphere 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 including Ne.

According to the bonding method of the present invention, long-term stability of a device such as a semiconductor device, an optical device, a MEMS, or a gas cell manufactured by sealing and bonding a device manufactured in a recess of a device forming substrate with a sealing substrate. Can be realized.

Further, in the case of a gas cell of an atomic clock in particular, Cs dispersed in Ne sealed in the cell is likely to react with the degassing component, so a Cs supply source and a gas getter material are disposed in the recess before sealing. Cs can be stably sealed in the cell by sealing after degassing treatment by annealing.

FIG. 1 is a schematic explanatory view showing a process of manufacturing an element by bonding of a wafer size substrate. FIG. 2 is a schematic explanatory view showing a bonding process according to a conventional method. FIG. 3 is a schematic explanatory view showing the effect of “baking out”. FIG. 4 is a schematic explanatory view showing a bonding process according to the present invention. FIG. 5 is an explanatory view showing the principle of the method of measuring the bonding strength (crack opening method). FIG. 6 is a table comparing the bonding strength according to the example of the present invention and the comparative example. FIG. 7 is a schematic view showing the gas cell manufacturing process. FIG. 8 is a schematic view showing a cross section of the gas cell. FIG. 9A is a photograph obtained by observing the cell portion of the gas cell subjected to baking out from above with a microscope. (B) is a spectrum of a transition frequency specific to Cs in Ne gas measured using the cell of (A). (C) is the photograph which observed the cell part of the gas cell which does not bake out with the microscope from the top. FIG. 10 is a view showing an atomic distribution in the depth direction by XPS after annealing treatment of the Au / Ti / Si laminated structure. FIG. 11 is a diagram showing an atomic distribution in the depth direction by XPS after annealing treatment of the Au / Cr / Si laminated structure. FIG. 10 is a view showing an atomic distribution in the depth direction by XPS after annealing treatment of the Au / Pt / Ti / Si laminated structure. FIG. 13 is a diagram showing atomic distribution in the depth direction by XPS after annealing treatment of the Au / Ta / Si laminated structure. FIG. 14 is a view showing the surface roughness after sputtering, after plasma processing, and annealing after measuring each laminated structure by AFM.

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

1. Substrate Bonding An overview of the method of substrate bonding according to the present invention is shown in FIG. As a first method, a metal having a diffusion coefficient of 1 × 10 ⁻⁶⁰ (m ² / s) or less is used as the underlayer. Table 1 shows self-diffusion coefficients at normal temperature of main metals used in the semiconductor process. ^{Examples of} metals having a diffusion coefficient of 1 × 10 ⁻⁶⁰ (m ² / s) or less include Hf, Mo, Nb, Ta, and W. Here, 10 nm of Ta as a base layer and 12 nm of Au as a bonding layer were formed on a 400 μm thick Si substrate by sputtering. Before bonding these samples to each other, annealing was performed at 200 ° C. for 10 minutes in a reduced pressure atmosphere of about 1 × 10 ⁻³ (Pa). Thereafter, the sample was cooled to 40 ° C., and a load of 123 kPa was applied to the sample to bond the two substrates. Bonding strength was measured by Maszara blade test. As shown in the outline of the principle of the Maszara blade test in FIG. 5, the blade (here, the teeth of the safety razor) is inserted between the bonded substrates, and the energy γ necessary for peeling the substrates is measured. is there. As a result, in this example, a large value of 5.0 (J / m ² ) was obtained.

The second method of substrate bonding according to the present invention is a method of sandwiching a diffusion barrier layer made of a material having a diffusion barrier effect between an underlayer and a bonding layer. The embodiment will be described next. A Ti film of 5 nm as a base layer and a Pt film of 10 nm as a diffusion barrier layer were formed on a 400 μm thick Si substrate, and a Au film of 12 nm was formed thereon as a bonding layer. Before bonding the samples produced in this manner, annealing was performed at 200 ° C. for 10 minutes in a reduced pressure atmosphere of about 1 × 10 ⁻³ (Pa). Thereafter, the sample was cooled to 40 ° C. and a load of 123 kPa was applied to the sample for bonding. The bonding strength measured by the Maszara blade test was 3.2 (J / m ² ).

A Ti film of 5 nm as a base layer and a Pt film of 10 nm as a diffusion barrier layer were formed on a 400 μm thick Si substrate, and a Au film of 12 nm was formed thereon as a bonding layer. Annealing treatment was performed at 250 ° C. for 10 minutes in a reduced pressure atmosphere of about 1 × 10 ⁻³ (Pa) before joining the samples produced in this manner. Thereafter, the sample was cooled to around 40 ° C., and a load of 123 kPa was applied to the sample to perform bonding. When the Maszara blade test was conducted, it broke from the Si substrate rather than from the bonding interface.

Comparative Example 1

Next, as a comparative example, the result in the case of using a metal having a diffusion coefficient larger than 1 × 10 ⁻⁶⁰ (m ² / s) as the underlayer will be described. Prepare a sample formed by sputtering 5 nm of Cr as a base layer and 12 nm of Au bonding layer on a 400 μm thick Si substrate by sputtering, and before bonding these samples to 1 × 10 ^-3 (Pa) Annealing was performed at 200 ° C. for 10 minutes in a somewhat reduced pressure atmosphere. Thereafter, the sample was cooled to 40 ° C. and a load of 123 kPa was applied to the sample for bonding. In this case, the bonding strength measured by the bonding Maszara blade test was a very small value of 0.04 (J / m ² ) (FIG. 6).

Comparative example 2

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

The structure of each layer thus obtained and the bonding strength (J / m ² ) according to the bonding conditions are summarized in FIG. Note that the column "Bonding (room temperature)" shows the results of conventional studies for comparison. "Base material failure" represents that the base material was broken because the bonding strength was much larger than the strength (2.5 J / m ² ) of the Si base material, and "bonding failure" represents that the bonding could not be made . 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. Fabrication of gas cell according to the present invention

FIG. 7 shows an outline of the process of manufacturing a gas cell using the present invention. A transparent sapphire is used as a substrate 21 for forming an element, and two cylindrical recesses 23 each having a diameter and depth 26 of 2 mm are formed at a distance of 4 mm from one surface of the substrate. It is connected by a groove 25 with 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 connected by a groove. In addition, a flat sapphire substrate was used as the upper sealing substrate 22.

Next, 5 nm of Ti as an underlayer, 10 nm of Pt as a diffusion barrier layer, and 12 nm of Au as a bonding layer are formed respectively on the flat outermost surface of the element forming substrate 21 by sputtering to form a multilayer metal structure 31 was produced. On the other hand, on the sealing substrate 22, similarly 5 nm of Ti as an underlayer and 10 nm of Pt as a diffusion barrier layer are formed on the surface of the element forming substrate other than the portions corresponding to the recess 23 and the groove 25. Au was formed into a film respectively by 12 nm sputtering as a joining layer on it, and the multilayer metal structure 32 was similarly produced.

Next, after irradiating Ar plasma with 200 W for 30 seconds on each Au surface on both substrates to remove the organic substance adhering to the bonding surface, the cesium dispenser 41 and the getter material 42 are formed in one concave portion 23 of the element formation substrate. Placed. Thereafter, the inside of the bonding apparatus was vacuumed to about 10 ^-4 Pa using a hermetic sealing bonding apparatus, and both substrates were heated to 200 ° C. and baked out for 10 minutes. Next, Ne gas was introduced to the inside of the apparatus to a pressure of 10 ⁴ Pa, and the inside of the recess was made into a Ne gas atmosphere to perform airtight bonding, and these recesses were made gas cells.

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. (A) of FIG. 9 shows a photograph of the cell thus produced and released from cesium and observed from above with a microscope. The gloss due to the deposition of Cs metal was observed on the side of the cylindrical recess. On the other hand, due to the presence of the groove 25 between the two recesses, Cs atoms are also diffused to the recesses where the cesium dispenser and the getter material were not disposed, and for the measurement of time, the recesses without the cesium dispenser and the getter material Use. That is, as shown in FIG. 8, the light from the light source 44 is allowed to pass through this cell and detected by the detector 46, whereby the transition frequency specific to Cs in Ne gas is obtained as shown in FIG. It measured.

Comparative example 3

Instead of using the multilayer metal structure (Au / Pt / Ti) of the above embodiment, Pt is not used, Ti is deposited to 5 nm, and an Au bonding layer is formed thereon by 12 nm sputtering. The sealing joint was tried by the same process. As a result, it was not possible to join.

Comparative example 4

The same multi-layered metal structure (Au / Pt / Ti) as in Example 3 was used, and sealing and bonding were performed at room temperature in Ne gas without performing the baking step. The other steps are the same as in Example 3. Cs release was attempted by irradiating a cesium dispenser with a laser under the same conditions as in Example 3, but as shown in (C) of FIG. 9, the metallic gloss due to cesium in the metallic state could not be confirmed on the side surface of the recess. The This is because water and other gases were generated inside the gas cell and cesium reacted with water to form cesium hydroxide because baking was not performed before bonding.
3. Component diffusion analysis

In order to investigate the factor of the difference in bonding strength due to the difference in layer structure described above, the depth direction distribution of the component elements of the film after annealing (baking out) was investigated by XPS (X-ray Photoelectron Spectroscopy). The results are shown in FIGS. FIG. 10 ultrasonically cleans a sample of the layer structure of Au (12 nm) / Ti (5 nm) / Si (400 μm) prepared by sputtering, and cleans the surface with Ar plasma and then 200 ° C. at 1 × 10 ⁻³ (Pa) 14 shows the depthwise distribution of component elements after annealing for 10 minutes. 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 distribution in the depth direction of the component elements of the sample of the layer structure of Au (12 nm) / Cr (5 nm) / Si (400 μm) prepared in the same manner 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 similarly reduces the bonding strength.

FIG. 12 shows the depths of component elements of the sample of the layer structure of Au (12 nm) / Pt (10 nm) / Ti (5 nm) / Si (400 μm) prepared in the same manner as the above sample. Indicates the direction distribution. Unlike FIG. 10, no Ti element is observed on the surface at all, and it can be seen that TiOx is not formed on the surface. For this reason, it can be inferred that the bonding strength does not decrease. It is considered that this is because Pt acts as a thermal diffusion barrier.

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

A factor other than the surface oxide that affects the bonding strength is a change in the surface smoothness due to the annealing treatment. Therefore, in order to examine the smoothness after each treatment of each sample, the result of measuring the root mean square (RMS) of the surface profile of an AFM (Atomic Force Microscope) is shown in FIG. . In the case of Au / Ti / Si and Au / Cr / Si in which an oxide film is formed on the surface, an oxide film is formed on the surface, while the RMS (roughness) of the surface is greatly increased after annealing. In the case of Au / Pt / Ti / Si and Au / Ta / Si, which were not performed, it is understood that the increase in RMS is small even after annealing, and is 0.6 nm or less. It is considered that one less deterioration of the surface smoothness due to the annealing treatment is one of the factors that the bonding strength does not decrease.

Although the above description is made for the examples, it is obvious to those skilled in the art that the present invention is not limited thereto, and various changes and modifications can be made within the spirit of the present invention and the scope of the appended claims. For example, in the above embodiment, the case where the layer structure of the surface of the element forming substrate in which the element is formed in the recess and the surface of the sealing substrate sealing the recess are the same has been described. May be For example, the element formation 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. Furthermore, the base layer and the bonding layer of the present invention are formed on both surfaces of the third substrate, and this is disposed between the element forming substrate and the sealing substrate, and arranged to bond three substrates. You may produce.

11, 21

Element Forming Substrate

12, 22

Sealing Substrate

13, 23 Recess 14 Chip 25 Groove 26

Recess Depth

31, 32 Multilayer Metal Structure 40 Gas Cell 41 Cesium Dispenser 42 Getter Material 44 Light Source 46 Detector

Claims

A method of forming a sealing structure by bonding two substrates,
Providing a first substrate having at least one recess and a flat surface therearound, and a second substrate having a flat surface,
Forming an underlayer on the planar surface of the first substrate and the planar surface of the second substrate;
Forming a metal bonding layer on the underlayers of the first and second substrates;
Degassing the first and second substrates on which the base layer and the metal bonding layer are formed by annealing;
The metal bonding layer of each of the first and second substrates after the degassing process by the annealing is atomic diffusion bonded to seal the recess of the first substrate with the second substrate. And bonding the substrate.
The method 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 normal temperature.
The method of claim 2, wherein the underlayer includes at least one of Ta, Ru, Mo, Hf, and W. 4.
The method according to claim 1, further comprising: forming a diffusion barrier layer between the underlayer and the metal bonding layer of the first and second substrates.
The underlayer according to claim 4, wherein the underlayer 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. Board bonding method.
The method of bonding substrates according to any one of claims 1 to 5, wherein the metal bonding layer contains Au.
7. The method according to claim 1, wherein the thickness of the metal bonding layer is 50 nm or less.
The method according to any one of claims 1 to 7, wherein a surface roughness of the metal bonding layer after degassing treatment by annealing is 0.6 nm or less in a root mean square roughness. .
The degassing treatment by annealing includes heating the first and second substrates at a temperature of 100 ° C. or more and 300 ° C. or less in a reduced pressure atmosphere or in an inert atmosphere. A method of bonding a substrate according to any one of the preceding claims.
10. The substrate according to claim 9, wherein the annealing degassing process comprises heating the first and second substrates at a temperature of 200 ° C. or more and 300 ° C. or less in a reduced pressure atmosphere or in an inert atmosphere. Bonding method.
11. The method of bonding a substrate according to claim 9, wherein the step of atomic diffusion bonding the first and second substrates includes performing at a temperature lower than the temperature of the degassing process.
A sealing structure comprising a gas cell formed by joining two substrates, the sealing structure comprising
A first substrate having at least one recess and a flat surface around the first substrate, the first substrate including an underlayer formed on the flat surface and a metal bonding layer formed thereon, and a flat surface A second substrate including an underlayer formed on the flat surface and a metal bonding layer formed thereon;
The underlayer and the metal bonding layer formed on the first and second substrates and the recess are annealed and degassed at a temperature of at least 100 ° C. and at a temperature of 300 ° C. or less, 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, and a plurality of the recesses are formed in the first substrate, and a metal atom source of Cs or Rb is disposed in advance in at least one of the recesses. And sealed in an external atmosphere of an inert gas to form the plurality of recesses as a plurality of cells, and each cell is in the inert gas atmosphere containing metal atoms generated from the metal atom supply source. The gas cell according to claim 12, wherein the light is transmitted to a cell in which the metal atom source is not disposed 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 that, or a mixed gas of Ne and a rare gas other than the Ne.
The sealing structure according to any one of claims 12 to 14, wherein the underlayer is made of a metal material having a diffusion coefficient of 1 × 10 -60 (m 2 / s) or less at normal temperature.
The sealing structure according to claim 15, wherein the underlayer includes 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 underlayer and the metal bonding layer of the first and second substrates.
The underlayer according to claim 17, wherein the underlayer 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. Sealed 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 a film thickness of the metal bonding layer is 50 nm or less.