Structure and method for sealing through-hole, and transfer substrate for sealing through-hole

Abstract

A sealing structure with a surface of a base material with a through-hole, an underlying metal film, and a sealing member bonded to the underlying metal film to seal the through-hole. The sealing member includes a compressed product of a metal powder including gold having a purity of 99.9% by mass or more and a lid-like metal film including a bulk-like metal including gold and having a thickness of not less than 0.01 m and not more than 5 m. The sealing material includes an outer periphery-side densified region in contact with an underlying metal film and a center-side porous region in contact with the through-hole. The shape of pores in the densified region is specified, and the horizontal length (l) of a pore in the radial direction at any cross-section of the densified region and the width (W) of the densified region satisfy the relationship of l0.1W.

Claims

1. A sealing structure comprising: a set of base materials forming a sealed space; at least one through-hole which is formed in at least one of the set of base materials, and communicates with the sealed space; and a sealing member that seals the through-hole, wherein the sealing structure comprises an underlying metal film on a surface of the base material in which the through-hole is formed, the underlying metal film comprising a bulk-like metal including at least one of gold, silver, palladium and platinum, the underlying metal film being formed so as to surround the peripheral part of the through-hole, the sealing member seals the through-hole while being bonded to the underlying metal film, the sealing member comprises: a sealing material which is bonded to the underlying metal film, and includes a compressed product of a metal powder including gold having a purity of 99.9% by mass or more; and a lid-like metal film which is bonded to the sealing material, and comprises a bulk-like metal including gold at least at a surface that is in contact with the sealing material, the bulk-like metal having a thickness of not less than 0.01 m and not more than 5 m, the sealing material comprises: an outer periphery-side densified region that is in contact with the underlying metal film; and a center-side porous region that is in contact with the through-hole, and a horizontal length (l) of a pore in a radial direction at any cross-section of the densified region and a width (W) of the densified region satisfy the relationship of 10.1 W.

2. The sealing structure according to claim 1, wherein the sealing material has a thickness of not less than 0.1 m and not more than 10 m.

3. The sealing structure according to claim 1, wherein the underlying metal film has a thickness of not less than 0.01 m and not more than 10 m.

4. The sealing structure according to claim 1, wherein a void content at any cross-section of the densified region is 20% or less in terms of an area ratio.

5. The sealing structure according to claim 1, wherein an area of a lateral cross-section of the sealing material is not less than 1.2 times and not more than 50 times the area of a lateral cross-section of the through-hole.

6. The sealing structure according to claim 1, wherein the sealing material is formed by compressing a sintered body of a metal powder including gold, the metal powder having a purity of 99.9% by mass or more and an average particle size of 0.01 m to 1.0 m.

7. A sealing method for forming the sealing structure defined in claim 1, comprising the steps of: providing a transfer substrate at a position corresponding to a position of a through-hole of the base material that forms a sealing space, the transfer substrate comprising the lid-like metal film including a bulk-like metal including gold at least at a surface that is in contact with the sealing material, the bulk-like metal having a thickness of not less than 0.01 m and not more than 5 m, and the metal powder sintered body obtained by sintering a metal powder including gold having a purity of 99.9% by mass or more, the metal powder having an average particle size of not less than 0.01 m and not more than 1.0 m; forming an underlying metal film on a surface of the base material provided with the through-hole, in such a manner that the underlying metal film surrounds at least the peripheral part of the through-hole; superposing the transfer substrate and the base material one on the other so as to be opposed to each other so that the through-hole is sealed with the metal powder sintered body being in contact with the underlying metal film; and pressing the transfer substrate to form the sealing material from the metal powder sintered body, and bond the sealing material to the underlying metal.

8. The sealing method according to claim 7, wherein the transfer substrate is pressed while at least one of the transfer substrate and the base material is heated at 80 C. to 300 C.

9. A transfer substrate to be used in the sealing method defined in claim 7, comprising; a substrate; a projection portion formed at a position corresponding to a position of a through-hole of the base material on the substrate; the lid-like metal film which is formed on at least the projection portion, and includes a bulk-like metal including gold at least at a surface that is in contact with the sealing material, the bulk-like metal having a thickness of not less than 0.01 m and not more than 5 m; and the metal powder sintered body which is formed on the lid-like metal film, and obtained by sintering a metal powder including gold, the metal powder having a purity of 99.9% by mass or more and an average particle size of 0.01 m to 1.0 m.

10. The transfer substrate according to claim 9, wherein the metal powder sintered body has a thickness of not less than 0.5 m and not more than 20 m.

11. The sealing structure according to claim 2, wherein the underlying metal film has a thickness of not less than 0.01 m and not more than 10 m.

12. A transfer substrate to be used in the sealing method defined in claim 8, comprising; a substrate; a projection portion formed at a position corresponding to a position of a through-hole of the base material on the substrate; the lid-like metal film which is formed on at least the projection portion, and includes a bulk-like metal including gold at least at a surface that is in contact with the sealing material, the bulk-like metal having a thickness of not less than 0.01 m and not more than 5 m; and the metal powder sintered body which is formed on the lid-like metal film, and obtained by sintering a metal powder including gold, the metal powder having a purity of 99.9% by mass or more and an average particle size of 0.01 m to 1.0 m.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 illustrates a configuration of a sealing material in a sealing structure according to the present invention, and dimension conditions for pores in the sealing material.

(2) FIGS. 2A, 2B, 2C, and 2D illustrates illustrate a process for producing a transfer substrate in a first embodiment.

(3) FIGS. 3A, 3B, and 3C illustrate a step of forming a base material (sealed space) in the first embodiment.

(4) FIGS. 4A, 4B, and 4C illustrate a step of sealing the sealed space by use of the transfer substrate in the first embodiment.

(5) FIG. 5 is a SEM photograph showing cross-section structures of the sealing material formed in the first embodiment.

DESCRIPTION OF EMBODIMENTS

First Embodiment

(6) Hereinafter, a preferred embodiment of the present invention will be described. In this embodiment, a transfer substrate including a sealing material obtained by using a metal powder including gold having a purity of 99.9% was produced, and a base material to be sealed was processed, followed by conducting a test in which a through-hole of the substrate was sealed.

(7) (i) Producing of Transfer Substrate

(8) FIGS. 2A, 2B, 2C, and 2D illustrate a process for producing a transfer substrate in this embodiment. First, a silicon wafer substrate was provided, and a projection portion was formed by dry etching in conformation to the diameter and position (pitch) of the through-hole of the base material to be sealed (FIG. 2A). The projection portion is a columnar projection having a diameter of 500 m and a height of 10 m.

(9) Next, as a lid-like metal film, gold (thickness: 0.5 m, FIG. 2B) was deposited on the top surface of the projection portion of the silicon wafer substrate by sputtering. In this embodiment, the lid-like metal film was deposited over the whole surface of the substrate so as to reliably deposit each thin film on the top surface of the projection portion. As described above, a gold film (lid-like metal film) may be formed on the top of the projection portion from a practical point of view.

(10) A metal paste was applied to form a metal powder sintered body on the lid-like metal film. As the metal paste, a metal paste prepared by mixing a gold powder (average particle size: 0.3 m, purity: 99.9%) produced by a wet reduction method with isobornyl cyclohexanol (Terusolve MTPH) as an organic solvent was used (mixing ratio of gold powder: 90% by mass). The metal paste was applied by a printing method with the substrate covered with a metal mask perforated so as to surround the projection portion (FIG. 2C). After application of the metal paste, the substrate was heated at 200 C. for 2 hours to sinter the metal powder, so that a 5 m-thick metal powder sintered body was formed on the top surface of the projection portion to complete the transfer substrate (FIG. 2D).

(11) (ii) Pretreatment of Base Material

(12) Base materials to be subjected to a sealing treatment in this embodiment include a silicon wafer (upper base material) and a glass wafer (lower base material), and a sealed space is formed by a set of two substrates (FIG. 3A). The base materials each have a thickness of 0.5 mm. A plurality of cavities (72 cavities) as sealed spaces are formed in the silicon wafer, and a through-hole communicating the cavities is formed. On the other hand, the glass wafer is a flat plate material. The cavity of the silicon wafer has a size of 2 mm square, and the through-hole is a hole having a circular cross-section with a diameter of 100 m.

(13) For formation off the sealed space, the silicon wafer and the glass wafer were positioned, and then bonded together by anodic bonding that is a conventional art (FIG. 3B). A bond aligner (BA 8 manufactured by SUSS MICROTEC SE.) was used for the positioning, and a wafer holder (SB 8e manufactured by SUSS MICROTEC SE.) was used for the anodic bonding. As bonding conditions, the wafers were bonded at 400 C. and 800 V for 30 minutes under a low pressing pressure in a vacuum atmosphere (10 Pa).

(14) After the base materials were bonded to form a sealed space, gold was deposited as an underlying metal film on the periphery of the through-hole of the silicon wafer (FIG. 3C). In this embodiment, the underlying metal film was formed over the whole surface of the wafer, a titanium/platinum film was formed for securing adhesion, and gold was then sputtered. The gold film has a thickness of 0.5 m.

(15) (iii) Airtight Sealing of Sealed Space of Base Material

(16) The through-hole of the sealed space of the base material was sealed by use of the transfer substrate produced in (i). For matching the positions of the transfer substrate and the base material, a bond aligner (BA 8 manufactured by SUSS MICROTEC SE.) was used to perform positioning so that the projection portion of the transfer substrate corresponded to the through-hole as in FIG. 4A. Thereafter, by use of a wafer holder (SB 8e manufactured by SUSS MICROTEC SE.), the pressure was reduced to a vacuum atmosphere (10 Pa), the transfer substrate was then abutted against the base material, and pressurized, and the transfer substrate and the base material were heated by a tool with a heater. Here, as a transfer condition, the pressure of the top surface of the projection portion was 100 MPa, and as a heating condition, heating was performed to 200 C. at a temperature elevation rate of 30 C./min.

(17) After the temperature of the transfer substrate reached 200 C., the transfer substrate was held for 10 minutes while being loaded (FIG. 4B). Thereafter, the transfer substrate was unloaded, and moved. Observation of the base material after removal of the transfer substrate revealed that the sealing member and the lid-like metal film of the transfer substrate were bonded to the through-hole (FIG. 4C). The sealing material was compressively deformed, and had a thickness of 1 m.

(18) FIG. 5 shows photographs of cross-sections of a densified region at an outer peripheral part and a porous region at a central part of the metal powder compressed product as a sealing material for the sealing structure formed in this embodiment. In this embodiment, the average value of the width W of the densified region was 200 m.

(19) The photographs of FIG. 5 are photographs where a cross-section of the prepared sealing structure is processed and polished by a focused ion beam apparatus (FIB), and observed by SEM (30,000 fold, inclined to 45). FIG. 5 revels a difference in densification degree between the densified region and the porous region. In the porous region, a plurality of pores (voids) are continuously connected. On the other hand, the densified region also has very small pores, but these pores are isolated from one another.

(20) The horizontal length of a pore at any cross-section is examined with reference to FIG. 5. For all of pores visible in the image, among pores observed in the photograph of the cross-section in FIG. 5, the horizontal length (l) between both ends was measured. The results showed that the horizontal length (l) and the width (W) of an outer periphery portion as the densified region met the relationship of l0.1 W for each measurement. In this embodiment, the minimum value of the measured length l was about 0.005 times the width W, and the maximum value of the measured length l was about 0.01 times the width W.

(21) In addition, from the image of a photographed cross-section, the porosity in the densified region was calculated by image analysis software (trade name AZOKUN (ver. 2.54) manufactured by Asahi Kasei Engineering Corporation.). In image analysis, the image was subjected to binarization processing analysis to separate pores from metal particles. As a specific procedure of the image analysis, the number of structures in a target image was set to 2 on the basis of the structure analysis tool of image analysis software, and the threshold for binarization was determined within a range of 150 to 160 while the original image was observed. The porosity was calculated by computation in the tool. The result showed that the porosity of the densified region in this embodiment was 10.4%.

(22) Next, airtightness of the sealed space was examined for the base material, the through-hole of which was sealed. For the examination, a helium leak test (bell jar method) was conducted. In this evaluation, samples having a helium leak rate of 10.sup.9 Pa.Math.m.sup.3/s were rated acceptable, and the helium leak rate in this embodiment was 10.sup.11 Pa.Math.m.sup.3/s to 10.sup.13 Pa.Math.m.sup.3/s. The result showed that in the sealing structure of this embodiment, the sealed space was airtightly sealed effectively.

Second Embodiment

(23) Here, with a base material (sealed space) identical to that in the first embodiment, a sealing structure was formed while the constituent material and the thickness of the sealing material, and the configuration and the thickness of the lid-like metal film are adjusted. A transfer substrate was prepared while the solvent and the metal content of the metal paste were the same as in the first embodiment, and the particle size, the type and the coating thickness of the metal powder were changed. In the same manner as in the first embodiment, a silicon wafer (upper base material) and glass wafer (lower base material) were bonded to form a sealed space, so that the through-hole of the base materials was sealed. Table 1 shows the configurations of various sealing structures produced by way of trial in this embodiment, the relationship between the horizontal length of the pore and the width W, and the results of the leak test.

(24) TABLE-US-00001 TABLE 1 Hole of densified region Sealing material Maximum Sealing test results Metal Particle Thickness Lid-like metal film Underling metal film 1 value of Void Leak rate Assess- No. powder size (D)*.sup.1 Metal Thickness Metal Thickness 0.1 W I/W*.sup.2 content (Pa .Math. m.sup.3/s) ment 1 Au 0.3 m 1 m Au 0.5 m Au 0.5 m 0.01 10.4% 10.sup.11 to 10.sup.13 Passed*.sup.3 2 0.01 m 0.005 9.3% 10.sup.11 to 10.sup.13 Passed 3 1.0 m 0.08 15.0% .sup.10.sup.9 to 10.sup.11 Passed 4 1.2 m X 0.12 22.5% 10.sup.6 to 10.sup.8 Not passed 5 0.3 m 5 m 0.01 10.5% 10.sup.11 to 10.sup.13 Passed 6 7 m 0.05 15.0% 10.sup.6 to 10.sup.8 Passed 7 10 m 0.10 19.8% 10.sup.6 to 10.sup.8 Passed 8 0.05 m X 0.11 21.0% 10.sup.6 to 10.sup.8 Not passed 9 1 m 5 m 0.01 m 0.01 10.6% 10.sup.11 to 10.sup.13 Passed 10 0.01 m 10 m 0.01 10.3% 10.sup.11 to 10.sup.13 Passed 11 7 m 0.5 m X 0.12 20.5% 10.sup.6 to 10.sup.8 Not passed 12 Au/Pt 0.5 m/ 0.5 m 0.005 9.0% 10.sup.11 to 10.sup.13 Passed 0.05 m 13 Au/Pd 0.5 m/ 0.005 8.5% 10.sup.11 to 10.sup.13 Passed 0.05 m 14 Ag 0.5 m Au 0.5 m X 0.11 21.0% 10.sup.6 to 10.sup.8 Not passed 15 Au 0.5 m None Sealing Not impossible*.sup.4 passed 16 None Au 0.5 m Sealing Not impossible*.sup.4 passed 17 Ag 0.3 m 1 m Au 0.5 m Ag 0.5 m X 0.13 25.0% 10.sup.6 to 10.sup.8 Not passed 18 Pd Au Pd X 0.12 24.5% 10.sup.6 to 10.sup.8 Not passed 19 Pt Au Au X 0.13 22.0% 10.sup.6 to 10.sup.8 Not passed *.sup.1Thickness of sealing material (metal powder compressed product) after sealing *.sup.2Based on radial horizontal length of largest hole in observed holes *.sup.3No. 1 indicates first embodiment *.sup.4It was impossible to conduct leak test because sealed space of base material was not brought into vacuum

(25) Table 1 indicates that favorable airtight sealing properties are obtained with sealing structures having a gold powder as a sealing material. When metal powders Nos. 17 to 19, that is powders of metals other than gold, were used as sealing materials, it was possible to perform sealing to some degree, but the samples were rated unacceptable in the leak test. Of course, even when a gold powder is applied, airtightness is reduced when the powder has an excessively large particle size (No. 4). This is because when the metal powder is coarse, a compressed product is formed with pores having an unpreferable shape even though an underlying metal film and a lid-like metal film exist.

(26) In the present invention, the type and the thickness of the lid-like metal influences sealing performance. When the lid-like formed film was more than 5 m (No. 11), a sufficient sealing structure was not formed. From the results of No. 1 to No. 4, it was determined that the lid-like metal film had a sufficient thickness of 5 m. In addition, when a metal other than gold was applied as a lid-like metal (No. 14), the sample was rated unacceptable as a sealing test result, and it was determined that application of gold was essential. However, regarding the configuration of the lid-like metal film, airtight sealing can be attained when a surface that is in contact with the sealing material is composed of Au.

(27) Examination the thickness of the sealing material revealed that a sufficient sealing effect was exhibited even when the sealing material had a relatively large thickness of 10 m. In addition, it was confirmed that even a sealing material having a small thickness of not more than 5 m was effective. Therefore, it was possible to confirm that the present invention was effective even when the sealing material or the like is thinned for reducing the height of a substrate and a product. However, it was not possible to perform airtight sealing when the sealing material was excessively thin (No. 8).

(28) The above results of examination have revealed that for exhibiting an appropriate airtight sealing property, it was important to control the maximum value of the horizontal length of the maximum pore in a radial direction as a shape of pores in the densified region of the sealing material.

INDUSTRIAL APPLICABILITY

(29) In the present invention, a predetermined metal powder compressed product is applied as a medium for sealing a through-hole in a method for sealing a sealed space including a through-hole. In the present invention, generation of an outgas, which is a problem in conventional arts such as brazing material welding and anodic bonding, and therefore the through-hole can be sealed at a relatively low temperature without possibility of contaminating the sealed space. A sealing structure according to the present invention can be appropriately formed as a transfer substrate, and is applicable even when a plurality of sealed spaces are set for one base material as in a wafer-level packaging because the sealing structure can be efficiently formed. The present invention is effective for airtight sealing of MEMS devices such as pressure sensors and acceleration sensors, and various semiconductor devices.