Secondary seal vacuum packaging process

Abstract

The application relates to a secondary sealing vacuum packaging process, which comprises the following steps: (1) adopting white tin as sealing solder to realize the first vacuum sealing; (2) placing the MEMS micro-cavity structure after the first sealing under normal temperature and pressure conditions; (3) second sealing alignment: aligning the MEMS micro-cavity structure after the placement with second sealing solder, installing the MEMS micro-cavity structure on a refrigeration and heating table, and placing a catalyst between the MEMS micro-cavity structure and the second sealing solder; (4) vacuumizing and keeping the vacuum packaging device, setting the temperature of the refrigeration and heating table to be lower than 13.2 DEG C, and converting the white tin into grey tin; (5) second vacuum sealing: vacuumizing the vacuum packaging device, increasing the temperature of the refrigeration and heating table to a welding temperature, melting the second sealing solder and the fallen grey tin powder, and welding the second sealing solder and the electrode together, and completing the second vacuum sealing after cooling. The sealing process can improve the production capacity of the vacuum packaging process and reduce the product cost.

Description

Technical Field

[0001] This invention belongs to the field of sensors, and specifically relates to a secondary sealing vacuum packaging process. Background Technology

[0002] Vacuum packaging is a key technology in microelectromechanical systems (MEMS) and is widely used in various MEMS devices such as micromechanical gyroscopes, silicon-based oscillators, and resonant sensors.

[0003] After decades of research and development, some effective vacuum packaging technologies have emerged, and various MEMS devices that achieve wafer-level vacuum packaging, such as micromechanical gyroscopes and high-precision resonant pressure sensors, have been mass-produced and commercialized. However, high-vacuum packaging remains a significant technical challenge in the MEMS field.

[0004] A major challenge in high-vacuum packaging of MEMS devices is the significant time required for gas desorption under vacuum, as the sensor surface inevitably adsorbs gases. Since MEMS devices are typically encapsulated in tiny cavities of a few cubic millimeters or even less than one cubic millimeter, even minute amounts of surface-adsorbed gas can significantly impact the ultimate vacuum level within the packaging cavity. Experiments show that even after 24 hours of vacuum evacuation in a vacuum packaging system, achieving high-vacuum packaging remains difficult. Furthermore, vacuum packaging equipment such as bonding machines is expensive, and the surface gas desorption process consumes substantial time, significantly reducing packaging throughput and increasing product costs. Summary of the Invention

[0005] The technical problem to be solved by the present invention is to provide a secondary sealing vacuum packaging process to solve the problem that surface gas desorption in the prior art requires a lot of machine time, thereby improving the production capacity of vacuum packaging process and reducing product cost.

[0006] This invention provides a secondary sealing vacuum packaging process, comprising the following steps:

[0007] (1) Using white tin as sealing solder to achieve the first vacuum sealing: Align the MEMS microcavity structure and white tin solder 7 and install them on the cooling and heating stage 8 of the vacuum packaging device; remove the natural oxide layer on the surface of white tin, then evacuate the vacuum packaging device, heat up until the white tin solder 7 melts and is welded together with the electrode 6, and cool down to complete the first vacuum sealing.

[0008] (2) Place the MEMS microcavity structure after the first vacuum sealing under normal temperature and pressure conditions;

[0009] (3) Second sealing alignment: after removing the natural oxidation layer of the second sealing solder 9 and the white tin solder 7 on the surface of the MEMS micro-cavity structure, the MEMS micro-cavity structure is aligned with the second sealing solder 9 and installed on the refrigeration and heating table 8, and the catalyst 10 is placed between the MEMS micro-cavity structure and the second sealing solder 9;

[0010] (4) Vacuum low-temperature treatment and opening of the seal: the vacuum packaging device is vacuumed and kept, the temperature of the refrigeration and heating table 8 is set to be lower than 13.2℃, and the white tin is converted into gray tin;

[0011] (5) Second vacuum sealing: the vacuum packaging device is vacuumed, the temperature of the refrigeration and heating table 8 is increased to the welding temperature, the second sealing solder 9 and the fallen gray tin powder are melted, and are welded with the electrode 6, and after cooling, the second vacuum sealing is completed, that is, the vacuum packaging process is completed.

[0012] Preferably, the MEMS micro-cavity structure in step (1) comprises a substrate 1, a MEMS movable structure 2 and a cap 3, the cap 3 is provided with a cavity 4 and a through hole 5, and an electrode 6 is arranged around the through hole 5 on the surface of the cap 3.

[0013] Preferably, the alignment method of the MEMS micro-cavity structure and the solder in steps (1) and (5) comprises but is not limited to using the positioning groove on the refrigeration and heating table 8 to realize the alignment.

[0014] Preferably, the method for removing the natural oxidation layer in steps (1) and (5) comprises but is not limited to formic acid vapor treatment.

[0015] Preferably, the placement time of the MEMS micro-cavity structure in step (2) is at least one week.

[0016] Preferably, the second sealing solder 9 in step (3) is a tin-containing solder.

[0017] More preferably, the second sealing solder 9 is a tin-lead solder containing 65%-95% of lead.

[0018] Preferably, the catalyst 10 in step (3) comprises gray tin particles or CdTe particles.

[0019] Preferably, the temperature of the refrigeration and heating table 8 in step (4) is set to be-30- -40℃.

[0020] Preferably, the welding temperature in step (5) is higher than the melting temperature of the second sealing solder 9 and the gray tin.

[0021] The flow chart of the secondary sealing vacuum packaging process of the application is as follows Figure 1The secondary vacuum sealing device used in the application can be formed by adding a refrigeration function to the heating table of a conventional vacuum welding furnace to form a refrigeration heating table. The minimum temperature of the refrigeration heating table must be lower than the threshold temperature (13.2℃) at which white tin is converted into gray tin. The refrigeration heating table can be realized in various ways such as by integrating semiconductor refrigeration sheets in the heating table or connecting the heating table with a refrigeration compressor.

[0022] Beneficial effects

[0023] (1) The application realizes the desorption of the gas adsorbed on the surface of the MEMS microcavity structure through the placement process after the first vacuum packaging. The placement process does not consume energy, and the desorption can be completed in a low vacuum environment, which facilitates the realization of a high vacuum degree through secondary vacuum sealing.

[0024] (2) The unsealing of the first vacuum packaging and the second welding are realized in the same vacuum welding device, and the whole process is always in a vacuum environment, which facilitates the realization of a high vacuum degree.

[0025] (3) The application utilizes the feature that white tin is converted into powder-shaped gray tin at low temperature to realize the unsealing of the first vacuum packaging. Only a welding furnace with refrigeration function is needed to realize the unsealing, and the requirement for the equipment is low. BRIEF DESCRIPTION OF DRAWINGS

[0026] Figure 1 The figure is a process flowchart of the application.

[0027] Figure 2 The figure is a structural cross-sectional view of the alignment during the first sealing of the application.

[0028] Figure 3 The figure is a structural cross-sectional view of the alignment during the second sealing of the application.

[0029] The figure is a structural cross-sectional view of the alignment during the second sealing of the application. DETAILED DESCRIPTION

[0030] The application will be further described below in conjunction with specific embodiments. It should be understood that these embodiments are only used to illustrate the application and not to limit the scope of the application. In addition, it should be understood that those skilled in the art can make various modifications or changes to the application after reading the content taught by the application, and these equivalent forms also fall within the scope defined by the appended claims.

[0031] EMBODIMENT

[0032] The application provides a secondary sealing vacuum packaging process, and the specific process flow is as follows:

[0033] (1) Adopting surface micro-mechanical process to make MEMS movable structure 2 on substrate 1, then adopting bonding process to bond cap 3, cavity 4, through hole 5 and electrode 6 surrounding through hole 5 with MEMS movable structure 2, and to form single MEMS micro-cavity structure by dicing;

[0034] (2) Installing MEMS micro-cavity structure and white tin solder 7 on refrigeration and heating table 8 of vacuum packaging device, and realizing alignment of MEMS micro-cavity structure and white tin solder 7 by positioning groove on refrigeration and heating table 8, and the structure cross-sectional view of alignment is shown in Figure 2 ; closing vacuum packaging device, introducing nitrogen and formic acid vapor, and heating refrigeration and heating table 8 to about 160℃ to remove natural oxidation layer on surface of white tin, then purging cavity with nitrogen, and then vacuumizing and keeping for a period of time, heating to melting of white tin and welding with electrode 6, and completing first vacuum sealing after cooling;

[0035] (3) Placing MEMS micro-cavity structure after first vacuum sealing at normal temperature and pressure for more than one week, and the gas adsorbed on surface of micro-cavity structure will be slowly released during the period of time, so as to reduce vacuum degree of micro-cavity structure;

[0036] (4) Second sealing alignment: after removing natural oxidation layer of second sealing solder 9 and white tin solder 7 on surface of MEMS micro-cavity structure by formic acid vapor treatment, placing second sealing solder 9, gray tin particle catalyst 10 and MEMS micro-cavity structure in sequence on refrigeration and heating table 8, and realizing alignment between MEMS micro-cavity structure and second sealing solder 9 by positioning groove on refrigeration and heating table 8, and in order to avoid falling of gray tin particles into MEMS micro-cavity structure, MEMS micro-cavity structure should be placed on top, and second sealing solder 9 should be placed on bottom, and the structure cross-sectional view of alignment is shown in Figure 3 ; second sealing solder 9 is tin-containing solder containing impurities, and after melting into gray tin, the content of each component reaches the ratio of conventional tin-containing solder, and in the embodiment, second sealing solder 9 is PbSn solder sheet with lead content of 95%, and the length and width of PbSn solder sheet are equal to those of white tin solder sheet, and the thickness satisfies that the lead content of lead-tin alloy after melting into white tin solder sheet is ≥35%. Since second sealing solder 9 contains impurities, tin epidemic of solder after second vacuum sealing can be avoided at low temperature; catalyst 10 is only used as nucleating agent for crystal structure conversion, and once the conversion starts, the gray tin formed by conversion has autocatalytic effect on white tin, so the amount of catalyst 10 only needs to be able to contact white tin solder 7 used in first sealing;

[0037] (5) Close the vacuum packaging device, vacuumize and keep the vacuum, set the temperature of the refrigeration heating platform 8 to be lower than the threshold temperature of white tin to gray tin conversion, i.e. 13.2℃, preferably, the temperature of the refrigeration heating platform 8 is set to be -30 to -40℃, keep the low temperature, make the white tin of the first vacuum sealing convert to powder gray tin, the MEMS microcavity structure is opened because the sealed white tin becomes powder gray tin, and the desorbed gas generated in the placement process in the microcavity is extracted;

[0038] (6) Continue to vacuumize the vacuum packaging device, raise the temperature of the refrigeration heating platform 8 to the welding temperature, make the PbSn solder and the gray tin powder melt, the gray tin melts into the PbSn solder, and is welded with the electrode 6, and the second vacuum sealing is completed after cooling.

[0039] In the present application Figure 2 and Figure 3 The schematic diagram of the secondary sealing vacuum packaging process of the single chip structure, in addition, the secondary sealing vacuum packaging process of the present application is also applicable to wafer level process. When the wafer level process is adopted, the second sealing solder needs to be temporarily made or placed on a wafer, and then the wafer level alignment is adopted to realize the alignment and lamination of the MEMS microcavity wafer and the wafer provided with the second sealing solder.

Claims

1. A secondary sealing vacuum packaging process, comprising the following steps: (1) First sealing by white tin solder: align and mount the MEMS micro-cavity structure and white tin solder on the refrigeration and heating table of the vacuum packaging device; remove the natural oxide layer on the surface of the white tin, then vacuumize the vacuum packaging device, and heat to melt the white tin solder and weld it with the electrode, and complete the first sealing after cooling; (2) Place the MEMS micro-cavity structure sealed by the first sealing under normal temperature and pressure conditions; (3) Second sealing alignment: after removing the natural oxide layer on the surface of the second sealing solder and the MEMS micro-cavity structure, align and mount the MEMS micro-cavity structure and the second sealing solder on the refrigeration and heating table, and place a catalyst between the MEMS micro-cavity structure and the second sealing solder; (4) Vacuum low-temperature treatment and opening of the sealing: vacuumize and maintain the vacuum packaging device, set the temperature of the refrigeration and heating table to be lower than 13.2℃, and convert the white tin into gray tin; (5) Second sealing: vacuumize the vacuum packaging device, and raise the temperature of the refrigeration and heating table to the welding temperature, so that the second sealing solder and the falling gray tin powder are melted and welded with the electrode, and the second sealing is completed after cooling, i.e. the vacuum packaging process is completed.

2. The secondary seal vacuum packaging process of claim 1, wherein, The MEMS micro-cavity structure in step (1) comprises a substrate, a MEMS movable structure and a cap, the cap is provided with a cavity and a through hole, and an electrode is arranged around the through hole on the surface of the cap.

3. The process of claim 1, wherein, The alignment method of the MEMS micro-cavity structure and the solder in steps (1) and (5) comprises using the positioning groove on the refrigeration and heating table to realize the alignment.

4. The process of claim 1, wherein, The method for removing the natural oxide layer in steps (1) and (5) comprises formic acid vapor treatment.

5. The process of claim 1, wherein, The placement time of the MEMS micro-cavity structure in step (2) is at least one week.

6. The process of claim 1, wherein, The second sealing solder in step (3) is a tin-containing solder.

7. The secondary seal vacuum packaging process of claim 6, wherein, The second sealing solder is a tin-lead solder containing 65%-95% of lead.

8. The process of claim 1, wherein, The catalyst in step (3) comprises gray tin particles or CdTe particles.

9. The process of claim 1, wherein, The temperature of the refrigeration and heating table in step (4) is set to -30 ~ -40℃.

10. The process of claim 1, wherein, The welding temperature in step (5) is higher than the melting temperature of the second sealing solder and the gray tin.

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