Secondary sealing vacuum packaging process

Abstract

The invention relates to a secondary sealing vacuum packaging process, which comprises the following steps of: (1) adopting white tin as a sealing solder to realize primary vacuum sealing; (2) placing the MEMS microcavity structure sealed for the first time under the conditions of normal temperature and normal pressure; (3) secondary sealing alignment: aligning the placed MEMS micro-cavity structure with a secondary sealing solder, mounting the aligned MEMS micro-cavity structure and the secondary sealing solder on a refrigerating and heating table, and placing a catalyst between the MEMS micro-cavity structure and the secondary sealing solder; (4) the vacuum packaging device is vacuumized and kept, the temperature of the refrigerating and heating table is set to be lower than 13.2 DEG C, and the white tin is converted into grey tin; and (5) secondary vacuum sealing is conducted, specifically, the vacuum packaging device is vacuumized, the temperature of the refrigerating and heating table is increased to the welding temperature, secondary sealing welding flux and falling grey tin powder are melted and welded together with the electrode, and secondary vacuum sealing is completed after cooling is conducted. According to the sealing process, the productivity of the vacuum packaging process can be improved, and the product cost is reduced.

Description

Technical Field

[0001] The invention belongs to the field of sensors, and in particular relates to a secondary sealing vacuum packaging process. Background Art

[0002] Vacuum packaging is a key technology in micro-electromechanical systems (MEMS) technology 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 a variety of MEMS devices that achieve wafer-level vacuum packaging, such as micromechanical gyroscopes and high-precision resonant pressure sensors, have been put on the market in large quantities. However, high vacuum packaging is still an important technical issue in the MEMS field.

[0004] A major problem with high vacuum packaging of MEMS devices is that since the sensor surface will inevitably adsorb gas, the gas desorption time under vacuum is very long. MEMS devices are generally packaged in a tiny cavity of several cubic millimeters or even less than 1 cubic millimeter. The trace gas adsorbed on the surface can significantly affect the ultimate vacuum degree in the packaging cavity. Experiments have shown that even if the vacuum is evacuated for 24 hours in the vacuum packaging equipment, it is still difficult to achieve high vacuum packaging. Vacuum packaging equipment such as bonding machines are expensive, and the surface gas desorption process takes up a lot of machine time, which will significantly reduce the packaging process capacity and significantly increase 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, which is used to solve the problem that surface gas desorption in the prior art requires a large amount of machine time, improve the production capacity of the vacuum packaging process, and reduce product costs.

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

[0007] (1) Using tin as the sealing solder to achieve the first vacuum sealing: aligning the MEMS micro-cavity structure and the tin solder 7 and mounting them on the cooling and heating table 8 of the vacuum packaging device; removing the natural oxide layer on the surface of the tin, and then evacuating the vacuum packaging device, heating it until the tin solder 7 melts and is welded to the electrode 6, and then cooling it down to complete the first vacuum sealing;

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

[0009] (3) Second sealing alignment: After removing the natural oxide layer of the second sealing solder 9 and the white tin solder 7 on the surface of the MEMS micro-cavity structure, align the MEMS micro-cavity structure with the second sealing solder 9 and install them on the cooling and heating table 8, and place a catalyst 10 between the MEMS micro-cavity structure and the second sealing solder 9;

[0010] (4) Vacuum low temperature treatment, opening the seal: evacuate the vacuum packaging device and maintain the vacuum, set the temperature of the cooling and heating stage 8 to below 13.2°C, so that the white tin is converted into gray tin;

[0011] (5) Second vacuum sealing: evacuate the vacuum packaging device, raise the temperature of the cooling and heating table 8 to the welding temperature, melt the second sealing solder 9 and the fallen gray tin powder, and weld them together with the electrode 6. After cooling down, the second vacuum sealing is completed, that is, the vacuum packaging process is completed.

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

[0013] Preferably, the alignment method between the MEMS microcavity structure and the solder in step (1) and step (5) includes but is not limited to using positioning grooves on the cooling and heating platform 8 to achieve alignment.

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

[0015] Preferably, the MEMS microcavity structure in step (2) is placed for 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 tin-lead solder with a lead content of 65%-95%.

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

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

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

[0021] The secondary sealing vacuum packaging process flow chart of the present invention is as follows Figure 1As shown. The secondary vacuum sealing equipment used in the present invention can be formed into a cooling and heating table by adding a cooling function to the heating table of a conventional vacuum welding furnace. The minimum temperature of the cooling and heating table must be lower than the threshold temperature (13.2°C) for the conversion of white tin to gray tin. The cooling and heating table can be realized in a variety of ways, such as integrating a semiconductor cooling sheet in the heating table, or connecting the heating table to a refrigeration compressor.

[0022] Beneficial Effects

[0023] (1) The present invention realizes the desorption of the adsorbed gas on the surface of the MEMS micro-cavity structure through the placement process after the first vacuum packaging, and the placement process does not consume energy; and the gas can be desorbed more completely in a low vacuum environment, which facilitates the realization of a higher vacuum degree through the secondary vacuum sealing;

[0024] (2) The first vacuum packaging unsealing and the second welding of the present invention are realized in the same vacuum welding device, and the whole process is always in a vacuum environment, which is convenient for achieving a higher vacuum degree;

[0025] (3) The present invention utilizes the characteristic that white tin will be transformed into powdered gray tin at low temperature to realize the first vacuum packaging unsealing, which can be achieved only by the welding furnace having a refrigeration function, and has low requirements on equipment. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 It is a process flow chart of the present invention.

[0027] Figure 2 It is a schematic diagram of the structural cross-section during the first sealing alignment of the present invention.

[0028] Figure 3 It is a schematic diagram of the structural cross-section during the second sealing alignment of the present invention.

[0029] Figure numerals: 1-substrate, 2-MEMS movable structure, 3-cap, 4-cavity, 5-through hole, 6-electrode, 7-white tin solder, 8-cooling and heating table, 9-second sealing solder, 10-catalyst. DETAILED DESCRIPTION

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

[0031] Example

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

[0033] (1) A MEMS movable structure 2 is fabricated on a substrate 1 by using a surface micromachining process, and then a cap 3, a cavity 4, a through hole 5, and an electrode 6 surrounding the through hole 5 are bonded to the MEMS movable structure 2 by using a bonding process, and a single MEMS microcavity structure is formed by dicing;

[0034] (2) The MEMS micro-cavity structure and the white tin solder 7 are mounted on the cooling and heating platform 8 of the vacuum packaging device, and the alignment of the MEMS micro-cavity structure and the white tin solder 7 is achieved by using the positioning grooves on the cooling and heating platform 8. The cross-sectional view of the aligned structure is shown in FIG. Figure 2 As shown; the vacuum packaging device is turned off, nitrogen and formic acid vapor are introduced, and the refrigeration and heating table 8 is heated to about 160°C to remove the natural oxide layer on the surface of the white tin, and then the cavity is purged with nitrogen, and then vacuumed and maintained for a period of time, and the temperature is raised until the white tin melts and is welded to the electrode 6, and the first vacuum sealing is completed after the temperature is lowered;

[0035] (3) The MEMS micro-cavity structure after the first vacuum sealing is placed at room temperature and pressure for more than one week. During this period, the gas adsorbed on the surface of the micro-cavity structure will be slowly released, causing the vacuum degree of the micro-cavity structure to decrease;

[0036] (4) Alignment of the second sealing: After removing the natural oxide layer of the second sealing solder 9 and the white tin solder 7 on the surface of the MEMS micro-cavity structure by formic acid vapor treatment, the second sealing solder 9, the gray tin particle catalyst 10 and the MEMS micro-cavity structure are quickly placed on the cooling and heating table 8 in sequence, and the alignment between the MEMS micro-cavity structure and the second sealing solder 9 is achieved by using the positioning groove of the cooling and heating table 8. In order to prevent subsequent gray tin particles from falling into the MEMS micro-cavity structure, the MEMS micro-cavity structure should be ensured to be on the top and the second sealing solder 9 on the bottom. The cross-sectional view of the aligned structure is shown in FIG. Figure 3 As shown; the second sealing solder 9 is a tin-containing solder containing impurities. After the gray tin is incorporated, the content of each component reaches the ratio of the conventional tin-containing solder. In this embodiment, the second sealing solder 9 is a PbSn solder sheet with a lead content of 95%. The length and width of the PbSn solder sheet are equal to those of the white tin solder sheet, and the thickness satisfies that the lead content of the lead-tin alloy after the white tin solder sheet is incorporated is ≥35%. Since the second sealing solder 9 contains impurities, tin disease can be avoided in the solder after the second vacuum sealing at low temperature; the catalyst 10 is only used as a nucleating agent for the transformation of the crystal structure. Once the transformation begins, the gray tin formed by the transformation has a self-catalytic effect on the white tin. Therefore, the amount of catalyst 10 only needs to be able to contact with the white tin solder 7 used for the first sealing;

[0037] (5) The vacuum packaging device is turned off, and the vacuum packaging device is evacuated and maintained, and the temperature of the cooling and heating stage 8 is set to be lower than the threshold temperature for the conversion of white tin to gray tin, that is, 13.2° C. Preferably, the temperature of the cooling and heating stage 8 is set to -30 to -40° C., and the low temperature is maintained so that the white tin in the first vacuum seal is converted into powdered gray tin. The MEMS micro-cavity structure is opened because the sealed white tin is converted into powdered gray tin, and the desorbed gas generated in the micro-cavity during the placement process is evacuated;

[0038] (6) Continue to evacuate the vacuum packaging device, raise the temperature of the cooling and heating stage 8 to the welding temperature, melt the PbSn solder and the gray tin powder, and integrate the gray tin into the PbSn solder and weld it to the electrode 6. After cooling down, complete the second vacuum sealing.

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

Claims

1. A secondary sealing vacuum packaging process, comprising the following steps: (1) Using white tin as a sealing solder to achieve the first vacuum sealing: aligning the MEMS micro-cavity structure and the white tin solder (7) and mounting them on a cooling and heating table (8) of a vacuum packaging device; removing the natural oxide layer on the surface of the white tin, then evacuating the vacuum packaging device, heating the white tin solder (7) until it melts and is welded to the electrode (6), and then cooling the device to complete the first vacuum sealing; (2) placing the MEMS microcavity structure after the first vacuum sealing under normal temperature and pressure conditions; (3) Second sealing alignment: after removing the natural oxide layer of the second sealing solder (9) and the white tin solder (7) on the surface of the MEMS micro-cavity structure, align the MEMS micro-cavity structure with the second sealing solder (9) and install them on the cooling and heating table (8), and place a catalyst (10) between the MEMS micro-cavity structure and the second sealing solder (9); (4) vacuum low temperature treatment, opening the seal: evacuate the vacuum packaging device and maintain the vacuum, set the temperature of the cooling and heating table (8) to below 13.2°C, so that the white tin is converted into gray tin; (5) Second vacuum sealing: evacuate the vacuum packaging device, raise the temperature of the cooling and heating table (8) to the welding temperature, melt the second sealing solder (9) and the fallen gray tin powder, and weld them together with the electrode (6). After cooling down, the second vacuum sealing is completed, that is, the vacuum packaging process is completed.

2. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The MEMS microcavity 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) surrounding the through hole (5) is provided on the surface of the cap (3).

3. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The method for aligning the MEMS microcavity structure with the solder in step (1) and step (5) comprises achieving alignment by using positioning grooves on a cooling and heating platform (8).

4. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The method for removing the natural oxide layer in step (1) and step (5) comprises formic acid vapor treatment.

5. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The MEMS microcavity structure in step (2) is placed for at least one week.

6. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The second sealing solder (9) in the step (3) is a tin-containing solder.

7. The secondary sealing vacuum packaging process according to claim 6, characterized in that: The second sealing solder (9) is tin-lead solder with a lead content of 65%-95%.

8. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The catalyst (10) in step (3) comprises gray tin particles or CdTe particles.

9. The secondary sealing vacuum packaging process according to claim 1, characterized in that: In the step (4), the temperature of the refrigeration and heating stage (8) is set to -30 to -40°C.

10. The secondary sealing vacuum packaging process according to claim 1, characterized in that: The soldering temperature in the step (5) is higher than the melting temperatures of the second sealing solder (9) and the gray tin.

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