Metal eutectic bonding method for packaging MEMS pressure sensor

Abstract

The invention relates to a metal eutectic bonding method for MEMS pressure sensor packaging, and belongs to the technical field of pressure sensor eutectic bonding. Sequentially depositing an adhesion layer, a gold diffusion barrier layer, a gold layer, a germanium layer and an indium layer on a substrate silicon layer of the MEMS pressure sensor; forming a back cavity structure of the MEMS pressure sensor; aligning the back cavity structure to a through hole of alloy steel for bonding, and simultaneously pressurizing and heating; after the temperature reaches the eutectic melting point of the ternary alloy of germanium, indium and gold, the gold layer, the germanium layer and the indium layer are subjected to a liquid-phase eutectic reaction, the temperature continues to rise to 520-550 DEG C, then cooling is conducted, liquid eutectic begins to be converted into solid eutectic, and therefore packaging of the MEMS pressure sensor by the alloy steel is achieved through metal eutectic bonding. An Au-Ge-In material system is used for connecting the pressure sensor and the alloy steel shell, the eutectic bonding temperature is reduced, and the bonding strength can reach 40 Mpa or above.

Description

Technical Field

[0001] The present invention relates to the technical field of eutectic bonding of pressure sensors, and more specifically, to a metal eutectic bonding method for MEMS pressure sensor packaging. Background Art

[0002] MEMS pressure sensors are widely used in the fields of aerospace and robotics. The precise measurement requirements impose strict requirements on the packaging technology, and the sensors are required to maintain excellent measurement accuracy in harsh environments. Generally, metal eutectic bonding is used for sensor bonding, that is, temperature and pressure are simultaneously applied on the metal interface to form an alloy, thereby realizing the bonding of the bonding interface. Compared with other bonding methods, the metal eutectic bonding process is relatively simple; it can ensure that the bonding interface has high mechanical strength; it is carried out at a relatively low temperature, reducing thermal stress; and it has good compatibility with materials such as Si, becoming the mainstream technology for current pressure sensor packaging.

[0003] Currently, the common metal eutectic material is Au-Sn, and the process is relatively mature. The following main problems currently exist in the Au-Sn alloy bonding: the bonding interface strength of Au-Sn eutectic bonding is relatively low; the Au-Sn eutectic alloy is relatively brittle and may crack under thermal cycling or mechanical stress, and intermetallic compounds will be formed at high temperatures, affecting long-term reliability. The document "Ricky W.Chuang, Dongwook Kim, Jeong Park, A Fluxless Au-Sn Bonding Process of Tin-Rich Compositions Achieved in Ambient Air. 2002 Electronic Components and Technology Conference. PP134-137." introduces two ratios of Au-Sn eutectic bonding, and the melting temperature of the eutectic is about 280°C. Its eutectic bonding strength is relatively low, not suitable for the current environment with higher airtightness requirements, and Sn is easily oxidized, unable to achieve large-area bonding, and the bonding quality is very poor. In addition, a relatively thick bonding layer is not beneficial to MEMS devices and integrated circuits. Although Au-Si has a relatively high bonding strength, it has high requirements for surface flatness during bonding, the process is complex, and the bonding temperature is relatively high.

[0004] For high-precision MEMS pressure sensors, large bonding strength and high-temperature stability are crucial. The eutectic bonding materials in the prior art have the following disadvantages: the eutectic bonding strength is relatively low and cannot meet the high airtightness requirements; the material is brittle at high temperatures and cannot work under harsh conditions of high temperature and chemical corrosion atmosphere. Summary of the Invention

[0005] To solve the problems existing in the above-mentioned prior art, the present invention proposes an Au-Ge-In eutectic system. Among them, Au-Ge has excellent high-temperature performance, high thermal conductivity and electrical conductivity, as well as excellent mechanical properties. The Ge-In system has a relatively low eutectic temperature, which can complete the bonding connection at a low temperature, and the bonding strength can reach more than 40 Mpa, thus solving the technical problems of high bonding temperature and low bonding strength in the prior art.

[0006] According to the purpose of the present invention, a metal eutectic bonding method for MEMS pressure sensor packaging is provided, including the following steps:

[0007] (1) Sequentially deposit an adhesion layer, a gold diffusion barrier layer, a gold layer, a germanium layer, and an indium layer on the substrate silicon layer of the MEMS pressure sensor;

[0008] (2) Use a wet etching method to form a back cavity structure;

[0009] (3) Align the back cavity structure with the through hole of the alloy steel used for bonding, and apply pressure and heat simultaneously; when the temperature reaches the eutectic melting point of the ternary alloy of germanium, indium, and gold, the gold layer, germanium layer, and indium layer undergo a liquid-phase eutectic reaction, continue to heat up to 520°C - 550°C, and keep it at this temperature range for 10 min - 20 min; then cool down. When the temperature drops below the eutectic solidus line of the ternary alloy of germanium, indium, and gold, the liquid eutectic begins to transform into a solid state, thereby realizing the packaging of the alloy steel to the MEMS pressure sensor through metal eutectic bonding.

[0010] Preferably, the adhesion layer is a Ti layer.

[0011] Preferably, the thickness of the Ti layer is 50 nm - 60 nm.

[0012] Preferably, the gold diffusion barrier layer is a Ni layer.

[0013] Preferably, the thickness of the Ni layer is 500 nm - 550 nm.

[0014] Preferably, the thickness of the gold layer is 600 nm - 650 nm.

[0015] Preferably, the thickness of the germanium layer is 100 nm - 150 nm.

[0016] Preferably, the thickness of the indium layer is 100 nm - 150 nm.

[0017] Preferably, the method for depositing the adhesion layer, gold diffusion barrier layer, germanium layer, and indium layer is magnetron sputtering, and the method for depositing the gold layer is evaporation.

[0018] Preferably, a Ti layer and a Au layer are sequentially deposited on the surface of the alloy steel.

[0019] Generally speaking, compared with the prior art, the above technical solution conceived by the present invention mainly has the following technical advantages:

[0020] (1) The present invention uses the Au-Ge-In material system for connecting the pressure sensor and the alloy steel shell. It has high thermal conductivity and electrical conductivity, excellent electromagnetic (EM) shielding ability and mechanical properties, and at the same time, its bonding temperature is relatively low.

[0021] (2) The present invention adopts the Au-Ge-In transient liquid-phase transition eutectic bonding process technology, and the alloy layer connection is stable, and the bonding strength can reach more than 40 Mpa.

[0022] (3) The pressure sensor-alloy steel system prepared by the present invention has strong high-temperature oxidation resistance and can work under harsh conditions of high temperature and chemical corrosion atmosphere. Brief Description of the Drawings

[0023] Figure 1 It is a sectional view of the sensor chip of the eutectic bonding layer for the application of the Au-Ge-In eutectic bonding technology in the MEMS pressure sensor of the present invention.

[0024] Figure 2 It is a schematic diagram of the eutectic bonding for the application of the Au-Ge-In eutectic bonding technology in the MEMS pressure sensor of the present invention.

[0025] Figure 3 It is the eutectic bonding process flow for the application of the Au-Ge-In eutectic bonding technology in the MEMS pressure sensor of the present invention.

[0026] Figure 4 It is the eutectic bonding process conditions for the application of the Au-Ge-In eutectic bonding technology in the MEMS pressure sensor of the present invention. Detailed Embodiments

[0027] In order to make the purpose, technical solution and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described here are only used to explain the present invention and are not used to limit the present invention. In addition, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0028] The purpose of the present invention is to provide an Au-Ge-In eutectic bonding process technology applicable to MEMS pressure sensors, and its high bonding strength and high-temperature performance can meet the application of pressure sensors.

[0029] To achieve the above object, the technical solution adopted by the present invention is:

[0030] An Au-Ge-In eutectic bonding technology, where the metals are all prepared as thin films. Provide an Au-Ge-In eutectic bonding process, and the process flow is as follows:

[0031] 1. Bond the sensing chip to the high-temperature resistant alloy steel packaging shell using the transient liquid-phase transition eutectic bonding process technology: First, deposit a multi-layer structure containing an adhesion layer, a gold diffusion barrier layer, a gold film layer, and ultra-thin indium and germanium layers on the back of the sensor chip by vacuum sputtering. Use Ti metal as the adhesion layer of the composite thin film with the substrate, which can effectively improve the adhesion of the composite thin film; use Ni for the gold diffusion barrier layer. The thermal expansion coefficient of Ni metal is between Ti metal and Au. In this way, Ni metal as the intermediate layer can not only block the excessive diffusion of Au and Si, but also reduce the internal stress of the composite film and enhance the stability of the composite film system.

[0032] 2. The high-precision forming of the chip back cavity is achieved through the anisotropic characteristics of KOH wet etching. Align the chip back cavity with the through-hole of the high-temperature resistant alloy steel used for bonding and paste it (as Figure 2 shown), apply a pressure of 0.3 MPa and place it in a high-temperature furnace for heating. The two will form a solid bond through the transient liquid-phase transition eutectic process. The eutectic layer formed by bonding is a solid layer. When the sample enters the bonding furnace, the temperature gradually rises to 488 °C, and the local area of the In-Ge and Au-Ge interfaces undergoes a liquid-phase eutectic reaction due to reaching the eutectic melting point. Continue to heat up to 520 - 550 °C to ensure sufficient reaction. Maintain a constant temperature stage at this temperature. The gold, germanium, and indium elements migrate to each other through the diffusion mechanism, resulting in a change in the composition distribution of the alloy system. When the temperature drops below the eutectic solidus line, the liquid eutectic begins to transform into a solid state. Finally, during the cooling process from 520 - 550 °C to room temperature, all liquid regions complete the phase change to form a solid structure, realizing the complete process of eutectic bonding. The pressure sensor chip described above successively includes a pressure-sensitive resistor layer with silicon dioxide as the dielectric layer, a single-crystalline silicon layer, and a substrate silicon layer.

[0033] The adhesion layer uses metal titanium Ti, and the sputtering thickness is 50 - 60 nm. The gold diffusion barrier layer uses metal Ni, and the sputtering thickness is 500 - 550 nm. The gold film layer uses metal gold Au, and the sputtering thickness is 600 - 650 nm. The ultra-thin layer uses metal germanium Ge and metal In. The thickness of metal germanium Ge is 100 - 150 nm, and the thickness of metal In is 100 - 150 nm.

[0034] The eutectic bonding temperature is 488 °C. The eutectic bonding vacuum degree is 4×10 -3Heat starts from room temperature. After 15 minutes, the temperature reaches the eutectic temperature point of 488 °C. Then continue heating for 13 minutes to 520 - 550 °C and hold for 10 - 20 minutes to ensure sufficient reaction. Turn off the heating and let it cool naturally. After 60 minutes, the temperature drops and the bonding process ends.

[0035] The alloy steel used is material 17 steel, with 50 - 60 nm of Ti and 800 - 850 nm of Au sputtered on the surface, and the Ti layer is connected to the alloy steel. The alloy steel 17 housing is integrally processed and has a central through-hole connected to the pressure sensor.

[0036] Example 1

[0037] See Figure 1 It is a sectional view of the sensor chip of the eutectic bonding layer for the application of an Au - Ge - In eutectic bonding technology in a MEMS pressure sensor according to the present invention. From top to bottom, they are the pressure sensor chip substrate, adhesion layer, diffusion barrier layer, gold layer, germanium layer, and indium layer. Specifically, the adhesion layer uses metal titanium Ti with a sputtering thickness of 50 nm. The gold diffusion barrier layer uses metal Ni with a sputtering thickness of 500 nm. The gold film layer uses metal gold Au with a sputtering thickness of 600 nm. The ultra-thin layer uses metals germanium Ge and In with sputtering thicknesses of 100 nm for Ge and 100 nm for In. The alloy steel used is material 17 steel, and 100 nm of Ti and 800 nm of Au are also sputtered on the surface, and the Ti layer is connected to the alloy steel. The alloy steel 17 housing is integrally processed and has a central through-hole connected to the pressure sensor.

[0038] Appendix Figure 3 and Figure 4 They are respectively the eutectic bonding process flow and process conditions for the application of an Au - Ge - In eutectic bonding technology in a MEMS pressure sensor according to the present invention.

[0039] Specifically, the process flow includes:

[0040] Sample pretreatment: Ultrasonically clean the sample with acetone, isopropanol, and deionized water for 5 minutes each, and blow dry with a nitrogen gun.

[0041] The magnetron sputtering conditions are as follows, vacuum 5×10 -3 Pa, and the sputtering thickness is controlled by the sputtering time. Sequentially sputter on the chip substrate: 50 nm adhesion layer Ti, 500 nm gold diffusion barrier layer Ni. Among them, the sputtering vacuum degree of the Ti layer is 1 Pa, the sputtering power is 150 W, and the sputtering time is 30 minutes. The sputtering vacuum degree of the Ni layer is 0.6 Pa, the sputtering power is 1000 W, and the sputtering time is 30 minutes.

[0042] Evaporate 600 nm of gold: After the gold wire is pre-melted, cool it for 10 min. Gently wipe the surface of the pre-melted gold with a lint-free cloth moistened with deionized water, and then dry the surface of the gold wire with a nitrogen gun. Clean the surface of the silicon wafer with deionized water. The process parameters are: background vacuum is 1×10 -4 Pa, the gold melting power is 1200 W, the evaporation power is 1000 W, and the evaporation time is 1.5 h. Then sputter 100 nm of germanium and 100 nm of indium layers. The sputtering vacuum degrees are 0.8 Pa and 0.6 Pa respectively, the sputtering power is 100 W, and the sputtering times are 30 min respectively.

[0043] Sputter a 50 nm adhesion layer of Ti on the alloy steel. The sputtering vacuum degree is 1 Pa, the sputtering power is 150 W, and the sputtering time is 30 min. Evaporate 800 nm of gold: The background vacuum is 1×10 -4 Pa, the gold melting power is 1200 W, the evaporation power is 1000 W, and the evaporation time is 2 h.

[0044] Sample alignment: Align the back cavity of the chip with the through-hole of the high-temperature alloy steel used for bonding and stick it on.

[0045] According to the attachment Figure 4 , the eutectic bonding temperature is 488 °C. The eutectic bonding vacuum degree is 4×10 -3 Pa. Start heating from room temperature. After 15 min, the temperature reaches the eutectic temperature point of 488 °C, and continue to heat up for 13 min to 520 - 550 °C, and hold for 10 - 20 min to ensure sufficient reaction. Turn off the heating and let it cool naturally. After 60 min, the temperature drops and the bonding process ends.

[0046] Using titanium (Ti) metal as the adhesion layer between the composite film and the substrate can significantly improve the adhesion strength of the composite film. This is because Ti metal has good wettability and excellent adhesion properties with various materials, which can effectively promote the tight bonding at the interface between the film and the substrate. The thermal expansion coefficient of Ni metal is between that of Ti metal and Au. It not only effectively blocks the excessive diffusion of gold (Au) and silicon (Si) elements in the chip substrate silicon layer, but also plays a role in relieving the internal stress of the composite film and enhancing the structural stability of the entire composite film system. Therefore, nickel (Ni) is selected as the gold diffusion barrier layer. The eutectic layer formed by bonding is a solid state layer. When the sample enters the bonding furnace, the temperature gradually rises to 488 °C, and the Au-Ge-In system undergoes a liquid phase eutectic reaction due to reaching the eutectic temperature point. The temperature is further raised to 520 - 550 °C to ensure sufficient reaction. At this temperature, during the constant temperature stage, the three elements of gold, germanium, and indium migrate mutually through the diffusion mechanism, resulting in a change in the composition distribution of the alloy system. When the temperature drops below the eutectic solidus line of 488 °C, the liquid eutectic begins to transform into a solid state. Finally, during the cooling process from 520 - 550 °C to room temperature, all the liquid regions complete the phase change to form a solid structure, and the eutectic bonding process is completed. This process ensures the firmness and reliability of the film structure.

[0047] The bonding strength and the oxidation resistance of the film layer are improved by using the Au-Ge-In eutectic material. At the same time, using this bonding process can have a greater stress transfer compared to the existing bonding methods, meeting the stress-sensitive response requirements of the pressure sensor. Its bonding strength can exceed 40 Mpa. This strength value has been significantly improved compared to the sealing strength of the traditional Au / Sn system, thus showing obvious advantages in ensuring the structural stability and reliability. In addition, the addition of In reduces the eutectic temperature, the formation of the intermetallic phase stabilizes the alloy layer, the addition of In also reduces the segregation of Au in the Au-Ge system, slows down the rapid nucleation of gold atoms at high temperatures, and improves its stability at high temperatures. The high-temperature chemical stability enables the MEMS pressure sensor to operate stably for a long time in a harsh environment with severe chemical corrosion without performance degradation or failure due to environmental effects. Therefore, the selection of the Au-Ge-In eutectic material and the eutectic bonding process provide an ideal choice for manufacturing high-performance and high-reliability pressure sensors.

[0048] Those skilled in the art can easily understand that the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent replacements, and improvements made within the spirit and principle of the present invention should be included in the protection scope of the present invention.

Claims

1. A metal eutectic bonding method for MEMS pressure sensor packaging, characterized in that: The following steps are involved: (1) depositing an adhesion layer, a gold diffusion barrier layer, a gold layer, a germanium layer, and an indium layer in sequence on a substrate silicon layer of a MEMS pressure sensor; (2) Using wet etching method to form a back cavity structure; (3) Aligning the back cavity structure with the through hole of the alloy steel used for bonding, pressurizing and heating are performed simultaneously; when the temperature reaches the eutectic melting point of the ternary alloy of germanium, indium and gold, the gold layer, the germanium layer and the indium layer undergo a liquid phase eutectic reaction, and the temperature is continued to rise to 520°C-550°C and kept within this temperature range for 10min-20min; then the temperature is lowered, and when the temperature drops below the eutectic solidus of the ternary alloy of germanium, indium and gold, the liquid eutectic begins to transform into a solid state, thereby realizing the packaging of the MEMS pressure sensor by the alloy steel through metal eutectic bonding.

2. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: The adhesion layer is a Ti layer.

3. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 2, characterized in that: The thickness of the Ti layer is 50nm-60nm.

4. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: The gold diffusion barrier layer is a Ni layer.

5. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 4, characterized in that: The thickness of the Ni layer is 500nm-550nm.

6. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: The thickness of the gold layer is 600nm-650nm.

7. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: The thickness of the germanium layer is 100nm-150nm.

8. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: The thickness of the indium layer is 100nm-150nm.

9. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: The adhesion layer, the gold diffusion barrier layer, the germanium layer and the indium layer are deposited by magnetron sputtering, and the gold layer is deposited by evaporation.

10. The metal eutectic bonding method for MEMS pressure sensor packaging according to claim 1, characterized in that: A Ti layer and an Au layer are sequentially deposited on the surface of the alloy steel.

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