Low-stress low-temperature bonding method based on multi-layer composite film structure

Abstract

The invention relates to a low-stress low-temperature bonding method based on a multi-layer composite film structure, and belongs to the technical field of semiconductor packaging and microelectronic manufacturing. Sequentially depositing an adhesion layer, an Au layer and a Bi layer on the element to be bonded; sequentially depositing an adhesion layer and an Au layer on the alloy steel; the Bi layer on the to-be-bonded element is aligned with the Au layer on the alloy steel, pressurization and heating are carried out at the same time, when the temperature reaches the eutectic melting point of the bismuth and gold binary alloy, the Bi layer and the Au layer on the alloy steel are subjected to a liquid phase eutectic reaction, and the temperature continues to rise to 280-300 DEG C; and then cooling is carried out, and when the temperature is reduced to be below the eutectic solidus of the bismuth and gold binary alloy, liquid eutectic begins to be converted into solid, so that eutectic bonding of the to-be-bonded element is realized. The Au-Bi material system is used for connecting the thermosensitive element and the alloy steel shell, and the good electromagnetic (EM) shielding capacity and mechanical performance are achieved. And due to the addition of Bi, the bonding temperature of the system is greatly reduced.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor packaging and microelectronics manufacturing, and particularly relates to a low-stress and low-temperature bonding method based on a multi-layer composite film structure, which is especially suitable for the high-reliability packaging of heat-sensitive components (such as flexible electronic devices, MEMS precision devices, and high-density integrated circuits). Background Art

[0002] As a high-precision and high-strength heterogeneous material connection method, the eutectic bonding technology is widely used in the fields of semiconductor packaging, MEMS device manufacturing, etc. Its core principle is to heat to the melting point of the eutectic alloy to cause a eutectic reaction between metals to form a liquid alloy, and then cool and solidify to achieve an atomic-level dense bond. However, the traditional eutectic bonding process faces key bottlenecks such as cost, temperature, interfacial strength, and stability and reliability.

[0003] Currently, the eutectic bonding technology generally adopts the Au-based system. Since Au can form stable bonds with various materials (such as silicon, glass, ceramics, and metals), it is especially suitable for three-dimensional integration and heterogeneous chip packaging. At the same time, the excellent electrical conductivity of Au (resistivity 2.44×10 -8 Ω·m) reduces the contact resistance of the bonding interface and is suitable for high-frequency and high-power devices. The Au-based eutectic bonding system has become the core technology of high-end packaging due to its high reliability, material compatibility, and electrical performance advantages. Among them, the Au-Au bond has become the preferred solution for extreme environments and high-precision packaging due to its unparalleled reliability, electrical performance, and high-temperature tolerance. However, its high cost and process complexity limit its large-scale application. Currently, the most mature eutectic bonding technology on the market is the Au-Sn eutectic bonding, which is widely used due to its low cost, low-temperature bonding ability, and wide process compatibility. However, its stability and reliability are poor. In a high-temperature or humid environment, Sn may be locally oxidized to form SnO2, resulting in a decrease in wettability or interface voids. At the same time, the phases such as AuSn4 formed at the Au-Sn bonding interface have poor ductility and are prone to microcracks under mechanical shock or thermal cycling, leading to interface failure. Another bonding technology with broad prospects is the Au-Ge eutectic bonding. Since the Au-Ge eutectic bonding interface has excellent mechanical strength, it is suitable for high-vibration or impact environments (such as aerospace devices). However, its high cost and material limitations seriously hinder its large-scale application. At the same time, the brittleness of Ge may cause microcracks at the bonding interface, and the difference in the diffusion rates of Au and Ge may also trigger the Kirkendall effect, resulting in voids at the bonding interface, reducing airtightness and mechanical strength.

[0004] Meanwhile, in the Au-Sn and Au-Ge bonding systems, due to the CTE mismatch, large thermal stresses will occur at the interface of the bonding film, resulting in poor film formation quality, and further causing a series of problems such as bonding strength and airtightness. There are many defects in the existing processes, that is, the problems of cost, temperature, CTE matching, and stability and reliability cannot be solved and balanced, which seriously restricts the development and large-scale application of the eutectic bonding technology in the high-reliability packaging field of heat-sensitive components. Summary of the Invention

[0005] To solve the problems existing in the above-mentioned prior art, the present invention provides a low-stress and low-temperature eutectic bonding process technology especially suitable for heat-sensitive components. The Au-Bi material system is used for connecting components (especially heat-sensitive components) and alloy steel casings, which has excellent electromagnetic (EM) shielding ability and mechanical properties. The addition of Bi significantly reduces the bonding temperature of the system to meet the requirements of low cost, low-temperature bonding, CTE matching, and high stability and reliability.

[0006] According to the first aspect of the present invention, a bonding method based on a multi-layer composite film structure is provided, including the following steps:

[0007] (1) Deposit an adhesion layer, an Au layer, and a Bi layer on the component to be bonded in sequence; deposit an adhesion layer and an Au layer on the alloy steel, and the alloy steel is used to bond the component to be bonded;

[0008] (2) Align the Bi layer on the component to be bonded with the Au layer on the alloy steel, and apply pressure and heat simultaneously. When the temperature reaches the eutectic melting point of the binary alloy of bismuth and gold, the Bi layer and the Au layer on the alloy steel undergo a liquid-phase eutectic reaction. Continue to heat up to 280 - 300 °C and keep it warm within this temperature range for 10 - 20 min; then cool down. When the temperature drops below the eutectic solidus line of the binary alloy of bismuth and gold, the liquid eutectic starts to transform into a solid state, thereby realizing the eutectic bonding of the component to be bonded.

[0009] Preferably, the component to be bonded is a heat-sensitive component.

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

[0011] Preferably, the thickness of the adhesion layer deposited on the component to be bonded is 50 - 100 nm.

[0012] Preferably, the thicknesses of the Au layer and the Bi layer deposited on the component to be bonded are 1 - 2 μm and 2 - 3 μm respectively.

[0013] Preferably, the thickness of the adhesion layer deposited on the alloy steel is 50 - 100 nm.

[0014] Preferably, the thickness of the Au layer deposited on the alloy steel is 1-2 μm.

[0015] Preferably, the pressure for pressurization is 0.3-0.6 MPa.

[0016] Preferably, the heating rate is 10-15 °C / min.

[0017] Preferably, the cooling rate is 5-10 °C / min.

[0018] Generally speaking, compared with the prior art, the above technical solutions conceived by the present invention mainly have the following technical advantages:

[0019] (1) The present invention adopts the Au-Bi material system for connecting the thermosensitive element and the alloy steel shell, which has excellent electromagnetic (EM) shielding ability and mechanical properties. The addition of Bi significantly reduces the bonding temperature of the system, and at the same time, Bi has rich reserves in nature, and its cost is greatly reduced compared with materials such as Au and Ge.

[0020] (2) The Au-Bi system adopted by the present invention can form a perfect CTE match to reduce thermal stress, and is more easily matched with ceramic or silicon substrates compared with traditional bonding systems, effectively avoiding problems such as non-uniform film formation and generation of microcracks caused by thermal stress.

[0021] (3) The present invention adopts the Au-Bi eutectic bonding process technology. The eutectic layer is stable without brittle compounds, the single Au2Bi phase reduces interface defects, has good oxidation resistance, has good long-term stability and reliability, and the bonding strength can reach more than 45 Mpa.

[0022] (4) The thermosensitive element-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.

[0023] (5) Compared with the traditional eutectic bonding process, the residual stress of the low-stress and low-temperature bonding based on the multi-layer composite film structure proposed by the present invention is reduced by 50% to 60%, providing an innovative solution for the packaging of high-reliability thermal sensitive elements. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 It is a structural sectional view of the Au-Bi eutectic bonding based on a thermosensitive element of the present invention.

[0025] Figure 2 It is a schematic diagram of the thermal stress simulation of an Au-Bi eutectic system of the present invention at 100 °C.

[0026] Figure 3Schematic diagram of the thermal stress simulation of the bonding interfaces of different eutectic system thin films at 100°C; among them: (a) is the schematic diagram of the thermal stress simulation of the Au-Bi thin film interface at 100°C, (b) is the schematic diagram of the thermal stress simulation of the Au-Sn thin film interface at 100°C, and (c) is the schematic diagram of the thermal stress simulation of the Au-Ge thin film interface at 100°C.

[0027] Figure 4 It is the binary phase diagram of the Au-Bi alloy.

[0028] In the above figures: 1 - substrate of the heat-sensitive element, 2 - adhesion layer, 3 - gold layer, 4 - bismuth layer, 5 - gold layer on the alloy steel, 6 - adhesion layer on the alloy steel. Specific implementation manner

[0029] In order to make the objectives, technical solutions and advantages of the present invention clearer and more understandable, 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 herein 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.

[0030] A Au-Bi eutectic bonding technology of the present invention, and the metals are all prepared by thin films. The transient liquid-phase transition eutectic bonding technology is used to realize the reliable connection between the heat-sensitive element and the high-temperature-resistant alloy steel packaging shell. The core innovation of this process lies in designing a multi-layer composite film structure and optimizing the bonding kinetic parameters. The specific implementation process is as follows:

[0031] The first step: (1) In the pre-treatment stage of the heat-sensitive element, first construct a composite film structure on its back through a magnetron sputtering vacuum coating system. This structure consists of three functional layers: (1) a 50-100 nm titanium (Ti) adhesion layer, which forms Ti-O covalent bonds with the substrate using the high chemical activity of titanium, significantly improving the film adhesion to more than 25 MPa; (2) a 1-2 μm gold (Au) transition layer, and high-ductility gold is selected as a buffer medium, which can effectively absorb stress deformation during the thermal cycle; (3) a 2-3 μm bismuth (Bi) layer, whose low melting point characteristic (271.4°C) creates conditions for subsequent low-temperature bonding, reducing the melting temperature during the eutectic process. At the same time, due to the small CTE difference between Au (14.2×10 -6 / K) and Bi (13.4×10 -6 / K), the Au-Bi system can form a perfect CTE match to reduce thermal stress. Compared with the traditional bonding system, it is easier to match with ceramic or silicon substrates, effectively avoiding problems such as uneven film formation and microcrack generation caused by thermal stress. It is worth emphasizing that the titanium intermediate layer plays three roles in this structure: as an adhesion layer, it significantly improves the film adhesion; using its low coefficient of thermal expansion (8.6×10-6 / K) regulates the thermal stress of the film system; at the same time, it inhibits the excessive diffusion of gold atoms by forming Ti-Au intermetallic compounds.

[0032] Step 2: Align the heat-sensitive element with the high-temperature alloy steel used for bonding and stick it on. Apply a certain pressure and heat it in a high-temperature furnace. The two will form a solid bond through a short liquid phase transition eutectic process. The eutectic layer after bonding is a solid layer with a bismuth atomic concentration of 81.1%. When the sample is placed in the furnace, as the temperature rises, the Au-Bi interface partially reaches the eutectic melting point and eutectic melting occurs. When the temperature rises to 241°C, the bismuth atoms melt and begin to diffuse. Continue to raise the temperature to 280-300°C, and the liquid Bi reacts with the solid Au to form Au2Bi. The reaction formula is:

[0033] 2Au(s)+Bi(L)→Au2Bi(s)

[0034] Maintain the highest temperature for 10-20 minutes to ensure sufficient reaction. Then cool slowly, and the eutectic begins to transform from liquid to solid. When the liquid alloy cools to the eutectic temperature, a eutectic reaction occurs, and (Au) and (Bi) phases precipitate simultaneously. The two phases are alternately distributed in lamellar or fibrous forms, forming a low-defect interface. At this point, the eutectic bonding operation is completed. The reaction formula is:

[0035] L→Au+Bi

[0036] The adhesion layer uses titanium Ti with a sputtering thickness of 50-100 nm. The bonding layer uses gold Au and bismuth Bi with a vapor deposition thickness of 1-2 μm and a sputtering thickness of 2-3 μm.

[0037] The eutectic bonding temperature is 241°C, and the actual operating temperature is 280-300°C. The eutectic bonding vacuum degree is 4×10 -3 Pa, the bonding process adopts a two-stage temperature control strategy: first, the temperature is raised to 280-300℃ at 10℃ / min under a nitrogen protection environment to melt the bismuth layer and undergo a eutectic reaction with the gold layer to form an Au2Bi intermetallic compound; then, it is slowly cooled at 5℃ / min to promote the equiaxed crystal growth of the compound, and the (Au) and (Bi) phases are precipitated at the same time, finally forming a eutectic bonded interface layer.

[0038] The alloy steel is made of stainless steel 17-4, and the surface is also sputtered with 50-100nm of Ti and evaporated with 1-2μm of Au. The stainless steel 17-4 shell is processed in one piece, and its upper surface is connected to the base of the thermal element.

[0039] See also Figure 1This is a cross-sectional view of the structure of the Au-Bi eutectic bond based on a thermal sensitive element of the present invention. From top to bottom, there are respectively a thermal sensitive element substrate 1, an adhesion layer 2, a gold layer 3, a bismuth layer 4, a gold layer 5 on the alloy steel, and an adhesion layer 6 on the alloy steel. The specific adhesion layer uses metal titanium Ti, which is prepared by vacuum sputtering with a sputtering thickness of 50-100nm. The gold film layer is prepared by evaporation with a evaporation thickness of 1-2μm. The bismuth layer is prepared by vacuum sputtering with a sputtering thickness of 2-3μm. The alloy steel is stainless steel 17-4, and 50-100nm of Ti and 1-2μm of Au are also sputtered on the surface. The stainless steel 17-4 shell is processed in one piece.

[0040] Figure 2 The figure is a schematic diagram of thermal stress simulation of an Au-Bi eutectic system at 100°C according to the present invention. Figure 2 It can be seen that different materials such as Au and Bi films will undergo different degrees of thermal expansion when heated due to their different thermal expansion coefficients CTE. The free expansion of the film is restricted, resulting in the generation of internal thermal stress. This thermal stress will affect the quality of the film and cause problems with bonding strength and airtightness.

[0041] Figure 3 Schematic diagram of thermal stress simulation at the bonding interface of different eutectic system films at 100°C.

[0042] Among them, (a) is a schematic diagram of thermal stress simulation of Au-Bi thin film interface at 100℃, (b) is a schematic diagram of thermal stress simulation of Au-Sn thin film interface at 100℃, and (c) is a schematic diagram of thermal stress simulation of Au-Ge thin film interface at 100℃. Figure 3 It can be seen that compared with Au-Sn film and Au-Ge film, the thermal stress of the Au-Bi film interface provided by the present invention at 100°C is the smallest, reduced by 50% to 60%, indicating the superiority of the Au-Bi bonding system under the same conditions.

[0043] The following are specific embodiments

[0044] Example 1

[0045] The low-stress and low-temperature bonding method of the multi-layer composite membrane structure of the present invention comprises the following process steps:

[0046] Sample pretreatment: The samples were ultrasonically cleaned with acetone, isopropanol, and deionized water for 5 min each, and then dried with a nitrogen gun.

[0047] The magnetron sputtering conditions were as follows: vacuum 5×10 -3Pa, the sputtering thickness is controlled by the sputtering time. The chip substrate is sequentially sputtered with: an adhesion layer Ti, an Au layer, and a Bi layer. The thickness of the adhesion layer Ti is 50 nm, the thickness of the Au layer is 1 μm, and the thickness of the Bi layer is 2 μm.

[0048] Sputtering of a 50 nm Ti layer: the sputtering vacuum is 1 Pa and the sputtering power is 150 W.

[0049] Evaporate a 1μm Au layer; pre-melt the gold wire and cool it for 10 minutes. Gently wipe the pre-melted gold surface clean with a dust-free cloth moistened with deionized water, then blow dry the gold wire surface with a nitrogen gun. Rinse the silicon wafer surface with deionized water. Process parameters: background vacuum 1×10 -4 Pa, gold melting power 1200W, evaporation power 1000W.

[0050] Sputtering of a 2 μm Bi layer: the sputtering vacuum is 0.6 Pa and the sputtering power is 100 W.

[0051] A 50nm adhesion layer Ti layer was sputtered on the alloy steel, the sputtering vacuum was 1Pa, and the sputtering power was 150W. A 1μm Au layer was evaporated: the background vacuum was 1x10 -4 Pa, gold melting power 1200W, evaporation power 1000W.

[0052] Align the heat-sensitive element with the high-temperature alloy steel used for bonding and stick it on. Apply a certain pressure of 0.5MPa and heat it in a high-temperature furnace. The two will form a solid bond through a short liquid phase transition eutectic process. The eutectic layer after bonding is a solid layer with a bismuth atomic concentration of 81.1%. When the sample is placed in the furnace, as the temperature rises, the Au-Bi interface partially reaches the eutectic melting point and eutectic melting occurs. When the temperature rises to 241°C, the bismuth atoms melt and begin to diffuse. Continue to raise the temperature to 280°C, and liquid Bi reacts with solid Au to form Au2Bi. The reaction formula is:

[0053] 2Au(s)+Bi(L)→Au2Bi(s)

[0054] Maintain the highest temperature for 15 minutes to ensure sufficient reaction. Then cool slowly, and the eutectic begins to transform from liquid to solid. When the liquid alloy cools to the eutectic temperature, a eutectic reaction occurs, and (Au) and (Bi) phases precipitate simultaneously. The two phases are distributed alternately in lamellar or fibrous forms, forming a low-defect interface. At this point, the eutectic bonding operation is completed. The reaction formula is:

[0055] L→Au+Bi

[0056] See also Figure 4 The eutectic bonding temperature is 241°C and the actual operating temperature is 280°C. The eutectic bonding vacuum degree is 4×10 -3Pa, the bonding process adopts a two-stage temperature control strategy: First, it is heated to 280 °C at a rate of 10 °C / min under a nitrogen protection environment to melt the bismuth layer and initiate a eutectic reaction with the gold layer to form the Au2Bi intermetallic compound; subsequently, it is slowly cooled at a rate of 5 °C / min to promote the growth of equiaxed crystals of the compound, and at the same time, two phases of (Au) and (Bi) are precipitated, finally forming an interfacial layer with eutectic bonding.

[0057] The present invention provides a low-stress and low-temperature bonding method based on a multi-layer composite film structure. Among them, the adhesion layer titanium (Ti) generates Ti-O covalent bonds with the substrate by virtue of its high chemical activity, greatly increasing the film adhesion force to above 25 MPa. Its low thermal expansion coefficient of 8.6×10 -6 / K can not only adjust the thermal stress of the film system, but also form an intermetallic compound with gold to inhibit the excessive diffusion of gold atoms. Gold (Au) is used as a highly ductile buffer medium to effectively absorb the stress deformation during the thermal cycle. The bismuth (Bi) layer not only reduces the melting temperature of the eutectic process due to its low melting point of 271.4 °C, creating favorable conditions for low-temperature bonding, and the thermal expansion coefficient of Au is 14.2×10 -6 / K, and the thermal expansion coefficient of Bi is 13.4×10 -6 / K. The difference between the two is extremely small, enabling a nearly perfect CTE match, reducing thermal stress, and effectively avoiding problems such as excessive internal stress and the generation of microcracks in the film. It is more easily matched with ceramic or silicon substrates, not only effectively avoiding the above-mentioned adverse phenomena caused by thermal stress, but also reducing the risk of thermal damage to components to a certain extent, significantly improving the performance and stability of thermally sensitive components. Compared with the traditional eutectic bonding system, the Au-Bi system shows broad application prospects due to its good match of the thermal expansion coefficient CTE, low-stress film interface, low eutectic temperature, and low cost.

[0058] 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 shall be included within the protection scope of the present invention.

Claims

1. A bonding method based on a multi-layer composite film structure, characterized in that, It includes the following steps: (1) Deposit an adhesion layer, an Au layer, and a Bi layer on the element to be bonded in sequence; deposit an adhesion layer and an Au layer on the alloy steel which is used to bond the element to be bonded; (2) Align the Bi layer on the element to be bonded with the Au layer on the alloy steel, and simultaneously apply pressure and heat. When the temperature reaches the eutectic melting point of the binary alloy of bismuth and gold, a liquid-phase eutectic reaction occurs between the Bi layer and the Au layer on the alloy steel. Continue to heat up to 280 - 300 °C and keep it warm within this temperature range for 10 - 20 min; then cool down. When the temperature drops below the eutectic solidus line of the binary alloy of bismuth and gold, the liquid eutectic starts to transform into a solid state, thus realizing the eutectic bonding of the element to be bonded.

2. The bonding method based on the multi-layer composite film structure according to claim 1, characterized in that, The element to be bonded is a heat-sensitive element.

3. The bonding method based on the multi-layer composite film structure according to claim 1, characterized in that, The adhesion layer is a Ti layer.

4. The bonding method based on the multi-layer composite film structure according to claim 1 or 3, characterized in that, The thickness of the adhesion layer deposited on the element to be bonded is 50 - 100 nm.

5. The bonding method based on the multi-layer composite film structure according to claim 1, characterized in that The thicknesses of the Au layer and the Bi layer deposited on the element to be bonded are 1 - 2 μm and 2 - 3 μm respectively.

6. The bonding method based on the multi-layer composite film structure according to claim 1, characterized in that, The thickness of the adhesion layer deposited on the alloy steel is 50 - 100 nm.

7. The bonding method based on the multi-layer composite film structure according to claim 1, characterized in that, The thickness of the Au layer deposited on the alloy steel is 1 - 2 μm.

8. The bonding method based on the multi-layer composite film structure according to claim 1, wherein The pressure applied is 0.3 - 0.6 MPa.

9. The bonding method based on the multi-layer composite film structure according to claim 1, wherein, The heating rate is 10 - 15 °C / min.

10. The bonding method based on the multi-layer composite film structure according to claim 1, wherein, The cooling rate is 5 - 10 °C / min.

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