A Cu-Cu low-temperature bonding method

Abstract

The present invention provides a low-temperature Cu-Cu bonding method. The method involves pre-treating the surface of a copper sheet to introduce a layer of an energetic compound onto the copper sheet, and then aligning and bonding two treated copper sheets under specific temperature and pressure conditions. The method introduces the energetic compound onto the copper surface through surface treatment. By utilizing the energetic compound's decomposition and heat release during Cu-Cu bonding, the diffusion of copper atoms is promoted, and no impurity elements remain at the bonding interface.

Description

Technical Field

[0001] The present invention relates to the technical field of three-dimensional integrated packaging, and in particular to a Cu-Cu low-temperature bonding method. Background Art

[0002] With the rapid development of emerging fields such as artificial intelligence, 5G, aerospace, and new energy vehicles, semiconductor devices are constantly developing towards integration, miniaturization, multifunctionality, and resistance to high voltage, high temperature, high frequency, and high power. In response to the challenges of high-density, high-reliability three-dimensional integrated packaging, people have proposed Cu-Cu bonding technology. Cu-Cu bonding avoids the use of Sn elements, remains solid throughout the bonding process, and does not have the problem of solder overflow, which can achieve narrow pitch interconnection. At the same time, copper has better electrical and thermal conductivity than tin, good anti-electromigration ability and thermo-mechanical reliability. Therefore, Cu-Cu bonding technology is a key technology for realizing three-dimensional integrated circuit interconnection and chip stacking, and is also an important technical means to achieve device integration and miniaturization. It directly affects the development prospects of three-dimensional integrated circuits and device packaging, and has important research value and significance.

[0003] In recent years, research on low-temperature Cu-Cu bonding technology has focused on low-temperature bonding of nano-copper / silver pastes and direct Cu-Cu bonding. The melting point of metal nanomaterials decreases with decreasing particle size, allowing them to fully diffuse and sinter at temperatures far below the melting point of bulk materials. This, to a certain extent, can meet the packaging goals of low-temperature bonding and high-temperature service. However, nano-copper / silver pastes are complex to prepare and expensive. Nano-copper has extremely high surface activity, and spontaneous oxidation in air to form surface oxides can severely hinder the diffusion and sintering of copper. Nano-silver, while relatively inert, has poor resistance to electromigration. Nano-copper / silver sintering results in certain porosity, which affects electrical and thermal conductivity. These deficiencies have restricted the application of nano-copper / silver pastes in high-power three-dimensional integrated packaging.

[0004] Cu-Cu direct bonding refers to the process of direct and tight bonding between Cu bumps without any intermediate layer. Under the action of hot pressing, the diffusion rate and contact area of ​​Cu atoms at the bonding interface increase, and after sufficient diffusion, a Cu-Cu bonding structure is formed. Due to the absence of an intermediate layer, Cu-Cu direct bonding provides the most essential interconnection structure between chips, with advantages such as strong resistance to electromigration, short signal delay, high current density and matching thermal expansion coefficients. However, Cu-Cu direct bonding also faces the problems of easy surface oxidation and slow mutual diffusion of copper atoms. Excessive bonding temperature and pressure (>350°C and >20MPa) can easily cause thermal and mechanical damage to the chip, seriously restricting chip performance and service life.

[0005] Therefore, it is necessary to provide a new Cu-Cu low-temperature bonding method to avoid the interference of the oxide layer and promote the diffusion of copper. Summary of the Invention

[0006] The purpose of the present invention is to overcome the defects of the prior art and provide a Cu-Cu low-temperature bonding method. By introducing an energetic compound on the copper surface through surface treatment, an anti-oxidation protective layer is formed. The decomposition and heat release properties of the energetic compound during Cu-Cu bonding are utilized to promote the diffusion of copper atoms, and no impurity elements remain at the bonding interface.

[0007] The present invention provides the following technical solutions:

[0008] The present invention provides a Cu-Cu low-temperature bonding method, which comprises pre-treating the surface of a copper sheet to introduce a layer of energetic compound on the surface of the copper sheet, and aligning and bonding two treated copper sheets under certain temperature and pressure conditions.

[0009] Furthermore, the pre-treatment includes the steps of preparing an electrolyte, and electrolytically depositing an energetic compound on the surface of the copper sheet under power-on conditions.

[0010] Furthermore, the energetic compound is copper azide, cuprous azide or a mixture thereof.

[0011] Furthermore, the electrolyte is a solution containing an azide salt, preferably sodium azide or potassium azide.

[0012] Furthermore, the electrolyte concentration is 0.001-10 mol / L.

[0013] Furthermore, the current density is 0.01-10A / dm 2 , electrolysis time is 0.1-200min.

[0014] Furthermore, the bonding temperature is 100-350° C., and the bonding pressure is 0.1-100 MPa.

[0015] The present invention has the following beneficial effects:

[0016] The present invention utilizes electrochemical reaction to deposit a layer of copper azide, cuprous azide or a mixture thereof on the copper surface through surface treatment to form an anti-oxidation protective layer. During the bonding process, the copper azide, cuprous azide or a mixture thereof is completely decomposed into nitrogen and highly active metallic copper under low temperature and low pressure conditions by utilizing the high activity and easy decomposition characteristics of the copper azide, cuprous azide or a mixture thereof, thereby achieving the purpose of leaving no impurity elements in the copper layer after bonding. At the same time, the decomposition generates a large amount of heat, which greatly promotes the diffusion of copper atoms at the bonding interface through the local thermal effect, thereby promoting the completion of bonding. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0018] Figure 1 This is the XRD result of the sample after pretreatment in Example 1 of the present invention;

[0019] Figure 2 This is a physical picture of the sample after bonding in Example 1 of the present invention;

[0020] Figure 3 The SEM and EDS results of the bonding cross section in Example 1 of the present invention are shown, wherein (a) is the SEM result, (b) is the EDS result, and (c) is the EDS elemental analysis diagram;

[0021] Figure 4 This is the XRD result of the sample after bonding in Example 1 of the present invention;

[0022] Figure 5 This is a graph showing the thrust machine test results of the bonded sample in Example 1 of the present invention;

[0023] Figure 6 This is a graph showing the thrust machine test results of the bonded sample in Example 2 of the present invention;

[0024] Figure 7 Graph showing thrust machine test results of the bonded sample in Example 3 of the present invention;

[0025] Figure 8 Graph showing the thrust machine test results of the bonded sample in Example 4 of the present invention;

[0026] Figure 9 This is the XRD result of the sample after pretreatment in Example 8 of the present invention;

[0027] Figure 10 Graph showing thrust machine test results of the bonded sample in Example 8 of the present invention;

[0028] Figure 11 This is a physical picture of the bonding in Comparative Example 1 of the present invention. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] An embodiment of the present invention provides a Cu-Cu low-temperature bonding method, which comprises pre-treating the surface of a copper sheet to introduce a layer of energetic compound on the surface of the copper sheet, and aligning and bonding two treated copper sheets under certain temperature and pressure conditions.

[0031] In a specific embodiment, the pretreatment method is to deposit energetic compounds on the copper surface by electrolysis, and the steps are: preparing an electrolyte, using a pure copper sheet as the cathode and a copper-plated silicon sheet as the anode, and depositing energetic compounds on the anode surface under power-on conditions.

[0032] In a specific embodiment, the energetic compound is copper azide, cuprous azide or a mixture thereof.

[0033] In a specific embodiment, the electrolyte contains N 3- A solution of, preferably sodium azide or potassium azide.

[0034] In a specific embodiment, the electrolyte concentration is 0.001-10 mol / L.

[0035] In a specific embodiment, the current density is 0.01-100 A / dm 2 , electrolysis time is 0.1-200min.

[0036] In a specific embodiment, the bonding temperature is 100-350° C., and the bonding pressure is 0.1-100 MPa.

[0037] The following is explained through specific embodiments:

[0038] Example 1

[0039] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 0.02 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, and the electrolyte is 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0040] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them at 250°C and 5 MPa for 60 min.

[0041] The XRD test was performed on the sample sheets dried after pretreatment. The test results are shown in Figure 1 As shown, it can be seen that a cuprous azide film is successfully deposited on the copper-plated silicon wafer.

[0042] The actual picture of the sample after Cu-Cu bonding is shown in Figure 2 As shown, the bonding cross section of the sample was analyzed by SEM and EDS. The results are shown in Figure 3 (a), 3(b), 3(c), it can be seen that after bonding, the cuprous azide film is completely converted into copper and nitrogen, the copper layer is tightly bonded, and no impurity elements remain. The XRD test analysis of the bonded sample is shown in Figure 4 shown.

[0043] The shear stress of the bonded samples was characterized using a push-pull tester. The results are as follows: Figure 5 As shown, the resulting shear stress is 6.0 MPa.

[0044] Example 2

[0045] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 0.02 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, and the electrolyte is 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0046] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them at 250°C and 10 MPa for 60 min.

[0047] The shear stress of the bonded samples was characterized using a push-pull tester. The results are as follows: Figure 6 As shown, the resulting shear stress is 10.82 MPa.

[0048] Example 3

[0049] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 0.02 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, and the electrolyte is 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0050] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them for 60 min at 300°C and 5 MPa.

[0051] The shear stress of the bonded samples was characterized using a push-pull tester. The results are as follows: Figure 7 As shown, the resulting shear stress is 6.5 MPa.

[0052] Example 4

[0053] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 0.02 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, and the electrolyte is 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0054] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them at 300°C and 10 MPa for 60 min.

[0055] The shear stress of the bonded samples was characterized using a push-pull tester. The results are as follows: Figure 8 As shown, the resulting shear stress is 11.65 MPa.

[0056] Example 5

[0057] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 0.2 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, use 3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0058] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them at 300°C and 10 MPa for 60 min.

[0059] The shear stress of the bonded sample was characterized using a push-pull testing machine, and the obtained shear stress was 9.87 MPa.

[0060] Example 6

[0061] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 2 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, use 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0062] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them at 300°C and 10 MPa for 60 min.

[0063] The shear stress of the bonded sample was characterized using a push-pull testing machine, and the obtained shear stress was 10.02 MPa.

[0064] Example 7

[0065] (1) Pretreatment: Prepare electrolyte: potassium azide concentration is 0.02 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, and the electrolyte is 0.3A / dm 2Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0066] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them for 60 min at 300°C and 5 MPa.

[0067] The shear stress of the bonded sample was characterized using a push-pull testing machine, and the obtained shear stress was 5.52 MPa.

[0068] Example 8

[0069] Preparation of copper azide: Copper-plated silicon wafers are placed in a 3 mol / L KOH electrolyte at 0.4 A / dm 2 Preparation of Cu(OH)2: Cu(OH)2 was heated to 200°C in a tube furnace under nitrogen and dried for 3 hours to generate CuO. Cu(N3)2 was prepared electrochemically in a NaN3 solution.

[0070] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 0.02 mol / L; the anode sample is copper oxide silicon wafer, the cathode is pure copper sheet, use 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0071] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them for 60 min at 300°C and 5 MPa.

[0072] The XRD test was performed on the sample after pretreatment. Figure 9 It can be seen that there is copper azide and cuprous azide on the surface of the silicon wafer.

[0073] The shear stress of the bonded samples was characterized using a push-pull tester. The results are as follows: Figure 10 As shown, the resulting shear stress is 6.14 MPa.

[0074] Example 9

[0075] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 8 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, use 5A / dm 2 Electrolyze at the current density for 10 min, then take out the sample and dry it;

[0076] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them for 60 minutes at 100°C and 100 MPa. The bonding surfaces are tightly bonded.

[0077] Example 10

[0078] (1) Pretreatment: Prepare electrolyte: sodium azide concentration is 2 mol / L; the anode sample is copper-plated silicon wafer, the cathode is pure copper sheet, use 0.3A / dm 2 Electrolyze at a current density of 5 min, then take out the sample and dry it;

[0079] (2) Bonding: Take two pre-treated sample sheets, the upper sheet is 3×3 mm and the lower sheet is 10×10 mm, and bond them for 60 minutes at 350°C and 0.1 MPa. The bonding surfaces are tightly bonded.

[0080] Comparative Example 1

[0081] Select two copper-plated silicon wafers without surface treatment, the upper plate is 3×3mm and the lower plate is 10×10mm, and bond them for 60 minutes at 300℃ and 10MPa. Figure 11 As shown, two copper-plated silicon wafers cannot be bonded.

[0082] Due to the high melting point of copper (1083°C) and its low self-diffusion rate, direct bonding requires a high temperature of over 400°C for sufficient atomic diffusion to occur. This will reduce the centering accuracy and place high demands on the equipment. In addition, Cu is easily oxidized during high-temperature bonding, which will hinder the diffusion of Cu atoms and affect the bonding effect.

[0083] The present invention utilizes electrochemical reaction to deposit a layer of copper azide, cuprous azide or a mixture thereof on the copper surface through surface treatment to form an anti-oxidation protective layer. During the bonding process, the copper azide, cuprous azide or a mixture thereof is completely decomposed into nitrogen and highly active metallic copper under low temperature and low pressure conditions by utilizing the high activity and easy decomposition characteristics of the copper azide, cuprous azide or a mixture thereof, thereby achieving the purpose of leaving no impurity elements in the copper layer after bonding. At the same time, the decomposition generates a large amount of heat, which greatly promotes the diffusion of copper atoms at the bonding interface through the local thermal effect, thereby promoting the completion of bonding.

[0084] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A Cu-Cu low temperature bonding method, characterized in that: The method comprises pre-treating the surface of a copper sheet to introduce a layer of energetic compound on the surface of the copper sheet, and aligning and bonding two treated copper sheets under certain temperature and pressure conditions; The pre-treatment comprises the steps of: preparing an electrolyte, and electrolytically depositing an energetic compound on the surface of the copper sheet under an electric condition; The energetic compound is copper azide, cuprous azide or a mixture thereof; Current density is 0.01-10A / dm 2 , the electrolysis time is 0.1-200 min; the bonding temperature is 100-350° C., and the bonding pressure is 0.1-100 MPa.

2. The Cu-Cu low-temperature bonding method according to claim 1, wherein: The electrolyte is a solution containing an azide salt.

3. The Cu-Cu low-temperature bonding method according to claim 1, wherein: The electrolyte concentration is 0.001-10 mol / L.

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