Copper cylinder surface metallization and bonding process based on current-assisted thermocompression bonding

Abstract

The invention discloses a copper cylinder surface metallization and bonding process based on current-assisted thermocompression bonding, and belongs to the technical field of semiconductor packaging. Comprising the following steps that a nickel layer (2.0 + / -0.3 mu m) and a gold layer (3.0 + / -0.4 mu m, Ra is smaller than or equal to 0.8 mu m) are sequentially electroplated on the surface of a copper cylinder, and then low-temperature bonding of the high-roughness gold layer is achieved under the synergistic effect of applied pulse current (the peak current is 1.2-2.1 A, the duty ratio is 25%-50%, and the frequency is 2 KHz) and gradient pressure (0-200 MPa) in the air environment. Through current-induced Joule heat and interface atom diffusion, the bonding deformation is remarkably reduced, the shearing strength is improved, meanwhile, chip thermal damage caused by a traditional high-temperature process is avoided, the method is suitable for high-density advanced packaging, through precise electroplating control (the total thickness error of a nickel / gold layer is smaller than or equal to 1 micron) and polishing-free design, the chemical mechanical polishing (CMP) procedure is reduced, and the production cost is reduced. And the process period is shortened by 40%.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of electronic packaging, in particular to a copper pillar surface metallization and bonding process based on current-assisted thermal compression bonding. BACKGROUND

[0002] In the field of semiconductor packaging and microelectronic interconnection, thermal compression bonding technology is the core process to realize solid-state diffusion connection of metal interface. Traditional thermal compression bonding usually needs to be carried out under high temperature (> 400 DEG C) and ultra-high pressure (> 150 MPa) conditions, and is maintained for 180-300 seconds to drive metal atoms to diffuse to form a dense interface. However, such process has the following significant defects: thermal damage and material degradation; mechanical stress concentration; high cost of surface treatment; energy consumption and efficiency bottleneck.

[0003] In recent years, some improved technologies attempt to reduce the process temperature by laser-assisted heating or ultrasonic vibration, but have the following limitations: the laser heat source is easy to cause local overheating or uneven heat distribution, causing non-uniform diffusion of the interface; the ultrasonic vibration has limited tolerance to interface roughness, and the equipment is complex; the above methods still need to be combined with higher pressure, and cannot fundamentally solve the mechanical stress problem.

[0004] Therefore, it is urgent to develop a technology that can realize efficient bonding under a medium-temperature and medium-pressure process window (such as 300 DEG C and 100 MPa) through an innovative mechanism, so as to balance the thermal-mechanical reliability, manufacturing cost and production efficiency. SUMMARY

[0005] The application aims to provide a copper pillar surface metallization and bonding process based on current-assisted thermal compression bonding, which realizes reliable bonding of high-roughness surfaces through current-assisted thermal compression bonding, and is suitable for packaging process of copper pillar interconnection structure.

[0006] To achieve the above-mentioned purpose, the application provides a copper pillar surface metallization and bonding process based on current-assisted thermal compression bonding, which comprises the following steps:

[0007] (1) Metal layer preparation: a nickel layer and a gold layer are successively electroplated on the surface of the copper pillar, the thickness of the nickel layer is 2.0+ / -0.2 microns, the thickness of the gold layer is 3.0+ / -0.3 microns, and the surface roughness of the gold layer is controlled to be Ra≤0.8 microns;

[0008] (2) Bonding process: two gold-plated copper pillars are aligned in an air environment, a gradient pressure is applied and a direct current pulse current is loaded synchronously, the local temperature of the bonding interface is instantaneously increased to the softening point of the gold layer through the Joule heating effect, while the overall chip temperature is maintained at 300 DEG C, and the bonding duration is 40-120 seconds.

[0009] Preferably, the total thickness error of the nickel layer and the gold layer in step (1) is ≤1 micron.

[0010] Preferably, the loading process of the gradient pressure in step (2) is linearly increased from 0 MPa to 80-200 MPa at a loading rate of 10-20 MPa / s, and a direct current pulse current is applied when the pressure reaches 50 MPa.

[0011] Preferably, the peak current of the direct current pulse current in step (2) ranges from 1.2 A to 2.1 A, the duty cycle ranges from 25% to 50%, and the frequency is 2 KHz.

[0012] Preferably, when the roughness Ra of the gold layer is greater than 0.5 μm, the duty cycle of the direct current pulse current is 40%-50%, and the bonding time lasts for 90-120 seconds.

[0013] Preferably, when the roughness Ra of the gold layer is less than or equal to 0.3 μm, the gradient pressure ranges from 80 MPa to 100 MPa, and the bonding time lasts for 40-60 seconds.

[0014] Further, a 2-5 nm titanium transition layer is sputtered and deposited between the nickel layer and the gold layer to further improve the copper atom diffusion barrier capability.

[0015] Technical mechanism of the present application:

[0016] Current thermal activation: high-density pulse current (2 KHz) induces violent collision of gold layer interface electrons, generates local high temperature softening gold layer, and accelerates atomic diffusion to form metallurgical bonding;

[0017] Rough surface compensation: 100-200 MPa pressure promotes micro-plastic flow of the gold layer with Ra≤0.8 μm to fill the interface gap;

[0018] Nickel layer functional strengthening: the nickel layer as a diffusion barrier layer inhibits copper atom migration, avoids the generation of Cu-Au brittle alloy at the bonding interface, and improves the bonding strength of the gold layer and the copper column.

[0019] Compared with the prior art, the present application has the following beneficial effects:

[0020] 1. High roughness compatibility: breaking through the strict requirement of traditional process on the surface roughness Ra of the gold layer (Ra≤0.1 μm), allowing the direct bonding of rough surfaces with Ra≤0.8 μm.

[0021] 2. Process cost optimization: through precise electroplating control (total thickness error of nickel / gold layer ≤1 μm) and polishing-free design, reducing the chemical mechanical polishing (CMP) process, and shortening the process cycle by 40%.

[0022] 3. Energy efficiency improvement: using local joule heat activation instead of global high-temperature bonding to reduce energy consumption.

[0023] 4. The structural reliability is enhanced: the nickel layer effectively inhibits the generation of intermetallic compounds, and the shear strength of the bonding interface is increased to ≥80 MPa (≤60 MPa in the traditional process). BRIEF DESCRIPTION OF DRAWINGS

[0024] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings needed in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present application, and therefore should not be regarded as a limitation on the scope. For those skilled in the art, other related drawings can also be obtained without creative labor on the basis of these drawings.

[0025] Figure 1 The bonding schematic diagram of Example 1 of the present application.

[0026] Figure 2 The scanning electron microscope diagram of the bonding interface of Example 1 of the present application.

[0027] Figure 3 The scanning electron microscope diagram of the bonding interface of Comparative Example 1 of the present application.

[0028] Figure 4 The scanning electron microscope diagram of the bonding interface of Example 2 of the present application.

[0029] Figure 5 The scanning electron microscope diagram of the bonding interface of Comparative Example 2 of the present application. DETAILED DESCRIPTION

[0030] Metal layer preparation: electroplating a nickel layer to obtain a dense nickel layer with a thickness of 2.0±0.2 μm; electroplating a gold layer to form a gold layer with a thickness of 3.0±0.3 μm, the total thickness error of the nickel layer and the gold layer is ≤1 μm, and the surface roughness Ra of the gold layer is ≤0.8 μm, without chemical mechanical polishing (CMP) pretreatment, direct bonding is performed.

[0031] Bonding process implementation: place two gold-plated copper pillars in the bonding machine clamp, realize interface contact through an optical alignment system (accuracy ±1 μm); load gradient pressure and simultaneously load direct current pulse current, the pressure is linearly increased from 0 to the target value (100-200 MPa) at a loading rate of 10-20 MPa / s; pressure maintenance: after reaching the target pressure, keep constant, and the pressure fluctuation is ≤±2%.

[0032] Pulse current application: parameter setting: peak current 1.2-2.1 A, duty cycle 25%-50%, frequency 2 KHz; the current path is that the current is conducted through the copper pillar body to the gold-gold contact interface.

[0033] Temperature synergistic control: combined with the poor contact due to the high roughness of the interface, the local temperature is increased due to the high resistance; the chip body is maintained at 300±10℃ (to avoid thermal damage to sensitive devices).

[0034] Bonding time: Total bonding time 60 seconds, with a time error of ≤±5%.

[0035] Process optimization plan:

[0036] (1) Rough surface compensation: When the roughness Ra of the gold layer is greater than 0.5 μm, the duty cycle of the pulse current can be increased to 40%-50%, and the bonding time can be extended to 90-120 seconds.

[0037] (2) Low-pressure rapid bonding: When the roughness of the gold layer Ra≤0.3μm, the pressure can be reduced to 80-100MPa and the bonding time can be shortened to 40-60 seconds.

[0038] (3) Anti-diffusion enhancement: Adding a 2-5nm titanium transition layer (sputter deposition) between the nickel layer and the gold layer can further enhance the diffusion barrier capability of copper atoms.

[0039] Example 1 (Basic Scheme of the Invention)

[0040] Steps: Metal layer preparation: Electroplating a nickel layer (2.0±0.2μm) and a gold layer (3.0±0.3μm) on the surface of a copper pillar with a diameter of 60μm, with a surface roughness Ra of 0.8μm for the gold layer;

[0041] Bonding process: Align two gold-plated copper pillars in air, apply a vertical pressure of 100MPa, and simultaneously apply a pulsed current (peak current 1.8A, duty cycle 40%, frequency 2KHz). Maintain the bonding temperature at 300℃ for 60 seconds.

[0042] Figure 1 This is a schematic diagram of the bonding process in Example 1.

[0043] Figure 2 The image shown is a scanning electron microscope (SEM) image of the bonding interface in Example 1. Figure 2 It can be seen that the bonding interface is well bonded with no obvious gaps and only a few tiny voids. The measured interface porosity is only 2.245%, and the shear strength is 147.6 MPa, which is a higher level of reliability than Comparative Example 1 and Comparative Example 2.

[0044] Comparative Example 1

[0045] Except for not applying a pulsed current, the other steps and parameters are the same as in Example 1.

[0046] Figure 3 The image shown is a scanning electron microscope image of the bonding interface in Comparative Example 1 of this invention. Gaps of about 10 μm in length appear on both sides, indicating poor bonding effect. The porosity is 38.33%, and 38.33% of the area has not formed a metallurgical bond. The shear strength is significantly reduced to only 31.1 MPa.

[0047] Conclusion: High-density pulse current (2KHz) induces the interface electron of gold layer to collide violently, which produces local high temperature to soften the gold layer and accelerate atomic diffusion to form metallurgical bonding; the pulse current significantly improves the interface atomic diffusion rate through Joule heating effect, porosity is reduced by 36.085%, and shear strength is increased by 116.56MPa.

[0048] Example 2 (high pressure and long time optimization)

[0049] The experimental steps are the same as in Example 1, except that the pressure is increased to 200MPa and the bonding time is extended to 120 seconds; the pulse current duty cycle is adjusted to 50%, and the remaining parameters are the same as in Example 1.

[0050] Test results: porosity is further reduced to 1.59%, and shear strength is increased to 187.6MPa; the plastic flow of the gold layer fills the interface gap, but the thermal stress of the chip increases, and the copper pillar deforms significantly.

[0051] From Figure 4 The SEM image of the bonding interface shows that there are almost no voids, and the bonding effect is better, but due to the increase of bonding pressure and time, the copper pillar is obviously bulging outward, and extrusion deformation occurs, which may affect the reliability of use in the later use process.

[0052] Conclusion: High pressure and long time process can optimize the interface density, but the risk of thermal stress needs to be considered.

[0053] Comparative Example 2 (low roughness comparison)

[0054] Traditional polishing process to prepare gold layer: the copper pillar is treated, first by cleaning to remove impurities such as particles, dirt and other impurities on the wafer surface, and then drying to avoid water affecting the activity of the polishing liquid; during polishing, the wafer is fixed on the stage and contacted with the rotating polishing pad under pressure, while the polishing liquid is continuously added, and through the combined action of mechanical friction of abrasive and chemical reaction of reagents, the surface protrusions and excess materials are removed; post-processing is carried out by cleaning to remove residual polishing liquid and debris, and after drying, detection is completed; after polishing, nickel and gold layers are electroplated as in Example 1, and finally the surface gold layer roughness Ra≤0.2μm is achieved.

[0055] Bonding parameters: pressure 150MPa, temperature 350℃, time 180 seconds, no current applied.

[0056] Test results: porosity 7.15%, shear strength 134.3MPa;

[0057] Figure 5The bonding interface scanning electron microscope graph of the present application comparative example 2, compared with comparative example 1, example 3 can realize better bonding interface metallurgical bonding, but there are more holes at the bonding interface, and the hole distribution area is wide, which may pose a certain threat to the use reliability in the later period.

[0058] Conclusion: Although the traditional process realizes low roughness bonding, the porosity and interface strength are worse than example 1, and the 100MPa pressure of the present application promotes the micro zone plastic flow of the gold layer with Ra≤0.8μm, fills the interface gap, and the porosity of the bonding interface is ≤3%.

[0059] All bonding parameters of the examples and comparative examples are shown in table 1.

[0060] Table 1 Bonding process parameters

[0061]

[0062] The test results can be known from table 1, example 1 and comparative example 1 are compared, the current auxiliary technology realizes reliable bonding of high roughness gold layer under the condition of low temperature and short time, the shear strength increases from 31.1MPa to 147.6MPa, and the porosity decreases from 38.33% to 2.245%; Example 1 and example 2 are compared, it can be known that increasing the pressure can further optimize the performance, but the thermal stress needs to be controlled; Example 1 and comparative example 2 are compared, it can be known that the traditional low roughness process has the lowest comprehensive performance, which verifies the technical advantages of the present application.

[0063] The above is only a specific embodiment of the present application, but the protection scope of the present application is not limited to this, any skilled person in the art can make any modification, equivalent replacement and improvement within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A surface metallization and bonding process for copper pillars based on current-assisted thermopressing bonding, characterized in that, Includes the following steps: (1) Metal layer preparation: A nickel layer and a gold layer are electroplated sequentially on the surface of the copper pillar. The thickness of the nickel layer is 2.0±0.2μm, the thickness of the gold layer is 3.0±0.3μm, and the surface roughness of the gold layer is controlled to Ra≤0.8μm. (2) Bonding process: Two gold-plated copper pillars are brought into contact in an air environment, gradient pressure is applied and DC pulse current is applied simultaneously. The local temperature of the bonding interface is instantly raised to the softening point of the gold layer through the Joule heating effect, while the overall chip temperature is maintained at 300℃. The bonding duration is 40-120 seconds.

2. The surface metallization and bonding process of copper pillars based on current-assisted thermopressing bonding according to claim 1, characterized in that, The total thickness error between the nickel layer and the gold layer in step (1) is ≤1μm.

3. The surface metallization and bonding process of copper pillars based on current-assisted thermopressing bonding according to claim 1, characterized in that, The gradient pressure loading process in step (2) is to linearly increase from 0 MPa to 80-200 MPa at a loading rate of 10-20 MPa / s, and to start applying DC pulse current when the pressure reaches 50 MPa.

4. The surface metallization and bonding process of copper pillars based on current-assisted thermopressing bonding according to claim 1, characterized in that, The peak current range of the DC pulse current in step (2) is 1.2-2.1A, the duty cycle range is 25%-50%, and the frequency is 2KHz.

5. The surface metallization and bonding process of copper pillars based on current-assisted thermo-press bonding according to claim 1, characterized in that, When the gold layer roughness Ra > 0.5 μm, the DC pulse current duty cycle is 40%-50%, and the bonding time lasts for 90-120 seconds.

6. The surface metallization and bonding process of copper pillars based on current-assisted thermopressing bonding according to claim 2, characterized in that, When the gold layer roughness Ra≤0.3μm, the gradient pressure ranges from 80 to 100MPa, and the bonding time lasts from 40 to 60 seconds.

7. The surface metallization and bonding process of copper pillars based on current-assisted thermo-press bonding according to claim 1, characterized in that, A 2-5 nm titanium transition layer is sputtered between the nickel and gold layers.

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