Low-temperature and low-voltage copper-copper metal bonding method

Abstract

The invention provides a low-temperature and low-voltage copper-copper metal bonding method, and belongs to the technical field of semiconductor packaging. The method comprises the following steps of: firstly, carrying out mechanochemical polishing on the copper column salient point formed by electroplating, reducing the surface roughness of the copper column salient point to a nanoscale to form a high-flatness bonding surface, and then applying a temperature of 290-300 DEG C and a pressure of 100-170 MPa in an air environment, and loading a constant current of 0.5-0.8 A to assist bonding for 30-90 seconds. The surface roughness is remarkably reduced through the CMP technology, the constant-current auxiliary induced electro-plastic effect is combined, rapid and reliable bonding of the copper pillar protruding points is successfully achieved under the low-temperature and low-pressure conditions, and the method is suitable for low-temperature and low-pressure rapid interconnection manufacturing of a three-dimensional integrated circuit (3D IC) and has good application prospects. And particularly, the bonding reliability problem of a heat-sensitive device in high-density packaging is solved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of semiconductor packaging, in particular to a copper-copper bonding method based on the synergistic effect of mechanical chemical polishing (CMP) and electroplastic effect, which is suitable for low-temperature and low-pressure rapid interconnection manufacturing of three-dimensional integrated circuits (3D IC), and in particular solves the bonding reliability problem of heat-sensitive devices in high-density packaging. BACKGROUND

[0002] The traditional copper bonding technology faces three major bottlenecks: high temperature (≥400℃) causes thermal damage, inert gas protection increases equipment cost by more than 30%, and high pressure (more than 200MPa) is required when the surface roughness is high; in the existing improved technology, it is impossible to simultaneously meet the four requirements of low temperature, low pressure, air environment adaptability and low roughness interface control, which seriously restricts the development of high-density three-dimensional packaging. Therefore, developing a copper-copper bonding method with low temperature, low pressure, rapidness and high reliability is still a technical difficulty that needs to be broken through in the field.

[0003] As the core technology of high-density three-dimensional integration, copper-copper thermal compression bonding technology has been limited by the high temperature and high pressure bottlenecks of traditional processes for a long time. In order to achieve reliable bonding, the conventional method needs to be operated at a high temperature of ≥400℃, which is far beyond the thermal budget (≤350℃) of silicon-based devices, and is prone to cause delamination of low-dielectric materials, warping of chips and thermal stress failure. At the same time, in order to prevent the oxidation of the copper surface, the bonding process must rely on inert gas protection such as nitrogen / argon, which leads to the complication of the sealing system of the equipment, an increase of more than 30% in the manufacturing cost of the equipment, and a significant shortening of the maintenance cycle. More importantly, the surface roughness (Ra>50nm) of the electroplated copper pillar greatly reduces the effective contact area, forcing the process to use ultra-high pressure (>200MPa) to make up for the interface gap, which in turn induces micro-bump collapse or substrate cracking, resulting in a yield loss of 15-20%.

[0004] In view of the above problems, the existing improved technology still has fundamental defects: the surface activation bonding requires an ultra-high vacuum environment (<10 -5 Pa), which is not compatible with mass production equipment; the solid-liquid phase diffusion bonding relies on a tin-silver intermediate layer, which introduces the risk of electromigration; none of these technologies can simultaneously meet the four core requirements of low temperature (≤300℃) to be compatible with heat-sensitive devices, low pressure (≤150MPa) to avoid mechanical damage, air environment adaptability to eliminate the cost of inert gas, and low roughness interface (Ra≤10nm) to ensure the uniformity of bonding strength.

[0005] The above multi-objective synergistic contradiction seriously restricts the development of high-density three-dimensional packaging towards ultra-thin chips and multi-layer stacking, and developing a copper-copper bonding method with low temperature, low pressure, air environment compatibility and high reliability has become a core technical difficulty that needs to be broken through in the field. SUMMARY

[0006] Therefore, the present application aims to provide a low-temperature and low-pressure copper-copper metal bonding method, which realizes high-reliability bonding under a low-temperature and low-pressure environment by synergistically optimizing surface treatment and bonding process parameters.

[0007] The present application provides a low-temperature and low-pressure copper-copper metal bonding method, comprising the following steps:

[0008] Step one: ultra-fine polishing of the surface of the copper column: the silicon substrate of the electroplated copper column is sequentially subjected to water washing and plasma cleaning; the cleaned copper column is polished, with SiO2alkaline colloid as a polishing liquid, and the copper layer removal amount is controlled to be 0.8-1.2 μm; after polishing, the copper column is subjected to water washing and drying, so as to obtain a bonding surface with a surface roughness of ≤10 nm (Ra);

[0009] Step two: bonding: the silicon substrate treated in step one is placed on a bonding machine stage, and the copper column bump is aligned and bonded; in an air environment, the double stages are synchronously heated to 290-300 ℃, a vertical pressure of 100-150 MPa is applied; a constant current direct current of 0.5-0.65 A is applied, and the bonding is completed after 30-90 seconds of temperature and pressure maintaining and power supply.

[0010] Preferably, in step one, the polishing is performed by using a rotary polisher.

[0011] Preferably, in step one, the SiO2alkaline colloid polishing liquid has a particle size of 50 nm and a pH of 10.5±0.5.

[0012] Preferably, in step two, the bonding machine stage has an optical alignment accuracy of ±1 μm, and the stage is preheated to 150 ℃.

[0013] Preferably, in step two, the copper column bump has a height of 10-50 μm, a diameter of 20-100 μm, and a pitch of 40-200 μm.

[0014] Preferably, in step two, the heating rate is 10 ℃ / s.

[0015] Preferably, in step two, the constant current direct current flows through the bonding interface in a vertical direction to induce an electroplastic effect and promote the diffusion of interface atoms.

[0016] Another object of the present application is that the copper-copper bonding interface prepared by the method can constitute a vertical interconnection channel in three-dimensional integrated packaging, as a chip interconnection structure.

[0017] Compared with the prior art, the present application has the following beneficial effects:

[0018] In a non-vacuum air environment, only 290-300℃ low temperature and 100-150MPa low pressure are needed to complete the bonding, which can greatly reduce the energy consumption cost and equipment requirement compared with the prior art of 400℃ high temperature and more than 200MPa high pressure; high reliability bonding level of 80MPa shear strength can still be achieved under low temperature and low pressure; air environment compatibility avoids the protection cost of vacuum cavity or inert gas, and provides a more economical solution for wafer level packaging, MEMS devices and other scenarios. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following will briefly introduce the drawings needed to be used in the embodiments. 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.

[0020] Figure 1 It is a schematic diagram of copper-copper bonding of the present application.

[0021] Figure 2 (a) is a three-dimensional profile diagram before polishing of the electroplated copper column, Figure 2 (b) is a three-dimensional profile diagram after polishing of the electroplated copper column.

[0022] Figure 3 It is an atomic force microscope (AFM) test diagram of the copper surface after CMP polishing (the measured Ra is 1.26nm).

[0023] Figure 4 It is a scanning electron microscope diagram of the bonding interface of Example 1.

[0024] Figure 5 It is a scanning electron microscope diagram of the bonding interface of Example 2.

[0025] Figure 6 It is a scanning electron microscope diagram of the bonding interface of the comparative example. DETAILED DESCRIPTION

[0026] The present application will be further described in detail below in combination with the drawings and specific embodiments.

[0027] The present application provides a low-temperature and low-pressure copper-copper metal bonding method, comprising the following steps:

[0028] Step one: super-fine polishing of the copper column surface

[0029] (a) the silicon substrate of the electroplated copper column, the copper column height is 10-50μm, the diameter is 20-100μm, the pitch is 40-200μm, and the water washing and plasma cleaning are carried out in sequence;

[0030] (b) The cleaned copper column is polished using a rotary polisher with SiO2alkaline colloid as polishing liquid (particle size 50 nm, pH = 10.5 ± 0.5) to control the copper layer removal amount to 0.8-1.2 μm;

[0031] (c) After polishing, water washing and drying, the bonding surface with a surface roughness ≤10 nm (Ra) and a surface roughness (Ra) of more than 70% of the area reduced to ≤10 nm is obtained.

[0032] Step two: pre-bonding treatment

[0033] The polished substrate is placed on the stage of the bonding machine with an optical alignment accuracy of ±1 μm, and the stage is preheated to 150 °C.

[0034] Step three: electric-thermal-force synergistic bonding

[0035] In an air environment, the double-stage is synchronously heated to 290-300 °C (heating rate 10 °C / s); a vertical pressure of 100-150 MPa (pressure uniformity >95%) is applied; a constant current of 0.5-0.65 A is applied with the current direction perpendicular to the bonding interface; and the bonding is completed by keeping the temperature, pressure and current for 30-90 seconds.

[0036] The polishing endpoint in step one (b) is monitored in real time by an online film thickness meter.

[0037] The surface roughness in step one (c) is detected by an atomic force microscope (AFM).

[0038] The bonding principle is that the directional current promotes the thermal diffusion of atoms and provides energy for the movement of defects such as dislocations and slips; the current flows through the copper column contact interface, the actual contact area is much smaller than the diameter of the original copper column, which leads to an increase in contact resistance, generates local Joule heat, softens the copper at the interface, and further promotes the diffusion of copper atoms to fill the void area, and finally realizes bonding.

[0039] Example 1: optimal parameter bonding

[0040] Step one: substrate preparation: provide an 8-inch silicon wafer with a surface plated copper column array (diameter 60 μm / height 10 μm / pitch 100 μm); sequentially perform deionized water washing and argon plasma cleaning (power 200 W, time 120 s).

[0041] Step two: CMP super-polishing

[0042] Equipment: rotary polisher;

[0043] Polishing liquid: SiO2alkaline colloid (pH = 10.5, particle size 50 nm);

[0044] Polishing removal amount: the copper layer removal amount is monitored by an online film thickness meter to 1.0 μm;

[0045] Post processing: rinse with deionized water for 30 seconds, dry;

[0046] AFM detection: surface roughness Ra = 1.26 nm, 80% area Ra≤10 nm.

[0047] Step three: bonding process

[0048] Environment: atmospheric environment, optical alignment accuracy ± 0.8 μm;

[0049] Preheating of the stage: 150℃;

[0050] Bonding parameters: temperature 300℃ (double-stage synchronous heating, rate 10℃ / s); pressure 150 MPa (pressure uniformity 98%); current 0.65 A (direct current constant current, vertical current through the interface); time: 90 seconds.

[0051] Bonding effect: shear strength 80.6 MPa as shown in Table 1.

[0052] As Figure 1 The schematic diagram of copper-copper bonding of the application, realizing bonding under the synergistic effect of electricity, heat and force.

[0053] As Figure 2 (a) is a three-dimensional profile diagram of the electroplated copper column before polishing, Figure 2 (b) is a three-dimensional profile diagram of the electroplated copper column after polishing, and after testing, more than 80% of the area of the copper column surface meets the surface roughness ≤10 nm (Ra) after mechanical and chemical polishing.

[0054] Figure 3 It is an atomic force microscope (AFM) topography diagram of the copper surface after CMP polishing, and the measured Ra = 1.26 nm.

[0055] Figure 4 It is a scanning electron microscope diagram of the bonding interface of Example 1; the interface SEM analysis has no void, and the open gap ratio is 15%.

[0056] Example 2: parameter boundary verification

[0057] The difference between Example 2 and Example 1 is that the bonding temperature is 290℃, the bonding pressure is 100 MPa, the bonding current is 0.5 A, and the bonding time is 30 seconds.

[0058] Bonding effect: shear strength 58.1 MPa as shown in Table 1.

[0059] Figure 5 It is a scanning electron microscope diagram of the bonding interface of Example 2; the interface SEM analysis has no void, and the open gap ratio is 22.5%.

[0060] Comparative Example: Traditional thermal compression bonding

[0061] The difference between the comparative example and Example 1 is: same copper pillar (not mechanically and chemically polished after electroplating); bonding temperature is 300℃, pressure is 150MPa, and time is 90s;

[0062] Bonding effect: shear strength is 42MPa as shown in Table 1.

[0063] Figure 6 The SEM image of the bonding interface of the comparative example; the interface SEM analysis has no cavity, and the open gap ratio is 47%.

[0064] Table 1: Comparison of test data of each group

[0065] Item Shear strength (MPa) Open slit ratio Example 1 80.6 15％ Example 2 58.1 22.5％ Comparative example 42 47％

[0066] As shown in Table 1, after the polishing treatment of the copper pillar, the shear strength is obviously improved from 42MPa to 80.6MPa; the open gap ratio is obviously reduced from 47% to 15%, and the bonding effect of the copper pillar after the polishing treatment is obviously better than that of the traditional thermal compression bonding; the application is suitable for the low-temperature, low-pressure and rapid interconnection manufacturing of three-dimensional integrated circuits (3D IC), and especially solves the bonding reliability problem of heat-sensitive devices in high-density packaging.

[0067] The above is only the preferred embodiment of the application, and is not used to limit the application, and the application can have various changes and variations for those skilled in the art, and any modification, equivalent replacement, improvement, etc. within the spirit and principle of the application should be included in the protection scope of the application.

Claims

1. A low-temperature, low-pressure copper-copper metal bonding method, characterized in that, Includes the following steps: Step 1: Ultra-fine polishing of copper pillar surface: The silicon substrate with electroplated copper pillar is sequentially washed with water and then cleaned with plasma; the cleaned copper pillar is polished with SiO2 alkaline colloid as polishing fluid, and the amount of copper layer removed is controlled to be 0.8-1.2μm. After polishing, it is washed with water and dried to obtain a bonding surface with a surface roughness Ra≤10nm. Step 2: Bonding: Place the silicon substrate processed in Step 1 on the bonding machine stage, align and bond the copper pillar bumps. In an air environment, the dual stages are simultaneously heated to 290-300°C, and a vertical pressure of 100-150 MPa is applied. A constant current of 0.5-0.65 A is applied, and the bonding is completed after maintaining the temperature and pressure for 30-90 seconds.

2. The low-temperature, low-pressure copper-copper metal bonding method according to claim 1, characterized in that, In step one, the SiO2 alkaline colloidal grinding slurry has a particle size of 50 nm and a pH of 10.5 ± 0.

5.

3. The low-temperature, low-pressure copper-copper metal bonding method according to claim 1, characterized in that, In step two, the optical alignment accuracy of the bonding machine stage is ±1μm, and the stage is preheated to 150℃.

4. The low-temperature, low-pressure copper-copper metal bonding method according to claim 1, characterized in that, In step two, the height of the copper pillar protrusions is 10-50 μm, the diameter is 20-100 μm, and the spacing is 40-200 μm.

5. The low-temperature, low-pressure copper-copper metal bonding method according to claim 1, characterized in that, In step two, the heating rate is 10℃ / s.

6. The low-temperature, low-pressure copper-copper metal bonding method according to claim 1, characterized in that: In step two, the constant current flows through the bonding interface in a vertical direction, inducing an electroplastic effect to promote the diffusion of interfacial atoms.

Images