Cu pillar bump direct bonding method based on high-frequency thermal induction and application

Abstract

The invention belongs to the field of integrated circuit packaging, particularly relates to a Cu column bump direct bonding method based on high-frequency thermal induction and application, and is applicable to packaging of chips containing Cu column bumps. According to the invention, a high-frequency electromagnetic induction heating technology is utilized, so that the Cu column bump part on the chip induces eddy current and is locally heated and interconnected. According to the method, the problem that the existing Cu pillar bump chip cannot bear overall high-temperature heating packaging is solved. The method has the beneficial effects that the bonding process is simple, and a plasma surface activation or additional nanocrystallization treatment process is not needed; the requirement on the flatness of the bonding surface is low, and the flatness is reduced from about 0.3 nm to a micron level; the bonding speed is high, and the bonding can be quickly completed within dozens of seconds; and the bonding quality is high, defects such as holes are few, and the shear strength can reach 60 MPa or above.

Description

Technical Field

[0001] The invention belongs to the field of integrated circuit packaging, and in particular relates to a Cu column bump direct bonding method based on high-frequency thermal induction and an application thereof, which is suitable for chip packaging containing Cu column bumps. Background Art

[0002] As chip electrical connections or signal I / O ports, Cu pillar bumps have the advantages of fast signal transmission, narrow package interconnection spacing, high mechanical properties and reliability. There is an increasing demand for their application in new technology fields such as high-speed communications, artificial intelligence, and autonomous driving.

[0003] However, the melting point of metal Cu (1083.4℃) is relatively high, and the chip is faced with the problem that it cannot withstand the overall high-temperature Cu-Cu direct bonding. At present, there are two main solutions in the electronics industry: one is to use plasma surface activation to make Cu pillar bumps perform Cu-Cu hot pressing bonding at a temperature far below its melting point or even at room temperature; the other is to nano-scale the surface of the Cu pillar bumps to form microstructures such as nanowires, nanopowders, and nanofilms, and use the nano effect of the material to perform low-temperature Cu-Cu sintering bonding. However, both methods belong to low-temperature overall heating bonding technology, which has the following problems: ① Surface treatment is required, the process is complicated, and hot pressing bonding has extremely high requirements for surface flatness (about 0.3nm); ② The bonding time is long (about 0.5~2h) and the efficiency is low; ③ The interface is prone to hole defects, poor quality, and low connection strength (about 15MPa, far lower than the theoretical value of Cu metal, about 295MPa), which seriously hinders the widespread application of Cu pillar bumps. Summary of the invention

[0004] In view of the current situation that the direct bonding process of Cu pillar bumps on chips is complicated, the flatness requirement is high, the efficiency is low, and the quality is poor, which restricts its further application, the present invention proposes a Cu pillar bump high-frequency induction direct bonding method with simple process, low flatness requirement, high speed and high quality, which uses high-frequency electromagnetic induction heating technology to induce eddy currents at the Cu pillar bumps on the chip and locally heat them for interconnection. This method overcomes the problem that the existing Cu pillar bump chips cannot withstand the overall high-temperature heating packaging.

[0005] The technical solution of the present invention is:

[0006] A Cu column bump direct bonding method based on high frequency induction heating, the steps are as follows:

[0007] (1) Substrate pretreatment: The substrate is cleaned with acetone, ethanol, and deionized water, and a Ti seed layer and a Cu seed layer are sequentially deposited on the substrate surface by magnetron sputtering or electron beam deposition. Then, a photoresist is spin-coated on the substrate surface with the Ti seed layer and the Cu seed layer deposited thereon, and then baked, photolithographed, and developed to obtain a patterned substrate covered with photoresist.

[0008] (2) Preparation of Cu column bumps: According to the size of the Cu column bumps, select the method of electroplating, chemical deposition, magnetron sputtering or electron beam deposition to prepare the Cu column bumps on the patterned substrate covered with photoresist obtained in step (1). When the height of the Cu column bump is greater than 1um, it is first prepared by electroplating or chemical deposition, such as using a copper sulfate system copper plating solution to electroplating the Cu column bumps, and then treating the surface of the electroplated or chemically deposited Cu column bumps with one of dilute sulfuric acid, dilute hydrochloric acid or citric acid solutions. When the height of the Cu column bump is between 0.1um and 1um, it is directly prepared by magnetron sputtering or electron beam deposition.

[0009] (3) Selective deposition of Cu-Ni alloy solder layer: A Cu-Ni alloy film is prepared on the surface of the Cu column bump by a dry method (magnetron sputtering, electron beam deposition) or a wet method (electrochemical deposition, chemical deposition) to obtain a nano-scale layered Cu-Ni alloy, and then the photoresist is removed and the seed layer is etched.

[0010] The sputtering background vacuum of Cu and Ni is less than 8×10 -4 Pa, Cu, Ni sputtering power is 50W-400w, gas is Ar, gas flow rate is 15-50sscm, sputtering pressure is 0.2-1Pa.

[0011] When Cu-Ni alloy is prepared by thin film deposition, nano-scale layered alloys with different mass fractions are obtained by controlling the deposition time of Cu and Ni, or by co-depositing Cu and Ni and adjusting the sputtering power of the two to obtain Cu-Ni solid solution alloys with different mass fractions. 2+ 、Ni 2+ The ion ratio, additive concentration and current density are adjusted to obtain Cu-Ni alloys with different component ratios, and the deposition time is controlled to control the thickness.

[0012] (4) Pre-alignment of bumps: Use a placement machine to align the copper pillar bumps with Cu-Ni alloy soldering layers on the two substrates, with an alignment deviation of less than 1um. Apply a force of 0 to 20N in the vertical direction to the two aligned substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue.

[0013] (5) Apply pressure and pass protective gas: The high-frequency induction heating system is mainly composed of a high-frequency induction coil and a frequency controller, a ceramic-based pressurized platform, and a gas atmosphere protective cabin. During operation, the frequency controller is adjusted to output a current of specific power and frequency, so that an alternating magnetic field is generated in the induction coil; the sample to be inductively heated is fixed to the middle of the induction coil through the ceramic-based pressurized platform. Before inductively heating the sample, a protective gas is passed in to prevent oxidation of the sample during the heating process. The two substrates with the Cu column bumps aligned are placed in the induction heating coil, and the upper and lower sides of the coil are ceramic-based platforms. A certain pressure is applied to the substrate through the ceramic platform to keep it constant, and a protective gas flow rate of 50 to 500sscm is passed into the cabin where the substrate is located to prevent metal oxidation.

[0014] (6) High-frequency induction heating: Adjust the frequency, power and time of high-frequency induction heating to perform bonding.

[0015] Further preferably, the substrate in step (1) is a silicon, silicon carbide, silicon nitride, silicon oxide, aluminum oxide or aluminum nitride substrate.

[0016] More preferably, the Cu column bumps in step (2) have a diameter ranging from 10 to 1000 μm and a height ranging from 0.1 to 500 μm, and are single or array structures.

[0017] Further preferably, the mass fraction content of Ni in the Cu-Ni alloy soldering layer in step (3) is 1% to 40% by weight; the thickness of the Cu-Ni alloy soldering layer ranges from 0.05 to 10 μm; and the Cu-Ni alloy soldering layer is prepared by one of the methods of electrochemical deposition, chemical deposition, magnetron sputtering, and electron beam deposition.

[0018] More preferably, the diameter of the Cu column bump in step (3) is in the range of 10 to 200 μm.

[0019] More preferably, the applied pressure in step (5) is in the range of 0 to 20 MPa.

[0020] More preferably, the protective gas in step (5) is an inert or reducing atmosphere such as argon, nitrogen, nitrogen-hydrogen mixed gas (hydrogen volume content 5% to 25%), formic acid vapor, etc.

[0021] More preferably, in step (6), the frequency range is 0.2 to 10 MHz, the power range is 2 to 30 kW, and the heating time is 1 to 300 s.

[0022] The application scope of the high-frequency electromagnetic induction method to achieve local heating of Cu pillar bump packaging interconnection is chip packaging in the integrated circuit manufacturing industry, especially suitable for chip packaging containing 10 to 1000μm diameter Cu pillar bump electrodes or array I / O interfaces.

[0023] The present invention adopts high-frequency electromagnetic induction technology to locally heat the bumps, and can quickly heat the surface of the Cu column bumps to above the recrystallization temperature (369-469° C.) within a few seconds without affecting other parts of the chip, so that the Cu column bumps can be bonded and interconnected in a highly softened state, without the need for plasma surface activation or additional nano-processing and other processes for the purpose of reducing the overall interconnection temperature. Compared with the 0.5-2h interconnection time required for Cu column bump bonding in the current electronics industry, the Cu column bumps in a highly softened state under high-frequency induction heating can quickly complete bonding within tens of seconds, and the interconnection efficiency is improved by dozens of times. Moreover, Cu in a highly softened state has a good "pit filling" ability and is no longer overly sensitive to the unevenness of the bonding bump surface, and the requirement for the surface flatness of the Cu column bump is reduced from about 0.3nm to micrometer level, which greatly reduces the process difficulty of direct bonding of the Cu column bumps. In addition, the obtained interconnection interface has fewer defects such as holes, the bonding quality is significantly improved, and the shear strength can reach more than 60MPa.

[0024] Therefore, the gain effect of the present invention is:

[0025] (1) Simple bonding process: no plasma surface activation or additional nano-processing is required;

[0026] (2) Low bonding surface flatness requirement: from about 0.3nm to micron level;

[0027] (3) Fast bonding speed: bonding can be completed quickly within tens of seconds;

[0028] (4) High bonding quality: few defects such as holes, and shear strength can reach over 60MPa. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 Schematic diagram of the Cu column bump preparation process flow.

[0030] Figure 2 It is a schematic diagram of an electric high-frequency induction heating system; in the figure, 1 is a pressure applicator, 2 is a pressure display, 3 is a ceramic-based platform, 4 is an induction coil, 5 is a gas protection wall, 6 is a control host, and 7 is a gas cylinder.

[0031] Figure 3 This is the stress-displacement curve of the shear strength test of the Cu-Cu joint in Example 3.

[0032] Figure 4 This is the SEM image of the cross section of the Cu-Cu joint in Example 3. DETAILED DESCRIPTION

[0033] The specific implementation of the present invention is further described below in conjunction with the accompanying drawings and technical solutions.

[0034] The basic process of the Cu column bump direct bonding method based on high frequency induction heating of the present invention is as follows: Figure 1 As shown, the composition and structure of the high-frequency induction heating system used are as follows Figure 2 shown.

[0035] Example 1

[0036] (1) The silicon wafer was cleaned with acetone, ethanol, and deionized water, and Ti and Cu seed layers were deposited on the surface in sequence by magnetron sputtering. Then, photoresist was spin-coated on the surface, baked, photolithographically processed, and developed to obtain a substrate with a pattern of circular holes with a diameter of 1000 μm, a pitch of 500 μm, and a height of 1 μm.

[0037] (2) Magnetron sputtering was used to prepare Cu column bumps with a height of 0.1 μm.

[0038] (3) The substrate on which the copper pillars are deposited is subjected to electrochemical deposition of a Cu-Ni soldering layer alloy. A 10 μm thick Cu-Ni soldering layer alloy with a 40% Ni (wt.%) content is prepared on the surface. The photoresist is then removed and the seed layer is etched.

[0039] (4) Use a high-precision placement machine to align the bumps of two copper pillar substrates with Cu-Ni solder layer alloy, with an alignment deviation of less than 1um. Apply a force of 20N in the vertical direction to the two aligned substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue.

[0040] (5) Place the two substrates with the bumps aligned in the induction heating coil, apply a 20 MPa pressure on the upper and lower sides of the substrates and keep it constant, and pass a protective N into the chamber where the substrates are located. 2 gas to prevent copper oxidation.

[0041] (6) The power of high-frequency induction heating was adjusted to 30Kw, the frequency to 10MHz, and the action time to 1s for bonding. The bonded sample was subjected to a shear strength test, and the connection strength was higher than 60Mpa.

[0042] Example 2

[0043] (1) The substrate was cleaned with acetone, ethanol, and deionized water, and Ti and Cu seed layers were deposited on the surface in sequence by electron beam deposition. Then, photoresist was spin-coated on the surface, baked, photolithographically processed, and developed to obtain a substrate with patterned photoresist. The pattern was an array of circular holes with a diameter of 10 μm, a spacing of 10 μm, and a height of 600 μm.

[0044] (2) Electrodeposition was used to prepare Cu pillar bumps, and the height of the resulting copper pillars was 500 μm.

[0045] (3) Place the substrate with copper pillars deposited in a magnetron sputtering device to deposit a Cu-Ni soldering layer alloy. Prepare a 0.05um thick Cu-Ni soldering layer alloy with a 1% Ni (wt.%) content on the surface. Then remove the photoresist and etch the seed layer.

[0046] (4) Use a high-precision placement machine to align the bumps of two copper pillar substrates with Cu-Ni solder layer alloy, with an alignment deviation of less than 1um. Apply a force of 1N in the vertical direction to the two aligned substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue.

[0047] (5) Place the two substrates with aligned bumps into an induction heating coil, apply a constant pressure of 0.1 MPa on the upper and lower sides of the substrates, and introduce protective Ar gas into the chamber where the substrates are located to prevent copper oxidation.

[0048] (6) The power of high-frequency induction heating was adjusted to 2Kw, the frequency to 0.2MHz, and the action time to 300s for bonding. The bonded sample was subjected to a shear strength test, and the connection strength was higher than 60Mpa.

[0049] Example 3

[0050] (1) The substrate is cleaned with acetone, ethanol, and deionized water, and Ti and Cu seed layers are sequentially deposited on the surface by magnetron sputtering or electron beam deposition. Then, photoresist is spin-coated on the surface, baked, photolithographically processed, and developed to obtain a patterned substrate covered with photoresist. The pattern is an array of circular holes with a diameter of 300 μm, a pitch of 200 μm, and a height of 15 μm.

[0051] (2) The Cu column bumps were prepared by electroplating method, and the height of the obtained copper column was 10 μm.

[0052] (3) The substrate on which the copper pillars are deposited is placed in a magnetron sputtering device to deposit a Cu-Ni soldering layer alloy, and then the photoresist is removed and the seed layer is etched.

[0053] (4) Use a high-precision placement machine to align the bumps of two copper pillar substrates with Cu-Ni solder layer alloy, with an alignment deviation of less than 1um. Apply a force of 10N in the vertical direction to the two aligned laminated substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue.

[0054] (5) Place the two substrates with aligned bumps into an induction heating coil, apply a constant pressure of 20 MPa on the upper and lower sides of the substrates, and introduce nitrogen into the chamber where the substrates are located to prevent copper oxidation.

[0055] (6) The high-frequency induction heating current was adjusted to 200A, the frequency was 800kHz, and the action time was 10s to perform bonding. The shear strength of the bonded joint was higher than 30Mpa. The bonded sample was subjected to a shear strength test, and its connection strength was higher than 60Mpa.

[0056] The stress-displacement curve of the shear strength test of the Cu-Cu joint in Example 3 is as follows: Figure 3 As shown, the SEM cross section of the Cu-Cu joint is as follows Figure 4 shown.

[0057] Example 4

[0058] (1) The substrate is cleaned with acetone, ethanol, and deionized water, and Ti and Cu seed layers are sequentially deposited on the surface by magnetron sputtering or electron beam deposition. Then, photoresist is spin-coated on the surface, baked, photolithographically processed, and developed to obtain a patterned substrate covered with photoresist. The pattern is an array of circular holes with a diameter of 300 μm, a pitch of 200 μm, and a height of 15 μm.

[0059] (2) The Cu column bumps were prepared by electroplating method, and the height of the obtained copper column was 10 μm.

[0060] (3) The substrate on which the copper pillars are deposited is placed in a magnetron sputtering device to deposit a Cu-Ni soldering layer alloy, and then the photoresist is removed and the seed layer is etched.

[0061] (4) Use a high-precision placement machine to align the bumps of two copper pillar substrates with Cu-Ni solder layer alloy, with an alignment deviation of less than 1um. Apply a force of 10N in the vertical direction to the two aligned laminated substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue.

[0062] (5) Place the two substrates with aligned bumps into an induction heating coil, apply a constant pressure of 20 MPa on the upper and lower sides of the substrates, and introduce a nitrogen-hydrogen mixed gas into the chamber where the substrates are located to prevent copper oxidation.

[0063] (6) The high-frequency induction heating current is adjusted to 200A, the frequency is adjusted to 800kHz, and the action time is adjusted to 15s for bonding. The shear strength of the bonded joint is higher than 60Mpa.

[0064] Example 5

[0065] (1) The substrate is cleaned with acetone, ethanol, and deionized water, and Ti and Cu seed layers are sequentially deposited on the surface by magnetron sputtering or electron beam deposition. Then, photoresist is spin-coated on the surface, baked, photolithographically processed, and developed to obtain a patterned substrate covered with photoresist. The pattern is an array of circular holes with a diameter of 300 μm, a pitch of 200 μm, and a height of 15 μm.

[0066] (2) The Cu column bumps were prepared by electroplating method, and the height of the obtained copper column was 10 μm.

[0067] (3) The substrate on which the copper pillars are deposited is placed in a magnetron sputtering device to deposit a Cu-Ni soldering layer alloy, and then the photoresist is removed and the seed layer is etched.

[0068] (4) Use a high-precision placement machine to align the bumps of two copper pillar substrates with Cu-Ni solder layer alloy, with an alignment deviation of less than 1um. Apply a force of 10N in the vertical direction to the two aligned laminated substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue.

[0069] (5) Place the two substrates with aligned bumps into an induction heating coil, apply a constant pressure of 20 MPa on the upper and lower sides of the substrates, and introduce a nitrogen-hydrogen mixed gas into the chamber where the substrates are located to prevent copper oxidation.

[0070] (6) The high-frequency induction heating current is adjusted to 200A, the frequency is adjusted to 800kHz, and the action time is adjusted to 20s for bonding. The shear strength of the bonded joint is higher than 100Mpa.

[0071] The results of Examples 3, 4, and 5 show that for the electrodeposited copper pillar bumps, for the same current and frequency of induction heating, the longer the bonding time, the higher the bonding strength, and the increased time is relatively short (within tens of seconds) but can greatly improve the bonding strength, reduce the requirements for surface flatness, and greatly improve the bonding efficiency.

Claims

1. A Cu column bump direct bonding method based on high frequency induction heating, characterized in that: Here are the steps: (1) Substrate pretreatment: The substrate is cleaned with acetone, ethanol, and deionized water, and a Ti seed layer and a Cu seed layer are sequentially deposited on the substrate surface by magnetron sputtering or electron beam deposition; Then, a photoresist is spin-coated on the surface of the substrate on which the Ti seed layer and the Cu seed layer are deposited, and the substrate is baked, photolithographically processed, and developed to obtain a patterned substrate covered with the photoresist; (2) Preparation of Cu column bumps: According to the size of the Cu column bumps, select electrodeposition, chemical deposition, magnetron sputtering or electron beam deposition methods to prepare Cu column bumps on the patterned substrate covered with photoresist obtained in step (1); when the height of the Cu column bumps is greater than 1 um, firstly use electrodeposition or chemical deposition methods to prepare, such as using a copper sulfate system copper plating solution to electrodeposit the Cu column bumps, and then treat the surface of the electrodeposited or chemically deposited Cu column bumps with one of dilute sulfuric acid, dilute hydrochloric acid or citric acid solutions; when the height of the Cu column bumps is between 0.1 um and 1 um, directly use magnetron sputtering or electron beam deposition methods to prepare; (3) Selective deposition of Cu-Ni alloy solder layer: a Cu-Ni alloy film is prepared on the surface of the Cu column bump by a dry method including magnetron sputtering and electron beam deposition or a wet method including electrochemical deposition and chemical deposition to obtain a nano-scale layered Cu-Ni alloy, and then the photoresist is removed and the seed layer is etched; (4) Pre-alignment of bumps: Use a placement machine to align the copper pillar bumps with Cu-Ni alloy solder layers on the two substrates, with an alignment deviation of less than 1 μm; apply a force of 0 to 20 N in the vertical direction of the two aligned substrates to ensure that the two substrates fit tightly, and then fix them with temporary bonding glue; (5) Applying pressure and passing protective gas: The high-frequency induction heating system is mainly composed of a high-frequency induction coil and a frequency controller, a ceramic-based pressurized platform, and a gas atmosphere protective cabin. During operation, the frequency controller is adjusted to output a current of specific power and frequency, so that an alternating magnetic field is generated in the induction coil. The sample to be inductively heated is fixed to the middle of the induction coil through the ceramic-based pressurized platform. Before inductively heating the sample, a protective gas is passed in to prevent oxidation of the sample during the heating process. Two substrates with Cu column bumps aligned are placed in the induction heating coil, and the upper and lower sides of the coil are ceramic-based platforms. A certain pressure is applied to the substrate through the ceramic platform to keep it constant, and a protective gas with a flow rate of 50 to 500 sscm is passed into the cabin where the substrate is located to prevent metal oxidation. (6) High-frequency induction heating: Adjust the frequency, power and time of high-frequency induction heating to perform bonding.

2. A Cu pillar bump direct bonding method based on high frequency induction heating according to claim 1, characterized in that: The substrate in step (1) is a silicon, silicon carbide, silicon nitride, silicon oxide, aluminum oxide or aluminum nitride substrate.

3. A Cu pillar bump direct bonding method based on high frequency induction heating according to claim 1, characterized in that: The Cu column bumps in step (2) have a diameter ranging from 10 to 1000 μm and a height ranging from 0.1 to 500 μm, and are single or array structures.

4. The method for direct bonding of Cu pillar bumps based on high frequency induction heating according to claim 1, characterized in that: The mass fraction of Ni in the Cu-Ni alloy soldering layer in step (3) is 1% to 40% by weight; the thickness of the Cu-Ni alloy soldering layer ranges from 0.05 to 10 μm; and the Cu-Ni alloy soldering layer is prepared by one of the methods of electrochemical deposition, chemical deposition, magnetron sputtering, and electron beam deposition.

5. The method for direct bonding of Cu pillar bumps based on high frequency induction heating according to claim 1, characterized in that: The diameter of the Cu column bump in step (3) ranges from 10 to 200 μm.

6. The method for direct bonding of Cu pillar bumps based on high frequency induction heating according to claim 1, characterized in that: The applied pressure in step (5) is in the range of 0 to 20 MPa.

7. The method for direct bonding of Cu pillar bumps based on high frequency induction heating according to claim 1, characterized in that: The protective gas in step (5) is argon, nitrogen, a nitrogen-hydrogen mixture or formic acid vapor atmosphere.

8. The method for direct bonding of Cu pillar bumps based on high frequency induction heating according to claim 1, characterized in that: The frequency range in step (6) is 0.2-10 MHz, the power range is 2-30 kW, and the heating time is 1-300 s.

9. A Cu pillar bump direct bonding method based on high frequency induction heating as described in any one of claims 1 to 8 is applied to chip packaging in the integrated circuit manufacturing industry.

Images