Process for using copper-nickel-gold as solder barrier layer for semiconductor packaging

Abstract

The invention discloses a process for using copper-nickel-gold as a solder barrier layer for semiconductor packaging, which comprises the following steps of: firstly, cleaning and pretreating a base material with a copper bonding pad, and then depositing a nickel-phosphorus alloy layer with the thickness of 3-8 microns on the surface of copper through chemical plating to serve as a main diffusion barrier layer; and then an ultra-thin gold layer with the thickness of 0.03-0.15 mu m is deposited on the nickel layer through a dip plating method to serve as an anti-oxidation and weldable layer. The method is simple in process, low in cost and good in environmental protection property, the prepared Cu / Ni / Au composite structure layer is uniform in thickness and good in binding force, solder diffusion can be effectively blocked, excellent weldability and welding spot reliability are provided, and the method is particularly suitable for high-density and high-reliability semiconductor packaging requirements.

Description

Technical Field

[0001] This invention relates to the field of semiconductor manufacturing technology, specifically to a process for using copper-nickel-gold as a solder barrier layer in semiconductor packaging. Background Technology

[0002] In semiconductor packaging, a reliable connection between the solder and the metal pads on the chip or substrate is crucial. To prevent the solder from over-reacting with the underlying metal (such as copper) to form brittle intermetallic compounds (IMCs) and to protect the pads from oxidation during storage and assembly, a solder barrier layer is typically fabricated on the pads.

[0003] Traditional barrier layer materials such as nickel (Ni) can effectively prevent copper from diffusing into the solder, but their surface is prone to oxidation, resulting in poor solderability. Therefore, a thin layer of gold (Au) is usually deposited on top of the nickel layer, utilizing gold's excellent oxidation resistance and solderability to provide a good wetting surface for the solder. However, if the gold layer is too thick, it can cause gold brittleness in the solder joint, affecting its reliability; if it is too thin, it may not be able to completely cover the nickel layer, leading to localized oxidation.

[0004] Therefore, in order to correct the above-mentioned defects, we propose a process for using copper-nickel-gold as a solder barrier layer in semiconductor packaging. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a semiconductor packaging chip mounting process with high reliability, low stress, low void ratio and good process controllability, solving the problems of difficulty in controlling thickness uniformity, complex process, high cost and possible "black nickel" in existing electroplating or chemical plating Ni / Au processes.

[0006] To achieve the above objectives, the present invention provides the following technical solution: a process for using copper-nickel-gold as a solder barrier layer in semiconductor packaging, comprising the following steps:

[0007] S1. Substrate: Provide a semiconductor packaging substrate or wafer with exposed copper pads on its surface. The surface of the copper pads shall be clean and free from oxidation and contamination.

[0008] S2. Cleaning and Pretreatment: The substrate is cleaned and micro-etched to thoroughly remove oxides and organic contaminants from the surface of the copper pads and to moderately roughen the surface to enhance the adhesion of subsequent metal layers. Cleaning can be performed using acidic cleaning solutions (such as dilute sulfuric acid or citric acid), and micro-etching can be performed using sodium persulfate or a hydrogen peroxide-sulfuric acid system solution.

[0009] S3. Electroless Nickel Plating: The pretreated substrate is immersed in an electroless nickel plating solution to deposit a dense nickel-phosphorus (Ni-P) alloy layer on the surface of the copper pads. The pH value of the electroless nickel plating solution is 4.5-5.5, and the operating temperature is 75-90℃. The thickness of the nickel-phosphorus alloy layer is controlled at 3-8 μm, and the phosphorus content is 8-12 wt%. This nickel layer serves as the main diffusion barrier layer.

[0010] S4. First wash: Thoroughly clean the substrate after nickel plating with deionized water to remove residual electroless nickel plating solution.

[0011] S5. Immersion Gold Plating (Displacement Gold Plating): The cleaned substrate is immersed in an acidic gold immersion solution. Through a chemical displacement reaction, a thin and continuous gold layer is deposited on the surface of the nickel layer. The main components of the gold immersion solution are potassium gold cyanide (or a non-cyanide gold salt), a complexing agent, and a pH adjuster. The pH value is maintained at 4.0-6.0, and the operating temperature is 80-90℃. The thickness of the gold layer is strictly controlled between 0.03-0.15 μm. This gold layer is used to prevent nickel oxidation and provides excellent solderability.

[0012] S6. Second wash: Thoroughly clean the gold-plated substrate with deionized water to remove all chemical residues.

[0013] S7. Drying: The substrate is completely dried by means of hot air drying or centrifugal drying to obtain a finished product with a Cu / Ni / Au composite solder barrier layer.

[0014] Preferably, the electroless nickel plating solution in step S3 comprises: 20-40 g / L nickel sulfate, 20-35 g / L sodium hypophosphite, an appropriate amount of complexing agent (such as sodium citrate, lactic acid, glycine, etc.), and a trace amount of stabilizer (such as lead ions, thiourea derivatives).

[0015] Preferably, the gold immersion solution in step S5 is a cyanide-free gold immersion solution, whose main components are sodium gold sulfite, complexing agent and buffer, so as to improve the environmental friendliness of the process.

[0016] Preferably, before the electroless nickel plating step S3, an activation step can be added, in which a palladium activating solution is used to activate the copper surface to initiate the electroless nickel plating reaction and ensure the uniformity and density of the nickel layer deposition.

[0017] Compared with the prior art, the beneficial effects of the present invention are:

[0018] 1. Excellent barrier properties: The 3-8μm nickel-phosphorus alloy layer can effectively block the diffusion of copper atoms into the solder, inhibit the formation of thick and brittle Cu-Sn intermetallic compounds, and improve the mechanical strength and thermal fatigue life of the solder joint.

[0019] 2. Excellent solderability and oxidation resistance: The 0.03-0.15μm thin gold layer perfectly balances the need for oxidation resistance with the risk of "gold brittleness". It ensures good wettability of the solder during welding, while avoiding the problem of reduced solder joint reliability caused by excessive gold layer.

[0020] 3. Excellent processability and uniformity: Utilizing chemical plating and dip plating processes, it exhibits excellent thickness uniformity and coverage, achieving uniform coverage even for pads with complex patterns and fine spacing, making it suitable for advanced packaging.

[0021] 4. High cost-effectiveness: The immersion gold plating process consumes very little gold, and the chemical nickel plating process is mature. Overall, the cost is more advantageous compared to some physical vapor deposition (PVD) or electroplating processes.

[0022] 5. High reliability: The Ni-P layer has good adhesion to both the Cu pad and the Au layer, and the resulting composite structure exhibits good stability in subsequent reflow soldering and aging tests. Attached Figure Description

[0023] Figure 1 This is a schematic diagram of the process flow in this invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] This invention provides a technical solution:

[0026] Please see Figure 1 A process for using copper-nickel-gold as a solder barrier layer in semiconductor packaging, taking a BT material packaging substrate as an example, has several copper pads with a diameter of 100μm on the surface.

[0027] 1. Clean the substrate with a 5% dilute sulfuric acid solution for 2 minutes at 60°C, then treat it with a micro-etching solution containing 2% sodium persulfate and 5% sulfuric acid for 1 minute, and then rinse with deionized water.

[0028] 2. Immerse the substrate in an activation solution containing palladium chloride (50 ppm) for 1 minute, then rinse with water.

[0029] 3. Immerse the substrate in a chemical nickel plating solution (nickel sulfate 30g / L, sodium hypophosphite 25g / L, sodium citrate 15g / L, pH=5.0, temperature 85℃) for 20 minutes to form a Ni-P layer with a thickness of about 5μm (P content about 10%).

[0030] 4. Clean with deionized water using ultrasonic cleaning for 2 minutes.

[0031] 5. Immerse the substrate in a cyanide-free gold immersion solution (sodium gold sulfite 1.5 g / L, appropriate amount of complexing agent, pH=5.5, temperature 85℃) for 8 minutes to form a gold layer with a thickness of about 0.08 μm.

[0032] 6. Rinse thoroughly with deionized water and centrifuge to dry.

[0033] Upon inspection, the gold layer on the surface of the pad was found to be uniform and continuous, with no exposed nickel.

[0034] The solder ball welding test showed that the solder (SAC305) spread area was greater than 90%, the shear strength test results were excellent, and after being stored at 155℃ for 1000 hours, the interface IMC growth was slow and uniform, and the solder joint reliability was high.

[0035] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A process for copper nickel gold as a solder barrier for semiconductor packaging, characterized by, The method comprises the following steps: S1, providing a substrate with a copper pad on the surface; S2, cleaning and pretreating the substrate; S3, depositing a layer of nickel-phosphorus alloy on the surface of the copper pad by electroless plating; S4, washing the substrate after step S3 for the first time; S5, depositing a layer of gold on the surface of the layer of nickel-phosphorus alloy by immersion plating; S6, washing the substrate after step S5 for the second time and drying to obtain a substrate with a copper-nickel-gold composite solder barrier layer.

2. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: The thickness of the layer of nickel-phosphorus alloy in step S3 is 3-8 μm, and the phosphorus content is 8-12 wt%.

3. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: The thickness of the layer of gold in step S5 is 0.03-0.15 μm.

4. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: The operating temperature for electroless plating of nickel in step S3 is 75-90 °C, and the pH value of the solution is 4.5-5.

5.

5. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: The operating temperature for immersion plating of gold in step S5 is 80-90 °C, and the pH value of the solution is 4.0-6.

0.

6. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: The pretreatment in step S2 comprises acid cleaning and micro-etching.

7. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: Before step S3, a step of palladium activation on the surface of the copper pad is further included.

8. The process for copper nickel gold as solder barrier layer for semiconductor package according to claim 1 wherein: The immersion gold solution used in step S5 is cyanide-free immersion gold solution.

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