KR100392498B1 - Method for Formation of Bump for conductive polymer flip chip interconnects using electroless plating - Google Patents

Abstract

The present invention relates to a method for forming electroless nickel / copper bumps for conductive polymer flip chip connection, and more particularly, to electroless nickel (Ni) plating, electroless copper (Cu) plating, and gold on aluminum (Al) pads. (Au) The present invention relates to a method of forming a nickel / copper bump formed by a substitution plating process, connecting a flip chip using an anisotropic conductive polymer, and then using the same in a high speed high frequency chip.

The electroless nickel / copper bump forming method of the present invention comprises the steps of washing the aluminum pads, pretreatment with zincate (zincate), plating the nickel by electroless plating, and back to the plated nickel. A process of plating copper by an electroless plating method, and a plating process of replacing a part of the electroless copper plating with gold.

An object of the present invention is to form electroless nickel / copper bumps and use them in electronic package technology to develop bumps for conductive polymer flip chip connection with a simple process and low cost as well as application to high speed high frequency chips.

Description

Method for Formation of Bump for conductive polymer flip chip interconnects using electroless plating}

The present invention relates to a method of forming electroless nickel / copper bumps for conductive polymer flip chip connection. (Au) The present invention relates to a nickel / copper bump forming method comprising a substitution plating process, and the bump and the anisotropic conductive polymer are flip-chip connected to each other for use in a high speed high frequency chip.

In recent years, the rapidly developing semiconductor technology has been developed with the trend of more than one million cell integration, large I / O pin count, large die size, large heat dissipation, and high performance of non-memory devices. Relatively, however, electronic packaging techniques for packaging such devices have not kept up with rapid semiconductor development.

Electronic packaging technology is a very wide variety of system fabrication techniques, covering all stages from semiconductor devices to final products, and is very important in determining the performance, size, price, and reliability of final electronic products. Particularly for the latest electronic products that pursue high performance, ultra small / high density, low power, multifunctional, ultra high speed signal processing, and permanent reliability, ultra small packaged parts are essential parts for computers, telecommunications, mobile communication, and high-end consumer electronics. Flip chip technology is currently expanding its use in display packaging such as smart cards, LCDs, and PDPs, computers, mobile phones, and communication systems.

This flip chip technology is being replaced by a connection using a conductive adhesive that has advantages such as low cost, extremely fine electrode pitch, a solventless environment friendly process, and a low temperature process in a conventional solder process. have.

Flip chip technology using a conductive adhesive consists of forming a bump having a uniform height on a pad, applying an adhesive containing conductive particles, and bonding a chip to a substrate.

Among the various processes constituting the flip chip technology, bump forming technology has a difficulty in forming a bump of a desired height selectively for each fine pad.

As a bump forming method, a bump forming method and a gold stud bump are mechanically formed by mixing evaporation, sputtering, electroplating, photolithography, and the like. However, these methods are mainly used, but since these methods are expensive and complicated, a process of forming bumps using an electroless plating method is introduced.

Bump forming technology using electroless plating is low cost and simple process because selective bump formation is possible on aluminum (Al) pad only by immersion in plating solution. In addition, wafer-based processes are possible and have great potential for microelectronic packaging due to advantages such as uniform height and excellent electrical and mechanical properties.

Currently, bump forming technology using electroless Ni plating is used as an under bump metallurgy (UBM) forming technology in solder flip chips, and is actively researching to expand the application field in the US and Japan, mainly in Germany. .

Meanwhile, US Pat. No. 5,583,073, which is a prior art, intends to form UBM layer for flip chip of solder bump using electroless Ni plating, and its role as a diffusion barrier is mainly mentioned. On the other hand, since the present invention uses electroless plating to form bumps for ACA flip chips, the application fields are different, and there are differences in applications to high-speed high frequency chips and application of electroless Cu plating.

Also published by Rolf Ascenbrenner, Andreas Ostmann, Ute Buetler abd H. Reichl, "Electroless Ni / Cu plating as a New Bump Metallization" IEES CPMT-B, 18, 2, 1995 Similar to the present invention in that -Cu bumps are used, in this paper, Ni-Cu bumps are TAB bonding (flip chip package as a kind of electronic package) by introducing soft electroless Cu to compensate for the shortcomings of hard electroless Ni. To use the performance of Cu (low inductance) for the electroless bump for ACA flip chip and the flip chip of high speed high frequency chip in the present invention. There is a difference in one point.

The present invention forms nickel / copper bumps on electroless nickel (Ni), electroless copper (Cu), and gold (Au) substitution plating on aluminum (Al) pads and uses them in electronic packaging technology. Therefore, the present invention aims to develop bumps for conductive polymer flip chip connection with low cost and simple process as well as application to high speed high frequency chip.

1 is a process diagram of electroless nickel plating-electroless copper plating-substitution gold plating formation,

1A shows a pretreatment process.

Figure 1b shows a zincate treatment process.

1C shows an electroless nickel plating process.

1D shows an electroless copper plating and gold substitution plating process.

Figure 2a is a photograph showing the aluminum pad of the high speed high frequency chip.

2b is a photograph showing electroless nickel plating.

2C is a photograph showing electroless copper plating.

Figure 2d is a photograph showing the gold substitution plating.

Figure 3a is a photograph showing the cross-section of the electroless nickel-copper-gold bump, the bottom of the bump represents nickel, the top represents copper.

Figure 3b is a photograph showing the interface of the electroless nickel-copper-gold bumps, the thick portion on the left represents nickel, the thin portion on the right represents copper.

4A is a photograph showing a flip chip connection section of an electroless nickel-copper-gold bump using a conductive polymer.

4B is a photograph showing flip chip connection of an electroless nickel-copper-gold bump using a conductive polymer.

The nickel / copper bump forming method using the electroless nickel (Ni), copper (Cu), and gold (Au) plating methods of the present invention will be described with reference to the accompanying drawings.

The method of forming the nickel / copper bumps of the present invention comprises the steps of washing the aluminum (Al) pad (FIG. 1A), pretreating the aluminum pad with zinc acid (zincate), and electroless plating method. The process of plating nickel (FIG. 1C), the process of plating copper on the plated nickel part by the electroless plating method, and the process of plating which replaces a part of electroless copper plating with gold (FIG. 1D).

In the above process, zincate (zincate) is a solution of 120 g / l of sodium hydroxide (NaOH) and 3 to 4 g / l of zinc oxide (ZnO) in distilled water and is used to improve adhesion to electroless nickel plating. A pretreatment step of 20 seconds of solution, 10 seconds of 50% nitric acid solution and 20 seconds of zincate solution.

Electroless nickel plating solution of nickel sulfate (Nickel Sulfate) is 10~30g / ℓ, and sodium hypophosphite (H ₂ PO ₂ ^-) is preferably the nickel sulfate 13g / ℓ, the car than in the range of 25~40g / ℓ It is recommended to use a solution containing 30 g / l of sodium phosphite. The reaction of electroless nickel plating with sodium hypophosphite can be divided into the following main reactions and secondary reactions.

Note reaction _{^{Ni +2 + H 2 PO 2 -}} + H 2 O → Ni + H 2 PO 3 - + 2H + (1)

^{_{H 2 PO 2 - + H 2}} O → H 2 PO 3 - + H 2 (2)

The second reaction _{^{2H + + 2e - → H 2}} (3)

_{^{H 2 PO 2 - + H +}} → P + OH - + H 2 O (4)

Reduction of nickel is performed by the formula (1) in the above reaction formula, where (2) represents the consumption of sodium hypophosphite, (3) represents the generation of hydrogen, and (4) represents the production of phosphorus (P). The reduction of nickel is caused by the zinc (Zn) particles that are substituted in the zincate treatment, so that the nickel grows only on the aluminum surface subjected to the zincate pretreatment. Therefore, only aluminum pads can be selectively plated by electroless nickel plating, and a uniform height (15 μm) can be obtained.

In electroless copper plating, formaldehyde (HCHO) is used as a reducing agent to reduce copper through the following reaction.

^{Cu 2+ + 2HCHO + 4OH - →} Cu + 2H 2 O + 2HCO 2 -

Electroless copper plating can cause nickel to nucleate, forming bumps of nickel / copper without pretreatment and achieving a uniform height (5 μm). Gold substitution plating is a step of replacing copper with gold (Au), and the surface of the copper is replaced with gold by the following reaction formula. If the surface of copper is replaced with gold, the plating of gold no longer occurs.

Cu + Au ²⁺ → Cu ²⁺ + Au

In the present invention, gold is substituted with a thickness of 1,000 to 5,000 kPa to prevent oxidation of copper.

Among various electroless plating methods, electroless nickel plating enables bump formation on aluminum pads and has advantages of high plating speed and excellent selectivity. However, due to the addition of phosphorus (P) during plating has a high specific resistance, such as cracks due to the difference in the coefficient of thermal expansion. While electroless copper plating has a very low resistivity, it cannot be formed directly on an aluminum pad and has a disadvantage of slow plating speed. However, nickel can be directly electroless copper plated to form bumps using these two electroless plating methods. Finally, the plating of gold (Au) is substituted to prevent the oxidation of copper.

When the conventional bumper system using only electroless nickel is applied to a high-speed high frequency chip, the resistivity is large and the delay time is increased because the inductance is increased due to the inductance due to the ferromagnetic property of nickel. However, by applying the electroless nickel-copper-gold bump of the present invention, the resistivity decreases as the copper layer replaces nickel and the inductance can be reduced due to the diamagnetic of copper. In addition, both nickel and copper gold plating use the electroless plating method so that the process is consistent.

2 (a) (b) (c) (d) show electroless nickel-copper-gold bumps formed on high-speed high-frequency chips, and show electroless nickel on high-speed high-frequency chips (Fig. 2a) made of aluminum pads with a 150 μm pitch. It is a photograph at the time of performing plating (FIG. 2B), electroless copper plating (FIG. 2C), and gold substitution plating (FIG. 2D) in order. It can be seen that the bumps are selectively formed only on the aluminum pad, and the shape of the bumps is also flat, which is advantageous when connecting the conductive polymer flip chip.

Figure 3a is a photograph showing the cross-sectional structure of the nickel / copper bumps, the bottom of the bumps represent nickel, the upper represents copper, it can be seen that the nickel / copper bumps having a flat surface. 3b is a nickel / copper bump interface photograph showing that the thick portion on the left side represents nickel, the thin portion on the right side represents copper, and that about 15 μm of nickel and about 5 μm of copper have a uniform height. .

FIG. 4A is a cross-sectional view illustrating flip chip connection of an electroless nickel / copper bump and a substrate on an IC chip using an anisotropic conductive adhesive (ACA). The bumped chip is flipped over and connected to the pad of the substrate through the ACA. The ACA contains conductive particles that connect through the conductive particles between the electroless nickel / copper bumps and the substrate pad. 4B is a photograph showing the entire ACA flip chip connection.

The electroless nickel / copper bumps for the conductive polymer flip chip of the present invention adopt electroless nickel plating and electroless copper plating together to achieve high plating speed, which is an advantage of electroless nickel plating, and low resistivity, which is an advantage of electroless copper plating. Have them all.

In addition, when connecting electroless nickel / copper bumps to conductive polymer flip chips, the inductance can be lowered by reducing the resistance due to the low resistivity of copper and converting the ferromagnetic of nickel to the diamagnetic of copper. It is possible to obtain an effect of reducing the delay time. Therefore, improved electrical performance can be obtained by applying the flip chip package using the bump to the electronic package of the high-speed high-frequency chip.

Claims (3)

In the bump forming method for conductive polymer flip chip connection, Pretreatment of the aluminum pad with zincate, forming a nickel plating layer on the pretreated aluminum pad by an electroless plating method, forming a copper plating layer on the nickel plating layer by an electroless plating method, and electroless copper Method for forming a bump for connecting a conductive polymer flip chip comprising the step of replacing a portion of the plating with gold. The zincate pretreatment process according to claim 1, wherein the zincate pretreatment step is a zincate solution containing 120 g / l sodium hydroxide (NaOH) and 3-4 g / l zinc oxide (ZnO) in distilled water to improve adhesion to the electroless nickel plating. 20 seconds-50% nitric acid solution 10 seconds-nitrate solution Process for forming a bump for conductive polymer flip chip connection using an electroless plating method characterized in that for 20 seconds. delete