Sulfite-free cyanide-free gold plating process applied to encapsulation bumps
Jiao Yu, Li Zhe, Ren Changyou, Deng Chuan, Wang Tong, Liu Zhiquan
Shenzhen Institute of Advanced Technology, Chinese Academy of Sciences, Shenzhen United Blue Ocean Applied Materials Technology Co., Ltd
Summary:
[Objective] Under the strategy of sustainable industrial development, the environmentally friendly cyanide-free gold plating technology is gradually replacing the traditional cyanide gold plating technology, and has been popularized and applied in the field of microelectronic packaging. [Method] A new sulfite-free cyanide-free gold plating formula and process was developed for the gold plating bump process of liquid crystal driver chip packaging wafers. [Result] Organic phosphonic acid additives and crystal adjusters were added to the self-developed cyanide-free gold plating solution, the former could fully inhibit the replacement of nickel-gold, and the latter helped to form a low-stress equiaxed crystal structure, which could avoid the extrusion deformation of domestic photoresist during the plating process. When applied to wafers, the self-developed cyanide-free gold plating process can obtain gold bumps with a flat and uniform microscopic surface and no defects. [Conclusion] The self-developed cyanide-free gold plating process can meet the requirements of wafer-level packaging and has good potential for popularization and application.
The electroplating gold layer has good electrical conductivity, thermal conductivity and chemical stability, and is an important surface treatment and metal interconnection material, which is widely used in the electronic information industry represented by microelectronic packaging. The deadly toxicity of traditional electroplating potions using cyanide ligands not only directly endangers the life and health safety of production personnel, but also brings a heavy burden to waste liquid treatment and environmental pollution. With the development of green industry and the continuous progress of formulation technology, people began to develop cyanide-free gold plating process to replace cyanide gold plating process. Feng et al. [1] reported a cyanide-free gold plating process using gold trichloride as the main salt and sodium sulfite as the coordination agent, and the obtained Au coating was finely crystallized, with good adhesion and corrosion resistance. Ye Renxiang et al. [2] developed a compound additive to prepare a bright Au coating with gloss higher than 656 gs and microhardness greater than 100 HV in order to solve the problems of poor stability of the plating solution, coarse crystallization and high porosity of the coating with multiphosphonate coordination and cyanide-free gold plating with multiple phosphonate coordination. In recent years, the cyanide-free gold plating process is gradually replacing the cyanide gold plating process in different fields, such as compound semiconductor RF chips, liquid crystal driver chip soft/hard gold bumps, and advanced packaging electroplated copper-nickel-gold rewiring layer (RDL). However, there are still deficiencies in the life of the plating solution, process performance, and microstructure of the plating layer, and there is a lack of effective control methods, which are also problems that need to be solved urgently for the further popularization of cyanide-free gold plating [3].
Gold plating is currently commonly used in the field of microelectronic packaging, such as liquid crystal screen (LCD) driver chip packaging (Figure 1). Liquid crystal driver chips are usually packaged by hot-pressed bonding process, which requires the hardness of the electroplated gold bump to be moderate (75 ~ 105 HV), when its hardness is too high, it can cause the chip to crack, and if it is too low, it is easy to deform and cause short circuit, and affect the circuit conduction on the anisotropic conductive adhesive (ACF). In order to reduce the manufacturing cost, the interconnect bump also tries to replace the pure gold with a Cu/Ni/Au coating structure, but the more active electroplating nickel layer is prone to a displacement reaction (Ni + 2Au + → Ni2+ + 2Au) when immersed in the electroplating solution, resulting in a loose chemical gold layer, resulting in nickel substrate corrosion and decreased Ni/Au interface adhesion [4-6]. Scholars have conducted a lot of research on the reliability problems caused by nickel-gold replacement. Liu et al. [7] found that PEI could be stably adsorbed at the active point on the surface of the nickel substrate, so that the activity of the nickel surface tended to be consistent, inhibited the local overcorrosion of nickel by the gold plating solution, and reduced the rate of early nickel-gold replacement reaction. However, PEI is easy to be mixed in the coating, resulting in problems such as unstable hardness and excessive internal stress. In addition to the reliability problems caused by nickel-gold replacement, sulfite electroplating also has problems such as coarse coating grains, unstable physical properties, and trace sulfur inclusions, which are difficult to meet the process requirements of precision electronic plating [8]. Wang Jicheng et al. [9] studied the effects of several mercaptocarboxylic acid organics on sulfite-free cyanide-free gold plating when they were used as coordination agents, and found that the stability of the plating solution could be improved when mercaptobutyric acid was used as the coordination agent, and the resulting Au coating was finely crystallized, uniform and bright in appearance. However, sulfhydryl compounds also have a strong adsorption effect on Au coating, which will also cause instability and reliability problems due to inclusions.
In view of the characteristics of the microelectronic packaging wafer electroplating bump process (see Figure 2), the author's team has developed a sulfite-free cyanide-free gold plating potion. The solution uses organic phosphonic acid as an additive. On the one hand, organic phosphonic acid will preferentially adsorb on the surface of nickel, effectively inhibiting nickel-gold replacement. On the other hand, organophosphonic acid, acting as a liganding agent for gold ions, does not cause inclusions. Compared with cyanide gold plating, the cyanide-free gold plating process can use a positive photoresist process that is easier to remove, and the electroplating process will not cause photoresist extrusion deformation, so as to meet the requirements of smooth and uniform appearance and no defects in the microstructure of the gold plating bump for wafer-level packaging, and has the potential for large-scale industrial application. In this paper, the self-developed cyanide-free gold plating potion is compared with the publicly available cyanide-free gold plating solution, so as to provide technical support for the application of cyanide-free gold plating in the field of microelectronic packaging.
1 Experiment
1. 1 Nickel plate electroplating experiment
A 20 mm × 20 mm commercial pure nickel sheet was used as the cathode for gold plating experiments. Before electroplating, the nickel sheet was degreased with 10% NaOH solution for 2 ~ 3 min, and then treated with 5% (mass fraction) sulfuric acid for 1 min to remove the oxide film on the surface.
The electroplating adopts Tektronix PWS4323 DC power supply and 4 L Yamamoto A-52 vertical electroplating tank, the anode is platinum mesh, the current density is 0.4 A/dm2, the plating time is 4 min, the whole process is applied to the circulation filtration and the surface of the cathode is stirred by scraping paddles.
自研亚硫酸盐体系无氰电镀金药水的组成及工艺条件为：金离子(以亚硫酸金钠形式加入)10 ~ 12 g/L，亚硫酸钠40 ~ 100 g/L，有机膦酸添加剂1 ~ 5 g/L，晶体调整剂60 ~ 80 mg/L，硬化剂2 ~ 10 mL/L，电流密度0.4 ~ 0.6 A/dm2，温度50 ~ 55 °C，pH 7.6 ~ 8.4。
公开的无氰亚硫酸盐电镀药水的组成为：金离子(以亚硫酸金钠形式加入)8 ~ 12 g/L，亚硫酸钠60 ~100 g/L，硫酸钠30 ~ 50 g/L，硫酸乙二胺10 g/L。
1. 2 Wafer gold plating experiment
The gold plating potions, processes, and equipment for the bumping on the wafer surface are the same as above. The difference is that the cathode of wafer plating is a silicon carbide wafer with a diameter of 4 in (about 10.16 cm), and the surface of the wafer is first magnetically sputtered with a 50 nm thick TiW barrier layer and a 50 nm thick Au seed layer, and the circuit patterns with a length, width, and height of 80 μm, 20 μm, and 15 μm are prepared by positive lithography. The bottom of some bump structures was evaporated with an aluminium passivation layer, and further photolithography was prepared to groove 1.2 μm deep and 4, 8 or 12 μm wide.
Before electroplating, the wafer was plasma activated by O2/Ar mixed gas, and the photoresist was ultrasonically dissolved in dimethyl sulfoxide for 5 min after electroplating, and annealed at 270 °C for 30 min after cleaning with pure water and cold air drying.
1. 3 Characterization of the microstructure of electroplated gold bumps
The Thermo Scientific FEI Nova NanoSEM 450 Field Emission Scanning Electron Microscope (FE-SEM) and its X-ray Energy Dispersive Spectrometer (EDS) were used to analyze the elemental distribution at the coating interface. KEYENCE VK-X1100 laser confocal microscope (LSCM) was used to observe the surface topography of electroplating gold and measure the roughness. Thermo Fisher Helios 5 UX Focused Ion Beam Scanning Electron Microscope (FIB-SEM) was used to observe the cross-sectional topography of electroplated gold films and bumps.
2 Results and Discussion
2.1 Inhibition of the substitution of nickel-gold by organophosphonic acid additives
The essence of nickel-gold replacement is the replacement reaction of active metals to inert metals, that is, in the process of immersion of nickel substrate in the plating solution, the more active nickel atoms lose electrons and become nickel ions, and gold ions are precipitated in the form of gold elemental after gaining electrons. Nickel-gold replacement will lead to corrosion of the nickel substrate and poor bonding force of the Ni/Au interface, resulting in huge mechanical defects and reliability risks of the product. The nickel flakes were soaked in the open and self-developed sulfite-free cyanide-free gold plating solutions for 2 min to investigate the nickel-gold replacement of the two plating solutions. As can be seen from Figure 3, the nickel sheet turns yellow after soaking in the publicly formulated sulfite gold plating solution for 2 min, indicating that nickel-gold displacement has occurred. On the contrary, the nickel sheet did not change color after soaking in the self-developed sulfite gold electroplating solution for 2 min, because the organic phosphonic acid additives in it could effectively inhibit the occurrence of nickel-gold replacement reaction.
The public and self-developed sulfite gold plating solution were used for plating at a current density of 0.4 A/dm2 for 4 min. As can be seen from Figure 4, the appearance of the Au coating obtained by the two systems is similar, indicating that the organic phosphonic acid additives will not adversely affect the appearance of the coating.
The energy dispersive spectrometer was used to analyze the cross-sectional element distribution of the self-developed cyanide-free gold-plated samples. As can be seen from Figure 5, there is no displacement interlayer, no P element between the Ni/Au layers, and no S element inclusion. The reason for this is that organophosphonic acids are also co-coordinating agents for gold ions, which not only do not have the risk of inclusion, but also improve the stability and service life of the plating bath [10-11].
2. 2 Morphology and structure analysis of electroplating gold layer on wafer surface
The 4-inch wafer was plated with sulfite-free cyanide-free gold plating solution with published and self-developed formulations, respectively, to explore the feasibility of its application in the field of microelectronic packaging. As can be seen from Figure 6, the surface roughness Ra of the sulfite-free cyanide-free gold plating layer under the public and self-developed formulations is 22 nm and 95 nm, respectively. The surface roughness of the Au coating obtained by the self-developed potion is significantly improved. On the one hand, the higher surface roughness (Ra = 90 ~ 110 nm) can improve the strength of the wire bonding, and on the other hand, the contact area with the sintered silver in the subsequent bonding process is large, which is more conducive to the heat dissipation of the power chip. Therefore, in terms of surface roughness, the self-developed potion is more in line with the needs of the microelectronic packaging industry.
The metal crystallization process is often accompanied by macroscopic stresses, which will cause a certain squeeze on the photoresist. However, domestic photoresists usually lack strength and are easy to be squeezed during the electroplating process and cause pattern deformation. As shown in Figure 7, when gold plating is done with open water particles, the plating extrudes the photoresist, resulting in deformation of the pattern. When the self-developed potion is used to plating the wafer pattern, the pattern is uniform, and there is no gold tumor or hole defect. This is mainly related to the difference in the microstructure of electrocrystallization between the two, the Au layer obtained by the electrocrystallization of the water is a columnar crystal structure (see Fig. 8a), which has a high macroscopic tensile stress, and a low-density twin layer is generated during the release of tensile stress. The self-developed potion contains a grain conditioner, and the resulting Au layer is an equiaxed crystal structure (see Fig. 8b). Different from the structure of the Au coating obtained by the open potion, this equiaxed grain structure has low macroscopic stress, uniform grain size and non-directional distribution, and the stress is not easy to concentrate.
LSCM and FIB-SEM were used to observe the appearance of gold plating on the surface of the wafer and the cross-sectional microstructure of the gold bump in the center position after annealing at 270 °C for 30 min, respectively, and the results are shown in Figs. 9 and Fig. 10.
As can be seen from Figure 9, when using the open cyanide-free solution, there are gold tumors and missing plating defects on the plating bumps, while when using the self-developed cyanide-free solution, the plating bumps on the entire wafer grow uniformly, and there are no macroscopic deformations or other defects. As can be seen from Figure 10, the shape of the electroplating gold bump is maintained well when the self-developed cyanide-free syrup is used, and with the increase of the slotting width of the aluminum passivation layer at the bottom, the center of the bump gradually becomes concave after electroplating, but the depression width on the surface of the bump is significantly lower than the bottom slotting width, and the surface depression value is also less than 1.0 μm, which has a flattening trend, which proves that the self-developed cyanide-free gold plating solution has a certain leveling ability and meets the requirements of wafer electroplating for the uniformity of the whole wafer. In addition, the shape of the bump is maintained well after annealing of the self-developed cyanide-free water plating sample, and the coating has no hole defects or abnormal crystallization, and the grain size distribution is uniform, which also meets the requirements of wafer electroplating for coating quality.
3 Conclusion
According to the characteristics of the microelectronic packaging wafer electroplating bump process, a new sulfite-free cyanide-free gold plating solution was developed. Organic phosphonic acid additives are used in this process, which can effectively inhibit the nickel-gold replacement reaction, and will not have the problems of uneven grain size and surface roughness of the coating. Crystal adjuster is also added to form a low-stress equiaxed crystal structure, which will not deform the domestic photoresist extrusion during the plating process, and the obtained electroplating bump meets the requirements of smooth and uniform microscopic morphology and no defects in the gold plating bump of wafer-level packaging. This self-developed potion has great potential for application and promotion.