Tokihiko Yokoshima

Interconnection of Micropad Electrodes by Controlled “Extraneous” Deposition of Electroless NiB Film

A method of interconnecting micropad electrodes was developed by using the electroless deposition of NiB for application to the flip-chip bonding technology. The phenomenon of bridge formation by the so-called extraneous deposition was utilized as a technique to perform selective deposition of NiB on noncatalytic surfaces. The process of extraneous electroless deposition was controlled by optimizing deposition conditions so that the deposit grew anisotropically along the surface between the facing pads. Practical feasibility of this technique was demonstrated by forming an interconnection between pads separated by 5 μm and measuring 5 μm in both height and width. This method was applied successfully to interconnecting the pads forming an array with a pitch of 20 μm.

Anisotropic Deposition of Localized Electroless Nickel for Preferential Bridge Connection

Preferential bridge connection by localized electroless NiB deposition was investigated. The phenomenon of bridge formation by so-called extraneous deposition was utilized as a technique to perform the selective deposition of NiB on organic surfaces. The films used for bridging were preferentially deposited in the area between facing pads. The deposition proceeded in the direction perpendicular to the surfaces of the facing pads. The occurrence of this anisotropic deposition strongly depends on solution diffusion, and the fabrication of a connection could be controlled by changing the pattern and pad dimensions. This method does not use a resist pattern for selective deposition; thus, it is a high throughput maskless fabrication process for the connection between separated pads. Using this technology, a preferential bridge connection for a pad array with a 5 μm pad width, a 5 μm pad height, a distance of 5 μm between the facing pads, and a 20 μm pitch was fabricated. Moreover, the combined use of localized electroless deposition and conventional electroless deposition on the bridging film is expected to realize for pad-to-pad connections with low electric resistance.

A Method of Fabricating Bump-Less Interconnects Applicable to Wafer-Scale Flip-Chip Bonding

A chemical flip-chip bonding method by electroless plating process has been developed. This method positively utilizes so-called bridge phenomenon between metal pads in electroless Ni-B plating, and enables bump-less interconnect without loading and/or heating at lower temperature (60°C). The interconnect behavior was examined using test chips and substrates with various pad-to-pad configurations. The result confirmed that effective pad width and a ratio of pad pitch to pad width determine the completeness of the interconnection under the condition that distance between facing pads are sufficiently close. A potential of improved method was also demonstrated for fabricating finer pitch flip-chip interconnect with a minimum pad-pitch of 20 ¿m.

Chemical flip-chip bonding method for fabricating 10-µm-pad-pitch interconnect

A flip-chip bonding method has been developed for fabricating ultrafine-pitch pad-to-pad interconnects. The method utilizes the preferential growth of Ni-B bridge layers on resin walls in a microscale cavity structure fabricated in the underfill resin between copper pads facing each other under some conditions of electroless Ni-B plating. In this method, the interconnect can be fabricated without loading and/or heating. By controlling the growth of the bridge layer on the resin walls in the microscale cavity under optimized plating conditions, the feasibility of ultrafine-pitch flip-chip bonding with a 10-µm pad pitch is experimentally demonstrated at a temperature of 60°C.

A method of “chemical flip-chip bonding” without loading and heating for ultra-fine chip-to-substrate interconnects

A method of chemical flip-chip bonding by electroless deposition process was proposed. This method positively utilizes preferential bridge deposition between metal pads in electroless Ni-B deposition and enables bump-less interconnect without loading and/or heating at lower temperature (60°C). Details of the deposition behavior for interconnection were investigated using fundamental test chips. The selection not only of various dimensions of pad design and pad-to-pad configurations but also of materials of base materials, was very important to achieve preferential bridge connection. Preferential bridge connections show high electric resistance because of using high resistivity materials with thin thickness. In the investigation of chip-to-substrate bonding, the low resistance interconnection could be achieved with combination usage of preferential bridge deposition and conventional electroless Au deposition from non-cyanide bath. The electric resistance of the interconnection decreased to less than one-20th, dramatically.

Maskless Fabrication for Micropad Interconnection using Electroless NiB Deposition and Application to "Chemical" Flip-Chip Bonding

In the field of electronics industry, highperformance flip-chip bonding is a one of the key requirement for realizing higher performance packaging. Thus, fine-pitch bonding of pads is strongly requested. However, realizing very fine bump allays with diameter and height less than 10 μm is not easy to realize connection with no failure. Furthermore, chip damages caused by pressure and heat treatment during bump bonding, are also becoming a problem. For solving these problems, we have investigated a method of micropad interconnection using localized electroless deposition [1] and have proposed a flip-chip bonding on the basis of this method [2]. In this method, we focused our attention on the bridge formation by extraneous deposition from the viewpoint of utilizing it as a novel technique to form metal deposits selectively on non-catalytic surfaces [1-3]. The one-on-one facing-pad connection, called preferential bridge interconnection, is realized by controlled anisotropic deposition of the localized electroless deposition. This formation of the interconnection was not used photolithographic technique, so that this method achieved a maskless fabrication process. On the other hand, insufficient alignment accuracy between the chip and the substrate during flip-chip bonding process is becoming a serious problem in dealing with finer pitch connections. This flip-chip bonding based on this method, called Chemical flip-chip bonding, has a potential to realize the bonding with poor-alignment accuracy of chipto-substrate bonding because micropad interconnection was realized by localized electroless deposition on the area of facing-pad. In this study, maskless fabrication of interconnection for facing pad with shifting was investigated using electroless NiB deposition. The test chips with the Cu pads facing each other were used for this study. Dimension of the pad array was summarized in table.1. The localized electroless NiB deposition carried out using the bath containing dimethylamineborane, NiSO4, and Na-citrate, at 60 C and pH=8. The details of the bath condition were shown in Ref.1. Chemical flip-chip bonding was carried out at the same conditions. Details of bonding condition were summarized in ref.2. Deposition behavior was examined by optical and microscopy scanning electron microscopy. Maskless fabrication of interconnection in an array of micropad electrodes was investigated using test chips. Figure 1 shows preferential bridge interconnection by facing anisotropic deposition of the localized electroless deposition. Interconnections were successfully obtained with the pad thickness of 1 μm that is 1/5 of the thickness of that in previous study (see fig.1a). Then, effect of shifting facing-pad location on preferential bridge interconnection was investigated. With the value of the location more than 5 μm, facing pads could not be interconnected. However, with the value of the location less than 5 μm, facing pads could be interconnected. From fig 1b, preferential bridge interconnection of the facing pad with 4μm shift was observed, that is same deposition behavior of the facing pad with no shift. Thus, chemical flip-chip bonding was carried out. Figure 2 shows results of chemical flip-chip bonding by preferential bridge interconnection. The pads on the chip and those on the substrate, which were shifted about 4 μm each other, were successfully interconnected. Alignment accuracy using conventional flip-chip bonder is about 5μm in general. From these results, the chip mounted on the substrate by conventional chip bonder could be interconnected by using this method. This method has a potential to apply to flip-chip bonding with ultra highdensity pad-interconnections.

Developing a leading practical application for 3D IC chip stacking technology: How to progress from fundamental technology to application technology

−1− Synthesiology English edition Vol.9 No.1 pp.1-15 (Jun. 2016) IC technologies, and the attempt to increase the integration density seemed to face the limit. The three-dimensional IC chip stacking technology whereby the IC devices are stacked vertically and packaged is one of the solutions, and expectation for it is rising recently as a technology for semiconductor device stacking that enables the increase of integration density for semiconductor ICs. Therefore, we established the fundamental technology for high-density high-integration electronic hardware construction required for 3D IC chip stacking, and we are working on the R&D of the application phase to create the flow of application system development, while engaging in technical support of massproduction technology that, in practice, should be undertaken by leading companies.