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

Abstract

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 m and measuring 5 m in both height and width. This method was applied successfully to interconnecting the pads forming an array with a pitch of 20 m.To meet the strong demand for high performance electronic packaging, interconnection technology involving chip-to-chip and chip-to-substrate bonds is a key element controlling the performance of electronic instruments. 1-3 In this packaging technology, the reliability of interconnection of high-density pad electrodes in three dimensions is very important. Currently, several methods, including wire bonding, tape automated bonding ͑TAB͒, and flip-chip bonding, are being used widely. Especially, the method of flip-chip bonding with small metal bumps is used more widely than other methods for fine-pitch bonding, as the former technology is more compatible with a higher packaging density with a shorter electrical path because of the use of a fine-bump array with a fine pitch. 4 Many researchers have investigated this method. 5,6 However, fine-pitch arrays of bumps of the order of micrometers in height and width are not easy to fabricate. Moreover, insufficient accuracy in alignment in the chip-to-substrate bonding is becoming a serious problem in dealing with finer bumps. 1 Furthermore, chip damages caused by pressure and heat treatment during bump bonding are also becoming a problem. 1,2 For solving these problems, several researchers have investigated the chip-to-substrate connection using the "surface activated bonding" method at room temperature 7 and the electroless deposition method. 8,9 In this article, we propose a method of pad interconnection using electroless metal deposition as a technique with a potential of application to flip-chip bonding technology. In this method, a metal is deposited electrolessly on organic resin to connect pads without using a seed layer and a resist pattern delineated by photolithography. No heat treatment nor pressure application is required, and the temperature during this fabrication process is as low as 60°C. This method is believed to be useful as a process for forming interconnections in technologies including three-dimensional system integration.Generally, it is known that electroless deposition, especially electroless NiB deposition, can occur not only on catalytic surfaces but also on noncatalytic surfaces near a catalytic area. 10-18 This phenomenon is called "extraneous deposition," and it is known to cause ...

Chemical flip-chip bonding method for fabricating 10-.MU.m-pad-pitch interconnect

“…Although the mechanism of the localized growth of the bridge layer on the cavity walls has not been fully elucidated at present, one explanation holds that the formation of bridge layer is associated with the presence of ionized products of side reactions, such as hydrogen, formed near the metal pads and the inhibition against ion diffusion of such ionized products [11]. According to this explanation, two factors govern our new method.…”

“…Finally, after surface treatment by using an alkali solution, 10% H 2 SO 4 solution, and PdCl 2 solution, the specimens were immersed in an electroless Ni-B plating solution containing dimethylamineborane (DMAB) as the reducing agent. The plating solution contained DMAB (0.025 mol dm −3 ), NiSO 4 .6H 2 O (0.1 mol dm −3 ), and sodium citrate (0.1 mol dm −3 ) [11]. The pH and temperature in the plating bath were 8 and 60 • C, respectively [11], so that the growth of the Ni-B bridge layers in the microscale cavity could occur within a reasonable limit.…”

“…The plating solution contained DMAB (0.025 mol dm −3 ), NiSO 4 .6H 2 O (0.1 mol dm −3 ), and sodium citrate (0.1 mol dm −3 ) [11]. The pH and temperature in the plating bath were 8 and 60 • C, respectively [11], so that the growth of the Ni-B bridge layers in the microscale cavity could occur within a reasonable limit. The completion of the fabrication of the interconnects was confirmed by SEM and electrical connection tests.…”

“…The difficulty in applying these methods to fine-pitch interconnects is mainly attributable to the isotropic growth of the plating film and/or insufficient pattern selectivity. To overcome the difficulty, we focus on determining a method to deposit the plating film selectively and efficiently in the connection region using electroless Ni-B plating; it is known that the so-called "extraneous deposition" easily occurs on the noncatalytic surfaces of an insulating material [9,10,11]. We find that under some conditions of electroless Ni-B plating, the growth of Ni-B film occurs preferentially even on the localized resin walls, which are placed vertically between a pair of copper pads facing each other in a cavity structure fabricated in the underfill resin.…”

Anisotropic Deposition of Localized Electroless Nickel for Preferential Bridge Connection

“…[8][9][10][11] We have proposed a method of forming pad-to-pad connections using electroless metal deposition as a technique with the potential of application to flip-chip bonding technology. [12][13][14][15][16] In this method, pad-to-pad bonding was performed using a metal film formed by electroless deposition. The metal film was deposited electrolessly on organic resin to connect pads without using a seed layer or a resist pattern delineated by photolithography.…”