Study of fine pitch micro-interconnections formed by low temperature bonded copper nanowires based anisotropic conductive film

Abstract

In this paper, Cu nanowire based anisotropic conductive film (ACF) has been systematically investigated in terms of its fine-pitch potential, electrical/ mechanical properties, process related issues and failure mechanisms. A test chip module including daisy-chain and 4-point structures was fabricated by electroplating and photolithography process, which has 160, 80, 40 and 30 µm bond pad pitches. The selection of membrane templates and the template based open/short failures have been discussed. Bond pads were finished with electroplated In to lower the bonding temperature and contact resistance. The electrical and mechanical performance of the interconnections has been studied in term of bonding force. The interconnect resistance of various pad sizes was 0.3-0.6 Ω per pad at the low bonding force of 1.5 N and dropped to 0.02-0.04 Ω per pad at the high bonding force of 10 N. The influence of bonding pressure on such nanowires formed interconnects was studied by microsectional analysis. For all pitch sizes, the electrical insulation was maintained for an applied voltage of 20 V. A shear strength of 1-5 MPa was achieved as a function of the bonding force and the fractural surface analysis verifies the Cu-In joint formed in such interconnections. IntroductionThe fabrication of stacked and vertically interconnected device layers (3D integration) with through Si via (TSV) technology [1] has provided a path to system scaling and performance enhancement. One of the key challenges in 3D integration is the fine pitch interconnection scheme, which has to accommodate various thermal and mechanical constraints, especially for the non-Si elements in the future 3D integrated package [2]. For advanced 3D stacking, various interconnection schemes and process technology have been proposed and most of them require a temperature higher than 250 °C and the underfill process during the bonding process [3,4]. As an alternative, anisotropic conductive film (ACF) can be a low-temperature, low-cost fine-pitch interconnection solution for future 3D integration. The conventional ACF, composed of randomly dispersed conductive micro particles in an adhesive polymer matrix, faces agglomeration problem when the size is reduced, resulting in potentially unstable electrical properties [5]. Moreover, the micro particles in the film have to be deformed to create electrical contact with high pressure, which constrains the minimum pitch size to ~ 100 µm for most commercial ACFs [6]. In order to further relax the thermal and mechanical constraints of interconnection process, nanowire based ACF (NW-ACF), containing metallic nanowire arrays vertically distributed in a porous polymer template, has been proposed as a low-temperature

Bumpless interconnects formed with nanowire ACF for 3D applications

“…While for the oxidationfree Au surfaces, a small amount of bonding force is sufficient for nanowires to form good contacts. Compared to the previous data of NW-ACF bonding to Cu-In dies [11], the minimum resistance of NW-ACF bonding with two bumped dies was 0.03 and 0.02 Ω for 80×80 and 40×40 µm 2 pad size. Without the In solder layer, Cu nanowires can still form a comparable low contact resistance joint with Au surfaces.…”

“…In our previous work, Cu NW-ACF has been demonstrated to bond with Cu-In bond pads [11]. Indium is applied as a layer of solder on the Cu bond pads to facilitate nanowire bonding, however, the question remains: will the NW-ACF be able to bond with the non-solder metal surface?…”