Reliability of through-silicon-vias (TSVs) with benzocyclobutene liners

Abstract

Through-silicon-vias (TSVs) using benzocyclobutene (BCB) as insulation layers (liners) are developed and the reliability related issues with regard to mechanical and electrical aspects are investigated. The BCB TSVs are fabricated using deep etching of annular trenches on a substrate, BCB cladding forming in the deep trenches, selective etching of the silicon posts in BCB claddings, and electroplating of copper plugs. The insulation capability and the configuration stability of BCB liners are evaluated by performing a thermal shock test and comparing the leakage currents before and after thermal shock. The results show that the BCB insulation capability does not change distinctly after thermal shock. Auger electronic spectroscopy (AES) is used to characterize copper diffusion through BCB thin films after high temperature treatment, and it is found that BCB is able to prevent copper diffusion at temperatures as high as 350 °C. Finite element analysis is performed to understand the thermal stress behaviors of BCB TSVs, and the simulation results show that BCB liners are able to reduce the thermal induced stresses in substrates, but increase the axial thermal expansion of copper plugs.

Introduction

Three-dimensional (3D) integration, which uses through-silicon-vias (TSVs) and wafer bonding to integrate multiple chips on the third direction, has been recognized as a promising technology for integrated circuits (ICs) to achieve further improvement on performance, heterogeneous integration of different technologies, and sophisticated functionalities [1], [2], [3], [4], [5]. The attractive foregrounds have promoted extensive research on 3D integration, and various fabrication technologies have been established for different applications [4], [5], [6], [7], [8]. Despite the significant advances in recent years, 3D integration is still facing several challenges, in particular the thermal related reliability issues.

Due to high density of active devices and complicated structures, the thermal problems, which have already been a tricky problem in 2D ICs, are even more serious in 3D integration. The large mismatch in the coefficients of thermal expansion (CTE) between silicon and copper causes significant thermal stresses inside TSVs and surrounding substrate [9]. Large thermal stress could lead to potential mechanical reliability problems such as die failure of cracks or breakage [10], copper pumping [11], and interfacial delamination [12], [13], [14]. Even the stress is not large enough to cause die failure, it can lead to mobility changes and thus parametric shifts of transistors, affecting the performance and tolerances of integrated circuits (ICs) [15], [16]. Besides, the scallops on the via sidewalls inherent to Bosch process may cause non-conformal deposition of silicon dioxide liners and barriers [17], which together with the high stress concentration at the sharp ridges, could induce reliability problems such as copper diffusion, electromigration, and current leakage [18], [19], [20], [21], [22].

To address these problems, polymers have been proposed as either part of TSVs [23], [24], [25], [26] or as an alternative to conventional silicon dioxide insulation layers (liners) [27], [28]. Thick polymers are expected to have the potential to address the mechanical and electrical reliability problems. For example for polymer liners, the large and uniform thickness (1–5 μm vs. SiO₂ 0.1–1 μm) allow to avoid the problems of copper diffusion by eliminating the discontinuity of the barrier layer and current leakage by increasing the thickness of liners. In addition, the low elastic moduli and soft feature of polymer liners enable them to deform more largely than silicon and metals, and thus allow polymer liners to act as a buffer layer between silicon substrates and copper plugs to alleviate the thermal stresses [21]. Besides, the low dielectric constants of polymers (normally around 3 [29] vs. SiO₂ 3.9), in conjunction with the large thickness, allows to achieve low capacitance and high electrical performance [27].

To exploit the advantages of polymers, this paper reports the implementation of TSVs with benzocyclobutene (BCB) polymer liners by developing a vacuum-assisted spin-coating method to fill BCB claddings in high aspect-ratio annular trenches. BCB has been chosen as the liner material due to some attractive features, such as low de-gassing property, low dielectric constant, low stress, good coverage, and excellent chemical resistance and high thermal stability (glass transition temperature T_g > 350 °C) [30], [31]. The reliability issues of BCB TSVs regarding to leakage current, thermal shock, BCB barrier capability, and thermal stress and thermal expansion are investigated experimentally and numerically.