Galvanostatic bottom-up filling of TSV-like trenches: Choline-based leveler containing two quaternary ammoniums

Highlights

• The choline-based leveler having two quaternary ammoniums was synthesized.

• The adsorption of this leveler with suppressor and accelerator was examined.

• Galvanostatic Cu bottom-up filling was achieved with three-additive system.

• The mechanism of gap-filling was elucidated based on the additive adsorption.

Abstract

Through Silicon Via (TSV) technology is essential to accomplish 3-dimensional packaging of electronics. Hence, more reliable and faster TSV filling by Cu electrodeposition is required. Our approach to improve Cu gap-filling in TSV is based on the development of new organic additives for feature filling. Here, we introduce our achievements from the synthesis of choline-based leveler to the feature filling using a synthesized leveler. The choline-based leveler, which includes two quaternary ammoniums at both ends of the molecule, is synthesized from glutaric acid. The characteristics of the choline-based additive are examined by the electrochemical analyses, and it is confirmed that the choline-based leveler shows a convection dependent adsorption behavior, which is essential for leveling. The interactions between the polymeric suppressor, accelerator, and the choline-based leveler are also investigated by changing the convection condition. Using the combination of suppressor, accelerator, and the choline-based leveler, the extreme bottom-up filling of Cu at trenches with dimensions similar to TSV are fulfilled. The mechanism of Cu gap-filling is demonstrated based on the results of electrochemical analyses and feature filling.

Introduction

Through silicon via (TSV) is considered one of the “More than Moore” technologies, enabling further advances in electronic devices [1], [2], [3], [4], [5], [6]. TSV is a vertical interconnection through the wafer, and it can realize 3-dimensional connections between multiple chips. 3-dimensional circuits fabricated by TSV have many advantages compared to previous packaging methods [1], [2], [3], [4], [5], [6]. TSV facilitates the production of multifunctional electronics by stacking semiconductor elements and improves the performance of electronics by increasing the surface wiring density.

In recent years, numerous research groups have dedicated their efforts towards advancing TSV with respect to the formation of TSV by the etching process [7], [8], [9], the deposition of barrier and seed layers [10], [11], [12], [13], [14], [15], [16], [17], electrodeposition of copper (Cu) for TSV filling [18], [19], [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30], [31], [32], [33], thermal stress caused by the difference in thermal expansion of Cu and silicon (Si) [34], [35], [36], chemical mechanical planarization (CMP) [37], [38], wafer thinning [39], [40], and so on. Uniform barrier/seed layers and excellent Cu electrodeposition process are required for the formation of superior conductor. Tantalum (Ta) and Cu deposited by physical vapor deposition (PVD) have been widely used as the barrier and seed layers, respectively. However, the formation of defects at the bottom corner of TSV has occasionally been reported. Therefore, alternative methods to deposit uniform and conformal layers have been researched, such as electroless deposition (ELD) [10], [11], [12], [13], chemical vapor deposition (CVD) [14], [15], and atomic layer deposition (ALD) [16], [17].

Cu electrodeposition is also used for TSV filling similar to damascene interconnection although the dimensions of TSV are much larger than those of damascene features. The diameter and depth of TSV are usually a few micrometers and a few tens of micrometers, respectively. Electrodeposition processes for TSV filling have been intensively reported; V-shaped filling using the convection dependent adsorption behavior of the leveler [18], [19], [20], [21], [22], the extreme bottom-up filling based on the bistability of suppressing adsorbate [23], [24], the reverse step electrodeposition for modifying the surface coverage of the accelerator [25], [26], the gap-filling assisted by Ta cap at the top surface and side wall near the opening of TSV [27], [28], and others. Previously, we also proposed the mechanism of galvanostatic bottom-up filling with pyridine-based levelers [29], [30]. In all of the filling procedures, the basic strategy for TSV filling is to selectively enhance electrodeposition at the bottom of the TSV with strongly inhibiting Cu deposition at the top surface and the side wall near the opening of TSV.

From previous literatures, it seems reasonable that the application of leveler (or suppressing adsorbates) is decisive for the defect-free filling of TSV based on the aforementioned strategy. The levelers introduced in the previous studies were Janus Green B (JGB) [18], [19], [22], diallylamines [20], [21], ethylenediamine tetrakis(propoxylate-block-ethoxylate) tetrol (Tetronic 701) [23], and pyridine-based additives [29], [30]. With advanced insight into the molecular structures of levelers, it was obvious that the levelers generally contain tertiary or quaternary ammoniums, and that adsorption of a leveler was definitely influenced by its molecular structure. Furthermore, TSV filling performance was determined by the adsorption behavior of the leveler. Therefore, further research on the characteristics of levelers is required for continuous progress in TSV filling.

In this study, we synthesized a new choline-based leveler from glutaric acid, and the molecular structure of this leveler is shown in Fig. 1. It was predicted that modulation of the molecular structure from choline made the adsorption on Cu surface much stronger and two quaternary ammoniums at both ends of the molecule played an important role in its adsorption. After synthesis, the adsorption characteristics of choline-based leveler and the interactions between leveler, accelerator, and suppressor were examined by electrochemical analyses. Finally, the choline-based leveler was applied to Cu gap-filling in the presence of a polymeric suppressor and an accelerator. The influences of leveler concentration and the evolution of deposition profiles according to the deposition amount were also investigated to elucidate the mechanism of the feature filling.