Synergy of mercaptosilane monolayer embedding and extremely dilute cobalt alloying for metallization of copper without a conventional metallic barrier

Highlights

• .Mercaptosilane monolayer as a barrier and seed adsorber for electroless metallization is studied.

• .Thin films of Cu with only 0.13 at.% of Co are prepared by the SAM-seeding electroless plating.

• .Synergy of the monolayer embedding and tiny cobalt alloying markedly strengthens Cu films.

• .Mechanism of seeding is clarified from solution chemistry and proton exchange mechanism.

• .Thin-film properties are studied elucidating film strengthening mechanisms.

Abstract

TaN/Ta bilayer is the standard barrier for the metallization of copper interconnects of current integrated circuit devices. However, thickness reduction of the barrier becomes a major bottleneck for the successful metallization of the downsizing copper interconnects in sub-7-nm technology nodes. Hence, the feasibility of a mercaptosilane self-assembled monolayer (SAM) as a barrier, as well as a seed adsorber for electroless copper metallization, is investigated. The mercaptosilane SAM after proper functionalization in a RCA-1 aqueous solution has the capacity to adsorb 3-nm-sized nickel particles, subsequently catalyzing the plating of extremely dilute Cu(Co) films with only 0.13 at.% of Co. Using the adhesion strength (13.1 MPa) and threshold temperature for thermal failure (450 °C) of pristine Cu/SiO₂ as a control, Cu/SAM/SiO₂ (pristine state) yields markedly enhanced adhesion strength and threshold temperature of 39.4 MPa and 500 °C, respectively. Cu(Co)/SAM/Cu exhibits even a much higher adhesion of 53.8 MPa (pristine state) and 74.6 MPa (annealed state). Seeding of the nickel particles by the functionalized mercaptosilane monolayer is clarified from the viewpoints of solution chemistry and proton exchange mechanism, and why only a tiny amount of cobalt is incorporated onto copper matrix is explained. The synergetic effect of the mercaptosilane monolayer embedding and extreme dilution of cobalt alloying on enhancing the adhesion and thermal stability of copper films is discussed considering the context of previous studies.

Introduction

Currently, Cu/TaN/Ta is a standard metallization stack for interconnects of microelectronic integrated circuit devices. TaN acts as a barrier to retard copper diffusion, and Ta serves as a liner to promote the adhesion of copper to an adjoining dielectric material. This bilayer combination leads to an enhanced barrier capacity over an individual TaN (and Ta) single layer [1]. In the sub-7-nm technology nodes, thickness reduction of the barrier and liner stack becomes a major bottleneck for the successful fabrication of copper interconnects for the downsizing integrated circuit devices, and the barrier/liner thickness should be reduced to <2 nm [2,3]. However, sputter deposited TaN exhibits a deteriorated barrier capacity (or thermal stability) as its thickness is reduced to 2 nm (or 3.5 nm) [4,5]. Atomic layer deposition can grow TaN with a conformal thickness of 1–2 nm, but it appears to be an unqualified barrier due to its exceptionally high electrical resistivity and loose film structure [6]. Moreover, as electrical resistivity of TaN/Ta is much higher than that of copper, the line resistance of copper interconnects for the sub-7-nm nodes abruptly increases by the occupation of a relatively thick TaN/Ta bilayer [7]. Therefore, it is important to develop an ultrathin (≤1 nm) barrier (also liner) as an alternative to the problematic TaN/Ta bilayer.

Self-assembled monolayers (SAMs) are ordered molecular layers with a thickness of 1 nm or less, which can be covalently tethered to a variety of metallic and oxide materials for surface engineering or interface modification [8,9]. Among the various SAMs used, alkylthiol SAMs, prepared typically by wet-solution methods, have received considerable attention because of the ability of their carbon-bonded thiol (R–SH) groups to coordinate noble metals. For example, (3-mercaptopropyl)trimethoxysilane (MPTMS) SAMs are often used as an intermediate layer to immobilize novel metallic (e.g., Au and Ag) particles on SiO₂ or mesoporous silica for catalysis [10,11], bionanotechnology [12,13], and noble metal recovery [14,15]. An MPTMS-SAM on SiO₂ enhances the quality of sputter [16] or chemical vapor [17,18] deposited copper films, and could serve as a barrier reducing in-situ diffusion of copper during metallization [19]; an MPTMS-SAM is capable of trapping silver atoms, thus preventing the silver atoms from out-diffusion through a titanium barrier upon oxygen plasma exposure [20]. However, MPTMS-SAMs appear to have a limited thermal stability and significantly decompose at 400 °C [21,22]. We thus revisit this topic by growing MPTMS-SAMs on SiO₂ hopefully to serve simultaneously as a liner and barrier, along with a metallurgical design of extremely dilute cobalt alloying of copper. The underlying concept is that MPTMS molecules contain methoxysilane [–Si(OCH₃)₃] and mercapto (R–SH) functional groups. The methoxysilane groups are hydrolyzable, capable of linking SiO₂ as a barrier by converting themselves into Si–O–Si chains through a hydrolysis-condensation reaction [23]. The R–SH groups form coordinate bonds with metallic elements, such as gold, silver and copper [24,25], and thus could be a liner to promote copper adhesion.

Our recent study has shown that (3-aminopropyl)trimethoxysilane (APTMS) SAMs after properly functionalizing with a RCA-1 solution effectively adsorb seeds (i.e., catalytic particles) for electroless copper metallization [26]. Here, we used this recipe to functionalize MPTMS-SAMs. Results presented suggest that an MPTMS-SAM, if properly grown and functionalized, indeed serves as not only a seed adsorber, but also an adhesion promoter and a reliable barrier, and hence holds promising as an alternative to the TaN/Ta bilayer barrier/liner. Moreover, cobalt has been widely used as a liner material to promote adhesion and enhance electrochemical copper metallization [27,28]. Considering the continuous downsizing of line widths, ultrathin CVD-Co is the trend of future studies [29]. Here. we also take it a step further by using electroless plating to fabricate copper films with an extremely dilute cobalt alloy (about 10⁻¹ at.%), instead of a separate cobalt liner, also showing capacity of further enhancing barrier capacity.