Electro-Less Deposition Using Metal Ion Based Reducing Agents: Potential Benefits and Applications in Semiconductor Manufacturing
Authors: Joi Aniruddha , Dordi Yezdi 
Published: February 2020

Further miniaturization of interconnects and devices to keep up with Moore’s law in the 21st century requires materials and process innovations. Electroplating, in a dual damascene process flow, has been the workhorse of the semiconductor industry for back-end-of-line metallization for the past two decades. As wire dimensions shrink below 20 nm in future technology nodes, electrochemical deposition faces a few challenges. Good gap-fill of <10 nm structures is increasingly difficult due to 1) the inability of conventional additives to provide bottom-up growth and 2) the terminal effect arising from resistive liners yielding poor nucleation. One process that becomes promising at such dimensions is electro-less deposition1. Electro-less deposition is a selective deposition process that can provide void-free metal fill in small dimensions. Since no external current is applied, electro-less deposition also does not suffer from terminal effects observed in electro-deposition on thin liners. Traditionally, electro-less deposition is performed using a class of reducing agents that include: boranes, hypophosphite, hydrazine etc. These processes have a few drawbacks. Firstly, presence of multi-step oxidation process for the reducing agents makes these systems difficult to study and characterize from a fundamental understanding standpoint2. Secondly, oxidation of the reducing agents often involve incorporation of B, P or H2 in the deposit which is undesirable for interconnect metals that require low resistivity and low stress.
Here we present an electro-less deposition process using metal ion reducing agents3,4, specifically trivalent titanium (Ti+3). We show that, in such systems, reduction and oxidation reactions can be studied separately and the overall redox process follows the mixed potential theory4, unlike the traditional electroless processes (Fig. 1). Based on electrochemical studies, electro-less deposition processes were developed logically for a group of metals that include: Cu, Co, Ni and Pt. We show that the resulting metal films are free of any impurities (such as B, P or H2) and possess very low resistivity, similar to those deposited by physical vapor deposition (Fig. 2). Processes were scaled up for deposition on 300 mm diameter wafers and challenges associated with them, particularly substrate pre-treatment and electrolyte stability, are discussed. Finally, few novel applications of such processes in semiconductor manufacturing are highlighted.

Figure 1: Measured mixed potential in a complete electro-less bath and those determined from cathodic and anodic polarization curves according to mixed potential theory for metal-ion and non metal-ion reducing agents processes.

Figure 2: Sheet resistances of cobalt and nickel films deposited on a Ru/TaN substrate using the Ti+3 based reducing agent. TEM image (inset) shows a representative cross-section of 4-nm thick cobalt film.

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Figure 1