Electroless UPD Monolayer Deposition - a Prelude to Highly Selective Electroless Atomic Layer Deposition Process

Abstract

The improved understanding of nucleation kinetics has led to various discoveries in which the thin film growth was manipulated to enhance the evolution of atomically flat epitaxial overlayers. Exploiting some of these results has led to invention of several new methods and protocols for electrodeposition where underpotentially deposited (UPD) monolayer is used as a mediator, surfactant or sacrificial template. The one successful example is so called “deposition via surface limited redox replacement (SLRR) of UPD monolayer,” which gained a lot of applications for synthesis of noble metal thin films with different functionalities. This protocol/method represents the combination of the potential controlled step (formation of UPD monolayer) and the electroless step (SLRR of UPD monolayer by more noble metal ions, i.e., galvanic displacement). However, our studies show that there is still a lot of room left for improvements and further simplifications. These should expand application of this deposition protocol in many areas where traditional atomic layer deposition (ALD) process has been used. In this talk we present results demonstrating an electroless ALD process in which a Pb UPD monolayer is used as a reducing agent and sacrificial material in SLRR reaction with noble metal ions such as Pt, Pd, and Ru and Cu. The full deposition cycle involves sequential exposure of the substrate such as Cu and Ru to the solution for electroless Pb UPD monolayer deposition and then to the solution for SLRR reaction and noble metal deposition. This results in an overall deposition of controlled amount of noble metal which is the function of the areal density of deposited Pb monolayer and stoichiometry of the SLRR reaction. Therefore, the process mimics, to a great extent, the standard ALD cycle where adsorption of the metal precursors and surface catalyzed reaction are replaced by electroless Pb UPD monolayer deposition and SLRR reaction. If the two-step SLRR cycle is repeated an arbitrary number of times, a highly compact, smooth, and conformal noble metal thin film can be grown. The deposition process is highly selective to the metal substrates at which Pb forms an UPD monolayer providing an advantage when certain integration requirements are considered. The process is designed for (but not limited to) aqueous solutions with fairly simple and stable chemistry that can be easily scaled up to any size and shape of the substrate surface. Results demonstrating details and underlying phenomena controlling this process will be discussed. In addition, an example of a high quality of Pt, Pd, Cu and Ru films grown on Cu and Ru substrate will be shown, as well as the applications of this process for metallization of structures relevant to semiconductor device fabrication.