Optimization of the SAM Coating Films As Resist Layers Utilized in the Area-Selective ALD Process
Authors: Hong Sungwook 
Published: December 2024

Atomic Layer Deposition (ALD) has garnered significant interest within the scientific community owing to its exceptional ability to deposit ultra-thin films with meticulous control over thickness and uniformity. By capitalizing on self-limited surface reactions, ALD represents a versatile technique for depositing a diverse array of materials encompassing semiconductors, insulators, and metals. However, traditional top-down methodologies, particularly those reliant on lithographic patterning, present inherent complexities characterized by a multitude of process steps, thereby giving rise to challenges such as misalignment, particularly pronounced in intricate three-dimensional (3D) and multilayer structures.
In response to these challenges, the emergence of Area-Selective ALD (AS-ALD) heralds a promising paradigm shift towards bottom-up approaches, offering the promising prospect of selective deposition on patterned substrates. The allure of AS-ALD lies in its potential to streamline manufacturing process steps by confining material deposition exclusively within predetermined regions, thereby refining pattern precision while concurrently curtailing process steps. The operationalization of AS-ALD hinges upon the deployment of a non-reactive polymer film as a strategic barrier layer, effectively impeding precursor penetration into undesired regions. In this context, Self-Assembled Monolayers (SAM) are organic monolayers films that are spontaneously formed via complex interactions of van der Waals forces and densely populated on the substrate surface, making them a pivotal component in achieving AS-ALD.
SAM, characterized by head, tail, and chain groups, confer a diverse functionality whereby the head group organized substrate reactivity modulation, the tail group modulates penetration of metal precursors into the substrate via end-group manipulation, and the chain group regulates SAM packing density. The strategic harnessing of SAM facilitates selective deposition by impeding precursor diffusion and penetration, thereby endowing a finely calibrated tool for SAM deposition. The controllability of surface properties through modification of molecular structure underscores ALD blocking efficiency as contingent upon factors such as alkyl chain length, with elongated chains fostering augmented Van der Waals attraction and packing density. Generally, empirical evidence substantiates the adequacy of a 12-carbon alkyl chain length for ALD blocking efficacy.
A pivotal facet of our investigation revolves around the optimization of SAM coatings on tungsten (W)-deposited silicon (Si) substrates, a crucial endeavor aimed at fortifying substrate adhesion while ensuring seamless compatibility with ALD processes. To this end, phosphonic acid-based SAM materials, boasting the capacity to form up to three bonds with metal surfaces, were judiciously selected to withstand the rigors of high-temperature, low-pressure ALD operations without decomposition. Furthermore, pretreatment of the metal surface with a basic solution served to enhance SAM coating uniformity and coverage, thereby affording a robust processability for subsequent ALD operations. Tailoring the alkyl chain length emerged as an additional aspect, with sophisticated adjustments tailored to fortify SAM coatings against structural degradation, thereby engendering enhanced stability and uniformity. Subsequent heat treatment emerged as a pivotal refinement step, efficaciously optimizing SAM coating uniformity and extending its application range, thereby endowing a versatile toolkit for material deposition.
Comprehensive characterization of SAM coatings via a suite of analytical techniques encompassing Atomic Force Microscopy (AFM), Water Contact Angle (WCA) measurements, and X-ray Photoelectron Spectroscopy (XPS) analyses provided invaluable insights into substrate coverage, adhesion, and thermal stability. Quantitative assessments facilitated by frictional force measurements, contact angle analyses, and bond reorganization studies emphasized the efficacy of SAM coatings in facilitating selective ALD, with enhancements in AS-ALD performance attributed to SAM optimization strategies. Collectively, these findings highlight the pivotal role of SAM coatings in advancing the frontiers of AS-ALD technology, offering the imperative optimization strategies in future process developments towards greater efficacy and efficiency.