(809a) Investigation of Growth Mechanisms in Molecular Layer Deposition of Polymer Films | AIChE

(809a) Investigation of Growth Mechanisms in Molecular Layer Deposition of Polymer Films


Bergsman, D. - Presenter, Stanford University
Zhou, H., Stanford University
Bent, S., Stanford University

In recent years, molecular layer deposition (MLD) has seen an increased interest for use in photoresists, chemical and biological sensors, and organic solar cells, due to its ability to deposit highly conformal polymer films with fine compositional control. Similar to its inorganic analogue, atomic layer deposition (ALD), MLD uses a sequence of self-limiting reactions of organic precursors to grow thin polymer films on a substrate surface. Using this technique, we have shown that polyurea films can be grown by reacting alternating precursors of phenylene diisocyanate (PDIC) and ethylenediamine (ED). These films exhibit characteristic qualities of films grown with MLD, such as precursor saturation in a given layer, low film roughness, indications of interlayer chemical reactions (polyurea linkages), and a linear growth rate. However, many questions still remain about the mechanism for growth in MLD films. For instance, because the occasional double reaction of precursors can reduce the number of surface reactive sites, deviation from constant growth rates is expected. Despite this, a majority of MLD reaction chemistries, including that of polyurea, have been experimentally found to exhibit linear growth. Here, we explore one possible mechanism for maintaining this linear behavior: that organic precursors have the ability to absorb into the deposited polymer film and re-initiate film growth. In this work, polyurea films are grown on silica substrates at thicknesses ranging from 1 to 20 nanometers. The available surface reactive isocyanate sites after a final PDIC dose are then terminated using a mono-functional amine precursor. Following the termination, continued MLD cycles of polyurea are performed on the terminated films and subsequent film growth is monitored. Ellipsometry and AFM measurements are used to measure film thicknesses, while infrared spectroscopy is used to confirm the formation of urea-linkage within the polymer films. Thickness results indicate that additional film growth is observed, suggesting that precursors are able to absorb into the film and act as new film growth sites. The extent of this precursor absorption into the film is monitored using fluorinated precursors in combination with x-ray photoemission spectroscopy (XPS) depth profiling. Implications for MLD growth mechanisms will be discussed.