AVS 72 Session TF1-TuA: VSHOP Fundamentals

Vapor phase infiltration (VPI) exposes polymers to gaseous inorganic precursors that dissolve into the polymer, become entrapped, and convert the polymer into an organic-inorganic hybrid material. VPI hybrids have unique mechanical, electrical, optical, and chemical properties that make them of interest for applications including EUV resists, membranes, and photocatalysts. However, fundamental predictions of which inorganic precursor chemistries will “successfully” infiltrate into which polymer chemistries is still elusive. Despite various systematic studies, no clear chemical “rules” have yet to be established to make these predictions. In this study, we conduct an experimental survey of “infiltration success” for a range of polymer chemistries, precursor chemistries, and infiltration temperatures to determine if any new insights for prediction can be gained. In total we study 12 different polymers, 3 different precursors, and 3 different infiltration temperatures. We include a range of different heteroatom functional groups on the polymers (e.g., nitrile, carboxylate, pyridine, etc.) as well as some differences in bending rigidity and free volume. Along the way, we note characterization tools best-suited for simple, fast detection and assessment of inorganic loading into polymer films, specifically SEM-EDX and XRF. This talk will show how analyzing the signal-to-noise ratio from these methods can then be used to provide a normalized assessment of inorganic loading. Broadly, the TiCl₄precursor is capable of successfully infiltrating into all the polymers that trimethylaluminum (TMA) can infiltrate, but just at a lower concentration, which is often not detectable with EDX alone. In contrast, the diethylzinc (DEZ) precursor does not successfully infiltrate many of the polymers that are successfully infiltrated with TiCl₄and TMA, like PMMA and P4VP. However, in certain instances, it appears that this lack of infiltration is not due to a lack of “dissolution” into the polymer, but rather a lack of permanent binding to the polymer. These insights along with efforts to train a machine learning model on this experimental data combined with other literature reports to better address the question of “What infiltrates what?” will be discussed.

Vapor phase infiltration (VPI) of polymers to form organic-inorganic hybrid materials has been demonstrated to improve the material’s chemical resistance to swelling and/or dissolution in organic solvents. Understanding the chemical stability of these hybrid organic-inorganic materials is important for developing new technologies, new membrane materials, and new photoresists. In this study, we examine the ability to use VPI of trimethylaluminum (TMA) and water (H2O) to alter the chemical stability of three different polymers: poly(vinylpyrrolidone) [PVP], poly(4-vinylpyridine) [P4VP], and polyacrylonitrile [PAN]. We find that all three of these polymers can be infiltrated with TMA, and we study their physicochemical structure and dissolution behavior after VPI at 70°C, 90°C, 120°C, and 150°C. For all polymers, the hybrids get thicker (swell) upon infiltration, indicative of inorganic loading into the film. This swollen thickness increases with VPI temperature, suggesting more inorganic loading at higher infiltration temperatures. This increased loading is further verified by combusting the hybrid at high temperatures (burn out) and measuring the residual inorganic layer’s thickness. After burnout, the infiltrated P4VP and PAN had inorganic film thickness of 19.2% and 17.0%, respectively, of their original hybrid thickness compared to an inorganic burnout thickness of only 8.8% for PVP , suggesting significantly less inorganic loading in PVP than the other two polymers. When these hybrid materials are then tested for dissolution in solvents known to dissolve the parent polymer, infiltrated PVP films dissolve almost immediately, showing no change in dissolution behavior after VPI. In contrast, infiltrated P4VP shows resistance to dissolution at VPI temperatures of 120°C and 150°C, and infiltrated PAN shows resistance to dissolution at VPI temperatures of 90°C, 120°C, and 150°C. These differences suggest that inorganic loading is likely an important predictor for chemical stability in these systems, however we will also discuss chemical structure changes interrogated with infrared spectroscopy (FTIR) and X-ray photoelectron spectroscopy (XPS) to provide more insights for how the chemical differences also contribute to chemical stability in these hybrid materials.

Vapor phase infiltration allows for the creation of hybrid organic-inorganic polymers through co-reaction with gas-phase small molecules. During VPI, polymers are isolated under vacuum and exposed to gas phase metal-organic precursors, which sorb into the bulk polymer and create hybrid materials possessing qualities of both the original polymer and the infiltrated inorganic. Despite the utility of VPI, well-defined reaction pathways, processing conditions, and resultant properties remain undefined for some of the most common commercial polymers. Herein, vapor phase infiltration of polyethylene terephthalate (PET) films via trimethlyl aluminum (TMA), in co-reaction with water, is explored as a function of both precursor dose time and subsequent isolation before introduction of water, in order to elucidate the process mechanisms. The rapid sorption of TMA is found to be followed by a slower, time-dependent desorption process that critically influences the resultant hybrid structure, chemical stability, and mechanical properties. In the case of inadequate desorption time, outdiffusing TMA reacts with subsequently dosed water vapor to form a brittle, aluminum oxide “coating” layer. If a longer desorption time is implemented between the TMA dose and water dose, then a homogeneous organic-inorganic hybrid can be achieved, resulting in the retainment of the PET film’s optical clarity and observed increases in the strength and resilience of the resultant surface when subjected to strain or bending. Between these extremes, however, we observe unusual transitory behavior with some hybrid character, but susceptibility to brittle cracking when exposed to certain levels of strain. This talk will examine this transitory behavior, and provide insights into how the physicochemical structure is changing at different levels of infiltration. Overall, this work elucidates new understanding of balancing reagent dosage and purge time in controlling the VPI process for infiltration of PET polymers.

Metal coatings on polymers are widely used in industries such as electronics, automotive, and packaging for their conductivity, barrier properties, and aesthetics. While strong adhesion between the metallic layer and the polymer is typically beneficial, it poses a significant challenge for their recycling. The separation of such composites is the critical step, requiring costly and limiting methods (e.g., shredding, pyrolysis, dissolving), often resulting in disposal of such waste, causing considerable environmental impact.

In this context, this study explores the use of sacrificial plasma polymer films (PPF) as intermediate layers between metals and polymers to improve the recyclability of the composite. The PPF would ensure strong adhesion with the metallic layer (i.e., Al) while embrittling the coating when exposed to a stimulus (e.g., water immersion, heating).

The PPFs are synthesized from acrylic acid, known to generate PPFs with a high content of carboxylic acid groups, ensuring a strong interaction with aluminum coatings.¹ The analysis of the XPS data reveals a higher retention of the carboxylic group at low plasma power (P_RF). ToF-SIMS analysis further shows that, at low P_RF, the reduced ionic bombardment during deposition, combined with the higher oxygen content in the PPFs, results in lower cross-linking density compared to PPF deposited at high P_RF. Consequently, low P_RF PPFs can be readily delaminated upon immersion in water, making them particularly suitable as sacrificial interlayers. Finally, aluminum thin film deposited by magnetron sputtering adheres well to this interlayer while its subsequent removal is significantly facilitated by water exposure.

References:

[1] Friedrich, J. F. et al. Surface & Coatings Technology 2005, 200, 565-568.