VUV-Driven Silica Formation from PHPS: A Low-Temperature Route to Dense and Structurally Robust Films

Creating high-quality silica thin films is fundamental to a wide range of modern technologies, from electronics and optical devices to protective coatings and barrier layers. These films are prized for their excellent thermal stability, chemical inertness, low dielectric constant, and optical clarity. Yet, despite their broad utility, traditional fabrication techniques—such as chemical vapor deposition (CVD) and physical vapor deposition (PVD)—come with inherent limitations. These processes often require elevated temperatures, vacuum environments, and sophisticated infrastructure. Such conditions make them energy-intensive, drive up material costs, and render them less suited for large-scale or cost-effective manufacturing. More critically, they’re often incompatible with thermally sensitive materials like plastics, paper substrates, or flexible electronic components. This becomes a real issue as the demand grows for lightweight, bendable, and compact devices. An alternative approach that has gained some attention involves using polymer-derived ceramics (PDCs). These materials offer a much simpler processing route, typically through solution-based methods like spin or dip coating, and they tend to be more economical. However, there’s a catch: converting PDCs into functional ceramic films usually still requires pyrolysis at elevated temperatures to drive off residual organics and form the final structure. This heating step doesn’t just limit what substrates you can use—it also leads to considerable volume shrinkage which introduced mechanical stress and resulted in surface defects like cracks, voids, or even film delamination. In high-precision applications, especially those at the micro- or nano-scale, such imperfections are unacceptable and can compromise both the performance and longevity of the device. To overcome these challenges, there’s been growing interest in developing a low-temperature, ambient-pressure method for converting solution-processed precursors into dense and uniform silica films. One particularly promising candidate for this purpose is perhydropolysilazane (PHPS), an inorganic polymer known for its highly reactive Si–H and N–H groups and lack of organic content. PHPS is typically converted into silica through high-temperature treatment in air, a process in which hydrogen and nitrogen atoms are replaced by oxygen atoms. Because of its ability to cleave chemical bonds and generate reactive oxygen species (ROS), high-energy light has opened up new possibilities for photochemical strategies for film fabrication under milder conditions. Among those strategies, vacuum ultraviolet (VUV) irradiation has emerged as especially compelling. Recognizing this potential, Senior Research Fellow Yasuhiro Naganuma at the Kanagawa Institute of Industrial Science and Technology (KISTEC), with valuable contributions from his colleagues—Senior Research Fellows Chihiro Kato, Toshiyuki Watanabe, Satoru Kaneko, and Satomi Tanaka—set out to explore whether VUV light, under well-controlled oxygen conditions, could drive the transformation of PHPS into silica films at or near room temperature. Their study, recently published in Thin Solid Films, was motivated by two key objectives: first, to unravel the underlying mechanisms of this photochemical conversion, and second, to rigorously compare the structural and physicochemical properties of the resulting films against those produced by more conventional high-temperature methods.

In their investigation into low-temperature processing of silica films, the researchers turned to VUV irradiation as a means to convert PHPS into silica without the need for high-temperature treatment. Their method began with spin-coating a carefully diluted PHPS solution onto pre-cleaned, mirror-polished silicon wafers. This step was important to ensure film uniformity and minimize variables across the experimental samples. Once coated and gently dried, the films were exposed to VUV light emitted from a xenon excimer lamp, operating at a wavelength of 172 nm. An important experimental design was the control of oxygen concentration (C_O) during irradiation and by adjusting the gas environment—introducing either nitrogen, dry air, or combinations thereof—they were able to precisely tune both the intensity of VUV light and the amount of ROS generated in situ, each of which played a major role in the conversion process. In parallel, careful control of atmospheric moisture was also crucial. Generally, PHPS undergoes hydrolysis and polycondensation to form a siloxane network—reactions that are often accompanied by volumetric shrinkage. To mitigate this, the water content was minimized, specifically by maintaining the relative humidity less than 2%, in order to suppress them. This approach potentially enables the formation of a dense and stable film with reduced shrinkage. Such a strategy is analogous to conversion reactions in which water involvement is deliberately minimized. Rather than limiting their observations to surface characteristics, the team conducted a thorough internal analysis using a suite of spectroscopic techniques. X-ray photoelectron spectroscopy (XPS) provided one of the first significant insights. The data showed that even at low oxygen concentrations, VUV exposure initiated the transformation of the PHPS surface into silica. However, the extent of this conversion varied with C_O. At 21% C_O, which closely mirrors atmospheric oxygen content, the conversion was nearly complete across the entire film thickness. In contrast, films irradiated in low-oxygen environments (0% and 5%) retained nitrogen within their interior, suggesting the formation of silicon oxynitride rather than pure silica. This was a key finding, revealing the influence of ROS concentration on the final chemical structure of the films. Fourier-transform infrared (FTIR) spectroscopy further reinforced these observations. The untreated PHPS films showed prominent absorption peaks associated with Si–H and N–H bonds. After VUV treatment, especially under higher oxygen conditions, these peaks diminished or disappeared entirely. In their place, the Si–O–Si stretching bands emerged—an unmistakable signature of silica network formation. Remarkably, even when the lamp was positioned at distances where the VUV light was largely absorbed by the surrounding oxygen before reaching the film, conversion still took place. This strongly pointed to ROS, rather than direct photon interaction alone, as the driving force behind the oxidation and crosslinking reactions. To evaluate the mechanical strength of the converted films, they carried out pencil hardness tests—a simple yet telling method. The unirradiated PHPS films showed poor resistance to scratching (below 6B), while the VUV-treated films demonstrated much greater resilience, scoring a hardness of 6H, on par with films produced by conventional heat treatment at 500°C. This was a clear sign that the structural integrity of the photochemically derived silica was not only intact but comparable to its thermally processed counterpart. Moreover, the research team used X-ray excited Auger electron spectroscopy (XAES) and found that the silicon in the VUV-treated films was predominantly bonded to oxygen, with Auger parameters matching closely to stoichiometric SiO₂. Through high-resolution scanning electron microscopy (SEM), the authors observed that both the VUV-irradiated and heat-treated films were smooth and free of cracks. However, X-ray reflectivity (XRR) measurements revealed a clear difference in thickness retention: while thermal treatment caused the films to shrink by nearly 20%, the VUV-treated films shrank by only about 3.4%, indicating that VUV irradiation more effectively preserved the original film thickness. This reduced shrinkage is a major advantage in applications requiring precise film dimensions, such as microelectronics or flexible displays. Interestingly, when they examined the films’ resistance to sputtering by ion beams, another pattern emerged. The VUV-converted films took considerably longer to etch through, suggesting they were denser and more cohesive. These observations lined up with other structural and compositional data, reinforcing the idea that the VUV process yields films that are chemically pure, physically robust, and structurally stable.

In conclusion, Senior Research Fellow Yasuhiro Naganuma and colleagues successfully developed a low-temperature, atmospheric-pressure method to convert PHPS into dense, high-quality silica films. In doing so, they’ve provided a practical alternative to the conventional high-heat processes that dominate the field. What’s striking is that the films produced using this VUV approach show properties—like hardness, chemical composition, and structural integrity—that closely match those of silica films formed at 500°C. Achieving that level of quality without exposing the substrate to extreme temperatures is a significant step forward for materials engineering. Perhaps one of the most impactful takeaways from this work is its relevance to heat-sensitive substrates. As the demand grows for flexible electronics, wearable sensors, and foldable displays, the need for gentle fabrication techniques has become more urgent. These substrates, often made of polymers or other soft materials, simply can’t tolerate the high thermal loads associated with traditional annealing or plasma-based treatments. This new photochemical approach offers a way around that limitation. It allows for the deposition of crack-free, dimensionally stable silica films directly onto delicate materials—making it much more compatible with the direction modern tech is heading.

The study also uncovered a fascinating detail about how the transformation from PHPS to silica occurs. Instead of following the expected route—where conversion proceeds from the surface downward—this process was found to initiate at both the surface and the substrate interface. That dual-front conversion challenges traditional assumptions about how light-driven reactions operate in thin films. It suggests that photon penetration and reactive oxygen species can trigger changes deeper in the material than previously assumed. Equally important is the finding that this method dramatically reduces film shrinkage. It is noteworthy to mention the new method has also implications for sustainable manufacturing since the process avoids high-temperature equipment, toxic byproducts, and the need for vacuum or inert gas chambers, it stands out as an energy-efficient and environmentally responsible alternative.