PECVD Silicon Nitride-Based Multilayers with Optimized Mechanical Properties
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Silicon nitride (SiN) based films deposited by plasma-enhanced chemical vapor deposition (PECVD) have interesting optical, mechanical, and chemical properties. They are used for applications such as anti-reflective coatings and surface passivation layers in solar cells. Amorphous SiN-based films also are frequently used to create multilayer structures of alternatingly high-index and low-index films. This approach is very promising to fabricate narrow-bandwidth notch filters on photovoltaic cells to reflect on demand a wide variety of colors over the entire visible spectrum [1]. However, these multilayer structures need to be highly durable and mechanically stable since the lifecycle of photovoltaic cells in some applications [2] is very long, and the coated surfaces are very large. To satisfy these specific functional requirements, we need to adjust the films' optical and mechanical properties by changing the deposition parameters of a PECVD reactor. For instance, it is possible to strongly influence the chemical composition of amorphous SiN-based films, such as their stoichiometry, by tuning the gas flows [3]. This work investigates the effect of the deposition pressure, power, and source gas ratio on SiN-based monolayers during plasma deposition. We have deposited SiN and silicon oxynitride (SiON) films in an electron cyclotron resonance (ECR) PECVD reactor using a SiH4/N2/O2/Ar precursor mixture. We have measured the refractive index and the absorption of the films using a variable angle spectroscopic ellipsometry (VASE) to assess their optical quality. The mechanical properties of the films, such as residual stress and coefficient of thermal expansion, were measured ex-situ on a KLA-Tencor FLX-2320 film stress measurement system. The Young's modulus and hardness of the films were evaluated using nanoindentation. After studying the properties of monolayers, we have made numerical simulations and further characterizations to optimize the design of multilayer structures considering both the optical and mechanical properties. Through this discussion, we try to better understand the interactions taking place when amorphous SiN-based films with different structural properties and compositions are stacked on top of each other. Finally, we suggest methods to predict and control the residual stress in multilayer structures without affecting the optical properties.References:[1] S. Lee et al., "RGB-Colored Cu(In,Ga)(S,Se) 2 Thin-Film Solar Cells with Minimal Efficiency Loss Using Narrow-Bandwidth Stopband Nano-Multilayered Filters," ACS Appl. Mater. Interfaces, vol. 11, no. 10, pp. 9994–10003, Mar. 2019.[2] G. Ban-Weiss, C. Wray, W. Delp, P. Ly, H. Akbari, and R. Levinson, "Electricity production and cooling energy savings from the installation of a building-integrated photovoltaic roof on an office building," Energy Build., vol. 56, pp. 210–220, 2013.[3] D. B. Bonneville, J. W. Miller, C. Smyth, P. Mascher, and J. D. B. Bradley, "Low-temperature and low-pressure silicon nitride deposition by ecr-pecvd for optical waveguides," Appl. Sci., vol. 11, no. 5, pp. 1–11, 2021.
