We aim to investigate the properties of multilayer structures made of Cu and SiNx, focusing on their optical properties in relation to thermal treatment characteristics. The films were deposited on various substrates, including glass, quartz, and silicon, using radio-frequency magnetron sputtering and high-power impulse magnetron sputtering techniques. After deposition, the multilayers were subjected to thermal treatments at temperatures ranging from 100°C to 500°C. The most significant changes in optical properties occurred within the first 24 h after deposition. The multilayers exhibited stable optical properties up to 400°C in a nitrogen atmosphere. However, optical properties deteriorate at temperatures above 400°C due to interdiffusion between the layers. XRD analysis confirmed the crystallization of SiNx/Cu/SiNx multilayer and the oxidation of Cu layers beyond these temperature thresholds. We also revealed that the type of substrate used had only a minor impact on the optical properties of the multilayers. The SiNx/Cu/SiNx multilayer demonstrated promising optical properties and thermal stability, making them suitable for transparent heat reflector applications. The layers’ integrity and optical properties are compromised only at temperatures above 400°C.