  Niobium-coated copper radio-frequency cavities are cost-effective
alternatives to bulk niobium cavities, given the lower material costs of copper
substrates and their operation in liquid helium at around 4.2 K. However, these
cavities historically exhibited a gradual degradation in performance with the
accelerating field. This phenomenon, not yet fully understood, limits the
application of niobium thin film cavities in accelerators where the real-estate
gradient needs to be maximized. Recent studies on niobium films deposited on
copper using high power impulse magnetron sputtering (HiPIMS) technique show
promising results in mitigating the performance degradation of niobium thin
film radio-frequency cavities. This paper examines the effect of film thickness
on the superconducting properties of niobium films deposited on copper using
HiPIMS. The study provides insights into how the critical temperature,
transition width, lower and upper critical fields, and critical current density
vary with the film thickness. Increasing the thickness of niobium films
deposited through HiPIMS is found to enhance superconducting properties and
reduce densities of defects and structural irregularities in the crystalline
lattice. This shows potential for enhancing overall performance and potentially
mitigating the observed performance degradation in niobium thin film
radio-frequency cavities. Additionally, the Ivry's scaling relation among
critical temperature, thickness, and sheet resistance at the normal state
appears applicable to niobium films up to approximately 4 $\mu$m. This extends
the previously confirmed validity for niobium films, which was limited to
around 300 nm thickness.
