Fig. 4: Process-induced material damage.
a X-ray reflectometry (XRR) measurements of LN thin films of varying thicknesses (63, 185, and 300 nm). Curves are shifted vertically for clarity. The 63 nm and 185 nm films are thinned from 300 nm using ion milling. The inset illustrates the measurement schematic and the multilayer structure, including the hypothesized damaged layer. b Schematic illustration of the selective wet etching process. A bare LN chip is first thinned using ion milling, and annealed at 200 °C for 24 h. Then half of the chip is masked with photoresist and immersed in 49% hydrofluoric (HF) acid for 3 min. c Thickness maps obtained from optical interferometry for LN chips with initial thicknesses of 70 nm and 180 nm, respectively. A height difference of  ~20 nm is observed between the HF-exposed and photoresist-protected regions, indicating the removal of a damaged surface layer. d Schematic illustration of the damaged surface layer. The scattering length density (SLD) gradually increases from the damaged (piezoelectrically inactive) surface to the crystalline LN. XRR primarily detects the upper region with a sharp SLD contrast (blue dashed line), while HF etching could remove chemically modified material deeper into the film (green dashed line).
