Force-modulated structured tool nanoimprinting: predictive depth control and spacing-driven plasticity

The efficient and accurate fabrication of large-area periodic nanostructure arrays is of great significance for numerous applications, yet it remains a substantial challenge. An enhanced nanoimprinting technique is presented that utilizes a structured indenter with constant-force modulation to achieve large-area patterning of nanostructured arrays on metallic surfaces. FIB-fabricated nanostructured indenters with three different tip spacings were employed for constant-load nanoimprinting of single-crystal copper along three different orientations. First, an elastic–plastic contact mechanics model is developed to predict multi-tip nanoimprint depths under a constant normal load. The model accounts for inter-tip interactions and material crystal orientation, and its accuracy and robustness are validated experimentally and against existing models. Second, compared with a single-tip indentation, multi-tip indentation exhibits a coupled load-depth response: the required load is lower than the sum of the single-tip elastic loads but higher than the sum of the plastic loads. Plastic events occur without pop-in, and unloading shows greater elastic recovery. Subsequently, at a small spacing ratio (K = 1.25), strong tip–tip interference results in a substantial overlap between plastic zones and stress fields. This, in turn, intensifies confined plastic flow, elevates strain gradients, and promotes the formation of pile-up and grain boundaries. The transition in plastic interference from strong (K = 1.25) to weak (K = 1.5) and negligible (K = 2) was analyzed by varying the tip spacing ratio (K). Moreover, crystallographic orientation appreciably modifies the depth response and surface morphology. The (110) surface exhibited the maximum imprint depth, reflecting orientation-dependent slip activity combined with tip interference. These results offer crucial insights into the nanoimprinting mechanisms of single-crystal metals and provide a solid theoretical foundation for the precise fabrication of high-quality nanostructure arrays.