Coating‐Free Nanoscratching for Liquid‐Crystal Alignment on PCB‐Compatible Millimeter‐Wave Reflective Unit Cells

ABSTRACT Liquid crystals (LCs) are attractive media for microwave and millimeter‐wave reconfigurable devices because their dielectric anisotropy enables continuous, low‐power RF phase modulation. However, conventional LC alignment generally requires polymer alignment layers, thermal baking, and rubbing, which can become process‐sensitive on PCB‐compatible Cu‐patterned RF substrates with thick metal topography and thermally sensitive laminates. Here, a polymer‐free and baking‐free nanoscratching strategy is introduced for LC alignment on PCB‐compatible Cu‐patterned RF substrates. A 500 nm diamond lapping film directly forms anisotropic nanoscale grooves on accessible Cu surfaces within minutes, producing a groove‐guided alignment interface without polymer coating. Pristine and commercially mechanically conditioned PCB surfaces are examined, showing that nanoscratching regularizes process‐inherited surface textures into directionally grooved interfaces. Surface morphology and LC alignment are evaluated by optical microscopy, SEM, AFM, polarization‐dependent reflection, apparent extinction‐ratio analysis, and dichroic absorption spectroscopy. As a proof‐of‐concept RF demonstration, the nanoscratched interface is integrated into a 28 GHz millimeter‐wave reflective unit‐cell platform, providing voltage‐programmable reflection‐phase modulation with an approximately 250° measured phase‐tuning range and reduced voltage‐sweep hysteresis compared with a polymer‐coated reference. These results establish direct nanoscratching as a coating‐free surface‐definition route for PCB‐compatible LC‐based millimeter‐wave reflective devices.