Fabrication process for creating functional nanostructures on sapphire and other hard optical ceramics

Background

Achieving precise high-aspect-ratio nanostructures on hard optical materials presents significant technical hurdles due to the intrinsic properties of these substrates, such as sapphire’s high hardness, chemical stability, and thermal tolerance. Traditional fabrication methods often struggle with generating uniformly patterned, tall, and sharply defined features because the robust chemical and physical properties that make these materials desirable for optical applica­tions also limit the efficacy of common etching and pattern transfer techniques. Inconsistencies like nonuniform mask thickness and imprecise etch endpoints compound the difficulty, while traditional masking materials and process configurations can lead to degraded nanoscale resolution, compromised aspect ratios, and reduced functional per­formance in areas such as anti-reflectivity and self-cleaning. These challenges drive the need for advanced, controlled fabrication strategies that ensure high fidelity, precise endpoint detection, and scalable production for a range of applications in nanophotonics and optoelectronics.

Technology overview

High-aspect-ratio sapphire nanostructures are created by depositing a thick polysilicon layer on sapphire, applying an antireflection coating, and then patterning a 200 nm photoresist layer using Lloyd’s mirror interference litho­graphy to produce a 2D nanopillar array. The pattern is transferred through inductively coupled plasma reactive ion etching (ICP‑RIE) where oxygen plasma removes the photoresist and ARC, and low-RF HBr plasma etches the polysilicon mask to form HAR nanopillars; subsequently, a BCl3/HBr plasma etch transfers the pattern into the sapphire substrate.

Optical emission spectroscopy (OES) combined with principal component analysis (PCA) is used to identify key emission wavelengths—most notably at 395.6 nm for Al and O—and to model the etch process with a first-order response for precise endpoint detection. The process achieves tapered sapphire nanostructures with controlled dimensions and an aspect ratio that can be increased through additional etching, resulting in surfaces with enhanced optical transmittance and functional properties such as antireflection, anti-fogging, anti-dust, and scratch resistance.

Benefits

• Highest aspect ratio achievement: The invention achieves a record aspect ratio of up to 2.1 for sapphire nanostructures, surpassing conventional methods that typically yield lower aspect ratios when patterning hard materials such as sapphire and alumina-based ceramics.
• Enhanced optical transmittance: By patterning the sapphire surface, the process increases optical transmittance (reported improvements from 86% to up to 96%), offering a significant performance boost over untreated sapphire substrates or traditional surface treatments.
• Real-time in situ process monitoring: The use of optical emission spectroscopy (OES) combined with principal component analysis (PCA) for endpoint detection provides precise, real-time monitoring of the etching process, a clear advantage over conventional, time-based or ex situ etching endpoint methods.
• Improved etch selectivity via low RF power processing: The method employs a low radio frequency power during the polysilicon mask etching, optimizing selectivity between the mask and sapphire substrate and ensuring high fidelity in the transfer of nanopatterns compared to standard reactive ion etching approaches.
• Multifunctional surface properties: The fabricated nanostructures impart multifunctional properties—including broadband and omni­directional antireflection, anti-fog, anti-dust, and scratch resistance—addressing performance challenges in harsh environments in ways not typically achieved with existing sapphire processing techniques.

Applications

• Consumer electronics display coatings: The nanostructured sapphire with anti-reflection, anti-fog, and scratch-resistant properties can be applied to high-end displays and touchscreens to enhance optical clarity and durability.
• Architectural and automotive glazing solutions: The technology’s ability to produce transparent, self-cleaning, and anti-dust surfaces is well-suited for windows, facades, and automotive glass that require high light transmission and environmental resilience.
• Aerospace and defense optical components: The fabrication process yields durable, thermally tolerant, and optically enhanced surfaces ideal for optical windows, sensors, and other critical components in harsh aerospace and defense environments.