Ultrafast Optical Scanning System for Real-Time Surface Profiling, Thickness, and Refractive Index Measurement

Background

Optical metrology and laser processing are critical fields in both research and industry, underpinning applications such as semiconductor manufacturing, thin-film characterization, and precision micromachining. Accurate measurement of surface position, film thickness, and refractive index is essential for quality control and device performance. Traditionally, these measurements are performed using optical microscopes equipped with confocal or interferometric modules, or through laser processing systems that require precise focus control. As the demand for higher throughput and finer resolution grows—driven by the miniaturization of devices and the complexity of modern materials—there is an increasing need for rapid, non-contact, and highly accurate 3D profiling and auto-focusing solutions that can keep pace with production and research requirements.

Current approaches to axial (z-axis) scanning and focus control in optical systems primarily rely on mechanical movement of either the sample stage or the objective lens. These mechanical methods are inherently limited by inertia, leading to relatively slow scanning speeds (typically below 100 Hz), which bottleneck the throughput of metrology and processing tasks. Moreover, mechanical systems are prone to wear, require regular maintenance, and can introduce vibrations or misalignments that degrade measurement accuracy. In the context of thin-film and multilayer samples, traditional confocal or interferometric techniques often require stepwise scanning or multiple measurements to resolve thickness and refractive index, further slowing down the process. These limitations make it challenging to achieve real-time, high-speed, and high-precision measurements, particularly for complex or non-flat surfaces, and hinder the integration of advanced metrology into fast-paced manufacturing environments.

Technology

This technology is an advanced optical system that leverages a tunable acoustic gradient index (TAG) lens to achieve high-speed, non-contact axial scanning for applications in laser processing, optical metrology, and microscopy. The system integrates both a probing laser and a fabrication laser into a shared optical path, using a confocal detector to monitor reflected light intensity as the TAG lens rapidly oscillates its focal length—up to 350 kHz. By analyzing the timing of intensity peaks generated during each scan, the system can determine surface position, thin-film thickness, and refractive index in real time. A synchronization controller precisely triggers the fabrication laser when the oscillating focus aligns with the detected surface, enabling real-time autofocusing and 3D profiling. The hardware is versatile, functioning as either a standalone laser processing tool or an add-on module for standard optical microscopes, thus facilitating ultrafast, non-contact metrology across a broad range of sample types and geometries.

What differentiates this technology is its elimination of traditional mechanical z-axis movement, which is typically slow, prone to wear, and limited in speed and accuracy. The use of a TAG lens allows for orders-of-magnitude faster scanning—up to hundreds of kilohertz—compared to the sub-100 Hz rates of mechanical systems. This dynamic z-scanning approach not only accelerates surface detection and focus adjustment but also enables simultaneous measurement of multiple parameters (position, thickness, refractive index) within a single scan period. The system’s adaptability to different objective lenses and TAG lens parameters allows it to be tuned for various detection ranges and accuracies, from sub-micron to millimeter scales. Its ability to process complex, non-flat surfaces in real time, combined with straightforward integration into existing microscopy setups, makes it a transformative solution for rapid, precise, and high-throughput 3D surface and thin-film characterization in both research and industrial environments.

Benefits

• Ultrafast axial scanning up to 350 kHz enables real-time, high-throughput surface detection and thickness measurement.
• Eliminates mechanical z-axis movement, reducing wear, inertia, and increasing system reliability.
• Simultaneous measurement of surface position, thin-film thickness, and refractive index with sub-micron accuracy.
• Precise synchronous triggering of fabrication lasers for optimized focus during laser processing on complex surfaces.
• Non-contact, confocal detection allows for rapid 3D profiling and autofocusing without physical sample disturbance.
• Modular design enables easy integration as an add-on to standard optical microscopes for enhanced metrology capabilities.
• Wide adaptability across different objective lenses and scan ranges, suitable for various sample types and thicknesses.
• Significantly faster and more accurate than traditional mechanical scanning or interferometric methods.

Applications

• Semiconductor wafer thickness metrology
• Real-time laser micromachining autofocus
• High-speed thin film inspection
• 3D surface profiling in microscopy
• Additive manufacturing layer measurement

Additional info

This optical system utilizes a tunable acoustic gradient index (TAG) lens for high-speed axial scanning up to 350 kHz, eliminating mechanical z-axis movement. By integrating probing and fabrication lasers, it uses confocal detection to analyze reflected intensity peaks. Processing these signals enables real-time calculation of surface position, film thickness, and refractive index. A synchronization controller triggers the laser at precise focal alignments, facilitating rapid 3D profiling and autofocusing.