Multi-mode interferometry for measuring the depth of deep silicon-etched microstructures

Deep silicon-etched microstructures are essential in both micro-electro-mechanical systems (MEMS) devices and advanced integrated-circuit packaging. Accurately acquiring the depth non-destructively is critically required. This work proposes a flexible multi-mode spectral interferometry applicable to microstructures of diverse geometry. First, the spectral reflectance (SR) mode is suited for narrow and high-aspect-ratio microstructures, requiring the light spot to be bigger than the width (width ≤10μm, aspect ratio ≥10:1). Second, the optical coherence tomography-like mode is more effective for microstructures with moderate or low aspect-ratios (width 10μm to 30μm, aspect ratio ≤10:1). Third, the image positioning-based spectroscopy (IP spectroscopy) mode is optimal while the light spot is smaller than the width. For demonstration, microstructures with widths ranging from 1 to 50μm, depths from 12 to 546μm, and aspect ratios between 1.2:1 and 51:1 are measured by the homemade apparatus. This method provides a metrology solution for multi-scale microstructure in MEMS and 3D-ICs.