Introducing SONUS Metrology and Defect Detection

Microelectronics industry trends continue to move rapidly towards 3D integration of semiconductor devices and the driving force remains the need for smaller, faster devices with optimized performance, enhanced data transfer speeds, minimum transmission loss, while maintaining reliability and meeting cost targets. Through Silicon Via (TSV) is an essential feature of the 3D process and the International Technology Roadmap for Semiconductors (ITRS) has identified unique metrology needs and challenges of TSV fabrication. Void-free copper fill in high aspect ratio TSV structures is critical to avoiding reliability problems and improving yield. Development of in-line metrology to detect voids in as-plated, post-CMP and annealed copper TSV has become a key metrology requirement. Current techniques such as FIB-SEM and X-ray microscopy are destructive or lack the resolution to detect defects typical to poorly-filled TSV structures.

Understanding these trends, Rudolph is excited to introduce a new acoustic measurement and defect detection technique. The SONUS™ system is a laser-based acoustic technique in which short laser pulses are used to generate broadband acoustic waves that propagate in the TSV structure. An optical interferometer detects the surface deformation caused by bulk acoustic waves reflecting from voids within the structure as well as other acoustic waves traveling near the surface. The range of acoustic frequencies generated can be up to 1GHz with acoustic penetration depths of hundreds of microns allowing measurements of high aspect ratio TSV structures, pre or post CMP. Measurements typically take a few seconds to a minute per site. Echo patterns returning from the bottom of the vias and those from the voids (stronger acoustic reflection) can be readily differentiated. Finite Element Modeling (FEM) techniques are used to model the acoustics with the sensitivity to detect sub-micron defects. Results from a number of customer applications studies have proven defect detection capability in vias as small as 3µm in diameter, >100µm in depth, and with a void resolution down to 0.5µm. In addition to via void detection, the technique has also demonstrated capability for measuring copper pillar film stacks, detecting delamination in stacked silicon, and performing metrology in complex MEMS structures.

The SONUS system is available now in a manual metrology platform for TSV and advanced metrology development. Beginning in Q2 of 2015, SONUS will be available in a fully automated 300mm platform for inline process control.

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