Shear Wave Ultrasound Inspection of Flaws in Silicon Wafers Using Focused Transducers

Area of Science:

• Materials Science
• Non-Destructive Testing
• Acoustics

Background:

• Micrometer-sized vertical cracks in silicon are difficult to detect using conventional methods.
• High-frequency focused ultrasound shows potential for inspecting such defects.
• Anisotropic media, like silicon, pose challenges for focused ultrasound due to directional wave propagation.

Purpose of the Study:

• To experimentally investigate the impact of silicon anisotropy on focused ultrasound inspection of back surface-breaking defects.
• To evaluate how different defect orientations affect wave focusing and scattering patterns.
• To correlate experimental findings with simulation results for a comprehensive understanding.

Main Methods:

• Experimental inspection of silicon wafers with defects at 0°, 15°, and 45° orientations using 100 MHz focused and unfocused shear waves in an immersion setup.
• Utilizing finite element (FE) modeling and ray tracing to simulate wave propagation and focusing.
• Analyzing scattering amplitude and defect detection sensitivity as a function of defect orientation.

Main Results:

• Defect orientation significantly impacts focused ultrasound scattering patterns and detection sensitivity.
• Defects at 45° orientation exhibited two-lobe scattering, while 0° orientation defects showed a one-lobe pattern, with reduced amplitude for the 45° defects.
• Ray tracing confirmed anisotropic focusing, producing butterfly wing and elliptical profiles for 45° and 0° orientations, respectively, aligning with experimental scattering patterns.

Conclusions:

• Anisotropic focusing behavior in silicon is the primary driver for orientation-dependent scattering patterns observed with focused ultrasound.
• Focused transducers are crucial for revealing these anisotropic effects, which are not apparent with unfocused transducers.
• The findings highlight the necessity of considering material anisotropy for accurate defect characterization in silicon using focused ultrasound.