Multilayer Thickness Measurements below the Rayleigh Limit Using FMCW Millimeter and Terahertz Waves

Area of Science:

• Physics
• Electrical Engineering
• Materials Science

Background:

• Traditional terahertz time-domain spectroscopy (TDS) has limitations in penetration depth and measurement speed.
• Millimeter and terahertz wave sensing are crucial for non-destructive material analysis.

Purpose of the Study:

• To present novel thickness measurement capabilities using frequency-modulated continuous-wave (FMCW) sensors in the millimeter and terahertz ranges.
• To overcome the Rayleigh resolution limit for sub-wavelength thickness determination.
• To analyze the impact of multiple reflections on measurement accuracy.

Main Methods:

• Utilized frequency-modulated continuous-wave (FMCW) sensors with up to 175 GHz bandwidth.
• Employed a model-based signal processing technique to resolve thicknesses below the Rayleigh limit.
• Adapted a modified transfer matrix method to analyze multiple reflections.
• Implemented brute force optimization processed in parallel on a GPU for rapid calculations (<1 s).

Main Results:

• Achieved high penetration depth and measurement rates of several kilohertz.
• Successfully resolved thicknesses below the Rayleigh resolution limit.
• Validated results against terahertz time-domain spectroscopy (TDS) on industrial samples.
• Demonstrated feasibility with reduced bandwidths for potential miniaturization.

Conclusions:

• FMCW sensors offer a promising alternative to TDS for high-speed, high-penetration thickness measurements.
• Model-based signal processing and advanced methods effectively address challenges like multiple reflections.
• Future miniaturization using monolithic microwave integrated circuit (MMIC) radar units is feasible.