Applying Multimodal Spectroscopy for Micro-LED Wafer Defect Detection

Multimodal spectroscopic metrology, integrating Photoluminescence (PL), Time-Resolved Photoluminescence (TRPL), and Raman spectroscopy, is a rapid, non-destructive, and high-resolution technique for comprehensively characterizing micro-LED mu (LED) epitaxial wafers. This approach provides atomic-level insights to monitor epitaxial growth quality and detect yield-killing microscopic defects that directly impact display brightness, color uniformity, and pixel functionality.

Electronic Device Failure Analysis (EDFA) recently published an article on this topic, titled Spectroscopic Characterization and Detection of Yield-Killing Defects in Micro-LED Wafers. The publication highlights how advanced metrology tools, specifically HORIBA's LabRAM Odyssey and SMS320 systems, enable the simultaneous optical characterization of wafers to identify structural imperfections like micro-pits and micro-cracks. Specifically, the article demonstrates the effectiveness of high-resolution PL and Raman spectroscopy in assessing residual stress and carrier lifetimes, offering a robust approach for ensuring die yield in the mass production of next-generation displays.

Read the full story on the ASM International website to explore the data and methodology. https://static.asminternational.org/edfa/202508/16/

Frequently Asked Questions About Spectroscopic Metrology for Micro-LEDs

The transition from OLED and LCD to micro-LED technology is driven by the demand for higher brightness and lower power consumption in devices like smartwatches and augmented reality (AR) glasses.

To achieve the high pixel densities required for these applications, microLEDs must be fabricated with dimensions of 3 μm or smaller. At this scale, even microscopic defects in the epitaxial wafer can lead to dead pixels or uneven color. With the mu LED display market projected to reach USD 21 billion by 2028, early-stage identification of these defects is financially critical to maintain high production yields and lower manufacturing costs.

The study utilizes a multimodal approach to fully characterize the material properties:

  1. Full Wafer PL Mapping: Used for rapid identification of nonuniformity across the entire wafer surface.
  2. High-Resolution PL Spectroscopy: Used to analyze specific defect regions with a spatial resolution down to 200 nm, identifying wavelength shifts caused by structural imperfections.
  3. Time-Resolved Photoluminescence (TRPL): Measures minority carrier lifetimes to determine if defects act as nonradiative recombination centers.
  4. Raman Spectroscopy: Maps the phonon mode of GaN to analyze residual stress (tensile or compressive) surrounding surface defects.

Defects present themselves through distinct optical signatures compared to non-defective regions:

  • Photoluminescence (PL): Defects appear as "cold spots" or low-intensity regions. A spectral shift in emission wavelength often occurs, indicating local strain or composition variations.
  • TRPL: Defect regions exhibit a significant reduction in carrier lifetime, indicating that the defects are trapping carriers and preventing radiative emission.
  • Raman: Structural defects like micro-pits and micro-cracks cause a redshift in the GaN peak, indicating the presence of tensile stress in the crystal lattice.

The resolution is adjusted based on the scale of the inspection:

  • Defect Characterization: For analyzing specific yield-killing defects on the epi-wafer, a high-resolution step size of 200 nm was employed.
  • Display Panel Inspection: For assessing brightness uniformity on a commercially available LED display panel, a step size of 5 μm was used to investigate local intensity variations within pixels.

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