Authors: Xiaohua Ye, Nanu Brates, Shaival Buch, William Grube, Huiling Zhu, Debbie Gustafson, Vikram Singh, Megan Dube, William Holber1
1. Plasmability Inc., 4715 Steiner Ranch Blvd, Austin, TX 78732
A broadband Laser-Driven Tunable Light Source (LDTLS®) powered by Energetiq’s Laser-Driven Light Source (LDLS®) is proposed for the application of diamond inspection. Experimental results of system performance and a summary of diamond sample inspections are presented.
Introduction
Diamond is an ultra-wide band gap semiconductor known for its numerous exceptional qualities including the highest thermal conductivity of any known material, high breakdown voltage, high carrier mobility (when doped) and high resistivity (when undoped) [1,2]. Unlike traditional semiconductor materials, such as silicon, diamond semiconductor devices can operate at much higher voltages and currents while providing low power dissipation. They also efficiently dissipate heat without compromising electrical performance. Outside of its coveted status in the jewelry industry, diamond could also play a crucial role in achieving carbon neutrality within the next 30 years by supporting the electrification of society.
While many people still think of diamond as a naturally occurring mineral that must be mined from the earth, it can now be produced more affordably and sustainably in laboratories. This makes diamond a viable and important semiconductor choice. Quality inspection of these lab-produced diamonds is especially critical for industrial applications, both for R&D efforts and for production. Various defects, such as dislocations and impurities, can occur in diamond material [3] and need to be carefully monitored and controlled in electronic and other applications. The following study presents a broadband LDTLS specifically designed to facilitate these types of inspections. The output wavelength of the LDTLS ranges from 200 nm to 770 nm, helping to generate fluorescence in diamond [4] and accurately identify various internal defects. The light-emitting xenon plasma in the LDLS driving an LDTLS is sustained by focusing a continuous-wave laser beam into the high-pressure Xe bulb. LDLS units offer several advantages over other light sources, including higher brightness, broader wavelength range, higher spatial and temporal stability, smaller plasma size, longer lifetime, and lower maintenance.
Schematic and Principles
A schematic for diamond inspection using an LDTLS is shown in Fig.1; the operation principle of the LDTLS can be seen in a previous publication [5]. In short, an EQ-77X model LDLS serves as the light source for the LDTLS. A spherical retroreflector (2) with enhanced UV coating is aligned to image the Xe plasma back onto itself for a higher forward-emitting radiance. Achromatic reflective optics, including two off-axis parabolic (OAP) mirrors (3, 4), are optimized to maximize optical throughput by etendue-matching the LDLS and monochromator (7). The UV monochromator selects a single wavelength which is then sent to the UV fiber (8). The order-sorting filter and width-adjustable slit are designed to suppress the second-order diffraction and control the wavelength bandwidth of the monochromatic spectral output. Images of the diamond sample (10), illuminated by the LDTLS output, are formed by the imaging system (9) for quality analysis.
Figure 1: Schematic diagram for diamond inspection using an LDTLS
Measurement Results and Analysis
Figure 2 illustrates the output performance of the LDTLS. Figures 2(a) and 2(b) display the in-band flux and full width at half maximum (FWHM) using a 1.5 mm core diameter fiber and 0.6 mm core diameter fiber for light collection, respectively. Figure 2(c) shows the normalized spectral output from 205 nm to 500 nm. As seen in Figure 2(a), the in-band flux average from 200 nm to 500 nm reaches 1.14mW with a 0.6 mm core diameter fiber and 2.79mW with a 1.5 mm core diameter fiber. The corresponding FWHM averages are 4.5 nm and 7.6 nm. The high in-band flux, adjustable bandwidth, and consistent spectral output across the entire wavelength range make the LDTLS suitable for various applications.
Figure 2: Performance for the LDTLS, (a) in-band flux and FWHM using a 1.5 mm core diameter fiber; (b) in-band flux and FWHM using a 0.6 mm core diameter fiber; (c) normalized spectral output from 205 nm to 500 nm.
Figure 3 shows examples of diamond inspection using LDTLS output illumination at different wavelengths. Diamond sample I was deliberately grown in multiple steps in order to introduce growth defects which could be observed using the LDTLS illumination. Images (a), (b), (c), and (d) show the inspection results for diamond sample I under illumination at 400 nm, 300 nm, 222 nm, and 205 nm. Images (e), (f), (g), and (h) show the inspection results for diamond sample II under illumination at 350 nm, 300 nm, 250 nm, and 222 nm. Fluorescence does not appear with visible light illumination but is useful for surface inspection, as shown in image (a). Fluorescence varies with different UV wavelengths, affecting the visibility of internal defects. The black circular line defect in sample I is only detectable using wavelengths that are 222 nm or shorter. The black circular line defect in sample II is detectable using wavelengths from 350 nm to 222 nm. This demonstrates the effectiveness of the LDTLS for detecting various internal defects.
Figure 3: Diamonds inspected with the illumination of LDTLS output, (a) sample I with 400 nm illumination; (b) sample I with 300 nm illumination; (c) sample I with 222 nm illumination; (d) sample I with 205 nm illumination; (e) sample II with 350 nm illumination; (f) sample II with 300 nm illumination; (g) sample II with 250 nm illumination; (h) sample II with 222 nm illumination.
Summary
A broadband LDTLS powered by LDLS technology is proposed for diamond inspection. Experimental results show that the in-band flux averages 1.14 mW in the range of 200 nm to 500 nm with a 0.6 mm core diameter fiber and 2.79 mW with a 1.5 mm core diameter fiber. The corresponding FWHM averages are 4.5 nm and 7.6 nm, respectively. Different internal defects can be detected by imaging and illumination with various spectral outputs of the LDTLS, as demonstrated by the inspection of two diamond samples.
Sources
- Kidalov, Sergey V., and Shakhov, Fedor M. (2009). Thermal conductivity of diamond composites. Materials 2.4, 2467-2495.
- Daniel Araujo, Mariko Suzuki, Fernando Lloret, Gonzalo Alba, and Pilar Villar (2021). Diamond for Electronics: Materials, Processing and Devices. Materials (Basel). 2021 Nov; 14(22): 7081.
- Christoph E. Nebel (2023) CVD diamond: a review on options ad reality. Functional Diamond 2023. 3.1 2201592.
- Luo,Y., et al. (2013). Fluorescence produced by optical defects in diamond: measurement, characterization, and challenges. Gems & Gemology 49.2.
- Ye, X., et al. (2018). LDLS Powered High Throughput Tunable Light Source. Frontiers in Optics. Optica Publishing Group.