The Bridgman method growth and spectroscopic characterization of calcium fluoride single crystals
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2016-01-01 |
| Journal | Science of Sintering |
| Authors | Ibrahim Elswie, Zorica LazareviÄ, Vesna RadojeviÄ, Martina GiliÄ, M.S. RabasoviÄ |
| Institutions | University of Tripoli, University of Belgrade |
| Citations | 7 |
| Analysis | Full AI Review Included |
Technical Documentation & Analysis: Next-Generation Optical Materials
Section titled âTechnical Documentation & Analysis: Next-Generation Optical MaterialsâThis document analyzes the requirements for high-quality optical crystals, as demonstrated by the $\text{CaF}_2$ research, and positions 6CCVDâs MPCVD diamond materials as the superior solution for next-generation DUV, high-power laser, and extreme environment applications.
Executive Summary
Section titled âExecutive SummaryâThe research successfully produced high-optical-quality Calcium Fluoride ($\text{CaF}_2$) single crystals via the Bridgman method for use in DUV-photolithography and excimer lasers. While $\text{CaF}_2$ is a traditional material, its limitations in thermal management, mechanical hardness, and high-flux stability necessitate a transition to advanced materials.
- Application Focus: The paper targets DUV optical components, steppers, and integrated circuit manufacturingâkey areas where diamond excels.
- Material Limitation: $\text{CaF}_2$ exhibits relatively high dislocation densities ($5 \times 10^4$ to $2 \times 10^5$ $\text{per cm}^2$) and low thermal conductivity, limiting its performance in high-power systems.
- 6CCVD Value Proposition: Single Crystal Diamond (SCD) offers superior optical transmission (from DUV/EUV to IR), extreme thermal conductivity (>2000 $\text{W/m}\cdot\text{K}$), and mechanical hardness, making it ideal for the high-flux environments described.
- Processing Expertise: The paper highlights the necessity of precise polishing using diamond paste; 6CCVD specializes in achieving ultra-low roughness (Ra < 1 nm) on diamond surfaces.
- Customization: 6CCVD provides custom SCD and PCD plates up to 125 mm diameter, exceeding the 20 mm diameter crystals achieved in this study.
- Conclusion: For engineers seeking to replicate or surpass the performance of $\text{CaF}_2$ in high-demand optical and thermal applications, 6CCVDâs MPCVD diamond is the definitive material solution.
Technical Specifications
Section titled âTechnical SpecificationsâThe following data points were extracted from the $\text{CaF}_2$ crystal growth and characterization study:
| Parameter | Value | Unit | Context |
|---|---|---|---|
| Crystal Diameter | 20 | mm | $\text{CaF}_2$ single crystal |
| Crystal Length | 90 | mm | $\text{CaF}_2$ single crystal |
| Crystal Orientation | <111> | N/A | Growth direction |
| Optimal Growth Rate | 6.8 | $\text{mm h}^{-1}$ | Vertical Bridgman method |
| Dislocation Density | $5 \times 10^4$ to $2 \times 10^5$ | $\text{per cm}^{2}$ | Measured via etching (Magnification 270x) |
| Raw Material Purity | 99.99 | % | $\text{CaF}_2$ powder used |
| Compaction Pressure | 3500 | $\text{kg cm}^{-2}$ | For tablet sintering |
| Sintering Temperature | 900 | °C | Under inert Argon atmosphere |
| Raman Optical Mode ($\text{T}_{2g}$) | 319.7 | $\text{cm}^{-1}$ | Room temperature measurement |
| IR Transverse Optical Mode ($\omega_{\text{TO}}$) | 272 | $\text{cm}^{-1}$ | Kramers-Kröning analysis |
| IR Longitudinal Optical Mode ($\omega_{\text{LO}}$) | 475 | $\text{cm}^{-1}$ | Kramers-Kröning analysis |
| Fluorescence Lifetime ($\tau$) | 33 | ns | Estimated lifetime of fluorescence |
Key Methodologies
Section titled âKey MethodologiesâThe experimental procedure focused on optimizing the Bridgman growth process and characterizing the resulting crystal quality using advanced spectroscopic techniques.
- Raw Material Preparation: $\text{CaF}_2$ powder (99.99% purity) was compacted under 3500 $\text{kg cm}^{-2}$ pressure and sintered at 900 °C under an inert argon atmosphere to form tablets.
- Crystal Growth: Crystals were grown using the BCG365 device via the vertical Bridgman method in a vacuum, testing various growth rates (6, 12, 24, 48 $\text{mm h}^{-1}$) and generator powers (3.8 kW to 3.94 kW).
- Optimal Growth: The best results were achieved at a crystal growth rate of 6.8 $\text{mm h}^{-1}$.
- Sample Processing: Selected single crystals were cut into tiles using a diamond saw. Plates were polished sequentially using silicon carbide, paraffin oil, and finally, diamond paste.
- Stress Reduction: Crystal annealing was performed on plates and bulk crystals at 1000 °C and 1080 °C for 1-3 hours in an inert argon atmosphere.
- Dislocation Analysis: Dislocation density was measured by observing the etched surface (using concentrated $\text{H}_2\text{SO}_4$ for 15 min) under a metallographic microscope (270x magnification).
- Spectroscopic Characterization:
- Raman Spectroscopy: Performed in backscattering geometry at room temperature using a Jobin-Yvon T64000 triple spectrometer, excited by a 514.5 nm ion laser (< 20 mW).
- IR Reflectivity: Measured using a BOMEM DA-8 FIR spectrometer with a DTGS pyroelectric detector (50 to 600 $\text{cm}^{-1}$).
- Photoluminescence (PL): Performed at room temperature using an Optical Parametric Oscillator (OPO) tuned at 350 nm excitation, recorded with a Hamamatsu streak camera.
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThe research highlights the critical need for high-quality, defect-free materials for DUV optics and integrated circuit manufacturing. 6CCVDâs MPCVD diamond offers superior performance characteristics necessary to advance these applications beyond the limits of traditional materials like $\text{CaF}_2$.
Applicable Materials: Replacing $\text{CaF}_2$ with Diamond
Section titled âApplicable Materials: Replacing $\text{CaF}_2$ with Diamondâ| Application Requirement (CaFâ) | 6CCVD Material Recommendation | Technical Advantage of Diamond |
|---|---|---|
| DUV/EUV Optical Components | Optical Grade SCD Diamond | Zero absorption in DUV/EUV range; superior radiation hardness compared to $\text{CaF}_2$. |
| High-Power Laser Windows | High Purity SCD (Thermal Grade) | Thermal conductivity >2000 $\text{W/m}\cdot\text{K}$ (10x higher than $\text{CaF}_2$), eliminating thermal lensing and stress fractures. |
| Integrated Circuit Wafers | Polycrystalline Diamond (PCD) | Excellent thermal spreading for high-density electronics and superior mechanical stability. |
| Electrochemical Sensing | Heavy Boron Doped Diamond (BDD) | Stable, wide potential window electrode material, ideal for harsh environments where $\text{CaF}_2$ is unsuitable. |
Customization Potential
Section titled âCustomization PotentialâThe $\text{CaF}_2$ study required precise material dimensions, orientation, and surface finishing. 6CCVDâs in-house capabilities directly address and exceed these requirements:
- Custom Dimensions: While the $\text{CaF}_2$ crystal was 20 mm in diameter, 6CCVD provides PCD plates/wafers up to 125 mm and large-area SCD, enabling scaling for industrial lithography and laser systems.
- Thickness Control: 6CCVD offers precise thickness control for both SCD (0.1 ”m - 500 ”m) and PCD (0.1 ”m - 500 ”m), allowing engineers to specify exact optical path lengths.
- Ultra-Low Defect Density: The $\text{CaF}_2$ dislocation density ($10^4$ to $10^5$ $\text{per cm}^2$) is high for demanding optics. 6CCVDâs high-quality SCD is engineered for dislocation densities orders of magnitude lower, ensuring superior optical homogeneity and reduced scattering.
- Advanced Polishing: The paper noted the use of diamond paste for final polishing. 6CCVD specializes in diamond surface preparation, guaranteeing Ra < 1 nm for SCD and Ra < 5 nm for inch-size PCD, critical for DUV/EUV applications where surface roughness dictates performance.
- Metalization Services: For integration into devices (e.g., thermal spreaders, electrodes), 6CCVD offers internal metalization capabilities, including Au, Pt, Pd, Ti, W, and Cu layers.
Engineering Support
Section titled âEngineering SupportâThe challenges faced in the $\text{CaF}_2$ researchâoptimizing growth conditions, minimizing defects, and achieving high optical qualityâare directly analogous to the challenges 6CCVD solves daily in MPCVD diamond growth.
6CCVDâs in-house PhD team provides expert consultation on:
- Material Selection: Assisting researchers in transitioning from traditional materials like $\text{CaF}_2$ to the optimal SCD or PCD grade for specific DUV-photolithography or high-power laser projects.
- Custom Recipe Development: Tailoring diamond growth parameters (thickness, doping, orientation) to meet unique device specifications.
- Advanced Characterization: Providing detailed analysis of thermal, electrical, and optical properties to ensure performance metrics are met.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
It must be noted that the main objective of this study was to obtain single crystals of calcium fluoride - CaF2, and after that the crystals were characterized with various spectroscopic methods. The crystals were grown using the Bridgman technique. By optimizing growth conditions, <111> oriented CaF2 crystals up to 20 mm in diameter were grown. Number of dislocations in CaF2 crystals was 5?104 - 2?105 per cm2. Selected CaF2 single crystal is cut into several tiles with the diamond saw. The plates were polished, first with the silicon carbide, then the paraffin oil, and finally with a diamond paste. The obtained crystal w?s studied by Raman and infrared -IR spectroscopy. The crystal structure is confirmed by X-ray diffraction (XRD). One Raman and two IR optical modes predicted by group theory are observed. A low photoluminescence testifies that the concentration of oxygen defects within the host CaF2 is small. All performed investigations show that the obtained CaF2 single crystal has good optical quality, which was the goal of this work.