Investigation of Surface Roughness of Single Point Diamond Turned Germanium Substrate by Coherence Correlation Interferometry and Image Processing
At a Glance
Section titled âAt a Glanceâ| Metadata | Details |
|---|---|
| Publication Date | 2016-09-01 |
| Journal | IOP Conference Series Materials Science and Engineering |
| Authors | Shivani Gupta, Neha Khatri, Vinod Karar, S. S. Dhami |
| Institutions | National Institute of Technical Teachers Training and Research, Central Scientific Instruments Organisation |
| Citations | 2 |
| Analysis | Full AI Review Included |
Technical Analysis and Documentation for 6CCVD
Section titled âTechnical Analysis and Documentation for 6CCVDâExecutive Summary
Section titled âExecutive Summaryâ- Application Focus: This research validates the capability of Single Point Diamond Turning (SPDT) to produce ultra-smooth surfaces on brittle materials, specifically single crystal Germanium (111) substrates used for high-end Infrared (IR) optics.
- Precision Achievement: A minimum surface roughness (Ra) of 5 nm was successfully achieved on the Germanium mirror by optimizing the SPDT process parameters.
- Methodology & Tooling: The key enabling technology is the precise geometry and material quality of the Single Point Diamond Tool. The experiment used a diamond tool with a negative rake angle (-25°) to facilitate the nanometric finish.
- Characterization: Surface morphology was quantitatively measured using non-contact Coherence Correlation Interferometry (CCI) and validated qualitatively through advanced image processing techniques (Canny, Sobel, Prewitt, and Roberts edge filters).
- Engineering Value: The documented optimal machining parameters (Set 6: 2000 rpm, 1 ”m/rev feed, 1.5 ”m depth of cut) provide a critical foundation for manufacturing high-quality diffractive optical elements and aspheric lenses requiring sub-10 nm surface finishes.
- 6CCVD Value Proposition: Replicating or exceeding this performance requires ultra-high quality Single Crystal Diamond (SCD) tooling materials. 6CCVD specializes in providing the MPCVD SCD required for such demanding ultraprecision machining applications.
Technical Specifications
Section titled âTechnical Specificationsâ| Parameter | Value | Unit | Context |
|---|---|---|---|
| Workpiece Material | Single Crystal Germanium (111) | N/A | Substrate for IR optics and semiconductor applications |
| Best Surface Roughness (Ra) | 5 | nm | Achieved using optimal SPDT parameters (Set 6) |
| Worst Surface Roughness (Ra) | 31 | nm | Achieved using sub-optimal SPDT parameters (Set 9) |
| Machining Method | SPDT | N/A | Single Point Diamond Turning on Nanoform250 |
| Tool Material | Diamond | N/A | Single Point Diamond Tool |
| Tool Nose Radius | 1.5 | mm | Critical parameter for surface finish |
| Rake Angle | -25 | ° | Negative angle used for brittle material machining |
| Relief Angle | 10 | ° | Tool specification |
| Spindle Speed (Best Case) | 2000 | rpm | Optimal rotational speed (Set 6) |
| Tool Feed Rate (Best Case) | 1 | ”m/rev | Optimal material removal rate (Set 6) |
| Depth of Cut (Best Case) | 1.5 | ”m | Optimal cutting depth (Set 6) |
| Primary Measurement | CCI | N/A | Coherence Correlation Interferometry (Non-contact) |
Key Methodologies
Section titled âKey MethodologiesâThe study utilized a rigorous ultraprecision machining process integrated with advanced non-contact measurement and validation:
- Workpiece Material: Single Crystal Germanium (111) substrate selected for its high refractive index and use in IR applications (thermal imaging).
- Fabrication Platform: Machining performed on a Taylor-Hobson Nanoform250 SPDT machine, operated in a wet condition.
- Tool Specification: A high-precision Single Point Diamond Tool was used with the following geometry:
- Nose Radius: 1.5 mm
- Rake Angle: -25°
- Relief Angle: 10°
- Experimental Design: A Central Composite Design (CCD) framework was used to systematically vary four input parameters (Spindle Speed, Tool Feed Rate, Depth of Cut, Tool Overhang) across 31 experimental sets.
- Optimal Machining Recipe (Set 6):
- Spindle Speed: 2000 rpm
- Tool Feed Rate: 1 ”m/rev
- Depth of Cut: 1.5 ”m
- Tool Overhang: 14 mm
- Primary Surface Metrology: Surface morphology and roughness were measured using non-contact Coherence Correlation Interferometry (CCI).
- Roughness Validation: CCI images were analyzed and validated using MATLAB-based image processing techniques, including:
- Edge Filters (Canny, Roberts, Sobel, Prewitt) to quantify discontinuities, where fewer irregularities indicate better finish.
- Histogram Analysis to correlate pixel value distribution width with surface quality (narrower distribution indicates smoother surface).
6CCVD Solutions & Capabilities
Section titled â6CCVD Solutions & CapabilitiesâThis research highlights the critical role of precision diamond materials in achieving nanometric surface finishes essential for next-generation optics. 6CCVD provides the specialized MPCVD diamond substrates necessary to replicate and advance this ultraprecision manufacturing capability.
| Research Requirement / Application | 6CCVD Material & Service | Engineering Solution |
|---|---|---|
| Ultraprecision Tooling Material | Single Crystal Diamond (SCD) Plates | We supply the high-purity, optical-grade SCD required for fabricating the precision diamond tools used in SPDT. SCD ensures exceptional wear resistance, thermal stability, and edge integrity necessary to consistently achieve Ra < 10 nm finishes on brittle materials like Ge. |
| Achieving Nanometric Finish | SCD Substrates with Ra < 1 nm | Our standard polishing processes guarantee surface roughness down to Ra < 1 nm on SCD wafers. This exceeds the 5 nm result achieved in the study, making our material ideal for creating primary optical master components or superior SPDT tool blanks. |
| Custom Tool Geometry | Custom Dimensions and Shaping Services | 6CCVD provides SCD plates up to 125 mm and custom thicknesses (0.1 ”m - 500 ”m). We offer in-house laser cutting and specialized shaping services to produce tool blanks that meet the exact rake, relief, and nose radius specifications (e.g., 1.5 mm radius, -25° Rake) required by SPDT engineers. |
| Advanced Optical Components | SCD/PCD Wafers for Diffractive Optics | For researchers extending this work to fabrication of diffractive optical elements or high-power thermal windows, we offer materials in both Single Crystal (SCD) and Polycrystalline (PCD) grades, ensuring mechanical and optical suitability. |
| Integration and Packaging | Custom Metalization Capabilities | If the resulting Ge component or diamond master requires electrical contact or bonding layers, 6CCVD offers in-house metalization services including Au, Pt, Pd, Ti, W, and Cu deposition, ensuring reliable integration into complex optical systems. |
Engineering Support: 6CCVDâs in-house PhD team can assist with material selection and parameter optimization for similar ultraprecision diamond turning or IR optical component projects. We ensure the diamond material provided maximizes tool life and final workpiece quality. We ship globally (DDU default, DDP available) to support international research and manufacturing efforts.
For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.
View Original Abstract
Germanium is a widely used material in the infrared range. Single crystal germanium is used as semiconductor and optical material due to its salient features like high refractive index and proper working in cryogenic conditions. Thus, germanium is an important substrate for infrared lens having many applications in thermal imaging cameras, optical telescopes and miniaturization of infrared optical elements. These applications require optical elements of excellent surface quality and high dimensional accuracy. In addition to fulfil the demands, ultraprecision machine is used to fabricate the optical components. In this work, single crystal germanium (111) mirror is fabricated by using single point diamond tool with, negative rake angle. A large number of experiments are performed to achieve the surface finish of nanometric range. The best and worst combinations of process parameters are found on the basis of surface roughness with the help of coherence correlation interferometry(CCI) measurement and image processing using Canny, Prewitt, Roberts and Sobel edge filters and histogram. These results can be used for fabrication of diffractive optical elements and aspheric lenses of germanium.