Characteristics of diamond turned NiP smoothed with ion beam planarization technique

At a Glance

Metadata	Details
Publication Date	2017-10-13
Journal	Journal of the European Optical Society Rapid Publications
Authors	Yaguo Li, Hideo Takino, Frank Frost
Institutions	Leibniz Institute of Surface Engineering, Chiba Institute of Technology
Citations	8
Analysis	Full AI Review Included

Diamond Substrate Finishing: Ion Beam Planarization (IBP) for Ultra-Smooth Optical Surfaces

An Analysis of Advanced Surface Modification Techniques for High-Precision Optics

This document analyzes the technical requirements and outcomes detailed in the research paper “Characteristics of diamond turned NiP smoothed with ion beam planarization technique.” We connect the demonstrated surface finishing capability—achieving ultra-low roughness crucial for EUVL and X-ray optics—to 6CCVD’s core MPCVD Single Crystal Diamond (SCD) and Polycrystalline Diamond (PCD) product lines.

Executive Summary

This paper validates Ion Beam Planarization (IBP) as a highly effective final processing step for achieving ultra-smooth surfaces necessary for high-performance optical systems.

Core Value Proposition: IBP successfully mitigates mid-spatial frequency turning marks on diamond-turned surfaces, crucial for optics operating at extreme ultraviolet (EUVL) and X-ray wavelengths.
Methodology: IBP utilizes a sacrificial photoresist layer (planarizing step) followed by precise Ion Beam Etching (IBE) at a matched removal rate angle (35°) to transfer the smooth resist surface to the substrate.
Roughness Reduction: A single IBP step reduced initial surface roughness (R_q) by 30% to 40% across various samples, regardless of initial mark amplitude (10 nm to 60 nm).
Exponential Improvement: Repeating the IBP process five times reduced the surface roughness exponentially, achieving an overall roughness reduction by one order of magnitude.
Key Achievement: Final surface roughness was reduced from an initial R_q ~ 6.5 nm to an exceptional final R_q ~ 0.7 nm (RMS) on diamond-turned substrates.
Material Efficiency: While thicker resist layers yield marginally smoother surfaces, the paper concludes that optimization of the coating process (viscosity, thickness uniformity) and repetition of IBP steps are more efficient than using excessively thick sacrificial films.

Technical Specifications

The following table summarizes the quantified results and parameters critical for replicating and extending this IBP process for diamond materials.

Parameter	Value	Unit	Context
Initial Surface Roughness (R_q Range)	4.99 to 19.91	nm	Diamond turned NiP samples before IBP
Final Achieved Surface Roughness (R_q)	0.70	nm	Sample A after 5 IBP steps (10% of initial)
Roughness Reduction (1^st Step)	30% - 40%	%	Reduction rate across all tested samples
Spatial Wavelength Range Tested (λ)	1.5 to 25	µm	Mid spatial frequency turning marks
Turning Mark Depth Range (Amplitude)	10 to 60	nm	Initial depth of surface defects
Ion Beam Planarization Angle (θ)	35	°	Angle for matched etching rates (NiP/Resist)
Beam Voltage (V_B)	700	V	Kaufman Ion Source Parameter
Beam Current (I_B)	70	mA	Kaufman Ion Source Parameter
Resist Etching Rate (Approx.)	10 to 12	nm/min	Rate achieved at planarization angle
Resist Thicknesses Tested	240, 380, 700	nm	Photoresist AZ1505
Typical Etching Time (Single Step)	30 + 5	min	Time to remove resist + 5 min additional etch

Key Methodologies

The IBP technique relies on precise preparation and controlled ion beam processing to ensure effective transfer of the planarized resist surface to the substrate.

Substrate Preparation: Electroless plated NiP samples were prepared via diamond turning, varying feed rate, cut depth, and tip radius to generate controlled spatial frequencies (1.5 µm to 25 µm) and mark depths (10 nm to 60 nm).
Resist Coating: Photoresist AZ1505 was applied via spin-coating (or spray/spin coating) and subsequently dried using an electric heater. Resist thickness was precisely controlled, typically around 300 nm for multi-step experiments.
Sample Mounting: Samples were mounted on a water-cooled, rotating substrate stage, maintained at room temperature (RT).
Ion Beam Etching Setup: A two-grid Kaufman-type ion source was used, employing Argon (Ar) as the process gas. The grids had a 180 mm aperture.
Process Parameters: The beam voltage was set to 700 V and the beam current to 70 mA.
Planarization Angle: The incident angle of the ion beam was set to 35° relative to the sample normal, chosen because this angle provided nearly identical etching rates for the photoresist and the NiP substrate (approximately 10-12 nm/min).
Etching Procedure: Etching time was determined by the resist thickness, typically 30 minutes to remove the resist, followed by an additional 5 minutes of etching into the substrate.
Multi-Step Repetition: The coating and etching steps were repeated up to five times to achieve exponential reduction in surface roughness and mitigate mid-spatial frequency features.

6CCVD Solutions & Capabilities

6CCVD is uniquely positioned to supply the advanced diamond materials and precision finishing services required for high-performance optical applications, particularly those requiring the ultra-low roughness demonstrated through IBP.

Applicable Materials for Replication and Extension

To replicate or advance research requiring surfaces smoothed below R_q = 1 nm (e.g., for EUVL/X-ray components), 6CCVD recommends materials that offer inherent purity and hardness suitable for demanding environments.

6CCVD Material	Recommended Grade	Application Relevance
Single Crystal Diamond (SCD)	Optical Grade, High Purity (N < 1 ppb)	Ideal replacement for NiP/metal optics due to high thermal conductivity, extreme hardness, and superior optical transmission across broad spectrums (including UV). Used when final R_a < 1 nm is mandatory.
Polycrystalline Diamond (PCD)	Optical Grade, Fine Grain	Recommended for large-area optics (up to 125 mm wafers) where the size constraints of SCD are prohibitive. High-density PCD minimizes grain boundary effects, making it suitable for subsequent polishing or IBP refinement.
Boron-Doped Diamond (BDD)	Electrode/Sensor Grade	If the application involves high-performance electrochemical electrodes requiring ultra-smooth surfaces, BDD provides the necessary conductivity, and 6CCVD can customize the doping level (heavy/light).

Customization Potential

The IBP process highlights the need for precise dimensional control, large substrate sizes, and compatibility with subsequent processing like metal contacts. 6CCVD’s in-house capabilities directly address these engineering requirements:

Dimensional Flexibility: 6CCVD provides custom dimensions for PCD wafers up to 125 mm, crucial for scaling up optical components, and SCD plates up to 10mm.
Precision Thickness Control: We offer tailored diamond layer thicknesses from 0.1 µm to 500 µm (SCD/PCD) and substrates up to 10 mm, ensuring material compatibility for demanding thermal or mechanical loads.
Ultra-Polishing Services: While the paper used IBP on NiP, 6CCVD can provide Polished SCD with native roughness R_a < 1 nm, often bypassing the need for complex, repetitive chemical planarization steps on the diamond surface itself. We also offer PCD polishing down to R_a < 5 nm for inch-sized wafers.
Integrated Metalization: If the final application requires patterning or contact layers (e.g., for mounting or electrical/thermal management), 6CCVD offers internal metalization capabilities including deposition of Ti, Pt, Au, Pd, W, and Cu layers, ensuring clean, optimized interfaces.

Engineering Support

Achieving final surface quality below R_q = 1 nm requires iterative process development. 6CCVD’s in-house PhD material science team specializes in customizing CVD growth parameters to minimize surface defects and optimize crystal orientation, providing the best possible starting surface for post-processing techniques like IBP.

We offer detailed consultation on material selection (SCD vs. PCD), crystallographic orientation, and optimized surface preparation techniques essential for replicating demanding EUVL and X-ray optical projects.

For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly.

View Original Abstract

\n Background: Diamond turning is widely used in machining metals and semiconductors but the turning marks are incurred on machined components due to the mechanics of the technology. The marks are generally harmful to the systems comprising of the machined components. Therefore, the capability of ion beam planarization (IBP) to reduce turning marks of diamond turned metal surfaces was investigated using NiP as an example.\n Methods: The turning marks and thereby roughness was reduced by IBP with respect to different spatial wavelengths and amplitudes of turning marks. Different thickness of coating resist was also examined in order to find out the potential effects of resist thickness on the reduction of turning marks and roughness. Additionally, the effect of multiple planarization steps was also analyzed.\n Results: The spatial wavelength and depth of turning marks have only minor impact on the degree of surface roughness reduction. Thicker coating tends to achieve smoother surface after coating turned NiP while ion beam etching can keep surface roughness almost unchanged in our experiments. The surface roughness of diamond turned NiP drops exponentially with processing steps under experimented conditions. Using up to five IBP steps, the surface roughness can be reduced up to one order of magnitude (from Rq ~ 6.5 nm to Rq ~ 0.7 nm).\n Conclusions: IBP technique performs very well in reducing turning marks on diamond turned NiP surfaces. The surface roughness can be further improved by optimizing the properties of planarizing resist layer and coating processes to enhance the IBP technique as a final surface finishing technology.\n

Tech Support

Original Source

DOI: https://doi.org/10.1186/s41476-017-0057-5