Large Area CVD Diamond Window Uniformity
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Solution Overview
Problem
Current methods for fabricating polycrystalline diamond windows using chemical vapour deposition (CVD) face challenges in achieving large, thick windows with extremely high optical quality due to defects and impurities like nitrogen, silicon, and non-diamond carbon, which affect the material's optical and thermal performance, and result in lower quality at the periphery and increased defects with thickness.
Innovation Solution
A microwave plasma reactor configuration with precise design tolerances and optimized process conditions, including high gas flow rates, high microwave power, and controlled nitrogen concentration, along with a gas inlet nozzle array to enhance atomic hydrogen distribution, addresses the issues of impurity incorporation and growth rate variations, enabling the production of large, thick polycrystalline diamond windows with uniform high optical quality across the entire area.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conventional CVD methods are used to fabricate polycrystalline diamond windows, then diamond material can be deposited on substrate, but defects and impurities (nitrogen, silicon, non-diamond carbon) are incorporated into the material reducing optical quality
Solution Approach 1:
The patent applies parameter changes by optimizing microwave power (20-40 kW), gas flow rates (total flow 2000-15000 sccm with H2 97-99%, CH4 0.1-1%), and substrate temperature (700-950°C) to control the deposition process. These parameter adjustments reduce impurity incorporation while maintaining high growth rates, achieving optical quality diamond windows with absorption coefficients ≤0.2 cm⁻¹
Solution Approach 2:
The patent employs periodic action through multi-stage deposition processes including nucleation, initial growth, and final growth stages with varying gas compositions and flow rates. This periodic variation in process conditions allows selective etching of non-diamond carbon by atomic hydrogen while promoting diamond growth, thereby reducing impurities
2Length of stationary object
If CVD growth continues to increase thickness, then larger thick windows can be produced, but defects increase with thickness reducing optical quality
Solution Approach 1:
The patent applies preliminary action by implementing a controlled nucleation stage followed by staged growth processes. The substrate is first prepared with controlled nucleation, then growth proceeds in stages with optimized parameters for each stage. This preliminary structuring ensures uniform grain structure and reduces defect formation even as thickness increases to 1.3 mm or more
Solution Approach 2:
The patent applies local quality by optimizing deposition conditions to achieve uniform optical properties throughout the bulk material. Through controlled gas flow distribution and microwave power coupling, the process maintains consistent impurity levels and grain structure from surface to surface, enabling large thick windows with uniform quality across the entire volume
3Area of stationary object
If CVD process is applied to large area substrates, then large area windows can be produced, but quality at periphery decreases due to growth rate variations
Solution Approach 1:
The patent applies dynamics by using a gas inlet nozzle array that can be positioned and oriented to optimize gas distribution across large substrate areas. The nozzles are arranged to create uniform atomic hydrogen flux across the entire substrate surface, compensating for edge effects and maintaining consistent growth rates and quality from center to periphery of large area windows
Solution Approach 2:
The patent uses atomic hydrogen as an intermediary species generated by microwave plasma dissociation of molecular hydrogen. This intermediary provides selective etching of non-diamond carbon while promoting diamond growth uniformly across large areas. The gas inlet nozzle array serves as another intermediary to distribute reactants uniformly, ensuring consistent optical quality across the entire window area
4Productivity
If high growth rates are achieved through increased power and gas flow, then productivity increases, but impurity incorporation and defects increase
Solution Approach 1:
The patent applies parameter changes by optimizing the balance between microwave power (20-40 kW), gas flow rates, and substrate temperature to achieve high growth rates while maintaining optical quality. The specific parameter window (H2 97-99%, CH4 0.1-1%, total flow 2000-15000 sccm, temperature 700-950°C) enables growth rates suitable for producing thick windows while keeping impurity incorporation low through enhanced atomic hydrogen etching activity
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The solution achieves polycrystalline diamond windows with enhanced tensile rupture strength, low absorption coefficients, high thermal conductivity, and reduced impurity levels, meeting the requirements for high-power laser applications and radiation-resistant windows by ensuring uniform quality across the entire window area.
Implementation Method 1
Source gases including a carbon source and molecular hydrogen are fed into the plasma reactor vessel and can be activated by the standing microwave to form a plasma in high field regions
Implementation Method 2
Chemical vapour deposition (CVD) processes for synthesis of diamond material are now well known in the art
Implementation Method 3
Atomic hydrogen is essential to the process because it selectively etches off non-diamond carbon from the substrate such that diamond growth can occur
Implementation Method 4
If a suitable substrate is provided in close proximity to the plasma, reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon
Implementation Method 5
reactive carbon containing radicals can diffuse from the plasma to the substrate and be deposited thereon
Data Source
Figure 1
Figure 2(a)~2(b)
Figure 3
AI summary
A polycrystalline chemical vapour deposited (CVD) diamond wafer comprising: a largest linear dimension equal to or greater than 70mm; a thickness equal to or greater than 1.3 mm; and one or both of the following characteristics measured at room temperature (nominally 298 K) over at least a central area of the polycrystalline CVD diamond wafer, said central area being circular, centred on a central point of the polycrystalline CVD diamond wafer, and having a diameter of at least 70% of the largest linear dimension of the polycrystalline CVD diamond wafer: an absorption coefficient ≤ 0.2cm-1 at 10.6 μm;and a dielectric loss coefficient at 145 GHz, of tan δ ≤ 2 x 10-4.