Diamond growth dynamics in a constrained system

At a Glance

Metadata	Details
Publication Date	2024-06-17
Journal	Frontiers in Carbon
Authors	Shengyuan Bai, Ramón D. Díaz, Matthias Muehle, Elias Garratt, Sergey V. Baryshev
Institutions	Michigan State University, Fraunhofer USA
Analysis	Full AI Review Included

Executive Summary

This research focuses on optimizing Epitaxial Lateral Outgrowth (ELO) in Microwave Plasma-Assisted Chemical Vapor Deposition (MPACVD) to scale Single Crystal Diamond (SCD) wafer size.

Core Challenge Addressed: Overcoming the inherent constraint of traditional pocket holders that limits ELO to an exponential decay regime and promotes parasitic Polycrystalline Diamond (PCD) rim formation.
Mechanism Identified: ELO is achieved by manipulating the pocket geometry to perturb the methyl radical (CH₃) flux near the growing diamond corners, thereby controlling gas-to-solid phase transformation kinetics.
Traditional Results: Straight-wall pocket designs yielded ELO kinetics that strictly followed an exponential decay (Avrami model), where the lateral growth rate slowed significantly over time.
Novel Achievement: A new generation of smooth, angled pocket holders (37° and 60°) was introduced, successfully transitioning the growth mode from exponential decay to a constant-rate, linear outgrowth.
Optimal Performance: The best configurations (Pockets #2 and #4) achieved a “perfect” growth scenario where the average lateral growth rate (R_AL) equaled the vertical growth rate (R_V), resulting in a R_AL/R_V ratio of 1.
Scaling Impact: This linear ELO process effectively doubled the linear size and quadrupled the useful area of the homoepitaxially grown SCD crystal, demonstrating a scalable path toward 1-2 inch SCD wafer manufacturing.

Technical Specifications

Parameter	Value	Unit	Context
Reactor Types Used	B and C	N/A	MPACVD configurations (B: TM₀₁₃/TEM₀₀₁; C: TM₀₀₁/TM_01n)
Operating Pressure	240	Torr	Standard growth condition
H₂ Flow Rate	400	sccm	Standard growth condition
CH₄ Flow Rate	20 (or 24)	sccm	Standard growth condition
Microwave Frequency	2.45	GHz	Plasma generation
Input Microwave Power	2 to 3	kW	Standard operating range
Substrate Temp (Reactor C)	980 ± 10 to 1,020 ± 20	°C	Measured using optical pyrometer (ε=0.1)
Substrate Temp (Reactor B)	760 ± 10 to 800 ± 5	°C	Measured using optical pyrometer (ε=0.6)
Vertical Growth Rate (R_V) Range	12.7 to 27.5	µm/hr	Typical rates for SB/ACH/AI samples
Max R_AL/R_V (Exponential Decay)	1.65	N/A	Sample SB A (shortest growth time, 9 hr)
R_AL/R_V (Linear Outgrowth)	1	N/A	Pockets #2 and #4 (Constant rate achieved)
Traditional Pocket Gap (G) Range	1.25 to 1.77	mm	Distance between substrate edge and pocket wall
Novel Pocket Inner Depth	1	mm	Fixed depth to expose 200-400 µm of substrate
Optimal Pocket Angles (α)	37° and 60°	Degrees	Angles used in linear ELO designs (#2 and #4)

Key Methodologies

Substrate Preparation: Type Ib High Pressure High Temperature (HPHT) diamond seeds were rigorously cleaned using a sequence of acids (sulfuric, nitric, hydrochloric) and solvents (acetone, methanol, isopropyl alcohol).
Reactor Conditioning: Substrates were placed in the designated pocket holders, loaded into the MPACVD chamber, and held under high vacuum (~10^-5 Torr) for a minimum of 12 hours.
Initial Etch: A 10-minute to 1-hour H₂ plasma etch (2.8 kW, 400 sccm H₂, 950 °C) was performed to ensure a clean, fresh surface for homoepitaxial growth.
CVD Growth Parameters: Growth was conducted at 240 Torr pressure using a CH₄/H₂ gas mixture (typically 20/400 sccm). Substrate temperatures were maintained between 760 °C and 1,020 °C depending on the reactor configuration (B or C).
Pocket Holder Design Iteration:
- Traditional Pockets: Rectangular, straight-wall designs were used to establish the baseline exponential decay ELO kinetics, varying the initial gap distance (G) and depth.
- Novel Pockets: Angled pockets (α=37°, 60°) were introduced, featuring a fixed inner depth (1 mm) and step length (1 mm) to manipulate the methyl radical flux profile.
Kinetics Modeling: Lateral gain L(t) in traditional pockets was quantified and fitted using the Avrami equation, L(t) = A(1 - e^-t/τ), where A is the maximum gain and τ is the characteristic time.
Linear Growth Verification: The success of the angled pockets was verified by achieving a constant lateral growth rate (linear L(t) profile) and confirming that the ratio of average lateral growth rate to vertical growth rate (R_AL/R_V) was approximately 1.

Commercial Applications

The successful demonstration of constant-rate, linear ELO for SCD growth directly impacts industries requiring large, high-quality diamond wafers:

High Power Electronics: Diamond’s wide bandgap and exceptional thermal conductivity (highest known) are critical for next-generation high-power RF transistors, high-voltage switches, and power conversion modules used in electric vehicles and smart grids.
Quantum Technology: Large, high-purity SCD substrates are essential for creating dense arrays of Nitrogen-Vacancy (NV) centers, fundamental components for quantum computing, quantum sensing, and secure communication.
Thermal Management: Diamond’s superior heat dissipation capabilities are leveraged in advanced microelectronics and optoelectronics where high heat flux management is necessary.
Microwave and 5G/6G Communication: Enabling high-frequency, high-power amplifiers and devices that require materials capable of operating under extreme electrical and thermal loads.
Industrial Manufacturing: The development of a self-replicating, scalable ELO process provides a viable industrial pathway for producing 1-2 inch SCD wafers, significantly reducing the cost barrier for diamond adoption in semiconductor manufacturing.

View Original Abstract

Single crystal diamond (SCD) is the most promising future semiconductor. However, it has not been able to make much inroad into the microelectronics industry due to its major disadvantage of the wafer size. Among a few contender technologies, epitaxial lateral outgrowth (ELO) using microwave plasma-assisted chemical vapor deposition (MPACVD) has shown early promise toward lateral area gain during epitaxial growth. While promising, significant wafer area enhancement remains challenging. This study explores the growth dynamics of SCD in a constrained system—a pocket holder—whose effect is twofold: linear dimension and area enhancement and polycrystalline diamond (PCD) edge rim suppression. A series of pocket-type holder designs were introduced that demonstrated that the depth and substrate-to-wall distance are the major means for optimizing and enhancing lateral outgrowth while still suppressing the PCD rim. When taken together with reactor modeling, the pocket effect on the extent of ELO could be understood as directly manipulating and perturbing methyl radical flux near the growing diamond surface, thereby directly manipulating gas-to-solid phase transformation kinetics. Because it was further discovered that simple box-like pockets limit the ELO process to an exponential-decay scenario, a new generation of angled pockets was proposed that allowed boosting ELO to its fullest extent where a constant rate, linear, outgrowth was found. Our results indicate that ELO by MPACVD could become an industrial means of producing SCD at scale.

Tech Support

Original Source

DOI: https://doi.org/10.3389/frcrb.2024.1367715

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