Stress engineering of high-quality single crystal diamond by heteroepitaxial lateral overgrowth

At a Glance

Metadata	Details
Publication Date	2016-02-01
Journal	Applied Physics Letters
Authors	Y.-H. Tang, B. Golding
Institutions	Michigan State University
Citations	42
Analysis	Full AI Review Included

Stress Engineering and Advanced SCD Growth via Heteroepitaxial Lateral Overgrowth (ELO)

Analysis of AIP Applied Physics Letters 108, 052101 (2016) by Y.-H. Tang and B. Golding

Executive Summary

This paper presents a highly effective method for producing high-quality, low-stress Single Crystal Diamond (SCD) films via Microwave Plasma Chemical Vapor Deposition (MPCVD) using Heteroepitaxial Lateral Overgrowth (ELO). This technique solves critical constraints facing large-area heteroepitaxial diamond used in demanding applications like particle detection.

Superior Stress Mitigation: Utilizing a ductile, patterned metal layer (Au) on an Ir-buffered a-plane sapphire substrate, the method virtually eliminates thermal mismatch stress between the diamond film and the non-diamond substrate.
Ultra-Low Surface Stress Achieved: A biaxial in-plane stress of 0.00 ± 0.16 GPa was achieved on a 180 µm thick SCD film, confirmed by averaged Raman shift (reference frequency 1332.4 cm^-1). This contrasts sharply with the calculated interfacial stress of -4.55 GPa in non-ELO systems.
Effective Dislocation Filtering: The ELO mechanism blocks the propagation of threading dislocations, confining them primarily to the unmasked aperture regions. This results in an extremely low dislocation density in the overgrown areas.
High Structural Quality: Dislocation density in the overgrown regions was quantified to be below 10⁸ cm^-2, accompanied by a 2x to 3x reduction in Raman linewidth relative to the unmasked area.
Simplified Substrate Separation (Lift-Off): The inherent thermal expansion mismatch, accommodated by the Au layer, promotes the delamination of thick diamond films (e.g., 180 µm) at the diamond-Ir interface, simplifying the creation of freestanding, high-quality SCD plates without extensive processing.
6CCVD Relevance: 6CCVD’s capability to supply thick (up to 500 µm), custom-sized SCD wafers and provide advanced in-house metalization (Au, Ti, Ir layering) positions us perfectly to replicate or scale this stress-engineering technique for next-generation detectors and optics.

Technical Specifications

Parameter	Value	Unit	Context
Final Diamond Thickness	180	µm	For stress measurement (average surface stress)
ELO Growth Time	10+	h	Minimum exposure for lateral overgrowth
Initial Diamond Thickness (Seed Layer)	550	nm	Diamond film 1, grown on Ir buffer
Ir Buffer Layer Thickness	300	nm	Grown on a-plane sapphire
Mask Material	Au (on 4 nm Ti adhesion layer)	nm	Au thickness: 85 nm
Mask Stripe Width	8	µm	Periodic metal-masked region
Aperture Width	4	µm	Exposed diamond growth area
Mask Pitch	12	µm	Combined aperture and mask width
Dislocation Density (Overgrown)	< 10⁸	cm^-2	Verified by SEM etch pit distribution
Surface Biaxial Stress	0.00 ± 0.16	GPa	For 180 µm film, derived from Raman shift
Interfacial Thermal Stress (Non-ELO)	-4.55	GPa	Calculated maximum stress without ELO
Raman Linewidth Reduction	2 to 3	Factor	Reduced in overgrown regions vs. apertures
Lateral Growth Rate	0.4	µm/h	Used during the lateral overgrowth step

Key Methodologies

The experiment utilized a two-step MPCVD process incorporating photolithography and a metal decoupling layer to achieve high-quality ELO:

Substrate Preparation:
- A 300 nm epitaxial Ir (001) buffer layer was deposited onto a 1x1 cm² a-plane sapphire substrate using electron-beam or sputtering deposition at temperatures above 750 °C.
- The Ir layer was biased at -180 V dc at 700 °C for 1 h in a 2% CH₄/H₂ plasma (18 Torr) to achieve uniform, high-density diamond nucleation.
Initial Diamond Film Growth (Diamond 1):
- Subsequent growth occurred at 650 °C for 3 h, forming a continuous, smooth, 550 nm SCD film (Diamond 1). No additional gases were intentionally added.
Patterning and Metal Mask Deposition:
- A two-step lithography process was used on the 550 nm SCD film.
- A standard photomask created 8 µm wide Au stripes along the diamond (110) direction on a 12 µm pitch, leaving 4 µm diamond apertures exposed.
- 4 nm Ti (adhesion layer) and 85 nm Au were thermally evaporated at room temperature and then lifted off.
Heteroepitaxial Lateral Overgrowth (ELO) (Diamond 2):
- The masked substrate was placed back into the MPCVD reactor.
- Growth was initiated at 750 °C - 50 °C lower than the final growth temperature to protect the Au mask from plasma damage.
- The growth gas mixture was 2%-3% CH₄/H₂ at a pressure of 58-60 Torr.
- Slow lateral growth (0.4 µm/h) was maintained until coalescence, reaching a thickness of approximately 10 µm (Diamond 2, where ELO overgrew the Au mask). Thicker films up to 180 µm were subsequently grown under similar conditions.

6CCVD Solutions & Capabilities

6CCVD is an expert provider of MPCVD diamond solutions, specializing in the materials and processes required to replicate and advance this stress-engineered ELO technique. Our capabilities directly address the complexity and scalability challenges highlighted in this research.

Applicable Materials

To replicate or extend this research for thicker, detector-grade applications (up to 1-2 mm mentioned in the paper), 6CCVD recommends:

Optical Grade SCD (Single Crystal Diamond): For high-quality, ultra-low defect films requiring exceptional purity, essential for high-efficiency energetic particle detection.
Custom Substrate Materials: We can source and process the Ir-buffered sapphire necessary for heteroepitaxial nucleation, or work with clients on alternative SCD/PCD seed materials if subsequent homoepitaxial growth is desired.
Heavy Boron Doped PCD/SCD (BDD): If the application requires electrically conductive diamond detectors or electrodes, we offer precise BDD materials with controlled doping profiles, suitable for integration into complex devices.

Customization Potential

The success of the ELO method is entirely dependent on precise patterning and the metal decoupling layer. 6CCVD provides comprehensive services essential for mastering this technique:

Requirement from Paper	6CCVD Capability	Value Proposition
Thick Film Requirements (1-2 mm)	SCD Thickness up to 500 µm; Substrates up to 10 mm	We exceed the thickness achieved in the paper (180 µm), enabling true detector-grade SCD plates.
Masking Layer Deposition (Ti/Au)	Internal Metalization (Au, Pt, Pd, Ti, W, Cu)	We provide high-purity, custom-patterned metal layers crucial for the ELO process and subsequent stress accommodation.
Precise Patterning & Mask Geometry	Custom Laser Cutting and Micromachining	We offer precise patterning of the diamond surface for aperture and mask creation, ensuring the required 12 µm pitch resolution.
Surface Finish (Post-Growth)	Precision Polishing (Ra < 1 nm on SCD)	We eliminate surface roughness issues noted in the paper, providing epi-ready surfaces for device integration or subsequent homoepitaxy.
Large Area Scale-Up	Custom Dimensions up to 125 mm plates	While the paper focused on 7.5 mm diameter regions, 6CCVD can scale production to industry-standard wafer sizes, facilitating commercial viability.
Global Logistics	DDU Default Global Shipping (DDP Available)	Secure and timely delivery of sensitive CVD diamond materials worldwide.

Engineering Support

The use of a metal decoupling layer to manage thermal stress and promote substrate lift-off is an advanced technique. 6CCVD’s in-house PhD team can assist with material selection, process optimization, and substrate transfer strategies for similar energetic particle detector or high-power electronic projects where ultra-low stress SCD is paramount. Our team supports clients in defining ideal MPCVD recipes and layer stacks (Ir/Ti/Au) tailored to specific stress-engineering goals.

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

View Original Abstract

Here, we describe a method for lateral overgrowth of low-stress single crystal diamond by chemical vapor deposition (CVD). The process is initiated by deposition of a thin (550 nm) (001) diamond layer on Ir-buffered a-plane sapphire. The diamond is partially masked by periodic thermally evaporated Au stripes using photolithography. Lateral overgrowth of the Au occurs with extremely effective filtering of threading dislocations. Thermal stress resulting from mismatch of the low thermal expansion diamond and the sapphire substrate is largely accommodated by the ductile Au layer. The stress state of the diamond is investigated by Raman spectroscopy for two thicknesses: at 10 μm where the film has just overgrown the Au mask and at 180 μm where the film thickness greatly exceeds the scale of the masking. For the 10-μm film, the Raman linewidth shows spatial oscillations with the period of the Au stripes with a factor of 2 to 3 reduction relative to the unmasked region. In a 180-μm thick diamond film, the overall surface stress was extremely low, 0.00 ± 0.16 GPa, obtained from the Raman shift averaged over the 7.5mm diameter of the crystal at its surface. We conclude that the metal mask protects the overgrown diamond layer from substrate-induced thermal stress and cracking. Lastly, it is also responsible for low internal stress by reducing dislocation density by several orders of magnitude.

Tech Support

Original Source

DOI: https://doi.org/10.1063/1.4941291