Direct Deposition of CVD Diamond Layers on Top of GaN Membranes

At a Glance

Metadata	Details
Publication Date	2020-12-30
Authors	Tibor Izsák, G. Vanko, Milan Držík, Stephan Kasemann, Johann Zehetner
Institutions	Czech Academy of Sciences, Institute of Physics, International Laser Center
Citations	2
Analysis	Full AI Review Included

Executive Summary

This research investigates the technological challenges associated with depositing Chemical Vapor Deposition (CVD) diamond layers directly onto Gallium Nitride (GaN) membranes to enhance thermal management and mechanical robustness in MEMS devices.

Core Value Proposition: Integrating diamond (high thermal conductivity) onto GaN MEMS significantly suppresses self-heating effects and improves device performance in high-temperature, harsh environments.
Mechanical Enhancement: Diamond coating dramatically increases the mechanical load tolerance of GaN membranes, boosting pressure resistance from 30 kPa (pure GaN) to 2.2 MPa.
Primary Challenge (Stress): Front-side deposition causes severe membrane wrinkling and bulging due to the mismatch in the Thermal Expansion Coefficient (TEC) between GaN and diamond.
TEC Mismatch: At room temperature (RT), the TEC of GaN is approximately 4x higher than that of diamond, leading to high intrinsic stress as the structure cools from the deposition temperature (~480 °C).
Thermal Deflection: White light interferometry confirmed that pure GaN membranes exhibit significant thermal bending, reaching a deflection of ~10 µm at 300 °C.
Proposed Solution: The study concludes by proposing an optimized technological procedure to achieve dual-sided diamond coating with minimized wrinkling and residual stress, addressing the current front-side deposition issues.

Technical Specifications

Parameter	Value	Unit	Context
Membrane Diameter	1	mm	GaN MEMS structure used in the study
Pure GaN Pressure Tolerance	30	kPa	Maximum pressure load before failure
Diamond-Coated Pressure Tolerance	2.2	MPa	Maximum pressure load (significant improvement)
Optimal Diamond Thickness (Simulated)	1	µm	Thickness required to double the maximum load
High Temperature TEC (GaN/Diamond)	4 to 5 x 10^-6	K^-1	Approximate TEC value for both materials above 500 °C
TEC Mismatch Ratio (RT)	4x higher	Ratio	GaN TEC compared to diamond TEC at room temperature
Deposition Temperature (T_dep)	~480	°C	Temperature where the bulging effect is maximized during reverse-heating
Membrane Deflection (300 °C)	~10	µm	Bulging measured on pure GaN membrane
Minimal Deflection Temperature	~100	°C	Temperature threshold where membrane deflection is minimal (~0.5 µm)

Key Methodologies

The study focused on front-side diamond deposition and subsequent analysis of thermal stress effects on the GaN membranes.

Diamond Deposition:
- Reactor Type: Ellipsoidal cavity microwave plasma chemical vapor deposition (CVD) reactor was used for diamond film growth.
- Target: Diamond was deposited on the front side (top) of 1 mm diameter GaN membranes.
- Nucleation: Previous work highlighted the importance of polymer-based nucleation methods for successful diamond growth on GaN, a technique likely adapted for this front-side deposition.
Stress and Deflection Analysis:
- Measurement Technique: Membrane deflection (bulging) was analyzed using white light interferometry.
- Bulging Method: The deflection was measured as a function of temperature (T) to quantify the thermal bending caused by the TEC mismatch.
- Thermal Cycling: Diamond-coated membranes were subjected to reverse-heating (from RT up to T_dep, ~480 °C) to observe the magnitude of the bulging effect as the temperature approached the stress-free deposition point.
- Observation: Wrinkles and thicker diamond layers were consistently observed at the membrane center, indicating non-uniform stress distribution and poor quality diamond outside the membrane area.

Commercial Applications

This technology is critical for developing robust, high-performance electronic and sensor systems that must operate reliably under extreme conditions.

High-Power/High-Frequency Electronics:
- Enabling efficient thermal management for GaN-based High Electron Mobility Transistors (HEMTs) and RF devices by integrating diamond heat spreaders directly onto the active semiconductor layer.
Harsh Environment Sensing (MEMS):
- Manufacturing highly durable pressure sensors and micro-diaphragms capable of surviving extreme mechanical loads (up to 2.2 MPa) and high operating temperatures, suitable for industrial process control or downhole drilling applications.
Aerospace and Automotive Systems:
- Creating robust sensors and control electronics that withstand rapid thermal cycling and high mechanical stress, where thermal buckling of traditional membranes is a failure risk.
Intrinsic Stress Engineering:
- The methodology developed for analyzing and mitigating thermal stress is crucial for the reliable fabrication of any composite diamond/semiconductor MEMS structure, ensuring long-term device stability.
Advanced Materials Integration:
- Paving the way for dual-sided diamond coating, which offers maximum thermal dissipation and mechanical protection for next-generation GaN microstructures.

View Original Abstract

We present technological issues in the deposition of diamond films on gallium nitride (GaN) membranes. Many wrinkles and thicker diamond layers were observed at the membrane center and poor quality diamond outside the membrane area. The deflection of the membranes was analyzed by a bulging method using white light interferometry. The membrane bending is discussed in the terms of temperature gradient and mismatch of thermal expansion coefficients of materials.

Tech Support

Original Source

DOI: https://doi.org/10.3390/proceedings2020056035

References

2015 - Diamond-coated three-dimensional GaN micromembranes: Effect of nucleation and deposition techniques [Crossref]