Research on the Mechanical Failure Risk Points of Ti/Cu/Ti/Au Metallization Layer

At a Glance

Metadata	Details
Publication Date	2023-11-23
Journal	Crystals
Authors	Mingrui Zhao, Xiaodong Jian, Si Chen, Minghui Chen, Gang Wang
Institutions	Xiamen University of Technology, Ministry of Industry and Information Technology
Analysis	Full AI Review Included

Technical Documentation & Analysis: Diamond Bonding Reliability

Executive Summary

This research successfully demonstrates a robust, room-temperature, low-pressure bonding technique for integrating high-thermal-conductivity diamond heat spreaders onto silicon substrates using a Ti/Cu/Ti/Au metal modification layer. The findings provide critical insights into the mechanical reliability of diamond-based thermal management assemblies.

Robust Bonding Achieved: Utilized Electron Beam Evaporation and low-pressure bonding to achieve strong adhesion between diamond and silicon using a Ti/Cu/Ti/Au (5/300/5/50 nm) stack.
High Shear Strength: Achieved a maximum shear bonding strength of 48.51 MPa at a bonding pressure of 6 MPa, confirming the strength of the bonding layer surpasses that of the bulk silicon substrate.
Interface Quality: Deposited layers exhibited excellent uniformity and ultra-low surface roughness (Ra < 2.23 nm on the diamond side), suitable for high-quality bonding.
Critical Failure Point Identified: Experimental and simulation results pinpointed the thin 5 nm Ti layer as the weakest link, suffering from severe lattice distortion and accumulating fatigue damage.
Thermal-Mechanical Reliability: Finite Element Analysis (FEA) confirmed that the large Coefficient of Thermal Expansion (CTE) mismatch between the Cu layer and the Ti layer concentrates stress, leading to a minimum predicted fatigue life of 21,340 cycles.
6CCVD Value Proposition: 6CCVD specializes in providing the high-purity Single Crystal Diamond (SCD) substrates and custom multi-layer metalization required to replicate and optimize this critical thermal management technology.

Technical Specifications

Parameter	Value	Unit	Context
Diamond Material Used	Single Crystal Diamond (SCD)	N/A	High thermal conductivity substrate
Diamond Dimensions	1 x 1 x 0.3	mm³	Component size
Substrate Material	Silicon (Si)	N/A	Wafer used for bonding
Metallization Stack	Ti/Cu/Ti/Au	nm	5/300/5/50 nm layer thicknesses
Total Film Thickness	360	nm	Total thickness of the modification layer
Bonding Pressure Range Tested	1 to 6	MPa	Applied during room-temperature bonding
Maximum Bonding Strength	48.51	MPa	Achieved at 6 MPa bonding pressure
Diamond Surface Roughness (Ra)	0.56 to 2.23	nm	Measured across D-Ti to D-Ti/Cu/Ti/Au layers
Si Surface Roughness (Ra)	0.52 to 2.17	nm	Measured across Si-Ti to Si-Ti/Cu/Ti/Au layers
Simulated Temperature (Si Block)	80	°C	High-temperature thermal load
Simulated Temperature (Diamond Block)	22	°C	Low-temperature thermal load
Minimum Fatigue Life	21,340	Cycles	Predicted under coupled thermal-mechanical load
Ti Layer Lattice Distortion	~0.3	° Shift	Observed in XRD (Ti (200) peak shift)

Key Methodologies

The research employed a precise sequence of deposition, bonding, and advanced characterization techniques to analyze the mechanical failure points:

Substrate Preparation: Diamond and silicon wafers were ultrasonically cleaned using isopropyl alcohol, acetone, and ethanol, followed by rinsing with ultra-pure water.
Thin-Film Deposition: Metal modification layers (Ti/Cu/Ti/Au) were deposited onto both substrates using Electron Beam Evaporation (Oxford Vapour Station 4).
Deposition Environment: The process was conducted at 25 °C under a low-pressure nitrogen atmosphere (10^-5 Pa).
Room-Temperature Bonding: Samples were bonded using a Fineplacer Lambda-controlled chip bonder at pressures ranging from 1 MPa to 6 MPa.
Interface Characterization: Non-destructive testing included X-ray inspection (XD7600NT) and Scanning Acoustic Microscopy (SAM, D9500) to confirm the absence of voids or delamination.
Failure Analysis: Fracture surfaces were analyzed using Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS) to determine the exact location and mechanism of failure (brittle fracture in Si and detachment at the Ti/Cu interface).
Crystallographic Analysis: X-ray Diffraction (XRD, Rigaku MinFlex600) was used to measure lattice distortion, confirming significant stress accumulation in the thin Ti layer.
Fatigue Simulation: Coupled thermal-mechanical fatigue analysis was performed using ANSYS Mechanical and nCode DesignLife, simulating alternating loads based on temperature cycling (80 °C/22 °C) and 10 MPa shear stress.

6CCVD Solutions & Capabilities

This research highlights the critical role of high-quality diamond substrates and precisely controlled metalization layers in achieving reliable thermal management assemblies. 6CCVD is uniquely positioned to supply the materials and engineering expertise required to replicate, scale, and optimize this bonding technology for commercial applications.

Applicable Materials

To replicate or extend this high-reliability bonding research, 6CCVD recommends the following materials:

Optical Grade Single Crystal Diamond (SCD): Required for applications demanding the highest thermal conductivity (up to 2000 W/m·K) and structural integrity, matching the material used in the study. 6CCVD supplies SCD plates up to 500 µm thick with superior crystal quality.
High-Purity Polycrystalline Diamond (PCD): For cost-sensitive or larger-area applications, 6CCVD offers PCD wafers up to 125 mm in diameter, providing excellent thermal properties and mechanical robustness.

Customization Potential

The study’s success relies heavily on precise material dimensions and controlled thin-film deposition. 6CCVD’s in-house capabilities directly address these requirements:

Research Requirement	6CCVD Customization Capability	Optimization Focus
Specific Metal Stack (Ti/Cu/Ti/Au)	Custom Metalization Services: We offer precise deposition of Ti, Cu, and Au layers. We can replicate the 5/300/5/50 nm stack or engineer new stacks using alternative metals (e.g., Pt, Pd, W) to mitigate CTE mismatch and improve fatigue life.	Stress Mitigation: Adjusting the thickness of the Ti adhesion layer or incorporating buffer layers (e.g., Pt) to reduce lattice distortion and compressive stress from the Cu layer.
Ultra-Smooth Interface (Ra < 2.23 nm)	Precision Polishing: Our SCD polishing achieves surface roughness of Ra < 1 nm. Our inch-size PCD polishing achieves Ra < 5 nm. This ensures optimal surface activation and bonding strength, exceeding the requirements of the study.	Enhanced Bonding Strength: Minimizing surface defects and roughness is crucial for achieving high shear strength (48.51 MPa).
Small Component Size (1x1 mm)	Custom Dimensions & Fabrication: While the study used small samples, 6CCVD can supply SCD or PCD wafers up to 125 mm in diameter. We offer precision laser cutting and dicing services for producing custom components down to sub-millimeter sizes.	Scalability: Providing large-area substrates for high-volume manufacturing of diamond heat spreaders.

Engineering Support

The research clearly identifies the thermal expansion coefficient mismatch between the Cu and Ti layers as the primary cause of mechanical failure and reduced fatigue life. 6CCVD’s in-house PhD team specializes in material science and thermal management engineering. We offer consultation services to help clients:

Optimize Metal Stacks: Design multi-layer metalization schemes that minimize internal stresses and maximize reliability under thermal cycling (e.g., for power electronics and high-power GaN HEMT applications).
Material Selection: Advise on the optimal diamond grade (SCD vs. PCD) and thickness for specific thermal loads and mechanical constraints.
Failure Analysis: Provide material characterization support (e.g., XRD, AFM) to validate bonding interfaces and predict long-term reliability.

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

View Original Abstract

The cohesive performance and durability of the bonding layer with semiconductor substrates are of paramount importance for realizing the high thermal conductivity capabilities of diamond. Utilizing electron beam evaporation and the room-temperature, low-pressure bonding process, robust adhesion between diamonds and silicon substrates has been achieved through the application of the metal modification layer comprised of Ti/Cu/Ti/Au (5/300/5/50 nm). Characterization with optical microscopy and atomic force microscopy reveals the uniformity and absence of defects on the surface of the deposited layer. Observations through X-ray and scanning acoustic microscopy indicate no discernible bonding defects. Scanning electron microscopy observation and energy-dispersive spectroscopy analysis of the fracture surface show distinct fracture features on the silicon substrate surface, indicating that the bonding strength of the Ti/Cu/Ti/Au metallization layer surpasses that of the base material. Furthermore, the fracture surface exhibits the presence of Cu and trace amounts of Ti, suggesting that the fracture also occurs at the interface between Ti and Cu. Characterization of the metal modification layer using X-ray diffraction reveals significant lattice distortion in the Ti layer, leading to noticeable stress accumulation within the crystalline structure. Thermal-mechanical fatigue simulations of the Ti/Cu/Ti/Au metal modification layer indicate that, owing to the difference in the coefficient of thermal expansion, the stress exerted by the Cu layer on the Ti layer results in the accumulation of fatigue damage within the Ti layer, ultimately leading to a reduction in its strength and eventual failure.

Tech Support

Original Source

DOI: https://doi.org/10.3390/cryst13121625

References

2023 - Chip-level thermal management in GaN HEMT: Critical review on recent patents and inventions [Crossref]
2020 - Application progress of diamond heat dissipation substrate in GaN-based power devices
2023 - Heterogeneous integration of high-quality diamond on aluminum nitride with low and high seeding density [Crossref]
2020 - Surface activated bonding of SiC/diamond for thermal management of high-output power GaN HEMTs [Crossref]
2021 - Diamond Schottky barrier diodes fabricated on sapphire-based freestanding heteroepitaxial diamond substrate [Crossref]
2021 - Two-inch high-quality (001) diamond heteroepitaxial growth on sapphire (1120) misoriented substrate by step-flow mode [Crossref]
2020 - Integration of GaN and diamond using epitaxial lateral overgrowth [Crossref]
2023 - Van der waals epitaxial GaN thin films on polycrystalline diamond substrate