Generation, Suppression, Application (Manipulation) and Measurement of Stress in CVD Diamond Films

Generation, Suppression, Application (Manipulation)  and Measurement of Stress in CVD Diamond Films

I. Origins and Classification of Stress in CVD Diamond Films

1. Growth Stress

Originates from crystal defects such as dislocations, grain boundaries, and vacancies, as well as impurities (e.g., hydrogen, nitrogen) causing lattice distortion.

Different types and sizes of defects contribute differently to macro or micro stress.

Ion bombardment from plasma can also induce additional stress during the deposition process.

2. Thermal Stress

Mainly caused by the mismatch in the coefficient of thermal expansion (CTE) between the diamond film and the substrate, particularly during the cooling phase.

Example: Mo substrates have a better CTE match with diamond, making them suitable for thick-film growth.

Uneven temperature distributions (e.g., radial gradients) can also generate thermal stress.

3. Comprehensive Stress Models

Various theoretical models exist to explain the origins of stress in diamond films, including:

Surface tension model

Lattice mismatch model

Impurity model

Atomic peening model (relating to plasma-ion bombardment)

II. Influence of Different CVD Techniques on Film Stress

CVD Method      Characteristics   Stress Behavior

HFCVD (Hot Filament CVD)          

Large deposition area, but metal contamination and high defect density               

High tensile/compressive stress (±2–5 GPa)

MPCVD (Microwave Plasma CVD)            

No contamination, high-quality films, small area                

Lower stress, good tunability (bias voltage can switch between tensile and compressive stress)

DAPCVD (Direct Arc Plasma CVD)              

High energy, fast deposition, suitable for large-area thick films   Free-standing films still maintain low compressive stress (~0.67–1.2 GPa)

III. Effects of Stress on Diamond Films

Cracking and Delamination: Stress accumulation during deposition or cooling can cause cracking or delamination.

Mechanical Performance Degradation: Residual stress weakens the film’s strength and dimensional stability.

Deformation and Warping: Warpage due to stress mismatch can lead to poor bonding, especially when integrating with materials like GaN.

Functional Performance Loss: Stress can degrade carrier mobility, optical properties, and thermal conductivity.

IV. Stress Control and Mitigation Measures (to be elaborated in following sections)

Optimizing deposition parameters (temperature, pressure, gas flow, etc.)

Choosing substrates with well-matched thermal expansion coefficients (e.g., Mo is better than Si)

Using stress compensation techniques (e.g., adjusting substrate bias voltage)

Improving process steps (e.g., interlayer design, post-treatment methods)

V. Future Trends

Stress-Function Integration: Using controlled stress to tune bandgap or enhance thermal conductivity.

Large-Area, Low-Stress Film Growth Breakthroughs: Enabling practical applications like optical windows and thermal management systems.

Quantitative Stress Modeling: Moving from qualitative to quantitative understanding with accurate simulation and prediction capabilities.