Research on Mechanical Properties and Reliability of AlN Thin Films +

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Aluminum nitride(AlN), as a III-V compound semiconductor material, has excellent properties such as stable wurtzite structure, wide bandgap (about 6.2eV), high electrical resistivity (d ₃₃ =5.3 pm/V), high resistivity (10¹¹– 10¹⁴Ω· cm), and high thermal conductivity (320 W/m·K). These characteristics make it widely applicable in fields such as microelectromechanical systems, bulk acoustic resonators, energy harvesters, and ultraviolet optoelectronic devices. However, although its electrical and piezoelectric properties have been widely studied, the mechanical properties of AlN thin films, especially fracture strength, Young’s modulus, and fatigue behavior, which are crucial for their reliability in devices, have not been fully explored.

1. Testing Methods and Challenges for Mechanical Properties of AlN Thin Films

Traditional micro nano mechanical testing methods such as nanoindentation, micro bending, micro stretching, and micro compression have significant limitations when testing high hardness and thin films such as AlN. For example, nanoindentation is susceptible to substrate effects, film cracking, and phase transitions, while micro bending and micro compression suffer from issues such as dislocation accumulation, FIB damage, and alignment difficulties.

In contrast, the bulge test method has been proven to be an effective means of evaluating the mechanical properties of AlN films due to its advantages such as no need for precise alignment, high sample preparation efficiency, and the ability to preserve the original stress state of the film. This method measures the maximum deflection of the film under pressure and fits the residual stress σ ₀ and Young’s modulus E using the following formula:

In the formula, c1 and c2 are geometric constants, t is film thickness, a is film radius, and v is Poisson’s ratio (taken as 0.207).

2. Effects of Different Deposition Methods on AlNMechanical Properties

Ö sterlund et al. compared the effects of sputtering, metal organic vapor phase epitaxy(MOVPE), and atomic layer deposition(ALD), on the mechanical properties of AlN thin films prepared.

Fig. 1 XRD diffraction patterns of AlN thin films prepared by different deposition methods: (a), (b) sputtered, (c) MOVPE, (d) ALD. The MOVPE film exhibits the sharpest (002) diffraction peak, indicating the best crystalline quality

The results show that:

MOVPE film has the best crystal quality due to its high deposition temperature (1085°C). Its (002) diffraction peak has the smallest full width at half maximum (0.35°), the highest Young’s modulus (346GPa), and the highest fracture strength (2.76GPa).

Sputtered films exhibit moderate performance, with a Young’s modulus of approximately 335-343GPa and a fracture strength of 1.42-1.54GPa.

ALD thin films have poor crystallinity, the lowest Young’s modulus (257GPa), and a fracture strength of only 0.61GPa.

These results validate the research hypothesis that films with higher crystalline quality have better mechanical properties and reliability.

Fig. 2 Thickness, residual stress, Young’s Modulus, and fracture strength of AlN thin films tested

3. Control of Process Parameters on the Properties of Sputtered AlN Thin Films

Talukder et al. systematically studied the influence of pulsed DC reactive sputtering process parameters on the structure and properties of AlN thin films and found that:

Process pressure: Lowering the pressure (such as 1mTorr) can increase the target voltage, enhance the kinetic energy of deposited particles, promote (002) orientation growth, thereby increasing the Young’s modulus (312GPa) and reducing surface roughness (1.8nm).

Fig. 3 Relationship between Young’s modulus of AlN thin film and process pressure

Nitrogen concentration: Increasing the N₂ concentration to 100% can enhance the (002) orientation, but has no significant effect on the Young’s modulus.

Pulse frequency and duty cycle: At 100kHz and 80% duty cycle, the film exhibits optimal crystallinity, the lowest roughness (1.2nm), and the highest Young’s modulus (335GPa), approaching the values of bulk AlN.

Fig. 4 Variation of Young’s modulus of AlN thin film with pulse frequency

Scanning electron microscopy images show that the AlN film deposited under optimized conditions exhibits a typical columnar growth structure.

Fig. 5 SEM cross-sectional image of 1μm thick AlN film deposited under optimized process parameters

4. Fatigue Behavior and Reliability Analysis of AlN Thin Films

Ö sterlund et al. conducted fatigue testing on sputtered AlN thin films for the first time, and after 10000 cycles under a cyclic load of 83% fracture strength, no significant changes in residual stress, Young’s modulus, or fracture strength were observed. The Weibull distribution shows that the strength of the sample slightly decreases in the high stress region after fatigue, indicating that cyclic loading may lead to slight expansion of micro defects, but overall AlN exhibits good fatigue resistance.

Fig. 6 Weibull distribution of fracture strength of AlN thin films prepared by sputtering, MOVPE, and ALD

The mechanical properties of AlN thin films strongly depend on their microstructure, which is significantly influenced by deposition methods and process parameters. MOVPE and optimized sputtering process can prepare high-strength and high modulus AlN thin films, which are suitable for MEMS devices with high reliability requirements. Although ALD thin films have good uniformity, their mechanical properties are poor and they are suitable for non structural functional layers. The bulge test method provides a reliable means for evaluating the mechanical properties of thin films, and future research can further explore a wider range of fatigue mechanisms and interface defect control strategies.

Whether you need AlN wafer for research or for industrial applications, please contact us email at [email protected] and [email protected].

 

References:

1. Österlund, E., Kinnunen, J., Rontu, V., Torkkeli, A., & Paulasto-Kröckel, M. (2019). Mechanical properties and reliability of aluminum nitride thin films. Journal of Alloys and Compounds, 772, 306-313.
2. Talukder, A. A., Baule, N., Steinhorst, M., Shrestha, M., Fan, Q. H., & Schuelke, T. (2022). Pulsed direct-current reactive sputtering of high Young’s modulus [002] oriented aluminum nitride thin films. Thin Solid Films, 751, 139239.