Invited Paper TF-TuA7
Mechanical Properties of ALD Thin Films

Tuesday, November 1, 2011, 4:00 pm, Room 107

Atomic Layer Deposition (ALD) is recognized as the preferred method to deposit technologically important thin films of novel high-k dielectric metal oxides or semiconducting metal oxides for CMOS and MEMS technology with Angstrom accuracy. Large bodies of extensive studies exist on the electrical characterization of ALD films, however, there exists a lack of systematic studies regarding the mechanical properties of ALD grown thin films. Elevated temperatures cause phase changes in many ALD metal oxide films, which affect the mechanical properties and surface morphologies. Little is known about the impact of those phase changes on the nanomechanical properties of ALD HfO₂. Phase change of ALD HfO₂ impacts the mechanical and electrical properties of high-k dielectric gate insulators depending on whether a Gate First or Gate Last process integration has been adopted. Nanoindentation is the most appropriate testing mechanism that accurately investigates the mechanical properties of extremely thin film specimens such as microcrystalline ALD thin films. Nanoindentation testing was conducted to investigate the impact of the different phase changes of HfO­₂ on the mechanical properties. We have deposited ALD HfO₂ at low temperature and measured the mechanical properties of the various phase changes of HfO₂ following various thermal annealing cycles. After crystallization by annealing in a rapid thermal annealing (RTA) chamber, the modulus was found to decrease from 370±20GPa to 240±20GPa as the HfO₂ films transition from amorphous to polycrystalline structure past the phase change transition temperature of 600°C. Similarly, the hardness measurements reveal a high value of 18±1GPa for amorphous HfO₂ films and a decrease to 15±1GPa following the transition temperature to polycrystalline HfO₂ films.

Piezoelectric films such as ALD ZnO are finding applications in microelectromechanical systems (MEMS), piezoelectric transducers and oscillators, micro-resonators, gyroscopes, and energy harvesters. Since piezoelectricity of ZnO involves internal generation of electrical charge resulting from an applied mechanical force deforming the static structure of the ZnO crystals, studying the mechanical properties of novel ALD thin films of ZnO is important for these technical applications. Obtaining a better understanding of the structural and mechanical properties of novel ALD ZnO films is essential to improve key performance parameters of MEMS micro-devices.

An overview of the measured mechanical properties of selected ALD thin films will be presented. The mechanical properties of ALD thin films differ significantly from published values of bulk material.