Full length articleMechanical behavior of ultrathin sputter deposited porous amorphous Al2O3 films
Graphical abstract
Introduction
Amorphous Al2O3 thin films are used in a variety of applications owing to an advantageous combination of hardness, transparency and electrically-insulating properties [1]. Most Al2O3 films are porous either with closed pores when produced by sputtering, e-beam evaporation, atomic layer deposition (ALD), and anodic oxidation of Al in non-acidic electrolytes, or with open pores when processed by anodic oxidation in acidic electrolytes. Al2O3 films with closed porosity are used among others as wear-resistant coatings [2], as passivation layers in metal-oxide-semiconductor (MOS) devices [3] or in solar cells [4]. Al2O3 films with open porosity find application as surface coatings on Al in different kinds of devices including smartphones, storage devices, etc. [5], as templates for coatings with high surface area [6], or as templates for the synthesis of nanowires [7]. In all cases, the mechanical performances of the films must be sufficient to preserve functionality. For instance, sufficient bendability is required in flexible devices [8], while scratch resistance is a major concern for a wide range of coatings [9]. Another interest of studying Al2O3 layers stems from the use of thin Al layers in micro and nano devices. The Al is always covered by a native Al2O3 film due to the oxide high thermodynamic stability [10]. Due to the small thickness, the elastic behavior [11] as well as the plastic behavior [12], [13], [14] of the metallic layers is altered by the presence of the oxide, which can limit the mechanical strength of the functional layer in some applications [15]. Proper control and optimization of the mechanical properties of Al2O3 layers in general requires to characterize, understand and model the relationships between deposition conditions, structure and porosity, and the deformation and failure mechanisms. This general objective has been partially addressed in the literature.
The mechanical properties of Al2O3 films produced by vapor deposition have been characterized in several earlier studies. Koski et al. [16] studied the influence of the sputtering deposition parameters, including the cathodic voltage, sputtering gas pressure and substrate bias voltage, on the internal stress (measured ex-situ), density, nano-hardness and elastic modulus. The elastic modulus was found to decrease with increasing deposition pressure, but without clear underlying explanation. Surprisingly, the density was found to increase with deposition pressure, whereas the density of films deposited by sputtering generally decreases with pressure. Wang et al. [17] studied the effects of the substrate bias and magnetic trap on the film crystallinity, hardness and refractive index. Crystalline alumina had a larger hardness (∼25 GPa) than amorphous alumina (∼10–12 GPa). Moghal et al. [9] performed uniaxial fragmentation and nano-scratch tests and qualitatively compared the adhesion and strain-to-failure of films deposited using various sputtering configurations (direct current, radio frequency and high power impulse magnetron sputtering). Delayed failure of the layer is promoted by direct current sputtering configuration, when compared to the two other configurations. Most of the experimental work was performed so far on Al2O3 layers thicker than 200 nm, essentially to avoid substrate effects when performing nanoindentation tests to characterize the elastic and plastic behavior. Also note that nanoindentation is not adapted to test fracture properties except for very brittle materials. Our experience is that Al2O3 films do not crack under the sharp tip of a nanoindentor. Only a few recent works have looked at films thinner than 200 nm using advanced microtesting methods. For instance, the so-called “push-to-pull” tensile test method [18] was used to extract the tensile strength evolution of ALD alumina layers 100 to 10 nm thick, exhibiting a strong size dependent strengthening with values increasing from 3.5 GPa to 5.2 GPa. Baumert et al. [8] studied the fatigue degradation properties of ALD alumina using silicon micro-resonators. Mueller et al. [19] measured the fracture toughness of nanocrystalline alumina fibres, with grain size ∼65 nm, using a nanoindenter to deflect chevron-notched type cantilever beams, leading to a fracture toughness equal to 2.3 ± 0.2 MPa m1/2. The relationship between the internal stress, stiffness and microstructure of amorphous alumina films was investigated by curvature-induced internal stress measurements [20]. Differences in internal stress for films thinner than 300 nm could not be related to the film microstructure. A comprehensive understanding of the relationship between the deposition conditions, the porosity and the mechanical properties, especially the plastic and fracture properties, in Al2O3 thin films is still missing in the literature.
The objective of this work is to contribute to a better understanding of the dependence of the effective and intrinsic mechanical properties of Al2O3 films, with thickness in the sub 200 nm range, on the microstructure, as controlled by the deposition conditions. This range is of interest for many applications listed earlier in this introduction. Reactive magnetron sputtering (RMS) is used to fabricate amorphous alumina with closed random porosity. The mechanical properties of interest are the internal stress, the elastic modulus, the hardness and the strain rate sensitivity, with an emphasis on the link with the porosity. Two advanced mechanical characterization methods are used in this study, i.e. the Multibeam Optical Stress Sensor (MOSS) [21] and the “lab-on-chip” technique [12], [22], [23], supplemented by more classical methods. The MOSS is employed to extract the internal stress evolution in the film, which is closely related to the microstructure of the film. Complementary measurements using ellipsometry provide the film porosity and roughness. Transmission Electron Microscopy (TEM) has been used to investigate the microstructure of the films. The elastic modulus is determined by using an “on-chip” uniaxial tensile test method adapted for the characterization of oxide films, and compared to nanoindentation data. The “on-chip” uniaxial tests also provide the fracture stress and fracture strain. Nanoindentation delivers information on the viscoplastic response of the Al2O3 thin films. All the data are analysed based on porous elasticity and plasticity mechanical models. Besides reporting novel measurements of fundamental quantities such as strain rate sensitivity and fracture stress of freestanding Al2O3 films, one of the main conclusions of the work is that not only the porosity and hence the effective properties of the layer change with deposition conditions, but also the intrinsic stiffness and strength of the matrix material around the pores.
The outline of the paper is as follows. Section 2 describes the deposition and characterization methods, as well as the mechanical test procedures. Section 3.1 presents the results about the internal stress and microstructure evolution, which are discussed and related to the deposition pressure in Section 4.1. Section 3.2 focuses on the results about the mechanical properties. The link between the microstructure and elastic properties is discussed in Section 4.2, and the link between the microstructure, hardness and viscoplastic response in Section 4.3.
Section snippets
Deposition and characterization methods
Al2O3 films involving closed porosity were grown by DC magnetron sputtering, from an Al target (99.9995% purity), with a target to substrate distance equal to 12.2 cm. The films were deposited on 3-inch single crystal silicon wafers. The samples were sputtered at ambient temperature and at pressures ranging from 1.2 to 8 mTorr, under a current of 300 mA and 350 mA in Ar/O2 gas mixtures. The sputtering rates were measured before deposition and kept constant, as indicated by the constant voltage
Internal stress and microstructure
Fig. 2 shows the evolution of the stress*thickness product as a function of thickness, measured in-situ during reactive cathodic pulverization of Al2O3. The internal stress is slightly negative (compressive) at the lowest sputtering pressure, and increases with deposition pressure, becoming positive (tensile) and equal to 353 MPa at 6 mTorr. Above this transition pressure, the internal stress decreases down to 50 MPa at 8 mTorr. The variation of the stress*thickness with respect to thickness
Effect of deposition pressure on internal stress, porosity and roughness
There are three other ways of evaluating the porosity, in addition to the one based on a direct ellipsometric measurement as presented in Section 2.1. Two methods use the following formula derived from the Lorentz-Lorenz equation [37] to calculate the pore volume fraction from the refractive index measurement:where is the refractive index of the film given in Table 1 and is the refractive index of the dense part of the film. The value of is then
Conclusions
The porosity and the roughness of amorphous Al2O3 films as well as the internal stress were modified by changing the deposition pressure during RMS sputtering. The dependence of the mechanical properties of Al2O3 layers on the film microstructure has been investigated. The main conclusions of the study are the following.
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The porosity of the film steadily increases with deposition pressure, as a result of a combination of different phenomena. A transition from atomic peening into the surface to
Acknowledgments
This work has been funded by the Belgian Science Policy through the IAP 7/21 project. The support of the ‘Fonds Belge pour la Recherche dans l’Industrie et l’Agriculture (FRIA)’ for A.v.d.R. is also gratefully acknowledged, as well as the support of FNRS through the grant PDR T.0122.13 “Mecano”.
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