Generating and measuring the anisotropic elastic behaviour of Co thin films with oriented surface nano-strings on micro-cantilevers
© Madurga et al; licensee Springer. 2011
Received: 5 November 2010
Accepted: 12 April 2011
Published: 12 April 2011
In this research, the elastic behaviour of two Co thin films simultaneously deposited in an off-normal angle method was studied. Towards this end, two Si micro-cantilevers were simultaneously coated using pulsed laser deposition at an oblique angle, creating a Co nano-string surface morphology with a predetermined orientation. The selected position of each micro-cantilever during the coating process created longitudinal or transverse nano-strings. The anisotropic elastic behaviour of these Co films was determined by measuring the changes that took place in the resonant frequency of each micro-cantilever after this process of creating differently oriented plasma coatings had been completed. This differential procedure allowed us to determine the difference between the Young's modulus of the different films based on the different direction of the nano-strings. This difference was determined to be, at least, the 20% of the Young's modulus of the bulk Co.
PACS: 62.25.-g; 81.16.Rf; 68.60.Bs; 81.15.Fg; 68.37.Ef; 85.85.+j
The study of the elastic and mechanical properties of thin films is of interest in basic and applied research because thin films are used extensively in micro-electronic and micro-electromechanical systems. Because the elastic constants of thin films are different from those of bulk material of the same composition, the elastic constants of the bulk material cannot be used to design thin film devices. Consequently, it is very important to accurately determine the elastic constants of thin films. These properties can be studied using a wide variety of techniques, including the analysis of the substrate curvature , micro-beam testing , micro-tensile testing , cantilever-bending resonance , nano-indentation , Rayleigh-wave velocity measurements  and Brillouin scattering . Among others, Young's modulus is an important parameter for thin-film technological applications.
Micro-cantilevers (MCLs) are mechanical devices with attractive applications; for instance, they are widely used as high-sensitivity sensors in different physical, chemical and biological technologies [8, 9]. Another use of MCLs is in the study of the mechanical properties of thin films . This type of analysis is possible because of the relation between the resonant frequency of MCLs and Young's modulus. If a MCL is coated with a thin film, a change results in the resonant frequency. By measuring this change, one can compute the Young's modulus of the thin film deposited on the MCL.
We conducted a study that demonstrated that the off-normal pulsed laser deposition (PLD) technique allows the simultaneous growth and sculpting of soft magnetic nano-strings with an orientation that is perpendicular to the incidence plane of the plasma and a medium width that can be selected between 8 and 30 nm by selecting an off-normal angle and the appropriate deposition time . Uniaxial in-plane magnetic anisotropy was then generated in the films that would have a value between 103 and 104 J/m3, depending on the deposition parameters . In addition to magnetic anisotropy, these nano-scale patterned Co films also presented controlled electrical, optical  and mechanical anisotropies . In an extension of the study, MCLs were coated with these magnetic nano-strings so that their magneto-mechanical properties were analysed .
In this study, we produced Co nano-strings over Si MCLs, validating a differential method of studying the elastic anisotropy of these Co thin films in connection with their nano-string morphology. This technique allowed us to determine the difference between the Young's modulus of the films depending on their nano-string direction.
This designed, homemade device allowed the incidence plane of the plasma to be parallel or perpendicular to the longitudinal direction of each MCL. Therefore, the nano-strings generated in the off-normal deposited film were perpendicular (transverse) or parallel (longitudinal) to the longitudinal direction of each MCL, as shown in the right part of Figure 1b. In addition, two glass circles that were 7 mm in diameter were situated on the cone's lateral surface in the same circumference of the two MCLs. This made it possible to perform magnetic measurements.
The two MCLs were selected after the resonant frequency of each, νo, had been determined. The two MCLs were similar because of their equal dimensions and because we did not allow differences between the frequencies of the two selected MCLs higher than 20 Hz in ≈10000 Hz. The two MCLs were simultaneously coated with Co in consecutive processes, either with the same coating time or with different coating times, whereas the rest of the parameters remained unchanged.
The same device was used to coat two MCLs with Au under the same conditions, which ensured that our device coated the two MCLs with the same amount of material.
The mechanical resonant frequency of the MCLs, νo prior to coating and ν(C-MCL) after coating, was determined through location as the working MCL in the head of an atomic force microscope (AFM) . The system performed a driving frequency scan for mechanical oscillation of the MCL, measuring the amplitude and the phase of the MCL's deflection. In this way, the MCL's resonant frequency, ν, was determined. The accuracy of the ν measurements was ± 1/10000.
Scanning tunnelling microscopy (STM) was performed to image the surface morphology of the coated glass circles and also the coated MCLs.
The magnetic hysteresis loops of the coated glass circles were determined using a vibrating sample magnetometer . The value of the measured magnetic moment of each film was used to deduce its thickness. A deposition rate of ≈1.02 nm/min was used in this study. The different films had thicknesses between 0.25 and 28 nm.
Results and discussion
This behaviour was also observed for other two simultaneously off-normal Co-coated MCLs with a consecutive deposition time of 4.0 min, as shown in Figure 6b.
Given the E 0 value, this difference is ≈20% of the Young's modulus of the micro-crystalline hcp bulk Co.
A specially designed homemade device combined with a PLD system allowed the off-normal simultaneous coating of two Si MCLs at different controlled locations with respect to the incidence plane of the plasma. For a fixed off-normal angle of θ = 55°, two positions were used for the two MCLs: a position parallel to the incidence plane of the plasma and one perpendicular to that plane. The two off-normal Au-coated MCLs exhibited equal mechanical behaviour, indicating the in-plane isotropic elasticity of these Au pulsed-laser deposited films. This equal mechanical behaviour ensured that the amount of material deposited on both simultaneously coated MCLs was equal and made it possible to conduct a differential analysis between both. The two simultaneously off-normal Co-coated MCLs exhibited the following behaviour. First, after percolation and nano-string generation, different mechanical behaviour occurred due to the increase in the spring constant for the MCL with Co nano-strings parallel to the longitudinal direction, whereas the MCL with Co nano-strings transverse to the longitudinal direction experienced changes in the resonant frequency mostly produced by the increase in mass. Secondly, these results were connected with the anisotropic elastic behaviour of the Co film with nano-strings morphology. Thirdly, the Young's modulus of the off-normal deposited Co film was 20% of the Young's modulus of the bulk Co higher for the film direction parallel to the nano-strings than for the film direction transverse to the nano-strings.
atomic force microscope
film deposited over the microcantilever parallel to the cone generatrix
film deposited over the microcantilever perpendicular to the cone generatrix
coated micro-cantilever parallel to the cone generatrix
coated micro-cantilever perpendicular to the cone generatrix
- C(PA or PE)-MCL:
coated micro-cantilever parallel to the cone generatrix or coated micro-cantilever perpendicular: to the cone generatrix
YAG: neodymium-doped yttrium aluminium garnet
micro-cantilever parallel to the cone generatrix
micro-cantilever perpendicular to the cone generatrix
pulsed laser deposition
scanning tunnelling microscopy.
This work was partially supported by the Spanish government under project MAT2007-66252.
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