2017 Volume 58 Issue 5 Pages 813-816
The Sm-Fe thin films were prepared by a DC magnetron sputtering system installed the Langmuir probe with various substrate bias voltage. In this work, internal stress of the Sm-Fe thin films was investigated considering ion bombardment. The influence of ion bombardment on internal stress in films was estimated by the ion bombardment parameter (Pi). The Pi increased with increasing negative substrate voltage. Internal stress of Sm-Fe thin films showed a larger compressive stress with increasing amount of the Pi. The magnetostrictive susceptibility of Sm-Fe thin films was improved by increasing compressive stress. The magnetostrictive susceptibility of the Sm-Fe thin film was dependent on the Pi.
The R-Fe alloys (R being Tb or Sm) exhibit giant magnetostriction at room temperature.1) At the present time, the Tb-Fe based alloys are used as devices for micro-machines, sensor systems and surface acoustic wave filters, whereas the Sm-Fe based alloys have a price advantage over the Tb-Fe alloys.1–4) The magnetostrictive susceptibility is the important factor for evaluation of the sensitivity of magnetostrictive materials.5) The devices which are installed the high magnetostrictive susceptibility material are low power consumption. The magnetostrictive properties of sputtered thin films are affected by a processing such as deposition conditions and the ion bombardment during sputter deposition.6) In previous study, Hoffman et al. suggested that an impingement ration (i/a); incident of ion and vapor particles on substrate was a key factor on the internal stress of thin films.7) On the other hand, Windischmann's reported the internal stress of a sputtered thin film depend on an ion energy.8) In former works, we proposed that the internal stress and magnetostrictive properties of Ni thin films can be controlled by the ion bombardment parameter Pi, which is based on magnitude of an ion momentum pG+ and an impingement ration of gas ions to metal particles iG+/aM.9–11) However, up to now few studies have been reported on the effects of the internal stress caused by the ion bombardment on giant magnetostrictive thin films with plasma diagnostics in consideration of secondary electron emission. In this study, effects of internal stress on magnetostrictive properties of Sm-Fe thin films are discussed.
Sm-Fe thin films with thickness range of 300 nm were prepared on single-crystal silicon (100) (5 mm × 25 mm) by the dc magnetron sputtering system install the Langmuir probe, as shown in Fig. 1. SmFe2 alloy bonding on 3 inch backing plate was applied as the target. The target to substrate distance was 60 mm. The chamber was evacuated by turbo molecule pump which the base pressure below 3.0 × 10−4 Pa. Argon (Ar with the purity of 99.999%) gas with the pressure of 1.0 × 10−1 Pa was applied as sputtering atmosphere. Then the pre-sputtering at 60 W (about 260 V × 0.23 A) was carried out for 600 seconds. Sputtering process has been conducted for 1200 seconds with a constant power of 60 W at the constant substrate temperature of 373 ± 5 K. A substrate bias voltage was set to be at 0 to −120 V.
Schematic diagram of the dc magnetron sputtering system with a single Langmuir probe.
To estimate the amount of ion bombardment on bias sputtering, the plasma potential and the ion density must be measured accurately. A single Langmuir probe was placed upward the sputtering target. The velocity of the Ar ion vAr+ is considered to be the acceleration caused by the difference between the plasma potential and the substrate bias voltage. The NAr+ is calculated to be equal to the electron density, Ne.12) This substitution holds under the assumption that the plasma is in a thermal equilibrium state. The Ne is estimated from the electron temperature, Te, and electron current at plasma potential, Ie, which are presumed by the plasma diagnostics. The Ne is calculated by following equation.
| \[N_{\rm e} = 3.73 \times 10^{11}(I_{\rm e}/\sqrt{T_e})\] | (1) |
| \[P_{\rm i} = (i_{{\rm Ar}^+}/a_{\rm f}) |p_{{\rm Ar}^+}|\] | (2) |
The iAr+ is a product of the vAr and the NAr+.The af is estimated using following equation:
| \[a_{\rm f} = (r_{\rm depo} \rho_{\rm f} N_{\rm A}/M_{\rm f})\] | (3) |
| \[p_{{\rm Ar}^+} = \sqrt{2m_{{\rm Ar}^+} e(V_{\rm s} - V_{\rm sub})}\] | (4) |
The curvature radius of the sample, R, was monitored using a cantilever form optical lever method.13) The internal stress, σ, was estimated applying Stoney's equation as follows.14)
| \[\sigma = E_{\rm s} t_{\rm s}^2/6t_{\rm f} R(1 - v_{\rm s})\] | (5) |
The magnetostriction of the sample, λ, was measured by using a cantilever method under applied magnetic fields and calculated by following formula.16,17)
| \[\lambda = dE_{\rm s} t_{\rm s}^2 (1 - v_{\rm f})/3(1 - v_{\rm s}) E_{\rm f} t_{\rm f} l^2\] | (6) |
In this study, the magnetostrictive susceptibility is defined as the average of the dλ/dH interval between 0 to 80 kA/m.
2.3.3 Sample analysisThe composition of Sm-Fe films was analyzed by the wavelength dispersive X-ray spectroscopy (WDX). Also, the crystal structure was analyzed by X-ray diffraction (XRD). The thicknesses of films were measured by a Dektak 3 stylus profilometer.
The changing of the electron density, Ne, with increasing the Vsub was shown in Fig. 2. The Ne was not influenced by the Vsub. The quantity of supplied Ar ions from sputtering plasma was constant however Ar ions were attracted by difference of potential between Vs and Vsub.
The Ne changing with the substrate bias voltage, Vsub.
Figure 3 shows the relationship of the Vsub to Pi. The increase of the Pi was caused by the increase of the driving force with expanding the difference between the Vs and the Vsub. Hence the iAr+ and the pAr+ were enlarged, according to eq. (1). Figure 4 shows the obtained σ compared to the Pi. σ obviously tends to decrease according to the increase of the Pi, this change caused by ion bombardment seems to be bring Ar ion peening effect on thin film.
The Pi as a function of the substrate bias voltage Vsub.
The calculated internal stress of the Sm-Fe thin films as a function of the Pi using eq. (4).
Figure 5 shows typical XRD spectrum of the Sm-Fe film prepared on the Si (100) substrate. The film, the substrate bias voltage was not applying condition, was assessed nanocrystal or amorphous. With increasing the substrate bias voltage, the film was impacted more by the Ar ion. All the samples showed almost same halo pattern. Accordingly, the substrate bias voltage is figured not effective parameter to change the crystallization of these films. The composition of the Sm-Fe films were about Sm22Fe78 determined by WDX. This composition on Sm-Fe system is presumed to form two phase alloy include Sm2Fe17 phase and SmFe3 phase by phase diagram. However, the films were supposed to be nanocrystal or amorphous because substrate temperature in this study (373 K) is not sufficient to crystallize due to surface diffusion.
The XRD spectrum of the Sm-Fe film on the Si(100) substrate.
Figure 6 shows the magnetostrictive susceptibility, dλ/dH, as a function of the Pi. The dλ/dH is improved with increasing the Pi. Magnetostrictive property showed same behavior as results of Makita et al. reported.19) Following Wang et al., magnetic anisotropy energy changes with the stress.20) Tsunashima et al. reported that internal stress of magnetostrictive thin films will affect the magnetostrictive susceptibility.21) Takeuchi et al. suggested that the magnetostrictive susceptibility of the thin films increased with decreasing magnetic anisotropy energy due to increasing the compressive stress.22) Accordingly, this improve of the dλ/dH can be caused by a development of the in-plane magnetic anisotropy induced by the increase of compressive stress.
The magnetostrictive susceptibility of the Sm-Fe thin films as a function of the Pi.
The electron density was almost constant regardless the substrate bias voltage. The ion bombardment parameter was increasing with increasing the substrate bias voltage. The internal stress of the Sm-Fe thin film on Si substrate showed large compressive stress with increasing the ion bombardment parameter Pi. Then, the crystal structure of the Sm-Fe thin film doesn't change with the ion bombardment parameter Pi. The magnetostrictive susceptibility of Sm-Fe film was improved due to increasing of compressive stress by ion peening. In conclusion, the magnetostrictive properties of giant magnetostrictive thin films could be controlled by using the ion bombardment parameter Pi.
With respect to the supply of microscopy equipment for sample check, Tokai University Imaging Center for Advanced Research (TICAR) and Nikon Instech co.ltd. are gratefully acknowledged.
