N ion implantation into Fe and Co films using plasma based ion implantation

Transcription

1 Trans. Mat. Res. Soc. Japan 41[3] (2016) N ion implantation into Fe and Co films using plasma based ion implantation Setsuo Nakao*, Tutomu Sonoda, Takeshi Kusumori and Kimihiro Ozaki National Institute of Advanced Industrial Science and Technology (AIST) - Chubu, Anagahora, Moriyama, Nagoya, Japan * Corresponding author: Fax: , and/or nakao.s@aist.go.jp Fe and Co films are prepared on glass substrates by DC magnetron sputtering and N ion implantation is carried out by a bipolar-type plasma based ion implantation (PBII) technique. Some samples are annealed at 150 o C in vacuum after N ion implantation. The structural and compositional changes are examined by thin film X-ray diffraction (XRD) measurements, scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The changes of magnetic properties are also examined by vibrating sample magnetometer (VSM). It is found that surface morphology is not changed significantly by N ion implantation and subsequently annealing as far as SEM observation, although N content is surely increased in the both films. XRD measurements reveal that nitride phases, -Fe 3-xN and -Co 3N, may be formed by N implantation. The results of VSM measurements indicate that the saturation magnetization of Fe films are slightly increased by N ion implantation. However, the saturation magnetization of Co films are decreased. After annealing at 150 o C, the magnetic properties are deteriorated significantly for the both samples. Key words: sputtering, plasma based ion implantation, iron nitride, cobalt nitride, magnetic property 1. INTRODUCTION Transition metal nitride films have attracted much attention because of the possibility of many applications such as hard coatings [1], magnetic and catalytic materials [2]. Fe-N and Co-N films are also expected for showing high magnetization property [3]. Many studies on preparation of Fe-N films have been done by physical vapor deposition, such as molecular beam epitaxy [4] and sputtering technique [5-8]. However, the magnetic property sensitively depends on the N concentration and the crystal structure of the films. It is known that the magnetic properties of the films are not always easy to control in the preparation by a conventional sputtering method. Ion implantation technique is a useful for the control of the impurity concentration in the host material. It is reported that Fe nitride surface layer is formed by N ion implantation into Fe target [9]. However, the use of ion accelerator is not always applicable in industry. On the other hand, plasma based ion implantation (PBII) is a conventional implantation technique and high cost performance may be expected as compared with the use of ion accelerator system. It is reported by J. Choi et al. [10] that effective nitriding could be achieved by a bipolar PBII system. In this study, Fe and Co films are prepared by DC magnetron sputtering and N ion implantation are carried out by the bipolar PBII system. After N implantation, the samples are annealed but the temperature is set at 150 o C to prevent decomposition of nitride phase. Structural changes of the films are examined by thin film X-ray diffraction (XRD) measurements. The surface morphology and composition of the films are examined by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Magnetic properties are also examined by vibrating sample magnetometer (VSM). 2. EXPERIMENTAL Fe and Co films are deposited on glass substrates by DC magnetron sputtering and the thickness of each film is controlled by sputtering time. The thickness of both films are estimated approximately 200 nm. The details on sputtering system are described elsewhere [11]. The N ion implantation is carried out using the bipolar-type PBII system [12]. The samples are set on water-cooled holder where the positive and negative pulse voltages are applied subsequently. The chamber is evacuated of approximately 3 x 10-3 Pa by turbo molecular pump connected with mechanical booster and rotary pumps. At first, Ar gas is inlet into the chamber at a flow rate of 10 sccm. Ar sputter cleaning is carried out at a pressure of 0.5 Pa for 10 min before N ion implantation. The positive and negative pulse voltages are applied at +2 kv and -5 kv, respectively. After surface cleaning, N2 gas is inlet into the chamber at a flow rate of 10 sccm. N2 gas discharge is generated at a pressure of 0.07 Pa by applying positive pulse voltage of +2 kv. The negative pulse voltage is increased at a step of -5 kv up to -20 kv. The implantation time is totally 60 min and the ion dose of N is estimated of the order of /cm 2, as reported previously [13]. The samples are annealed at 150 o C for 30 min in vacuum by golden image furnace (ULVAC Sinku-riko RHL-E410P). Surface observation and EDX analysis are carried out using SEM-EDX system (ERA-8900FE). Thin film XRD (Rigaku RAD-2X) measurements are performed in order to check the formation of nitride phases. The changes of magnetic property are measured by VSM system (Riken Denshi VT-800). In the N2 gas plasma, a dominant ion species is N2 + so that maximum ion energy is equivalent to 10 kev/ion when the negative potential is -20 kv. Figure 1 shows the depth profile of N ion at an energy of 10 kev in the Fe and 313

2 314 N ion implantation into Fe and Co films using plasma based ion implantation Co films which is calculated by the transport of ions in matter (TRIM98) code [14]. The projected ranges are similar for both films and estimated within approximately nm, at most assuming maximum ion energy of 10 kev. Therefore, N ions should impinge on the surface of the films and enter into the near surface region of the films. Therefore, little effect is expected at deeper part of the films by ion implantation. signal of C may be contamination of the films which are incorporated during deposition because the same chamber is usually used for the preparation of carbon films. After N implantation, N signal is apparently increased in intensity for each sample. The signal is not changed by thermal annealing at 150 o C in vacuum. These results suggest that N atoms are presented in the films due to N implantation and they are not reduced in amount by thermal annealing at 150 o C. Fig. 1. N distribution in Fe or Co calculated by TRIM RESULTS AND DISCUSSION 3.1 SEM observation The surface morphology of the Fe and Co films deposited on glass substrates is observed by FE-SEM, as shown in Fig. 2. The surfaces are very smooth and there is no significant difference between before and after N ion implantation. Also, the annealing process at 150 o C in vacuum causes no significant difference in surface image as far as SEM observation. Fig. 2. FE-SEM images of (a) Fe, (b) N implanted Fe (FeN), (c) FeN annealed at 150 o C in vacuum (FeN-150C), (d) Co, (e) N implanted Co (CoN) and (f) CoN annealed at 150 o C in vacuum (CoN-150C). 3.2 EDX measurements Compositional changes of the films are examined by EDX measurements. Figure 3 shows EDX spectra of the samples before and after N implantation, and followed by annealing at 150 o C in vacuum. The signals of O, Si and Ca comes from glass substrate. The strong signals of Fe and Co are observed for respective films. In the higher magnification of the low energy region, the signals from C and N are observed for respective Fe and Co films. The Fig. 3. EDX spectra of Fe and Co samples of Fig. 2, which are denoted as Fe, FeN, FeN-150C, Co, CoN and CoN-150C. The signals of O K, SiK and CaK come from glass substrate. 3.3 XRD measurements Crystallographic structure of the films is examined by thin film XRD measurements. Figure 4 shows the XRD patterns of the samples of Fig. 2. For as-deposited Fe samples, the peaks are observed at 44.8 o, 65.8 o, 82.6 o, 117 o and 137 o. These peaks originate from -Fe(110), (200), (211), (310) and (222), respectively. After N ion implantation, the peaks appear at 30.5 o, 35.5 o, 57.2 o and 62.8 o. These peaks are assigned to Fe3O4(220), (311), (511) and (440), respectively. In addition, small bumps are also observed at 43.1 o, 50.2 o and 69.5 o. These bumps are assigned to -Fe3-xN(111), (201) and (300), respectively. The peaks at approximately 57 o and 63 o, arising from -Fe3-xN(112) and (211), are also expected but they may be overlapped on the peaks assigned to Fe3O4(511) and (440), respectively. These features are not significantly changed by the annealing at 150 o C in vacuum. For as-deposited Co samples, the peaks are observed at 41.6 o, 44.1 o, 47.7 o, 51.0 o, 76.2 o and 92.0 o. These peaks originate from -Co(100), -Co(002) + cubic-co(111), -Co(101), cubic-co(200), -Co(110) + cubic-co(220) and -Co(112) + cubic-co(311), respectively. After N ion implantation, new bumps appear at 58.0 o, 62.0 o, 78.1 o, 88.0 o and 96.5 o. These bumps are assigned to -Co3N(102), -Co(102), -Co3N(110), -Co3N(201) and cubic-co(222), respectively. The diffraction peaks of -Co3N(002) and (101) are expected which may be overlapped on the peaks of -Co(100) and -Co(002) + cubic-co(111), respectively. It is also seen that the diffraction patterns are not changed significantly by the annealing at 150 o C in vacuum as similar to the case of the Fe samples.

3 Setsuo Nakao et al. Trans. Mat. Res. Soc. Japan 41[3] (2016) 315 Fig. 4. XRD patterns of the Fe samples a c (upper) and the Co samples d - f (lower). The alphabets indicate as follows: a: Fe as grown film (Fe), b: N implanted Fe film (FeN), c: N implanted Fe film after annealed at 150 o C (FeN-150C), d: Co as grown film (Co), e: N implanted Co film (CoN) and f: N implanted Co film after annealed at 150 o C (Co-150C). Fig. 5. VSM spectra of the samples of (a) Fe, (b) FeN, (c) FeN-150C, (d) Co, (e) CoN and (f) CoN-150C which correspond to the samples of Fig. 4.

4 316 N ion implantation into Fe and Co films using plasma based ion implantation 3.4 VSM measurements Figure 5 shows the results of VSM measurements of the samples; (a) as-grown Fe film, (b) after N implantation (c) followed by thermal annealing, (d) as-grown Co film, (e) after N implantation, and (f) followed by thermal annealing. It is indicated that the saturation magnetization of Fe film is slightly increased from to emu by N ion implantation. However, the magnetic property of the film is deteriorated by thermal annealing. On the other hand, the saturation magnetization of Co film is decreased from to emu. As similar to the case of Fe films, the magnetic property of Co film is also deteriorated by thermal annealing. 3.5 Discussion The results of the EDX analysis clearly show that N implantation into Fe and Co films is successively achieved by PBII system. Considering with the results of XRD measurements, it is found that the crystal structure of nitride phase of Fe and Co is formed by N ion implantation without extra annealing process. However, ferromagnetic properties of both films are not always improved by N ion implantation. In the Fe films, the crystal structure of oxide phase (magnetite: Fe3O4) is also observed after N implantation. Fe is easy to be oxidized in the air so that amorphous oxide may be presented near the surface region of the films before N implantation. It is expected that N implantation causes not only the formation of nitride phase but also the crystallization of amorphous oxide phase. In current experiments, -Fe3-xN phase (0 x 1) are formed by N implantation. In the case of gas nitriding process, the temperature of 400 o C is at least necessary for the formation of -Fe3-xN phase in NH4 atmosphere [15]. The samples are set on water-cooled holder so that the temperature during implantation is estimated to less than 100 o C. These results suggest that N implantation using PBII is useful for the formation of nitride phase. It is reported that -Fe3N (x=0) shows weak ferromagnetic property but it is decreased with increasing x, i.e., N concentration. The ferromagnetic property disappears at x>0.6 [5]. It is believed that the slight increase in the saturation magnetization is caused by the formation of -Fe3-xN and the crystallization of Fe3O4. On the other hand, magnetic property of Co nitride is not always clarified. It is predicted by computer simulation that -Co4N has strong ferromagnetic property [3]. Therefore, the challenges of the formation of -Co4N phase are carried out by gas nitriding process, such as multi-step process using NH3 [16] and nitridation via hexa-ammine cobalt nitrate route [17]. As similar to Fe nitrides, however, the increase in N concentration causes the decrease in ferromagnetic property. Considering with the results, it is reasonable that the formation of -Co3N phase reduces the ferromagnetic property of N-implanted Co films. According to J. Choi et al. [10], the conditions, such as high positive pulse voltage, low negative pulse voltage, high N2 gas pressure and addition of H2 to the precursor gas, are effective for steel nitriding in the bipolar PBII system. The -Fe3N phase can be formed over 500 nm in thickness due to thermal diffusion enhanced by high positive pulse voltage application. In contrast to their results, the negative pulse voltage application is only adjusted in order to lower the substrate temperature in this experiment. However, other conditions are not always optimized as pointed out by J. Choi et al. [10]. Therefore, it is considered to admit of improvement in the nitriding process in the case of thin film nitriding at relatively low temperature. After annealing at 150 o C, the surface morphology and the intensity of N signals in the EDX spectra are not changed significantly. In addition, XRD patterns are also similar to those before annealing. These results suggest that the nitride phase is stable at 150 o C. However, ferromagnetic properties of both films are rapidly weakened by the annealing. The reason is not clear. It is possible consideration that the contamination, such as C, in the films may affect the magnetic properties of Fe and Co films at 150 o C. In any event, further investigation is necessary in future. 4. SUMMARY N ion implantation into Fe and Co films prepared on glass substrates by DC magnetron sputtering is carried out by bipolar-type plasma based ion implantation (PBII) technique. The samples are annealed at 150 o C for 30 min in vacuum. The changes of the microstructure and composition are examined by SEM, EDX and XRD measurements. Magnetic properties of the samples are also examined by VSM. It is found that the formation of -Fe3-xN and the crystallization of Fe3O4 are occurred by N ion implantation. The saturation magnetization of the samples is slightly increased by N implantation, which may be related to the structural changes, the nitride formation and the oxide crystallization. In Co films, -Co3N phase are formed by N implantation, although excess N reduces ferromagnetic property. These results suggest that the N implantation using PBII is useful for the formation of nitride phase in Fe and Co films. After annealing at 150 o C, the surface morphology and N concentration are not changed significantly. The nitride phases of Fe and Co are also stable. However, the ferromagnetic properties of both samples are significantly deteriorated after the annealing, although the reason is unclear. Further study is underway. 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