- Email: [email protected]
Low damage smoothing of magnetic materials using off-normal gas cluster ion beam irradiation
Surface & Coatings Technology 201 (2007) 8632 – 8636 www.elsevier.com/locate/surfcoat
Low damage smoothing of magnetic materials using off-normal gas cluster ion beam irradiation S. Kakuta a,b,⁎, S. Sasaki a,b , K. Furusawa a,b , T. Seki b,c , T. Aoki b,c , J. Matsuo c a
Central Research Laboratory, Hitachi, Ltd., 292 Yoshida-cho Totsuka-ku Yokohama, 244-0817, Japan b Collaborative Research Center of Advanced Quantum Beam Process Technology, Japan c Quantum Science Engineering Center, Kyoto University, Gokasho Uji, 611-0011, Japan Available online 13 March 2007
Abstract A damage-free smoothing process for magnetic materials using a gas cluster ion beam (GCIB) has been studied. As the areal density of hard disk drives (HDDs) has increased, the flying height of the magnetic recording head above the disk has been decreasing. In order to further reduce the flying height and improve the sensitivity of sensors, a damage-free smoothing technology for magnetic materials is imperative. A GCIB process has been proposed as a novel smoothing technique for various materials. With this process it is expected that the damage will be low, since the gas clusters are aggregations of a few to thousands of atoms or molecules. In this paper, oblique GCIB irradiation has been studied in order to achieve low damage smoothing of PtMn and NiFe thin films. The surface morphology after GCIB irradiation was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A smooth surface with an average roughness of less than 1 nm was obtained by using oblique GCIB irradiation. Irradiation damage, such as fluctuations in the ratio of the material components and oxidation of the surface, were investigated by secondary ion mass spectrometry (SIMS). The fluctuations were suppressed by using oblique GCIB irradiation with incident angles larger than 80°. Consequently, irradiation from a GCIB at grazing incidence can be applied to achieve low-damage smoothing of magnetic materials. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas cluster; Low damage smoothing; Off-normal irradiation; Magnetic materials
1. Introduction With the increasing areal density of hard disk drives (HDDs), the flying height of the magnetic recording head over the disk has been decreasing. The air bearing surface (ABS), which faces onto the disk, needs to be very smooth in order to enable high density data to be read/written. Therefore, in order to further reduce the flying height, a surface smoothing process, involving minimal damage to the magnetic material, is extremely important, since the sensitivity of the sensor can easily be degraded by damage induced by the smoothing process. Traditionally, a polishing process using diamond abrasives has been utilized for surface smoothing of magnetic recording ⁎ Corresponding author. Central Research Laboratory, Hitachi, Ltd., 292 Yoshida-cho Totsuka-ku Yokohama, 244-0817, Japan. Tel.: +81 45 860 2444; fax: +81 45 860 2433. E-mail address: [email protected] (S. Kakuta). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.03.064
heads. However, this polishing process is, by nature, liable to leave scratches on the surface, and these can cause severe damage to magnetic recording heads. Recently, a gas cluster ion beam (GCIB) technology has been proposed as a novel smoothing technique [1]. Gas cluster ions are aggregations of between a few and several thousands of atoms or molecules. Extremely low energy ion irradiation can easily be achieved since the atoms or molecules in cluster ions share the kinetic energy. Surface smoothing of materials can be performed by “lateral sputtering”, which is one of the unique characteristics of the GCIB process. Thus GCIB can be utilized to obtain a low-damage and ultra-smooth surface finish on magnetic materials. 2. Experimental Fig. 1 shows a schematic diagram of the GCIB apparatus. The GCIB is generated by adiabatic expansion of Ar gas at high
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
8633
Fig. 1. Schematic diagram of the GCIB apparatus.
pressure passing through a nozzle into a vacuum and subsequent ionization by electron bombardment. Monomer ions, which are generated simultaneously with the cluster ions, are eliminated by a magnetic field perpendicular to the beam. Measurement of the beam current and dose control are effected using a Faraday cup. In order to avoid charging the equipment is fitted with a neutralizer which emits electrons. In this study, both normal and off-normal GCIB irradiation was carried out. The angle of incidence (as measured from the normal) was varied from 0° to 90°. Samples were placed on a rotational stage and its rotation period was 0.5 s. One of the most important parameters in the GCIB process is the cluster size (i.e. the number of aggregated atoms). The cluster size distribution can be controlled by the source pressure, the ionization voltage and the ionization electron current [2]. The cluster size distribution was measured by the time of flight (TOF) method in a separate experiment. In this paper, a GCIB whose size distribution has a maximum at around 3000 was used.
In order to investigate the effects of GCIB irradiation on magnetic materials, 50 nm-thick NiFe and 60 nm-thick PtMn films, deposited onto Si wafers by Ar sputtering, were prepared for use as test samples. The film thicknesses before and after irradiation were measured by the X-ray fluorescence (XRF) method. The thickness calibration was performed by the X-ray reflection (XRR) method. The surface morphology of the GCIBirradiated surfaces was observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Depth profiles of Ni, Fe and O for NiFe films and Pt, Mn and O for PtMn films were obtained using secondary ion mass spectrometry (SIMS). 3. Results and discussion Fig. 2 shows typical surface morphologies of NiFe films after GCIB irradiation without sample rotation at incident
Fig. 2. SEM images of NiFe film surfaces after GCIB irradiation without sample rotation at an acceleration voltage of 20 kV and an ion dose of 1 × 1016 cm− 2. The surface morphology after (a) normal irradiation and oblique irradiation at incident angles of (b) 45°, (c) 65° and (d) 85° are shown. The arrows on the pictures indicate the direction of the beam.
8634
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
angles of 0°, 45°, 65° and 85°. Irradiation was done at an acceleration voltage of 20 kV and an ion dose of 1 × 1016 cm− 2. The arrows shown in the SEM images represent the beam direction. It was found that the surface morphology varied drastically with the incident angle. Many small hillocks with a diameter of about 10 nm were observed at an incident angle of 0°. This surface morphology was also observed for various materials after normal GCIB irradiation [1]. Yamada et al. investigated the angular distributions of sputtered atoms for various angles of incidence. In the case of normal irradiation, sputtered atoms are ejected radially and are likely to have launch angles about 30° from the surface. Surface smoothing obtained by GCIB irradiation was achieved by this angular distribution of sputtered atoms, which is quite different from the angular distribution due to monomer ion irradiation. At an incident angle of 45°, periodic ripples perpendicular to the GCIB direction were formed on the surface as shown in Fig. 2(b). In contrast, the ripple orientation was parallel to the GCIB at an incident angle of 85° as shown in Fig. 2(d). As the angle of incidence was increased from 45° to 85°, the ripples perpendicular to the GCIB disappeared and those parallel to the GCIB gradually became clearer. A critical angle where the ripple orientation changes from perpendicular to parallel to the GCIB should be at about 65°. The formation of ripples by monomer ions arriving at a glancing angle of incidence is a well-known phenomenon that has been explained by thermal surface diffusion and the dependence of the sputtering yield on curvature [3]. Koponen et al. simulated ripple formation by off-normal irradiation with a monomer ion beam, and they determined that the wavelengths of the ripples perpendicular and parallel to the beam produced on a C surface were 14 and 19 nm, respectively [4]. Chan et al. demonstrated ripple patterns produced on a Cu surface
perpendicular and parallel to a monomer ion beam, with wavelengths of 250 and 400 nm, respectively [5]. These results indicate that the wavelengths of ripples that lie both perpendicular and parallel to a monomer ion beam are similar. The wavelengths of the ripples perpendicular and parallel to the GCIB on NiFe surfaces, however, were about 120 and 20 nm, respectively. The wavelength of the ripples perpendicular to the GCIB is similar to that observed in previous work [1]. Ripples with a wavelength of 200 nm were produced on a Cu surface by off-normal irradiation with incident angles ranging from 45° to 60°. It is speculated that the ripples parallel to the GCIB are tracks of cluster ions traveling on the surface since the diameter of the craters due to cluster ion impacts is also about 10–20 nm measured by scanning tunneling microscopy (STM) and calculated by molecular dynamics simulation [1,6]. On the other hand, the large wavelength of the ripples perpendicular to the GCIB might result from piled-up debris produced by oblique cluster ion impacts. The debris, being formed in front of the penetration point along the beam direction, has been observed by STM [1]. Fig. 3 shows typical surface morphologies of NiFe films after GCIB irradiation at incident angles of 20°, 45°, 65° and 85° with sample rotation. The irradiation was done at an acceleration voltage of 20 kV and the ion doses were 2.8 × 1016, 2.1 × 1016, 1.3 × 1016 and 3 × 1015 cm− 2, respectively. Small hillocks with a diameter of about 10 nm were observed at incident angles of 20° and 85° (Fig. 3(a) and (d)). Similar surfaces were obtained at incident angles of less than 40° and more than 80°. On the other hand, relatively large mounds with small protrusions were formed at incident angles of 45° and 60° (Fig. 3(b) and (c)). These mounds were also observed at incident angles ranging from 45° to 75°. In the case of oblique GCIB irradiation with incident angles of less than 40°, the majority of
Fig. 3. SEM images of NiFe film surfaces after GCIB irradiation with sample rotation at an acceleration voltage of 20 kV. The surface morphology after oblique irradiation at incident angles of (a) 20°, (b) 45°, (c) 65° and (d) 85° are shown.
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
Fig. 4. Average roughness of GCIB irradiated PtMn film as a function of incident angle. Squares and diamonds denote the roughness obtained after irradiation with and without sample rotation.
sputtered atoms were ejected in the forward direction with a launch angle of 20–30° [1]. Observed from the impact point, the ion beam precesses during oblique irradiation and simultaneous rotation. Therefore, the accumulated distribution should be similar to that of normal irradiation. In contrast, large mounds at incident angles from 45° to 75° and small hillocks at incident angles larger than 80° clearly originate from the ripples observed for oblique irradiation without rotation. These mounds and hillocks are produced by sputtering of ripples from all directions due to sample rotation. Consequently, although the configurations displayed in Fig. 3(a) and (d) are similar, the origin of the small hillocks is quite different. Fig. 4 shows the average roughness of GCIB irradiated PtMn films as a function of incident angle. Measurements both with and without rotation are plotted. The irradiation was done at an acceleration voltage of 20 kV and ion doses of 5 × 1015 cm− 2 with rotation and 1 × 1016 cm− 2 without rotation. No significant difference was observed between the results with and without
8635
rotation. Similar results for Au surfaces were obtained in a previous paper [7]. For normal irradiation, a smooth surface with an average roughness of 1.3 nm was obtained due to lateral sputtering. The average roughness increases slightly with incident angles of less than 40°. Above this, because of the formation of ripples or mounds, a dramatic increase is seen, reaching a maximum at an angle of 60°. Finally, a fairly smooth surface with an average roughness of 0.9 nm was obtained at an incident angle of 80°. In order to realize a manufacturing process for magnetic devices using GCIB, the damage remaining on the irradiated surface has to be minimized. Fig. 5 shows the SIMS data of Pt, Mn and O in a PtMn film before and after GCIB irradiation. The irradiation was done at normal incidence with an acceleration voltage of 5 kV and a dose of 1 × 1016 cm− 2 (Fig. 5(b)). Oblique irradiation at incident angle of 80° was performed at an acceleration voltage of 20 kV and a dose of 1 × 1016 cm− 2 (Fig. 5(c)). The dotted lines on the SIMS data denote the boundary between Si substrate and the PtMn film. Oxygen signal intensity maxima at the dotted lines signify the existence of native oxide on the Si substrates. The Pt signals intensity obtained by XRF for normal and offnormal irradiations show that the etched depth were 0.1 and 6.3 nm, respectively. On the other hand, the corresponding results for Mn show these to be 4.7 and 8.8 nm, respectively. This indicates that normal GCIB irradiation gives rise to fluctuations in the composition ratio. The profiles shown in Fig. 5(b) also demonstrate that a Pt-rich/Mn-poor layer exists on the surface of the film after normal GCIB irradiation. This is the reason why the horizontal axis is expressed as the etch time instead of the depth. From a separate experiment for measurement of the Pt-rich/Ma-poor layer thickness using XRR method, the etch rate of the Pt-rich/Mn-poor layer should be slower than the original PtMn film. Fig. 5(b) also shows that a surface oxide layer exists. From the sequence of experiments under conditions with various ion doses and acceleration
Fig. 5. SIMS data of Pt, Mn and O in PtMn films before and after GCIB irradiation. (a) As deposited, (b) with normal irradiation at an acceleration voltage of 5 kV and (c) oblique irradiation with an incident angle of 80° at an acceleration voltage of 20 kV. The ion dose in each case was 1 × 1016 cm− 2. The dotted line shown on each graph represents the boundary between PtMn film and the native oxide on the Si substrate.
8636
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
voltages, it seems that there is a strong relationship between the thickness of the layer in which fluctuations in the composition ratio occur and the thickness of the surface oxide layer. Therefore, the fluctuation in the composition ratio might be the result of surface oxidation by the GCIB. It can be speculated that the source of this oxygen is residual moisture in the vacuum chamber or impurities in the Ar source gas. In contrast, the layer in which fluctuations in the composition occur and the oxidized layer on the surface of the film irradiated at an incident angle of 80° are quite a thin, as shown in Fig. 5(c). Damage formation due to cluster ion impact is controlled by the kinetic energy component perpendicular to the surface. The normal component of the kinetic energy of irradiated cluster ions should be proportional to the square of the cosine of the incident angle. Therefore, this is an order of magnitude smaller than normal irradiation at an acceleration voltage of 5 kV as shown in Fig. 5(b). Surprisingly, as described above, the etched depth by oblique irradiation with an incident angle of 80° at an acceleration voltage of 20 kV is larger than that due to normal irradiation at an acceleration voltage of 5 kV, although the kinetic energy perpendicular to the surface is smaller. Similar results can be obtained on NiFe films. To summarize the above discussion, grazing irradiation from a GCIB can be applied to achieve low-damage smoothing of magnetic materials. Fairly smooth surfaces can be obtained by off-normal irradiation with incident angles larger than 80°, and fluctuations in the component ratio and surface oxidation can be successfully suppressed. 4. Conclusions In order to achieve low-damage smoothing of magnetic materials, normal and oblique irradiation using a GCIB was studied. Quite smooth surfaces were obtained, independent of sample rotation, using both normal- and grazing-incidence irradiation. After oblique irradiation without rotation at incident
angles larger than 45°, periodic ripples were produced. By increasing the angle of incidence above 45°, the orientation of the ripples rotated from perpendicular to parallel to the irradiation beam when the incident angle exceeded a critical angle. On the other hand, when the sample is rotated, large mounds with small protrusions were produced by oblique GCIB irradiation with incident angles ranging from 45° to 75°. Fluctuations in the composition ratio and oxidation of the surface were observed for normal GCIB irradiation. The fluctuation in the composition ratio might have resulted from oxidation of the surface by the GCIB. It might be speculated that the source of this oxygen is residual moisture in the vacuum chamber or impurities in the Ar source gas. Fluctuations in the composition ratio and surface oxidation can be suppressed by employing oblique GCIB irradiation with incident angles larger than 80°. Consequently, grazing incidence irradiation from a GCIB can be applied to achieve low-damage smoothing of magnetic materials. Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng., R Rep. 34 (2001) 231. [2] T. Seki, J. Matsuo, G.H. Takaoka, I. Yamada, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 206 (2003) 902. [3] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol., A 6 (1988) 2390. [4] I. Koponen, M. Hautara, O.-P. Sievänen, Phys. Rev. Lett. 78 (1997) 2612. [5] W.L. Chanm, N. Pavenayotin, E. Chason, Phys. Rev. B 69 (2004) 245413. [6] T. Aoki, J. Matsuo, G. Takaoka, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 202 (2003) 278. [7] N. Toyoda, I. Yamada, Mater. Res. Soc. Symp. Proc. 849 (2005) 109.
Low damage smoothing of magnetic materials using off-normal gas cluster ion beam irradiation S. Kakuta a,b,⁎, S. Sasaki a,b , K. Furusawa a,b , T. Seki b,c , T. Aoki b,c , J. Matsuo c a
Central Research Laboratory, Hitachi, Ltd., 292 Yoshida-cho Totsuka-ku Yokohama, 244-0817, Japan b Collaborative Research Center of Advanced Quantum Beam Process Technology, Japan c Quantum Science Engineering Center, Kyoto University, Gokasho Uji, 611-0011, Japan Available online 13 March 2007
Abstract A damage-free smoothing process for magnetic materials using a gas cluster ion beam (GCIB) has been studied. As the areal density of hard disk drives (HDDs) has increased, the flying height of the magnetic recording head above the disk has been decreasing. In order to further reduce the flying height and improve the sensitivity of sensors, a damage-free smoothing technology for magnetic materials is imperative. A GCIB process has been proposed as a novel smoothing technique for various materials. With this process it is expected that the damage will be low, since the gas clusters are aggregations of a few to thousands of atoms or molecules. In this paper, oblique GCIB irradiation has been studied in order to achieve low damage smoothing of PtMn and NiFe thin films. The surface morphology after GCIB irradiation was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM). A smooth surface with an average roughness of less than 1 nm was obtained by using oblique GCIB irradiation. Irradiation damage, such as fluctuations in the ratio of the material components and oxidation of the surface, were investigated by secondary ion mass spectrometry (SIMS). The fluctuations were suppressed by using oblique GCIB irradiation with incident angles larger than 80°. Consequently, irradiation from a GCIB at grazing incidence can be applied to achieve low-damage smoothing of magnetic materials. © 2007 Elsevier B.V. All rights reserved. Keywords: Gas cluster; Low damage smoothing; Off-normal irradiation; Magnetic materials
1. Introduction With the increasing areal density of hard disk drives (HDDs), the flying height of the magnetic recording head over the disk has been decreasing. The air bearing surface (ABS), which faces onto the disk, needs to be very smooth in order to enable high density data to be read/written. Therefore, in order to further reduce the flying height, a surface smoothing process, involving minimal damage to the magnetic material, is extremely important, since the sensitivity of the sensor can easily be degraded by damage induced by the smoothing process. Traditionally, a polishing process using diamond abrasives has been utilized for surface smoothing of magnetic recording ⁎ Corresponding author. Central Research Laboratory, Hitachi, Ltd., 292 Yoshida-cho Totsuka-ku Yokohama, 244-0817, Japan. Tel.: +81 45 860 2444; fax: +81 45 860 2433. E-mail address: [email protected] (S. Kakuta). 0257-8972/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.03.064
heads. However, this polishing process is, by nature, liable to leave scratches on the surface, and these can cause severe damage to magnetic recording heads. Recently, a gas cluster ion beam (GCIB) technology has been proposed as a novel smoothing technique [1]. Gas cluster ions are aggregations of between a few and several thousands of atoms or molecules. Extremely low energy ion irradiation can easily be achieved since the atoms or molecules in cluster ions share the kinetic energy. Surface smoothing of materials can be performed by “lateral sputtering”, which is one of the unique characteristics of the GCIB process. Thus GCIB can be utilized to obtain a low-damage and ultra-smooth surface finish on magnetic materials. 2. Experimental Fig. 1 shows a schematic diagram of the GCIB apparatus. The GCIB is generated by adiabatic expansion of Ar gas at high
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
8633
Fig. 1. Schematic diagram of the GCIB apparatus.
pressure passing through a nozzle into a vacuum and subsequent ionization by electron bombardment. Monomer ions, which are generated simultaneously with the cluster ions, are eliminated by a magnetic field perpendicular to the beam. Measurement of the beam current and dose control are effected using a Faraday cup. In order to avoid charging the equipment is fitted with a neutralizer which emits electrons. In this study, both normal and off-normal GCIB irradiation was carried out. The angle of incidence (as measured from the normal) was varied from 0° to 90°. Samples were placed on a rotational stage and its rotation period was 0.5 s. One of the most important parameters in the GCIB process is the cluster size (i.e. the number of aggregated atoms). The cluster size distribution can be controlled by the source pressure, the ionization voltage and the ionization electron current [2]. The cluster size distribution was measured by the time of flight (TOF) method in a separate experiment. In this paper, a GCIB whose size distribution has a maximum at around 3000 was used.
In order to investigate the effects of GCIB irradiation on magnetic materials, 50 nm-thick NiFe and 60 nm-thick PtMn films, deposited onto Si wafers by Ar sputtering, were prepared for use as test samples. The film thicknesses before and after irradiation were measured by the X-ray fluorescence (XRF) method. The thickness calibration was performed by the X-ray reflection (XRR) method. The surface morphology of the GCIBirradiated surfaces was observed using scanning electron microscopy (SEM) and atomic force microscopy (AFM). Depth profiles of Ni, Fe and O for NiFe films and Pt, Mn and O for PtMn films were obtained using secondary ion mass spectrometry (SIMS). 3. Results and discussion Fig. 2 shows typical surface morphologies of NiFe films after GCIB irradiation without sample rotation at incident
Fig. 2. SEM images of NiFe film surfaces after GCIB irradiation without sample rotation at an acceleration voltage of 20 kV and an ion dose of 1 × 1016 cm− 2. The surface morphology after (a) normal irradiation and oblique irradiation at incident angles of (b) 45°, (c) 65° and (d) 85° are shown. The arrows on the pictures indicate the direction of the beam.
8634
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
angles of 0°, 45°, 65° and 85°. Irradiation was done at an acceleration voltage of 20 kV and an ion dose of 1 × 1016 cm− 2. The arrows shown in the SEM images represent the beam direction. It was found that the surface morphology varied drastically with the incident angle. Many small hillocks with a diameter of about 10 nm were observed at an incident angle of 0°. This surface morphology was also observed for various materials after normal GCIB irradiation [1]. Yamada et al. investigated the angular distributions of sputtered atoms for various angles of incidence. In the case of normal irradiation, sputtered atoms are ejected radially and are likely to have launch angles about 30° from the surface. Surface smoothing obtained by GCIB irradiation was achieved by this angular distribution of sputtered atoms, which is quite different from the angular distribution due to monomer ion irradiation. At an incident angle of 45°, periodic ripples perpendicular to the GCIB direction were formed on the surface as shown in Fig. 2(b). In contrast, the ripple orientation was parallel to the GCIB at an incident angle of 85° as shown in Fig. 2(d). As the angle of incidence was increased from 45° to 85°, the ripples perpendicular to the GCIB disappeared and those parallel to the GCIB gradually became clearer. A critical angle where the ripple orientation changes from perpendicular to parallel to the GCIB should be at about 65°. The formation of ripples by monomer ions arriving at a glancing angle of incidence is a well-known phenomenon that has been explained by thermal surface diffusion and the dependence of the sputtering yield on curvature [3]. Koponen et al. simulated ripple formation by off-normal irradiation with a monomer ion beam, and they determined that the wavelengths of the ripples perpendicular and parallel to the beam produced on a C surface were 14 and 19 nm, respectively [4]. Chan et al. demonstrated ripple patterns produced on a Cu surface
perpendicular and parallel to a monomer ion beam, with wavelengths of 250 and 400 nm, respectively [5]. These results indicate that the wavelengths of ripples that lie both perpendicular and parallel to a monomer ion beam are similar. The wavelengths of the ripples perpendicular and parallel to the GCIB on NiFe surfaces, however, were about 120 and 20 nm, respectively. The wavelength of the ripples perpendicular to the GCIB is similar to that observed in previous work [1]. Ripples with a wavelength of 200 nm were produced on a Cu surface by off-normal irradiation with incident angles ranging from 45° to 60°. It is speculated that the ripples parallel to the GCIB are tracks of cluster ions traveling on the surface since the diameter of the craters due to cluster ion impacts is also about 10–20 nm measured by scanning tunneling microscopy (STM) and calculated by molecular dynamics simulation [1,6]. On the other hand, the large wavelength of the ripples perpendicular to the GCIB might result from piled-up debris produced by oblique cluster ion impacts. The debris, being formed in front of the penetration point along the beam direction, has been observed by STM [1]. Fig. 3 shows typical surface morphologies of NiFe films after GCIB irradiation at incident angles of 20°, 45°, 65° and 85° with sample rotation. The irradiation was done at an acceleration voltage of 20 kV and the ion doses were 2.8 × 1016, 2.1 × 1016, 1.3 × 1016 and 3 × 1015 cm− 2, respectively. Small hillocks with a diameter of about 10 nm were observed at incident angles of 20° and 85° (Fig. 3(a) and (d)). Similar surfaces were obtained at incident angles of less than 40° and more than 80°. On the other hand, relatively large mounds with small protrusions were formed at incident angles of 45° and 60° (Fig. 3(b) and (c)). These mounds were also observed at incident angles ranging from 45° to 75°. In the case of oblique GCIB irradiation with incident angles of less than 40°, the majority of
Fig. 3. SEM images of NiFe film surfaces after GCIB irradiation with sample rotation at an acceleration voltage of 20 kV. The surface morphology after oblique irradiation at incident angles of (a) 20°, (b) 45°, (c) 65° and (d) 85° are shown.
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
Fig. 4. Average roughness of GCIB irradiated PtMn film as a function of incident angle. Squares and diamonds denote the roughness obtained after irradiation with and without sample rotation.
sputtered atoms were ejected in the forward direction with a launch angle of 20–30° [1]. Observed from the impact point, the ion beam precesses during oblique irradiation and simultaneous rotation. Therefore, the accumulated distribution should be similar to that of normal irradiation. In contrast, large mounds at incident angles from 45° to 75° and small hillocks at incident angles larger than 80° clearly originate from the ripples observed for oblique irradiation without rotation. These mounds and hillocks are produced by sputtering of ripples from all directions due to sample rotation. Consequently, although the configurations displayed in Fig. 3(a) and (d) are similar, the origin of the small hillocks is quite different. Fig. 4 shows the average roughness of GCIB irradiated PtMn films as a function of incident angle. Measurements both with and without rotation are plotted. The irradiation was done at an acceleration voltage of 20 kV and ion doses of 5 × 1015 cm− 2 with rotation and 1 × 1016 cm− 2 without rotation. No significant difference was observed between the results with and without
8635
rotation. Similar results for Au surfaces were obtained in a previous paper [7]. For normal irradiation, a smooth surface with an average roughness of 1.3 nm was obtained due to lateral sputtering. The average roughness increases slightly with incident angles of less than 40°. Above this, because of the formation of ripples or mounds, a dramatic increase is seen, reaching a maximum at an angle of 60°. Finally, a fairly smooth surface with an average roughness of 0.9 nm was obtained at an incident angle of 80°. In order to realize a manufacturing process for magnetic devices using GCIB, the damage remaining on the irradiated surface has to be minimized. Fig. 5 shows the SIMS data of Pt, Mn and O in a PtMn film before and after GCIB irradiation. The irradiation was done at normal incidence with an acceleration voltage of 5 kV and a dose of 1 × 1016 cm− 2 (Fig. 5(b)). Oblique irradiation at incident angle of 80° was performed at an acceleration voltage of 20 kV and a dose of 1 × 1016 cm− 2 (Fig. 5(c)). The dotted lines on the SIMS data denote the boundary between Si substrate and the PtMn film. Oxygen signal intensity maxima at the dotted lines signify the existence of native oxide on the Si substrates. The Pt signals intensity obtained by XRF for normal and offnormal irradiations show that the etched depth were 0.1 and 6.3 nm, respectively. On the other hand, the corresponding results for Mn show these to be 4.7 and 8.8 nm, respectively. This indicates that normal GCIB irradiation gives rise to fluctuations in the composition ratio. The profiles shown in Fig. 5(b) also demonstrate that a Pt-rich/Mn-poor layer exists on the surface of the film after normal GCIB irradiation. This is the reason why the horizontal axis is expressed as the etch time instead of the depth. From a separate experiment for measurement of the Pt-rich/Ma-poor layer thickness using XRR method, the etch rate of the Pt-rich/Mn-poor layer should be slower than the original PtMn film. Fig. 5(b) also shows that a surface oxide layer exists. From the sequence of experiments under conditions with various ion doses and acceleration
Fig. 5. SIMS data of Pt, Mn and O in PtMn films before and after GCIB irradiation. (a) As deposited, (b) with normal irradiation at an acceleration voltage of 5 kV and (c) oblique irradiation with an incident angle of 80° at an acceleration voltage of 20 kV. The ion dose in each case was 1 × 1016 cm− 2. The dotted line shown on each graph represents the boundary between PtMn film and the native oxide on the Si substrate.
8636
S. Kakuta et al. / Surface & Coatings Technology 201 (2007) 8632–8636
voltages, it seems that there is a strong relationship between the thickness of the layer in which fluctuations in the composition ratio occur and the thickness of the surface oxide layer. Therefore, the fluctuation in the composition ratio might be the result of surface oxidation by the GCIB. It can be speculated that the source of this oxygen is residual moisture in the vacuum chamber or impurities in the Ar source gas. In contrast, the layer in which fluctuations in the composition occur and the oxidized layer on the surface of the film irradiated at an incident angle of 80° are quite a thin, as shown in Fig. 5(c). Damage formation due to cluster ion impact is controlled by the kinetic energy component perpendicular to the surface. The normal component of the kinetic energy of irradiated cluster ions should be proportional to the square of the cosine of the incident angle. Therefore, this is an order of magnitude smaller than normal irradiation at an acceleration voltage of 5 kV as shown in Fig. 5(b). Surprisingly, as described above, the etched depth by oblique irradiation with an incident angle of 80° at an acceleration voltage of 20 kV is larger than that due to normal irradiation at an acceleration voltage of 5 kV, although the kinetic energy perpendicular to the surface is smaller. Similar results can be obtained on NiFe films. To summarize the above discussion, grazing irradiation from a GCIB can be applied to achieve low-damage smoothing of magnetic materials. Fairly smooth surfaces can be obtained by off-normal irradiation with incident angles larger than 80°, and fluctuations in the component ratio and surface oxidation can be successfully suppressed. 4. Conclusions In order to achieve low-damage smoothing of magnetic materials, normal and oblique irradiation using a GCIB was studied. Quite smooth surfaces were obtained, independent of sample rotation, using both normal- and grazing-incidence irradiation. After oblique irradiation without rotation at incident
angles larger than 45°, periodic ripples were produced. By increasing the angle of incidence above 45°, the orientation of the ripples rotated from perpendicular to parallel to the irradiation beam when the incident angle exceeded a critical angle. On the other hand, when the sample is rotated, large mounds with small protrusions were produced by oblique GCIB irradiation with incident angles ranging from 45° to 75°. Fluctuations in the composition ratio and oxidation of the surface were observed for normal GCIB irradiation. The fluctuation in the composition ratio might have resulted from oxidation of the surface by the GCIB. It might be speculated that the source of this oxygen is residual moisture in the vacuum chamber or impurities in the Ar source gas. Fluctuations in the composition ratio and surface oxidation can be suppressed by employing oblique GCIB irradiation with incident angles larger than 80°. Consequently, grazing incidence irradiation from a GCIB can be applied to achieve low-damage smoothing of magnetic materials. Acknowledgements This work is supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. References [1] I. Yamada, J. Matsuo, N. Toyoda, A. Kirkpatrick, Mater. Sci. Eng., R Rep. 34 (2001) 231. [2] T. Seki, J. Matsuo, G.H. Takaoka, I. Yamada, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 206 (2003) 902. [3] R.M. Bradley, J.M.E. Harper, J. Vac. Sci. Technol., A 6 (1988) 2390. [4] I. Koponen, M. Hautara, O.-P. Sievänen, Phys. Rev. Lett. 78 (1997) 2612. [5] W.L. Chanm, N. Pavenayotin, E. Chason, Phys. Rev. B 69 (2004) 245413. [6] T. Aoki, J. Matsuo, G. Takaoka, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 202 (2003) 278. [7] N. Toyoda, I. Yamada, Mater. Res. Soc. Symp. Proc. 849 (2005) 109.