- Email: [email protected]
Development of a compact angle-resolved secondary ion mass spectrometer for Ar+ sputtering
ARTICLE IN PRESS
Vacuum 80 (2006) 768–770 www.elsevier.com/locate/vacuum
Development of a compact angle-resolved secondary ion mass spectrometer for Ar+ sputtering Shinichi Kawaguchia, Masaki Tanemuraa,, Masato Kudoa, Nobumasa Handaa, Naokazu Kinoshitaa, Lei Miaoa, Sakae Tanemuraa, Yasuhito Gotohb, Meiyong Liaob, Satoko Shinkaic a
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan c Takuma National College of Technology, 551 Kohda, Takuma-cho, Mitoyo-gun, Kagawa 769-1192, Japan
Abstract A compact angle-resolved secondary ion mass spectrometer with a special geometrical configuration, composing of a differentially pumped micro-beam ion gun, a tiltable sample stage and a time-of-flight (TOF) mass spectrometer, was newly developed. This system enables the measurement of angular distribution (AD) of secondary ions, which are ejected by oblique Ar+ sputtering, by a simple tilt operation of the sample stage for ejection angles ranging from 01 to 601 with keeping the ion incidence angle constant 621721 from the normal to the surface. Using this system, AD of secondary ions from an HfN film by 3 keV Ar+-ion bombardment was measured at room temperature. Since the yield of HfN+ dimer ions was almost independent of Hf+ and N+ monomer ions, it was concluded that the HfN+ dimer ions were generated via the ‘‘as such’’ direct emission process. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sputtering; Secondary ion mass spectrometer; Angular distribution; Cluster; Hafnium; Nitride
1. Introduction Ion sputtering is widely used for thin film deposition of various kinds of materials and the surface analysis such as secondary ion mass spectrometry (SIMS). For the deposition of composition-controlled thin films from alloy targets by sputtering and the precise SIMS analysis, the elucidation of the sputtering behavior for multicomponent targets is indispensable. Although the angular distribution (AD) of sputtered particles provides key information on the sputtering mechanism, experimental data on the AD of sputtered particles from multicomponent targets are still insufficient in quantity [1–10]. This is due to the difficulty in the experimental setup. During the AD measurement, the incidence angle of the primary ion beam should be kept constant. No commercially available SIMS, however, Corresponding author.
E-mail address: [email protected] (M. Tanemura). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.016
satisfies this requisite, because a change in the detection angle (ejection angle of ions) by a tilt operation of a sample always entails a change in the ion-incidence angle. Ideally, ADs of sputtered particles should be measured by a mass spectrometer moving around a specimen during sputtering. This method, however, is not realistic, because a huge vacuum chamber, in which a mass spectrometer is installable, is necessary. Alternatively, ADs have been measured by a so-called collector method [1–10]. The sputtered material is collected onto a thin cylindrical foil located just above a specimen, and the amount of the deposit onto the foil at various ejection angles is quantified after sputtering. Although this collection method is very simple, sputter-ejected clusters are not distinguishable from monomers. Very recently, we noticed that a special geometrical configuration of a primary ion gun, a sample and a detector enables the AD measurement of sputtered ions by a simple tilt operation of the sample with almost no change in the ion-incidence angle. Based on this idea, we have
ARTICLE IN PRESS S. Kawaguchi et al. / Vacuum 80 (2006) 768–770
constructed a compact angle-resolved SIMS composing of a micro-focus ion gun, a tiltable sample stage and a timeof-flight (TOF) mass spectrometer. In the present study, we will deal with the details about this system and also ADs of sputter-ejected ions from an HfN film measured using this system. 2. Experimental Our experimental system (Fig. 1), whose size is 90 cm 140 cm 120 cm (height), comprises a differentially pumped micro-beam ion gun (JEOL; MIED-III), a TOF mass spectrometer (Jordan Co. Ltd.; D-850 AREF), and a tiltable sample stage (JEOL; AP-UHVGS). Fig. 2 shows their geometrical configuration. The ion gun is located in the X–Z plane with an angle of 601 between the ion-gun axis and Z-axis. The TOF detector is set in the Y–Z plane with an angle of 301 between the TOF detector
Ion Gun
Primary ion
VXY1
TOF
FDC
θout
Sample
1
VR2
VXY2
2
L MCP
GND
Fig. 1. Schematic diagram of the developed system.
769
axis and Z-axis. The sample is tiltable around the X-axis from y ¼ 301 to 301. At the tilt angle y ¼ 0, the detection angle (ejection angle of ions) from the normal to the sample surface [1] is yout ¼ 301 (Fig. 2(a)). The ion-incidence angle, yin , which is generally defined as the angle between the iongun axis (~ a) and the normal to the sample surface (~ n), and the ejection angle, yout , defined as the angle between the detector axis and ~ n are calculated by scalar products of respective vectors (Fig. 2(b)): yin ¼ cos1 ðcos 60 cos yÞ
(1)
and yout ¼ 30 y,
(2)
respectively. For tilt angles y ranging from 301 to 301, ion-incidence angle yin is almost constant, 621721, while the ejection angle yout varies from 01 to 601. Thus, AD of sputtered ions can be measured for oblique sputtering at an almost constant ion-incidence angle by a simple tilt operation of a sample. The polar representation of directions of the primary ion beam and the detector is shown in Fig. 2(c). In order to check the performance of the system, an HfN film deposited onto Si [11] was prepared. The sample was ion-bombarded with 3 keV Ar+ ions in an ultra-high vacuum (UHV) condition. The ion beam was focused into a microbeam of 350 mm in diameter. The base pressure in the chamber was 6.0 108 Pa, while the operating pressure during sputtering was maintained in the 106 Pa region, due to differential pumping of the ion gun. An entrance aperture, 2.5 mm in diameter, was mounted in front of the TOF system for the AD measurement. The distance between the sputtering spot on the sample surface and the entrance aperture was set to be 20 mm. The angular resolution was 7.151. 3. Results and discussion
(a)
(b)
(c) Fig. 2. Geometrical configuration of an ion gun, a sample and a detector for measuring the angular distribution of sputtered ions: (a) tilt angle ¼ 0; (b) tilt angle ¼ y. (c) The polar representation of directions of the primary ion beam and the detector.
In order to obtain reproducible data, the acquisition of mass spectrum was repeated 2000 times at respective ejection angles. The time necessary for this 2000 times acquisition and averaging was about 1 min. Fig. 3(a) shows a typical mass spectrum thus obtained at an ejection angle yout ¼ 0. Hf naturally should display peaks at 176, 177, 178, 179 and 180 amu in a mass spectrum due to the stable isotopes. In order to enhance the signal intensity and hence for the reproducible data, however, the measurement of mass spectra was carried out at a low-resolution mode with which isotope peaks are not separated. We will hereafter refer to the peaks at 180 and 194 amu as Hf+ and HfN+ peaks, respectively. As shown in Fig. 3(a), an intense Hf+ peak together with N+ and HfN+ peaks were detected. No trimer or larger sized ion was observed. Fig. 3(b) shows the dependence of Hf+, N+ and HfN+ intensities on the ejection angle yout , revealing that the respective peak intensities decreased with an increase in the ejection angle. It should be noted that the angular
ARTICLE IN PRESS 770
S. Kawaguchi et al. / Vacuum 80 (2006) 768–770
In the present case, if the HfN+ dimer forms via the recombination process, Eq. (3) is written as Y ðHfNÞ Y ðHfÞ Y ðNÞ / ¼ Y ðNÞ. Y ðHfÞ Y ðHfÞ
(4)
Thus, the intensity ratio of HfN+ to Hf+ should be directly proportional to the intensity of N+. This implies that the angular dependence of HfN+/Hf+ should show the identical tendency with that of N+. As shown in Figs. 3(b) and (d), however, experimental results do not agree with this theoretical prediction; N+ intensity decreased monotonously with an increase in the ejection angle, while HfN+/Hf+ intensity ratio increased gradually with the ejection angle. Thus, HfN+ dimer ions were thought to be generated via the ‘‘as such’’ direct emission process. Fig. 3. (a) Typical mass spectrum obtained from a HfN sample. yout : 01. (b) Angular distribution of ejected Hf+, N+ and HfN+ ions from the HfN sample bombarded with 3 keV Ar+ ions. (c) and (d) Angular dependence of intensities of N+ and HfN+ ions normalized with Hf+ ion intensities, respectively.
dependence of monomer-ion intensity seems to be more prominent than that of dimer-ion intensity. In order to highlight this, peak intensities were normalized with Hf+ intensities for respective ejection angles. The angular dependence of normalized N+ and HfN+ intensities, N+/Hf+ and HfN+/Hf+, respectively, is shown in Figs. 3(c) and (d). It is manifest from Figs. 3(c) and (d) that the normalized N+ intensities are almost independent of the ejection angle, implying that the AD of Hf+ is identical with that of N+, whereas the normalized HfN+ intensities increase with an increase in the ejection angle. Based on these facts, in what follows, we will discuss the formation mechanism of dimer ions. On the cluster formation, two models have been proposed [12]; (i) direct or ‘‘as such’’ emission from the ion bombarded surface, and (ii) recombination of independently emitted atoms or ions after leaving the surface [13–15]. Although experimental and theoretical effort has been devoted mainly for pure elemental targets, it is still controversial which mechanism dominates the formation of small clusters in sputtering. In the former case, (i), the cluster-ion yield is independent of the monomer-ion yield, whereas in the latter case, (ii), the cluster-ion yield, Y ðX n Þ, for an n-atom cluster X n is known to be in proportion to the nth power of the number, Y ðX Þ, of sputtered atom X , because the recombination is a statistical process [13–16] Y ðX n Þ / fY ðX Þgn .
(3)
4. Conclusion A compact angle-resolved SIMS composing of a differentially pumped micro-beam ion gun, a tiltable sample stage and a TOF detector was constructed. Using this system, ADs of sputtered ions from an HfN film by 3 keV Ar+-ion bombardment at the ion-incidence angle of 621721 were measured in a range of the ejection angle of 01–601. Since the angular dependence of HfN+/Hf+ intensity ratio was independent of that of N+ intensity, it was concluded that HfN+ dimer ions were generated via the ‘‘as such’’ direct emission process. References [1] Hofer WO. In: Behrisch R, Wittmaack K, editors. Sputtering by particle bombardment III. Berlin: Springer; 1991 [chapter 2]. [2] Sigmund P, Oliva A, Falcone G. Nucl Instr Meth 1982;194:541. [3] Biersack JP, Eckstein W. Appl Phys A 1984;34:73. [4] Huang W. Surf Interface Anal 1983;14:469. [5] Nagatomi T, Min K-Y, Shimizu R. Jpn J Appl Phys 1994;33:6675. [6] Kang HJ, Shimizu R. Surf Sci 1986;169:337. [7] Andersen HH, Stenum B, Sorensen T, Whitlow HJ. Nucl Instr Methods 1987;209/210:487. [8] Aoyama T, Tanemura M, Okuyama F. Appl Surf Sci 1996;100/ 101:351. [9] Tanemura M, Aoyama T, Otani O, Ukita M, Okuyama F, Chini TK. Surf Sci 1997;376:163. [10] Tanemura M, Ukita M, Okuyama F. Surf Sci 1999;426141. [11] Gotoh Y, Liao MY, Tsuji H, Ishikawa J. Jpn J Appl Phys 2003;42:L778. [12] Gnaser H, Hofer WO. Appl Phys 1989;A48:261. [13] Oechsner H, Gerhard W. Surf Sci 1974;44:480. [14] Gerhard W, Oechsner H. Z Phys B 1975;22:41. [15] Konnen GP, Tip A, de Vries AE. Radiat Eff 1974;21:269; 1975;26:23. [16] Gnaser H, Oechsner H. Surf Sci 1991;251/252:696.
Vacuum 80 (2006) 768–770 www.elsevier.com/locate/vacuum
Development of a compact angle-resolved secondary ion mass spectrometer for Ar+ sputtering Shinichi Kawaguchia, Masaki Tanemuraa,, Masato Kudoa, Nobumasa Handaa, Naokazu Kinoshitaa, Lei Miaoa, Sakae Tanemuraa, Yasuhito Gotohb, Meiyong Liaob, Satoko Shinkaic a
Nagoya Institute of Technology, Gokiso-cho, Showa-ku, Nagoya 466-8555, Japan b Kyoto University, Kyotodaigaku-Katsura, Nishikyo-ku, Kyoto 615-8510, Japan c Takuma National College of Technology, 551 Kohda, Takuma-cho, Mitoyo-gun, Kagawa 769-1192, Japan
Abstract A compact angle-resolved secondary ion mass spectrometer with a special geometrical configuration, composing of a differentially pumped micro-beam ion gun, a tiltable sample stage and a time-of-flight (TOF) mass spectrometer, was newly developed. This system enables the measurement of angular distribution (AD) of secondary ions, which are ejected by oblique Ar+ sputtering, by a simple tilt operation of the sample stage for ejection angles ranging from 01 to 601 with keeping the ion incidence angle constant 621721 from the normal to the surface. Using this system, AD of secondary ions from an HfN film by 3 keV Ar+-ion bombardment was measured at room temperature. Since the yield of HfN+ dimer ions was almost independent of Hf+ and N+ monomer ions, it was concluded that the HfN+ dimer ions were generated via the ‘‘as such’’ direct emission process. r 2005 Elsevier Ltd. All rights reserved. Keywords: Sputtering; Secondary ion mass spectrometer; Angular distribution; Cluster; Hafnium; Nitride
1. Introduction Ion sputtering is widely used for thin film deposition of various kinds of materials and the surface analysis such as secondary ion mass spectrometry (SIMS). For the deposition of composition-controlled thin films from alloy targets by sputtering and the precise SIMS analysis, the elucidation of the sputtering behavior for multicomponent targets is indispensable. Although the angular distribution (AD) of sputtered particles provides key information on the sputtering mechanism, experimental data on the AD of sputtered particles from multicomponent targets are still insufficient in quantity [1–10]. This is due to the difficulty in the experimental setup. During the AD measurement, the incidence angle of the primary ion beam should be kept constant. No commercially available SIMS, however, Corresponding author.
E-mail address: [email protected] (M. Tanemura). 0042-207X/$ - see front matter r 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2005.11.016
satisfies this requisite, because a change in the detection angle (ejection angle of ions) by a tilt operation of a sample always entails a change in the ion-incidence angle. Ideally, ADs of sputtered particles should be measured by a mass spectrometer moving around a specimen during sputtering. This method, however, is not realistic, because a huge vacuum chamber, in which a mass spectrometer is installable, is necessary. Alternatively, ADs have been measured by a so-called collector method [1–10]. The sputtered material is collected onto a thin cylindrical foil located just above a specimen, and the amount of the deposit onto the foil at various ejection angles is quantified after sputtering. Although this collection method is very simple, sputter-ejected clusters are not distinguishable from monomers. Very recently, we noticed that a special geometrical configuration of a primary ion gun, a sample and a detector enables the AD measurement of sputtered ions by a simple tilt operation of the sample with almost no change in the ion-incidence angle. Based on this idea, we have
ARTICLE IN PRESS S. Kawaguchi et al. / Vacuum 80 (2006) 768–770
constructed a compact angle-resolved SIMS composing of a micro-focus ion gun, a tiltable sample stage and a timeof-flight (TOF) mass spectrometer. In the present study, we will deal with the details about this system and also ADs of sputter-ejected ions from an HfN film measured using this system. 2. Experimental Our experimental system (Fig. 1), whose size is 90 cm 140 cm 120 cm (height), comprises a differentially pumped micro-beam ion gun (JEOL; MIED-III), a TOF mass spectrometer (Jordan Co. Ltd.; D-850 AREF), and a tiltable sample stage (JEOL; AP-UHVGS). Fig. 2 shows their geometrical configuration. The ion gun is located in the X–Z plane with an angle of 601 between the ion-gun axis and Z-axis. The TOF detector is set in the Y–Z plane with an angle of 301 between the TOF detector
Ion Gun
Primary ion
VXY1
TOF
FDC
θout
Sample
1
VR2
VXY2
2
L MCP
GND
Fig. 1. Schematic diagram of the developed system.
769
axis and Z-axis. The sample is tiltable around the X-axis from y ¼ 301 to 301. At the tilt angle y ¼ 0, the detection angle (ejection angle of ions) from the normal to the sample surface [1] is yout ¼ 301 (Fig. 2(a)). The ion-incidence angle, yin , which is generally defined as the angle between the iongun axis (~ a) and the normal to the sample surface (~ n), and the ejection angle, yout , defined as the angle between the detector axis and ~ n are calculated by scalar products of respective vectors (Fig. 2(b)): yin ¼ cos1 ðcos 60 cos yÞ
(1)
and yout ¼ 30 y,
(2)
respectively. For tilt angles y ranging from 301 to 301, ion-incidence angle yin is almost constant, 621721, while the ejection angle yout varies from 01 to 601. Thus, AD of sputtered ions can be measured for oblique sputtering at an almost constant ion-incidence angle by a simple tilt operation of a sample. The polar representation of directions of the primary ion beam and the detector is shown in Fig. 2(c). In order to check the performance of the system, an HfN film deposited onto Si [11] was prepared. The sample was ion-bombarded with 3 keV Ar+ ions in an ultra-high vacuum (UHV) condition. The ion beam was focused into a microbeam of 350 mm in diameter. The base pressure in the chamber was 6.0 108 Pa, while the operating pressure during sputtering was maintained in the 106 Pa region, due to differential pumping of the ion gun. An entrance aperture, 2.5 mm in diameter, was mounted in front of the TOF system for the AD measurement. The distance between the sputtering spot on the sample surface and the entrance aperture was set to be 20 mm. The angular resolution was 7.151. 3. Results and discussion
(a)
(b)
(c) Fig. 2. Geometrical configuration of an ion gun, a sample and a detector for measuring the angular distribution of sputtered ions: (a) tilt angle ¼ 0; (b) tilt angle ¼ y. (c) The polar representation of directions of the primary ion beam and the detector.
In order to obtain reproducible data, the acquisition of mass spectrum was repeated 2000 times at respective ejection angles. The time necessary for this 2000 times acquisition and averaging was about 1 min. Fig. 3(a) shows a typical mass spectrum thus obtained at an ejection angle yout ¼ 0. Hf naturally should display peaks at 176, 177, 178, 179 and 180 amu in a mass spectrum due to the stable isotopes. In order to enhance the signal intensity and hence for the reproducible data, however, the measurement of mass spectra was carried out at a low-resolution mode with which isotope peaks are not separated. We will hereafter refer to the peaks at 180 and 194 amu as Hf+ and HfN+ peaks, respectively. As shown in Fig. 3(a), an intense Hf+ peak together with N+ and HfN+ peaks were detected. No trimer or larger sized ion was observed. Fig. 3(b) shows the dependence of Hf+, N+ and HfN+ intensities on the ejection angle yout , revealing that the respective peak intensities decreased with an increase in the ejection angle. It should be noted that the angular
ARTICLE IN PRESS 770
S. Kawaguchi et al. / Vacuum 80 (2006) 768–770
In the present case, if the HfN+ dimer forms via the recombination process, Eq. (3) is written as Y ðHfNÞ Y ðHfÞ Y ðNÞ / ¼ Y ðNÞ. Y ðHfÞ Y ðHfÞ
(4)
Thus, the intensity ratio of HfN+ to Hf+ should be directly proportional to the intensity of N+. This implies that the angular dependence of HfN+/Hf+ should show the identical tendency with that of N+. As shown in Figs. 3(b) and (d), however, experimental results do not agree with this theoretical prediction; N+ intensity decreased monotonously with an increase in the ejection angle, while HfN+/Hf+ intensity ratio increased gradually with the ejection angle. Thus, HfN+ dimer ions were thought to be generated via the ‘‘as such’’ direct emission process. Fig. 3. (a) Typical mass spectrum obtained from a HfN sample. yout : 01. (b) Angular distribution of ejected Hf+, N+ and HfN+ ions from the HfN sample bombarded with 3 keV Ar+ ions. (c) and (d) Angular dependence of intensities of N+ and HfN+ ions normalized with Hf+ ion intensities, respectively.
dependence of monomer-ion intensity seems to be more prominent than that of dimer-ion intensity. In order to highlight this, peak intensities were normalized with Hf+ intensities for respective ejection angles. The angular dependence of normalized N+ and HfN+ intensities, N+/Hf+ and HfN+/Hf+, respectively, is shown in Figs. 3(c) and (d). It is manifest from Figs. 3(c) and (d) that the normalized N+ intensities are almost independent of the ejection angle, implying that the AD of Hf+ is identical with that of N+, whereas the normalized HfN+ intensities increase with an increase in the ejection angle. Based on these facts, in what follows, we will discuss the formation mechanism of dimer ions. On the cluster formation, two models have been proposed [12]; (i) direct or ‘‘as such’’ emission from the ion bombarded surface, and (ii) recombination of independently emitted atoms or ions after leaving the surface [13–15]. Although experimental and theoretical effort has been devoted mainly for pure elemental targets, it is still controversial which mechanism dominates the formation of small clusters in sputtering. In the former case, (i), the cluster-ion yield is independent of the monomer-ion yield, whereas in the latter case, (ii), the cluster-ion yield, Y ðX n Þ, for an n-atom cluster X n is known to be in proportion to the nth power of the number, Y ðX Þ, of sputtered atom X , because the recombination is a statistical process [13–16] Y ðX n Þ / fY ðX Þgn .
(3)
4. Conclusion A compact angle-resolved SIMS composing of a differentially pumped micro-beam ion gun, a tiltable sample stage and a TOF detector was constructed. Using this system, ADs of sputtered ions from an HfN film by 3 keV Ar+-ion bombardment at the ion-incidence angle of 621721 were measured in a range of the ejection angle of 01–601. Since the angular dependence of HfN+/Hf+ intensity ratio was independent of that of N+ intensity, it was concluded that HfN+ dimer ions were generated via the ‘‘as such’’ direct emission process. References [1] Hofer WO. In: Behrisch R, Wittmaack K, editors. Sputtering by particle bombardment III. Berlin: Springer; 1991 [chapter 2]. [2] Sigmund P, Oliva A, Falcone G. Nucl Instr Meth 1982;194:541. [3] Biersack JP, Eckstein W. Appl Phys A 1984;34:73. [4] Huang W. Surf Interface Anal 1983;14:469. [5] Nagatomi T, Min K-Y, Shimizu R. Jpn J Appl Phys 1994;33:6675. [6] Kang HJ, Shimizu R. Surf Sci 1986;169:337. [7] Andersen HH, Stenum B, Sorensen T, Whitlow HJ. Nucl Instr Methods 1987;209/210:487. [8] Aoyama T, Tanemura M, Okuyama F. Appl Surf Sci 1996;100/ 101:351. [9] Tanemura M, Aoyama T, Otani O, Ukita M, Okuyama F, Chini TK. Surf Sci 1997;376:163. [10] Tanemura M, Ukita M, Okuyama F. Surf Sci 1999;426141. [11] Gotoh Y, Liao MY, Tsuji H, Ishikawa J. Jpn J Appl Phys 2003;42:L778. [12] Gnaser H, Hofer WO. Appl Phys 1989;A48:261. [13] Oechsner H, Gerhard W. Surf Sci 1974;44:480. [14] Gerhard W, Oechsner H. Z Phys B 1975;22:41. [15] Konnen GP, Tip A, de Vries AE. Radiat Eff 1974;21:269; 1975;26:23. [16] Gnaser H, Oechsner H. Surf Sci 1991;251/252:696.