Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation

Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx

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Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation Naresh T. Deoli ⇑, Lucas C. Phinney 1, Duncan L. Weathers Ion Beam Modification and Analysis Laboratory, Department of Physics, University of North Texas, Denton, TX 76203, USA

a r t i c l e

i n f o

Article history: Available online xxxx Keywords: Sputtering yields Angular distributions Beam fluence Heavy ion Rutherford backscattering spectrometry

a b s t r a c t The dependences of the total sputtering yield of Bi and the differential angular distribution of these sputtered Bi atoms on the fluence of 50 keV Ar+ ions at normal incidence have been experimentally measured. Polycrystalline Bi targets were used for these purposes. The collector technique and accurate current integration methods were adopted for the determination of angular distributions of sputtered Bi atoms. The ion fluence was varied from 1.9  1019 to 3.1  1020 ions/cm2. The sputtered atoms were collected on high purity aluminum foils under ultra-high vacuum (5  109 Torr). The collector foils were subsequently analyzed using heavy ion Rutherford backscattering spectroscopy. The shape of the angular distribution of sputtered atoms was found not to change significantly with the fluence, but the sputtering yield increased significantly from 2.2 ± 0.2 to 9.6 ± 0.6 atoms/ion over the fluence range studied. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Sputtering, or the removal of surface atoms by energetic particle bombardment, has been studied in detail over several decades for various ion–target combinations; even so, it still remains a subject of considerable interest. Quantities of specific interest are the sputtering yield Y, defined as the average number of target atoms sputtered per incident projectile particle, and the angular distributions of the particles sputtered from a surface; these quantities reveal important information about the sputtering mechanism [1]. For solid targets, one of the many factors that controls the sputtering yield for a given ion–target combination is the fluence of the bombarding ions at low energy (<100 keV). A surface generally will become rougher during sputtering, and the roughness in turn affects the sputtering yield. The target composition will also change with the implantation of the projectile species. Both of these effects can cause the sputtering yield to change with increasing projectile fluence until steady state conditions are achieved. Bismuth, a semi-metallic element with specific properties such as low surface binding energy and high sensitivity to electronic energy deposition is much less studied as compared to other elements [2]. Bi lends itself to a range of applications and is an

⇑ Corresponding author. Address: 1155 Union Circle # 311427, Denton, TX 76203, USA. Tel.: +1 9402937365. E-mail address: [email protected] (N.T. Deoli). 1 Present address: Amethyst Research Incorporated, 123 Case Circle, Ardmore, OK 73401, USA.

interesting element for materials modification. Recently, thin films from Bi based compounds produced by thermal evaporation, sputtering and electrochemical deposition techniques have been topics of active research [3–5]. Bi along with other elements like Ga, Au, and Li forms a liquid metal alloy which is used as a high beam brightness liquid metal ion source (LMIS) for focused ion beam application [6–8]. Despite the availability of theoretical and computational models, experimental data on the angular distribution of sputtered atoms are needed by modern technologies concerned with etching processes [9,10] and high quality thin film production [11]. The effect of surface morphology on sputtering yields of materials, which strongly depends on incident ion beam fluence, is still an area of active research [12–14]. The effects of ion fluence and surface morphology on sputtering yields for different ion–target combinations (Ar–Au; Ar–Cu; He–Nb and W; noble gases–Si; noble gases and Zn, Ag, Au–Au) have previously been reported [15–19]; however, there has been no report so far for the ion fluence dependence of angular distributions of sputtered atoms from the surface of solid polycrystalline Bi. In this paper, we report the measured differential angular sputtering yields and total sputtering yields of the heaviest stable element, Bi, sputtered by normally incident 50 keV Ar+ ions for four different fluences. Argon, being one of the noble gases, is widely used as a projectile for sputtering studies to avoid chemical sputtering. Also, the Ar-induced sputtering yield of Bi reaches a maximum around 50 keV for a given incidence; hence the choice for ion energy [20]. The findings from the current experiment not only enrich the existing sputtering yield database but also provide

http://dx.doi.org/10.1016/j.nimb.2014.02.079 0168-583X/Ó 2014 Elsevier B.V. All rights reserved.

Please cite this article in press as: N.T. Deoli et al., Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.079

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a baseline for future planned experiments to measure the angular and isotopic sputtering yields from a Bi:Ga alloy that exhibits extreme surface segregation [21].

2. Experiment

3. Results and discussion Fig. 2 shows a representative Si backscattering spectrum of sputtered Bi on an Al collector foil. The breadth of the backscattered peak is attributed to the pulse height defect for the heavy Si ions in the solid state detector [27]. The measured Bi areal densities on the collector foil were converted to a differential sputtering yield dY/dX; this conversion included a small correction for misalignment in the sputtering geometry. The differential sputtering yield as a function of polar angle h in the sputtering geometry is plotted in Fig. 3 for 50 keV Ar+ on Bi for beam fluences of 1.9  1019, 8.0  1019, 1.6  1020, and 3.1  1020 ions/cm2. The resulting differential angular sputtering yield data were fitted with a function:

dY ¼ A cosB h dX

ð1Þ

where A (atoms/ion) and B are the fit parameters. The fitted differential angular sputtering yields obtained using Eq. (1) as a function of polar angle h are also plotted in Fig. 3. This fitted function, when integrated over the 2p-steradian solid angle in front of the target surface, gives the total sputtering yield:

Y¼

2pA Bþ1

ð2Þ

The error analysis includes the statistical uncertainty in the areal densities from the RBS measurements, uncertainties in current integration and RBS detector solid angle, and uncertainty

4000

← Bi Peak 3000

Counts

High purity (99.9999%) Bi shot was flattened on a tantalum disc using a mechanical press to produce a sputtering target 60 mm2 in area and 1 mm thick. Before being inserted into the UHV sputtering chamber, the target was etched in a diluted H2SO4 ultrasonic bath for 2 min, rinsed in deionized water and methanol, and warmair dried to remove any surface contaminants and passivate the surface with a thin layer of oxide. The 50 keV Ar+ ion beam used for sputtering was extracted from a 200 kV Cockcroft Walton accelerator with a radio-frequency plasma ion source. Before reaching the target, the ion beam passed through an electrostatic quadrupole lens, collimating horizontal and vertical slits, a 45° analyzing magnet, and another set of collimating slits [22]. At the entrance to the UHV chamber, the beam passed through a custom designed electrostatic quadrupole triplet lens [23] that focused the beam to a diameter of 50 lm on target. The beam currents in the beamline were measured using two Faraday cups with secondary electron suppression, located after the collimating slits. The beam current on target was between 10 and 50 nA. The target and collector foil holders were introduced in the UHV chamber through a load-lock without breaking the vacuum. The chamber was then baked for more than 24 h at 150 °C. An ion pump and Ti sublimation pump were used to further improve the vacuum in the chamber and maintain it at the base pressure of 5  1010 Torr. During the sputtering experiment, the chamber pressure rose to as high as 5  109 Torr due to the relatively higher pressure in the beamline and loading of Ar in the chamber, and stayed stable for the entire sputtering process. A target bias of +36 V for the 50 keV Ar+ beam was chosen after a series of measurements to suppress any secondary electrons and tertiary electrons reaching the collector foil holder. A cross-section of the sputtering geometry and collector foil holder are shown in Fig. 1. To collect the sputtered materials, 99.995% pure aluminum foil was cut into 9  71 mm strips, which were mounted in cylindrical holders that could be placed in front of the target. A hole was cut in the center of each strip to allow passage of the ion beam. A spot was sputtered-cleaned using 3.2  1017 ions/cm2 to avoid the collection of any contamination on the Bi target surface. The target was then sputtered at the same spot with the beam fluence of 1.9  1019 ions/cm2. This procedure was repeated at different spots for beam fluences of 8.0  1019, 1.6  1020, and 3.1  1020 ions/cm2. After sputtering, the collector foils were removed from the UHV chamber and mounted on a target holder in an analysis chamber attached to the Ion Beam Modification and Analysis Laboratory’s

3 MV National Electrostatics Corporation Tandem Pelletron (9SDH-2) accelerator facility [24]. A 2.0 MeV Si+ beam was then used to perform Rutherford backscattering spectroscopy (RBS) to measure the areal density of the Bi along the length of the Al collector foil. A projectile species more massive than Al was desired, and Si was a convenient choice. For these measurements, a 2 mm diameter, 5 nA beam was used, and an integrated charge of 2 lC was accumulated at each position. Many factors affect the accuracy of current integration measurements when fast ions impinge upon a solid target [25]. The experimental set up for RBS and the methods for accurate current integration used for the collector foil analysis are described in detail elsewhere [26].

2000

1000

0 0.0

0.5

1.0

1.5

2.0

2.5

3.0

Backscattered Energy (MeV)

Fig. 1. Schematic of the sputtering collector geometry. The target was bias to +36 V to suppress secondary and tertiary electrons. The foil radius is 23 mm.

Fig. 2. RBS spectrum of Bi on Al collector foil using 2.0 MeV Si+ beam. The distance of the foil to the detector face was 8.0 cm and the backscattering angle was 157°. The spectrum shown is for collected Bi atoms at an angle 15.7° from target normal due to the sputtering by 50 keV Ar+ beam of a fluence of 3.1  1020 ions/cm2. Because Si is more massive than Al, there is no backscattering signal from Al.

Please cite this article in press as: N.T. Deoli et al., Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.079

3

11

6

5

A

10

B

9

C

8

D

4

Y (atoms/ion)

Differential Sputtering Yield (atoms/ion)

N.T. Deoli et al. / Nuclear Instruments and Methods in Physics Research B xxx (2014) xxx–xxx

E F

3

G H

2

7 6 5 4 3

Measured yield Extrapolated to zero fluence

2

1

1

0 -90

-60

-30

0

30

60

90

0 0.0

19

7.0x10

1.4x1020

2.1x1020

20

2.8x10

3.5x1020

2

Fluence(ions/cm )

Polar Angle (degrees) Fig. 3. Plot of differential sputtering yield (atoms/ion) vs. polar angle (degrees) due to 50 keV Ar+ on Bi for different beam fluences. Series G, E, C and A are measured for bombarding ion fluences of 1.9  1019, 8.0  1019, 1.6  1020, and 3.1  1020 ions/ cm2, respectively. H, F, D and B are the fitted differential yields obtained from Eq. (1) for bombarding ion fluences of 1.9  1019, 8.0  1019, 1.6  1020, and 3.1  1020 ions/cm2, respectively. Random uncertainties (±1r) in the measured values are smaller than the size of the plotted symbols.

from the nonlinear curve fit. In Fig. 3, series G and H show the measured and fitted differential sputtering yield for 50 keV Ar+ ion at 1.9  1019 ions/cm2, respectively. The values for the fitting parameter for series H are provided in Table 1. The over-cosine shape of the angular sputtering distribution is typical of that observed for other polycrystalline metals [28]. The measured total sputtering yield of 2.2 ± 0.2 Bi atoms/Ar ion for normally incident 50 keV Ar+ ions at the fluence of 1.9  1019 ions/cm2 is low compared to values from sputtering models: 5.5 atoms/ion obtained from the Monte Carlo computer simulation code SRIM [20], 11.5 atoms/ion obtained from Sigmund’s theory [29], and 10.4 atoms/ion obtained from the semi-empirical formula for ion-induced sputtering yields from monatomic solids at normal incidence developed by Yamamura et al. [30]. Series E, C, and A in Fig. 3 show the measured differential sputtering yield of Bi for normally incident 50 keV Ar+ ions at the beam fluences of 8.0  1019, 1.6  1020, and 3.1  1020 ions/ cm2, respectively. The over-cosine distribution is more prominent in series E, C, and A as compared to series G. Series F, D, and B show the fitted differential sputtering yields of Bi for normally incident 50 keV Ar+ ions for the beam fluences of 8.0  1019, 1.6  1020, and 3.1  1020 ions/cm2, respectively. The values of the overall sputtering yield and the fitting parameters for series F, D, and B along with their relative uncertainties are provided in Table 1. Fig. 4 shows a plot of sputtering yield Y for Bi as a function of incident ion beam fluence. The line in the plot is extrapolated to zero fluence sputtering yield and is to guide the eye. As seen in Figs. 3 and 4 and Table 1, the differential and total sputtering yield for Bi by normally incident 50 keV Ar+ ions

Fig. 4. Plot of total sputtering yield Y (atoms/ion) vs. ion fluence (ions/cm2) for normally incident 50 keV Ar+ on Bi. The dots represent measured Bi sputtering yield and the line is to guide the eye and extrapolate to zero fluence.

increases with increasing ion fluence for the measured fluence range. The present measurements of the angular sputtering yield of Bi in the high fluence regime were made starting from similar surface conditions. Recently Mammeri et al. [31] measured the fluence dependence of sputtering yield of Bi using 60 and 120 keV Ar+ projectiles. The targets used in those investigations were Bi evaporated onto Si wafer, and the incident ion beam fluences were in the range of 1015–1016 ions/cm2. The sputtering yield for both Ar ion energies on Bi/Si was reported to be 8 Bi atoms/ion. Mammeri et al. reported a decrease in the sputtering yield of Bi with increase in bombarding beam fluence. The difference between the results of their investigation and those of the current study is attributed to the target used and the higher bombarding fluence used here. The increase and decrease in sputtering yield with increasing bombarding ion beam fluence has previously been reported [15,19,32]. Also, it has been reported previously that inert gas implantation into solid surfaces has a significant effect on the sputtering yield as compared to other implanted elements [19]. We attribute the increase in sputtering yield with fluence in the current study to a change in the surface roughness and pitting caused by the bombarding ions, changes in surface binding energy, and implantation of Ar+ ions into the sample while sputtering. The interesting result from the current measurements is that the sputtering yield was not observed to saturate as a function of ion fluence in the measured fluence regime, although the value for the sputtering yield at the largest fluence was 9.6 atoms/ion, close to the predicted yield of 10.4 atoms/ion one obtains from the semi-empirical formula of Yamamura and Tawara [30]. There are no measurements available for the sputtering yield of Bi in the fluence regime of 1019–1020 ions/cm2 to compare with the current results.

4. Summary and conclusions Table 1 Total sputtering yield (atoms/ion) as a function of ion fluence for 50 keV Ar+ ions normally incident on solid polycrystalline Bi. Values for fitting parameters A (atoms/ ion) and B along with their relative uncertainties are also provided. Ion fluence (ions/cm2)

Sputtering yield (atoms/ion)

A (atoms/ion)

B

1.9  1019 8.0  1019 1.6  1020 3.1  1020

2.2 ± 0.2 3.9 ± 0.3 5.5 ± 0.4 9.6 ± 0.6

1.0 ± 0.1 2.2 ± 0.1 3.2 ± 0.2 4.9 ± 0.3

1.9 ± 0.2 2.5 ± 0.1 2.6 ± 0.1 2.2 ± 0.1

We have measured the sputtering yield of solid polycrystalline Bi surface for normally incident 50 keV Ar+ ions at four different fluences, along with the angular distribution of sputtered Bi atoms with respect to target normal. It was observed in the current study that the sputtering yield increased with increasing bombarding ion fluence over the range that we have investigated. The shape of angular distribution was found not to be strongly dependent on bombarding ion fluence for the measured fluence range, and the fitted power of the over-cosine distributions varied from a

Please cite this article in press as: N.T. Deoli et al., Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.079

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Please cite this article in press as: N.T. Deoli et al., Ion fluence dependence of the total sputtering yield and differential angular sputtering yield of bismuth due to 50 keV argon ion irradiation, Nucl. Instr. Meth. B (2014), http://dx.doi.org/10.1016/j.nimb.2014.02.079