Elastic recoil detection analysis for large recoil angles (LA-ERDA)

Nuclear Instruments and Methods in Physics Research B 170 (2000) 163±170

www.elsevier.nl/locate/nimb

Elastic recoil detection analysis for large recoil angles (LA-ERDA) I. Bogdanovic Radovic a

a,b,*

, E. Steinbauer a, O. Benka

a

Institut fur Experimentalphysik, Johannes Kepler-Universit at Linz, Altenbergerstraûe 69, A-4040 Linz, Austria b Ruder Boskovi c Institute, P.O. Box 180, 10002 Zagreb, Croatia Received 29 December 1999; received in revised form 15 February 2000

Abstract In this paper, elastic recoil detection (ERD) measurements at recoil angle of 60° using ion-induced electron emission (IEE) for particle identi®cation are presented. In our IEE system for particle identi®cation, recoiled target atoms and scattered projectiles penetrate a set of thin carbon foils before their energy is analyzed in a solid state detector. Particle identi®cation is based on the fact that the total number of electrons emitted from the foils depends on the particle nuclear charge. This method is characterized by its low minimum detectable energy, which stimulated us to study ERDA at 60°. Due to collision kinematics and due to the angular dependence of the scattering cross-sections, it is expected that the sensitivity can be signi®cantly improved. In this work, the detection eciency of the IEE particle identi®cation system for H recoils at energies below 1 MeV was determined. LA-ERDA measurements were performed with 4 He and 12 C projectiles using two di€erent types of samples with a well-known amount and depth distribution of H atoms near the surface. Sample 1 consisted of a 50 lg/cm2 melamine layer evaporated on a ¯at Si substrate, sample 2 was a Si wafer with implanted H. Sensitivity and depth resolution were measured using LA-ERDA with a recoil angle of 60° and ERDA with recoil angles of 30° and 45°. The results for di€erent recoil geometries and projectiles are discussed and compared with theoretical predictions. Ó 2000 Elsevier Science B.V. All rights reserved. PACS: 34.50.)s; 07.81.+a; 68.55.Jk Keywords: Large angle elastic recoil detection analysis; Sensitivity; Depth pro®ling

1. Introduction One of the most useful ion-beam techniques developed for quantitative analysis and depth pro®ling of light elements in surface layers and

* Corresponding author. Tel.: +385-1-4561-161; fax: +385-14680-239. E-mail address: [email protected] (I. Bogdanovic RadovicÂ).

thin ®lms is elastic recoil detection analysis (ERDA). In contrast to Rutherford backscattering, where only backscattered projectiles can reach the energy detector, in ERDA both scattered projectiles and recoiled target atoms are simultaneously detected in the forward direction. In the last two decades, di€erent methods for separating scattered projectiles from the recoiled atoms based on particle identi®cation or discrimination have been developed [1±5]. The most simple and widely used is ERDA with a stopping foil.

0168-583X/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 0 ) 0 0 0 7 7 - X

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A new system for ERDA was developed a few years ago at the University of Linz. The system uses ion-induced electron emission (IEE) for particle identi®cation [6]. Scattered ions and recoiled target atoms pass through a set of thin carbon foils before they reach a solid state (energy) detector. The number of electrons emitted from the foils is roughly proportional to the stopping power of the particles in the foil. The emitted electrons are accelerated toward a microchannel plate (MCP). The signal from the MCP is measured in coincidence with the energy signal from the solid state detector and recorded in a two-dimensional multichannel analyzer. The number of detected particles is displayed as a function of solid state detector pulse height (energy) and MCP pulse height (number of emitted electrons). Compared to the standard ERDA setup with a stopping foil or with gas telescopes, which also have a foil at the entrance window, this system gives better depth resolution due to the large reduction of energy loss straggling. Because the carbon foils are thin, their in¯uence on total energy resolution was considered as negligible. This setup was already tested for H depth pro®ling with MeV 4 He ions. It was shown that it is possible to separate 1 H and 2 H using a 4.8 MeV He beam and a scattering angle of 45° [7]. The energy of the recoiled atom strongly depends on the recoil angle h (proportional to cos2 h). From that, larger recoil angles are not possible for systems with a high minimum detectable energy. The low threshold energy of our system (70 keV) stimulated us to study the advantages and disadvantages of LA-ERDA. One of the biggest advantage is the increase of recoil cross-sections (proportional to cosÿ3 h) and therefore an enhanced sensitivity is expected. A highly sensitive setup minimizes possible target damage caused by the incoming projectiles. The optimum conditions concerning sensitivity, depth resolution and maximum probing depth were ®rst studied by computer simulations using the program DEPTH [8]. According to these calculations the experimental parameters have been chosen to achieve maximum sensitivity for H detection near the sample surface. ERDA at 60° was tested using two di€erent types of samples with a well-known distribution and amount of H near the sample surface. The results

were compared with theoretical predictions and with ERDA measurements at 30° and 45°. 2. Experimental setup All measurements were done using the 1.6 MV 5SDH Pelletron tandem accelerator at the Johannes Kepler University in Linz. Projectiles were 3 MeV 4 He as well as 5.5 and 6 MeV 12 C ions. The base pressure in the chamber was about 5 ´ 10ÿ8 mbar. The existing scattering chamber for ERDA measurements at 30° and 45° was modi®ed so that scattering angles of 60° and 75° can be selected. Targets can be transferred to a sample holder using a load-lock system without breaking the vacuum. The number of 6 lg/cm2 thick carbon foils inside the particle identi®cation system was reduced from 6 to 4 without signi®cantly deteriorating the height of the electron signal and, thus, mass separation capability. The electron emission yield was enhanced by coating the carbon foils with a thin CaF2 ®lm (thickness 4 lg/cm2 ) on both sides and tilting them by 45° to the beam direction. A precision manipulator can move the sample holder. The angle between the sample surface and the beam direction can be set with 1° accuracy. Using an optical microscope the position of the target can be reproduced to 0.2 mm. The microscope can also be used during the measurements to monitor target changes caused by beam irradiation. The beam spot on the target is de®ned by a narrow beam entrance slit, whose width was kept below 0.2 mm to minimize geometrical e€ects. The width of the slit can be adjusted by a manual feedthrough without breaking the vacuum. More details about the scattering chamber and the particle identi®cation system based on IEE can be found elsewhere [6,7]. The LA-ERDA system was compared with the ERDA measurements at lower angles using two di€erent types of samples. As a sample, which is rather sensitive to beam intensity and total ¯uence, a 50 lg/cm2 thick melamine layer was deposited by vacuum evaporation of 99% pure melamine (C3 H6 N6 ) on a crystalline Si substrate. The second sample, which is more stable to beam irradiation but which contains a rather low amount of H, was

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prepared by implanting 2  1016 H atoms/cm2 into a Si wafer. In order to achieve a small implantation depth, 30 keV H‡ ions were used and the sample was tilted by 50° to the beam direction during implantation. The projected range and the implanted pro®le were calculated using the Monte Carlo simulation program TRIM [9]. According to the simulation the maximum concentration of implanted H is expected at a depth of 265 nm. 3. Results and discussion 3.1. Theoretical predictions Depth resolution and sensitivity of our 60° ERDA setup were studied by computer simulations using the programs DEPTH [8] and SIMNRA [10]. In SIMNRA it is possible to choose a user-de®ned H recoil cross-section database. Since there are no experimental cross-sections available for 45° and 60°, theoretical cross-sections calculated by Yansen et al. [11] were used. The results for 60° were compared with those for a frequently used setup (standard setup: 3 MeV 4 He projectiles, scattering angle h ˆ 30°, angle between the ion beam and the sample surface a ˆ 15°). For 4 He projectiles it can be seen that the H yield emerging from a thin H layer at the sample surface, thus avoiding matrix e€ects, is about 2 times higher at 60° than that at 30°. Even larger improvements concerning sensitivity could be obtained with heavier ions (12 C, 16 O). Using 5.5 MeV 12 C ions at 60° instead of 30° will give about 5 times higher yield for the same number of impinging projectiles. Sensitivity can be even more improved by using ions with lower energies. However, a lower limit for the ion energy is given by scattering kinematics because the energy of the recoiled H atoms must exceed the minimum detectable energy, which is as low as 70 keV in our case. Due to the strong angular dependence of the recoil energy this limitation is more restrictive at large scattering angles. The depth resolution dx is the ability to separate, in the spectrum along the energy axis, the signal coming from recoiled atoms at di€erent depths in the sample and it is de®ned as [12]

dx ˆ

dEd ; dEd =dx

165

…1†

where dEd is the total energy resolution and dEd =dx is the e€ective stopping power [12]. There are several factors contributing to dEd [8], but close to the sample surface geometrical contributions arising from the ®nite beam spot and the detector solid angle dominate. Calculations show that for 4 He projectiles the depth resolution for H in a Si matrix close to the surface is more than two times better at 30° than at 60°, mainly because of the larger e€ective stopping power. The in¯uence of other contributions to dEd (except the detector energy resolution and energy straggling in the IEE foils) at two di€erent recoil angles can be seen from Fig. 1. For larger recoil angles and at larger depths multiple scattering along the outgoing trajectories becomes more important due to the fact that the energy of the recoiled atoms is much lower at 60° than at 30°. Use of heavier ions improves

Fig. 1. Contributions to the energy spread dEd (keV) for hydrogen depth pro®ling in a Si matrix with 3 MeV 4 He ions just after the sample (before the detector and IEE foils). The energy spread was calculated for two di€erent recoil angles, h ˆ 30° and h ˆ 60°. Solid line: total energy resolution, dashed line: geometrical resolution, dotted line: energy straggling, dash± dotted line: multiple scattering along the incident path and dash±dot±dotted line: multiple scattering along the outgoing path.

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the depth resolution due to the larger e€ective stopping power. For depth pro®ling, it is important to distinguish between H atoms at the surface and H in the bulk. From the DEPTH program, it can be seen that for 60° LA-ERDA with 3 MeV 4 He ions the depth resolution is larger than the depth for all depths 6 350 nm. The situation is much better for the 60° ERDA with 5.5 MeV 12 C ions, where the minimum depth after which the H distribution can be resolved is about 100 nm. This minimum depth is comparable with the standard setup (30° ERDA using 3 MeV 4 He ions) for which the minimum depth is about 70 nm. All calculations were made for H in a Si matrix with the angle between the ion beam and the sample surface at 15°. Therefore, in LA-ERDA signi®cant improvements for the H sensitivity with minor deterioration of depth resolution can be expected only if heavier projectiles are used. 3.2. Detection eciency of the IEE particle identi®cation system As it has been pointed out, our IEE system for particle identi®cation is characterized by its low threshold energy and therefore it is suitable to detect low energy recoils. In LA-ERDA this is often the case since the energy of the recoiled atoms decreases strongly with increasing scattering angle. Energy loss and multiple scattering of recoiled H atoms in the IEE foils, which are negligible at higher energies, must be taken into account. TRIM calculations reveal that for 1 MeV H ions the energy loss in the foils is about 12 keV, whereas 200 keV H ions lose 32 keV. Due to the foil con®guration (four carbon foils coated with CaF2 on both surfaces and tilted by 45° to the beam direction), it is not so easy to calculate the ®nal angular distribution of the ions caused by the multiple scattering e€ect in the foils. The FWHM of the multiple scattering angular distribution has been estimated using the theory of Sigmund and Winterborn [13]. The calculations show that the FWHM increases rapidly for H recoils at energies below 1 MeV. Therefore, it is possible that some part of the low energy H recoils is not detected by the energy detector.

In order to measure this e€ect, two spectra of 2 MeV protons scattered from a thick Si wafer were collected and compared. The ®rst spectrum was taken with the IEE particle identi®cation system (foils) in its position. After that, the foils were removed and a second spectrum was recorded. After correction for the energy loss in the foils, both spectra were normalized to the same number of counts in the high-energy region where multiple scattering e€ects should be negligible. Fig. 2 shows the resulting detection eciency obtained from the ratio of the two spectra. In the low energy region the eciency drastically decreases if the foils are present. This correction was applied to the H recoil spectra at energies below 1 MeV. 3.3. Melamine sample It has been reported in the literature [14] that for 10±25 lg/cm2 thick melamine layer irradiated by 1.5 MeV 4 He ions, no signi®cant loss of H has been observed for ion currents lower than 1 pnA (particle nA) up to a total collected charge of 4 plC [14]. If currents higher than 1 pnA were used, loss of H started immediately. In our work, melamine samples were irradiated with 3 MeV 4 He and 5.5 MeV 12 C ions. To reduce H loss from the sample as much as possible, current density was kept low during all measurements. This was especially important for the measurements with 12 C ions where beam-induced damage was visible on the target after only a few

Fig. 2. Experimentally determined H detection eciency (%) of the IEE particle identi®cation system as a function of H energy (keV).

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seconds of measurement in all cases except for extremely low current densities. Starting with a fresh melamine target, spectra were recorded repetitively for constant ion ¯uence. The number of H counts was monitored and the spectra were added until the loss of H from the sample exceeded 10%. Since both H recoils and forward scattered projectiles were detected in the same solid state detector, the ratio between the H recoils and the total number of projectiles scattered from Si could be used as a H stability monitor. At the same time, the spectra of the forward scattered projectiles were used to calculate the total number of impinging projectiles. The energy spectrum of the recoiled H was obtained by integrating the two-dimensional spectrum in the direction of the MCP signal. After that, an energy calibration of the spectrum was done. To correlate the detected H energies with the depths from which the H atoms have emerged, a slab analysis was performed. Fig. 3(a) shows the H distribution in a 318 nm thick melamine layer which was used for the simulation. Fig. 3(b)±(d) shows the measured H depth pro®les for recoil angles of 30°, 45° and 60°, obtained with 3 MeV 4 He projectiles (full line). The same set of measurements performed with 5.5 MeV 12 C ions is shown in Fig. 4(a)±(c). The best depth resolution with 4 He ions is obtained for 30° recoil angle. At 45° the depth resolution becomes worse and at 60° the depth pro®le cannot be recognized at all. It was observed that for 12 C projectiles beam-induced H loss is much stronger than that induced by 4 He projectiles. This increase is much larger than expected from the assumption that H loss is proportional to nuclear stopping alone [15]. Therefore, for 12 C projectiles, only a very low number of counts could be collected for low recoil angles. Only at 60° a satisfactory counting statistics and a depth resolution comparable with the standard setup (30° ERDA with 4 He ions) were obtained. The experimental H spectra in Fig. 3(b)±(d) and 4(a)±(c) were compared with the simulations using the programs DEPTH [8] (dotted lines) and SIMNRA [10] (dashed lines). The most important di€erence between these two programs is that DEPTH takes into account multiple scattering of

167

Fig. 3. H depth pro®le for a 318 nm thick melamine layer on a ¯at Si substrate obtained with 3 MeV 4 He ions. (a) Theoretical pro®le used for SIMNRA and DEPTH simulations; (b) h ˆ 30°; a ˆ 15°; (c) h ˆ 45°; a ˆ 15°; (d) h ˆ 60°; a ˆ 15°. In ®gures (b)±(d) experimental depth pro®les (full line) are compared with the depth pro®les obtained from SIMNRA (dashed line) and DEPTH (dotted line) simulations.

incident ions and emitted recoils in the target, while SIMNRA neglects those e€ects. The total yields calculated with SIMNRA are in good agreement with the experimental ones (deviations < 15%), although the theoretical cross-sections from [11] were used. In DEPTH, it is not possible to change or modify the cross-section database. Therefore, total yields calculated by DEPTH were normalized to the experimental yields to allow for the comparison with the measured pro®le. From Fig. 3, it can be seen that at 45° and 60° H depth pro®les calculated with SIMNRA are signi®cantly narrower than the measured ones. Almost perfect agreement between experiment and theory for all three recoil angles and both ions is obtained by DEPTH. This again

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studied in these experiments. For both projectiles (4 He and 12 C) no signi®cant change of implanted H was observed for the total collected charge in all measurements. However, a completely di€erent behavior was found for the surface H. It was observed that the amount of surface H di€ers at di€erent locations on the target in the range from 1 to 2.5 ´ 1016 H atoms/cm2 . In all measurements with 12 C ions, the ratio between surface and implanted H was decreasing exponentially with ion ¯uence. In measurements with 4 He ions, no signi®cant loss of surface H was observed. In all simulations with SIMNRA, with the same crosssection database as in the case of melamine, the simulated yields were about 1.5 times larger than the measured yields. As the results are very con-

Fig. 4. H depth pro®le for a 318 nm thick melamine layer on a ¯at Si substrate obtained with 5.5 MeV 12 C ions. (a) h ˆ 30°; a ˆ 15°; (b) h ˆ 45°; a ˆ 15°; (c) h ˆ 60°; a ˆ 15°. Experimental depth pro®les (full line) are compared with the depth pro®les obtained from SIMNRA (dashed line) and DEPTH (dotted line) simulations.

con®rms that multiple scattering e€ects dominate ERDA energy resolution at large recoil angles and at larger depths in the sample. 3.4. Implanted sample Our LA-ERDA setup was ®nally used for the depth pro®ling of a Si sample implanted with 2  1016 H atoms/cm2 . Measurements were performed with 3 MeV 4 He and 6 MeV 12 C ions. The angle between the beam direction and the sample surface was 12° for all measurements. In all measured spectra two H peaks were present: one at the surface and one corresponding to the implanted H. We believe that the surface peak is due to contamination, either induced during the implantation process from the vacuum pumping system or due to water adsorbed at the surface during sample handling. The behavior of the implanted as well as the surface H under the beam irradiation was also

Fig. 5. H depth pro®le for the H-implanted Si-sample obtained with 3 MeV 4 He ions. (a) Theoretical pro®le used for SIMNRA and DEPTH simulations; (b) h ˆ 30°; a ˆ 12°; (c) h ˆ 45°; a ˆ 12°; (d) h ˆ 60°; a ˆ 12°. In ®gures (b)±(d) experimental depth pro®les (full line) are compared with the depth pro®les obtained from SIMNRA (dashed line) and DEPTH (dotted line) simulations.

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sistent for all geometries and both ions (6 independent measurements), we conclude that the actual amount of implanted H is lower than speci®ed. For an actual concentration of 1.35 ´ 1016 atoms/cm2 the simulations give best agreement with the measurements. Fig. 5(a) shows the depth pro®le of the implanted H calculated by the program TRIM. The depth pro®les measured with 4 He ions are presented in Fig. 5(b)±(d) and those measured with 12 C ions in Fig. 6(a)±(c). The depth resolution obtained with 3 MeV 4 He ions at 60° is very poor and the implanted H pro®le can hardly be seen. For the measurement with 6 MeV 12 C ions at 60° satisfactory depth resolution and signal intensity for a total collected charge of 1 plC was obtained. Although the resolution at 30° with He ions is slightly better, a very low number of counts was collected per 1 plC. Therefore, in that case, LA-ERDA at 60° using 12 C projectiles is most suitable.

169

Table 1 Experimental depth resolution at the surface calculated from the FWHM of the surface H peak (nm)a Ion, energy

30°

45°

60°

He, 3 MeV C, 6 MeV

46 (48) 20 (10)

77 (64) 35 (34)

140 (122) 65 (54)

a

Theoretical estimates calculated with DEPTH are given in parenthesis.

The measured depth pro®les at all scattering angles again show very good agreement with the predictions of the DEPTH and SIMNRA programs for 12 C ions. In the case of 4 He, correct treatment of the multiple scattering e€ects is essential to achieve good agreement between experiment and simulations. Since the surface peak is very narrow, it can be used to estimate the depth resolution close to the surface. The experimental values are compared with the values calculated with the DEPTH program in Table 1. Again, good agreement between experiment and theory is obtained. 4. Conclusions

Fig. 6. H depth pro®le for the H-implanted Si-sample obtained with 6 MeV 12 C ions. (a) h ˆ 30°; a ˆ 12°; (b) h ˆ 45°; a ˆ 12°; (c) h ˆ 60°; a ˆ 12°. Experimental depth pro®les (full line) are compared with the depth pro®les obtained from SIMNRA (dashed line) and DEPTH (dotted line) simulations.

In this paper, ERDA measurements of H at a recoil angle of 60° are presented. They are compared with measurements at 30° and 45° using 4 He and 12 C ions. Two types of samples were used: a thin melamine layer on Si and hydrogen implanted in Si. Due to the increase of recoil cross-sections with increasing recoil angle, two times higher H yields are obtained at 60° ERDA with 4 He ions. However, in that case the depth resolution is signi®cantly worse due to smaller e€ective stopping power, lower recoil energy and, thus, more pronounced geometrical and multiple scattering effects. Very good agreement is found between experimental spectra and theoretical depth pro®les calculated by the program DEPTH in all cases. According to the present measurements LA-ERDA with 4 He ions can be useful in cases where the main interest is to measure H at the surface (e.g., surface contamination of a sample with H) and where high sensitivity is required. However, measurements with 4 He ions at 60° should not be used for

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problems where depth pro®ling is important because of the poor depth resolution. If LA-ERDA is performed using heavier ions, the achievable depth resolution is comparable with that for the standard setup. At the same time, the sensitivity can be signi®cantly improved. However, special care must be taken during the measurements with heavier projectiles concerning possible H loss. Acknowledgements This work was supported by the Austrian Science Foundation (FWF) project number M518TPH.

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