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Investigation of deep implanted fluorine channeling profiles in silicon using resonant NRA
Nuclear Instruments and Methods in Physics Research B 201 (2003) 623–629 www.elsevier.com/locate/nimb
Investigation of deep implanted fluorine channeling profiles in silicon using resonant NRA M. Kokkoris a,*, G. Perdikakis a,b, R. Vlastou b, C.T. Papadopoulos b, X.A. Aslanoglou c, M. Posselt d, €tzschel d, S. Harissopulos a, S. Kossionides a R. Gro a
d
Institute of Nuclear Physics, NCSR ÔDemokritosÕ, Laboratory for Material Analysis, P.O. Box 60228, GR-153 10 Aghia Paraskevi, Athens, Greece b Department of Physics, National Technical University of Athens, GR-157 80, Greece c Department of Physics, The University of Ioannina, GR-451 10 Ioannina, Greece Institute of Ion Beam Physics and Material Analysis, Forschungszentrum Rossendorf, Postfach 510119, D-01314, Dresden, Germany Received 26 August 2002; received in revised form 27 December 2002
Abstract Si(1 0 0) and (1 1 1) crystals were irradiated in the random as well as in the channeling direction, using 5 MeV 19 Fþ ions, to a maximum fluence of approximately 1 1017 particles/cm2 . The occurring deep implanted profiles were subsequently investigated using the Resonant Nuclear Reaction Analysis technique in the energy range Ep ¼ 950–1200 keV. The reaction 19 F(p; ac)16 O reaction exhibits a strong resonant behavior in the above mentioned energy range, thus providing an excellent tool for the depth profiling of fluorine, yielding minimum detection limits of the order of a few ppm. The occurring profiles are analyzed with SRIM and c-TRIM codes and an attempt is made to explain the characteristics of the experimental spectra, as well as to compare with results already existing in literature. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.85.+p Keywords: High-energy implantation; Channeling; Nuclear resonance; Fluorine profiling; Resonant NRA; c-TRIM
1. Introduction The effects of irradiation on the properties of solids are of significant interest in the scientific and
* Corresponding author. Tel.: +30-1-4288217/6518770; fax: +30-1-6511215. E-mail address: [email protected] (M. Kokkoris).
technological context. Several studies have been presented concerning irradiations with ions having an energy of more than 100 keV/nucleon [1–3]. These ions find an ever increasing application in the modification of the properties of metals and semiconductors. There exists a growing interest in the physical nature of the effect of this high-energy ion implantation as it is considered to be a promising way of increasing the microelectronic chip integration by the formation of multilayer
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00448-8
624
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629
three-dimensional structures [4]. Indeed, nowadays, implantation at MeV energies is frequently used in VLSI fabrication, with the most common application being the formation of deep isolated regions among individual devices and of tubs in CMOS structures. Moreover, there are strong indications that the increase of ion energy does not lead exclusively to quantitative changes of the ion implanted layer parameters, but also to qualitative changes of the defect-impurity structure of the whole irradiated area [5]. This type of implantation is mainly characterized by the high linear density of the energy contribution of ions into the electronic subsystem of the target, resulting in electronic energy losses of the order of MeV/lm. It is also characterized by the fact that this type of losses strongly prevail over the direct transmission of the ion energy to the nuclear subsystem of the target, which is predominant at the end of the mean ion projected range, but almost negligible otherwise (less than 3% in our case as shown with the use of the SRIM code [6]). It should be noted here, however, that the data on defect production and annealing at depths of the order of a few lm inside a target are sparse, often contradictory and by no means conclusive as to the very nature of the mechanism of the phenomenon. On the other hand, to the authorsÕ best knowledge there is a lack in literature, concerning data in the case of high-energy irradiations and their effects in the channeling mode. It is well known that the channeling procedure provides an easy means for placing ions deep inside the target, while minimizing lattice damage. However, the channeled atom profile is sensitive to the beam divergence, as well as to scattering by amorphous surface films and residual damage from previous processing steps. Interesting works presented recently concern low-energy channeling implantations and examinations of the depth profiles using SIMS [7–10]. Nevertheless, despite its superb mass, depth resolution and sensitivity (108 atoms/cm2 ), this technique is not suitable for the examination of channeling profiles that extend over a few lm inside the target. In the present work Si(1 0 0) and (1 1 1) crystals were irradiated in the channeling as well as in
the random direction, using 5 MeV 19 Fþ ions, to a maximum fluence of 1 1017 atoms/cm2 . The occurring implantation profiles were subsequently examined using the Resonant NRA technique via the 19 F(p; ac) reaction. Resonant NRA provides a reliable and accurate means for the investigation of several light ion depth profiles (H, Li, B, Be, C, O, N, Al and F) and has been successfully applied in the past for the investigation of amorphous materials (rocks, archeological flints) and biological samples [11]. This work extends its applicability in the case of deep implanted channeling profiles.
2. Experimental procedure The experiments were performed at Forschungszentrum, Rossendorf, Dresden, Germany, using the 3 MV TANDETRON Accelerator, as well as at NCSR ‘‘Demokritos’’, Athens, Greece, using the 5.5 MV TN11 TANDEM Accelerator. The experiment proceeded in two steps. At Rossendorf, the Si crystal wafers having a relatively high resistivity (15 kX cm for the (1 1 1) and 20 kX cm for the (1 0 0) crystal) and thus a low dopant concentration, were initially aligned using 2 MeV a-particles. The a-particles were lead to a scattering chamber, which included a four-motor goniometer system capable of determining the target orientation with an accuracy of 0.01°. The detection system consisted of a single Si surface barrier detector having an overall resolution of 12 keV for a-particles (8 keV for protons). The beam divergence was less than 0.05° due to the long collimation system (two 1 mm collimator apertures having a distance of 2.5 m between them and 20 cm between the antiscatterer and the target). The current on target did not exceed 5 nA in order to avoid dead-time corrections. The vacuum pressure was kept constant during the measurements (5 107 mbar). The channeling angle was found to vary between as low as 0.2° and as high as 0.6°. For the polar scan, the targets were tilted by 3° and rotated around the beam axis. The fine tuning of the channeling position was finally achieved via angular scans, which revealed the excellent crystalline behavior of both crystals, exhibiting a vmin
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629
of the order of 2–3% (in an integrated region of 2 channels after the surface peak). After determining the channeling position, both wafers were irradiated at virgin spots with 5 MeV 19 þ F ions, up to a maximum fluence of 1 1017 atoms/cm2 . The beam spot size was approximately 1:5 1:5 mm2 and the beam current did not exceed 50 nA on target, in order to avoid overheating of the samples and subsequent in-beam annealing. For the random irradiation, the Si(1 1 1) wafer was tilted at 7° and was irradiated to the same maximum fluence. The normalization of the accumulated fluence was achieved via charge integration, since the whole goniometric chamber acted as a Faraday cup. A typical RBS spectrum from the irradiation in the random mode is shown in Fig. 1. The carbon formation on the targetÕs surface was not negligible, but relatively small, as will be analyzed in the following section. Nevertheless there was a clear change of color, or rather shade, in the irradiation spots. After the irradiation, the samples were put into a simple goniometric chamber in Athens, tilted by 45° with respect to the proton beam. A HPGe detector, having 80% relative efficiency, was placed at a distance of 20 cm from the sample, at 45° with respect to the target, thus normal to the beam axis. Such a detector is mandatory for the investigation of the relatively high-energy c-rays originating
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from the 19 F(p; ac)16 O reaction (particularly the strong 6129 keV line that was utilized in the present work). The current on target did not exceed 20 nA and the dead time was kept below 5% during the acquisition. The proton beam energy varied between 950 and 1200 keV in steps of 10–20 keV. The well-collimated proton beam formed a 2 2 mm2 spot, thus covering completely, but not excessively, the fluorine irradiated area, in order to avoid extensive background contributions originating from HF traces, always present, due to the application of HF in the manufacturing process of practically all metals and semiconductors. For the final assessment of the inflicted damage, the targets were placed in a four-motor goniometric chamber similar to the one used in Rossendorf, they were aligned using 1.2 MeV protons and the RBS/channeling spectra of virgin and irradiated spots were measured. The beam divergence was less than 0.07° in Athens due to the long collimation system (consisting of two 1.5 mm collimator apertures having a distance of 1.5 m between them and 60 cm between the antiscatterer and the target). The current on target did not exceed 1 nA. The rest of the experimental conditions were kept similar to the ones adopted in Rossendorf.
3. Results and discussion RBS spectrum 15000 19 +
F in Si, E=5 MeV
Counts
10000
5000
0 50
100
150
200
Channel
Fig. 1. RBS spectrum of 5 MeV 19 Fþ ions in Si along the random direction, used as monitor during the high-energy implantation.
In order to investigate the depth profile of the deep implanted fluorine ions, the 19 F(p; ac)16 O reaction has been used, due to its high cross-section and strong resonant behavior. For these reasons it is ideal for depth profiling and such an application was first introduced in the late 60s, as reported in [11]. The c-rays of interest from this reaction pertain to the a1 , a2 , a3 groups, with energies 6.129, 6.917 and 7.117 MeV, respectively. In the present work they were detected using a HPGe detector with 80% relative efficiency, having a resolution of 4.7 keV and they were clearly isolated. From these three c-rays only the 6129 keV transition was used, unlike works reported in the past [12], since it is the only one which exhibits a constant cross-section in the energy range under interest (Ep ¼ 950–1200 keV) [13,14]. Methods for
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629
extracting the depth profile from a nuclear excitation curve have been described in numerous publications [15–19], since the experimental yield Y ðEÞ represents the convolution of the fluorine depth profile concentration CðxÞ with the excitation peak GðE; xÞ, according to the equation Y ðEÞ ¼ k
Z
R
CðxÞGðE; xÞ dx;
ð1Þ
0
where R is the range of the incident ion and k is a calibration constant which depends on the reaction cross-section at the resonance energy and on the efficiency of the detection system [15]. In general, the centroid and width of GðE; xÞ varies with depth x, due to energy loss and energy straggling. In the present work, the relative invariance of GðE; xÞ for the 6129 keV transition in the energy range Ep ¼ 950–1200 keV [13,14], allows, at first order approximation, the implementation of simple equations rather than numerical approximation techniques, rendering Y ðEÞ / CðxÞ. For the determination of the proton energy loss and the corresponding transformation of the energy scale into depth scale, ZieglerÕs data were adopted [6]. The beam energy step was set to either 10 or 20 keV, which corresponded to an equivalent distance of 0.2 and 0.4 lm, respectively (depending on the proton stopping power in Si). The experimental curves for the depth profile of implanted fluorine ions are presented in Fig. 2 for the random, as well as the channeling directions (1 0 0) and (1 1 1). The results are expressed in yield/lCb. The same resonant NRA technique was applied to non-irradiated Si(1 1 1) and (1 0 0) wafers in order to determine the background level of fluorine concentration at the same proton beam energies. The extracted values were subsequently subtracted from the ones determined for the irradiated samples. The errors indicated in the graph correspond not only to the combined statistical (through error propagation) but also to the systematic ones (charge collection efficiency, fluctuations of the beam on target and uncertainties in the beam energy due to the formation of carbon on the targetÕs surface). For all experimental points the total estimated error ranged, from as low as 5% in the high fluorine concentration area, to as high as 90%
Fluorine depth profile 220
(Ep=950-1200 keV)
200
Si [random] Si[100] Si[111]
180
Normalized Yield/ µCb
626
160 140 120 100 80 60 40 20 0 2
3
4
5
6
7
8
Depth (µm)
Fig. 2. Experimental fluorine depth profiles using the resonant 19 F(p; ac)16 O reaction in yield/lCb versus depth (in lm). Both the random and the (1 0 0), (1 1 1) channeling orientations are presented for the same accumulated fluence of 1 1017 atoms/ cm2 . The total estimated errors are indicated in the graph.
in the fluorine trace region and in the high-energy part of the spectrum (which corresponded to the channeling tails caused by the best channeled fluorine ions). The first interesting result of this analysis was the profound broadening of the random peak, as shown in Fig. 2. This can be partially justified by the change of density in the end-of-range region of the implanted fluorine ions. Due to the high inflicted fluence, in this region, the density of Si with respect to F ions is comparable. Thus, a severe reduction in the actual density of the material is expected, ranging between d ¼ 2:32 and 1.11 g/cm3 (in the case of the formation of large fluorine defect clusters). This density change is a dynamic process, strongly dependent on the inflicted fluence and its impact becomes significant after 1 1016 particles/cm2 (1/10 of the total accumulated fluence). The results of SRIM-2000, assuming a thickness of 1 lm for the end-of-range region (3 lm Si/1 lm Si1 F1x /5 lm Si) and including estimates concerning the density of this heavily distorted layer are presented in Fig. 3. This change of density, however, could explain the broadening of the random peak only up to a depth of 4.8 lm, but the high-energy tail that extends up to 6–6.2 lm
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629 +
SRIM Results
627 17
2
F into Si, 5 MeV, 10 ions/cm
300 0
21
(100) Si, 0 tilt 0 (111) Si, 0 tilt
3
200
Number of Ions
0
1.5x10
Concentration (ions/cm )
250
0
Si, 7 tilt, 0 rotation
3
d=2.32 g/cm 3 d=1.72 g/cm 3 d=1.11 g/cm
150
100
21
1.0x10
20
5.0x10
50 19 +
5 MeV F in Si 2
0 0
1
2
3
4
5
6
Depth (in µm)
Fig. 3. Results obtained using SRIM-2000 (for the random case), examining the fluorine ion range according to the density of the end-of-range region of implanted ions, assuming a structure of the form: 3 lm Si/1 lm Si1 F1x /5 lm Si.
cannot be accounted for. This tail implies the existence of a channeling phenomenon. Indeed, irradiating with the sample positioned at 7° (and not randomly rotated during the process), (1 1 0) planar channeling interferences cannot be excluded. For the examination of the above mentioned hypothesis and the investigation of the channeling profiles, c-TRIM was implemented [20]. This algorithm simulates ion implantation into single-crystalline silicon with up to 10 amorphous overlayers of arbitrary composition. It comprises dynamic simulation of damage accumulation in the crystalline substrate, including the formation of amorphous layers. Thus, implanted range distributions as a function of depth can be generated. The algorithm has been proven quite successful in the past in reproducing low-energy experimental channeling SIMS profiles, therefore it was slightly modified to suit the high-energy case as well. For practical applications and since the wafers were given no prior treatment with HF (only the biological components were removed after boiling in acetone for 15 min), a 1.5 nm thick native oxide (average thickness of the amorphous surface layer) on Si was assumed and was taken into account in the simulation procedure. The results are presented in Fig. 4.
3
4
5
6
7
8
Depth (µm)
Fig. 4. Monte-Carlo simulation results using the c-TRIM algorithm. The simulation results were obtained without using any damage accumulation model included in the algorithm.
The ÔshoulderÕ obtained in the simulation for the random case, is due to (1 1 0) planar channeling and extends up to 6 lm, thus verifying the remarks mentioned above concerning the broadening of the random peak. As a result, for the correct assessment of the broadening, both changes in density and planar channeling effects have to be taken into consideration. A comparison between Figs. 2 and 4 reveals a qualitative agreement in both the random peaks and the channeling tails. The agreement is quite satisfactory for both (1 0 0) and (1 1 1) channeling orientations. Nevertheless, the relative ÔgapÕ that appears between 4 and 5.5 lm in the simulated spectra (Fig. 4) does not exist in the experimental ones (Fig. 2). Although the dechanneling process is taken into consideration in the c-TRIM algorithm, due to the high inflicted fluence, the damage of the crystal lattice in the trace region must be analyzed. This constantly accumulated damage modifies the Ômean channeling distanceÕ of fluorine ions towards lower values. The existence of a relative ÔplateauÕ of the fluorine concentration in the experimental channeling spectra thus corresponds to the overlapping of channeling peaks towards smaller depths. On the other hand, the absence of a particularly strong random peak clearly implies that despite the high inflicted fluence the crystal was not amorphized in the trace region and that only the crystalline quality was affected to a certain
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M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629 RBS/Channeling Spectra 1400
Ep=1.2 MeV 1200
1000
C
Counts
800
600
F
400
Irradiated spot Virgin spot Random
200
Si edge
0 100
200
300
400
500
Channel
Fig. 5. RBS/C spectra of the irradiated and a virgin spot, using 1.2 MeV protons. The F and C (due to carbon buildup) edges are clear in the channeling spectrum of the irradiated spot, as well as the changes in the vmin and vðxÞ values (near the surface or at greater depths inside the crystal). The spectrum refers to the Si(1 0 0) wafer.
extent. Indeed, as shown in Fig. 5, RBS/channeling spectra taken at a virgin and an irradiated spot, using 1.2 MeV protons, demonstrate that, up to a depth of 3–4 lm below the crystal surface, the only significant change in the irradiated area was the increase of the vmin and vðxÞ values (vmin 3% in the virgin spot, while 15–20% in the irradiated one). The fluorine signal appears strong in the channeling spectrum of the irradiated spot, denoting the existence of an amorphous layer at the end-of-range region, after which the backscattering spectra of protons in the channeling and in the random mode of incidence are almost identical (vðxÞ 90–95%). The carbon edge that also appears in the channeling spectrum is due to the carbon buildup during the implantation process, with an estimated thickness of 0.2 lm. The strong crystalline behavior of the irradiated sample at small depths, despite the existence of an additional amorphous carbon surface layer, proves conclusively that the inflicted damage in the trace region of the fluorine ions was minimal. It has to be noted that the best simulation results obtained with the c-TRIM code excluded the usage of any damage accumulation model. The main reason for this behavior lies in the saturation
of defect creation, which has been observed in the past [21,22], not taken into account by the simulation code, which can be explained mainly by defect annealing due to high electronic energy loss [23]. The subsequent passage of an ion along the path of another, which can lead to the breaking of stable defects and therefore to their annealing, is considered to be a second order effect. The amount of energy released during ion penetration (up to 800 keV/lm) is sufficient to produce heating of the material near the ion trajectory (thermal spike induced crystallization and partial restoration of the lattice [24]). Thus, the utilization of any damage accumulation model, disregarding dynamic defect annealing, especially in the case of a high inflicted fluence, can lead to the subsequent suppression of the channeling effects in the simulated spectra. Nevertheless, it is evident that a series of effects related to the high-energy implantations in the channeling direction are not clearly understood. A thorough study of the dechanneling and stopping powers of several medium mass ions in different Si crystal orientations is imperative before a complete understanding of the damage mechanism is accomplished.
4. Conclusions The present work presents the first attempt to study channeling profiles of deep implanted fluorine ions in Si(1 0 0) and (1 1 1). Resonant NRA proves to be a powerful tool for such an analysis when high-energy implantations are concerned and can be applied to the case of many light and medium mass ion beams. In the case of fluorine depth profiling, the reaction used, 19 F(p; ac)16 O, provides excellent depth resolution and minimum detection limits of the order of a few ppm. Although the inflicted fluence in this series of experiments was high (1 1017 atoms/cm2 ), resonant NRA could in principle allow the investigation of fluences lower by approximately two orders of magnitude in the most favorable case. In this series of experiments Si(1 0 0) and (1 1 1) crystals were irradiated in both random and channeling directions with 5 MeV 19 Fþ ions, up to a fluence of 1017 ions/cm2 . The implanted profiles
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629
were investigated using the 6129 keV c-ray of the 19 F(p; ac)16 O reaction, in the proton beam energy range of 950–1200 keV. The measured profiles were compared with simulation predictions of the SRIM and c-TRIM algorithms and the results were found to be in reasonable agreement. The occurring differences between the experimental and simulated results were explained by means of density variations in the end of range of the implanted F ions and dynamic annealing due to the high electronic energy loss of the incoming ions, deposited in the material. Another interesting aspect of this high-energy channeling implantation was the minimal damage caused in the crystal lattice in the trace region of incoming fluorine ions, as implied by the simulations using the c-TRIM algorithm and finally experimentally verified by the RBS/C spectra. Although many problems concerning this type of implantation remain unsolved (channeling energy loss, dechanneling, dynamic annealing), it seems to provide an interesting means of creating multiple deep implanted layers in Si wafers, with a minimum of inflicted damage in the crystal lattice.
Acknowledgement Part of the present work was accomplished in the framework of the LSF in Rossendorf, Dresden.
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Investigation of deep implanted fluorine channeling profiles in silicon using resonant NRA M. Kokkoris a,*, G. Perdikakis a,b, R. Vlastou b, C.T. Papadopoulos b, X.A. Aslanoglou c, M. Posselt d, €tzschel d, S. Harissopulos a, S. Kossionides a R. Gro a
d
Institute of Nuclear Physics, NCSR ÔDemokritosÕ, Laboratory for Material Analysis, P.O. Box 60228, GR-153 10 Aghia Paraskevi, Athens, Greece b Department of Physics, National Technical University of Athens, GR-157 80, Greece c Department of Physics, The University of Ioannina, GR-451 10 Ioannina, Greece Institute of Ion Beam Physics and Material Analysis, Forschungszentrum Rossendorf, Postfach 510119, D-01314, Dresden, Germany Received 26 August 2002; received in revised form 27 December 2002
Abstract Si(1 0 0) and (1 1 1) crystals were irradiated in the random as well as in the channeling direction, using 5 MeV 19 Fþ ions, to a maximum fluence of approximately 1 1017 particles/cm2 . The occurring deep implanted profiles were subsequently investigated using the Resonant Nuclear Reaction Analysis technique in the energy range Ep ¼ 950–1200 keV. The reaction 19 F(p; ac)16 O reaction exhibits a strong resonant behavior in the above mentioned energy range, thus providing an excellent tool for the depth profiling of fluorine, yielding minimum detection limits of the order of a few ppm. The occurring profiles are analyzed with SRIM and c-TRIM codes and an attempt is made to explain the characteristics of the experimental spectra, as well as to compare with results already existing in literature. Ó 2003 Elsevier Science B.V. All rights reserved. PACS: 61.85.+p Keywords: High-energy implantation; Channeling; Nuclear resonance; Fluorine profiling; Resonant NRA; c-TRIM
1. Introduction The effects of irradiation on the properties of solids are of significant interest in the scientific and
* Corresponding author. Tel.: +30-1-4288217/6518770; fax: +30-1-6511215. E-mail address: [email protected] (M. Kokkoris).
technological context. Several studies have been presented concerning irradiations with ions having an energy of more than 100 keV/nucleon [1–3]. These ions find an ever increasing application in the modification of the properties of metals and semiconductors. There exists a growing interest in the physical nature of the effect of this high-energy ion implantation as it is considered to be a promising way of increasing the microelectronic chip integration by the formation of multilayer
0168-583X/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0168-583X(03)00448-8
624
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629
three-dimensional structures [4]. Indeed, nowadays, implantation at MeV energies is frequently used in VLSI fabrication, with the most common application being the formation of deep isolated regions among individual devices and of tubs in CMOS structures. Moreover, there are strong indications that the increase of ion energy does not lead exclusively to quantitative changes of the ion implanted layer parameters, but also to qualitative changes of the defect-impurity structure of the whole irradiated area [5]. This type of implantation is mainly characterized by the high linear density of the energy contribution of ions into the electronic subsystem of the target, resulting in electronic energy losses of the order of MeV/lm. It is also characterized by the fact that this type of losses strongly prevail over the direct transmission of the ion energy to the nuclear subsystem of the target, which is predominant at the end of the mean ion projected range, but almost negligible otherwise (less than 3% in our case as shown with the use of the SRIM code [6]). It should be noted here, however, that the data on defect production and annealing at depths of the order of a few lm inside a target are sparse, often contradictory and by no means conclusive as to the very nature of the mechanism of the phenomenon. On the other hand, to the authorsÕ best knowledge there is a lack in literature, concerning data in the case of high-energy irradiations and their effects in the channeling mode. It is well known that the channeling procedure provides an easy means for placing ions deep inside the target, while minimizing lattice damage. However, the channeled atom profile is sensitive to the beam divergence, as well as to scattering by amorphous surface films and residual damage from previous processing steps. Interesting works presented recently concern low-energy channeling implantations and examinations of the depth profiles using SIMS [7–10]. Nevertheless, despite its superb mass, depth resolution and sensitivity (108 atoms/cm2 ), this technique is not suitable for the examination of channeling profiles that extend over a few lm inside the target. In the present work Si(1 0 0) and (1 1 1) crystals were irradiated in the channeling as well as in
the random direction, using 5 MeV 19 Fþ ions, to a maximum fluence of 1 1017 atoms/cm2 . The occurring implantation profiles were subsequently examined using the Resonant NRA technique via the 19 F(p; ac) reaction. Resonant NRA provides a reliable and accurate means for the investigation of several light ion depth profiles (H, Li, B, Be, C, O, N, Al and F) and has been successfully applied in the past for the investigation of amorphous materials (rocks, archeological flints) and biological samples [11]. This work extends its applicability in the case of deep implanted channeling profiles.
2. Experimental procedure The experiments were performed at Forschungszentrum, Rossendorf, Dresden, Germany, using the 3 MV TANDETRON Accelerator, as well as at NCSR ‘‘Demokritos’’, Athens, Greece, using the 5.5 MV TN11 TANDEM Accelerator. The experiment proceeded in two steps. At Rossendorf, the Si crystal wafers having a relatively high resistivity (15 kX cm for the (1 1 1) and 20 kX cm for the (1 0 0) crystal) and thus a low dopant concentration, were initially aligned using 2 MeV a-particles. The a-particles were lead to a scattering chamber, which included a four-motor goniometer system capable of determining the target orientation with an accuracy of 0.01°. The detection system consisted of a single Si surface barrier detector having an overall resolution of 12 keV for a-particles (8 keV for protons). The beam divergence was less than 0.05° due to the long collimation system (two 1 mm collimator apertures having a distance of 2.5 m between them and 20 cm between the antiscatterer and the target). The current on target did not exceed 5 nA in order to avoid dead-time corrections. The vacuum pressure was kept constant during the measurements (5 107 mbar). The channeling angle was found to vary between as low as 0.2° and as high as 0.6°. For the polar scan, the targets were tilted by 3° and rotated around the beam axis. The fine tuning of the channeling position was finally achieved via angular scans, which revealed the excellent crystalline behavior of both crystals, exhibiting a vmin
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of the order of 2–3% (in an integrated region of 2 channels after the surface peak). After determining the channeling position, both wafers were irradiated at virgin spots with 5 MeV 19 þ F ions, up to a maximum fluence of 1 1017 atoms/cm2 . The beam spot size was approximately 1:5 1:5 mm2 and the beam current did not exceed 50 nA on target, in order to avoid overheating of the samples and subsequent in-beam annealing. For the random irradiation, the Si(1 1 1) wafer was tilted at 7° and was irradiated to the same maximum fluence. The normalization of the accumulated fluence was achieved via charge integration, since the whole goniometric chamber acted as a Faraday cup. A typical RBS spectrum from the irradiation in the random mode is shown in Fig. 1. The carbon formation on the targetÕs surface was not negligible, but relatively small, as will be analyzed in the following section. Nevertheless there was a clear change of color, or rather shade, in the irradiation spots. After the irradiation, the samples were put into a simple goniometric chamber in Athens, tilted by 45° with respect to the proton beam. A HPGe detector, having 80% relative efficiency, was placed at a distance of 20 cm from the sample, at 45° with respect to the target, thus normal to the beam axis. Such a detector is mandatory for the investigation of the relatively high-energy c-rays originating
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from the 19 F(p; ac)16 O reaction (particularly the strong 6129 keV line that was utilized in the present work). The current on target did not exceed 20 nA and the dead time was kept below 5% during the acquisition. The proton beam energy varied between 950 and 1200 keV in steps of 10–20 keV. The well-collimated proton beam formed a 2 2 mm2 spot, thus covering completely, but not excessively, the fluorine irradiated area, in order to avoid extensive background contributions originating from HF traces, always present, due to the application of HF in the manufacturing process of practically all metals and semiconductors. For the final assessment of the inflicted damage, the targets were placed in a four-motor goniometric chamber similar to the one used in Rossendorf, they were aligned using 1.2 MeV protons and the RBS/channeling spectra of virgin and irradiated spots were measured. The beam divergence was less than 0.07° in Athens due to the long collimation system (consisting of two 1.5 mm collimator apertures having a distance of 1.5 m between them and 60 cm between the antiscatterer and the target). The current on target did not exceed 1 nA. The rest of the experimental conditions were kept similar to the ones adopted in Rossendorf.
3. Results and discussion RBS spectrum 15000 19 +
F in Si, E=5 MeV
Counts
10000
5000
0 50
100
150
200
Channel
Fig. 1. RBS spectrum of 5 MeV 19 Fþ ions in Si along the random direction, used as monitor during the high-energy implantation.
In order to investigate the depth profile of the deep implanted fluorine ions, the 19 F(p; ac)16 O reaction has been used, due to its high cross-section and strong resonant behavior. For these reasons it is ideal for depth profiling and such an application was first introduced in the late 60s, as reported in [11]. The c-rays of interest from this reaction pertain to the a1 , a2 , a3 groups, with energies 6.129, 6.917 and 7.117 MeV, respectively. In the present work they were detected using a HPGe detector with 80% relative efficiency, having a resolution of 4.7 keV and they were clearly isolated. From these three c-rays only the 6129 keV transition was used, unlike works reported in the past [12], since it is the only one which exhibits a constant cross-section in the energy range under interest (Ep ¼ 950–1200 keV) [13,14]. Methods for
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extracting the depth profile from a nuclear excitation curve have been described in numerous publications [15–19], since the experimental yield Y ðEÞ represents the convolution of the fluorine depth profile concentration CðxÞ with the excitation peak GðE; xÞ, according to the equation Y ðEÞ ¼ k
Z
R
CðxÞGðE; xÞ dx;
ð1Þ
0
where R is the range of the incident ion and k is a calibration constant which depends on the reaction cross-section at the resonance energy and on the efficiency of the detection system [15]. In general, the centroid and width of GðE; xÞ varies with depth x, due to energy loss and energy straggling. In the present work, the relative invariance of GðE; xÞ for the 6129 keV transition in the energy range Ep ¼ 950–1200 keV [13,14], allows, at first order approximation, the implementation of simple equations rather than numerical approximation techniques, rendering Y ðEÞ / CðxÞ. For the determination of the proton energy loss and the corresponding transformation of the energy scale into depth scale, ZieglerÕs data were adopted [6]. The beam energy step was set to either 10 or 20 keV, which corresponded to an equivalent distance of 0.2 and 0.4 lm, respectively (depending on the proton stopping power in Si). The experimental curves for the depth profile of implanted fluorine ions are presented in Fig. 2 for the random, as well as the channeling directions (1 0 0) and (1 1 1). The results are expressed in yield/lCb. The same resonant NRA technique was applied to non-irradiated Si(1 1 1) and (1 0 0) wafers in order to determine the background level of fluorine concentration at the same proton beam energies. The extracted values were subsequently subtracted from the ones determined for the irradiated samples. The errors indicated in the graph correspond not only to the combined statistical (through error propagation) but also to the systematic ones (charge collection efficiency, fluctuations of the beam on target and uncertainties in the beam energy due to the formation of carbon on the targetÕs surface). For all experimental points the total estimated error ranged, from as low as 5% in the high fluorine concentration area, to as high as 90%
Fluorine depth profile 220
(Ep=950-1200 keV)
200
Si [random] Si[100] Si[111]
180
Normalized Yield/ µCb
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160 140 120 100 80 60 40 20 0 2
3
4
5
6
7
8
Depth (µm)
Fig. 2. Experimental fluorine depth profiles using the resonant 19 F(p; ac)16 O reaction in yield/lCb versus depth (in lm). Both the random and the (1 0 0), (1 1 1) channeling orientations are presented for the same accumulated fluence of 1 1017 atoms/ cm2 . The total estimated errors are indicated in the graph.
in the fluorine trace region and in the high-energy part of the spectrum (which corresponded to the channeling tails caused by the best channeled fluorine ions). The first interesting result of this analysis was the profound broadening of the random peak, as shown in Fig. 2. This can be partially justified by the change of density in the end-of-range region of the implanted fluorine ions. Due to the high inflicted fluence, in this region, the density of Si with respect to F ions is comparable. Thus, a severe reduction in the actual density of the material is expected, ranging between d ¼ 2:32 and 1.11 g/cm3 (in the case of the formation of large fluorine defect clusters). This density change is a dynamic process, strongly dependent on the inflicted fluence and its impact becomes significant after 1 1016 particles/cm2 (1/10 of the total accumulated fluence). The results of SRIM-2000, assuming a thickness of 1 lm for the end-of-range region (3 lm Si/1 lm Si1 F1x /5 lm Si) and including estimates concerning the density of this heavily distorted layer are presented in Fig. 3. This change of density, however, could explain the broadening of the random peak only up to a depth of 4.8 lm, but the high-energy tail that extends up to 6–6.2 lm
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SRIM Results
627 17
2
F into Si, 5 MeV, 10 ions/cm
300 0
21
(100) Si, 0 tilt 0 (111) Si, 0 tilt
3
200
Number of Ions
0
1.5x10
Concentration (ions/cm )
250
0
Si, 7 tilt, 0 rotation
3
d=2.32 g/cm 3 d=1.72 g/cm 3 d=1.11 g/cm
150
100
21
1.0x10
20
5.0x10
50 19 +
5 MeV F in Si 2
0 0
1
2
3
4
5
6
Depth (in µm)
Fig. 3. Results obtained using SRIM-2000 (for the random case), examining the fluorine ion range according to the density of the end-of-range region of implanted ions, assuming a structure of the form: 3 lm Si/1 lm Si1 F1x /5 lm Si.
cannot be accounted for. This tail implies the existence of a channeling phenomenon. Indeed, irradiating with the sample positioned at 7° (and not randomly rotated during the process), (1 1 0) planar channeling interferences cannot be excluded. For the examination of the above mentioned hypothesis and the investigation of the channeling profiles, c-TRIM was implemented [20]. This algorithm simulates ion implantation into single-crystalline silicon with up to 10 amorphous overlayers of arbitrary composition. It comprises dynamic simulation of damage accumulation in the crystalline substrate, including the formation of amorphous layers. Thus, implanted range distributions as a function of depth can be generated. The algorithm has been proven quite successful in the past in reproducing low-energy experimental channeling SIMS profiles, therefore it was slightly modified to suit the high-energy case as well. For practical applications and since the wafers were given no prior treatment with HF (only the biological components were removed after boiling in acetone for 15 min), a 1.5 nm thick native oxide (average thickness of the amorphous surface layer) on Si was assumed and was taken into account in the simulation procedure. The results are presented in Fig. 4.
3
4
5
6
7
8
Depth (µm)
Fig. 4. Monte-Carlo simulation results using the c-TRIM algorithm. The simulation results were obtained without using any damage accumulation model included in the algorithm.
The ÔshoulderÕ obtained in the simulation for the random case, is due to (1 1 0) planar channeling and extends up to 6 lm, thus verifying the remarks mentioned above concerning the broadening of the random peak. As a result, for the correct assessment of the broadening, both changes in density and planar channeling effects have to be taken into consideration. A comparison between Figs. 2 and 4 reveals a qualitative agreement in both the random peaks and the channeling tails. The agreement is quite satisfactory for both (1 0 0) and (1 1 1) channeling orientations. Nevertheless, the relative ÔgapÕ that appears between 4 and 5.5 lm in the simulated spectra (Fig. 4) does not exist in the experimental ones (Fig. 2). Although the dechanneling process is taken into consideration in the c-TRIM algorithm, due to the high inflicted fluence, the damage of the crystal lattice in the trace region must be analyzed. This constantly accumulated damage modifies the Ômean channeling distanceÕ of fluorine ions towards lower values. The existence of a relative ÔplateauÕ of the fluorine concentration in the experimental channeling spectra thus corresponds to the overlapping of channeling peaks towards smaller depths. On the other hand, the absence of a particularly strong random peak clearly implies that despite the high inflicted fluence the crystal was not amorphized in the trace region and that only the crystalline quality was affected to a certain
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M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629 RBS/Channeling Spectra 1400
Ep=1.2 MeV 1200
1000
C
Counts
800
600
F
400
Irradiated spot Virgin spot Random
200
Si edge
0 100
200
300
400
500
Channel
Fig. 5. RBS/C spectra of the irradiated and a virgin spot, using 1.2 MeV protons. The F and C (due to carbon buildup) edges are clear in the channeling spectrum of the irradiated spot, as well as the changes in the vmin and vðxÞ values (near the surface or at greater depths inside the crystal). The spectrum refers to the Si(1 0 0) wafer.
extent. Indeed, as shown in Fig. 5, RBS/channeling spectra taken at a virgin and an irradiated spot, using 1.2 MeV protons, demonstrate that, up to a depth of 3–4 lm below the crystal surface, the only significant change in the irradiated area was the increase of the vmin and vðxÞ values (vmin 3% in the virgin spot, while 15–20% in the irradiated one). The fluorine signal appears strong in the channeling spectrum of the irradiated spot, denoting the existence of an amorphous layer at the end-of-range region, after which the backscattering spectra of protons in the channeling and in the random mode of incidence are almost identical (vðxÞ 90–95%). The carbon edge that also appears in the channeling spectrum is due to the carbon buildup during the implantation process, with an estimated thickness of 0.2 lm. The strong crystalline behavior of the irradiated sample at small depths, despite the existence of an additional amorphous carbon surface layer, proves conclusively that the inflicted damage in the trace region of the fluorine ions was minimal. It has to be noted that the best simulation results obtained with the c-TRIM code excluded the usage of any damage accumulation model. The main reason for this behavior lies in the saturation
of defect creation, which has been observed in the past [21,22], not taken into account by the simulation code, which can be explained mainly by defect annealing due to high electronic energy loss [23]. The subsequent passage of an ion along the path of another, which can lead to the breaking of stable defects and therefore to their annealing, is considered to be a second order effect. The amount of energy released during ion penetration (up to 800 keV/lm) is sufficient to produce heating of the material near the ion trajectory (thermal spike induced crystallization and partial restoration of the lattice [24]). Thus, the utilization of any damage accumulation model, disregarding dynamic defect annealing, especially in the case of a high inflicted fluence, can lead to the subsequent suppression of the channeling effects in the simulated spectra. Nevertheless, it is evident that a series of effects related to the high-energy implantations in the channeling direction are not clearly understood. A thorough study of the dechanneling and stopping powers of several medium mass ions in different Si crystal orientations is imperative before a complete understanding of the damage mechanism is accomplished.
4. Conclusions The present work presents the first attempt to study channeling profiles of deep implanted fluorine ions in Si(1 0 0) and (1 1 1). Resonant NRA proves to be a powerful tool for such an analysis when high-energy implantations are concerned and can be applied to the case of many light and medium mass ion beams. In the case of fluorine depth profiling, the reaction used, 19 F(p; ac)16 O, provides excellent depth resolution and minimum detection limits of the order of a few ppm. Although the inflicted fluence in this series of experiments was high (1 1017 atoms/cm2 ), resonant NRA could in principle allow the investigation of fluences lower by approximately two orders of magnitude in the most favorable case. In this series of experiments Si(1 0 0) and (1 1 1) crystals were irradiated in both random and channeling directions with 5 MeV 19 Fþ ions, up to a fluence of 1017 ions/cm2 . The implanted profiles
M. Kokkoris et al. / Nucl. Instr. and Meth. in Phys. Res. B 201 (2003) 623–629
were investigated using the 6129 keV c-ray of the 19 F(p; ac)16 O reaction, in the proton beam energy range of 950–1200 keV. The measured profiles were compared with simulation predictions of the SRIM and c-TRIM algorithms and the results were found to be in reasonable agreement. The occurring differences between the experimental and simulated results were explained by means of density variations in the end of range of the implanted F ions and dynamic annealing due to the high electronic energy loss of the incoming ions, deposited in the material. Another interesting aspect of this high-energy channeling implantation was the minimal damage caused in the crystal lattice in the trace region of incoming fluorine ions, as implied by the simulations using the c-TRIM algorithm and finally experimentally verified by the RBS/C spectra. Although many problems concerning this type of implantation remain unsolved (channeling energy loss, dechanneling, dynamic annealing), it seems to provide an interesting means of creating multiple deep implanted layers in Si wafers, with a minimum of inflicted damage in the crystal lattice.
Acknowledgement Part of the present work was accomplished in the framework of the LSF in Rossendorf, Dresden.
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