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Channeling effects in high energy ion implantation: Si(N)
Nuclear Instruments and Methods in Physics Research B80/81 (1993) 58-61 North-Holland
Besam Interactions with Materials &Atoms
Channeling effects in high energy ion implantation : Si(N) MI . Berti, G. Bnisatin, A. Cesrnera and A. Gasparotto i/mità INFM, Dtparrbnento di Tisica
G. Gaklei ", Via Maaolo 8, I-35131 Padova, Italy
R . Fesbbri
CNR-Istituto LAMEL, Ka Caa.'agnoli 1, 1-40126 Bologna, Italy
Nitrogen implantation in Si single crystals in the 600-1400 keV energy range in random, (100) and (110) alignment conditions as performed. The 6x10 3-2x1016 fluence range was investigated. Double crystal diffraction (DCD) and RBS-channel; 'ig analysis were performed in order to extract the damage profiles and the damage production rate as a function of the implanted ion fluence . A strong dependence of the damage on the implantation geometry has been observed and discussed . 1 . Introduction Ion implantation is a well established process m semiconductor technology . Most of the work has been done in the energy range from few keV to 100-200 keV. However with the advent of high rr.crgt ion implanters, a great effort has been spent to investigate new possible applications [1] Channeling effects are generally regarded as undesired, because of the tails they can introduce in the ion distribution, and ion implantations are usually performed by tilting the surface normal a few degrees with respect to the beau. direction. However it is difficult to eliminate channeling effects when implantations are performed in crystals characterized by a high degree of symmetry like silicon [2] . On the contrary, it is possible to intentionally use v'ianneling effects to modify implantation profiles and to obtain deep implants at relatively low enr,,gies [3,4] by taking advantage of the ^nergy loss red, ;tion occutrmg for incidence parallel to maim-sta axes. Moreover the energy transfer mechanisms bt ween the ions and the lattice arc strongly affected by the c. .anneling phenomena . It is therefore very important to obtain a good knowledge of the implanted ion distributions as well as of the damage profiles and defect production rate. In fact, although great efforts have been payed to measure and to simulate the essential features of "random" implantation a !ot of woYk remains to be done for channeling implantations. In this paper we repo,t part of the enp--rimental resuits obtained by studying N implantation in silicon single crystals in random and in many axial and planar directions . The shape of the damage distributions as well as its
dependence on the ion fluenec are presented and discussed.
2. Experimental (001) silicon wafers were implanted with 14N ions at doses ranging from 6 X 10 13 to 2 X 10 16 ions/cm` . The ion implantation was performed in the scattering chamber for channeling rrcasurcments available at the high voltage AN-2000 Van de Graaff accelerator at the Laboratori Nazianali di Legnaro (Padova, Italy). The goniomerric assembly which was used to perform the ion implantations was originally developed for high precision channeling measurements and has veen described elsewhere [5]. The main features of this anniometer are the possibility of three axis rotations and the presence of an x-y translation stage parallel to the sample surface . All the goniometer's movements are computer controlled as well as the translation of the sample holder. In this way a continuous scan of the implantation area in front of the beam can be obtained, thus no beam deflection is necessary in order to achieve homogeneous implanted areas i,f the order of a few square centimeters . The alignment of the sample to the beam was performed by using the same N beam further used for the implantations. The results reported in this paper concern implantations in random geometry as well as along the (100 P,,d (110) axial directions of the silicon crystal. The energy of the N beam ranged from 600 to 1400 keV . Conventional 4 He+ T:BS-channeling measurements were successively performed in order to measure the defect profiles.
0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B .V. All rights reserved
M. Berti et al. / Channeling effects in SIMS profiles of the implanted nitrogen were performed with the CAMI;CA IMS-4f spectrometer at the Dipartimento di Fisica deil'Universitü di Padova. A 14.5 kV Cs' beam having 150 nA intensity was scanned over 175 x 175 Wm` area in the central part of the implantation spot. The 30 Si - and the 42 SiN - ion yields were monitored. High resolution X-ray curves (RCs) were obtained by means of double crystal diffractometry (DCD) in parallel (nondispersive) configuration with 004 reflection and Cu K ., radiation [6]. The RCs were computer simulated by means of the dynamical model of X-ray diffraction from imperfect crystals following the method de~cribt'd in ref. [7] . Tw" r-Otc ;,f these simulations are the depth profiles of the lattice strain and t .-.e displacements of the atoms from the sites of the deformed lattice (static Debye-Waller factor). Since no mention is made in this paper to the static disorder, the latter profiles will not be reported. 3 . Results and discussion The implanted ion profiles show a marked dependence upon the implantation direction at all the implantation energies and doses. Fig. 1 shows the superpos?tion of the SIMS nitrogen profiles obtained after implantation at 1 .4 MeV and 1 x 10 15 at./cm 2 fluence with the beam in random geometry and parallel either to the (10(1) or to the (110) axial directions of the silicon sample. The "random profile" shows a rather sharp leading edge and a long tail towards the sample surface . The (100) and (110) aligned implants generate N concentration profiles which are deeper and broader than the random ore, due to the presence of a channeled component of th°, beam which suffers a lower
random
10'8
loi' F foie 0[
1
2
Wü 1 I,I
3
Depth (pm) Fig. 1. Typical nit ogen profiles after random, (100) and (tiJ) implantations.
energy ion implantation
high
3
59
700k-Rzndaan
2.5 4,2 0 J5 a
1 0.5 0
Fig . 2 . Damage profiles for different nitrogen fluentes at 700 keV as measured by DCD (left scale) and RBS-channeling (right scale), 'He' 2 .3 MeV. energy loss. The channeled component dominates in the case of the (110) implants leading to a id concentration profile which is more than 2 times deeper and noticeably broader than in the case of random implants . These profiles have been analyzed in terms of three components of the ion beam, a random, a channeled and a dechanueled component [8] . By simply assuming a Gaussian shape for each component the SIMS concentration profiles were satisfactorily fitted . It was in particular found that in the case of (100) implants the channeled fraction saturates at fluences of 2 x 10 15 N/cm2 whereas no saturation could be seen for (110) implants up to 1 .5 x 10 16 N/cm 2. For samples implanted with doses ranging from 6 x 10 13 to 2 ;c 10 1 -s N/cm 2, DCD was used in order to extract the damage profile, while for samples implanted with doses ranging from 2 X 10 15 to 2 x 10 16 N/cm 2 the 4 He + RBS-channeling technique was used. The calculation procedure is described in ref. [8] and was firstly suggested by Feldman and Rodgers [9]. In this scheme the dechannelirg probability can be evalu-aiwring evonts and the ated by assuming unit' critical angle must be scaled in order to account for the slowing down of the ions. Fig. 2 shows the damage profiles as a function of the depth for three samples implanted at 700 keV it, random geometry. The left scale refers to the samples implanted at lower doses and gives the strain (Aala) of the lattice parameter as measured by DCD. The right scale refers to the sample implanted at a higher dose and, as measured by RBS-channeling, gives the ratio of the displaced atoms to the atomic density. The figure shows that the damage peak increases with the implanted dose while its width remains substantially constant. Similar results were obtained for (100) implantations (not shown) . For (110) implantations the analysis Ia . BASIC INTERACTIONS (a)
M. Berri et al / Channeling effects in high energy ion implantation
60
DCD Strain Meewresnente a) random
810"
a
Vg
210"
0 210 o 1Q' 1101, t' Implanted hone (nt'em'1
310t°
700 keV random implantation l0 %1
I I
q
1
Lmesr Regime
e0 0
I
I
., Regime Sub-
T I
Amorphous I-er Growth
0 0 gds loll 10 4 Implanted dose (at, 2,
Fig . 3. 700 keV nitrogen implantations: (a) strain integral as a function o1 nitrogen fluence for the three implantation geometries; (b) total damage for random implantations as mea sured by DCD (left scale) and 1Y13S,hanneling (right scale), ° He` 2 .3 MeV.
of the RBS-channeling spectra could not be performed witlb good accuracy due to the vu-)j iocv dcmagc pioductiot, rate. For instance, after S x 10 15 N/CM2 implantation only a small change in the dechanneling rate, with respect t, ) a virgin crystal, is detectable . On the contrary after implantation of the same N doss both in random and (100) geometries an evident damage peak has been produced [8] . The strain integral, i .e . the area of the damage profiles as measured by DCD, is reported in fig . 3a as a function S die impiv,ttcû nitrogen dose . In the case of random implants the t<-31 measured damage is more than 2 times higher than the c wresponding one for aligned implants . Moreovc , -, cwh `-ir random and (100) implants the :strain integral increases with a high rate, as a function of the implanted dose, for doses up to 2-3 x 10 1° N/cm 2, while for higher doses its growth rate is lower. The data reported in fig. 3n do not allow to reach any conclusion about (110) implants. Fig. 3b summarizes all the DCD and RBS-channeling results for random implants . The left scale refers to DCD
measurements, as in fig . 3a, while the right scale refers to RBS-channeling measurements and the damage is reported in terms of displaced lattice atoms. Fig . 3b shows that three different regimes of damage production can be evidenced: a first linear regime up to doses of about 2 x 10 1° N/em 2 , a second, sublinear regime, for doses ranging from 2 x 10 1 ° to about 9 x 10 15 N/cmz while for higher implanted doses a buried amorphous layer is formed . In the case of (100) implants (not shown) again three regimes can be evidenced but shifted towards higher implantation doses: 3-4 x 10" N/cm` for the end of the linear regime and more than 10 1`' N/cm` for the amorphouL iaycr growth. In (110) implants only two regions were fuund for the fluences reported in this paper . The linear regime extends up to doses of the order of 10 15 N/cm= , it is followed by a sublinear regime and, as already pointed out, no amorphous growth zone could be evidenced . The presence of a linear regime of damage production followed by a sublinear one was already evidenced in the case of random implantations both at low and high b:i :rtl energies [10,11]. In the linear regime collision cascades mainly develop in an undamaged crystal and each cascade results, on average, in tba swine amount of damage . By increasing the dose, point defect., from different collision cascades, in predamaged regions, can interact, so the damage behavior deviates from a simple linear rate. In particular two competing mechanisms can be invoked to explain the sublinear regime [10] . The first one is the annealing of Frenkel defects of opposite sign within spatially isolated cascades, the second one is the production of Frenkel pais in adiacent ion tracks . Our results about the damage production rate can be interpreted as follows : even thn,r21i i : iltu e,tclgy ratlgc. ~tutiied the main enerRv toss mechanism is elt,cironic, the damage production is at least in principle, due to nuclear collisions. In channeling implants the presence of a channeled fraction of the beam increases the relative amount of energy loss via electronic losses at the expenses of the energy loss via nuclear collisions, thus reducing the total amount of damage within the crystal . The rows characterized by a low atomic density are those for which the channeling effect is strongest, this is, for instance, the case of (110) direction, and, therefore, the damage production rate is lower with respect to other crystallographic directions . 4 . Conclusions We have demonstrated that the usual "random" conditions implantation is performed in Not only the distribution of
strong de'riations from can be o()servc:d when channeling conditions . the implanted ions is
M. Berti et al. /Channeling effects in high energy ionimplantation
affected by the implantation geometry but also the shape of the damage and its production rate . The strongest channeling effects have been observed for (110) implantation. Wcrk is in progress to define the amorphization threshold along this direction as well as to relate the damage production rate to the atomic structure of the axial and planar channels within the crystal .
References 1 1] AN . Saxena and D. Pramanik, Mater. Sci . Eng . B2 (1989) 1 . [2] V. Rainen, G . Galvagno, E. Rimini, S . Capizzi, A. La Ferla, A . Carnera and G . Ferla, Semicand . Sci . Technol . 7 (1990) 1007 . [3] R . Scl,rcutelkamp, F .W . Saris, J .F.M . Westendorp, R .E.
61
Kaim, G .B . Odlum and K.T.F . Janssen, Mater. Sci Eng . B2 (1989) 139. [4] L. Eriksson, J.A . Davies and P. Jespersgaard, Phys . Rev. 161 (1967) 219. [5] A. Camera and A.V . Drigo, Nuci . Instr. and Meth.'1344 (1990) 357. [6] M . Servidori and F. Cembali, J. Appl . Crystallogr. 21 (1988) 176 . 17] C .R. Wie, 'I' .A. Tombrello and T . Vreeland, J . Appl . Phys . 59 (1986) 3743 . [8] A. Gasparotto, A. Carnera, S. Acco and A.M. La Ferla, Nucl. Instr. and Meth. 1362 (1992) 356. [9] L.C. Feldman and J .W. Rodgers, J . Appl. Phys . 41 11970) 3776. [10] R Fabbri, G. Lulli, R. Nipoti and M. Servidori, these Proacediags (8th Int . Conf. on Ion Beam Modification of Materials, Heidelberg, Germany, 1992) Nucl . Instr. and Meth. B80/81 (1993) 624. [111 J .K .N. Lindner, R . Zuschlag and E .H. te Kaat, Nucl. Instr. and Meth. B62 (1992) 314.
la. BASIC INTERACTIONS (a)
Besam Interactions with Materials &Atoms
Channeling effects in high energy ion implantation : Si(N) MI . Berti, G. Bnisatin, A. Cesrnera and A. Gasparotto i/mità INFM, Dtparrbnento di Tisica
G. Gaklei ", Via Maaolo 8, I-35131 Padova, Italy
R . Fesbbri
CNR-Istituto LAMEL, Ka Caa.'agnoli 1, 1-40126 Bologna, Italy
Nitrogen implantation in Si single crystals in the 600-1400 keV energy range in random, (100) and (110) alignment conditions as performed. The 6x10 3-2x1016 fluence range was investigated. Double crystal diffraction (DCD) and RBS-channel; 'ig analysis were performed in order to extract the damage profiles and the damage production rate as a function of the implanted ion fluence . A strong dependence of the damage on the implantation geometry has been observed and discussed . 1 . Introduction Ion implantation is a well established process m semiconductor technology . Most of the work has been done in the energy range from few keV to 100-200 keV. However with the advent of high rr.crgt ion implanters, a great effort has been spent to investigate new possible applications [1] Channeling effects are generally regarded as undesired, because of the tails they can introduce in the ion distribution, and ion implantations are usually performed by tilting the surface normal a few degrees with respect to the beau. direction. However it is difficult to eliminate channeling effects when implantations are performed in crystals characterized by a high degree of symmetry like silicon [2] . On the contrary, it is possible to intentionally use v'ianneling effects to modify implantation profiles and to obtain deep implants at relatively low enr,,gies [3,4] by taking advantage of the ^nergy loss red, ;tion occutrmg for incidence parallel to maim-sta axes. Moreover the energy transfer mechanisms bt ween the ions and the lattice arc strongly affected by the c. .anneling phenomena . It is therefore very important to obtain a good knowledge of the implanted ion distributions as well as of the damage profiles and defect production rate. In fact, although great efforts have been payed to measure and to simulate the essential features of "random" implantation a !ot of woYk remains to be done for channeling implantations. In this paper we repo,t part of the enp--rimental resuits obtained by studying N implantation in silicon single crystals in random and in many axial and planar directions . The shape of the damage distributions as well as its
dependence on the ion fluenec are presented and discussed.
2. Experimental (001) silicon wafers were implanted with 14N ions at doses ranging from 6 X 10 13 to 2 X 10 16 ions/cm` . The ion implantation was performed in the scattering chamber for channeling rrcasurcments available at the high voltage AN-2000 Van de Graaff accelerator at the Laboratori Nazianali di Legnaro (Padova, Italy). The goniomerric assembly which was used to perform the ion implantations was originally developed for high precision channeling measurements and has veen described elsewhere [5]. The main features of this anniometer are the possibility of three axis rotations and the presence of an x-y translation stage parallel to the sample surface . All the goniometer's movements are computer controlled as well as the translation of the sample holder. In this way a continuous scan of the implantation area in front of the beam can be obtained, thus no beam deflection is necessary in order to achieve homogeneous implanted areas i,f the order of a few square centimeters . The alignment of the sample to the beam was performed by using the same N beam further used for the implantations. The results reported in this paper concern implantations in random geometry as well as along the (100 P,,d (110) axial directions of the silicon crystal. The energy of the N beam ranged from 600 to 1400 keV . Conventional 4 He+ T:BS-channeling measurements were successively performed in order to measure the defect profiles.
0168-583X/93/$06.00 0 1993 - Elsevier Science Publishers B .V. All rights reserved
M. Berti et al. / Channeling effects in SIMS profiles of the implanted nitrogen were performed with the CAMI;CA IMS-4f spectrometer at the Dipartimento di Fisica deil'Universitü di Padova. A 14.5 kV Cs' beam having 150 nA intensity was scanned over 175 x 175 Wm` area in the central part of the implantation spot. The 30 Si - and the 42 SiN - ion yields were monitored. High resolution X-ray curves (RCs) were obtained by means of double crystal diffractometry (DCD) in parallel (nondispersive) configuration with 004 reflection and Cu K ., radiation [6]. The RCs were computer simulated by means of the dynamical model of X-ray diffraction from imperfect crystals following the method de~cribt'd in ref. [7] . Tw" r-Otc ;,f these simulations are the depth profiles of the lattice strain and t .-.e displacements of the atoms from the sites of the deformed lattice (static Debye-Waller factor). Since no mention is made in this paper to the static disorder, the latter profiles will not be reported. 3 . Results and discussion The implanted ion profiles show a marked dependence upon the implantation direction at all the implantation energies and doses. Fig. 1 shows the superpos?tion of the SIMS nitrogen profiles obtained after implantation at 1 .4 MeV and 1 x 10 15 at./cm 2 fluence with the beam in random geometry and parallel either to the (10(1) or to the (110) axial directions of the silicon sample. The "random profile" shows a rather sharp leading edge and a long tail towards the sample surface . The (100) and (110) aligned implants generate N concentration profiles which are deeper and broader than the random ore, due to the presence of a channeled component of th°, beam which suffers a lower
random
10'8
loi' F foie 0[
1
2
Wü 1 I,I
3
Depth (pm) Fig. 1. Typical nit ogen profiles after random, (100) and (tiJ) implantations.
energy ion implantation
high
3
59
700k-Rzndaan
2.5 4,2 0 J5 a
1 0.5 0
Fig . 2 . Damage profiles for different nitrogen fluentes at 700 keV as measured by DCD (left scale) and RBS-channeling (right scale), 'He' 2 .3 MeV. energy loss. The channeled component dominates in the case of the (110) implants leading to a id concentration profile which is more than 2 times deeper and noticeably broader than in the case of random implants . These profiles have been analyzed in terms of three components of the ion beam, a random, a channeled and a dechanueled component [8] . By simply assuming a Gaussian shape for each component the SIMS concentration profiles were satisfactorily fitted . It was in particular found that in the case of (100) implants the channeled fraction saturates at fluences of 2 x 10 15 N/cm2 whereas no saturation could be seen for (110) implants up to 1 .5 x 10 16 N/cm 2. For samples implanted with doses ranging from 6 x 10 13 to 2 ;c 10 1 -s N/cm 2, DCD was used in order to extract the damage profile, while for samples implanted with doses ranging from 2 X 10 15 to 2 x 10 16 N/cm 2 the 4 He + RBS-channeling technique was used. The calculation procedure is described in ref. [8] and was firstly suggested by Feldman and Rodgers [9]. In this scheme the dechannelirg probability can be evalu-aiwring evonts and the ated by assuming unit' critical angle must be scaled in order to account for the slowing down of the ions. Fig. 2 shows the damage profiles as a function of the depth for three samples implanted at 700 keV it, random geometry. The left scale refers to the samples implanted at lower doses and gives the strain (Aala) of the lattice parameter as measured by DCD. The right scale refers to the sample implanted at a higher dose and, as measured by RBS-channeling, gives the ratio of the displaced atoms to the atomic density. The figure shows that the damage peak increases with the implanted dose while its width remains substantially constant. Similar results were obtained for (100) implantations (not shown) . For (110) implantations the analysis Ia . BASIC INTERACTIONS (a)
M. Berri et al / Channeling effects in high energy ion implantation
60
DCD Strain Meewresnente a) random
810"
a
Vg
210"
0 210 o 1Q' 1101, t' Implanted hone (nt'em'1
310t°
700 keV random implantation l0 %1
I I
q
1
Lmesr Regime
e0 0
I
I
., Regime Sub-
T I
Amorphous I-er Growth
0 0 gds loll 10 4 Implanted dose (at, 2,
Fig . 3. 700 keV nitrogen implantations: (a) strain integral as a function o1 nitrogen fluence for the three implantation geometries; (b) total damage for random implantations as mea sured by DCD (left scale) and 1Y13S,hanneling (right scale), ° He` 2 .3 MeV.
of the RBS-channeling spectra could not be performed witlb good accuracy due to the vu-)j iocv dcmagc pioductiot, rate. For instance, after S x 10 15 N/CM2 implantation only a small change in the dechanneling rate, with respect t, ) a virgin crystal, is detectable . On the contrary after implantation of the same N doss both in random and (100) geometries an evident damage peak has been produced [8] . The strain integral, i .e . the area of the damage profiles as measured by DCD, is reported in fig . 3a as a function S die impiv,ttcû nitrogen dose . In the case of random implants the t<-31 measured damage is more than 2 times higher than the c wresponding one for aligned implants . Moreovc , -, cwh `-ir random and (100) implants the :strain integral increases with a high rate, as a function of the implanted dose, for doses up to 2-3 x 10 1° N/cm 2, while for higher doses its growth rate is lower. The data reported in fig. 3n do not allow to reach any conclusion about (110) implants. Fig. 3b summarizes all the DCD and RBS-channeling results for random implants . The left scale refers to DCD
measurements, as in fig . 3a, while the right scale refers to RBS-channeling measurements and the damage is reported in terms of displaced lattice atoms. Fig . 3b shows that three different regimes of damage production can be evidenced: a first linear regime up to doses of about 2 x 10 1° N/em 2 , a second, sublinear regime, for doses ranging from 2 x 10 1 ° to about 9 x 10 15 N/cmz while for higher implanted doses a buried amorphous layer is formed . In the case of (100) implants (not shown) again three regimes can be evidenced but shifted towards higher implantation doses: 3-4 x 10" N/cm` for the end of the linear regime and more than 10 1`' N/cm` for the amorphouL iaycr growth. In (110) implants only two regions were fuund for the fluences reported in this paper . The linear regime extends up to doses of the order of 10 15 N/cm= , it is followed by a sublinear regime and, as already pointed out, no amorphous growth zone could be evidenced . The presence of a linear regime of damage production followed by a sublinear one was already evidenced in the case of random implantations both at low and high b:i :rtl energies [10,11]. In the linear regime collision cascades mainly develop in an undamaged crystal and each cascade results, on average, in tba swine amount of damage . By increasing the dose, point defect., from different collision cascades, in predamaged regions, can interact, so the damage behavior deviates from a simple linear rate. In particular two competing mechanisms can be invoked to explain the sublinear regime [10] . The first one is the annealing of Frenkel defects of opposite sign within spatially isolated cascades, the second one is the production of Frenkel pais in adiacent ion tracks . Our results about the damage production rate can be interpreted as follows : even thn,r21i i : iltu e,tclgy ratlgc. ~tutiied the main enerRv toss mechanism is elt,cironic, the damage production is at least in principle, due to nuclear collisions. In channeling implants the presence of a channeled fraction of the beam increases the relative amount of energy loss via electronic losses at the expenses of the energy loss via nuclear collisions, thus reducing the total amount of damage within the crystal . The rows characterized by a low atomic density are those for which the channeling effect is strongest, this is, for instance, the case of (110) direction, and, therefore, the damage production rate is lower with respect to other crystallographic directions . 4 . Conclusions We have demonstrated that the usual "random" conditions implantation is performed in Not only the distribution of
strong de'riations from can be o()servc:d when channeling conditions . the implanted ions is
M. Berti et al. /Channeling effects in high energy ionimplantation
affected by the implantation geometry but also the shape of the damage and its production rate . The strongest channeling effects have been observed for (110) implantation. Wcrk is in progress to define the amorphization threshold along this direction as well as to relate the damage production rate to the atomic structure of the axial and planar channels within the crystal .
References 1 1] AN . Saxena and D. Pramanik, Mater. Sci . Eng . B2 (1989) 1 . [2] V. Rainen, G . Galvagno, E. Rimini, S . Capizzi, A. La Ferla, A . Carnera and G . Ferla, Semicand . Sci . Technol . 7 (1990) 1007 . [3] R . Scl,rcutelkamp, F .W . Saris, J .F.M . Westendorp, R .E.
61
Kaim, G .B . Odlum and K.T.F . Janssen, Mater. Sci Eng . B2 (1989) 139. [4] L. Eriksson, J.A . Davies and P. Jespersgaard, Phys . Rev. 161 (1967) 219. [5] A. Camera and A.V . Drigo, Nuci . Instr. and Meth.'1344 (1990) 357. [6] M . Servidori and F. Cembali, J. Appl . Crystallogr. 21 (1988) 176 . 17] C .R. Wie, 'I' .A. Tombrello and T . Vreeland, J . Appl . Phys . 59 (1986) 3743 . [8] A. Gasparotto, A. Carnera, S. Acco and A.M. La Ferla, Nucl. Instr. and Meth. 1362 (1992) 356. [9] L.C. Feldman and J .W. Rodgers, J . Appl. Phys . 41 11970) 3776. [10] R Fabbri, G. Lulli, R. Nipoti and M. Servidori, these Proacediags (8th Int . Conf. on Ion Beam Modification of Materials, Heidelberg, Germany, 1992) Nucl . Instr. and Meth. B80/81 (1993) 624. [111 J .K .N. Lindner, R . Zuschlag and E .H. te Kaat, Nucl. Instr. and Meth. B62 (1992) 314.
la. BASIC INTERACTIONS (a)