- Email: [email protected]
Channeling phenomena in off-axis ion implanted (001) silicon
Nuclear Instruments and Methods North-Holland, Amsterdam
CHANNELING F. CEMBALI,
PHENOMENA M. SERVIDORI
CNR - Istituto LAMEL, Received
4 March
in Physics
Research
225
B12 (1985) 225-228
IN OFF-AXIS ION IMPLANTED
(001) SILICON
and A.M. MAZZONE
Via Castagnoli 1, 40126 Bologna, Italy
1985 and in revised form 20 May 1985
(001) silicon wafers were implanted in random conditions with 8 X 10” cmm2, 60 keV silicon ions. The samples were given a tilt angle of 8” from the [OOI] direction to avoid axial channeling and rotation angles of 0”, 20”, 70” and 90” to evidence (110) planar channeling effects, the O”, 90” conditions denoting parallelism between (110) planes and the ion beam direction. An unforeseen more marked ion channeling was detected by double-crystal X-ray diffraction for the 70” rotated wafer compared with the ones observed for the 0” and 90” cases. Considering the errors in the sample alignment procedure and the deviations of wafer reference flats from (110) directions, the experimental results could be explained with the simple concept of crystal transparency to the ion beam for the incidence conditions used.
1. Introduction
that unexpected channeling phenomena can occur even when (110) planar channeling is avoided.
The main advantage of the ion implantation technique as a doping process over conventional thermal diffusion (i.e. the control and reproducibility of the dopant profile) is subjected to the requirement of a careful orientation of the sample with respect to the incident ion beam. It is known, in fact, that a misalignment of the crystal axis by about 8” from the beam direction is sufficient to suppress axial channeling. However, planar channeling can still occur in these conditions and, even though its influence on the doping profile is not as marked as that of axial channeling, some attention has recently been given to this effect in order to avoid the presence of deep tails of dopant. One of the most effective ways to do this when commercial (001) Si or GaAs single crystals are implanted is to rotate the 8” tilted wafers at such an angle as to prevent parallelism between the beam and a low-index crystallographic plane. The reference flat of the wafers, which identifies the (110) direction, is used for this operation [l-3]. In the quoted works, the authors verified the effectiveness of the sample rotation at fixed tilt by determining the carrier concentration profiles after annealing and came to the conclusion that (110) planar channeling is fully suppressed for rotation angles @ > 7” (@ = 0 denotes parallelism between (110) planes and the ion beam direction) and that the carrier profile is insensitive to orientation until, with increasing @, another (110) plane is approached. In this paper, the variations in the lattice damage profiles induced by the wafer rotation angle are analyzed by double-crystal X-ray diffraction (DCD) in off-axis self-ion implanted silicon samples and it will be shown
0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
2. Experimental (001) silicon wafers by Wacker were implanted with 8 x 1013 cm-’ dose, 60 keV energy, - 1 mA current silicon ions, within a Lintott Series III implanter. The shape of the beam can be approximated by an ellipse with semiaxes 13 mm vertical and 5 mm horizontal. The density current is therefore 0.5 mA/cm*. The implanter is provided with a mechanical-scan system, in the sense that the cylindrical surface of the caroussel, covered by a series of rectangular sample-holder plates, rotates with a speed of 2 m/s and moves up and down with a period of 6 min. Due to this feature, each point of the sample is exposed to the beam every 0.75 s and receives a dose of 1.5 X 1013 cm-* in 5 X 10m3 s. Considering the good thermal contact between sample and holder, the specimen temperature reaches the value of about 30°C at the end of the described implant. Four wafers were mounted on the same rectangular plate with their flats making angles @ = 0”, 20”, 70°, 90”, respectively, with the horizontal direction of the holder (fig. 1). The accuracy in Q, was essentially that of the mechanical devices used to orient the flats of the wafers with respect to the reference sides of the holder. While for @ = 0” and 90” no error was found in the rectangular device, for @ = 20” and 70” the acute angle of the triangular device resulted in 19.4” k 0.1” instead of 20’ by goniometric measurements. In addition, an error of 0.8” + O.l”, equal for all wafers, was found by X-ray diffraction between the flat and the (110) direction. Consequently, the nominal angular values @ =
F. Cembafi et a/. / Off-axis im~ian~a~ionin (001) silicon
226
3. Results and discussion
ITilt axis
D
I
,Tllt axis
Fig. 1. Sketch of wafer alignment procedure by making use of the mechanical devices D. Only two parts of the rectangular holder are shown, in which 1, 2, 3 and 4 refer to nominal rotation angles i[, = 0”. 20”, 70” and 90”, respectively. The beam direction is normal to the drawing and the tilt axis lies on the holder.
0”, 20”, 70”, 90” became Q,= 0.8”, 20.2”, 71.4’ and 90.8”. The plate holder was then placed inside the implant chamber and tilted by an angle a: equal to 8” around an axis parallel to the vertical direction of the plate itself. The accuracy of the tilt angle is determined by the intrinsic mechanical precision of the implanter and by the horizontal divergence of the beam. While the former can reasonably be of the order of O.l”, the latter was evaluated as 0.5” from the distance crossover-sample. By means of the same procedure, the resulting value of the vertical divergence was found to nearly the same. It is relevant to point out that the tilt angle does not vary during the implant, since the caroussel is buiit in such a way that the rotation motion of the holder is transformed in a pure translational one, in the direction parallel to the wafer surface, when it crosses the region of the beam. After implantation, the samples were analyzed by DCD arranged in a parallel configuration. The experimental X-ray intensity profiles (rocking curves (RC)) were simulated by computer through application of the dynamical theory of X-ray diffraction from imperfect crystals. By this simulation, the depth profiles of the component of the lattice strain normal to the wafer surface (e L) and the standard deviation of the displacement of Si atoms (U) from their lattice sites (static disorder) were obtained, according to the procedure previously described [4,5]. The static disorder depth profiles are not reported here since they are nearly coincident with those of e I . The maximum standard deviation of the atomic displacements the maxima of the strain distributions U = 0.014 nm for the three cases.
corresponding was evaluated
to
as
Owing to the 4-fold symmetry of the (001) oriented wafers, the RC curves obtained after implantation are expected to be identical for the specimens characterized by nominal rotation angles Cp= 0” and 90”, and @ = 20’ and 70”. Fig. 2 shows the experimental and the calculated intensity profiles. While the above hypothesis is verified for the case @ = 0’ and 90”, a marked difference is observed for @ = O* and 70”. The lattice damage in the implanted samples is represented in fig. 3. Three distinct strain profiles were obtained, showing a decrease of the maximum value, a shift towards greater depths and deeper tails on passing from Qt= 20” to Cg= 70’. Also the integral strain diminishes according to the same trend. The finding that the implants characterized by a nominal @ = 0” and 90” were identical and not the most channeled ones, as expected, but intermediate between the nominal Q,= 20” and @ = 70°, could be accounted for by considering the actual rotation angles. In the h~othesis that only the errors in the alignment device and in the flat directions were operating, i.e. excluding those intrinsic to the ion implanter, in the condition 0 = 0.8’, 20.2’, 71.4’, 90.8’, the ion beam is
20
15
$ 10
5
-200
0
200
A r? [arcsec] Fig. 2. Experimental (lines) and calculated (symbols) RCs obtained from 8 X 1Or3 Si+/cm’, 60 keV implants performed with a nominal tilt angle a = 8” and nominal rotation angles Cp= 0”, 20”, 70” and 90”.
F. Cembali et al. /
Off-axis implantation in (001) silicon
224
221
OCCUPIED CELLS •1 up to a between gj above 73% and 87% 87% 73%
21.6 20.8
0 0
200
100 Depth
[nm]
Fig. 3. Strain profifes obtained from the RC simulations of fig. 2.
very likely to see different atomic distributions in the crystal. To show the differences in the crystal transparency to the beam, the coordinates of the atoms included in a parallelepiped 1 X 1 X 84 nm3 (the projected ion range) were generated by computer and projected on its square base. After dividing this base in square cells with sides equal to twice the Thomas-Fermi screening radius (r = 0.0137 nm), the number of cells occupied by the pointlike atoms was evaluated for the three cases (due to the crystal symmetry, the Q, = 0.8” and @ = 90.8” cases are still equivalent) in a small range of rotation angles Q, and tilt angles cx Here, the assumption is that the channeling probability increases with a decrease of occupied cells. Fig. 4 shows the maps of occupied cells for the three cases. For clarity, only three ranges are shown, corresponding to occupied cells up to 73% between 73% and 87% and above 87%, respectively. By allowing for a horizontal and vertical beam divergence of about 0.5” and for the effect on a of an error of about f0.5” in the wafer surface from the (001) plane, the possible incidence conditions of the beam for the three cases are indicated as rectangular dashed windows in the figure. The average percentages of occupied cells resulting are 76X, 83% and 94% in these regions for the implants at nominal cx= 8” and CJ= 70°, 0’(90*) and ZOO,respectively. These values, which are indicative of how open the crystal is with respect to the beam, could explain the observed channeling phenomena. In particular, the difference between the strain profiles (b) and (c) shown in fig. 3 can be due to a partial reduction of (110) planar channeling and a fortuitous incidence parallel to the
X0
a0
a0
Tilt angle (r [degrees) Fig. 4. Crystal transparency to the beam, as a function of tilt angle a and rotation angle (9, based on three percent ranges of elemental cells occupied by atoms. The windows opened in these maps represent the probable incidence conditions of the beam on the crystals. Q,= 0” and Cp= 71.6’ correspond to (110) and (ZiO) planar channeling, respectively.
(2iO) planes. Strictly speaking, in agreement with the results reported in ref. [6], these planes do not allow the same channeling effect as that relative to, e.g., the (110) planes, since the percentage of occupied cells along them, though low, changes if the tilt angle is varied.
4. Conclusions
The present work shows that ~sali~ment of the wafer from the beam direction is effective in reducing channeling tails in implant experiments, provided that the wafer itself is properly oriented with an accuracy of the order of a tenth of a degree. In practice, the modern ion implanters used for industrial applications do not allow such a high alignment precision. Moreover, the commercial wafers are supplied with errors in the surface cut and in the reference flats, so that the control of their exact o~entation in the implant chamber is impossible. For these reasons, unexpected in-depth shifts of the doping profiles can be obtained even when rotation is
F. Cembali et al. / Off -axis implantation
228
imposed
on the wafer,
in addition
to tilting,
to avoid
(110)planar channeling. Finally, it is to be pointed out that the conditions of beam incidence reported in fig. 4 should be considered as probable, since slightly different situations could arise if other sources of error in Q, and (Y are concurrently operating.
References [l] P. Blood, Phys. Stat. Sol. (a) 25 (1974) K151.
in (001) silicon
PI R.T. Blunt, I.R. Sanders and J.F. Singleton, Proc. 4th Int. Conf. on Ion Implantation: Equipment and Techniques, Berchtesgaden (FGR), eds., H. Ryssel and H. Glawischnig (Springer, Berlin, 1983) p. 443. [31T. Hara and T. Inada, Sol. Stat. Tech. 22 (1979) 69. [41 F. Cembali, M. Servidori, E. Gabilli and R. Lotti, Phys. Stat. Sol. (a) 87 (1985) 527. [51F. Cembali, M. Servidori and A. Zani, Sol. Stat. Electron. (1985) in press. 161J.F. Ziegler and R.F. Lever, Appl. Phys. Lett. 46 (1985) 358.
CHANNELING F. CEMBALI,
PHENOMENA M. SERVIDORI
CNR - Istituto LAMEL, Received
4 March
in Physics
Research
225
B12 (1985) 225-228
IN OFF-AXIS ION IMPLANTED
(001) SILICON
and A.M. MAZZONE
Via Castagnoli 1, 40126 Bologna, Italy
1985 and in revised form 20 May 1985
(001) silicon wafers were implanted in random conditions with 8 X 10” cmm2, 60 keV silicon ions. The samples were given a tilt angle of 8” from the [OOI] direction to avoid axial channeling and rotation angles of 0”, 20”, 70” and 90” to evidence (110) planar channeling effects, the O”, 90” conditions denoting parallelism between (110) planes and the ion beam direction. An unforeseen more marked ion channeling was detected by double-crystal X-ray diffraction for the 70” rotated wafer compared with the ones observed for the 0” and 90” cases. Considering the errors in the sample alignment procedure and the deviations of wafer reference flats from (110) directions, the experimental results could be explained with the simple concept of crystal transparency to the ion beam for the incidence conditions used.
1. Introduction
that unexpected channeling phenomena can occur even when (110) planar channeling is avoided.
The main advantage of the ion implantation technique as a doping process over conventional thermal diffusion (i.e. the control and reproducibility of the dopant profile) is subjected to the requirement of a careful orientation of the sample with respect to the incident ion beam. It is known, in fact, that a misalignment of the crystal axis by about 8” from the beam direction is sufficient to suppress axial channeling. However, planar channeling can still occur in these conditions and, even though its influence on the doping profile is not as marked as that of axial channeling, some attention has recently been given to this effect in order to avoid the presence of deep tails of dopant. One of the most effective ways to do this when commercial (001) Si or GaAs single crystals are implanted is to rotate the 8” tilted wafers at such an angle as to prevent parallelism between the beam and a low-index crystallographic plane. The reference flat of the wafers, which identifies the (110) direction, is used for this operation [l-3]. In the quoted works, the authors verified the effectiveness of the sample rotation at fixed tilt by determining the carrier concentration profiles after annealing and came to the conclusion that (110) planar channeling is fully suppressed for rotation angles @ > 7” (@ = 0 denotes parallelism between (110) planes and the ion beam direction) and that the carrier profile is insensitive to orientation until, with increasing @, another (110) plane is approached. In this paper, the variations in the lattice damage profiles induced by the wafer rotation angle are analyzed by double-crystal X-ray diffraction (DCD) in off-axis self-ion implanted silicon samples and it will be shown
0168-583X/85/$03.30 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
2. Experimental (001) silicon wafers by Wacker were implanted with 8 x 1013 cm-’ dose, 60 keV energy, - 1 mA current silicon ions, within a Lintott Series III implanter. The shape of the beam can be approximated by an ellipse with semiaxes 13 mm vertical and 5 mm horizontal. The density current is therefore 0.5 mA/cm*. The implanter is provided with a mechanical-scan system, in the sense that the cylindrical surface of the caroussel, covered by a series of rectangular sample-holder plates, rotates with a speed of 2 m/s and moves up and down with a period of 6 min. Due to this feature, each point of the sample is exposed to the beam every 0.75 s and receives a dose of 1.5 X 1013 cm-* in 5 X 10m3 s. Considering the good thermal contact between sample and holder, the specimen temperature reaches the value of about 30°C at the end of the described implant. Four wafers were mounted on the same rectangular plate with their flats making angles @ = 0”, 20”, 70°, 90”, respectively, with the horizontal direction of the holder (fig. 1). The accuracy in Q, was essentially that of the mechanical devices used to orient the flats of the wafers with respect to the reference sides of the holder. While for @ = 0” and 90” no error was found in the rectangular device, for @ = 20” and 70” the acute angle of the triangular device resulted in 19.4” k 0.1” instead of 20’ by goniometric measurements. In addition, an error of 0.8” + O.l”, equal for all wafers, was found by X-ray diffraction between the flat and the (110) direction. Consequently, the nominal angular values @ =
F. Cembafi et a/. / Off-axis im~ian~a~ionin (001) silicon
226
3. Results and discussion
ITilt axis
D
I
,Tllt axis
Fig. 1. Sketch of wafer alignment procedure by making use of the mechanical devices D. Only two parts of the rectangular holder are shown, in which 1, 2, 3 and 4 refer to nominal rotation angles i[, = 0”. 20”, 70” and 90”, respectively. The beam direction is normal to the drawing and the tilt axis lies on the holder.
0”, 20”, 70”, 90” became Q,= 0.8”, 20.2”, 71.4’ and 90.8”. The plate holder was then placed inside the implant chamber and tilted by an angle a: equal to 8” around an axis parallel to the vertical direction of the plate itself. The accuracy of the tilt angle is determined by the intrinsic mechanical precision of the implanter and by the horizontal divergence of the beam. While the former can reasonably be of the order of O.l”, the latter was evaluated as 0.5” from the distance crossover-sample. By means of the same procedure, the resulting value of the vertical divergence was found to nearly the same. It is relevant to point out that the tilt angle does not vary during the implant, since the caroussel is buiit in such a way that the rotation motion of the holder is transformed in a pure translational one, in the direction parallel to the wafer surface, when it crosses the region of the beam. After implantation, the samples were analyzed by DCD arranged in a parallel configuration. The experimental X-ray intensity profiles (rocking curves (RC)) were simulated by computer through application of the dynamical theory of X-ray diffraction from imperfect crystals. By this simulation, the depth profiles of the component of the lattice strain normal to the wafer surface (e L) and the standard deviation of the displacement of Si atoms (U) from their lattice sites (static disorder) were obtained, according to the procedure previously described [4,5]. The static disorder depth profiles are not reported here since they are nearly coincident with those of e I . The maximum standard deviation of the atomic displacements the maxima of the strain distributions U = 0.014 nm for the three cases.
corresponding was evaluated
to
as
Owing to the 4-fold symmetry of the (001) oriented wafers, the RC curves obtained after implantation are expected to be identical for the specimens characterized by nominal rotation angles Cp= 0” and 90”, and @ = 20’ and 70”. Fig. 2 shows the experimental and the calculated intensity profiles. While the above hypothesis is verified for the case @ = 0’ and 90”, a marked difference is observed for @ = O* and 70”. The lattice damage in the implanted samples is represented in fig. 3. Three distinct strain profiles were obtained, showing a decrease of the maximum value, a shift towards greater depths and deeper tails on passing from Qt= 20” to Cg= 70’. Also the integral strain diminishes according to the same trend. The finding that the implants characterized by a nominal @ = 0” and 90” were identical and not the most channeled ones, as expected, but intermediate between the nominal Q,= 20” and @ = 70°, could be accounted for by considering the actual rotation angles. In the h~othesis that only the errors in the alignment device and in the flat directions were operating, i.e. excluding those intrinsic to the ion implanter, in the condition 0 = 0.8’, 20.2’, 71.4’, 90.8’, the ion beam is
20
15
$ 10
5
-200
0
200
A r? [arcsec] Fig. 2. Experimental (lines) and calculated (symbols) RCs obtained from 8 X 1Or3 Si+/cm’, 60 keV implants performed with a nominal tilt angle a = 8” and nominal rotation angles Cp= 0”, 20”, 70” and 90”.
F. Cembali et al. /
Off-axis implantation in (001) silicon
224
221
OCCUPIED CELLS •1 up to a between gj above 73% and 87% 87% 73%
21.6 20.8
0 0
200
100 Depth
[nm]
Fig. 3. Strain profifes obtained from the RC simulations of fig. 2.
very likely to see different atomic distributions in the crystal. To show the differences in the crystal transparency to the beam, the coordinates of the atoms included in a parallelepiped 1 X 1 X 84 nm3 (the projected ion range) were generated by computer and projected on its square base. After dividing this base in square cells with sides equal to twice the Thomas-Fermi screening radius (r = 0.0137 nm), the number of cells occupied by the pointlike atoms was evaluated for the three cases (due to the crystal symmetry, the Q, = 0.8” and @ = 90.8” cases are still equivalent) in a small range of rotation angles Q, and tilt angles cx Here, the assumption is that the channeling probability increases with a decrease of occupied cells. Fig. 4 shows the maps of occupied cells for the three cases. For clarity, only three ranges are shown, corresponding to occupied cells up to 73% between 73% and 87% and above 87%, respectively. By allowing for a horizontal and vertical beam divergence of about 0.5” and for the effect on a of an error of about f0.5” in the wafer surface from the (001) plane, the possible incidence conditions of the beam for the three cases are indicated as rectangular dashed windows in the figure. The average percentages of occupied cells resulting are 76X, 83% and 94% in these regions for the implants at nominal cx= 8” and CJ= 70°, 0’(90*) and ZOO,respectively. These values, which are indicative of how open the crystal is with respect to the beam, could explain the observed channeling phenomena. In particular, the difference between the strain profiles (b) and (c) shown in fig. 3 can be due to a partial reduction of (110) planar channeling and a fortuitous incidence parallel to the
X0
a0
a0
Tilt angle (r [degrees) Fig. 4. Crystal transparency to the beam, as a function of tilt angle a and rotation angle (9, based on three percent ranges of elemental cells occupied by atoms. The windows opened in these maps represent the probable incidence conditions of the beam on the crystals. Q,= 0” and Cp= 71.6’ correspond to (110) and (ZiO) planar channeling, respectively.
(2iO) planes. Strictly speaking, in agreement with the results reported in ref. [6], these planes do not allow the same channeling effect as that relative to, e.g., the (110) planes, since the percentage of occupied cells along them, though low, changes if the tilt angle is varied.
4. Conclusions
The present work shows that ~sali~ment of the wafer from the beam direction is effective in reducing channeling tails in implant experiments, provided that the wafer itself is properly oriented with an accuracy of the order of a tenth of a degree. In practice, the modern ion implanters used for industrial applications do not allow such a high alignment precision. Moreover, the commercial wafers are supplied with errors in the surface cut and in the reference flats, so that the control of their exact o~entation in the implant chamber is impossible. For these reasons, unexpected in-depth shifts of the doping profiles can be obtained even when rotation is
F. Cembali et al. / Off -axis implantation
228
imposed
on the wafer,
in addition
to tilting,
to avoid
(110)planar channeling. Finally, it is to be pointed out that the conditions of beam incidence reported in fig. 4 should be considered as probable, since slightly different situations could arise if other sources of error in Q, and (Y are concurrently operating.
References [l] P. Blood, Phys. Stat. Sol. (a) 25 (1974) K151.
in (001) silicon
PI R.T. Blunt, I.R. Sanders and J.F. Singleton, Proc. 4th Int. Conf. on Ion Implantation: Equipment and Techniques, Berchtesgaden (FGR), eds., H. Ryssel and H. Glawischnig (Springer, Berlin, 1983) p. 443. [31T. Hara and T. Inada, Sol. Stat. Tech. 22 (1979) 69. [41 F. Cembali, M. Servidori, E. Gabilli and R. Lotti, Phys. Stat. Sol. (a) 87 (1985) 527. [51F. Cembali, M. Servidori and A. Zani, Sol. Stat. Electron. (1985) in press. 161J.F. Ziegler and R.F. Lever, Appl. Phys. Lett. 46 (1985) 358.