39
Nuclear Instruments and Methods in Physics Research B55 (1991) 39-44 North-Holland
Channeling control for large tilt angle implantation in Si ( 100) Robert
B. Simonton,
Dennis
E. Kamenitsa
and Andrew
M. Ray
Eaton Corp. Semiconductor Equipment Division, 2433 Rutland Drive, Austin, TX 78758, USA
Changhae
Park, Kevin M. Klein and Al F. Tasch
Microelectronics Research Center, University of Texas at Austin, Austin, TX 78712, USA
This investigation will present measurements of silicon (100) wafers, implanted with tilt angles in the range 7-60°, which identify combinations of tilt and azimuthal(twist) angles that avoid major channeling zones. The orientations identified in this study minimizechanneling effects even for very low dose implantation. A stereographicprojection demonstrates that all major variations in observed channeling behavior are explained by channeling in the six major (low Miller index) crystallographic axes and planes. The implanted wafers were characterizedusing modulated reflectance and SIMS measurements. We investigated the relative severity of ion channeling in major poles and planes and the effect of energy and species variations on channeling behavior. The physical basis for the observed variations is explained by employing the concepts of critical channeling angles and average distance traveled within a channel.
1. Introduction Ion implantation processes employing large tilt angles (7-60 o ) have stimulated great interest in the recent past due to the advanced device fabrication capability this implantation technique offers, particularly when used in conjunction with in situ wafer rotational repositioning [l]. Previous investigations of optimum silicon wafer orientation for channeling (dopant profile) control have typically described the range of tilt angles from 0 to about 10’ [2], although the range O-20” has been characterized by Ziegler and Lever with backscattering techniques [3]. In order to obtained dopant profile control when using larger tilt angles, optimum wafer orientations must be identified which will avoid major channeling zones. 2. Experimental methods 150 mm, prime silicon (100) wafers without a screen oxide were implanted using a systematic matrix of tilt and azimuthal (twist) angle combinations. Tilt is defined as the angle between the incident ion beam vector and a vector perpendicular to the wafer’s surface at the wafer’s center. Twist is defined as the rotational angle between a projection of the beam vector onto the wafer’s surface and the (110) planes perpendicular to the wafer’s major flat. All implants were performed on an Eaton NV0168-583X/91/$03.50
6200AV, a modern, electrostatically scanned medium current ion implanter which allows any combination of tilt angles 0-60° with twist angles O-360” [4]. Direct measurements on this implanter system revealed that both the accuracy and precision of the tilt and twist orientation were < f 0.25 O and < + 2.0 O, respectively. The tilt accuracy includes the (100) alignment error of the silicon substrates. The advanced digital scan system of this implanter effectively eliminates the cross-wafer nonuniformity resulting from geometric effects at any tilt angle 0 to 60 O, allowing a typical implant uniformity of < 0.5%, one sigma [5] throughout the available tilt range. The majority of the wafers were implanted with a boron, 100 keV, 5 x 1O’l cm-* implant. Smaller groups of wafers were implanted using boron at energies of lo-400 keV and phosphorus at 50-600 keV. After implantation, all wafers were characterized by modulated reflectance measurements in a Therma-Wave Inc. TP-300 [6], using two modes, high spatial density conformal mapping and diameter scans. The cross-wafer uniformity (%, one sigma) and the average value from modulated reflectance measurements on each wafer were plotted as a function of the tilt and twist angle setting for that wafer. The measurements are in therma-wave units (TW), an arbitrary unit of modulated reflectance, which is proportional to the damage produced by the implanted ions [7]. Therefore, increased channeling would cause a reduction in TW signal, because channeled ions produce less damage in the substrate.
0 1991 - Elsevier Science Publishers B.V. (North-Holland)
II. REAL TIME PROCESSING
R.B. Simonton et al. / Channeling control
40
The therma-wave modulated reflectance measurement is quite sensitive to the distortions of the as-implanted profile which result from ion channeling during implantation; this is demonstrated in fig. 1. This figure contains an overlay of seven implant profiles, measured using secondary ion mass spectroscopy (SIMS) [Xl, from different locations on a radius of the same silicon (100) wafer; the wafer had also been mapped in TW units. This wafer had been implanted (with no screen oxide) using 180 keV, 5 x 1011 cmP2 boron in a variable scan angle implanter at a tilt angle of 0’ (ion beam aligned with the (100) pole at the wafer center). This resulted in a range of known ion beam orientation angles to the (100) pole at different locations on the wafer’s radius. SIMS profiles from the seven locations were correlated with the known ion beam orientation angle to the (100) and the measured TW value for each location, resulting in fig. 1. As indicated there, a variation of a few percent in TW units means a fairly dramatic shift in the as-implanted profile has occurred.
3. Results The plots of average TW value and cross-wafer TW uniformity (%, one sigma) for the boron, 100 keV, 5 X 1O1l cmm2 implant condition are presented in fig. 2. Fig. 2a presents this data for a constant twist angle (23” ) at various tilts from 0 to 60 ‘. Figs. 2b-f provide this data as a function of twist angle for various (fixed) tilt angles. The ion beam is aligned with major channeling features at those orientations in fig. 2 which exhibit significantly decreased average TW value and degraded TW uniformity. Conversely, orientations which display the best TW uniformities and local maxima in average TW values are orientations where the ion beam is not aligned
10IS
BORON ISOkeV, 5X10"
UT? -222.3
0.0
-236.2
0.6
-250.1
0.8
-264.0
1.0
.....277.9
1.2
.....291.6
1.5
.....291.6
1.9
*NOTE: TWTANGCEIS
DEPTH
(microns)
Fig. 1. Therma-wave (TW) modulated reflectance signal sensitivity to as-implanted profile changes from (100) axial channeling variations, determined by SIMS measurements for a boron, 180 keV, 5 x 10" crne2 implant without a screen oxide.
with major channeling features; profile control should be optimum there. Table 1 summarizes the optimum combinations of tilt and twist angles, obtained from the data in fig. 2, for the boron, 100 keV, 5 x 1011 cmP2 implant condition.
4. Discussion All major variations of average TW value and TW uniformity observed in fig. 2 can be explained by considering channeling in six major (low Miller index) poles and planes. This is demonstrated by considering fig. 3, a stereographic projection [9] for the silicon lattice as viewed along a (100) pole. This projection represents the entire twist range O-360” because of the mirror symmetry about the (110) planes and the 90 o rotational symmetry about the (100) poles. Note that the 0 o twist references in figs. 2 and 3 are the (110) planes because these planes correspond to the major wafer flat, which is the twist orientation reference in most implanter equipment. The major, low Miller index poles and planes are represented in fig. 3 by the largest filled circles and heaviest lines, respectively. Higher Miller index poles are represented by smaller filled circles; for simplicity, many of these poles are not indicated here. The regions of angle space in the stereographic projection which were investigated by the implants whose TW measurements are plotted in fig. 2 are identified by light dashed lines. The electrostatic scan system of the NV-6200AV ion implanter used in this study produces small variations in ion beam orientation as the beam is scanned across the wafer. The tilt and twist angle space “sampled” by the scanned ion beam depends on the tilt angle (and wafer size) employed for the implant; the space sampled at higher tilt angles is greatly reduced from that at lower tilt angles. The angle space sampled by the area of one 150 mm wafer is visualized by the small ellipses in fig. 3. By considering each part of fig. 2 in the light of fig. 3, all major variations are explained. Wherever the dotted lines (representing the tilt/ twist implant matrix) in fig. 3 encounter a major plane or pole, a relatively major decrease in TW value and increase in TW nonuniformity appears in the corresponding plot in fig. 2 which contains the tilt/ twist angles of the encounter. For example, in fig. 2a, the large TW and uniformity variations which occur at 0 o tilt, at about 37” tilt, and about 47O tilt are caused by ion beam alignment with the (100) pole, (111) planes, and (110) planes, respectively. (The relatively minor TW variations at about 25 o tilt are caused by higher Miller index poles, previously reported by Ziegler and Lever [3], which are not indicated in fig. 3 but which were obvious in the TW conformal maps of these wafers. The effect of these
R.B. Simonton et al. / Channeling control A. XI’ TWIST vs TILT
B.
5 -5
0
5
10
15
20
25
30
TILT ANGLE
C.
20°,
25’
3.5 40
45
50
55
60
65
7’.
41
lo”,
12”
1
310
0 -10-5
300 0
5
(DEGREES)
10
15
nwr
TILT vs TWIST
TILT vs TWIST
D.
35O,
45’
20
25
30
ANGLE
35
40
45
50
55
60
(DEGREES)
TILT vs TWIST
F~~,i -,0-s
0
5
10
15
20
25
TWIST ANGLE
E.
5s”
30
35
40
45
50
55
60
-10-5
0
5
10
(DEGREES)
15
nwsr
F.
TILT vs TWIST
20
25
ANGLE
30
35
40
45
50
55
60
(DEGREES)
1
SO” TILT vs TWIST
%
341 351 356 346 361 366
g E z
-10-5
0
5
10
15
TWlSi
20
25
ANGLE
30
35
40
45
50
55
60
(DEGREES)
0.0-336 -10-5
0
5
10
15
20
25
Swiss ANGLE
30
35
40
45
50
55
60
(DEGREES)
Fig. 2. TW average value and cross-wafer ~ifor~ty (%, one sigma) for various tilt and twist angle combinations, as noted above. (AU implants are boron, 100 keV, 5 x 10”’ cm-‘, without screen oxides.) poles is also evident at about 20 ’ twist for 25 o tilt, and less so for 20° tilt, in fig. 2c.) In this manner, direct comparison of the channeling behavior in fig. 2 and the location of poles and planes in fig. 3 explains all major channeling variations. The relative magnitude of TW variations in fig. 2 from channeling into the different major features is expIained by the data presented in fig. 4. The information in this figure also reveals the effect of energy variations on channeling behavior. This data was obtained by appropriately oriented TW diameter scans of wafers which had been deliberately oriented during implantation so that the ion beam was aligned with a
selected channeling zone over a portion of the target surface during scanning. Correlation of the measured TW value with position on the measurement diameter and accurate knowledge of the ion beam scan angles on the target plane allowed plotting of the measured TW values as a function of ion beam alignment with the channeling zone, in degrees. The measured TW values for each wafer have been normalized to the minimum (channeled) value, and are presented as a percent variation in the plots in fig. 4. By examining the plots in fig. 4, the relative magnitude for channeling of boron ions in several major zones is revealed. Channeling from poles clearly overshadows II. RE4L TIME PROCESSING
R.B. Simonton et al. / Channeling control
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Table 1 Optimum wafer orientations for channeling control for boron, 100 keV, 5 x 10” cm-’ implants without screen oxide Tilt angles *)
I0 10"
12” 20” 25” 35” 45O 55” 60”
Table 2 Boron critical angle and average channeled distance Critical angle a1
Average
Optimum twist angles a1
Id%4
distance b, [A]
25-33O 23-35’ 15-37O 12” or 26-32” 12” or 28-38” 28-38° 18-26’ 14-23” or 32-38” 18-20’ or 40-45 o
2.2 1.92 1.74 1.04 0.94 0.79
3400 1750 1180 720 800 780
a) Experimental orientation accuracy for tilt and twist angles is =z+ 0.25 o and i t 2.0 O, respectively.
Channel (110) (111) (100) jlll) jll.0)
pole pole pole plane plane
PI
plane
a’ Critical angles for 100 keV boron calculated using a ZBL specific boron-silicon interatomic potential [ll]. b, Average distance traveled in the channel for boron ions entering at 1” off perfect alignment, calculated using ~RLOWE code (at 100 keV for planes and 80 keV for
poles). 23’
TWIST
60’
TILT
V5
55’
TILT
”
45’
TILT
25”
TILT
20’
TILT
w-T
VS TILT TWIS
” =
(DEGREES)
FROM
va01,
POLE --
Fig. 3. Stereographic projection of the silicon lattice for the tilt range O-90 o from the (100) pole and the twist range O-45 o from the { 110) planes toward the { 100) planes.
that from planes; this observation has been reported in previous investigations [3]. Also, (110) planes manifest significantly more severe channeling than (100) planes. This hierarchy explains the relative magnitude for most of the major TW variations in fig. 2, particularly near 0 * tilt in fig. 2a and for 0 o twist relative to 45 * twist at the various tilts in figs. 2b and c.
(100)
(1 IO)
POLE
ANGLE FROM ALIGNMENT
(DEGREES)
.4w-E
The severity of channeling in a pole or plane (ignoring dose effects) depends strongly on its critical acceptance angle. This angle is dependent on the atomic number (Z) and energy (E) of the implant species; it is proportional to (Z/E)1/2 [lo]. A summary of critical angles (for boron at 100 keV) for all major features affecting the measurements in fig. 2 is presented in table 2. These angles were calculated using a ZBL interatomic potential specific to boron and silicon [ll], which yields critical angles smaller than previous calculations. Also contained in table 2 are the average distances traveled by a channeled ion in these same major poles and planes, calculated using MARLOWE. As is clear from this table, major poles will have significantly greater channeling effects than major planes, and {llO} planes will have worse channeling effects than (100) planes, as is experimentally verified in fig. 4. Consideration of the critical channeling angles and average channeled distance indicates the degree of channeling will be greatest for (110) poles, then (111) poles, followed closely by (100) poles; this is consistent with the behavior observed for these poles in figs. 2a, d, and e. Furthermore, the table predicts that (111) planes manifest channeling
PLANE
FROM +WGNMENT
{IOO)
(DEGREES)
PLANE
ANGLE FROM ALIGNMENT
Fig. 4. The energy dependence of the sensitivity to ion beam orientation for channeling of boron, 5 X 10” CIII-~, 7’ (without a screen oxide) into (100) poles, (110) planes, and (100) planes.
(DEGREES)
tilt implants
43
R.B. Simonton et al. / Channeling control slightly worse than (110) planes, which will clearly be worse than {loo} planes; this is consistent with the relative variations observed for these planes in figs. 2a and f ((111) vs (llo}), fig. 2b, and fig. 4 ((110) vs {100)). Fig. 4 also demonstrates the variation in channeling behavior as the energy of a 5 X 1O’i cme2 boron implant is varied from 400 down to 10 keV. The highest energy implants have the smallest critical angles, but channeling variations are greater at high energy because the sensitivity to small changes in ion beam orientation near alignment is (greater. This sensitivity is reduced by reducing the energy, because the critical channeling angle depends on (Z/E)‘/‘, so that lower energy will produce larger channel~g acceptance angles. Althou~ this means that acceptance into all major channels is easier, it also means reduced sensitivity to small variations near alignment with any particular channel. For example, in fig. 4, when the boron implant energy is below about 20 keV, the manifestation of channeling variation caused by (100) planes is eliminated, even at 5 x 10” cm-’ dose. When increasing the physical tilt angle (0) while keeping the beam aligned with a planar channel, the average distance traveled by a channeled ion (perpendicular to the wafer’s surface) is reduced by a factor cos B 1121; consequently, there is a reduction of channeling variations when 8 is large. This explains the absence of (100) planar channeling effects for 60” tilt at the 45O twist position and the diminished effect of (110) planar channeling at O” twist in fig. 2f. In the same manner that boron channeling behavior is characterized in fig. 4, we also characterized channeling in phosphorus 5 x 101’ crnw2 impl~tation. The same general channeling tendencies were observed with phosphorus as with boron, except that the sensitivity of channeling to orientation was reduced by the selection of the heavier species, and reductions in implant energy more quickly reduced this sensitivity for phosphorus than for boron. These effects are expected when the critical acceptance angle is increased by selection of phosphorus (Z = 15) instead of boron (2 = 5). The changes in channeling behavior from energy and species variations we observed were consistent with previous reports [ 31. When comparing our data for phosphorus and boron, we observed the channeling behavior for specific implant conditions with the same (Z/E)“’ ratios is essentially identical. This occurs because the 5 X 101’ cm-* dose used here is too low to allow significant channeling differences from accumulated crystal damage to appear in a comparison of these two species. Since channeling is not effectively reduced by increasing the implant dose until an amorphous layer is formed, consideration of critical channeling angles and characterization of orientation sensitivity (as presented in fig. 4)
should allow reasonably accurate general predictions of channeling behavior for a wide range of species and
energy, at doses up to the formation layers.
of amorphous
5. Conclusion Optimum wafer o~entations (tilt/ twist combinations) for the tilt range 7-60” have been identified for a boron, 100 keV, 5 X 101i cm-* implant (table 1). These orientations should provide adequate channeling (profile) control, even for implantations employing very low doses. A stereographic projection reveals that all major variations in the experimentally observed channeling behavior from 7-60° can be explained by considering channeling in the six major, low Miller index crystallographic poles ((loo), (110) and (111)) and planes ({llo}, (111) and (100)). The relative severity of ion ch~e~~g in these major poles and planes, and the effect of energy, species and tilt angle variations on the manifestation of channeling can be explained using the concepts of critical channeling angle and average distance traveled for channeled ions. The orientations identified here will apply to a broad range of implant conditions. However, implantations employing conditions with a significantly larger ratio (lower energy/higher mass) and/or (z/%)1’* much higher doses ( >> 5 X lOi cmb2) will have a larger set of acceptable orientations. Conversely, implants with a significantly smaller (Z/E)1’2 ratio (higher energy/ lower mass} using low doses (* 1 X 1014 cm-‘) will have a more constrained set of allowable orientations, due to increased sensitivity to ion beam orientation of channeling effects for all crystallographic features in the silicon lattice, particularly higher Miller index poles [3].
Acknowledgements This work was supported in part by the Semiconductor Research Corporation and Sematech.
lZeferences [l] Y. Akasaka, Nucl. Instr. and Meth. B37/38 (1989) 9. [2] K. Klein, C. Park, A. Tasch, R. Simonton and S. Novak, Ext. Abstr. Electrochem. Soe. Conf. 90-I (Pennington, NJ) 359. [3] J. Ziegler and R. Lever, Appl. Phys. Lett. 46 (1985) 358. [4] J. Dykstra, A. Ray and R. Simonton, these Proceedings (8th Int. Conf. on Ion Implantation Technology, GuildIord, UK, 1990) Nucl. Instr. and Meth. I355 (1991) 478. II. REAL TIME PROCESSING
44 [5] R. Simonton,
R.B. Simonton et al. / Channeling control
D. Kamenitsa and A. Ray, these Proceedings. (8th Int. Conf. on Ion Implantation Technology, Guildford, UK, 1990) Nucl. Instr. and Meth. B55 (1991) 39. [6] Therma-Wave, Inc., 47734 Westinghouse Drive, Fremont, CA 94534, USA. [7] W. Smith, A. Rosencwaig, D. Willenborg, J. Opsal and M. Taylor, Solid State Technol. 29 (1986) 85. [8] Charles Evans & Assoc., 301 Chesapeake Dr., Redwood City, CA 94063, USA.
[9] L. Azaroff, Elements of X-Ray Crystallography (McGraw-Hill, 1968) p. 23. [lo] D. Van Vliet, in: Channeling: Theory, Observation, and Experiment, ed. D. Morgan (Wiley, New York, 1973). [ll] C. Park, K. Klein, A. Tasch and J. Ziegler, Electrochem. Sot. Conf. 90-l (Pennington, NJ) 357. [12] G. Fuse, H. Umimoto, S. Odanaka, M. Wakabayashi, M. Fukumoto and T. Ohzone, Electrochem. Sot. 133 (1986) 996.