- Email: [email protected]
Amorphization of silicon by high dose germanium ion implantation with no external cooling mechanism
s __
__ B
applied surface science
ELSEVIER
Applied Surface Science 78 (1994) 321-330
Amorphization of silicon by high dose germanium ion implantation with no external cooling mechanism 2. Xia *,a, J. Saarilahti
a, E. Ristolainen b, S. Ergnen a, H. Ronkainen P. Kuivalainen a, D. Paine ‘, T. Tuomi b
a,
’ KT Electronics, Olarinluoma 9, P.O. Box 11012, FIN-02044 VTT, Finland b Helsinki University of Technology, FIN-02150 Espoo, Finland ’ Brown Uniuersity, Providence, RI 02912, USA
(Received 5 January 1994; accepted for publication 28 March 1994)
Abstract Si(100) wafers were implanted by using three different methods: single-energy Ge+ ion implantation, double-energy Ge+ and Ge*+ ion implantation, and double-energy Si+ and Ge’ ion implantation. The single-energy implantations were performed at energies from 50 to 180 keV, over the range of 8.38 x 1015 to 5.80 X 1016 ions/ cm’. By keeping the ion beam power density below 0.09 W/cm’, full surface amorphization could be achieved in the single-energy Ge+ implanted samples. Also beam heating was suppressed during implantation, although the implanter had no external cooling. In addition, a two-step single-energy implant technique using sequentially high and low power densities was further developed in order to reduce implantation times. In order to locate the amorphous/crystalline (a/c) interfaces far away from the concentration peak positions of the implanted Ge+ ions, the double-energy Ge+ and Ge*+, and Si+ and Get implantations were carried out. Three Ge+ implanted wafers were either pre-implanted with 180 keV Si+ ions, or post-implanted with 360 keV Ge2+ ions,
respectively, in order to locate deeper a/c interfaces. Channelling effect measurements indicate that the double-energy Ge+ and Ge*+ implantation is a preferable technique for wilfully tailoring the amorphous depth and the Ge peak position.
1. Introduction Recently Si, _,Ge,/ Si heterostructure formed by germanium ion implantation has stimulated increasing interests due to its high compatibility with current IC processing technology. For recovering the crystalline quality and realizing the
* Corresponding author. Tel.: +358 0 4566624; Fax: +358 0 4522593; E-mail: [email protected].
strained regrowth of the implanted layers, some techniques have been attempted besides essential thermal processes, such as low-temperature Ge implantation [1,2], carbon doping into the SiGe layers [3], Ge and B co-implantation [4]. No matter what techniques can be used, the implanted Ge ion doses must be in the range from lOI to 10” ions/cm*, so that the peak contents of Ge in the Si,_,Ge, layers can reach 5-20 at%, otherwise no effective band-gap shrinkage can be achieved [5]. Furthermore, such high dose implants are time-consuming with the medium flux
0169-4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00129-O
322
2. Xia et al. /Applied
Surface Science 78 (1994) 321-330
implanters. For instance, the implants can take over 10 to 100 min in this dose range even using a high beam current density of 2.5 pA/cm2. Simply the way to enhance the throughput is to raise beam currents, but it in turn may be self-defeating by causing harmful dynamic annealing of amorphous layers. It has been well documented that a Wayflow-type end station with no external cooling mechanism may result in significant wafer heating and ion-beam-induced epitaxial crystallization (IBIEC) during high ion flux implantation, and more defects may remain at the surface regions and in the vicinity of the original amorphous/crystalline (a/c) interfaces after regrowth due to dynamic annealing [6-101. Therefore, the formation of totally amorphized layers and the repression of dynamic annealing is a prerequisite for satisfying regrowth. Using a Waycool fixture with Freon or water cooling is helpful to repress beam heating. However, implanters without external cooling are still widely used today. The present work deals with the amorphization of Si substrates by Ge ion implantations using a Wayflow fixture. For all the samples implanted with Gef ions, the peak contents of Ge are kept at 7-9 at% by determining the implant doses in combination with the implant energies. Remaining end-of-range (EOR) damage is often found near the original a/c interfaces after annealing, especially with increasing the implant energies and doses. As a result, the EOR damage can significantly degrade the device performance. It is well established that the reverse bias leakage current can be considerably reduced by increasing the distance between p+-n junction and EOR damage [ll]. The simulation results for Si, _-x Ge,/ Si heterojunction bipolar transistors (HBTs) indicate that in order to get high current gain and short base transit time the Ge profile maximum should be close to the base-collector junction [12-141. Single-energy Ge + implants can only lead to the fixed and narrow junction-EOR damage separations, while double-energy implants with Ge*+ or Sif ions can make the tailoring of EOR damage position and the Ge profile maximum possible. In the present work, three wafers implanted with Gef ions were either Si+ pre-implanted or Ge2+ post-implanted in order to widen
the wanted spatial separation. In addition, channelling effect measurements were used to evaluate all the implanted samples.
2. Experimental In the present study 10 cm diameter (100) n-type (l-6 R. cm) Czochralski wafers were used. Implantations were carried out with the commercial ion implanter Eaton-Nova 200 (acceleration voltage: 20-200 kV) with no external cooling mechanism. The implantation schedules are listed in Table 1. First, single-energy Ge implantations were performed with 74Gef ions at energies of 50, 70, 100 and 180 keV with doses between 8.38 X 1015 and 5.80 X lO”j ions/cm2. Furthermore, three wafers implanted with 180 keV 74Gef ions up to 9 at% were either 28Si+ pre-implanted
Table 1 Implantation sequences employed to amorphize Si(100) with single-energy 74Ge+ ions, double-energy 28Sit and 74Gef ions, and double-energy 74Gef and 74GeZ+ ions Sample
Ion
Al A2 Gl D7 B2 B3 El6 E21
74Ge+
E22 El3 El5 E4
2nsi+ 2xsi2+ 2asi
+
74Ge + E5
*asi+
74Ge +
U
D
I
P
(kV)
(lo”/ cm*)
(/LA/ cm*)
W/ cm’)
;min)
50 50 70 100 180 180 180
0.838 1.70 2.00 2.60 5.80 4.50 3.30 0.50 4.72 0.40 6.50 0.40 0.70 0.40 1.00 2.60 1 .oo 4.72
0.83 0.83 1.21 0.89 1.66 1.66 1.50 0.32 2.55 0.25 2.55 0.25 0.38 0.25 0.34 0.89 0.34 0.38
0.041 0.041 0.085 0.089 0.30 0.30 0.27 0.058 0.46 0.090 0.46 0.090 0.068 0.076 0.061 0.089 0.061 0.068
27 55 44 78 93 73 59 42 49 85 68 85 49 85 79 78 79 332
180 360 180 360 180 300 180 100 180 180
Samples El3 and El5 were amorphized by single-energy 2sSi+ and *sSi2+ respectively, as a control to the double-energy 28Sic and 74Gef implants. U, D, I, P and t are the ion acceleration voltage, the implanted ion dose, the ion beam current density, the ion beam power density, and the implantation time, respectively.
323
Z. Xia et al. /Applied Surface Science 78 (I 994) 321-330
at 180 keV with 1 X 1016 ions/cm2, or 74Ge2+ post-implanted at 360 keV with 4 X 101’ ions/ cm’. During implantations, the beam power densities (the product of the acceleration voltage and the beam current density) were kept constant over the range of 0.041 to 0.46 W/cm2 except one sample, E16, which was first implanted by Ge+ ions at 180 keV with the beam power den-
s
sity of 0.27 W/cm2 while the dose was 3.3 X 1016 ions/ cm’, and then by using Gef ions at 180 keV with the reduced power density of 0.058 W/cm2 having the total dose of 3.8 X lOI ions/ cm’. All the implanted samples were characterized by performing RBS channelling measurements, which were carried out with 1.5 and 2.0 MeV
1000
-
-
E z s u z F ._F $ = 3
Al random Al aligned A2aligned
800
600
400
y” “m 200
m
0
600
600
1000
1200
Energy (keV)
Fig. 1. (a) Channelling spectra for samples Al and A2; (b) cross-sectional bright-field micrograph of the sample to 650°C for 4 h; and (c) high resolution HRTEM micrograph of the same sample (A: surface; B: original a/c
A2 after annealing interface).
324
Z. Xia et al. /Applied Surface Science 78 (1994) 321-330
Fig. 1 (continued).
He+ ions at normal incidence with the detector mounted at 170” to the incident beam direction. The aligned measurements reported in this work were made along the (100) direction. Also transmission electron microscopy (TEM) was used for the studies on the dynamic annealing effect, and all the cross-sectional XTEM studies were performed with a high voltage of 200 kV avoiding defects caused by the electron beams. Cross-sectional samples were prepared in the standard way by using argon ion milling.
3. Results and discussion 3.1. Single-energy implantation
With the implantation parameters listed in Table 1, channelling properties of 50 keV Ge+ implanted samples Al and A2 are shown in Fig. la. In the case of the aligned spectra of Ale and A2c, implanted regions have the same backscattering yields as for random incidence, which confirms that the surface layers of the two samples have been fully disordered with amorphous thick-
nesses of 90 and 95 nm, and with Ge peak concentrations about 4 and 8 at%, respectively. The spectra of the 70 and 100 keV Ge+ implanted samples Gl and D7 present continuous amorphous layers initiated from the surfaces with thicknesses of 130 and 150 nm and Ge peak concentrations of about 8 at%, respectively. Figs. lb and lc are the TEM micrographs of the sample A2 treated by furnace annealing at 650°C for 4 h, which do not detect any extended defects in the regrown region but clearly show residual EOR defects (dislocation loops) beyond the original a/c interface at a depth of 90-100 nm. These depth profile measurements were well demonstrated with RBS. Furthermore, with the rapid thermal annealing at 1100°C for 10 s, the high resolution HRTEM studies of the sample A2 show no EOR damage or any other obvious crystallographic defects. Note that all the single-energy implanted samples show full surface disorder when the ion beam power densities were below 0.09 W/cm’, and also the dynamic annealing was suppressed. The random and aligned channelling spectra for the sample B2 implanted by Ge+ ions with
Z. Xia et al. /Applied
the random spectrum, which clearly indicates that during the implantation the implanted layer was annealed due to beam heating. This implanted layer is heavily damaged, but no clear amorphous
the beam power density of 0.30 W/cm* are plotted in Fig. 2a. The aligned He ion backscattering yields from Si and Ge atoms in the implanted region are obviously lower than those shown in
1200 37 E s s u s > .?
32.5
Surface Science 78 (I 994) 321-330
--
B2aligned
--
B2random
1000
600
600
‘i 5 2
400
2 s
m
200
600
1000
1200
Energy (keV)
Fig. 2. (a) Channelling spectra for the sample B2; (b) cross-sectional bright-field micrograph of the sample B3 after annealing to 590°C for 24 h and (c) high resolution HRTEM micrograph of the same sample.
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Z. Xia et al. /Applied Surface Science 78 (1994) 321-330
Fig. 2 (continued).
region or a/c interface can be observed in Fig. 2a. Under the same beam power density, similar dynamic annealing behavior is observed in the sample B3, although the implant dose and the implant time have been reduced. The calculated average temperature rise due to beam heating is about 340°C for implantation with the beam power density of 0.3 W/cm2 [151. Figs. 2b and 2c show the TEM micrographs of the sample B3 after furnace annealing at 590°C for 24 h. This sample is too heavily defected to observe EOR damage, and it contains a lot of amorphous regions of 5 nm wide and lo-20 nm long. Thus, it also confirms that this sample was not amorphized during implantation and the dynamic annealing effect is detrimental to the recrystallization of implanted layers. Clearly, suppressing the
dynamic annealing effect during implantation is critical for achieving good epitaxial regrowth of implanted layers. The sample El6 was implanted by single-energy 180 keV Ge+ ions with a dose of 3.80 x 1016 ions/cm2. With a high beam power density of 0.27 W/cm2, this sample was first implanted by Ge+ ions with a dose of 3.30 X 1016 ions/cm”; and then the rest of the total dose was implanted with a low beam power density of 0.058 W/cm2. Note, however, that dynamic annealing probably occurred during the first-step implantation with the high beam power density. Therefore, the second-step implantation with the low power density was expected to repress the beam heating and amorphize the surface region. The channelling spectra for the El6 exhibit that a full surface disorder has been obtained with the amorphous thickness of 265 nm shown in Fig. 3, which confirms that the adopted second-step implantation is effective for amorphizing the implanted region and avoiding the excess beam heating. This twostep implantation procedure sequentially using high and low power densities can significantly improve the throughput compared with the onestep implantation using a low power density. For instance, the one-step implantation with the beam power density of 0.058 W/cm* will take about 318 min, but the two-step implanted sample El6 with the same implant energy and dose took only 101 min. Finally, the parameters for the secondstep implantation should be designed as follows: the power density and the dose used should be low enough to repress beam heating and save implant time, yet the dose has to be high enough to avoid the formation of any buried amorphous layers. 3.2. Double-energy implantation As listed in Table 1, samples E21 and E22 were prepared by double-energy Ge+ and Ge2+ ion implantations. Note that the Gef ion implantations with a high power density of 0.46 W/cm2 were expected to form Si,_,Ge, alloy layers and locate Ge peak positions as far from the surface as in the sample E16. In this case the implanted wafers were annealed by the beam heating effect,
Z. Xia et al. /Applied
1200
I
I
Surface Science 78 (1994) 321-330 I
I
I
I
E21 aligned
800
-
E21 random
-
E22
-
ElGaligned
aligned
600
0 400
600
800
-1200
1000 Energy
Fig. 3. Channelling
spectra
while the estimated temperature rise was about 380°C at this power density [I5]. Following Ge*+ ion implantations with a low power density of 0.09 W/cm* were expected to form a continuous amorphous layer initiated from the surface and locate the a/c interfaces much deeper than in the sample E16. Channelling measurements shown in Fig. 3 indicate that the double-energy implantations used for samples E21 and E22 have reached the goal: the Ge peak positions are the same as that of the sample E16, both a/c interfaces are at a depth of about 410 nm, which is about 145 nm deeper than that of the sample E16. Corresponding to the doses used, the measured Ge peak concentrations for samples E16, E21 and E22 are 6, 8 and 11 at%, respectively. In addition, the Ge*+ implantations shown in Fig. 4 produced slightly widening profiles of Ge in the tail parts.
for samples
_-__ 1400
1600
1800
CkeV)
E16, E21 and E22.
Double-energy Sif and Gef implantations were studied following the similar train of thoughts used for samples E21 and E22: the Si+ implantations determined the location of the a/c interfaces and Gef implantations formed Si, ox Ge, alloys. In order to check the ability of amorphization by Si ion implantations, samples El3 and El5 were implanted with Sif and Si*+ ions only. As evidently shown in Fig. 4, buried amorphous layers were formed and surface regions were of long-range order in the two samples. For the sample E13, the front a/c interface was at a depth of 110 nm and the thickness of the buried amorphous layer was 230 nm. In the case of the sample E15, they were 190 and 340 nm, respectively. Comparing the implantation doses, implant times and power densities used for the two samples with those for samples E16, E21 and E22,
Z. Xia et al. /Applied
328
Surface Science 78 (1994) 321-330
one can find that when using Si ion implantations it is more difficult to form continuous amorphous layers. With such Si ion implantations, the Gef ion implantations have to be carried out with low power densities in order to amorphize the surface regions which were not amorphized by using the Si ion implantations. Samples E4 and E5 exhibit such examples: they were first implanted with Si+ ions having similar implant parameters to the sample E13, and then implanted with Ge+ ions at energies of 100 and 180 keV, respectively. All the Si and Ge implantations were done with beam power densities smaller than 0.09 W/cm2. Fig. 5 shows that continuous amorphous layers in these samples were formed, and the a/c interface layers were at a depth of about 390 nm. Furthermore, the Ge peaks have been observed to be located at different depth for different implant energies. As a result, the double-energy Si+ and 1200
I
I
800
Ge+ implantation technique is also effective for locating the spatial separation between the a/c interface and the Ge profile peak. With this double-energy implant recipe, the implant sequence for Si+ and Ge+ ions is of little importance because all the implantations must be performed with low beam power densities to avoid any dynamic annealing. Consequently, it is much more time-consuming than the double-energy Ge2+ and Gef implant recipe. For instance, for locating the similar spatial separations and getting similar Ge depth profiles, the implantation used for the sample E21 took about 134 min, while for the sample E5 at least 400 min. Furthermore, this double-energy Si+ and Ge+, or Si2+ and Gef implant recipe may not facilitate the spatial separation between the a/c interface and the Ge profile maximum, since the Ge+ implantation has to localize its damaged layer to
1000 Energy
Fig. 4. Channelling
spectra
1200 CkeV)
for samples
I
I
I
1
El3 and E15.
1400
---a-
El5 random
--o-
El3
aligned
*
El5
aligned
Z. Xia et al. /Applied
1200
I
I
329
Surface Science 78 (1994) 321-330
I
I
I
I
_
E5 aligned
-o-
E5 random E4 aligned
Ge
0
400
600
800
1000 Energy
1200 CkeV)
1400
1600
1800
Fig. 5. Channelling spectra for samples E4 and ES.
cover the crystalline surface where it cannot be disordered by Si+ or Si2+ implantation. As seen in Fig. 4 and Table 1, it is also predictable that the double-energy Si2+ and Ge+ implantation will result in less spatial freedom and is more time-consuming than the Sif and Ge+ implantation, although the a/c interface can be located much deeper. Finally, comparing all the double-energy implant recipes, the Gef and Ge2+ implantation is of advantage: more spatial freedom, higher production output, and easier implant operation (only a Ge ion source is needed).
4. Conclusions By the control of the ion beam power density, the dynamic annealing effect caused by beam
heating can be repressed during high dose Ge implantation even in the case of a Wayflow fiiture with no external cooling. Below the power density of 0.09 W/cm2, Ge ion implantations have resulted in continuous amorphous layers in all the studied samples and dynamic annealing was repressed. Moreover, it has been shown that it is more difficult to form continuous amorphous layers by using Si ion implantations than by Ge. For the single-energy Ge+ ion implantation, a two-step implant recipe using sequentially high and low power densities has been developed, which can significantly improve the throughput. Furthermore, the double-energy Gef and Ge2+, and Si+ and Ge+ implantation recipes have also been presented, which effectively can locate the spatial separations between the a/c interfaces and the Ge peak positions. Thus, although there are many factors affecting the Ge ion implanta-
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Z. Xia et al. /Applied Surface Science 78 (1994) 321-330
tion, the present study demonstrates that the double-energy Ge+ and Ge*+ implant recipe is preferable in processing the wanted spatial separation, saving the implant time and simplifying the implantation operation.
[3] [4] [5]
Acknowledgements
[6] [7]
The authors wish to thank R. Korkeamaki for technical assistance, and T. Wiik and I. Suni for helpful discussions. This research was supported by the Technical Research Centre of Finland (VTT). One of us (D.P.) acknowledges support from NSF contract MRG-DMR-9223683.
[8] [9] [lo] [ll] [12]
References [I] K.-I. Shoji, A. Fukami,
T. Nagano, T. Tokuyama and C.Y. Yang, Appt. Phys. Lett. 60 (1992) 451. [2] A. Gupta, C. Cook, L. Toyoshiba, J. Qiao, C. Yang, K.
[13] [14] [15]
Shoji, A. Fukami, T. Nagano and T. Tokuyama, J. Electron. Mater. 22 (1993) 125. A. Fukami, K.-I. Shoji and T. Nagano, Extended Abstracts 22nd Conf. SSDM, Sendai, August 1990, p. 1163. K. Ohta, J. Sakano and S. Furukawa, Nucl. Instr. Meth. B 59/60 (1991) 1113. Roosevelt People, IEEE J. Quantum Electron. QE-22 (1986) 1696. S. Prussin, D. Margolese and R. Tauber, J. Appt. Phys. 54 (1983) 2316. S. Prussin, D. Margolese and R. Tauber, J. Appl. Phys. 57 (1985) 180. S. Prussin and K. Jones, J. Electrochem. Sot. 137 (1990) 1912. K. Jones and D. Venables, J. Appl. Phys. 69 (1991) 2931. K. Jones, S. Prussin and E.R. Weber, Appl. Phys. A 45 (1988) 1. M.E. Lunnon, J.T. Chen and J.E. Baker, J. Electrochem. Sot. 132 (1985) 2473. K. Grahn, Z. Xia, P. Kuivalainen, M. Karlsteen and M. Willander, Electron. Lett. 29 (1993) 1621. G.-B. Gao and H. MorkoG, Electron. Lett. 27 (1991) 1409. S.S. Winterton, B.M. Manning, D.J. Walkey and N.G. Tarr, Electron. Lett. 27 (1991) 1338. P. Parry, J. Vat. Sci. Technol. 13 (1976) 622.
__ B
applied surface science
ELSEVIER
Applied Surface Science 78 (1994) 321-330
Amorphization of silicon by high dose germanium ion implantation with no external cooling mechanism 2. Xia *,a, J. Saarilahti
a, E. Ristolainen b, S. Ergnen a, H. Ronkainen P. Kuivalainen a, D. Paine ‘, T. Tuomi b
a,
’ KT Electronics, Olarinluoma 9, P.O. Box 11012, FIN-02044 VTT, Finland b Helsinki University of Technology, FIN-02150 Espoo, Finland ’ Brown Uniuersity, Providence, RI 02912, USA
(Received 5 January 1994; accepted for publication 28 March 1994)
Abstract Si(100) wafers were implanted by using three different methods: single-energy Ge+ ion implantation, double-energy Ge+ and Ge*+ ion implantation, and double-energy Si+ and Ge’ ion implantation. The single-energy implantations were performed at energies from 50 to 180 keV, over the range of 8.38 x 1015 to 5.80 X 1016 ions/ cm’. By keeping the ion beam power density below 0.09 W/cm’, full surface amorphization could be achieved in the single-energy Ge+ implanted samples. Also beam heating was suppressed during implantation, although the implanter had no external cooling. In addition, a two-step single-energy implant technique using sequentially high and low power densities was further developed in order to reduce implantation times. In order to locate the amorphous/crystalline (a/c) interfaces far away from the concentration peak positions of the implanted Ge+ ions, the double-energy Ge+ and Ge*+, and Si+ and Get implantations were carried out. Three Ge+ implanted wafers were either pre-implanted with 180 keV Si+ ions, or post-implanted with 360 keV Ge2+ ions,
respectively, in order to locate deeper a/c interfaces. Channelling effect measurements indicate that the double-energy Ge+ and Ge*+ implantation is a preferable technique for wilfully tailoring the amorphous depth and the Ge peak position.
1. Introduction Recently Si, _,Ge,/ Si heterostructure formed by germanium ion implantation has stimulated increasing interests due to its high compatibility with current IC processing technology. For recovering the crystalline quality and realizing the
* Corresponding author. Tel.: +358 0 4566624; Fax: +358 0 4522593; E-mail: [email protected].
strained regrowth of the implanted layers, some techniques have been attempted besides essential thermal processes, such as low-temperature Ge implantation [1,2], carbon doping into the SiGe layers [3], Ge and B co-implantation [4]. No matter what techniques can be used, the implanted Ge ion doses must be in the range from lOI to 10” ions/cm*, so that the peak contents of Ge in the Si,_,Ge, layers can reach 5-20 at%, otherwise no effective band-gap shrinkage can be achieved [5]. Furthermore, such high dose implants are time-consuming with the medium flux
0169-4332/94/$07.00 0 1994 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00129-O
322
2. Xia et al. /Applied
Surface Science 78 (1994) 321-330
implanters. For instance, the implants can take over 10 to 100 min in this dose range even using a high beam current density of 2.5 pA/cm2. Simply the way to enhance the throughput is to raise beam currents, but it in turn may be self-defeating by causing harmful dynamic annealing of amorphous layers. It has been well documented that a Wayflow-type end station with no external cooling mechanism may result in significant wafer heating and ion-beam-induced epitaxial crystallization (IBIEC) during high ion flux implantation, and more defects may remain at the surface regions and in the vicinity of the original amorphous/crystalline (a/c) interfaces after regrowth due to dynamic annealing [6-101. Therefore, the formation of totally amorphized layers and the repression of dynamic annealing is a prerequisite for satisfying regrowth. Using a Waycool fixture with Freon or water cooling is helpful to repress beam heating. However, implanters without external cooling are still widely used today. The present work deals with the amorphization of Si substrates by Ge ion implantations using a Wayflow fixture. For all the samples implanted with Gef ions, the peak contents of Ge are kept at 7-9 at% by determining the implant doses in combination with the implant energies. Remaining end-of-range (EOR) damage is often found near the original a/c interfaces after annealing, especially with increasing the implant energies and doses. As a result, the EOR damage can significantly degrade the device performance. It is well established that the reverse bias leakage current can be considerably reduced by increasing the distance between p+-n junction and EOR damage [ll]. The simulation results for Si, _-x Ge,/ Si heterojunction bipolar transistors (HBTs) indicate that in order to get high current gain and short base transit time the Ge profile maximum should be close to the base-collector junction [12-141. Single-energy Ge + implants can only lead to the fixed and narrow junction-EOR damage separations, while double-energy implants with Ge*+ or Sif ions can make the tailoring of EOR damage position and the Ge profile maximum possible. In the present work, three wafers implanted with Gef ions were either Si+ pre-implanted or Ge2+ post-implanted in order to widen
the wanted spatial separation. In addition, channelling effect measurements were used to evaluate all the implanted samples.
2. Experimental In the present study 10 cm diameter (100) n-type (l-6 R. cm) Czochralski wafers were used. Implantations were carried out with the commercial ion implanter Eaton-Nova 200 (acceleration voltage: 20-200 kV) with no external cooling mechanism. The implantation schedules are listed in Table 1. First, single-energy Ge implantations were performed with 74Gef ions at energies of 50, 70, 100 and 180 keV with doses between 8.38 X 1015 and 5.80 X lO”j ions/cm2. Furthermore, three wafers implanted with 180 keV 74Gef ions up to 9 at% were either 28Si+ pre-implanted
Table 1 Implantation sequences employed to amorphize Si(100) with single-energy 74Ge+ ions, double-energy 28Sit and 74Gef ions, and double-energy 74Gef and 74GeZ+ ions Sample
Ion
Al A2 Gl D7 B2 B3 El6 E21
74Ge+
E22 El3 El5 E4
2nsi+ 2xsi2+ 2asi
+
74Ge + E5
*asi+
74Ge +
U
D
I
P
(kV)
(lo”/ cm*)
(/LA/ cm*)
W/ cm’)
;min)
50 50 70 100 180 180 180
0.838 1.70 2.00 2.60 5.80 4.50 3.30 0.50 4.72 0.40 6.50 0.40 0.70 0.40 1.00 2.60 1 .oo 4.72
0.83 0.83 1.21 0.89 1.66 1.66 1.50 0.32 2.55 0.25 2.55 0.25 0.38 0.25 0.34 0.89 0.34 0.38
0.041 0.041 0.085 0.089 0.30 0.30 0.27 0.058 0.46 0.090 0.46 0.090 0.068 0.076 0.061 0.089 0.061 0.068
27 55 44 78 93 73 59 42 49 85 68 85 49 85 79 78 79 332
180 360 180 360 180 300 180 100 180 180
Samples El3 and El5 were amorphized by single-energy 2sSi+ and *sSi2+ respectively, as a control to the double-energy 28Sic and 74Gef implants. U, D, I, P and t are the ion acceleration voltage, the implanted ion dose, the ion beam current density, the ion beam power density, and the implantation time, respectively.
323
Z. Xia et al. /Applied Surface Science 78 (I 994) 321-330
at 180 keV with 1 X 1016 ions/cm2, or 74Ge2+ post-implanted at 360 keV with 4 X 101’ ions/ cm’. During implantations, the beam power densities (the product of the acceleration voltage and the beam current density) were kept constant over the range of 0.041 to 0.46 W/cm2 except one sample, E16, which was first implanted by Ge+ ions at 180 keV with the beam power den-
s
sity of 0.27 W/cm2 while the dose was 3.3 X 1016 ions/ cm’, and then by using Gef ions at 180 keV with the reduced power density of 0.058 W/cm2 having the total dose of 3.8 X lOI ions/ cm’. All the implanted samples were characterized by performing RBS channelling measurements, which were carried out with 1.5 and 2.0 MeV
1000
-
-
E z s u z F ._F $ = 3
Al random Al aligned A2aligned
800
600
400
y” “m 200
m
0
600
600
1000
1200
Energy (keV)
Fig. 1. (a) Channelling spectra for samples Al and A2; (b) cross-sectional bright-field micrograph of the sample to 650°C for 4 h; and (c) high resolution HRTEM micrograph of the same sample (A: surface; B: original a/c
A2 after annealing interface).
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Z. Xia et al. /Applied Surface Science 78 (1994) 321-330
Fig. 1 (continued).
He+ ions at normal incidence with the detector mounted at 170” to the incident beam direction. The aligned measurements reported in this work were made along the (100) direction. Also transmission electron microscopy (TEM) was used for the studies on the dynamic annealing effect, and all the cross-sectional XTEM studies were performed with a high voltage of 200 kV avoiding defects caused by the electron beams. Cross-sectional samples were prepared in the standard way by using argon ion milling.
3. Results and discussion 3.1. Single-energy implantation
With the implantation parameters listed in Table 1, channelling properties of 50 keV Ge+ implanted samples Al and A2 are shown in Fig. la. In the case of the aligned spectra of Ale and A2c, implanted regions have the same backscattering yields as for random incidence, which confirms that the surface layers of the two samples have been fully disordered with amorphous thick-
nesses of 90 and 95 nm, and with Ge peak concentrations about 4 and 8 at%, respectively. The spectra of the 70 and 100 keV Ge+ implanted samples Gl and D7 present continuous amorphous layers initiated from the surfaces with thicknesses of 130 and 150 nm and Ge peak concentrations of about 8 at%, respectively. Figs. lb and lc are the TEM micrographs of the sample A2 treated by furnace annealing at 650°C for 4 h, which do not detect any extended defects in the regrown region but clearly show residual EOR defects (dislocation loops) beyond the original a/c interface at a depth of 90-100 nm. These depth profile measurements were well demonstrated with RBS. Furthermore, with the rapid thermal annealing at 1100°C for 10 s, the high resolution HRTEM studies of the sample A2 show no EOR damage or any other obvious crystallographic defects. Note that all the single-energy implanted samples show full surface disorder when the ion beam power densities were below 0.09 W/cm’, and also the dynamic annealing was suppressed. The random and aligned channelling spectra for the sample B2 implanted by Ge+ ions with
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the random spectrum, which clearly indicates that during the implantation the implanted layer was annealed due to beam heating. This implanted layer is heavily damaged, but no clear amorphous
the beam power density of 0.30 W/cm* are plotted in Fig. 2a. The aligned He ion backscattering yields from Si and Ge atoms in the implanted region are obviously lower than those shown in
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Fig. 2. (a) Channelling spectra for the sample B2; (b) cross-sectional bright-field micrograph of the sample B3 after annealing to 590°C for 24 h and (c) high resolution HRTEM micrograph of the same sample.
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Fig. 2 (continued).
region or a/c interface can be observed in Fig. 2a. Under the same beam power density, similar dynamic annealing behavior is observed in the sample B3, although the implant dose and the implant time have been reduced. The calculated average temperature rise due to beam heating is about 340°C for implantation with the beam power density of 0.3 W/cm2 [151. Figs. 2b and 2c show the TEM micrographs of the sample B3 after furnace annealing at 590°C for 24 h. This sample is too heavily defected to observe EOR damage, and it contains a lot of amorphous regions of 5 nm wide and lo-20 nm long. Thus, it also confirms that this sample was not amorphized during implantation and the dynamic annealing effect is detrimental to the recrystallization of implanted layers. Clearly, suppressing the
dynamic annealing effect during implantation is critical for achieving good epitaxial regrowth of implanted layers. The sample El6 was implanted by single-energy 180 keV Ge+ ions with a dose of 3.80 x 1016 ions/cm2. With a high beam power density of 0.27 W/cm2, this sample was first implanted by Ge+ ions with a dose of 3.30 X 1016 ions/cm”; and then the rest of the total dose was implanted with a low beam power density of 0.058 W/cm2. Note, however, that dynamic annealing probably occurred during the first-step implantation with the high beam power density. Therefore, the second-step implantation with the low power density was expected to repress the beam heating and amorphize the surface region. The channelling spectra for the El6 exhibit that a full surface disorder has been obtained with the amorphous thickness of 265 nm shown in Fig. 3, which confirms that the adopted second-step implantation is effective for amorphizing the implanted region and avoiding the excess beam heating. This twostep implantation procedure sequentially using high and low power densities can significantly improve the throughput compared with the onestep implantation using a low power density. For instance, the one-step implantation with the beam power density of 0.058 W/cm* will take about 318 min, but the two-step implanted sample El6 with the same implant energy and dose took only 101 min. Finally, the parameters for the secondstep implantation should be designed as follows: the power density and the dose used should be low enough to repress beam heating and save implant time, yet the dose has to be high enough to avoid the formation of any buried amorphous layers. 3.2. Double-energy implantation As listed in Table 1, samples E21 and E22 were prepared by double-energy Ge+ and Ge2+ ion implantations. Note that the Gef ion implantations with a high power density of 0.46 W/cm2 were expected to form Si,_,Ge, alloy layers and locate Ge peak positions as far from the surface as in the sample E16. In this case the implanted wafers were annealed by the beam heating effect,
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Fig. 3. Channelling
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while the estimated temperature rise was about 380°C at this power density [I5]. Following Ge*+ ion implantations with a low power density of 0.09 W/cm* were expected to form a continuous amorphous layer initiated from the surface and locate the a/c interfaces much deeper than in the sample E16. Channelling measurements shown in Fig. 3 indicate that the double-energy implantations used for samples E21 and E22 have reached the goal: the Ge peak positions are the same as that of the sample E16, both a/c interfaces are at a depth of about 410 nm, which is about 145 nm deeper than that of the sample E16. Corresponding to the doses used, the measured Ge peak concentrations for samples E16, E21 and E22 are 6, 8 and 11 at%, respectively. In addition, the Ge*+ implantations shown in Fig. 4 produced slightly widening profiles of Ge in the tail parts.
for samples
_-__ 1400
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CkeV)
E16, E21 and E22.
Double-energy Sif and Gef implantations were studied following the similar train of thoughts used for samples E21 and E22: the Si+ implantations determined the location of the a/c interfaces and Gef implantations formed Si, ox Ge, alloys. In order to check the ability of amorphization by Si ion implantations, samples El3 and El5 were implanted with Sif and Si*+ ions only. As evidently shown in Fig. 4, buried amorphous layers were formed and surface regions were of long-range order in the two samples. For the sample E13, the front a/c interface was at a depth of 110 nm and the thickness of the buried amorphous layer was 230 nm. In the case of the sample E15, they were 190 and 340 nm, respectively. Comparing the implantation doses, implant times and power densities used for the two samples with those for samples E16, E21 and E22,
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one can find that when using Si ion implantations it is more difficult to form continuous amorphous layers. With such Si ion implantations, the Gef ion implantations have to be carried out with low power densities in order to amorphize the surface regions which were not amorphized by using the Si ion implantations. Samples E4 and E5 exhibit such examples: they were first implanted with Si+ ions having similar implant parameters to the sample E13, and then implanted with Ge+ ions at energies of 100 and 180 keV, respectively. All the Si and Ge implantations were done with beam power densities smaller than 0.09 W/cm2. Fig. 5 shows that continuous amorphous layers in these samples were formed, and the a/c interface layers were at a depth of about 390 nm. Furthermore, the Ge peaks have been observed to be located at different depth for different implant energies. As a result, the double-energy Si+ and 1200
I
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Ge+ implantation technique is also effective for locating the spatial separation between the a/c interface and the Ge profile peak. With this double-energy implant recipe, the implant sequence for Si+ and Ge+ ions is of little importance because all the implantations must be performed with low beam power densities to avoid any dynamic annealing. Consequently, it is much more time-consuming than the double-energy Ge2+ and Gef implant recipe. For instance, for locating the similar spatial separations and getting similar Ge depth profiles, the implantation used for the sample E21 took about 134 min, while for the sample E5 at least 400 min. Furthermore, this double-energy Si+ and Ge+, or Si2+ and Gef implant recipe may not facilitate the spatial separation between the a/c interface and the Ge profile maximum, since the Ge+ implantation has to localize its damaged layer to
1000 Energy
Fig. 4. Channelling
spectra
1200 CkeV)
for samples
I
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I
1
El3 and E15.
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1200 CkeV)
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Fig. 5. Channelling spectra for samples E4 and ES.
cover the crystalline surface where it cannot be disordered by Si+ or Si2+ implantation. As seen in Fig. 4 and Table 1, it is also predictable that the double-energy Si2+ and Ge+ implantation will result in less spatial freedom and is more time-consuming than the Sif and Ge+ implantation, although the a/c interface can be located much deeper. Finally, comparing all the double-energy implant recipes, the Gef and Ge2+ implantation is of advantage: more spatial freedom, higher production output, and easier implant operation (only a Ge ion source is needed).
4. Conclusions By the control of the ion beam power density, the dynamic annealing effect caused by beam
heating can be repressed during high dose Ge implantation even in the case of a Wayflow fiiture with no external cooling. Below the power density of 0.09 W/cm2, Ge ion implantations have resulted in continuous amorphous layers in all the studied samples and dynamic annealing was repressed. Moreover, it has been shown that it is more difficult to form continuous amorphous layers by using Si ion implantations than by Ge. For the single-energy Ge+ ion implantation, a two-step implant recipe using sequentially high and low power densities has been developed, which can significantly improve the throughput. Furthermore, the double-energy Gef and Ge2+, and Si+ and Ge+ implantation recipes have also been presented, which effectively can locate the spatial separations between the a/c interfaces and the Ge peak positions. Thus, although there are many factors affecting the Ge ion implanta-
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tion, the present study demonstrates that the double-energy Ge+ and Ge*+ implant recipe is preferable in processing the wanted spatial separation, saving the implant time and simplifying the implantation operation.
[3] [4] [5]
Acknowledgements
[6] [7]
The authors wish to thank R. Korkeamaki for technical assistance, and T. Wiik and I. Suni for helpful discussions. This research was supported by the Technical Research Centre of Finland (VTT). One of us (D.P.) acknowledges support from NSF contract MRG-DMR-9223683.
[8] [9] [lo] [ll] [12]
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