Drive current, Ion (mA/um)
Figure 2.4.12 Drive current, Ion-Lmin was affected by the implanted As and P induced damage effect in the strained-SiGe layer, which causes various degrees of strain relaxation upon the RTA and MSA.
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2.4.3 Implant sequence effect on strain relaxation of S/D SiGe and device performance
On the other hand, changing the sequence of the implantation species, As, from post-SiGe implantation process to Pre-SiGe implantation process, as revealed Flow-2 in Fig.2.4.14, enables a less relaxed strained-SiGe to be achieved, increasing the drive current gain. Figures 2.4.15 and 2.4.16 present the device performance of PMOSFETs and junction leakage current of pre-SiGe implantation sequence. As implantation and RTA process prior to strained-SiGe layer were conducted to present a low level of strain relaxation. MSA did not cause low degree of strained-SiGe relaxation to reveal large wafer warpage change and did not form defects in the underlying Si substrate. Then, PMOSFET devices had an Id,sat gain of
0.1%
Figure 2.4.13 Improvement in the P+/n well junction leakage by the post-SiGe implantation of species, P, reduces the relaxation of strained-SiGe and yields a defect-free underlying Si substrate upon RTA+MSA.
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10%
Drive current, Ion (mA/um)
Isoff(nA/um)
0.35 0.40 0.45 0.50 0.55 0.60 0.65
IMP(As)+RTA+SiGe+MSA
Drive current, Ion (mA/um)
Isoff(nA/um)
0.35 0.40 0.45 0.50 0.55 0.60 0.65
IMP(As)+RTA+SiGe+MSA
STI and Well implant formationSTI and Well implant formation
EOT 12A Gate oxide formationEOT 12A Gate oxide formation
Poly gate patterningPoly gate patterning
Post SiGe implant Post SiGe implant (As 25Kev 3e13cm (As 25Kev 3e13cm--2)2) Implant
RTA (1000C) anneal RTA (1000C) anneal
MSA anneal (1200C, 500us)MSA anneal (1200C, 500us)
TEOS formationTEOS formation
S/D SiGe recess and formationS/D SiGe recess and formation
STI and Well implant formationSTI and Well implant formation
EOT 12A Gate oxide formationEOT 12A Gate oxide formation
Poly gate patterningPoly gate patterning
Post SiGe implant Post SiGe implant (As 25Kev 3e13cm (As 25Kev 3e13cm--2)2) Implant
RTA (1000C) anneal RTA (1000C) anneal
MSA anneal (1200C, 500us)MSA anneal (1200C, 500us)
TEOS formationTEOS formation
S/D SiGe recess and formationS/D SiGe recess and formation
Figure 2.4.15 Improvement of Ion-Ioff by the change in sequence of SiGe and implantation. Implantation prior to SiGe deposition exhibited less strain relaxation and a higher channel stress.
Figure 2.3.14 The Process flow of implantation followed by the S/D SiGe, then subsequent MSA was conducted.
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around 11% due to the low degree of strained-SiGe relaxation because of non-implantation in the strained-SiGe layer and a significantly improved junction leakage than that of those post-SiGe implantation process.
2.5 Mechanism
To understand the formation of defects in the underlying Si during the MSA of implanted strained-SiGe, three implantation conditions - low energy (15keV) of shallower implantation Rp, medium energy (50keV) of medium implantation Rp and high energy (70keV) of deep implantation Rp, made implanted species, As, reach the surface, medium and
60%
1.E-12 1.E-10 1.E-08 1.E-06 Junction leakage (A)
1.E-12 1.E-10 1.E-08 1.E-06 Junction leakage (A)
Probability
SiGe+IMP(As)+RTA+MSA
Figure 2.4.16 Upon MSA, implantation following the SiGe process sequence results in low strain relaxation and few defects in the underlying Si, and consequently improved junction leakage.
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bottom of the 100nm-thick strained-SiGe blanket wafer, respectively. The three samples with different implantation conditions were followed by RTA and MSA, causing 11%, 51% and 75% relaxation of strained-SiGe. The relaxation percentage of the strained SiGe was calculated from bow height change ratio before implant as well as after implant and RTA.
High-resolution X-ray diffraction reciprocal space maps (HR XRD RSMs) [19] were obtained to characterize the relaxation of strain SiGe and the defects in the underlying Si, as shown in Fig.2.4.17(a)(b)(c). For the case of 11% relaxation of strained-SiGe, the MSA did not significantly degrade the lightly relaxed strained-SiGe and did not produce defects in the underlying Si substrate, presented in the TEM image, and as revealed by results of the tight XRD RSM patterns of the Si. In Fig.2.4.17 (b), the medium implantation Rp, RTA process and subsequent MSA caused a 51% strained-SiGe relaxation, which was consistent with a weak SiGe diffraction peak in RSM pattern. MSA of such remarkably relaxed strained-SiGe formed many defects in the underlying Si, as shown in the TEM image, corresponding to the broadening of the Si RSM pattern in the most {111} direction by the defects. Finally, in Fig.
2.4.17 (c), a more deeply implanted Rp, the RTA process and subsequent MSA caused an almost 75% relaxation of strained-SiGe, and produced numerous dislocations in SiGe, which result is related to the very weak SiGe diffraction RSM pattern. MSA applying to a high relaxation of strained-SiGe wafers does not form defects in the underlying Si, as presented in the TEM image, indicating a slightly broadened Si RSM pattern, due to the clarity of the Si.
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Hence, the degree of strained-SiGe relaxation determines whether defects are formed in the underlying Si substrate under millisecond annealing. Figure 2.4.18 the TEM image reveals that upon MSA, the formation of defects or their injection into underlying Si was observed because strained-SiGe underwent moderately medium strain relaxation. Neither excessively low nor excessively high strained-SiGe relaxation was associated with any defect injection into the underlying Si.
Figure 2.4.17 High-resolution X-ray diffraction reciprocal space maps (HR XRD RSMs) and TEM of strained-SiGe and underlying Si following implantation under various energy and RTA and MSA, associated with various strained-SiGe relaxation (a) No defect formation for 11% relaxation of strain SiGe. (b) Defects in the underlying Si for 51% strained-SiGe relaxation. (c) 75% relaxation shows the almost completely relaxed strained-SiGe with weak diffraction peak, and defect-free in the underlying Si.
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Therefore, Fig. 2.4.19 plots a correlation between the relaxation of strain SiGe and the injection of defects, obtained from the bow height of the SiGe wafer and the corresponding full width at half maximum (FWHM) of the Si XRD peak, respectively [20]. In designing a high performance of CMOS devices with strained-SiGe and MSA, pseudomorphic relaxation of strain SiGe of less than 11% must be maintained to ensure favorable channel stress to boost the drive current and an absence of defect formation in the Si to ensure a low junction leakage current.
Figure 2.4.18 Upon MSA, TEM image showed that relaxed strained-SiGe caused injection of defects into underlying Si.
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The stress of the strained-SiGe layer on the Si substrate can be calculated using Stoney’s well-known equation, which assumes uniform biaxial film stress:[21]
d k thickness of the Si substrate; df is the thickness of the strained-SiGe layer, and k is the wafer curvature.
When MSA was performed using pulsed surface heating with a millisecond peak width at a temperature of around 1200°C at the surface of the Si wafer without SiGe layer, as shown
-100
Strain Relaxation of SiGe (%)
Wafer Bow Height (um)
0
FWHM in Si peak (sec-1)
Slightly
Strain Relaxation of SiGe (%)
Wafer Bow Height (um)
0
FWHM in Si peak (sec-1)
Slightly
Relaxation of strain SiGe (%)
-100
Strain Relaxation of SiGe (%)
Wafer Bow Height (um)
0
FWHM in Si peak (sec-1)
Slightly
Strain Relaxation of SiGe (%)
Wafer Bow Height (um)
0
FWHM in Si peak (sec-1)
Slightly
Relaxation of strain SiGe (%)
Figure 2.4.19. Correlation between relaxation of strain SiGe and injection of defects, determined from wafer bow height and full width at half maximum (FWHM) of Si XRD peak.