Dopant and Carrier Profiling in FinFET-based devices with sub-nanometer resolution
J. Mody , A.K. Kambham , G. Zschätzsch1, 2, P. Schatzer2, T. Chiarella2, N.Collaert2, L. Witters2, M.Jurczak2, N. Horiguchi2, M. 2 1, 2 1,2 2 1, 2 Gilbert1, 2, P. Eyben , S, Kölling , A. Schulze , T-Y. Hoffmann , W. Vandervorst 1, 2
1, 2
1
K.U.Leuven, Department of Physics and Astronomy, B-3001, Leuven, Belgium, 2IMEC, Kapeldreef 75, B-3001 Leuven, Belgium email:
[email protected]
Abstract Atom probe tomography (APT) in conjunction with scanning spreading resistance microscopy (SSRM) is demonstrated for the first time to profile dopant and carrier distributions in FinFET-based devices with sub-nanometer resolution. These two techniques together provide information on the degree of conformality, the dose retention and the dopant activation. These results are also compared with a methodology involving secondary ion mass spectrometry (SIMS). Ion implantation for increased conformality of source/drain extensions is demonstrated for tilted implants, which clearly leads to improved device performance. Introduction Despite the interest in FinFET-based devices for enhanced electrostatic control and performance [1], there remains a major challenge for the dopant profile optimization (conformal doping) of the source/drain extension as this significantly influences the sub-threshold swing and short channel effects [2]. To optimize the doping profiles in FinFETbased devices, one can use traditional ion implantation [2], vapor phase doping (VPD) [3] or plasma doping [4,5]. However, adequate development of these processes requires the availability of 3D-dopant and carrier profiling techniques with sub-nm resolution. In this work ion implantation using different tilt angles (45° and 10°) has been considered to obtain conformal doping. To optimize and understand the dose retention and activation of dopants in FinFET-based structures we demonstrate the use of APT [6] to probe the 3D dopant distribution and thus the relevant vertical and lateral (conformal) dopant profiles. As the atom probe only provides the (chemical) dopant concentration, to obtain the respective active carrier concentration, the APT results are complemented with 2D-SSRM [7]. To validate the results obtained using APT we also used a SIMS-based methodology for probing dopants in fins [7]. Based on the understanding of dose retention and activation we look at the p-FinFET device performance where the extensions are doped using ion implantation at different tilt angles (45° and 10°).
Experiment To enable the experiment for dopant and carrier profiling special test structures were designed. They consist of repeated arrays of 10 parallel fins as shown in Figure 1. The fin height was targeted around 200nm and the fin width around 80nm and 40nm. These dimensions were intentionally taken larger than the ones used for device fabrication (60 nm height, 10-20 nm width) [8] to separate top/sidewall profile overlaps, thus providing more insight in the individual processes (sidewall incorporation, lateral in-diffusion, vertical diffusion). The fin arrays were implanted using 2-quad ion implantation at different tilt conditions (45° and 10°) and planarized using CMP after Si-deposition to enable the SIMS measurements [7]. High Angle Annular Dark Field (HAADF) TEM images of the test structures (Fig. 1) reveal the fin dimensions and the presence of silicon-oxide (~ 1-3nm) at the deposited-Si/Si-fin interface (inset Fig. 1). However, for the fabrication of device a standard process flow was followed (Fig. 2).
Results and discussion 1. APT analysis (Dopant Profiling): The APT-analysis was performed using a laser assisted wide angle atom probe (LAWATAP) from CAMECA [9]. Fig. 3 illustrates the APT-analysis for the 40nm fins where the 3D dopant distribution of boron (green) inside a fin and the native oxide (orange) covering the sidewalls of the fin are shown. Due to the size of the fins (80 nm) relative to the APT-tip (50-100 nm) (Fig. 1(a) dashed line), it becomes virtually impossible to image the fin with one APT-tip completely and separate APT-measurements are used to probe the top and side profiles (Fig. 3 & 4). Although focused ion beam (FIB) preparation is used to ion mill the APT-tip, intermixing of the dopant profile and Ga-incorporation is avoided by working consecutively at reduced Ga+-beam energies, 30, 16, 5 and 2kV which prevents the milling of the cap layer. Comparing the lateral profiles at mid height in the fins (Fig. 3 (a) & (b)), the different sidewall incorporation efficiencies for the 45° and 10° case is revealed (Fig. 5). The implant angle has a clear impact on the lateral B-distribution (Fig. 5) translating to very different conformalities (table 1). Further detailed analysis of APT measurements in Fig. 6 provides a comparison of the dopant profiles laterally through the fin and vertical at the top of the fin and the area in
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between two fins (trench). Note the large difference in junction depth in the trench and top (36 nm) versus sidewall (11 nm) as a result of the 10° tilt during the implantation. When a lateral profile (Fig. 7) for the 45° case clearly shows an almost uniform B-distribution within the fin, its location relative to the native oxide (represented here by the SiO2 cluster) and the F-distribution (shown here as SiF) which exhibits an accumulation within the native oxide as well. To validate our results we performed SIMS analysis on these samples to obtain a vertical dopant distribution profile as shown in Fig. 8 based on the methodology described in [7]. The central part (apparent depth 50-200 nm) of the SIMS profile corresponds to the sidewall dose and the apparent SIMS concentration can be converted to sidewall dose using the methodology discussed in [7]. The experimental dose retention values are compared to the theoretical calculations [10] in table 1. It must be noted that this is for the first time such detailed lateral dopant distribution in a FinFET-based structure can be observed. 2. SSRM analysis (Active dopant profiling): An improvement of the 2D-SSRM technique has led to a sub-nm resolution, enabling us to probe carrier distribution (activation) within the fin. The SSRM images of 40 nm wide fins for both tilts (45° & 10°) are shown in Fig. 9. The fins implanted at 45° appear completely doped with a higher concentration in the top. However, in the 10° case, a shallow highly doped region is observed at the sidewall whereas the top surface implant is deeper (Fig. 9). Comparisons of the retained sidewall dose obtained by APT, SIMS & SSRM (table 1) show a good agreement suggesting nearly 100 % activation agreement (within the error margin of 20-30%) for the sidewall (Figs. 10, 11, 12) which is also in agreement with 2D-simulation results (Fig. 12). The latter is in contrast with the top dose (Table 1) where APT and SIMS indicate a much higher incorporation than SSRM. A large inactive fraction is present here as evidenced also by the difference in near-surface peak concentration. The latter implies that the conformality (sidewall dose/top dose) of the “electrical carriers” is higher (upto 23% even at 10° tilt) than the “chemical dopant” conformality. For the 45° implant case, 80nm fin, SSRM reveals an extra in-diffusion at the foot of the fins as well as a curvature in the top surface distribution which is also observed in the APT results (Fig. 13 & 14 circled). Evidence that this indiffusion in the foot originates from shadowing by nearby fins is confirmed by 2D-simulations. The in-diffusion at the top of the fins as seen by SSRM & APT has been observed previously and can be explained by the proximity of the surface acting as a sink for the interstitials driving the B-diffusion. As such the B-diffusion near the sidewall surface is reduced relative to the diffusion in the center of the fin [11]. 3. p-FinFET device results: Figure. 15 represents the Ioff vs. Ion curve for p-FinFET devices where the extensions are implanted at 45° and 10°. We see approximately 25% improvement in the drive current at Ioff = 100nA/µm with the 45° tilted implant as compared to the 10° tilt implanted case. In 10° case, the lower sidewall concentration (~ 4 x 1019 cm-3) and the reduced underlap degrades the drive current by a higher extension resistance. The latter is in agreement with theoretical simulations [2] which show a similar dependence of the drive current on the side wall concentration (Fig. 16). A further increase of the dose for the 10° tilted implants is required to compensate this effect and to account for the reduced underlap under the gate.
Conclusions Detailed dopant distributions within a fin have been resolved for the first time using APT and 2D-SSRM analysis. Conformality and sidewall retention for tilted implants appear consistent with process simulations whereas some anomalies can be observed as well. Electrical conformality is clearly higher than dopant conformality. p-FinFET devices were fabricated and it was seen that conformality is needed to obtain better performance of the devices.
Acknowledgements IMEC acknowledges the collaboration with CAMECA and A.S. thanks IWT for his PhD fellowship.
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M. J. H. Van Dal et al, VLSI Technol. Symp., pp. 110 – 111, 2007. “CAMECA LA-WATAP 3D Atom Probe http://www.cameca.com/html/product_atom_probe.html. 10. W. Vandervorst et al, J. Vac. Sci. Technol. B 26(1), pp. 396-401, 2008. 11. S. M. Kluth et al, J. Vac. Sci. Technol. B 23(1), pp. 76-79, 2005.
S. Takeuchi et al, ECS Meeting Abstracts, vol. 802, p. 2432, 2008. D. Lenoble et al, IWJT, pp. 78 – 83, 2006. Y. Sasaki et al, IEDM Tech. Digest, pp. 917 – 920, 2008. S. Kölling et al, Ultramicroscopy, vol. 109, pp. 486-491, 2009. J. Mody et al, J. Vac. Sci. Technol. B 28(1), 2010.
Fin patterning Gate stack Gate patterning Extensions Spacer SEG HDD + anneal Silicidation
Figure 2: Process description for FinFET devices
Figure 1: Schematic & XTEM image of the fin array test structure used for characterization. The final APT-tip shape is indicated by the dashed line.
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Figure 11: SSRM vs. SIMS (10° implant). SSRM profile is calculated from figure 10. ([8] for the procedure)
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Figure 10: SSRM vs. SIMS (45° implant). Figure 9: 2D-SSRM map of active carrier concentration of BF2 implanted SSRM profile is calculated from figure 10. at 45° and 10° ([7] for the procedure)
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Figure 6: Comparison of boron SIMS-thru fin profiles implanted at 45° and 10°. (SIMS)
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Figure 12: Comparison of lateral profiles as Figure 13: 2D-SSRM map of 80nm fin extracted from APT, SSRM and 2D-simulations 45° tilted implant. Note the in-diffusion for both 45° & 10° tilted implants in the top surface and the foot of the fin.
Figure 14: APT measurement on a 80nm fin showing in-diffusion on the top surface of the fin consistent with SSRM results (figure 13)
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Figure 8: Lateral dopant profile of 45° Figure 7: Boron profiles in lateral and vertical (top, bottom trench) direction for a showing Boron, Oxygen (SiO2) and F (SiF) 10° implanted fin. (Atom Probe) profiles for 45° implanted fin. (Atom Probe)
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Figure 5: Lateral dopant profile of 45° and 10° implanted fins (figs. 2 (a) & (b)) (Atom Probe)
Figure 4: Undistorted top surface of the fin by using reduced Ga+ beam energies
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Figure 3: 3D dopant distribution data of 40nm fin implant at (a) 45° & (b) 10°. (Green → Boron, Orange → SiO2)
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Figure 15: Comparison of p-FinFET device Figure 16: Comparison of normalized drain performance for extensions ion implanted at current vs. sidewall surface concentration and the top /side surface concentration ratio 45° and 10° tilts. indicating the degree of conformality.
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Conformality (Sidewall/top) (%)
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Table 1: Comparison of conformalities obtained for dose retention using APT, SIMS and theoretical values. Also conformality values for active percentage of retained dose are mentioned as measured with SSRM.
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