A comparative study on ultra-shallow junction

Materials Science and Engineering B 124–125 (2005) 188–191

A comparative study on ultra-shallow junction formation using co-implantation with fluorine or carbon in pre-amorphized silicon Houda Graoui ∗ , Majeed A. Foad Applied Materials Inc., Front End Products Group, 974 E. Arques Ave, Sunnyvale, CA 94086, USA

Abstract The main driver in ultra-shallow formation for the 65 nm technology node and beyond is to find solutions that both reduce boron transient enhanced diffusion and can be integrated in the CMOS process flow. To this end, many studies have recently focused on using co-doping techniques with fluorine and most recently with carbon. In most cases, one or both of these is co-implanted with a dopant specie in pre-amorphized silicon. In this work, we show a comparative study of fluorine or carbon co-implanted with low-energy boron to form source and drain extension junctions for PMOS devices. We will show that by a systematic optimization of germanium, boron, fluorine or carbon energies and doses, spike annealing technology can be extended to the 65 nm node. These results will be used to discuss how the different formed junctions offer potential solutions for either low-power or high-performance PMOS device fabrication. © 2005 Elsevier B.V. All rights reserved. Keywords: Ultra-shallow formation; PMOS devices; Pre-amorphized silicon

1. Introduction Source and drain extension engineering is becoming increasing challenging as the semiconductor industry moves towards the 65 nm technology node and beyond. New processes based on materials modification have had to be developed. To this end, materials modification through ion implantation represents a key technology enabling the transition from the 90 to the 65 nm node, while extending the use of spike annealing technology. Fluorine co-implanted with boron in germaniumpre-amorphized (Ge PAI) silicon substrate has been reported [1–3]. Theses studies have shown that the fluorine energy and dose need to be well optimized and that possible mechanisms responsible of fluorine reduction of boron transient enhanced diffusion (TED) could be related to the fluorine interaction with interstitials, reducing their ability to pair with B and diminishing, as a result, the boron TED. On the other hand, could be a F–B chemical interaction causing a drop of B diffusion. However, the exact role of fluorine in boron TED is still controversial. On the other hand, many studies have shown that the TED of the boron delta layers can be almost completely suppressed due to SiC or SiGeC layers in the presence of excess silicon interstitials [7,8]. These layers are primarily formed by molecular beam ∗

Corresponding author. Tel.: +1 408 584 1109; fax: +1 408 584 1193. E-mail address: houda [email protected] (H. Graoui).

0921-5107/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2005.08.040

epitaxy. The above studies have also shown that carbon needs to be incorporated into substitutional sites to be effective in reducing boron TED. The reported mechanism of carbon behaviors is related to the trapping of silicon interstitials by substitutional carbon during annealing through the formation of (SiC) clusters [9]. In this work, we will present a comparative study of fluorine co-implantation versus carbon co-implantation for ultra-shallow junction formation. Examples of process optimization of fluorine and carbon co-implanted junctions will be shown. We will also highlight the tremendous reduction of boron TED, and hence improvement, both in junction depth and abruptness in carbon co-implanted junctions, while maintaining a low sheet resistance. 2. Experimental Wafers used for this study were 300 mm, N-type, 10–30 cm resistivity. Implants were performed using Applied Materials’ Quantum® X single-wafer high current ion implanter. All wafers were first amorphized with germanium at energies between 5 to 80 keV and a dose of 1 × 1015 ions/cm2 . A range of different carbon energies from 2 to 10 keV was used while maintaining the peak atomic concentration constant at 0.5 at.% in order to maintain the same carbon concentration and only vary the position of the carbon profile peak. A second set of wafers

H. Graoui, M.A. Foad / Materials Science and Engineering B 124–125 (2005) 188–191

was implanted with fluorine with energies ranging from 6 to 20 keV at a dose of 2 × 1015 ions/cm2 . In addition, these wafers were P-type doped with boron at 0.5 keV, 1 × 1015 ions/cm2 . All wafers were subsequently annealed using Applied Materials’ RadiancePlusTM RTP annealing chamber. Spike anneals were performed at 1050 ◦ C with 250 ◦ C/s ramp-up and 90 ◦ C/s cool-down. Secondary ion mass spectrometry (SIMS) and fourpoint probe sheet resistance measurements were carried out on most samples. 3. Results and discussion Fluorine and carbon co-implantation processes are key in reducing boron TED. When fluorine is co-implanted with boron in pre-amorphized silicon, dose and energy optimization of fluorine implant step is critical to efficiently reduce TED. Fig. 1 shows this effect. When silicon is pre-amorphized with germanium at 20 keV, an amorphous layer of ∼40 nm is formed, as measured by Rutherford backscattering spectrometry (RBS) [1]. Positioning the fluorine very close to the surface (at 1 keV), and therefore closer to the boron peak concentration, fails to efficiently reduce boron TED while both maintaining a low sheet resistance and improving junction abruptness. SIMS analysis on this sample after anneal has shown that ∼94% of the fluorine has out-diffused, and therefore did not participate in junction formation. This suggests that the possible mechanism of fluorine reducing boron TED is not predominantly linked to the formation of boron–fluorine pairs with low diffusion coefficient as postulated previously [3–6]. Fluorine co-implanted junctions at least 6 keV energy at 2 × 1015 ions/cm2 to being observing improvements in sheet resistance, junction depth, and abruptness. We found that the optimum fluorine energy to minimize all these factors is 10 keV. This condition resulted in a junction depth of 32.7 nm (at 1 × 1018 atoms/cm3 ), sheet resistance of 479 /sq, and abruptness of 4.5 nm/decade. When a similar fluorine condition was used to coimplant boron in pre-amorphized silicon at only 2 keV, 1 × 1015 ions/cm2 Ge+ , similar junction depth and abruptness values were obtained. However, an improvement in sheet resis-

Fig. 1. Junction concentration profiles for fluorine implanted at different energies from 1 to 20 keV in silicon substrates pre-amorphized with germanium at 20 keV Ge+ , 1 × 1015 ions/cm2 .

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tance of about 15% was observed. This may be due to two effects. First, since boron activation is known to enhance with the presence of high germanium concentration close to boron, when germanium PAI is implanted at 2 keV, 1 × 1015 ions/cm2 , germanium concentration in the first 5 nm surface layer is much higher and closer to the boron than when Ge is implanted at the same dose at 20 keV. Second, since the fluorine is amorphizing at 10 keV, 2 × 1015 ions/cm2 as observed by RBS. It forms the same amorphous depth of about 40 nm as the 20 keV Ge+ 1 × 1015 ions/cm2 . However, in the case of 2 keV germanium, the silicon interstitials supersaturation at the end of range (EOR) area will be primarily controlled by fluorine implanted at 10 keV. While when both deep Ge PAI (20 keV) and fluorine (10 keV) are used, the silicon supersaturation at the EOR area should be more pronounced than in the first case (shallow Ge PAI and 10 keV F+ ), leading to less boron clustering and an increase in sheet resistance. The effect of different fluorine doses ranging from 5 × 1014 to 3 × 1015 ions/cm2 has also been investigated at 10 keV energy [1]. The measured sheet resistance and junction depth results showed that the optimum fluorine dose was 2 × 1015 ions/cm2 . It is important to note that for fluorine doses above 2 × 1015 ions/cm2 , boron TED is reduced less, possibly due the formation of fluorine bubbles in the silicon. However, carbon co-implantation with boron in pre-amorphized silicon with germanium at 20 keV 1 × 1015 ions/cm2 has shown to be more sensitive than fluorine to energy and dose conditions. A variation of only 0.5 keV in the carbon energy was found to significantly change the final boron profiles both in term of abruptness and junction depth, as shown in Fig. 2. The measured sheet resistance of these junctions varied only between 921 and 957 /sq (Fig. 3) with errors in the order of 2.5%. We have also investigated the role of different preamorphization implants while keeping the carbon and boron implant conditions constant. The SIMS profiles (Fig. 4) from the formed junctions show that the pre-amorphization implant step, which dictates the depth of the amorphous layer, and the level of silicon interstitials supersaturation are important and have to be optimized in carbon co-implanted junctions. Fig. 4 compares the case of using a shallow Ge PAI of 5 keV (energy-1) versus

Fig. 2. SIMS profiles of boron co-implanted with carbon at different carbon energies in Ge preamorphized Si (after 1050 ◦ C spike anneal).

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Fig. 3. Sheet resistance and junction depth (at 1 × 1018 atoms/cm3 ) as functions of different carbon energies for carbon co-implanted with boron at 0.5 keV B+ , 1 × 1015 ions/cm2 .

a medium (energy-2) or a deep (energy-3) pre-amorphization implant. The corresponding amorphous depths are in the range of 10, 20 and 100 nm, respectively. In the case of shallow PAI, the damage created by pre-amorphization is close to the surface and can thereby act as a sink for silicon interstitials, thus minimizing the overall junction leakage. Although this may be attractive, the data in Fig. 4 show that deep PAI (energy-3) was needed to significantly reduce boron TED. Therefore, carbon energy optimization will be needed for the different pre-amorphization conditions used in forming ultra-shallow junctions. Optimized junctions using fluorine and carbon co-implants are shown in Fig. 5 and compared to a non-co-implanted boron junction in pre-amorphized silicon at the same depth for all the junctions. It is found that by carefully optimizing the carbon co-implant conditions, junctions as shallow as 21 nm (at 5 × 1018 atoms/cm3 ) can be obtained with sheet resistance of ∼570 /cm2 and abruptness of 2.5 nm/decade. Compared with the best optimized fluorine co-implanted junction (see Table 1), this junction shows ∼27% improvement in junction depth and 44% improvement in abruptness, while maintaining a low sheet resistance of 570 /sq, which is well bellow the upper limit of 760 /sq as set by the ITRS [10] for the 65 nm technology node. To better understand the diffusion suppression mechanisms involved in co-implants, physics-based simulations have been

Fig. 4. Boron co-implanted with carbon in pre-amorphized silicon at PAI energies.

Fig. 5. Junction formation using optimized Ge + B only, Ge + F + B and Ge + C + B. All the junctions were formed using a 1050 ◦ C spike anneal.

Table 1 Junction depth (Xj ), sheet resistance (Rs ) and abruptness results for optimized junctions under different implant conditions Implant conditions

Xj (nm)a

Rs (/sq)

Abruptness (nm/dec)

PAI + B PAI + F + B PAI + C + B

35.6 29.0 21.0

435 479 573

9.4 4.5 2.5

a At

5 × 1018 atoms/cm3 .

performed using TSuprem-4 [11] to account for the impact of carbon-interstitial clusters on boron TED. Fig. 6 shows a good fit to the measured data for 2 keV carbon when a small fraction of the implant damage is assumed to be captured by the carboninterstitial clusters. However, in the case of 5 keV carbon, all of the implant damage appears to be captured by carbon, thus eliminating boron TED. Furthermore, boron solubility is apparently increased by ∼20% above its thermodynamic equilibrium value. This increase is extremely important in reducing sheet resistance of the junction and is achieved because of the reduction of interstitials concentration around the boron peak, thus minimizing boron clustering.

Fig. 6. Simulation and measured SIMS profiles showing good fitting of the simulated data for two different carbon co-implant energies.


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4. Conclusions

References

Co-implantation of fluorine or carbon has proven to be a key enabler in reducing boron TED. Both fluorine and carbon co-implantation must be optimized for the different preamorphization depths used in the fabrication of PMOS junctions. Carbon co-implantation has proven to be more sensitive to small energy variations, on the order of 0.5 keV, while fluorine co-implantation optimization appears to be less sensitive. Overall carbon is found to reduce boron diffusion more efficiently than fluorine, thus junctions depths as low as 21 nm at 5 × 1018 atoms/cm3 , with an abruptness of 2.5 nm/decade and a sheet resistance of 570 /sq, can be achieved with a standard spike anneal, therefore extending the use of spike annealing technology to the 65 nm node. Such junctions formed with optimized carbon co-implant are currently key candidates to source and drain extension formation for high-performance 70 nm transistors, while fluorine co-implanted junctions may be suitable for low-power devices.

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Acknowledgment The authors would like to acknowledge the contribution from Victor Moroz of Synopsys, Inc. in developing the carbon coimplant simulation model.