Results and Discussion

2.3.1 The C I/I samples

Figure 2-1 shows the Rs values of the CIA samples and control1 samples as a function of annealing temperature at a constant annealing time of 30 sec. As the silicide formation temperature is lower than 700 °C, it can be observed that the Rs

values of the CIA samples are slightly higher than those of the control1 samples due to the existence of C atoms in the Ni-silicide film. Therefore, low-energy or high-dose C I/I results in slightly higher Rs values. According to the Rs values measured after deposition of the Ni film, we found that the thickness of the Ni film on the CIA samples implanted at 30 keV with a dose of 5x10¹⁵ cm^-2 is thinner than those of the other samples, so its Rs values after Ni silicide formation are higher than the other

samples. The Rs values of the control1 samples gradually increase after 700 °C because of serious agglomeration and phase transformation. At the same time, the Rs

values of the CIA samples implanted at 40 keV with a dose of 1x10¹⁵ cm^-2 are almost stable until 700 °C and start to increase at 800 °C because lightly agglomeration and phase transformation simultaneously occur, as shown in Fig. 2-2 and 2-3, respectively.

As the C I/I dose increases up to 5x10¹⁵ cm^-2, the Rs values are stable without increasing until 850 °C. For the CIA samples implanted at 30 keV with a dose of 5x10¹⁵ cm^-2, the Rs values increase slightly at 850 °C. However, for the control1 samples, the Rs values increase greatly at 850 °C due to serious agglomeration. For the CIA samples implanted at 40 keV with a dose of 5x10¹⁵ cm^-2, the Rs values keep unchanged even at 850 °C. The NiSi film is still continuous without agglomerating after 850 °C, as shown in Fig. 2-4(d). According to the XRD spectra shown in Fig.

2-5, the phase transformation dose not occur as well. Hence, the function of C atoms within the Ni-silicide film is suppressing agglomeration and phase transformation.

Figure 2-6 and 2-7 show the cross-sectional TEM image and SIMS depth profile, respectively, of the CIA sample implanted at 40 keV with a dose of 5x10¹⁵ cm^-2 after silicide formation at 500 °C for 30 sec. The thickness of the NiSi film is 52 nm.

According to SIMS analysis, it is clearly observed that C atoms are distributed within the NiSi film and pile-up at the NiSi/Si interface.

Figure 2-8 shows the Rs values of the w/o CIA samples as a function of the annealing temperature for 30 sec. For the w/o CIA samples implanted to a dose of 1x10¹⁵ cm^-2, lightly agglomeration and phase transformation simultaneously take place at 800 °C, as shown in Fig. 2-9(b) and Fig. 2-10, respectively. The agglomeration and phase transformation temperatures of 800 °C are the same as those of the CIA samples under the same implantation conditions. When the C I/I dose

increases to 5x10¹⁵ cm^-2, as comparing between Fig. 2-1 and Fig. 2-8, the temperature dependence of the Rs values of the w/o CIA samples is more apparent than those of the CIA samples. We speculate that this phenomenon arises from the grain size variation of the Ni-silicide film. Because the Si surface is amorphized by the high-dose C I/I, the grain size of the Ni-silicide film of the w/o CIA samples is smaller than that of the CIA samples. Furthermore, the Rs values of the w/o CIA samples decrease with increasing grain size at higher silicide formation temperatures.

For the w/o CIA samples with a dose of 5x10¹⁵ cm^-2, the effect of grain growth dominates the Rs values at the temperature range of 500-800 °C, as shown in Fig. 2-8.

However, the agglomeration and phase transformation dominate the Rs values at 850

°C, as shown in Fig. 2-11(d) and 2-12, respectively. In Table 2-2, we summarize the agglomeration and phase transformation temperatures of all samples. It is clear that the agglomeration and phase transformation temperatures of the NiSi film are raised to 800 °C as the C I/I dose is equal to 1x10¹⁵ cm^-2. As the C I/I dose is increased to 5x10¹⁵ cm^-2, the agglomeration and phase transformation temperatures of the NiSi film can be further raised up to 850 °C.

Figure 2-13 shows the Rs values of the TNS samples and control2 samples as a function of annealing temperature at a constant annealing time of 30 sec. For thinner Ni-silicide films, the agglomeration mechanism plays an important role in the thermal degradation of the Ni-silicide film. Figure 2-14 and 2-15 show the plan-view SEM micrographs of the TNS samples and control2 samples, respectively. The control2 samples agglomerate at a temperature as low as 500 °C, however, the agglomeration temperature of the TNS samples keeps at 800 °C. Therefore, C atoms in Si can effectively suppress the agglomeration phenomenon of the Ni-silicide film, especially for thinner Ni-silicide films. At silicide formation temperature of 500 °C, the Rs value

of the TNS samples is higher than that of the control2 samples due to the existence of C atoms in the Ni-silicide film. During silicide formation at 600-800 °C, the Rs values of the control2 samples are much higher than those of the TNS samples because of severe agglomeration, as shown in Fig. 2-15(b), 2-15(c), and 2-15(d).

Figure 2-16 shows the measured Rs values as a function of silicide formation temperature with As I/I energy at 35 keV. The C5As35 samples have slightly higher Rs values at a lower silicide formation temperature (≦ 700 °C) owing to the larger number of C atoms within the Ni-silicide film. Figure 2-17, 2-18, and 2-19 display SEM images of the C0As35, C1As35, and C5As35 samples, respectively. The Rs

values of the C0As35 samples increase from 700 °C due to the agglomeration. Many broken holes in the NiSi film can be seen in Fig. 2-17(b). On the other hand, few holes can be observed at 750 °C in the C1As35 samples, as shown in Fig. 2-18(c), and a continuous Ni-silicide film without any holes is obtained even at 800 °C in the C5As35 samples, as shown in Fig. 2-19(d). Hence, it is confirmed that sufficient C atoms can effectively suppress the agglomeration of the Ni-silicide film.

To understand the reason for the increased Rs values of the C1As35 samples at 750 °C and the C5As35 samples at 800 °C, XRD analysis was used to identify the Ni-silicide phase. The XRD spectra of the C1As35 and C5As35 samples are shown in Fig. 2-20 and 2-21, respectively. In Fig. 2-20, it is observed that the NiSi2(400) phase appears at 750 °C in the C1As35 samples, and the NiSi(304) phase coexists in the Ni-silicide film at the same time. Furthermore, the NiSi(304) phase disappears, and only the NiSi2(400) phase can be detected at 800 °C. In the C5As35 samples, the NiSi2(400) phase does not occur until 800 °C, as shown in Fig. 2-21. According to these observations, it is clear that increasing the C I/I dose to 5x10¹⁵ cm^-2 can retard the phase transformation of the NiSi film. In summary, performing the C I/I process

with a higher C dose of 5x10¹⁵ cm^-2 is a feasible method for enhancing the thermal stability of the NiSi film even if As dopants exist.

Figure 2-22 shows the measured Rs values as a function of silicide formation temperature with As I/I energy at 85 keV, and the corresponding SEM images and XRD spectra are shown in Fig. 2-23 and 2-24, respectively. The information revealed from these figures is similar to that discussed previously. Moreover, the phase transformation temperature of the C0As85 samples is 750 °C while that of the C0As35 samples is 700 °C. On the basis of the experimental results of Ahmet et al.

[14], we consider that the higher As I/I energy results in fewer As dopants within the NiSi film. Thus, the phase transformation temperature of the C0As85 samples is 50

°C higher than that of the C0As35 samples.

According to Table 2-2, it is clear that C I/I with a dose of 1x10¹⁵ cm^-2 can raise the agglomeration and phase transformation temperatures of the Ni-silicide film to 750 °C. By increasing the C I/I dose to 5x10¹⁵ cm^-2, the phase transformation temperature of the Ni-silicide film can be further raised to 800 °C, and the agglomeration temperature becomes higher than 800 °C. The C I/I process successfully enhances the thermal stability of the Ni-silicide film by at least 100 °C.

2.3.2 The C PIII samples

Figure 2-25 shows the cross-sectional TEM image of the C PIII sample implanted only with C ions at 3 kV for 5 min. A-27-nm-thick implantation-induced amorphous Si layer is clearly observed. Furthermore, the 4-nm-thick diamond-like carbon (DLC) film is simultaneously deposited on the Si substrate surface during the C PIII process [15]. The input-voltage pulse waveform of the C PIII process is not a perfect square wave, which produces many low energy ions or neutral radicals during

the rising time and falling time of a square wave. While the energy of ions or neutral radicals in the CH4 plasma environment is smaller than 100 eV, they would accumulate and deposit on the Si substrate surface to form the DLC film. The effect of the DLC layer on the formation of Ni silicide will be discussed later.

Figure 2-26 shows the measured Rs values of the C PIII samples as a function of silicide formation temperature for 30 sec. The Rs values of the C3K1M samples show abnormal high values as the silicide formation temperature is equal to or lower than 700 °C. Based on the diffraction peaks of XRD analysis at the temperature range from 500 °C to 700 °C, as shown in Fig. 2-27, it does not contain any detected Ni-silicide phase. In other words, Ni silicide can not be formed owing to the existence of the DLC film. The DLC film acts like the blocking layer between the Ni film and the Si substrate surface to stop the formation of the Ni-silicide film. Ni atoms need higher silicide formation temperature (≧ 800 °C) to break through the DLC film and react with the Si substrate. Therefore, we can observe the diffraction peaks of the NiSi2

phase at the temperature range from 800 °C to 900 °C, as shown in Fig. 2-27.

However, the Rs values of the C3K1M samples are still high owing to severe agglomeration, even if the Ni-silicide film is successfully formed at 800-900 °C.

Figure 2-28 displays the plan-view SEM images of the C3K1M samples. NiSi2 islands can be clearly observed, which cause the high Rs values at 800-900 °C, as shown in Fig. 2-28 (d) and 2-28(e). The severe agglomeration at 800 °C could be explained by insufficient C atoms at the Ni-silicide grain boundary and Ni-silicide/Si interface.

On the contrary, the Rs values of the C5K1M samples are normal at low silicide formation temperature. It represents the Ni-silicide formation is successful and not affected by the DLC film. The different experimental results of the C3K1M samples and the C5K1M samples are attributed to the thickness of the DLC film. The lower

input voltage pulse of 3 kV results in the thicker DLC film [15]. The XRD spectra of the C5K1M samples also show different patterns from those of the C3K1M samples, as shown in Fig. 2-29. Figure 2-29 indicates the NiSi film is already formed at 600°C, and the NiSi phase transforms into the NiSi2 phase at 700 °C. Figure 2-30 shows the agglomeration of the Ni-silicide film of the C5K1M samples occurs at 700 °C. It could be attributed to insufficient C atoms at the Ni-silicide grain boundary and Ni-silicide/Si interface. Although the C PIII dose may be increased by increasing the implantation time, the thickness of the DLC film is also increased to prohibit implantation.