Time-Dependent Cyclic Modulation Process of CH₄ Gas Flow Rate in CH₄-H₂-O₂ Plasma for the Enhancement of Diamond Film Characteristics

다이아몬드 막 특성 향상을 위한 CH₄-H₂-O₂ 플라즈마 내의 CH₄ 기체 유량의 시간 의존 싸이클릭 변조 프로세스

EXPERIMENTAL SECTION

Diamond films were deposited on the 10.0×10.0 mm2 pretreated (100) Si substrate in a horizontal-type MPECVD system. The substrate was merely heated by the plasma. We deposited diamond film under substrate temperature=ca. 850 ℃, microwave power=800W, and total pressure=5.33 kPa condition. The pretreatment was carried out by an ultrasonic treatment for 30 minutes using diamond powders in ethanol solution. Before the deposition reaction, we cleaned the substrate with H2 plasma for a few minutes. CH4, H2 and O2 were used as source gases. Total flow rate was fixed at 100 standard cm3 per minute (sccm). Concentrations of CH4 and O2 were fixed at 1.5% and 0.5%.

To elucidate the effect of the cyclic process on the enhancement of the diamond nucleation density and the diamond quality, we deposited the diamond film via two different ways, namely, the cyclic process and the normal process. For the cyclic process, we incorporated the cyclic modulation process of CH4 flow during the initial deposition stage. The cyclic modulation was carried out through on/off control of CH4 flow. Namely, it was started from H2+CH4+O2 plasma (CH4 flow on) and ended in H2+O2 plasma (CH4 flow off). Actually, it was proceeded as H2+CH4+O2 (120 s)→H2+O2 (120 s)→H2+CH4+O2 (120 s)→H2+O2 (120 s), then 32 min depositing the diamond film under H2+CH4 plasma condition. So, the total on/off CH4 flow modulation time was 8 min and the total reaction time was 40 minutes. For the normal process, we deposited the diamond films for 40 min without the incorporation of the CH4 flow cyclic modulation and the oxygen incorporation. The detailed reaction flow condition for the different samples was shown in Table 1.

We investigated the detailed surface states, and the grain morphologies using field emission scanning electron microscopy (FESEM). The qualities of diamond grains on the pretreated glass substrate were investigated by a micro-Raman spectrometer (Renishaw 2000) with ca. 1 mm spot size using an Ar laser source.

Table 1.Reaction flow conditions for different samples

 

RESULTS AND DISCUSSION

First of all, we investigated the surface images of as-deposited diamond films without the oxygen incorporation in the source gas. To reduce the measurement error, we set the measuring position as ca. 1 mm apart from the outermost edge of the substrate. Fig. 1a shows FESEM image of as-deposited diamond films, in case of the normal process (sample A). The diamond nucleation density on the pretreated Si substrate surfaces was counted about 1.8×1010 (nuclei/cm2). Fig. 1b shows FESEM image of as-deposited diamond films, in case of the CH4 flow rate on/off cyclic modulation process under CH4+H2 plasma condition (sample B). The number density of diamond nuclei on Si substrate was counted more than 8.5×1010 (nuclei/cm2). Figs. 1c and d show FESEM images of as-deposited diamond grains, in case of CH4 flow on/off cyclic modulation process (sample C) and O2 flow on/off cyclic modulation process (sample D) under the condition of the oxygen incorporation. The number densities of nuclei on Si substrate were counted about 7.8×1010 (Fig. 1c) and 2.0×1010 (Fig. 1d) (nuclei/cm2), respectively. Based on the results of Fig. 1, we suggest that the CH4 flow on/off cyclic modulation process with or without the incorporation of oxygen can increase the diamond nucleation densities on the pretreated Si substrate. In addition it is also derived, from the comparing results of Figs. 1a with d, that the oxygen flow on/off cyclic modulation process doesnt give rise to the noticeable increase in the diamond nucleation density, compared with the normal process.

Fig. 1.FESEM images of as-deposited diamond grains of (a) sample A, (b) sample B, (c) sample C and (d) sample D.

To investigate the enhancement of the diamond quality by the oxygen incorporation under the condition of the cyclic modulation process, we investigated the diamond grains on Si substrate using micro-Raman spectroscopy with ca. 1 μm spot size. Fig. 2 shows micro-Raman spectra of the different samples. It reveals the enhancement of the diamond quality by the cyclic modulation process of CH4 flow rate (Figs. 2b and c) or O2 flow rate (Fig. 2), dcompared with the normal process. To analyze the extent of the diamond quality enhancement according to the different samples we measured the variation of the relative intensity ratio (Id/Ia) of diamond (at 1,332 cm−1) to amorphous carbon (around 1,500 cm−1) as shown in Table 2.

Fig. 2.Micro-Raman spectra for (a) sample A, (b) sample B, (c) sample C and (d) sample D.

Table 2.Id/Ia values of micro-Raman spectra for (a) sample A, (b) sample B, (c) sample C and (d) sample D

Based on the results of the Id/Ia intensity according to the samples A through D, it is suggested that the cyclic modulation process would enhance the diamond quality with or without the incorporation of oxygen (compare samples A with B, C and D). Previously, it was reported that the cyclic modulation process could produce more atomic hydrogen during the plasma reaction.9,10 These results reveal that the abundant amount of etching source gas, such as the atomic hydrogen and the oxygen, in the plasma during the cyclic process seems to be associated with the diamond quality enhancement, because the atomic hydrogen and oxygen can more readily etch away amorphous carbon than the diamond component in the grain.

By comparing Id/Ia of sample B with that of sample C, we observed the increase in Id/Ia at sample C. It reveals the relative increase of diamond component in the grain by the oxygen incorporation even under the similar process condition of the CH4 flow on/off cyclic modulation.

In addition, the combined results of Figs. 1 and 2 for samples C and D indicate that the diamond nucleation density of sample C would be enhanced while the diamond quality was maintained. These results reveal the possibility to enhance not only the diamond quality but also the diamond nucleation density by the on/off cyclic modulation of CH4 flow under CH4-H2-O2 flow condition.

The decrease in the diamond nucleation density by the on/off cyclic modulation of O2 flow (sample D) seems to be due to the removal of sub-critical size diamond nuclei as well as nucleation sites on a silicon surface by the excess etching source gases, such as atomic hydrogen and the oxygen related components.8

 

CONCLUSIONS

Both the diamond nucleation density and the diamond quality could be enhanced by the on/off cyclic modulation process of CH4 flow under the condition of the oxygen incorporation. Regardless of the oxygen incorporation in the source gas, not only the diamond quality but also the diamond nucleation density was enhanced by the on/off cyclic modulation of CH4 flow.