□Wet Colloid 방법□을 이용하여 투명한 SiC 박막의 제조

조규진, 문일식

순천대학교 화학공학과

Preparation and Characterization of Transparent SiC Thin Films through Wet Colloid Method

Gyoujin Cho, Il-Shik Moon

Department of Chemical Engineering Sunchon National University

Introduction

Cubic silicon carbide (□-SiC) is a well known ceramic in the field of high- temperature and resistant engineering applications.1 Therefore, there has been a growing interest in producing □-SiC coatings for a wide range of applications.2 For the fabrication of SiC on the substrate, there are three major requirements should be considered. First, the adhesion should be strong and durable. Second, there should be low porosity or crack. Finally, the temperature during the fabrication should not be too high in order to prevent detrimental modifications of the substrate properties due to structural changes or excessive chemical reactions. Although there are a variety of techniques to fabricate SiC films, including chemical vapor deposition (CVD), plasma process, and ion beam techniques,3 those methods require high deposition temperature (usually above 500 ℃) and have the problem of the cost and complex process for the industrial application.

To keep pace with the advanced technologies, a new processing which can fabricate SiC thin films with strong adhesion and crack-free at low temperature is highly desirable. Therefore we have paid attention on "wet" colloid chemical methods which have been intensively investigated recently.4 Since colloidal materials are usually in the range of micrometer-to-submicrometer in size and are well dispersed in solution, the spontaneous adsorption of colloidal particles onto a substrate could lead to micrometer- or submicrometer-structured materials at low temperature.

In this paper, as an obvious extension of self-assembly, we first report the wet colloid method in the fabrication of SiC thin films at room temperature using aqueous dispersions of nano-sized SiC particles using self-assembled sodium dodecylsulfate (SDS) arrays and self-assembled monolayers (SAM) of polyethylene glycol on substrate. The resulting SiC thin films were characterized using scanning electron microscopy (SEM) and X-ray diffraction (XRD).

Experiment

□-SiC nanoparticles (SUMITOMO OSAKA CEMENT CO) with 48.0 m2/g BET specific surface area and 30 nm of average particle size were re-dispersed in deionized water without further treatment. When the SiC particles were redispersed in deionized water, no precipitation was observed due to the Brownian motions of the nanoparticles. Sodium dodecylsulfate was attained from Aldrich and recrystallized from anhydrous ethanol before using. Doubly distilled deionized water was used in preparing SiC thin films. For the deposition of SiC thin films, both the PEG self-assembled and bare Si wafers were placed into the synthesis vessel containing 10 mL of SiC colloidal solution (0.005 wt% of SiC) at pH 3, and then added SDS (5 mM; determined from adsorption isotherm as a minimum concentration to form self-assembled bilayers) kept in a thermostated bath at 25 ℃ over a period of 1 to 72 h. The resulting Si plates were rinsed with stirring water for 5 min and dried at 80 ℃. The morphology and thickness of the resulting SiC thin films was studied using SEM (JEOL 200CX attached ASDID4D apparatus). The crystallinity of the resulting films were investigated using XRD (D/MAX, Rikagu, Japan: CuK radiation).

Results and discussion

To confirm the critical role of SDS in SiC deposition process, we carried out the experiment under various conditions (Table 1). Without SDS, no coagulation and film formation of SiC were observed. Furthermore, even in the presence of SDS, coagulation and film formation only can be observed at lower pH (< 3). This may be due to the generation of positive surface binding sites of SiC to SDS. An interesting thing was that defect-free and dense SiC films can be formed only with PEG self-assembled (PSA) Si substrate while rough SiC films with many defects were formed on bare Si substrate under the same condition to the PSA Si substrate (Fig. 1).

There are two essential features in the fabrication of SiC thin films on Si substrate. The first is the SDS adsorption on SiC and the formation of self-assembled bilayers (SAB) on SiC under low pH. The SAB of SDS will render the attractive interaction and reduce the repulsive interaction between SiC particles. Therefore, the interaction between self-assembled bilayers of SDS on SiC in aqueous medium may be sufficient to cause the SiC particles to coagulate and settle out of the solution to deposit on the substrate as a thin film when the SiC particles coated with SDS self-assembled bilayers collide each other. The second is the role PSA on Si substrate. Ethylene oxide unit of PSA will favorably attract SDS self-assembled SiC at low pH. In fact, PEG can attract SDS in aqueous phase and formed enlarged micelle of SDS even under the CMC of SDS.5

The resulting SiC thin films on PSA Si plate do not have the problem of desorption on rinsing because SDS molecules adsorbed on SiC and PSA of SiC have fairly strong interaction. However, the adhesion is not strong enough, and the film can be removed with a Scotch tape. We are further investigate to enhance the adhesion of SiC films on the substrate using chemical reaction at the interface. XRD results of the films are identical to the SiC powders (Fig.2). Although the relatively thick films were coated on the substrate, the coated films were transparent (Fig. 3).

In conclusion, our results show that transparent and dense SiC films with the thickness of 4000 nm can be formed on a PEG self-assembled Si wafer through the process of self-assembly of SDS molecules on the two kind surfaces. The thickness of the film can be controlled by repeating the film deposition process. Further study is being made to enhance adhesion of the films. It is hoped that this method would become valuable in the preparation of novel ceramic thin films.

Table 1. Conditions for the coagulation and deposition of SiC nanoparticles onto a silicon substrate.

SDS (mM)	SiC nanoparticles (wt%)	pH	Temp. (℃)	Rxn. Hour	Substrate	Coagulation	Film formation
4.8	0.001	10	25	60	Bare	O	X
4.8	0001	10	25	60	PSA	O	X
4.8	0.001	6.8	25	60	Bare	O	X
4.8	0.001	6.8	25	60	PSA	O	X
4.8	0.001	3.2	25	60	Bare	O	O
4.8	0.001	3.2	25	60	PSA	O	O
4.8	0.003	3.2	25	60	Bare	O	O
4.8	0.003	3.2	25	60	PSA	O	O
X	0.001	6.8	25	60	Bare	X	X
X	0.001	6.8	25	60	PSA	X	X

(a)

(b)

Figure 1. SEM images of deposited SiC thin films on bare(a) and PSA(b) Si substrate.

Figure 2. XRD data for the SiC films deposited on PSA Si substrate.

Figure 3. Photograph of transparent SiC films on PSA glass.

References

1. J. P. Revice, J. Delafond, P. Misaelides, F. Noli, Surf. Coat. Technol. 1998, 100, 243 and references there in.

2. H. S. Kong, J. T. Glass, R. F. Davis, Appl. Phys. Lett., 1986, 49, 1074

3. S. Nishino, J. A. Powell, H. A. Will, Appl. Phys. Lett., 1983, 42, 460

4. S. Herminghaus, Science, 1998, 282, 916 and references there in.

5. P. Favoriti, C. Treiner, Langmuir, 1998, 14, 7493