자기응집막을 이용한 세라믹 박막 제조

조규진*, 김선미*, 문일식*, 민경신**, 이재석**, 정경택***, 설용건***

*국립 순천대학교, 화학공학과, 전남 순천, 540-742

**광주과학기술원(K-JIST), 신소재공학과, 광주, 506-712

***연세대학교, 화학공학과, 서울, 120-749

Preparation of Ceramic Thin Films Using Self-Assembled Arrays.

Gyoujin Cho*, Sun-Mi Kim*, Il-Shik Moon*, Kyung-Shin Min**,

Jae-Suk Lee**, Kyeong-Tack Jung***, Yong-Gun Shul***

*Dept. of Chem. Eng., Sunchon Nat. Univ., Sunchon, Chonnam, 540-742, Korea

**Dept. of Mater. Sci. and Eng., Kwangju Inst. of Sci. and Tech.(K-JIST), Kwangju 506-712, Korea

***Dept. of Chem. Eng., Yonsei Univ., Seoul 120-749,Korea

Introduction

Ceramic thin films are in highly demand as active and passive components in microelectronic circuits as capacitors, memories, and insulating or passivating layers. However, conventional ceramic processing can not be used in many of the above applications because the process often involves high-temperature sintering. The high-temperature sintering at the formation of ceramic thin films on substrates can impede the further applications of the ceramic films in the advanced technologies. Even the advanced "sol-gel" techniques for producing ceramic layers on substrate require processing temperatures in excess of 400℃. Thermal chemical vapor deposition1 and photochemical vapor deposition2 methods give high-quality of ceramic thin films at somewhat lower temperatures, i. e., 110-500℃. To keep pace with the advanced technologies, a new ceramic processing which can control the microstructural and macrostructrural morphologies of the film at the lower temperature (< 50℃) is highly demanded.

One route successfully employed is the imitation of nature in which organisms deposit highly oriented, detect-free ceramics from aqueous solutions under ambient condition on substrates such as living tissues3. Biologically controlled mineralization occurs within isolated environments created by vesicles or insoluble macromolecules (proteins and/or polysaccarides) in which the nucleation and growth processes can be controlled4. While detailed mechanisms for the role of the functional groups of vesicles or insoluble macromolecules at interfaces between the aqueous solutions and the substrate are still poorly understood, the natural phenomena can be mimicked by the introduction of functional groups onto substrate surfaces. Processes mimicking biomineralization have now been used to prepare materials including iron oxides apatite, and CdS. Those processes employed self-assembled monolayers or Langmuir monolayers at solid/solution or solid/air interfaces to direct the orientation of the growing mineralization. Usually an epitaxial relationship between crystal of the grown mineral and monolayer lattices has been demonstrated for various systems.

We here investigate the growth of silicon oxide thin films on self-assembled monolayers (SAMs) of  nonionic surfactant on mica using contact angle measurement, x-ray diffraction (XRD) and atomic force microscopy (AFM). A major reason in using silicon  oxide as the ceramic material and mica as a substrate is respectively that silicon oxide materials are a particularly important class of cerarmics since most silicon semiconductor technology is based on the ability to form thin SiO2 layers and that  mica has convenience properties such as atomic size, smoother surface, low cost, and ease in cleaning. In this paper, we are presenting new results  in the preparation of SAMs and their use as a well ordered media for the growth of SiO2 thin films.

Experimental Section

Materials. HPLC grade p-xylene (99+%) and 3-(triethoxysilyl)propyl isocyanate (TPC, 95+%) were purchased from Aldrich and used without further purification. Muscovite mica disks were obtained from Asheville-Schoonmaker Mica Co. and used only after peeling off the  contaminated top surface layers. Methylene chloride (anhydrous, 99.8%), polyethylene oxide (PEO, Mw 3400), and acetonitril (99.9+%) were used as received from Aldrich. Stannous 2-ethyl-hexanoate as a catalyst and tetraethyl orthosilicate (TEOS, 98%) were purchased respectively from Sigma and Fluka. Doubly-distilled water was used in preparing SiO2 thin films.

SAMs preparation.  Surface modification of mica disks using TPC has been carried out based on previously reported method5. Those surface modified mica disks are characterized using AFM and contact angle meter. All samples were kept under N2 atmosphere. SAMs of PEO were simply prepared by adding PEO onto the surface modified mica immersed methylene chloride solution under N2 atmosphere. The resulting SAMs of PEO on mica were characterized using AFM and contact angle meter.

SiO2 thin film formation. SAMs of mica disks were simply added into water containing various amounts of TEOS, and then, kept at 50℃ thermosetted bath for 10 h. The resulting mica disks were rinsed with distilled water and dried at 50℃. The morphology of the resulting SiO2 thin films was studied using AFM.

Results and Discussion

The contact angles of solid surfaces with water provide a useful insight into wettability, roughness, and molecular motions at interfaces. The results for the advancing (□a) and receding (□r) contact angles of the bare mica, TPC modified mica, and PEO self-assembled mica are give in Table 1.

Table 1. Advancing and receding contact angle for mica sample.

	Bare Mica	TPC Modified Mica	PEO Self-assembled Mica
□a	0o	100o	30o
□r	0o	30o	10o

While the advancing contact angles of the bare mica is 0o, the advancing contact angle of the TPC modified mica is 100o. If we consider that the contact angle of the completely hydrophobic surface of polyethylene is 120o and the end group of TPC on mica in cyanate group (NCO), it is reasonable to conclude that almost perfect packing of TPC on mica was achieved. In addition, after PEO self-assembled on TPC modified mica, the advancing contact angle was lower (30o) than that of TPC modified mica. By deliberating upon the hydrophilicity of PEO, the observed contact angle for the PEO self-assembled mica is reasonable one. The observed large value in hysteresis is commonly taken to indicate a system not at equilibrium. In fact, the hysteresis in these kinds of systems is often very large, and □r may be zero even when □a is large. All reported values are the average of at least five measurements taken at different locations on the mica surfaces.

For this work, it is more practical to investigate the structure of TPC modified and self-assembled monolayers of PEO on the mica surfaces by using AFM. First, the fresh bare mica surface was scanned to reveal the image of the bare mica surface. The  AFM image of bare mica was a clean surface with an □-range roughness.  The TPC modified mica yields AFM images quite different from that of bare mica surface. The AFM image of  TPC modified mica reveals that slightly ordered features are observed with nm-range ronghness. Furthermore, the AFM image of PEO self-assembled monolayers on mica shows the ordered features of pillars. The AFM results obtained for this work are thus shown to be consistent with the results of the contact angle measurements. The preliminary results represented in this work therefore provide a reasonable basis to postulate that the SAMs of PEO actually exists in a 2-dimensional ordered liquid state on the mica surface.

To investigate the influence of SAMs of PEO on thinly-deposited SiO2 films, it is interesting to note the change in morphology between the SiO2 films deposited on bare mica and SiO2 films deposited on mica modified by SAMs of PEO. From the AFM topological images and the cross-sectional slices of AFM images, the formation of SiO2 thin films can be easily detected from the both bare and SAMs of PEO mica. The AFM image of SiO2 deposited on bare mica (Fig 1) is close to that of bare mica. However, the AFM image of SiO2 deposited on mica (Fig 1) modified by SAMs of PEO is quite different from those of both bare and PEO self-assembled mica. Based on those results, the mechanism of surface polymerization of TEOS at the interfaces of the PEO self-assembled systems may be speculated up on to be a lateral growth of SiO2 films along the interface between PEO self-assembled mica and water because TEOS molecule may be condensed in the SAMs of PEO on mica. The further investigation to elucidate the film growth mechanism is underway in our Lab.

(a)                                  (b)

Fig 1. AFM images for the SiO2 deposited on bare mica (a) and mica        modified by SAMs of PEO (b).

Acknowledgment

Author gratefully acknowledge the financial support by the Korea Science and Engineering Foundation (#971-1104-023-1).

References

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4. Lowenstam, H. A.; Weiner, S. On Biomineralization; oxford University         Press: Oxford, 1989.

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