Synthesis and characterization of BN thin films prepared by plasma MOCVD with organoboron precursors
Introduction
Very large compressive and shearing stresses act on dies and punches used for cold forging and deep drawing. Such dies are, therefore, coated with TiC or TiN films using the thermal CVD method, which is believed to assure high adhesion. Thermal CVD processes at elevated temperature (about 1000 °C), therefore, require heat treatment of a base metal after processing. For this reason, such processes are not widely used for precision dies that do not allow dimensional change.
On the other hand, because BN films have excellent heat resistance and sliding attributes, and because BN can be produced at lower temperatures than those of thermal CVD method, they are applicable to precision dies without any dimensional changes. Nevertheless, because present BN films have inadequate adhesion, they have not been practically applied to dies, etc. For that reason, development of low-temperature and high-adhesion BN films is desirable.
In fact, BN, which consists of boron and nitrogen, both neighbors of carbon on the periodic table, has allotropes with a similar structure to that of carbon. It can have a hexagonal lamellar structure (hBN), cubic zincblende type (cBN), and hexagonal wurtzite type (wBN), which correspond, respectively, to carbon structures of graphite, diamond, and hexagonal diamond. One among those, cBN, which is produced with high pressure synthesis, has been industrialized and used as a tool material.
Conventional CVD synthesis processes of BN films include CVD enhanced using high-density plasma with negative bias applied to the substrate and ion bombardment by Chayahara et al. [1] and Ichiki et al. [2] for cBN films, and thermal CVD [3], [4] and atmospheric pressure CVD [5] for hBN films. Respective PVD processes include ion beam assisted deposition [6], [7], activated reactive evaporation [8], and ion plating [9], [10] for cBN films.
For this study, we sought to develop BN films of novel properties and structure that can eventually replace conventional TiC and TiN films using thermal CVD. We used a technique that we have researched and developed [11]: producing films of high adhesion and high hardness at low temperatures (500 °C and below).
Commonly used raw materials for conventional BN films typically have hazardous characteristics: toxicity, corrosiveness, and explosivity of diborane (B2H6), decarburize (B10H14), and trimethyl boron (B(CH3)3). Those characteristics have prevented their practical use. Accordingly, this study used trimethyl borate (TMB: B(OCH3)3), which is a safer and less expensive liquid organic metal, as a raw material. Because of the necessity of using a liquid organic metal, we chose MOCVD for BN film preparation, we investigated the preparation conditions of BN films of good adhesion at low temperature.
Section snippets
Experimental
Fig. 1 depicts the instrument used for the experiment. It consists of a vacuum chamber (ϕ 350 mm × L 400 mm), an RF power source for pulse plasma generation (RF: 13.56 MHz), a microwave generator (MW: 2.45 GHz), and a negative high voltage pulse power source. The BN films were synthesized on Si wafer and cemented-carbide (WC) substrates. In addition, Ar gas was used for bombardment, and TMB and N2 gases were used for deposition.
First, cleaning treatment of a sample surface was performed as follows:
Results and discussion
The FT-IR measurements of BN films were carried out. The BN films deposited onto a transparent Si wafer substrate that is transparent in the infrared region were measured using the transmission method. The measurement results are shown in Fig. 2. Five distinctive peaks were apparent at 800, 921, 1193, 1380, 1450, and 3220 cm−1. Peaks occur in the IR spectra of BN films at 780 and 1380 cm−1 for hBN and 1070 cm−1 for cBN [12]. Our BN films do not produce cBN peak but produce peaks at 800 and 1400 cm−1
Conclusion
Hard BN films, which are applicable to deposition onto dies, were prepared using the MOCVD method with trimethyl borate, a liquid organic metal, as a raw material. A composite BN film, the mixture of boric acid with the hBN film, was obtained. Composite BN films, a mixture film of hBN film and boric acid, were also obtained. The resultant BN films' characteristics are the following.
- (1)
The hBN peaks and the oxide peaks were obtained as a result of infrared-spectroscopic measurement of BN films.
- (2)
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