Deposition behavior of TiB₂ by microwave heating chemical vapor deposition (CVD)

1 Introduction

Titanium diboride (TiB₂) has a number of superior performances, such as a high hardness, high melting point, excellent anticorrosive characteristics, low bulk resistivity and good wear resistance. These excellent performances make TiB₂ a promising candidate for extensive applications in aerospace facilities, military field, industrial production and our daily life [1].

At the same time, there is increasing interest in the green process of preparing materials on account of the environmental protection needs as well as the demand of industrial development [2–4]. As a green process, microwave energy supply method can selectively transfer the required energy to the reaction molecules or atoms by the means of dialectic loss of the material themselves. Jones, Lelyveld, Mavrofidis, Kingman and Miles [5] concluded that microwave heating offers a number of advantages over conventional heating such as: non-contact heating, energy transfer, no heat transfer, rapid heating, material selective heating, volumetric heating, quick start-up and stopping, heating starting from the interior of the material body, higher level of safety and automation, and no waste gas. Thus, a lot of research has been conducted in the fields of pre-treatment [6, 7], drying [8, 9], roasting [10, 11], reduction [12] and the leaching strengthening of the processes by microwave energy [13, 14].

In this paper, the microwave heating chemical vapor deposition (CVD) process of TiB₂ according to eq. (1) will be described:

The microwave heats the graphite substrate locally which induces a surface reaction. Results of the growth rate as a function of the temperature and the microwave power will be presented. As will be shown, under the fixed gas flow ratio of TiCl₄ and BCl₃, the deposition temperature and vacuum degree, that the growth rates and the morphologies depend to a large extent on the microwave power. The deposition processes using different microwave powers were performed on graphite substrate under the same conditions of temperature, reagent concentration and vacuum, while X-ray diffraction (XRD) and microstructure will be presented.

2 Materials and methods

The equipment is self-made and its schematic diagram is shown in Figure 1. It is composed of a gas supply system, a microwave heating system, a temperature control system, a vacuum system and a tail gas treatment device. TiB₂ films are deposited on a high purity graphite substrate by microwave heating CVD using a gas mixture of TiCl₄, BCl₃, H₂ and Ar. Ar is used as the carrier gas for the TiCl₄. The gas lines are heated with heating tape to avoid the condensation of the reagent gases on the way. The gases are mixed in a gas distributor by passing multistage baffles, and then piped into the vacuum chamber through a gas distributor. A microwave with a 2.45 GHz frequency is generated by magnetron and radiates out into the vacuum chamber through a waveguide tube. The graphite substrate is heated by the microwave radiation and the temperature is measured by the thermocouple. Off-gases are removed through a suction arrangement full with metal filing.

Figure 1:

Schematic diagram of the microwave heating CVD system.

The base pressure is <5×10^-6 mTorr, while the deposition is carried out at 70 mTorr. The deposition temperature and microwave power range from 800 to 1000°C and from 800 to 1500 W, respectively. Table 1 summarizes the deposition conditions.

The TiB₂ films are characterized using X-ray diffraction (XRD) (X′ Pert Pro, PANalytical Corporation). Moreover, the microstructures are analyzed by field emission scanning electron microscopy (FESEM) and EDS (Ultra Plus, Zeiss Company). The micro-hardness is evaluated from the loading/unloading curves measured using a computer controlled micro-hardness tester (Fischerscope H100).

3 Results and discussion

As shown in Figure 2, under the fixed conditions of vacuum degree, temperature and boron-hydrogen-chlorine ratio, the TiB₂ films deposited at different microwave power have different XRD patterns. The comparison of width values at half maximum of the diffraction peaks for all the crystallized films indicates that the smaller grain size comes from the film deposited at lower microwave power. Figure 3A–D display the micrographs of the TiB₂ films deposited at different microwave powers. As shown in Figure 3A–D, the grain size of the TiB₂ film deposited at low microwave power is 0.5–1 μm, while the high power gets a size of 1–2 μm.

Figure 2:

XRD patterns of TiB₂ films deposited at 800 and 1500 W of power (70 mTorr, 900°C and TiCl₄/BCl₃=1/2).

Figure 3:

SEM micrographs of TiB₂ films. (A, B) and (C, D) are the microstructure of TiB₂ films deposited at 800 and 1500 W of power, respectively.

Figure 3A and B displays that the film deposited at 800 W of power has a (surface) morphology of granular structure with uneven grain sizes and most of these grains are about 0.5 μm in size, which is probably caused by insufficient energy supplied for the deposition reaction. Meanwhile, Figure 3C and D shows that the film deposited at 1500 W of power has a (surface) morphology of granular structure with uniform grain sizes of about 1 μm. It is related closely to the nucleation and crystal growth of TiB₂, which is improved with the increasing of microwave power.

The SEM micrographs and EDS results of deposited films at 800 W and 1500 W of microwave power are shown in Figures 4 and 5, respectively. As shown in Figures 4 and 5, the thickness of the film deposited at 800 W for 1 h has reached 15 μm, while the film deposited at 1500 W has achieved 22 μm thickness, which revealed that the growth rate is improved with the microwave power. The X-ray EDS mappings show that the boron and titanium in deposited films have the same location. The EDS patterns shown in Figure 4 and the results of spot 1 in Table 2 provide evidence of an excess of boron (B) in the film deposited at a low microwave power. As the microwave power is increased, the B/Ti ratio of spot 2 shown in Table 2 approached the stoichiometric value of 2. Boron rich TiB_x (XN₂) films are deposited at a low microwave power, because insufficient energy was supplied for the reaction. The enhanced ion density and ion mobility at high microwave power produced stoichiometric TiB₂ films.

Figure 4:

SEM micrograph and the EDS results of TiB₂ film (70 mTorr, 900°C, TiCl₄/BCl₃=1/2, 800 W, 1 h).

Figure 5:

SEM micrograph and the EDS results of TiB₂ film (70 mTorr, 900°C, TiCl₄/BCl₃=1/2, 1500 W, 1 h).

Figure 6 shows that the micro-hardness of the TiB₂ films increases with increasing deposition temperature and microwave power in the range of the experimental conditions. The highest hardness is obtained for the bulk TiB₂ because of a strong bonding between graphitic B-layers along the c-axis [15, 16].

Figure 6:

Hardness of TiB₂ films deposited at the various microwave power (70 mTorr, TiCl₄/BCl₃=1/2, 1 h).

4 Conclusion

TiB₂ films with superior mechanical properties could be prepared using microwave heating CVD. The film hardness was determined by the deposition temperature, the microwave power and the gas flow ratio of TiCl₄/BCl₃. With a fixed TiCl₄/BCl₃ gas flow ratio, the grain size and the growth rate of TiB₂ films are improved with increasing microwave power, and the bulk TiB₂ has the highest hardness.