유기금속화학증착법에 의한 Boron Nitride의 제조

진 용기*, 주 재백*, 이 중기, 박 달근

홍익 대학교 화학공학과*

한국과학기술연구원 청정기술연구센터

Preparation of Boron Nitride Thin Films By MOCVD

Yong-Gi Jin*, Jeh-Beck Ju, Joong Kee Lee and Dalkeun Park

Dept. of Chem. Eng., Hong-Ik University*

Clean Technology Research Center, KIST

INTRODUCTION

Boron Nitride(BN) has attracted a greate deal of interest in recent year for its outstanding properties such as optical transparency, chemical inertness, hardness, high electrical resistivity, high thermal conductivity and low dielectric loss, etc. Many investigators have studied on the BN thin films prepared by chemical vapor deposition(CVD) method. Now it is widely used in industry because of the advantage of high growth rate, good step coverage and good control of film morphology and stoichiometry by adjustment of deposition conditions. BN films can crystallize in various phases. Hexagonal BN (h-BN) is similar to graphite in structure showing analogous properties. Cubic boron nitride (c-BN) has become the focus of considerable attention because of the properties which are closely similar to those of diamond, particularly extreme hardness. C-BN is generally prepared by CVD and PVD(physical vapor deposition) and it is known that growth of c-BN is possible only when film growing surface is bombarded with energetic ions. Therefore, stress built-up is inevitable and adhesion of the film is very poor. Although thermal energy alone does not seem to conduce to the deposition of c-BN, CVD can be used for the deposition of versatile BN thin films.

In CVD various physical and chemical parameters are involved in the formation of thin films. It has a difficulty to measure profiles of temperature and gas flow in the reactor as well as concentration of all the chemical species involved in the deposition conditions. In spite of the fact, MOCVD(CVD using metal organic precursor) is an often preferred method in preparation of industrially much important III-V group thin films. Therefore, extensive efforts have been made to elucidate kinetics and mechanisms in the MOCVD of those materials.

In this study, we investigated the effects of deposition conditions on deposition rate and characteristics of BN films. BN thin films were prepared on Si(100) substrate by MOCVD.

EXPERIMENTAL

The reactor is made of 20mm I.D. quartz tube which is jacketed with a larger diameter quartz tube and air was fed into the annulus for the cooling of reactor wall. Substrate could be mounted on graphite holder which was located in the center of the reactor. Reactor is heated by a three zone electrical furnace. The temperature of substrate was measured with k-type thermocouple in contact with it. Hydrogen is used as a carrier gas for TEB of high purity(99.999%, Morton). The properties of TEB are described in Table 1. TEB was put into a bubbler which was submerged in a thermostat. The distance between the feeding tube for TEB and the substrate could be varied by linear movement of the holder. Ammonia and argon were fed into the reactor through a separate line to avoid formation of adducts in the gas feed line. All the gases were fed into the reactor through mass flow controllers and the moisture and oxygen being contained in gases are removed by using traps. Working pressure of the reactor could be regulated through a control loop system consisting of a throttling valve in the gas exhaust and a mechanical pump. Exhaust gases from the reactor was passed through a scrubbing unit before evacuation.

Single crystal silicon (100) wafers were cut into 10x10x0.53 mm pieces and put in boiling solution of H2SO4/H2O2/H2O for ten minutes to remove organic material and then rinsed with HF/H2O solution to remove the impurities of oxide metals. After drying with nitrogen gas it was placed into the reactor. It was baked at 600 oC for ten minutes under 10-3torr. Then argon was introduced to adjust reactor pressure to the desired value and reactor temperature was raised at the rate of 6.7 oC/min up to the deposition temperature. Deposition was carried out by feeding reactant gases. Deposition conditions are summarized in Table 2. The characteristics of deposited BN films were investigated by FT-IR, XPS, SEM, AES, XRD and the measurement of substrate weight difference before and after the deposition.

Table 1. Physical Properties of TEB

Chemical Name	Triethylboron
Formula	(C2H5)3B
Fomula Weight	97.9951
Melting Point, oC	-92.9
Boiling Point, oC	95
Density	0.6961 gram/cc (20oC)
Appearance	Clear colorless liquid
Vapor Pressure	Log(P)=9.837-3193.7/T (P; torr, T; Kelvin)

Table 2. Experimental conditions of MOCVD

Deposition temperature	850∼1100℃
Reactor pressure	7.4∼740torr
Gas flow rate of TEB	0.06∼1.5ml/min
NH3	10∼300ml/min
Diluent Ar	0∼700ml/min
H2	9∼50ml/min
Bubbler pressure	100∼760torr
Bubbler temperature	-18.5∼-1.25℃

RESULT AND DISCUSSION

It was found that deposition rate is highest for a optimum value of the distance between the inlet tube of TEB and substrate. If deposition of BN is through intermediates of TEB-ammonia adducts, deposition of BN is possible only when contact between TEB and ammonia is made. When the distance is short, much of reactant gases leave the reactor without being very well mixed. Thus, it resulted in low probability of forming adducts and low conversion to BN. On the other hand, films were deposited on reactor wall upstream substrate before deposition on substrate when the distance from the TEB inlet tube to the substrate is long.

Deposition rate was increased with total gas flow rate but it was not clear in the experimental ranges of our study. It was also found that effects of hydrogen  and ammonia molar fraction was insignificant. This is expected because  ammonia feed rate is in excess as is common in MOCVD of III-V compounds. While, total gas feed rate was constant deposition rate was increased with TEB feed rate.

Fig. 1 shows the effect of total reactor pressure on deposition rate at the fixed values of TEB partial pressures, respectively. It showed that deposition rate increases with decrease of total reactor pressure. As pressure was reduced, the diffusivities of gases increased and transport rate of film forming materials toward substrate surface increased. When the flow rates of all the gases were constant and only total reactor pressure was varied, deposition rate was increased dramatically, but reached a constant value after 100torr (Fig. 2).

Fig. 3 showed typical FT-IR spectra of deposited BN thin film. B-N stretching mode was observed at wave number 1,380 ∼ 1,400 cm-1 resulting from in-plane vibration. B-N-B bending mode was observed at wave number 790 ∼ 800 cm-1 resulting from out-of-plane vibration. It could be construed that the film is of h-BN structure with sp2 orbital.

Fig. 2 Effect of reactor pressure

on the deposition rate at

various total gas flow rates.

Fig. 1 Effect of reactor pressure

on the deposition rate at two

different TEB partial pressures.

Fig. 3 FT-IR transmission spectra of BN thin                   films deposited on Si(100).

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