Decomposition of Boron Trifluoride in the RF Plasma Environment

1 Department of Environmental Engineering and Science, Chia-Nan University of Pharmacy and Science, No. 60, Sec. 1, Erh-Jen Rd., Tainan 717, Taiwan 2 Department of Chemical Engineering, National Kaohsiung University of Applied Sciences, No. 415, Chien Kung Road, 807 Kaohsiung, Taiwan 3 Department of Environmental Engineering and Science National Pingtung University of Science and Technology, 1Hseuh Fu Rd., Nei Pu Hsiang, Ping Tung, 91201, Taiwan

Boron trifluoride (BF 3 ) is an inorganic fluorinated, Figure 1.Schematic of the BF 3 RF plasma system highly toxic, colorless and nonflammable has used to catalyze several chemical reactions, including polymerization, alkylation and acylation.It is also used to detector neutrons and is often used in ion implantation in the semiconductor industry (Schmidt et al., 1998;Boenig, 1988).Ion implantation is a process by which ionized atoms are accelerated directly into a substrate to selectively add dopant atoms.Although the requirements of clearance for semiconductor manufacturing, the generation of fine particles during the process of ion implantation warrants much more attention.The emissions of BF 3 must be controlled because at an immediate danger to live and health (IDLH) of 10 ppm and at a threshold level value (TLV) of 1 ppm, respectively (Josep et al., 1999).
A radio-frequency (RF) plasma environment with a high electrical and thermal conductivity constitutes an excellent energy conversion and heat transfer medium for reactants and products (Hsieh et al., 1998).The technology provides a more complete and a lower-temperature reaction environment for gas molecules than other methods.
However, RF plasma exists out of equilibrium and is often called cold plasma (Boenig, 1988).The kinetic energy of electrons and ions exceeds that of molecules in the cold plasma system.The apparent operating temperature in an RF plasma reactor is generally below 400℃, while the real temperature of the electrons therein exceeds 2000 ℃ .
Consequently, conventional reactions that must proceed at a very high temperature can be completed at a lower temperature in the RF plasma reactor (Hsieh et al., 1998).Besides, RF plasma does not cause the erosion or corrosion of electrodes by by-products such as HCl or HF, unlike DC plasma (Breitbarth et al., 1997).
Although controlling and reducing the emission of perfluorocompounds (PFCs) has received considerable attention, the decomposition of BF 3 has seldom been addressed.This work compares

Experimental Apparatus
Figure 1 schematically depicts the experimental apparatus used in this study.The BF 3 /CH 4 /Ar or BF 3 /O 2 /Ar mixing gas was metered using Brooks type 5850E mass flow controllers, at a total flow rate of 700 sccm : the gas entered a mixing vessel and was introduced perpendicularly into a 4.14 × 15 cm cylindrical glass reactor.The RF plasma discharge was produced using a plasma generator (PFG 600 RF, Fritz Huttinger Elektronik Gmbh) at 13.56 MHz and with a matching network (Matchbox PFM).RF power was delivered through the power meter and the matching unit to an outer copper electrode that was wrapped around the reactor, the other electrode was earthed.The system was inductively coupled : the external electrode and the glass reactor wall beneath it, together with the conductive plasma inside the reactor, generated a capacitor that enabled capacitive coupling of RF power into the discharge (Biederman et al., 1992).
Before the experiment, a diffusion pump was used to maintain the pressure of the system below 0.001 Torr to clean up contamination.Under each designed experimental condition, the input power, the CH 4 /BF 3 or O 2 /BF 3 ratio, the operational pressure and the BF 3 feeding concentration were measured more than three times within five minutes to ensure that steady-state conditions had been reached.Both reactants and final products were first identified by gas chromatography/mass spectrometry (HP5890A PLUS GC/MS).Then, all species were identified and quantified using an on-line Fourier Transform Infrared (FTIR) spectrometer (Thermo Nicolet AVATRA 360).
Gaseous reactants and products were calibrated

Results and Discussion
Experiments were performed to determine the dependence of the BF 3 decomposition fraction (η BF3 ).Theη BF3 was defined as follows: The SiF 4 was converted into CaF 2 according to the results proposed by Breitbarth, et al. (1997).
Additionally, the produced O 2 will react further with CH 4 , resulting in the formation of CO and CO 2 in the RF plasma system.HF was formed because of the high bond strength of H-F (567.9 ± 0.1 D 0 298 /kJ mol -1 ), and reduces the opportunities for reaction between
Accordingly, for a given input power, η BF3 of the BF 3 /CH 4 /Ar plasma system exceeded that of the BF 3 /O 2 /Ar or BF 3 /O 2(glass) /Ar plasma system.
Moreover, η BF3 increased when a catalyst (glass beads) was added to the system, when the input power exceeded 120 Watts.The value of η BF3 increased with the input power, η BF3 also increased from 47.1% to 52.6% as the CH 4 /BF 3 ratio increased from 1.0 to 4.0 (Fig 6  The value of F SiF4 increased from 0.61% to 0.83%, from 2.11% to 2.90% and from 1.03% to The value of η BF3 was around 50% during mixing with the CH 4 , but near only 29% during mixing with O 2 , even when the input power exceeded 120 Watts.Although the value of η BF3 in the BF 3 /O 2 /Ar plasma system was lower than that in the BF 3 /CH 4 /Ar plasma system, the generation of fine particles and their deposition were more serious in the BF 3 /O 2 /Ar plasma system, which fact warrants further investigation. by withdrawing unreacted gases and passing them through the sampling line connected to the FTIR.The mass of species was calculated by comparing the response factor (absorbance height/ concentration) of standard gas and reaction gas at given IR wave number.Each run of the experiment took 20 min and the results indicated that the steady-state conditions were reached in the effluent after 10 min.The data presented herein are mean values measured after a steady-state condition had been reached.

FFigure 9 .Figure 10 .
Figure 9.Comparison of the fraction of total input fluorine converted into SiF 4 with input power among BF 3 /CH 4 /Ar, BF 3 /O 2 /Ar and BF 3 /O 2(glass) /Ar RF plasma systems glass) /Ar plasma systems, respectively, as the input power increased from 70 to 150 Watts (Fig9).The F SiF4 was higher in the BF 3 /O 2 /Ar plasma system than in the BF 3 /CH 4 /Ar plasma system, because of the competition between the etching process and the formation of HF.Additionally, the value of F SiF4 declined from 0.74% to 0.70% as the CH 4 /BF 3 ratio increased from 1.0 to 4.0 (Fig10).

Detecting Products in the BF 3 /CH 4 /Ar Plasma System
ratio increased from 1.0 to 4.0.The value of F CO+CO2 declined from 92.8% to 73.2% as the