published at the ISPC 14, Prague, CZ, August 2 nd - 6 th 1999

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1 published at the ISPC 14, Prague, CZ, August 2 nd - 6 th 1999 In-situ characterization of plasma chemical reactions during the deposition of Si-C (-N) coatings in a D.C. plasma jet by means of emission spectroscopy Johannes Wilden 1, Andreas Wank 1, Wolfram Scharff 2, Till Wallendorf 2, Swen Marke 2 1 Inst. of Composite Materials, Chemnitz Univ. of Technology, Chemnitz, Germany 2 IfU GmbH, Heinrich-Heine-Str. 5, Flöha, Germany Abstract By means of emission spectroscopy the complete defraction of the liquid precursor hexamethyldisiloxane inside the nozzle is proved. Atomic silicon, carbon and C 2 -clusters are detected. The concentration of the excited species increases inside the expanding plasma jet before an exponential slope occurs. As the slope of the concentration of excited species is much steeper for silicon and carbon atoms than for C 2 -clusters only few activated silicon and carbon atoms reach the substrate surface. Emission spectroscopy is proved to be a useful tool to examine the plasma chemical reactions in plasma jet CVD processes. 1. Introduction The synthesis of silicon based ceramic layers is of interest for many industrial applications with respect to wear protection. Silicon carbide has a high hardness (3.500 HV), low density (3.2 g/cm 3 ), high thermal conductivity (100 W/m K at room temperature; 25 W/m K at 1,400 C) and excellent corrosion and oxidation resistance. Si-C-N composites are expected to show excellent thermal shock resistance combined with an outstanding wear resistance, especially at elevated temperatures. Today research has been done on the deposition of these materials by various CVD processes. Because of the rather low deposition rates the main application fields are in the optical and electronical industry where only thin (about 1 µm) layers are necessary [1-4]. Conventional thermal spraying cannot be employed for pure SiC, Si 3 N 4 or Si-C-N composites as these materials have no melting point. Applying the D.C. plasma jet CVD process makes it possible to reach deposition rates higher than 1,000 µm/h. In contrast to earlier investigations with mixtures of gaseous silicon and carbon precursors [5] liquid mono-precursors that emerge as waste in silicon industries, hexamethyldisiloxane (HMDSO) or hexamethyldisilazane (HMDSZ) are used. The structure and morphology of the Si-C (-N) coatings depend strongly on the process parameters, especially substrate temperature and distance. To produce coatings with defined properties, structure and morphology it is necessary to control the chemical reactions in the plasma jet. An emission spectrometer is employed to image the reactions. The precursors are evaporated and disintegrated when they are vaporized in the plasma jet. Decomposition of the precursors leads to molecular and atomic fragments which are accelerated and gradually deposited on the surface of the substrate. Optical emission spectrometry is used to examine the plasma chemical reactions in order to develop a control unit for the deposition process.

2 The change of the species density on the way to the substrate shows the chemical reactions dependent on the process parameters and the place inside the plasma. Especially the reactions on the surface have to be controlled for defined properties of the coatings. 2. Experimental procedure The coating process (figure 1) is carried out in a Plasmatechnik A 3000 S vacuum plasma spraying system. The liquid precursor is atomized with the help of argon into an argon/hydrogen plasma jet inside the nozzle. To keep the temperature of the substrate at a certain level it is fixed on a water cooled substrate holder. The optical sensor can be moved along an axis covering the whole distance between nozzle exit and substrate surface. The light is led by an optical fibre to the IfU AcoustoOptic Spectrometer (AOS). This special kind of spectrometer allows a random wavelength access in less than 1 ms with a good wavelength resolution 250nm, 800nm). Based on the random wavelength access and the large aperture (about 50mm 2 ) the AOS is suitable for plasma-monitoring. Figure 1: Experimental setup First experiments were carried out with chlorosilanes as precursors (SiCl 3 CH 3 and SiCl(CH 3 ) 3 ). No process parameters could be found to get chlorine free coatings, which results in severe corrosion of the substrate. Therefore in a next step chlorine free precursors were applied (hexamethyldisiloxane: (CH 3 ) 3 Si-O-Si(CH 3 ) 3 and later hexamethyldisilazane: (CH 3 ) 3 Si-NH-Si(CH 3 ) 3 ). The process parameters used are shown in table 1. To prevent the sensor from getting dusty an argon flow is applied to scavenge the optics continuously during the deposition process. The angle between the axes of the sensor and the plasma jet is fixed at 90.

3 parameter values power [kw] 11,5-17 argon [slpm] hydrogen [slpm] 2-3 chamber pressure [hpa] precursor flow rate [10-3 slpm] atomizing gas flow rate [slpm] 2-3 distance nozzle substrate [mm] substrates Silicon-Wafer, S235JR, S316L, Cu, Ti, Al Mg 1,5 Table 1: Process parameters 3. Results The coatings synthesized mostly show a morphology consisting of nanosized spheroidal particles (figure 2), but dense or columnar morphology can be achieved as well. Although the precursor HMDSO contains oxygen in its structure its content in the coatings is negligible. Figure 2: SEM picture of a cross-section and EDX-spectrum of an SiC coating synthesized from HMDSO on Al Mg 1,5 By emission spectrometry the plasma gases argon and hydrogen are verified in the plasma as well as silicon and C 2 -clusters in the visible and atomic carbon and silicon in the ultraviolet range of wave lengths (figures 3 and 4). For all process parameters used no further ionic or atomic species are detected. To investigate the development of the concentration of excited species characteristic wave lengths are chosen and their intensity dependent on the distance from the nozzle is determined. For this purpose the jets profile is recorded and the area under the curve is integrated. By this way the lateral expansion of the jet does not influence the result. For the C 2 -clusters band spectra the highest peaks that are not superimposed by further peaks are chosen. Figure 5 contains the intensities of the detected species normalized in a logarithmic plot dependent on the distance from the nozzle exit for free expansion of the jet. For each location profile measurements for upward and downward movement of the torch are carried out. In the ultraviolet range the results fluctuate much stronger than in the visible range, but smooth curves of the type f(x) = (a x b) e -c x can be fitted to the measured points.

4 I = 400 A P = 13 kw V Ar = 35 l/min V H2 = 2 l/min V Ar,at. = 2 l/min V Prec. = 10 ml/min p = 70 hpa 10 mm to nozzle Figure 3: Emission spectrum of the process in the visible range of wavelengths I = 400 A P = 13 kw V Ar = 35 l/min V H2 = 2 l/min V Ar,at. = 2 l/min V Prec. = 10 ml/min p = 70 hpa artifact artifact 10 mm to nozzle Figure 4: Emission spectrum of the process in the ultraviolet range of wavelengths While the concentration of the plasma gas species hydrogen and argon decrease constantly the concentration of silicon, carbon and C 2 -clusters increases to a maximum value before an exponential slope (linear decrease in the logarithmic plot). The increase is distinctive for silicon and carbon; their maximum values are reached at close distances to the nozzle exit (about 50 mm for silicon and 60 mm for carbon). The concentration of excited C 2 - Clusters increases and vanishes rather slightly with an maximum at about 70 mm. Even at a distance of 140 mm the C 2 concentration is nearly the same as directly at the nozzle exit.

5 Figure 5: Relative intensity of excited species dependent on the distance from the nozzle for a free expanding jet (fit curves; solid line: carbon, dashed lines: silicon) Figure 6: Relative intensity of excited species between nozzle and substrate surface When a substrate (distance to nozzle exit: 120 mm) hinders the free expansion of the plasma jet the development of the concentration of excited species changes significantly. The slope of the plasma gas species is steeper and the increase of the concentration of excited silicon and C 2 -clusters is less than for the free expanding plasma jet. The maximum of the relative concentration of species shifts closer to the nozzle exit (about 40 mm for silicon and 60 mm for C 2 ). An increase of the plasma power (17 kw) does not result in a significant change of the local distribution of the excited species concentration in the plasma jet. 4. Discussion No more species that are due to the precursor injection but silicon, carbon and C 2 - clusters are detected. The formation of C 2 -clusters has already been verified in low pressure plasma coating processes of SiC [6]. The same band spectra are detected in acetylene/oxygen flames as well.

6 As only atomic silicon and carbon, the formation of C 2 -clusters and a very small CHband are detected directly at the nozzle exit a complete defraction of HMDSO inside the nozzle can be concluded. At a distance of about 50 mm behind the nozzle exit the concentration of the excited silicon falls exponentially. This is due to a natural deactivation process without further excitement. Carbon shows a similar slope behavior as silicon. The C 2 - clusters also show an exponential decrease at nozzle distances exceeding 60 mm, which is especially obvious for the case of the hindered plasma jet expansion due to a substrate. The deposition efficiency depends on the concentration of excited species. That means, that the aim of further work has to be to influence the distance of the maximum concentration of excited silicon and carbon atoms in a way, that the substrate temperature will be less than its transformation temperature. As the concentration of excited species mainly depends on the transition probability to the ground state, the location of maximum concentration can be moved downstream by high gas velocities and higher injection velocities of the precursor. To examine the surface reactions the optics will be turned to focus on the substrate surface. By now it is not possible to ascertain whether the carbon atoms or the C 2 -clusters are dominant in the formation reaction of SiC. In most specimen no graphite is found by XRD. Therefore the excess of carbon (silicon/carbon ratio in HMDSO 1:3) results in the formation of amorphous carbon or leaves the process through the exhaust. Species due to the oxygen content in the precursor cannot be detected neither as atomic nor as molecular species like CO or as SiO. But as the oxygen content of the coatings is negligible it can be assumed that the oxygen leaves the process as OH, H 2 O, CO or CO Summary First results of the characterization of the D.C. plasma jet CVD synthesis of SiC coatings from the liquid precursor HMDSO show that the precursor is completely disintegrated inside the nozzle. In addition to atomic silicon and carbon C 2 -clusters occur as a result of the plasma chemical reactions inside the nozzle. The relative concentration of the excited species of the precursor products increase behind the nozzle exit as a consequence of activation by the hot plasma jet, before an exponential slope begins at a respective distance for all species of the precursor products. The C 2 -clusters show a high stability inside the plasma jet as their slope is much less steeper than for atomic silicon and carbon. All in all the acoustooptic emission spectroscopy proves to be a useful tool to display and investigate plasma jet CVD processes. Thereby a basis for the development of a process control is made possible. References [1] A. Klumpp, Sensors and Actuators A (1994), [2] N.V. Novikov, Diamond and Related Materials (1992), 1, [3] M. Lelogeais, Surface and Coatings Technology 48 (1991), [4] H. Windischmann, Journal of Vacuum Science and Technology A9 (1991), 4, [5] Y. Kojima, ISIJ International 35 (1995), 11, [6] P. Joeris, Surface and Coatings Technology 59 (1993),