Plasma processes and film deposition using tetraethoxysilane

Surface and Coatings Technology 131 Ž2000. 102᎐108

Plasma processes and film deposition using tetraethoxysilane M. Nothe ¨ a,U , H. Bolt b a

Forschungszentrum Julich, Institute for Materials and Processes in Energy Systems, D-52425 Julich, Germany ¨ ¨ b Max-Planck-Institut fur ¨ Plasmaphysik, D-85748 Garching, Germany

Abstract The fragmentation of tetraethoxysilane, SiŽC 2 H 5 O.4 ŽTEOS. has been investigated in a hollow cathode arc discharge plasma. Measurements of the molecular ions were performed with an energy dispersive mass spectrometer having an orifice in the plane of the substrate holder. During the measurements the plasma density and the electron temperature as well as the composition of the background plasma ŽAr, Arq He, Ar q N2 , Ar q C 2 H 2 , Ar q H 2 , H 2 q Ar. were systematically varied. The change of the molecular ion composition which mainly consisted of TEOS fragments was measured as a function of the parametrical changes. The results of these measurements were compared with the chemical composition, the deposition rate and the structure of films deposited under nearly identical conditions. A change in the magnetic field and thus a variation of the electron temperature caused a change of the film composition showing a significant increase in carbon content with an increase in electron temperature. It is assumed that this change in composition is caused by the increased production of lower molecular weight molecule fragments containing carbon with a high sticking coefficient. 䊚 2000 Elsevier Science B.V. All rights reserved. Keywords: Plasma process; Deposition; Tetraethoxysilane; Mass spectrum

1. Introduction The structure and quality of plasma deposited coatings is very strongly dependent on the specific choice and value of the process parameters. In a complex way these process parameters influence the plasma parameters and the plasma composition. This has effect on the deposition process via selective deposition of species from the plasma and surface processes. Thus the composition, the dominant chemical co-ordination, the presence of crystalline phases, and the porosity are results of these parametric dependencies. Hence the application of plasma processes for the deposition of such coatings relies on a proper process control. External parameters like gas composition, magnetic field and applied power give the opportunity to adjust plasma parameters. The chemical gas composition gives the possible reaction paths leading to decomposition and rearrangement of the precursor substances. The reacU

Corresponding author.

tion rates and the dominant reaction paths are determined by the activation energy provided by the plasma Ži.e. electron energy.. The spatial plasma density distribution determines the distribution of ions with a high sticking coefficient and thus the deposition efficiency and uniformity during coating deposition. The energy distribution of the species in the plasma and the potential of the substrate against the plasma has a major influence on the interaction of the impinging ions with the substrate and the surface reactions wmobility of surface species, fragmentation due to impact energy, Žpreferential . sputtering and ion implantationx and by this on the coating structure w1x. Compared to CH 4 and C 2 H 2 which are mostly used for a-C:H deposition often the precursor molecules for other PACVD processes are far more complex. The plasma processes and the deposition of films from tetraethoxysilane wTEOS, SiŽC 2 H 5 O.4 x have been studied in the present work as an example for a complex precursor molecule. Research on the plasma processes with TEOS has been performed with optical emission spectroscopy w2x and mass spectrometry w2,3x. In a re-

0257-8972r00r$ - see front matter 䊚 2000 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 0 . 0 0 7 4 7 - 7

M. Nothe, ¨ H. Bolt r Surface and Coatings Technology 131 (2000) 102᎐108

cent study the fragmentation of TEOS under defined electron impact has been investigated and a comparison was made with the behaviour of TEOS in a r.f.-discharge w4x. In the present work the plasma processes and the formation of molecular ions from TEOS precursor were investigated in a low pressure arc discharge under variation of the external parameters background plasma composition, plasma density and temperature, and applied magnetic field. Energy selective mass spectrometry was performed by measuring the ion flow at the substrate position. In parallel, deposition experiments were performed under comparable conditions and the film composition and film structure was determined. 2. Experimental In the present work the deposition of silicon containing coatings was investigated. The precursor substance used was tetraethoxysilane ŽTEOS, SiŽC 2 H 5 O.4 , Fig. 1.. The decomposition of the TEOS molecule in various background plasmas with variation of the applied magnetic field was measured with a plasma monitor system ŽBalzers PPM 421.. The results of the gas phase analysis were compared with the composition of Si-containing coatings deposited under similar conditions. Fig. 2 shows the plasma facility Plasma Engineering and Research Assembly ŽPETRA. with the plasma monitor mounted in the substrate plane. The plasma source is attached to a process chamber and a hollow cathode arc discharge is generated using a LaB 6 cathode as a thermal electron emitter. Within the process chamber two capillaries were used as inlets for reactive gases and TEOS. The anode plate at the end of the process chamber has a hole in the centre. The plasma monitor was mounted behind the anode plate on the axis of the process chamber. The tube containing the

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Fig. 1. Structural formula of TEOS Žtetraethoxysilane ..

cylindrical mirror energy analyzer and the quadrupole mass analyzer reached through the hole in the anode plate and the orifice of the spectrometer ended approximately at the substrate position. To reduce the influence of the magnetic field on the ion optics a ␮-metal shielding was build around the plasma monitor. The plasma was confined and shaped using the magnetic coils of the process chamber. A more detailed description of the plasma facility is given in w5x. The discharge current during the plasma monitor measurements was 20 A to avoid thermal damage to the spectrometer parts during the application of strong magnetic fields. For the deposition experiments a current of 30 A was applied. The electron densities were in the range of 10 10 ᎐10 12 cmy3 and the electron temperature was 5᎐10 eV. In the presented work six background gas mixtures were used: Ž1. Pure Ar with a flow of 100 sccm; Ž2. 140 sccm He mixed with 20 sccm Ar; Ž3. 70 sccm N2 with 35 sccm Ar; Ž4. 100 sccm Ar with 5 sccm C 2 H 2 ; Ž5. 100 sccm Ar mixed with 5 sccm H 2 ; and Ž6. 100 sccm H 2 with 20 sccm Ar. The resulting neutral gas pressure was in the range of 0.33᎐0.9 Pa. Homogenous magnetic fields of 0, 3, 6, 9 and 12 mT were used. An inhomogeneous magnetic field configu-

Fig. 2. Schematic drawing of the plasma facility PETRA.

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M. Nothe, ¨ H. Bolt r Surface and Coatings Technology 131 (2000) 102᎐108

Fig. 3. Correction factors for plasma monitor measurements in the presence of weak magnetic fields.

ration with a coil current of 40 A in the coil next to the plasma source and the other coil at no current was used as well as two magnetic cusp configurations with coil currents of 40 Ary20 A, respectively, 20 Ary10 A. For the operation of the shielded mass spectrometer under the applied external fields a calibration had to be performed. In general the suppression factor of the transmission of the ion optics in presence of a magnetic field is proportional to the square root of Žion chargerion mass. 3. To determine the correction factor for the reduced transmission mass spectra of neutral gas were

measured for all magnetic field configurations. The spectrum without magnetic field was divided by the other mass spectra. It is assumed that the correction factor is one for all masses higher than the lowest mass with an intensity ratio of one Žwithin the error of measurement.. The measured correction factors for lower masses Žexcept m - 3. were fitted using a potential fit curve. The measured correction factors of the masses 1᎐2 were used without the application of a fit procedure. The resulting correction factors for weak magnetic fields are shown in Fig. 3. Before the plasma monitor measurements the isotopic ratio of Ar 36 and Ar 40 was checked by a neutral gas measurement. After switching on the background gas plasma energy spectrum for the respective magnetic field configuration was measured to obtain the energy of the intensity maximum. Then the mass spectra of the background gas plasma were measured as reference using these determined energies. This procedure was repeated with plasma mixed with evaporated TEOS using a TEOS flow of approximately 1.1 sccm. For further processing the measured spectra with 64 Ptsramu were transformed to spectra with 1 Ptramu. As shown in Fig. 4 this transformation is performed for peaks with a width of more than 0.3 amu assuming that the particular integer mass number enclosed by the peak represents the mass of the measured species. The average of 5 Pts around the peak maximum is used in the transformed spectrum. If several integer masses are

Fig. 4. Transformation of measured mass spectra with 64 ptsramu to mass spectra with 1 ptsramu.

M. Nothe, ¨ H. Bolt r Surface and Coatings Technology 131 (2000) 102᎐108

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Fig. 5. Mass spectrum of Ar plasma mixed with TEOS without magnetic field.

included in one broader distribution of signals it was necessary to find the maxima manually. To determine the maximum intensity of very narrow or extensively subdivided signal distribution the widest gaps above and below the nominal mass on a 2 amu section were attributed to be the limits of the peak. The reference spectra were subtracted from the mass spectra with TEOS using: I s Im y Iref ) Ž Im 36rIref 36 .

䢇

䢇

䢇

䢇

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I: intensity of the TEOS spectrum after reference subtraction of the selected mass. Im : intensity of the selected mass from the measured TEOS spectrum. Iref : intensity of the selected mass from the reference spectrum. Im36 : intensity of Ar 36 from the measured TEOS spectrum. Iref36 : intensity of Ar 36 from the reference spectrum.

For comparison the spectra were normalized to a common ratio of Ar 36 intensity and Ar flow. The Si Ž100. substrates for the deposition of silicon containing coatings were cleaned using ultrasonic bath with ethanol and acetone and by plasma etching using a background gas mixture of 100 sccm Ar and 5 sccm H 2 . Following the pretreatment the background gas mixture selected for deposition was turned on. After the magnetic field was adjusted 11 sccm of evaporated TEOS were introduced into the plasma. The coatings deposited on the substrate were analyzed by SNMS ŽThermoquest SIMSLAB 410..

3. Results A mass spectrum of Ar plasma mixed with TEOS

without magnetic field applied is shown in Fig. 5. The masses found below 40 amu are molecular ions known mostly from methane plasmas w6x. The species found are of the composition CH x , C 2 H x and COH x . The peak intensity ratios at the upper border of the peak group approximately 30 amu do not match the isotopic ratios of Si. Therefore we have no evidence, that Si species without bonding to oxygen are formed. This matches the results of Foest w4x. For masses higher than 80 amu the TEOS fragment spectrum consists of a series of peak groups divided by gaps with no TEOS fragments and even undissociated TEOS is found Žmass 208.. The most intensive peaks of two subsequent species groups usually have a distance of approximately 12᎐16 amu. This supports the assumption that the fragments are formed by breaking of C᎐C, C᎐O or Si᎐O bonds. The species without an odd number of electrons are suppressed. This electron configuration is probably reached by the preferred abstraction of species containing a number of hydrogen atoms, that leads to an odd number of hydrogen atoms of the remaining fragment. Another mechanism can be the addition hydrogen atoms. In Fig. 6 a mass spectrum of TEOS in Ar plasma without magnetic field Žtop. is compared with the spectrum measured, if a magnetic field of 12 mT is applied Žbelow.. The TEOS molecules are dissociated more extensively, if the magnetic field is applied. The Si species remaining in the plasma almost exclusively consist of SiO x H y Ž xF 3.. In Fig. 7 the mass of the heaviest species found to be present in significant amounts are shown as function of the applied magnetic field. Starting from 0 up to 6 mT a fast decrease of the mass of the heaviest significant species is observed. With the further increase of the magnetic field this decrease of the mass of the heaviest species is less significant. As shown by Foest w4x the appearance potentials of the Si-containing TEOS fragments decrease with increasing mass of the fragment for the higher

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M. Nothe, ¨ H. Bolt r Surface and Coatings Technology 131 (2000) 102᎐108

Fig. 6. Mass spectrum of Ar plasma mixed with TEOS without magnetic field Žtop. compared with the spectrum under a magnetic field of 12 mT.

mass part of the dissociation spectrum. In our experiment the electron temperature increases with the strength of the applied magnetic field from approximately Te s 5 eV to approximately Te s 8 eV w7x. Thus, also, the population of higher energy electrons strongly increases with magnetic field and causes the enhanced fragmentation of the TEOS molecule by the dissociation also of the stronger molecular bonds. Measurements with different background gases showed in general only minor differences compared with Ar ŽFig. 8.. Of course the presence of reactive background gas components like nitrogen leads to the formation of TEOS fragments with N adatoms and the addition of acetylene increases some of the heavier polymeric C-species. Fig. 9 shows the comparison of Ar plasma mixed with TEOS and of hydrogen plasma mixed with Ar and TEOS. The addition of hydrogen improves the formation of the preferred electron configuration by the addition of hydrogen and therefore reduces further separation of carbon. In the low mass area the oxygen species with a low hydrogen content and the mono- and diatomic carbon species are suppressed. These results have to be compared with the results of the deposition experiments. In Fig. 10 the elemental composition of a-Si:C,O,N,H coatings produced by plasma deposition are compared with respect to the background gas composition and the applied magnetic field. The fact that nitrogen is only present in the coating with N2 among the background gas components needs no further discussion. The most outstanding feature is the reduced Si and the increased C content of the coating, if a magnetic field is applied. In absence of a magnetic field the C-atoms are dominantly bound in large TEOS fragments which are less dissociated than in the case of an applied magnetic field. It appears that during the impact of these large fragments onto the growing surface the carbon is Žpartly. removed. In addition, in the case without magnetic field the lower degree of dissociation leads to less

low mass carbon species which have a high sticking coefficient and thus to a lower deposition of carbon.

4. Conclusions External parameters of a PACVD process using a hollow cathode arc discharge have been varied systematically and the influence on the plasma processes have been studied by ion mass spectrometry. TEOS was introduced as precursor substance into background plasmas of Ar, and mixtures of Ar with H 2 , He, N2 , C 2 H 2 . The composition of the molecular ion fragment population changed, depending on the addition of H 2 , N2 and C 2 H 2 . Considerable change of the fragment population was found under the application of magnetic field, which is ascribed to an increase in electron temperature and thus a stronger dissociation under applied magnetic fields. Deposition of films from TEOS were carried out under conditions similar to the mass spectrometric

Fig. 7. Masses of the heaviest species found in significant amounts in Ar plasma mixed with TEOS under different magnetic fields.

M. Nothe, ¨ H. Bolt r Surface and Coatings Technology 131 (2000) 102᎐108

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Fig. 8. Comparison of the mass spectra using different background plasma gases mixed with TEOS.

experiments. It was found that the carbon content of the films increases with applied magnetic field. It ap-

pears that highly fragmented C x H y species which may be responsible for the carbon deposition are predomi-

Fig. 9. Comparison of Ar plasma mixed with TEOS and of hydrogen plasma mixed with Ar and TEOS.

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M. Nothe, ¨ H. Bolt r Surface and Coatings Technology 131 (2000) 102᎐108

Acknowledgements The authors appreciate the SNMS analyses of Dr Breuer, ZCH, Forschungszentrum Julich. ¨ References

Fig. 10. Elemental composition of coatings deposited from plasma mixed with TEOS.

nantly formed in a highly dissociative plasma of high electron temperature. Depending on the applied background plasma also the composition of the deposited films can be varied.

w1x R. Messier, A.P. Giri, R.A. Roy, J. Vac. Sci. Technol. A2 Ž1984. 500. w2x M. Latreche, Y. Segui, R. Delsol, P. Raynaud. Proc. 11th Int. Symp. on Plasma Chemistry, Loughborough, 22᎐27 August 1993, 1284. w3x C. Charles, P. Garcia, B. Grolleau, G. Turban, J. Vac. Sci. Technol. A 10 Ž1992. 4. w4x R. Foest, Massenspektrometrische Untersuchungen der Ionen und Neutralteilchen von RF-Entladungen mit siliziumorganischen Zumischungen, Dissertation, Greifswald, 1998. w5x H. Bolt, V. Hemel, H. Nickel, S. Tanaka, Surf. Coat. Technol. 74r75 Ž1995. 188. w6x A. Buuron, H. Bolt, Ph. Nizot, F. Koch, in: Proc. Surface Modification Technologies XI, Sept. 8᎐10, 1997, 1998 pp. 844᎐859. w7x A. Wirtz, Probe Measurements in a Hollow Cathode Arc Plasma, Diploma Thesis, FZ Julich, 1994. ¨