Spectroscopic Electron Temperature Measurement in Methane/helium Plasma During Diamond-like Carbon Coating

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1 248 Chiang Mai J. Sci. 2015; 42(1) Chiang Mai J. Sci. 2015; 42(1) : Contributed Paper Spectroscopic Electron Temperature Measurement in Methane/helium Plasma During Diamond-like Carbon Coating Artit Chingsungnoen*[a], Vittaya Amornkitbamrung [b] [a] Department of Physics, Faculty of Science, Mahasarakham University, Maha Sarakham 44150, Thailand. [b] Integrated Nanotechnology Research Center, Department of Physics, Faculty of Science, Khon Kaen University, Khon Kaen 40002, Thailand. *Author for correspondence; artit.c@msu.ac.th Received: 15 August 2013 Accepted: 29 January 2014 ABSTRACT The electron temperature (T e ) was extracted from the capacitively-coupled radio frequency (CCRF) plasma reactor by the spectral line intensity ratio method. The measured intensity ratio of 587.6/706.5 nm from CH 4 /He plasma was used to calculate the electron temperature under the steady state corona (SSC) approximation. Results were in good agreement with Langmuir probe measurement. This analysis method was then used to determine the electron temperature profile of CH 4 /He plasma. The increase in power density from ~980 to ~1715 mw/cm 2 resulted in a decrease in the electron temperature from ~0.93 ev to ~0.65 ev. The highest electron temperature appeared at the discharge radial center and ~4 mm from the RF powered electrode. From the radial electron temperature measurement, the temperature gradient inside the discharge region tended to decrease with the increase in power density, while in the axial direction, the electron temperature changed sharply near the powered electrode. Keywords: electron temperature, CCRF reactor, steady state corona approximation 1. INTRODUCTION Methane-containing plasmas are nowadays generally used for the production of amorphous carbon (a-c:h) films which are mainly deposited as protective layers, on a variety of substrates [1]. Of the various possible geometries, capacitively-coupled plasmas (CCP) which are generally sustained by radio-frequency (RF) excitation are preferred because they allow the deposition of uniform films over large areas [2, 3]. One of the physical parameters defining the state of a neutral gas in thermodynamic equilibrium is its temperature, which represents the mean translational energy of the particles in the system. In low-temperature RF discharges, plasmas are mainly characterized by the electron (T e ) temperature. The electron temperature corresponds to the translatory kinetic energy of the electrons that can excite atoms from the ground states to the excited

2 Chiang Mai J. Sci. 2015; 42(1) 249 states. This parameter plays a very important role for thin film growth mechanisms because the electron collisions with neutral gas molecules control most behavior of plasma [4]. Therefore, techniques for measuring the electron temperature are also essential for identifying the key chemical mechanisms that can link to the surface processes. These could provide a better understanding about the basic mechanism that leads to film formation. Optical emission spectroscopy (OES) techniques have been widely used for measurement of electron temperature [5] because they can probe from a long distance, without disturbing the light source. The OES techniques based on the spectral line intensity ratio methods are often used since they require only measurements of relative line intensity of spectral lines of the same atomic or ionic species. However, the appropriate models for extracting are generally dependent on the electron density. In the case when the electron density is too low to complete local thermal equilibrium (LTE) and none of the lines is affected by self-absorption [6, 7] the steady-state corona (SSC) model can be used. This means that all upward transitions can be considered collisional from ground state and all downward transitions are radiative. As a result, densities in various excited states are proportional to the products of statistical weight and the exponentials of the ratio of the electron thermal energy and excitation energy. Therefore, the electron temperature is directly calculated from inversion of the logarithm of the ratio of the total intensities of lines arising from different upper levels. In the present work, line intensity measurements are presented from RFgenerated methane/helium plasmas. The steady-state corona model is used to evaluate the electron temperature by using the line intensities of He I (λ = and nm). Results are compared with the RFcompensated Langmuir probe measurement. Spatially resolved spectral line intensity ratio was also measured to investigate the electron temperature distribution in the discharge area. The temperature profile along axial and radial directions could give useful information about the species concentration and the correlation with the thin film uniformity. 2. MATERIALS AND METHODS 2.1 Apparatus The setup used for recording the spectral line ratio and a Hiden Analytical RFcompensated Langmuir Probe inserted into the middle of the plasma, are shown in figure 1. The geometry of the CCRF reactor (MDC Vacuum Products, SSAC-12) consisted of a stainless steel chamber of internal diameter ~290 mm and height ~350 mm with a quartz window 150 mm in diameter, used for optical diagnostics. The spacing between the parallel-plate stainless steel electrodes was held at 20 mm for the experiments reported here. The plasma discharge chamber was pumped by a turbomolecular pump to a background pressure of 10-5 Pa. The CH 4 and He gases were introduced into the chamber and the MHz RF power was supplied to the lower electrode with the area of cm 2. The same-size upper electrode was grounded together with the wall of the reactor.

3 250 Chiang Mai J. Sci. 2015; 42(1) Figure 1. Schematic diagram of CCRF system and apparatus involved. In the present experiment, an optical spectrometer (getspec-3648-vis) is used for the analysis of optical emission from the plasma. The detector is a 3648 pixel charge-coupled device (CCD) array with 75 mm focal length. It can be used for measurements from nm for channel 1 and nm for channel 2, with spectral resolution of 0.07 nm FWHM. The local emission intensities are collected via a collimator lens (focal length 100 mm, 25.6 mm diameter) which is connected via a fiber optic (600 μm diameter) to an entrance aperture having a 10m slit width. The CCD accepts light transmitted through a single optical fiber and dispersed via a fixed 1200 grooves/mm holographic grating. The wavelength scale was calibrated with a standard H 2 light source and the spectral sensitivity of the whole system was measured with a spectral lamp of known color temperature. The Abel inversion method [8] was used to transform the measured intensity into the correct intensity. 2.2 Spectroscopic Electron Temperature Measurement When considering the atoms in the excited state, which decay spontaneously to a level under the emission radiation, the intensity of irradiation (radiant flux, measured in Watts/(cm 3 -steradian)) is defined by I ij = A ij E ij N j, (1) where N j is the population density (cm -3 ) in the state j, and E ij, A ij, are respectively the energy of the quantum of radiation and the Einstein coefficient (the probability per second) for spontaneous emission associated with the transition from upper to lower states. Therefore, the spectral line intensity ratio for the selected two emission lines which correspond to wavelengths λ ij and λ lk is measured as I ij E ij A ij N j (2) I = lk E lk A lk N k

4 Chiang Mai J. Sci. 2015; 42(1) 251 In low density plasmas and under thermodynamic equilibrium in which the plasmas stay in the steady-state corona (SSC) discharge, the population ratio can be subject to the Boltzmann distribution [9]: N j g j E jk N = k g exp - k T, (3) e where g i, g k and E jk are the statistical weights and the energy difference of the state j and state k in electron volts, respectively, and T e is the electron temperature in electron volts. By putting (3) into (2) and taking the natural logarithm, the electron temperature can be extracted from that expression. This is justified on the basis that the optical emissions of both spectral lines are proportional to the ground state population and the transitions are not radiation trapped. 2.3 Langmuir Probe Diagnosis The compensated Langmuir probe was a tungsten wire of radius mm with exposed length of 10 mm inserted into the plasma at the middle of the electrodes for collecting plasma current at the swept potential from -20 V to 50 V. In RF plasma, a usual problem in probe diagnostics is a change in the probe surface conditions due to the continuous sputtering of the RF electrode. As a result the probe sheath voltage may not correspond to the applied probe voltage. To ensure that the plasma being monitored did not contaminate its surface, the probe was continually cleaned at regular intervals. In this experiment, having intermediate-pressure and weakly ionized plasma, the conventional probe theory based on the thin collisionless sheath approximation was used. Under the assumption that the free electrons have a Maxwellian velocity distribution, the electron density and the average electron temperature can be obtained from [10] d(lni e ) -1, T ev dv (4) where I e is the electron current. In the case where the sheath in a plasma is larger than the probe radius, the collection of the charge particles by an attracting cylindrical probe is determined by their orbital motion [11]. For large sheaths and for large negative bias potential, a plot of the square of the ion saturation current versus bias voltage may be a straight line with a slope S that is related to the ion density (n i ) by equation [12] π Sm i n i = A p e 3/2 2, (5) where m i is the ion mass, A p is the exposure area of the probe tip, and e is the electronic charge of electron. For a quasi-neutral plasma, an ionized gas in which the electron density and ion density are present in approximately equal number, n e n i. 3. RESULTS AND DISCUSSION The overall discharge was investigated at the center between the power and grounded electrodes and emission spectra were recorded over the spectral range nm. Spectra were accumulated by using 100 ms integration time and 100 times averaging to provide stability of line intensities. The relative intensity of each line is determined directly when the background intensity has been subtracted. Figure 2 shows the principle features of helium and methane/helium emission spectra together with identification of the dominant species under discharge conditions of gas pressure of 400 Pa, power density of 980 mw/cm 2, and electrode gap of 20 mm. It is apparent that the most prominent helium

5 252 Chiang Mai J. Sci. 2015; 42(1) emission lines in the visible range were generated from transitions ending at the metastable levels 2 1 S, 2 1 P (singlet state) and 2 3 S, 2 3 P (triplet state). One emission band from CH radicals together with the atomic hydrogen Balmer series can also be observed. Figure 5 shows the electron temperature and electron density as a function of power density obtained from CH 4 /He plasma. As seen in figure 3, the ln(i e ) versus V characteristic has the property that the contribution of Maxwellian electrons to I e is proportional to exp(-ev/t ev ) for sufficiently positive V. Therefore, it is possible to determine the bulk-electron temperature T e by measuring the slope of the Maxwellian lines of free electrons in the transition region. With the increase in RF power the slopes of Maxwellian lines also increase. This means that the average electron energy decreases with the increase of power consumption. A deviation from linearity of this semi-logarithmic plot also indicates deviation of EEDF from Maxwellian, and in such a case it may be misleading to refer to an electron temperature. Under the assumption of quasi-neutral plasma and using the OML technique of Pletnev and Laframboise [13], the plasma electron density n e or ion density n i are proportional to the square root of a slope S in the ion saturation region, as shown in figure 4. With the analysis shown in figures 3 and 4, the electron temperature and electron density as a function of power density can be obtained. The result is shown in figures 5. It is seen that with the increase of power density from 590 mw/cm 2 to 1760 mw/cm 2, the n e increases from cm -3 to cm -3 while the T e decreases from 1.7 ev to 0.8 ev. The change in slope of the T e in the higher power density regime indicates that the discharge maintenance is changed from the RF-oscillatory movement of the sheath to the electrons emitted from the electrodes under ion bombardment. Figure 2. Typical emission spectra obtained in pure helium and in CH 4 /He plasmas.

6 Chiang Mai J. Sci. 2015; 42(1) 253 Figure 3. Maxwellian line of the free electrons in a semi-logarithmic I-V plot. Figure 4. A plot of the square of ion current versus probe voltage in the ion saturation region. Figure 5. The electron temperature and electron density as a function of power density.

7 254 Chiang Mai J. Sci. 2015; 42(1) Figure 6. Spectral line intensities of (a) 412.1, (b) 443.8, (c) 471.3, (d) 504.8, (e) 667.8, and (f) nm as a function of radial (-57.5 R 57.5 mm) and vertical (0 Z 20 mm) positions. Figure 7. Comparing the excitation temperature obtained from the SSC model of 587.6/ nm ( ), with the electron temperature obtained from a probe method ( ) as a function of power density for considered pressures. From now on, the temperatures obtained under the SSC approximation and measured by a single Langmuir probe will be assigned as T e SSC and T e prob, respectively. For checking the validity of the selected line ratios, these three temperatures were compared as a function of power density with different gas pressures (0.13, 0.40, 0.67, 0.93, 1.12, and 1.47 kpa). As shown in figures 7 (a) - (f), the T e SSC is in good agreement with a single Langmuir probe method over the experimental conditions used. It decreases as the power

8 Chiang Mai J. Sci. 2015; 42(1) 255 density increases, in agreement with the results of Ray and Chaudhury [14]. The close relation of T e SSC and T e prob observed in this experiment indicates that the electron temperature obtained from SSC model of 587.6/706.5 nm can be used in conditions for which the original excitation temperature dominates. The next purpose of this work is the quantitative determination of electron temperature profile from the emission line identifications. With regards to T e SSC, the spectral line intensity ratio of 587.6/706.5 nm was then applied for measurement of the electron temperature in CH 4 /He plasmas. For observing radial profile, the collimator lens or optical probe was scanned from the left hand side (r = mm) to the end of the other side (r = 57.5 mm) with step increment of 5 mm. In the vertical axis, it was started from mm to mm with the incremental steps of 2 mm, where represents distance from RF electrode. In order to investigate the role of power density in the electron temperature profile, the power densities were increased from 980 to 1715 mw/cm 2 with the step increment of 245 mw/cm 2. All other discharge parameters such as gas pressure, total gas flow, and electrode gap were maintained constant (at Pa, 60 sccm and 20 mm) during the entire experiment. In order to avoid missing data near the plasma sheath boundaries, sets of data at and mm were omitted. Figure 8. Excitation temperature profiles with power densities of (a) 980 mw/cm 2, (b) 1225 mw/cm 2, (c) 1470 mw/cm 2, and (d) 1715 mw/cm 2. As shown in figure 8, the highest intensity appears near the discharge center above the power electrode around z = 4 mm. This result shows that the emission intensity profile is generally stronger in the center of the electrode gap than at the electrode edge and tends to appear nearer the power electrode due to the asymmetrical capacitively coupled discharge in which a large, self-bias voltage develops on the power electrode with respect to the plasma. This potential gives a higher electric field near the plasma/sheath boundary above the power electrode. As a result, a greater discharge occurs around this

9 256 Chiang Mai J. Sci. 2015; 42(1) area. The increase of power density leads to a decreasing of the T e from 0.93 ev to 0.65 ev. It decreases by more than 60% near the electrode edge and consequently high temperature gradients occur near these areas. The temperature gradient inside the discharge region tends to decrease with the increase of power density and/or electron density. This means that the temperature uniformity is expandable which certainly affects the film growth since it will shorten the distance between the growing film surface and the point where film precursors are initially produced. 4. CONCLUSIONS In conclusion, spectroscopic temperature measurement has been performed in the methane/helium discharge by spectral line intensity ratio method. The steady-state corona model is used to extract the electron temperature from the absolute line intensity ratio of 587.6/706.5 nm. The electron temperature obtained from this method was compared with the electrostatic probe method. It is found that the spectral line ratio of 587.6/706.5 nm under the SSC approximation gives the electron temperature in good agreement with a single Langmuir probe. The increase of power density leads to a decrease in the electron temperature. This result can be explained by more RF power generating more fast electrons. As a result, many species will transit into excited states or ionic states depending on the energy of electron impact and the threshold energy of reaction. There are many new electrons that can reduce the average absorbed energy. This is the reason why the electron energy detected by probe tends to decrease with power. For spatially resolved T e, we found that the highest appeared near the discharge center around 4 mm above the power electrode and the temperature gradient inside the discharge region tends to decrease with the increase of power density; as a result the temperature uniformity is enhanced and expanded along the radial direction. In the axial direction, the electron temperature has to change sharply near the power electrode due to a large negative self-bias voltage developing on the power electrode with respect to the plasma. The temperature gradient can give a picture of plasma potential because this potential tends to retain the electrons leaving the plasma by attracting them back into the plasma. The shape of the temperature profile which corresponds to the regions with the highest density of electrons, having the appropriate minimum energy to generate a particular species, gives an important indication of the uniformity of coatings that will be produced from methane-containing plasmas. ACKNOWLEDGEMENTS This work has been partially supported by the Thailand Research Fund (MRG ) and the National Nanotechnology Center (NANOTEC), NSTDA, Ministry of Science and Technology, Thailand, through its program of Center of Excellence Network. REFERENCES [1] Elidiane C.R., Nilson C.daC., Milton E.K., Rita C.C.R., Nazir M. and Steven F.D., Optical and electrical properties of polymerizing plasmas and their correlation with DLC film properties, Plasmas Polym., 2004; 9: DOI /B:PAPO c6. [2] Roth J.R., Industrial Plasma Engineering vol. 1 Principles, IOP Publishing, 1995; [3] Smith D.L., Thin-Film Deposition: Principles and Practice, McGraw-Hill, New York, 1995;

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