The effect of self-absorption in hollow cathode lamp on its temperature

Plasma Science and Applications (ICPSA 2013) International Journal of Modern Physics: Conference Series Vol. 32 (2014) 1460349 (9 pages) The Author DOI: 10.1142/S2010194514603494 The effect of self-absorption in hollow cathode lamp on its temperature Samad Sobhanian Department of atomic and molecular physics, East Azerbaijan Science and Research Branch, Islamic Azad University, Tabriz, Iran sobhanian@tabrizu.ac.ir Hamid Naghshara Department of solid state physics, University of Tabriz, Tabriz, Iran naghshara@tabrizu.ac.ir Published 13 August 2014 It has been shown experimentally that even a small error in the calculation of the temperature inside the hollow-cathode lamp (HCL) and the current applied to the lamp, may cause a tremendous error in determination of the absorption ratio in optical resonance absorption (ORA) method. This effect is intensified nonlinearity for large absorption ratios. If a higher current is applied to a copper hollow cathode lamp, the copper density inside the lamp is increasing rapidly. Due to the cylindrical (axisymmetric) form of the lamp, the density of atoms around the main axis of the lamp becomes greater than that near the internal wall. In this case the auto-absorption (or self-absorption) is occurred and as its result, the emission spectrum produced by copper atoms is locally absorbed before going out from the lamp. This absorption is stronger near the main axis compared with the areas near the wall because of the Gaussian profile of the spectral line. Two different Cu atoms ground state lines with the similar lower state (327.4 nm and 324.7 nm) are used in this work as optical resonance absorption and the absorption coefficient is obtained for three different pressures (0.6, 4.5 and 14 µbar). The best values for copper HCL temperature and for maximum HCL current were found respectively 450 K, and 5mA. Keywords: Optical resonance absorption; hollow-cathode lamp; magnetron sputtering. 1. Introduction Thin film deposition is evidently an important key in recent micro-electronic technology. 1,2 Beside this technique is widely used for surface modification and surface treatment in semiconductor industries. 3,4 Low temperature plasmas are used for plasma sputtering. Especially in magnetron discharges; the magnetic field produced by permanent magnets behind the target (cathode) helps to confine the plasma near the target area. In DC plasma sputtering devices, plasma ions, normally from noble gases like argon, are accelerated towards the target which is used as the cathode for the system. The This is an Open Access article published by World Scientific Publishing Company. It is distributed under the terms of the Creative Commons Attribution 3.0 (CC-BY) License. Further distribution of this work is permitted, provided the original work is properly cited. 1460349-1

S. Sobhanian and H. Naghshara sputtered atoms from the target then are directed towards the substrate and are deposited on its surface. Thus, one can realize thin films of different conducting materials using magnetron plasma sputtering system. The exact knowledge of the sputtering yield and its variation with plasma s main parameter and also the number of sputtered atoms reaching the substrate is a very important problem. One of the most applied method to diagnose the plasma and determine the sputtered atom density is so-called resonance optical absorption (ROA). 5, 6 Having information on the sputtering yield, one can optimize the deposition rate with a great precision by controlling the number of atoms moving toward the substrate and by adjusting the gas pressure. Diffusion of the sputtered atoms towards the chamber walls decreases the deposition rate. Beside the vacuum chamber walls are polluted rapidly and some fraction of the input energy and also the working gas is lost by this unwanted diffusion. This will be very important specially if the target material is expensive. Thus the knowledge of total number of atoms moving in the plasma is necessary for adjusting plasma operation conditions. There are many technique for measuring the number of metal atoms or ions in plasma sputtering devices, Laser absorption (LA), 7 Laser induced fluorescent (LIF), 8 Optical absorption spectroscopy (OAS) 9 and Resonance optical absorption (ROA). Resonance optical absorption (ROA) is among the others the simplest and cheapest way. In this method, only a hollow cathode lamp (HCL) is used as the light source. Using this method one can determine the absolute (total) density of particles absorbing the photons (the density of atoms in the ground state and metastable state). Here for the case of copper target we use a copper hollow cathode lamp. The quantity Io/Ii, the ratio of the output to input light intensities from HCL at dedicated line is very sensitive to the gas temperature. If the applied current to HCL is exceed some given high value, the copper atoms density inside the lamp will be also increased rapidly. In this case because of the cylindrical symmetry of the lamp, the density of atoms near the principal axis of the lamp wills higher than near the internal walls of the copper cylinder inside the HCL. At this point a phenomena known as the auto-absorption or self-absorption is occurred during which the produced emission spectrum of copper is reabsorbed by next copper atoms before they leave the HCL. This absorption will be stronger in the middle pail of the cylinder since there are more Cu atoms. In section II, the experimental arrangement for the ROA of Cu atoms density measurement is given and in section III the effect of self-absorption is explained and the effect of internal temperature of HCL and lamp current is clarified. A brief conclusion from the experimental results will be finally presented. 2. Experimental The resonance optical absorption (ROA) method is used to measure the density of atoms sputtered in plasma sputtering unit. The main system, MECA-2000 is co-sputtering (DC & RF) plasma source in operation at Thin Film Deposition laboratory of Tabriz University. The vacuum chamber is a cylinder of 60 cm in diameter and 50 cm height made of stainless steel. This system has two separate magnetron using water cooled metallic target 1460349-2

The effect of self-absorptionin hollow cathode lamp of 7.5 cm diameter. One of the magnetrons is connected to a 13.56 MHz RF source via a matching box and the other one to a DC supply. The number of magnets used in each magnetron is 22 and the magnetic field produced by these magnets on the surface of the target is 40mT. In this research we used the DC magnetron and a target made of 99% copper. The chamber is kept by an Alkatel turbomolecular pump (model ATP900) at a pressure of ~5x10-5 mbar. Argon (99.999%) and nitrogen (99%) gases are used as the working gases in our experiment. The pressure range was 0.3 mbar- 14 mbar. To measure the gas temperature from nitrogen molecular rotational temperature, an admixture of 5% nitrogen is used. The HV power supply used in this system can produce a DC 1000 Volts with the final power of 1 KW. The substrate holder has a diameter of 10 cm and is located at 18 cm from the target. The deposition rate on the substrate is controlled and measured by a quartz micro balance cell. The final precision for the deposition rate measurement is 0.1 A/sec. The vacuum chamber has several observation ports and windows for spectroscopic purposes. The detailed description of the experimental setup is given in the Ref. 10 and 11. Fig. 1. Experimental setup. There exist possibility in the system to heat the substrate holder up to 500 C and one can locate simultaneously 6 substrates on the holder. Each can be brought in front of the target by a motor driven from outside. The experimental layout is shown in Fig. 1. As shown in this figure, the light from the HCL collimated by a quartz lens with a focal length of 10 cm enters the vacuum chamber through a quartz window (6) and after passing through a 5 cm tube (9) enters the plasma medium. The copper atoms existing inside the plasma, absorbs some percentages of the light. On the other hand, the copper atoms themselves emit similar spectral light. The remaining light from HCL together with the light produced by Cu atoms inside the 1460349-3

S. Sobhanian and H. Naghshara vacuum chamber after passing the 35 cm tube (9) enters the monochromator (10), through a second quartz lens with 5 cm focal distance. The spectral light is separated inside the monochromator from other lights and enters the photomultiplier tube (11). The detector counts the number of arriving photons and sends counts via RS-232 interface to the computer (12). The turbomolecular pump is connected to the chamber via the port (15). The unit (13) in the figure is a pulse generator that triggers signal for the setup. The power supply provides a 650 DC voltage (4) for supplying of the HCL with a ballast resistor in order to control the current. The detail of the method and required formulation for measuring Cu atoms density are given in Ref. 10 and 11. The temperature of Cu atoms inside the plasma is supposed to be equal to the working gas (Ar-N2) temperature. Thus the gas temperature is measured via N 2 molecule rotational emission light. The result of the measured variation of the number density with the input power and for three different pressures (0.3 µbar, 4.5 µbar and 14 µbar) are given in the reference 10,11 for Cu atoms in ground state and also in metastable state. For the ground state of Cu, the 327.4 nm line and for the metastable state the 510.6 nm lines are used. 3. Gas temperature As it has been pointed out earlier, the gas temperature is obtained by adding 5% nitrogen to the working gas. For this purpose, the experimental arrangement of Fig. 2 is used. The result for gas temperature for different applied magnetron powers and for three Fig. 2. Gas temperature measurement. pressures (0.6, 4.5 and 14 µbar) are shown in Fig. 3. As expected we see from this figure that by increasing the applied power to magnetron, the gas temperature is increased, but it decreases by decreasing the gas pressure. As an example, the results for the variation of 1460349-4

The effect of self-absorptionin hollow cathode lamp Cu atoms density with the input power for a pressure of 4.5µbar and also the variation of the deposition rate with the applied power is shown in Fig. 4. Fig. 3. Temperatures as a function of magnetron power. One can conclude that by increasing the pressure the mean free path of particles decreases as λ = 1 2πnd 2, thus leading to an increase in the collision frequency, so the number of atoms excited to metastable state is also increased. We tried to see the effect of the addition of nitrogen gas to the working gas (argon) temperature. The results are shown in Fig. 5. Fig. 4. Variation of density with power for 4.5 µbar. As is seen from this figure, the addition of nitrogen gas, especially for higher pressures causes the reduction in the working gas temperature. This phenomenon is due 1460349-5

S. Sobhanian and H. Naghshara to the molecular nature of the nitrogen. The rotational levels of the nitrogen molecule in collision with plasma electrons can acquire some energy from them and the corresponding Boltzmann equilibrium will give a higher temperature. Fig. 5. Temperatures vs. nitrogen percentage. Since there is equilibrium and balance between the rotational and translation degree of freedom of the molecule, some fraction of the additional rotational energy goes to the translation motion. So by addition of some percentages of nitrogen to the working gas, we will have some increase in the temperature. Fig. 6. Absorption for different Te. 1460349-6

The effect of self-absorptionin hollow cathode lamp The profile of lines emitted by of Cu atoms of HCL is considered to be Gaussian. As it can be seen from Fig. 6, an error would be generated in the exact internal temperature of the lamp when is not used and this may cause a major error in the calculation of I o I i. Fig. 7. Measured density for different lamp current. If the applied current to HCL exceeds the normal value, then Cu atoms density inside the lamp will increase and because of the cylindrical symmetry of the lamp, the density around the main axis becomes greater than that near the internal wall of the copper cylinder inside the HCL. Here the phenomenon called self-absorption (or auto absorption) takes place. In this process, the emission spectra produced by Cu atoms are before leaving the HCL reabsorbed by other Cu atoms, but this absorption is stronger near the middle part of the cylinder, since the density of Cu atoms is higher. Fig. 8. Current influence on absorption rate in this part. 1460349-7

S. Sobhanian and H. Naghshara This reabsorption distorts the Gaussian profile of the spectral line, because the Cu atoms inside the lamp are not isothermal. So, the calculated internal temperature of the lamp, considering the pro-gaussian profile of the line will give the true temperature. Figure7 shows the difference in the calculated density for two HCL currents 5 ma and 12 ma in a pressure of 14 µbar. This diagram belongs to Cu atoms in the ground state. Giving attention to this curve, we note that for low absorption rates where Cu atom density is decreased, the obtained results for two currents emerge to one similar value, but in the case of high absorption rates, the difference becomes relatively large. This is consistent with the result shown in Fig. 6. So, one must be careful in applying the current to HCL. To find the appropriate current to be used in HCL we carried out an experiment where for a high absorption value inside the plasma, the lamp current is reduced gradually. As Fig. 8 shows the absorption becomes very low and constant for the lamp currents smaller than 5 ma. 4. The effective temperature inside the HCL Considering the above mentioned facts one can conclude that the substitution of the exact value for HCL lamp temperature is very important and major errors might be resulted in the calculated values, especially for high absorption rates. One common method to measure the temperature inside the lamp is to repeat the ROA method used in the experiment for finding the density of atoms for the same conditions and parameters. In the first step, the light from the HCL is passed through the plasma, but in the second stage, the outgoing light is reflected by a mirror and is passed again through the plasma. In these two cases all conditions are similar; except that the absorption length is doubled in the second stage. Thus by doing calculations and solving the equation for the lamp temperature, one can obtain the real temperature. This is a difficult way to determine the effective temperature. To avoid this difficulty we used here the following simple method. Using the experimental arrangement of Fig. 1, we considered two spectral lines of wavelengths 327.4 nm and 324.8 nm. Since the population of Cu atoms in the lower level of these two lines are the same and only their oscillation strengths are different, we should obtain the same results for the number density of Cu atoms in the ground state if we use the correct internal temperature of the HCL. The absorption coefficient is defined as A = 1 I o I i where the values close to zero represents low absorption. To obtain the most appropriate value for the internal temperature of HCL, the quantity A(324)-A(327) is plotted against A(327) for same estimated (test) temperatures in figure 9. The experimental values for three different pressure (0.6, 4.5 and 14 µbar) are then superimposed on them. By careful study of this figure the best fitted value for the gas temperature of the HCL is found as 450 K. 1460349-8

The effect of self-absorptionin hollow cathode lamp 5. Conclusion As it is important to use the exact value of the temperature inside the HCL to get the density of Cu atoms in ground state or exited metastable state moving inside the plasma and thus affecting the deposition rate. Using two main spectral lines of copper in ground states (327 and 324 nm lines), we could calculate the absorption fraction. By fitting the experimental values the absorbed fraction A for different pressures; a value around 450 K is obtained for the temperature of HCL. References 1. Liberman M.A. and Lichtenberg A.J., Principles of Plasma Discharges and Material Processing, (John Wiley & Sons, New York, 1994). 2. Lerner Eric, (The Industrial Physicist, AIP, 1999), pp 18-21. 3. Reece J., Industrial Plasma Engineering, (Institute of Physics Publishing, Bristol, 1995). 4. Mia L., Tanemura S., Watanabe H., Mori Y., Kaneko K. and Tahshoichi, Journal of Crystal Growth 260, (2004). 5. Mitchell A.C.G. and Zemansky M. W., Resonance Radiation and Exited Atoms, (Cambridge University Press, London, 1971). 6. Kang N., Oh S., Gaboriau F. and Ricard A., Rev. Sci. Instrument 81, 013102 (2010). 7. Horikana Y., Kunihara K. and Sasaki K., Journal of Physics: Conference Series 227 (1), 012012 (2010). 8. Britum N., Gaillard M. and Han J.G., J. of Phys. D: Applied Physics 41, 185201(2008). 9. Xu L., Sadeghi N., Donnelly V.M. and Economou D.J., Journal of Applied Physics 101, 013304 (2007). 10. Naghshara H., Ph-D Thesis, Dept. of Atomic and Molecular Physics, Tabriz University, Tabriz, IRAN. 11. Naghshara H., Shobhanian S., Khorram S. and Sadeghi N., J. of Phys. D: Applied Physics 44, 025202 (2011). 1460349-9