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Measurement of total energy flux density at a substrate during TiOx thin film deposition by using a plasma jet system
Vacuum 83 (2009) 738–744
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Measurement of total energy flux density at a substrate during TiOx thin film deposition by using a plasma jet system ˇ . Kment, Z. Hubicˇka *, P. Virostko, S M. Cada Institute of Physics of the ASCR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic
a b s t r a c t Keywords: Hollow cathode Plasma jet Sputtering Pulsed DC Energy influx on substrate TiO2
The total energy flux density delivered to an electrically isolated substrate in a low-pressure pulsed DC hollow cathode plasma jet sputtering system during TiO2 thin film deposition has been quantified. The plasma source was operated in constant average current mode and in a mixture of argon and oxygen or only in pure argon working gas. A titanium nozzle served as the hollow cathode. The total energy flux density measurements were made using a planar calorimeter probe. The main results from the calorimeter probe showed clearly that the total energy flux density at the electrically isolated substrate decreases significantly with duty cycle from 100% (DC mode) to 10% at a given pulsing frequency 2.5 kHz. A local maximum at duty cycle 60% for only pure argon operation has been observed. In addition, the voltage waveforms on the hollow cathode and before the ballast resistor have been saved for pulsed DC measurements for both pure argon and argon þ oxygen mixture. A similar transient phenomenon on the cathode voltage and discharge current as observed recently in mid-frequency pulsed DC magnetron discharge has been discovered in the hollow cathode plasma jet sputtering system. We can conclude from these preliminary measurements that the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving of the same plasma system lies in the possibility to reduce or to increase energy influx on the floating substrate within the change of duty cycle. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Thin film deposition utilizing a sputtering technique is a widely used method in semiconductor industry, for the preparation of tribological and optical coatings and for the modification of surfaces. In addition to well-known magnetron sputtering utilizing the planar cathode [1], alternative sputtering techniques, as in hollow cathode plasma jet system, have been developed [2]. The lowpressure hollow cathode plasma jet sputtering system has been successfully used for the deposition of different types of thin films. For example, perpendicularly oriented ZnO thin films [3] produced during reactive sputtering of metallic Zn hollow cathode in DC or RF discharge, PbZrxTi1xO3 (PZT) thin films deposited on polymer substrates during reactive sputtering of cathode made of PZT ceramics in an RF pulse-modulated discharge [4] or BaxSr1xTiO3 (BSTO) dielectric films reactively deposited by double hollow cathode RF pulse-modulated plasma jet system using two cathodes made of BaTiO3 and SrTiO3 ceramics inserted into a single nozzle or into two separate nozzles [5]. Recently, the hollow cathode plasma jet deposition system has been used for the preparation of nanoscaled TiO2 photoactive catalytic thin films [6]. Besides thin film * Corresponding author. Tel.: þ420 266 052 995; fax: þ420 286 581 448. E-mail address: [email protected] (M. Cada). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.05.014
deposition, the plasma diagnostics by means of Langmuir probe and optical emission spectroscopy in the hollow cathode plasma jets have been carried out. The pulsed DC-driven magnetron discharge was introduced almost 20 years ago as a possible solution for arcing suppression [7]. Lately, the pulsed DC or pulse-modulated driving of the cathode voltage in the magnetron sputtering systems is becoming a more often used method in comparison with DC or CW driving of the cathode voltage. This results in production of thin films of better quality, improves discharge stability and enables us to prepare thin films with new properties, which cannot be deposited by conventional sputtering techniques in required quality [8]. The pulsed DC or pulse-modulated RF driving of the hollow cathode discharge has been used during deposition of PZT, BSTO and TiO2 thin films. Introducing pulse driving of discharge results in much more effective sputtering of ceramic cathodes, increasing of mean power delivered to discharge without overheating of the hollow cathode, significant reduction of substrate heating, enhancement of thin film quality and last but not least significant enhancement in plasma parameters (such as electron temperature, plasma density, plasma potential, etc.) [5,9]. Therefore, these facts demonstrate the necessity to investigate plasma parameters in connection with new thin film properties prepared by pulsed discharges.
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One of the plasma parameters representing relation between plasma and thin film properties is the total energy flux density (TEFD) at the substrate. This parameter comprises the particle flux towards the substrate, by the thermal radiation flux from plasma photons, by the chemical reactions at the substrate and by the elementary processes taking place at the substrate. On the contrary, there are also cooling effects caused by thin film re-sputtering, a secondary emission or substrate thermal radiation [10]. Measurement of the TEFD at the substrate is then the basic method to characterize the interaction between plasma and substrate – the energy balance at substrate. The most recent results of TEFD measurements in a mid-frequency pulsed DC magnetron discharge equipped with titanium target showed a significant influence of pulse frequency and duty cycle on energy conditions on a substrate [11]. Our investigation presented in this paper deals with preliminary measurements of the total energy flux density at the substrate during the deposition process of TiO2 thin films in the pulsed DC hollow cathode plasma jet sputtering system by using a calorimeter probe. Our study is focused on the characterization of energy conditions at the floating substrate for various duty cycles at pulsing frequency typical for deposition process.
2. Experiment The low-pressure plasma jet configuration for TiO2 thin film deposition is shown in Fig. 1. The UHV reactor chamber was continuously pumped by a turbo-molecular pump backed by combination of roots and rotary vane pumps. After achieving a base pressure of <105 Pa, the chamber was backfilled with pure argon (flow rate 120 sccm) or with a mixture of argon and oxygen (flow rates of 120 sccm and 60 sccm, respectively) to an operating pressure of 2.8 Pa or 7.3 Pa, respectively. The cylindrical nozzle placed on the top of the chamber was made-up of pure titanium and acted as RF hollow cathode and simultaneously as inlet nozzle for argon gas only. The internal diameter of the nozzle was 5 mm and the length of the nozzle was 30 mm. The oxygen gas was fed into the
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reactor chamber via a separate port located sideways. This configuration ensured that the inner surface of the titanium nozzle was not covered by a TiO2 thin film and then the sputtering system was able to reach higher deposition rates. The hollow cathode plasma jet discharge was driven using a pulsed DC and an RF (operating at 13.56 MHz) power supply operating simultaneously. The pulsed DC supply comprises an Advanced Energy MDX-1K (Umax ¼ 1 kV and Imax ¼ 1 A) and a pulse modulator with high input parallel capacity. A ballast resistor of 177 U was inserted between pulsed DC source and hollow cathode to stabilize a discharge current. The RF power was kept on 3 W due to better stabilization of the pulsed DC discharge. After ignition, an intensive DC hollow cathode discharge was generated inside the nozzle, on the background of the capacitive RF discharge excited in the volume of the reactor. Incoming working gas forced this DC hollow cathode discharge out of the nozzle into the reactor chamber. The calorimeter probe was placed perpendicularly to the plasma jet axis at a distance of 50 mm from the hollow cathode outlet. A schematic diagram of the calorimeter probe, as fitted to the ceramic block made of Lava (a type of ceramic), is shown in Fig. 1. This configuration ensured that: (1) the probe can stay at the floating potential or can be independently biased and (2) the loss of heat through conduction was minimized. The calorimeter probe consists of 8 mm diameter by 2 mm thick copper disc. The tip consisting of a spherical junction (0.4 mm in diameter) of a K-type thermocouple was soldered into a slot machined into the center of the copper disc. The solder (a silver alloy) was chosen as it had both a thermal conductivity and specific heat similar to that of copper. The calorimeter probe construction was designed to minimize thermal losses by conduction, and so to ensure the temperature measurements are as precise as possible. The thermal capacity of the probe was calculated using CP ¼ m cP, where m ¼ 0.803 g is the mass of the probe and cP is the specific heat of copper, which yielded a value of 0.31 J K1 with maximal error about 10%. The method used for determination of the heat transfer to the substrate is based on the measurement of temporal evolution of a small calorimeter probe located at the position of the substrate [10]. This method is in detail described elsewhere and from previous work the effective total energy flux density is determined with maximum error of w5% [11]. Digital sampling oscilloscope Agilent 54830B has been used for acquisition of voltage waveforms measured before and behind the ballast resistor – see Fig. 1. The discharge current has been then calculated from the voltage drop on the ballast resistor. 3. Results The TEFD has been calculated according to Eq. (1):
P ¼
Fig. 1. Experimental arrangement.
CP dT S dt TS
(1)
where S is the area of calorimetric probe, TS is a temperature not more than 40 C and ðdT=dtÞTS has been replaced by the slope of a linear fit of periodically measured probe temperatures during the first 60 s after stabilization of the discharge. Previous detailed studies of calorimeter probe technique revealed that probe temperature increases linearly with time with a maximum error of 5% up to a probe temperature of about 40 C. The first set of measurements were made for plasma jet driving by DC voltage only with power supply set in constant current mode for pure argon as working gas. The measured values of total energy flux density (TEFD) of a floating substrate at a distance 50 mm from titanium nozzle outlet and at constant pressure 2.8 Pa are plotted
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versus discharge current in the range from 100 mA to 600 mA in Fig. 2. Furthermore, the power density delivered into the discharge has been calculated and ratio between the plasma power density and the TEFD has been determined and is plotted in Fig. 2. The calculated ratio enables us to compare how big part of the discharge power density is transferred into the substrate for different plasma conditions. The TEFD values measured for constant discharge current 500 mA are plotted in Fig. 3 versus working pressure in the range from 1 Pa to 10 Pa. Further measurements have been carried out for a constant pulsing frequency 2.5 kHz and different duty cycles in the range from 10% to 90%. The pulsing frequency of 2.5 kHz has been set for reason of its usage during TiO2 thin film deposition. In Fig. 4, a plot of TEFD versus duty cycle for pure argon working gas (working pressure 2.8 Pa, mean discharge current 500 mA) is shown. The value for duty cycle 10% is not shown here because strong interference of thermocouple transducer by pulsing discharge did not allow us to reliably determine a temperature course with time. A plot of TEFD versus duty cycle for the same experimental conditions but for the working gas mixture of argon and oxygen with the working gas pressure 7.3 Pa is shown in Fig. 5. During the calorimeter probe measurements, voltage waveforms on the hollow cathode and before the ballast resistor have been saved for only pulsed DC measurements for both pure argon and argon þ oxygen mixture. As an example, the waveform of cathode voltage and pulsed DC supply output voltage are plotted in Fig. 6 for argon working gas and all duty cycles. A discharge current ID has been calculated as a difference between cathode and pulsed DC supply output voltage waveforms over value of the ballast resistor (177 U). The power fed PD into the plasma has been estimated as a product of cathode voltage and discharge current waveforms. Both values ID and PD as calculated from data in Fig. 6 for argon working gas are plotted in Fig. 7 for all the duty cycles. 4. Discussion The TEFD at the floating substrate consists of contributions of individual particles and radiation. Detailed discussion on the radiation contribution in the low-temperature plasma sputtering systems revealed that radiation energy influx on the substrate could be neglected because it comprises roughly a few percent of the total energy influx [10,11]. Then we can ascribe the main contributions to
Fig. 2. TEFD versus DC discharge current at constant pressure 2.8 Pa and pure argon as working gas. A small graph (inset) represents ratio between power density delivered into the plasma and the TEFD.
Fig. 3. TEFD versus working pressure for constant DC discharge current 500 mA and pure argon as a working gas. Inset: the cathode voltage versus the working pressure.
the TEFD at the substrate to electrons, ions and neutral particles with their kinetic and potential energies. Previous measurements in a magnetron sputtering device with titanium target showed that the contribution from kinetic potential energy of electrons and ions comprises approximately 80% of the TEFD at the floating substrate [11]. The kinetic energy contribution of the charged particles is given by the flux density Ge h Gi of the high-energy electrons overcoming negative floating potential and the ions accelerated both in ‘‘presheath’’ and in the floating potential of the substrate. The potential energy contribution of plasma electrons is given by electron work function of film surface reduced by an energy released by secondary electrons. For the plasma ions, the potential energy contribution is given by the ion recombination energy reduced by the electron work function and by the average energy of emitted secondary electrons [11]. The neutral particles such as sputtered titanium and argon atoms contribute to the TEFD at the substrate by their kinetic and condensation energies. The contribution of titanium atoms represents the magnetron system around 10% of the TEFD [11]. The flux density of sputtered titanium atoms in the plasma jet system could be relatively high in comparison with the magnetron sputtering
Fig. 4. TEFD versus duty cycle for constant mean pulsed DC discharge current 500 mA, pulsing frequency 2.5 kHz, and pressure of pure argon 2.8 Pa.
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Fig. 5. TEFD versus duty cycle for constant mean pulsed DC discharge current 500 mA, pulsing frequency 2.5 kHz and pressure of argon þ oxygen mixture 7.3 Pa.
Fig. 7. The waveforms of (a) power delivered to discharge and (b) discharge current as calculated by using data from Fig. 6.
Fig. 6. The measured waveforms for the experimental condition same as on Fig. 4 for (a) cathode voltage, and (b) pulsed DC supply output voltage.
device due to estimated higher plasma density in the hollow cathode. On the other hand, a relatively very high pressure inside of the nozzle together with one order in magnitude higher pressure in the rig leads to a very small fraction of non-thermalised Ti atoms
reaching the substrate surface. Gras-Marti and Valles-Abarca created analytical model describing the energy distribution of the sputtered particles at distance x from the target based on simplified assumption of continuous slowing down of particles travelling along straight-line trajectories [12]. According to this model applied to our experimental conditions, the 95% of sputtered Ti atoms will be thermalised before reaching the substrate surface during their path outside of the nozzle. If we take into account a one order in magnitude shorter path through the nozzle outlet where the pressure is almost two orders in magnitude higher then we can conclude that practically 100% of sputtered Ti particles will be thermalised on the substrate. Hence, the kinetic energy contribution from the titanium atoms is negligible. For evaluation of the Ti atoms’ condensation energy contribution to the TEFD, the Ti particle flux density on the substrate must be known. If we assume that the microscopic sticking probability gn ¼ 1 then the energy released by a one titanium atom is given by the binding energy of titanium surface UB ¼ 4.85 eV. Most of the argon atoms in our plasma are thermalised and only a certain fraction of the argon species has the high kinetic energy. These atoms are produced by Auger neutralization of reflected ions on the cathode surface and by charge transfer in an inelastic collision of a fast argon ion with the argon neutral. Eckstein and Biersack investigated in detail the ion reflection coefficient RN and the mean energy of reflected ions ER by TRIM code working with
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the Monte–Carlo simulation of incident ions on a matter [13]. This simulation gives us the values of RN z 0.03 for the argon ions bombarding titanium surface with energy up to 1 keV and the ER is comparable with average energy of the sputtered titanium particles. With respect to the above mentioned facts concerning sputtered titanium particles, the contribution of the high-energy reflected argon atoms must be much lower than the contribution of sputtered Ti particles and hence again negligible. If we want to consider an influence of a beam-like high-energy neutrals created by a capture of an electron during inelastic collision of Arþ and Ar, we have to know a cross-section for this elementary process, the plasma density and the sheath thickness. A recent study carried out by Martı´nez et al. demonstrated that the cross-section for this process for energy range over six orders is in the order of magnitude 1.1019–4.1019 m2 [14]. Considering that the high-energy ions are predominantly produced in the cathode sheath, which thickness around the hollow cathode is smaller than the mean free path of the resonant charge transfer collision (z8 mm for pressure 3 Pa) then the amount of produced fast neutrals will be relatively low. Even if the scattering cross-section of the fast neutrals with the background gas is low we assume that the total energy influx on the substrate will be insignificantly low in comparison with charged particles’ contribution. For more precise calculation of the high-energy neutrals’ contribution, it is necessary to measure plasma parameters (Te, ne) and to assess a number of high-energy argon neutrals created in the sheath. As expected, the data plotted in Fig. 2 reveals a direct relationship between TEFD at the substrate and the discharge current. The rate of increase is w43.1 mW cm2 A1. Previously made Langmuir probe measurements in the same system indicated that the concentration of charged particles within the vicinity of the substrate was approximately linearly proportional to the discharge current [15]. These measurements further showed that electron temperatures and floating probe sheath potentials do not significantly change over the range of the investigated discharge currents. This may suggest that within this system, the energy contributions from the charged particles, from high-energy argon atoms (produced by the Auger neutralization of backscattered ions from the target), from the beam-like high-energy argon neutrals and from the sputtered titanium atoms may be directly related to their concentration in the discharge. The inset in Fig. 2 shows that from 2.7& to 1.2& of the power density delivered into the discharge through the hollow cathode is transformed into the TEFD on the floating substrate. Furthermore, an increase in the discharge current from 100 mA to 600 mA results in decrease of ratio of TEFD to discharge power density by one-half. It means that an effectiveness of energy transfer from the hollow cathode discharge to the floating substrate decreases with increasing discharge current. On one hand this can be beneficial, for example temperature sensitive substrates, and on the other hand supplied energy is probably ineffectively consumed in cathode heating, radiation, etc. The increase in the working pressure, shown in Fig. 3, leads to an approximately linear increase in TEFD in pressure range from 1 Pa to 5 Pa followed by saturation region, where TEFD is roughly invariable. The high-energy argon neutrals and fast sputtered titanium particles lose their kinetic energy due to collisions with the background gas atoms as they travel from hollow cathode to substrate. For a given cathode to substrate separation, the decay in their kinetic energy is exponentially proportional to the increasing pressure [16]. The one-dimensional gas dynamic theory predicts that the local gas pressure at the nozzle outlet is much higher than working pressure inside the vacuum vessel due to high flow rate of working gas through the nozzle [17]. In our case, the pressure at the nozzle outlet attains values in the order of magnitude w100 Pa. Therefore, the beam-like high-energy argon atoms and sputtered titanium particles lose the main part of their kinetic energy just
inside the nozzle. Then their kinetic energy contribution to the TEFD at the substrate will be practically constant in our investigated range of pressures 1–10 Pa. As the DC power supply was operating in constant current mode, the plasma density had to decrease with increasing gas pressure inside the rig. Simultaneously, we can observe rapid decrease in cathode voltage followed by approximately constant value from 4 Pa to 10 Pa – see Fig. 3. Therefore sputtering rate must decrease with increasing pressure because the sputtering yield is approximately linearly proportional to argon ions energy in the range corresponding to our cathode voltages. Then we can expect lower amount of titanium particles hitting the substrate when working pressure is increasing and consequently lower energy contribution from sputtered titanium particles [18]. This inference is in contradiction with experimental results of the TEFD increasing with pressure. Previously made TEFD measurements carried out in unbalanced magnetron sputtering system equipped with Ti target revealed exponential decrease in TEFD with increasing pressure [19]. This fact indicates that the hollow cathode plasma jet system generates wholly different plasma conditions for deposition of the same thin films than in industry used techniques. Thus, the missing energy contribution must be searched in possible increment of plasma density due to delivered small RF power, in unknown behavior of electron temperature with increasing working pressure or in unknown influence of radiation from the plasma. At this time, the reason for this contradiction is unclear and needs further investigation. When pulsing has been used for pure argon working gas we can observe with decreasing duty cycle a low increase in the TEFD with maximal value at duty cycle 60% followed by gradual decrease – see Fig. 4. A similar trend in TEFD has been observed for mid-frequency (100–350 kHz) pulsed DC magnetron sputtering system [11]. The model of the contributions to the TEFD at the substrate for pulsed DC magnetron discharge introduced by Cada et al. considers a substantial influence of high-energy charged particles created during on–off and off–on transitions in the cathode voltage [11]. The cathode waveforms similar to them in pulsed magnetron discharge have been observed in our study on the hollow cathode and it suggests itself the possibility to apply modified model used for pulsed DC magnetron system for pulsed DC hollow cathode sputtering system. Similar transient phenomenon on the cathode voltage and discharge current as observed in the mid-frequency pulsed DC magnetron discharge can be seen in the hollow cathode plasma jet system. During the off–on transition a negative peak in the cathode voltage gradually decreases with increasing duty cycle in range from approximately 700 V to 300 V – see Fig. 5. The same trend is observed for steady cathode voltage during the ON phase. On the contrary, the output voltage of the pulsed DC supply during the ON phase is void of transient phenomenon. Then we can suppose that ballast resistor having a big inductance will be responsible for transient phenomena on the hollow cathode voltage. During the on–off transition a massive positive overshoot taking w2 ms on the pulsed DC supply output springs up to 700 V and is followed by attenuated ringing. Furthermore, time length of the overshoot for duty cycle 10% and 20% is significantly longer than for other duty cycles. The magnitude of the positive overshoot at the beginning of the OFF phase decreases gradually except for the value for duty cycle 10%, which is lower than the value for duty cycle 40%. On the hollow cathode, the positive voltage overshoot is much smaller (about 100 V). After approximately 20 ms, both hollow cathode and supply output voltage start to decrease gradually on the value about 60 V. As during the OFF phase of a pulse cycle the cathode is at a floating potential and via a capacitor connected to the continually running RF power supply a self-bias negative DC voltage is created on the cathode. It is evident from Fig. 6 that self-bias cathode
M. Cada et al. / Vacuum 83 (2009) 738–744
voltage 60 V is formed for two times longer time for duty cycles 10% and 20% than for other higher duty cycles. Then we can suppose that for duty cycle 10% and partly for 20% the hollow cathode discharge operates under a slightly different mode, which may result in generally different plasma parameters. RT The calculated mean power PD ¼ ð1=TÞ 0 PD ðtÞdt delivered to the pure argon discharge for all the duty cycles is plotted in Fig. 8. In the first approximation, decrease of the mean power with increasing duty cycle can be fitted by an exponential decay. Then we should expect higher TEFD at the substrate for lower duty cycles but experimental results demonstrate exact opposite. It is evident from a comparison of curve shapes plotted in Figs. 4 and 8 that energy delivered to the substrate is not directly related to the mean energy delivered to discharge when pulse driving of plasma is in operation. Therefore, the higher mean power delivered to discharge can lead to denser and hotter plasma, higher sputtering rate and simultaneously to reduce energy influx on the substrate if pulsed DC driving of the hollow cathode is used. When argon and oxygen mixture has been used, the pressure in the rig was more than two times higher than in case when only pure argon was used. As follows from Fig. 3, an increase in the TEFD by approximately 20% has been observed in DC discharge with pure argon working gas when pressures rose from 3 Pa to 7 Pa. Such a high increase in the TEFD due to higher working gas pressure has not been observed for DC driving of the plasma when argon and oxygen mixture was used (see a comparison between Figs. 4 and 5). The explanation of this fact may lie in both different gas composition (pure argon versus Ar þ O2 mixture) and error of measurements of the TEFD. A gradual decrease in the TEFD with decreasing duty cycle has been measured. An unclear small variation (much less than error of measurements) in the TEFD for duty cycles 50% and 60% from systematic decrease has been observed. Furthermore we can observe slower decrease in the TEFD than in the case of pure argon working gas. An inflow of oxygen by port located sideways of the rig has probably a minimal influence on the plasma parameters inside of the hollow cathode because of high flow rate of argon gas through the nozzle and impossibility of oxygen molecules to reach high-intensive plasma region inside the hollow cathode. Then the oxygen gas can interact only with plasma jet plume blown out of the nozzle in vicinity of a substrate. Therefore, plasma parameters in vicinity of the substrate can be influenced by presence of the oxygen.
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In addition to the contributions from the kinetic and potential energies of the charged particles, sputtered titanium atoms and high-energy argon neutrals to the TEFD, we have to take into consideration the contributions given by energy released during TiO2 formation on the substrate and by a energy released during the recombination, adsorption and desorption of oxygen atoms and molecules on the substrate surface. Besides, we suppose the possible contribution from the negative oxygen ions can be neglected because most of the negative ions are not able to overcome the negative floating potential. The energy contribution released during TiO2 formation on the substrate can be estimated from known deposition rate, from knowledge of oxygen adsorption energy, energies released during adsorption and reactions of individual atoms. Eventually, we can see from comparison of Figs. 4 and 5 that different reactions for thin film formation, recombination or adsorption of oxygen atoms or ions lead to higher TEFD on the floating substrate. 5. Conclusion Our experimental results demonstrate that total energy flux density on the electrically isolated substrate strongly depends on duty cycle if the hollow cathode plasma jet sputtering system operates under pulse DC conditions. For duty cycle lower than 50% the TEFD on the substrate is even lower (up to 50% of the value for DC mode) than the TEFD when the plasma jet operating in DC mode. Furthermore, for duty cycles in range 60–90% the total energy influx at substrate increases up to by 15%. Therefore, the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving lies in the possibility to reduce or increase total energy flux density on the substrate within the change of duty cycle. This complex behavior of total energy influx on the electrically isolated substrate under different pulsing conditions may have an influence on deposited thin films. The knowledge of individual contributions to the TEFD is then essential for possible impact of change of duty cycle on thin film properties. In order to more precisely calculate the individual contributions from the charged particles we need to carry out in-depth investigation of plasma parameters such as the plasma density, the electron temperature, the plasma or the floating potential. It seems that condensation energy of Ti atoms, energy released during TiO2 formation or adsorption and recombination energy of oxygen atoms can comprise an appreciable part of the TEFD on the floating substrate. Hence, it will be necessary in future to measure titanium and oxygen particle flux density on the substrate and subsequently to calculate the energy contribution from these processes taking place on the substrate. Acknowledgment This work was supported by projects KAN301370701, KJB100100701 and KJB100100707 of the Academy of Sciences of the Czech Republic. References
Fig. 8. The calculated mean power delivered to the discharge from waveforms provided by data from Fig. 7a. Furthermore, the mean power data were fitted by exponential decay of third order.
[1] Thornton JA, Penfold A. In: Vossen JL, Kern W, editors. Thin film processes. New York: Academic Press; 1987. p. 76. [2] Ba´rdosˇ L, Berg S. Surf Coat Technol 1993;54:91–5. [3] Cada M, Hubicˇka Z, Ada´mek P, Pta´cˇek P, Sˇı´chova´ H, Sˇı´cha M, et al. Surf Coat Technol 2003;174:627–31. M, Ada´mek P, Virostko P, Olejnı´cˇek J, Deyneka A, et al. Surf [4] Hubicˇka Z, Cada Coat Technol 2005;200:940–6. [5] Hubicˇka Z, Chichina M, Deyneka A, Kudrna P, Olejnı´cˇek J, Sˇı´chova´ H, et al. J Optoelectron Adv Mater 2007;9:875–80. ˇ P, Ba´rtkova´ H, Kry´sa J, Churpita O, Cada [6] Kment Sˇ Kluson M, Virostko P, et al. Surf Coat Technol 2008;220:2379–83.
744
M. Cada et al. / Vacuum 83 (2009) 738–744
[7] Schneider JM, Sproul WD. Reactive pulsed dc magnetron sputtering and control. In: Glocker DA, Shah SI, editors. Handbook of thin film processing technology, vol. A.5:1. Bristol: IOP; 1998. [8] Arnell RD, Kelly PJ. Surf Coat Technol 1999;112:170–6. [9] Olejnı´cˇek J, Hubicˇka Z, Virostko P, Deyneka A, Jastrabı´k L, Ada´mek P, et al. Czech J Phys 2006;56B:1283–9. [10] Kersten H, Deutsch H, Steffen H, Kroesen GMW, Hippler R. Vacuum 2001;63: 385–431. [11] Cada M, Bradley JW, Clarke GCB, Kelly PJ. J Appl Phys 2007;102:063301. [12] Gras-Marti A, Valles-Abarca JA. J Appl Phys 1983;54(2):1071–5. [13] Eckstein W, Biersack JP. Z Phys B 1986;63(4):471–8.
[14] Martı´nez H, Castillo F, Reyes PG, Santibanez FG. Int J Mass Spectrom 2003;228: 107–16. [15] Virostko P, Hubicˇka Z, Ada´mek P, Cada M, Olejnı´cˇek J, Tichy´ M, et al. Contrib Plasma Phys 2006;46:445–50. [16] Ekpe SD, Dew SK. J Vac Sci Technol A 2003;21:476–83. [17] Hubicˇka Z, Pribil G, Soukup RJ, Ianno NJ. Surf Coat Technol 2002;160:114–23. [18] Matsunami N, Yamamura Y, Itikawa Y, Itoh N, Kazumata Y, Miyagawa S, et al. At Data Nucl Data Tables 1984;31:1–80. [19] Cada M, Clarke GCB, Kelly PJ, Bradley JW. Investigation of energy flux density at a substrate in a pulsed DC magnetron discharge. In: Proceedings of the 27th ICPIG, Prague, Czech Republic; 15–20 July, 2007. ISBN: 978-80-87026-01-4.
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Measurement of total energy flux density at a substrate during TiOx thin film deposition by using a plasma jet system ˇ . Kment, Z. Hubicˇka *, P. Virostko, S M. Cada Institute of Physics of the ASCR, v.v.i., Na Slovance 2, 182 21 Prague 8, Czech Republic
a b s t r a c t Keywords: Hollow cathode Plasma jet Sputtering Pulsed DC Energy influx on substrate TiO2
The total energy flux density delivered to an electrically isolated substrate in a low-pressure pulsed DC hollow cathode plasma jet sputtering system during TiO2 thin film deposition has been quantified. The plasma source was operated in constant average current mode and in a mixture of argon and oxygen or only in pure argon working gas. A titanium nozzle served as the hollow cathode. The total energy flux density measurements were made using a planar calorimeter probe. The main results from the calorimeter probe showed clearly that the total energy flux density at the electrically isolated substrate decreases significantly with duty cycle from 100% (DC mode) to 10% at a given pulsing frequency 2.5 kHz. A local maximum at duty cycle 60% for only pure argon operation has been observed. In addition, the voltage waveforms on the hollow cathode and before the ballast resistor have been saved for pulsed DC measurements for both pure argon and argon þ oxygen mixture. A similar transient phenomenon on the cathode voltage and discharge current as observed recently in mid-frequency pulsed DC magnetron discharge has been discovered in the hollow cathode plasma jet sputtering system. We can conclude from these preliminary measurements that the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving of the same plasma system lies in the possibility to reduce or to increase energy influx on the floating substrate within the change of duty cycle. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Thin film deposition utilizing a sputtering technique is a widely used method in semiconductor industry, for the preparation of tribological and optical coatings and for the modification of surfaces. In addition to well-known magnetron sputtering utilizing the planar cathode [1], alternative sputtering techniques, as in hollow cathode plasma jet system, have been developed [2]. The lowpressure hollow cathode plasma jet sputtering system has been successfully used for the deposition of different types of thin films. For example, perpendicularly oriented ZnO thin films [3] produced during reactive sputtering of metallic Zn hollow cathode in DC or RF discharge, PbZrxTi1xO3 (PZT) thin films deposited on polymer substrates during reactive sputtering of cathode made of PZT ceramics in an RF pulse-modulated discharge [4] or BaxSr1xTiO3 (BSTO) dielectric films reactively deposited by double hollow cathode RF pulse-modulated plasma jet system using two cathodes made of BaTiO3 and SrTiO3 ceramics inserted into a single nozzle or into two separate nozzles [5]. Recently, the hollow cathode plasma jet deposition system has been used for the preparation of nanoscaled TiO2 photoactive catalytic thin films [6]. Besides thin film * Corresponding author. Tel.: þ420 266 052 995; fax: þ420 286 581 448. E-mail address: [email protected] (M. Cada). 0042-207X/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2008.05.014
deposition, the plasma diagnostics by means of Langmuir probe and optical emission spectroscopy in the hollow cathode plasma jets have been carried out. The pulsed DC-driven magnetron discharge was introduced almost 20 years ago as a possible solution for arcing suppression [7]. Lately, the pulsed DC or pulse-modulated driving of the cathode voltage in the magnetron sputtering systems is becoming a more often used method in comparison with DC or CW driving of the cathode voltage. This results in production of thin films of better quality, improves discharge stability and enables us to prepare thin films with new properties, which cannot be deposited by conventional sputtering techniques in required quality [8]. The pulsed DC or pulse-modulated RF driving of the hollow cathode discharge has been used during deposition of PZT, BSTO and TiO2 thin films. Introducing pulse driving of discharge results in much more effective sputtering of ceramic cathodes, increasing of mean power delivered to discharge without overheating of the hollow cathode, significant reduction of substrate heating, enhancement of thin film quality and last but not least significant enhancement in plasma parameters (such as electron temperature, plasma density, plasma potential, etc.) [5,9]. Therefore, these facts demonstrate the necessity to investigate plasma parameters in connection with new thin film properties prepared by pulsed discharges.
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One of the plasma parameters representing relation between plasma and thin film properties is the total energy flux density (TEFD) at the substrate. This parameter comprises the particle flux towards the substrate, by the thermal radiation flux from plasma photons, by the chemical reactions at the substrate and by the elementary processes taking place at the substrate. On the contrary, there are also cooling effects caused by thin film re-sputtering, a secondary emission or substrate thermal radiation [10]. Measurement of the TEFD at the substrate is then the basic method to characterize the interaction between plasma and substrate – the energy balance at substrate. The most recent results of TEFD measurements in a mid-frequency pulsed DC magnetron discharge equipped with titanium target showed a significant influence of pulse frequency and duty cycle on energy conditions on a substrate [11]. Our investigation presented in this paper deals with preliminary measurements of the total energy flux density at the substrate during the deposition process of TiO2 thin films in the pulsed DC hollow cathode plasma jet sputtering system by using a calorimeter probe. Our study is focused on the characterization of energy conditions at the floating substrate for various duty cycles at pulsing frequency typical for deposition process.
2. Experiment The low-pressure plasma jet configuration for TiO2 thin film deposition is shown in Fig. 1. The UHV reactor chamber was continuously pumped by a turbo-molecular pump backed by combination of roots and rotary vane pumps. After achieving a base pressure of <105 Pa, the chamber was backfilled with pure argon (flow rate 120 sccm) or with a mixture of argon and oxygen (flow rates of 120 sccm and 60 sccm, respectively) to an operating pressure of 2.8 Pa or 7.3 Pa, respectively. The cylindrical nozzle placed on the top of the chamber was made-up of pure titanium and acted as RF hollow cathode and simultaneously as inlet nozzle for argon gas only. The internal diameter of the nozzle was 5 mm and the length of the nozzle was 30 mm. The oxygen gas was fed into the
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reactor chamber via a separate port located sideways. This configuration ensured that the inner surface of the titanium nozzle was not covered by a TiO2 thin film and then the sputtering system was able to reach higher deposition rates. The hollow cathode plasma jet discharge was driven using a pulsed DC and an RF (operating at 13.56 MHz) power supply operating simultaneously. The pulsed DC supply comprises an Advanced Energy MDX-1K (Umax ¼ 1 kV and Imax ¼ 1 A) and a pulse modulator with high input parallel capacity. A ballast resistor of 177 U was inserted between pulsed DC source and hollow cathode to stabilize a discharge current. The RF power was kept on 3 W due to better stabilization of the pulsed DC discharge. After ignition, an intensive DC hollow cathode discharge was generated inside the nozzle, on the background of the capacitive RF discharge excited in the volume of the reactor. Incoming working gas forced this DC hollow cathode discharge out of the nozzle into the reactor chamber. The calorimeter probe was placed perpendicularly to the plasma jet axis at a distance of 50 mm from the hollow cathode outlet. A schematic diagram of the calorimeter probe, as fitted to the ceramic block made of Lava (a type of ceramic), is shown in Fig. 1. This configuration ensured that: (1) the probe can stay at the floating potential or can be independently biased and (2) the loss of heat through conduction was minimized. The calorimeter probe consists of 8 mm diameter by 2 mm thick copper disc. The tip consisting of a spherical junction (0.4 mm in diameter) of a K-type thermocouple was soldered into a slot machined into the center of the copper disc. The solder (a silver alloy) was chosen as it had both a thermal conductivity and specific heat similar to that of copper. The calorimeter probe construction was designed to minimize thermal losses by conduction, and so to ensure the temperature measurements are as precise as possible. The thermal capacity of the probe was calculated using CP ¼ m cP, where m ¼ 0.803 g is the mass of the probe and cP is the specific heat of copper, which yielded a value of 0.31 J K1 with maximal error about 10%. The method used for determination of the heat transfer to the substrate is based on the measurement of temporal evolution of a small calorimeter probe located at the position of the substrate [10]. This method is in detail described elsewhere and from previous work the effective total energy flux density is determined with maximum error of w5% [11]. Digital sampling oscilloscope Agilent 54830B has been used for acquisition of voltage waveforms measured before and behind the ballast resistor – see Fig. 1. The discharge current has been then calculated from the voltage drop on the ballast resistor. 3. Results The TEFD has been calculated according to Eq. (1):
P ¼
Fig. 1. Experimental arrangement.
CP dT S dt TS
(1)
where S is the area of calorimetric probe, TS is a temperature not more than 40 C and ðdT=dtÞTS has been replaced by the slope of a linear fit of periodically measured probe temperatures during the first 60 s after stabilization of the discharge. Previous detailed studies of calorimeter probe technique revealed that probe temperature increases linearly with time with a maximum error of 5% up to a probe temperature of about 40 C. The first set of measurements were made for plasma jet driving by DC voltage only with power supply set in constant current mode for pure argon as working gas. The measured values of total energy flux density (TEFD) of a floating substrate at a distance 50 mm from titanium nozzle outlet and at constant pressure 2.8 Pa are plotted
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versus discharge current in the range from 100 mA to 600 mA in Fig. 2. Furthermore, the power density delivered into the discharge has been calculated and ratio between the plasma power density and the TEFD has been determined and is plotted in Fig. 2. The calculated ratio enables us to compare how big part of the discharge power density is transferred into the substrate for different plasma conditions. The TEFD values measured for constant discharge current 500 mA are plotted in Fig. 3 versus working pressure in the range from 1 Pa to 10 Pa. Further measurements have been carried out for a constant pulsing frequency 2.5 kHz and different duty cycles in the range from 10% to 90%. The pulsing frequency of 2.5 kHz has been set for reason of its usage during TiO2 thin film deposition. In Fig. 4, a plot of TEFD versus duty cycle for pure argon working gas (working pressure 2.8 Pa, mean discharge current 500 mA) is shown. The value for duty cycle 10% is not shown here because strong interference of thermocouple transducer by pulsing discharge did not allow us to reliably determine a temperature course with time. A plot of TEFD versus duty cycle for the same experimental conditions but for the working gas mixture of argon and oxygen with the working gas pressure 7.3 Pa is shown in Fig. 5. During the calorimeter probe measurements, voltage waveforms on the hollow cathode and before the ballast resistor have been saved for only pulsed DC measurements for both pure argon and argon þ oxygen mixture. As an example, the waveform of cathode voltage and pulsed DC supply output voltage are plotted in Fig. 6 for argon working gas and all duty cycles. A discharge current ID has been calculated as a difference between cathode and pulsed DC supply output voltage waveforms over value of the ballast resistor (177 U). The power fed PD into the plasma has been estimated as a product of cathode voltage and discharge current waveforms. Both values ID and PD as calculated from data in Fig. 6 for argon working gas are plotted in Fig. 7 for all the duty cycles. 4. Discussion The TEFD at the floating substrate consists of contributions of individual particles and radiation. Detailed discussion on the radiation contribution in the low-temperature plasma sputtering systems revealed that radiation energy influx on the substrate could be neglected because it comprises roughly a few percent of the total energy influx [10,11]. Then we can ascribe the main contributions to
Fig. 2. TEFD versus DC discharge current at constant pressure 2.8 Pa and pure argon as working gas. A small graph (inset) represents ratio between power density delivered into the plasma and the TEFD.
Fig. 3. TEFD versus working pressure for constant DC discharge current 500 mA and pure argon as a working gas. Inset: the cathode voltage versus the working pressure.
the TEFD at the substrate to electrons, ions and neutral particles with their kinetic and potential energies. Previous measurements in a magnetron sputtering device with titanium target showed that the contribution from kinetic potential energy of electrons and ions comprises approximately 80% of the TEFD at the floating substrate [11]. The kinetic energy contribution of the charged particles is given by the flux density Ge h Gi of the high-energy electrons overcoming negative floating potential and the ions accelerated both in ‘‘presheath’’ and in the floating potential of the substrate. The potential energy contribution of plasma electrons is given by electron work function of film surface reduced by an energy released by secondary electrons. For the plasma ions, the potential energy contribution is given by the ion recombination energy reduced by the electron work function and by the average energy of emitted secondary electrons [11]. The neutral particles such as sputtered titanium and argon atoms contribute to the TEFD at the substrate by their kinetic and condensation energies. The contribution of titanium atoms represents the magnetron system around 10% of the TEFD [11]. The flux density of sputtered titanium atoms in the plasma jet system could be relatively high in comparison with the magnetron sputtering
Fig. 4. TEFD versus duty cycle for constant mean pulsed DC discharge current 500 mA, pulsing frequency 2.5 kHz, and pressure of pure argon 2.8 Pa.
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Fig. 5. TEFD versus duty cycle for constant mean pulsed DC discharge current 500 mA, pulsing frequency 2.5 kHz and pressure of argon þ oxygen mixture 7.3 Pa.
Fig. 7. The waveforms of (a) power delivered to discharge and (b) discharge current as calculated by using data from Fig. 6.
Fig. 6. The measured waveforms for the experimental condition same as on Fig. 4 for (a) cathode voltage, and (b) pulsed DC supply output voltage.
device due to estimated higher plasma density in the hollow cathode. On the other hand, a relatively very high pressure inside of the nozzle together with one order in magnitude higher pressure in the rig leads to a very small fraction of non-thermalised Ti atoms
reaching the substrate surface. Gras-Marti and Valles-Abarca created analytical model describing the energy distribution of the sputtered particles at distance x from the target based on simplified assumption of continuous slowing down of particles travelling along straight-line trajectories [12]. According to this model applied to our experimental conditions, the 95% of sputtered Ti atoms will be thermalised before reaching the substrate surface during their path outside of the nozzle. If we take into account a one order in magnitude shorter path through the nozzle outlet where the pressure is almost two orders in magnitude higher then we can conclude that practically 100% of sputtered Ti particles will be thermalised on the substrate. Hence, the kinetic energy contribution from the titanium atoms is negligible. For evaluation of the Ti atoms’ condensation energy contribution to the TEFD, the Ti particle flux density on the substrate must be known. If we assume that the microscopic sticking probability gn ¼ 1 then the energy released by a one titanium atom is given by the binding energy of titanium surface UB ¼ 4.85 eV. Most of the argon atoms in our plasma are thermalised and only a certain fraction of the argon species has the high kinetic energy. These atoms are produced by Auger neutralization of reflected ions on the cathode surface and by charge transfer in an inelastic collision of a fast argon ion with the argon neutral. Eckstein and Biersack investigated in detail the ion reflection coefficient RN and the mean energy of reflected ions ER by TRIM code working with
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the Monte–Carlo simulation of incident ions on a matter [13]. This simulation gives us the values of RN z 0.03 for the argon ions bombarding titanium surface with energy up to 1 keV and the ER is comparable with average energy of the sputtered titanium particles. With respect to the above mentioned facts concerning sputtered titanium particles, the contribution of the high-energy reflected argon atoms must be much lower than the contribution of sputtered Ti particles and hence again negligible. If we want to consider an influence of a beam-like high-energy neutrals created by a capture of an electron during inelastic collision of Arþ and Ar, we have to know a cross-section for this elementary process, the plasma density and the sheath thickness. A recent study carried out by Martı´nez et al. demonstrated that the cross-section for this process for energy range over six orders is in the order of magnitude 1.1019–4.1019 m2 [14]. Considering that the high-energy ions are predominantly produced in the cathode sheath, which thickness around the hollow cathode is smaller than the mean free path of the resonant charge transfer collision (z8 mm for pressure 3 Pa) then the amount of produced fast neutrals will be relatively low. Even if the scattering cross-section of the fast neutrals with the background gas is low we assume that the total energy influx on the substrate will be insignificantly low in comparison with charged particles’ contribution. For more precise calculation of the high-energy neutrals’ contribution, it is necessary to measure plasma parameters (Te, ne) and to assess a number of high-energy argon neutrals created in the sheath. As expected, the data plotted in Fig. 2 reveals a direct relationship between TEFD at the substrate and the discharge current. The rate of increase is w43.1 mW cm2 A1. Previously made Langmuir probe measurements in the same system indicated that the concentration of charged particles within the vicinity of the substrate was approximately linearly proportional to the discharge current [15]. These measurements further showed that electron temperatures and floating probe sheath potentials do not significantly change over the range of the investigated discharge currents. This may suggest that within this system, the energy contributions from the charged particles, from high-energy argon atoms (produced by the Auger neutralization of backscattered ions from the target), from the beam-like high-energy argon neutrals and from the sputtered titanium atoms may be directly related to their concentration in the discharge. The inset in Fig. 2 shows that from 2.7& to 1.2& of the power density delivered into the discharge through the hollow cathode is transformed into the TEFD on the floating substrate. Furthermore, an increase in the discharge current from 100 mA to 600 mA results in decrease of ratio of TEFD to discharge power density by one-half. It means that an effectiveness of energy transfer from the hollow cathode discharge to the floating substrate decreases with increasing discharge current. On one hand this can be beneficial, for example temperature sensitive substrates, and on the other hand supplied energy is probably ineffectively consumed in cathode heating, radiation, etc. The increase in the working pressure, shown in Fig. 3, leads to an approximately linear increase in TEFD in pressure range from 1 Pa to 5 Pa followed by saturation region, where TEFD is roughly invariable. The high-energy argon neutrals and fast sputtered titanium particles lose their kinetic energy due to collisions with the background gas atoms as they travel from hollow cathode to substrate. For a given cathode to substrate separation, the decay in their kinetic energy is exponentially proportional to the increasing pressure [16]. The one-dimensional gas dynamic theory predicts that the local gas pressure at the nozzle outlet is much higher than working pressure inside the vacuum vessel due to high flow rate of working gas through the nozzle [17]. In our case, the pressure at the nozzle outlet attains values in the order of magnitude w100 Pa. Therefore, the beam-like high-energy argon atoms and sputtered titanium particles lose the main part of their kinetic energy just
inside the nozzle. Then their kinetic energy contribution to the TEFD at the substrate will be practically constant in our investigated range of pressures 1–10 Pa. As the DC power supply was operating in constant current mode, the plasma density had to decrease with increasing gas pressure inside the rig. Simultaneously, we can observe rapid decrease in cathode voltage followed by approximately constant value from 4 Pa to 10 Pa – see Fig. 3. Therefore sputtering rate must decrease with increasing pressure because the sputtering yield is approximately linearly proportional to argon ions energy in the range corresponding to our cathode voltages. Then we can expect lower amount of titanium particles hitting the substrate when working pressure is increasing and consequently lower energy contribution from sputtered titanium particles [18]. This inference is in contradiction with experimental results of the TEFD increasing with pressure. Previously made TEFD measurements carried out in unbalanced magnetron sputtering system equipped with Ti target revealed exponential decrease in TEFD with increasing pressure [19]. This fact indicates that the hollow cathode plasma jet system generates wholly different plasma conditions for deposition of the same thin films than in industry used techniques. Thus, the missing energy contribution must be searched in possible increment of plasma density due to delivered small RF power, in unknown behavior of electron temperature with increasing working pressure or in unknown influence of radiation from the plasma. At this time, the reason for this contradiction is unclear and needs further investigation. When pulsing has been used for pure argon working gas we can observe with decreasing duty cycle a low increase in the TEFD with maximal value at duty cycle 60% followed by gradual decrease – see Fig. 4. A similar trend in TEFD has been observed for mid-frequency (100–350 kHz) pulsed DC magnetron sputtering system [11]. The model of the contributions to the TEFD at the substrate for pulsed DC magnetron discharge introduced by Cada et al. considers a substantial influence of high-energy charged particles created during on–off and off–on transitions in the cathode voltage [11]. The cathode waveforms similar to them in pulsed magnetron discharge have been observed in our study on the hollow cathode and it suggests itself the possibility to apply modified model used for pulsed DC magnetron system for pulsed DC hollow cathode sputtering system. Similar transient phenomenon on the cathode voltage and discharge current as observed in the mid-frequency pulsed DC magnetron discharge can be seen in the hollow cathode plasma jet system. During the off–on transition a negative peak in the cathode voltage gradually decreases with increasing duty cycle in range from approximately 700 V to 300 V – see Fig. 5. The same trend is observed for steady cathode voltage during the ON phase. On the contrary, the output voltage of the pulsed DC supply during the ON phase is void of transient phenomenon. Then we can suppose that ballast resistor having a big inductance will be responsible for transient phenomena on the hollow cathode voltage. During the on–off transition a massive positive overshoot taking w2 ms on the pulsed DC supply output springs up to 700 V and is followed by attenuated ringing. Furthermore, time length of the overshoot for duty cycle 10% and 20% is significantly longer than for other duty cycles. The magnitude of the positive overshoot at the beginning of the OFF phase decreases gradually except for the value for duty cycle 10%, which is lower than the value for duty cycle 40%. On the hollow cathode, the positive voltage overshoot is much smaller (about 100 V). After approximately 20 ms, both hollow cathode and supply output voltage start to decrease gradually on the value about 60 V. As during the OFF phase of a pulse cycle the cathode is at a floating potential and via a capacitor connected to the continually running RF power supply a self-bias negative DC voltage is created on the cathode. It is evident from Fig. 6 that self-bias cathode
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voltage 60 V is formed for two times longer time for duty cycles 10% and 20% than for other higher duty cycles. Then we can suppose that for duty cycle 10% and partly for 20% the hollow cathode discharge operates under a slightly different mode, which may result in generally different plasma parameters. RT The calculated mean power PD ¼ ð1=TÞ 0 PD ðtÞdt delivered to the pure argon discharge for all the duty cycles is plotted in Fig. 8. In the first approximation, decrease of the mean power with increasing duty cycle can be fitted by an exponential decay. Then we should expect higher TEFD at the substrate for lower duty cycles but experimental results demonstrate exact opposite. It is evident from a comparison of curve shapes plotted in Figs. 4 and 8 that energy delivered to the substrate is not directly related to the mean energy delivered to discharge when pulse driving of plasma is in operation. Therefore, the higher mean power delivered to discharge can lead to denser and hotter plasma, higher sputtering rate and simultaneously to reduce energy influx on the substrate if pulsed DC driving of the hollow cathode is used. When argon and oxygen mixture has been used, the pressure in the rig was more than two times higher than in case when only pure argon was used. As follows from Fig. 3, an increase in the TEFD by approximately 20% has been observed in DC discharge with pure argon working gas when pressures rose from 3 Pa to 7 Pa. Such a high increase in the TEFD due to higher working gas pressure has not been observed for DC driving of the plasma when argon and oxygen mixture was used (see a comparison between Figs. 4 and 5). The explanation of this fact may lie in both different gas composition (pure argon versus Ar þ O2 mixture) and error of measurements of the TEFD. A gradual decrease in the TEFD with decreasing duty cycle has been measured. An unclear small variation (much less than error of measurements) in the TEFD for duty cycles 50% and 60% from systematic decrease has been observed. Furthermore we can observe slower decrease in the TEFD than in the case of pure argon working gas. An inflow of oxygen by port located sideways of the rig has probably a minimal influence on the plasma parameters inside of the hollow cathode because of high flow rate of argon gas through the nozzle and impossibility of oxygen molecules to reach high-intensive plasma region inside the hollow cathode. Then the oxygen gas can interact only with plasma jet plume blown out of the nozzle in vicinity of a substrate. Therefore, plasma parameters in vicinity of the substrate can be influenced by presence of the oxygen.
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In addition to the contributions from the kinetic and potential energies of the charged particles, sputtered titanium atoms and high-energy argon neutrals to the TEFD, we have to take into consideration the contributions given by energy released during TiO2 formation on the substrate and by a energy released during the recombination, adsorption and desorption of oxygen atoms and molecules on the substrate surface. Besides, we suppose the possible contribution from the negative oxygen ions can be neglected because most of the negative ions are not able to overcome the negative floating potential. The energy contribution released during TiO2 formation on the substrate can be estimated from known deposition rate, from knowledge of oxygen adsorption energy, energies released during adsorption and reactions of individual atoms. Eventually, we can see from comparison of Figs. 4 and 5 that different reactions for thin film formation, recombination or adsorption of oxygen atoms or ions lead to higher TEFD on the floating substrate. 5. Conclusion Our experimental results demonstrate that total energy flux density on the electrically isolated substrate strongly depends on duty cycle if the hollow cathode plasma jet sputtering system operates under pulse DC conditions. For duty cycle lower than 50% the TEFD on the substrate is even lower (up to 50% of the value for DC mode) than the TEFD when the plasma jet operating in DC mode. Furthermore, for duty cycles in range 60–90% the total energy influx at substrate increases up to by 15%. Therefore, the main asset of the pulsed DC hollow cathode plasma jet discharge as distinct from the DC driving lies in the possibility to reduce or increase total energy flux density on the substrate within the change of duty cycle. This complex behavior of total energy influx on the electrically isolated substrate under different pulsing conditions may have an influence on deposited thin films. The knowledge of individual contributions to the TEFD is then essential for possible impact of change of duty cycle on thin film properties. In order to more precisely calculate the individual contributions from the charged particles we need to carry out in-depth investigation of plasma parameters such as the plasma density, the electron temperature, the plasma or the floating potential. It seems that condensation energy of Ti atoms, energy released during TiO2 formation or adsorption and recombination energy of oxygen atoms can comprise an appreciable part of the TEFD on the floating substrate. Hence, it will be necessary in future to measure titanium and oxygen particle flux density on the substrate and subsequently to calculate the energy contribution from these processes taking place on the substrate. Acknowledgment This work was supported by projects KAN301370701, KJB100100701 and KJB100100707 of the Academy of Sciences of the Czech Republic. References
Fig. 8. The calculated mean power delivered to the discharge from waveforms provided by data from Fig. 7a. Furthermore, the mean power data were fitted by exponential decay of third order.
[1] Thornton JA, Penfold A. In: Vossen JL, Kern W, editors. Thin film processes. New York: Academic Press; 1987. p. 76. [2] Ba´rdosˇ L, Berg S. Surf Coat Technol 1993;54:91–5. [3] Cada M, Hubicˇka Z, Ada´mek P, Pta´cˇek P, Sˇı´chova´ H, Sˇı´cha M, et al. Surf Coat Technol 2003;174:627–31. M, Ada´mek P, Virostko P, Olejnı´cˇek J, Deyneka A, et al. Surf [4] Hubicˇka Z, Cada Coat Technol 2005;200:940–6. [5] Hubicˇka Z, Chichina M, Deyneka A, Kudrna P, Olejnı´cˇek J, Sˇı´chova´ H, et al. J Optoelectron Adv Mater 2007;9:875–80. ˇ P, Ba´rtkova´ H, Kry´sa J, Churpita O, Cada [6] Kment Sˇ Kluson M, Virostko P, et al. Surf Coat Technol 2008;220:2379–83.
744
M. Cada et al. / Vacuum 83 (2009) 738–744
[7] Schneider JM, Sproul WD. Reactive pulsed dc magnetron sputtering and control. In: Glocker DA, Shah SI, editors. Handbook of thin film processing technology, vol. A.5:1. Bristol: IOP; 1998. [8] Arnell RD, Kelly PJ. Surf Coat Technol 1999;112:170–6. [9] Olejnı´cˇek J, Hubicˇka Z, Virostko P, Deyneka A, Jastrabı´k L, Ada´mek P, et al. Czech J Phys 2006;56B:1283–9. [10] Kersten H, Deutsch H, Steffen H, Kroesen GMW, Hippler R. Vacuum 2001;63: 385–431. [11] Cada M, Bradley JW, Clarke GCB, Kelly PJ. J Appl Phys 2007;102:063301. [12] Gras-Marti A, Valles-Abarca JA. J Appl Phys 1983;54(2):1071–5. [13] Eckstein W, Biersack JP. Z Phys B 1986;63(4):471–8.
[14] Martı´nez H, Castillo F, Reyes PG, Santibanez FG. Int J Mass Spectrom 2003;228: 107–16. [15] Virostko P, Hubicˇka Z, Ada´mek P, Cada M, Olejnı´cˇek J, Tichy´ M, et al. Contrib Plasma Phys 2006;46:445–50. [16] Ekpe SD, Dew SK. J Vac Sci Technol A 2003;21:476–83. [17] Hubicˇka Z, Pribil G, Soukup RJ, Ianno NJ. Surf Coat Technol 2002;160:114–23. [18] Matsunami N, Yamamura Y, Itikawa Y, Itoh N, Kazumata Y, Miyagawa S, et al. At Data Nucl Data Tables 1984;31:1–80. [19] Cada M, Clarke GCB, Kelly PJ, Bradley JW. Investigation of energy flux density at a substrate in a pulsed DC magnetron discharge. In: Proceedings of the 27th ICPIG, Prague, Czech Republic; 15–20 July, 2007. ISBN: 978-80-87026-01-4.