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Effects of plasma in nucleation process of thin film growth
Surface and Coatings Technology 169 – 170 (2003) 61–64
Effects of plasma in nucleation process of thin film growth N. Imamuraa, S. Asatania, S. Hashiguchia, N. Teradaa, Y. Furukawab, K. Obaraa,* a Kagoshima University, Korimoto 1-21-40, Kagoshima 890-0065, Japan Hokkaido University, Kita 19, Nishi 8, Kita-ku, Sapporo 060-0819, Japan
b
Abstract Physical surface phenomena, which occur under pulse type plasma techniques, were investigated by using optical emission spectra from gas phase species. In order to extend the time scale, mercury was used. From the temperature dependence of emission intensity at the steady state, the condensate of mercury in plasma showed two-dimensional state above 310 K, and threedimensional state below 310 K. In the transient state, time dependence of emission intensity showed ‘overshoot’ when the offtime period was longer than 64 s at 333 K, which were related to the transition from two-dimensional condensation to three-dimensional condensation in plasma. These results showed a proof that the duty cycle in the pulse plasma techniques changed the structure of the condensate. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Pulse plasma; Surface phenomena; Condensation; Mercury; Transient phenomena
1. Introduction Surface morphology of thin films depends on growth conditions. Especially, surface states of substrates are important factors to dominate the final surface morphology of the films. From technological view points, it is important to control the nucleation process of condensed matters in nano-scale w1x. In this report, we present time dependence of condensed states of mercury atoms in pulse plasma, and discuss the interaction between plasma and surface. Mercury is only metal which is in liquid state at room temperature and its vapor pressure is enough to monitor the conditions of growth processes from the local equilibrium between condensed phase and gas phase. Therefore we used mercury as an experimental substance for developing growth procedure to control initial surface morphology.
state in plasma w2x. The size of the cell was 18.5=15.0=1.5 mm3. The height of the growth cell was decreased below the critical thickness of the thermal convection for avoiding complex gas flow due to the convection. Fig. 1 shows the experimental system. The cell contains 32 Torr argon, 8 Torr xenon and approximately 1 mg mercury. An electric heater installed in a copper plate to control the temperature of the growth cell. Electrical input power for plasma was supplied from an inverter-type power source with repetition frequency 13.25 kHz, pulse width 1 ms, pulse voltage 770 V and pulse current 45 mA. Emission spectra from gaseous elements were measured by using multi-channel spectrometer with an optical fiber in the range from 330 to 850 nm. Temperature control system and data acquisition system are perfectly controlled by a computer. 3. Experimental results
2. Experiment In vapor phase crystal growth processes, the mass transport of constituent atoms depends on gas flow in the growth cell. We developed a special thin cell in order to observe growth processes in non-equilibrium *Corresponding author. Tel.yfax: q81-99-285-8401. E-mail address: [email protected] (K. Obara).
3.1. Temperature dependence of emission spectra in steady state Fig. 2 shows temperature dependence of emission spectra from mercury and xenon in steady state. The logarithmic emission intensities were proportional to the inverse of temperature and their gradients changed at 310 K. The intensity of inherent emission light from the
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 1 5 0 - 6
N. Imamura et al. / Surface and Coatings Technology 169 – 170 (2003) 61–64
62
Fig. 2. Temperature dependence of emission spectra from mercury atoms in steady state. The emission lines were classified into two groups by the magnitude of activation energies. The activation energies changed at 310 K.
Fig. 1. Schematic diagram of discharge cell and system.
excited individual mercury atoms is proportional to its density w3x. Then if the electron temperature of plasma and the excitation rate are constants in the range of experimental conditions, the emission intensity, I, from mercury atoms is expressed as follows w4,5x, B
IANexpCy D
EHg E F, kT G
(1)
where N is density of evaporation sites in unit area, EHg is the activation energy of a mercury atom for evaporation process. k and T are Boltzmann constant and temperature, respectively. Since the total of mercury atoms in the cell is constant anytime, the quantity of condensate was obtained by subtracting the quantity in total gas vapor phase from the total; NCond Hg sNHg yNHg . The activation energy, EHg, was estimated from the gradients of temperature dependence of the intensity. The estimated activation energies were classified into two groups. Group A consists of emission lines 365 and 579 nm, which have an activation energy 0.45 eV below 310 K and 0.24 eV above 310 K. Group B consists of emission lines 404, 435 and 546 nm which have an activation energy 0.68 eV below 310 K and 0.34 eV above 310 K. The activation energy of mercury estimated from saturated vapor pressure was estimated as 0.64 eV from the data in Fig. 2. The classification of activation energies was related to the initial state of the optical transition. As shown in Fig. 3, the emission lines (365, 579 nm) in group A were the transition from 61D2 and 63D3 of mercury, respectively, in which the energy is 8.83 eV. On the other hand, the emission lines (404, 435, 546 nm) in
group B were all the transition from 73S1, in which the energy is 7.73 eV. The smaller activation energies in group A suggested the interaction between excited xenon atoms and mercury atoms because the initial energy of group B located in the middle of the optical transition of xenon 823 nm emission line w6x. Since the activation energy of xenon was negative as shown in Fig. 2, if excited xenon atoms interact with mercury atoms, the activation energies of group A decrease from that of group B. On the other hand, the group B shows the same activation energy with that of mercury vapor. We, therefore, can use the emission lines of group B as tools to monitor the mercury vapor pressure in the cell. Since, below 310 K, the activation energy, 0.68 eV, of group B was approximately the same activation energy, 0.64 eV, from mercury droplets, the condensate below 310 K was considered as 3D-like droplets. Assuming the close-packed structure, energy of one bond was estimated as 0.08 eV. From this result, the number of bonds in the state with energy 0.34 eV above
Fig. 3. Energy-level diagram of Hg and Xe.
N. Imamura et al. / Surface and Coatings Technology 169 – 170 (2003) 61–64
Fig. 4. Time dependence of emission intensity of mercury 546 nm as a function of time after switching on the power at 333 K. When the off-time period was longer than 64 s, overshoot of emission intensity was observed.
310 K was estimated as four, which is the same number of bonds at the edge of mono-layer structure. 3.2. Time dependence of emission spectra in transient state In order to investigate the effects of energetic particles from plasma, time dependence of condensation states was measured at a constant temperature. After the mercury system approached the steady state, the power of plasma was turned off for a constant period. In this period, the system in the cell transits from the steady state in plasma to the thermal equilibrium state. After the off-time period, the power of plasma was switched on again. Fig. 4 shows time dependence of emission intensity of mercury 546 nm at 333 K as a function of time after switching on the plasma power. As increasing the offtime period, the emission intensity at the first point after switching on the power decreased gradually because of the increase of condensation of mercury atom in the off-time period. The time dependence of the emission intensity after switching on the plasma power showed unique time dependence due to the evaporation of excess condensate. When the off-time period was longer than 64 s, ‘the overshoot’ was observed as shown in Fig. 4. Then, the overshoot sharply decreased through the maximum, which is due to the re-condensation process from vapor phase to solid phase in plasma. The duration of the overshoot increased as increasing the off-time. As shown in the inserted graph in Fig. 4, the time dependence of emission intensity up to the maximum was well fitted by following equation; IsI`yI(0)exp(ytyt),
(2)
63
where I`, I(0) and t are expectable emission intensity at the steady state, expectable emission intensity at the end of off-time period, and time constant which reflects the bonding state of a mercury atom, respectively. Fig. 5 shows off-time dependence of estimated parameters I`, I(0) and t at 333 K. The time constant was approximately constant value 8 s up to off-time 32 s and increased sharply to 32 s above off-time period 512 s. Parameter I` showed similar off-time dependence to the time constant as shown in Fig. 5. Parameter I(0) gradually decreased at the off-time period 128 s, and then approached to the constant value above off-time period 256 s. These estimated parameters sharply changed at the off-time period 64 s. According to the result of Fig. 2, the condensate under plasma at 333 K was in two-dimensional state. Condensation process just after switching off the plasma power kept two-dimensional growth because of the thermodynamic continuity of surface system. After 64 s, however, the two-dimensional condensates transformed to three-dimensional condensates due to asymptotic approach to the thermal equilibrium state without plasma. Strong correlation between t and I` supported these considerations and the changes of the values above off-time period 64 s showed excess condensation due to three-dimensional condensates. Constant value of I(0) above off-time period 256 s suggested the equilibrium between surface condensates and gas phase mercury atoms. Asymptotic curves of t and I` above off-time period 256 s suggested the change of size distribution of three-dimensional condensates. 4. Conclusion From the temperature dependence of emission intensity at the steady state, the condensate of mercury in
Fig. 5. Off-time dependence of parameters I`, I(0) and t estimated from the data of Fig. 4. I` and I(0) sharply changed at off-time period 64 s.
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N. Imamura et al. / Surface and Coatings Technology 169 – 170 (2003) 61–64
plasma was two-dimensional state above 310 K, and three-dimensional condensates below 310 K. The existence of the ‘overshoot’ indicates the different type condensate from the steady state in plasma. From the analysis of the transient characteristics after switching on the plasma power, the transition point of the bonding state in plasma was estimated as the point at off-time period 64 s at 333 K. These characteristics show that the adjustment of the duty cycle of the pulse plasma technique is important factor to control the structure of the condensate.
References w1x M. Schneider, S. Rohde, W.D. Sproul, A. Matthews, J. Phys. D: Appl. Phys. 33 (2000) R173. w2x K. Obara, N. Imanura, J. Sakaguchi, R. Higashizono, Displays 21 (2000) 105. w3x T.J. Cotle, M.L. Passow, J.P. Fournier, M.L. Brake, M. Elta, J. Appl. Phys. 65 (5) (1991) 2885. w4x R. Agotisno, F. Cramarossa, S.D. Benedictis, G. Ferraro, J. Appl. Phys. 52 (3) (1991) 1259. w5x T. Pech, J.P. Chabrerie, A. Ricard, J. Vac. Sci. Technol. A 6 (1988) 5. w6x B. Elisasson, B. Gellert, J. Appl. Phys. 68 (1990) 2026.
Effects of plasma in nucleation process of thin film growth N. Imamuraa, S. Asatania, S. Hashiguchia, N. Teradaa, Y. Furukawab, K. Obaraa,* a Kagoshima University, Korimoto 1-21-40, Kagoshima 890-0065, Japan Hokkaido University, Kita 19, Nishi 8, Kita-ku, Sapporo 060-0819, Japan
b
Abstract Physical surface phenomena, which occur under pulse type plasma techniques, were investigated by using optical emission spectra from gas phase species. In order to extend the time scale, mercury was used. From the temperature dependence of emission intensity at the steady state, the condensate of mercury in plasma showed two-dimensional state above 310 K, and threedimensional state below 310 K. In the transient state, time dependence of emission intensity showed ‘overshoot’ when the offtime period was longer than 64 s at 333 K, which were related to the transition from two-dimensional condensation to three-dimensional condensation in plasma. These results showed a proof that the duty cycle in the pulse plasma techniques changed the structure of the condensate. 䊚 2003 Elsevier Science B.V. All rights reserved. Keywords: Pulse plasma; Surface phenomena; Condensation; Mercury; Transient phenomena
1. Introduction Surface morphology of thin films depends on growth conditions. Especially, surface states of substrates are important factors to dominate the final surface morphology of the films. From technological view points, it is important to control the nucleation process of condensed matters in nano-scale w1x. In this report, we present time dependence of condensed states of mercury atoms in pulse plasma, and discuss the interaction between plasma and surface. Mercury is only metal which is in liquid state at room temperature and its vapor pressure is enough to monitor the conditions of growth processes from the local equilibrium between condensed phase and gas phase. Therefore we used mercury as an experimental substance for developing growth procedure to control initial surface morphology.
state in plasma w2x. The size of the cell was 18.5=15.0=1.5 mm3. The height of the growth cell was decreased below the critical thickness of the thermal convection for avoiding complex gas flow due to the convection. Fig. 1 shows the experimental system. The cell contains 32 Torr argon, 8 Torr xenon and approximately 1 mg mercury. An electric heater installed in a copper plate to control the temperature of the growth cell. Electrical input power for plasma was supplied from an inverter-type power source with repetition frequency 13.25 kHz, pulse width 1 ms, pulse voltage 770 V and pulse current 45 mA. Emission spectra from gaseous elements were measured by using multi-channel spectrometer with an optical fiber in the range from 330 to 850 nm. Temperature control system and data acquisition system are perfectly controlled by a computer. 3. Experimental results
2. Experiment In vapor phase crystal growth processes, the mass transport of constituent atoms depends on gas flow in the growth cell. We developed a special thin cell in order to observe growth processes in non-equilibrium *Corresponding author. Tel.yfax: q81-99-285-8401. E-mail address: [email protected] (K. Obara).
3.1. Temperature dependence of emission spectra in steady state Fig. 2 shows temperature dependence of emission spectra from mercury and xenon in steady state. The logarithmic emission intensities were proportional to the inverse of temperature and their gradients changed at 310 K. The intensity of inherent emission light from the
0257-8972/03/$ - see front matter 䊚 2003 Elsevier Science B.V. All rights reserved. PII: S 0 2 5 7 - 8 9 7 2 Ž 0 3 . 0 0 1 5 0 - 6
N. Imamura et al. / Surface and Coatings Technology 169 – 170 (2003) 61–64
62
Fig. 2. Temperature dependence of emission spectra from mercury atoms in steady state. The emission lines were classified into two groups by the magnitude of activation energies. The activation energies changed at 310 K.
Fig. 1. Schematic diagram of discharge cell and system.
excited individual mercury atoms is proportional to its density w3x. Then if the electron temperature of plasma and the excitation rate are constants in the range of experimental conditions, the emission intensity, I, from mercury atoms is expressed as follows w4,5x, B
IANexpCy D
EHg E F, kT G
(1)
where N is density of evaporation sites in unit area, EHg is the activation energy of a mercury atom for evaporation process. k and T are Boltzmann constant and temperature, respectively. Since the total of mercury atoms in the cell is constant anytime, the quantity of condensate was obtained by subtracting the quantity in total gas vapor phase from the total; NCond Hg sNHg yNHg . The activation energy, EHg, was estimated from the gradients of temperature dependence of the intensity. The estimated activation energies were classified into two groups. Group A consists of emission lines 365 and 579 nm, which have an activation energy 0.45 eV below 310 K and 0.24 eV above 310 K. Group B consists of emission lines 404, 435 and 546 nm which have an activation energy 0.68 eV below 310 K and 0.34 eV above 310 K. The activation energy of mercury estimated from saturated vapor pressure was estimated as 0.64 eV from the data in Fig. 2. The classification of activation energies was related to the initial state of the optical transition. As shown in Fig. 3, the emission lines (365, 579 nm) in group A were the transition from 61D2 and 63D3 of mercury, respectively, in which the energy is 8.83 eV. On the other hand, the emission lines (404, 435, 546 nm) in
group B were all the transition from 73S1, in which the energy is 7.73 eV. The smaller activation energies in group A suggested the interaction between excited xenon atoms and mercury atoms because the initial energy of group B located in the middle of the optical transition of xenon 823 nm emission line w6x. Since the activation energy of xenon was negative as shown in Fig. 2, if excited xenon atoms interact with mercury atoms, the activation energies of group A decrease from that of group B. On the other hand, the group B shows the same activation energy with that of mercury vapor. We, therefore, can use the emission lines of group B as tools to monitor the mercury vapor pressure in the cell. Since, below 310 K, the activation energy, 0.68 eV, of group B was approximately the same activation energy, 0.64 eV, from mercury droplets, the condensate below 310 K was considered as 3D-like droplets. Assuming the close-packed structure, energy of one bond was estimated as 0.08 eV. From this result, the number of bonds in the state with energy 0.34 eV above
Fig. 3. Energy-level diagram of Hg and Xe.
N. Imamura et al. / Surface and Coatings Technology 169 – 170 (2003) 61–64
Fig. 4. Time dependence of emission intensity of mercury 546 nm as a function of time after switching on the power at 333 K. When the off-time period was longer than 64 s, overshoot of emission intensity was observed.
310 K was estimated as four, which is the same number of bonds at the edge of mono-layer structure. 3.2. Time dependence of emission spectra in transient state In order to investigate the effects of energetic particles from plasma, time dependence of condensation states was measured at a constant temperature. After the mercury system approached the steady state, the power of plasma was turned off for a constant period. In this period, the system in the cell transits from the steady state in plasma to the thermal equilibrium state. After the off-time period, the power of plasma was switched on again. Fig. 4 shows time dependence of emission intensity of mercury 546 nm at 333 K as a function of time after switching on the plasma power. As increasing the offtime period, the emission intensity at the first point after switching on the power decreased gradually because of the increase of condensation of mercury atom in the off-time period. The time dependence of the emission intensity after switching on the plasma power showed unique time dependence due to the evaporation of excess condensate. When the off-time period was longer than 64 s, ‘the overshoot’ was observed as shown in Fig. 4. Then, the overshoot sharply decreased through the maximum, which is due to the re-condensation process from vapor phase to solid phase in plasma. The duration of the overshoot increased as increasing the off-time. As shown in the inserted graph in Fig. 4, the time dependence of emission intensity up to the maximum was well fitted by following equation; IsI`yI(0)exp(ytyt),
(2)
63
where I`, I(0) and t are expectable emission intensity at the steady state, expectable emission intensity at the end of off-time period, and time constant which reflects the bonding state of a mercury atom, respectively. Fig. 5 shows off-time dependence of estimated parameters I`, I(0) and t at 333 K. The time constant was approximately constant value 8 s up to off-time 32 s and increased sharply to 32 s above off-time period 512 s. Parameter I` showed similar off-time dependence to the time constant as shown in Fig. 5. Parameter I(0) gradually decreased at the off-time period 128 s, and then approached to the constant value above off-time period 256 s. These estimated parameters sharply changed at the off-time period 64 s. According to the result of Fig. 2, the condensate under plasma at 333 K was in two-dimensional state. Condensation process just after switching off the plasma power kept two-dimensional growth because of the thermodynamic continuity of surface system. After 64 s, however, the two-dimensional condensates transformed to three-dimensional condensates due to asymptotic approach to the thermal equilibrium state without plasma. Strong correlation between t and I` supported these considerations and the changes of the values above off-time period 64 s showed excess condensation due to three-dimensional condensates. Constant value of I(0) above off-time period 256 s suggested the equilibrium between surface condensates and gas phase mercury atoms. Asymptotic curves of t and I` above off-time period 256 s suggested the change of size distribution of three-dimensional condensates. 4. Conclusion From the temperature dependence of emission intensity at the steady state, the condensate of mercury in
Fig. 5. Off-time dependence of parameters I`, I(0) and t estimated from the data of Fig. 4. I` and I(0) sharply changed at off-time period 64 s.
64
N. Imamura et al. / Surface and Coatings Technology 169 – 170 (2003) 61–64
plasma was two-dimensional state above 310 K, and three-dimensional condensates below 310 K. The existence of the ‘overshoot’ indicates the different type condensate from the steady state in plasma. From the analysis of the transient characteristics after switching on the plasma power, the transition point of the bonding state in plasma was estimated as the point at off-time period 64 s at 333 K. These characteristics show that the adjustment of the duty cycle of the pulse plasma technique is important factor to control the structure of the condensate.
References w1x M. Schneider, S. Rohde, W.D. Sproul, A. Matthews, J. Phys. D: Appl. Phys. 33 (2000) R173. w2x K. Obara, N. Imanura, J. Sakaguchi, R. Higashizono, Displays 21 (2000) 105. w3x T.J. Cotle, M.L. Passow, J.P. Fournier, M.L. Brake, M. Elta, J. Appl. Phys. 65 (5) (1991) 2885. w4x R. Agotisno, F. Cramarossa, S.D. Benedictis, G. Ferraro, J. Appl. Phys. 52 (3) (1991) 1259. w5x T. Pech, J.P. Chabrerie, A. Ricard, J. Vac. Sci. Technol. A 6 (1988) 5. w6x B. Elisasson, B. Gellert, J. Appl. Phys. 68 (1990) 2026.