- Email: [email protected]
H density in a hydrogen plasma post-glow reactor
Vacuum 71 (2003) 201–205
H density in a hydrogen plasma post-glow reactor Miran Mozetic$ a,*, Alenka Vesela, Virginie Monnab, Andre Ricardb a
Institute of Surface Engineering and Optoelectronics, Teslova 30, 1000 Ljubljana, Slovenia b CPAT, University Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
Received 16 June 2002; received in revised form 8 August 2002; accepted 24 September 2002
Abstract A nickel catalytic probe was used to determine the density of neutral hydrogen atoms in a hydrogen post-glow reactor. The plasma was created in a side quartz tube by a surfatron microwave generator. The charged particles were recombined on the quartz surface before reaching the post-glow reactor, so that the gas entering the reactor was just a mixture of molecular and atomic hydrogen. The density of hydrogen atoms in the post-glow reactor was measured at different output power of the microwave generator and different flow of hydrogen through the vacuum system. The H density increased linearly with increasing microwave power, and it did not depend much on the hydrogen flow. At the lowest power tested, i.e. 40 W, the H density was 5 1020 m3, while at the highest power of 145 W it was about 2 1021 m3. The degree of dissociation of hydrogen molecules, on the other hand, depended on the hydrogen flow. At the flow of 80 cm3 min1 the degree of dissociation was between 4% (at the power of 40 W) and 14% (at 145 W), while at 300 cm3 min1 it dropped to between 1.5% (at 40 W) and 5% (at 145 W). The results were explained taking into account the collision phenomena in ionized gases. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Plasma; Hydrogen; Atoms; Characterization; Catalytic probe
1. Introduction Low-pressure hydrogen plasma has been used in many modern technological processing of materials including deposition of thin films of a-H:C [1–3], passivation of wafers [4–6] and discharge cleaning of different components [7–10]. It was found that the main reactant in hydrogen plasma is neutral atomic hydrogen. It is created during inelastic collision between fast electrons and hydrogen molecules. The rate of production *Corresponding author. Tel.: +386-61-1264592; fax: +38661-1264593. E-mail address: [email protected] (M. Mozeti$c).
depends on the electron density and the electron energy distribution function, and the two parameters depend on the plasma generator power. The density of H atoms depends on the production and the loss rate. At low pressure (say below 1 mbar), gas phase recombination is negligible and the main loss mechanism is heterogeneous recombination on the walls of the discharge vessel. The probability of the recombination is often expressed in terms of the recombination coefficient, which has been defined as the ratio between the number of atoms recombined at a unit surface in unit time and the flux of atoms towards the surface. The recombination coefficient has been studied extensively [11]. It was found that some glasses and
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00737-6
202
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
ceramics have a very low recombination coefficient (of the order of 104) while many metals have a recombination coefficient of 0.1 or even higher. In some cases the recombination coefficient does not depend on the surface temperature significantly, while for some other materials it was found that the recombination coefficient is a strong function of the surface temperature: it often increases steeply with increasing temperature. A metal with a constant recombination coefficient over a range of temperature between 300 and 800 K is nickel. Not surprisingly, nickel is the most common material used as a catalyst for H atoms [12–14]. Processing of materials with hydrogen plasma often takes place in pyrex vessels. Since the material is not at all that inert to H atoms, the density of H atoms in the reactor may not be very high. According to a recent paper by Ricard et al. [14] the recombination coefficient increases with increasing surface temperature. Since the walls exposed to the plasma are always heated due to a variety of mechanisms, it is often better to mount the plasma source apart from the pyrex chamber and leak only a mixture of well accommodated molecules and atoms into the chamber. Since the loss of H atoms on the way from discharge vessel to the chamber is generally not known it is wise to measure the H density right in the chamber.
volume flow meter. Hydrogen flow is always measured on the high-pressure side. A nickel catalytic probe is mounted in the center of the pyrex chamber. It consists of a catalytic surface that is a nickel disc with the thickness of 0.2 mm and the radius of 1 mm. The temperature of the disc is measured with a chromel–alumel thermocouple, which is insulated with narrow thin glass tubes and connected to the probe holder. The probe is shown in Fig. 2. The H density ðno Þ is calculated from the first derivative of the probe temperature just after the plasma is turned off [12,14]: no ¼
4Mcp dT ; gWD pr2 dt
ð1Þ
where M is the mass of the nickel disc, cp its heat capacity, g the recombination coefficient, pr2 the disc area, WD is the dissociation energy
5 6
4 3 7 2
2. Experimental Experiments were performed in a vacuum system presented in Fig. 1. The reactor is a pyrex cylinder with the diameter of 15 cm and the length of 20 cm. It is pumped with a two stage rotary pump with the pumping speed of 48 m3 h1. The pressure is measured in the chamber prior to plasma experiments (when hydrogen is not yet dissociated) with a baratron. The ultimate pressure is about 1 Pa. The discharge vessel is a quartz tube with the internal diameter of 5 mm and the length of 20 cm. The plasma is generated in the discharge tube by a surfatron microwave generator with the frequency of 2.45 GHz and the output power is adjustable up to 300 W. Commercially available hydrogen is leaked into the system through a
8
1
Fig. 1. Schematic of the experimental system: 1—rotary pump; 2—valve; 3—catalytic probe; 4—pyrex chamber; 5—vacuum gauge; 6—discharge chamber; 7—valve with flow meter; 8—hydrogen flask.
Fig. 2. Schematic of the catalytic probe: 1—nickel disc; 2—chromel–alumel thermocouple wires; 3—thin glass tube; 4—kovar wire; 5—glass tube.
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
of a hydrogen molecule (4.5 eV) and dT=dt the derivative. Experiments were performed at different output power of the microwave generator and five different volume flows of hydrogen, i.e. 80, 100, 140, 220 and 300 cm3 min1. The corresponding pressure in the pyrex chamber was 25, 30, 42, 61 and 85 Pa, respectively. The pressure versus flow is plotted in Fig. 3, indicating excellent linearity. The H density versus the power of plasma generator is shown in Fig. 4. Taking into account the accuracy of the method that has been estimated to 20% [14]
203
the H density does not depend much on the hydrogen flow. The H density increases linearly with increasing power. The slope of the line is 451—doubling the power causes doubling of H density. Fig. 5 is a plot of the degree of dissociation of hydrogen molecules versus the power of plasma generator. The degree of dissociation is calculated as Z ¼ no kT=2p; where no is the density of H atoms, k the Boltzmann constant and T the gas temperature (in our case T=300 K). p is the pressure in the chamber and the factor 2 comes
90 80
Pressure [Pa]
70 60 50 40 30 20 10 0 0
50
100
150
200
250
300
350
Hydrogen flow [cm3/min]
Fig. 3. Pressure as a function of hydrogen flow.
0.16 300 cm3/ min
0.14
220 cm3/ min 140 cm3/ min
0.12
100 cm3/ min 80 cm3 /min
η
0.10 0.08 0.06 0.04 0.02 0.00 0
50
100
150
200
P [W] Fig. 4. Density of hydrogen atoms in the pyrex chamber versus the power of the microwave generator. The parameter is the hydrogen flow.
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
204 2.5E+21
300 cm3/min 220 cm3/min
2.0E+21
140 cm3/min 100 cm3/min
nH [m-3]
80 cm3/min
1.5E+21
1.0E+21
5.0E+20
0.0E+00 0
20
40
60
80
100
120
140
160
P [W] Fig. 5. Degree of dissociation of hydrogen molecules in the pyrex chamber versus the power of the microwave generator. The parameter is the hydrogen flow.
as a molecule consists of 2 atoms. The degree of dissociation increases linearly with increasing microwave power and it also increases with decreasing hydrogen flow through the system.
3. Discussion and conclusions The degree of dissociation of hydrogen molecules is reasonably high (of the order of 0.1). Both the observed increase with increasing power and decrease with increasing flow can be explained taking into account collision phenomena in ionized gases. The production of H atoms in the plasma depends on the density of fast electrons in the plasma (those having the kinetic energy higher than the dissociation threshold). The density of fast electrons increases rather linearly with increasing microwave power, but does not depend much on the hydrogen flow. The density of H atoms therefore linearly increases with increasing power as observed in Fig. 4. Since the degree of dissociation is well below unity (Fig. 5), no saturation effect is observed at our experimental conditions, i.e. the output power up to 145 W. The slope of the =(P) curves (Fig. 5) would probably become less steep at higher power and would eventually become a constant at very high power.
Since the pressure is proportional to the flow of hydrogen through the vacuum system, the time molecules spend in the discharge region is independent of the flow. The density of H atoms (Fig. 4) does not depend on the flow indicating that the major mechanism controlling the H density is dissociation by electron impact. The loss rate due to gas phase recombination can obviously be neglected. Otherwise, the H density should decrease with increasing hydrogen flow. The degree of dissociation (Fig. 5) is just the H density divided by the hydrogen pressure. Since the H density is independent of the flow and the pressure increases with increasing flow, the degree of dissociation decreases with increasing flow. The numerical values of the degree of dissociation are between 0.015 and 0.14, depending on the experimental conditions. The highest dissociation degree is observed at the highest power and lowest hydrogen flow. The value is rather typical for vacuum chambers made of a material with a low recombination coefficient for the reaction H+HH2. The values obtained at low power and high flow are rather low. However, the poor degree of dissociation obtained at that experimental conditions are not due to extensive recombination, but rather poor production of H atom in the discharge region.
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
Acknowledgements The experiments described in the paper were performed during the visit of the Slovenian partner in France sponsored by NATO Science Programme—Expert Visit Grant PST.EV.976469.
References [1] Cuomo J, Tzeng Y, Yoshikawa M, Murakawa M, Feldman A. Application of Diamond Films and Related Materials. Elsevier: Amsterdam, 1991. [2] Ricard A. Reactive Plasmas. Paris: Societe Francais du Vide, 1996. [3] Piekarczyk W, Prawer S. Diamond Relat Mater 1993;2:41. [4] Somashekar A, Ying H, Smith PB, Aldrich DB, Nemanich RJ. J Electrochem Soc 1999;146:2318.
205
[5] King SW, Barnak JP, Bremser MD, Tracky KM, Ronning C, Davis RF, Nemanich JR. J Appl Phys 1998;84:5248. [6] Montgomery JS, Schneider TP, Carter RJ, Barnak JP, Chen YL, Hauser JR, Nemanich JR. Appl Phys Lett 1995;67:2194. [7] Veprek S, Elmer J, Eckermann C, Jur$cik M. J Electrochem Soc 1987;134:2398. [8] De Graaf MJ, Severens R, Van de Sanden MJF, Schram DC, Meijers HJM, Kars H. J Nucl Mater 1993;200:380. [9] Mozeti$c M. Vacuum 2001;61:376. $ [10] Sorli I, Patasch W, Kegel B, Smid H, Liebl G. Inform Midem 1996;26:35. [11] Wise H, Wood BJ. In: Bates DR, Estermann I, editor. Advances in atomic and molecular physics. New York: Academic Press, 1967. [12] Mozeti$c M, Drobni$c M, Pregelj A, Zupan K. Vacuum 1996;47:943. [13] Mozeti$c M. Ann NY Acad Sci 1999;891:325. [14] Ricard A, Gaillard M, Monna V, Vesel A, Mozeti$c M. Surf Coat Technol 2001;143–144:333.
H density in a hydrogen plasma post-glow reactor Miran Mozetic$ a,*, Alenka Vesela, Virginie Monnab, Andre Ricardb a
Institute of Surface Engineering and Optoelectronics, Teslova 30, 1000 Ljubljana, Slovenia b CPAT, University Paul Sabatier, 118 route de Narbonne, 31062 Toulouse, France
Received 16 June 2002; received in revised form 8 August 2002; accepted 24 September 2002
Abstract A nickel catalytic probe was used to determine the density of neutral hydrogen atoms in a hydrogen post-glow reactor. The plasma was created in a side quartz tube by a surfatron microwave generator. The charged particles were recombined on the quartz surface before reaching the post-glow reactor, so that the gas entering the reactor was just a mixture of molecular and atomic hydrogen. The density of hydrogen atoms in the post-glow reactor was measured at different output power of the microwave generator and different flow of hydrogen through the vacuum system. The H density increased linearly with increasing microwave power, and it did not depend much on the hydrogen flow. At the lowest power tested, i.e. 40 W, the H density was 5 1020 m3, while at the highest power of 145 W it was about 2 1021 m3. The degree of dissociation of hydrogen molecules, on the other hand, depended on the hydrogen flow. At the flow of 80 cm3 min1 the degree of dissociation was between 4% (at the power of 40 W) and 14% (at 145 W), while at 300 cm3 min1 it dropped to between 1.5% (at 40 W) and 5% (at 145 W). The results were explained taking into account the collision phenomena in ionized gases. r 2003 Elsevier Science Ltd. All rights reserved. Keywords: Plasma; Hydrogen; Atoms; Characterization; Catalytic probe
1. Introduction Low-pressure hydrogen plasma has been used in many modern technological processing of materials including deposition of thin films of a-H:C [1–3], passivation of wafers [4–6] and discharge cleaning of different components [7–10]. It was found that the main reactant in hydrogen plasma is neutral atomic hydrogen. It is created during inelastic collision between fast electrons and hydrogen molecules. The rate of production *Corresponding author. Tel.: +386-61-1264592; fax: +38661-1264593. E-mail address: [email protected] (M. Mozeti$c).
depends on the electron density and the electron energy distribution function, and the two parameters depend on the plasma generator power. The density of H atoms depends on the production and the loss rate. At low pressure (say below 1 mbar), gas phase recombination is negligible and the main loss mechanism is heterogeneous recombination on the walls of the discharge vessel. The probability of the recombination is often expressed in terms of the recombination coefficient, which has been defined as the ratio between the number of atoms recombined at a unit surface in unit time and the flux of atoms towards the surface. The recombination coefficient has been studied extensively [11]. It was found that some glasses and
0042-207X/03/$ - see front matter r 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0042-207X(02)00737-6
202
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
ceramics have a very low recombination coefficient (of the order of 104) while many metals have a recombination coefficient of 0.1 or even higher. In some cases the recombination coefficient does not depend on the surface temperature significantly, while for some other materials it was found that the recombination coefficient is a strong function of the surface temperature: it often increases steeply with increasing temperature. A metal with a constant recombination coefficient over a range of temperature between 300 and 800 K is nickel. Not surprisingly, nickel is the most common material used as a catalyst for H atoms [12–14]. Processing of materials with hydrogen plasma often takes place in pyrex vessels. Since the material is not at all that inert to H atoms, the density of H atoms in the reactor may not be very high. According to a recent paper by Ricard et al. [14] the recombination coefficient increases with increasing surface temperature. Since the walls exposed to the plasma are always heated due to a variety of mechanisms, it is often better to mount the plasma source apart from the pyrex chamber and leak only a mixture of well accommodated molecules and atoms into the chamber. Since the loss of H atoms on the way from discharge vessel to the chamber is generally not known it is wise to measure the H density right in the chamber.
volume flow meter. Hydrogen flow is always measured on the high-pressure side. A nickel catalytic probe is mounted in the center of the pyrex chamber. It consists of a catalytic surface that is a nickel disc with the thickness of 0.2 mm and the radius of 1 mm. The temperature of the disc is measured with a chromel–alumel thermocouple, which is insulated with narrow thin glass tubes and connected to the probe holder. The probe is shown in Fig. 2. The H density ðno Þ is calculated from the first derivative of the probe temperature just after the plasma is turned off [12,14]: no ¼
4Mcp dT ; gWD pr2 dt
ð1Þ
where M is the mass of the nickel disc, cp its heat capacity, g the recombination coefficient, pr2 the disc area, WD is the dissociation energy
5 6
4 3 7 2
2. Experimental Experiments were performed in a vacuum system presented in Fig. 1. The reactor is a pyrex cylinder with the diameter of 15 cm and the length of 20 cm. It is pumped with a two stage rotary pump with the pumping speed of 48 m3 h1. The pressure is measured in the chamber prior to plasma experiments (when hydrogen is not yet dissociated) with a baratron. The ultimate pressure is about 1 Pa. The discharge vessel is a quartz tube with the internal diameter of 5 mm and the length of 20 cm. The plasma is generated in the discharge tube by a surfatron microwave generator with the frequency of 2.45 GHz and the output power is adjustable up to 300 W. Commercially available hydrogen is leaked into the system through a
8
1
Fig. 1. Schematic of the experimental system: 1—rotary pump; 2—valve; 3—catalytic probe; 4—pyrex chamber; 5—vacuum gauge; 6—discharge chamber; 7—valve with flow meter; 8—hydrogen flask.
Fig. 2. Schematic of the catalytic probe: 1—nickel disc; 2—chromel–alumel thermocouple wires; 3—thin glass tube; 4—kovar wire; 5—glass tube.
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
of a hydrogen molecule (4.5 eV) and dT=dt the derivative. Experiments were performed at different output power of the microwave generator and five different volume flows of hydrogen, i.e. 80, 100, 140, 220 and 300 cm3 min1. The corresponding pressure in the pyrex chamber was 25, 30, 42, 61 and 85 Pa, respectively. The pressure versus flow is plotted in Fig. 3, indicating excellent linearity. The H density versus the power of plasma generator is shown in Fig. 4. Taking into account the accuracy of the method that has been estimated to 20% [14]
203
the H density does not depend much on the hydrogen flow. The H density increases linearly with increasing power. The slope of the line is 451—doubling the power causes doubling of H density. Fig. 5 is a plot of the degree of dissociation of hydrogen molecules versus the power of plasma generator. The degree of dissociation is calculated as Z ¼ no kT=2p; where no is the density of H atoms, k the Boltzmann constant and T the gas temperature (in our case T=300 K). p is the pressure in the chamber and the factor 2 comes
90 80
Pressure [Pa]
70 60 50 40 30 20 10 0 0
50
100
150
200
250
300
350
Hydrogen flow [cm3/min]
Fig. 3. Pressure as a function of hydrogen flow.
0.16 300 cm3/ min
0.14
220 cm3/ min 140 cm3/ min
0.12
100 cm3/ min 80 cm3 /min
η
0.10 0.08 0.06 0.04 0.02 0.00 0
50
100
150
200
P [W] Fig. 4. Density of hydrogen atoms in the pyrex chamber versus the power of the microwave generator. The parameter is the hydrogen flow.
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
204 2.5E+21
300 cm3/min 220 cm3/min
2.0E+21
140 cm3/min 100 cm3/min
nH [m-3]
80 cm3/min
1.5E+21
1.0E+21
5.0E+20
0.0E+00 0
20
40
60
80
100
120
140
160
P [W] Fig. 5. Degree of dissociation of hydrogen molecules in the pyrex chamber versus the power of the microwave generator. The parameter is the hydrogen flow.
as a molecule consists of 2 atoms. The degree of dissociation increases linearly with increasing microwave power and it also increases with decreasing hydrogen flow through the system.
3. Discussion and conclusions The degree of dissociation of hydrogen molecules is reasonably high (of the order of 0.1). Both the observed increase with increasing power and decrease with increasing flow can be explained taking into account collision phenomena in ionized gases. The production of H atoms in the plasma depends on the density of fast electrons in the plasma (those having the kinetic energy higher than the dissociation threshold). The density of fast electrons increases rather linearly with increasing microwave power, but does not depend much on the hydrogen flow. The density of H atoms therefore linearly increases with increasing power as observed in Fig. 4. Since the degree of dissociation is well below unity (Fig. 5), no saturation effect is observed at our experimental conditions, i.e. the output power up to 145 W. The slope of the =(P) curves (Fig. 5) would probably become less steep at higher power and would eventually become a constant at very high power.
Since the pressure is proportional to the flow of hydrogen through the vacuum system, the time molecules spend in the discharge region is independent of the flow. The density of H atoms (Fig. 4) does not depend on the flow indicating that the major mechanism controlling the H density is dissociation by electron impact. The loss rate due to gas phase recombination can obviously be neglected. Otherwise, the H density should decrease with increasing hydrogen flow. The degree of dissociation (Fig. 5) is just the H density divided by the hydrogen pressure. Since the H density is independent of the flow and the pressure increases with increasing flow, the degree of dissociation decreases with increasing flow. The numerical values of the degree of dissociation are between 0.015 and 0.14, depending on the experimental conditions. The highest dissociation degree is observed at the highest power and lowest hydrogen flow. The value is rather typical for vacuum chambers made of a material with a low recombination coefficient for the reaction H+HH2. The values obtained at low power and high flow are rather low. However, the poor degree of dissociation obtained at that experimental conditions are not due to extensive recombination, but rather poor production of H atom in the discharge region.
M. Mozeti$c et al. / Vacuum 71 (2003) 201–205
Acknowledgements The experiments described in the paper were performed during the visit of the Slovenian partner in France sponsored by NATO Science Programme—Expert Visit Grant PST.EV.976469.
References [1] Cuomo J, Tzeng Y, Yoshikawa M, Murakawa M, Feldman A. Application of Diamond Films and Related Materials. Elsevier: Amsterdam, 1991. [2] Ricard A. Reactive Plasmas. Paris: Societe Francais du Vide, 1996. [3] Piekarczyk W, Prawer S. Diamond Relat Mater 1993;2:41. [4] Somashekar A, Ying H, Smith PB, Aldrich DB, Nemanich RJ. J Electrochem Soc 1999;146:2318.
205
[5] King SW, Barnak JP, Bremser MD, Tracky KM, Ronning C, Davis RF, Nemanich JR. J Appl Phys 1998;84:5248. [6] Montgomery JS, Schneider TP, Carter RJ, Barnak JP, Chen YL, Hauser JR, Nemanich JR. Appl Phys Lett 1995;67:2194. [7] Veprek S, Elmer J, Eckermann C, Jur$cik M. J Electrochem Soc 1987;134:2398. [8] De Graaf MJ, Severens R, Van de Sanden MJF, Schram DC, Meijers HJM, Kars H. J Nucl Mater 1993;200:380. [9] Mozeti$c M. Vacuum 2001;61:376. $ [10] Sorli I, Patasch W, Kegel B, Smid H, Liebl G. Inform Midem 1996;26:35. [11] Wise H, Wood BJ. In: Bates DR, Estermann I, editor. Advances in atomic and molecular physics. New York: Academic Press, 1967. [12] Mozeti$c M, Drobni$c M, Pregelj A, Zupan K. Vacuum 1996;47:943. [13] Mozeti$c M. Ann NY Acad Sci 1999;891:325. [14] Ricard A, Gaillard M, Monna V, Vesel A, Mozeti$c M. Surf Coat Technol 2001;143–144:333.