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Reduction of byproduct particle size using low-pressure plasmas generated by a cylindrical-shaped electrode
Vacuum 86 (2012) 1834e1839
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Reduction of byproduct particle size using low-pressure plasmas generated by a cylindrical-shaped electrode M. Hur a, *, J.O. Lee a, H.A. Yoo a, W.S. Kang a, Y.H. Song a, D.G. Kim b, S.Y. Lee b a b
Plasma Engineering Laboratory, Korea Institute of Machinery & Materials, 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea Head Office & Factory, LOTVACUUM Co., Ltd., 59-7 Shingeonji-dong, Anseong-si, Gyeonggi-do 456-370, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 3 April 2012 Accepted 3 April 2012
In the case of a plasma-enhanced chemical vapor deposition, large amounts of byproduct particles arrive at a vacuum pump in spite of the use of cleaning process. As these particles are accumulated on internal components of a vacuum pump, its performance is reduced. A plasma reactor located right before a vacuum pump is proposed with an aim to reduce the size and quantity of byproduct particles, thereby to improve the durability of a vacuum pump. The plasma reactor and electrode all have a concentric cylindrical shape, which allows the device to be easily connected to pre-existing vacuum pipelines without any disturbance to the exhaust stream. The sizes of byproduct particles before the vacuum pump are quite diverse in the range of tens of nanometers to tens of micrometers. The plasma reactor makes the size and quantity of byproduct particles much smaller, and the particles larger than 1 mm are not observed anymore. This result indicates that the vacuum pump durability can be significantly improved by using plasmas. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Low-pressure plasma Byproduct particle size reduction Vacuum pump durability
1. Introduction Dust particles are observed in all plasmas used for etching, deposition, and sputtering, and they are considered as an important source of device contamination in the semiconductor industry [1,2]. Chamber cleaning based on fluorine chemistries is usually carried out to convert dust particles into volatile gaseous species that can be exhausted from a processing chamber. Cleaning process, however, can’t completely remove dust particles so that residual particles always survive outside of the processing chamber. These byproduct particles head for a vacuum pump, where a part of them are accumulated, which causes another side effect: reduction in vacuum pump durability. The build-up of byproduct particles on internal components of a vacuum pump leads to the decrease in pumping speed, and this is currently a serious concern in the semiconductor industry. Fig. 1 shows a typical shape of internal surface of a pipeline connected to a vacuum pump used for the SiO2 film formation by a plasma-enhanced chemical vapor deposition (PECVD). This confirms that a lot of solid residual particles arrive at a vacuum pump in spite of the use of cleaning process. The use of low-pressure plasmas is known to be an attractive approach for the treatment of gaseous pollutants, because the
* Corresponding author. Tel.: þ82 42 868 7634; fax: þ82 42 868 7284. E-mail address: [email protected] (M. Hur). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.04.002
abundant reactive species in plasmas can easily breakdown chemical bonds. For this reason, many researchers have developed plasma reactors placed between a processing chamber and a vacuum pump for the abatement of the greenhouse gases emitted by semiconductor manufacturing processes. It was already verified in many research works that low-pressure plasmas are effective in abating even CF4 which is most difficult perfluorinated compounds to destroy because of the strong CeF covalent bond [3e6]. However, no study of plasma technology has been published so far from the view point of vacuum pump durability. This work aims to investigate the applicability of low-pressure plasma technology to the increase in durability of vacuum pumps used in the semiconductor industry. Particles much smaller than the gap between the rotors, typically tens of micrometers, are expected to be easily penetrated through the vacuum pump, so that the durability of vacuum pump is closely related to the size and quantity of byproduct particles. Similar to the case of chamber cleaning, byproduct particles can be converted into volatile gaseous products by passing through the fluorine-included plasma. The fluorine-included plasma can also influence the size of residual particles by etching their surface. If the plasma can make the size and quantity of byproduct particles much smaller, the vacuum pump durability is expected to be greatly enhanced. With an aim to suppress the decrease in pumping speed by byproduct particles, we propose a cylindrical-shaped plasma reactor placed right before a vacuum pump. In this work, the electrical and discharge
M. Hur et al. / Vacuum 86 (2012) 1834e1839
Fig. 1. Image of a vacuum pipeline covered with SiO2 byproduct particles generated in SiO2 film formation process.
characteristics of the plasmas generated in cylindrical-shaped reactor are investigated based on the voltage-power curve in conjunction with the plasma image. The sizes of byproduct particles for plasma on and off are compared using the scanning electron microscope (SEM). Finally, the applicability of low-pressure plasmas to the extension of vacuum pump lifetime is discussed. 2. Experimental Fig. 2 shows a schematic of the experimental system, which consists of a gas supply unit, two plasma systems, and a vacuum pump. For the convenience of explanation, the two plasma systems used for film deposition and particle treatment are expressed as the
Fig. 2. Schematic of the experimental system.
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PECVD chamber and the plasma reactor, respectively. A more detailed reactor design is given in Fig. 3. The plasma reactor consists of a quartz tube wrapped in a metal electrode. A sinusoidal high voltage signal with a frequency of 60 kHz (Dawonsys, AP-61k0714) is applied to the electrode on the quartz tube, whereas the vacuum pipelines connected to the reactor are grounded. The powered electrode and the grounded pipelines are separated by small gaps, so that a kind of capacitively-coupled plasma is generated between them. The reactor and electrode have a simple cylindrical shape, which allows them to be easily connected to the vacuum pipelines already existing in the semiconductor industry without any disturbance to the exhaust stream. The plasma reactor is placed before a vacuum pump (Lotvacuum, HD 1200) which consists of a dry pump and a mechanical booster. The maximum pumping speed of the vacuum pump is 1200 m3/h. The inner diameter of the quartz tube and the width of the powered electrode are 16 and 40 cm, respectively. The grounded pipelines have the same diameter as the powered electrode at their interfaces. To obtain more uniform plasmas in space, the diameter of the pipelines slightly decreases with increasing distance from the interfaces [6]. Semiconductor manufacturing process is simulated by using a commercialized PECVD equipment (Applied Materials, P-5000 Mark II), where SiO2 films are deposited using tetraethylorthosilicate (TEOS) and O2. As given in Table 1, PECVD process consists of three steps (deposition - cleaning I e cleaning II). During deposition process, a 1.0 mm thick SiO2 film is formed on a 150 mm silicon wafer while dust particles are generated as a byproduct. During cleaning processes, NF3, O2, and He are injected into the CVD chamber and dust particles are removed from the interior surfaces of the chamber by reactions with fluorine radicals decomposed from NF3 plasma. A gaseous product SiF4 and SiO2 residual particles are pumped out of the CVD chamber and move toward the vacuum pump. Sampling of byproduct particles is carried out at two positions. First, a sampling collector is placed 60 cm downstream of the plasma reactor with a view to investigating the change in size of byproduct particles by plasmas. As shown in Fig. 3, a silicon wafer with a diameter of 3 cm is on the bottom of the sampling collector. The size and morphology of byproduct particles on the wafer are analyzed with a scanning electron microscope (Philips-XL30S FEG). Second, influence of plasma reactor on the quantity of byproduct particles are indirectly investigated by using a stainless steel flange placed downstream of the sampling collector. Byproduct particles passing through the plasma reactor are accumulated on the sampling flange with time, so that their quantity can be evaluated. Processes in Table 1 iterate three times to collect enough particles to observe.
Fig. 3. Schematic of the plasma reactor with a cylindrical-shaped electrode.
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M. Hur et al. / Vacuum 86 (2012) 1834e1839
Table 1 Processing conditions used to form SiO2 films in the PECVD chamber.
Pressure (Torr) RF power I (W) RF power II (W) Gas flow rate (sccm)
Duration (s)
Deposition
Cleaning I
Cleaning II
14 190 90 He: 300 O2: 460 TEOS: 460 190
5 600 0 NF3: 600 O2: 500 He: 200 100
2 600 0 NF3: 500 O2: 400 He:150 170
3. Results and discussion Fig. 4 shows the voltage-power characteristics of the plasma reactor measured for deposition and cleaning conditions. For the two cases the power increases with raising the applied voltage, but the voltage-power curves have different trends. For deposition condition, the power increases monotonically with a rise in voltage, and the values of the power at the rising stage of the voltage almost coincide with those of the power at the falling stage. For cleaning condition, the power is more greatly influenced by the variation of voltage. In addition, the values of the power at the rising stage of the voltage are different from those of the power at the falling stage: voltage-power curve has a hysteresis. The discharge images are closely related to the voltage-power characteristics. For deposition condition, the plasmas fill the entire volume of the reactor regardless of applied voltage. For cleaning condition, the plasmas at low voltages are observed only near the electrode surface as shown in Fig. 4. With increasing the voltage, the plasmas are expanded in the radial direction and occupy the entire volume of the reactor only above 2.9 kV. NF3 is widely used as a cleaning gas because of its short lifetime, high destruction efficiency, and fast cleaning time. NF3 plasmas are, however, known to be susceptible to the instability, which is caused by their electronegative property. Because stable and uniform plasmas are necessary for in situ chamber cleaning, the relationship between plasma stability and cleaning parameters were investigated in many works [7e9]. For example, Ji et al. [7,8] observed that NF3 plasmas sometimes contract into part of the chamber while leaving the other part of the chamber unexposed to the plasma. At a specific condition, NF3 plasmas severely oscillate and even collapse. They reported that NF3 plasmas can be stabilized by
Fig. 4. Voltage-power curves of the plasma reactor for deposition and cleaning conditions of the PECVD chamber. For cleaning condition, the plasmas at low voltages are generated only near the electrode surface whereas the plasmas occupy the entire volume of the reactor above 2.9 kV.
diluting with an inert gas and adjusting the power and flow rate. The peculiar voltage-power characteristic for cleaning condition shown in Fig. 4 seems to be caused by electronegative property of NF3 plasmas. The relationships between the voltage-power curve and discharge image for cleaning condition are similar to those found in the discharge mode transition in other structures (a-g mode transition for the capacitively-coupled plasmas in parallel plates and E-H mode transition for the inductively coupled plasmas). Further studies such as effect of working gas species and pressure on the voltageecurrent characteristics and the discharge evolutions in time and space would be required for understanding of the discharge mode transition observed in this work. The low bonding energy of NF3 (2.47 eV) allows for its rapid dissociation in plasmas [7,8],
NF3 þ e/NF2 þ F þ e:
(1)
NF3 can also release fluorine atoms via dissociative ionization (15 eV) [10],
NF3 þ e/NFþ 2 þ F þ e:
(2)
The F atoms are partly recombined into molecular fluorine via the threeebody reaction,
2F þ M/F2 þ M:
(3)
Chamber cleaning use the atomic and molecular fluorine radicals generated from reactions (1)e(3). The F and F2 react with dust particles via
SiO2 ðsÞ þ 4FðgÞ/SiF4 ðgÞ þ O2 ðgÞ;
(4)
and
SiO2 ðsÞ þ 2F2 ðgÞ/SiF4 ðgÞ þ O2 ðgÞ:
(5)
The gaseous product, SiF4, is produced as a result of cleaning process, but solid SiO2 residues also release from the CVD chamber as well. Fig. 5 shows the SEM images of the byproduct particles obtained on the silicon wafer in the sampling collector for plasma off. Particle sizes are quite diverse in the range of tens of nanometers to tens of micrometers. These particles are composed of smaller primary particles. During deposition process, the primary particles act as precursors for dust particles, which are agglomerated by coagulation. The agglomerated particles are further grown by sticking on their surface. Using an in situ laser light scattering technique and an ex situ SEM analysis, Setyawan et al. [11] and Shimada et al. [12] measured the size of particles formed in the deposition chamber working in TEOS/O2 plasmas. According to their results, the sizes of particles generated in the deposition chamber are estimated to be below a few micrometers. Deposition process is followed by cleaning process, when dust particles are changed into the gaseous product, SiF4. However, Fig. 5 clearly shows that large amounts of SiO2 byproduct particles remain even after cleaning process. The boundary among the byproduct particles is not clear, which indicates that the surface deposition among them has occurred downstream of the CVD chamber. The sizes of the particles in Fig. 5 are much larger than those of dust particles observed in Setyawan et al. and Shimada et al.’s works. The size of big particles reaches tens of micrometers. This indicates that the size of SiO2 particles has increased after the CVD chamber. Fig. 6 shows the influence of plasma reactor on the quantity of byproduct particles. To avoid the additional particle generation from the reaction between TEOS and O2, the plasma reactor is turned on only during the cleaning processes. When SiO2 particles pass through the plasma, small-sized particles are converted into
M. Hur et al. / Vacuum 86 (2012) 1834e1839
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Fig. 5. SEM images of byproduct particles accumulated on the particle collector for plasma off.
Fig. 6. Comparison of the quantity of byproduct particles accumulated on the sampling flange between plasma on and off. A part of the sampling flanges are covered with carbon tapes used for attaching a silicon wafer.
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M. Hur et al. / Vacuum 86 (2012) 1834e1839
Fig. 7. SEM images of byproduct particles accumulated on the particle collector for plasma on.
gaseous SiF4 by the reactions (4) and (5). Fig. 6 clearly indicates that the quantity of byproduct particles is significantly reduced by the plasma. Fig. 7 shows the SEM images of the byproduct particles for plasma on. It is clearly seen that the plasma greatly influences the size of byproduct particles. When the plasma reactor is turned on, the particle surfaces agglomerated by coagulation are etched away by F and F2 radicals generated in plasmas. As a result largesized byproduct particles are changed into small-sized particles. As expected, the sizes of byproduct particles at 2.6 kV are bigger than those at 3.0 kV. For both cases, the particles larger than 1 mm, however, are not observed anywhere. Fig. 7 shows that the Si wafer surface is slightly etched although it is unexposed to the plasma. The etched surface is more clearly seen for the wafer at 3.0 kV. This is probably caused by the F2 radicals that have longer lifetime than the F radials [7,8]. The F2 radicals can survive even outside of the plasma as verified in Fig. 7. Therefore, NF3 plasmas not only directly reduce the size of byproduct particles, but also may remove residues from the surfaces of internal components of the vacuum pump. This can be particularly advantageous for other processes that leave bigger byproduct particles.
4. Conclusions We demonstrated the feasibility of a low-pressure plasma reactor for reduction of the size and quantity of residual particles remaining after cleaning process. In spite of the use of cleaning process, large amounts of byproduct particles are still observed upstream of the vacuum chamber. Before the vacuum pump particle sizes are quite diverse in the range of tens of nanometers to tens of micrometers. When the plasma reactor is turned on, smallsized particles are converted into gaseous SiF4 while large-sized particles are changed into small particles. As these results the quantity and size of byproduct particles are significantly reduced. The particles larger than 1 mm are not observed anywhere. This result indicates that low-pressure plasma technology has the great potential to increase the lifetime of the vacuum pumps. Acknowledgments This work is supported by the Korea Small & Medium Business Administration and Korea Research Council for Industrial Science & Technology.
M. Hur et al. / Vacuum 86 (2012) 1834e1839
References [1] Boufendi L, Bouchoule A. Particle nucleation and growth in a low-pressure argon-silane discharge. Plasma Sources Sci Technol 1994;3:262e7. [2] Boufendi L, Jouanny MC, Kovacevic E, Berndt J, Mikikian M. Dusty Plasma for nanotechnology. J Phys D Appl Phys 2011;44:174035e40. [3] Chang MB, Chang JS. Abatement of PFCs from semiconductor manufacturing processes by nonthermal plasma technologies. Ind Eng Chem Res 2006;45: 4101e9. [4] Kiehbauch MW, Graves DB. Temperature resolved modeling of plasma abatement of perfluorinated compounds. J Appl Phys 2001;89:2047e57. [5] Hur M, Lee JO, Song YH. AC low-pressure plasmas generated by using annularshaped electrodes for abatement of pollutants emitted during semiconductor manufacturing processes. J Korean Phys Soc 2011;59:2742e9. [6] Hur M, Lee JO, Song YH, Yoo HA. Abatement of CF4 and CHF3 byproducts using low-pressure plasmas generated by annular-shaped electrodes. J Vac Sci Technol A 2012;30:021305e12.
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[7] Ji B, Elder DL, Yang JH, Badowski PR, Karwacki EJ. Power dependence of NF3 plasma stability for in situ chamber cleaning. J Appl Phys 2004;95:4446e51. [8] Ji B, Yang JH, Badowski PR, Karwacki EJ. Optimization and analysis of NF3 in situ chamber cleaning plasmas. J Appl Phys 2004;95:4452e62. [9] Entley WR, Langan JG, Felker BS, Sobolewski MA. Optimizing utilization efficiencies in electronegative discharges: the importance of the impedance phase angle. J Appl Phys 1999;86:4825e35. [10] Hsueh HP, McGrath RT, Ji B, Felker BS, Langan JG, Karwacki EJ. Ion energy distributions and optical emission spectra in NF3-based process chamber cleaning plasmas. J Vac Sci Technol B 2001;19:1346e57. [11] Setyawan H, Shimada M, Hayashi Y, Okuyama K, Yokoyama S. Particle formation and trapping behavior in a TEOS/O2 plasma and their effects on contamination of a Si wafer. Aerosol Sci Technol 2004;38:120e7. [12] Shimada M, Setyawan H, Shimada H, Hayashi Y, Kashihara N, Okuyama K, et al. Incorporation of dust particles into a growing film during silicon dioxide deposition from a TEOS/O2 plasma. Aerosol Sci Technol 2005;39: 408e14.
Contents lists available at SciVerse ScienceDirect
Vacuum journal homepage: www.elsevier.com/locate/vacuum
Reduction of byproduct particle size using low-pressure plasmas generated by a cylindrical-shaped electrode M. Hur a, *, J.O. Lee a, H.A. Yoo a, W.S. Kang a, Y.H. Song a, D.G. Kim b, S.Y. Lee b a b
Plasma Engineering Laboratory, Korea Institute of Machinery & Materials, 104 Sinseongno, Yuseong-gu, Daejeon 305-343, Republic of Korea Head Office & Factory, LOTVACUUM Co., Ltd., 59-7 Shingeonji-dong, Anseong-si, Gyeonggi-do 456-370, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 February 2012 Received in revised form 3 April 2012 Accepted 3 April 2012
In the case of a plasma-enhanced chemical vapor deposition, large amounts of byproduct particles arrive at a vacuum pump in spite of the use of cleaning process. As these particles are accumulated on internal components of a vacuum pump, its performance is reduced. A plasma reactor located right before a vacuum pump is proposed with an aim to reduce the size and quantity of byproduct particles, thereby to improve the durability of a vacuum pump. The plasma reactor and electrode all have a concentric cylindrical shape, which allows the device to be easily connected to pre-existing vacuum pipelines without any disturbance to the exhaust stream. The sizes of byproduct particles before the vacuum pump are quite diverse in the range of tens of nanometers to tens of micrometers. The plasma reactor makes the size and quantity of byproduct particles much smaller, and the particles larger than 1 mm are not observed anymore. This result indicates that the vacuum pump durability can be significantly improved by using plasmas. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: Low-pressure plasma Byproduct particle size reduction Vacuum pump durability
1. Introduction Dust particles are observed in all plasmas used for etching, deposition, and sputtering, and they are considered as an important source of device contamination in the semiconductor industry [1,2]. Chamber cleaning based on fluorine chemistries is usually carried out to convert dust particles into volatile gaseous species that can be exhausted from a processing chamber. Cleaning process, however, can’t completely remove dust particles so that residual particles always survive outside of the processing chamber. These byproduct particles head for a vacuum pump, where a part of them are accumulated, which causes another side effect: reduction in vacuum pump durability. The build-up of byproduct particles on internal components of a vacuum pump leads to the decrease in pumping speed, and this is currently a serious concern in the semiconductor industry. Fig. 1 shows a typical shape of internal surface of a pipeline connected to a vacuum pump used for the SiO2 film formation by a plasma-enhanced chemical vapor deposition (PECVD). This confirms that a lot of solid residual particles arrive at a vacuum pump in spite of the use of cleaning process. The use of low-pressure plasmas is known to be an attractive approach for the treatment of gaseous pollutants, because the
* Corresponding author. Tel.: þ82 42 868 7634; fax: þ82 42 868 7284. E-mail address: [email protected] (M. Hur). 0042-207X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2012.04.002
abundant reactive species in plasmas can easily breakdown chemical bonds. For this reason, many researchers have developed plasma reactors placed between a processing chamber and a vacuum pump for the abatement of the greenhouse gases emitted by semiconductor manufacturing processes. It was already verified in many research works that low-pressure plasmas are effective in abating even CF4 which is most difficult perfluorinated compounds to destroy because of the strong CeF covalent bond [3e6]. However, no study of plasma technology has been published so far from the view point of vacuum pump durability. This work aims to investigate the applicability of low-pressure plasma technology to the increase in durability of vacuum pumps used in the semiconductor industry. Particles much smaller than the gap between the rotors, typically tens of micrometers, are expected to be easily penetrated through the vacuum pump, so that the durability of vacuum pump is closely related to the size and quantity of byproduct particles. Similar to the case of chamber cleaning, byproduct particles can be converted into volatile gaseous products by passing through the fluorine-included plasma. The fluorine-included plasma can also influence the size of residual particles by etching their surface. If the plasma can make the size and quantity of byproduct particles much smaller, the vacuum pump durability is expected to be greatly enhanced. With an aim to suppress the decrease in pumping speed by byproduct particles, we propose a cylindrical-shaped plasma reactor placed right before a vacuum pump. In this work, the electrical and discharge
M. Hur et al. / Vacuum 86 (2012) 1834e1839
Fig. 1. Image of a vacuum pipeline covered with SiO2 byproduct particles generated in SiO2 film formation process.
characteristics of the plasmas generated in cylindrical-shaped reactor are investigated based on the voltage-power curve in conjunction with the plasma image. The sizes of byproduct particles for plasma on and off are compared using the scanning electron microscope (SEM). Finally, the applicability of low-pressure plasmas to the extension of vacuum pump lifetime is discussed. 2. Experimental Fig. 2 shows a schematic of the experimental system, which consists of a gas supply unit, two plasma systems, and a vacuum pump. For the convenience of explanation, the two plasma systems used for film deposition and particle treatment are expressed as the
Fig. 2. Schematic of the experimental system.
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PECVD chamber and the plasma reactor, respectively. A more detailed reactor design is given in Fig. 3. The plasma reactor consists of a quartz tube wrapped in a metal electrode. A sinusoidal high voltage signal with a frequency of 60 kHz (Dawonsys, AP-61k0714) is applied to the electrode on the quartz tube, whereas the vacuum pipelines connected to the reactor are grounded. The powered electrode and the grounded pipelines are separated by small gaps, so that a kind of capacitively-coupled plasma is generated between them. The reactor and electrode have a simple cylindrical shape, which allows them to be easily connected to the vacuum pipelines already existing in the semiconductor industry without any disturbance to the exhaust stream. The plasma reactor is placed before a vacuum pump (Lotvacuum, HD 1200) which consists of a dry pump and a mechanical booster. The maximum pumping speed of the vacuum pump is 1200 m3/h. The inner diameter of the quartz tube and the width of the powered electrode are 16 and 40 cm, respectively. The grounded pipelines have the same diameter as the powered electrode at their interfaces. To obtain more uniform plasmas in space, the diameter of the pipelines slightly decreases with increasing distance from the interfaces [6]. Semiconductor manufacturing process is simulated by using a commercialized PECVD equipment (Applied Materials, P-5000 Mark II), where SiO2 films are deposited using tetraethylorthosilicate (TEOS) and O2. As given in Table 1, PECVD process consists of three steps (deposition - cleaning I e cleaning II). During deposition process, a 1.0 mm thick SiO2 film is formed on a 150 mm silicon wafer while dust particles are generated as a byproduct. During cleaning processes, NF3, O2, and He are injected into the CVD chamber and dust particles are removed from the interior surfaces of the chamber by reactions with fluorine radicals decomposed from NF3 plasma. A gaseous product SiF4 and SiO2 residual particles are pumped out of the CVD chamber and move toward the vacuum pump. Sampling of byproduct particles is carried out at two positions. First, a sampling collector is placed 60 cm downstream of the plasma reactor with a view to investigating the change in size of byproduct particles by plasmas. As shown in Fig. 3, a silicon wafer with a diameter of 3 cm is on the bottom of the sampling collector. The size and morphology of byproduct particles on the wafer are analyzed with a scanning electron microscope (Philips-XL30S FEG). Second, influence of plasma reactor on the quantity of byproduct particles are indirectly investigated by using a stainless steel flange placed downstream of the sampling collector. Byproduct particles passing through the plasma reactor are accumulated on the sampling flange with time, so that their quantity can be evaluated. Processes in Table 1 iterate three times to collect enough particles to observe.
Fig. 3. Schematic of the plasma reactor with a cylindrical-shaped electrode.
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M. Hur et al. / Vacuum 86 (2012) 1834e1839
Table 1 Processing conditions used to form SiO2 films in the PECVD chamber.
Pressure (Torr) RF power I (W) RF power II (W) Gas flow rate (sccm)
Duration (s)
Deposition
Cleaning I
Cleaning II
14 190 90 He: 300 O2: 460 TEOS: 460 190
5 600 0 NF3: 600 O2: 500 He: 200 100
2 600 0 NF3: 500 O2: 400 He:150 170
3. Results and discussion Fig. 4 shows the voltage-power characteristics of the plasma reactor measured for deposition and cleaning conditions. For the two cases the power increases with raising the applied voltage, but the voltage-power curves have different trends. For deposition condition, the power increases monotonically with a rise in voltage, and the values of the power at the rising stage of the voltage almost coincide with those of the power at the falling stage. For cleaning condition, the power is more greatly influenced by the variation of voltage. In addition, the values of the power at the rising stage of the voltage are different from those of the power at the falling stage: voltage-power curve has a hysteresis. The discharge images are closely related to the voltage-power characteristics. For deposition condition, the plasmas fill the entire volume of the reactor regardless of applied voltage. For cleaning condition, the plasmas at low voltages are observed only near the electrode surface as shown in Fig. 4. With increasing the voltage, the plasmas are expanded in the radial direction and occupy the entire volume of the reactor only above 2.9 kV. NF3 is widely used as a cleaning gas because of its short lifetime, high destruction efficiency, and fast cleaning time. NF3 plasmas are, however, known to be susceptible to the instability, which is caused by their electronegative property. Because stable and uniform plasmas are necessary for in situ chamber cleaning, the relationship between plasma stability and cleaning parameters were investigated in many works [7e9]. For example, Ji et al. [7,8] observed that NF3 plasmas sometimes contract into part of the chamber while leaving the other part of the chamber unexposed to the plasma. At a specific condition, NF3 plasmas severely oscillate and even collapse. They reported that NF3 plasmas can be stabilized by
Fig. 4. Voltage-power curves of the plasma reactor for deposition and cleaning conditions of the PECVD chamber. For cleaning condition, the plasmas at low voltages are generated only near the electrode surface whereas the plasmas occupy the entire volume of the reactor above 2.9 kV.
diluting with an inert gas and adjusting the power and flow rate. The peculiar voltage-power characteristic for cleaning condition shown in Fig. 4 seems to be caused by electronegative property of NF3 plasmas. The relationships between the voltage-power curve and discharge image for cleaning condition are similar to those found in the discharge mode transition in other structures (a-g mode transition for the capacitively-coupled plasmas in parallel plates and E-H mode transition for the inductively coupled plasmas). Further studies such as effect of working gas species and pressure on the voltageecurrent characteristics and the discharge evolutions in time and space would be required for understanding of the discharge mode transition observed in this work. The low bonding energy of NF3 (2.47 eV) allows for its rapid dissociation in plasmas [7,8],
NF3 þ e/NF2 þ F þ e:
(1)
NF3 can also release fluorine atoms via dissociative ionization (15 eV) [10],
NF3 þ e/NFþ 2 þ F þ e:
(2)
The F atoms are partly recombined into molecular fluorine via the threeebody reaction,
2F þ M/F2 þ M:
(3)
Chamber cleaning use the atomic and molecular fluorine radicals generated from reactions (1)e(3). The F and F2 react with dust particles via
SiO2 ðsÞ þ 4FðgÞ/SiF4 ðgÞ þ O2 ðgÞ;
(4)
and
SiO2 ðsÞ þ 2F2 ðgÞ/SiF4 ðgÞ þ O2 ðgÞ:
(5)
The gaseous product, SiF4, is produced as a result of cleaning process, but solid SiO2 residues also release from the CVD chamber as well. Fig. 5 shows the SEM images of the byproduct particles obtained on the silicon wafer in the sampling collector for plasma off. Particle sizes are quite diverse in the range of tens of nanometers to tens of micrometers. These particles are composed of smaller primary particles. During deposition process, the primary particles act as precursors for dust particles, which are agglomerated by coagulation. The agglomerated particles are further grown by sticking on their surface. Using an in situ laser light scattering technique and an ex situ SEM analysis, Setyawan et al. [11] and Shimada et al. [12] measured the size of particles formed in the deposition chamber working in TEOS/O2 plasmas. According to their results, the sizes of particles generated in the deposition chamber are estimated to be below a few micrometers. Deposition process is followed by cleaning process, when dust particles are changed into the gaseous product, SiF4. However, Fig. 5 clearly shows that large amounts of SiO2 byproduct particles remain even after cleaning process. The boundary among the byproduct particles is not clear, which indicates that the surface deposition among them has occurred downstream of the CVD chamber. The sizes of the particles in Fig. 5 are much larger than those of dust particles observed in Setyawan et al. and Shimada et al.’s works. The size of big particles reaches tens of micrometers. This indicates that the size of SiO2 particles has increased after the CVD chamber. Fig. 6 shows the influence of plasma reactor on the quantity of byproduct particles. To avoid the additional particle generation from the reaction between TEOS and O2, the plasma reactor is turned on only during the cleaning processes. When SiO2 particles pass through the plasma, small-sized particles are converted into
M. Hur et al. / Vacuum 86 (2012) 1834e1839
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Fig. 5. SEM images of byproduct particles accumulated on the particle collector for plasma off.
Fig. 6. Comparison of the quantity of byproduct particles accumulated on the sampling flange between plasma on and off. A part of the sampling flanges are covered with carbon tapes used for attaching a silicon wafer.
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M. Hur et al. / Vacuum 86 (2012) 1834e1839
Fig. 7. SEM images of byproduct particles accumulated on the particle collector for plasma on.
gaseous SiF4 by the reactions (4) and (5). Fig. 6 clearly indicates that the quantity of byproduct particles is significantly reduced by the plasma. Fig. 7 shows the SEM images of the byproduct particles for plasma on. It is clearly seen that the plasma greatly influences the size of byproduct particles. When the plasma reactor is turned on, the particle surfaces agglomerated by coagulation are etched away by F and F2 radicals generated in plasmas. As a result largesized byproduct particles are changed into small-sized particles. As expected, the sizes of byproduct particles at 2.6 kV are bigger than those at 3.0 kV. For both cases, the particles larger than 1 mm, however, are not observed anywhere. Fig. 7 shows that the Si wafer surface is slightly etched although it is unexposed to the plasma. The etched surface is more clearly seen for the wafer at 3.0 kV. This is probably caused by the F2 radicals that have longer lifetime than the F radials [7,8]. The F2 radicals can survive even outside of the plasma as verified in Fig. 7. Therefore, NF3 plasmas not only directly reduce the size of byproduct particles, but also may remove residues from the surfaces of internal components of the vacuum pump. This can be particularly advantageous for other processes that leave bigger byproduct particles.
4. Conclusions We demonstrated the feasibility of a low-pressure plasma reactor for reduction of the size and quantity of residual particles remaining after cleaning process. In spite of the use of cleaning process, large amounts of byproduct particles are still observed upstream of the vacuum chamber. Before the vacuum pump particle sizes are quite diverse in the range of tens of nanometers to tens of micrometers. When the plasma reactor is turned on, smallsized particles are converted into gaseous SiF4 while large-sized particles are changed into small particles. As these results the quantity and size of byproduct particles are significantly reduced. The particles larger than 1 mm are not observed anywhere. This result indicates that low-pressure plasma technology has the great potential to increase the lifetime of the vacuum pumps. Acknowledgments This work is supported by the Korea Small & Medium Business Administration and Korea Research Council for Industrial Science & Technology.
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