- Email: [email protected]
Basic study of a glass substrate in dry etching system
ARTICLE IN PRESS
Vacuum 81 (2006) 344–346 www.elsevier.com/locate/vacuum
Basic study of a glass substrate in dry etching system Hee-Hwan Choe School of Electronics, Telecommunications and Computer Engineering, Hankuk Aviation University, 200-1 Hwajeon-dong, Goyang-city, Gyeonggi-do, Republic of Korea Received 10 January 2006; received in revised form 19 May 2006; accepted 1 June 2006
Abstract Electrostatic Chucks (ESCs) in the dry etchers for the TFT-LCD fabrications have been investigated briefly. The behaviors of glass encountering electric fields in the presence of plasma were studied. In some conditions, it has been shown that the electric force and the pressure by He backside cooling acting on the glass might be larger than the gravitational force. With a simple model, the stability condition of glass substrate was obtained. r 2006 Elsevier Ltd. All rights reserved. Keywords: Glass; ESC; Plasma; Dry etching
1. Introduction Substrate holding systems are used in the microelectronics fabrication processes such as dry etching, photo lithography, and chemical vapor deposition (CVD). For the vacuum processes, it is hard to use the vacuum holding system to protect substrate from some forces. Therefore, the mechanical system has been used, until the ESC (Electrostatic Chuck) technology was introduced [1]. There were some reports of wafer substrates handling problem in using ESC [2,3]. However, there were rare reports about the force on the glass substrate. A glass substrate is widely used for display devices such as organic light emitting device (OLED), thin film transistor liquid crystal display (TFT-LCD). Fig. 1 shows the simple structure of ESC. The electrode is connected with DC power supply, and there is dielectric material between the glass substrate and electrode. We have focused our problem on the dry etching system of capacitively coupled plasma (CCP). Among the CCP systems, Reactive Ion Etching (RIE) modes are widely used for dry etching process. For RIE mode, the RF (radio frequency) power is applied from the lower electrode which contacts with the substrate. In the plasma system, the electrons out of plasma move to the chamber walls and the Tel.: +82 1191845170; fax: +82 231599257.
E-mail address: [email protected]. 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.06.010
substrate. The electrons are accumulated in the substrate if it is a dielectric material. This accumulated charge with the electric field produced at the electrode by the current from the power will produce the electric force. Fig. 2 is the schematic diagram of the usual RIE system. The RF power is applied from the electrode under the glass substrate. In addition, He backside cooling is widely used to prevent the heating of glass during the dry etching process, which causes another force on the glass. ESC is widely used to hold the substrate from these forces. ESC should hold the substrate against some forces and have little effect to the plasma state or substrate. As a start for the study of glass holding systems, in this paper, we have briefly studied the forces on the glass in RIE mode. 2. Theory and results The charge density at the electrode is determined from the displacement current at the electrode. If the current at the electrode is given by I ¼ I 0 cos ot, the charge (Q) at the electrode is Q¼
I0 sin ot. o
If we assume the dimension of the electrode is sufficiently large compared to that of the gap between the upper and
ARTICLE IN PRESS H.-H. Choe / Vacuum 81 (2006) 344–346
345
Therefore, the charge in the sheath is VC. If we assume the electrons out of plasma is accumulated at the glass, the charge at the glass can be estimated as VC, enAs0 3 4 sin ot cos 2ot . (2) 1 sin ot 4 From Eqs. (1) and (2), the electric force acting on the glass becomes. 2 1 I 0 sin otð3 4 sin ot cos 2otÞ qE ¼ . 8A0 o 1 sin ot
q ¼ VC ¼
Fig. 3 shows the change of electric force on the glass. For most time during the period of RF, the calculation shows that the electric force exerted on the glass is attractive. However, repulsive force exists for some interval. The electric force depends on the discharge conditions. If the repulsive electric force overcomes the weight of the glass, the glass begins to move from the electrode if there are no other methods to hold the glass. If the electric and gravitational force are considered, the force acting on the glass is given by
Fig. 1. Schematic diagram of ESC.
F XqE rG Ad G g,
Fig. 2. Schematic diagram of the dry etching system for the glass substrate.
lower electrodes, then the magnitude of electric field due to the charge at the lower electrode is
1
(1)
where A is the area of the lower electrode. A simple homogeneous capacitive discharge model is used to calculate the charge at the glass, [4]. At the lower electrode, voltage from the electrode to the plasma-sheath boundary is en 3 2 1 2 2 V¼ s 2s0 sin ot s0 cos 2ot , 20 2 0 2 where s0 ¼ I 0 =enoA is the average sheath length, and the capacitance at the lower electrode is A . C ¼ 0 s0 ð1 sin otÞ
where rG is the density, d G is the thickness of the glass, and g is the acceleration due to gravity. This indicates ffi that the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi net force becomes upward if IX4Ao 0 rG d G g: For example, If a typical glass of rG ¼ 2:37 g=cm3 and d G ¼ 0:7 mm is used, IX300:2Af RF 106 , where f RF ¼ o=2p. If a glass of size of 1870 2200 mm2 is used, A ¼ 4:114 m2 . For a typical RIE reactors, o=2p ¼ 13:56 MHz or o=2p ¼ 2 MHz is used. In this case, the electric force overcomes the weight of the glass if IX16747 A for o=2p ¼ 13:56 MHz, and IX2470 A for o=2p ¼ 2 MHz. For the large size OLED or poly-Si TFT application, a glass of size of 730 920 mm2 is commonly used. In this case, the electric force overcomes the weight of the glass if IX2734 A for o=2p ¼ 13:56 MHz, and IX403 A for o=2p ¼ 2 MHz. This shows that, if low frequency or high current conditions are used, ESC voltage must be increased,
0
8Aε0F/(I0/ω)2
Q , E¼ 2A0
(3)
-1 -2 -3 -4 -5 0
5
10 t
Fig. 3. Electric force on the glass.
15
20
ARTICLE IN PRESS H.-H. Choe / Vacuum 81 (2006) 344–346
346
for o=2p ¼ 13:56 MHz, and IX2021 A for o=2p ¼ 2 MHz. For a glass of size of 730 920 mm2 , the net force becomes upward for IX2237 A for o=2p ¼ 13:56 MHz, and IX330 A for o=2p ¼ 2 MHz. 3. Conclusion
Fig. 4. Schematic diagram of ESC with He backside cooling.
because force due to ESC proportional to the ESC voltage (F ESC / V 2ESC ). Fig. 4 shows schematic diagram when a He backside cooling is applied. If the pressure of backside He is kept pHe and the area of hole is AHe , the upward force becomes F He ¼ AHe pHe . Here we assumed that the pressure of He is larger than that of process gases. Thus, Eq. (3) becomes F XqE þ F He rG Ad G g.
(4)
If the total area of He cooling hole compared to that of electrodepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi is 1%, the net force becomes upward for IX4Ao 0 ðrdg 1:333pHe Þ, where pHe in Torr. If we use rG ¼ 2:37 g=cm3 , d G ¼ 0:7 mm and pHe ¼ 4 Torr, IX245:6Af RF 106 . If a glass of size of A ¼ 4:114 m2 , the net force becomes upward for IX13 701 A
There were rare study on the glass holding systems using ESC. In the TFT fabrication, if RIE systems are used, it is shown that glass may experience the upward force due to the electric field against and He backside cooling its weight. It is shown that the electric force depends on the frequency and the current which are determined from RIE system configurations and process conditions. The calculation results explains that ESC voltage should have minimum value to hold the glass. However, it should be noticed that large ESC voltage may cause some problems to the thin film on glass in the process and during the dechucking process. Acknowledgments This work was supported by System IC 2010 program under the contract M103BY010043-05B2501-04311. References [1] Wardly GA. Rev Sci Intrum 1973;44:1506. [2] Tossel D, Powell K, Bourke M, Song Y. The international conference on compound semiconductor manufacturing technology; 2000. [3] Asano K, Hadakeyama F, Yatsuzuka K. IEEE Trans Ind Appl 2002;38:840. [4] Lieberman MA, Lichtenberg A. Principles of plasma discharge and materials processing. New York: Wiley; 2005.
Vacuum 81 (2006) 344–346 www.elsevier.com/locate/vacuum
Basic study of a glass substrate in dry etching system Hee-Hwan Choe School of Electronics, Telecommunications and Computer Engineering, Hankuk Aviation University, 200-1 Hwajeon-dong, Goyang-city, Gyeonggi-do, Republic of Korea Received 10 January 2006; received in revised form 19 May 2006; accepted 1 June 2006
Abstract Electrostatic Chucks (ESCs) in the dry etchers for the TFT-LCD fabrications have been investigated briefly. The behaviors of glass encountering electric fields in the presence of plasma were studied. In some conditions, it has been shown that the electric force and the pressure by He backside cooling acting on the glass might be larger than the gravitational force. With a simple model, the stability condition of glass substrate was obtained. r 2006 Elsevier Ltd. All rights reserved. Keywords: Glass; ESC; Plasma; Dry etching
1. Introduction Substrate holding systems are used in the microelectronics fabrication processes such as dry etching, photo lithography, and chemical vapor deposition (CVD). For the vacuum processes, it is hard to use the vacuum holding system to protect substrate from some forces. Therefore, the mechanical system has been used, until the ESC (Electrostatic Chuck) technology was introduced [1]. There were some reports of wafer substrates handling problem in using ESC [2,3]. However, there were rare reports about the force on the glass substrate. A glass substrate is widely used for display devices such as organic light emitting device (OLED), thin film transistor liquid crystal display (TFT-LCD). Fig. 1 shows the simple structure of ESC. The electrode is connected with DC power supply, and there is dielectric material between the glass substrate and electrode. We have focused our problem on the dry etching system of capacitively coupled plasma (CCP). Among the CCP systems, Reactive Ion Etching (RIE) modes are widely used for dry etching process. For RIE mode, the RF (radio frequency) power is applied from the lower electrode which contacts with the substrate. In the plasma system, the electrons out of plasma move to the chamber walls and the Tel.: +82 1191845170; fax: +82 231599257.
E-mail address: [email protected]. 0042-207X/$ - see front matter r 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.vacuum.2006.06.010
substrate. The electrons are accumulated in the substrate if it is a dielectric material. This accumulated charge with the electric field produced at the electrode by the current from the power will produce the electric force. Fig. 2 is the schematic diagram of the usual RIE system. The RF power is applied from the electrode under the glass substrate. In addition, He backside cooling is widely used to prevent the heating of glass during the dry etching process, which causes another force on the glass. ESC is widely used to hold the substrate from these forces. ESC should hold the substrate against some forces and have little effect to the plasma state or substrate. As a start for the study of glass holding systems, in this paper, we have briefly studied the forces on the glass in RIE mode. 2. Theory and results The charge density at the electrode is determined from the displacement current at the electrode. If the current at the electrode is given by I ¼ I 0 cos ot, the charge (Q) at the electrode is Q¼
I0 sin ot. o
If we assume the dimension of the electrode is sufficiently large compared to that of the gap between the upper and
ARTICLE IN PRESS H.-H. Choe / Vacuum 81 (2006) 344–346
345
Therefore, the charge in the sheath is VC. If we assume the electrons out of plasma is accumulated at the glass, the charge at the glass can be estimated as VC, enAs0 3 4 sin ot cos 2ot . (2) 1 sin ot 4 From Eqs. (1) and (2), the electric force acting on the glass becomes. 2 1 I 0 sin otð3 4 sin ot cos 2otÞ qE ¼ . 8A0 o 1 sin ot
q ¼ VC ¼
Fig. 3 shows the change of electric force on the glass. For most time during the period of RF, the calculation shows that the electric force exerted on the glass is attractive. However, repulsive force exists for some interval. The electric force depends on the discharge conditions. If the repulsive electric force overcomes the weight of the glass, the glass begins to move from the electrode if there are no other methods to hold the glass. If the electric and gravitational force are considered, the force acting on the glass is given by
Fig. 1. Schematic diagram of ESC.
F XqE rG Ad G g,
Fig. 2. Schematic diagram of the dry etching system for the glass substrate.
lower electrodes, then the magnitude of electric field due to the charge at the lower electrode is
1
(1)
where A is the area of the lower electrode. A simple homogeneous capacitive discharge model is used to calculate the charge at the glass, [4]. At the lower electrode, voltage from the electrode to the plasma-sheath boundary is en 3 2 1 2 2 V¼ s 2s0 sin ot s0 cos 2ot , 20 2 0 2 where s0 ¼ I 0 =enoA is the average sheath length, and the capacitance at the lower electrode is A . C ¼ 0 s0 ð1 sin otÞ
where rG is the density, d G is the thickness of the glass, and g is the acceleration due to gravity. This indicates ffi that the pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi net force becomes upward if IX4Ao 0 rG d G g: For example, If a typical glass of rG ¼ 2:37 g=cm3 and d G ¼ 0:7 mm is used, IX300:2Af RF 106 , where f RF ¼ o=2p. If a glass of size of 1870 2200 mm2 is used, A ¼ 4:114 m2 . For a typical RIE reactors, o=2p ¼ 13:56 MHz or o=2p ¼ 2 MHz is used. In this case, the electric force overcomes the weight of the glass if IX16747 A for o=2p ¼ 13:56 MHz, and IX2470 A for o=2p ¼ 2 MHz. For the large size OLED or poly-Si TFT application, a glass of size of 730 920 mm2 is commonly used. In this case, the electric force overcomes the weight of the glass if IX2734 A for o=2p ¼ 13:56 MHz, and IX403 A for o=2p ¼ 2 MHz. This shows that, if low frequency or high current conditions are used, ESC voltage must be increased,
0
8Aε0F/(I0/ω)2
Q , E¼ 2A0
(3)
-1 -2 -3 -4 -5 0
5
10 t
Fig. 3. Electric force on the glass.
15
20
ARTICLE IN PRESS H.-H. Choe / Vacuum 81 (2006) 344–346
346
for o=2p ¼ 13:56 MHz, and IX2021 A for o=2p ¼ 2 MHz. For a glass of size of 730 920 mm2 , the net force becomes upward for IX2237 A for o=2p ¼ 13:56 MHz, and IX330 A for o=2p ¼ 2 MHz. 3. Conclusion
Fig. 4. Schematic diagram of ESC with He backside cooling.
because force due to ESC proportional to the ESC voltage (F ESC / V 2ESC ). Fig. 4 shows schematic diagram when a He backside cooling is applied. If the pressure of backside He is kept pHe and the area of hole is AHe , the upward force becomes F He ¼ AHe pHe . Here we assumed that the pressure of He is larger than that of process gases. Thus, Eq. (3) becomes F XqE þ F He rG Ad G g.
(4)
If the total area of He cooling hole compared to that of electrodepffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi is 1%, the net force becomes upward for IX4Ao 0 ðrdg 1:333pHe Þ, where pHe in Torr. If we use rG ¼ 2:37 g=cm3 , d G ¼ 0:7 mm and pHe ¼ 4 Torr, IX245:6Af RF 106 . If a glass of size of A ¼ 4:114 m2 , the net force becomes upward for IX13 701 A
There were rare study on the glass holding systems using ESC. In the TFT fabrication, if RIE systems are used, it is shown that glass may experience the upward force due to the electric field against and He backside cooling its weight. It is shown that the electric force depends on the frequency and the current which are determined from RIE system configurations and process conditions. The calculation results explains that ESC voltage should have minimum value to hold the glass. However, it should be noticed that large ESC voltage may cause some problems to the thin film on glass in the process and during the dechucking process. Acknowledgments This work was supported by System IC 2010 program under the contract M103BY010043-05B2501-04311. References [1] Wardly GA. Rev Sci Intrum 1973;44:1506. [2] Tossel D, Powell K, Bourke M, Song Y. The international conference on compound semiconductor manufacturing technology; 2000. [3] Asano K, Hadakeyama F, Yatsuzuka K. IEEE Trans Ind Appl 2002;38:840. [4] Lieberman MA, Lichtenberg A. Principles of plasma discharge and materials processing. New York: Wiley; 2005.