Electrification of glass substrate surface by plasmas

Journal of

ELECTROSTATICS ELSEVIER

Journal of Electrostatics40&41 (1997) 103-108

Electrification of glass substrate surface by plasmas Hiroyoshi KitabayashP, Haruhisa FujiP and Takayuki Ooishib "Advanced Technology R&D Center, Mitsubishi Electric Corporation 1-1, Tsukaguchi-Honmachi, 8-Chome, Amagasaki, Hyogo, 661 JAPAN bAdvanced Display Inc.(ADI) 997, Miyoshi, Nishigohshi, Kikuchi, Kumamoto, 861-11 JAPAN

It is probable that charge-up of a glass substrate during plasma processing, such as ashing a n d / o r etching, deteriorates the production yield of TFT-LCD devices. In order to control the production yield, it is necessary to understand the electrification mechanism of glass surface in plasma and then offer countermeasure to the process. We measured the surface potential of the glass substrate during discharge. We exposed the glass substrate to plasma in a parallel-plate electrode system by introducing Ar, 02 or SF6 gas into the plasma reactor and exciting with 13.56MHz RF or DC power supply. As a result, we found that the charging of the glass substrate in plasma was considerably affected by gas species and power sources. We discussed the phenomenon from a viewpoint of the ionizatiation and the movement of molecular ions in plasma. 1. INTRODUCTION Recently charging of a glass substrate in plasma processes has become a serious concern for the production of thin film transistor-liquid crystal display (TFT-LCD) devices. This electrification phenomenon is propable to cause the damage a n d / o r breakdown of gate dielectrics and the attachment of dusts to the suhstrates and then to lower the production yield. Therefore it is necessary to understand the electrification mechanism and to provide the countermeasures to the process. Although there have been many reports concerned with the electrification of the SiO2/Si interface states due to plasma exposure by evaluating capacitance-voltage(C-V) curves for MOS capacitors[I,2], however the electrification phenomena of the glass surface used for TFT-LCD substrate in plasma process has not been reported yet. We measured the surface potential of a glass substrate after exposure to the plasma by using a non-contact electrost0tic voltmeter. On the other hand, during discharge we measured the surface potential transient through an electrode deposited on the substrate surface directly by an oscilloscope. In this paper, we will describe the experimental results and discuss the electrification mechanism of the glass surface by plasmas. 0304-3886/97/$17.00© ElsevierScienceB.V. All fights reserved. S0304-3886(97)00022-3

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2. EXPERIMENTAL PROCEDURE A schematic diagram of the experimental arrangement for measurement of the glass substrate surface potential in plasma is shown i n Figure 1. We used the glass substrates of the diameter of 150mm and the thickness of 1.0mm. 13.56 MHz radiofrequency(RF) power was supplied to the one electrode on which the glass substrate was placed, and the grounded electrode was indium tin oxide(ITO) deposited glass electrode. The gap distance between the electrodes was 50mm. We also used DC power supply for comparison with the RF power. Ar, 02 or SF6 was introduced into the reactor after a preliminary evacuation of the reactor up to 1.5X10 a Torr. The pressure and the flow rate of gas were also monitored by a Pirani guage and a mass flow meter, respectively. After discharge, we measured the surface potential of a glass substrate in vacuum by using an electrostatic voltmeter(Monroe Inc. ;MODEL174) in the chamber(Figure l(a)). We also measured the surface potential of the glass substrate during RF discharge. The potential difference between the output potential of the RF power supply and the A1 electrode (2X2mm2) deposited with 200nm thickness on the glass substrate was measured by the digital oscilloscope(Tektronics;TDS524) (Figure 1(lo)). The lead wire conected with the AI electrode was encapsulated by silicone to prevent it from the direct exposure to the plasma[3].

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H. Kitabayashi et al. /Journal o f Electrostatics 40&41 (1997) 103-108

3. EXPERIMENTAL RESULTS 3.1 The surface potential after discharge Figures 2(a),(b) and (c) show the glass substrate surface potential after discharge as functions of the gas flow rate or the pressure for Ar, O2 and SF6plasmas, respectively. Figure 3 shows the RF input power dependences of the surface potentials for Ar, 02 and SF6 plasmas. In case of Ar plasma, the glass surface potential increases with increasing the RF input power and the gas pressure, until saturated at about +150V. On the other hand, the surface potential after O 2 or SF6 plasma exposure was nearly 0V under all condition of the RF power and the pressure tested. Figure 4 shows the glass substrate surface potentials after exposure to 02 and SF6 DC plasma. The surface potential by DC plasma exposure was higher than that of RF plasma exposure(Figures 2(b) and (c)).

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1-I. Kitabayashi et al./Journal o f Electrostatics 40&41 (1997) 103-108

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3.2 The substrate surface potential during RF discharge In order to study the electrification mechanism of a glass substrate during discharge, we measured the potential differences between A1 electrode deposited on the glass substrate and the RF electrode by using the measuring system as shown in Figure l(b). Typical transient waveforms observed by the digital oscilloscope are shown in Figure 5. The transient waveform of glass substrate surface potential during Ar discharge was shown in Figure 5(a). The potential of A1 electrode was nearly 0V while RF electrode was biased negatively. Figure 5(b) shows the case of Ol discharge. The transient waveform of the surface potential shows the same tendency with Ar discharge, On the other hand, Figure 5(c) shows the case of SF6 discharge. The glass substrate surface potential was nearly 0V and RF electrode was not biased negatively during discharge. potential of AI electrode on the gl ss 0V ~

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H. Kitabayashi et al./Journal of Electrostatics 40&41 (1997) 103-108

107

4. DISCUSSION We propose a model of a electrification mechanism from the experimental results mentioned above. Electrification model in Ar plasma was shown in Figure 6(a). In case of Ar plasma the potential of RF electrode was more negative than that of A1 electrode on the glass(Figure 5(a)). Therefore the positive ions in plasma, Ar*[4], can be attracted to and attached on the glass substrate. This phenomenon is maintained during and also after termination of plasma. As the number of At* increases with increase of the RF input power, the potential on the substrate depends on the input power. Figure 6(b) shows the case of 02 plasma exopsure. It is reported that O 2is a weakly electronegative gas and 02÷, O ÷, 02, O and electrons exist in 02 discharge plasma[5]. The potential of RF electrode was more negative than that of A1 electrode on the glass during plasma exposure(Figure 50))). Therefore the positive ions can be deposited on the glass substrate. As negative ions, O', increase at the time of tume-off[6], they are attracted toward the positive potential and neutralize the positive ions on the glass. Then the surface potential after O 2 plasma exposure becomes nearly 0V. On the other hand, Figure 6(c) shows the case of SF6 plasma exposure. It is reported that SF6 is a strongly electronegative gas and the dominant positive ion are SFs~ and negative ions are SF~',SFs and F" in SF6discharge plasma[7]. As the potential of the RF electrode was not biased(Figure 5(c)), the ions with opposite polarity to the electrode potential were attached to the glass substrate repeatedly. Therefore charge accumulation on the glass surface does not occur after the turn-off of the power supply. And the surface potential after SF6plasma exposure was nearly 0V. The positive ions are accumulated on the glass in a moment when DC power supply is turned on. By this accumulation the density of discharge plasma near the substrate becomes tenuous. Then the charge-up is maintained after DC discharge is turned off. 5. CONCLUSION We measured the surface potential on the glass substrate in plasma process. The potential depended on gas species or power supplies. In case of RF plasma, the charging potential on the glass after Ar plasma exposure increased with the RF input power and the gas pressure, until saturated with about +150V. While the surface potential after 02 or SF6 plasma exposure was nearly 0V under any condition of the RF power and the pressure. However the charging potential was higher in PC plasma exposure than that in RF plasma exposure. We explained these phenomena from the viewpoint of the ionization and the movement of ions in plasma.

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H. Kitabayashi et aL /Journal of Electrostatics 40&41 (1997) 103-108

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