Chemistry: electrical and wave energy – Processes and products – Coating – forming or etching by sputtering
Reexamination Certificate
1999-03-16
2002-04-30
Huff, Mark F. (Department: 1756)
Chemistry: electrical and wave energy
Processes and products
Coating, forming or etching by sputtering
C204S192290, C204S192100, C204S192120, C204S298080
Reexamination Certificate
active
06379508
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for forming a thin film on a substrate. More particularly, the present invention relates to a method for forming a thin film that is suitable for forming a transparent conductive film used in an image display apparatus such as a liquid crystal display.
2. Description of the Prior Art
Generally, a pixel electrode substrate used in a liquid crystal panel is produced by the following method. A transparent conductive substrate is first produced by forming a thin film made of indium tin oxide (hereinafter, referred to as “ITO”) by reactive sputtering as a transparent conductive film on the entire surface of a transparent substrate. Then, pixel electrode patterning is performed with respect to the ITO thin film by etching so as to produce a pixel electrode substrate.
The reactive sputtering for the ITO thin film used in this method is performed by introducing inert gas and reactive gas into a sputtering apparatus. A sputtering target for discharge power application, a substrate holder for holding a substrate to be treated, a shutter located between the sputtering target and the substrate holder are provided in the sputtering apparatus.
Prior to the formation of a thin film by reactive sputtering, sputtering discharge is performed with gas introduction and discharge power application while the shutter is closed before a substrate to be treated is introduced into the sputtering chamber (hereinafter, the sputtering discharge without a substrate to be treated being introduced in the sputtering chamber is referred to as “idling discharge”), in order to stabilize a plasma discharge state in sputtering discharge.
Conventionally, it has been common that this idling discharge is performed for a short time such as 3 to 5 minutes and at most about 10 minutes.
Furthermore, the discharge power during the formation of a thin film is set to a low power value in order to stabilize reactive sputtering. On the other hand, the discharge power during the idling discharge is set to a power higher than the discharge power applied for the formation of a thin film in order to speed up the cleaning of the surface of the target for shortening the treatment time and to reduce variations in the plasma discharge state caused by opening and closing the shutter.
Conventionally, a black and white liquid crystal panel has been displayed by determining at each pixel whether the pixel is bright or dark, i.e., whether or not light is transmitted. For this reason, the quality of a transparent conductive film is determined by whether or not light is transmitted, and does not significantly depend on the transmittance of the transparent conductive film.
However, since a color liquid crystal panel is displayed with three primary colors of red, green and blue, a non-uniform transmittance in the transparent conductive substrate results in unintended reproduction of colors in the liquid crystal panel. Furthermore, when transparent conductive substrates have significantly different transmittances from each other, liquid crystal panels that have been produced with the transparent conductive substrates reproduce colors differently. Therefore, in the production of color liquid crystal panels, when transparent conductive films are formed on a large number of substrates, it is important for each of the transparent conductive substrates to have a uniform transmittance within its own substrate. Moreover, it is important for all the transparent conductive substrates to have a uniform transmittance.
Generally, the sheet resistance and the transmittance of a transparent conductive film formed of a metal oxide are correlated closely. Stable sheet resistance and low levels of dispersion in the sheet resistance result in stable transmittance. However, as a result of study in detail by the inventors as to conventional methods for forming a thin film such as a transparent conductive film, the inventors found a first problem in that the dispersion in the sheet resistance of transparent conductive films successively formed on a plurality of substrates by reactive sputtering and the sheet resistance in each of the transparent conductive substrates depends on the order of the formation of the films, namely, the order of the introduction of substrates to be treated to the sputtering chamber.
The first problem in conventional methods for forming a thin film will be described with reference to
FIGS. 6
,
7
,
8
and
9
.
FIG. 6
shows a sequence of discharge power applied for formation of ITO thin films on 50 substrates to be treated. First, gas is introduced into a sputtering chamber, a discharge power of 450 W is applied for idling discharge and a first substrate to be treated is transported into a sputtering chamber and positioned on a substrate holder (during a period shown as T
1
in FIG.
6
). Then, the discharge power is dropped to 400 W and the shutter is opened, and an ITO thin film is formed on the first substrate to be treated so that a first transparent conductive substrate is produced (during a period shown as T
2
in FIG.
6
). The shutter is closed and the discharge power is raised to 450 W, and the first transparent conductive substrate is transported out from the sputtering chamber before a second substrate to be treated is transported into the sputtering chamber and positioned in the substrate holder (during a period shown as T
3
in FIG.
6
). Then, the discharge power is dropped to 400 W and the shutter is opened, and an ITO thin film is formed on a second substrate to be treated (during a period shown as T
4
in FIG.
6
). The same film-formation cycles are repeated until an ITO thin film is formed on a 50
th
transparent conductive substrate (during a period shown as T
100
in FIG.
6
). Then, the shutter is closed and the discharge power is raised to 450 W, and the 50
th
transparent conductive substrate is transported out from the sputtering chamber (during a period shown as T
101
in FIG.
6
). Then, the discharge power is stopped and thus the process for forming the transparent conductive films is completed.
FIG. 7
shows changes in the sheet resistance of the transparent conductive substrate according to the order of the ITO thin film-forming process by the conventional method for forming a thin film as described above.
FIG. 8
shows the dispersion in the sheet resistance in each transparent conductive substrate. As seen from
FIGS. 7 and 8
, the sheet resistance is high and the dispersion in the sheet resistance is large with respect to the ITO thin films on the first to fifth transparent conductive substrates, i.e., the substrates treated with reactive sputtering in an early stage of the sequential forming process. The dispersion in the sheet resistance represents a difference from an average value of values obtained by measuring a sheet resistance at 5 points on a surface of a transparent conductive substrate with a 150 mm diameter.
The gas state in the sputtering chamber during the formation of ITO thin films by the conventional method shown in
FIG. 6
is analyzed with a quadruple mass spectrometer.
FIG. 9
shows pressure changes of introduced gases, i.e., argon (Ar), hydrogen (H
2
) and oxygen (O
2
), and water (H
2
O) calculated with mass numbers obtained by the mass analysis.
FIG. 9
shows the results with respect to a period before start of discharge, an idling discharge period
60
, and a period until a thin film is formed on a 10
th
substrate in the period for sequentially forming ITO thin films on the 50 substrates. Sharp falling portions seen in the pressure change result of each gas are caused by the switching of discharge power between 450 W for idling discharge and 400 W for formation of the thin films and the opening and closing of the shutter. The ITO thin films are formed on sequentially introduced substrates during periods segmented by the falling portions.
As seen from
FIG. 9
, the pressures of Ar, H
2
and O
2
are substantially constant except for the initial period with gas introduction and the sta
Ishihara Tomoaki
Kobayashi Kazunori
Nakamura Akira
Nobusada Toshihide
Chacko-Davis Daborah
Huff Mark F.
Matsushita Electric - Industrial Co., Ltd.
Merchant & Gould P,C,
LandOfFree
Method for forming thin film does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for forming thin film, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for forming thin film will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-2838953