METHOD FOR PRODUCING A METAL FILM, A THIN FILM DEVICE HAVING SUCH METAL FILM AND A LIQUID CRYSTAL DISPLAY DEVICE HAVING SUCH THIN FILM DEVICE
 The invention relates to a method for producing metal films and, in particular, an improved method for producing such metal films as light shutter films, which are to be incorporated in thin film transistors within a liquid crystal display device.
BACKGROUND OF THE INVENTION
 In recent years, the demand for liquid crystal display devices (LCD) has been increasing in accordance with the common usage of computer displays, digital cameras, portable telephones, car-navigation devices and so on. In order to drive pixels of the active matrix type of LCD that is the majority of such LCDs, thin film transistors (TFT), which are thin film devices as active elements for the LCDs, are particularly required. Thus, in order to meet the strong demand for the LCDs, it is a key challenge in the art to increase the efficiency of producing the TFTs and also to improve their quality.
 As for the TFTs used in the LCDs, there are two types of TFTs, a bottom-gate type and a top-gate type, both of which contain metal films to be used for wiring members and/or light shutter films. The Japan Patent Application NO. 1997-263974 discloses a production process (a fine metal work technique) for forming a metal light shutter film so as to increase the efficiency of producing the latter type (topgate type) of the TFT and improve its quality. Following will briefly explain the fine metal work technique disclosed in the above-referenced patent application.
 Referring to FIG. 12 (a) as a plan view and FIG. 12 (b) as a cross-section view, an array substrate 100 comprising the top-gate type of TFT elements is shown. As illustrated, gate electrodes (Y electrodes) 101 and data electrodes (X electrodes) 102 are disposed in a matrix manner on the array substrate 100, and TFTs 103 are located at intersectional points of the electrodes. Besides, sub-pixel electrodes 104 comprising transparent conductive films (ITO) 104 are connected to source electrodes (or drain electrodes) 105 of the TFTs 103 and electrodes for capacitors Cs 106 for accumulating the data are located in a part (approximately in the center in FIG. 12) of the sub-pixel electrodes 104. In the periphery of the array substrate 100, there provided pad electrodes 101', 102' that are connected with external devices (e.g., electronic circuits) so that subpixel electrodes 104 can be interfaced with such external devices to communicate data and control signals.
 Referring to FIG. 12 (b), following will explain the cross-sectional structure of one unit among a plurality of units, each of which comprises a TFT element 103, a sub-pixel electrode 104 and a data accumulating capacitor Cs 106 that are disposed upon the array substrate 100. In order to construct the basic cross-sectional structure of the top-gate type of the LCD as illustrated in FIG. 12 (b), a light shutter film 108, which is a Cr metal film, is first formed on the glass substrate 107 and then an insulation layer SiOx 109 is formed on the light shutter film 108. Then a drain electrode 110 and a source electrode 111 are formed through
ITO on the insulation layer SiOx 109. Besides, N+a-Si layer 112, which contains N+ as an impurity constituent to reduce the bonding resistance, is formed on the drain electrode 110 and the source electrode 111 and thereupon aSi layer 113 and SiNx layer 115 are formed, on which a gate electrode 113 comprising, for example, molybdenum tantalum (MoTa) is further formed. Finally, a protection film 114 comprising a nitrification material (SiNx) is formed on the gate electrode 113 so as to protect the a—Si layer 112, the gate electrode 113 and the SiNx layer 115. It should be noted that the protection layer 114 is not necessarily prerequisite, but rather not necessary if the SiNx exists above the layer corresponding to the ITO because any SiNx material remaining on pixels may cause some burnout problem on the pixel that would be displayed for a certain consecutive time period. In this way, one unit of TFT is formed on the array substrate 100. A display part (not shown herein) of the LCD incorporating the active matrix type of the TFTs is built up by bonding the array substrate (TFT substrate) 100 with an opposite substrate (not shown herein) having common electrodes in such manner as they are sandwiching the liquid crystal. Within the array substrate, a series of the TFT are located on the matrix of display electrodes. Respective opposite parts between the display electrode and the common electrode form a pixel capacity with the liquid crystal as an dielectric layer and will be serially selected by the TFT to be charged with a proper voltage. The charged voltage against the pixel capacity may be maintained by the OFF resistance of the TFT for a time period of one field unit. Liquid crystals have a characteristic of electrooptic anisotropy, so that the amount of transmitted lights may be finely adjusted according to the strength of the electric field formed by the pixel capacity. Thus a color distribution in which respective transmission rates are controlled for each pixel may pass through each color filter of the RGB, and as a result the desired image can be seen according to the principle of additive mixture of color stimuli on the display screen of the LCD.
 Now the method for forming the Cr metal film 108 in accordance with the above-referenced patent application will be briefly introduced in conjunction with FIG. 13 (a), (b) and (c). At first, as illustrated in FIG. 13 (a), a Cr metal film with a thickness of about 1,500 angstrom is formed on the glass substrate 1 by means of spattering and then a resist film R is formed above the metal film. Thereafter, a desired pattern is developed by means of a known photolithography method.
 As a next process, the portion of the Cr metal film 2 that extends outside below the portion covered by the resist R is removed using an appropriate etchant so that the resulting pattern of the Cr metal film 2 might be the same with that of the resist R as illustrated in FIG. 13 (b). With this process, in particular, such etched sidewalk of the Cr metal film 2 may be formed in a perpendicular shape as seen in FIG. 13 (b).
 Furthermore, as illustrated in FIG. 13 (c), an RIE (reactive ion etching) process using an etchant gas comprising, for example, mixed gas of oxygen and either Cl2 or HC1 is performed on the resist R so as to be etched with the oxygen. Then another etching process using the same mixed gas is performed on the sidewalk of the Cr metal film 2 that has remained below the resist R. When this etching process is performed, predetermined etching conditions such as the