Thin film transistor with an amorphous carbon light protection and method to make this light protection.

p0001The invention relates to thin film transistors, such as are for example used for controlling of liquid crystal cells in flat screens, and a process for applying a sunscreen to these thin film transistors.

p0002Flat screens consist of a large number of individual pixels whose light transmittance is controlled by means of liquid crystal cells. To control the individual liquid crystal cell, there are two possibilities, one of which namely the active addressing is discussed in the following only one. In this type of addressing, a thin film transistor is used, which works as a field effect transistor. In the conducting state, the transistor performs the necessary information for the control as the electric charge in the liquid crystal cell or an additional storage capacitor. Thereafter, the transistor is blocked and the liquid crystal cell acts as a capacitor. The stored information is now in the form of electric charge in the liquid crystal or an additional storage capacitor. If this semiconductor material made of light, in him electron-hole pairs are generated and the capacitor is discharged. In order to prevent discharge by light, the layer of semiconductor material is covered with a light-protection, usually with a thin metal layer. Since metal is electrically conductive, the semiconductor layer has to be provided before the application of the metal film with an insulating intermediate layer.

p0003The well-known thin film transistor has the disadvantage that sunscreen is only accessible by means of two layers, an insulating layer and a metal layer. Object of the invention is to find a sunscreen that is realized in a single layer. Another object of the invention is to provide a simple method for producing such a sunscreen.

p0004The thin film transistor according to the invention has the advantage that its light protection is both electrically isolated and opaque. It consists of amorphous carbon is deposited in a CVD process with the aid of methane (CH₄).

p0005An embodiment of the invention will be described below and explained in more detail with reference to FIGS 1 and 2. FIG.

p0006Show it:<ul><li>FIG. 1 shows a thin film transistor with metal sunscreen,</li><li>FIG. 2 is a thin film transistor with an amorphous carbon film.</li></ul>

p0007In Fig. 1, 10, a glass substrate referred to, on which a thin film transistor is formed of a tantalum electrode 11, a gate insulator 12, a source electrode 13, a drain electrode 14, a light-sensitive semiconductor 15, a transparent insulating layer 16 and a metal light shield 17 consists. Such a thin film transistor is a component of a large number of thin film transistors which are arranged in an mxn matrix. These thin film transistors used for active addressing of liquid crystal cells, which in turn are in turn part of a flat screen. The light transmittance of the liquid crystal cell is controlled by means of thin-film transistors. Of the source electrode 13 enter the charge carriers across the photosensitive semiconductor material 15 to the drain electrode 14 and thus to the liquid crystal. Thereafter, the source-drain path is made highly resistive, and the liquid crystal functions as a capacitor. The information stored in this capacitor charge is a measure of the optical information (brightness), which is proposed for the single pixel. It takes the Fig. 1, the semiconductor layer 15 is only partially covered by the two electrodes 13 and 14. Therefore, light entering the semiconductor material 15th If light enters the semiconductor material 15, there electron-hole pairs are generated, which lead to a discharge of the capacitor. Such an effect is not desired. the semiconductor material 15 must be protected from light. Known versions of TFTs use to a metal light shielding, which however must be separated from the semiconductor by an insulating layer because of its electrical conductivity. The insulating layer is here with 16, of the designated metal light shielding with the seventeenth In the manufacture of such a transistor, two processes are thus required: firstly, the application of a transparent insulation layer 16, and secondly the application of a metallic sunscreen 17th

p0008FIG. 2 shows a thin film transistor, which corresponds with one exception in FIG. 1. Like reference numerals designate like elements. Instead of a transparent insulating layer 16 and a metal light shield 17, a film 20 is used consisting of amorphous carbon here. Amorphous carbon is non-conductive and has an absorption of 99.96%. By using amorphous carbon, the application of an additional insulation layer is accounted for by the transistor.

p0009The film of amorphous carbon is deposited using a CVD process. As the source of the carbon methane (CH₄) can be used. The patterning of the film is carried out either after the lift-off method or etching in an oxygen plasma.