Thin film transistor array having optical shield layer
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a thin film transistor (TFT) array used in an active matrix-type liquid crystal display (LCD) apparatus. 
     2. Description of the Related Art 
     An active matrix-type LCD apparatus is thin, so that it is often used in various display apparatuses. In this active matrix-type LCD apparatus, since individual pixel electrodes are independently driven, the contrast is not reduced based upon the reduction of a duty ratio, and also, the angle of visibility is not reduced, even when the capacity of display is increased to increase the number of lines. 
     In the active matrix-type LCD apparatus, one TFT is provided as a switching element for each pixel. A prior art TFT is constructed by a gate electrode formed on a transparent insulating substrate, a semiconductor active layer made of amorphous silicon opposing the gate electrode, a drain electrode and a source electrode connected to a transparent pixel electrode, which will be later explained in detail. 
     In the above-mentioned TFT used in an active matrix-type LCD apparatus, however, there is a large optical transmission area surrounding the semiconductor active layer. Therefore, light is incident from a backlight source through this optical transmission area to the semiconductor active layer. As a result, even when the TFT is in an OFF state, carriers are generated within the semiconductor active layer of the TFT to thereby increase an OFF current (leakage current) flowing therethrough. This may reduce the performance of the active matrix-type LCD apparatus. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to reduce an OFF current (leakage current) flowing through a TFT. 
     Another object is to provide a high performance active matrix-type LCD apparatus. 
     According to the present invention, in a thin film transistor formed by a gate electrode formed on a transparent insulating substrate, a semiconductor active layer opposing the gate electrode, a drain electrode, and a source electrode connected to a transparent pixel electrode, an optical shield layer is located so as to approximately surround the semiconductor active layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be more clearly understood from the description as set forth below, with reference to the accompanying drawings, wherein: 
     FIG. 1 is a circuit diagram illustrating a prior art active matrix-type LCD apparatus using a TFT array; 
     FIG. 2 is a partial plan view of the apparatus of FIG. 1; 
     FIG. 3 is a cross-sectional view taken along the line III--III of FIG. 2; 
     FIG. 4 is a cross-sectional view taken along the line IV--IV of FIG. 2; 
     FIG. 5 is a partial plan view illustrating a first embodiment of the TFT array according to the present invention; 
     FIG. 6 is a cross-sectional view taken along the line VI--VI of FIG. 5; 
     FIGS. 7A and 7B are graphs showing the amount of light incident to the amorphous silicon layer; 
     FIGS. 8 and 9 are partial plan views illustrating second and third embodiments of the TFT array according to the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before the description of the preferred embodiments, a prior art TFT array will be explained with reference to FIGS. 1 through 4. 
     In FIG. 1, which illustrates a prior art active matrix-type LCD apparatus, references GL 1 , GL 2 , . . . , GL 6  designate gate bus lines (or scan lines) driven by a gate bus line driver DR1, and DL 1 , DL 2 , . . . , DL 6  designate drain bus lines (or signal lines) driven by a drain bus line driver DR2. A plurality of TFTs Q 11 , Q 12 , . . . , Q 66  are provided at intersections of the gate bus lines GL 1 , GL 2 , . . . , GL 6  and the drain bus lines DL 1 , DL 2 , . . . , DL 6 . That is, the TFT Q ij  (i, j=1, 2, . . . , 6) has a gate electrode connected to one of the gate bus lines GL 1 , GL 2 , . . . , GL 6 , a drain electrode connected to one of the drain bus lines DL 1 , DL 2 , . . . , DL 6 , and a source electrode connected to a transparent pixel electrode E ij  (i, j=1, 2, . . . , 6). 
     In FIG. 1, the gate bus lines GL 1 , GL 2 , . . . , GL 6  are sequentially made high by the gate bus line driver DR1. For example, when the gate bus line GL 1  is made high, all of the TFTs Q 11 , Q 12 , . . . , Q 16  connected to the gate bus line GL 1  are turned ON. Therefore, the pixel electrodes E 11 , E 12 , . . . , E 16  are charged or discharged in accordance with the potentials at the data bus lines DL 1 , DL 2 , . . . , DL 6 . After the gate bus line GL 1  is made low, the potentials at the pixel electrodes E 11 , E 12 , . . . , E 16  are maintained until the gate bus line GL 1  is again made high to thereby change them. 
     In order to enhance the display performance of the LCD apparatus of FIG. 1, the high potentials at the pixel electrodes have to be sufficiently high until they are changed. If some of the high potentials at the pixel electrodes are reduced, the display is not uniform so as to reduce the performance. 
     In order to maintain the high potentials at the pixel electrodes for a long time, the following conditions are required. 
     i) The area of each pixel electrode is as large as possible; and 
     ii) The ratio of an ON current to an OFF current (leakage current) of each TFT is as large as possible. Particularly, it is essential to reduce the OFF current of each TFT. 
     In FIG. 2, which illustrates an enlargement of one pixel of FIG. 1, one TFT Q ij  is constructed by a gate electrode G shunted from the gate bus line GL i , a semiconductor active layer A made of amorphous silicon opposing the gate electrode G, a drain electrode D shunted from the drain bus line DL j , and a source electrode S connected to the pixel electrode E ij . One end of the semiconductor active layer A is connected to the drain electrode D, and the other end of the semiconductor active layer A is connected to the source electrode S. 
     In more detail, referring to FIGS. 3 and 4 which are cross-sectional views taken along the line III--III and the line IV--IV, respectively, of FIG. 2, the gate electrode G associated with the gate bus line GL i  made of Cr is deposited and patterned on a glass substrate 1. Also, formed on the entire surface, a gate insulating layer 2 made of SiN x  is deposited. Then, the semiconductor active layer A, the drain electrode D associated with the drain bus line DL j , the source electrode S, and the pixel electrode E ij  are formed. Further, a passivation layer 3 is formed on the entire surface. 
     On the other hand, formed on another glass substrate 4 are an optical shield layer (so-called black matrix) 5 and a color filter 6. Also, an insulating layer 7 made of SiN x  and a transparent counter electrode 8 are formed. 
     The two above-mentioned glass substrates 1 and 4 are assembled with a space therebetween into which liquid crystal is filled, thus creating a liquid crystal layer 9. Also, two polarizer plates 11 and 12 are mounted outside of the glass substrates 1 and 4, respectively. 
     When no voltage is generated between the pixel electrode E ij  and the counter electrode 8, light passes through both of the polarizer plates 11 and 12. On the contrary, when a voltage is generated between the pixel element E ij  and the counter electrode 8, light does not pass through the polarizer plates 11 and 12. 
     In the TFT Q ij  as illustrated in FIGS. 2, 3 and 4, however, there is a large optical transmission area surrounding the semiconductor active layer A. That is, as indicated by an arrow X in FIG. 3, backlight is reflected by the light shield layer 5 and is incident to the semiconductive active layer A, to thereby create carriers therein. Therefore, even if the TFT is in an OFF state, a leakage current is increased, to reduce the display performance. 
     In FIG. 5, which illustrates a first embodiment of the present invention, optical shield layers S1 and S2 made of Cr, for example, are added to the elements of FIG. 2. The optical layers S1 and S2 are located between the semiconductor active layer A and the pixel electrode E ij . As illustrated in FIG. 6, which is a cross-sectional view taken along the line VI--VI of FIG. 5, backlight is reflected by an arrow Y and incident light to the semiconductor active layer A is reduced thereby. Therefore, when the TFT is in an OFF state, the leakage current is reduced, thus improving the display performance. 
     Note that the optical shield layers S1 and S2 can be formed independently of the source electrode S, but, as illustrated in FIGS. 5 and 6, the optical shield layers S1 and S2 are extensions of the source electrode S, thus reducing the manufacturing cost. 
     The manufacturing process of the TFT of FIGS. 5 and 6 is explained below. 
     First, a 0.2 μm thick Cr layer is deposited by sputtering on the glass substrate 1, and patterned by a photolithography process and a wet etching process to form the gate electrode G and the gate bus line GL i . 
     Next, a 0.3 μm thick silicon nitride layer is deposited in a plasma chemical vapor deposition (CVD) chamber using a gas of SiH 4 , NH 3  and N 2 , to form the gate insulating layer 2. Subsequently, a gas of SiH 4  and H 2  is introduced into the plasma CVD chamber to grow amorphous silicon having a thickness of 0.2 μm. Further, a gas of PH 3  is introduced into the plasma CVD chamber to grow a 150 nm thick N +  -type amorphous silicon (not shown). Then, the phosphous silicon including N +  -type phosphous silicon is patterned by a photolithography process and a reactive ion etching (RIE) process to form the semiconductive active layer A. 
     Next, a 0.5 μm thick Cr layer is deposited by sputtering, and is patterned by a photolithography process and a wet etching process to form the drain electrode D associated with the drain bus line DL j , the source electrode S, and the optical shield plates S1 and S2. Then, the N +  -type amorphorous silicon on the amorphous silicon is etched by an RIE process to separate the drain electrode D from the source electrode S. 
     Next, a 0.2 μm thick Indium Tin Oxide (ITO) layer is formed by sputtering and is patterned by a photolithography process and a wet etching process, to form the transparent pixel electrode E ij . 
     Next, a 0.5 μm thick silicon nitride layer is formed by plasma CVD using a gas of SiH 4 , NH 3  and N 2 , to form the passivation layer 3. Thus, the TFT is completed. 
     In FIGS. 7A and 7B, which show the amount of light incident to the semiconductor active layer A, the abscissa indicates a distance between the gate electrode G and a backlight source (not shown). In the present invention where the gap between the optical shield layer S1 and the gate G is 3 μm, as shown in FIG. 7A, the total incident light amount is 131.5. Conversely, in the prior art, as shown in FIG. 7B, the total incident light amount is 216.5. Thus, the amount of light incident to the semiconductor active layer A can be reduced by 40% as compared with the prior art. 
     In FIG. 8, which illustrates a second embodiment of the present invention, the gate electrode G completely covers the semiconductor active layer A from the bottom. In this second embodiment, an optical shield layer S1&#39; is smaller than the optical shield layer S1 of FIG. 5. Also, an optical shield layer S2&#39; is located between the pixel electrode E ij  and the gate bus line GL i . 
     In FIG. 9, which illustrates a third embodiment of the present invention, the gate electrode G is not shunted from the gate bus line GL i , but is a part of a wider portion of the gate bus line GL i . In this third embodiment, optical shield layers S1&#34; and S2&#34; are located between the semiconductor active layer A and the pixel electrode E ij , Contrary to this, an optical shield layer S3&#34; is located on the side of the drain electrode D, i.e., between the drain electrode D and the drain bus line DL j+1 . 
     In the above-described embodiments, although the pixel electrode E ij  is located over the source electrode S, the pixel electrode E ij  can be located beneath the source electrode S. 
     As explained hereinbefore, according to the present invention, since the optical shield layers reduce the amount of light incident to the semiconductor active layer of the TFT, the OFF current of the TFT can be reduced. Therefore, the display performance of an active matrix-type LCD apparatus using this type of TFT can be improved.