Abstract:
An improved process for manufacturing a thin film transistor uses two masks for etching and therefore one mask alignment. The technical effect of said process is to provide the thin film transistor with low cost and enhanced yield.

Description:
FIELD OF THE INVENTION 
     The present invention relates to a process for manufacturing a thin film transistor (hereinafter referred to as TFT) usable for an active matrix type liquid crystal display device. 
     BACKGROUND OF THE INVENTION 
     An active matrix type liquid crystal display device, placing TFT on a display cell substrate in a matrix form, is a device which makes possible mass storage display of high quality. The device has been intensively applied to televisions and the like. 
     A conventional process for manufacturing TFT array substrates suitable for liquid crystal televisions will be illustrated based on FIGS. 14(A) and (B). FIG. 14(A) shows a plane view of one picture element of the TFT array manufactured by a conventional process. FIG. 14(B) shows an X--X&#39; sectional view of the picture element. A metal layer such as Al and the like is formed on a transparent insulation substrate 70 and patterned by photo-etching to form a gate electrode bar 71. A gate insulation layer 72 made from an oxide film or a nitride film and a semiconductor layer 73 made from Si, CdS and the like are successively laminated and the semiconductor layer 73 is etched. On the semiconductor 73, a transparent electrode layer is laminated and etched to form a source electrode bar 76 and a drain electrode or display electrode 77. 
     As described above, the conventional process requires at least two mask alignments, because it employs at least three for etching. This complicates the manufacturing process of the TFT array substrate and causes high cost and low yield. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to simplify a process for TFT array substrates by way of using two mask layers and one mask alignment to improve producibility. The present invention is to provide a process for manufacturing a thin film transistor comprising; 
     forming four layers by successively laminating on a insulation substrate a metal layer as a gate electrode, a first insulation layer as a gate insulation layer, a semiconductor layer and an electrode layer which makes an ohmic contact with said semiconductor layer, 
     etching said four layers by a photoresist to form a pattern, 
     laminating a second insulation layer with remaining said photoresist, 
     removing the remaining photoresist to form a pattern by a lift-off process, 
     laminating a transparent conductive layer as a source and drain electrode and a display electrode, etching said transparent conductive layer and said electrode layer. 
     Another object of the present invention is to provide a process for manufacturing a thin film transistor comprising; 
     forming four layers by successively laminating on a insulation substrate a metal layer as a gate electrode, a first insulation layer as a gate insulation layer, a semiconductor layer and an electrode layer which makes an ohmic contact with said semiconductor layer, 
     etching said four layers by a photoresist to form a pattern, 
     anodic-oxidazing said metal layer in its pattern edge to form a second insulation layer, 
     laminating a transparent conductive layer as a source and drain electrode and a display electrode, etching said transparent conductive layer and said electrode layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(A) and (B) show a plane view and an X--X&#39; sectional view of the first embodiment of the TFT of the present invention. 
     FIGS. 2(A) and (B) to FIGS. 7(A) and (B) show plane views and X--X&#39; sectional views, explaining the first embodiment of the process for manufacturing the TFT of the present invention. 
     FIGS. 8(A) and (B) show a plane view and an X--X&#39;  sectional view of the second embodiment of the TFT of the present invention. 
     FIGS. 9(A) and (B) to FIGS. 13(A) and (B) show plane views and X--X&#39; sectional views, explaining the second embodiment of the process for manufacturing the TFT of the present invention. 
     FIGS. 14(A) and (B) show a plane view and an X--X&#39; sectional view of a conventional TFT array substrate. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will be illustrated by examples based on the drawings. 
     EXAMPLE 1 
     FIGS. 1(A) and (B) show a plane view and an X--X&#39; sectional view of one picture element of the TFT array substrate produced by the process of the present invention. Two masks are employed in this embodiment, one is for patterning a gate electrode bar 11, a gate insulation layer 12, a semiconductor layer 13 and an electrode layer 14 which forms an ohmic contact with a semiconductor layer 13. The other is for patterning a source electrode bar 16, a drain electrode or display electrode 17 and the electrode layer 14. The manufacturing process and the concrete construction are explained by FIGS. 2 to 7. 
     Step I (see FIGS. 2(A) and (B)) 
     An Al layer 11&#39;, which will make the gate electrode bar 11, is deposited on a glass substrate 10 in a thickness of 2,000A by a sputtering. The gate insulation layer 12&#39; of Si 3  N 4 , the semiconductor layer 13&#39; of amorphous hydrogenated silicon (a-Si:H) and the electrode layer 14&#39; of phosphorus-doped a-Si:H (n +  a-Si:H) which forms an ohmic contact with a-Si:H 13&#39; are successively laminated on the Al layer 11&#39; by way of a plasma CVD. The thickness of the layers are respectively 2,000A, 2,000A and 1,000A. After forming four layers, a photoresist 18 is coated and exposed by using the first mask to develope. 
     Step II (see FIGS. 3(A) and (B)) 
     The four layers manufactured by the step I are etched to form a pattern. The etchant of the n +  a-Si:H 14&#39; and the a-Si:H 13&#39; is a mixture solution of HF and HNO 3  and the etchant of Si 3  N 4  is a 5% HF solution. The etchant of the Al layer 11&#39; is an aqueous H 3  PO 4  solution. The etching can be conducted by immersing the substrate in the etchants and etching with the same pattern. 
     Step III (see FIGS. 4(A) and (B)) 
     In this step, the second insulation layer 15 of Si 3  N 4  is laminated over the remaining photoresist 18 and covers the gate electrode bar 11 with the insulation layer. The purpose of this step is to prevent an electrical contact of the gate electrode bar 11 with the source electrode bar 16 and the drain electrode or display electrode 17. The second insulation layer 15 can be formed by laminating the Si 3  N 4  layer in a thickness of 5,000A at 100° C. by a plasma CVD, whereby the covering is completed. 
     Step IV (see FIG. 5 (A) and (B)) 
     In this step, the Si 3  N 4  insulation layer 15 on the photoresist 18 is immersed in a resist remover to remove it together with the photoresist 18, that is, it is patterned by a so-called lift-off process. By removing the photoresist, the n +  a-Si:H 14&#39; surface appears. 
     Step V (see FIG. 6 (A) and (B)) 
     In this step, the transparent conductive layer 17&#39; is laminated on all surfaces, containing the n 30  a-Si:H 14&#39; in a thickness of 3,000A by a vacuum evaporation for making the source electrode bar 16 and the drain electrode or display electrode 17. After laminating it, a photoresist 19 is coated on it and exposed by using the second mask to develop to a desirable shape. The mask alignment is conducted only once in this step, which makes the process simple and leads low cost products. 
     Step VI (see FIG. 7 (A) and (B)) 
     In this step, the transparent conductive layer 17&#39; is etched with the photoresist 19 to make a pattern of the source electrode bar 16 and the drain electrode or display electrode 17, followed by etching the n +  a-Si:H 14&#39; which forms an ohmic contact. The etchant of the transparent conductive 17&#39; is a HCl solution and the etchant of the n +  a-Si:H 14&#39; is a mixture solution of HF and HNO 3 . Etching of the transparent conductive layer 17&#39; is conducted by immersing the laminated substrate in the etchants to form the source electrode bar 16 and the drain electrode or display electrode 17. Further, the n +  a-Si:H layer 14&#39; is etched to the electrode 14 which forms an ohmic contact of the a-Si:H semiconductor layer 13 with the source electrode bar 16 and the drain electrode 17. 
     Step VII 
     The photoresist 19 is removed to form a TFT as shown in FIG. 7 (A) and (B). 
     The resultant TFTs are placed in a matrix form on the substrate and the gate electrode bar 11 and the source electrode bar 16 are extended in a matrix direction to contact with the other TFTs, which constitutes TFT array substrate. If the TFT array substrate is used as a cell substrate of a liquid crystal display device, mass storage display information can appear in its display as a clear picture. 
     According to the present invention, TFTs are produced by using two masks and therefore mask alignment, which is a most complicated treatment, can be done only once. This makes low cost and causes high yield. 
     EXAMPLE 2 
     FIG. 8 (A) and (B) show a plane view and an X--X&#39; sectional view of one picture element of the TFT array substrate produced by the process of the present invention. Two masks are employed in this embodiment, one is for patterning a gate electrode bar 11, a gate insulation layer 12, a semiconductor layer 13 and an electrode layer 14 which forms an ohmic contact with a semiconductor layer 13. The other is for patterning a source electrode bar 16, a drain electrode or display electrode 17 and the electrode layer 14. The manufacturing process and the concrete construction are explained by FIGS. 9 to 13. 
     Step I (see FIG. 9 (A) and (B)) 
     An Al layer 11&#39; which will make the gate electrode bar 11, is deposited on a glass substrate 10 in a thickness of 2,000A by a sputtering. The gate insulation layer 12&#39; of Si 3  N 4 , the semiconductor layer 13&#39; of amorphous hydrogenated silicon (a-Si:H) and the electrode layer 14&#39; of phosphorusdoped a-Si:H (n +  a-Si:H) which forms an ohmic contact with a-Si:H 13&#39;, are successively laminated on the Al layer 11&#39; by way of a plasma CVD. The thickness of the layers are respectively 2,000A, 2,000A and 1,000A. After forming four layers, a photoresist 18 is coated and exposed by using the first mask for developing. 
     Step II (see FIG. 10 (A) and (B)) 
     The four layers manufactured by the above step I are etched to form a pattern. The etchant of the n +  a-Si:H 14&#39; and the a-Si:H 13&#39; is a mixture solution of HF and HNO 3  and the etchant of Si 3  N 4  is a 5% HF solution. The etchant of the Al layer 11&#39; is an aqueous H 3  PO 4  solution. The etching can be conducted by immersing the substrate in the etchants and etching with the same pattern. 
     Step III (see FIG. 4 (A) and (B) 
     In this step, the Al layer 11&#39; which will give the gate electrode bar 11 is anodic-oxidized in the pattern edge portion. The purpose of this step is to prevent an electrical contact of the gate electrode bar 11 with the source electrode bar 16 and the drain electrode or display electrode 17. The anodic oxidation of the pattern edge portion is conducted by treating it in an ammonium borate solution at 40 volts to form Al 2  O 3  15 in the pattern edge portion of the gate electrode bar 11. 
     In the above process, the gate electrode 11 employs Al and forms Al 2  O 3  in the pattern edge, but the gate electrode bar 11 can employ so-called bulb metals such as Ta, Nb, Hf and the like, which forms insulation layer by anodic-oxidation. Among the metals mentioned above, in case of Ta, it is necessary that a Ta 2  O 5  layer be formed before Ta layer is laminated in order to avoid damage of the glass substrate during etching. 
     Step IV (see FIG. 12 (A) and (B)) 
     In this step, the transparent conductive layer 17&#39; is laminated on all surfaces, containing the n +  a-Si:H 14&#39; in a thickness of 3,000A by a vacuum evaporation for giving form to the source electrode bar 16 and the drain electrode or display electrode 17. After laminating it, a photoresist 19 is coated on it and exposed by using the second mask to develop to a desirable shape. The mask alignment is conducted only once, in this step, which makes the process simple and leads to low cost products. 
     Step V (see FIG. 13 (A) and (B)) 
     In this step, the transparent conductive layer 17&#39; is etched with the photoresist 19 to make a pattern of the source electrode bar 16 and the drain electrode or display electrode 17, followed by etching the n +  a-Si:H 14&#39; which form an ohmic contact. The etchant of the transparent conductive layer 17&#39; is a HCl solution and the etchant of the n +  a-Si:H 14&#39; is a mixture solution of HF and HNO 3 . Etching of the transparent conductive layer 17&#39; is conducted by immersing the laminated substrate in the etchants to give the source electrode bar 16 and the drain electrode or display electrode 17. Further, the n +  a-Si:H layer 14&#39; is etched to the electrode 14 which forms an ohmic contact of the a-Si:H semiconductor layer 13 with the source electrode bar 16 and the drain electrode 17. 
     Step VI 
     The photoresist 19 is removed to form a TFT as shown in FIG. 8 (A) and (B). 
     The resultant TFTs are placed in a matrix form on the substrate and the gate electrode bar 11 and the source electrode bar 16 are extended in a matrix direction to contact with the other TFTs, which constitutes TFT array substrate. If the TFT array substrate is used as a cell substrate of a liquid crystal display device, mass storage display information can appear in its display as a clear picture. 
     According to the present invention, TFTs are produced by using two masks and therefore mask alignment, which is a most complicated treatment, can be done only once. This makes for low cost and causes high yield.