Patent Document

BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method for fabricating a thin film transistor. More particularly, the present invention relates to a method for fabricating a thin film transistor having a double metal layer and a fine source/drain dimension using a highly conductive metal. 
   2. Description of the Related Art 
   With the rapid development of opto-electronic technologies, digital video and imaging devices have become some of the most common electrical appliances in our daily life. One of the most important man-machine communication interfaces for the digital video or imaging device is the display. A user can easily read information from the display to perform controlling operations. 
   Thin film transistor (TFT) is a driving device commonly deployed inside a display. Typically, the thin film transistor comprises a gate, a channel and a source/drain. In recent years, the process of forming the source/drain includes depositing a plurality of metallic layers (for example, chromium/aluminum/chromium composite layer or a molybdenum/aluminum/molybdenum composite layer) and performing a wet etching process to pattern the multi-layered metallic layer. However, with the continuous reduction of line widths, a source/drain interconnection using the aforementioned materials often leads to an increase in the resistor-capacitor (RC) delay that the operating speed of the thin film transistor will be slowed down. Thus, using a metallic material with good electrical conductivity to form the source/drain interconnect can minimize the RC delay effect significantly. 
   Due to the high electrical conductivity, interconnection using copper wires will be a major trend in the future. Yet, the fabrication of copper wires has a few problems: (1) it is difficult to control the dimension of copper patterns in a wet etching operation, and yet, it is hard to etch copper in a dry etching operation; (2) copper ions are easily diffused into surrounding areas leading to a change in the electrical properties of channel layers and/or the contamination of equipment. As a result, copper is often combined with other metals (for example, molybdenum) to form a multi-layered metallic layer in the fabrication of the source/drain. 
     FIG. 1  is a schematic cross-sectional view of a conventional thin film transistor having a source/drain fabricated using a molybdenum/copper/molybdenum multi-layered metallic layer. As shown in  FIG. 1 , the thin film transistor  100  comprises a substrate  110 , a gate  120 , a gate-insulating layer  130 , a semiconductor layer  140  and a pair of source/drain  150 . The semiconductor layer  140  comprises a channel layer  142  and an ohmic contact layer  144  and the source/drain  150  comprises a molybdenum layer  152 , a copper layer  154  and another molybdenum layer  156 . 
   To form the source/drain  150 , a patterned photoresist layer  160  is formed over the thin film transistor  100 . Then, using the patterned photoresist layer  160  as a mask, a wet etching of the underlying molybdenum layer  156  and the copper layer  154  is carried out. However, because copper has an etching rate greater than molybdenum, the effect of side undercuts  170  is occurred in the wet etching operation as shown in  FIG. 1 . Thus, the possibility of forming a line cut or a thin film transistor  100  that does not match the dimensional specifications is increased. Furthermore, since the copper layer  154  is dipped inside a pool of etching solution in the wet etching operation, copper ions may diffuse into the nearby semiconductor layer  140  by hitching a ride with the etching solution. Ultimately, the electrical performance of the thin film transistor  100  is affected. 
   Furthermore, after performing the wet etching operation, a dry etching operation is carried out to remove the molybdenum layer  152  above the gate  120  and a back channel etching (BCE) operation is carried out to remove the ohmic contact layer  144  and a portion of the channel layer  142  above the gate  120 . 
     FIG. 2  is a schematic cross-sectional view showing the dry etching operation for removing the molybdenum layer above the gate and the back channel etching operation. In the dry etching operation, the copper atoms can diffuse into areas in the neighborhood of the semiconductor layer  140  and affect the electrical properties of the semiconductor layer  140  because the copper layer  154  is still exposed in the etching environment. Moreover, since the patterned photoresist layer  160  still serves as a mask in the dry etching operation and the back channel etching operation, a side surface  180  of the molybdenum layer  152 , the ohmic contact layer  144  and the channel layer  142  aligns with the edge of the patterned photoresist layer  160 . It should be noted that there is a large difference in the dimension between the side surface  180  of the molybdenum layer  152  and the side undercut  170  of the copper layer  154 . This causes an unfavorable condition for forming a source/drain  150  with a fine dimension. 
   SUMMARY OF THE INVENTION 
   Accordingly, at least one objective of the present invention is to provide a method for fabricating a thin film transistor suitable for forming a source/drain having a multi-layered metallic layer structure and a fine dimension using a metallic material with high conductivity. 
   To achieve these and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, the invention provides a method for fabricating a thin film transistor comprising the following steps. First, a gate is formed on a substrate. Then, a gate-insulating layer is formed on the substrate to cover the gate. Next, a patterned semiconductor layer is formed on the gate-insulating layer above the gate. After that, a first conductive layer and a second conductive layer are sequentially formed on the patterned semiconductor layer. The second conductive layer is patterned such that each side of the second conductive layer above the gate has a taper profile and the first conductive layer is exposed. Then, a first plasma processing is performed to transform the surface and the taper profile of the second conductive layer into a first protection layer. After that, the first conductive layer not covered by the first protection layer and the second conductive layer is removed to form a source/drain. 
   In one embodiment of the present invention, the material of the second conductive layer includes copper. 
   In one embodiment of the present invention, the material of the first protection layer includes copper oxide or copper nitride. 
   In one embodiment of the present invention, the material of the first conductive layer is selected from a group consisting of molybdenum, tungsten molybdenum and tantalum or a combination of them. 
   In one embodiment of the present invention, the reactive gas for performing the plasma treatment is selected from a group consisting of oxygen, nitrogen, nitrogen dioxide and ammonia or a combination of them. 
   In one embodiment of the present invention, the method of patterning the second conductive layer includes the following steps. First, a patterned photoresist layer is formed on the substrate to expose the second conductive layer above the gate. Then, using the patterned photoresist layer as a mask, a wet etching operation is performed to etch the second conductive layer until the first conductive layer is exposed and the etched second conductive layer has a taper profile. 
   In one embodiment of the present invention, the step of removing the first conductive layer not covered by the protection layer and the second conductive layer includes performing a dry etching operation. The gas used in the dry etching operation is selected from a group consisting of hexafluorosulfide (SF 6 ), oxygen (O 2 ), chlorine (Cl 2 ), hydrogen chloride (HCl) and trifluoromethane (CHF 3 ) or a combination of them. 
   In one embodiment of the present invention, the patterned semiconductor layer comprises a patterned channel layer and a patterned ohmic contact layer. Furthermore, the patterned ohmic contact layer is disposed on the patterned channel layer. 
   In one embodiment of the present invention, the method of fabricating the thin film transistor also includes performing a back channel etching operation to remove the patterned ohmic contact layer and a portion of the patterned channel layer above the gate. 
   In one embodiment of the present invention, the method of forming a gate on the substrate includes the following steps. First, a third conductive layer and a fourth conductive layer are sequentially formed on the substrate. Then, the fourth conductive layer is patterned. After that, a second plasma treatment is carried out to transform the surface of the fourth conductive layer into a second protection layer. Then, the third conductive layer not covered by the second protection layer and the fourth conductive layer is removed to form the gate. 
   In one embodiment of the present invention, the material of the fourth conductive layer includes copper. 
   In one embodiment of the present invention, the material of the second protection layer includes copper oxide or copper nitride. 
   In one embodiment of the present invention, the material of the third conductive layer is selected from a group consisting of molybdenum, tungsten molybdenum and tantalum or a combination of them. 
   In one embodiment of the present invention, the reactive gas for performing the second plasma treatment is selected from a group consisting of oxygen, nitrogen, nitrogen dioxide and ammonia or a combination of them. 
   In one embodiment of the present invention, the step of removing the third conductive layer not covered by the second protection layer and the fourth conductive layer includes performing a dry etching operation. The gas used in the dry etching operation is selected from a group consisting of hexafluorosulfide (SF 6 ), oxygen (O 2 ), chlorine (Cl 2 ), hydrogen chloride (HCl) and trifluoromethane (CHF 3 ) or a combination of them. 
   In one embodiment of the present invention, the method of patterning the fourth conductive layer includes performing a photolithographic process and a wet etching operation. 
   In the present invention, materials such as copper and molybdenum having good electrical conductivity are used to fabricate a double metal layer source/drain. Furthermore, a plasma treatment is carried out to process the surface of the copper layer so that the copper layer on the upper layer can be used as a mask for dry-etching the underlying molybdenum layer and the channel layer. Thus, the method of fabricating the thin film transistor according to the present invention can prevent the occurrence of side undercuts on the copper layer and thus produce source/drain with a fine dimension. Moreover, the higher electrical conductivity of a copper layer is able to minimize resistor-capacitor (RC) delay so that the thin film transistor can have a higher operating speed. 
   It is to be understood that both the foregoing general description and the following detailed description are exemplary, and are intended to provide further explanation of the invention as claimed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. In the drawings, 
       FIG. 1  is a schematic cross-sectional view of a conventional thin film transistor having a source/drain fabricated using a molybdenum/copper/molybdenum multi-layered metallic layer. 
       FIG. 2  is a schematic cross-sectional view showing the dry etching operation for removing the molybdenum layer above the gate and the back channel etching operation. 
       FIGS. 3A through 3J  are schematic cross-sectional views showing the steps for fabricating a thin film transistor according to one preferred embodiment of the present invention. 
       FIGS. 4A through 4E  are schematic cross-sectional views showing the steps for fabricating a gate according to one embodiment of the present invention. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Reference will now be made in detail to the present preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers are used in the drawings and the description to refer to the same or like parts. 
     FIGS. 3A through 3J  are schematic cross-sectional views showing the steps for fabricating a thin film transistor according to one preferred embodiment of the present invention. First, a gate  310  is formed on a substrate  300  as shown in  FIG. 3A . In one embodiment of the present invention, the method of forming the gate  310  is described in  FIGS. 4A through 4E . First, as shown in  FIG. 4A , a first conductive layer  210  and a second conductive layer  220  are sequentially formed on the substrate  300 . The method of forming the conductive layer  210  and  220  includes, for example, performing a sputtering or an evaporation process. The conductive layer  210  is fabricated using a low contact resistant, high adhesion metal. In one embodiment, the material constituting the conductive layer  210  is selected from a group consisting of molybdenum, tungsten molybdenum and tantalum or a combination of them. On the other hand, the conductive layer  220  is fabricated using a low resistant, highly conductive metal. In one embodiment, the conductive layer  220  is fabricated using copper, for example. However, the conductive layer  220  can also be fabricated using silver or gold. 
   As shown in  FIG. 4B , the conductive layer  220  is patterned. In one embodiment of the present invention, the method of patterning the conductive layer  220  includes performing a photolithographic process and a wet etching operation. First, a photoresist layer  230  is formed over the conductive layer  220  by performing a photolithographic process. Then, using the photoresist layer  230  as a mask, a wet etching operation of the conductive layer  220  is carried out to form the patterned conductive layer  220  as shown in  FIG. 4C . 
   Next, as shown in  FIG. 4C , a plasma treatment  240  is carried out to form a protection layer  250  on the surface of the conductive layer  220 . In one embodiment of the present invention, the reactive gas used in the plasma treatment  240  is selected from a group consisting of oxygen, nitrogen, nitrogen dioxide and ammonia or a combination of them. The plasma generated from the reactive gas can react with the surface of the conductive layer  220 . As a result, the material of the protection layer  250  can be copper oxide or copper nitride, for example. 
   As shown in  FIG. 4D , the conductive layer  210  not covered by the protection layer  250  and the conductive layer  220  is removed to form a gate  310  as shown in  FIG. 4E . In one embodiment of the present invention, the method of removing the conductive layer  210  not covered by the protection layer  250  and the conductive layer  220  includes performing a dry etching operation  260 , for example. Furthermore, the gas used in the dry etching operation  260  is selected from a group consisting of hexafluorosulfide (SF 6 ), oxygen (O 2 ), chlorine (Cl 2 ), hydrogen chloride (HCl) and trifluoromethane (CHF 3 ) or a combination of them. 
   After forming the gate  310 , a gate-insulating layer  320  is formed on the substrate  300  to cover the gate  310  as shown in  FIG. 3B . In one embodiment of the present invention, the method of forming the gate-insulating layer  320  includes performing a chemical vapor deposition (CVD) process. Furthermore, the gate-insulating layer  320  is fabricated using silicon nitride or silicon oxide, for example. 
   As shown in  FIG. 3C , a patterned semiconductor layer  330  is formed on the gate-insulating layer  320  above the gate  310 . In one embodiment of the present invention, the method of forming the patterned semiconductor layer  330  includes sequentially depositing channel material and ohmic contact material and patterning the ohmic contact material layer (not shown) and the channel material layer (not shown) thereafter. The patterned semiconductor layer  330  in  FIG. 3C  comprises a patterned channel layer  332  and a patterned ohmic contact layer  334 , for example. Moreover, the patterned ohmic contact layer  334  is disposed on the patterned channel layer  332 . 
   As shown in  FIG. 3D , a first conductive layer  340  and a second conductive layer  350  are sequentially formed over the patterned semiconductor layer  330 . In one embodiment of the present invention, the method of forming the conductive layers  340  and  350  includes performing a sputtering or an evaporation process, for example. The first conductive layer  340  is fabricated using a low contact resistant, high adhesion metal. In one embodiment, the material constituting the first conductive layer  340  is selected from a group consisting of molybdenum, tungsten molybdenum and tantalum or a combination of them. On the other hand, the second conductive layer  350  is fabricated using a low resistant, high conductivity metal. In one embodiment, the material constituting the second conductive layer  350  includes copper, for example, but can be silver or gold. 
   Then, the conductive layer  350  is patterned so that the two sides of the conductive layer  350  above the gate  310  have a taper profile  352  and expose the conductive layer  340  as shown in  FIG. 3G . In one embodiment of the present invention, the process of patterning the conductive layer  350  is shown in  FIGS. 3E to 3G . First, as shown in  FIG. 3E , a patterned photoresist layer  360  is formed on the substrate  300 . The patterned photoresist layer exposes the conductive layer  350  above the gate  310 . Then, as shown in  FIG. 3F , a wet etching operation is carried out using the patterned photoresist layer  360  as a mask until the conductive layer  340  is exposed and the taper profile  352  is formed in the etched conductive layer  350 . Finally, the patterned photoresist layer  360  is removed as shown in  FIG. 3G . 
   As shown in  FIG. 3H , a plasma treatment  370  is carried out to form a protection layer  380  on the surface of the conductive layer  350  and the taper profile  352 . In one embodiment of the present invention, the reactive gas for performing the plasma treatment  370  is selected from a group consisting of oxygen, nitrogen, nitrogen dioxide and ammonia or a combination of them. As a result, the material of the protection layer  380  is copper oxide or copper nitride, for example. Furthermore, the plasma treatment  370  is carried out at a temperature between room temperature (25° C.)˜380° C., and the power rating for performing the plasma treatment  370  is set between 50 W˜5 KW. They changes with process pressure, mixture gas rate, and gas flow rate, respectively. 
   It should be noted that the step of transforming the surface of the conductive layer  350  into the protection layer  380  in the plasma treatment  370  permits the direct use of the protection layer  380  as a mask in dry etching the conductive layer  340  underneath the conductive layer  350 . Hence, the pattern dimension between the first conductive layer  340  and the second conductive layer  350  will be very close and resolve the problem of having large dimensional difference between the layers in the conventional multi-layered metallic layer. 
   As shown in  FIG. 3I , the first conductive layer  340  not covered by the protection layer  380  and the second conductive layer  350  is removed to form source/drain  390 . In one embodiment of the present invention, the process of removing the conductive layer  340  not covered by the protection layer  380  and the conductive layer  350  includes performing a dry etching operation  400 . The gas used in the dry etching operation  400  is selected from a group consisting of hexafluorosulfide (SF 6 ), oxygen (O 2 ), chlorine (Cl 2 ), hydrogen chloride (HCl) and trifluoromethane (CHF 3 ) or a combination of them, for example. It should be noted that the material (such as copper) in the conductive layer  350  would not be released in the dry etching operation  400  because the protection layer  380  covers the conductive layer  350 . Therefore, the electrical performance of the patterned semiconductor layer  330  will not be affected. 
   As shown in  FIG. 3J , in one embodiment of the present invention, the aforementioned method of fabricating the thin film transistor further includes performing a channel back etching operation  410 . The channel back etching operation removes the patterned ohmic contact layer  334  above the gate  310  and a portion of the patterned channel layer  332 . Up to this point, the process of fabricating a thin film transistor  500  is completed. 
   In summary, the method of fabricating thin film transistors in the present invention has the following advantages. 
   1. The effect of side undercuts on the copper conductive layer is prevented so that the source/drain can have a finer dimension. 
   2. By forming a protection layer over the copper conductive layer in a plasma treatment, copper is prevented from releasing and contaminating the channel layer; ultimately, electrical performance of the semiconductor layer will not be affected because of copper. 
   3. The present invention uses the copper wire interconnection technique to fabricate the source/drain of a thin film transistor. The better electrical conductivity of copper can solve the prior art resistor-capacitor time delay problem so that the thin film transistor can operate at a higher speed. 
   It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.

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