Abstract:
In an active matrix liquid crystal display (LCD) device a conductor line interconnecting a drain of each thin-film transistor and a corresponding pixel electrode constructed with indium tin oxide (ITO) is formed in a three-layer structure in which an aluminum film is sandwiched between a pair of titanium films. This construction prevents poor contact and deterioration of reliability because electrical contact is established between one titanium film and semiconductor and between the other titanium film and ITO. The aluminum film has low resistance which is essential for ensuring high performance especially in large-screen LCDs.

Description:
This is a continuation of U.S. application Ser. No. 08/683,241, filed Jul. 18, 1996, which claims the benefit of a foreign priority application filed in Japan, serial number 7-211195, filed Jul. 27, 1995. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to the construction of an active matrix display device such as an active matrix liquid crystal display (LCD) device. 
     2. Description of the Related Art 
     In an active matrix LCD device, thin-film transistors are arranged on a sheet of quartz or glass at a high density. It is increasingly required in recent years to raise the level of integration of the thin-film transistors. On the other hand, it is more and more required for the LCD device to provide larger display areas to meet the growing demand for large-screen displays. This is where the LCD device greatly differs from large-scale integrated (LSI) circuits which are required to provide higher integration levels and smaller physical sizes. 
     Besides the requirement for large-screen display, it is desired to make conductor lines of the LCD device as narrow as possible so as to provide an increased aperture ratio. If, however, narrow conductor lines are used, problems arising from their increased resistances will become evident. 
     The active matrix LCD device requires means for masking the thin-film transistors arranged in individual pixels as well as masking means, which is known as a black matrix, for masking edges of individual pixel electrodes. Generally, these masking means including the black matrix are separately arranged from the conductor lines. Such a construction is not preferable though because it complicates processing steps needed for the production of the active matrix LCD device. 
     One method for reducing conductor line resistance is to use aluminum as a wiring material. However, the use of aluminum could give rise to reliability problems because electrical contacts between aluminum and semiconductor and between aluminum and a transparent conductive coating, which is usually a thin layer of conductive oxide such as indium tin oxide (ITO), are liable to become unstable. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide a construction of an active matrix LCD device, by which an increased aperture ratio is obtainable with a small number of steps in the overall manufacturing process. It is a further object of the invention to provide a construction which can eliminate instability of electrical contact caused by specific wiring materials. 
     According to the invention, a semiconductor device comprises a conductor line for interconnecting a semiconductor element and a conductive oxide film, the conductor line having a layer structure including an aluminum film sandwiched between a pair of titanium films, wherein one of the titanium films is held in contact with the semiconductor element while the other titanium film is held in contact with the conductive oxide film. 
     In one specific form of implementation of this construction, a drain region of a thin-film transistor is connected to a pixel electrode constructed with ITO by the conductor line which is formed by sandwiching the aluminum film with the pair of titanium films. More particularly, the drain region made of a semiconductor material is held in contact with one titanium film while the pixel electrode constructed with ITO is held in contact with the other titanium film. This construction provides a solution to the reliability problem arising from the instability of electrical contact between aluminum and semiconductor. 
     This construction also serves to improve the quality of contact between ITO and the conductor line as the latter is covered by the titanium film. In general terms this means that the invention provides a solution to the reliability problem arising from the instability of electrical contact between aluminum and ITO (or a conductive oxide film in general) as well. Needless to say, an additional advantage comes from the use of aluminum of which resistance is remarkably low. 
     In a varied form of the invention, a semiconductor device comprises a conductive oxide film which constitutes a pixel electrode, a conductor line for interconnecting the conductive oxide film and a drain region of a thin-film transistor, a first masking film constructed with the same material as the conductor line for masking the thin-film transistor, and a second masking film constructed with the same material as the conductor line for masking edges of the pixel electrode, wherein the conductor line has a layer structure including an aluminum film sandwiched between a pair of titanium films. 
     As will be discussed later in greater detail, the semiconductor device thus constructed has the pixel electrode constructed with ITO. the conductor line connecting the pixel electrode to the drain region of the thin-film transistor and the first masking film for masking the thin-film transistor. The conductor line has a three-layer structure in which the aluminum film is sandwiched between the pair of titanium films. In addition, the second masking film, or the black matrix, constructed with the same material as used for producing the conductor line masks the edges of the pixel electrode. 
     What is significant in this construction is that the conductor line, first masking film and black matrix are obtained simultaneously by patterning the same three-layer structure. This serves to simplify the overall manufacturing process, improve production yield and reduce manufacturing costs. 
     The titanium films are most preferable for sandwiching aluminum from the viewpoint of electrical characteristics in this invention. Chromium films may be employed instead of the titanium films, if desired. However, titanium is more desirable since the titanium film can be more easily patterned by dry etching than the chromium film. Furthermore, a few percent by weight of appropriate impurities may be added to the titanium, to adjust their optical and/or electrical characteristics. 
     In yet varied form of the invention, a semiconductor device comprises a conductive oxide film which constitutes a pixel electrode, a first conductor line for interconnecting the conductive oxide film and a drain region of a thin-film transistor, a masking film constructed with the same material as the first conductor line for masking the thin-film transistor, and a masking film constructed with the same material as the first conductor line for masking edges of the pixel electrode, a second conductor line connected to a source region of the thin-film transistor, an outgoing conductor line constructed with the same material as the first conductor line, the outgoing conductor line being connected to the second conductor line, wherein the first conductor line has a layer structure including an aluminum film sandwiched between a pair of titanium films. 
     In one specific form of implementation of this construction, the first conductor line has a three-layer structure in which the aluminum film is sandwiched between the pair of titanium films while the second conductor line is constructed with a titanium film and an aluminum film. The first conductor line, first masking film, black matrix and outgoing conductor line can be formed by patterning the same three-layer structure. 
     Accordingly, the invention provides a highly reliable construction of semiconductor devices, eliminating instability of contact between wiring materials. The three-layer conductor line configuration is particularly effective in view of low resistance of the aluminum film and good electrical contact between the titanium film and semiconductor and between the titanium film and conductive oxide film. 
     Remarkable features of the invention can be summarized as follows: 
     (1) The stacked two-layer structure including the aluminum and titanium films helps reduce voltage drop in individual source lines, and this is particularly evident in large-screen LCD devices; 
     (2) The stacked two-layer structure including the aluminum and titanium films enhances the reliability of connections between the individual source lines and source regions; 
     (3) The three-layer structure of individual first conductor lines interconnecting drain regions and pixel electrodes can be used for producing the masking film for masking each thin-film transistor without requiring any additional processing step; 
     (4) Individual outgoing conductor lines used for connecting to surrounding circuit elements can be created at the same time with the first conductor lines, and the reliability of connections between the individual first conductor lines and source lines can be improved; 
     (5) The reliability of connections between the first conductor lines and drain regions and between the first conductor lines and ITO pixel electrodes can be improved; and 
     (6) The black matrix can be formed at the same time with the first conductor lines. 
     As will be recognized from the foregoing discussion, the invention makes it possible to simultaneously produce various elements of the active matrix LCD device without requiring additional processing steps. Moreover, the invention provides improvements in product performance and reliability as well as a reduction in manufacturing costs. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS.  1 (A)- 1 (D) illustrate processing steps for the production of an active matrix circuit according to a first embodiment of the invention; 
     FIGS.  2 (A)- 2 (C) illustrate processing steps for the production of the active matrix circuit which follow the steps shown in FIGS.  1 (A)- 1 (D); 
     FIG. 3 is a top view generally illustrating the configuration of a single pixel of the active matrix circuit; and 
     FIGS.  4 (A)- 4 (D) illustrate the general construction of a gate electrode according to a second embodiment of the invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIRST EMBODIMENT 
     FIGS. 1 and 2 generally illustrate successive processing steps for the production of an active matrix LCD device according to a first embodiment of the present invention. First, silicon dioxide is coated to a thickness of 3000 Å on a substrate  101  which is constructed with a sheet of glass or quartz to form an undercoat  102  thereupon. This undercoat  102  may be formed by a plasma chemical vapor deposition (CVD) process or a sputtering process. 
     The silicon dioxide undercoat  102  helps suppress diffusion of impurities from the substrate  101  and make mechanical stresses occurring between the substrate  101  and a later-produced semiconductor layer less severe. When a sheet of quartz is used as the substrate  101 , it is preferable to produce a thick anodic oxidization film which forms the undercoat  102 . This is because the quartz substrate shrinks much less than the silicon dioxide undercoat  102  when heated so that stresses are likely to occur between the substrate  101  and the semiconductor layer. 
     After the undercoat  102  has been completed, there is formed an amorphous silicon film which will serve as a starting layer for producing an active layer of each thin-film transistor. This amorphous silicon film is made 500 Å thick, for instance. Either the plasma CVD process or low-pressure thermal CVD process may be used to produce the amorphous silicon film. 
     Thin-film transistors may be produced on top of the amorphous silicon film as it is if they are not required to provide high performance. If, however, high-quality display is desired, the amorphous silicon film should be converted into a crystalline silicon film. Described below is an example of a processing step by which the amorphous silicon film is converted into a crystalline silicon film. 
     Specifically, a crystalline silicon film having high crystallinity is obtained with the aid of a metallic element which accelerates crystallization of silicon. In this process, a solution of acetate nickel adjusted to a specific level of concentration is applied to the surface of the amorphous silicon film at first. Excess solution is then removed by means of a spinner. Now that a layer of nickel is held directly on the surface of the amorphous silicon film, the substrate  101  is subjected to a temperature of 620° C. for a period of four hours. The crystalline silicon film is obtained through this heat treatment. 
     Besides the above crystallization method, the crystalline silicon film can be obtained by a laser beam irradiation method, a simple heat treatment method, a high-energy infrared heating method, or a combination thereof. 
     The crystalline silicon film is then patterned to obtain a structure as shown in FIG.  1 (A), in which the glass substrate  101  is covered with the silicon dioxide undercoat  102  which carries active layers  103  (island semiconductor layers) for producing individual thin-film transistors. The following discussion is based on the assumption that the active layers  103  are constructed of crystalline silicon. 
     When the structure illustrated in FIG.  1 (A) has been obtained, a silicon dioxide film which will serve as a gate insulation layer  104  is deposited to a thickness of 1000 Å by the plasma CVD process or sputtering process. Further, a layer of aluminum containing 0.2% scandium by weight is formed on top of the silicon dioxide film to a thickness of 6000 Å. This aluminum layer is then patterned to produce individual gate electrodes  105 . The gate electrodes  105  thus produced constitute a first circuit layer. 
     It is important that the gate electrodes  105  is constructed with aluminum. As illustrated in FIG. 3, each gate electrode  105  branches out from one of gate buses which are arranged in a matrix form. In a case where resistances associated with these bus lines can not be disregarded, signal delays and/or operational errors may result. This problem becomes particularly evident in large-screen LCD devices. The use of aluminum, which is a low-resistance material, for constructing the gate electrodes  105  and gate buses in this embodiment therefore offers remarkable advantages. 
     After the gate electrodes  105  have been completed, the gate electrodes  105  are anodized in an electrolytic solution of ethylene glycol of about pH 7 containing 3 to 10% of tartaric acid. In this anodic oxidization process, an anodic oxide film  106  having fine and dense composition is deposited to a thickness of 2500Å to cover each gate electrode  105 . The anodic oxide films  106  thus created serve to prevent abnormal growth of aluminum as well as development of cracks. The anodic oxide films  106  also serve as a mask when producing offset gate regions through a dopant ion implantation process. 
     When the structure illustrated in FIG.  1 (B) has been obtained, dopant ions for producing source and drain regions are implanted. In this embodiment, a plasma doping process is used to implant phosphor ions for creating n-channel thin-film transistors. 
     By the implantation of phosphor ions, source regions  107  and drain regions  110  are created through a self-alignment process. Conducting channel regions  109  and offset gate regions  108  are also created through a self-alignment process, as shown in FIG.  1 (C). 
     Upon completion of the implantation of dopant ions shown in FIG.  1 (C), a laser beam is irradiated for annealing the source regions  107  and drain regions  110 . The annealing process serves to activate the implanted phosphor ions and remove damages to the crystalline lattice caused by the implantation of the phosphor ions. 
     Then, a first interlayer dielectric film  111  is produced as shown in FIG.  1 (D) by depositing a layer of silicon dioxide to a thickness of 5000Å by the plasma CVD process. Then, contact holes reaching to the individual source regions  107  are created. It is to be noted that in a case where silicon dioxide is used for forming the first interlayer dielectric film  111 , a titanium film of a later-produced circuit may react with the silicon dioxide film, resulting in formation of titanium oxide. Should this happen, it is preferable to produce a silicon nitride film instead of the silicon dioxide film or a two-layer structure including silicon nitride and silicon dioxide films. 
     Source lines  112  connecting to the individual source regions  107  are formed as illustrated in FIG.  2 (A). Each source line  112  is constructed with a stacked deposition of a titanium film and an aluminum film. In this embodiment, the titanium film is made 500Å thick while the aluminum film is 4000Åmade thick by the sputtering process. The source lines  112  thus produced constitute together a second circuit layer. 
     The titanium film is provided to prevent direct contact between aluminum and silicon. This is because if they are allowed to come into mutual contact, a catalytic reaction will take place, resulting in poor contact or a variation in contact resistance with the lapse of time. Each source line  112  branches out and connects to the source region  107  arranged in each individual pixel as shown in FIG.  3 . 
     A second interlayer dielectric film  113  is then produced to a thickness of 4000Å as shown in FIG.  2 (B). The second interlayer dielectric film  113  is produced by depositing silicon dioxide by the plasma CVD process. It may be constructed with a silicon nitride film instead of a silicon dioxide film to prevent conversion of the later-produced titanium film into a titanium oxide film. Alternatively, a two-layer structure including silicon dioxide and silicon nitride films or a three-layer structure including silicon nitride, silicon dioxide and silicon nitride films may be employed. 
     Next, an ITO layer which constitutes pixel electrodes  114  is produced. Tin oxide (SnO 2 ) can also be used as an alternative to ITO. What is essential for materials for constructing the pixel electrodes  114  is their capability to create a transparent conductive coating. 
     Contact holes  115  and  116  are then produced as shown in FIG.  2 (B). The contact holes  115  are openings in which wiring to surrounding circuit elements is formed while the contact holes  116  are openings through which the individual drain regions  110  are connected to the respective pixel electrodes  114 . 
     Subsequently, a three-layer structure including a titanium film, an aluminum film and another titanium film which form together a third circuit layer is produced by sputtering or evaporation. This third circuit layer is patterned to create the following elements: 
     (1) Conductor lines  117  for connecting to surrounding circuit elements and external circuitry; 
     (2) Masking films  118  for masking the thin-film transistors; 
     (3) Conductor lines  119  for connecting outputs from the individual drain regions  110  to the respective pixel electrodes  114 ; and 
     (4) A black matrix which is not illustrated in FIGS.  2 (A)- 2 (C). (The black matrix is designated by the numeral  301  in FIG. 3.) 
     The aforementioned three-layer structure in which the aluminum film is sandwiched between the two titanium films offers the following advantages: 
     Good electrical contact with the drain regions  110 ; 
     Good electrical contact with the source lines  112  in the second circuit layer; and 
     Good electrical contact with the ITO pixel electrodes  114 . 
     FIG. 3 is a top view generally illustrating the configuration of a single pixel of the active matrix circuit of FIGS.  2 (A)- 2 (C), where FIG.  2 (C) is a cross-sectional view taken along lines A—A′ shown in FIG.  3 . FIG. 3 depicts part of the black matrix  301  which encloses the individual pixel electrodes  114 . As is apparent from FIG. 3, the black matrix  301  and the masking film  118  of each thin-film transistor are connected to each other by a continuous film structure in this embodiment. In one variation of the embodiment, the black matrix  301  and the individual masking films  118  may be constructed as separate elements. It is not, however, preferable to electrically connect the masking films  118  to the conductor lines  119  because such an arrangement will develop undesirable stray capacitance. 
     It is to be noted that FIG. 3 does not show the conductor line  117  which is illustrated in FIG.  2 (C). The conductor line  117  is actually connected to an end of the source line  112  at an edge of each pixel electrode  114 . 
     Second Embodiment 
     The second embodiment of the invention features a different construction of gate electrodes in comparison with the first embodiment. More particularly, each gate electrode is formed in a three-layer structure including a titanium film, an aluminum film and another titanium film. 
     FIGS.  4 (A)-(D) illustrate how a gate electrode of the second embodiment is produced. Shown in FIG.  4 (A) is an unfinished gate electrode which has been produced by forming a titanium film  402  to a thickness of about 100 Å on top of a gate insulation layer  401 , a aluminum film  403  containing a small amount of scandium to a thickness of 5000 Å on top of the titanium film  402 , and a titanium film  404  to a thickness of about 100 Å on top of the aluminum film  403 , and then patterning this three-layer structure of the titanium film  402 , aluminum film  403  and titanium film  404  to the shape of a gate electrode. 
     After the structure shown in FIG.  4 (A) has been obtained, it is subjected to an anodic oxidization process, in which an anodic oxide film  405  having fine and dense composition is deposited to a thickness of 200 Å around the gate electrode which comprises the titanium film  402 , aluminum film  403  and titanium film  404 , as shown in FIG.  4 (B). Since the anodic oxide film  405  created in this process is made of titanium oxide and aluminum oxide, it is difficult to form the anodic oxide film  405  to a thickness of more than a few hundred Å. 
     Next, a silicon nitride film  406  is formed as a first interlayer dielectric film to a thickness of 4000 Å by the plasma CVD process, as shown in FIG.  4 (C). 
     Then, a contact hole is created for forming an aluminum conductor line  407  which connects to the titanium film  404  of the gate electrode. and the aluminum conductor line  407  is produced down to the titanium film  404  as shown in FIG.  4 (D). It is to be noted that the aluminum conductor line  407  is formed in a peripheral area away from where a thin-film transistor is located. 
     According to this construction, there is no chance of direct contact between the gate insulation layer  401  and aluminum film  403  so that intrusion of abnormal growth of aluminum into the gate insulation layer  401  does not occur at all. Furthermore, it is possible to achieve desirable interface characteristics of the junction between the gate insulation layer  401  and the gate electrode which comprises the titanium film  402 , aluminum film  403  and titanium film  404 . This also ensures improved operation of the individual thin-film transistors. 
     In addition, the second embodiment facilitates etching of the anodic oxide film  405  on the top surface of the gate electrode for making the contact hole in which the aluminum conductor line  407  is created. It is difficult to selectively remove the anodic oxide film  405  alone if it is formed directly on aluminum. The three-layer structure of the gate electrode of this embodiment provides a solution to this problem in etching. 
     The structure of the second embodiment may be combined with that of the first embodiment in order to achieve further improvements in production yield and manufacturing costs as well as device reliability.