Liquid crystal cells – elements and systems – Particular structure – Having significant detail of cell structure only
Reexamination Certificate
2002-03-20
2004-05-04
Parker, Kenneth (Department: 2871)
Liquid crystal cells, elements and systems
Particular structure
Having significant detail of cell structure only
C349S147000
Reexamination Certificate
active
06731364
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an active matrix type liquid crystal display device (AM-LCD) driven by thin film transistors (TFT) and a method of manufacture thereof.
In an effort to provide image display devices that have a reduced thickness and weight and are able to attain high definition, the market has moved toward the provision of thin film transistor-driven liquid crystal display devices (TFT-LCD), instead of the existing cathode ray tubes. The conventional TFT-LCD comprises scanning signal lines, image signal lines, thin film transistors formed in the vicinity of intersections between the scanning signal lines and the image signal lines, pixel electrodes connected with the thin film transistors, a gate insulative film and a protection film formed on a glass substrate; and, a counter substrate is provided with a liquid crystal layer disposed between the glass substrate and the counter substrate. In recent years, along with an increased size and an improved definition of the screen of a TFT-LCD, specifications required for a reduction in the resistance of scanning signal lines and image signal lines and an increase in the production yields have become more and more severe. Further, there has also been a demand to reduce the production cost by simplifying the production steps.
For the image signal lines of the bottom gate type amorphous silicon TFT-LCD, those comprising a single layer of a metal film, such as Ti, Ta, Cr, Mo and CrMo, or those made of laminated metal films, such as Mo/Cr, Al/Ti, CrMo/Cr, Mo/Al/Mo, Ti/Al/Ti, Cr/Al/Cr and MoCr/Al/MoCr, have been adopted. In the foregoing, the slash (/) represents the constitution of laminated films in which an upper layer is indicated on the left and a lower layer is indicated on the right of the slash. Such wiring constitutions are properly selected depending on the wiring resistance specifications required for liquid crystal driving, the production performance of a sputtering step, the performance of an etching apparatus, and so on.
Among the various possibilities, the constitution capable of obtaining the lowest wiring resistance can include films employing Al, Al/Ti, Mo/Al/Mo, Ti/Al/Ti, Cr/Al/Cr and MoCr/Al/MoCr, and a similar constitution capable of obtaining the second lowest wiring resistance can include employing pure Mo, Mo and Mo/Cr. Incidentally, Ti or Mo can be laminated as an upper or a lower layer on Al, such as in Ti/Al/Ti or Mo/Al/Mo, in order to improve the contact with the silicon constituting a thin film transistor and a pixel electrode comprising indium oxide or the like as the main ingredient. Further, those simple constitutions providing the lowest load on the sputtering step can include single-layered constitutions of Ti, Ta, Cr, Mo or CrMo, and similar constitutions for providing the second lowest load can include a two-layered constitution of Mo/Cr, Al/Ti. Further, the constitutions capable of selective wet etching for a gate insulative film without the use of a hydrofluoric system etching solution can include Cr, Mo, CrMo, Al/Cr, Mo/Al/Mo, Cr/Al/Cr and MoCr/Al/MoCr. Further, those constitutions capable of forming through holes in the protection insulative film substantially covering a drain electrode and a source electrode without eliminating the wiring film can include Cr, CrMo, Mo/Cr, CrMo/Cr, Cr/Al/Cr and MoCr/Al/MoCr.
The constitutions of the image signal lines described above have respective features, as described previously, but constitutions capable of simultaneously satisfying requirements for reduced wiring resistance, a reduced load on the sputtering step, selective wet etching for the gate insulative film, and formation of the through hole in the protection insulative film by dry etching, have not yet been known.
For example, Ti has a high resistivity, and since buffered hydrofluoric acid is used for wet etching for Ti, selective wet etching with respect to the gate insulative film is difficult. Further, Ta also has a high resistivity, and since buffered hydrofluoric acid is used for wet etching for Ta, selective wet etching with respect to the gate insulative film is also difficult. Further, since it is etched with an SF
6
, gas, formation of the through holes in the protection insulative film by dry etching is difficult. Further, Cr, CrMo, and CrMo/Cr each have a high resistivity. Also, since Mo is etched by SF
6
, gas, formation of the through hole in the protection insulative film by dry etching is difficult. Since a Mo layer is rapidly dissolved abnormally upon wet etching in Mo/Cr, patterning is difficult. Since it is necessary for Al/Ti that the through hole has to be formed by dry etching in the protection insulative film and Al has to be etched, the step is complicated. Further, for Mo/Al/Mo, since Mo as a cap layer is etched by SF
6
gas, and formation of the through hole in the protection insulative film by dry etching is difficult and also requires three-layered deposition, the sputtering step is subjected to a large load. Similarly, since Ti/Al/Ti, Cr/Al/Cr and MoCr/Al/MoCr each require the deposition of three layers, they impose a great load on the sputtering step.
SUMMARY OF THE INVENTION
A first object of this invention is to attain a constitution of image signal lines capable of simultaneously satisfying the requirements of reduced wiring resistance, a reduced load on a sputtering step, selective wet etching relative to a gate insulative film, formation of a through hole in a protection insulative film by dry etching, and favorable contact with silicon of a thin film transistor and a transparent conductive film constituting a pixel electrode, and to provide a liquid crystal display device using the same. An target value for the wiring resistance is assumed to be 170 n&OHgr;/m or lower of an average conductivity defined as a product of sheet resistance and overall film thickness. This average conductivity is at a level lower than 180 n&OHgr;/m that can be attained with a single Cr layer film or single CrMo layer film.
A target for reduction in the load imposed on the sputtering step involves setting the number of laminated layers to two or less. The selective wet etching with respect to the gate insulative film is such that etching can be conducted with a chemical solution excluding those capable of chemically attacking the gate insulative film, such as a buffered hydrofluoric acid or an alkali solution giving damage to a resist. Further, from the viewpoint of the simplicity of the steps, it is a necessary condition that upper and lower layers can be etched simultaneously on a wiring constituted of laminated layers. Further, it is a necessary condition that the cross section of the wirings can be fabricated into a tapered shape. For the formation of the through hole in the protection insulative film by dry etching, it is necessary that the contact layer of wirings has a durability to dry etching using an SF
6
gas for establishing contact between the wirings and the pixel electrode. As a matter of fact, it is difficult to use an alloy comprising Al as a main ingredient for the contact layer.
A second object of this invention is, in addition to attaining the foregoing first object, to provide a good matching property with a process for forming scanning signal lines. Since a particularly lowered resistance is required for the scanning signal lines, it is desirable that a film constituting them is a laminated film containing an alloy comprising aluminum as the main ingredient. When the etching solution for fabricating the same and that for fabricating the image signal lines can be used in common, this is preferred in view of the consequent reduction in the manufacturing cost.
According to a first aspect of the present invention, a liquid crystal display device comprises: a pair of substrates; a liquid crystal layer disposed between the pair of substrates; a plurality of scanning signal lines formed on one of the pair of substrates; a plurality of image signal lines crossing the scanning signal lines in a matrix arrangement; thin film transistors
Kaneko Toshiki
Onisawa Ken-ichi
Takahashi Takuya
Tamura Katsumi
Antonelli Terry Stout & Kraus LLP
Chung David
Hitachi , Ltd.
Parker Kenneth
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