1.  Field of Invention
The present invention relates to a semiconductor device structure and manufacturing method thereof.  More particularly, the present invention relates to the contact structure of a semiconductor device and manufacturing method thereof. 
2.  Description of Related Art
In the fabrication of a liquid crystal display, the conductive layers above and below an insulating layer are electrically connected through a contact in the insulating layer fabricated by performing a photolithographic and an etching process.  For example, the pixel electrode and the drain terminal of a thin film transistor in a pixel structure are electrically connected together by performing a photolithographic and etching process to form a contact opening in the dielectric layer that exposes the underlying drain terminal.  Thereafter, the pixel electrode is deposited over the dielectric layer and electrically connects to the drain terminal through the contact opening. 
In the process of fabricating the thin film transistor, the first metallic layer (comprising circuit elements such as the gates and the scanning lines) and the second metallic layer (comprising the circuit elements such as the source/drain terminals and the data lines) are fabricated using aluminum or aluminum alloy.  Aluminum is often selected because it has the preferred conductive properties.  On the other hand, the pixel electrode is fabricated using indium-tin oxide.  Aside from having an electrical connection with the drain terminal through a contact, the pixel electrode also connects electrically with the upper electrode of a pixel storage capacitor.  However, if the first metallic layer and the second metallic layer are made from aluminum, the indium-tin film over the aluminum layer may initiate a galvanic reaction under the influence of a chemical developer.  As a result, the indium-tin oxide film may gradually peel off from the first metallic layer or the second metallic layer. 
To prevent the indium-tin oxide film from peeling off the first or the second metallic layer, a molybdenum or titanium layer is formed over the exposed first or second metallic layer within the contact opening to serve as a buffer layer.  In other words, current flows from the indium-tin oxide electrode to the first or the second metallic layer via the molybdenum or titanium layer.  Although the method is effective in stopping the indium-tin oxide film from peeling, molybdenum or titanium is expensive metal to work with.  Moreover, sputtering molybdenum or titanium material over a contact opening to form the required buffer layer is a rather inefficient process.  Hence, the conventional method often leads to a waste of material and an increase in production cost.