Abstract:
A method is provided to increase the contact area of a contact window. In this method, the contact area is mainly increased by a concavity which is formed by first forming a thin oxide layer in the contact region using local oxidation, then further by removing the thin oxide layer. Additionally, in order to reduce the contact resistance, a metal oxide layer can be selectively formed at the contact interface.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates in general to a method of fabricating a semiconductor device, and more particularly, to a method of increasing the contact area of contact window using local oxidization. 
     2. Description of the Related Art 
     At present, in many highly integrated semiconductor devices, more than two levels of interconnecting metal layers are necessary to form a three dimensional network, called mutilevel metal interconnects (MLM), because of the increasing integration density of semiconductor devices. In the first level, the metal line is commonly polysilicon or metal, which can be electrically connected by contact plugs to a device. 
     In the conventional method, first the transistor is formed in the active region insulated by a field oxide (FOX). Then, the substrate is covered by an insulator, and the contact plug is formed in the insulator and connects to the source/drain of the transistor. 
     In the contact interface between contact plug and substrate, junction resistance induced by the contact of different materials results in slowing the transmission of the electric current. The larger the junction resistance and the longer the resistance-capacitance time delay, the slower the electric current transmits. 
     In the integrated circuit technology at the deep sub-micron level, the planar area which can be used by the contact plug is continuously decreasing and the contact area of the contact window is decreasing, as well. Thus, in this situation of increasing junction resistance, a method which can promote the operation quality of the device, reduce the contact resistance and resistance-capacitance time delay, and effectively increase the contact area without increasing the planar area is demanded. 
     Therefore, the purpose of the invention is to provide a method which can meet the requirements described above, without adding extra steps to the process. 
     SUMMARY OF THE INVENTION 
     The invention provides a method of increasing the contact area of a contact window. 
     In the invention, a substrate is patterned with a mask to define the field oxide region, the active region and the local oxide region. The active region is surrounded by the field oxide region. In the subsequent oxidation process, the field oxide region and the local oxide region are oxidized. Thus, the field oxide region is transformed into a field oxide layer for insulating devices and the local oxide region is transformed into a local oxide layer. Then, the mask layer and the local oxide layer are removed, so that a concavity is formed and the contact area is increased. The concavity results from removing the oxidized part of the substrate in the contact window. Thus, a concavity appears when the oxidized part is removed from the local oxide layer. In the subsequent process, the fabrication of a device such as a transistor is undertaken. A source/drain region in the substrate is formed, and then a self-aligned process forms a silicide layer on the surface of the source/drain region to reduce the contact resistance. Finally, an insulator layer and a contact plug are formed on the substrate. The contact plug is located within the insulator layer and couples with the substrate. 
     The invention mainly forms a concavity in the contact window by removing the local oxide region of the substrate formed in the local oxidation process. The concavity increases the contact area and reduces the contact resistance, but does not increase the planar area of the contact window. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1A to 1E are schematic, cross-sectional views showing a process for increasing the contact area of a contact window in a preferred embodiment according to the invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     In FIG. 1A, a substrate 100 is provided, and a mask layer 103 is formed on the substrate 100 to define a field oxide region 101 and an active region 102. The active region 102 is surrounded by the field oxide region 101. The mask layer has openings 108 and 109. The contact window 150 exposing the substrate 100 is a position reserved for a contact plug and is directly below the opening 108. Additionally, the opening 109 is in the field oxide region 101. 
     A method for forming the mask layer 103 is, for example, first forming a silicon oxide layer 104 by silicon oxidation on the substrate 100 and then depositing a silicon nitride layer 106 by chemical vapor deposition (CVD) in order. Then, openings 108 and 109 are formed using photolithography and etching to pattern the mask layer 103 and to define the field oxide region 101, the active region 102, and the contact window 150 in the active region 102. 
     In FIG. 1B, the exposed field oxide region 101 and contact window 150 on the substrate 100 are oxidized into silicon oxide layers, including the field oxide layer 112 for insulating devices and the local oxide layer 110, respectively. 
     Since the open area of the opening 108 is much smaller than the opening 109, the smaller area of the opening 108 affects the local oxidation. Thus, the local oxide layer 110 is much thinner than the field oxide layer 112. 
     In FIGS. 1B and 1C, the mask layer 103 and the local oxide layer 110 are removed, for example, by wet etching to form a concavity 114 in the contact window 150. 
     The concavity 114 results from the oxidation in the silicon-substrate 100. Since silicon is a reactant, the oxidation occurs in the local oxide layer 110 and consumes some silicon in the substrate 100, and the concavity 114 is formed. In addition, the size of the concavity 114 is controllable by the size of the opening 108; thus the increase of the contact area is controllable, as well. 
     When the local oxide layer 110 is removed, some of the field oxide layer 112 on the surface is also removed. However, the field oxide layer 112 is much thicker than the local oxide layer 110. Thus, the thinning does not affect the insulation ability of the field oxide layer 112, even though some material is removed from the surface of the field oxide layer 112. Alternatively, the subsequent processes still include removing oxide layers such as portions of a gate oxide layer, a sacrificial oxide layer, etc. Therefore, the local oxide layer 110 can be removed with these subsequently formed oxide layers as long as the removal does not affect the subsequent processes. 
     In FIG. 1D, an active device such as a metal oxide semiconductor (MOS) and a conductive region such as a source/drain region 130 are formed. Typically, the MOS device consists of the gate oxide layer 122, the gate including the gate conductive layer 124 and the spacer 126, the lightly doped drain (LDD) 128, and the source/drain region 130 in the contact window 150. The fabrication of MOS is a well-known technology for people in this field; it is therefore not described here. 
     As shown in FIG. 1D, whether the source/drain region 130 needs to form a silicide layer 132 on the surface is dependent upon the product requirements such as cost, etc. This process is completely optional. The method to form a silicide layer 132 is, for example, a self-aligned process. It can be a titanium silicide or a cobalt silicide layer. 
     In FIG. 1E, an insulator layer 140 is, for example, a silicon oxide layer formed by chemical vapor deposition. A window opening 141 is formed within the insulator layer 140 to expose the substrate 100 (the silicide layer 132 or the source/drain region 130) in the contact window 150 by, for example, conventional photolithography and etching. Then, the window opening 141 is filled with a conductive layer such as a doped polysilicon layer or a metal layer to form a conductive contact plug 142. 
     Since the concavity 114 is formed in the contact window 150, the contact area of the contact window can increase effectively and reduce the contact resistance, but the planar area of the contact window 150 does not increase. 
     In conclusion, when the insulating field oxide layer is formed, the contact window is formed into a local oxide layer. After removing the local oxide layer, a concavity in the contact window is created, so that the contact area of the contact window increases and the contact resistance is reduced. 
     Other embodiments of the invention will appear to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims.