MOS structure and method for fabricating the structure

The invention is about a method for forming a MOS device. A substrate is provided first. A field oxide layer is formed on the substrate to define an active region. A gate structure is formed on the active region, where the gate structure has a gate oxide layer, a first gate layer, and a cap layer on the gate layer. The field oxide layer has a height substantially equal to the cap layer. The cap layer is thicker than the first gate layer, such as about three times of the first gate layer. A lightly doped region is formed in the substrate. A spacer is formed on a sidewall of the gate structure. A source/drain region is formed at each side of the gate. An epitaxial silicon layer is selectively formed on the source/drain region with a height substantially equal to the height of the first gate layer. The cap layer is removed to form a trench that exposes the first gate layer. A conductive layer is deposited on the first gate layer and the epitaxial silicon layer within the source/drain region. The conductive layer has a thickness of about equal to the cap layer, so that a portion of the conductive layer with the trench forms a second gate layer.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention is directed to a formation of a metal-oxide semiconductor (MOS) device. The method allows a cap layer on the gate layer to be removed and replaced with a conductive layer having higher conductivity. A two-layer conductive layer also fills the cavity space between the spacer and the field oxide layer, so that the two-layer structure is also formed on the source/drain region of the MOS transistor with sufficient thickness without consuming the junction depth. Each layer of the two-layer conductive layer has thickness about respectively equal to the two-layer gate structure. An embodiment is provided in the follow for descriptions. FIGS. 1 A- 1 C are cross-sectional views, schematically illustrating the process to form a MOS semiconductor device, according to one preferred embodiment of this invention. In FIG. 1A, a gate structure is formed at an active region on a substrate. This structure can be formed by a few of steps. First, a substrate 100 is provided. A field oxide (FOX) layer 102 is formed on the substrate 100 to define the active region. The thickness of the FOX layer is about 2000 angstroms. A typical gate structure is formed on the substrate at the active region. The gate structure includes a gate oxide layer 104 on the substrate 100 , a gate layer 106 on the gate oxide layer 104 , and a cap layer 108 on the gate layer 106 . The gate oxide layer usually is about 50-300 angstroms, the gate layer 106 usually is about 500 angstroms, and the cap layer 108 usually is about 1500 angstroms. In general, the cap layer 108 is thicker than the gate layer 106 . The gate layer 106 usually includes, for example, polysilicon and the cap layer 108 usually includes, for example, silicon nitride layer. Usually, a spacer 112 is formed on a sidewall of the gate structure. A source/drain region 114 with a doped extension region under the spacer 112 is also formed in the substrate 100 at each side of the gate structure. The spacer 112 can be formed by depositing a dielectric layer with a thickness of about 300-2000 angstroms, and etching back the dielectric layer to expose the cap layer 108 . The material of the spacer 112 is chosen to be different the material of the cap layer 108 . Preferably, the spacer 112 includes silicon oxide. It should be noted that the cap layer 108 has a height substantially equal to the FOX layer 102 . This can be done by, for example, depositing the gate oxide layer 104 , the gate layer 106 , and the cap layer 108 in a blanket deposition manner. Before pattering them to form the gate structure, a CMP process is performed to have the FOX layer 102 and the cap layer 108 with about the same height. The cap layer 108 includes material different from the FOX layer 102 . Preferably, the cap layer 108 includes silicon nitride. The spacer 112 includes also silicon oxide different from the material of the cap layer 108 . A cavity space between the spacer 112 and the FOX layer 102 is naturally formed after the MOS transistor is formed. The structure as shown in FIG. 1A can be achieved by various manners. The foregoing manner is only an example. In general, the gate structure can be any conductive structure layer. The source/drain region 114 are not absolutely necessary to be included. In FIG. 1 B, an epitaxial silicon layer 116 is selectively formed on the source/drain region 114 between the FOX 102 and the spacer 112 . The thickness of the epitaxial silicon layer 116 preferably is about equal to the gate layer 106 . However, a height of the epitaxial silicon layer 116 is about equal to the height of the gate layer 106 . In FIG. 1 C, the cap layer 108 of FIG. 1B is removed by for example, wet etching. Since the material of the cap layer is properly chosen, a proper etching selective ratio on the cap layer 108 can be set. After the cap layer 108 is removed, a trench originally occupied by the cap layer 108 is formed. Then, a conductive layer is selectively formed to fill the trench, resulting in the conductive layer 128 by, for example, chemical mechanical deposition (CVD). The selective deposition can be, for example, achieved due to the epitaxial silicon and the poly-silicon that provide the seed surface. Simultaneously, the conductive layer also fills the cavity between the FOX layer 102 and the spacer 112 , resulting in the conductive layer 120 . In other words, the conductive layer 128 and the conductive layer 120 are simultaneously formed, both of which has a height about equal to the original height of the cap layer 108 of FIG. 1B . It also means that the thickness of the conductive layers 120 , 128 is about equal to the thickness of the cap layer 108 . The total height of the gate structure is about 2000 angstroms. Alternatively, the conductive layer 120 , 128 can be formed through a conventional manner by depositing the preliminary conductive layer, and polishing the conductive layer. As shown in FIG. 1 C, the gate structure now includes the gate oxide layer 104 , the polysilicon gate layer 106 and the conductive layer 128 . If the gate structure is modified into a conductive structure layer, the gate oxide layer 104 may be not necessary. Moreover, the conductive layers 120 , 128 can include tungsten, aluminum, metal or any material with sufficient conductivity. The conductive layers can be formed by, for example, selective deposition. Actually, the conductive layer 120 and 128 can be converted into conductive silicide layer by performing a further thermal process. The conductive silicide layer can include, for example, tungsten silicide, titanium silicide, or cobalt silicide. It still has several subsequent processes to accomplish the intended device, but those process are known by the skilled artisans. No further descriptions are provided here. In summary, the invention has several features as follows: 1. The invention uses the cap layer 108 to reserve a trench space, which can filled with a conductive layer having higher conductivity. The conductive layer 128 on the gate layer 104 can have sufficient thickness to effectively reduce the resistance without consume the polysilicon material of the gate. As a result, the gate structure includes two conductive layers. 2. Through the CMP process using the FOX layer as a polishing stop, the cap layer can have a height substantially equal to the FOX layer, so that the FOX can be used as a polishing stop when the silicide layer is polished. 3. The two-layer conductive layer is formed on the source/drain region with the same height as the gate structure. 4. Thickness of each layer of the two-layer conductive layer is respectively equal to thickness of each of the gate structure. 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 covers modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.