Abstract:
An SOI semiconductor and method for making the same includes a substrate and dielectric support structures that support a silicon body above the substrate. This creates a void underneath the silicon body and thereby reduces the capacitance between the source/drain regions on body and the substrate.

Description:
FIELD OF THE INVENTION 
     The present invention relates to the field of semiconductor manufacturing, and more particularly, to the formation of a silicon-on-insulator structure with improved capacitance characteristics. 
     BACKGROUND OF THE INVENTION 
     Silicon-on-insulator (SOI) metal-oxide semiconductor field effect transistors (MOSFETs) are well known in the field of semiconductors. SOI MOSFETs have been demonstrated to be superior to bulk silicon MOSFETs in low-power, high-speed, very large scale integration (VLSI) applications. Some of the advantages include (1) less junction capacitance so that higher circuit speed can be achieved; (2) better device isolation; and (3) sufficient radiation hardness. 
     A conventional SOI structure comprises a substrate made of silicon, for example. An insulator layer is formed over the substrate, and is typically an oxide, such as silicon oxide. A silicon body, or silicon island, is formed on the insulator layer. This causes the insulator layer to be a “buried oxide” layer or BOX layer. The silicon bodies are isolated from one another by shallow trench isolation (STI) regions or other isolation regions. The source/drain regions are formed in the silicon body and the gate electrode is formed on top of the silicon body, thus forming the MOSFET device. 
     One of the limiting factors in transistor performance in SOI devices is the capacitance that exists from the source/drain regions to the substrate. The typical dielectric constant of the oxide that is conventionally used as the insulator layer of SOI devices is approximately 3.9. Reduction in the capacitance from the source/drain regions to the substrate will improve overall performance of the device by lowering the RC time constant. 
     SUMMARY OF THE INVENTION 
     There is a need for a SOI device that exhibits reduced capacitance between the source/drain regions and the substrate. 
     These and other needs are met by embodiments of the present invention which provide a semiconductor device comprising a substrate, dielectric support structures, and a silicon body held between the dielectric support structures and above the substrate such that a void is formed between the silicon body and the substrate. 
     The void between the silicon body and the substrate, as provided by the present invention, has the advantage of presenting a reduced dielectric constant and thereby reduction in the capacitance between source/drain regions and the substrate. For example, the dielectric constant of air is equal to one, which is significantly lower than the dielectric constant for silicon dioxide (approximately 3.9). The reduced capacitance improves the overall performance of the device of the present invention. 
     The earlier stated needs are also met by embodiments of the present invention which provide a method of forming a semiconductor device comprising the steps of forming a silicon-on-insulator precursor including a substrate, a buried oxide layer on the substrate, a silicon body on the buried oxide layer, and isolation regions surrounding the periphery of the silicon body. The portions of the buried oxide layer that are under the silicon body are etched to create a void between the substrate and the silicon body. 
     The etching of portions of the buried oxide layer in accordance with the embodiments of the present invention serves to create the void that provides the reduced dielectric constant between the source/drain regions and the substrate. The etching allows the creation of the void in a production-worthy method. 
     The earlier stated needs are also met by embodiments of the present invention which provide a method of forming a semiconductor device comprising the steps of forming a SOI structure having a substrate, an insulator on the substrate, a silicon body on the insulator, and isolation regions surrounding the periphery of the silicon body. In this method, a void is formed in the insulator layer between the silicon body and the substrate. 
     The foregoing and other features, aspects and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1A is a cross-section of a silicon-on-insulator (SOI) precursor formed in accordance with embodiments of the present invention. 
     FIG. 1B is a top view of the SOI precursor of FIG.  1 A. 
     FIG. 2A depicts the structure of FIG. 1A following the formation of an etch mask in accordance with embodiments of the present invention. 
     FIG. 2B depicts the top view of the structure of FIG. 2A after the wet etch mask has been formed. 
     FIG. 3A shows the structure of FIG. 2B following a wet etch procedure formed in accordance with embodiments of the present invention. 
     FIG. 3B depicts a top view of the structure of FIG. 3A with a mask in place. 
     FIG. 4A depicts the structure of  3 A following the formation of dielectric support structures in accordance with embodiments of the present invention. 
     FIG. 4B shows the structure of FIG. 4A, with the mask in place. 
     FIG. 5 shows the structure of FIG. 4A following further processing steps to form a MOSFET, in accordance with embodiments of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention addresses problems related to the formation of SOI devices, and the capacitance between the source/drain regions and the substrate in SOI devices. The present invention improves upon the conventional devices by reducing the capacitance between the source/drain regions and the substrate. This is achieved, in part by the formation of a void underneath the silicon body of an SOI structure. The silicon body is supported by dielectric support structures, maintaining the void between the silicon body and the substrate. Since the air in the void has a much lower dielectric constant than the typical oxide employed in the insulator layer of an SOI structure, the capacitance between the source/drain regions and the substrate is significantly reduced. 
     FIG. 1A depicts the cross-section of a silicon-on-insulator (SOI) precursor constructed in accordance with embodiments of the present invention. A precursor  10  may be formed in a conventional manner and includes a silicon substrate  12 , formed of bulk silicon. An insulator layer  14 , or buried oxide layer, is provided on the substrate  12 . A silicon body  16  is provided on the buried oxide (BOX) layer  14 . This silicon body  16  forms an island, or a silicon island, as it is surrounded on its periphery by isolation regions  18 . These isolation regions  18  may be formed of oxide, for example. A shallow trench isolation (STI) technique may be employed to form the isolation regions  18 . 
     Conventional methodologies may be employed to create the precursor  10 , such as SIMOX and others. A top view of the SOI precursor  10  is provided in FIG.  1 B. 
     FIG. 2A shows the SOI precursor  2 A as it is exposed by a mask  20 , depicted in FIG.  2 B. The mask  20  is a wet etch mask. 
     The isolation regions  18  that are not covered by the mask  20  will not be etched during the wet etching procedure. The wet etch mask  20  exposes portions  19  of the isolation regions  18  to the etchant. The silicon body  16  is also exposed to the etchant. A suitable etchant is employed that is highly selective to etch the oxide in the portions  19  of the isolation regions  18  that are exposed by the mask  20 , and not etch the silicon body  16 . A suitable exemplary etch is a buffered oxide etch (BOE) well known to those of ordinary skill in the art for preferentially etching oxide and maintaining the silicon intact. 
     The results of the wet etch step are depicted in FIG.  3 A and in FIG.  3 B. The isolation regions  18  exposed by the mask  20  are etched through to create a void  22  that also extend underneath the silicon body  16 . 
     The silicon body  16  is suspended by the isolation regions  18  that have not been etched. This can best be seen in FIG.  3 B. At this point in the formation process, only two sides of the periphery of the silicon body  16  are contacted by the isolation regions  18 , rather than all four sides. 
     In order to provide enhanced structural stability, dielectric support structures  24  are formed, as depicted in FIG.  4 A. The dielectric support structures  24  are formed by deposition of a dielectric material, such as silicon dioxide, into the isolation regions  18  that were etched through. A conventional deposition technique, such as chemical vapor deposition, may be employed to deposit the dielectric support structures  24 . 
     The dielectric support structures  24 , together with the isolation regions  18  that were not etched, securely support the silicon body  16  above the substrate  12 , with a void  26  formed between the silicon body  16  and the substrate  12 . The void  26  will contain air, which has a dielectric constant of one. This dielectric constant is much lower than that of the buried oxide material that was previously underneath the silicon body  16 . This reduces the capacitance between the source/drain regions that will be formed in the silicon body  16  and the substrate  12 . Improved performance of the SOI device is therefore a result. A top view of the SO structure and mask  20  is provided in FIG.  4 B. 
     Following the formation of the dielectric support structures  24  and the void  26  underneath the silicon body  16 , further processing may be performed in a conventional manner to complete the formation of an SOI device. An exemplary embodiment of an SOI device constructed in accordance with the present invention is provided in FIG.  5 . Source/drain regions  28  are formed in the silicon body  16 . A gate electrode  30  is provided on the silicon body  16 , over a gate dielectric. Dielectric material  32  is formed over the gate electrode  30 . Silicide regions  38  are provided on the source/drain regions  28  and the gate electrode  30 . Sidewalls  34  are formed on the sides of the gate electrode  30 . Contacts  36  may be formed through the dielectric layer  32  to the silicide regions  38 . 
     The completed SOI device depicted in FIG. 5 exhibits reduced capacitance between the source/drain regions  28  and the substrate  12 , due to the lower dielectric constant in the void  26  underneath the silicon body  16 , as compared to conventional SOI structures in which the silicon body is on an oxide layer. 
     Although the present invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only, and is not to be taken by way of limitation, the scope of the present invention be limited only by the terms of the appended claims.