Abstract:
Disclosed is a semiconductor device having low voltage characteristic and advantageous integrity simultaneously. The semiconductor device comprises a silicon-on-insulator (SOI) substrate of a stack structure comprising a base layer as a means for supporting, a buried oxide layer, and a semiconductor layer providing an active region; and a first transistor and a second transistor formed on the active region of the SOI substrate, wherein the first and second transistors are formed as a stack structure on one active region and they share one gate electrode, a drain region of the second transistor is electrically connected to the gate electrode and a source region of the second transistor is electrically connected to the active region.

Description:
This application is a division of U.S. Ser. No. 09/348,572 filed Jul. 7, 1999, now U.S. Pat. No. 6,168,979, which U.S. application is hereby incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a semiconductor device, and more particularly to a semiconductor device having low voltage characteristic and advantageous integrity, and method for fabricating the same. 
     2. Description of the Related Art 
     Due to the fast development in semiconductor device industry such as high integration, fast speed and low voltage, a semiconductor device using the SOI substrate instead of a silicon substrate made of bulk silicon, has been proposed. The SOI substrate has a stack structure comprising a base layer as a means for supporting, a buried oxide layer disposed on the base layer, and a semiconductor layer disposed on the buried oxide layer and providing an active region. 
     Compared to general semiconductor device integrated on a silicon substrate, the semiconductor device integrated on the SOI substrate (hereinafter “SOI device”) has advantages such as a high speed due to lower junction capacitance, and a reduction in driving voltage due to lower threshold voltage and latch-up prevention due to the complete isolation. 
     In the meantime, as the demand of portable electric products increases, it is required a reduction not only in the device size but also in the voltage supplied thereto. Currently, a method for lowering the threshold voltage of the transistor is frequently used so as to obtain the low-voltage device. There have been suggested those method for lowering the threshold voltage such as a structure in which two transistors are provided at one device. Herein, those two transistors consist of a main transistor and an auxiliary transistor being connected to the main transistor. 
     When the low-voltage device is embodied on the silicon substrate with above described structure, there is however occurred limitations such as an increase in the leakage current simultaneously although the threshold voltage is reduced. On the other hand, when the low-voltage device is embodied on the SOI substrate, since the SOI device has basically characteristics of low threshold voltage and low leakage current, the SOI device is probably suitable for manufacturing process of the portable electronics products requiring small size and low voltage rather than the semiconductor device embodied on the silicon substrate. 
     However, the low-voltage device having two transistors as above also incurs limitation in view of integrity since active regions for the respective transistors should be provided. 
     SUMMARY OF THE INVENTION 
     Accordingly, it is one object of the present invention to provide a semiconductor device having low voltage characteristic and advantageous integrity. 
     Further, it is another embodiment of the present invention to provides a method for fabricating said semiconductor device. 
     To accomplish foregoing objects, the present invention provides a semiconductor device comprising: a silicon-on-insulator (SOI) substrate of a stack structure comprising a base layer as a means for supporting, a buried oxide layer, and a semiconductor layer providing an active region; and a first transistor and a second transistor formed on the active region of the SOI substrate, wherein the first and second transistors are formed as a stack structure on one active region and they share one gate electrode, a drain region of the second transistor is electrically connected to the gate electrode and a source region of the second transistor is electrically connected to the active region. 
     The present invention further provides a semiconductor device comprising: an SOI substrate of a stack structure comprising a base layer as a means for supporting, a buried oxide layer, and a semiconductor layer providing an active region; a first transistor formed on the active region of the semiconductor layer, and comprising a gate electrode having a first gate oxide layer, source and drain regions formed in the semiconductor layer at both sides of the gate electrode respectively; a second transistor sharing the gate electrode with the first transistor, and comprising a second gate oxide layer formed on the gate electrode, source and drain regions formed on the second gate oxide layer; an intermediate insulating layer formed on the SOI substrate to cover the first and second transistors; source and drain electrodes being contacted with the source and drain regions of the first transistor respectively; a first metal wiring for electrically connecting the gate electrode and the drain region of the second transistor; and a second metal wiring for electrically connecting the active region of the first transistor and the source region of the second transistor. 
     The present invention provides a method for fabricating a semiconductor device comprising the steps of: providing an SOI substrate of a stack structure comprising a base layer as a means for supporting, a buried oxide layer, and a semiconductor layer providing an active region; forming a first oxide layer, a first conduction layer, a second oxide layer and a second conduction layer on the semiconductor layer successively; forming a conduction layer pattern, a second gate oxide layer, a gate electrode and a first gate oxide layer on the active region of the semiconductor layer by patterning the first oxide layer, the first conduction layer, the second oxide layer and the second conduction layer; forming first source and drain regions in the semiconductor layer at both sides of the gate electrode, and second source and drain regions at both ends of the conduction layer pattern, wherein a first transistor comprises the gate electrode, the first source and drain regions, and a second transistor comprises the gate electrode, the second source and drain regions; forming an intermediate insulating layer on a resultant; etching-back the intermediate insulating layer until the second source and drain regions of the second transistor are exposed; forming a first contact hole and a second contact hole exposing the first source and drain regions of the first transistor respectively, and a third contact hole and a fourth contact hole exposing the gate electrode and the active region respectively by selectively etching selected portions of the intermediate insulating layer; forming a metal layer on the intermediate insulating layer with a sufficient thickness to fill the first, second, third and fourth contact holes entirely; and forming source and drain electrodes being contacted with the first source and drain regions of the first transistor through the first and second contact holes respectively, a first metal wiring for electrically connecting the gate electrode and the drain region of the second transistor through the third contact hole, and a second metal wiring for electrically connecting the active region and the second source region of the second transistor through the fourth contact hole on the intermediate insulating layer by patterning the metal layer. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The object, spirit and advantages of the present invention will be readily understood by the accompanying drawings and detailed descriptions. 
     FIG. 1 is a layout of a semiconductor device according to the embodiment of the present invention. 
     FIG. 2 is a perspective view showing the semiconductor device according to the embodiment of the present invention. 
     FIG. 3 is a circuit diagram of the semiconductor device according to the embodiment of the present invention. 
     FIGS. 4 to  11  are cross-sectional views showing the fabricating process according to the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, preferred embodiment of the present invention will be described in detail with reference to accompanying drawings. 
     FIG. 1 is a layout of a semiconductor device according to the embodiment of the present invention. The reference numeral  11  stands for an active region,  14   a  for gate electrodes being shared by first and second transistors,  16  and  18  for a source region and a drain region of the first transistor respectively,  26  for a source region of the second transistor,  32   a  and  34   a  for a contact of the source and drain regions of the first transistor respectively,  40   a  for a contact of the gate electrode,  42   a  for a contact of the source region of the second transistor and  42   b  for a contact of the active region. 
     As shown in the drawing, the semiconductor device of this invention comprises first and second transistors stacked on one active region  11 . The first and second transistors share one gate electrode  14   a , and especially the gate electrode  14   a  is arranged perpendicular to the source and drain regions  16 ,  18  of the first transistor. 
     FIG. 2 is a perspective view showing a semiconductor device according to the embodiment of the present invention. As shown in the drawing, the semiconductor device of the present invention has an SOI substrate  10  of stack structure comprising a base layer  1  as a means for supporting, a buried oxide layer  2  and a semiconductor layer  3  providing an active region, and a first transistor  100  and a second transistor  200  formed on the active region  11  of the semiconductor layer  3 . Herein, the active region  11  of the semiconductor layer  3  is defined by a field oxide film (not shown). 
     The first transistor  100  and the second transistor  200  are formed as a stack structure on one active region  11 . The first transistor  100  and the second transistor  200  share one gate electrode  14   a . The first transistor  100  is constituted by including a gate electrode  14   a , a first gate oxide layer  12   a  formed in a lower portion of the gate electrode  14   a , a first source region  16  and drain region  18  formed within regions of the semiconductor layer  3  in both sides of the gate electrode  14   a . The second transistor  200  is constituted by including the gate electrode  14   a , a second gate oxide layer  22   a  formed on the gate electrode  14   a  and the second source region  26  and drain region  28  formed on the second gate oxide layer  22   a.    
     The source electrode  17  and the drain electrode  19  are contacted with the first source region  16  and the drain region  18  of the first transistor  100  respectively. A first metal wiring  40  is formed such that the gate electrode  14   a  being shared by the first and second transistors  100 ,  200  is electrically connected to the drain region  28  of the second transistor  200 . A second metal wiring  42  is formed such that the active region  11  is electrically connected to the source region  26  of the second transistor  200 . 
     FIG. 3 is a circuit diagram of the semiconductor device according to the present invention. As shown in the drawing, the semiconductor device of the present invention comprises a first transistor  100  as a main transistor, a second transistor  200  as an auxiliary transistor. Gate electrodes G 1 , G 2  of the first transistor  100  and the second transistor  200  are connected each other. Also, a drain region D 2  of the second transistor  200  is connected with the gate electrode G 2 . Accordingly, an equal voltage is applied to the gate electrode G 1  of the first transistor  100 , the gate electrode G 2  and the drain electrode D 2  of the second transistor  200 . Furthermore, a source electrode S 2  of the second transistor  200  is contacted with a body of the first transistor  100 . 
     In the semiconductor device of the present invention as described above, the body charge in the channel region of the main transistor, i.e. the first transistor  100  is reduced by a positive bias of the auxiliary transistor, i.e. the second transistor  200 . Herein, the reduction in the body charge signifies a reduction of voltage that is applied to the gate electrode to generate a channel region on a surface of the active region. Accordingly, the threshold voltage of the first transistor  100  as a main transistor is reduced. 
     Further, in the semiconductor device of the present invention, the positive bias applied to the body of the first transistor  100  by the second transistor  200 , reduces an electric field that should be applied to the gate electrode G 1  of the first transistor, compared to the general transistor structure. Therefore, a mobility in the first transistor  100  increases. 
     Moreover, the semiconductor device of this invention has less leakage current compared to the general transistor structure since the positive bias is not applied to the body of the first transistor  100  before the second transistor  200  is turned on. 
     Accordingly, the semiconductor device according to the present invention has low threshold voltage and low leakage current characteristics. Especially, the semiconductor device of the present invention has superior integrity property to the property of conventional devices since two transistors are formed in one active region. 
     FIGS. 4 to  11  are cross-sectional views showing the fabricating process according to the embodiment of the present invention. Herein, FIGS. 4 to  8  and FIG. 10 are cross-sectional views taken along the line A-A′ of FIG. 1, and FIGS. 9 and 11 are cross-sectional views taken along the line B-B′ of FIG. 1 according to fabricating steps shown in FIGS. 8 and 10. 
     Referring to FIG. 4, there is provided an SOI substrate  10  of a stack structure comprising a base layer  1  as a means for supporting, a buried oxide layer  2  and a semiconductor layer  3  providing an active region. A field oxide film (not shown) defining its active region, is formed in the semiconductor layer  3 . A first oxide layer  12 , a first conduction layer  14 , a second oxide layer  22  and a second conduction layer  24  are formed on the semiconductor layer  3  of the SOI substrate  10  successively. The first conduction layer  14  is a layer that becomes a gate electrode being shared by the first and second transistors, and the first conduction layer  14  is made of doped polysilicon layer. The second conduction layer is a layer in which a source region, a drain region and a channel region of the second transistor  200  are formed later, and is made of undoped polysilicon layer. 
     Referring to FIG. 5, the second conduction layer, the second oxide layer, the first conduction layer and the first oxide layer is patterned thereby obtaining a conduction layer pattern  24   a  in which a source region, a drain region and a channel region are formed later, a gate oxide layer  22   a  of the second transistor, a gate electrode  14   a  being shared by the first and second transistors and a gate oxide layer  12   a  of the first transistor. 
     Next, an impurity of selected conductivity type, for example, N-type impurity is ion-implanted with high concentration into the semiconductor layer region of both sides of the gate electrode  14   a  and into both ends of the conduction pattern  24   a  respectively. Next, the N-type impurity-implanted resultant is annealed thereby forming a first source region  16  and a drain region  18  of the first transistor, and a second source region and drain region (not shown) of the second transistor. Herein, the conduction pattern  24   a  signifies a channel region disposed between the second source region and the drain region of the second transistor. Accordingly, hereinafter the reference numeral  24   a  stands for the channel region of the second transistor. 
     Referring to FIG. 6, an intermediate insulating layer  30  is coated on the resultant. The intermediate insulating layer  30  has a two-story structure comprising of a TEOS oxide layer  30   a  and a BPSG layer  30   b . The BPSG layer  30   b  is coated, and then is flowed at temperature range of 600˜800° C. so that a surface planarization may be obtained. 
     Referring to FIG. 7, the intermediate insulating layer  30  is etched-back until the channel region  24   a  including the second source region and the drain region (not shown) of the second transistor is exposed. 
     Referring to FIGS. 8 and 9, selected portion of the intermediate insulating layer  30  is selectively etched according to a forming process of a photosensitive layer pattern on the intermediate insulating layer  30  and an etching process using the photosensitive layer pattern as an etching barrier. As a result, as shown in FIG. 8, a first contact hole  32  and a second contact hole  33  for respectively exposing the source and drain regions  16 ,  18  of the first transistor, are formed in the intermediate insulating layer  30 , and as shown in FIG. 9, there are also formed a third contact hole  34  for exposing the gate electrode  14   a  and a fourth contact hole  35  for exposing the active region  11  of the semiconductor layer  3  in the intermediate insulating layer  30 . 
     In FIG. 9, the reference numeral  4  stands for a field oxide film defining a active region,  26  and  28  for the second source and drain regions of the second transistor that is formed in the step of FIG.  5 . 
     Referring to FIGS. 10 to  11 , a metal layer is coated with a sufficient thickness such that the first to fourth contact holes  32 ,  33 ,  34 ,  35  are filled entirely. Afterward, the metal layer is patterned in a predetermined configuration. Therefore, as shown in FIG. 10, a source region  17  and a drain electrode  19  being contacted with the source and drain regions of the first transistor respectively, are formed on the intermediate insulating layer  30 . Also, as shown in FIG. 11, on the intermediate insulating layer  30 , a first metal wiring  40  is formed such that the gate electrode  14   a  being shared by the first and second transistors is electrically connected to the drain region  28  of the second transistor and a second metal wiring  42  is formed such that the active region  11  of the semiconductor layer  3  is electrically connected to the source region  26  of the second transistor  200 . 
     As discussed above, the semiconductor device of the present invention is advantageous in view of the integrity since two transistors are formed in one active region. Further, the semiconductor device is probably suitable for manufacturing process of the portable electric products requiring small size and low voltage since the semiconductor device has the low threshold voltage and low leakage current characteristics. 
     Various other modifications will be apparent to and can be readily made by those skilled in the art without departing from the scope and spirit of the present invention.