Abstract:
A unit cell of a static random-access memory includes a laminated gate electrode structure adjacent to a diffusion layer. A top surface of the gate electrode structure is coated with a first silicide layer and the diffusion layer includes a second silicide layer. The second silicide layer is separated from the gate electrode structure by a distance that is the same as a width of a sidewall spacer on an opposite side of the gate electrode structure. The portion of the diffusion layer that is exposed between the second silicide layer and the gate electrode structure has a higher impurity concentration than the remainder of the diffusion layer to reduce or eliminate undesired leakage voltage.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a division of application Ser. No. 09/188,378, now U.S. Pat. No. 6,031,271, filed Nov. 10, 1998. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to semiconductors, and more particularly to a semiconductor device and a method of fabricating the same so that the yield of such semiconductor devices is improved. 
     2. Description of the Related Art 
     In known semiconductor devices where a laminated structure of an insulating layer and a conductive layer is formed on a first semiconductor body, spacers are provided on sidewalls of the laminated structure and a diffusion layer is then formed in the first semiconductor body so that one edge of the diffusion layer is adjacent to one of the spacers. A second semiconductor body is formed and a shared contact hole is then created in the second semiconductor body for establishing an ohmic contact between the diffusion layer and the conductive layer. It is the usual practice to position the shared contact hole so that its vertical center axis is aligned with the edge of the conductive layer. 
     However, the yield of the prior art semiconductor devices is low due to device failures revealed at the inspection stage of manufacture. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to increase the yield of semiconductor devices. 
     The present invention is based on a discovery that the hole contact resistance that varies as a function of distance from the center axis of the hole is asymmetric with respect to that axis. 
     According to a first aspect of the present invention, there is provided a semiconductor device comprising a first semiconductor body, a diffusion layer in the first semiconductor body, a laminated structure of an insulating layer on the first semiconductor body adjacent to the diffusion layer and a conductive layer on the insulating layer, an insulating spacer on a sidewall of the laminated structure, the spacer having a width W, and a second semiconductor body having a shared contact hole for establishing an ohmic contact between the diffusion layer and the conductive layer, the shared contact hole having a center axis located a distance W/ 2  from an edge of the conductive layer so that portions of the diffusion layer and the conductive layer which are exposed to the outside through the shared contact hole have substantially equal areas. 
     According to a second aspect, the present invention provides a semiconductor device comprising a first semiconductor body, a diffusion layer in the first semiconductor body, a laminated structure of an insulating layer on the first semiconductor body adjacent to the diffusion layer, a polysilicon layer on the insulating layer and a first silicide layer on the polysilicon layer, an insulating spacer on a sidewall of the laminated structure, a second silicide layer in the diffusion layer, the second silicide layer having an edge spaced from an edge of the laminated structure by an amount equal to a width of the spacer, and a second semiconductor body having a shared contact hole for establishing an ohmic contact between the first and second silicide layers, the shared contact hole having a center axis located at equal distances from the edges of the first and second silicide layers so that portions of the first and second silicide layers which are exposed to the outside through the shared contact hole have substantially equal areas. 
     According to a third aspect, the present invention provides a method for fabricating a semiconductor device, comprising the steps of forming a first semiconductor body, forming a laminated structure of an insulating layer on the first semiconductor body and a conductive layer on the insulating layer, forming insulating spacers on sidewalls of the laminated structure, each of the spacers having a width W, forming a diffusion layer in the first semiconductor body so that an edge of the diffusion layer is adjacent to one of the spacers, forming a second semiconductor body, and forming a shared contact hole in second semiconductor body for establishing an ohmic contact between diffusion layer and conductive layer, a center axis of the shared contact hole being located a distance W/ 2  from an edge of laminated structure so that portions of diffusion layer and conductive layer which are exposed to the outside through shared contact hole have substantially equal areas. 
     According to a fourth aspect, the present invention provides a method for fabricating a semiconductor device, comprising the steps of forming a first semiconductor body, forming a laminated structure of an insulating layer on the first semiconductor body, a polysilicon layer on the insulating layer and a first silicide layer on the polysilicon layer, forming insulating spacers on sidewalls of the laminated structure, forming a diffusion layer in the first semiconductor body so that an edge of the diffusion layer is adjacent to one of the spacers, forming a second silicide layer in the diffusion layer so that an edge of the second silicide layer is adjacent to one of the spacers, forming a second semiconductor body, and forming a shared contact hole in the second semiconductor body for establishing an ohmic contact between the first and second silicide layers, a center axis of the shared contact hole being located at equal distances from the edges of the first and second silicide layers so that portions of the diffusion layer and the conductive layer which are exposed to the outside through the shared contact hole have substantially equal areas. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be described in further detail with reference to the accompanying drawings, in which: 
     FIG. 1 is a circuit diagram of a unit cell of a conventional static random-access memory; 
     FIG. 2 is a layout view of the unit cell of FIG. 1; 
     FIGS. 3A and 3B are cross-sectional views of a portion of the unit cell taken along the lines  3 — 3  of FIG. 1 for illustrating successive processes of forming that portion of the cell; 
     FIG. 4 is a graphic representation of the shared contact resistance plotted as a function of off-center displacement of the contact hole of the prior art unit cell from the intended reference point; 
     FIG. 5 is a layout view of the unit cell according to the present invention; 
     FIG. 6 is a cross-sectional view taken along the lines  6 — 6  of FIG. 5; 
     FIG. 7 is a graphic representation of the shared contact resistance plotted as a function of off-center displacement of the contact hole of the unit cell of the present invention from the intended reference point; and 
     FIG. 8 is a cross-sectional view of a modified embodiment of this invention taken along the lines  6 — 6  of FIG.  5 . 
    
    
     DETAILED DESCRIPTION 
     Before proceeding with the detailed description of the present invention, it may prove helpful to provide an explanation of the prior art problem using a static RAM as a typical example with reference to FIGS. 1 to  4 . 
     As shown in FIG. 1, a 6-transistor unit cell of the conventional static RAM is made up of a pair of driver gates  1  and  1 ′ and a pair of transfer gates  3  and  3 ′. Each of the driver gates  1  and  1 ′ consists of a PMOS transistor “p” and an NMOS transistor “n”. The gate electrodes of driver-gate transistors  1   p  and  1   n  are connected together and cross-coupled to a circuit node, or shared contact  5 ′ where the drains of the driver-gate transistors  1 ′ p  and  1 ′ n  are connected, and the gate electrodes of driver-gate transistors  1 ′ p  and  1 ′ n  are connected together and cross-coupled to a circuit node, or shared contact  5  where the drains of the driver-gate transistors  1   p  and  1   n  are connected. The sources of both driver-gate PMOS transistors  1   p  and  1 ′ p  are connected to a voltage supply V DD  and the sources of both driver-gate NMOS transistors  1   n  and  1 ′ n  are connected to ground. Each of the transfer gates  3  and  3 ′ is made up of an NMOS transistor whose gate is connected to a word line  7 . The transfer-gate NMOS transistor  3   n  has its source connected to a bit line  8  at a circuit node  9  and its drain connected to the circuit node  5 , and the transfer-gate NMOS transistor  3 ′ n  has its source connected to a bit line  8 ′ at a circuit node  9 ′ and its drain to the shared contact  5 ′. 
     The layout of the unit memory cell is illustrated in FIG.  2 . In the driver gate  1 , the PMOS transistor  1   p  is formed by a p-type diffusion layer  11 , with the source region of transistor  1   p  being connected through a contact  15  to the voltage source V DD  and the drain region being connected through a contact  17  to a metallic layer  18 . The NMOS transistor  1   n  is formed by the horizontally extending portion of an inverted-L shaped n-type diffusion layer  12 . The source region of transistor  1   n  is connected through a contact  16  to ground and its drain region is connected to the metallic layer  18  through a contact which forms the circuit node  5  of FIG.  1 . The channel regions of transistors  1   p  and  1   n  are both connected to a polysilicon gate  13 . 
     The layout of driver gate  1 ′ is similar to that of driver gate  1 . The PMOS transistor  1 ′ p  is formed by a p-type diffusion layer  11 ′, with the source region of transistor  1 ′ p  being connected through a contact  15 ′ to the voltage source V DD  and the drain region being connected through a contact  17 ′ to a metallic layer  18 ′. The NMOS transistor  1 ′ n  is formed by the horizontally extending portion of an inverted-L shaped n-type diffusion layer  12 ′. The source region of transistor  1 ′ n  is connected through a contact  16 ′ to ground and its drain region is connected through the shared contact  5 ′. The channel regions of transistors  1 ′ p  and  1 ′ n  are both connected to a polysilicon gate  13 ′. 
     Polysilicon gate  13 , to which the drains of transistors  1   p  and  1   n  are connected together, is so shaped that it extends horizontally into the region of the other driver gate  1 ′ and connects to the shared contact  5 ′. Similarly, the polysilicon gate  13 ′, to which the drains of transistors  1 ′ p  and  1 ′ n  are connected together, has a horizontal extension which connects to the shared contact  5 . 
     NMOS transfer-gate transistor  3   n  is formed by the vertically extending portion of the n-type diffusion layer  12 , with the source region of transistor  3   n  being connected to word line  7  through a contact  9  and the drain region being connected to the shared contact  5  as well as to contact  17 . NMOS transfer-gate transistor  3 ′ n  is similarly formed by the vertically extending portion of the n-type diffusion layer  12 ′, with the source region of transistor  3 ′ n  being connected to word line  7  through a contact  9 ′. The drain region of this transistor  3 ′ n  is connected to the contact  17 ′ as well as to the shared contact  5 ′. 
     A vertical cross-section of a portion of the unit cell that encompasses the shared contact  5  is formed in successive stages as illustrated in FIGS. 3A and 3B. As shown in FIG. 3A, the cross-sectioned portion has a p-type well  22  formed on a p-type silicon substrate  21 . The p-type well  22  is ion-injected to form the n-type diffusion layer  12  and the local oxidation technique is used to form a channel stopper  23  in the well  22 . A silicon-dioxide layer  24  is then formed on the p-type well  22  so that it covers an area of the well  22  extending from the edge of the n-type diffusion layer  12  to a point beyond the edge of channel stopper  23 . A portion of the polysilicon gate  13 ′ is formed on the silicon-dioxide layer  24 . 
     In order to reduce the sheet resistance of the memory cell, the surface of the polysilicon gate  13 ′ and the n-type diffusion layer are coated with a silicide layer. Polysilicon gate  13 ′ is first coated with a silicide layer  25  and insulating sidewall spacers  26   a  and  26   b  are deposited on sidewalls of the laminated structure of the layers  24 ,  13 ′ and  25 . A silicide layer  27  is then deposited on the n-type diffusion layer  12  as well as on the p-type diffusion layer  11 . Sidewall spacers  26   a  and  26   b  electrically isolate the silicide layer of all regions of the diffusion layers from polysilicon gate  13 ′. 
     The memory cell is then coated with an interlayer film  28  and a photoresist  29  is provided to expose a portion of the interlayer film  25 . The memory cell is then subjected to radiation through the photoresist  29  and then etched to form a shared contact hole for the shared contact  5  as shown in FIG.  3 B. With this etching process, the sidewall spacer  26   a  is also removed, leaving a portion of the n-type diffusion layer  12 , as well as portions of the silicide layers  25 ,  27 , to be exposed to the outside through the hole of shared contact  5 . Hole of the shared contact  5  is then filled with conductive material so that the drain region of the n-type diffusion layer  12  is brought into ohmic contact with the polysilicon gate  13 ′. 
     It is the usual practice that the photoresist  29  is positioned so that the center axis of the shared contact hole  5  is aligned with the edge of the silicide layer  25 . Since the semiconductor structure around the shared contact  5 ′ is identical to that of the shared contact  5 , the description is only limited to the structure around the shared contact  5 . Since the areas of the silicide layers  25  and  27  that are exposed within the shared contact hole  5  are not equal to each other, the resistance of the shared contact  5  exhibits a significantly asymmetric characteristic. 
     As shown in a resistance curve of FIG. 4 which is plotted as a function of off-center displacement and in which the intended hole center is located at a reference point  0  on the horizontal axis of the graph, a 0.05-μm displacement of the contact hole  5  to the right from the reference point  0  shows a marked increase in resistance, while an almost steady state is maintained until the hole  5  is displaced a distance of 0.14 micrometers to the left from the reference point. 
     Due to the inherent manufacturing tolerances, this asymmetric contact resistance characteristic is a cause of device failures, and hence low yield. 
     In addition, it is found that the surface portion of diffusion layer  12  where the sidewall spacer  26   a  was existent is depleted of the necessary impurities and is tended to be physically worn out due to the absence of the silicide layer  27 . As a result, the memory cell produces an undesired leakage current. 
     FIG. 5 is a diagram showing the layout of a static RAM unit cell according to the present invention. In FIG. 5, parts corresponding in significance to those in FIG. 2 are marked with the same numerals as those used in FIG.  2 . In the present invention, contacts  30  and  30 ′ are provided corresponding to the shared contacts  5  and  5 ′ of the prior art. Shared contacts  30  and  30 ′ are positioned slightly away from the polysilicon gates  13 ′ and  13 , respectively. 
     FIG. 6 shows the structure around the shared contact  30  in detail. It is seen that the shared contact hole  30  is positioned so that its center is located a distance W/ 2  from the edge of the silicide layer  25 , where W is the width of each of the sidewall spacers  26   a  and  26   b . Note that this width is equal to the width of the removed sidewall spacer  26   a,  and hence to the spacing between the edges of silicide layers  25  and  27 . Therefore, it is intended in the present invention that the center of the contact hole  30  be located at equal distances from the edges of the two silicide layers. The areas of the silicide layers  25  and  27  which are exposed to the outside through the shared contact hole  30  are of the same size. 
     A resistance curve of the unit cell of FIG. 5 is plotted as a function of off-center displacement as shown in FIG.  7 . The intended hole center (i.e., W/ 2  from both edges of the silicide layers  25  and  27 ) is indicated as reference point  0  on the horizontal axis of the graph. It is seen that off-center displacement of the shared contact hole  30  shows a substantially symmetric ohmic relationship with respect to the reference point over distances of 0.09 micrometers. Thus, the positioning of shared contact hole  30  can be tolerated in a range of ±0.09 micrometers. 
     In a preferred embodiment, it is useful to inject n-type impurity such as phosphorus is doped into the exposed area of the shared contact hole  30  at an energy level of 30 to 70 keV with a dose of 5×10 14  cm −2  to 5×10 15  cm −2 . This impurity doping results in the creation of an n-type diffusion region  31  which, in effect, increases the impurity concentration of the area n-type diffusion layer  12  which is not covered by the silicide layer  27 . The undesired leakage current is eliminated in this way. As a result, the resistance at the shared contacts  30  and  30 ′ is reduced, making possible high speed read/write operation of the static RAM. 
     The leakage current problem can also be solved by an embodiment shown in FIG.  8 . In this embodiment, use is made of an anti-etching material when the sidewall spacers  26   a  and  26   b  are formed before the shared contact hole  30  is created, so that the spacer  26   a  resists corrosion by the etching material and remains intact as shown at  32 . Such an anti-etching material may be of a substance dissimilar to the interlayer film  28 . If silicon dioxide is used for the interlayer film  28 , silicon nitride can be used for the sidewall spacers  26   a  and  26   b  and the film  28  is etched by using a process in which the oxide-to-nitride selection ratio of 20 percent is employed.