Abstract:
A semiconductor device comprises static random access memory (SRAM) cells formed in a semiconductor substrate, first deep trenches isolating each boundary of an n-well and a p-well of the SRAM cells, second deep trenches isolating the SRAM cells into each unit bit cell, and at least one or more contacts taking substance voltage potentials in regions isolated by the first and second deep trenches. Then, the device becomes possible to improve a soft error resistance without increasing the device in size.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-263798 filed on Sep. 12, 2005, the entire contents of which are incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a semiconductor device and a manufacturing method thereof with a static random access memory (SRAM) mounted thereon.  
         [0004]     2. Description of the Related Art  
         [0005]     In recent years, influence of cosmic rays such as a rays and neutron rays on a semiconductor device has been highlighted as miniaturization of the semiconductor device. This includes the fact that an occurrence of a soft error phenomenon to be a false operation of a device caused by generating pairs of electrons and holes in a semiconductor substrate and becoming a current to flow into an electrode when the cosmic rays are incident to the semiconductor substrate with an element formed thereon.  
         [0006]     For instance, in a semiconductor element shown in  FIG. 1 , when an a ray  120  with high energy is incident to a semiconductor substrate  101 , the semiconductor element generates pairs of electrons and holes  121  in the semiconductor substrate along with an incident locus. The generated electric charges flow out as a current depending on an electric potential distribution in the semiconductor substrate. Here, a diffusion layer n+ isolated by element isolating dielectric films  108  having been biased to a power supply voltage Vdd and a P-well region  102  of a substrate part having been biased to a ground voltage GND, the generated electrons and holes flow into the diffusion layer n+  105  and the P-well region  102 , respectively.  
         [0007]     In a dynamic random access memory (DRAM) one memory cell is comprised of one MOSFET and one capacitor storing data as electric charges therein; however the DRAM poses a problem such that an electric charge quantity of the capacitor is easily varied due to the electric charges generated from the cosmic rays in the manner described above. Therefore, a method has been used, in which the structure of the DRAM is varied from a planer capacitor into a deep trench capacitor forming an electrode on the inner wall of a deep trench formed in a substrate so as to maintain a capacity of the capacitor as well as improve a soft error resistance.  
         [0008]     In contrast, in the SRAM, a bit cell is comprised of a P-channel MOSFET and an N-channel MOSFET combining alternatively. For example, as a cross-sectional view and an top view are shown in  FIG. 2  and  FIG. 3 , respectively, in an N-well  102   a  and a P-well  102   b  formed in the p-semiconductor substrate  101 , a P-channel MOSFET  107   a  and an N-channel MOSFET  107   b  are comprised of gate electrodes  104   a  and  104   b , respectively, and high-density diffusion layers  105  and low-density diffusion layers  106 ′ formed so as to sandwich the directly under part of the gate electrodes  104   a  and  104   b , respectively. These P-channel and N-channel MOSFETs  107   a  and  107   b  are isolated by the element isolating dielectric films  108  and connected to metal wirings  110  such as word lines, bit lines and Vdds, respectively.  
         [0009]     As a circuit diagram of an SRAM cell structured as mentioned above and shown in  FIG. 4 , the SRAM cell is biased even in storing data and the data is stored owing to balance of a voltage applied to a MOSFET circuit, so that the influence caused by the cosmic rays has not posed a large problem. As the semiconductor device becomes minute, however, the inner capacity of the SRAM, etc., becomes small to about 1 fF and a breakdown of stored data due to a soft error becomes no longer allowed to be neglected.  
         [0010]     Up to this day, to improve the soft error resistance, a variety of methods have been disclosed in, for instance, JP 7-183400. However, a plurality of electric charges generated from the cosmic rays incident into a substrate; disturb data-storing currents of a plurality of bit cells. So that even with a data correction conducted in a circuit, the method has posed a problem that it is hard to conduct accurate data-storing.  
       SUMMARY OF THE INVENTION  
       [0011]     According to an embodiment of the present invention, there is provided a semiconductor device, comprising static random access memory (SRAM) cells formed in a semiconductor substrate, first deep trenches isolating each boundary between an n-well and a p-well of the SRAM cells, second deep trenches isolating the SRAM cells into each unit bit cell, and at least one or more contacts taking substance voltage potentials in regions isolated by the first and second deep trenches.  
         [0012]     According to an embodiment of the present invention, there is provided a manufacturing method of a semiconductor device, comprising forming first deep trenches isolating each boundary between an n-well and a p-well of the SRAM cells, forming second deep trenches isolating the SRAM cells into each unit bit cell, forming at least one or more contacts taking substrate voltage potentials, and forming trench capacitors of DRAM cells at the same time of forming the first and/or second deep trenches. 
     
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0013]     The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate embodiments of the invention, and together with the general description given above and the detailed description of the embodiments given below, serve to explain the principles of the invention.  
         [0014]      FIG. 1  shows influence of a rays on a semiconductor element;  
         [0015]      FIG. 2  shows an exemplary cross-sectional view of a conventional SRAM cell;  
         [0016]      FIG. 3  shows an exemplary top view of the conventional SRAM cell;  
         [0017]      FIG. 4  shows an exemplary circuit diagram of an SRAM cell;  
         [0018]      FIG. 5  shows an exemplary cress-sectional view of an SRAM cell relating to an embodiment of the present invention;  
         [0019]      FIG. 6  shows an exemplary top view of a unit bit cell of the SRAM cell relating to an embodiment of the present invention;  
         [0020]      FIG. 7  shows an exemplary top view of the SRAM cell relating to an embodiment of the present invention; and  
         [0021]      FIG. 8  shows an exemplary schematic top view of DRAM elements of a semiconductor device with DRAM mixedly mounted thereon relating to an embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS  
       [0022]     Hereinafter, embodiments of the present invention will be described with reference to the drawings.  
         [0023]      FIG. 5  shows a cross-sectional view of an SRAM cell in a semiconductor device relating to an embodiment of the present invention. An N-well  2   a  and a P-well  2   b  formed in a p-semiconductor substrate  1  are, as shown in  FIG. 5 , isolated by a deep trench  3 . In the deep trench  3  (for example, with depth of 7-10 μm), a SiO 2  film  3   a  is formed on a side wall and a bottom of the deep trench  3 , and a polysilicon film  3   b  is formed inside the SiO 2  film  3   a.    
         [0024]     Gate electrodes  4   a  and  4   b  composed of, for instance, polysilicon layers  42   a  and  42   b  and silicide layers (for instance, NiSi layer)  43   a  and  43   b  are formed on the N-well  2   a  and P-well  2   b  through gate dielectric films  41   a  and  41   b  composed of, for instance, SiO 2 , respectively. Dielectric films  44   a  and  44   b  are formed on the surfaces and side walls of the Gate electrodes  4   a  and  4   b , respectively. A p-channel MOSFET  7   a  and an N-channel MOSFET  7   b  are constituted together with deep diffusion layers  5   a ,  5   a ′,  5   b  and  5   b ′ (for example, with depths of about 0.1-0.2 μm) and shallow diffusion layers  6   a ,  6   a ′,  6   b  and  6   b ′ (for example, with depths of not more than about 0.1 μm) formed so as to sandwich directly under part of the gate electrodes  4   a  and  4   b , respectively. Element isolating dielectric films  8  isolate the P-channel MOSFET and the N-channel MOSFET, and contacts  9  connect these parts to metal wirings  10  through each silicide layer (for example, NiSi layer)  51  on the surfaces of the diffusion layers  5   a ,  5   a ′  5   b  and  5   b ′, respectively.  
         [0025]      FIG. 6  shows a top view of the unit bit cell of the SRAM cell. As shown in  FIG. 6 , like a conventional one, each diffusion layer  5  formed on the N-well  2   a  and P-well  2   b  is connected to metal wirings  10 , such as word lines, bit lines and a Vdds through the contacts  9 , respectively.  
         [0026]     Then, like the top view of the SRAM cell shown in  FIG. 7 , substrate contacts (N-contact  12   a , P-contact  12   b ) taking substrate electric potentials are each disposed between bit cells  11  constituted by alternately combining the p-channel MOSFET and the N-channel MOSFET. Further, three bit cells  11  are set to a unit bit cell  13 , and each unit bit cell  13  is isolated by deep trenches  14 .  
         [0027]     Such semiconductor device is formed as follows. The deep trenches  3  are firstly formed in the p-semiconductor substrate  1 , then the SiO 2  film  3   a  is formed on the side wall and bottom of deep trench  3 , and furthermore, the polysilicon film  3   b  is formed thereinside. Next to this, after forming the N-well  2   a  and P-well  2   b  by injecting impurities, shallow trenches are formed and plugged with a dielectric film, and then, the element isolating dielectric film  8  is formed. Then, the polysilicon layer and silicide layer (for instance, NiSi layer) are formed and the gate electrodes  4   a  and  4   b  are formed by patterning in a usual method, the shallow diffusion layers  6   a ,  6   a ′,  6   b  and  6   b ′ and the deep diffusion layers  5   a ,  5   a ′,  5   b  and  5   b ′ are formed by injecting impurities therein.  
         [0028]     Further, in a self-aligning method, etc., the dielectric films  44   a  and  44   b  are formed on the surfaces and side walls of the gate electrodes  4   a  and  4   b  and dielectric film such as TEOS is formed on the whole surfaces. After forming the contact holes at the prescribed positions of the dielectric film, the contact holes are plugged with W, Mo, etc. to form the contacts  9  connecting the silicide layers of the surfaces of the deep diffusion layers  5   a ,  5   a ′,  5   b  and  5   b ′, respectively. Moreover, the metal wirings  10  such as word lines, bit lines and Vdds are formed at the prescribed positions in their upper layers.  
         [0029]     As mentioned above, the well isolating boundary and the unit bit cell boundary of the SRAM cell are isolated to sufficient deep regions by the deep trenches  3  and  14  not by the shallow trenches like conventional ones. Conventionally, electric charges, which are generated in the unit bit cell by for instance, incidence of a rays with high energy flow into the diffusion layers of a plurality of SRAM cells closed to one another to cause a destructive phenomenon of information stored in a plurality of cells. However, according to such structure, the electric charges generated within the regions isolated by the deep trenches  3  and  14  having been absorbed in the closest diffusion layers to enable suppressing destructive phenomena of data within limited sells, it becomes possible to prevent the data stored in the adjacent cells from being destroyed.  
         [0030]     With the deep trenches  3  and  14  disposed in the lower layer of the element isolating dielectric film  8 , the bit cell may be isolated without increasing the element in size. Further, disposing correcting functions in circuit for each unit bit cell makes it possible, as a semiconductor device, to suppress a functional deterioration caused from the soft error.  
         [0031]     Moreover, with the substrate contacts  12   a  and  12   b  disposing; it becomes possible to suppress a glitch such as latch-up due to a variation in substrate voltage potential. In this case, to achieve a latch-up resistance and miniaturization of the element, it is preferable to shorten distances among each bit cell (MOSFET) and each substrate contact as much as possible, for instance, not more than 10 μm of the 0.1 μm generation. It is necessary to form one or more substrate contacts for each unit bit cell; two or more thereof can be formed.  
         [0032]     In the present embodiment, the unit bit cells isolated by the deep trenches  3  and  14  having been set as a pair of bit cells  11  sandwiching the substrates contacts  12   a  and  12   b  therebetween, it is not limited to one pair of them, and the unit bit cell may be one which includes plural pieces of bit cells therein. It is preferable for the unit bit cell to be smaller in number of pairs (for instance, one pair of bit cells) to suppress the functional deterioration due to the soft error. Depending on the size of the element, it is desirable for the interval between the deep trenches to suppress to, for example, not more than 10-20 μm of the 0.1 μm generation.  
         [0033]     The deep trenches  3  and  14  are formed by usual etching processes, it is possible to use such as a method forming trench capacitors of DRAM. Accordingly, for a semiconductor device, such as an embedded DRAM system LSI, of which the top surface schematic view is shown in  FIG. 8 , on forming the trench capacitors of DRAM, similarly forming the deep trenches  3  and  14  in accordance with the same design rule as those of the trench capacitors of DRAM allows forming the deep trenches without increasing the number of processes. Like this manner, with the deep trenches  3  and  14  forming in the same design rule as those of the trench capacitors of DRAM, it is possible to sufficiently suppress movements of electric charges generated in the unit bit cell to the outside of the bit cell.  
         [0034]     In the present embodiment, forming the SiO 2  film and the polysilicon film in the deep trenches in the same way as that of the trench capacitor of DRAM, it is enough for a structure to enable suppressing the movements of the electric charges to the outside of the bit cell and it is not always have to form the polysilicon film thereinside.  
         [0035]     Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.