Structure of semiconductor memory device
BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present invention relates to the structure of a memory cell in a dynamic random access memory (DRAM), and in particular to a stacked capacitor cell structure thereof. 
     (2) Description of the Related Art 
     FIG. 4 shows the structure of a stacked capacitor cell. In the figure, there are shown an element isolation region 1 formed by a selective oxidation process, a gate oxide film 2 formed by thermal oxidation, a polycrystalline silicon gate electrode 3 for a memory transistor formed by a chemical vapor deposition process, impurity diffusion layers 4, 4&#39; formed by ion implantation and having a conductive type opposite to that of a silicon substrate 10, a layer insulation film 5 formed by a CVD process, a contact hole 6 formed by etching, a conductive polycrystalline silicon layer 7, a capacitor insulation film 8, and a conductive polycrystalline silicon layer 9. With this arrangement, a memory cell comprising one transistor and one capacitor is formed, as shown by an equivalent circuit in FIG. 3. However, the above-mentioned memory cell structure suffers a decrease in accumulated charge capacity since the capacitance of the capacitor decreases as the level of integration of the device becomes higher thus making the area of the cell extremely small. Accordingly, a problem arises in that the so-called hold time, that is, the time of holding the accumulated charge becomes shortened. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to eliminate the problem in which the accumulated charge hold-time of a memory cell becomes short due to a decrease in the accumulated charge capacity. 
     The structure of a semiconductor memory device according to the present invention includes a MIS transistor formed between a charge accumulation electrode, and a substrate and opposite conductive type diffusion layer regions, thereby increasing the hold-time of accumulated charges. 
     According to the present invention, since the MIS transistor is provided, in a stacked capacitor cell, between the charge accumulation electrode, and the reverse conductive type diffusion layer domains and the substrate of a transfer gate for controlling the flow-in and -out of accumulated charges, the flow-out of the charges from the above-mentioned diffusion layer domains, that is, leak current can be reduced. Thus, it is possible to eliminate the above-mentioned problem. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIGS. 1(a) to 1(d) are sectional views showing a manufacturing process in one embodiment of the present invention; 
     FIG. 2 is an equivalent circuit diagram showing the structure of a semiconductor memory device according to the present invention; 
     FIG. 3 is an equivalent circuit diagram showing the structure of a conventional semiconductor memory device; and 
     FIG. 4 is a sectional view showing the structure of a conventional semiconductor memory device. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     With reference to FIGS. 1(a) to 1(d) an explanation will be made of a process of manufacturing a semiconductor memory device structure according to the present invention. 
     First, an element isolation region 11 is selectively formed on a P-type silicon substrate 10 by a known LOCOS process. A gate oxide film 12 is selectively formed on the substrate by thermal oxidation in an atmosphere of N 2  :O 2  :H 2  =1:1:2 at a temperature of 850 deg. C. for about ten minutes. A gate electrode (a first conductive polycrystalline silicon layer) 13 is formed on the gate oxide film 12 by a CVD process at a temperature of 620 deg. C. and at a pressure of about 0.2 torr. Thereafter, an N-type diffusion layer 14 is formed in the substrate 10 by ion implantation in which As +  ions are implanted under the condition of 5×10 15  ions/cm 2  at 40 keV. Further, an insulation film 15 is formed on the structure obtained above by a CVD process at a temperature of 350 deg. C. and at a normal pressure (FIG. 1a). 
     Next, the insulation film 15, the gate electrode 13 and the gate oxide film 12 are selectively removed by a well-known photolithography process or etching process to form a contact hole 16. Further, N-type impurities such as As +  ions are implanted through the contact hole 16 into the exposed surface of the substrate 10 under the condition of 5×10 15  ions/cm 2  at 40 keV so that an N-type diffusion layer 14&#39; is formed in the substrate 10 (FIG. 1b). 
     Then, the exposed polycrystalline silicon of the gate electrode 13 which is located at the side wall of the contact hole 16 is thermally oxidized under an atmosphere of N 2  :O 2  :H 2  =1:1:2 at a temperature of 850 deg. C. for about ten minutes so that a gate insulation film 110 for a MIS transistor mentioned below is formed. The gate insulation film 110 may be formed of Ta 2  O 5  film, a silicon nitride film, PZT film or the like which are formed by a CVD process or a sputter process. When the gate insulation film 110 is formed, an insulation film is also formed in the bottom part of the contact hole 16. The insulation film formed in the bottom of the contact hole 16 is selectively removed by anisotropic etching or the like. Accordingly, the gate insulation film 110 remains at only the side wall part of the contact hole 16. 
     Thereafter, the contact hole 16 is filled up with polycrystalline silicon containing P-type impurity at a low density to form a silicon layer 111. This silicon layer 111 is obtained by the following steps. At the first step, polycrystalline silicon containing P-type impurity at a low density, is deposited on the entire surface of the structure shown in FIG. 1b up to a thickness of about 1 μm by a CVD process or the like. In this step, the surface of the deposited polycrystalline silicon is made to be flat. In the second step, the entire surface of the deposited polycrystalline silicon is then etched back to obtain the silicon layer 111 (FIG. 1c). The silicon layer 111 may be also formed in such a way that polycrystalline silicon containing P-type impurity at a low density is grown up on the substrate 10 by an epitaxial growth process in an atmosphere of 100% SiH 4  at 2 torr at a temperature of about 900 deg. C. for more than 30 minutes. Further, the silicon layer 111 may be replaced by a monocrystalline silicon layer. The monocrystalline silicon layer is formed as follows: Polycrystalline silicon containing P-type impurity is filled in the contact hole 16. Then, polycrystalline silicon is converted into monocrystalline silicon by laser-annealing in which Ar laser having a power of 15 W is applied at a substrate temperature of 400 deg. C. and at a speed of 50 cm/sec. 
     Then, a conductive polycrystalline silicon layer is formed on the entire surface of the structure obtained in the above step by a CVD process or the like so that it is electrically connected to the silicon layer 111. Further, the above polycrystalline silicon layer is subjected to patterning so as to form a charge accumulation electrode 17. When the polycrystalline silicon layer is formed by a low pressure CVD process at a temperature of about 570 deg. C. the resultant polycrystalline silicon layer has many microneous concavities and convexities on its outer surface. If there are many microneous concavities and convexities on the outer surface of the polycrystalline silicon layer, an outer surface of the polycrystalline silicon layer becomes larger, so that accumulated charge capacity thereof increases. 
     In this step, the MIS transistor is obtained. The MIS transistor is composed of the electrode 17 and diffusion layer 14&#39; as a source/drain, the gate insulation film 110 and the gate electrode 13. 
     Then, a capacitor insulation film 18 is formed on the charge accumulation electrode 17. This capacitor insulation film 18 is formed of a thin film having a high dielectric constant, such as a silicon oxide film, a silicon nitride film or the like. 
     Next, a third polycrystalline silicon layer 19 is formed on the capacitor insulation film 18. This third silicon layer 19 is connected to the ground potential or a stationary potential (FIG. 1d). 
     Thus, the structure of the stacked capacitor cell has been completed. 
     FIG. 3 is an equivalent circuit diagram which shows a conventional capacitor cell. In this figure, it is found that only one transistor for connecting a memory capacitor with a bit line (BL) is present. FIG. 2 is an equivalent circuit diagram which shows a capacitor cell according to the present invention. As understood from this figure, two transistors are substantially present for connecting between the capacitor and the bit line (BL). Thus, it is found that charge accumulated in the capacitor can hardly leak in comparison with the conventional one.