Method of fabricating a stacked capacitator DRAM cell

Abstract: A method of fabricating capacitor electrodes in a stacked type of a dynamic random access memory (DRAM) is disclosed. This method of fabricating the capacitor electrodes of a storage capacitor in the semiconductor memory device having a memory cell comprising a MOS transistor and a storage capacitor includes the steps of forming a highly doped polysilicon film (10), patterning the highly doped polysilicon film in a predetermined shape, forming a lightly doped polysilicon film (11) so as to cover the surface of the highly doped polysilicon film having the predetermined shape, and anisotropically etching the lightly doped polysilicon film. A cylindrical capacitor electrode having a bottom portion (10a) in a stacked type of memory cell can be easily formed. By increasing the height of the cylindrical portion, the area of the electrodes (10a, 11a; 13) opposing with each other of the storage capacitor can be also easily increased.

Description: BACKGROUND OF THE INVENTION (1) Field of the Invention The present invention relates to a method of fabricating a semiconductor memory device and, more particularly, to a method of fabricating a storage capacitor in a stacked type of a dynamic random-access memory (hereinafter referred to as "DRAM"). (2) Description of the Related Art Conventionally, in many cases, DRAM adopts a memory cell composed of one transistor serving as a switch and one storage capacitor. The DRAM designed in such a structure includes a stacked type memory cell. Figs. 1A to 1E are sectional views for explaining the method of fabricating a conventional stacked type memory cell in the order of its fabricating steps. First, as seen from Fig. 1A, formed on a silicon substrate 1 are a field oxide film 2, a gate insulating film 3, gate electrodes 4, source/drain regions 5 and first insulating films 6 covering the gate electrodes 4. As seen from Fig. 1B, a second insulating film 7 is formed on the entire outer (upper) surface using chemical vapor deposition (CVD) techniques (hereinafter simply referred to as "CVD"), and thereafter a contact hole 8 is opened at the position corresponding to a storage capacitor by photolithography and reactive ion-etching (RIE) techniques (hereinafter simply referred to as "RIE"). As seen from Fig. 1C, a polysilicon (also called polycrystalline silicon) film is formed by the CVD, and further a capacitor storage electrode 20 of the polysilicon film is formed by the RIE using a photoresist film 9 as a mask. As seen from Fig. 1D, a dielectric film 12 is formed on the resultant surface, and subsequently a capacitor opposing electrode 13 of a polysilicon film is formed. The capacitor opposing electrode 13 is formed so as to cover the capacitor storage electrode 20, and the dielectric film 12 is etched using the capacitor opposing electrode 13 as a mask. Finally, as seen from Fig. 1E, a third insulating film 14 serving as an inter-layer insulating film is formed by the CVD, and thereafter the third insulating film 14 and the second insulating film 7 on the source/drain region 5 are selectively removed to open a contact hole for a bit followed by the formation of a metal wiring 15. The above conventional stacked type memory cell has the following defects. For the storage capacitor to have a large capacitance, the level difference in the gate electrodes 4 is used to increase the surface area of the capacitor storage electrode 20. Further, the thickness of the polysilicon film of the capacitor storage electrode 20 is made as thin as possible in order to facilitate easy etching on the portion having the level difference. For example, the polysilicon film having the thickness in the order of 200 nm was used at an early stage. However, if the capacitor storage electrode 20 is thin, its side area counts small in terms of an increase in the surface area. As the memory cell area is made smaller, the pattern of the capacitor storage electrode 20 must be made smaller. Therefore, it is not possible to secure a sufficient area for the capacitor if only the surface of the capacitor storage electrode 20 is used in such a structure. A method of making a memory capacitor whose lower electrode comprises a base portion and a cylindrical sidewall portion in known from JP-A-2 260 453. SUMMARY OF THE INVENTION It is an object of the invention to overcome the above defects in the conventional method and to provide an improved method of fabricating a semiconductor memory device which provides an increased area for a storage capacitor. In order to attain the above object there is provided a method of fabricating capacitor electrodes as set out in the attached claim. BRIEF DESCRIPTION OF THE DRAWINGS The above and other objects, features and advantages of the present invention will be apparent from the following description of preferred embodiments of the invention. with reference to the accompanying drawings, in which: Figs. 1A to 1E are sectional views showing the steps in the semiconductor memory device fabricating process according to the prior art; Figs. 2A to 2E are sectional views showing the steps in the semiconductor memory device fabricating process according to a first embodiment of the present invention; and Figs. 3A to 3C are sectional views showing the steps in the semiconductor memory device fabricating process useful for understanding the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Now, referring to the drawings, an embodiment of the present invention will be explained below. Please note that, throughout the specification of the invention, the same reference numerals refer to the same or like elements in all the figures of the drawings. Figs. 2A to 2E show the fabricating process according to the embodiment in the order of its steps. First, as seen from Fig. 2A, a field oxide film 2 is formed on, e.g., a p-type silicon substrate 1. After a gate insulating film 3 is formed on an element area, gate electrodes 4 of a polysilicon film are formed on predetermined areas, N⁺-type source/drain regions 5 are formed by ion-implantation using the gate electrodes 4 as masks. After a silicon oxide film is grown on the entire upper surface, it is etched back by the RIE using mixed gas of, e.g., CHF₄ and O₂ thereby to form a first insulating film 6 covering the gate electrodes 4. Further, after a second insulating film 7 is formed on the entire upper surface by the CVD, a contact hole 8 is opened. Subsequently, after a polysilicon film having the thickness of, e.g., 600 nm is grown, it is highly doped with n-type impurities such as phosphorus (P) by diffusion or ion-implantation. A photoresist 9 having a predetermined pattern is formed on the surface of the above n⁺-type polysilicon film. The n⁺-type polysilicon film is etched by the RIE using the photoresist 9 as a mask thereby to form a first capacitor storage electrode 10 of the n⁺-type polysilicon film. As seen from Fig. 2B, after the photoresist 9 is removed, a polysilicon film is grown to a thickness of, e.g., 100 nm on the entire upper surface. Further, the polysilicon film is lightly doped with n-type impurities such as phosphorus (P) by diffusion or ion-implantation thereby to form a second capacitor storage electrode 11 of the n-type polysilicon film. In this case, it should be noted that with respect to this second capacitor storage electrode 11, doping of n-type impurities is carried out at a lower concentration than in the first capacitor storage electrode 10 so that dry etching by, e.g., RIE is to be executed at a lower speed than in the first capacitor storage electrode 10. As seen from Fig. 2C, anisotropic etching by, e.g., the RIE is carried out for a time longer than the time required to remove the second capacitor storage electrode 11 which has been formed on the upper surface of the first capacitor storage electrode 10. Thus, after the initial second capacitor storage electrode 11 formed on the first initial capacitor storage electrode 10 is removed, etching for the first capacitor storage electrode 10 proceeds at a relatively high speed. As a result, only a part 10a of the first capacitor storage electrode 10 and a part 11a of the second capacitor storage electrode 11 which is formed at each of the two sides of the first storage electrode 10 are left. Accordingly, the capacitor storage electrode is constituted by both the first capacitor storage electrode 10a and the second capacitor storage electrode 11a in such a structure that the former defines a bottom portion and the latter defines a cylindrical portion of the storage capacitor. As seen from Fig. 2D, a dielectric film 12 and a capacitor opposing electrode 13 of a polysilicon film are successively formed on the above composite capacitor storage electrode for them to have predetermined shapes. Finally, as seen from Fig. 2E, after a third insulating film 14 is deposited on the resultant surface, a contact hole for a bit is opened to reach the source/drain region 5 and a metal wiring 15 of, e.g., aluminum is provided on the contact hole. Figs. 3A to 3C show a fabricating process useful for understanding the present invention the order of its steps. First, as seen from Fig. 3A, a field oxide film 2 is formed on, e.g., a p-type silicon substrate 1. After a gate insulating film 3 is formed on element areas, gate electrodes 4 of a polysilicon film are formed on predetermined areas. N⁺-type source/drain regions 5 are formed by ion-implantation using the gate electrodes 4 as masks. After a silicon oxide film is grown on the entire upper surface, it is etched back by the RIE using a mixed gas of, e.g., CHF₄ and O₂ thereby to form a first insulating film 6 covering the gate electrodes 4. Further, after a second insulating film 7 is formed on the entire upper surface by the CVD, a contact hole 8 is opened. Subsequently, a tungsten (W) film is formed on the source/drain region 5 with the contact hole 8, and further a deposition film 21 of a polysilicon film is grown to a predetermined thickness thereon. A photoresist 9 having a predetermined pattern is applied to the resultant surface. Using the photoresist 9 as a mask, the deposition film 21 and the tungsten film are etched by the RIE so that the first capacitor storage electrode 10a of the tungsten film is formed. Next, as seen from Fig. 3B, after the photoresist 9 is removed, another tungsten film having a thickness of, e.g., 100 nm is deposited to form an initial second capacitor storage electrode 11. As seen from Fig. 3C, anisotropic etching by the RIE using mixed gas containing fluorine (F) is carried out to remove the deposition film 21 and to form an actual second capacitor storage 11a. Accordingly, the capacitor storage electrode is constituted by both the first capacitor storage electrode 10a and the second capacitor storage electrode 11a in such a structure that the former defines a bottom portion and the latter defines a cylindrical portion of the storage capacitor. Here, it should be noted that the etching speed of the polysilicon film is higher than the tungsten film. For this reason, in this anisotropic etching process, immediately after the second capacitor storage electrode 11 on the deposition film 21 is removed, the deposition film 21 will be etched at a high speed. In this case, the first capacitor storage electrode 10 serves as a stopper for the etching for the deposition film 21 so that the composite capacitor storage electrode according to this example having a predetermined shape can be formed more easily than in the embodiment of the present invention. Additionally, although in this example, tungsten (W) is used as the material for the first and second capacitor storage electrodes 10a and 11a, other refractory metals such as molybdenum (Mo) and titanium (Ti) may well be used. Further, the first and second capacitor storage electrodes may not be made of the same material but may be made of a combination of these materials. Although the deposition film is made of a polysilicon film, it may be made of any other material as long as its etching speed is higher than that of the first and the second capacitor storage electrode. As readily understood from the explanation hitherto made, in accordance with the method of fabricating a semiconductor memory device according to the present invention, a cylindrical capacitor storage electrode having a bottom portion in a stacked type of memory cell can be easily formed. Therefore, by increasing the height of the cylindrical portion, the area of the electrodes opposing with each other of a capacitor can also be easily increased so that capacitance value is effectively increased.

Claims: A method of fabricating capacitor electrodes in a semiconductor memory device having a memory cell comprising a MOS transistor serving as a switch and a storage capacitor connected with one of its source and drain regions (5), wherein by said method a lower electrode of two opposing capacitor electrodes is produced, the lower electrode comprising a base portion and a cylindrical sidewall portion, said method being characterized by comprising the steps of: forming a highly doped polysilicon film (10); patterning said highly doped polysilicon film into a predetermined shape; forming a lightly doped polysilicon film (11) on the surface of said highly doped polysilicon film to form a composite structure, said highly doped polysilicon film having an etching rate faster than that of said lightly doped polysilicon film; and anisotropically etching said composite structure so that only sidewall portions of the lightly doped polysilicon film remain as said cylindrical sidewall portion.

Inventors: YAMANAKA KOJI C O NEC CORPORAT, YAMANAKA, KOJI, C/O NEC CORPORATION
Applicants: NIPPON ELECTRIC CO, NEC CORPORATION
Application year: 1991
Main classification: H01L27
IPC codes: H01L27, H01L21