Stacked capacitor and method for making same

Abstract: The stacked capacitor has a first electrode (3) with a fin structure formed on a semiconductor device and a second electrode (7) formed over the first electrode via a dielectric film (6). The first electrode consists of an electrically conductive material different from polycrystalline silicon and a polycrystalline silicon film (4) containing am impurity covering the electrically conductive material. According to this structure, the film thickness of the storage electrode of the fin capacitor can be reduced and corrugation of the surface of a memory device can be mitigated.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention This invention relates to a capacitor and a method of fabricating same. More particularly, it relates to a capacitor suitable for application to dynamic random access memories (DRAMs), and a method of fabricating same. 2. Description of the Related Art Remarkable progress has been made in technologies required for acdomplishing a miniaturization and a multi-layered structure, in semiconductor processes for semiconductor devices having a higher integration density and higher operation speed. In memory devices, however, an area occupied by a capacitor, and a capacitance hereof, must be further improved. Figure 4(A) is a sectional view of a storage electrode of a conventional capacitor, and Figure 4(B) is a sectional view of a conventional fin-type stacked capacitor. In these drawings, reference numeral 22 denotes a silicon (Si) substrate; 23 is a silicon nitride (Si 3 N 4 film; 24 is a polycrystalline silicon (poly-Si) film; 25 is a storage electrode; 26 is a silicon nitride (Si 3 N 4 ) film; 27 is a poly-Si film; 28 is an opposed electrode; and 29 is a fin-type stacked capacitor. Conventionally, the storage electrode 25 of the fin-type stacked capacitor is composed of a single layer of the polycrystalline silicon (poly-Si) film 24, as shown in Fig. 4(A). Such a poly-Si film 24, however, is easily deformed or broken during etching or a thermal oxidation treatment, and therefore, to make it suitable for practical applications, the poly-Si film 24 must be at least 1,000 Å thick (1Å = 0.1 nm). If the thickness of the storage electrode 25 is great in a three-dimensional structure such as fin-type stacked capacitor 29, which is widely applied in DRAMS, etc., as shown in Fig. 4(B), the height of the multi-layered fin becomes even greater, and thus a multi-layered wiring film such as alumium (A1) is easily broken due to a step of a gate electrode, etc., and consequently, it is difficult to improve the production yield and quality. Various other types of electrode structures are known in the art, as briefly outlined below. JP-A-1 120 050 explains how a simplified memory cell can be obtained with a stacked storage electrode formed of polycrystalline silicon and covered with a material having good conductivity, such as titanium nitride. JP-A-63 240 057 discloses a stacked type of capacitor comprising a laminate structure of alternate semiconductor and insulator layers. A doped semiconductor layer is shaped onto the surface of the laminate at an etched end face of the structure. JP-A-2 076 258 relates to a memory having a reduced bit line capacity by disposing unit cells in adjacent arrays in a staggered disposition so as to constitute an open bit line. JP-A-1 278 061 discloses a stacked capacitor formed of a first polysilicon film, a dielectric film, a silicon carbide film, a dielectric film and a second polysilicon film. A capacitor electrode of one polarity is formed from the silicon carbide film, while the other electrode is formed on the first and second polycrystalline films. SUMMARY OF THE INVENTION In view of the problems of the prior art described above, an object of the present invention is to mitigate any corrugation of the surface of a memory device by reducing the film thickness of the storage electrode of a fin-type stacked capacitor. In a stacked capacitor having a fin structure formed on a semiconductor substrate, the object of the invention described above can be accomplished by providing a stacked capacitor comprising a first electrode with a fin structure comprising fin portions and fin-connecting portions formed on a semiconductor device and a second electrode formed over the first electrode via a dielectric film, characterized in that each fin portion and each fin-connecting portion of the first electrode consists of an electrically conductive material different from polycrystalline silicon, and a polycrystalline silicon film containing an impurity covering the electrically, conductive material. A method of forming a stacked capacitor comprising a first electrode with a fin structure comprising fin portions and fin-connecting portions formed on a semiconductor device and a second electrode formed over the first electrode via a dielectric film according to the present invention comprises a) a step of alternately laminating an insulating film and a first electrically conductive film different from polycrystalline silicon on a substrate; b) a step of forming an opening reaching a surface of said substrate from the laminate surface to expose the first electrically conductive film in the opening; c) a step of forming a second electrically conductive film different from polycrystalline silicon on at least all surfaces of the opening, said second conductive film connecting to the first conductive film; d) a step of removing said insulating film; e) a step of covering the entire surface of the conductive films with a polycrystalline silicon film containing an impurity; f) a step of forming a dielectric film on the polycrystalline silicon film; and g) a step of forming a second electrode on the dielectric film. BRIEF DESCRIPTION OF THE DRAWINGS The object and advantages of the present invention will become more apparent from the following description of a preferred embodiment, taken in connection with the accompanying drawings, in which: Figures 1(A) and 1(B) are explanatory views explaining the principle of the present invention; Figure 2 is a sectional view showing steps in the process according to one embodiment of the present invention; Figure 3 is a graph showing the capacitor capacitance of a capacitor according to the present invention, and that of a capacitor of a prior art example; and Figures 4(A) and 4(B) are sectional views showing a storage electrode of a conventional capacitor and a fin-type stacked capacitor, respectively. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a view for explaining the principle of the present invention. In the drawing, reference numeral 1 denotes a semiconductor substrate; 2 is a capacitor; 3 is a storage electrode; 4 is a polycrystalline silicon (poly-Si) film; 5 is a compound containing SiC; 6 is a dielectric film; and 7 is an opposed electrode. As shown in Figure 1(A), a stacked capacitor 2 having a fin structure according to the present invention is characterized in that the storage electrode 3 of the stacked capacitor 2 has a structure wherein an electrically conductive material 5 other than poly-Si is coated with poly-Si and an impurity. As shown in Figure 1(B), in the stacked capacitor 2 having a fin structure according to the present invention, the surface of the storage electrode 3 is coated with the dielectric film 6, and the opposed electrode 7 is formed on this dielectric film. In the present invention, the conductive material comprises a compound containing silicon carbide (SiC) and an impurity. The term "compound containing silicon carbide" as used herein means not only silicon carbide (SiC) but also other compounds such as SiCH, SiCHF, and SiCF. The resistance of compounds containing these silicon carbides can be remarkably reduced by doping with an impurity, and they can then be rendered conductive. In the present invention, the electrically conductive material is made from a material having a higher strength than polysilicon. Also, the higher strength material, for example, can be a mixed crystal containing silicon carbides. In addition to the materials described above, the conductive material in the present invention can be constituted by a silicide, and in this case, the doping with an impurity is not necessary. Examples of the silicides that can be used in the present invention include tungsten silicide, tantalum silicide, molybdenum silicide, and titanium silicide. The doping of the impurity into the poly-Si film 4 described above can be accomplished by a thermal diffusion from the SiC-containing compound 5; it also can be accomplished by an in - situ doping by which the impurity is doped simultaneously with the growth of the poly-Si film. Preferred examples of the impurities to be doped in the present invention include metals of the Group III (such as gallium, boron, indium, etc.) and metals of the Group V (such as antimony, arsenic, phosphorus, etc.) of the Periodic Table. As described above, the storage electrode in the present invention is constituted by the material obtained by coating the thermally stable compound, such as the SiC-containing compound, with the poly-Si film, and therefore can solve the problems of the prior art as described above. Namely, the SiC-containing compound has a higher strength and thermal stability than the poly-Si film, and has a sufficient strength even during an oxidation process of above 1,000°C, for example, and therefore, the thickness of the electrode can be reduced. Even in the multi-layered fin structure, therefore, corrugation of the device surface is lessened and a wiring process for Al, etc., can be simplified. Since a coating by the Si film is carried out, the capacitor electric characteristics can be made equal to those of the poly-Si film single layer electrode. Next, the present invention will be explained in further detail with reference to examples, but the invention is not limited to these examples. Example The present invention will be explained as an embodiment thereof applied to a fin-type stacked capacitor of a DRAM, with reference to Figure 2. As shown in Figure 2(A), a MOS device is formed on the Si substrate 8, up to the gate electrode thereof, and the silicon nitride (Si 3 N 4 ) film 9 is deposited on the entire surface of the Si substrate 8. Next, as shown in Figure 2(B), a silicon dioxide (SiO 2 ) film 10 and an n-type polycrystalline SiC film 11 are alternately deposited to a thickness of 1,000 Å and 500 Å, respectively, by CVD, under the usual conditions for the former and under the following conditions for the latter, such that the upper and lower surfaces thereof become the SiO 2 film 10. Next, an opening 12 is formed by patterning the SiO 2 film 10, the SiC film 11, and the Si 3 N 4 film 9 in such a manner that the Si substrate 8 is exposed in a capacitor formation region (Figure 2(C)). Here, the SiC film 11 is grown by supplying acetylene (C 2 H 2 ) at a rate of 10 sccm, trichlorosilane (SiHCl 3 ) at a rate of 50 sccm, hydrogen (H 2 ) at a rate of 15 slm, phosphine at a rate of 0.1 sccm and hydrogen chloride (HCl) at a rate of 30 sccm, at a substrate temperature of 1,000°C and a pressure of 20 Torrs (1Torr = 133.32 Pa). In this example, phosphine (PH 3 ) is simultaneously supplied as an impurity to be doped into the SiC film 11, at a rate of 0.1 sccm. A SiC film 13 is deposited on the entire surface of the Si substrate 8 as shown in Figure 2(D). Furthermore, as shown in Figure 2(E), the SiC films 11, 13, and the SiO 2 in regions other than the capacitor formation region, are etched and removed by a patterning process as shown in Figure 2(E). The SiO 2 film 10 is etched and removed by a usual dry etching, and the SiC film 11, 13 are etched and removed by RIE by supplying chlorine (Cl 2 ) at a rate of 10 sccm and carbon tetrachloride (CCl 4 ) at a rate of 5 sccm, at a chamber internal pressure of 0.3 Torr and a discharge output of 800 W at 13.56 MHz. The SiO 2 film 10 between the Si 3 N 4 film and the SiC film 11, and the SiO 2 film 10 between the SiC film 11 and the SiC film 13 are etched and removed, to thereby form an storage electrode 14 having a multi-layered fin structure. Thereafter, as shown in Figure 2(G), poly-Si film 15 is selectively grown to a thickness of 200 Å, only on the SiC film 11, by CVD while supplying SiH 2 Cl 2 at a rate of 50 sccm, H 2 at a rate of 3 slm, PH 3 at a rate of 0.1 sccm and HCl at a rate of 30 sccm, at a substrate temperature of 800°C and a pressure of 2 Torrs. Subsequently, the surface of the poly-Si film 15 is oxidized to a depth of about 50 Å as shown in Figure 2(H) to cause a change thereof to the SiO 2 film 16. At this time, phosphorus (P), as the impurity doped into the n-type SiC film 11, is thermally diffused into the poly-Si film 15 of the surface, whereby the resistance of the poly-Si film 15 is also reduced to a low level. Next, a Si 3 N 4 film 17 is deposited to a thickness of 50 Å on this SiO 2 film 15 by CVD, as shown in Figure 2(I), to form a dielectric film 18. The conversion film thickness of this dielectric film 18 consisting of the SiO 2 film 16 and the Si 3 N 4 film 17 of the capacitor 21 is 60 Å. As shown in Figure 2(J), a poly-Si film 19 is deposited to a thickness of 300 Å, to form an opposed electrode 20, and accordingly, a small area large capacitance fin-type stacked capacitor 21 is completed. For a comparison with the present invention, a prior art example is obtained by forming the fin of the storage electrode of a 900 Å-thick poly-Si film alone without using the SiC film, the capacitor capacitances therefore compared, and the results are shown in Figure 3. As shown in Figure 3(A), the storage electrode using the SiC film according to the present invention can provide a capacitor having a fin made of a thin storage electrode, whereas the prior art example using the conventional poly-Si alone exhibits a large variance of the capacitance and is very unstable (Figure 3(B)). As described above, the present invention can stably form a large capacitance capacitor, and greatly contributes to the realization of very-large scale integration DRAMs of above 1 G.

Claims: A stacked capacitor (21) comprising a first electrode (11, 13) with a fin structure comprising fin portions and fin-connecting portions formed on a semiconductor device (8) and a second electrode (19) formed over the first electrode via a dielectric film (18), characterized in that each fin portion and each fin-connecting portion of the first electrode (11, 13) consists of an electrically conductive material different from polycrystalline silicon and of a polycrystalline silicon film (15) containing an impurity and covering the electrically conductive material. A stacked capacitor (21) according to claim 1, in which the electrically conductive material is made from a material having a higher mechanical strength than polysilicon. A stacked capacitor (21) according to claim 2, in which the higher mechanical strength material is a mixed crystal of a compound containing silicon carbide. A method of forming a stacked capacitor (21) comprising a first electrode (11, 13) with a fin structure comprising fin portions and fin-connecting portions formed on a semiconductor device (8) and a second electrode (19) formed over the first electrode (11, 13) via a dielectric film (18), comprising: a) a step of alternately laminating an insulating film (10) and a first electrically conductive film (11) different from polycrystalline silicon on a substrate; b) a step of forming an opening (12) reaching a surface of said substrate (8) from the laminate surface to expose the first electrically conductive film (11) in the opening; c) a step of forming a second electrically conductive film (13) different from a polycrystalline silicon on at least all surfaces of the opening, said second conductive film (13) connecting to the first conductive film (11); d) a step of removing said insulating film (10); e) a step of covering the entire surface of the conductive films (11, 13) with a polycrystalline silicon film (15) containing an impurity; f) a step of forming a dielectric film (18) on the polycrystalline silicon film (15); and g) a step of forming a second electrode (19) on the dielectric film (18). A method according to claim 4, in which the electrically conductive films (11, 13) are made from a material having a higher mechanical strength than polysilicon. A method according to claim 5, in which the higher mechanical strength material is a mixed crystal of a compound containing silicon carbide. A method according to claim 4, in which the impurity is simultaneously doped when forming the polycrystalline silicon film (15). A method according to claim 4, wherein the impurity is doped by thermal diffusion into the polycrystalline silicon when forming the polycrystalline silicon film (15).

Inventors: FUJIOKA HIROSHI, FUJIOKA, HIROSHI, Fujioka, Hiroshi, c/o Fujitsu Limited
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1992
Main classification: H01L27
IPC codes: H01L27, H01L21