A capacitor and method for making a capacitor

Abstract: The present invention provides a semiconductor device having a capacitor that is formed through: a first step of forming a polysilicon layer (23) having a rough surface after a nonconductive layer (22) is applied to a base substrate (21); a second step of etching back away the polysilicon layer (23) to expose the nonconductive layer (22) and thus remaining islandlike polysilicon layers (24); a third step of etching the nonconductive layer (22), using the remained polysilicon layers (24) as an etching mask; a fourth step of etching the base substrate (21) of the capacitor, using the nonconductive layer (22) as a mask; a fifth step of forming a pattern (37) of the base substrate of the capacitor after the removal of the remained nonconductive layer (22); a sixth step of forming an upper substrate (39) of the capacitor after the formation of a dielectric film (38) of the capacitor.

Description: The present invention relates generally to a capacitor and a method for making a capacitor. More particularly, this invention relates to a capacitor having a conductive layer with a plurality of microscopic trenchlike sections and a relatively simple method for making a capacitor having an increased capacitance. Remarkable progress has been made in the manufacture of Dynamic Random Access Memories (DRAM) using high integration technology. Accordingly, as a semiconductor device has a small size and the problem of securing a high capacitance becomes an issue. For example, main stream production has changed from 1Mbit DRAMs to 4M bit DRAMs has been achieved. In such a DRAM with high integration, a predetermined capacitance of the cell storage capacitor must be kept constant, in spite of the decrease in the area of the cells. For example, each area of a cell and a capacitor in a 64M bit DRAM decreases to about 0.8 µm 2 and 1.0 µm 2 . In the case where the areas of charge storage capacitors are also decreased and the capacitance becomes small, a soft error occurs on exposure to α-light, and the problem of reliability on a semiconductor device becomes an issue, too. Accordingly, the capacitance of cell storage capacitors must be kept constant, in spite of the decrease in the areas of the capacitors, in order to obtain an improved integration of a semiconductor device. In a recent DRAM whose cells are based on transistor-stacked capacitor combinations, one of a pair of electrodes of a storage capacitor has been formed to have a three-dimensional structure. This makes the capacitance larger by 30 to 40% than that of a two-dimensional storage capacitor having the same size as the three-dimensional one. In the case of 64M bit DRAMs having high integration, the capacitance needs to increase without the increase of cell areas or storage area, and various three-dimensional structures or high dielectric constant have been studied. One of technologies for increasing the capacitance of a storage electrode without the increase of cell areas or storage area is producing high-performance capacitors with a rough surface polysilicon film as a storage electrode. A method for obtaining a high capacitance in defined small areas of capacitors, such as the above three-dimensional DRAMs, is described in "Solid state Device & Material No. 90-167", page 49, published December 1990. As a prior art, FIG. 1 shows a sectional view of the structure of a capacitor formed with a polycrystalline silicon film having a rough surface. A storage electrode 11 that serves as a first electrode of the capacitor, e.g. a first polycrystalline silicon film is deposited at 550°C. At these temperatures, amorphous and polycrystalline structures coexist, and the surface areas of silicon grains having a hemispherical shape are maximized. Consequently the surface morphology strongly depends on the deposition temperature. In the surface area increase on 550°C deposited film, the area of the surface covered with hemispheres is about twice as large as that of the flat surface. After that, first electrodes are defined using conventional photolithography and etching technique. A capacitor dielectric film 12 of oxide film/nitride film is applied thereon, and a plate electrode 13 that becomes a second electrode, i.e. a polycrystalline silicon is deposited. In the above method, however, since close attention to the control of temperatures should be paid and the polysilicon film has hemispherical grains, there is a limit on the increase of capacitance. Accordingly, it is an object of the present invention to increase the capacitance of a capacitor. It is another object of the invention to provide a capacitor having capacitance and a manufacturing technique for the capacitor, by including an electrode substrate of a plurality of microscopic trenchlike sections. According to the present invention there is provided a method for making a capacitor comprising the steps of: forming and patterning a first conductive layer, forming a first oxide film thereon, and then forming a silicon nitride film of thickness in the range 3 to 50nm thereon; forming a second oxide film by oxidizing said silicon nitride film thereby forming holes or cracks in the silicon nitride film; etching the second oxide film to expose said silicon nitride film; etching the first oxide film to expose the first conductive layer using the silicon nitride film as a mask; forming a plurality of trenches by etching said first conductive layer, using as a mask the silicon nitride film and the remaining first oxide film; removing said first oxide film and silicon nitride film; and forming a dielectric film along the surface of said first conductive layer and forming a second conductive layer thereon. The above and further objects, features and advantages of the present invention will become more apparent from the following detailed description of embodiments taken in conjunction with the accompanying drawings, in which; Figure 1 is a sectional view of a known stacked capacitor type DRAM (Dynamic Random Access Memory); Figures 2A to 2F depict the steps in the manufacture of a capacitor including an electrode substrate of a plurality of microscopic trenchlike sections in accordance with a preferred embodiment of this invention; Figures 2D' to 2F' depict partially the steps in the manufacture of a capacitor including an electrode substrate of a plurality of microscopic trenchlike sections in accordance with a preferred embodiment of this invention; Figures 3A to 3G depict the steps in the manufacture of a DRAM having a capacitor formed in accordance with a preferred embodiment of this invention, and; Figures 3D' to 3G' depict partially the steps in the manufacture of a DRAM having a capacitance formed in accordance with a preferred embodiment of this invention; A detailed description of the present invention will now be made referring to the accompanying drawings. As an embodiment in the manufacture of a capacitor in accordance with the present invention, a polysilicon layer that becomes a first conductive layer is deposited to a thickness of 2000 to 4000 angstroms by means of a low pressure chemical vapor deposition process, and then doped with impurity. In this process, a single crystalline silicon doped with impurity can be utilized, instead of polysilicon doped with the impurity. Subsequently, the polysilicon layer is etched, using a photoresist pattern, and a first conductive layer 41 is formed as shown in Figure 2A, a first oxide film 42 is formed to a thickness of 50 to 2000 angstroms, and a silicon nitride film 43 is then formed thereon to a thickness of 30 to 500 angstroms. As shown in FIG. 2B, the silicon nitride film 43 is thermally oxidized to form a second oxide film 44 on the surface. Due to this oxidation process, pinholes are increased in the thin silicon nitride film 43, and silicon of the silicon nitride film is consumed at the time of the oxidation whereby new pinholes are created or the film is cracked partially. Referring now to FIG. 2C, the second oxide film 44 and the first oxide film 42 are removed by a wet-etching method to expose the silicon nitride film 43 and the first conductive layer, using the silicon nitride film 43 that is cracked or has pinholes as an etching mask such that undulated profiles made of the silicon nitride film 43 and oxide film 42 are created. If the first conductive layer is etched through an anisotropic etching, using as a mask the remained silicon nitride film 43 and oxide film 42 passed through the process of FIG. 2C, trenches having uneven configurations are formed on a region where the silicon nitride film 43 and oxide film 42 are not formed. The depth of the trench is determined by controlling the etching time according to a predetermined capacitance of capacitor. And, since the anisotropic etching process has been carried out, these trenches are formed vertically just on the upper surface of the first conductive layer 41. After the process of FIG. 2D, by etching the silicon nitride film 43 and oxide film 42 used as a mask in serial order through wet etching, just the first conductive layer 41 having a plurality of microscopic trenchlike sections are remained, as shown in FIG. 2E. After the process of FIG. 2E, a dielectric film 45 is formed along the surface of the first conductive layer 41, and a second conductive layer 46 is formed thereon, whereby the capacitor in accordance with this invention is completely formed, as depicted in FIG. 2F. The dielectric film 45 is preferably formed of two layers of nitride and oxide film, three layers of oxide, nitride and oxide film, or high dielectric film such as Ta 2 O 5 . The second conductive layer 46 that is the upper substrate of the capacitor is formed of polysilicon doped with impurities or single crystalline silicon doped with impurities. As a further preferred embodiment, in the manufacture of a capacitor of the present invention, the processes of FIGS. 2A to 2C in this embodiment are carried out equally to those of the second embodiment. After these processes, the first conductive layer is etched through isotropic etching, instead of the anisotropic one, using the remained silicon nitride film 43 and oxide film 42 as a mask. According to this isotropic etching, trenches are formed over the whole upper and side surfaces of the first conductive layer 41, as depicted in FIG. 2D'. The depth of the trench is determined by controlling the etching time according to a predetermined capacitance of capacitor. After the process of FIG. 2D', by etching the silicon nitride film 43 and oxide film 42 used as a mask in serial order through wet etching, just the first conductive layer 41 having a plurality of microscopic trenchlike sections are remained, as shown in FIG. 2E'. After the process of FIG. 2E', a dielectric film 45 is formed along the surface of the first conductive layer 41, and a second conductive layer 46 is formed thereon, whereby the capacitor in accordance with this invention is completely formed, as depicted in FIG. 2F'. Accordingly, the capacitor in accordance with this invention has a a first conductive layer having a plurality of microscopic trenchlike sections, and, thus, the surface area of the capacitor electrode is remarkably enhanced such that the sufficient capacitance of the capacitor is secured in the microscopic defined area and the margin of process becomes broad. FIGS. 3A to 3G depict the steps in the manufacture of a DRAM having a capacitor formed in accordance with the first preferred embodiment of this invention, and FIGS. 3E' to 3G' depict partially the steps in the manufacture of a DRAM having a capacitance formed in accordance with the second preferred embodiment of this invention. The manufacture of a DRAM having a capacitor according to a first preferred embodiment of this invention begins with defining an isolation region on a p-type semiconductor substrate 50 with a field oxide film 51. After gate electrodes 53 are formed, source/drain impurity regions 52 and 52' are formed. Insulating films 54 are formed, and the source region is then opened. After that, referring to FIG. 3B, a polysilicon layer, a first conductive layer that serves as a storage electrode is deposited to a thickness of 2000 to 4000 angstroms thereon by the LPCVD process, and impurities are then doped. Single crystalline silicon doped with impurities can be used in this process. Subsequently, a first conductive layer 55 is formed by etching the polysilicon layer by means of the photoresist pattern, and a first oxide film 56 is formed to a thickness of 50 to 2000 angstroms thereon. On the first oxide film, a silicon nitride film 57 is formed to a thickness of 30 to 500 angstroms. Referring now to FIG. 3C, a second oxide film 56' is formed by oxidizing the silicon nitride film 57. Due to the oxidation process, pinholes are increased in the thin silicon nitride film 43 and the silicon of the silicon nitride film is consumed, which results in the occurrence of new pinholes or cracks in the film. As shown in FIG. 3D, after the second oxide film 56' is removed by a wet etching process, the first oxide film 56 is etched using the silicon nitride film 57 having pinholes as an etching mask, and the undulated profiles comprising of the silicon nitride film 57 and oxide film 56 are then formed. In FIG. 3E, where the first conductive layer placed underneath is etched by an anisotropic etching method, using as a mask the silicon nitride film 57 and oxide film 56 remained after the process of FIG. 3D, trenches are formed on a region where the silicon nitride film 57 and oxide film 56 are not formed. The depth of the trench is determined by controlling the etching time according to a predetermined capacitance of the capacitor. And since the anisotropic etching was performed in the above process, these trenches are formed vertically just on the first conductive layer 55. After the process of FIG. 3E, by removing the silicon nitride film 57 and oxide film 56 used as a mask in serial order through wet etching, just the first conductive layer 55 having a plurality of microscopic trenchlike sections are remained, as shown in FIG. 3F. After the process of FIG. 3F, a dielectric film 58 is formed along the surface of the first conductive layer 55, and a second conductive layer 59 is formed thereon thereby forming a semiconductor device having the capacitor according to the second embodiment of this invention, as shown in FIG. 3G. The dielectric film 58 is preferably formed of two layers comprising of nitride and oxide film, three layers comprising of oxide, nitride and oxide film, or a high dielectric film such as Ta 2 O 5 . The second conductive layer 59 that is a second conductive layer of the capacitor is formed of polysilicon doped with impurities or single crystalline silicon doped with impurities. In the manufacture of a DRAM having a capacitor in accordance with a further embodiment of this invention, the processes depicted in FIG. 3A to FIG. 3D are carried out in this embodiment, equally to those of the second embodiment. When it comes to the performance of a process for etching the first conductive layer placed underneath, using the remained silicon nitride film 57 and oxide film 56 as a mask, an isotropic etching method is used instead of the anisotropic etching one. According to this isotropic etching method, trenches are formed on the whole upper and side surfaces of the first conductive layer 55, as depicted in FIG. 3E'. The depth of the trench is determined by controlling the etching time according to a predetermined capacitance of the capacitor. After the process of FIG. 3E', the silicon nitride film 57 and oxide film 56 that were used as a mask are removed in serial order by a wet etching process, and just the first conductive layer 55 having a plurality of microscopic trenchlike sections on its whole upper and side surfaces is remained, as shown in Figure 3F'. After the process of Figure 3F', the dielectric film 58 is formed along the surface of the first conductive layer 55, and a second conductive layer 59 is formed on the dielectric film 58 thereby completing the capacitor in accordance with the second embodiment as depicted in Figure 3G'. Accordingly, the capacitor in accordance with this invention has an electrode substrate having a plurality of microscopic trenchlike sections, and, thus, the surface area of the capacitor electrode is remarkably enhanced such that the sufficient capacitance of the capacitor is secured in the microscopic defined area and the margin of process becomes broad. The manufacture of a capacitor where trenches are formed deeply not to expose the bottom of the base substrate of the capacitor and the surface area of the capacitor is then enhanced, can have an increased capacitance compared to the case where a storage electrode is formed of polysilicon having a rough surface. The method for making a capacitor in accordance with this invention is applied to stack/trench capacitors or trench ones and a structure of trenchlike sections as well as stacked capacitors. In conclusion, according to the advantages of this invention, since a storage electrode is formed of combination of countless trenchlike sections compared to a capacitor made by a conventional method, the surface area of the storage electrode can be enhanced, and if an etch rate is controlled at the time of etching the storage electrode, using an oxide film as a mask, a predetermined capacitance can be controlled. Furthermore, the capacitor of this invention has an electrode substrate of a plurality of microscopic trenchlike sections, and therefore, the surface area of the capacitor electrode is remarkably enhanced such that the sufficient capacitance of the capacitor is secured in the microscopic defined area and the margin of process becomes broad, whereby the integrity of DRAMs is more improved.

Claims: A method for making a capacitor comprising the steps of: forming and patterning a first conductive layer (41), forming a first oxide film (42) thereon, and then forming a silicon nitride film (43) of thickness in the range 3 to 50nm thereon; forming a second oxide film (44) by oxidizing said silicon nitride film (43) thereby forming holes or cracks in the silicon nitride film (43) ; etching the second oxide film (44) to expose said silicon nitride film (43); etching the first oxide film (42) to expose the first conductive layer (41) using the silicon nitride film (43) as a mask; forming a plurality of trenches by etching said first conductive layer (41), using as a mask the silicon nitride film (43) and the remaining first oxide film (42); removing said first oxide film (42) and silicon nitride film (43); and forming a dielectric film (45) along the surface of said first conductive layer (41) and forming a second conductive layer (46) thereon. The method as set forth in Claim 1, wherein said first conductive layer (41) has a thickness of 200-400nm (2000 to 4000 angstroms). The method as set forth in Claim 1, wherein said first conductive layer (41) is etched through anisotropic etching or isotropic etching. The method as set forth in claim 1, wherein said second conductive layer (46) is formed of polysilicon doped with impurities or single crystalline silicon doped with impurities.

Inventors: KANG SEONGHUN, KIM SUNGTAE, KO JAEHONG, SHIN HYUNBO, KANG, SEONGHUN, KIM, SUNGTAE, KO, JAEHONG, SHIN, HYUNBO
Applicants: SAMSUNG ELECTRONICS CO LTD, SAMSUNG ELECTRONICS CO. LTD.
Application year: 1992
Main classification: H01L21
IPC codes: H01L27, H01L21, H01L29