Semiconductor memory device and method for producing the same

Abstract: A semiconductor memory device including a semicon­ ductor substrate (1) comprising a field oxide layer (10) selectively formed on the semiconductor substrate, a capaci­ tor including a insulating layer (7) formed on the surface of a trench (6) formed in such a manner that at least an edge portion of the field oxide layer is removed, a conductive layer (8) formed on the insulating layer (7), a dielectric layer (9) formed on the conductive layer (8) and an electrode (11) formed on the dielectric layer (9).

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a semiconductor memory device and a method for producing the same. More particularly, it relates to a dynamic random access memory (DRAM) cell having a trench capacitor and a method for producing the DRAM cell. 2. Description of the Related Art Conventionally, to miniaturize a DRAM cell, the cell area is increasingly reduced and in order to increase storage capacity trench capacitors are formed to obtain a larger substantial capacitor area than that provided in a conventional plane area. The above process, however, is disadvantageous in that when the trench capacitors are arranged close to each other, data stored therein is lost due to a punch-through phenomenon occurring between storage electrodes and that the capture of carriers generated in a semiconductor substrate by alpha (α) ray irradiation is large so that soft errors occur. To alleviate the above problems it is necessary to form impurity-implanted regions, having a concentration of an order 1 to 2 degrees higher than the concentration in the substrate, around the trench capacitor, thus decreasing the width of the depletion layer and forcing both capacitors to be located close together. However, in the above, an ion implantation process cannot be effectively used as the process for implanting impurities into the side surfaces of the trench capacitors. To prevent the occurrence of the above problems, a semiconductor integrated circuit device is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 59-191373 (hereinafter referred to as "the publication-373") filed on May 15, 1983. According to the structure of the trench capacitor disclosed in the publication-373, a storage electrode plate and an insulating film for the inner wall or the trench capacitor are formed in that order around the capacitor dielectric film. By forming the insulating film on the inner wall of the trench capacitor, the generation of undesired depression regions, which extend from the respective trench capacitor to the semiconductor substrate and cause operational errors, can be avoided. However, the above conventional trench capacitors are positioned on each side of a field oxide film having a so-called bird's beak formed at both ends thereof. The field oxide film having the bird's beak has a width of at least about 2 µm even if the most superior lithography techniques are used in the etching thereof. Therefore, trench capacitors must be formed with a space having a width of at least 2 µm as for the oxide film. Thus, the structure disclosed in the publication-373 has insufficient miniaturization effects, even though the above punch-through phenomenon is prevented by the formation of the storage electrode plate and the insulating film for the inner wall of the trench. Document JP-A-59 63757 discloses another example of a prior art device comprising trench capacitors. The substrate is prepared with a bird beak field oxide region, and the trench is formed by reactive sputtering over a region that includes the bird beak. The trench is thereby configured with an inclined side wall corresponding to the portion partially masked by the bird beak region, and a vertical side wall on the other side. The capacitor is formed on the the inclined wall and the base of the trench, while the vertical wall serves to define an isolation region. SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to obviate the above mentioned problems by providing a semiconductor memory device wherein the punch-through phenomenon between storage electrodes is avoided. It is a further object of the present invention to provide a semiconductor memory device wherein the capture of carriers due to α-ray irradiation also can be avoided. It is a still further object of the present invention to provide a method for producing the semiconductor memory device. Therefore, according to the present invention, there is provided a method for producing a semiconductor memory device comprising the steps defined in claim 1. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross sectional view of a conventional trench capacitor cell; Fig. 2 is a cross sectional view of another conventional trench capacitor cell; Fig. 3 is a plan view of Fig. 1; Fig. 4 is a cross sectional view of a DRAM cell according to the present invention; Fig. 5 is a plan view of Fig. 4; Fig. 6 is a cross sectional view of another DRAM cell according to the present invention; Fig. 7 is a plan view of Fig. 6; Figs. 8A to 8H are cross sectional view explaining steps of an example falling outside the scope of the present invention; Fig. 9 is a plan view of Fig. 8B; Fig. 10 is a plan view of Fig. 8C; Fig. 11 is a plan view of Fig. 8F; Fig. 12 is a plan view of Fig. 8H; and Fig. 13 is a comparative example wherein a capacitor storage electrode plate is electrically connected to a source and a drain of a transfer gate transistor by using a mask alignment process not according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Before describing the preferred embodiments of the present invention, a further description of the prior art will be made. Figure 1 is a cross-sectional view of a conventional trench capacitor cell and Figure 3 is a plan view of Figure 1. In Figs. 1 and 3 on a surface of a p-type silicon substrate 1, drain and source regions 2 and 3 of n-type silicon are formed. Further, at a portion 31 (Fig. 3) of a word line 5a, a gate is formed. Thus, a transfer transistor is formed. A capacitor is formed by forming a trench 6 in the p-type silicon substrate 1, forming a capacitor dielectric film 9 therein, and forming a cell plate 11 of polycrystalline silicon as shown in Fig. 1. A capacitor is realized by forming the capacitor dielectric film positioned between an inversion layer 27 in the p-type silicon substrate 1 and a cell plate 11. However, when the distance between the trenches 6 becomes too small, in the trench capacitor a depletion layer (not shown) extending from a capacitor portion is in contact with another depletion layer extending from a capacitor portion of an adjacent cell, and thus an injection current is caused to flow between the capacitors closely positioned, resulting in a loss of information (charges) stored in the capacitors. This is known as the so-called punch-through phenomenon. The punch-through phenomenon occurs more often in capacitors having a deep trench. To prevent the punch-through phenomenon, the trench capacitor shown in Fig. 2 has been provided which is described in the above-mentioned publication-373. As shown in Fig. 2, in a trench 6, an insulating layer 7 and a storage electrode 8 are provided. The capacitor is realized by forming a capacitor dielectic film 9 positioned between an insulating layer 7 and a cell plate 11. The insulating layer 7 prevents the occurrence of a punch-through and the capture of carriers due to α-ray irradiation. However, the distance W between the trenches 6 is still about 2 µm. Thus, although the punch-through problem is solved, the miniaturization of the capacitor cannot be attained by the structure of Fig. 2. Figure 4 shows a cross sectional view of a DRAM cell according to the present invention and Figure 5 shows a plan view of Fig. 4. As shown in Fig. 4 the structure of the DRAM cell according to the present invention is substantially similar to that of Fig. 2 except that the trenches 6 are positioned closer together. Namely, the feature of the present invention resides in the fact that each trench 6 is formed by removing, for example, a p-type silicon substrate 1 and a part of a field oxide layer 10. As shown in Figs. 4 and 5, to form a transfer transistor, drain and source regions 2 and 3 of n-type silicon are formed on the surface of the p-type silicon substrate 1. The drain and source regions 2 and 3 are positioned below a portion surrounded by a field oxide layer portion 10 in Fig. 2. Further, above a portion between the drain and source regions 2 and 3, a word line 5a of, for example, polycrystalline silicon, is formed through a field oxide layer 12. The word line 5a is oriented in the vertical direction, as seen in the drawing and acts as a gate electrode together with the drain and source regions 2 and 3 at portions 16 in Fig. 5, so that a transfer transistor is operated. The word lines 5b are formed only to connect transfer transistors closely positioned to each other. In the trenches 6 having a square shape, an insulating layer 7 of, for example, SiO₂ , and a storage electrode 8 on the insulating layer 7 are formed. The storage electrode 8 is connected to the source region 3 of the transfer transistor by a contact 15. On the surface of the storage electrode 8 are formed a capacitor dielectric layer 9 of oxide of silicon and a cell plate 11 of polycrystalline silicon. Further, above the cell plate 11, a bit line 13 of, for example, aluminum, is formed through a field oxide layer 12, which is connected to the drain 2 of the transfer transistor by a contact 14. Capacitors formed in the closely positioned trenches are electrically connected to the cell plates 11 but physically isolated by the thick field oxide layer 10. In the DRAM cell shown in Figs. 4 and 5 a capacitor is formed by the capacitor dielectric layer 9 between the storage electrode 8 and the cell plate 11, and is electrically connected to the p-type silicon substrate 1 through the insulating layer 7. As explained in Fig. 2 the punch-through phenomena and the capture of carriers due to α ray irradiation are also prevented in the structure of Fig. 4. Furthermore, since the trench is also formed by removing the thick field oxide layer, the distance W₂ between trenches 6 is decreased. Thus, the cell area can be miniaturized to 3 µm x 5.52 µm = 16.56 µm² as shown in Fig. 5. Figures 6 and 7 show a cross sectional view of another DRAM cell according to the present invention and a plan view of Fig. 6. The structure shown in Figs. 6 and 7 is very similar to that of Figs. 4 and 5 except that, on the thick field oxide layer 10, an extended portion (having a distance S′) of the storage electrode 8 is made to align with the trench 6. Therefore the distance between the capacitors in the respective trenches 6 amounts to 2S′ + S, which is larger than that of Figs. 4 and 5. Namely, the cell area becomes 3 µm x 6.25 µm = 18.75 µm². Figures 8A to 8 are cross sectional views explaining steps of a method according to the present invention. As shown in Fig. 8A, over a p-type silicon substrate 1, an insulating layer is entirely formed by a well-known process. The layer is composed of double layers of SiO₂42 (having a thickness of about 500 Å) and Si₃N₄43 (having a thickness of about 2000 Å). The Si₃N₄ layer 43 is patterned to differentiate a circumferential circuit of the memory and an active region of a forwarding transistor of a memory cell from an isolation region. Then, an ion implanted layer 4 is formed. As shown in Fig. 8B the obtained wafer is entirely oxided to form a field oxide layer 10 of SiO₂ (having a thickness of about 5000 Å). The above explained process is a conventional LOCOS process. As shown in Fig. 8C, a part of the oxidation resistance layer in which a trench capacitor is to be formed is selectively removed by using a mask, and subsequently, the substrate positioned below the part of the insulating layer is removed by an etching process, using the same mask mentioned above, to form a trench having a depth of 5 µm. Thus, the trench 46 is formed so that it is positioned at a part including a thin SiO₂ layer and thick field oxide layer 10 having a bird's beak B at the edge thereof. In the formation of the trench, at first, both the silicon substrate and the field oxide layer 10 are simultaneously etched at substantially the same rate by using, a reactive ion etching which has a strong sputtering quality, using, for example, Ar or CHF₃ , etc. As shown in Fig. 8D, the obtained structure is then oxided again. After that an insulating layer (SiO₂)7 (having a thickness of about 1500 Å) is selectively formed on the inner wall surface of the trench. Then, as shown in Fig. 8E the insulating layer (SiO₂42, Si₃N₄43) is removed to expose the surface 8 of the silicon substrate 1 and the surface of the trench is oxidized so that a capsule shaped insulating layer 7 is formed in the trench 6. By the process for removing the oxidation resistance layer, the insulating layer (SiO₂)7 of the trench 46 is removed to some extent so that the thickness of the insulating layer 7 becomes about 800 Å. In order to form a capacitor surrounded by the capsule-shaped insulating layer 7, as shown in Fig. 8F, a polycrystalline silicon 9 is then deposited to a thickness of about 1500 Å and patterned to the shape of the capcitor. The polycrystalline silicon 9 is doped to make it a conductive type opposite to that of the Si substrate. For example, if the Si substrate is p-type the polycrystalline silicon 9 is doped to an n-type, so that the polycrystalline silicon 9 acts as a charge storage electrode plate. Then, as shown in Fig. 8G, a capacitor dielectric layer 9 is formed on the polycrystalline silicon 8 by oxidizing the surface thereof by a thermal oxidation process, to, for example, about 150 Å, while forming an n-type diffusion layer 4 in an Si substrate at the edge of the trench. The polycrystalline silicon 11 is then deposited in such a manner that the trench is buried so that an electrode, i.e., so-called self plate, is formed. Then, as shown in Fig. 8H on an insulating layer positioned on the polycrystalline silicon and at the gate portion of a transfer gate transistor, word lines 5b and 5a are respectively formed by a well known process. After the formation of the word lines 5a and 5b a drain region and a source region 3 are formed by an ion implantation process and activating heat treatment process. In this heat treatment n-type impurities are further diffused from the n-type diffusion region 4 and an n⁺ layer of the drain 3 is connected thereto without mask alignment, i.e., in self-alignment, at the edge of the trench 46. Further, a bit line 13 is also formed with the result that a DRAM cell shown in Fig. 8H can be obtained. Figures 9 to 12 show plan views of Figures 8B, 8C, 8F, and 8K respectively. Figure 13 shows an comparative example wherein a capacitor storage electrode plate is electrically connected to a source and a drain of a transfer gate transistor by using a mask alignment process not according to the present invention. As shown in Fig. 13 if a contact hole is formed between the source and the drain regions in an insulating layer (SiO₂)7 extending to the surface of the Si substrate the margin (ℓ in Fig. 13) required in the mask alignment between the contact hole and the edge surface of the trench causes an increase in the size of the memory cell. When the contact hole is positioned very near to the trench, the insulating layer (SiO₂)7 of the inner surface of the trench is likely to be broken. Thus, it is necessary to leave that margin to make the contact hole.

Claims: A method for producing a semiconductor memory device, comprising the steps of: selectively forming, on the surface of a semiconductor substrate (1), a field oxide layer (10) having an edge portion in the form of a bird's beak; etching simultaneously the edge portion of the field oxide layer and the semiconductor substrate to remove the bird's beak and to form a trench (46) having some of its side walls almost perpendicular to the surface of the substrate; forming an insulating layer (7) in the trench (46); and forming, in the trench, a capacitor including a conductive layer (8) formed on the insulating layer (7), a dielectric layer (9) formed on the conductive layer (8) and an electrode (11) formed on the dielectric layer (9), characterized in that the etching is performed at substantially the same etching rate through said field oxide layer and said semiconductor substrate to form under the region of bird's beak a side wall of the trench (46) which is almost perpendicular to the surface of the substrate. A method for producing a semiconductor memory device according to claim 1, wherein the etching for the formation of the trench (46) is made by using a reactive ion etching which has a strong sputtering quality.

Inventors: TAGUCHI MASAO, TAGUCHI, MASAO
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1985
Main classification: H01L27
IPC codes: H01L27, H01L21