Method of fabricating a semiconductor device having a trench for device isolation

Abstract: A device-isolating trench (28) having a taper at its upper portion is formed in a silicon semiconductor substrate (22). Then, a silicon oxide film (32) is formed on the inner wall of the trench (28) and the surface of the semiconductor substrate (22) near the trench (28) by an oxidizing method, and polycrystalline silicon (33) is buried in the trench (28).

Description: The present invention relates to a method of fabricating a semiconductor device in which device isolation is made by a trench formed in a semiconductor substrate. IEDM, December 1982, pp 62-65, discloses a method for forming a trench in a substrate. According to said method, an SiO 2 film is overetched using an upper Si 3 N 4 layer as a mask to form an Si 3 N 4 overhang and then silicon is etched. When the silicon is etched, an end portion of the SiO 2 film is also etched and retreated. Therefore, an opening of the upper portion of a trench becomes larger than that transferred to the Si 3 N 4 . The trench is formed deeply by RIE (reactive ion etching) using the Si 3 N 4 layer as a mask. SiO 2 is not deposited on the inner wall of the trench during the etching so that the trench may be hollowed out in the lateral direction, preventing uniform formation of a deep trench. The Journal of the Electrochemical Society, Vol 134, No. 2, pp 481-490, and Vol. 135, No. 3, pp 726-730, discloses formation of the radius of curvature at the bottom corner of a trench. Japanese Journal of Applied Physics Supplement 18th Conference on Solid State Devices, 1986, pp. 303--306, discloses the ratio of the curvature of a radius of Si at an upper corner portion of a trench to the thickness of an insulating film formed on the upper corner portion. DE-A-3 829 015 discloses a method for forming a semiconductor device, said method including forming on a substrate a mask of oxide, nitride and oxide, etching said mask to allow forming a trench with anisotropic etching (RIE) in the substrate, isotropically etching the substrate with a buffer HF to remove an oxide film formed on the surface of the trench, the top oxide film and to undercut the oxide, isotropically etching the substrate to form a taper at the upper portion of the trench, removing the mask, and oxidizing the surface of the trench. EP-A-0 181 188 discloses a method comprising forming a mask of oxide/nitride/oxide, isotropically etching a shallow trench and forming a deep trench before oxidizing the walls of the trench. Moreover, according to a conventional method, in a semiconductor device a trench for device isolation is carried out in the following manner. As shown in Fig. 1A, a trench 11 is formed in a semiconductor substrate 12, and a thin oxide film 13 is formed on the inner wall of the trench 11. A nonoxide film 14 is then formed only on a device region 15. Then, with the nonoxide film 14 used as a mask, the resultant structure is subjected to an oxidization treatment to form a thick oxide film 16 on the surface of the device region 15 except on a part thereof, as shown in Fig. 1B. Thereafter, polycrystalline silicon is buried in the trench 11 where the oxide film 16 is formed, followed by flattening of the top surface of the polycrystalline silicon, deposition of a cap oxide film, and so forth, thereby providing the device isolation. According to this conventional method, the wall of the trench 11 and the surface of the substrate 12 define almost right angles at an upper corner portion 17 of the trench 11, as shown in Fig. 1B. At the time the thick oxide film 16 is formed, therefore, thermal stress is concentrated on the corner portion 17 at the oxidization time, or so is stress originated from expansion of volume or the like. This causes dislocation in the oxide film 16 near the corner portion. This dislocation impairs the device isolation characteristic and the characteristic of a device formed in the device region. For instance, if bipolar transistors are respectively formed in multiple device regions, the dislocation results in an increase in leak current between the collectors of the individual transistors and deterioration of the current amplification factor. In other words, since the presence of dislocation to a certain degree increases the recombination current, the device characteristic and the device isolation characteristic are deteriorated. It is therefore an object of the present invention to provide a method of fabricating a semiconductor device, which can suppress the occurrence of dislocation in an oxide film at the upper corner portion of a trench for device isolation and can thus improve the characteristic of a device formed in a device region as well as the device isolation characteristic. According to the present invention, this object is attained by the method according to claim 1. Further improvements according to the present invention are specified in the sub-claims. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Figs. 1A and 1B are cross-sectional views of individual steps of a conventional method of fabricating a semiconductor device; Figs. 2A to 2H are cross-sectional views of individual steps of a conventional method of fabricating a semiconductor device according to a preferred embodiment of the present invention; Fig. 3 is a perspective view illustrating the structure of a bipolar transistor fabricated by the method according to the first embodiment; and Fig. 4 is a diagram illustrating the characteristic of the semiconductor devices fabricated by the method according to said embodiment. A preferred embodiment of the present invention will now be described referring to the accompanying drawings. First, a silicon oxide film 21 is formed on the whole surface of a silicon semiconductor substrate 22 by a thermal oxidizing method, and a silicon nitride film 23 is then formed on the oxide film 21, as shown in Fig. 2A. Subsequently, a silicon oxide film 24 is deposited on the silicon nitride film 23 by a CVD (Chemical Vapor Deposition) method, followed by coating of a photoresist on the entire surface of the resultant structure. The photoresist is then subjected to pattern exposure and developing processing, thus forming photoresist mask 25. Next, by using an anisotropic etching method, such as RIE (Reactive Ion Etching), with the mask 25, the silicon oxide film 24 and silicon nitride film 23 are selectively removed. Subsequently, by using an isotropic etching method, such as a wet etching method involving a mixture of a hydrofluric acid and a nitric acid, the underlying substrate 22 is further removed about 0.5 µm, forming a trench 26 in the substrate 22. It is to be noted that, as the isotropic etching method, a plasma etching method called "CDE (Chemical Dry Etching)" may replace the wet etching method. Then, by using the anisotropic etching method, such as a wet etching method involving a mixture of isopropyl alcohol, KOH and water, with the mask 25, the substrate 22 at the upper portion of the trench 26 is etched, forming a taper portion 27 in the substrate 22, as shown in Fig. 2B. Next, the trench 26 is further etched by anisotropic etching, such as RIE, thereby forming a trench 28, 5 to 7 µm deep, as shown in Fig. 2C. Fig. 2D illustrates multiple trenches 28 formed through the above-described individual steps. These trenches 28 isolate individual device regions 29 from one another. Then, after the resist mask 25, silicon oxide film 24, silicon nitride film 23 and silicon oxide film 21 are removed, the resultant structure is subjected to thermal oxidization at 950°C in the environment of a gas mixture of H 2 and O 2 , forming a silicon oxide film 30 having a thickness of about 0.05 µm on the surface of the semiconductor substrate 22 and the inner wall of the trench 28, as shown in Fig. 2E. Subsequently, a silicon nitride film 31 is deposited 0.05 to 0.15 µm thick on the entire surface of the resultant structure at 780°C by a low pressure CVD method. The silicon nitride film 31 is selectively etched to remain on each device region 29 by a plasma etching method. The remaining silicon nitride film 31 is to be used as a nonoxide film in the subsequent step. Next, as shown in Fig. 2F, by a wet oxidizing method using the nonoxide film (silicon nitride film 31) as an oxidation-resistant mask, a silicon oxide film 32 is formed about 0.8 µm thick on part of each device region 29 where the silicon nitride film 31 is not formed, and the inner wall of each trench 28. Fig. 2G presents an enlarged illustration of one of the trenches after the above step has been completed. Then, after the silicon nitride film 31 is removed, polycrystalline silicon 33 is buried in each trench 28 and its surface is flattened, followed by formation of a thin cap oxide film 34 on the trench 28, as shown in Fig. 2H. Thereafter, a bipolar transistor having, for example, an N type emitter region E, a P type base region B and an N type collector region C, is formed in each device region 29 by a known method, as shown in Fig. 3. Referring to Fig. 3, the substrate 22 has a three-layer structure including a P type region 41, an N + type buried region 42 and an N type epitaxial region 43. It is to be noted that a P + type region 44 is formed in the bottom of each trench 28 by an ion implantation method or diffusion method before the polycrystalline silicon 33 is buried therein. According to the method of the first embodiment, the taper portion 27 is formed in the substrate 22 at the upper portion of the trench 28 at the time the step illustrated in Fig. 2B is executed, and the resultant structure is subjected to oxidization in the subsequent step, thereby forming the silicon oxide film 32. In forming the silicon oxide film 32, therefore, an upper corner portion of the trench 28 will be shaped round with some radius of curvature, not square, as shown in Fig. 2G. It is therefore possible to relax the concentration of thermal stress on the corner portion at the oxidization time, or the concentration of stress originated from expansion of volume or the like thereon. This suppresses the occurrence of dislocation near the corner portion of the trench, which has been a bottleneck in the prior art, thus ensuring improvement of the device isolation characteristic and the characteristic of a device formed in each device region. Fig. 4 is a characteristic diagram illustrating the relationship between the radius of curvature, R, expressed in Angstroms Å (1 Angstrom = 0.1 nanometre), at the upper corner portion of the trench 28 and the dislocation density when bipolar transistors are formed in the device regions of the semiconductor substrate isolated by the methods of the first and second embodiments. As illustrated, with the radius of curvature, R, set 0.1 µm or larger, the dislocation can be suppressed as compared with the prior art in which R is almost 0. As a result, the device isolation characteristic and the characteristic of devices formed in the device regions can be improved. The radius of curvature, R, at the upper corner portion of the trench 28 is not limited to about 0.1 µm, but can be set larger. It is to be noted that the larger the radius of curvature, R, the greater the effect of suppressing the dislocation.

Claims: A method of fabricating a semiconductor device, comprising the successive steps of: forming a first silicon oxide film (21) on a silicon semiconductor substrate (22), a silicon nitride film (23) on said first silicon oxide film, a second silicon oxide film (24) deposited on said silicon nitride film, and a photoresist film on the surface of the resultant structure and subjected to pattern exposure for forming a photoresist mask (25); selectively removing said second silicon oxide film (24), said silicon nitride film (23) and said first silicon oxide film (21) by an anisotropic etching method using the photoresist as a mask (25), and forming a shallow trench (26) in said silicon semiconductor substrate (22); etching said substrate (22) at an upper portion of said shallow trench by an anisotropic wet etching method and forming a taper portion (27) in said substrate (22); forming a deep trench (28) in said shallow trench by an anisotropic etching method using the photoresist as a mask (25); removing said photoresist mask (25), said second silicon oxide film (24), said silicon nitride film (23) and said first silicon oxide film (21) from said semiconductor substrate (22); and forming an oxide film (32) on an inner wall of said trench (28) and a surface of said semiconductor substrate (22) near said trench by an oxidizing method. The method according to claim 1, wherein said deep trench (28) is formed so as to be 10 times as deep as said shallow trench (26) or deeper. The method according to claim 1 or 2, wherein said shallow trench (26) is formed so as have a depth of 0.5 µm or less. The method according to any of the preceding claims, wherein said deep trench (28) is formed so as have a depth of 5 µm or more. The method according to any of the preceding claims, comprising a further step of burying polycristalline silicon in said deep trench (28).

Inventors: MIYASHITA NAOTO, TAKAHASHI KOICHI, MIYASHITA, NAOTO, TAKAHASHI, KOICHI, MIYASHITA, NAOTO, C/O INTELLECTUAL PROPERTY DIV., TAKAHASHI, KOICHI, C/O INTELLECTUAL PROPERTY DIV.
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1991
Main classification: H01L21
IPC codes: H01L21