Manufacturing method of a field effect semiconductor device

Abstract: A manufacturing method for a semiconductor device, which can attain a low ON-state voltage in a manufacturing method for a semiconductor device involving a process for forming a groove by etching prior to selective oxidation, selectively oxidizing a region including the groove and thereby making a channel part of the groove, is disclosed. A groove part (64) is thermally oxidized by using a silicon nitride film (63) as a mask. A LOCOS oxide film (65) is formed by this thermal oxidation, and concurrently a U-groove (50) is formed on the surface of an n - -type epitaxial layer (2) eroded by the LOCOS oxide film (65), and the shape of the U-groove (50) is fixed. A curve part (709) formed during a chemical dry etching process remains as a curve part (710) on the side surface of the U-groove (50). Then, an n + -type source layer (4) is formed by means of thermal diffusion to a junction thickness of 0.5 to 1 µm, and a channel (5) is set up as well. The junction depth obtained by this thermal diffusion is set up more deeply than the curve part (710) which is formed during the above etching and remains on the side surface of the U-groove (50) after the above selective thermal oxidation.

Description: The present invention generally relates to a manufacturing method for a semiconductor device. More particularly, the present invention relates to a manufacturing method for a semiconductor device used as a power semiconductor device and can suitably be employed for, for example, a vertical type MOSFET (metal oxide semiconductor field effect transistor), an IGBT (insulated gate bipolar transistor) or a MOSIC as a single unit or with the incorporation of a power semiconductor device. EP-A-0 583 023 discloses a method of manufacturing a trenched DMOS transistor wherein trenches are formed in a semiconductor substrate by reactive ion dry etching (RIE) using a mask layer. The trench walls and corners are smoothened by an isotropic plasma round hole etch and the growth of a sacrificial oxide which is subsequently stripped. Adjacent the trenches, base layers and source regions are formed by implanting impurities. Finally, source, gate and drain electrodes are produced. H.-R. Chang et al: IEEE Transactions on Electron Devices, volume ED-34, No. 11 (1987), pages 2329 to 2334, is directed to the manufacturing of self-aligned UMOSFETs wherein trenches are formed by RIE using a mask. A thin layer of sacrificing oxide is grown after trench formation, and is then etched off to remove the damaged surfaces of the trenches as well as to partially smoothen the trench corners. Finally, electrodes are produced for forming the source electrodes, gate electrodes and drain electrode. EP-A-550 770 discloses a method of manufacturing a semiconductor device comprising the features of the preamble of claim 1. Recently, a vertical type power MOSFET has been used in many industrial fields for various features thereof such as high frequency characteristics, fast switching speed and low power driving. The Nikkei Electronics (Nikkei-McGraw-Hills, Inc., May 19, 1986, pp. 165 - 188) says that the focus of the development of the power MOSFET has shifted to the low withstand voltage type and the high withstand voltage type, and that the ON-resistance of the power MOSFET having a withstand voltage of 100V or less has been lowered down to a level of 10 mΩ for the reason that channel width per unit area has been widened by using the microprocessing of LSI for the manufacture of the power MOSFET or by improving the shape of the power MOSFET. The description of this magazine centers around the prevailing vertical type power MOSFET using DMOS type (double diffusion type) cells, reasoning that the DMOS type has productional advantages of high yield and low cost owing to the use of the planar process featured by the use of a flat main surface of a silicon wafer for the channel part. On the other hand, along with the spread of the vertical power MOSFET, there is an increasing demand for lower loss and lower cost, while the ON-resistance reduction by means of microprocessing and cell shape improvement has reached the limits. According to the Japanese Unexamined Patent Publication No. 63-266882, for example, the DMOS type has the minimal point from which the ON-resistance will not decrease however small the unit cell dimension is made by microprocessing, and it has been known that the major cause of the existence of the minimal point is an increase in the JFET (junction field effect transistor) resistance constituting a component of the ON-resistance. Furthermore, as disclosed in the Japanese Unexamined Patent Publication No. 2-86136, the unit cell dimension with which the ON-resistance takes the minimal point is approximately 15 µm under the current microprocessing technology. Various constructions have been proposed in an attempt to break through this limit. Common to these proposals is a construction in which a groove is formed on an element surface and a channel part is formed on a side surface of the groove, and this construction can substantially reduce the above JFET resistance. Furthermore, in a construction in which the channel part is formed on the groove side surface, even if the unit cell dimension is reduced, the consequent increase in the JFET resistance is negligible. Therefore, there is no limits that the ON-resistance takes the minimal point against the reduction in the unit cell dimension unlike the description in the Japanese Unexamined Patent Publication No. 63-266882, and the unit cell dimension can further be reduced to the limits of the microprocessing downwardly exceeding 15 µm. Examples of the conventional manufacturing method with the above construction with the channel part on the groove side surface have been disclosed in the International Publication No. PCT WO93/03502 and the Japanese Unexamined Patent Publication No. 62-12167, for example, and the results of investigation have been disclosed in the ISPSD '93 pp.135 - 140. FIG. 25(a) and FIG. 25(b) are a plane view and a cross-sectional view of the MOSFET disclosed in the International Publication No. PCT WO93/03502, and FIGs. 26 though 37 are cross-sectional views of the MOSFET illustrating the manufacturing process thereof according to the same publication. The above manufacturing process will now be described. First of all, as illustrated in FIG. 26, a wafer 21 is prepared with an n - -type epitaxial layer 2 developed on the main surface of a semiconductor substrate 1 made of n + -type silicon. This semiconductor substrate 1 has an impurity concentration of approximately 10 20 cm -3 , while the n - type epitaxial layer 2 is developed to a thickness of approximately 7 µm and an impurity concentration of approximately 10 16 cm -3 . A field oxide film 60 is formed to a thickness of approximately 60 nm by thermally oxidizing the main surface of the wafer 21, and then a resist film 61 is deposited and patterned by the publicly known photolithographic technique with the central part thereof open, the location of which being coincided with a location of cell formation. Then, boron ions (B + ) are implanted through the field oxide film 60 by using the resist film 61 as a mask. After removing the resist film 61, as illustrated in FIG. 27, a p-type diffusion layer 62 is formed to a junction depth of approximately 3 µm. The p-type diffusion layer 62 ultimately becomes a part of a p-type base layer 16 (described herein later), and plays a role of improving the surge resistance by stably causing breakdown to the bottom part of the p-type diffusion layer 62 when a high voltage is applied to between a drain electrode and a source electrode. Next, as illustrated in FIG. 27, a silicon nitride film 63 is deposited to a thickness of approximately 200 nm on the main surface of the wafer 21. The silicon nitride film 63 is patterned into a lattice-like open pattern with openings with a pitch (dimension of a unit cell 15) a. Incidentally, mask alignment is applied to the open pattern so that the p-type diffusion layer 62 can be positioned in the central part of the pitch. Following the above, as illustrated in FIG. 28, the field oxide film 60 is etched by using the silicon nitride film 63 as a mask, and then a groove 64 is formed by etching the n - -type epitaxial layer 2 to a depth of approximately 1.5 µm. As illustrated in FIG. 29, this time, the groove 64 is thermally oxidized by using the silicon nitride film 63 as a mask, which is a well known oxidizing technique as LOCOS (local oxidation of silicon). By this oxidation, a selective oxide film, i.e., a LOCOS oxide film 65 is formed, and concurrently a U-groove 50 is formed on the surface of the n - -type epitaxial layer 2 eroded by the LOCOS oxide film 65 and the shape of the groove 50 is fixed. Then, as illustrated in FIG. 30, boron ions are implanted through a thin field oxide film 60 by using the LOCOS oxide film 65 as a mask to form the p-type base layer 16. In this process, the boundary surface between the LOCOS oxide film 65 and the field oxide film 60 is coincided with a self-alignment position, and accordingly a region into which boron ions are to be implanted can exactly be defined. In the next process, as illustrated in FIG. 31, thermal diffusion is performed to obtain a junction depth of approximately 3 µm. By this thermal diffusion, the p-type diffusion layer 62 previously formed in the process illustrated in FIG. 27 and the diffusion layer of the boron ions implanted in the process illustrated in FIG. 30 are integrated into the single p-type base layer 16, and both end surfaces of the p-type base layer 16 are self-alignedly defined in the position of the side walls of the U-groove 50. Now, as illustrated in FIG. 32, phosphorous ions are implanted through the thin field oxide film 60 by using both a resist film 66 patterned by a pattern left on the central part of the surface of the p-type base layer 16 surrounded by the LOCOS oxide film 65 formed on the surface of the wafer 21 by a lattice-like pattern and the LOCOS oxide film 65 to form the n + -type source layer 4. In this process as well, like the ion implantation performed with boron ions in the process illustrated in FIG. 30, the boundary part between the LOCOS oxide film 65 and the field oxide film 60 is coincided with a self-alignment position, and accordingly a region into which phosphorous ions are to be implanted can exactly be defined. After the above, as illustrated in FIG. 33, the n + -type source layer 4 is formed and concurrently the channel 5 is set up by thermal diffusion to a junction depth of 0.5 to 1 µm. By this thermal diffusion, the end surface being in contact with the U-groove 50 in the region of the n + -type source layer 4 is self-alignedly defined. As a result of the processes illustrated in FIGs. 30 through 33, the junction depth and shape of the p-type base layer 16 can be fixed. As illustrated in FIG. 34, now, the LOCOS oxide film 65 is removed by wet etching to expose an inside wall 51 of the U-groove 50, and then a gate oxide film 8 is formed to a thickness of approximately 60 nm by thermal oxidation. Next, as illustrated in FIG. 35, a polysilicon film is deposited to a thickness of approximately 400 nm on the main surface of the water 21. Following the above, as illustrated in FIG. 36, boron ions are implanted through the oxide film 67 by using the patterned resist film 68 as a mask in preparation for forming a p + -type base contact layer 17. Then, as illustrated in FIG. 37, the p + -type base contact layer 17 is formed by the thermal diffusion to a junction depth of approximately 0.5 µm. Subsequently, as illustrated in FIG. 25(b), an interlayer insulating film 18 is formed with BPSG (boron phosphate silicate glass) on the main surface of the wafer 21, and contact holes are made in parts of the interlayer insulating film 18 to expose the p + -type base contact layer 17 and the n + -type source layer 4. Furthermore, a source electrode 19 is formed with an aluminum film to achieve an ohmic contact between the source electrode 19 and the p + -type base contact layer 17 and n + -type source layer 4 through the contact holes. In addition, a passivation film (not illustrated) for protecting the aluminum film is formed by the plasma CVD method or the like using silicon nitride. On the other hand, on the back surface of the wafer 21 is formed a drain electrode 20 with three layers of a Ti film, a Ni film and a Au film and ohmically contacted with the n + -type semiconductor substrate 1. However, according to the above manufacturing method for a semiconductor device, a specified region of the low impurity concentration semiconductor layer is removed by the etching technique prior to the selective oxidation. As a result, a curve part is formed with the side surface of the etched groove and the surface of the semiconductor layer (709 in FIG. 8(a)). This curve part had been expected to become smooth during the subsequent selective oxidation process, but it was proved through experiments that this curve part remained even after the elective oxidation as a curve part with a small radius of curvature (710 in FIG. 8(b)). According to this conventional manufacturing method, therefore, the channel region is formed including this curve part, and the carrier mobility is lowered by this curve part and a low ON-state voltage can not be achieved. With the above problem in view, a primary object of the present invention is to obtain a manufacturing method for a semiconductor device which can achieve a low ON-state voltage. In order to achieve the above objective, the manufacturing method according to the present invention comprises the features as defined in the claims. In the manufacturing method for a semiconductor device as composed above, it is preferable that the impurity introduction process should comprise a process in which the second conductivity type and first conductivity impurities are doubly diffused from the side of the main surface in a successive manner in self-alignment with the selective oxide film to form the channel on the surface of the semiconductor layer being in contact with a side surface of the selective oxide film and the channel is formed by the double diffusion on the part deeper than the curve part located on the inlet part of the groove and concurrently the semiconductor layer is turned to be the first type conductivity drain layer by forming the second conductivity type base layer and the first conductivity type source layer. According to the present invention constructed as described above, a specified region of the surface of the low impurity concentration semiconductor layer is removed by etching prior to selective oxidation. In this process, a curve part is formed with the side surface of the etched groove and the surface of the semiconductor layer, and this curve part remains as a curve part even after the subsequent selective oxidation process. To counter this curve part, the channel region is formed in a region deeper than the curve part. As a result, the channel region in which electrons flow through a very thin inversion layer becomes flat, and the disturbance to the electron flow due to the influence of the curve part can be prevented, whereby the manufacturing method for a semiconductor device which can achieve a low ON-state voltage can be obtained. BRIEF DESCRIPTION OF THE DRAWINGS These and other objects, features and characteristics of the present invention will be appreciated from a study of the following detailed description, the appended claims, and drawings, all of which form a part of this application. In the drawings: FIG. 1(a) is a plane view illustrating a part of the vertical type power MOSFET of the first embodiment according to the present invention, and FIG. 1(b) is a cross-sectional view taken along line A-A of FIG. 1(a); FIG. 2 is a view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 3 is a cross-sectional view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 4 is a cross-sectional view illustrating the manufacturing process of an important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 5 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 6 is a plane view illustrating the manufacturing process of an important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 7 is a view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIGs. 8(a) and 8(b) are views illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 9 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 10 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 11 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 12 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 13 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 14 is an enlarged view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 15 is a view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 16 is a view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 17 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 18 is a view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 19 is a view illustrating the manufacturing process of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 20 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 21 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 22 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 23 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 24 is a cross-sectional view illustrating the manufacturing process of the important part of the vertical type power MOSFET of the first embodiment according to the present invention; FIG. 25(a) is a plane view illustrating a part of a conventional power MOSFET, and FIG. 25(b) is a cross-sectional view taken along line A-A of FIG. 25(a); FIG. 26 is a cross-sectional view illustrating the manufacturing process of an important part of the conventional vertical type power MOSFET; FIG. 27 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 28 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 29 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 30 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 31 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 32 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 33 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 34 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 35 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; FIG. 36 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; and FIG. 37 is a cross-sectional view illustrating the manufacturing process of the important part of the conventional vertical type power MOSFET; DETAILED DESCRIPTION OF THE PRESENTLY PREFERRED EXEMPLARY EMBODIMENTS An embodiment according to the present invention will now be described referring to the appended drawings. FIG. 1(a) is a plan view of a vertical type power MOSFET composed of a numerosity of square unit cells according to the first embodiment of the present invention, and FIG. 1(b) is a cross-sectional view taken along line A-A of FIG. 1(a). FIGs. 2 through 24 are descriptive views of the respective stages of the manufacturing method for a vertical type power MOSFET according to the first embodiment, wherein FIG. 4 is a cross-sectional view of a wafer into which boron ions have been implanted to form a central part of a p-type base layer; FIG. 5 is a cross-sectional view of the wafer whose silicon nitride film has been patterned at a pitch which is equal to the dimension of a unit cell a for the LOCOS (localized oxidation of silicon); FIG. 9 is a cross-sectional view of the wafer on which the LOCOS oxide film has been formed; FIG. 10 is a cross-sectional view of the wafer into which boron ions have been implanted to form a p-type base layer by using the LOCOS oxide film as a mask; FIG. 11 is a cross-sectional view of the wafer on which the p-type base has been formed by means of thermal diffusion; FIG. 12 is a cross-sectional view of the wafer into which phosphorous ions have been implanted to form an n + -type source layer by using the LOCOS oxide film as a mask; FIG. 13 is a cross-sectional view of the wafer on which the n + -type source layer has been formed by means of thermal diffusion; FIG. 20 is a cross-sectional view of the wafer on which a gate oxide film has been formed by means of thermal oxidation since the removal of the LOCOS oxide film; FIG. 21 is a cross-sectional view of the wafer with a gate electrode formed on the gate oxide film; FIG. 23 is a cross-sectional view of the wafer into which boron ions have been implanted to form a p + -type base contact layer; FIG. 24 is a cross-sectional view of the wafer on which the p + -type base contact layer has been formed by means of thermal diffusion; and FIG. 1(b) is the cross-sectional view of the completed wafer on which an interlayer insulating film, a source electrode and a drain electrode have been formed. The main part (unit cell part) of the vertical type power MOSFET according to this embodiment is so constructed as illustrated in FIGs. 1(a) and 1(b) in which a numerosity of unit cells 15 are laid out lengthwise and breadthwise with ordination at a pitch (unit cell dimension) of a. In FIGs. 1(a) and 1(b), a wafer 21 is composed of a semiconductor substrate 1 comprising an n + -type silicon having an impurity density of 10 20 cm -3 and a thickness of 100 to 300 µm, an n - -type epitaxial layer 2 having an impurity density of approximately 10 16 cm -3 and a thickness of approximately 7 µm formed on the semiconductor substrate 1, and a unit cell 15 formed on the main surface of the wafer 21. On the main surface of the wafer 21 is formed a LOCOS oxide film to a thickness of approximately 3 µm to form a U-groove 50 at a pitch of the unit cell dimension a (= approximately 12 µm), and by using this oxide film are formed a p-type base layer 16 to a junction depth of approximately 3 µm and an n + -type source layer 4 to a junction depth of approximately 1 µm, whereby a channel 5 is set on the side wall part 51 of the U-groove 50. Incidentally, the p-type base layer 16 is set to a junction depth which does not permit the p-type layer 16 to be fractured due to breakdown at an edge part 12 on the bottom of the U-groove 50. Furthermore, boron ions have been implanted into the central part of the p-type base layer 16 beforehand to deepen the junction depth in the central part of the p-type layer 16 down from the circumference, so that a breakdown can be caused when a high voltage is applied to between the drain electrode and the source electrode. After the double diffusion, the diffusion mask and the LOCOS oxide film used for forming the U-groove 50 are removed, a gate oxide film 8 is formed to a thickness of approximately 60 nm on the inner wall of the U-groove 50, and on the gate oxide film 8 are formed a gate electrode 9 with a polysilicon to a thickness of approximately 400 nm and an interlayer insulating film 18 with a BPSG to a thickness of approximately 1 µm. Furthermore, a p + -type base contact layer 17 is formed to a junction depth of approximately 0.5 µm in the central part surface of the p-type base layer 16, and ohmic contact is made between a source electrode 19 formed on the interlayer insulating film 18 and the n + -type source layer 4 and p + -type base contact layer 17 through contact holes. On the other hand, a drain electrode 20 is formed on the back surface of the semiconductor substrate 1 to make ohmic contact therewith. An example of the manufacturing method according to this embodiment will now be described. A wafer 21 is prepared in which, as illustrated in FIGs. 2 and 3, an n - -type epitaxial layer 2 is formed by means of epitaxial growth on the main surface of the semiconductor substrate 1 having the index of plane of (100). The impurity concentration of this semiconductor substrate 1 is approximately 10 20 cm -3 , and the thickness of the epitaxial layer 2 is approximately 7 µm and the impurity concentration thereof is approximately 10 16 cm -3 . Then, as illustrate in FIG. 4, a field oxide film 60 is formed to a thickness of approximately 60 nm by thermally oxidizing the main surface of the wafer 21, and subsequently a resist film 61 is deposited on the field oxide film 60 and the resist film is patterned to form a pattern opened in the central part of a region where a cell is to be formed by means of the publicly known photolithographic technique. Then, boron (B + ) ions are implanted into the field oxide film 60 by using the resist film 61 as a mask. As illustrated in FIG. 5, after removing the resist, a p-type diffusion layer 62 is formed to a junction depth of approximately 3 µm by means of thermal diffusion. This p-type diffusion layer 62 ultimately constitutes a part of the p-type base layer 16 (described herein later) and plays a role of improving the surge resistance of the device by stably causing a breakdown at the bottom part of the p-type diffusion area when a high voltage is applied to between the drain electrode and the source electrode. Furthermore, as illustrated in FIG. 5, a silicon nitride film 63 is deposited to a thickness of approximately 200 nm on the main surface of the wafer 21. Then, as illustrated in FIG. 6, the silicon nitride film 63 is patterned to be vertical and parallel to the direction <011> to form a lattice-like open pattern opening at a pitch of a dimension of unit cell 15. This open pattern is mask-aligned so that the above p-type diffusion layer 62 can be positioned in the central part of the pitch. The field oxide film 60 is etched by using the silicon nitride film 63 as a mask. Following this, as illustrated in FIG. 7, a chemically active species is made by generating plasma within a discharge chamber 703 containing a carbon tetrafluoride and oxygen gas, and the active species is transported into a reaction chamber 703, and a groove 64 is formed by isogonally applying chemical dry etching to the n - -type epitaxial layer 2 within the reaction chamber 703. In this process, as illustrated in FIG. 8(a), a curve part 709 is formed with the surface of the semiconductor substrate 1 and the groove 64. Then, as illustrated in FIGs. 8(b) and 9, the groove 64 is thermally oxidized by using the silicon nitride film 63 as a mask, which is an oxidizing method well known as the LOCOS (local oxidation of silicon) method. By this oxidation, a LOCOS oxide film 65 is formed and concurrently the U-groove 50 is formed on the surface of the n - type epitaxial layer 2 eroded by the LOCOS oxide film 65, whereby the U-groove shape is fixed, leaving the curve part 709 formed during the chemical dry etching process as a curve part 710 on the side surface of the U-groove 50. In the above process, the conditions of the chemical dry etching and LOCOS oxidation are selected so that the index of plane of the channel forming part on the side surface of the U-groove 50 can be approximated to (111). The inside wall surface 51 of the U-groove 50 formed by the LOCOS oxidation as described above has a high flatness and a few defects, and the surface condition thereof is as good as the main surface of the wafer 21 in the initial stage illustrated in FIG. 2. Now, as illustrated in FIG. 10, boron ions are implanted through the thin field oxide film 60 by using the LOCOS oxide film 65 as a mask to form the p-type base layer 16. As a result, the boundary part between the LOCOS oxide film 65 and the field oxide film 60 constitutes a self-alignment position to exactly define a region into which boron ions are implanted. Then, as illustrated in FIG. 11, thermal diffusion is applied to the field oxide film 60 to a junction depth of approximately 3 µm. As the result of this thermal diffusion, the p-type diffusion layer 62 previously formed in the process illustrated in FIG. 5 and the boron diffusion region into which boron ions have been implanted in the process illustrated in FIG. 10 are integrated into the p-type base layer 16, both ends of which are self-aligned and defined by the positions of the side walls of the U-groove 50. Subsequently, as illustrated in FIG. 12, phosphorous ions are implanted through the thin field oxide film 60 by using a resist film 66 patterned according to the pattern left on the central part of the surface of the p-type base layer 16 surrounded by the LOCOS oxide film 65 formed on the surface of the wafer 21 according to the lattice-like pattern and the LOCOS oxide film 65 as masks to form the n + -type source layer 4. In this process, like the process illustrated in FIG. 9 in which boron ions have been implanted, the boundary part between the LOCOS oxide film 65 and the field oxide film 60 constitutes a self-alignment position, whereby the ion implantation region can exactly be defined. The next process is, as illustrated in FIG. 13, to apply thermal diffusion to a junction depth of 0.5 to 1 µm to form the n + -type source layer 4 and set the channel 5 as well. The junction depth obtained by this thermal diffusion obtained during the previous etching which is set to be deeper than the curve part 710 survived on the side surface of the U-groove 50 through the previous selective oxidation as illustrated in FIG. 14. The end surface being in contact with the U-groove 50 in the region of the n + -type source layer 4 is self-aligned and defined in the position of the side wall of the U-groove 50. The processes illustrated in FIGs. 10 to 13 fix the junction depth and shape of the n-type base layer 16. Here, a point is that the shape of the p-type base layer 16 is perfectly symmetric with respect to the U-groove 50 due to the self-alignment and thermal diffusion. Next, as illustrated in FIG. 15, an inside wall 51 of the U-groove 50 is exposed by removing the LOCOS oxide film 65 by way of applying an end effect to the silicon surface with hydrogen within an aqueous solution containing hydrofluoric acid and adjusted to an acidity/alkalinity of around pH5 with ammonium fluoride. This process should be performed by applying an shading cloth over the wafer 21 to shade the surface with the selective oxide film from light. After the completion of the above process, the wafer 1 is taken out of the aqueous solution and dries in a clean air. Then, as illustrated in FIG. 17, an oxide film is formed on a side surface of the U-groove 50 of the p-type base layer 16 on which the channel 5 is to be formed until the (111) plane is formed. By this thermal oxidation process, the flatness of the surface on which the channel 5 is to be formed is improved in terms of atomic order. As illustrated in FIG. 16, this thermal oxidation process is performed by slowly inserting the wafer 21 into an oxidation furnace 601 which is maintained within the oxygen atmosphere at a temperature of approximately 1000°C. In this thermal oxidation process, as the initial stage of the oxidation is performed at a comparatively low temperature, the scattering of impurities within the p-type base layer region 16 and n + -type source region 4 to the outside of the wafer 21 during the thermal oxidation process can be controlled. Then, an oxide film 600 thus formed is removed as illustrated in FIG. 18. The removal of the oxide film 600 is also performed by way of applying an end effect to the exposed silicon surface with hydrogen within an aqueous solution containing hydrofluoric acid and adjusted to an acidity/alkalinity of around pH5 with ammonium fluoride. The inside wall surface 51 of the U-groove 50 formed by this method is a good silicon surface with a high flatness and a few defects. As illustrated in FIG. 20 this time, a gate oxide film 8 is formed to a thickness of approximately 60 nm on the side surface and bottom surface of the U-groove 50 by thermal oxidation. This thermal oxidation process is, as described above, performed by slowly lowering the wafer 21 into an oxidation furnace 601 which is maintained within the oxygen atmosphere 602 (FIG. 19) and at a temperature of approximately 1000°C. In this thermal oxidation process, as the initial stage of the oxidation is performed at a comparatively low temperature, the scattering of impurities within the p-type base layer region 16 and n + -type source region 4 to the outside of the wafer 21 during the thermal oxidation process can be controlled. The film quality and thickness uniformity of the gate oxide film 8, the interface interfacial sequence density of the interface of the channel 5 and the carrier mobility are as high as those of the conventional DMOS. Now, as illustrated in FIG. 21, the gate electrode 9 is formed by depositing a polysilicon film on the main surface of the wafer 21 to a thickness of approximately 400 nm and patterning the polysilicon film so as to be separated by a distance of c which is shorter by 2β than the distance b between the two U-grooves 50 adjacent to each other. Then, the gate oxide film 8 is further oxidized so as to be thicker at the end part of the gate electrode 9. Here, when the length of a part where the gate oxide film 8 is thickened is x as illustrated in FIG. 22, β is set to be longer than x (β > x). The processes illustrated in FIGs. 10 through 22 are the most important stages of the manufacturing process according to this embodiment, in which the n-type base layer 16, the n + -type source layer 4 and the channel 5 are formed by using the LOCOS oxide film 65 as a mask for self-alignment type double diffusion, then the LOCOS oxide film 65 is removed, and then the gate oxide film 8 and the gate electrode 9 are formed. Next, as illustrated in FIG. 23, boron ions are implanted through an oxide film 67 by using a patterned resist film 68 as a mask to form the p + -type base contact layer 17. Following the above, as illustrated in FIG. 24, the p + -type base contact layer 17 is formed to a thickness of approximately 0.5 µm by thermal diffusion. Then, as illustrated in FIG. 1(b), the interlayer insulating film 18 is formed with a BPSG on the main surface of the wafer 21, and contact holes are made in parts of the interlayer insulating film 18 to expose the p + -type base contact layer 17 and the n + -type source layer 4. Furthermore, the source electrode 19 is formed with an aluminum film, and ohmic contact is made between the source electrode 19 and the p + -type base contact layer 17 and n + -type source layer 4 through the contact hole. Subsequently, a passivation film (not illustrated) is formed with silicon nitride, etc. for protecting the aluminum film by the CVD (chemical vapor deposition) technique or the like. On the back surface of the wafer 21 is formed the drain electrode 20 with three layers of a Ti film, a Ni film and a Au film, an ohmic contact is made between the drain electrode 20 and the n + -type semiconductor substrate 1. According to this embodiment of the present invention, the channel region is formed during the etching process to be deeper than the curve part survived on the groove side surface through the selective oxidation. This flattens the channel region in which electrons flow through the very thin inversion layer, whereby disturbance to the flow of electrons due to the influence of the curve part can be prevented and a low ion voltage can be obtained. In the above embodiment, the lattice-like pattern was used for describing the present invention. The present invention, however, is not limited to the lattice-like pattern, but may be applied to, for example, a stripe-like pattern as well and the same effect as the lattice-like pattern can be obtained. Furthermore, the present invention is not limited to the vertical type MOSFET, but may be applied to other gate construction such as a power MOSIC with an incorporation of the MOSFET referred to in the above and an IGBT (insulated gate bipolar transistor). In addition, in the above embodiment, description was given to only the n-type channel, but needless to say, the present invention can obtain the same effect as the n-type channel by a p-type channel in which the semiconductor type (n type) is replaced with p type.

Claims: A manufacturing method for a semiconductor device comprising the sequential steps of: a) forming a mask (63) having an opening part within a specified region on a main surface of a first conductivity type semiconductor substrate (1, 2); b) forming a groove (64) on said semiconductor substrate having an inlet part wider than said opening part by etching said semiconductor substrate through said opening part; c) forming a selective oxide film (65) on the surface of said groove and between said mask and said semiconductor substrate by selectively thermally oxidizing a region including said groove; d) forming a second conductivity type base layer (16) adjacent a side surface of said groove and extending from said main surface into said semiconductor substrate by diffusing second conductivity type impurities from said main surface; e) forming a first conductivity type source layer (4) by diffusing first conductivity type impurities from said main surface into said base layer so as to form a channel region (5) adjacent said side surface in a region within said base layer; f) removing said selective oxide film (65) to expose said groove (50); g) forming a source electrode (19) that electrically contacts both said source layer and said base layer, forming a gate electrode (9) on a gate oxide film (8) on a surface of said groove, and forming a drain electrode (20) that electrically contacts the other main surface of the semiconductor substrate (1, 2); characterized in that said etching comprises chemical dry etching; a curved part (710) is formed on said side surface of said groove (50) as a result of steps b) and c); said first conductivity type impurities are diffused into said base layer to a region deeper than said curved part such that said channel region (5) is formed in a region deeper than the curved part within said base layer. The manufacturing method for a semiconductor device according to claim 1, wherein, in step c), a part of said selective oxide film is formed between said mask and said semiconductor substrate in such a way that the thickness of said part of said selective oxide film decreases as the distance from said curved part in a direction away from said groove increases. The manufacturing method for a semiconductor device according to claim 1 or 2, wherein step g) comprises forming said gate oxide film by oxidizing an inside wall of said groove and forming said gate electrode on said gate oxide film. The manufacturing method for a semiconductor device according to any one of claims 1 to 3, wherein, in step d), said base layer is formed by diffusing said second conductivity impurities in self-alignment with the selective oxide film used as a mask and, in step e), said source layer is formed by diffusing said first conductivity impurities in self-alignment with said selective oxide film used as a mask.

Inventors: KATAOKA MITSUHIRO, TAKAHASHI SHIGEKI, YAMAMOTO TSUYOSHI, KATAOKA, MITSUHIRO, TAKAHASHI, SHIGEKI, YAMAMOTO, TSUYOSHI
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1995
Main classification: H01L21
IPC codes: H01L21, H01L29