Vertical insulated gate semiconductor device and method of manufacturing the same

Abstract: A P - layer (51) is formed between a P base layer (43) and an N - layer (42) so as to be in contact with the P base layer (43), facing an insulating film (46) of a trench (45) with the N - layer (42) between. In the configuration, a depletion layer extends to the P - layer (51) to relieve an electric field at a tip end portion of the trench (45) and a channel length can be lessened. Therefore, it is possible to provide an insulated gate semiconductor device of high breakdown voltage and low On-resistance.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to an insulated gate semiconductor device and a method of manufacturing the same, and more particularly to a device structure to prevent a reduction in breakdown voltage of a power semiconductor device which has a trench MOS gate where a channel is formed in a surface of a trench buried in a semiconductor substrate and a method of manufacturing the device. Description of the Background Art Fig. 22 is a cross-sectional view of an insulated gate semiconductor device in a background art. Referring to this figure, now a vertical MOS (metal oxide semiconductor) transistor (hereinafter referred to as "UMOS") of trench-gate structure will be discussed as an example. This UMOS is used for low breakdown voltage of 200 V or below, for example, used for an electrical equipment system of automobile. Fig. 22 shows an N + substrate 1, an N - layer 2, a P base layer 3, an N + source region 4, a trench 5, a gate insulating film 6, a gate 7, an interlayer insulating film 8, a source electrode 9, a drain electrode 10 and a channel region 11. Reference character L represents a spacing between the adjacent trenches. Next, an operation of the UMOS will be discussed. When a prescribed drain voltage V DS is applied across the source electrode 9 and the drain electrode 10 and a prescribed gate voltage V GS is applied across the source electrode 9 and the gate 7, the channel region 11 of the P base layer 3 in the vicinity of the gate insulating film 6 is inverted to N type, thus forming a path for electric charges, i.e., a channel. The channel produces continuity between the source and drain, and the resistance of the UMOS produced at this time is called an ON-resistance. In an On state, that is, a state where there is continuity between the source and drain, if the gate voltage V GS applied across the source electrode 9 and the gate 7 is changed to 0 V or a negative voltage, i.e., reverse bias, the gate is turned off and the channel region 11 of N type is inverted again to P type, entering an OFF state where there is no continuity between the source and drain. Thus, the control over the gate voltage V GS makes it possible to control a current flow between the source and drain, and therefore the UMOS can be used as a power switching device. The drain voltage V DS applied to the UMOS when the gate is in the OFF state is referred to as a breakdown voltage of a device. The breakdown voltage of a device is generally determined by an impurity concentration and thickness of the N - layer 2 and in the case of the UMOS, further depends on a structure of a surface region of the device. Especially in the UMOS of trench structure, since the trench 5 protrudes into the N - layer 2 through the P base layer 3, the breakdown voltage of the device is dependent on the condition of a tip portion of the trench 5 protruding into the N - layer 2. Fig. 23 is a simulation graph showing an electric field of the UMOS of trench structure. In the simulation, it is assumed that the impurity concentration of the N - layer 2 is 1 Ωcm and the thickness from a surface of the P base layer 3 to a bottom surface of the N - layer 2 is 8.5 µm. The upper-left rectangular portion of Fig. 23 represents a portion of trench structure. As can be seen form Fig. 23, when the drain voltage V DS is applied to the UMOS, a depletion region extends from the P base layer 3 to the N - layer 2, but the equipotential lines form loops to be discontinuous at the portion where the trench 5 protrudes into the N - layer 2, indicating that the electric field becomes stronger at the tip end portion of the trench 5. The breakdown voltage of the device depends on the electric field strength at the tip end portion. The breakdown voltage of the device, which is dependent on a condition of the tip end portion of the trench 5, undergoes a more strict situation when the gate voltage V GS is changed into the reverse bias. Specifically, the sum of the drain voltage V DS applied to the UMOS and the reverse-biased gate voltage V GS is applied to the tip portion of the trench 5 protruding into the N - layer 2. Accordingly, the breakdown voltage of the device when the gate voltage V GS is reversely biased is lower by a reverse-bias of the gate voltage V GS as compared with the case where the gate voltage V GS is 0 V. As a countermeasure for reduction in breakdown voltage of the UMOS having trench structure, for example, a proposed device structure is disclosed in US 5,072,266. Fig. 24 is a cross-sectional perspective view of the device disclosed in US 5,072,266. The device has a structure in which a central portion of the P base layer 3 extends deeper than the trench 5 does as shown in Fig. 24. Other structure of the device is the same as that of the background-art UMOS of Fig. 22. When the drain voltage V DS is applied to the UMOS of Fig. 24, the depletion region extends from the P base layer 3 to the N - layer 2 in the same manner as the UMOS of Fig. 22. In the UMOS of Fig. 24, however, since the central portion of the P base layer 3 extends deeper than the trench 5, the equipotential lines form fewer loops at the tip end portion of the trench 5 and a gradual change of the equipotential lines can be found, from those which extend to an adjacent trench to those which enclose the trench 5. Accordingly, the discontinuity of the depletion region at the tip end portion of the trench 5 is relieved and the reduction in breakdown voltage of the device at the tip end portion of the trench 5 can be suppressed. Furthermore, since the distance from the central portion of the P base layer 3 to the N + substrate 1 is shorter than that from the tip end portion of the trench 5 to the N + substrate 1, a breakdown occurs at the central portion of the P base layer 3 and the breakdown voltage is dependent on the central portion of the P base layer 3. Accordingly, even if the gate voltage V GS is reversely biased, it is possible to keep the breakdown voltage of the device from becoming lower by the reverse bias of the gate voltage V GS . To ensure the required breakdown voltage of the device, however, in the device structure of the UMOS as shown in Fig. 24, the distance from the central portion of the P base layer 3 to the N + substrate 1 should be long, that is, the N - layer 2 should be thick, since the central portion of the P base layer 3 extends deeper than the trench 5. In the case of the UMOS having a trench structure, a current path is found from the tip end portion of the trench 5 through the channel region 11 to the drain electrode in the ON state, and therefore the N + layer 2 of greater thickness causes a higher On-resistance. In the device structure of the UMOS as shown in Fig. 24, the trench 5 extends deeper than the P base layer 3 at a portion where the trench 5 and the P base layer 3 come into contact although the central portion of the P base layer 3 extends deeper than the trench 5. In order to form the P base layer 3, which is generally obtained by diffusion, the distance between the adjacent trenches 5 requires, at least, the sum of the length of an opening from which the impurity is implanted and twice the depth of the central portion of the P base layer 3, because the impurity is diffused over the same range in vertical and horizontal directions. An evaluation of the spacing between the adjacent trenches on the basis of a commonly-used device size, assuming that the length of the opening at the central portion of the P base layer 3 is 2 µm, the depth of the trench is 2 µm and the depth of the central portion of the P base layer 3 is 3 µm, determines the spacing L between the trenches to be about 8 µm. As the spacing L between the trenches becomes larger than 8 µm, the ON-resistance of the UMOS becomes higher. A test reveals that the ON-resistance becomes higher by about 0.18 Ωcm 2 as a cell pitch (the spacing between the central portions of the adjacent trenches) becomes wider by 1 µm. Document EP-A-0 345 380, on which the preamble of claim 1 is based discloses a conventional insulated gate semiconductor device with a semiconductor substrate and a first semiconductor layer disposed on the semiconductor substrate, wherein at the surface at least one trench is provided reaching to the surface of the semiconductor substrate in a vertical direction. Over an inside wall of the trench a first insulating film is provided and a conductor is disposed inside the trench which is electrically connected to a control electrode. A further semiconductor layer is disposed in a portion of the surface of the first semiconductor layer next to the insulating film of the trench. A second insulating layer covers the surface of the conductor and a portion of the surface of the second semiconductor layer. Finally, first and second main electrodes are disposed at the main surfaces of the semiconductor device. In "IEEE ED-34, 11; pp. 2329-2334" a conventional self-aligned U-MOSFET is disclosed. According to this document a self-aligned process for producing a plurality of closely-spaced vertical trench gates is known, thereby increasing the channel density and reducing the on-resistance pair unit area. In detail, a base and a source layer are provided on a substrate, while the whole structure includes a plurality of vertical trenches serving as a gate and extending into the substrate. SUMMARY OF THE INVENTION It is an object of the present invention to provide a semiconductor device and a method of manufacturing the same showing an improved breakdown voltage. According to the invention this object is achieved by independent claims 1 and 5. The dependent claims 2 to 4 and 6 to 8 describe particular embodiments of the invention. Since the third semiconductor layer of the second conductivity type, the impurity concentration of which is lower than that of the first semiconductor layer, is disposed between the first major surface of the semiconductor substrate and the first semiconductor layer, the depletion layer extends to the third semiconductor layer to relieve the electric field concentration at the tip end portion of the trench when a voltage is applied across the first main electrode and the second main electrode, and therefore an insulated gate semiconductor device of high breakdown voltage can be provided. Moreover, since the third semiconductor layer is disposed facing the trench with a portion of the semiconductor substrate between, the channel region of short length is formed on the first semiconductor layer and accordingly the ON-resistance can be reduced. Therefore, it is possible to provide an insulated gate semiconductor device of high breakdown voltage and low power consumption. Since the fourth semiconductor layers adjoin a plurality of trenches along both the sides of the opening and the two fourth semiconductor layers existing between the adjacent trenches sandwich the first semiconductor layer, a plurality of cell regions are formed and therefore an element of large current capacity can be provided. Since a plurality of fifth semiconductor layers are arranged in parallel, intersecting the openings of a plurality of trenches, a large number of cell regions are obtained and therefore an element of large current capacity can be provided. Since the method of manufacturing the insulated gate semiconductor device comprises the step of implanting and diffusing the impurity of the second conductivity type in low concentration in the major surface of the semiconductor substrate to form the first semiconductor layer and the step of implanting and diffusing the impurity of the second conductivity type in higher concentration in the surface of the first semiconductor layer, the first semiconductor layer of the second conductivity type, the impurity concentration of which is lower than that of the second semiconductor layer, is formed between the major surface of the semiconductor substrate and the second semiconductor layer. Therefore an insulated gate semiconductor device of high breakdown voltage can be provided. Since the method of manufacturing the insulated gate semiconductor device further comprises, after the step of removing the semiconductor, the step of forming the oxide film over the inside wall of the trench to remove the impurity of the second conductivity type in the surface region of the inside wall of the trench in the first semiconductor layer and the step of removing the oxide film, the third semiconductor layer can be so disposed as to be opposed to the trench with a portion of the semiconductor substrate therebetween. Therefore, it is possible to provide an insulated gate semiconductor device of high breakdown voltage and low power consumption. Since the method of manufacturing the insulated gate semiconductor device comprises the implantation step using the resist pattern with a plurality of third openings arranged in parallel and the removing step using the shield film with a plurality of fourth openings of narrower stripe shape positioned correspondingly to the third openings along the direction of the length, the second semiconductor layer is so formed as to extend along the both sides of a plurality of trenches and two second semiconductor layers between the adjacent trenches sandwich the first semiconductor layer. Therefore, an insulated gate semiconductor device of large current capacity, consisting of elements of high current density, can be provided. Since the method of manufacturing the insulated gate semiconductor device comprises the implantation step using the resist pattern with a plurality of third openings of stripe shape arranged at prescribed pitches and the removing step using the shield film with a plurality of fourth openings of stripe shape in such stripe shapes as to intersect the third openings, it is possible to form the second semiconductor layer including regions which extend in parallel, intersecting the trenches. Therefore, an insulated gate semiconductor device having a cell configuration of high density can be provided. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a plan view of an insulated gate semiconductor device in accordance with a first preferred embodiment of the present invention; Fig. 2 is a fragmentary plan view of cells in the insulated gate semiconductor device of Fig. 1; Fig. 3 is a fragmentary cross-sectional view taken along the section of A-A of Fig. 2; Fig. 4 is a graph showing an electric field when a drain voltage is applied to the insulated gate semiconductor device of the present invention; Figs. 5 to 14 are fragmentary cross-sectional views each showing a device in each step of a process of manufacturing the insulated gate semiconductor device of the present invention; Fig. 15 is a fragmentary cross-sectional view of an insulated gate semiconductor device in accordance with a second preferred embodiment of the present invention; Fig. 16 is a fragmentary cross-sectional view of a device in a process of manufacturing the insulated gate semiconductor device of the present invention; Fig. 17 is a fragmentary plan view of an insulated gate semiconductor device in accordance with a third preferred embodiment of the present invention; Fig. 18 is a fragmentary cross-sectional view of the insulated gate semiconductor device taken along the section of A-A of Fig. 17; Fig. 19 is a fragmentary cross-sectional view of the insulated gate semiconductor device taken along the section of B-B of Fig. 17; Fig. 20 is a fragmentary cross-sectional view of an insulated gate semiconductor device in accordance with a fourth preferred embodiment of the present invention; Fig. 21 is a fragmentary cross-sectional view of the insulated gate semiconductor device taken along another section of Fig. 20; Fig. 22 is a cross-sectional view of an insulated gate semiconductor device in the background art; Fig. 23 is a graph showing an electric field when a drain voltage is applied to the insulated gate semiconductor device in the background art; and Fig. 24 is a fragmentary cross-sectional perspective view of the insulated gate semiconductor device in the background art. DESCRIPTION OF THE PREFERRED EMBODIMENTS < The First Preferred Embodiment > Fig. 1 is a plan view of an insulated gate semiconductor device in accordance with the first preferred embodiment of the present invention. A UMOS will be discussed as an example of the insulated gate semiconductor device. Fig. 2 is a fragmentary plan view of cells of the UMOS of Fig. 1, and Fig. 3 is a fragmentary cross-sectional view taken along the section of A-A of Fig. 2. In Fig. 2, a source electrode 49 and an interlayer insulating film 48 of Fig. 3 are omitted. A UMOS 30 as shown in Fig. 1 comprises a source electrode 31 as a first main electrode, a gate interconnection 32, a gate pad 33 as a control electrode and a fragment 34 of the cells. Figs. 2 and 3 show a semiconductor substrate consisting of an N + substrate 41 and an N - layer 42, a P base layer 43 as a first semiconductor layer, an N + source region 44 as a second semiconductor layer, a trench 45, an insulating film 46, a polysilicon 47 as a conductor, an interlayer insulating film 48 as a insulating layer, a source electrode 49 as a first main electrode which is part of the source electrode 31 of Fig. 1, a drain electrode 50 as a second main electrode, a P - layer 51 as a third semiconductor layer and a channel region 52. The insulating film 46 is generally a thermal oxide film formed of SiO 2 , and the polysilicon 47 is doped with an impurity to serve as a gate. The interlayer insulating film 48 is formed of a silicate glass (hereinafter referred to as "BPSG") containing boron and phosphorus, the source electrode 49 is formed of a silicon-containing aluminum (hereinafter referred to as "Al-Si) and the drain electrode 50 is formed of TiNiAu. The gate interconnection 32, which is connected to the gate of the cell, serves to reduce polysilicon portions in a path from the gate to the gate pad thus suppressing the electrical resistance between the gate and the gate pad, and further makes it possible to uniformly perform a control operation over the device on the whole. In the UMOS of the first preferred embodiment, the N - layer 42 is formed on a surface of the N + substrate 41, the P - layer 51 is formed on the N - layer 42 and further the P base layer 43 is formed on the P - layer 51. The N + source regions 44 of stripe shape are formed on a surface of the P base layer 43, being spaced. Along the direction of the length of the stripe-shaped N + source region 44, the trench 45 is provided, extending in a vertical direction from the surface of the N + source region 44 to the N - layer 42. The insulating film 46 is formed over the inside wall of the trench 45 and the polysilicon 47 fills the trench 45 up to the opening portion at the surface of the N + source region 44. Between the adjacent trenches 45, there exists two N + source regions 44 which adjoin the trenches 45 respectively and an exposed portion of the P base layer 43 between the N + source regions 44. The surface of the polysilicon 47 is covered with the interlayer insulating film 48. Over the surface of the device where the N + source regions 44 and the P base layer 43 are found, the source electrode 49 is provided so that the N + source region 44 and the P base layer 43 may be short circuited and the gate interconnection 32 connected to the polysilicon 47 and the gate pad 33 are further provided. The drain electrode 50 is provided on the other surface of the N + substrate 41. For example, a device with a breakdown voltage of 60 V class has sizes as follows: a cell pitch is about 5 µm when the spacing between the adjacent trenches is about 3 µm, the thickness from the surface of the P base layer 43 to the bottom surface of the N - layer 42 is about 8.5 µm and the depth of the trench 45 is 2 to 3 µm. The tip portion of the trench 45 extends into the N - layer 42 at a depth of about 0.5 µm, and the N + substrate 41 has a thickness of about 340 µm. As (arsenic) is generally used as an N-type impurity. Now, an operation of the UMOS will be discussed. When a prescribed drain voltage V DS is applied across the source electrode 49 and the drain electrode 50 and a prescribed gate voltage V GS is applied across the source electrode 49 and the gate 47, the insulating film 46 serves as a gate insulating film and the channel region 52 in the P base layer 43 and the P - layer 51 is inverted to N type in the vicinity of the insulating film 46, thus providing a path for electric charges, i.e., a channel. The channel produces continuity between the source and drain. In the On state, if the gate voltage V GS applied across the source electrode 49 and the gate 47 is changed to 0 V or reversely biased, the gate is turned off and the channel region 52 of N type is inverted again to P type, entering an OFF state where there is no continuity between the source and drain. When the drain voltage V DS is applied in the OFF state, few depletion layer extends to the P base layer 43 without the P - layer 51. In this case, however, since the P - layer 51 is provided between the P base layer 43 and the N - layer 42, the depletion layer extends even to the P - layer 51. The depletion layer extending to the P - layer 51 stretches the potential distribution at the tip end portion of the trench 45 to relieve an electric field concentration, thus increasing the breakdown voltage of the device. Fig. 4 is a graph showing a result of a simulation of the electric field in the UMOS of the first preferred embodiment when the drain voltage is applied thereto. In the simulation, it is assumed that the impurity concentration of the N - layer 42 is 1 Ωcm and the thickness from the surface of the P base layer 43 to the bottom surface of the N - layer 42 is 8.5 µm. The upper-left rectangular portion of Fig. 4 represents a portion of trench structure. As compared with Fig. 23 showing the electric field in the background art, it can be seen from Fig. 4 showing the electric field in the first preferred embodiment that the depletion layer extends to the P - layer 51 and the electric field is stretched in a horizontal direction to relieve the electric field concentration at the tip end portion of the trench 45. The simulation reveals that the breakdown voltage of the device rises to 98 V with P - layer 51, while it is 82 V without the P - layer 51. Moreover, since the above structure has no need to widen the spacing between the adjacent trenches 45 and makes it possible to narrow the spacing as far as the manufacturing process permits, the ON-voltage can be suppressed. Furthermore, having an arrangement where the N + source region 44 is so disposed as to be adjacent to the trench 45 with the insulating film 46 between and extend along the trench 45 in the direction of its length, the device of the first preferred embodiment has a broadened gate width and a high current density. Next, an exemplary method of manufacturing the UMOS of the first preferred embodiment will be discussed. Figs. 5 to 14 are fragmentary cross-sectional views each showing a device in each step of a process of manufacturing the insulated gate semiconductor device of the present invention. First, an N + substrate 60 is prepared and an N - layer 61 is formed thereon (see Fig. 5). Next, a P-type impurity is implanted into the N - layer 61 and diffused therein to form a P - layer 62 (see Fig. 6). The P-type impurity is further implanted into a surface of the P - layer 62 and so diffused as to come into contact with the N - layer 61 and keep a prescribed thickness of the P - layer 62, thus forming a P base layer 63 (see Fig. 7). A resist is layered over a surface of the P base layer 63 and then a photolithographic process is performed to form a resist pattern 64 having a plurality of stripe-shaped openings arranged in parallel as first openings. An N-type impurity is implanted in high concentration to the surface of the P base layer 63 through the resist pattern 64 as a mask and diffused therein to form N + source regions 65 (see Fig. 8). After that, an oxide film 66 is formed over the surfaces of the P base layer 63 and the N + source regions 65 as a shield film, and a silicon etching mask is formed of the oxide film 66, having stripe-shaped openings on the surfaces of the N + source regions 65 as second openings each of which is narrower in width than each N + source region 65. An etching is performed through RIE (Reactive Ion Etching) by using the silicon etching mask to form trenches 67 each of which extends from the surface of the N + region 65 to the N - layer 61 (see Fig. 9). Next, a thermal oxide film is formed over the surfaces of the trenches 67. A polysilicon 69 doped with the P-type impurity is layered on the oxide film over the surfaces of the P base layer 63, the N + source regions 65 and the trenches 67, to fill the trenches 67. (see Fig. 10). The layered polysilicon 69 is etched back to the opening portion of the trench 67 so as to leave the polysilicon 69 filling the trench 67 (see Fig. 11). After that, a BPSG 70 is layered over the surface of the oxide film 68 formed over the surfaces of the P base layer 63 and the N + source regions 65 and over the surface of the polysilicon 69 buried in the trench 67 (see Fig. 12). Next, a resist 71 is layered over the BPSG 70 and then a photolithographic process is performed to form a resist pattern having stripe-shaped openings arranged in parallel with the trench 67 to cover the P base layer between the adjacent trenches 67 and part of the N + source regions 65. The BPSG 70 and the oxide film 68 are etched by using the resist pattern as a mask, to form an interlayer insulating film 70 over the surface of the polysilicon 69 buried in the trench 67. (see Fig. 13). An Al-Si 72 is layered over the surface of the device where the P base layer 63, the N + source regions 65 and the interlayer insulating film 70 are found so that an exposed portion of the P base layer 63 which is made by etching and the N + source region 65 may be short circuited. At the same time, a sdurce- electrode and a gate interconnection connected to the polysilicon 69 and a gate pad are formed (see Fig. 14). Further, an drain electrode is formed over the other surface of the N + substrate 60. < The Second Preferred Embodiment > Fig. 15 is a fragmentary cross-sectional view of a UMOS as an example of in accordance with the second preferred embodiment of the present invention. In Fig. 15, the P - layers 51 is opposed to the insulating film 46 of the trench 45 with the N - layer 42 therebetween. Other structural features are the same as those of the UMOS of the first preferred embodiment. As discussed with respect to the operation in the first preferred embodiment, the prescribed drain voltage V DS is applied across the source electrode 49 and the drain electrode 50 and the prescribed gate voltage V GS is applied across the source electrode 49 and the gate 47, the P-type semiconductor layer in vicinity of the insulating film 46 is inverted to N type thus forming a path for electric charges, i.e., a channel that produces continuity between the source and drain. Specifically, the P base layer 43 and the P - layer 51 are inverted to N type as the channel region 52 to form a channel serving as a path for electric charges in the first preferred embodiment. In the second preferred embodiment, only the P base layer 43 is inverted to N type to serve as the channel region 52 and accordingly the channel region 52 becomes shorter, not including the P - layer 51, the impurity concentration of which is lower than that of the P base layer 43. Therefore, the ON-resistance decreases by the P - layer 51 of lower impurity concentration not included in the channel region 52 as compared with the UMOS of the first preferred embodiment. Fig. 16 is a fragmentary cross-sectional view of a UMOS in a manufacturing process in accordance with of the second preferred embodiment. In a method of manufacturing the UMOS in the second preferred embodiment, a certain step is added to the method of manufacturing the UMOS in the first preferred embodiment. Specifically, the manufacturing process goes in a like manner until the step of forming the trench 67 as shown in Fig. 9, and subsequently an oxidation step is performed after removing the silicon etching mask, to draw a P-type impurity from the surface of the P - layer 62 which is exposed in the side wall of the trench 67 into an oxide film. An etching is performed after the oxidation step to remove the oxide film which includes the P-type impurity. By repetition of the oxidation step and the etching step, a portion 80 adjacent to the trench 67, which is formerly part of the P - layer 62, is cleared of the P-type impurity to become the same as the N - layer 61 which does not experience the P-type impurity implantation (see Fig. 16). Subsequently, in the same manner as the method of manufacturing the UMOS of the first preferred embodiment, the thermal oxide film is formed over the surfaces of the trenches 67, and then the polysilicon 69 doped with the P-type impurity is layered on the oxide film 68 over the surfaces of the P base layer 63, the N + source regions 65 and the trenches 67, to fill the trench 67. The following steps are the same as those in the first preferred embodiment. < The Third Preferred Embodiment > Fig. 17 is a fragmentary plan view of a UMOS in accordance with the third preferred embodiment of the present invention. Fig. 18 is a fragmentary cross-sectional view taken along the section of A-A shown in Fig. 17, while Fig. 19 is a fragmentary cross-sectional view taken along the section of B-B shown in Fig. 17. In Fig. 17, the source electrode 49 and the interlayer insulating film 48 of Figs. 18 and 19 are omitted. In the first and second preferred embodiments, the N + source region 44 is so disposed as to be adjacent to the trench 47 and extend along the trench 47 in the direction of its length. In the third preferred embodiment, the N + source region 44 intersects the trench 47 and is arranged alternately with the exposed face of the P base layer 43. As shown in Figs. 18 and 19, the P - layer 51 is disposed between the P base layer 43 and the N - layer 42. The N + source region 44 is disposed on the surface region of the P base layer 43 and the exposed P base layers 43 are contiguous to each other with the P base layer 43 under the N + source region 44. This structure allows the trenches 45 to be densely arranged in a configuration of a plurality of cells, as well as a higher breakdown voltage like the first preferred embodiment. In the case where the N + source region 44 and the trench 45 are disposed in parallel with each other, since the polysilicon 47 in the trench 45 is covered with the interlayer insulating film 48 and there is a need to establish connection of the source region 49 formed on the interlayer insulating film 48 with the N + source region 44 and the P base layer 43, the two N + regions 44 between the adjacent trenches 45 should be so disposed as to sandwich the P base layer 43. The N + source region 44, which is usually formed by diffusion, needs a width equivalent to the diffused layer depth. Moreover, an extra width, e.g., of about 0.5 to 1 µm in each side of the trench for mask alignment is required in forming the interlayer insulating film 48 to cover the polysilicon 47 in the trench 45 over the exposed surface of the N + source regions 44 since there arises a need to establish a short circuit between the N + source region 44 and the P base layer 43 in the later step. For these reasons, it is difficult to reduce the width of the N + source region 44, and the N + source region 44 of larger width imposes a restriction on reduction in the spacing L between the trenches, in consideration of the manufacturing process. In the third preferred embodiment, however, since the N + source region 44 intersects the trenches 47 and is disposed alternately with the P base layer 43, there is no need for an arrangement where the N + source regions 44 between the adjacent trenches 47 should sandwich the exposed P base layer 43 and consequently the spacing L between the trenches can be sufficiently reduced. Therefore, the cells can be densely arranged to achieve a size reduction of the device. The process of manufacturing the UMOS of the third preferred embodiment is the same as shown in association with the UMOS of the first preferred embodiment. < The Fourth Preferred Embodiment > Fig. 20 is a fragmentary cross-sectional view of a UMOS in accordance with the fourth preferred embodiment of the present invention. Fig. 21 is a fragmentary cross-sectional view taken a section including the N + source region 44. A plan view of the same portion should be like Fig. 17 in association with the third preferred embodiment. Fig. 20 is a fragmentary cross-sectional view taken along the section of A-A shown in Fig. 17, while Fig. 21 is a fragmentary cross-sectional view taken along the section of B-B shown in Fig. 17. The UMOS of the fourth preferred embodiment has an arrangement where the N + source region 44 intersects the trench 47 and is disposed alternately with the exposed face of the P base layer 43, and moreover, the P - layer 51 is opposed to the insulating film 46 of the trench 45 with the N - layer 42 therebetween. Other structural features are the same as those of the third preferred embodiment. This structure allows the cells to be densely arranged, keeping the ON-resistance low, as well as a higher breakdown voltage, to achieve a size reduction of the device. The process of manufacturing the UMOS of the fourth preferred embodiment is the same as shown in association with the UMOS of the second preferred embodiment. Although an N-channel UMOS has been discussed above, the present invention may be naturally applied to an P-channel UMOS. While the invention has been shown and described in detail, the foregoing description is in all aspects illustrative and not restrictive. It is therefore understood that numerous modifications and variations can be devised without departing from the present claims.

Claims: An insulated gate semiconductor device, comprising: a semiconductor substrate (41, 42) of a first conductivity type, having a first major surface and a second major surface; a first semiconductor layer (43) of a second conductivity type disposed on said first major surface of said semiconductor substrate (41, 42), having a surface and at least one trench (45) with an opening at said surface reaching said first major surface of said semiconductor substrate (41, 42) in a direction vertical to said surface of said first semiconductor layer; a first insulating film (46) formed over an inside wall of said at least one trench; a conductor (47) disposed inside said at least one trench, being opposed to said first semiconductor layer (43) with said first insulating film (46) therebetween, and electrically connected to a control electrode; a second semiconductor layer (44) of the first conductivity type disposed in a portion of said surface of said first semiconductor layer (43) being opposed to said conductor (47) with said insulating film (46) therebetween; a second insulating layer (48) so disposed as to cover a portion of a surface of said second semiconductor layer (44) and a surface of said conductor (47); a first main electrode (49) disposed on said surfaces of said first and second semiconductor layers (43, 44); and a second main electrode (50) disposed on said second major surface of said semiconductor substrate (41, 42), characterized in that a third semiconductor layer (51; 62) of the second conductivity type is disposed between said first major surface of said semiconductor substrate (41, 42) and said first semiconductor layer (43), the impurity concentration of which is lower than that of said first semiconductor layer (43). The insulated gate semiconductor device of claim 1, characterized in that said third semiconductor layer (51) is opposed to said at least one trench with a portion (53; 80) of said semiconductor substrate therebetween. The insulated gate semiconductor device of claim 1 or 2, characterized in that said at least one trench (45) includes a plurality of trenches, said second semiconductor layer (44) includes a plurality of fourth semiconductor layers adjoining said plurality of trenches along said both sides of said opening, and two of said fourth semiconductor layers existing between adjacent ones of said plurality of trenches are arranged so that said first semiconductor layer (43) may be laid between said two fourth semiconductor layers. The insulated gate semiconductor device of claim 1 or 2, characterized in that said at least one trench (45) includes a plurality of trenches, and said second semiconductor layer (44) includes a plurality of fourth semiconductor layers which extend in parallel with each other, intersecting openings of said plurality of trenches. A method of manufacturing an insulated gate semiconductor device, comprising the steps of: (a) preparing a semiconductor substrate (60, 61) of a first conductivity type which has a first major surface and a second major surface; (b) implanting and diffusing an impurity of a second conductivity type in low concentration in said first major surface of said semiconductor substrate to form a third semiconductor layer (62); (c) further implanting and diffusing an impurity of the second conductivity type in higher concentration than that of said step (b) in a surface of said third semiconductor layer (62) to form a first semiconductor layer (63); (d) forming a resist pattern over a surface of said first semiconductor layer (63) with at least one first opening made by removing a portion of said surface of said first semiconductor layer (63); (e) still further implanting and diffusing an impurity of the first conductivity type in said surface of said first semiconductor layer (63) through said at least one first opening by using said resist pattern as a mask, to form a second semiconductor layer (65); (f) forming a shield film (66) over said surface of said first semiconductor layer (63) and said second semiconductor layer (65) with at least one second opening to define a portion of said surface of said second semiconductor layer (65); (g) removing a semiconductor in a vertical range from said surface of said second semiconductor layer (65) to said semiconductor substrate (60, 61) through said at least one second opening by using said shield film (66) as a mask, to form at least one trench (67); (h) forming an insulating film (68) over a surface of said at least one trench (67); (i) layering a conductor (69) over said insulating film (68) to fill said at least one trench (67); (j) removing said conductor (69) to be left in said at least one trench flatly at an opening portion thereof; (k) forming an insulating layer (70) over said surfaces of said first and second semiconductor layers (63, 65) and a surface of said conductor buried in said at least one trench (67); (l) forming a resist pattern (71) over said insulating layer (70) with an opening on said surface of said first semiconductor layer (63) and a portion of said surface of said second semiconductor layer (65); (m) removing said insulating layer (70) through said opening of said resist pattern (71) by using said resist pattern as a mask; (n) layering a conductor (72) over said surface of said first semiconductor layer (63) and said portion of said surface of said second semiconductor layer (65) which are exposed by said step (m) to form a first main electrode; (o) layering a conductor over said second major surface of said semiconductor substrate (60, 61) to form a second main electrode; and (p) forming a control electrode to be electrically connected to said conductor buried in said at least one trench (67). The method of manufacturing the insulated gate semiconductor device of claim 5, wherein said at least one first opening of said resist pattern formed in said step (d) includes a plurality of third openings of stripe shape arranged in parallel with each other; and said at least one second opening of said shield film (66) formed in said step (f) includes a plurality of fourth openings disposed on said second semiconductor layers (65) of stripe shape which are formed through said third openings in said step (e), along the direction of the length in such stripe shapes as to be narrower than said second semiconductor layers (65). The method of manufacturing the insulated gate semiconductor device of claim 5, wherein said at least one first opening of said resist pattern formed in said step (d) includes a plurality of third openings of stripe shape arranged in parallel with each other at prescribed pitches; and said at least one second opening of said shield film (66) formed in said step (f) includes a plurality of fourth openings disposed on said second semiconductor layers (65) which are formed through said third openings in said step (e), in such stripe shapes as to intersect said second semiconductor layers (65). The method of manufacturing the insulated gate semiconductor device of any of claims 5 to 7, further comprising, after said step (g) before said step (h), the steps of: (q) forming an oxide film over an inside wall of said at least one trench (67) to remove said impurity of the second conductivity type existing in a surface region of said inside wall of said at least one trench (67) in said third semiconductor layer (62); and (r) removing said oxide film formed over said inside wall of said at least one trench (67).

Inventors: TAKAHASHI HIDEKI, TAKAHASHI, HIDEKI, Takahashi, Hideki, c/o Mitshubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1995
Main classification: H01L29
IPC codes: H01L29, H01L21