Semiconductor device including insulated gate bipolar transistor and method of fabricating the same

Abstract: A high-concentration n-type buffer layer (12) and a low-concentration n-type buffer layer (11) are provided between a p-type collector layer (19) and a high-resistance n-type base layer (1), and respective impurity concentrations of the low-concentration n-type buffer layer (11) and the high-concentration n-type buffer layer (12) are so set that concentrations of carriers propagated through the low-concentration n-type buffer layer (11) and the high-concentration n-type buffer layer (12) are in excess of the respective impurity concentrations thereof in an ON state. Thus, an insulated gate bipolar transistor having excellent withstand voltage, ON-state voltage and turn-off characteristics is obtained.

Description: The present invention relates to an insulated gate bipolar transistor (IGBT) according to the preamble of claim 1 and a method of fabricating the same. Fig. 17 is a sectional view showing the sectional structure of a conventional transversal IGBT. This IGBT is fabricated as follows: First, an impurity is diffused in a part of an n-type base layer 1 having a relatively low impurity concentration, to form an n-type buffer layer 2 having a relatively high impurity concentration. Then, a gate insulating film 24 and a polysilicon film 3 serving as a gate are selectively formed on the n-type base layer 1. Then, the polysilicon film 3 is employed as a mask for depositing and thereafter diffusing an impurity from the left side in the figure into an upper layer part of the n-type base layer 1, for forming a p-type base layer 4. At the same time, a p-type collector layer 19 for serving as collector is selectively formed on a surface of the n-type buffer layer 2 by diffusion. Then, an n-type emitter layer 17 for serving as an emitter is formed on a surface of the p-type base layer 4. Fig. 19 is a plan view showing the plane structure of the conventional transversal IGBT. A section taken along the line A - A in Fig. 19 corresponds to Fig. 17. As shown in Fig. 19, the p-type base layer 4, the n-type emitter layer 17, the gate electrode 3, the n-type base layer 1, the n-type buffer layer 2 and the p-type collector layer 19 have elliptic plane shapes respectively, so that the n-type emitter layer 17 is formed inside the p-type base layer 4, and the gate electrode 3, the n-type base layer 1, the n-type buffer layer 2 and the p-type collector layer 19 are successively formed inside the n-type emitter layer 17 in a similar manner to the above. Finally, an emitter electrode 20, a gate electrode 21 and a collector electrode 22 are formed on parts of the p-type base layer 4 and the n-type emitter layer 17, the polysilicon film 3, and the p-type collector layer 19 respectively. Thus, the transversal IGBT is formed as shown in Fig. 17. In sectional views which are hereinafter referred to, one-dot chain lines may be drawn along centers of p-type collector layers 19, to indicate that the p-type collector layers 19 are planarly positioned at the centers. According to the conventional IGBT structure, the gate electrode 21 and the emitter electrode 20 are grounded, and the n-type buffer layer 2 suppresses spreading of a depletion layer when a positive voltage is applied to the p-type collector layer 19. When a voltage which is positive with respect to the emitter electrode 20 is applied to the gate electrode 21, an n-type channel is formed on a surface of the p-type base layer 4 under the gate 3, and electrons flow into the p-type collector layer 19 through the n-type base layer 1. At this time, holes are injected from the p-type collector layer 19, in order to satisfy a charge neutrality condition. Thus, conductivity modulation is increased and an ON-state voltage is reduced as compared with a power MOSFET. Switching loss is generally expressed by the product of an ON-state voltage and a turn-off time. In order to reduce power consumption, therefore, desired is an IGBT having a low ON-state voltage and a short turn-off time for reducing switching loss. An IGBT which is expected as a high withstand voltage element has an important object of improvement in withstand voltage as a matter of course, and IGBTs which are aimed at improving withstand voltages have been announced in various structures, while these IGBTs have such disadvantages that absolutely no consideration is made on ON-state voltages. In a power IC, it is necessary to form a low withstand voltage element such as a CMOS element for forming a logic circuit on the same substrate as a high withstand voltage element such as an IGBT. Fig. 18 is a sectional view showing the sectional structure of an n-channel MOSFET. First, a p-type well diffusion layer 7 is formed in the same n-type base layer 1 as a high withstand voltage element. A gate insulating film 23 and a polysilicon film 8 serving as a gate are selectively formed on a surface of the p-type well diffusion layer 7. Then, the polysilicon film 8 is employed as a mask to form n-type diffusion layers 9 and 10 of relatively high concentrations for serving as source and drains on both sides of the polysilicon film 8 on the surface of the p-type well diffusion layer 7. Then, a drain electrode 64, a source electrode, and a gate 66 are formed on the n-type diffusion layer 9, the n-type diffusion layer 10, and the polysilicon film 8 respectively. Thus, a low withstand voltage MOSFET shown in Fig. 18 is formed. In the aforementioned fabrication steps of the high withstand voltage element (IGBT) and the low withstand voltage element (MOSFET), both the p-type base layer 4 of the high withstand voltage element and the p-type well diffusion layer 7 of the low withstand voltage element are diffusion layers for forming channel parts. However, these must be formed through separate steps, for the following reason: The p-type base layer 4 for serving as a p-type channel part of the high withstand voltage element is formed by diffusing an impurity after deposition from the left side of a position A through the polysilicon film 3 serving as a mask, and hence the p-type channel part provided under the polysilicon film 3 is formed by transversal diffusion. Namely, there is such a tendency that the impurity concentration is rightwardly reduced from the position A. On the other hand, the p-type well diffusion layer 7 for serving as a p-type channel part of the low withstand voltage element is formed before formation of the polysilicon film 8, and hence the same is formed by longitudinal diffusion from the surface of the n-type base layer 1, whereby a uniform impurity concentration can be attained in the transversal direction. Thus, there is an essential difference between the methods of forming the channel parts in the high withstand voltage element and the low withstand voltage element, and hence the implanation doses of the impurities are different from each other in the p-type base layer 4 and the p-type well diffusion layer 7. Further, the p-type channel part of the high withstand voltage element utilizes transversal diffusion of the p-type base layer 4 as described above, whereby a channel length L is inevitably decided by the depth of the diffusion. Thus, it is necessary to design the depth of the diffusion in the high withstand voltage element independently of the low withstand voltage element. Thus, the fabrication steps for the conventional IGBT which is employed for a power IC are independent of those for the field effect transistor which is a simultaneously integrated low withstand voltage element, and the fabrication steps are inevitably complicated in order to integrate both on one chip. From JP-A-4 320 377 a vertical or longitudinal insulated gate bipolar transistor (IGBT) is known which uses buffer layers in order to reduce the turn OFF loss of the device. In detail, the buffer layers consist of a layer having a comparatively low impurity concentration and a layer having a comparatively high impurity concentration. The collector is provided on a bottom side of a substrate and the buffer layers are located successively on a collector layer. An insulated gate bipolar transistor according to the preamble of claim 1 is known from T. Matsudai et al.: Proceedings of the International Symposium on Power Semiconductor Devices and IC's, Davos Switzerland, May 1994, pages 399 to 403. In more detail, there is known an insulated gate bipolar transistor comprising a base layer, a base region formed in the base layer, an emitter region formed in the base region, an insulating film being formed on the base region between the base layer and the emitter region, and a control electrode which is formed on the insulating film. Apart from the base region a resurf structure is formed. It is an object of the present invention to obtain an insulated gate bipolar transistor which is improved in withstand voltage, reduced in ON-state voltage and reduced in turn-off time to have excellent withstand voltage, ON-state voltage and turn-off characteristics and a method of manufacturing the same. As regards the insulated bipolar transistor this object is solved by the subject matter of claim 1 and as regards the method of manufacturing the same this object is solved by the subject matter of claim 6. Further advantageous modifications of the present invention are subject matter of the dependent claims. The foregoing and other objects, features and advantages of the present invention will become more fully apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings. In the drawings: Fig. 1 is a sectional view showing the structure of a transversal IGBT according to a first embodiment of the present invention; Fig. 2 is a sectional view showing a method of fabricating the transversal IGBT according to the first embodiment; Fig. 3 is a sectional view showing the method of fabricating the transversal IGBT according to the first embodiment; Fig. 4 is a sectional view showing the method of fabricating the transversal IGBT according to the first embodiment; Fig. 5 is a diagram showing impurity concentration distribution of the transversal IGBT according to the first embodiment; Fig. 6 is a sectional view showing the structure of a semiconductor device according to a second embodiment of the present invention; Fig. 7 is a sectional view showing a method of fabricating the semiconductor device according to the second embodiment; Fig. 8 is a sectional view showing the method of fabricating the semiconductor device according to the second embodiment; Fig. 9 is a sectional view showing the method of fabricating the semiconductor device according to the second embodiment; Fig. 10 is a sectional view showing the method of fabricating the semiconductor device according to the second embodiment; Fig. 11 is a sectional view showing the method of fabricating the semiconductor device according to the second embodiment; Fig. 12 is a sectional view showing the method of fabricating the semiconductor device according to the second embodiment; Fig. 13 is a sectional view showing the method of fabricating the semiconductor device according to the second embodiment; Fig. 14 is a graph showing the relation between a distance of implantation of an impurity and concentration distribution of an n-type buffer layer; Fig. 15 is a sectional view showing the structure of a transversal IGBT according to a third embodiment of the present invention; Fig. 16 is a sectional view showing the structure of a transversal IGBT according to a fourth embodiment of the present invention; Fig. 17 is a sectional view showing the structure of a conventional transversal IGBT; Fig. 18 is a sectional view showing the structure of a conventional NMOS transistor; Fig. 19 is a plan view showing the plane structure of the conventional transversal IGBT; Fig. 20 is a plan view showing the plane structure of the transversal IGBT according to the first embodiment; Fig. 21 is a sectional view showing the structure of a transversal IGBT according to a first mode of a fifth embodiment of the present invention; Fig. 22 is a sectional view showing a method of fabricating the transversal IGBT according to the first mode of the fifth embodiment; Fig. 23 is a sectional view showing the method of fabricating the transversal IGBT according to the first mode of the fifth embodiment; Fig. 24 is a sectional view showing the method of fabricating the transversal IGBT according to the first mode of the fifth embodiment; Fig. 25 is a plan view showing the plane structure of the transversal IGBT according to the first mode of the fifth embodiment; Fig. 26 is a plan view showing a part of Fig. 25 in detail; Fig. 27 is a plan view showing the method of fabricating the transversal IGBT according to the first mode of the fifth embodiment; Fig. 28 is a sectional view showing the structure of a transversal IGBT according to a second mode of the fifth embodiment of the present invention; Fig. 29 is a plan view showing the plane structure of the transversal IGBT according to the second mode of the fifth embodiment; Fig. 30 is a sectional view showing the structure of a transversal IGBT according to a sixth embodiment of the present invention; and Fig. 31 is a plan view showing the plane structure of the transversal IGBT according to the sixth embodiment. Fig. 1 is a sectional view showing the sectional structure of a transversal IGBT according to a first embodiment of the present invention. As shown in Fig. 1, a p-type base layer 13 is selectively formed in an upper layer part of a high-resistance n-type base layer 1, an n-type emitter layer 17 and a high-concentration p-type layer 18 are formed on a surface of the p-type base layer 13 to be adjacent to each other, a low-concentration n-type buffer layer 11 is formed in the upper layer part of the n-type base layer 1 independently of the p-type base layer 13, a high-concentration n-type buffer layer 12 is selectively formed in an upper layer part of the low-concentration n-type buffer layer 11, and a p-type collector layer 19 is selectively formed on a surface of the high-concentration n-type buffer layer 12. Namely, the high-concentration buffer layer 12 and the low-concentration n-type buffer layer 11 are provided between the p-type collector layer 19 and the high-resistance n-type base layer 1. Impurity concentrations of the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 satisfy conditional expressions 1 and 2 described later. Fig. 20 is a plan view showing the plane structure of the transversal IGBT according to the first embodiment. A section taken along the line B - B in Fig. 20 corresponds to Fig. 1. As shown in Fig. 20, the high-concentration p-type layer 18, the n-type emitter layer 17, a polysilicon film 16, the n-type base layer 1, the low-concentration n-type buffer layer 11, the high-concentration n-type buffer layer 12 and the p-type collector layer 19 have elliptic plane shapes respectively, so that the n-type emitter layer 17 is formed inside the high-concentration p-type layer 18, and the polysilicon film 16, the n-type base layer 1, the low-concentration n-type buffer layer 11, the high-concentration n-type buffer layer 12 and the p-type collector layer 19 are successively formed inside the n-type emitter layer 17 in a similar manner to the above. The polysilicon film 16 is necessarily formed on the p-type base layer 13 (not shown in Fig. 20). A gate insulating film 15 is formed on a part of the n-type emitter layer 17, a surface of the p-type base layer 13 and a part of the n-type base layer 1, while a polysilicon film 16 serving as a gate electrode part is formed on the gate insulating film 15. Further, an emitter electrode 20 is formed on the n-type emitter layer 17 and the high-concentration p-type layer 18, a gate electrode 21 is formed on the polysilicon film 16, and a collector electrode 22 is formed on the p-type collector layer 19. Numeral 14 denotes a field oxide film. <Fabrication Method> Figs. 2 to 4 are sectional views showing a method of fabricating the transversal IGBT according to the first embodiment. The method of fabricating the IGBT according to the first embodiment is now described with reference to Figs. 2 to 4. First, n-type impurity ions are selectively implanted (first impurity implantation) into the high-resistance n-type base layer 1 at an injection rate of 1 x 10 13 cm -2 , for example, and diffused to form the low-concentration n-type buffer layer 11, as shown in Fig. 2. Then, n-type impurity ions are selectively implanted (second impurity implantation) into the low-concentration n-type buffer layer 11 and diffused, to form the high-concentration n-type buffer layer 12. Therefore, the high-concentration n-type buffer layer 12 is formed by double diffusion. The impurity concentrations of the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 satisfy conditional expressions 1 and 2 described later. Further, a p-type impurity is selectively implanted into the upper layer part of the high-resistance n-type base layer 1 independently of the low-concentration n-type buffer layer 11 and diffused, to form the p-type base layer 13. Then, the field oxide film 14 is formed by a LOCOS method as shown in Fig. 3, and thereafter the gate insulating film 15 is formed so that the polysilicon film 16 is formed on the field oxide film 14 and the gate insulating film 15. Then, the polysilicon film 16 is worked into the shape of a gate electrode part, as shown in Fig. 4. Then, the n-type emitter layer 17 is selectively formed on the surface of the p-type base layer 13 through resist, similarly to a general method of fabricating a low withstand voltage CMOS transistor. Then, the high-concentration p-type layer 18 is selectively formed on the surface of the p-type base layer 13 through resist to be adjacent to the n-type emitter layer 17, while the p-type collector layer 19 is selectively formed on the surface of the high-concentration n-type buffer layer 12. Then, an unnecessary portion of the oxide film is removed, the emitter electrode 20 is formed on parts of the n-type emitter layer 17 and the high-concentration p-type layer 18 respectively, the gate electrode 21 is formed on the polysilicon film 16, and the collector electrode 22 is formed on the p-type collector layer 19, thereby completing the structure of the transversal IGBT shown in Fig. 1. Fig. 5 is a graph showing impurity concentration distribution and carrier concentration distribution in an ON state in the transversal IGBT according to the first embodiment. Referring to this graph, lines L1 and L2 show concentrations of electrons and holes respectively, while the emitter electrode 20, the gate electrode 21 and the collector electrode 22 are set at the ground level, 5 V and 5 V in an ON time respectively. In the transversal high withstand voltage IGBT according to the first embodiment, an n-type buffer layer is divided into two stage concentrations by the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12, whereby the concentrations of the carriers (electrons and holes) are attenuated from the p-type collector layer 19 toward the high-resistance n-type base layer 1, as shown in Fig. 5. Therefore, the impurity concentrations of the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 are so set that the concentrations of the carriers propagated through the n-type buffer layers 11 and 12 respectively are in excess of the impurity concentrations thereof. Consequently, it is possible to prevent increase of an ON-state voltage by attenuating the concentrations of the n-type buffer layers 11 and 12 in two stages between the p-type collector layer 19 and the high-resistance n-type base layer 1. Further, turn-off ability is also improved since carrier lives are in inverse proportion to the impurity concentrations in a turn-off time. However, the concentrations of the n-type buffer layers 12 and 13 must be set in a range capable of maintaining the withstand voltage. Therefore, it is preferable to set the concentration of the n-type buffer layer 11 at the maximum in a range below the carrier concentration in the ON time. While a buffer region is formed by the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 having different impurity concentrations according to the first embodiment, the buffer region may alternatively be formed by three or more buffer layers. Namely, the buffer region may be formed by first to n-th (n ≥ 2) buffer layers over the n-type base layer 1 and the p-type collector layer 19. According to the first embodiment, n = 2 and first and second buffer layers correspond to the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 respectively. Assuming that B1 to Bn represent impurity concentrations of the first to n-th buffer layers respectively, and C1 to Cn represent concentrations of carriers which are propagated through the first to n-th buffer layers respectively in an ON state, conditional expression 1: B1 < B2 < ... < Bn and conditional expression 2: Bi < Ci (1 ≤ i ≤ n) may be satisfied. In order to attain excellent withstand voltage and turn-off characteristics, it is preferable to maximize the respective impurity concentrations Bi in a range satisfying the aforementioned conditional expressions 1 and 2. <<Concentration of n-Type Buffer Layer>> Table 1 shows the relation between the element withstand voltage, the ON-state voltage and the turn-off time in the transversal IGBT according to the first embodiment. Specification of SOI Substrate Second Phosphorous Injection Rate (E12) First Phosphorous Injection Rate (E13) nch-HVMOS nch-IGBT tSOI (um) tox (um) Id(mA) Vg = Vd = 5V BVds (V) Ic(mA) Vg = Vc = 5V BVces (V) tf(ns) Vc = 50V Ic = 10mA 10.0 2.0 4.5 0.0 6.5 363 185 100 Unmeasured 1.0 7.8 309 182 284 590 20.0 8.1 274 17.5 274 390 Referring to Table 1, tSOI denotes the thickness of an n-type active layer 33 and tox denotes the thickness of an oxide film 31, while HVMOS denotes a high withstand voltage n-type MOS transistor having such a structure that the p-type collector layer 19 is replaced by an n-type high-concentration region, and Id denotes its drain current, Vg denotes its gate voltage, Vd denotes its drain voltage, and BVds denotes its drain-to-source withstand voltage. Further, VC denotes the collector voltage of an n-type IGBT corresponding to the transversal IGBT according to the first embodiment, BVces denotes a collector-to-emitter withstand voltage, tf denotes a turn-off time, and IC denotes a collector current. The impurity concentration of the low-concentration n-type buffer layer 11 is decided by the first phosphorus implanation dose in a first impurity implantation step, while that of the high-concentration n-type buffer layer 12 is decided by the sum of first and second phosphorus implanation doses in first and second impurity implantation steps. In view of improvement of the withstand voltage in the transversal IGBT, the first phosphorus implanation dose must be at least 1 x 10 13 cm -2 in order to attain a withstand voltage comparing with that of a high withstand voltage HVMOS, while the collector current IC of the IGBT is disadvantageously reduced from 182 to 17.5 mA by one digit to increase the ON-state voltage if the first phosphorus implanation dose is set at 2 x 10 14 cm -2 . As to the implanation doses capable of maintaining a high withstand voltage and reducing the ON-state voltage, therefore, the ideal first phosphorus implanation dose is about 1 x 10 13 cm -2 when the second phosphorus implanation dose is 4.5 x 10 12 cm -2 . Also as to the turn-off time tf, it is possible to attain 590 ns of the allowable range. Namely, the ideal first and second ion implanation doses are 1.0 x 10 13 cm -2 and 4.5 x 10 12 cm -2 respectively in the IGBT having the structure of the first embodiment from Table 1. Table 2 shows the relation between phosphorus implanation doses and peak values of the impurity concentration of the n-type buffer layer. In this case, the ion implantation energy is 80 keV, and the depth of implantation is 0.1 µm. Phosphorus Injection Rate (cm -2 ) yi (µm) Peak Concentration of n-Type Buffer Layer (cm -3 ) 4.5 × 10 12 1.5 3.4 × 10 16 1 × 10 13 1.8 8.2 × 10 16 1 × 10 14 2.6 8.0 × 10 17 1 × 10 15 3.5 7.8 × 10 18 1 × 10 16 4.8 5.2 × 10 19 Referring to Table 2, yi denotes depths where the impurity concentrations reach 1 x 10 16 cm -3 . The relation with respect to the concentration distribution of the n-type buffer layer after ion implantation and heat treatment is as shown in a characteristic curve L3 of Fig. 14. Referring to Fig. 14, y shows depths from the surface of the n-type base layer 1. From Table 2, the peak concentration in case of ion implantation at 4.5 x 10 12 cm -2 corresponding to the ideal second phosphorus implanation dose is 3.4 x 10 16 cm -3 , and that in case of ion implantation at 1 x 10 13 cm -2 corresponding to the ideal first phosphorus implanation dose is 8.2 x 10 16 cm -3 . In the structure of the IGBT according to the first embodiment, therefore, all of the withstand voltage, the ON-state voltage and the turn-off time can be satisfied when the impurity peak concentration of the low-concentration n-type buffer layer 11 is 8.2 x 10 16 cm -3 and that of the high-concentration n-type buffer layer 12 is 11.6 x 10 16 cm -3 . Further, it is inferred from Table 1 that the limit of the first phosphorus implanation dose is about 1.0 x 10 14 cm -2 in order to maintain the turn-off time and the ON-state voltage, whereby the range of phosphorus implanation doses by the first and second phosphorus implantation operations is about 4.5 x 10 13 to 1.0 x 10 14 cm -2 . Therefore, it is conceivable that an impurity concentration setting range for the n-type buffer layers in an actual level is conceivably about 3.4 x 10 16 to 8.0 x 10 17 cm -3 from Table 2. Fig. 6 is a sectional view showing a semiconductor device according to a second embodiment, in which the high withstand voltage transversal IGBT according to the first embodiment shown in Fig. 1 and a low withstand voltage CMOS element are formed on the same dielectric isolated substrate. The substrate or a wafer has a support layer 32 and an n-type active layer 33 which are insulated from each other by an oxide film 31. A substrate of this type can be obtained by a cladding SOI method or an SIMOX method. An oxide film 48 and a polysilicon buried layer 63 are arranged in the n-type active layer 33 through a surface and a back surface thereof, in order to dielectric-isolate a formation region 60 for the high withstand voltage transversal IGBT from a formation region 61 for another high withstand voltage element or a low withstand voltage element. An isolation structure by a trench or a V groove can be employed as the dielectric isolation structure. The transversal IGBT which is formed in the formation region 60 has the same structure as the transversal IGBT according to the first embodiment shown in Fig. 1, as hereafter described. A p-type base layer 13 is selectively formed in an upper layer part of the n-type active layer 33, an n-type emitter layer 17 and a high-concentration p-type layer 18 are formed on a surface of the p-type base layer 13 to be adjacent to each other, a low-concentration n-type buffer layer 11 is formed in an upper layer part of an n-type base layer 1 independently of the p-type base layer 13, a high-concentration n-type buffer layer 12 is selectively formed in an upper layer part of the low-concentration n-type buffer layer 11, and a p-type collector layer 19 is selectively formed on a surface of the high-concentration n-type buffer layer 12. A gate insulating film 15 is formed on a part of the n-type emitter layer 17, a surface of the p-type base layer 13 and a part of the n-type base layer 1, while a polysilicon film 16 serving as a gate electrode part is formed on the gate insulating film 15. Further, an emitter electrode 20 is formed on the n-type emitter layer 17 and the high-concentration p-type layer 18, a gate electrode 21 is formed on the polysilicon film 16, and a collector electrode 22 is formed on the p-type collector layer 19. Numeral 14 denotes a field oxide film. Impurity concentrations of the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 satisfy the conditional expressions 1 and 2 described with reference to the first embodiment. The low withstand voltage CMOS element which is formed in the formation region 61 has the following structure: An n-type well layer 5 and a p-type well layer 6 are selectively formed in the n-type active layer 33. A gate insulating film 25 and a polysilicon film 26 serving as a gate electrode part are selectively formed on a surface of the n-type well layer 5, while a gate insulating film 27 and a polysilicon film 28 serving as a gate electrode part are selectively formed on a surface of the p-type well layer 6. P-type diffusion layers 34 and 35 having relatively high impurity concentrations are formed on the surface of the n-type well layer 5 on both sides of the polysilicon film 26 to serve as a source and a drain respectively, while n-type diffusion layers 36 and 37 having relatively high impurity concentrations are formed on a surface of the p-type well layer 6. A source electrode 71, a gate electrode 72 and a drain electrode 73 are formed on the p-type diffusion layer 34, the polysilicon film 26 and the p-type diffusion layer 35 respectively, while a source electrode 74, a gate electrode 75 and a drain electrode 76 are formed on the n-type diffusion layer 36, the polysilicon film 28 and the n-type diffusion layer 37 respectively. At this time, the semiconductor device according to the second embodiment sets the impurity concentration of the n-type well layer 5 of the low withstand voltage PMOS transistor at a level which is set by only a second phosphorus implanation dose for formation of the high-concentration n-type buffer layer 12 of the high withstand voltage transversal IGBT. In addition, the p-type base layer 13 of the high withstand voltage transversal IGBT and the p-type well layer 6 of the low withstand CMOS element are set at the same impurity concentrations. As hereinabove described, the transversal IGBT of the semiconductor device according to the second embodiment presents the same structure as that of the first embodiment including setting of the impurity concentrations of the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12, whereby excellent ON-state voltage, withstand voltage and turn-off characteristics can be attained similarly to the first embodiment. Figs. 7 to 13 are sectional views showing a method of fabricating the semiconductor device according to the second embodiment. The method of fabricating the semiconductor device according to the second embodiment is now described with reference to Figs. 7 to 13. First, a surface of the n-type active layer 33 of a cladding SOI substrate consisting of the support layer 32, the oxide film 31 and the n-type active layer 33 is oxidized to form an oxide film 40, a nitride film 41 is deposited on the oxide film 40, and a resist layer 42 is formed on the nitride film 41 and patterned, as shown in Fig. 7. The patterned resist layer 42 is employed as a mask to remove the nitride film 41 from regions for forming the n-type buffer layers 11 and 12, and thereafter phosphorus is implanted at an implanation dose of 3 x 10 13 cm -2 (first phosphorus implanation dose). As shown in Fig. 8, the resist layer 42 is removed, heat treatment is performed at 1200°C for forming the low-concentration n-type buffer layer 11, thereafter the nitride film 41 and the oxide film 40 are removed, surfaces of the n-type active layer 33 and the low-concentration n-type buffer layer 11 of the SOI substrate are again oxidized to form an oxide film 56, a nitride film 43 is deposited on the oxide film 56, and a resist layer 44 is formed on the nitride film 43 and patterned. The patterned resist layer 44 is employed as a mask to remove a part for forming the high-concentration n-type buffer layer 12 inside the low-concentration n-type buffer layer 11 and a part of the nitride film 43 for forming an n-type well of a PMOS transistor of a CMOS element, and phosphorus is implanted at an implanation dose of 4.5 x 10 12 cm -2 (second phosphorus implanation dose). Thereafter the resist layer 44 is removed, heat treatment is performed at 1100°C to form the n-type well layer 5 and the high-concentration n-type buffer layer 12, and thereafter the nitride film 43 is removed, as shown in Fig. 9. Then, a resist layer 46 is formed on the oxide film 56, and patterned. Thereafter the resist layer 46 which is so patterned that a portion for forming a p-type well of an NMOS transistor of the CMOS element and a portion for forming a p-type base of the transversal IGBT are removed is employed as a mask to implant boron at 1 x 10 13 cm -2 . Then, the resist layer 46 is removed, and thereafter heat treatment is performed at 1100°C to form the p-type well layer 6 and the p-type base layer 13, as shown in Fig. 10. Then, an oxide film 47 is formed on the overall surface, and the oxide film 47 is patterned to remove only regions for forming the thermal oxide film 48 and the polysilicon buried layer 63 between the formation regions 60 and 61 for the transversal IGBT and the low withstand voltage CMOS element. The patterned oxide film 47 is employed as a mask to etch the n-type active layer 33, thereby forming a trench to reach the oxide film 31. Thereafter the oxide film 47 is removed and a side wall of the trench is thermally oxidized to form the thermal oxide film 48, thereafter polysilicon is deposited on the overall surface including the surface in the trench, and the polysilicon is etched back to be flush with the surface of the n-type active layer 33, for forming the polysilicon buried layer 63. As shown in Fig. 11, the field oxide film 14 is formed by a LOCOS method, to complete element isolation. Then, a gate oxide film 51 is formed, a polysilicon film 52 is deposited on the overall surface, and a resist layer 53 is formed on the polysilicon film 52. The resist layer 53 is patterned to leave only gate electrode parts of the respective devices. Thereafter the patterned resist layer 53 is employed as a mask to perform anisotropic etching on the polysilicon film 52, and the resist layer 53 is removed. Thus, the gate insulating film 15, the polysilicon film 16, the gate insulating film 25, the polysilicon film 26, the gate insulating film 27 and the polysilicon film 28 are formed as shown in Fig. 12. Thereafter a resist layer 54 is formed on the overall surface, and patterned to cover a PMOS part of the CMOS element and a p-type collector part and a p-type base lead part of the transversal IGBT. The patterned resist layer 54 is employed as a mask to implant arsenic at an implanation dose of 4 x 10 15 cm -2 , and thereafter the resist layer 54 is removed. Then, a resist layer 55 is formed on the overall surface and patterned to cover an NMOS part of the CMOS element and an n-type emitter part of the IGBT, as shown in Fig. 13. The patterned resist layer 55 is employed as a mask to implant boron at an implanation dose of 4 x 10 15 cm -2 , and thereafter the resist layer 55 is removed. Then, heat treatment is performed to form the p-type diffusion layers 34 and 35, the n-type diffusion layers 36 and 37, the high-concentration p-type layer 18, the n-type emitter layer 17 and the n-type base layer 1, a source electrode 71, a gate electrode 72, a drain electrode 73, a source electrode 74, a gate electrode 75, a drain electrode 76, an emitter electrode 20, a gate electrode 21 and a collector electrode 22 are formed on the p-type diffusion layer 34, the polysilicon film 26, the p-type diffusion layer 35, the n-type diffusion layer 36, the polysilicon film 28, the n-type diffusion layer 37, parts of the n-type emitter layer 17 and the p-type layer 18, the polysilicon film 16, and the p-type collector layer 19 respectively through a passivation film, and the electrodes of the respective devices are taken out for completing the semiconductor device according to the second embodiment in which the transversal IGBT according to the first embodiment and the low withstand voltage CMOS element are formed on the same substrate. In the semiconductor device according to the second embodiment, the n-type well layer 5 of the low withstand PMOS transistor is set at an impurity concentration which is set only by the second phosphorus implanation dose for formation of the high-concentration n-type buffer layer 12 of the high withstand voltage transversal IGBT while the p-type base layer 13 of the high withstand voltage transversal IGBT and the p-type well layer 6 of the low withstand voltage MOS are set at the same impurity concentrations, as hereinabove described. Therefore, the impurity concentration of the n-type well layer 5 of the low withstand voltage PMOS transistor can be set by the second phosphorus implanation dose for formation of the high concentration n-type buffer layer 12 of the high withstand voltage transversal IGBT in fabrication of the semiconductor device according to the second embodiment, whereby the high-concentration n-type buffer layer 12 and the n-type well layer 5 can be formed in the same step. In addition, the p-type base layer 13 of the high withstand voltage transversal IGBT and the p-type well layer 6 of the low withstand voltage CMOS element are formed at the same impurity implanation dose, whereby the p-type base layer 13 and the p-type well layer 6 can be formed in the same step. Consequently, the high-concentration n-type buffer layer 12 and the n-type well layer 5 as well as the p-type base layer 13 and the p-type well layer 6 can be formed in the same steps respectively, whereby the fabrication steps can be shortened. Further, the channel length can be shortened through a fine working technique which is practicable in ordinary low withstand CMOS element fabrication steps, whereby it is also possible to reduce the ON-state resistance of the high withstand voltage IGBT. While the buffer region is formed by the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layer 12 of different impurity concentrations in the transversal IGBT of the semiconductor device according to the second embodiment, the buffer region may alternatively be formed by three or more buffer layers, similarly to the transversal IGBT according to the first embodiment. Namely, the buffer region may be formed by first to n-th (n ≥ 2) buffer layers over the n-type base layer 1 and the p-type collector layer 19, so that the respective buffer layers satisfy the conditional expressions 1 and 2 of the first embodiment. The first to n-th buffer layers are formed by successively performing first to n-th impurity introducing processings for selectively introducing an n-type impurity. At this time, the impurity concentration of an i-th (1 ≤ i ≤ n) is set by the first to i-th impurity introducing processings. According to the second embodiment, n = 2, and the first buffer layer, the second buffer layer, the first impurity introducing processing and the second impurity introducing processing correspond to the low-concentration n-type buffer layer 11, the high-concentration n-type buffer layer 12, implantation and diffusion of phosphorus ions at the first phosphorus implanation dose, and implantation and diffusion of phosphorus ions at the second phosphorus implanation dose respectively. Assuming that SBk (k = 2 to n) = Bk - B(k - 1) and SB1 = B1 in respective impurity concentrations B1 to Bn of the first to n-th buffer layers, impurity concentrations which are set by the first to n-th impurity introducing processings respectively are SB1 to SBn. When the impurity concentration of the n-type well region 5 is set at any one of the values SB1 to SBn, therefore, this layer can be fabricated simultaneously with any one of the first to n-th impurity introducing processings, whereby the fabrication steps can be shortened. While the n-type buffer structure of the transversal IGBT according to the second embodiment has the n-type active layer 33 under the p-type collector layer 19, an effect which is basically similar to those of the transversal IGBTs according to the first and second embodiments can be attained also when a low-concentration n-type buffer 11 passes through a surface and a back surface of an n-type active layer 33 to be in contact with a buried oxide film 31 in a transversal IGBT according to a third embodiment of the present invention as shown in Fig. 15. According to the structure of the transversal IGBT of the third embodiment, turn-off characteristics can be improved since the n-type active layer 33 can be formed in a smaller thickness as compared with that in the second embodiment. While Fig. 15 is illustrated in correspondence to the IGBT of the semiconductor device according to the second embodiment, the low-concentration n-type buffer layer 11 passes through a surface and a back surface of an n-type base layer 1 when the third embodiment is rendered to correspond to the IGBT according to the first embodiment. The transversal IGBT according to the third embodiment has an elliptic plane shape about the p-type collector layer 19, similarly to the plane shape of the first embodiment shown in Fig. 20. Further, a similar effect can be expected also when a high-concentration n-type buffer layer 12 is also formed to pass through an n-type active layer 33 in addition to a low-concentration n-type buffer layer 11 while a portion immediately under a p-type collector layer 19 is formed only by the high-concentration n-type buffer layer 12, as shown in Fig. 16. According to the structure of a transversal IGBT of a fourth embodiment, a turn-off time can be improved since the n-type active layer 33 can be formed in a smaller thickness as compared with that in the second embodiment, similarly to the transversal IGBT according to the third embodiment. While Fig. 16 is illustrated in correspondence to the IGBT of the semiconductor device according to the second embodiment, the high-concentration n-type buffer layer 12 also passes through a surface and a back surface of an n-type base layer 1 in addition to the low-concentration n-type buffer layer 11 and the portion immediately under the p-type collector layer 19 is formed only by the high-concentration n-type buffer layer 12 when the fourth embodiment is rendered to correspond to the IGBT according to the first embodiment. The transversal IGBT according to the fourth embodiment has an elliptic plane shape about the p-type collector layer 19, similarly to the plane shape of the first embodiment shown in Fig. 20. Fig. 21 is a sectional view showing the sectional structure of a transversal IGBT according to a first mode of a fifth embodiment of the present invention. As shown in Fig. 21, a p-type base layer 13 is selectively formed in an upper layer part of a high-resistance n-type base layer 1, while an n-type emitter layer 17 and a high-concentration p-type layer 18 are formed on a surface of the p-type base layer 13 to be adjacent to each other, a low-concentration n-type buffer layer 11 is formed in the upper layer part of the n-type base layer 1 independently of the p-type base layer 13, high-concentration n-type buffer layers 12' are selectively discretely formed in the low-concentration n-type buffer layer 11, and a p-type collector layer 19 is selectively formed on surfaces of the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layers 12'. Namely, the high-concentration n-type buffer layers 12' and the low-concentration n-type buffer layer 11 are provided between the p-type collector layer 19 and the high-resistance n-type base layer 1 in a mixed manner, so that the low-concentration n-type buffer layer 11 and the high-concentration n-type buffer layers 12' are in connection with the p-type collector layer 19. Fig. 25 is a plan view showing the plane structure of the transversal IGBT according to the first mode of the fifth embodiment. Fig. 26 is a plan view showing a region A1 of Fig. 25 in an enlarged manner. A section taken along the line C - C in Fig. 25 corresponds to Fig. 21. As shown in Fig. 25, the high-concentration p-type layer 18, the n-type emitter layer 17, a polysilicon film 16, the p-type base layer 13, the low-concentration n-type buffer layer 11 and the p-type collector layer 19 have elliptic plane shapes respectively, so that the n-type emitter layer 17 is formed inside the high-concentration p-type layer 18, and the polysilicon film 16, the p-type base layer 13, the low-concentration n-type buffer layer 11 and the p-type collector layer 19 are successively formed inside the n-type emitter layer 17 in a similar manner to the above. The polysilicon film 16 is necessarily formed on the p-type base layer 13. In the low-concentration n-type buffer layer 11, the high-concentration n-type buffer layers 12' having comb-type plane shapes (12A) and having cog-type plane shapes (12B) are formed longitudinal across a central portion and end circular portions of the p-type collector layer 19 respectively in a discrete manner. On the other hand, a gate insulating film 15 is formed on a part of the n-type emitter layer 17, a surface of the p-type base layer 13 and a part of the n-type base layer 1, while the polysilicon film 16 is formed on the gate insulating film 15 to serve as a gate electrode part. An emitter electrode 20, a gate electrode 21 and a collector electrode 22 are formed on the n-type emitter layer 17 and the high-concentration p-type layer 18, the polysilicon film 16 and the p-type collector layer 19 respectively. Numeral 14 denotes a field oxide film. Figs. 22 to 24 are sectional views showing a method of fabricating the transversal IGBT according to the first mode of the fifth embodiment. Fig. 27 is a plan view showing the plane structure of a portion corresponding to Fig. 26 during the fabrication steps. The method of fabricating the transversal IGBT according to the first mode of the fifth embodiment is now described with reference to Figs. 22 to 24 and 27. As shown in Fig. 22, n-type impurity ions are selectively implanted into the high-resistance n-type base layer 1 (first impurity implantation) at an injection rate of 1.4 x 10 13 cm -2 , for example, and diffused to form the low-concentration n-type buffer layer 11. Then, n-type impurity ions are selectively implanted into the low-concentration n-type buffer layer 11 (second impurity implantation), to discretely form the high-concentration n-type buffer layers 12', as shown in Fig. 27. Thus, the high-concentration n-type buffer layers 12' are formed by double diffusion. Further, a p-type impurity is selectively implanted into and diffused in the upper layer part of the high-resistance n-type base layer 1 independently of the low-concentration n-type buffer layer 11, to form the p-type base layer 13. Then, the field oxide film 14 is formed by a LOCOS method, followed by formation of the gate insulating film 15, and the polysilicon film 16 is formed on the field oxide film 14 and the gate insulating film 15, as shown in Fig. 23. Then, the polysilicon film 16 is worked into the form of a gate electrode part, as shown in Fig. 24. Then, the n-type emitter layer 17 is selectively formed on the surface of the p-type base layer 13 through resist, similarly to an ordinary method of fabricating a low withstand voltage CMOS transistor. Then, the high-concentration p-type layer 18 is selectively formed on the surface of the p-type base layer 13 through resist to be adjacent to the n-type emitter layer 17, while the p-type collector layer 19 is selectively formed on the surfaces of the low-concentration n-type buffer -layer 11 and the high-concentration n-type buffer layers 12'. Unnecessary portions of the oxide film are removed, and the emitter electrode 20, the gate electrode 21 and the collector electrode 22 are formed on parts of the n-type emitter layer 17 and the high-concentration p-type layer 18, the polysilicon film 16 and the p-type collector layer 19 respectively, thereby completing the structure of the transversal IGBT shown in Fig. 21. In the transversal IGBT according to the first mode of the fifth embodiment, a low-concentration part (the low-concentration n-type buffer layer 11) and high-concentration parts (the high-concentration n-type buffer layers 12') are formed in a mixed manner as n-type buffer layers, and a part of the low-concentration n-type buffer layer 11 and the p-type collector layer 19 are necessarily connected with each other, whereby electrons are directed toward the p-type collector layer 19 upon application of a positive voltage to the gate electrode 21, so that injection of holes is first started from the p-type collector layer 19 into the low-concentration n-type buffer layer 11 to facilitate electron injection from the emitter side. Thus, a synergistic effect acts to increase the carrier concentration. While it is necessary to set the impurity concentrations of the high-concentration n-type buffer layers 12' below the carrier concentration in a stationary state in order to prevent increase of the ON-state voltage, the carrier concentration in the stationary state can be sufficiently highly set due to the presence of the part of the low-concentration n-type buffer layer 11 which is connected with the p-type collector layer 19, as described above. Consequently, it is possible to improve the withstand voltage and the turn-off ability without increasing the ON-state voltage by sufficiently highly setting the impurity concentrations of the high-concentration n-type buffer layers 12'. Figs. 28 and 29 are a sectional view and a plan view showing the sectional and plane structures of a transversal IGBT according to a second mode of the fifth embodiment of the present invention respectively. Fig. 28 shows a section taken along the line D - D in Fig. 29. As shown in Fig. 29, high-concentration n-type buffer layers 12' consist of central regions (12C) which are positioned on a central portion of a p-type collector layer 19 and end regions (12D) which are radially formed from the central regions 12C, and have plane shapes radially extending from portions under the central portion of the p-type collector layer 19. The remaining structure of this mode is similar to that of the first mode, and hence redundant description is omitted. A method of fabricating the transversal IGBT according to the second mode is similar to that for the first mode except setting of the plane shapes of the high-concentration n-type buffer layers 12'. In the transversal IGBT according to the second mode of the fifth embodiment having the aforementioned structure, a part of a low-concentration n-type buffer layer 11 and the p-type collector layer 19 are necessarily electrically connected with each other similarly to the first mode, whereby the carrier concentration in a stationary state can be increased. Consequently, impurity concentrations of the high-concentration n-type buffer layers 12' can be sufficiently highly set below the carrier concentration in the stationary state, whereby it is possible to improve the withstand voltage and the turn-off ability while preventing increase of the ON-state voltage. In addition, the high-concentration n-type buffer layers 12' are necessarily present under the central portion of the p-type collector layer 19, whereby the high-concentration n-type buffer layers 12' suppress extension of a depletion layer extending from a lower portion toward the p-type collector layer 19, so that it is possible to reliably avoid the depletion layer from reaching the central portion of the p-type collector layer 19 and to maintain a higher withstand voltage as compared with the first mode. Figs. 30 and 31 are a sectional view and a plan view showing the sectional and plane structures of a transversal IGBT according to a sixth embodiment of the present invention respectively. Fig. 30 shows a section taken along the line E - E in Fig. 31. As shown in these figures, high-concentration n-type buffer layers 12" are not connected with a p-type collector layer 19, but elliptically discretely formed to be separated from the outer periphery of the p-type collector layer 19 at prescribed distances in plane shape. The remaining structure is similar to that of the first mode of the fifth embodiment, and hence redundant description is omitted. In the transversal IGBT according to the sixth embodiment having the aforementioned structure, a part of a low-concentration n-type buffer layer 11 and the p-type collector layer 19 are necessarily electrically connected with each other similarly to the first and second modes of the fifth embodiment, whereby the carrier concentration is increased. Thus, it is possible to improve the withstand voltage and the turn-off ability without increasing the ON-state voltage, similarly to the fifth embodiment. In particular, the p-type collector layer 19 is formed only on the surface of the low-concentration n-type buffer layer 11 to attain a wide connection region between the p-type collector layer 19 and the low-concentration n-type buffer layer 11, whereby the impurity concentrations of the high-concentration n-type buffer layers 12", which must be set below the carrier concentration in a stationary state in order to prevent increase of the ON-state voltage, can be set to be higher than those in the fifth embodiment. 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 scope of the invention as defined in the appending claims.

Claims: Insulated gate bipolar transistor comprising: a first conductivity type base layer (1); a second conductivity type base region (13) being selectively formed in an upper layer part of said base layer (1); a first conductivity type emitter region (17) being selectively formed on an upper surface of said base region (13); an insulating film (15) being formed on said base region (13) between said base layer (1) and said emitter region (17); a control electrode part (16) being formed on said insulating film (15); and a second conductivity type collector region (19); characterized by: first conductivity type first to n-th buffer regions (11, 12), with n ≥ 2, having a higher impurity concentration than that of said base layer (1) and being selectively formed in said upper layer part of said base layer (1) distantly of said base region (13); and said second conductivity type collector region (19) being selectively formed on an upper surface of at least one of said first to n-th buffer regions (11, 12), said first to n-th buffer regions (11, 12) being provided between: said collector region (19) and said base layer (1) to be adjacent to each other in the transversal direction, and satisfying conditional expression: B1 < B2 < ... < Bn wherein B1 to Bn represent first conductivity type impurity concentrations of said first to n-th buffer regions (11, 12) respectively. Insulated gate bipolar transistor in accordance with claim 1, characterized in that n = 2, said impurity concentration B1 of said first buffer region (11) is 3.4 x 10 16 cm -3 , and said impurity concentration B2 of said second buffer region (12) is 1.16 x 10 17 cm -3 . Insulated gate bipolar transistor in accordance with claim 1, characterized in that said impurity concentrations B1 to Bn of said first to n-th buffer regions (11, 12) are set in the range of 3.4 x 10 16 to 8.0 x 10 17 cm -3 respectively. Insulated gate bipolar transistor in accordance with claim 1, characterized in that the plurality of buffer regions (11, 12) is formed to pass from the top surface to the back surface of said base layer (1). Insulated gate bipolar transistor in accordance with claim 4, characterized in that n = 2, and said first buffer region (11) is not formed under said collector region (19). Method of fabricating an insulated gate bipolar transistor in accordance with claim 1 comprising the following steps: (a) preparing a first conductivity type base layer (1); (b) selectively forming a second conductivity type base region (13) in an upper layer part of said base layer (1); (c) selectively forming a first conductivity type emitter region (17) on an upper surface of said base region (13); (d) forming an insulating film (15) on said base region (13) between said base layer (1) and said emitter region (17); (e) forming a control electrode part (16) on said insulating film (15); (f) selectively forming first conductivity type first to n-th buffer regions (11, 12), with n ≥ 2, in said upper layer part of said base layer (1) distantly of said base region (13); and (g) selectively forming a second conductivity type collector region (19) on an upper surface of at least one of said first to n-th buffer regions (11, 12), said step (f) being adapted to form said first to n-th buffer regions between said collector region (19) and said base layer (1) to be adjacent to each other in the transversal direction by successively carrying out first to n-th impurity introduction processings for selectively introducing a first conductivity type impurity, conditional expression: B1 < B2 < ... < Bn being satisfied assuming that B1 to Bn represent first conductivity type impurity concentrations of said first to n-th buffer regions respectively. Method of fabricating an insulated gate bipolar transistor in accordance with claim 6, characterized in that n = 2, an impurity concentration being set by said first impurity introduction processing is 3.4 x 10 16 cm -3 , and an impurity concentration being set by said second impurity introduction processing is 8.2 x 10 16 cm -3 . Method of fabricating an insulated gate bipolar transistor in accordance with claim 7, characterized in that said first conductivity type is an n-type, and said second conductivity type is a p-type.

Inventors: WATABE KIYOTO, WATABE, KIYOTO, Watabe, Kiyoto, c/o Mitsubishi Denki K.K.
Applicants: MITSUBISHI ELECTRIC CORP, MITSUBISHI DENKI KABUSHIKI KAISHA
Application year: 1995
Main classification: H01L29
IPC codes: H01L29, H01L27, H01L21