Vertical semiconductor device

Abstract: This invention aims at suppressing a parasitic transistor operation of a vertical MOS device at the time of application of a noise current and improving the limitations of withstanding against destruction of the device. P base layers 3 constituting each unit cell of an n-channel DMOS device are partially connected by p extraction regions between the unit cells so as to short-circuit the p base layers to source electrodes 9 in regions Z2 through the extraction regions. Accordingly, an applied noise branches to a conventional path extending from a region Z1 to the source electrodes 9 through an n© source layer 5 and a path extending from regions Z2 to the source electrodes 9 through the p extraction regions 4. Since the p regions form one continuous region throughout the device as a whole, a local potential rise of the p base layer can be limited. Accordingly, the parasitic transistor operation can be suppressed and a breakdown voltage of the device can be improved.

Description: This invention relates to a vertical semiconductor device according to the preamble of claim 1 which is used as a power semiconductor device. Double diffusion type MOS transistors (hereinafter referred to merely as "DMOS") have been proposed in the past as power semiconductor devices. An n-channel type DMOS device, for example, has a basic structure such as shown in Fig. 13. Fig. 13(a) is a schematic plane view of principal portions of the device and Fig. 13(b) is a sectional view taken along a line C - C in Fig. 13(a). Like reference numerals are used to identify like constituent members in these drawings. The DMOS device uses the structure shown in Fig. 13(b) as a unit cell; this device controls a current flowing through a drain terminal D to a source terminal S disposed on both sides of a chip by a potential applied to a gate terminal G. When a potential exceeding a threshold value is applied to a gate electrode 27 relative to a source electrode 29, a surface of a p base layer 23 (channel) below the gate electrode 27 undergoes inversion, and becomes conductive with the drain side by the source electrode 29 through an n source layer 25 and the channel. When the device is turned OFF, a pn junction formed by the p base layer 23 and an n - drain layer 22 enters a backward bias state and obtains a withstand voltage which is determined by the thickness and an impurity concentration of the n - drain layer 22 as a drift region. In this DMOS, a parasitic npn bipolar transistor exists. An electrical equivalent circuit corresponding to Fig. 13(b) is shown in Fig. 14, where R D is a diffusion resistance inside the p base layer 23 immediately below the n + source layer 25, C 1 is a junction capacitance between the n - drain layer 22 and the p base layer 23, C 2 is a parasitic capacitance between the gate electrode 27 and the n - drain layer 22, and C 3 is a parasitic capacitance between the gate electrode 27 and the p base layer 23. In other words, there is constituted a parasitic npn transistor using the n + source layer 25 as its emitter, the p base layer as its base and the n drain layers 21, 22 as its collector. When a voltage pulse such as a noise voltage is applied across the drain electrode 30 and the source electrode 29, a noise current I B flows into the p base layer 23 through the junction capacitance C 1 between the n - drain layer 22 and the p base layer 23, the parasitic capacitance C 2 between the gate electrode 27 and the n - drain layer 22 and the parasitic capacitance C 3 between the gate electrode 27 and the p base layer 23, and reaches a source contact window region ZZ1 while flowing through a portion immediately below the n + source layer 25. In this instance, a potential of a vicinity portion ZZ2 of the p base layer 23 and the n - drain layer 22 becomes higher by a potential V B expressed by the following equation (1) due to the diffusion resistance R D inside the p base layer 23: V B = I B × R D When this potential V B becomes greater than a forward voltage of the pn junction formed by the n + source layer 25 and the p base layer 23, the noise current I B flowing into the p base layer 23 at the vicinity portion ZZ2 of the p base layer and the n - drain layer flows as such into the n + source layer 25 and becomes a base current of a parasitic transistor. Then, this parasitic transistor enters the ON state and is operated. In an insulated gate bipolar transistor (IGBT), the operation of the parasitic transistor results in latchup of an internal thyristor. When a current flows through the parasitic transistor, the temperature of this parasitic transistor rises. In a bipolar transistor, the temperature rise of the device itself reduces the resistance value of the device itself, so that positive feedback operates in a direction which increases the current. Therefore, currents flowing through other portions of the chip concentrate on one position of the parasitic transistor turned ON. In other words, a so-called "hot spot" occurs and eventually, the semiconductor device is destroyed. Normally, the device is constituted by the use of a plurality of unit cells shown in Fig. 13(b) and in this case, the operation of the parasitic transistor occurs locally in most cases. In such cases, noise energy concentrates on such a local portion and destroys the device without waiting for positive feedback by heat. T improve the limitations of withstanding such a destruction, the operation of this parasitic transistor must be suppressed. In a prior art insulated gate bipolar transistor (IGBT), a structure equipped with independent extraction layers shown in Fig. 15(a), for example, has been proposed in order to improve latchup resistance at the time of turn-ON of the gate (see for example JP-A- 63-164473). Generally, the vertical MOS gate device consists of a plurality of unit cells as described above, and the structure shown in Fig. 15(a) can be expressed by an equivalent circuit diagram shown in Fig. 15(b). This circuit arrangement suppresses the rise of the base potential of the parasitic transistor by branching a part of the current flowing through each unit cell to the extraction layer. However, since the parasitic transistor in each unit cell is mutually independent, there remains the problem that when the parasitic operation occurs, this operation becomes a local operation as described above and the device undergoes destruction even when the impressed energy is small. In EP-A-0 118 336, there is disclosed a semiconductor device in which a MOSFET is integrally coupled to a bipolar transistor; therefore, the overall structure of this known semiconductor device cannot be compared to the structure as claimed in the present invention. According to the preamble of claim 1 the present invention starts out from a prior art as it is known from FR-A-2 641 417. This document discloses a vertical MOS-semiconductor device comprising a semiconductor substrate having a drift region of a first conductivity type on a main plane side thereof. Furthermore, there are provided a plurality of base regions of a second conductivity type formed in the drift region, wherein each base region constitutes a unit cell. Furtheron, this known device comprises a plurality of source layers of the first conductivity type which are formed in each of the base regions. A plurality of gate electrodes are formed on a gate insulation film upon a channel region which is formed between the source layers and the drift region inside the base regions. There are further provided a source electrode which is electrically connected to the source layers and to the base regions, and a drain electrode which is disposed on the other main plane side of the substrate. In this known semiconductor device, there are further provided a plurality of extraction regions which have the same conductivity type as the base regions and which have the same impurity concentration as the base regions. These extraction regions are formed in the center region between four neighbouring unit cells. The extraction regions, consequently, are formed separately from the base regions so that a part of the drain current flowing therethrough is drawn from the main drain current to the respective extraction region. The semiconductor device known from FR-A-2 641 417, thus, basically corresponds to the semiconductor device which has been discussed with reference to Fig.15. It is the object of the present invention to improve a semiconductor device according to the preamble of claim 1 in such a way that the the latch-up voltage can be improved. This object, according to the present invention, is solved by the advantageous measures indicated in the characterizing part of claim 1. Hence, acccording to the present invention, the extraction regions are partially connected to the base regions in such a way that the extraction regions and the base regions are formed into one continuous region. Due to this advantageous measure it is possible to greatly improve the latch-up voltage. Further advantageous developments of the present invention are the subject-matter of the subclaims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1(a) is a plane view showing a surface pattern of principal portions of a first embodiment of the present invention; Fig. 1(b) is a plane view showing a continuous p region of Fig. 1(a); Fig. 2(a) is a sectional view taken along a line A A of Fig. 1(a); Fig. 2(b) is a sectional view taken along a line B - B of Fig. 1(b); Figs. 3(a) and 3(b) are sectional views useful for explaining step-wise a fabrication process in the first embodiment of the present invention; Figs. 4(a) and 4(b) are sectional views useful for explaining step-wise the fabrication process of the first embodiment; Figs. 5(a) and 5(b) are sectional views useful for explaining step-wise the fabrication process of the first embodiment; Figs. 6(a) and 6(b) are sectional views useful for explaining step-wise the fabrication process of the first embodiment; Figs. 7(a) and 7(b) are sectional views useful for explaining step-wise the fabrication process of the first embodiment; Figs. 8(a) and 8(b) are sectional views useful for explaining step-wise the fabrication process of the first embodiment; Figs. 9(a) and 9(b) are sectional views useful for explaining step-wise the fabrication process of the first embodiment; Fig. 10 is circuit diagrams each showing a structure of a model used for simulation; Fig. 11 is a diagram showing a voltage waveform applied to each of the models during the simulation; Fig. 12 is a diagram showing the result of each model obtained by the simulation; Fig. 13(a) is a plan view showing a surface pattern at the principal portions of a conventional n-channel DMOS; Fig. 13(b) is a sectional view taken along a line C - C of Fig. 13(a); Fig. 14 is an equivalent circuit diagram of the device shown in Fig. 13; Fig. 15(a) is a sectional view showing an example of a conventional vertical MOS gate device; Fig. 15(b) is an equivalent circuit diagram of the vertical MOS gate device shown in Fig. 15(b); Fig. 16(a) is an equivalent circuit diagram of a vertical MOSFET according to the present invention; and Fig. 16(b) is an equivalent circuit diagram of a vertical insulated gate bipolar transistor (IGBT) according to the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT Hereinafter, a preferred embodiment of the present invention will be explained with reference to the accompanying drawings. Figs. 1 and 2 show an n-channel DMOS according to the first embodiment of the present invention. Fig. 1(a) in its plan view, Fig. 2(a) is a sectional view taken along a line A - A in Fig. 1(a), and Fig. 2(b) is a sectional view taken along a line B - B in Fig. 1(a). Fig. 1(b) is a plan view showing p-type semiconductor diffusion regions corresponding to the surface pattern shown in Fig. 1(a). Throughout these drawings, like reference numerals will be used to identify like constituent elements. In the DMOS structure of this embodiment, a high concentration n + drain layer 1, an n - drain layer 2 having a relatively low concentration as a drift region, a p base layer 3 on the main plane side of the n - drain layer 2 and an n + source layer 5 inside the p base layer 3 are disposed inside a silicon substrate in the side direction of a unit cell as shown in Fig. 2(a). A gate electrode 7 is formed on the main plane side of the silicon substrate through a gate oxide film 6. An inter-level insulating film 8 is formed on the gate electrode 7, and a source electrode 9 is connected electrically to the p base layer 3 and to the n + source layer 5 at a window Z1 on the this inter-level insulating film 8. On the other hand, a drain electrode 10 is formed on the other plane of the silicon substrate. As shown in Fig. 2(b), a p extraction layer 4, which has the same conductivity type as the p base layer 3 and which is connected electrically to the source electrode 9 through a window Z2, is formed in a corner direction of the cell. This p extraction layer is connected partially to the p base layer 3 so that the p base layers 3 can be connected continuously with one another by the extraction layers 4 between a plurality of unit cells. In the case of the square unit cells shown in Fig. 1(a), for example, the grid-like continuous p semiconductor regions are formed by the p extraction layers 4 and the individual base layers 3 as shown in Fig. 1(b). Next, this embodiment will be explained in the sequence of fabrication steps with reference to Figs. 3 to 9. In these drawings, (a) is a sectional view of the section A - A in Fig. 1(a) and (b) is a sectional view of the section 3 - 3 in Fig. 1(a), respectively. To begin with, an n - silicon substrate is prepared as shown in Figs. 3(A) and (b), and an n - layer is grown on one of the main plane sides by epitaxy. These n + and n - layers 1 and 2 together constitute the drain of the n channel DMOS. Next, as shown in Figs. 4(a) and (b), a p-type impurity is introduced into positions at which the p base layers are to be formed and into positions at which the p extraction layers are to be formed, using the mask pattern shown in Fig. 4(c), to thereby form deep p well layers 3a and 4a on the surface of the n - drain layer 2. At this time, a p layer for shielding is simultaneously formed in a pad region, not shown in the drawings. Next, the wafer surface is oxidized by thermal oxidation, for example, and is converted to the gate oxide film 6, and a polysilicon film is deposited on the gate oxide film 6 by a known method such as LPCVD. The polysilicon film is then etched by known photolithography in such a manner as to bore windows for exposing the p well layers 3a and 4a and to form the gate electrode 7 as shown in Figs. 5(a) to (c). A p-type impurity is then doped into the entire wafer surface using the gate electrode 7 as the mask, so as to form relatively shallow channel p layer 3b and extraction layer 4b as shown in Figs. 6(a) and (b). In this instance, the p layers 3a and 3b form the p base layer 3, and the p layers 4a and 4b form the p extraction region 4. If the dimension L of the gate electrode shown in Figs. 5(c) and 6(b) and the diffusion depth Xj of the shallow p layer satisfy the following relation at this time, the p base layer 3 continues the p extraction layer 4 and together form a grid-like continuous semiconductor region below the gate electrode 7 (see Fig. 1(b)): L ≤ 2 × R × Xj where R is a lateral diffusion ratio = 0.85 Next, a resist pattern (see Fig. 7(c) having windows along the gate electrodes 7 inside the p base layer 3 forming the unit cell is formed, and P ion implantation is carried out to form the n + source layer 5 using the resist film and the gate electrode 7 as the mask. After the resist film is peeled, drive-in is effected (see Figs. 7(a) and (b)). Here, the resist film that should be left on the gate electrode 7 is to such an extent that it can bury at least the windows of the gate electrodes 7 to the p extraction region 4. Therefore, the resist film may be etched to a width smaller than the line width of the gate electrode 7. Thereafter, a BPSG film is deposited by CVD to form the inter-level insulating film 8, and contact holes opening to both of the p base layer 3 and the n + source layer 5 and contact holes opening to the p extraction region 4 are bored in the inter-level insulating film 8 as shown in Fig. 8(c) to form a source contact region Z1 and an extraction port contact region Z2 as shown in Figs. 8(a) and (b). Vacuum deposition of an aluminum film and its patterning are then carried out to form the source electrode 9, a source pad (not shown) and a gate pad (not shown) as shown in Figs. 9(a) and (b). A Ti/Ni/Au metal film, for example, is vacuum deposited on the back side to form the drain electrode, and the DMOS shown in Fig. 12 is fabricated. Though a passivation film is formed on the source electrode 9, this film is omitted form the drawings. According to the structure described above, the p base layer 3 is short-circuited to the source electrode 9 through the p extraction region 4 set in the corner direction of the unit cell, that is, in the diagonal direction, at the channel portion Z3 which is set in the proximity of the p base layer 3. Therefore, in the region Z3 in which the potential is most likely to rise in the p base layer in the prior art when a noise current is applied, the potential rise can be suppressed. Furthermore, the p base layers 3 of a plurality of unit cells together form a continuous p-type diffusion region through the extraction regions 4. This arrangement connects the potential of the p base layer of each unit cell to one another, and limits the local potential rise. As described above, this structure can suppress the npn parasitic transistor operation of each unit cell and even when such an npn transistor operation occurs, the structure can limit it to the local operation, and can improve the limitations of withstanding against destruction and the maximum current value occuring latchup phenomena in IGBT. The extraction region 4 in the present invention must be disposed between two adjacent unit cells, and a part of this extraction region 4 and a part of the unit cell must be connected to each other. In the present invention, the extraction region 4 is disposed preferably between at least two unit cells that are disposed adjacent to each other in the diagonal direction among a plurality of unit cells. The shape of the unit cell in the vertical semiconductor device according to the present invention is not limited, in particular, but is preferably square or rectangular and particularly preferably, square. Accordingly, the extraction region 4 is disposed preferably between at least two unit cells disposed in the diagonal direction. The planar area of the extraction region in the vertical semiconductor device according to the present invention is set preferably to be smaller than the planar area of the base layer. According to such a structure, the dead spaces at the corners of the square unit cell, for example, can be utilized effectively. Therefore, even when this extraction region 4 is formed, it is possible to avoid the problem that the gap between the unit cells is expanded substantially or the number of the unit cells is reduced. Furthermore, it is preferred in the present invention that the diffusion depth of the extraction region 4 be set to be substantially equal to that of the base layer. When such a structure is employed, the depletion layer that extends from the P-type base towards the n + substrate inside the n - region at the time of turn-off of the semiconductor device extends substantially flatly, and the occurrence of the field effect distribution can be suppressed. In the vertical semiconductor device according to the present invention, the base region described above may consist of the well region 3a having a high withstand voltage and the shallow channel setting region 3b, and the extraction region may consist of the deep well region which is formed simultaneously with the well region 3a and the connection region which is formed simultaneously with the channel setting region 3b and is connected to the channel setting region 3b. In the vertical semiconductor device described above, the planar area of the connection region 4b is set preferably to be smaller than the planar area of the channel setting region 3b. Next, the potential rise suppression effect in the region Z3 in the vertical semiconductor device according to the present invention will be explained on the basis of a simulation result. Fig. 10 shows the models used for simulation. The model A represents the structure of this embodiment having the extraction layer, the model B represents the structure in which the extraction layer is set independently of the p base layer, and the model C represents the prior art structure. The external electrode of the n + source layer and the external electrode of the p base layer are hereby arranged to be independent so that the value of a current flowing through the n + source layer can be confirmed during the parasitic transistor operation. Fig. 11 shows an impressed voltage (noise current). First of all, a source electrode voltage Vs is applied constantly so as to forwardly bias the PN junction consisting of the p base layer and n - drain layer and to allow the injection of large quantities of positive holes into the drain layer. In this state, the drain electrode voltage Vs is raised at a rate of 4 V/ns. Since the positive holes injected into the drain layer are pulled back to the p base layer in this instance, a large noise current Is flows toward the source electrode. The state at this time is analyzed transiently. Fig. 12 shows the result. The drawing shows the change with time of the total noise current I B and the bipolar operation current I E flowing through the n + source layer. In the model A representative of the structure of this embodiment, current flow through the n + source layer cannot be confirmed, and parasitic transistor operation can also not be confirmed. In contrast, in the models B and C, the current flow through the n + source layer, and the occurrence of the parasitic transistor operation can both be confirmed. It can be understood from the result given above that this embodiment is effective for suppressing the parasitic transistor operation. When the device is constituted by the use of a plurality of unit cells shown in Figs. 1 and 2, the local potential rise of the p base layer can be suppressed much more than in the prior art structure having the independent p base layers between the unit cells, and the embodiment is effective for suppressing the local parasitic transistor operation because the p base layers form the continuous region through the extraction layers in addition to the higher breakdown voltage of each unit cell described above. In the explanation given above, the gate electrode is used as the mask for obtaining the continuous p base layer and the p extraction layer and the continuous region is formed by utilizing the lateral diffusion at the time of the formation of the p base layer and the extraction layer. However, it is possible to form the p-type continuous region using a mask having a continuous window pattern before the formation of the gate electrode. It is also possible to form the p-type continuous region by removing the gate electrode operating as the mask at the junction between the p base layer and the extraction layer and without using the lateral diffusion. In this case, however, an arrangement must be made or the gate electrode in each unit cell will be mutually independent electrically, and electrical connection is established using separate wirings, whenever necessary. In the first embodiment described above, the n + source layer is so formed as to extend along the periphery of the p base layer, but it is also possible to locally form the n + source layer in such a manner as to leave the surface of the p base layer at the corners, for example. According to this arrangement, another current path extending toward the source contact region can be formed in the p base layer constituting the unit cell, too, and the parasitic transistor operation can be further suppressed. Although an explanation has been given of the n-channel type MOSFET, by way of example, the present invention can be applied similarly not only to the p-channel type MOSFET but also to an IGBT using the p + silicon substrate and using the n + drain layer for the p + drain layer in the embodiment shown in Fig. 2.

Claims: A vertical semiconductor device, comprising: [ a ] a semiconductor substrate comprising a drift region of a first conductivity type (n) on a main plane side thereof; [ b ] a plurality of base regions (3) of a second conductivity type (p) formed in said drift region, each base region (3) constituting a unit cell; [ c ] a plurality of extraction regions (4) of the same conductivity type (p) as said base regions (3) and of the same impurity concentration as said base regions (3); [ d ] a plurality of source layers (5) of the first conductivity type (n) which are formed in each of said base regions (3); [ e ] a plurality of gate electrodes (7) which are formed on a gate insulation film (6) upon a channel region which is formed between said source layers (5) and said drift region inside said base regions (3); [ f ] a source electrode (9) electrically connected to said source layers (5) and to said base regions (3); and [ g ] a drain electrode (10) disposed on the other main plane side of said semiconductor substrate; characterized in that [ c2 ] said extraction regions (4) are partially connected to said base regions (3) in such a way that said extraction regions (4) and said base regions (3) are formed into one continuous region. Semiconductor device according to claim 1, characterized in that each of said extraction regions (4) is disposed between an adjacent pair of said unit cells. Semiconductor device according to claim 1 or 2, characterized in that each unit cell is a rectangular cell, wherein said extraction region (4) is disposed between an adjacent pair of said unit cells in a diagonal direction of said unit cells. Semiconductor device according to one of claims 1 through 3, characterized in that a diffusion depth of said extraction region (4) is substantially equal to a diffusion depth of said base regions (3). Semiconductor device according to one of claims 1 through 4, characterized in that a plane area of said extraction region (4) is smaller than a plane area of said base regions (3). Semiconductor device according to one of claims 1 through 5, characterized in that said base regions (3) consist of a deep well region (3a) for setting a withstand voltage and a shallow channel setting region (3b), and said extraction regions (4) consist of a deep well region (4a) and a connection region (4b) which is connected to a respective channel setting region (3b). Semiconductor device according to claim 6, characterized in that the plane area of said connection region (4b) is smaller than the plane area of said channel setting region (3b). Method of producing a vertical semiconductor device according to one of claims 1 through 7, comprising the steps of: [ a ] producing a semiconductor substrate equipped with a drift region of a first conductivity type (n) on a main plane side thereof; [ b ] forming a plurality of base regions (3) of a second conductivity type (p), each constituting a unit cell, and a plurality of extraction regions (4) of the same conductivity as said base regions (3) inside a plurality of regions in said drift region; [ c ] forming source layers (5) of the first conductivity type (n) in each of said base regions (3); [ d ] forming gate electrodes (7) on a gate insulation film (6) upon a channel region which is formed between said source layers (5) and said drift region inside said base regions (3) [ e ] injecting an impurity of a second conductivity type (p) into said extraction regions (4) and said base regions (3) using said gate electrodes (7) as a mask and laterally diffusing said impurity in order to partially connect said extraction regions (4) and said base regions (3) and to form them into one continuous region; [ f ] forming a source electrode (9) which is connected electrically to said source layers (5) and to said base regions (3); and [ g ] disposing a drain electrode (10) on the other main plane side of said semiconductor substrate. Method according to claim 8, characterized in that said base regions (3) and said extraction regions (4) forming step includes a step of forming each extraction region between an adjacent pair of said unit cells. Method according to claim 9, characterized in that said base regions (3) and extraction regions (4) forming step includes a step of forming said unit cells into rectangular cells, and each extraction region is formed between a pair of said unit cells which adjoin each other in their diagonal direction. Method according to one of claims 8 through 10, characterized in that said injecting and diffusing step includes a step of diffusing an impurity of the second conductivity type (p) to a depth substantially equal to a diffusion depth of said base regions (3). Method according to one of claims 8 through 11, characterized in that said base regions (3) and extraction regions (4) forming step includes a step of forming each extraction region (4) so that a planar area thereof is smaller than a planar area of said base region (3). Method according to one of claims 8 through 12, characterized in that said base regions (3) and extraction regions (4) forming step includes: a step of forming deep well regions (3a) of said base regions (3) which are able to withstand a high voltage and shallow channel setting regions (3b) in said base regions (3); and a step of forming deep well regions (4a) of said extraction regions (4) which are formed simultaneously when said deep well regions (3a) of said base regions (3) are formed, and connection regions (4b) which are formed in said extraction regions (4) simultaneously when said shallow channel setting regions (3b) are formed and which are connected to said shallow channel setting regions (3b). Method according to claim 13, characterized in that each planar area of said connection regions (4b) is formed smaller than each planar area of said shallow channel setting regions (3b).

Inventors: NAOHITO KATO, NAOHITO, KATO
Applicants: DENSO CORP, DENSO CORPORATION
Application year: 1992
Main classification: H01L29
IPC codes: H01L29, H01L27