Integrated structure active clamp for the protection of power devices against overvoltages, and manufacturing process thereof

Abstract: An integrated structure active clamp for the protection of a power device against overvoltages comprises a plurality of serially connected diodes (D1-D4,SD1-D4,DF1,DF2), each having a first and a second electrodes, obtained in a lightly doped epitaxial layer (2;2,2') of a first conductivity type in which the power device (M) is also obtained; a first diode (D1;SD1) of said plurality of diodes has the first electrode (12,13;31,32) connected to a gate layer (7) of the power device (M) and the second electrode (14,15;35) connected to the second electrode (16,17;21,22;27,52,28) of at least one second diode (D2-D4) of the plurality whose first electrode (18,20,24,29) is connected to a drain region (D) of the power device (M); said first diode (D1;SD1) has its first electrode (12,13;31,32) comprising a heavily doped contact region (12;32) of the first conductivity type included in a lightly doped epitaxial layer region (13;31) of the first conductivity type which is isolated from said lightly doped epitaxial layer (2;2,2') by means of a buried region (14;33) of a second conductivity type and by a heavily doped annular region (15;34) of the second conductivity type extending from a semiconductor top surface to said buried region (14;33).

Description: The present invention relates to an integrated structure active clamp for the protection of power devices, particularly high-voltage MOSFETs and IGBTs, against overvoltages, and to a manufacturing process thereof. The term "active clamp" refers to a circuit which is integrated on the same chip with a semiconductor power device to protect it from overvoltages. The problems concerning the integration of active clamps in power devices have been discussed in the European Patent Application EP-A-0 624 806 falling under Art. 54(3) EPC. In this document an integrated structure protection circuit comprising a plurality of serially connected junction diodes between the gate and the drain of the power device is disclosed. Parasitic components associated with this structure could generate its uncorrect functioning. For example, the first diode of the chain, connected to the gate of the power device, has a parasitic bipolar transistor associated with it; this bipolar transistor has a collector-emitter breakdown voltage (BV CES ) equal to the drain-source breakdown voltage (BV DSS ) of the power device. However, when the protection circuit operates the parasitic transistor is biased in the active region, the base current being equal to the current flowing through the protection circuit. This causes the collector-emitter voltage across the parasitic transistor, and thus the clamping voltage (Vclamp) of the protection circuit, to drop to a value (LV CEO ) which is much lower than the BV CES , while the desired Vclamp should be just a little bit lower than the BV DSS . Consequently, if the power device is a power MOSFET, it is necessary to increase the thickness of the epitaxial layer, i.e. its BV DSS , with the consequence of an undesired increase in the "on" resistance value (R DS(on) ). In the case of an Insulated Gate Bipolar Transistor (IGBT), due to the presence of a P+ substrate, the parasitic component is no longer a bipolar transistor, but an SCR, which can trigger a degenerative condition which could lead to the device destruction. Different techniques for the integration of active clamps are known. One of said technique provides for the integration of a series of polysilicon diodes connected in parallel between the gate and the drain of the power device. According to another technique the active clamp features a polysilicon diode in series to a junction diode. In US-A-5 162 966 there is disclosed an N-channel MOSFET with gate shortcircuited to the drain and channel region connected to the source of the power MOSFET in series to a series of junction diodes. In view of the state of the art just described, the object of the present invention is to accomplish an integrated structure active clamp in which the effects of parasitic components are minimized. According to the present invention, such object is attained by means of an integrated structure active clamp as defined in claim 1, processes of manufacturing the integrated structure active clamp according to claim 1 are defined in claims 11 and 12. According to an embodiment of the present invention, said first diode is a junction diode, and its second electrode is represented by said buried region and by said annular region of the second conductivity type. According to another embodiment of the present invention, said first diode is a Schottky diode, and its second electrode comprises a metallization strip in contact with said lightly doped epitaxial layer region, said metallization strip being also connected to the second electrode of said at least one second diode. Thanks to the present invention, the injection of carriers from the first electrode of the first diode into its second electrode is minimized, and the gain of the parasitic transistor having emitter, base and collector represented by the first electrode and the second electrode of said diode and the lightly doped epitaxial layer is minimized. The features of the present invention shall be made more evident by the following detailed description of two embodiments, illustrated as non-limiting examples in the annexed drawings, wherein: Figure 1 is a cross-sectional view of a power device with an integrated structure active clamp according to one embodiment of the invention; Figure 2 is a cross sectional view of said power device with an intergrated structure active clamp according to another embodiment of the invention; Figures 3 to 6 are cross-sectional views of intermediate steps of a manufacturing process of the active clamp according to the invention, for the fabrication of the device of Figure 1; Figures 7 to 10 are cross-sectional views of another manufacturing process of the active clamp according to the invention, for the fabrication of the device of Figure 2. With reference to Figure 1, an integrated power device M, for example a N-channel power MOSFET or an IGBT, comprises a bidimensional array of elementary cells 1 (only one of which is shown in Figure 1) obtained in a lightly doped epitaxial layer 2 of the N conductivity type, grown over a heavily doped semiconductor substrate 3 having a low resistivity value. In the case of a power MOSFET the substrate 3 is of the N type, whereas in the case of an IGBT it is of the P type. Each cell 1 comprises a heavily doped P type deep body region 4, surrounded by a lightly doped P type body region 5; a heavily doped N type region 6 partially overlaps both the deep body region 4 and the body region 5, and constitutes a source region of the elementary cell 1. A polysilicon gate layer 7, isolated from the semiconductor top surface by a thin gate oxide layer 8, determines, when a proper bias voltage is applied to it, the formation of a conductive channel in a surface portion of the body region 5, which therefore represents a channel region of the cell 1. The polysilicon gate layer 7 is covered by an insulating oxide layer 9, a contact area is provided to allow a superimposed metal strip 10 to contact the source region 6 and the deep body region 4, thus providing a source contact to the cell 1. The metal strip 10 similarly contacts all the other cells 1 of the bidimensional array (not shown). The gate layer 7 is contacted by another metal strip 11 to provide a gate contact for the cells 1. This same metal strip 11 also contacts a heavily doped N type region 12 which represents a contact region to a cathode region 13 of a first diode D1 belonging to a plurality of serially connected diodes D1-D4 and DF1, DF2; this plurality of diodes as a whole constitutes an integrated structure active clamp suitable to protect the power device M against overvoltages. Even if in the example of Figure 1 only six diodes are shown, the number of diodes connected in series can be higher, being determined by the desired value of Vclamp. The cathode region 13 of diode D1 is constituted by a portion of the epitaxial layer 2, which is electrically isolated from the bulk epitaxial layer 2 by means of a P type buried region 14, forming an anode region of diode D1, and by a P+ annular region 15; the region 13 is therefore lightly doped, this reduces the emitter efficiency of a parasitic bipolar transistor T1 having emitter, base and collector respectively represented by the cathode region 13, the buried layer 14 and the epitaxial layer 2. The resistor R shown in the figure is deliberately introduced by extending the gate layer 7 before contacting it to the metal strip 11, to increase if needed the series resistance of the active clamp. The P+ region 15 is merged with another P+ annular region 16 which surrounds a P- region 17 constituting an anode region of a second diode D2 of said plurality. Inside the P- region 17, an N+ region 18 forms a cathode for diode D2, and is connected by means of a metal strip 19 to a third diode D3 of the plurality of diodes. The diode D3 is almost identical in its structure to the diode D1, having a cathode region comprising a portion 20 of the epitaxial layer, wherein an N+ contact region 24 is obtained, and an anode region represented by a buried region 21. The only difference with diode D1 is given by the fact that the annular P+ region 22 not only allows to isolate the portion 20 from the bulk epitaxial layer 2, but also constitutes an anode for a diode DF1, whose cathode is represented by an N+ region 23. As described in the cited Patent Application, the diode DF1 is one of a number of intermediate diodes (two of such diodes are shown in Fig. 1) forward biased during the active clamp operation. The presence of such forward biased diodes, due to the negative thermal coefficient of their forward voltage, allows to compensate the positive thermal coefficient of the breakdown voltage of those diodes, such as D2-D5, which operate in reverse bias condition, so that a stable value of Vclamp can be achieved. Furthermore, since the forward voltage of a diode is generally much lower than its breakdown voltage, the insertion of forward biased diodes makes it possible to vary the value of Vclamp almost continuosly, and not only in step corresponding to one breakdown voltage, as it would be if only reverse biased diodes were present. As shown in Figure 1, the intermediate diodes such as DF1 and DF2 have a structure which allows, simply by modifying the metal interconnections mask, to bypass those intermediate diodes which are not necessary in the specific application (as DF1 in Fig. 1, wherein the metal strip 19 contacts both the anode region 22 and the cathode region 23 of diode DF1). The contact region 24 of diode D3 is connected by a metal strip 25 to the N+ cathode region 26 of the diode DF2 which, differently from DF1, has not been bypassed; the anode of DF2 is a P+ annular region 27, a P- annular region 28 inside it constitutes the anode of diode D4; an N+ region 29 forms a cathode for diode D4 and is connected to the drain D of the power device M. A P type buried region 52 is also provided under that portion of the P+ annular region 27 inside which the N+ cathode region 26 is obtained, in order to further reduce parasitic effects. A P type buried region 30 is also defined under the cells 1 to reduce the gain and the base resistance of the parasitic transistor associated with each of them. Figure 2, referred to another embodiment of the invention, shows again the power device M with an active clamp structure constituted by a plurality of serially connected diodes D1-D4 and DF1, DF2, but the first diode SD1, is a Schottky diode. Its cathode comprises a portion 31 of the lightly doped epitaxial layer 2, as for diode D1 of the previous embodiment, and is connected to the polysilicon gate layer 7, through a heavily doped N type region 32 corresponding to the contact region 12 of Figure 1, by the metal strip 11. The portion 31 is isolated from the epitaxial layer 2 by a P type buried region 33 and by a P+ annular region 34, which also constitutes a P+ deep body region of an elementary cell in the periphery of the bidimensional array. This cell has a slight different topology, but is functionally identical to the other cells 1 of the array. A metal strip 35 contacts the portion 31, and since this is lightly doped a rectifying contact is formed which allows to obtain the diode SD1. The anode of SD1 is represented by the metal strip 35, and is connected to the anode of the diode D2. The remaining part of the structure of Figure 2 is identical to that of Figure 1. The structure of the diode D3 is again substantially identical to that of the diode D1 in Figure 1, but diode D3 could be as well identical to diode SD1. A manufacturing process suitable to obtain both the embodiments of the invention, e.g. the structure of Figure 1, will be now described with reference to Figures 3 to 6. The lightly doped epitaxial layer 2 is initially grown over the substrate 3. After the oxidation of the entire semiconductor surface, a masked implant of P type dopant ions is performed into selected areas of the epitaxial layer 2 to obtain, after diffusion, heavily doped P type deep body regions 4, 15, 16, 22 and 27 (Fig. 3). A selective implant of P type dopant ions into the epitaxial layer 2 allows the formation of medium doped P type buried regions 30, 14, 21 and 52. The energy of the implanted ions must be high enough so that the dopant concentration peak is located below the semiconductor surface and the concentration of acceptor impurities near the surface is lower than the concentration of donor impurities of the epitaxial layer 2, so that portions 13 and 20 of the epitaxial layer isolated from the bulk epitaxial layer 2 can be obtained (Fig. 4). Active areas are then defined on the semiconductor surface, and the thin gate oxide layer 8 is grown over them. The polysilicon gate layer 7 is successively deposited over the entire surface of the semiconductor, and is doped to reduce its resistivity. The polysilicon layer 7 is then selectively removed outside those regions which will become gate regions of the elementary cells 1; a low concentration of P type dopant ions is then implanted and diffused to obtain lightly doped P type body regions 5 at the sides of and under said gate regions to form a channel regions of the elementary cells 1; this step also allows the formation of anode regions 17 and 28 of some diodes of the active clamp, in the example D2 and D4 (Fig. 5). A high concentration of N type dopant ions is then selectively implanted at the sides of said gate regions to form the source regions 6 of the elementary cells 1; this same step also provides for the formation of cathode regions 18, 23, 26 and 29 for diodes D2, DF1, DF2 and D4, and for the formation of cathode contact regions 12 and 24 for the diodes D1 and D3. An insulating oxide layer 9 is then deposited over the semiconductor surface (Fig. 6). Contact areas are then opened in the insulating oxide layer 9 to allow a superimposed metal layer to electrically interconnect the various components. The metal layer is then selectively etched, to define the metal strips 10, 11, 19 and 25. The semiconductor surface is then covered by a passivating material, and a metal layer (not shown in the drawings) is deposited on the bottom surface of the substrate 3 to provide an electrical drain contact for the power device. Another manufacturing process suitable to obtain both the embodiments of the invention is shown in Figures 7 to 10, which by way of example relates to the fabrication of the structure of Figure 2. After the growth of the lightly doped epitaxial layer 2 over the substrate 3, and the oxidation of the entire semiconductor surface, P type dopant ions are selectively implanted and diffused into the epitaxial layer 2 to form the medium doped P type buried regions 30, 33, 21 and 52 (Fig. 7). The previously grown oxide layer is then removed from the semiconductor surface, and another lightly doped N type epitaxial layer 2' is grown over the epitaxial layer 2; the two epitaxial layers 2 and 2' can have the same concentration of dopants. Field oxidation, masked implant and diffusion of a high concentration of P type dopant ions are carried out to form heavily doped P type deep body regions 4, 34, 16, 22 and 27 (Fig. 8). Active areas are then defined on the surface of the semiconductor, a thin gate oxide layer 8 is grown over said active areas, and a polysilicon layer is deposited over the entire surface of the semiconductor, and is doped to reduce its resistivity. The polysilicon layer is then selectively etched outside those regions wich will become gate regions of the elementary cells 1, and the masked implant and diffusion of a low concentration of P type dopant ions allows the creation of lightly doped P type body regions 5 at the sides of and under said gate regions to form channel regions of the cells 1; this same step allows the formation of anode regions 17 and 28 of some diodes (D2 and D4 in the example) of the active clamp (Fig. 9). A high concentration of N type dopant ions are then selectively implanted and diffused at the sides of said gate regions to form source regions 6 of the cells 1; this same step also provides for the formation of cathode regions 18, 23, 26 and 29 for diodes D2, DF1, DF2 and D4, and for the formation of cathode contact regions 32 and 24 for the diodes D1 and D3. An insulating oxide layer 9 is then deposited over the semiconductor surface (Fig. 10). The process continues with the same steps described in connection with the previously described fabrication process.

Claims: An integrated structure active clamp for the protection of a power device against overvoltages, comprising a plurality of serially connected diodes (D1-D4,SD1-D4,DF1,DF2), each having a first and a second electrode, defined in a lightly doped epitaxial layer (2;2,2') of a first conductivity type in which the power device (M) is also obtained, a first diode (D1;SD1) of said plurality of diodes having the first electrode (12,13;31,32) connected to a gate layer (7) of the power device (M) and the second electrode (14,15;35) connected to the second electrode (16,17;21,22;27,52,28)) of at least one second diode (D2-D4) of the plurality whose first electrode (18,20,24,29) is connected to a drain region (D) of the power device (M), wherein the first electrode (12,13;31,32) of said first diode (D1;SD1) comprises a heavily doped contact region (12;32) of the first conductivity type extending from a semiconductor top surface into a lightly doped epitaxial layer region (13;31) of the first conductivity type which is isolated from said lightly doped epitaxial layer (2;2,2') by means of a buried region (14;33) of a second conductivity type and by a heavily doped annular region (15;34) of the second conductivity type extending from said top surface to said buried region (14;33), said contact region (12;32) having a depth smaller than said epitaxial layer region (13;31) so as to be separated from said buried region (14;33) by the epitaxial layer region (13;31). An active clamp according to claim 1, wherein said first diode (D1) is a junction diode, and its second electrode is represented by said buried region (14) and by said annular region (15) of the second conductivity type. An active clamp according to claim 1, wherein said first diode (SD1) is a Schottky diode, and its second electrode comprises a metallization strip (35) in contact with said lightly doped epitaxial layer region (31) of the first conductivity type, said metallization strip (35) being also connected to the second electrode (16,17;21,22;27,52,28) of said at least one second diode (D2-D4). An active clamp according to claim 1, wherein said at least one second diode (D3) has a structure similar to that of the first diode (D1;SD1), having its first electrode comprising a second heavily doped contact region (24) of the first conductivity type included in a second lightly doped epitaxial layer region (20) of the first conductivity type which is isolated from said lightly doped epitaxial layer (2;2,2') by means of a second buried region (21) of the second conductivity type and by a second heavily doped region (22) of the second conductivity type extending from the semiconductor top surface to the second buried region (21). An active clamp according to claim 1, wherein said plurality of serially connected diodes (D1-D4,SD1-D4,DF1,DF2) comprises a series of said second diodes (D2-D4) interposed between the second electrode (14,15;35) of the first diode (D1;SD1) and the drain region (D) of the power device (M). An active clamp according to anyone of the preceding claims, wherein said lightly doped epitaxial layer (2;2,2') is superimposed over a heavily doped semiconductor substrate (3). An active clamp according to claim 6, wherein said semiconductor substrate (3) is of the first conductivity type, so that the power device (M) is a power MOSFET. An active clamp according to claim 6, wherein said semiconductor substrate (3) is of the second conductivity type, so that the power device (M) is an IGBT. An active clamp according to anyone of the preceding claims, wherein said first conductivity type regions are semiconductor regions doped with donor impurities, while said second conductivity type regions are semiconductor regions doped with acceptor impurities. An active clamp according to anyone of claims 1 to 8, wherein said first conductivity type regions are semiconductor regions doped with acceptor impurities, while said second conductivity type regions are semiconductor regions doped with donor impurities. A process of manufacturing the integrated structure active clamp according to claim 1, comprising the following steps: a) growth of a lightly doped epitaxial layer (2) of a first conductivity type over a semiconductor substrate (3); b) formation of a thick field oxide layer; c) masked implant and diffusion of a high concentration of dopants of a second conductivity type into said epitaxial layer (2) to form heavily doped deep body regions (4,15,16,22,27); d) masked implant at high energy of a medium concentration of dopants of the second conductivity type into the epitaxial layer (2) to obtain at least one medium doped buried region (14) defining, together with a respective deep body region (15), an insulated epitaxial layer surface portion (13) constituting the first electrode of said first diode (D1); e) definition of active areas on the surface of said epitaxial layer (2); f) growth of a thin gate oxide layer (8) over said active areas; g) deposition and doping of a polysilicon layer (7) over the semiconductor surface; h) selective etching of the polysilicon layer (7) to form gate regions of elementary cells (1) of the power device (M); i) masked implant and diffusion of a low concentration of dopants of the second conductivity type to form lightly doped channel regions (5) of the elementary cells (1) at the sides of and under said gate regions, and to form the second electrode (17,28) of said at least one second diode (D2,D4) of said plurality of diodes; j) masked implant and diffusion of a high concentration of dopants of the first conductivity type to form highly doped source regions (6) of the elementary cells (1) at the sides of said gate regions, the first electrode (18,29) of said at least one second diode, and a contact region (12) to the insulated surface portion (13) to form the first electrode (12,13) of said first diode (D1). A process of manufacturing the integrated structure active clamp according to claim 1 comprising the following steps: a) growth of a first lightly doped epitaxial layer (2) of a first conductivity type over a semiconductor substrate (3); b) oxidation of the surface of the first epitaxial layer (2); c) masked implant and diffusion of a medium concentration of dopants of a second conductivity type into the first epitaxial layer (2) to form at least one medium doped buried region (33); d) removal of the oxide layer from the surface of the first epitaxial layer (2); e) growth of a second lightly doped epitaxial layer (2') of the first conductivity type over the first epitaxial layer (2); f) growth of a thick field oxide layer over the surface of the second epitaxial layer (2'); g) masked implant and diffusion of a high concentration of dopants of the second conductivity type into the second epitaxial layer (2') to form heavily doped deep body regions (4,34,16,22,27), one of which defines, together with said at least one medium doped buried region (33), an insulated epitaxial layer region (31); h) active areas definition on the surface of said second epitaxial layer (2'); i) growth of a thin gate oxide layer (8) over said active areas; j) deposition and doping of a polysilicon layer (7) over said thin gate oxide layer (8); k) selective etching of the polysilicon layer (7) to form gate regions of the power device (M); l) masked implant of a low concentration of dopants of the second conductivity type to form lightly doped body regions (5) at the sides of and under said gate regions representing channel regions of elementary cells (1) of the power device (M), and to form the second electrode (17,28) of said at least one second diode (D2,D4) of the plurality; m) masked implant and diffusion of a high concentration of dopants of the first conductivity type to form heavily doped source regions (6) at the sides of said gate regions, to form the first electrode (18,29) of said at least one second diode (D2,D4) of the plurality, and to form a contact region (32) to the insulated portion (31) of epitaxial layer (2').

Inventors: ZAMBRANO RAFFAELE, ZAMBRANO, RAFFAELE
Applicants: CONS RIC MICROELETTRONICA, CONSORZIO PER LA RICERCA SULLA MICROELETTRONICA NEL MEZZOGIORNO
Application year: 1993
Main classification: H01L27
IPC codes: H01L29, H01L27, H01L21