High voltage driver circuit for inductive loads

Abstract: The invention relates to a structure for a driver circuit (10) for an inductive load monolithically integrated on a semiconductor substrate (11) doped with a first type (P) of doping agent and on which is grown an epitaxial well (16) having a second type (N) of doping agent. An insulated well (17) doped with the same type (P) of doping agent as the substrate (11), which houses at least one power transistor (M1) of the driver circuit (10), is provided within the epitaxial well (16). The epitaxial well (16) also houses a first (20) and a second (21) active area which form the cathode terminal (CA) and anode terminal (AN) of a protection diode (D), respectively. The invention also relates to the method of manufacturing such a structure.

Description: Field of application This invention relates to a high voltage driver circuit protected against polarity reversals, in particular for inductive loads. More specifically, the invention relates to a driver circuit for an inductive load according to claim 1 which is monolithically integrated on a semiconductor substrate doped with a first type of doping agent and on which is grown an epitaxial well with a second type of doping agent, bounded by insulating regions, which has a second type of doping agent and in which is provided a further well forming a body region of at least a power transistor of the driver circuit, the circuit further including a protection diode. The invention also relates to a manufacturing process of a driver circuit integrated on a semiconductor substrate according to claim 5 having a first type of doping agent and of a type comprising at least one power transistor provided over an epitaxial well provided on the substrate with a second type of doping agent, said transistor having a body region formed by a further epitaxial well including active regions obtained through implantation and corresponding diffusion phases of the first and second type of doping agent. As is well known, continous innovation in the manufacturing technology of semiconductor integrated circuits has made it possible to achieve levels of integration which reduce the semiconductor area accupied by each individual circuit to a minimum, thus increasing the number of circuits which can be integrated on a single chip. One of the limiting factors for the application of such reduced levels of integration is that the circuits obtained in this way are only able to operate with low levels of current and voltage. These circuits cannot therefore be directly interfaced with power systems or load devices operating with high levels of voltage or current. In the last few years a technology similar to reduced scale integration has been developed which is capable of obtaining integrated power circuits of reduced size, but operating with high levels of current and voltage. The devices obtained by this technology are known as Integrated Power Circuits (IPC). Integrated Power Circuits are currently used in any machine or device which requires a power supply which is different from the primary power supply and in which a high level of voltage, current and/or output power control is desired. A particular category of IPC circuits known as High Voltage Integrated Circuits or HVIC includes interfaces with high voltage and relatively low current which are capable of connecting for example logic circuits and discrete high power devices. Integrated circuits, and in particular high voltage transistors, can be realized using IPC technology with vertical or lateral structures. Vertical integration of high voltage transistors and of the corresponding driver circuitry however requires thick epitaxial layers or multiple layers, as well as complicated and expensive insulating dielectrics. Lateral integration of power devices is generally preferred, as this only requires a thin epitaxial layer (thickness 5-10 µm), and this is therefore more compatible with the manufacturing processes of low voltage circuits. In addition to this, in all applications in which inductive loads have to be controlled it is of fundamental importance to be able to limit if not in fact eliminate parasitic transistors which might originate in the vicinity of layers having the opposite type of doping. Figure 1 diagrammatically illustrates a circuit architecture 1 which includes a driver circuit 2 inserted between a first supply reference voltage Vcc and a second reference voltage, for example a ground GND, and connected to an inductive load L which is in turn connected to ground GND. In order to simplify the description, driver circuit 2 in Figure 1 will be illustrated with only two components, specifically a power transistor T1 and a bias transistor T2, cascade inserted between the supply reference voltage Vcc and,the ground GND, and having control terminals B1 and B2 respectively. Transistors T1 and T2 may be of the IGBT type. Terminals B1 and B2 are connected to a circuit portion which is not shown in Figure 1 as it is not pertinent to the operating of architecture 1. Figure 2 shows the structure of power transistor T2 in driver circuit 2 in cross-section, comprising a substrate 3 of the P - type on which are provided a deep layer 4 of the N + type and an epitaxial layer 5. Within the epitaxial layer 5 a first diffusion zone 6 of the N - type and a second diffusion zone 7 of the P + type, together with a third diffusion zone 8 of the N + type located on the deep layer 4, are obtained by implantation and subsequent diffusion of the doping agent. Diffusion zone 6 comprises collector terminal C1 for power transistor T2 of the driver circuit 2 and is therefore directly connected to the inductive load L. As the current recycles within the inductive load, the potential of collector C1 is forced carried to a value less than the potential of ground GND. In this way a parasitic transistor P, of which an electrical diagram is indicated superimposed as dashed lines on the section of the integrated circuit shown in Figure 2, is thus turned on. This parasitic transistor P has an emitter terminal Ep which coincides with the third diffusion zone 8, and a collector terminal Cp which coincides with the first diffusion zone 6, and a control or base terminal Bp which coincides with the substrate 3 of the integrated circuit. Parasitic transistor P is therefore a lateral transistor of the NPN type. The current gain in this transistor may vary according to the arrangement and the area of the diffusion wells in the device. The presence of this parasitic transistor generally interferes with correct operation of the integrated circuit. This becomes a very serious problem for applications with high supply voltages, i.e. values in excess of 500 V. A known technical solution for eliminating this parasitic transistor is illustrated in Figure 3. In addition to power transistor T2, this figure shows other diffusion zones 7' and 8', as well as a deep region 4', realized within the epitaxial layer 5, which include components of the integrated circuit adjacent to power transistor T1. The known solution provides for the use of a structure 9, a so-called barrier structure, placed between the area in which power transistor T1 is formed and the areas which include the adjacent circuits. Barrier 9 collects most of the current provided by emitter terminal Ep of parasitic transistor P. Although fulfilling its object, this solution needs an appreciable area of integrated circuit for its implementation, thus countering the efforts made to obtain a high integration density for power devices too. Other known solution is disclosed in the U.S. patent N. 4,979,001. In this document is disclosed a diode structure comprising an integrated diode connected to a vertical transistor. Other known solutions are obtained using a thin epitaxial technique, which presents problems with the integration of structures which are operating at high voltage levels and are at the same time safe as regards polarity reversals. In thin epitaxial technology it is not in fact possible to obtain integrated structures with a low loss toward the substrate. The technical problem underlying this invention is to provide an integrated structure for a driver circuit which is capable of withstanding high voltage levels and ensuring correct operation in the presence of polarity reversals, in such a way as to overcome the limitations which nevertheless affect integrated circuits realized in accordance with the known art. Summary of the invention The concept underlying this invention is to provide an integrated structure comprising a LDMOS transistor capable of withstanding high voltage levels, and a PN diode protecting the circuit against polarity reversal, provided in a single epitaxial well which also includes the bonding terminal. Based on this solution idea, the technical problem is resolved by an integrated structure of the type indicated above and defined in claim 1 and subsequent claims. The problem is likewise solved by a manufacturing process for an integrated structure of the type indicated above and defined in claim 5 and subsequent claim. The features and advantages of the integrated structure according to the invention will be apparent from the following description of an embodiment given by way of a non-restrictive example with reference to the enclosed drawings. Brief description of the drawings In the drawings: Figure 1 shows in diagrammatical form a driver circuit for an inductive load according to the known art, Figure 2 shows a diagrammatical view in vertical cross-section of a semiconductor on which a driver circuit manufactured in accordance with the known art is integrated, Figure 3 shows a diagrammatical view in vertical cross-section of a semiconductor on which a driver circuit with a device for reducing the effects of parasitic transistors manufactured in accordance with the known art is incorporated, Figure 4 shows diagrammatically a high voltage driver circuit for an inductive load protected against polarity reversal according to the invention, Figure 5 shows a diagrammatical view in vertical cross-section of a semiconductor in which a driver circuit according to the invention is integrated. Detailed description With reference to these figures, and in particular to the example in Figure 4, 10 indicates as a whole a driver circuit for an inductive load L manufactured according to the invention. Driver circuit 10 comprises a transistor M1 of the LDMOS type and a diode D of the PN type, together cascade inserted between inductive load L and a reference voltage, for example a ground GND. Figure 5 shows a diagrammatical view in vertical cross-section of the integrated structure CI incorporating driver circuit 10 according to the invention. This figure does not show inductive load L, which we consider to be outside the driver circuit. On a semiconductor substrate 11 having a reduced concentration of a first type of doping agent, for example of P type, a first implantation phase of a first 12 and a second deep region 13 of the first type of doping agent is performed. A second implantation phase of a third deep region 14 of a second type of doping agent, for example of type N, is provided simultaneously to the production of regions 12 and 13. There follows a phase of the growth of an epitaxial layer which is lightly doped through implantation. Upper insulating regions 15' are provided to define a well 16 in said epitaxial layer. Above epitaxial well 16 an implantation and subsequent diffusion phase of the first type of doping agent is performed, forming a P-body well 17 within epitaxial well 16. A subsequent oxidation phase makes it possible to obtain a first oxide layer 22, using a mask which keeps the P-body well 17 and active regions 18, 19, 20 and 21 free from oxide. Subsequent thermal diffusion causes fixing of the amount of the doping agents in P-body well 17. P-body well 17 then suffers a further implantation and diffusion phase of a high concentration of the second type of doping agent N + to form a first active region 18 and a subsequent implantation and diffusion phase with a high concentration of the first type of doping agent P + to form a second active region 19. A third active region 20 is also formed within epitaxial well 16 during the implantation and diffusion phase of the N + doping agent, while a further fourth active region 21 is also formed, again within epitaxial well 16, during the implantation and diffusion phase of doping agent P + . A thin layer of oxide is grown on the surface of the semiconductor. Above this a layer 25 of suitably doped polysilicon is deposited to form the gate terminal G1 for LDMOS transistor M1. At this point a second oxide layer 23 is deposited using a mask which keeps part of first active region 18 and second active region 19 and fourth active region 21 free from oxide. The manufacturing process is completed by a definition and metallisation phase of the contact areas 24 for gate G1 and source S1 of transistor M1, of the anode terminal AN of diode D and of the bonding terminal 26. Advantageously, and in accordance with the invention, drain region D1 of transistor M1 coincides with the cathode region CA of diode D, i.e. with active region 20 contained within epitaxial well 16. The anode terminal AN of diode D is accessible to the user. This makes it possible to limit the turning on of NPN parasitic transistor P already illustrated with reference to the known art, and in particular in Figure 2, up to the maximum reverse voltage of diode D, typically a value of 9 V. Advantageously, and in accordance with the invention, this anode terminal AN is connected directly to the contact pin with a reference voltage for the integrated circuit. In this way the high voltage metallisation is prevented from passing over upper insulating zone 15', which is connected to ground. It should also be noted that the integrated structures shown in Figure 5 comprises a buried region 14 which avoids the phenomenon of anode terminal AN of diode D punching through substrate 11 and at the same time minimises the injection of carriers in substrate 11 when anode AN itself is directly biased, i.e. in the normal operating condition of LDMOS transistor M1. The operation of transistor M1 may be compared to that of a lateral LIGBT transistor. In reality, the integrated structure according to the invention makes use of the special features of the diode-transistor connection in reverse biasing, while the IGBT transistors base their operation on the capacity of the diode to carry current in the zone of direct biasing. For this reason the size of driver circuit 10 differs from that of an LIGBT transistor. In particular, buried zone 14 is enriched in such a way as to reduce the efficiency and gain of the PNP parasitic transistor constituted by the anode AN of diode D (diffusion zone 21 of P type), epitaxial layer 16 (N type layer) and the substrate (P type layer). To conclude, the integrated CI structure according to the invention makes it possible to eliminate the problems associated with the presence of parasitic elements and polarity reversal, overcoming the disadvantages of circuits realized in accordance with the prior art.

Claims: A driver circuit (10), for an inductive load, monolithically integrated on a semiconductor substrate (11) doped with a first type (P) of doping agent and on which is grown an epitaxial well (16) with a second type (N) of doping agent, bounded by insulating regions (15'), in which is provided a further well (17) forming a body region of at least one power transistor (M1) of the driver circuit (10), the circuit further including a protection diode, wherein said power transistor is a lateral DMOS transistor and that said protection diode (D) is formed by a first (20) and a second (21) active region having opposite doping agents in said epitaxial well (16) to form respectively the cathode region (20) and the anode region (21) of the protection diode (D) , said cathode region coinciding with the drain region of said power transistor (M1), a buried region (14) of a second type (N) of doping agent between the substrate and the epitaxial well (16) under and separated from said first and second additional active regions (20, 21), said buried region (14) has a greater amount of the second type of doping agent (N) in comparison with the epitaxial well (16). A driver circuit according to claim 1, characterised in that said cathode active region (20) surrounds the anode active region (21). A driver circuit according to claim 2, characterised in that said anode active region (21) of diode (D) is connected to anode terminal (AN) accessible to the output of the circuit. A driver circuit according to claim 3, characterised in that said anode terminal (AN) is connected directly to the contact pin with a reference voltage for the circuit. A manufacturing process of a driver circuit (10) according to claim 1, integrated on a semiconductor substrate (11) having a first type of doping agent (P) and of the type comprising at least one power transistor (M1) provided over an epitaxial well (16) with a second type (N) of doping agent provided on said substrate (11), said power transistor (M1) having a first active region (18, 19) obtained through a double implantation and corresponding diffusion phases of both types of doping agent, and wherein a buried region (14) of a second type is provided between said substrate and said epitaxial well (16) and that during the implantation and diffusion phases of doping agent of the first (P) and second (N) type, first and second active regions (20, 21) of a protection diode are respectively formed within said epitaxial well (16) and over said buried region (14) to provide the cathode region and the anode region of said protection diode connected to the transistor (M1), said first and second active regions (20, 21) having opposite doping agents, said first active region (20) of said protection diode coinciding with the drain region of said power transistor (M1). A process according to claim 5, characterised in that said first active region (20) surrounds the second active region (21) of the diode.

Inventors: DEPETRO RICCARDO, NOVELLI ALDO, DEPETRO, RICCARDO, NOVELLI, ALDO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H01L27
IPC codes: H01L27, H01L21, H01L29