INTEGRATED DEVICE FOR SHIELDING CHARGE INJECTION INTO THE SUBSTRATE, IN PARTICULAR IN DRIVING CIRCUITS FOR INDUCTIVE AND CAPACITIVE LOADS

Abstract: This integrated device for shielding injected charges in driving circuits for inductive and/or capacitive loads comprises four integrated structures including a first barrier region (26) with high resistivity which surrounds the buried layer (25) of the epitaxial flyback pocket (4) which may be brought at a potential lower than ground on the side of the buried layer (25) which faces the driving circuit pocket (20); a first charge collecting region (29-31) provided in the epitaxial flyback pocket (4); a third low-loss diode structure (35-37, 39), formed in an epitaxial pocket (15) which is insulated from the flyback pocket (4) and is arranged between the latter and the driving circuit, and connected so as to clamp the voltage between the epitaxial flyback pocket (4) and the substrate (1) to the diode direct conduction voltage; and, finally, a last barrier structure formed by a charge collecting region (53-55) connected to the supply voltage (V cc ).

Description: The present invention relates to an integrated device for shielding charge injection into the substrate, in particular in driving circuits for inductive and capacitive loads. As is known, during the driving of inductive and/or capacitive loads, in order to achieve a correct behaviour of the system it is necessary to discharge the current accumulated in the load towards the ground or towards the supply. When load driving is performed by means of integrated devices, the substrate of the device is generally clamped to the ground and the current is discharged through the diode formed by the junction of epitaxial pocket connected to the load (which pocket defines the cathode) and substrate (which defines the anode of the diode). In order to allow flyback of the current upon switching off of the load driving devices, this diode is directly biased and the epitaxial pocket reaches a negative voltage with respect to the ground. In this condition, the epitaxial pocket with negative voltage injects current into the substrate, behaving like the emitter of a parasitic NPN transistor, and every other epitaxial pocket of the same device which is at a greater voltage with respect to the substrate collects the injected electrons behaving like the collector of the parasitic transistor having the substrate for base and indeed the negative epitaxial pocket for emitter. This injection of charges in epitaxial pockets which are at high voltage entails some problems and therefore must be avoided. In fact, in particular, it causes an undesired dissipation of power; if the epitaxial pocket which is at high voltage is the base of a PNP transistor, it causes switching-on of said lateral PNPs (including, for example, the driving ones); it furthermore causes an increase in the base current of the lateral PNP transistors, thus causing a malfunction in the device. In order to avoid the above described problems, solutions are currently used having the purpose of creating a path which has lower impedance than the other epitaxial pockets of the circuit and therefore such as to collect all the current injected by the epitaxial pocket which is at a negative voltage with respect to the ground. Such solutions have a similar structure which comprises an insulated epitaxial pocket which surrounds the entire epitaxial pocket which can reach a negative voltage with respect to the ground. Said insulated pocket comprises, in its interior, a buried layer, as well as a sinker layer, which are more doped than the epitaxial layer. This structure is illustrated for the sake of clarity in figure 1 and can be connected to the ground or to the supply (cf. e.g. H.S. Lee, IBM TDB 22 (7), 2689 (1979)). Though these solutions partially solve the problem, they are however still unsatisfactory since they are unable to completely eliminate the current towards other active regions of the circuit, and they do not reduce the dissipated power. Moreover in particular the solution comprising the connection to the ground is scarcely competitive with respect to high-voltage epitaxial pockets which tend to provide a better collecting of the charges injected by the negative voltage epitaxial pocket, thus producing results which are not fully satisfactory. On the contrary, the solution wherein the described structure is connected to the supply has a better behaviour as to collection of the injected current, but it is disadvantageous in terms of power dissipation since the current of the parasitic transistor is collected towards the point at the highest voltage. A device which enhances the isolation withstand voltage is described in Patent Abstracts of Japan, vol. 10, no. 343 (E-456)[2399], 19th November 1986 & JP-A-61 147 546, where an N⁻ buried layer is formed between a N⁺ buried layer and a p-type substrate. Given this situation, the aim of the present invention is to provide an integrated device for shielding charge injection into the substrate, in particular in driving circuits for inductive and/or capacitive loads, which elimi nates the disadvantages of the prior art. Within this aim, a particular object of the present invention is to provide an integrated device which is highly efficient in collecting the current injected by the region which has reached a negative voltage with respect to the ground so as to completely exclude triggering of other transistors of the device, or their increase in current, thus eliminating this cause of circuit malfunction. Another object of the present invention is to provide an integrated device which reduces the power dissipated in the circuit, thus improving the reliability and increasing the applications of the circuit. Not least object of the present invention is to provide an integrated device which has a conceptually simple structure which can be easily manufactured using the methods and the machines currently in use for the production of integrated electronic devices, so as to not entail a substantial complication in the production of the entire circuit and therefore such as to have modest manufacturing costs. The aim described, the objects mentioned, and others which will become apparent hereinafter, are achieved by an integrated device for shielding charge injection into the substrate, in particular in driving circuits for inductive and capacitive loads, as defined in the appended claims. The features and advantages of the invention will become apparent from the description of a preferred, but not exclusive, embodiment, illustrated only by way of non-limitative example in the accompanying drawings, wherein: figure 1 is a transverse cross section view through a small semiconductor wafer, illustrating a protection structure according to the prior art; and figure 2 is a transverse cross section view through a silicon wafer accommodating the integrated device according to the invention. Initially, reference is made to figure 1, which illustrates the structure of the known protection solutions. In detail, figure 1 illustrates a substrate 1, here having P-type conductivity, as well as an epitaxial layer 2, here of N-type conductivity. Inside the substrate 2, insulating regions 5 and 6 having a P-type conductivity are provided and extend from the main face of the device to the substrate 1 and divide the epitaxial layer 2 into a plurality of insulated pockets or tubs, whereof the figure illustrates the pocket 3, which accommodates the protection device, and the pocket 4, which constitutes for example the pocket connected to the load and intended to reach a lower voltage than the substrate in certain operating states of the circuit to be protected. A buried layer 7, with N⁺ conductivity, extends along a part of the junction between the substrate and the insulated epitaxial pocket 3. Above and in contact with the buried layer 7, a sinker region 8, having N⁺ conductivity, is provided and extends up to the main surface of the device. At said face, inside the sinker region, a region 9 with N⁺ conductivity is provided, and is produced during the emitter diffusion phase of the associated circuit. Above the main face of the device, an oxide layer 10 is deposited, such layer 10 having a through hole overlying the region 9, to allow formation of a contact 11 for connecting the protective structure to the ground or to the supply. Though it is not illustrated, the insulated structure formed by the epitaxial pocket 3 and including the layers 7 to 9 completely surrounds the epitaxial pocket the potential whereof may become lower than the ground. Figure 2 instead illustrates the structure of the integrated protection device according to an embodiment of the invention. In this figure, the substrate with P-type conductivity and the epitaxial layer with N-type conductivity have again been respectively indicated at 1 and 2. Here, too, 4 indicates the epitaxial pocket connected to the load and therefore intended to discharge the flyback current of the load and thus to rea ch a lower voltage with respect to the ground in certain operating phases of the circuit to be protected. Thus from now on this pocket 4 will be also indicated as "under-ground pocket" or "flyback pocket". In the present case, the under-ground epitaxial pocket 4 is separated from the rest of the circuit by the insulating region 17, having P-type conductivity, while further insulating regions 18, 19 with P-type conductivity define further insulated epitaxial pockets, indicated by the reference numerals 15, 16 and 20. Let's suppose that the epitaxial pocket 20 accommodates the components forming, together with the under-ground epitaxial pocket 4, the driving circuit. Thus in the present embodiment, the epitaxial pocket 20 is the part of the circuit which should be shielded from the charges injected by the under-ground epitaxial pocket 4, during the flyback operation of the driving circuit. As can be noted, the present embodiment comprises four structures appropriately formed and arranged so as to completely eliminate the flow of charges from the epitaxial pocket 4 towards further epitaxial pockets of the device, possibly at high voltage. In detail, the first protection structure consists of the region 26 with N⁻ - type conductivity formed by diffusion before forming the buried layer 25 with N⁺ -conductivity and extending between the under-ground epitaxial pocket 4 and the substrate 1. In particular, the region 26, having a smaller doping level than the buried layer 25, has a greater resistivity and extends all around the buried layer 25 on the side thereof looking towards the circuit to be shielded (that is, as above indicated, the epitaxial pocket 20, on the right of Fig. 2). In fact, by virtue of the greater resistivity of the layer 26 (which constitutes a bottom N-well region and is hereinafter also defined as barrier region), a decrease in the charge injection efficiency from the epitaxial flyback pocket 4 layer which may form the emitter of parasitic transistors NPN having the substrate 1 as base and regions with N-type conductivity with appropriate voltage as collector. The second structure constituting the protection device according to the invention is formed by the buried layer 29, with N⁺ - type conductivity, extending on the junction between the epitaxial flyback pocket 4 and the substrate 1, and by the sinker layer 30, also having N⁺ - type conductivity and extending from the buried layer 29 up to the main surface of the device. At said surface, the sinker layer 30 accommodates a region 31 with N⁺ - type conductivity diffused during the emitter diffusion step for manufacturing the driving circuit. Said region 31 is electrically connected to the third structure, as will be described hereinafter. The structure formed by the regions 29 and 30 acts as a first picking-up collector for collecting the electrons injected by the epitaxial flyback pocket 4 towards the ground, contributing to the absorption of the injected charges. The third protective structure of the device according to the invention is formed in an insulated epitaxial pocket, indicated at 15 in the drawing and delimited by the regions 17 and 18. Said structure is provided to the side of the epitaxial flyback pocket, between the latter and the circuit to be shielded (pocket 20). In detail, the third structure constitutes a low-loss diode used to clamp the base-emitter voltage of the NPN parasitic transistor to its direct operating voltage, and is also used as a second charge collector. In particular, said per se known structure (see e.g, WO-A-8300776) comprises a buried layer 35 with N⁺ - type conductivity extending inside the region delimited by the insulating layers 17 and 18 on the junction between the epitaxial pocket 15 and the substrate 1. Above the buried layer 35, an implanted insulation layer 36 with P-type conductivity is provided, extending adjacent to the buried layer 35 and in contact therewith along a part of its extension. Above the insulating la yer 36, an insulating layer 37 is furthermore formed and extends inside the epitaxial pocket 15 between the main surface of the device and the insulating layer 36. In practice the region 37, which, in top plan view, forms an insulating ring, divides an inner portion of the epitaxial pocket 15, here indicated by 15′, from the rest of the pocket. In turn the inner portion 15′ of the epitaxial pocket accommodates, at the main surface of the device, a layer 39 with N⁺ - type conductivity which, together with the pocket 15′, defines the cathode of the low-loss diode having its anode formed by regions 36, 37 and is intended to be connected, by means of a metal layer schematically illustrated at 49 in figure 2, to the region 31 and thereby to the first collecting collector defined by the structure 29-30. The diode furthermore comprises a sinker layer 45 with N⁺ - type conductivity extending from the main surface of the device to the buried layer 35 to the side of the insulating layer 36. Inside the sinker layer 45, on the main face of the device, the region 46 with N⁺ - type conductivity is furthermore present and, by means of the short-circuit contact 50, is electrically connected to the insulating regions 37 and 18. Accordingly, by virtue of the configuration of the low-loss diode and of its connection, the base-emitter voltage of any parasitic NPN transistor is clamped to the direct conduction voltage of the low-loss diode. In this manner, therefore, an excessive increase of said base-emitter voltage is prevented, therefore reducing the efficiency of such parasitic transistors, with a reduction of the charge injected by the under-ground epitaxial pocket 4. Moreover, the sinker structure 45 acts as a second collector for collecting the injected charges, operating like a collector of a parasitic transistor also formed by the substrate 1 as base and by the flyback pocket 4 as emitter. Finally, the fourth structure which constitutes the shielding integrated device according to the invention comprises a buried layer 53 with N⁺ - type conductivity extending between the junction formed, on the one side by the substrate 1 and, on the other side, by the insulated epitaxial pocket 16 and a sinker layer 54 with N⁺ - type conductivity, extending from the main surface of the device to the buried layer 53. Also in this case, inside the sinker layer 54 a region 55 is provided having N⁺ - type conductivity, also formed during the emitter diffusion step so as to reduce the thickness of the oxide at the terminal. In fact said region 55 is connected, by means of an adapted metal layer schematically indicated by 56 in the figure, to the supply voltage V cc . In practice this last structure acts as a third collector for collecting or picking up charges and has the function of collecting the residual charges which have escaped the structure formed by the layers 29 and 30 and the structure formed by the layers 35 and 45. The circuit is completed by a ground contact at the edge of the wafer which accommodates the complete system formed by the driving circuit plus protection or shielding device, the latter being normally provided at the edge of the wafer. Said ground contact is formed on the insulating region 58 and is connected to the ground by means of a suitable metal layer, as schematically shown in Fig. 2 at 60. In this manner it is possible to reduce the series resistance of the substrate diode, so as to increase its efficiency in injection towards the edge of the wafer. In practice, as mentioned earlier, the four mutually adjacent structures cooperate so as to effectively absorb the injected charges and to reduce the injection efficiency of the parasitic transistors which form. In particular, by means of the illustrated structure, the maximum efficiency of the shielding device is obtained, by virtue whereof the problem of the charges injected by the epitaxial flyback layer towards the substrate is completely solved, since said charges are "picked up" by three adjacent structures so as to prevent, in practice, the migration thereof towards the high-voltage epitaxial pockets forming the associated driving circuit. By means of the protection device according to the invention, a decrease in dissipated power is furthermore obtained by virtue of the reduction of the efficiency of the formed parasitic transistors, and of the collection of most of the injected charges towards the ground through paths (scribing lines) which do not pass close to the driving circuit. The shielding effect is besides completed by the fourth structure connected to the supply voltage, which however does not dissipate greatly since the level of the current which reaches said structure is by then reduced by virtue of the preceding structures, arranged between the flyback pocket and this high voltage structure. The invention thus conceived is susceptible to numerous modifications and variations, all of which are within the scope of the inventive concept. In particular, it should be stressed that, although the best results have been achieved with a protection device formed by all four structures, it is howeveer possible to provide a less number of structures and in whatever combination. Furthermore the shapes and dimensions may be any according to the requirements.

Claims: An integrated device for shielding charge injection into the substrate, in particular in driving circuits for inductive and capacitive loads, comprising a substrate (1) having a first conductivity type, an epitaxial layer (2) superimposed on said substrate, having a second conductivity type and defining a main face of the device, a plurality of regions (58, 17, 18, 19) having a first conductivity type extending transversely to said epitaxial layer from said main face to said substrate and forming, in said epitaxial layer, at least one protecting structure comprising an insulated epitaxial flyback pocket (4) and at least one circuit insulated epitaxial pocket (20) accommodating integrated components, said insulated epitaxial flyback pocket (4) being intended for connection to a load and forming a flyback diode together with said substrate, said integrated device further comprises a buried layer (25) of a second conductivity type and a barrier region (26) of a second conductivity type between said epitaxial flyback pocket (4) and said substrate (1), said barrier region (26) being less doped than said buried layer and extending laterally to, and in contact with, said buried layer (25) at least on a side thereof looking towards said circuit insulated epitaxial pocket (20). An integrated device according to claim 1, characterized in that said epitaxial flyback pocket (4) accommodates a further protecting structure comprising a first charge collecting region (29, 30) of a second conductivity type, extending transversely to said epitaxial flyback pocket (4) between said main face and said substrate (1). An integrated device according to claim 1, characterized in that it furthermore comprises a further protecting structure comprising a second insulated epitaxial pocket (15) arranged between said epitaxial flyback pocket (4) and said circuit insulated epitaxial pocket (20), said second insulated epitaxial pocket (15) accommodating regions (36, 37, 15′, 39) with first conductivity types defining a low-loss diode having one terminal electrically connected (50) to at least one (18) of said plurality of regions, said low-loss diode comprising a charge collecting region (35, 45) of a second conductivity type, extending between said substrate (1) and said main face and being electrically connected to one of said plurality of regions (18). An integrated device according to claim 3, characterized in that said low-loss diode has another terminal thereof connected to said first charge collecting re gion (29, 30). An integrated device according to claim 1, characterized in that it furthermore comprises a further protecting structure comprising a further insulated epitaxial pocket (16), arranged between said flyback epitaxial pocket (4) and said circuit insulated epitaxial pocket (20), said further insulated epitaxial pocket (16) accommodating a charge collecting region (53-55) of a second conductivity type, extending transversely to said further epitaxial pocket (16) between said substrate (1) and said main face, said collecting region being connected to a circuit reference voltage (V cc ). An integrated device according to claims 3 and 5, characterized in that said further insulated epitaxial pocket (16) as defined in claim 5 is arranged between said second insulated pocket (15) as defined in claim 3 and said circuit insulated epitaxial pocket (20). An integrated device according to claims 5 or 6, characterized in that said charge collecting regions comprise each a buried layer (29, 35, 53) which extends between the related epitaxial pocket (4, 15, 16) and the substrate (1) and a sinker region (30, 45, 54), which extends between said buried layer and said main face, said buried layer and said sinker region having a greater doping concentration than said epitaxial pockets. An integrated device according to claim 3, characterized in that said low-loss diode comprises a buried layer (35) extending between said substrate (1) and said second epitaxial pocket (15) and having a second conductivity type, a layer (36) of a first conductivity type extending in said second epitaxial pocket (15) in contact with said buried layer (35), a region (37) of a first conductivity type, extending between said layer (36) and said main face, said layer (36) and said region (37) delimiting and insulating a portion (15′) of said second epitaxial pocket (15) from the remaining part and forming with said portion (15′) the junction which defines said low-loss diode, and said charge collecting region being formed by a sinker layer (45) which extends laterally to said layer (36) and said region (37), between said main face and said buried layer (35). An integrated device according to one or more of the preceding claims, characterized in that one of said plurality of regions (58) delimiting said epitaxial flyback pocket (4) is connected to ground (60). An integrated device according to one or more of the preceding claims, characterized in that said first conductivity type is the P type and said second conductivity type is the N type.

Inventors: BERTOTTI, FRANCO, FERRARI, PAOLO, GATTI, MARIA TERESA
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1987
Main classification: H01L27
IPC codes: H01L27, H01L21