Buried Avalanche Diode Having Laterally Adjacent Semiconductor Layers

1. Field of the Invention

The present invention relates to the manufacturing of integrated circuits and more particularly of BICMOS integrated circuits.

2. Discussion of the Related Art

Integrated circuit manufacturers use various technological processes to manufacture integrated circuits capable of meeting specific requirements. These technological processes are especially characterized by the number of diffusion (or implantation) steps and of masking steps. In a given technology, the manufacturer of integrated circuits desires to be capable of providing the highest possible number of basic components.

In the following description, the BICMOS process as described in European patent application 401,135 filed by the applicant in the name of the same inventor and claiming a priority of Jun. 2, 1989, will be more particularly considered. This process has the advantage of providing complementary MOS transistors and high and low voltage bipolar transistors on a single integrated circuit.

As will be described in relation with FIGS. 1A and 1B, this technological process provides three types of buried layers realized in a silicon substrate before growing an epitaxial layer in which will be formed the MOS and bipolar transistors.

FIG. 1A is a simplified diagram of an intermediate step of this technological process and schematically corresponds to FIG. 4 of the above mentioned patent application.

From the upper surface of a P-type low doped semiconductor substrate 1, have been formed, generally by implantation and annealing steps, N-type highly doped regions 2, N-type low doped regions 3, and P-type highly doped regions 4-1 and 4-2. The highly doped P regions can be realized inside regions 3, as shown by region 4-1.

The state of the device during a subsequent manufacturing step is illustrated in FIG. 1B which schematically corresponds to FIG. 5 of the above mentioned patent application. During this step, an N-type low doped epitaxial layer 5 is grown above the structure of FIG. 1A.

Transistors can be conventionally formed in the epitaxial layer 5 from the upper surface of the chip. Similarly, those skilled in the art know how to form avalanche diodes into layer 5 by using diffused regions from the upper surface. In this case, it is known that impurity diffusions from the surface or polarizations of surface isolating layers generate a drift of the breakdown voltage of the junctions during the lifetime of the diode. Additionally, in known devices, to reach a relatively high threshold voltage (12-20 volts) several diodes in series are used.

An object of the present invention is to provide an avalanche diode in accordance with the above technology.

Another object of the invention is to provide such an avalanche diode that is easy to manufacture, has a simple structure, and has breakdown voltage characteristics that are well determined and reliable in time.

A further object of the invention is to provide such an avalanche diode that is suitably isolated from the other components provided in the chip.

To achieve this object, the invention provides an avalanche diode structure incorporated in an integrated circuit having a lateral junction between two adjacent buried layers having opposite conductivity types and a high doping level.

According to an embodiment of the invention, the diode includes a first highly doped buried layer of the same first conductivity type as the substrate of the integrated circuit; a second highly doped buried layer of the second conductivity type, surrounding the first buried layer and laterally contacting the first layer; and a third low doped buried layer of the second conductivity type disposed beneath the first buried layer and extending beyond the first layer so as to also contact a portion of the second buried layer. The diode is coated with a low doped epitaxial layer of the second conductivity type. First and second highly doped and deeply diffused regions, of first and second conductivity types, respectively contact first and second buried layers from the upper surface of the integrated circuit. The upper surfaces of the first and second deeply diffused regions are provided with contacts.

The foregoing and other objects, features, aspects and advantages of the invention will become apparent from the following detailed description of the present invention which should be read in conjunction with the accompanying drawings.

FIGS. 1A and 1B, above described, disclose intermediate manufacturing steps of an integrated circuit in a BICMOS technology; and

FIG. 2 is a schematic cross-sectional view of an avalanche diode according to an embodiment of the invention.

The drawings are schematic and not drawn to scale. In addition, the shapes, and more particularly the lateral shapes, of the various layers are represented for the sake of simplicity with right angles whereas, in practice, as is well known by those skilled in the art, the diffusion and annealing steps form rounded shapes.

As shown in FIG. 2, the invention uses a structure obtained with the technology illustrated in FIGS. 1A and 1B, comprising on a substrate 1 an epitaxial layer 5. A highly doped (P.sup.+) P-type buried region 11 is surrounded by a highly doped (N.sup.+) N-type buried region 12 so that there is a continuous lateral contact between regions 11 and 12. A low doped (N.sup.-) N-type buried region 13 is disposed beneath region 11 and also extends beneath a portion of region 12. This N.sup.- region 13, which is at the voltage of N.sup.+ region 12, insulates the P.sup.+ region 11 from the substrate. With the conventional doping levels used for buried layers P.sup.+ and N.sup.+, the avalanche voltage of N.sup.+ P.sup.+ junction between regions 12 and 11, when this junction is reverse biased, is approximately 10-20 volts. Such an avalanche diode having such a breakdown voltage is particularly useful in practice to serve as a protection diode against overvoltages on the supply line, when the supply voltage is normally 12 volts.

It will be noted that, for the operation of the circuits usually embodied in the considered technology, the accuracy of the doping level of the N.sup.+ and P.sup.+ regions is not critical. In fact, the N.sup.+ region is always much more highly doped than the P.sup.+ region. By modulating the doping level of the P.sup.+ region, while maintaining the same at a high level, it is possible to adjust the avalanche voltage of the N.sup.+ P.sup.+ junction to a selected value, for example 12 to 16 volts.

In order to provide from the above described N.sup.+ P.sup.+ junction, an avalanche diode connected to external terminals, or to other contacts of the integrated circuit, contacts are established with the junction from the upper surface of the substrate. This can be achieved by deep diffusions, such as an N-type diffused region 14 which extends between the surface of the substrate and region 12, and a P-type deeply diffused region 15 which extends between the surface of the substrate and region 11. Contacts 16 and 17, such as metallizations, can be provided on the upper surface of regions 14 and 15. If the deep diffusions have an insufficient diffusion level at the surface, an overdoped region can be provided beneath the metallization. Such an overdoped P-type region 18 is represented beneath metallization 17.

In an application to provide a protection diode, metallization 16 is connected to the supply terminal and metallization 17 to ground.

Additionally, an advantage of the avalanche diode realized from the buried layers according to the invention is that its breakdown voltage is unlikely to drift during its lifetime, unlike surface diodes.

Of course, the invention is liable of numerous structure modifications and of various applications, which will appear to those skilled in the art.

Having thus described one particular embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements as are made obvious by this disclosure are intended to be part of this disclosure though not expressly stated herein, and are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.