Protective Device for a Semiconductor Integrated Circuit Including Double Polysilicon Resistor

1. Field of the Invention

This invention relates to a protective device for a semiconductor integrated circuit. More particularly, it relates to a device for protecting a semiconductor integrated circuit from the destruction phenomena caused by a high voltage applied to the input terminal of the semiconductor integrated circuit.

2. Description of the Prior Art

Conventionally, at the input stage of a semiconductor integrated circuit, a protective MIS (metal-insulator semiconductor) transistor is provided to protect the input transistor in an internal integrated circuit (hereinafter referred to as an internal IC) from a high voltage input caused by static electricity. The protective MIS transistor has a gate oxidation film thicker than that in the internal IC so that the protective MIS transistor does not operate under a normal operating voltage and operates only when a high voltage is applied. The diffusion region, that is, the drain region of the MIS transistor, is extended so as to have a resistance region which also protects the internal IC from a high voltage. At the end of the resistance region, the diffusion region is directly and electrically in contact with an input electrode wiring layer.

In such a conventional structure, however, since the diffusion region directly contacts the input electrode wiring layer, the tolerance voltage at the input electrode is relatively low. That is, when a high voltage of, for example, 200 V is applied to the input electrode wiring layer, the electrode metal and the diffusion region contacting the electrode metal generates an excessive alloy which easily penetrates through the diffusion region and short circuits the semiconductor substrate with the diffusion region. As a result, the internal IC cannot be driven even when a normal signal voltage is applied to the input electrode wiring layer because the input signal is directly conducted to the semiconductor substrate. Also, due to the presence of the resistance region in the diffusion region, the diffusion region in the protective MOS transistor is large. Further, the junction capacitance between the contact portion of the diffusion region and the semiconductor substrate causes the input impedance of the internal IC to be large so that the operating speed of the semiconductor integrated circuit is deteriorated.

Accordingly, an object of the present invention is to provide a protective device for a semiconductor integrated circuit in which the tolerance voltage for the destruction phenomena between the input electrode wiring layer and the semiconductor substrate is increased.

Another object of the present invention is to provide such a protective device as described above in which the protective input resistance region in the diffusion region is decreased.

Still another object of the present invention is to provide such a protective device as described above in which the junction capacitance between the diffusion region and the semiconductor substrate is decreased.

In order to accomplish the above objects, there is provided a protective device for a semiconductor integrated circuit comprising a protective input MIS transistor having a diffusion region and a semiconductor substrate under the diffusion region for protecting the semiconductor integrated circuit from a high input voltage and an input electrode metal layer electrically connected to the diffusion region of the protective input MOS transistor. According to the present invention, the protective device further comprises an insulating film formed on the surface of the semiconductor substrate and arranged adjacent to the diffusion region, a first polysilicon MIS layer having a high concentration of impurities formed on the insulating film and electrically contacting the input electrode metal layer, and a second polysilicon layer having an impurity concentration lower than the concentration of the first polysilicon layer and electrically contacting the peripheral portion of the first polysilicon layer and the diffusion layer.

The above objects and advantages as well as other features of the present invention will be better understood from the following more detailed description with reference to the accompanying drawings in which:

FIG. 1 is a conventional example of a circuit diagram of a protective circuit connected to an input stage of a semiconductor integrated circuit;

FIG. 2 is a plan view of a main portion of the protective circuit of the conventional device realizing the circuit of FIG. 1;

FIG. 3 is an expanded cross-sectional view taken along line III--III' of FIG. 2;

FIG. 4 is a plan view of a main portion of a protective device according to a first embodiment of the present invention;

FIG. 5 is an enlarged plan view of part of the plan view of FIG. 4;

FIG. 6 is a cross-sectional view taken along line VI--VI' of FIG. 5;

FIG. 7 is a plan view of a main portion of a protective device according to a second embodiment of the present invention; and

FIG. 8 is a cross-sectional view taken along line VIII--VIII' of FIG. 7.

Before describing the embodiments of the invention, a conventional device and the problem involved with this device will first be described with reference to FIGS. 1 through 3.

FIG. 1 is a circuit diagram of a conventional device of a protective circuit connected to an input stage of a semiconductor integrated circuit. In FIG. 1, IC is a semiconductor integrated circuit, T.sub.1 is a MOS transistor at the input stage of the semiconductor integrated circuit IC, R is a protective resistor, and T.sub.2 is a protective MOS transistor. The gate of the input stage MOS transistor T.sub.1, one end of the protective resistor R, and the drain D of the protective MOS transistor T.sub.2 are connected at a first contact portion N.sub.1. The other end of the protective resistor R and the gate of the protective MOS transistor T.sub.2 are connected at a second contact portion N.sub.2. The second contact portion N.sub.2 is connected to an input terminal IN. The source S of the protective MOS transistor T.sub.2 is grounded.

FIG. 2 is a plan view of the protective circuit of a conventional device realizing the circuit of FIG. 1. In FIG. 2, the protective MOS transistor T.sub.2, the protective resistor R, and the input terminal IN are illustrated. S and D represent impurity diffusion regions which are the source and the drain of the protective MOS transistor T.sub.2. A gate electrode metal layer G is arranged so as to cover a part of one side of the impurity diffusion region D and a part of one side of the impurity diffusion region S. The metal layer G is connected to the input terminal IN. At a first contact portion N.sub.1, the diffusion region D, that is, the drain of the protective transistor T.sub.2, electrically contacts another metal layer M. The metal layer M is connected to the gate of the input stage transistor T.sub.1 in the semiconductor integrated circuit IC (FIG. 1). The diffusion region S, that is, the source of the protective transistor T.sub.2, electrically contacts a grounding electrode GND at contact portions N.sub.3, N.sub.4, and N.sub.5. The diffusion region D is extended to form the protective resistor R. At an end portion D.sub.1 of the protective resistor R, a second contact portion N.sub.2 is provided. At the second contact portion N.sub.2, the end portion D.sub.1 of the resistor region R, that is, the other end of the protective resistor R, electrically contacts the gate electrode metal layer G. An insulation film ISO.sub.1 is formed on the surface of the semiconductor substrate (not shown in FIG. 2) except for the diffusion regions D and S and an active region between the diffusion regions D and S. The insulation film ISO.sub.1 insulates the protective transistor T.sub.2 and the protective resistor R from other elements.

In the structure of the conventional device illustrated in FIG. 2, a problem exists in that the diffusion region D must be large since the diffusion region must be extended in order to incorporate the protective resistor R. Also, if a high voltage of, for example, 200 V or more, is applied to the second contact portion N.sub.2, the electrode metal destroys the diffusion region under it and contacts with the semiconductor substrate under the diffusion region. This destruction phenomena will now be described with reference to FIG. 3.

FIG. 3 is an expanded cross-sectional view taken along line III--III' of FIG. 2. In FIG. 3, the diffusion region D.sub.1, that is, the end portion D.sub.1 of the protective resistor R (FIG. 2), is formed beneath the surface of a semiconductor substrate SUB. The type of conductivity of the semiconductor substrate SUB is opposite to that of the diffusion region D.sub.1. The periphery of the diffusion region D.sub.1 is insulated from other elements (not shown) by a first insulating film ISO.sub.1. An oxidation film L for insulating between layers is formed so as to cover the diffusion region D.sub.1, except for the contact portion N.sub.2 a metal layer G is formed on the insulating oxidation film L and on the contact portion N.sub.2.

During normal operation, a sufficiently low voltage of, for example, 5 V is applied to the input terminal IN (see FIGS. 1 and 2). In this case, the electrode metal does not destroy the diffusion region D.sub.1 at the second contact portion N.sub.2 and the input voltage is definitely applied through the metal layer G, the protective resistor R, the diffusion region D, and the metal layer M to the gate of the input stage MOS transistor T.sub.1 (see FIGS. 1 and 2). In contrast, if an extremely high voltage of, for example, 200 V or more is applied, due to static electricity, to the input terminal IN, a part M.sub.0 of the electrode metal destroys the diffusion region D.sub.1 and contacts the semiconductor substrate SUB. As a result, the electrode metal is short-circuited to the semiconductor substrate SUB so that the semiconductor integrated circuit IC (FIG. 1) cannot be driven. In addition, due to a PN junction capacitor between the diffusion region D.sub.1 under the contact portion N.sub.2, and the semiconductor substrate, the input carriers are stored in or discharged from the PN junction capacitor so that the operating speed of the semiconductor integrated circuit is deteriorated.

The present invention will now be described in detail with reference to FIGS. 4 through 6, in which the above-mentioned destruction phenomena is prevented and the necessary space for the diffusion region of the protective resistor is decreased, as is the PN junction capacitance at the contact portion.

FIG. 4 is a plan view of a main portion of a protective device according to a first embodiment of the present invention. In FIG. 4, the same or similar portions are denoted by the same reference symbols as the reference symbols employed in FIG. 2.

The difference between the plan view of FIG. 2 and the plan view of FIG. 4 is that in FIG. 2 the protective resistor R having the end portion D.sub.1 is formed by extending the diffusion region D while in FIG. 4 the diffusion region D.sub.10 is not extended so as to form a protective resistor having an end portion. Therefore, the length of the gate electrode metal layer G.sub.10 of the protective transistor T.sub.2 in FIG. 4 is greatly reduced in comparison with the conventional structure shown in FIG. 2. In FIG. 4, ISO.sub.2 represents a second insulating film formed in the diffusion region D.sub.10. A contact portion N.sub.20 is provided in the diffusion region D.sub.10 so that the gate electrode metal layer G.sub.10 electrically contacts, through a first polysilicon layer PS.sub.1 and a second polysilicon layer PS.sub.2, the diffusion layer D.sub.10. The diffusion region D.sub.10 not only provides a space for the contact portion N.sub.1 to make the gate electrode metal layer G.sub.10 contact the diffusion region D.sub.10, but also provides a space for the drain of the protective MOS transistor T.sub.2. The structure of FIG. 4 will be described in more detail with reference to FIGS. 5 and 6.

FIG. 5 is an enlarged plan view illustrating the contact portion N.sub.20 and the periphery of the contact portion N.sub.20 of FIG. 4. FIG. 6 is a cross-sectional view taken along line VI--VI' of FIG. 5. In FIGS. 5 and 6, the second insulating film ISO.sub.2 is provided in the portion of the diffusion region D.sub.10 near the gate electrode metal layer G.sub.10. Therefore, the second insulating film ISO.sub.2 is surrounded by the diffusion region D.sub.10 on all sides. In other words, the second insulating film ISO.sub.2 is formed as an island in the diffusion region D.sub.10. The center of the surface of the second insulating film ISO.sub.2 substantially corresponds to the contact portion N.sub.20. Thus, the central portion of the diffusion region D.sub.10 is insulated from other layers by the second insulating film ISO.sub.2. The periphery of the diffusion region D.sub.10 is also insulated from other elements (not shown) by the first insulating film ISO.sub.1. On the second insulating film ISO.sub.2, a first polysilicon layer PS.sub.1 having a high concentration of impurities is formed. A second polysilicon layer PS.sub.2, having a concentration of impurities lower than the concentration of the first polysilicon layer PS.sub.1, is formed so as to electrically contact the first polysilicon layer PS.sub.1 and the diffusion region D.sub.10. That is, the second polysilicon layer PS.sub.2 surrounds a part of the periphery of the second insulating layer ISO.sub.2, a part of the periphery of the first polysilicon layer PS.sub.1, and a part of the upper surface of the diffusion region D near the periphery of the second insulating film ISO.sub.2. A contact portion N.sub.20 is provided on the upper surface of the first polysilicon layer PS.sub.1. An oxidation film L.sub.1 for insulating between layers is also formed on the first polysilicon layer PS.sub.1, except for the contact portion N.sub.20, on the second polysilicon layer PS.sub.2, on the remainder of the portion of the diffusion region D.sub.10 not covered by the second polysilicon layer PS.sub.2, and on the first insulating film ISO.sub.1. A gate electrode metal layer G.sub.10 is formed on the oxidation film L.sub.1 and the contact portion N.sub.20.

In the structure illustrated in FIGS. 4, 5, and 6, even when a high voltage of up to 500 V is applied to the gate electrode metal layer G.sub.10 connected to an imput terminal, the electrode metal does not destroy through the diffusion region D.sub.10 and contact the semiconductor substrate because of the presence of the first polysilicon layer PS.sub.1 and the second insulating film ISO.sub.2. Also, because the concentration of impurities in the first polysilicon layer PS.sub.1 is higher than the concentration of impurities in the second polysilicon layer PS.sub.2, the electrical resistance of the first polysilicon layer PS.sub.1 is lower than the electrical resistance of the second polysilicon layer PS.sub.2. Therefore, a good electric contact is effected between the gate electrode metal layer G.sub.10 and the first polysilicon layer PS.sub.1. The second polysilicon layer PS.sub.2 acts as a protective resistor similar to the protective resistor R shown in FIG. 1. Therefore, during normal operation, an electrical path is formed between the gate electrode metal layer G.sub.10 and the diffusion region D.sub.10 through the first and the second polysilicon layers. Thus, extending the conventional diffusion region D and the resistance region R as illustrated in FIG. 2 is not necessary according to the present invention. Further, under the contact portion N.sub.20, there is only the first polysilicon layer PS.sub.1 and the insulating film ISO.sub.2. Therefore, almost no PN junction capacitance is present at the contact portion N.sub.20. Accordingly, the input impedance for the semiconductor integrated circuit is decreased in comparison with the conventional device.

FIG. 7 is a plan view of a main portion of a protective device according to a second embodiment of the present invention. FIG. 8 is a cross-sectional view taken along line VIII--VIII' of FIG. 7. The main difference between the first embodiment and the second embodiment is that instead of forming the second insulation film ISO.sub.2 in the portion of the diffusion region D.sub.10 near the gate electrode metal layer G.sub.10, no such second insulation film ISO.sub.2 is formed in the diffusion region D.sub.20 (FIG. 7), that is, in the drain region of the protective transistor T.sub.2. In FIGS. 7 and 8, a contact portion N.sub.30 is formed in an area adjacent to the diffusion region D.sub.20. More precisely, as is apparent from FIGS. 2, 4, and 7, the first insulating film ISO.sub.1 which is for insulating the drain diffusion region D.sub.20, the source diffusion region S, and the active region between the regions D.sub.20 and S from other elements, is adjacent to the first diffusion region. In the second embodiment, on the first insulating film ISO.sub.1 near the left-hand side of the diffusion region D.sub.20, a first polysilicon layer PS.sub.10, having a high concentration of impurities, is formed as illustrated in FIGS. 7 and 8. A second polysilicon layer PS.sub.20, having a concentration of impurities lower than the concentration of the first polysilicon layer PS.sub.10 is formed so as to electrically contact the first polysilicon layer PS.sub.10 and the diffusion region D.sub.20. The contact portion N.sub.30 is provided on the upper surface of the first polysilicon layer PS.sub.10. An oxidation film L.sub.2 for insulating between layers also covers the first insulating film ISO.sub.1, the first polysilicon layer PS.sub.10 except for the contact portion N.sub.30, the second polysilicon layer PS.sub.20, the diffusion region D.sub.20, a gate oxide film OX for the transistor T.sub.2, the source diffusion region S, and the remainder of the surface of the semiconductor substrate SUB. The gate electrode metal layer G.sub.20 electrically contacts the first polysilicon layer PS.sub.10 at the contact portion N.sub.30.

With the structure illustrated in FIG. 7 or 8, the same or similar effects can be obtained as the effects obtained with the first embodiment.

It will be apparent to those skilled in the art that the present invention is not restricted to the illustrated embodiments but that many other changes and modifications are possible without departing from the spirit of the present invention. For example, instead of forming the contact portion on the left-hand side of the diffusion region in FIG. 7, the contact portion may be formed on the upper side of the diffusion region. Also, a polysilicon wiring layer may be substituted for the input electrode metal layer.

From the forgoing description, it will be apparent that, according to the present invention, at the contact portion of the input electrode wiring layer in a semiconductor integrated circuit, the electrode material is prevented from destroying the diffusion region even when a high voltage of up to 500 V is applied to the input terminal of the semiconductor integrated circuit. Also, the protective resistor region is reduced in comparison with the conventional art. Further, the junction capacitance between the contact region and the diffusion region is reduced so that a low input impedance is obtained, resulting in high-speed operation of the semiconductor integrated circuit.