Semiconductor Circuit Device With Voltage Clamp

1. Field of the Invention

The present invention relates to a semiconductor circuit device formed by a plurality of metal oxide semiconductor (MOS) transistors. More particularly, it relates to a large scale integrated circuit (LSI) having a circuit structure which prevents bonding pads on the MOS LSI chip from being subjected to an electrolytic corrosion action caused by moisture or the water content of the atmosphere.

2. Description of the Related Art

As is well-known, an N-channel MOS transistor is, in general, formed by a p-type substrate, n.sup.+ diffusion regions formed in the substrate, and aluminum (A1) electrodes. These electrodes are connected to input or output pads (A1 pads) on a semiconductor chip. In this case, generally one end of a gold wire is bonded to the A1 pad and the other end of the gold wire is connected to an external terminal of a lead frame of the LSI package which encapsulates the semiconductor chip.

In an LSI chip, particularly an LSI chip formed by a plastic package, when moisture or the water content of the atmosphere sinks into the LSI chip through a very narrow gap between the plastic mold and the lead frame, a kind of "battery" is formed between the bonding pad (anode side) and the substrate electrode or the LSI chip stage (cathode side) based on a difference in the tendency toward ionization caused by the moisture or water content acting as an electrolyte. In this case, since a large current flows from the p-type substrate to the bonding pad through the n.sup.+ diffusion region, the bonding pad is gradually corroded by this large current caused by the "battery". Consequently, contact faults occur between the gold wire and the bonding pad.

Moreover, this electrolytic corrosion action frequently occurs in an MOS transistor type that is activated by a back gate voltage applied to the substrate. This is because the substrate potential gradually rises when the back gate voltage is cut off, so that the threshold voltage of the MOS transistor rises and the input/output MOS transistor for preventing electrostatic breakdown is turned on. This results in a large electrolytic corrosion current flowing from the substrate to the bonding pad.

The object of the present invention is to provide a semiconductor circuit device having a circuit structure which prevents bonding pads from being subjected to an electrolytic corrosion action caused by moisture or water content of the atmosphere.

In accordance with the present invention, there is provided a semiconductor circuit device comprising a semiconductor substrate, a plurality of metal oxide semiconductor (MOS) transistors formed on the semiconductor substrate and a plurality of ground side power source lines formed on the semiconductor substrate. In addition, a back gate bias generating circuit, formed between the substrate and the ground side power source line, for supplying a back gate voltage to the substrate is provided. A clamp circuit including a MOS diode formed on the substrate is provided between the substrate and the ground side power source line, for clamping a potential of the substrate to a predetermined level when the back gate bias generating circuit is not operated.

FIG. 1 is a sectional view of a conventional semiconductor circuit device;

FIG. 2 is a typical input circuit of a conventional MOS transistor circuit;

FIG. 3 is a typical output circuit of a conventional MOS transistor circuit;

FIG. 4 is a conventional back gate bias generating circuit;

FIG. 5 is a clamp circuit according to an embodiment of the present invention;

FIG. 6 is an input circuit having a clamp circuit as shown in FIG. 5;

FIG. 7 is an output circuit having a clamp circuit as shown in FIG. 5;

FIG. 8A is a plan view of a clamp circuit according to an embodiment of the present invention;

FIG. 8B is a sectional view taken along the line A--A' of FIG. 8A;

FIG. 9A is a plan view of an input circuit according to an embodiment of the present invention;

FIG. 9B is a sectional view taken along the line B--B' of FIG. 9A;

FIG. 10A is a plan view of an output circuit according to an embodiment of the present invention;

FIG. 10B is a sectional view taken along the line C--C' of FIG. 10A; and,

FIG. 11 is a perspective partial view of the semiconductor circuit device according to an embodiment of the present invention.

Before describing the preferred embodiments, an explanation will be given of a conventional semiconductor circuit device formed by a plurality of MOS transistors.

Referring to FIG. 1, reference number 1 is a stage of a lead frame, 2 is a gold (Au) plating layer on stage 1, 3 is a p-type silicon (Si) substrate, 4 is a substrate electrode having a Au evaporation layer, and 5 is a Su-Si solder for bonding the chip 3 to the stage 1. Reference numbers 6, 7, and 8 are n.sup.+ l diffusion regions.

As is well-known, aluminum electrodes 60, 70, and 80 are provided on the n.sup.+ diffusion regions 6, 7, and 8 and gate electrodes are provided on the silicon oxide film (SiO.sub.2) shown by a dotted area. In FIG. 2, the aluminum layer 60 is a ground wiring for supplying the ground voltage to the inner circuit, and generally, the n.sup.+ diffusion region 6 connected to the ground wiring is large enough to allow a large current to flow therein. On the other hand, the other n.sup.+ diffusion regions 7, 8 are relatively small regions which are connected to the aluminum electrodes 70 and 80, for example. In the p-n junction shown in FIG. 1, an internal resistance r in the substrate 3 is relatively small (about several tens of ohms) because the corresponding p-n junction area is wide, as shown by reference number 6. The other internal resistance R.sub.i (R.sub.O) is large (about several hundreds of ohms) because the corresponding p-n junction area is narrower than that of the resistance r. Accordingly, a large electrolytic corrosion current I.sub.D1 flows from the substrate electrode 4 or Au plating layer 2 to the bonding pad (see FIG. 2) connected to the A1 electrode through the p-type substrate 3 and n.sup.+ diffusion region 6. The other electrolytic corrosion currents I.sub.D2 and I.sub.D3 also flow, but these currents are so small that they are considered to be negligible. The electrolytic corrosion current will be explained in detail with reference to FIGS. 2 and 3.

In FIGS. 2 and 3, reference number 9 is a bonding pad for a second power source V.sub.SS (usually, ground potential), 10 is a bonding pad for an input signal, 11 a bonding pad for an output signal, 12 is a back gate bias generating circuit V.sub.BB GEN for supplying a reference voltage V.sub.BB to the substrate 3 in FIG. 1, and 13 is a bonding pad for a first power source V.sub.DD. A protection transistor T.sub.1 is provided for preventing electrostatic breakdown. The source region of the transistor T.sub.1 corresponds to the n.sup.+ diffusion region 6 in FIG. 1 since the source region is connected to the V.sub.SS pad 9. The drain region of the transistor Y.sub.1 corresponds to the n.sup.+ diffusion region 7 in FIG. 1. Q.sub.1 and Q.sub.2 are input transistors. The output of this circuit is supplied to an internal circuit (not shown). The back gate bias generating circuit V.sub.BB GEN supplies the substrate potential (usually, a minus voltage) against ground (GND) V.sub.SS for proper operation of the MOS transistors. The detailed circuit of V.sub.BB GEN is shown in FIG. 4.

As shown in FIG. 4, this circuit operates as a so-called charge pump. That is, when the first power source V.sub.DD is supplied to the V.sub.BB GEN, an oscillator generates a pulse to a capacitor C so that the charges in the substrate are always pulled to the capacitor C through the transistor T.sub.a. The charges in the capacitor C are always supplied to ground V.sub.SS. Consequently, when V.sub.DD is supplied, the substrate potential V.sub.BB is maintained at the predetermined constant minus voltage as the reference value.

Problems arise, however, when the first power source V.sub.DD is cut OFF, and thus the circuit V.sub.BB GEN is rendered inoperative. That is, when the circuit V.sub.BB GEN does not operate, the substrate potential V.sub.BB gradually rises because the substrate is in a floating state and the charges in the substrate cannot be pulled out therefrom. Accordingly, the back gate voltage V.sub.BG of the MOS transistor T.sub.1 is raised, so that the threshold level of the MOS transistor becomes low, and then the transistor T.sub.1 is turned ON. Consequently, the large electrolytic corrosion current I.sub.D1 flows from the substrate 3 to the input pad 10 through the internal resistance r, the junction diode Dir, and the protection transistor T.sub.1 (see FIG. 2). This corrosion current causes a gradual corrosion of the bonding pad which destroys the contact between the pad and the wire. In FIG. 3, similarly, the output transistor T.sub.3 is turned ON so that the large electrolytic corrosion current flows from the substrate 3 to the output pad 11 through the internal resistance r, the junction diode Dir, and the output transistor T.sub.3 , causing a gradual corrosion of the output pad.

A semiconductor circuit device according to an embodiment of the present invention will be explained in detail below.

FIG. 5 is a clamp circuit according to an embodiment of the present invention. Referring to FIG. 5, a clamp circuit CC comprises only one MOS transistor T (MOS diode) having a source S connected to the ground V.sub.SS and a gate G and drain D connected to the substrate at point P.

When the back gate bias generating circuit V.sub.BB GEN shown in FIG. 4 is operated, this clamp circuit CC is not activated because the substrate potential V.sub.BB is maintained at a lower level than the ground potential V.sub.SS , so the transistor T is cut OFF.

When the back gate bias circuit V.sub.BB GEN does not operate, the substrate potential V.sub.BB gradually rises so that the transistor T is turned ON, because the gate voltage of the transistor T becomes high and the threshold level becomes relatively low. Accordingly, the V.sub.SS and V.sub.BB are short-circuited through the transistor T so that the voltage V.sub.SS becomes almost equivalent to the voltage V.sub.BB. In this case, the protection transistor T.sub.1 is not turned ON because the threshold voltage of transistor T.sub.1 is maintained at a relatively high level. Therefore, the electrolytic corrosion current I.sub.D1 shown in FIG. 2 no longer flows from the substrate to the bonding pad 10.

Referring to FIG. 6, the clamp circuit CC including MOS diode is provided between the ground V.sub.SS and the substrate V.sub.BB. Accordingly, when the back gate bias circuit V.sub.BB GEN does not operate, since the clamp circuit CC begins to operate, the electrolytic corrosion current no longer flows because the substrate voltage V.sub.BB becomes approximately equal to the ground voltage V.sub.SS .

Referring to FIG. 7, since the clamp circuit CC is already provided for in the input circuit, the electrolytic corrosion current does not flow from the substrate to the output pad 11.

FIG. 8A is a plan view of a clamp circuit including the MOS diode T in FIG. 5 according to an embodiment of the present invention. FIG. 8B is a sectional view taken along the line A--A' of FIG. 8A. In FIGS. 8A and 8B, G is a gate of the transistor T, n.sup.+ l (D) is a drain of the transistor T formed in the n.sup.+ diffusion region, and n.sup.+ (S) is a source of the transistor T formed in the n.sup.+ diffusion region. CH are contact holes for connecting between the Al electrode wires (V.sub.BB and V.sub.SS ) and the n.sup.+ diffusion region or the gate of the transistor. PSG is a phosphor-silicate glass layer, and F-OX is a field oxide layer.

In FIG. 8A, the contact holes CH.sub.1 and CH.sub.2 are used for connecting the substrate voltage aluminum layer V.sub.BB and the gate of the transistor T. That is, the contact point P shown in FIG. 5 is formed by the contact holes CH.sub.1 and CH.sub.2.

FIG. 9A is a plan view of input circuits based on FIG. 6 according to an embodiment of the present invention. FIG. 9B is a sectional view taken along the line B--B' of FIG. 9A. Although the clamp circuit CC is not shown in FIG. 9A, FIGS. 8A and 9A are combined with each other through the ground wiring V.sub.SS , as is obvious from the circuit shown in FIG. 6.

FIG. 10A is a plan view of output circuits shown in FIG. 7 according to an embodiment of the present invention. FIG. 10B is a sectional view taken along the line C--C' of FIG. 10A.

As is obvious from FIGS. 8A to 10A, the semiconductor circuit device according to an embodiment of the present invention is formed by a combination of circuits shown in FIGS. 8A to 10A. In this case, an internal circuit (not shown) is omitted because an arbitrary internal circuit can be connected between the input circuit and the output circuit.

FIG. 11 is a perspective view of the structure between the V.sub.BB pad and the terminal chip. The V.sub.BB pad is connected to the terminal chip by a gold wire. The ground wiring V.sub.SS on the substrate 3 is connected by way of the V.sub.SS pad to the clamp circuit and to the back gate bias circuit V.sub.BB GEN. In FIG. 11, the back gate voltage V.sub.BB generated by the circuit V.sub.BB GEN is provided through the terminal chip which connects the lead frame stage and the V.sub.BB bonding pad on the substrate 3. In another embodiment, the output of the back gate bias circuit V.sub.BB GEN is provided directly to the substrate 3 through a well known structure.