Electrostatic protection circuit and semiconductor...

Details Electrostatic protection circuit and semiconductor... Electrostatic protection circuit and semiconductor...

2000-12-19

2003-12-30

Toatley, Jr., Gregory J. (Department: 2836)

Electricity: electrical systems and devices

Safety and protection of systems and devices

Load shunting by fault responsive means

C361S111000

Reexamination Certificate

active

06671146

ABSTRACT:

TECHNICAL FIELD
The present invention relates to an electrostatic protection circuit and a semiconductor integrated circuit using the same, particularly to an electrostatic protection circuit for an integrated circuit using an insulated gate field effect transistor (hereinafter referred to as a MOSFET).
BACKGROUND ART
FIG. 9
shows a typical electrostatic protection circuit having a commonly-used signal terminal
1509
and power supply terminals
1
and
2
of a MOS integrated circuit using a bulk substrate. In
FIG. 9
, there are two paths for absorbing static electricity charge applied between the signal terminal
1509
and the power supply terminals
1
and
2
. One is a path in which discharge current flows from the signal terminal
1509
to the first power supply terminal
1
designated as potential +V
DD
through a diode
1503
and the other is a path in which discharge current flows from the second power supply terminal
2
designated as potential −V
SS
to the signal terminal through a diode
1504
. There is another path in which discharge current flows from the second power supply terminal
2
through the diode
1501
to the first power supply terminal
1
. To be more practical, an input signal from a pad terminal
1506
is fed to gate electrodes of a p-type MOSFET
1507
and an n-type MOSFET
1508
which form an internal circuit inverter, through a resistor
1505
and the connection of the terminals of the diodes
1503
and
1504
, as shown in FIG.
9
.
If the pad
1506
is directly connected to the gate of the p-type MOSFET
1507
or n-type MOSFET
1508
of the internal circuit in
FIG. 9
, the gate electrodes of the p-type MOSFET
1507
and the n-type MOSFET
1508
are often broken when static electricity is applied from the pad
1506
. To prevent this, a resistor
1505
for buffering the shock of static electricity and the diodes
1503
and
1504
for absorbing the charge are used. In addition, the diode
1501
serves as a charge-absorbing path not only for static electricity applied between the first and second power supply terminals but also for static electricity applied to the above-described signal terminals as discussed later.
In the conventional electrostatic protection circuit, the above-mentioned diode element
1503
for absorbing charges is connected so as to conduct the charges to the first power supply terminal
1
, and the diode element
1504
is connected so as to conduct the charges from the second power supply terminal
2
to the signal terminal
1509
. Further, the diode
1501
is connected in a reverse direction between the first power supply terminal
1
and the second power supply terminal
2
. This is because, if the diodes
1501
,
1503
, and
1504
are connected in a forward direction so as to conduct the current in the opposite direction to that shown in
FIG. 9
, leakage currents may flow through the forward-biased diodes when the integrated circuit is connected to a power supply.
Further, in integrated circuits using a silicon-on-insulator substrate (hereinafter abbreviated as “SOI integrated circuit”), there are no wells such as those in a bulk substrate, the bottom is insulated by a buried oxide film, and the side is also covered with a local oxide film formed by LOCOS (local oxidation of silicon) method. For this reason, there is generally no equivalent to the diode
1501
between the first and second power supply terminals shown in FIG.
9
. In other words, there is no diode formed by a p well and an n well in a conventional substrate between the first and second power supply terminals as shown in FIG.
11
. However, such a diode is required from the viewpoint of electrostatic protection as discussed later. Therefore, in an SOI integrated circuit, a diode
1801
is added between the first and second power supply terminals as shown in
FIG. 12
, or a MOSFET
1901
with the source and gate electrodes connected together is connected between the first and second power supply terminals for causing the MOSFET
1901
to perform the function of the reverse-biased diode as shown in FIG.
13
. Alternatively, a p-type MOSFET
2001
and n-type MOSFET
2000
each having the source and gate electrodes connected together are connected in parallel between the first and second power supply terminals as shown in
FIG. 14
to cause them perform the function of the reverse-biased diodes. SOI integrated circuits are thus provided with electrostatic protection based on the same principle as for bulk-substrate integrated circuits.
If static electricity is added between the power supply terminals or between the signal terminal and one of the power supply terminals, electrostatic breakdown may occur inside the integrated circuit. In the case where static electricity is added to signal terminals, the internal circuit to which the signal terminals are connected may frequently be damaged unless the charges are quickly absorbed by an electrostatic protection circuit. In
FIG. 9
, the gates of p-type MOSFET
1507
and n-type MOSFET
1508
may be broken down. The gate insulating film of a MOSFET is made of a very thin film from several hundred angstrom to several tens angstrom formed between the substrate and the gate electrode, and the substrate or the source electrode is finally connected to the power supply. As a result, a high voltage is applied across the thin gate silicon oxide film to produce a strong electric field, resulting in breakdown of the gate film. Therefore, when static electricity is added, the electrostatic protection circuit in
FIG. 9
or a similar means is used to absorb the charges quickly. In the circuit of
FIG. 9
, the following four cases are possible for the flow of charges between the signal terminal
1509
and the first and second power supply terminals
1
and
2
:
(A) Signal terminal: positive charges, First power supply terminal: negative charges
(B) Signal terminal: negative charges, First power supply terminal: positive charges
(C) Signal terminal: positive charges, Second power supply terminal: negative charges
(D) Signal terminal: negative charges, Second power supply terminal: positive charges
In the conventional circuit shown in
FIG. 9
, the diode
1503
or diode
1504
operates in a forward direction in the cases (A) and (D) above. Accordingly, electrostatic charges that comes in can be quickly absorbed, thereby preventing electrostatic destruction. In cases (B) and (C), the diodes
1503
and
1504
are both in a reverse direction with respect to the polarity of electrostatic charges. In case (B), negative charge forces its path through the diode
1503
in a reverse direction. Otherwise, the negative charges first flow through the diode
1504
in a forward direction, then flow from the second power supply terminal
2
to the first power supply terminal
1
through the reverse diode
1501
in the substrate. In case (C), positive charges force its path through the diode
1504
in a reverse direction. Alternatively, the positive charges first flow through the diode
1503
in a forward direction, then flow from the first power supply terminal
1
to the second power supply terminal
2
through the reverse diode
1501
in the substrate. Therefore, in cases (B) and (C), because charges must necessarily flow through a diode in a reverse direction, the circuit is vulnerable to static electricity and can be broken by even comparatively low voltages. A path through which electrostatic charges flow out in the case (C) is shown in
FIG. 10
as an example.
The function of the reverse-biased diode between the power supplies in the case where static electricity is applied to the signal terminals has been described above. If static electricity is applied between the power supply terminals in the same polarity as the power supply voltage, the charges flow through the diode between the power supply terminals in a reverse direction. Since even this diode does not exist in SOI integrated circuits, electrostatic charges cannot be absorbed, and hence breakdown easily occurs at the most vulnerable part among the parts involved in the p

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