Electricity: electrical systems and devices – Safety and protection of systems and devices – With specific voltage responsive fault sensor
Reexamination Certificate
1999-06-01
2001-08-14
Sherry, Michael J. (Department: 2836)
Electricity: electrical systems and devices
Safety and protection of systems and devices
With specific voltage responsive fault sensor
C361S056000, C361S111000
Reexamination Certificate
active
06275367
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor circuit device, and more particularly to an electrostatic protection circuit with high reliability.
2. Description of the Related Art
In a first example of a conventional electrostatic protection circuit shown in
FIG. 1
, a common wiring pattern
110
is provided on a semiconductor chip. Each of all terminals such as a power supply (Vdd) terminal
101
, a ground (GND) terminal
102
, and input/output terminals
103
and the common wiring pattern are connected by a parallel protection element composed of a parasitic bipolar element and a diode element. In the electrostatic protection circuit having such a structure, a completely equivalent discharge path can be established in any combination of any terminals and also any positive or negative application voltage mode. Therefore, the electrostatic protection circuit having high reliability can be easily provided.
The operation of the electrostatic protection circuit will be described with reference to FIG.
1
. Referring to
FIG. 1
, all terminals including the power supply (Vdd) terminal
101
and the ground (GND) terminal
102
are connected to the common wiring pattern
110
by the protection element, i.e., a parallel circuit composed of a bipolar element and a diode. When a positive electrostatic pulse is applied to the input terminal
103
with respect to the GND terminal as a reference point, a discharge path is the path of the input terminal
103
→the bipolar element Q
103
of the input terminal→the common wiring pattern
110
→the diode D
102
of the GND terminal→GND terminal
102
. Current flows through this discharge path to protect an internal circuit
120
. Also, in a case where a negative electrostatic pulse is applied to the input terminal
103
with respect to the GND terminal
102
as the reference point, current flows through the discharge path of the GND terminal
102
→the bipolar element Q
102
of the GND terminal the common wiring pattern
110
→the diode D
103
of the input terminal→the input terminal
103
, to protect the internal circuit.
FIG. 2
shows the current (I)-voltage (V) characteristics between the ground terminal
102
and one of the terminals other than the ground terminal
102
, e.g., the input terminal
103
, when the impedance of the internal circuit
120
is infinite in the first conventional example shown in
FIG. 1
, that is, when only the electrostatic protection circuit and the common wiring pattern are connected to each of these terminals. In this case, when an over-voltage is applied between the ground terminal
102
and the other terminal such as the input terminal
103
and the over-voltage reaches Vtp (Vtm in case of the negative over-voltage), a trigger current starts to flow through the protection element. When the trigger current higher than a threshold value flows, the bipolar element operates to clamp the applied voltage to a predetermined voltage of Vsbp (Vsbm). In this case, the current flows through the bipolar element and the diode, even when the over-voltage is a positive polarity or a negative polarity. Therefore, if a parasitic resistance can be ignored, the clamped voltage can be expressed as
Vsbp=|Vsbm|=Vc+Vbi,
where Vc is the clamped voltage by the bipolar element and Vbi is the built-in voltage of the diode. For example, when a bipolar element is the parasitic bipolar transistor manufactured in a MOSLSI process in accordance with the 0.6-&mgr; rule, Vc is about 7 V and Vbi is about 0.9 V. As seen from these values, the clamped voltage Vsbp and |Vsbm| are about 8 V. In this way, when the internal circuit
120
is not connected to the terminals
102
and
103
, the electrostatic protection circuit operates completely symmetrically and operates ideally.
However, actually, there is a case that the internal circuit
120
has the impedance characteristic shown in
FIG. 3
, resulting in degradation of protection capability. The reason of the degradation of the protection capability will be described below in detail.
FIG. 3
shows the I-V characteristic of the internal circuit
110
with a voltage being applied between the power supply terminal
101
and the ground terminal
102
. When a positive voltage is applied to the terminal
101
or
103
with respect to the ground terminal
102
, current does not flow, because the impedance of the internal circuit
120
is high. In accordance with, the current first starts to flow through the protection element as mentioned above. When the current reaches the trigger current, the bipolar element starts to operate to clamp the applied voltage. In this case, because most of the current flows through the protection element and the common wiring pattern
110
, the internal circuit
120
can be protected.
On the other hand, when a negative voltage is applied to the internal circuit
120
with respect to the ground terminal
102
, there is a case that the impedance of the internal circuit
120
is low, as shown in FIG.
3
. In this case, because an absolute value |Vsbm| of the clamped voltage is larger than the applied negative voltage even if the protection element enters the operation state, a lot of current flows through the internal circuit
120
. When this current centers on a small area of the internal circuit
120
, an element in the small area is sometimes damaged or destroyed. Further, because the impedance of the internal circuit
120
is low, there is a case that the current does not flow sufficiently through the protection element so that the bipolar element does not operate.
As a consequence, in the conventional electrostatic protection circuit shown in
FIG. 1
, there is the following problem. That is, a sufficient protection performance sometimes cannot be achieved, depending on the structure of the internal circuit
120
, when the negative over-voltage is applied to the other terminal with respect to the ground terminal
102
.
Next, an example of the internal circuit
120
will be described with reference to FIG.
1
. Referring to
FIG. 1
, an inverter Inv
1
is composed of a P-type MOS transistor TP
1
and an N-type MOS transistor TN
1
, and an inverter Inv
2
is composed of a P-type MOS transistor TP
2
and an N-type MOS transistor TN
2
in the same way. The input of the inverter Inv
1
is selectively connected to one of an internal signal &PHgr; and the ground (GND) potential in accordance with selection of a wiring pattern master slice switch SW.
For example, when the wiring pattern master slice switch SW is fixedly switched to the ground (GND) potential, the above-mentioned problem occurs. That is, when a positive voltage with respect to the Vdd terminal
101
is applied to the GND terminal
102
, the N-type MOS transistor TN
1
is set to the conductive state. Further, a PN junction between a P
+
-type impurity diffusion layer of the source/drain region of the P-type MOS transistor and an N well, i.e., a diode DS is set to a forward direction bias state, and a current flows to the Vdd terminal
101
via the N well. In this case, when the threshold voltage of the N-type MOS transistor TN
1
is, for example, 0.7 V, the voltage with which the current abruptly increases in
FIG. 3
, i.e., a threshold voltage is about 1.6 V, because the built-in voltage of the diode DS is 0.9 V.
The threshold voltage of the electrostatic protection circuit is generally determined based on a breakdown voltage of the N
+
-type impurity diffusion layer, and is about 14 V. Therefore, when a positive surge voltage is applied to the GND terminal
102
, large current flows through the inverter Inv
1
so that degradation of the junction is caused or a gate insulating film is damaged.
In the above-mentioned example, whether or not it results in the destruction of the internal circuit
120
is greatly dependent upon the circuit structure of the internal circuit
120
. Also, it depends on the dimension and layout of each element in the discharge p
Fujii Takeo
Narita Kaoru
NEC Corporation
Sherry Michael J.
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