Active solid-state devices (e.g. – transistors – solid-state diode – Avalanche diode
Reexamination Certificate
2003-04-11
2004-08-31
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
Avalanche diode
C257S106000, C257S605000, C257S606000
Reexamination Certificate
active
06784520
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention generally relates to semiconductor devices for use in semiconductor integrated circuits, and more particularly relates to semiconductor devices that are formed in integrated circuits and constitute constant voltage devices used to raise internal voltage, for example. The present invention also relates to methods for fabricating the semiconductor devices.
In a booster provided inside an integrated circuit, a constant voltage device, which is called a “clamping diode”, has been conventionally used to fix an increased voltage at a given voltage. Such a constant voltage device, also known as a Zener diode, uses the reverse breakdown phenomenon that occurs at a pn junction formed by a semiconductor substrate and a doped layer formed in the semiconductor substrate in order to obtain a given constant voltage.
(First Conventional Case)
Hereinafter, a constant voltage device in accordance with a first conventional case will be described with reference to FIG.
7
. As shown in
FIG. 7
, the constant voltage device of the first conventional case includes an n-type doped layer
103
and a p-type doped layer
104
. The n-type doped layer
103
is formed by doping with n-type impurity ions a part of the upper portion of a p-type semiconductor substrate
101
, in an active region
100
surrounded by an isolating oxide film
102
. The p-type doped layer
104
is formed by doping the remaining part thereof with p-type impurity ions. In this case, a pn junction is formed in the substantially central portion of the active region
100
by the n-type and p-type doped layers
103
and
104
.
An interlevel dielectric film
105
is formed on the semiconductor substrate
101
. In the interlevel dielectric film
105
, plugs
106
of tungsten are formed to be electrically connected to the n-type and p-type doped layers
103
and
104
, respectively. Further, interconnects
107
made of aluminum are formed on the interlevel dielectric film
105
in such a manner that the interconnects
107
establish connection with the respective plugs
106
.
In the constant voltage device of the first conventional case, the constant voltage of the device is determined by the reverse breakdown voltage at the pn junction formed by the n-type and p-type doped layers
103
and
104
. Specifically, when a reverse voltage exceeding the constant voltage is applied to the region between the n-type and p-type doped layers
103
and
104
, the reverse current resulting from the Zener effect or the avalanche effect flows between the n-type and p-type doped layers
103
and
104
. This phenomenon allows the voltage between the n-type and p-type doped layers
103
and
104
to be substantially maintained at a constant voltage, even if a large voltage is applied.
(Second Conventional Case)
Next, a constant voltage device in accordance with a second conventional case will be described with reference to FIG.
8
. The constant voltage device shown in
FIG. 8
has a structure in which a pn junction is formed between a p-type semiconductor substrate
101
and an n-type doped layer
103
formed in an upper portion of an active region
100
in the p-type semiconductor substrate
101
.
In the constant voltage device of the second conventional case, the constant voltage of the device is determined by the reverse breakdown voltage at the pn junction formed by the n-type doped layer
103
and the semiconductor substrate
101
. Specifically, when a reverse voltage exceeding the constant voltage is applied between an aluminum interconnect
107
and the p-type semiconductor substrate
101
, the resulting reverse current due to the Zener effect or the avalanche effect flows between the semiconductor substrate
101
and the n-type doped layer
103
. Even if a large voltage is applied, the voltage between the aluminum interconnect
107
and the semiconductor substrate
101
is thus substantially maintained at a constant value.
The constant voltage devices of the first and second conventional cases both have the following problems, however.
First, in the constant voltage device of the first conventional case, the doped layers
103
and
104
of the mutually differing conductivity types are both formed side by side in the semiconductor-substrate
101
active region
100
in a planar direction of the substrate principal surface, leading to the problem that the constant voltage device accounts for a large area in an integrated circuit. Further, since the constant voltage of the device is determined by the reverse breakdown voltage applied to the pn junction that is the interface formed between the n-type and p-type doped layers
103
and
104
, impurity concentration in at least one of the n-type and p-type doped layers
103
and
104
has to be adjusted in order to obtain a desired constant voltage.
In the constant voltage device of the second conventional case, on the other hand, the doped layer is of n-type alone, allowing the area occupied by the device in an integrated circuit to be reduced. As mentioned above, however, the constant voltage of the device is determined by the reverse breakdown voltage applied to the pn junction, that is, the interface formed between the n-type doped layer
103
and the p-type semiconductor substrate
101
. Thus, as in the first conventional case, impurity concentration in at least one of the semiconductor substrate
101
and the n-type doped layer
103
has to be adjusted in order to obtain a desired constant voltage.
Nevertheless, generally, in integrated circuits, an n-type doped layer
103
, a p-type doped layer
104
, or a semiconductor substrate
101
is often used as a well shared by other semiconductor devices. Thus, the level of freedom at which impurity concentration in the n-type doped layer
103
, p-type doped layer
104
, or semiconductor substrate
101
may be independently adjusted is low. Consequently, it is very difficult to establish an intended constant voltage for the constant voltage device.
Moreover, the constant voltage devices of the first and second conventional cases also have the problem that the reverse breakdown voltage changes with time.
FIG. 9A
shows the relationship between duration of applied constant current stress and variation in reverse breakdown voltage in the constant voltage devices of the first and second conventional cases.
FIG. 9B
shows the relationship between time for which the constant voltage devices are left to stand under high temperature conditions after constant current stress has been applied, and variation in reverse breakdown voltage. It should be understood that the conditions under which the measurements shown in
FIG. 9A
were carried out include an applied current of 200 &mgr;A and an evaluation temperature of 125° C. The measurements shown in
FIG. 9B
were made under conditions in which a current of 2 mA was applied for 3.5 hours, and thereafter the constant voltage devices were left to stand under a temperature of 150° C. The line plotted with circles represents the first conventional case, while the line plotted with triangles represents the second conventional case. As can be seen from
FIGS. 9A and 9B
, the variation in the constant voltage is as much as 1 to 1.2 V in the first conventional case, and the variation in the constant voltage is about 0.7 to 0.9 V in the second conventional case.
SUMMARY OF THE INVENTION
The present invention was made in view of the conventional problems, and it is therefore an object thereof to facilitate obtaining a desired constant voltage in a semiconductor device, while reducing the area occupied by the device, and at the same time to prevent reverse breakdown voltage therein from varying with time.
In order to achieve the object, a semiconductor device according to the present invention has a structure in which in at least one of p-type and n-type doped layers that form a pn junction, a portion adjacent to (in the vicinity of) an isolation film has a lower impurity concentration than the other portion.
Such a structure allows the position at which reverse breakdown o
Matsushita Electric - Industrial Co., Ltd.
McDermott & Will & Emery
Ngo Ngan V.
LandOfFree
Semiconductor devices constitute constant voltage devices... does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Semiconductor devices constitute constant voltage devices..., we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Semiconductor devices constitute constant voltage devices... will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3330743