Increasing the susceptability of an integrated circuit to...

Details Increasing the susceptability of an integrated circuit to... Increasing the susceptability of an integrated circuit to...

2000-06-09

2004-09-21

Nadav, Ori (Department: 2811)

Active solid-state devices (e.g., transistors, solid-state diode

With means to control surface effects

Insulating coating

C257S635000, C257S921000

Reexamination Certificate

active

06794733

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to semiconductor processing and integrated circuits in general, and, more particularly, to integrated circuits that are susceptible to ionizing radiation.
BACKGROUND OF THE INVENTION
As is well known in the prior art, older semiconductor processing technologies produced integrated circuits that were highly susceptible to damage from ionizing radiation. Such ionizing radiation is emitted from galactic sources (e.g., the Sun, stars, pulsars, quasars, black holes, etc.) and exists above the ionosphere. Ionizing radiation is also emitted when nuclear weapons are detonated.
The mechanism by which ionizing radiation affects the electrical characteristics of CMOS and NMOS transistors and other semiconductor devices is described in detail below in conjunction with
FIGS. 1A through 1C
. A summary of the mechanism is as follows.
Under normal operating conditions, the threshold voltage, V
T
, of an n-type transistor is normally and advantageously high. When an n-type transistor is exposed to ionizing radiation, the ionizing radiation causes the threshold voltage, V
T
, of the transistor to fall, which can cause the transistor to operate abnormally. If an n-type transistor is exposed to a sufficiently large total dose of ionizing radiation, then the threshold voltage can fall so low that the transistor continuously conducts current between the source and drain. This causes the transistor to fail completely and typically destroys the operation of the integrated circuit of which it is a part.
A detailed understanding of the mechanism by which ionizing radiation affects CMOS and NMOS transistors requires and understanding of the physical and electrical structure of an n-type transistor.
An n-type transistor, such as that depicted in
FIGS. 1A through 1C
, comprises two distinct “transistors” that are electrically in parallel: (1) an “operating” transistor, and (2) a “parasitic” transistor.
The operating transistor is the intended ideal structure for regulating the flow of current between the source and drain. The parasitic transistor is an unintended, but real transistor structure that results from artifacts in the manufacture of the operating transistor and from the fact that the materials used to build the operating transistor do not function perfectly. The operating transistor and the parasitic transistor each have their own threshold voltage, V
T
. Because the operating transistor and the parasitic transistor are in parallel, the one with the lower threshold voltage, V
T
, is the one that effectively overrules the operation of the other. For the purposes of this specification, the “effective threshold voltage” of a transistor is defined as the lower of (i) its operating transistor's threshold voltage and (ii) its parasitic transistor's threshold voltage.
Under normal operating conditions, the threshold voltage of the parasitic transistor is higher than the threshold voltage of the operating transistor, and, therefore, the current flow between the source and the drain is regulated by the operating transistor. This is the desired condition.
In contrast, when the transistor has been exposed to a high dose of ionizing radiation, the threshold voltage of the parasitic transistor can fall below that of the operating transistor. In fact, the threshold voltage of the parasitic transistor can fall so low that it becomes a “closed” circuit, which effectively shorts the source and drain regardless of the state of the operating transistor. This is the abnormal condition. As stated above, when the source and drain are effectively shorted together regardless of the state of the operating transistor, the transistor fails completely and most likely destroys the operation of the integrated circuit of which it is a part.
FIG. 1A
depicts a plan view of n-type transistor
102
, which comprises: n-type drain region
104
, n-type source region
106
, and gate electrode
108
, (e.g., polysilicon, etc.). Drain region
104
and source region
106
are bounded by field oxide
112
, (e.g., silicon dioxide, etc.), which help electrically isolate transistor
102
from other transistors that might be near it (and are not shown in FIG.
1
A).
FIG. 1B
is a cross-section of transistor
102
along line I—I (as shown in
FIG. 1A
) as viewed in the direction indicated and depicts the germane portions of the operating transistor. The operating transistor comprises: gate electrode
108
which overlies gate dielectric
118
and channel region
110
in p-type substrate
114
between drain region
104
and source region
106
.
FIG. 1C
is a cross-section of transistor
102
along line II—II (as shown in
FIG. 1A
) as viewed in the direction indicated and depicts the germane portions of the parasitic transistor. The parasitic transistor comprises: gate electrode
108
which overlies field oxide
112
/p-type substrate
114
at regions
120
, wherein field oxide
112
forms a shape similar to a bird's beak. Typically, p-type regions
116
are more heavily-doped than substrate
114
to increase the threshold voltage of the parasitic transistor, which is advantageous because it helps to ensure that the threshold voltage of the operating transistor is higher than the threshold voltage of the parasitic transistor.
As stated above, the exposure of an n-type transistor to ionizing radiation can change the threshold voltage, V
T
, of both the operating transistor and the parasitic transistor. The threshold voltage, V
T
, of either is theoretically predicted by the following equation:
V
T
=&phgr;−(&sgr;/&egr;)
d−F
  (Eq.1)
where: &phgr; is the work function of the gate region; &sgr; is the total charge at the dielectric(insulator)-semiconductor interface; &egr; is the dielectric constant of the insulator; d is the insulator thickness; and F is a term that can be considered a constant.
If, somehow, positive interface charge, &sgr;, is added at the dielectric(insulator)-semiconductor interface, then the threshold voltage, V
T
, decreases. One way of adding positive interface charge, &sgr;, to the device is to expose it to ionizing radiation. It can be seen from Equation 1 that with a sufficient increase in the positive interface charge, &sgr;, an n-type transistor can have an effective threshold voltage, V
T
, of zero.
When an n-type transistor is exposed to ionizing radiation, electron-hole pairs are formed in the gate dielectric and the field oxide. Some of the holes become trapped in the gate dielectric and field oxide as various gate-induced fields sweep out the electrons as part of normal circuit operation. Because holes behave like positive charge, this phenomenon is referred to as positive-charge trapping. Although the electrons are swept out of the circuit, the trapped “positive” charges migrate toward the dielectric(insulator)-semiconductor interface, which adds positive interface charge, &sgr;, and decreases the effective threshold voltage, V
T
, of the transistor.
Because the field oxide traps more positive charge than the gate dielectric, and because the threshold voltage decreases as the positive interface charge increases, the threshold voltage of the parasitic transistor at regions
120
will therefore shift downwardly more than will the threshold voltage of the operating transistor (i.e., the transistor having gate dielectric
118
) when exposed to the same amount of ionizing radiation. If the dose of radiation is sufficiently great, the parasitic transistor will conduct at regions
120
(i.e., under the edge of field oxide
112
) when the operating transistor normally conducts. Therefore, the parasitic transistor is more susceptible to ionizing radiation than is the operating transistor.
The important consequence of the susceptibility of integrated circuits to ionizing radiation is that they are not well suited for use in satellites or in military applications, and, therefore, they can be freely sold and exported without creating the fear that they can be used militarily against the United States or its allies.
In contrast,

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