Doping methods for fully-depleted SOI structures, and device...

Semiconductor device manufacturing: process – Making field effect device having pair of active regions... – Charge transfer device

Reexamination Certificate

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C438S151000, C438S206000

Reexamination Certificate

active

06780686

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates generally to semiconductor fabrication technology, and, more particularly, to doping methods for fully-depleted SOI structures, and a device comprising the resulting doped regions.
2. Description of the Related Art
There is a constant drive within the semiconductor industry to increase the operating speed of integrated circuit devices, e.g., microprocessors, memory devices, and the like. This drive is fueled by consumer demands for computers and electronic devices that operate at increasingly greater speeds. This demand for increased speed has resulted in a continual reduction in the size of semiconductor devices, e.g., transistors. That is, many components of a typical field effect transistor (FET), e.g., channel length, junction depths, gate insulation thickness, and the like, are reduced. For example, all other things being equal, the smaller the channel length of the transistor, the faster the transistor will operate. Thus, there is a constant drive to reduce the size, or scale, of the components of a typical transistor to increase the overall speed of the transistor, as well as integrated circuit devices incorporating such transistors.
As transistors are continually scaled in keeping with the requirements of advancing technology, device reliability dictates a concomitant reduction in the power supply voltage. Hence, every successive technology generation is often accompanied by a reduction in the operating voltage of the transistor. It is known that transistor devices fabricated on silicon-on-insulator (SOI) substrates exhibit better performance at low operating voltages than do transistors of similar dimensions fabricated in bulk silicon substrates. The superior performance of SOI devices at low operating voltage is related to the relatively lower junction capacitances obtained on an SOI device compared to a bulk silicon device of similar dimensions. The buried oxide layer in an SOI device separates active transistor regions from the bulk silicon substrate, thus reducing junction capacitance.
FIG. 1
depicts an example of a transistor
10
fabricated on an illustrative silicon-on-insulator substrate
11
. As shown therein, the SOI substrate
11
is comprised of a bulk substrate
11
A, a buried oxide layer
11
B, and an active layer
11
C. The transistor
10
is comprised of a gate insulation layer
14
, a gate electrode
16
, sidewall spacers
19
, a drain region
18
A, and a source region
18
B. A plurality of trench isolation regions
17
are formed in the active layer
11
C. Also depicted in
FIG. 1
are a plurality of conductive contacts
20
formed in a layer of insulating material
21
. The conductive contacts
20
provide electrical connection to the drain and source regions
18
A,
18
B. As constructed, the transistor
10
defines a channel region
12
in the active layer
11
C beneath the gate insulating layer
14
. The bulk substrate
11
A is normally doped with an appropriate dopant material, i.e., a P-type dopant such as boron or boron difluoride for NMOS devices, or an N-type dopant such as arsenic or phosphorous for PMOS devices. Typically, the bulk substrate
11
A will have a doping concentration level on the order of approximately 10
15
ions/cm
3
. The buried oxide layer
11
B may be comprised of silicon dioxide, and it may have a thickness of approximately 200-360 nm (2000-3600 Å). The active layer
11
C may be comprised of a doped silicon, and it may have a thickness of approximately 5-30 nm (50-300 Å).
Transistors fabricated in SOI substrates offer several performance advantages over transistors fabricated in bulk silicon substrates. For example, complementary-metal-oxide-semiconductor (CMOS) devices fabricated in SOI substrates are less prone to disabling capacitive coupling, known as latch-up. In addition, transistors fabricated in SOI substrates, in general, have large drive currents and high transconductance values. Also, the sub-micron SOI transistors have improved immunity to short-channel effects when compared with bulk transistors fabricated to similar dimensions.
Although SOI devices offer performance advantages over bulk silicon devices of similar dimensions, SOI devices share certain performance problems common to all thin-film transistors. For example, the active elements of an SOI transistor are fabricated in the thin-film active layer
11
C. Scaling of thin-film transistors to smaller dimensions requires that the thickness of the active layer
11
C be reduced. However, as the thickness of the active layer
11
C is reduced, the electrical resistance of the active layer
11
C correspondingly increases. This can have a negative impact on transistor performance because the fabrication of transistor elements in a conductive body having a high electrical resistance reduces the drive current of the transistor
10
. Moreover, as the thickness of the active layer
11
C of an SOI device continues to decrease, variations in the threshold voltage (V
T
) of the device occur. In short, as the thickness of the active layer
11
C decreases, the threshold voltage of the device becomes unstable. As a result, use of such unstable devices in modern integrated circuit devices, e.g., microprocessors, memory devices, logic devices, etc., becomes very difficult if not impossible.
Additionally, off-state leakage currents are always of concern in integrated circuit design, since such currents tend to, among other things, increase power consumption. Such increased power consumption is particularly undesirable in many modern consumer devices employing integrated circuits, e.g., portable computers. Lastly, as device dimensions continue to decrease in fully depleted SOI structures, increased short channel effects may occur. That is, in such fully depleted devices, at least some of the field lines of the electric field of the drain
18
A tend to couple to the channel region
12
of the transistor
10
through the relatively thick (200-360 nm) buried oxide layer
11
B. In some cases, the electric field of the drain
18
A may act to, in effect, turn on the transistor
10
. Theoretically, such problems may be reduced by reducing the thickness of the buried oxide layer
11
B and/or increasing the doping concentration of the bulk substrate
11
A. However, such actions, if taken, would tend to increase the junction capacitance between the drain and source regions
18
A,
18
B and the bulk substrate
11
A, thereby negating one of the primary benefits of SOI technology, i.e., reducing such junction capacitance.
The present invention is directed to a device and various methods that may solve, or at least reduce, some or all of the aforementioned problems.
SUMMARY OF THE INVENTION
The present invention is generally directed to doping methods for fully-depleted SOI structures, and a device comprising such resulting doped regions. In one illustrative embodiment, the device comprises a transistor formed above a silicon-on-insulator substrate comprised of a bulk substrate, a buried oxide layer and an active layer, the transistor being comprised of a gate electrode, the bulk substrate being doped with a dopant material at a first concentration level. The device further comprises a first doped region formed in the bulk substrate, the first doped region being comprised of a dopant material that is the same type as the bulk substrate dopant material and having a greater concentration level of dopant material than the first concentration level of the bulk substrate, the first doped region being substantially aligned with the gate electrode.
In another illustrative embodiment, the device comprises a transistor formed above a silicon-on-insulator substrate comprised of a bulk substrate, a buried oxide layer and an active layer, the transistor being comprised of a gate electrode, the bulk substrate being doped with a dopant material at a first concentration level. The device further comprises first, second and third doped regions formed in the bulk substrate, the first, second and third regions being

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