Semiconductor device having an oxygen-rich punchthrough region extending through the length of the active region

A semiconductor device having an oxygen-rich punchthrough region under the channel region, and a process for fabricating such a device are disclosed. In accordance with one embodiment, a semiconductor device is formed by forming an oxygen-rich punchthrough region in a substrate, and forming a channel region over the oxygen-rich punchthrough region. The use of an oxygen-rich punchthrough region may, for example, inhibit the diffusion of dopants used in forming the channel region.

FIELD OF THE INVENTION 
The present invention is directed generally to semiconductor devices and, 
more particularly, to a semiconductor device having an oxygen-rich 
punchthrough region and a process for fabricating such a device. 
BACKGROUND OF THE INVENTION 
Over the last few decades, the electronics industry has undergone a 
revolution by the use of semiconductor technology to fabricate small, 
highly integrated electronic devices. The most common and important 
semiconductor technology presently used is silicon-based. A large variety 
of semiconductor devices have been manufactured having various 
applications in numerous disciplines. One such silicon-based semiconductor 
device is a metal-oxide-semiconductor (MOS) transistor. 
The principal elements of a typical MOS semiconductor device are 
illustrated in FIG. 1. The device generally includes a semiconductor 
substrate 101 on which a gate electrode 103 is disposed. The gate 
electrode 103 acts as a conductor. An input signal is typically applied to 
the gate electrode 103 via a gate terminal (not shown). Heavily-doped 
source/drain regions 105 are formed in the semiconductor substrate 101 and 
are connected to source/drain terminals (not shown). As illustrated in 
FIG. 1, the typical MOS transistor is symmetrical, which means that the 
source and drain are interchangeable. Whether a region acts as a source or 
drain depends on the respective applied voltages and the type of device 
being made (e.g., PMOS, NMOS, etc.). Thus, as used herein, the term 
source/drain region refers generally to an active region used for the 
formation of a source or drain. 
A channel region 107 is formed in the semiconductor substrate 101 beneath 
the gate electrode 103 and separates the source/drain regions 105. The 
channel is typically lightly doped with a dopant of a type opposite to 
that of the source/drain regions 105. A punchthrough region 111 is 
typically formed beneath the channel region 107. The punchthrough region 
111 is typically moderately doped with a dopant of a type opposite to that 
of the source/drain regions 105. 
The gate electrode 103 is generally separated from the semiconductor 
substrate 101 by an insulating layer 109, typically an oxide layer such as 
SiO.sub.2. The insulating layer 109 is provided to prevent current from 
flowing between the gate electrode 103 and the source/drain regions 105 or 
channel region 107. 
The source/drain regions 105, illustrated in FIG. 1, are 
lightly-doped-drain (LDD) structures. Each LDD structure includes a 
lightly-doped, lower conductivity region 106 near the channel region 107 
and a heavily-doped, higher conductivity region 104 typically connected to 
the source/drain terminal. Generally, the LDD structures are typically 
formed by implanting a first dopant into active regions adjacent the gate 
electrode 103 at relatively low concentration levels to form the 
lightly-doped regions 106; forming spacers 102 on sidewalls of the gate 
electrode 103; and implanting a second dopant into the active regions at 
higher concentration levels to form the heavily-doped regions 104. The 
substrate is typically annealed to drive the dopant in the heavily-doped 
regions deeper into the substrate 101. 
In operation, an output voltage is typically developed between the source 
and drain terminals. When an input voltage is applied to the gate 
electrode 103, a transverse electric field is set up in the channel region 
107. By varying the transverse electric field, it is possible to modulate 
the conductance of the channel region 107 between the source region and 
the drain region. In this manner, an electric field controls the current 
flow through the channel region 107. This type of device is commonly 
referred to as a MOS field-effect-transistor (MOSFET). Semiconductor 
devices, like the one described above, are used in large numbers to 
construct most modern electronic devices. 
The punchthrough region 111 plays an important role in the operation of a 
semiconductor device by reducing unwanted current flow between the source 
and the substrate 101, typically when the gate voltage is below the 
threshold voltage. The punchthrough region 111 is typically formed by 
implanting a dopant of the opposite conductivity type as that of the 
source/drain region 105. The punchthrough implant is typically performed 
before forming the gate insulating layer 109, and is typically implanted 
to a depth ranging from about 300 to 1,500 .ANG.. 
SUMMARY OF THE INVENTION 
Generally, the present invention relates to a semiconductor device having 
an oxygen-rich punchthrough region under the channel region, and a process 
for fabricating such a device. The use of an oxygen-rich punchthrough 
region may, for example, inhibit the diffusion of dopants used in forming 
the channel region. 
In accordance with one embodiment of the invention, a semiconductor device 
is formed by forming an oxygen-rich punchthrough region in a substrate, 
and forming a channel region over the oxygen-rich punchthrough region. 
The above summary of the present invention is not intended to describe 
every incrementation of the present invention. The figures and the 
detailed description which follow exemplify the embodiments more 
particularly.

While the invention is amenable to various modifications and alternative 
forms, specifics thereof have been shown by way of example in the drawings 
and will be described in detail. It should be understood, however, that 
the intention is not to limit the invention to the particular embodiments 
described. On the contrary, the intention is to cover all modifications, 
equivalents, and alternatives falling within the spirit and scope of the 
invention as defined by the appended claims. 
DETAILED DESCRIPTION OF THE VARIOUS EMBODIMENTS 
The present invention is believed to be applicable to a number of 
semiconductor devices, including in particular MOS, CMOS, and BiCMOS 
structures. While the present invention is not so limited, an appreciation 
of various aspects of the invention will be gained through a discussion of 
the fabrication process and characteristics of such a device in connection 
with the examples provided below. 
FIGS. 2A-2D illustrate an exemplary process for fabricating a semiconductor 
device with an oxygen-rich punchthrough region under a channel region. In 
accordance with this exemplary process, isolation regions 202 may be 
formed in a substrate 201, as illustrated in FIG. 2A. The isolation 
regions 202 may be formed using, for example, well-known LOCOS or trench 
isolation techniques. The isolation regions 202 are used to separate 
active areas (only one of which is shown) in which active devices, such as 
transistors, are formed. After forming the isolation regions, a well 
implant may be performed to form well regions in the substrate 201. The 
well implants are typically performed using a dopant of a conductivity 
type opposite to that of the substrate. The well implant may be performed 
using, for example, well-known techniques. The well regions are typically 
used to enable the fabrication of, for example, both PMOS and NMOS devices 
on the same substrate. Typically, the devices of one type are formed in 
the well regions and the devices of another type are formed outside of the 
well regions. 
An oxygen-rich punchthrough region 203 is formed in the substrate 201, as 
shown in FIG. 2B. The oxygen-rich punchthrough region 203 may be formed, 
for example, by implanting an oxygen-bearing species in the substrate 201. 
Suitable oxygen-bearing species include O or O.sub.2, for example. 
A channel region will be formed over the oxygen-rich punchthrough region 
203. The oxygen-rich punchthrough region 203 will be used to inhibit 
diffusion of dopants used to form the channel region and generally confine 
the channel dopants between the oxygen-rich punchthrough region 203 and 
the substrate surface. 
The implant energy is chosen depending on the oxygen-bearing species and on 
the desired depth of the oxygen-rich punchthrough region 203. Suitable 
depths of the oxygen-rich punchthrough region 203 range from about 300 to 
1,500 .ANG.for many applications. Suitable implant energies for oxygen 
range from about 50 to 250 keV for many applications. Suitable implant 
energies for an O.sub.2 implant range from about 100 to 500 keV for many 
applications. The concentration of the implanted oxygen-bearing species is 
chosen depending on the desired concentration of the oxygen-rich 
punchthrough region 203. For example, an oxygen-bearing species, such as O 
or O.sub.2, may be implanted with a concentration ranging from about 5E12 
to 5E15 atoms/cm.sup.2 (i.e. 5.times.10.sup.12 to 5.times.10.sup.15 
atoms/cm.sup.2). 
The oxygen-bearing species implant generally forms an oxygen-rich region in 
the substrate. The oxygen-rich region 203 may be an oxide region depending 
on the implant dosage. For example, at lower dosages (e.g., 5E13 
atoms/cm.sup.2 or less) typically little or no oxide region is formed. At 
these dosage levels, the oxygen-bearing layer 203 remains mildly 
conductive. At higher dosages (e.g. 5E13 atoms/cm.sup.2 and above) the 
oxygen-rich region becomes increasingly an oxide, such as silicon dioxide, 
region. As the oxide content of the region 203 increases, the conductive 
nature of the region 203 falls dramatically. 
A lightly-doped region 205 is formed over the oxygen-rich punchthrough 
region 203, as shown in FIG. 2B. Portions of the lightly-doped region 205 
will be used to form a channel region over the oxygen-rich punchthrough 
region 203. The lightly-doped region 205 is typically formed by implanting 
a dopant of the opposite conductivity type than that of the active region 
dopant. The lightly-doped region 205 and the associated implant are 
commonly referred to as a threshold voltage region and a threshold voltage 
implant respectively. 
A gate electrode 207 is formed over the substrate 201 after forming the 
oxygen-rich punchthrough region 203 and the lightly-doped region 205, as 
illustrated in FIG. 2C. The gate electrode 207 is typically separated from 
the substrate 201 by a gate insulating layer 206, such as an oxide layer, 
for example. It should be appreciated that the gate electrode structure 
depicted in FIG. 2C may be formed using a number of different known 
techniques. 
Active regions 209 may are formed in the substrate 201, adjacent the gate 
electrode 207, as illustrated in FIG. 2D. The active regions 209 may, for 
example, be LDD (lightly-doped-drain) source/drain regions. An LDD 
source/drain region may be formed by making two implants in the substrate 
201. Typically, the first implant is a low dose, low energy implant of a 
dopant (of a conductivity type opposite that of the channel region), and 
the second implant is a heavy dose, higher energy implant of a 
similar-type or the same dopant. The first dose is typically implanted 
into the substrate 201 when the semiconductor device has the configuration 
shown in FIG. 2C. Before the second dose is implanted, spacers (not shown) 
are formed on sidewalls of the gate electrode 207. The spacers are used to 
space the second dopant implant from the gate electrode 207 and the 
channel region below the gate electrode 207. The dopant implants, as well 
as spacer formation, may be done using well-known techniques. 
The conductivity type of the dopant(s) used to form the active regions 209 
depends on the type (e.g. NMOS or PMOS) of device being formed. For 
example, in an NMOS transistor the source/drain regions are formed by 
implanting an n-type dopant, such as arsenic or phosphorus. 
Correspondingly, in an PMOS transistor the source/drain regions are formed 
by implanting a p-type dopant, such as boron. The junction depth of the 
active region 209 will depend on the depth of the heavy dose dopant 
implant as well as the depth and type of oxygen-rich region 203. For 
example, when the oxygen-rich region 203 is substantially an oxide, the 
active region junction depth will be defined by the oxygen-rich region 
203. With a mildly conductive oxygen-rich region 203 (e.g., one formed at 
dosage less than about 5E13 atoms/cm.sup.2) the active region junction 
depth may be deeper than within, or at the interface of the oxygen-rich 
region 203. In either event, the oxygen-rich region 203 will serve to 
inhibit diffusion of the active region dopant. 
After forming the active regions 209, the substrate 201 is typically heated 
to activate the dopants in the active regions 209 and to drive the dopants 
deeper into the substrate 201. The substrate 201 may be heated using, 
e.g., well-known anneal techniques such as rapid thermal anneal (RTA). 
During this heating, the dopants in the active regions 209 as well as the 
channel dopants tend to diffuse further into the substrate 201. However, 
the oxygen-rich punchthrough region 203 will advantageously act to inhibit 
this diffusion. 
Using the above process, a semiconductor device can be fabricated having an 
oxygen-rich punchthrough region. The use of an oxygen-rich punchthrough 
region can, for example, inhibit the diffusion of channel dopants. This 
can, for example, increase the drive current of the transistor. The 
oxygen-rich punchthrough region may also, for example, inhibit the flow of 
leakage currents from the channel region. 
FIGS. 3A-3D illustrate another exemplary process for fabricating a 
semiconductor device with an oxygen-rich punchthrough region under the 
channel region, and oxygen-rich liners under the active regions. In 
accordance with this process, isolation regions 302 and well regions (not 
shown) are formed in a substrate 301, as illustrated in FIG. 3A. The 
isolation regions 302 and well regions may be formed in a similar manner 
as discussed above. 
An oxygen-rich punchthrough region 303 is formed in the substrate 301, as 
shown in FIG. 3B. The oxygen-rich punchthrough region 303 may be formed, 
for example, by implanting an oxygen-bearing species in the substrate 301. 
Suitable oxygen-bearing species include O or O.sub.2, for example. A 
channel region will be formed over the oxygen-rich punchthrough region 
303. The oxygen-rich punchthrough region 303 will be used to inhibit 
diffusion of dopants used to form the channel region and generally confine 
the channel dopants between the oxygen-rich punchthrough region 303 and 
the substrate surface. 
The implant energy is chosen depending on the oxygen-bearing species and on 
the desired depth of the oxygen-rich punchthrough region 303. Suitable 
depths of the oxygen-rich punchthrough region 303 range from about 0.03 to 
0.05 microns for many applications. Suitable implant energies for oxygen 
range from about 50 to 250 keV for many applications. Suitable implant 
energies for an O.sub.2 implant range from about 100 to 500 keV for many 
applications. The concentration of the implanted oxygen-bearing species is 
chosen depending on the desired concentration of the oxygen-rich 
punchthrough region 303. For example, a low dose of an oxygen-bearing 
species, such as O or O.sub.2, may be implanted with a concentration 
ranging from about 5E12 to 5E13 atoms/cm.sup.2. At these dosages the 
region 303 will remain mildly conductive. 
A lightly-doped region 305 is formed in the substrate 301, as illustrated 
in FIG. 3B. Portions of the lightly-doped region 305 will be used to form 
a channel region over the oxygen-rich punchthrough region 303. The 
lightly-doped region 305 and the associated implant are commonly referred 
to as a threshold voltage region and a threshold voltage implant 
respectively. The lightly-doped region 305 may be formed in accordance 
with the description of the lightly-doped region 205 in the previous 
embodiment. 
A gate electrode 307 is formed over the substrate 301, as illustrated in 
FIG. 3C. The gate electrode 307 is typically separated from the substrate 
301 by a gate insulating layer 306. The gate electrode 307 and the gate 
insulating layer 306 may be formed using, for example, well-known 
techniques. 
An oxygen-rich liner 308 is formed in the substrate 301 adjacent the gate 
electrode 307, as shown in FIG. 3C. The oxygen-rich liner 308 may, for 
example, be formed by implanting an oxygen-bearing species in the 
substrate 301. Suitable oxygen-bearing species include O and O.sub.2, for 
example. 
The oxygen-bearing species is generally implanted deeper into the substrate 
301 than the oxygen-rich punchthrough region 303. The implant energy is 
chosen depending on the oxygen-bearing species and on the desired depth of 
the oxygen-rich liner 308. The oxygen-rich liner generally inhibits the 
active region dopants from diffusing deeper into the substrate 301, and in 
this manner defines the depth of the active regions 309. The oxygen-rich 
region depth and thickness are selected in consideration of the desired 
active region depth. Suitable depths of the oxygen-rich liner 308 range 
from about 0.1 to 0.12 microns. Suitable implant energies for oxygen range 
from about 100 to 500 keV for many applications. Suitable implant energies 
for an O.sub.2 implant range from about 200 to 1,000 keV for many 
applications. 
The concentration of the implanted oxygen-bearing species is chosen 
depending on the desired concentration of the oxygen-rich liner 308. An 
oxygen-bearing species may, for example, be implanted with a concentration 
ranging from about 5E13 to 5E15 atoms/cm. As discussed above, at these 
dosages the oxygen-rich region is typically a silicon dioxide region. 
Active regions 309 are formed in the substrate 301, as illustrated in FIG. 
3D. The active regions 309 may, for example, be LDD (lightly-doped-drain) 
source/drain regions. LDD source/drain regions may be formed in a similar 
manner as discussed above. After forming the active regions 309, the 
substrate 301 may be heated to activate the dopants in the active regions 
309 and to drive the dopants deeper into the substrate 301. The substrate 
301 may be heated using, e.g., well-known anneal techniques such as rapid 
thermal anneal (RTA). 
During the anneal, the dopants in the active regions 309 as well as the 
channel dopants tend to diffuse further into the substrate 301. However, 
the oxygen-rich punchthrough region 303 and the oxygen-rich liner 308 will 
inhibit diffusion of dopants used in forming the channel region 205 and 
active regions 309 respectively. In particular, a silicon dioxide liner 
308 provides a sharp junction depth. Fabrication of the semiconductor 
device may continue with well-known processing steps such as silicidation, 
contact formation, and so forth, to complete the device structure. 
Using the above process, a semiconductor device can be fabricated having an 
oxygen-rich punchthrough region and an oxygen-rich liner beneath the 
active regions. The use of an oxygen-rich punchthrough region can, for 
example, inhibit the diffusion of dopants used to form the channel region 
and enhance the performance of the device. The use of an oxygen-rich liner 
can, for example, allow the sharp definition of active region junction 
depths and inhibit the flow of hot charge carriers between, for example, 
the drain and a grounded portion of the device. 
The present invention is applicable to the fabrication of a number of 
different devices including, but not limited to, MOS, CMOS, and BiCMOS 
structures. It should be noted, therefore, that when fabricating a CMOS 
device, for example, it will be necessary to form different source/drain 
regions using both n-type and p-type dopants. Accordingly, the present 
invention cannot be considered limited to the particular examples 
described above, but rather should be understood to cover all aspects of 
the invention as fairly set out in the attached claims. Various 
modifications, equivalent processes, as well as numerous structures to 
which the present invention may be applicable will be readily apparent to 
those of skill in the art to which the present invention is directed, upon 
review of the present specification. The claims are intended to cover such 
modifications and devices.