U.S. patent application number 12/940304 was filed with the patent office on 2011-02-24 for single transistor memory device having source and drain insulating regions and method of fabricating the same.
This patent application is currently assigned to Samsung Electronics Co., Ltd.. Invention is credited to Woo-Yeong Cho, Chang-Woo Oh, Ki-Whan Song, Nam-Kyun Tak.
Application Number | 20110042746 12/940304 |
Document ID | / |
Family ID | 38988094 |
Filed Date | 2011-02-24 |
United States Patent
Application |
20110042746 |
Kind Code |
A1 |
Tak; Nam-Kyun ; et
al. |
February 24, 2011 |
SINGLE TRANSISTOR MEMORY DEVICE HAVING SOURCE AND DRAIN INSULATING
REGIONS AND METHOD OF FABRICATING THE SAME
Abstract
A single transistor floating-body dynamic random access memory
(DRAM) device includes a floating body located on a semiconductor
substrate and a gate electrode located on the floating body, the
floating body including an excess carrier storage region. The DRAM
device further includes source and drain regions respectively
located at both sides of the gate electrode, and leakage shielding
patterns located between the floating body and the source and drain
regions. Each of the source and drain regions contact the floating
body, which may be positioned between the source and drain regions.
The floating body may also laterally extend under the leakage
shielding patterns, which may be arranged at outer sides of the
gate electrode.
Inventors: |
Tak; Nam-Kyun; (Bucheon-si,
KR) ; Song; Ki-Whan; (Seoul, KR) ; Oh;
Chang-Woo; (Suwon-si, KR) ; Cho; Woo-Yeong;
(Suwon-si, KR) |
Correspondence
Address: |
VOLENTINE & WHITT PLLC
ONE FREEDOM SQUARE, 11951 FREEDOM DRIVE SUITE 1260
RESTON
VA
20190
US
|
Assignee: |
Samsung Electronics Co.,
Ltd.
Suwon-si
KR
|
Family ID: |
38988094 |
Appl. No.: |
12/940304 |
Filed: |
November 5, 2010 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11829113 |
Jul 27, 2007 |
7851859 |
|
|
12940304 |
|
|
|
|
Current U.S.
Class: |
257/348 ;
257/E29.242 |
Current CPC
Class: |
H01L 29/7841 20130101;
H01L 27/10802 20130101; H01L 29/7834 20130101; H01L 29/1045
20130101; H01L 27/108 20130101; H01L 29/0653 20130101 |
Class at
Publication: |
257/348 ;
257/E29.242 |
International
Class: |
H01L 29/772 20060101
H01L029/772 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 1, 2006 |
KR |
10-2006-0107345 |
Nov 29, 2006 |
KR |
10-2006-0119087 |
Claims
1. A single transistor floating-body dynamic random access memory
(DRAM) device, comprising: a floating body located on a
semiconductor substrate, the floating body comprising an excess
carrier storage region; a gate electrode located on the floating
body; source and drain regions respectively located at both sides
of the gate electrode, each of the source and drain regions
contacting the floating body; and leakage shielding patterns
located between the floating body and the source and drain
regions.
2. The device according to claim 1, wherein the leakage shielding
patterns are arranged at outer sides of the gate electrode.
3. The device according to claim 1, wherein the leakage shielding
patterns contact bottom surfaces of the source and drain
regions.
4. The device according to claim 3, wherein the floating body is
positioned between the source and drain regions, and laterally
extends under the leakage shielding patterns.
5. The device according to claim 1, wherein the leakage shielding
patterns comprise at least one of a silicon oxide layer, a silicon
nitride layer and a silicon oxynitride layer.
6. The device according to claim 1, wherein a width of the excess
carrier storage region is larger than a width of the gate
electrode.
7. The device according to claim 1, wherein the floating body
comprises a single crystal semiconductor layer having p-type
impurity ions.
8. The device according to claim 1, further comprising: a buried
insulating layer located between the semiconductor substrate and
the floating body.
9. The device according to claim 1, further comprising: an
isolation layer defining the floating body, the leakage shielding
patterns contacting the isolation layer.
10. The device according to claim 1, wherein the semiconductor
substrate is configured to serve as a back gate electrode.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a divisional application of application
Ser. No. 11/829,113 filed on Jul. 27, 2007, which claims priority
from Korean Patent Application Nos. 10-2006-0107345 and
10-2006-0119087, filed on Nov. 1, 2006, and Nov. 29, 2006,
respectively, the subject matters of which are hereby incorporated
herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to a semiconductor device and
a method of fabricating the same. More particularly, the present
invention relates to a single transistor memory device having
insulating regions associated with the source and drain regions,
and a method of fabricating the same.
[0004] 2. Description of the Related Art
[0005] A conventional dynamic random access memory (DRAM) cell,
which is a type of volatile memory, typically incorporates a
capacitor, a transistor and interconnection units. In response to
demands for electronic devices to be increasingly lightweight,
small and thin, DRAM cells have become more highly integrated. That
is, as many DRAM cells as possible are formed within a restricted
space. However, the technology of highly integrating DRAM cells
faces several limitations.
[0006] For example, a typical DRAM cell capacitor includes upper
and lower electrodes, and a capacitor dielectric layer. The upper
and lower electrodes share an overlapping region, and the capacitor
dielectric layer is positioned between the upper and lower
electrodes. The capacitance of the capacitor is directly
proportional to the size of the overlapping region, and inversely
proportional to a thickness of the overlapping region. A minimum
area for forming a capacitor is therefore required.
[0007] A single transistor floating-body DRAM cell has been
developed, which includes a floating body region for storing data.
Because there is no capacitor, the single transistor floating-body
DRAM cell may be more highly integrated than a common DRAM cell
having a capacitor.
[0008] A capacitor-less, single transistor DRAM cell is described,
for example, in an article entitled "Scaled IT-bulk Devices Built
with CMOS 90 nm Technology for Low-cost eDRAM Applications" by R.
RANICA (IEEE Symposium on VLSI Technology Digest of Technical
Papers, pp. 38-39 (2005)). FIG. 1 herein is a cross-sectional view
of a single transistor DRAM cell like that disclosed in the RANICA
article.
[0009] Referring to FIG. 1, a semiconductor substrate 1 includes a
deep n-well 3 and a pocket p-well 5 located in the deep n-well 3.
An isolation layer 7 is located within a predetermined region of
the pocket p-well 5, defining an active layer 5a of the pocket
p-well 5. The isolation layer 7 is in contact with the deep n-well
3 through the pocket p-well 5. As a result, the active region 5a
functions as an electrically floated bulk region, which is
surrounded by the isolation layer 7 and the deep n-well 3.
[0010] Source and drain regions 16s and 16d are respectively
located in both ends of the bulk region 5a, and a gate pattern 10
is located on the bulk region 5a between the source and drain
regions 16s and 16d. The gate pattern 10 includes a gate insulating
layer 8 and a gate electrode 9, which are sequentially stacked. A
spacer 13 may be located on a sidewall of the gate pattern 10. The
source region 16s may include a heavily-doped source region 15s
spaced apart from the gate pattern 10, and a lightly-doped source
region 11s extending from the heavily-doped source region 15s.
Likewise, the drain region 16d may include a heavily-doped drain
region 15d spaced apart from the gate pattern 10, and a
lightly-doped drain region 11d extending from the heavily-doped
drain region 15d. The lightly-doped source and drain regions 11s
and 11d may be located beneath the spacer 13.
[0011] According to RANICA, the source and drain regions 16s and
16d are shallower in thickness than the active region 5a, i.e., the
bulk region, as illustrated in FIG. 1. Thus, the bulk region 5a may
also extend under the source and drain regions 16s and 16d. As a
result, during a program operation, the number of holes stored in
the bulk region 5a is maximized. However, the holes stored in the
bulk region 5a may be recombined with electrons in the source and
drain regions 16s and 16d after program operation, and erased in a
short period of time. In other words, the single transistor DRAM
cell illustrated in FIG. 1 has poor data retention
characteristics.
[0012] Furthermore, when the source and drain regions 16s and 16d
have large junction areas, junction capacitances Cs and Cd of the
source and drain regions 16s and 16d also increase. Thus, a loading
capacitance of a bit line electrically connected to the drain
region 16d increases, which may lead to a decrease in a data
sensing margin and operating speed of the single transistor DRAM
cell.
[0013] Another example of a single transistor floating-body DRAM
device is disclosed in U.S. Patent Application Publication No.
2006/0049444, entitled "Semiconductor Device and Method of
Fabricating the Same" by SHINO. According to SHINO, a floating body
having a single crystal structure is located on a semiconductor
substrate. The floating body has an expanded structure capable of
storing excess holes, although the excess holes may be easily
erased through source and drain regions.
SUMMARY OF THE INVENTION
[0014] An aspect of the present invention provides a single
transistor floating-body dynamic random access memory (DRAM)
device, including a floating body located on a semiconductor
substrate, the floating body having an excess carrier storage
region, and a gate electrode located on the floating body. Source
and drain regions are respectively located at both sides of the
gate electrode, each of the source and drain regions contacting the
floating body. Leakage shielding patterns are located between the
floating body and the source and drain regions. The leakage
shielding patterns may arranged at outer sides of the gate
electrode, and may contact bottom surfaces of the source and drain
regions. The leakage shielding patterns may include at least one of
a silicon oxide layer, a silicon nitride layer and a silicon
oxynitride layer.
[0015] The floating body may be positioned between the source and
drain regions, and may laterally extend under the leakage shielding
patterns. The floating body may include a single crystal
semiconductor layer having p-type impurity ions. A width of the
excess carrier storage region of the floating body may be larger
than a width of the gate electrode.
[0016] The DRAM device may further include a buried insulating
layer located between the semiconductor substrate and the floating
body, and an isolation layer defining the floating body, where the
leakage shielding patterns contact the isolation layer. Also, the
semiconductor substrate may be configured to serve as a back gate
electrode.
[0017] Another aspect of the present invention provides a method of
fabricating a single transistor floating-body DRAM device,
including defining a floating body in a semiconductor substrate,
forming a gate pattern on the floating body, and forming leakage
shielding patterns in the floating body at both sides of the gate
pattern. The floating body includes an excess carrier storage
region, which may extend beneath the leakage shielding patterns and
between the source and drain regions. Forming the leakage shielding
patterns may include injecting oxygen ions into the floating body
using the gate pattern as a mask to form temporary patterns, and
annealing the temporary patterns. Forming the leakage shielding
patterns may also include injecting germanium (Ge) ions into the
floating body using the gate pattern as a mask to form temporary
patterns, etching the temporary patterns to form gaps, and filling
the gaps with an insulating layer.
[0018] The method of fabricating the DRAM device may further
include forming source and drain regions in the floating body above
the gaps, and forming source and drain regions in contact with the
leakage shielding patterns. Also, a gate dielectric layer may be
formed on the floating body prior to forming the gate pattern.
Forming the gate pattern may include sequentially stacking a gate
electrode, a pad oxide layer and a mask nitride layer. Also,
defining the floating body in the semiconductor substrate may
include forming an isolation layer.
[0019] Another aspect of the present invention provides a single
transistor memory cell, including an active semiconductor pattern
having a bulk region and an impurity region, stacked in sequence on
a semiconductor substrate, and insulated from the semiconductor
substrate. The memory cell also includes a recessed region passing
through the impurity region, dividing the impurity region into a
source region and a drain region, which are separated from each
other, where the recessed region includes first and second
sidewalls adjacent to the source and drain regions, respectively. A
first insulating region is positioned between the source region and
the bulk region, and separated from the first sidewall of the
recessed region, and a second insulating region is positioned
between the drain region and the bulk region, and separated from
the second sidewall of the recessed region. Each of the first
insulating region and the second insulating region may include one
of an empty space or an insulating layer pattern. A gate electrode
is positioned within the recessed region.
[0020] A conductivity type of the impurity region may be different
than a conductivity type of the bulk region. The bulk region may
include a lower bulk region and an upper bulk region, the lower
bulk region having a higher impurity concentration than the upper
bulk region. In addition, the bulk region may include a first bulk
region extending between the first insulating region and the first
sidewall, and a second bulk region extending between the second
insulating region and the second sidewall. Also, the gate electrode
may include a projecting portion that extends above an upper
surface of the impurity region, and a spacer may cover sidewalls of
the projecting portion of the gate electrode. The first and second
bulk regions may be aligned with the spacer. A gate insulating
layer may be positioned between the gate electrode and the first
and second sidewalls of the recessed region. Also, an insulating
layer may cover the semiconductor substrate, the source and drain
regions and the gate electrode. The memory cell may further include
a back gate interconnection located on the insulating layer, and
electrically connected to the semiconductor substrate through a
back gate contact hole passing through the insulating layer.
[0021] Yet another aspect of the present invention provides a
method of fabricating a single transistor memory cell, including
forming an active semiconductor pattern surrounded by an isolation
layer on a semiconductor substrate, the active semiconductor
pattern including a bulk region and an impurity region, which are
sequentially stacked. The method further includes forming a gate
pattern passing through the impurity region to divide the impurity
region into a source region and a drain region, which are separated
from each other, and forming spacers on sidewalls of the gate
pattern. Sacrificial impurity ions are injected into the bulk
region, using the gate pattern and the spacers as ion injection
masks, to form first and second sacrificial impurity layers under
the source and drain regions, respectively. The sacrificial
impurity ions may be silicon germanium ions. The isolation layer is
etched to expose the first and second sacrificial impurity layers,
and the first and second sacrificial impurity layers are removed to
form first and second undercut regions, exposing bottom surfaces of
the source and drain regions, respectively. An insulating layer is
formed on the semiconductor substrate having the first and second
undercut regions.
[0022] Forming the active semiconductor pattern may include
preparing a silicon on insulator (SOI) substrate comprising a
supporting substrate, a buried insulating layer and a semiconductor
body layer, which are sequentially stacked; forming an isolation
layer in contact with the buried insulating layer in a
predetermined region of the semiconductor body layer to define an
isolated semiconductor body pattern; and forming the impurity
region in an upper portion of the semiconductor body pattern to
define the bulk region under the impurity region. Also, forming the
active semiconductor pattern may include preparing an SOI substrate
having a supporting substrate, a buried insulating layer and a
semiconductor body layer, which are sequentially stacked; forming
the impurity layer in an upper portion of the semiconductor body
layer to define the bulk layer under the impurity layer; and
forming an isolation layer contacting the buried insulating layer
in predetermined portions of the impurity layer and the bulk layer
to define an isolated semiconductor body pattern comprising the
bulk region and the impurity region.
[0023] Forming the gate pattern may include forming a mask pattern
covering the impurity region and the isolation layer, the mask
pattern defining an opening over the impurity region; forming a
recessed region by etching the impurity region and the bulk region
using the mask pattern as an etch mask, the recessed region
dividing the impurity region into the source and drain regions;
forming a gate insulating layer covering a bottom surface and
sidewalls of the recessed region; forming a gate electrode and a
capping insulating layer pattern, which are sequentially stacked,
on the gate insulating layer within the recessed region; and
removing the mask pattern to expose the gate electrode and the
capping insulating layer pattern.
[0024] The first and second sacrificial impurity layers may be
formed to have bottom surfaces above the bulk region, and may
define a first bulk region between the first sacrificial impurity
layer and the recessed region and a second bulk region between the
second sacrificial impurity layer and the recessed region. The
insulating layer may be formed to leave empty spaces in the first
and second undercut regions, or to fill the first and second
undercut regions. The method may further include patterning the
insulating layer and forming a back gate contact hole to expose the
semiconductor substrate, and forming a back gate interconnection
electrically connected to the semiconductor substrate through the
back gate contact hole.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] The embodiments of the present invention will be described
with reference to the attached drawings, not necessarily drawn to
scale, in which:
[0026] FIG. 1 is a cross-sectional view of a conventional single
transistor memory cell;
[0027] FIG. 2 is a cross-sectional view of a single transistor
floating-body DRAM device, according to a first exemplary
embodiment of the present invention;
[0028] FIG. 3 is a cross-sectional view of a single transistor
floating-body DRAM device, according to a second exemplary
embodiment of the present invention;
[0029] FIG. 4 is a plan view of a single transistor memory cell,
according to a third exemplary embodiment of the present
invention;
[0030] FIG. 5 is a cross-sectional view taken along line I-I' of
FIG. 4;
[0031] FIG. 6 is a cross-sectional view illustrating a method of
programming a single transistor memory cell, according to the third
exemplary embodiment of the present invention;
[0032] FIG. 7 is a cross-sectional view illustrating a method of
erasing a single transistor memory cell, according to the third
exemplary embodiment of the present invention;
[0033] FIGS. 8 to 11 are cross-sectional views illustrating a
method of fabricating a single transistor floating body DRAM
device, according to the first exemplary embodiment of the present
invention;
[0034] FIGS. 12 to 15 are cross-sectional views illustrating a
method of fabricating a single transistor floating body DRAM
device, according to the second exemplary embodiment of the present
invention; and
[0035] FIGS. 16 to 24 are cross-sectional views illustrating a
method of fabricating a single transistor memory cell, according to
the third exemplary embodiment of the present invention.
DETAILED DESCRIPTION OF THE EMBODIMENTS
[0036] The present invention will now be described more fully with
reference to the accompanying drawings, in which exemplary
embodiments of the invention are shown. The invention, however, may
be embodied in various different forms, and should not be construed
as being limited only to the illustrated embodiments. Rather, these
embodiments are located as examples, to convey the concept of the
invention to one skilled in the art. Accordingly, known processes,
elements, and techniques are not described with respect to some of
the embodiments of the present invention. Throughout the drawings
and written description, like reference numerals will be used to
refer to like or similar elements.
[0037] In the drawings, the thickness of layers and regions may be
exaggerated for clarity. Also, when a layer is described to be
formed on other layer or on a substrate, this means that the layer
may be formed on the other layer or on the substrate, or a third
layer may be positioned between the layer and the other layer or
the substrate.
[0038] FIG. 2 is a cross-sectional view of a single transistor
floating-body dynamic random access memory (DRAM) device, according
to a first exemplary embodiment of the present invention.
[0039] Referring to FIG. 2, a floating body 55 is on a
semiconductor substrate 51. A gate electrode 63 may be located on
the floating body 55, and source and drain regions 73 may be
positioned at both sides of the gate electrode 63. The source and
drain regions 73 may be in contact with the floating body 55.
Leakage shielding patterns 71' may be located between the floating
body 55 and the source and drain regions 73. An excess carrier
storage region 55S may be included in the floating body 55.
[0040] The semiconductor substrate 51 may be a single crystal
silicon wafer, for example, and the floating body 55 may be a
semiconductor layer formed from single crystal silicon, for
example. P- or n-type impurity ions may have been injected into the
floating body 55. Also, the impurity ions may show a graded ion
profile in the floating body 55. For example, the p-type impurity
ions may gradually decrease in concentration toward a surface of
the floating body 55.
[0041] Assuming that the floating body 55 has p-type impurity ions,
the device will be described below. A buried insulating layer 52
may be positioned between the semiconductor substrate 51 and the
floating body 55. In this case, the buried insulating layer 52 may
cover the semiconductor substrate 51. The buried insulating layer
52 may be an insulating layer, such as a silicon oxide layer. The
buried insulating layer 52 insulates the floating body 55 from the
semiconductor substrate 51.
[0042] An isolation layer 53, defining the floating body 55, is
positioned on the buried insulating layer 52, and may surround the
floating body 55. The isolation layer 53 may be an insulating
layer, such as a silicon oxide layer, a silicon nitride layer, a
silicon oxynitride layer, or a combination thereof.
[0043] A gate dielectric layer 61 may be positioned between the
floating body 55 and the gate electrode 63. The gate dielectric
layer 61 may be a silicon oxide layer or a high-k dielectric layer,
for example.
[0044] A mask nitride layer 67 may be located on the gate electrode
63. A pad oxide layer 65 may be positioned between the gate
electrode 63 and the mask nitride layer 67. The gate electrode 63,
the pad oxide layer 65 and the mask nitride layer 67, which are
sequentially stacked, may constitute a gate pattern. The gate
electrode 63 may be formed from a conductive layer, such as a
polysilicon layer, a metal silicide layer, a metal layer, or a
combination thereof. The mask nitride layer 67 may be a nitride
layer, such as a silicon nitride layer. The pad oxide layer 65 may
be a silicon oxide layer.
[0045] The source and drain regions 73 may include impurity ions
with a different conductivity type from the floating body 55. For
example, when the floating body 55 has the p-type impurity ions,
the source and drain regions 73 may include n-type impurity ions.
The source and drain regions 73 may be located at both sides of the
gate electrode 63. Also, the source and drain regions 73 may be
positioned to face each other over a space or distance, in which
case, at least a portion of the floating body 55 may be positioned
between the source and drain regions 73 within the space.
[0046] The leakage shielding patterns 71' may be located under the
source and drain regions 73. In this case, the floating body 55 may
extend beneath the leakage shielding patterns 71'. The leakage
shielding patterns 71' may be arranged at outer sides of the gate
electrode 63. The leakage shielding patterns 71' may be insulating
layers, such as silicon oxide layers.
[0047] As a result, the floating body 55 may be positioned between
the source and drain regions 73 facing each other, and extend
beneath the leakage shielding patterns 71'. Also, the floating body
55 may have a larger width than the gate electrode 63. The excess
carrier storage region 55S may likewise have a larger width than
the gate electrode 63.
[0048] One of the source and drain regions 73 is electrically
connected to a source line S, and the other of the source and drain
regions 73 is electrically connected to a drain line D. The gate
electrode 63 is electrically connected to a gate line G. Also, the
semiconductor substrate 51 may be electrically connected to a back
gate line BG. In an embodiment, the source line S may be connected
to a ground terminal, the drain line D may be a bit line, and the
gate line G may be a word line.
[0049] Storing data, i.e., a write operation, in a floating body
DRAM device according to the first exemplary embodiment of the
present invention will now be explained. A gate program voltage
higher than a threshold voltage (Vt) may be applied to the gate
electrode 63, and a bit program voltage may be applied to the drain
line D. The source line S may be grounded. The bit program voltage
may be a positive voltage.
[0050] In this case, electron-hole pairs (EHP) are generated in the
floating body 55 by impact ionization. The electrons generated by
impact ionization may flow through the drain line D, whereas the
holes generated by impact ionization may be accumulated in the
excess carrier storage region 55S. That is, excess holes are
accumulated in the excess carrier storage region 55S. The excess
holes accumulated in the excess carrier storage region 55S change
the threshold voltage (Vt).
[0051] However, a contact surface of the source and drain regions
73 and floating body 55 may be minimized by the leakage shielding
patterns 71'. In other words, the leakage shielding patterns 71'
may serve to block a leakage path of the excess holes. Accordingly,
the leakage shielding patterns 71' may serve to extend the time
during which the excess holes are retained in the excess carrier
storage region 55S. As a result, according to the first exemplary
embodiment of the present invention, the retention time of the
excess holes accumulated in the excess carrier storage region 55S
is significantly increased.
[0052] Furthermore, a back gate voltage may be applied to the back
gate line BG, enabling the semiconductor substrate 51 to serve as a
back gate, further extending the retention time of the excess
holes.
[0053] The erase operation of the floating body DRAM device may be
performed by applying a gate program voltage higher than the
threshold voltage (Vt) to the gate electrode 63, and applying a bit
erase voltage to the drain line D. The source line S may be
grounded. The bit erase voltage may be a negative voltage. The
excess holes accumulated in the excess carrier storage region 55S
may be erased by the erase operation.
[0054] A read operation of the floating body DRAM device may be
performed by applying a gate read voltage lower than the gate
program voltage to the gate electrode 63, and applying a bit read
voltage to the drain line D. The source line S may be grounded. An
amount of current flowing between the source line S and the drain
line D may be different depending on the existence or nonexistence
of the excess holes. The data stored in the floating body DRAM
device may be read out by sensing the amount of current flowing
between the source line S and the drain line D.
[0055] FIG. 3 is a cross-sectional view of a single transistor
floating-body DRAM device according to a second exemplary
embodiment of the present invention.
[0056] Referring to FIG. 3, a floating body 155 is on a
semiconductor substrate 151. A gate electrode 163 may be located on
the floating body 155. Source and drain regions 173 may be located
at both sides of the gate electrode 163. The source and drain
regions 173 may be in contact with the floating body 155. Leakage
shielding patterns 171' may be positioned between the floating body
155 and the source and drain regions 173. An excess carrier storage
region 155S may be located in the floating body 155.
[0057] The semiconductor substrate 151 may be a single crystal
silicon wafer, for example. The floating body 155 may be a
semiconductor layer, for example, formed of single crystal silicon.
P-type impurity ions may be injected into the floating body 155.
The p-type impurity ions may decrease in concentration toward a
surface of the floating body 155.
[0058] A buried insulating layer 152 may be positioned between the
semiconductor substrate 151 and the floating body 155. An isolation
layer 153 defining the floating body 155 may be on the buried
insulating layer 152. A gate dielectric layer 161 may be positioned
between the floating body 155 and the gate electrode 163. A pad
oxide layer 165 and a mask nitride layer 167, which are
sequentially stacked, may be located on the gate electrode 163.
[0059] The source and drain regions 173 may have impurity ions with
a different conductivity type from the floating body 155. For
example, when the floating body 155 has the p-type impurity ions,
the source and drain regions 173 may include n-type impurity ions.
The source and drain regions 173 may be located at both sides of
the gate electrode 163, respectively. Also, the source and drain
regions 173 may be positioned to face each other over a space or
distance. In this case, at least a portion of the floating body 155
may be positioned between the source and drain regions 173 within
the space.
[0060] An interlayer insulating layer 177 covering sidewalls of the
source and drain regions 173 may be included above the
semiconductor substrate 151. In an embodiment, top surfaces of the
interlayer insulating layer 177 and the mask nitride layer 167 may
be exposed on the same plane. The interlayer insulating layer 177
may be an insulating layer, such as a silicon oxide layer, a
silicon nitride layer, a silicon oxynitride layer, or a combination
thereof.
[0061] The interlayer insulating layer 177 may extend between the
source and drain regions 173 and the floating body 155, and thus
serve as the leakage shielding patterns 171', located under the
source and drain regions 173. In the depicted embodiment, the
floating body 155 may extend through the leakage shielding patterns
171'. The leakage shielding patterns 171' may be arranged on outer
sides of the gate electrode 163. The leakage shielding patterns
171' may be formed of an insulating layer, such as a silicon oxide
layer, a silicon nitride layer, a silicon oxynitride layer, or a
combination thereof.
[0062] As a result, the floating body 155 may be positioned between
the source and drain regions 173 facing each other and extending
beneath the leakage shielding patterns 171'. The floating body 155
may be larger in width than the gate electrode 163. The excess
carrier storage region 155S may likewise be larger in width than
the gate electrode 163.
[0063] One of the source and drain regions 173 is electrically
connected to a source line S, and the other of the source and drain
regions 173 is electrically connected to a drain line D. The gate
electrode 163 is electrically connected to a gate line G. Also, the
semiconductor substrate 151 may be electrically connected to a back
gate line BG. In an embodiment, the source line S may be connected
to a ground terminal, the drain line D may be a bit line, and the
gate line G may be a word line.
[0064] A write operation in the floating body DRAM device according
to the second exemplary embodiment of the present invention may
include applying a gate program voltage higher than a threshold
voltage (Vt) to the gate electrode 163, and applying a bit program
voltage to the drain line D. The source line S may be grounded. The
bit program voltage may be a positive voltage.
[0065] In this case, electron-hole pairs (EHPs) may be generated in
the floating body 155 by impact ionization. The electrons generated
by the impact ionization may flow through the drain line D, whereas
the holes generated by the impact ionization may be accumulated in
the excess carrier storage region 155S. That is, excess holes are
accumulated in the excess carrier storage region 155S. The excess
holes accumulated in the excess carrier storage region 155S change
the threshold voltage (Vt).
[0066] However, a contact surface of the source and drain regions
173 and the floating body 155 may be minimized by the leakage
shielding patterns 171'. In other words, the leakage shielding
patterns 171' may serve to block a leakage path of the excess
holes. Accordingly, the leakage shielding patterns 171' extend the
time during which the excess holes are retained in the excess
carrier storage region 155S. As a result, according to the second
exemplary embodiment of the present invention, the retention time
of the excess holes accumulated in the excess carrier storage
region 155S is significantly increased.
[0067] FIG. 4 is a plan view of a single transistor memory cell
according to a third exemplary embodiment of the present invention,
and FIG. 5 is a cross-sectional view taken along line I-I' of FIG.
4.
[0068] Referring to FIGS. 4 and 5, a buried insulating layer 253 is
stacked on a semiconductor substrate 251, and an active
semiconductor pattern 255a is on a portion of the buried insulating
layer 253. The semiconductor substrate 251 may be formed, for
example, from a supporting substrate of a silicon on insulator
(SOI) substrate. The active semiconductor pattern 255a may include
a bulk region 255c and an impurity region (for providing a source
region 261s and a drain region 261d, discussed below), which are
sequentially stacked. The impurity region may have a different
conductivity type from the bulk region 255c. For example, when the
bulk region 255c is formed of p-type semiconductor, the impurity
region may be formed of n-type semiconductor.
[0069] A recessed region R passes through the impurity region and
extends into the bulk region 255c. The recessed region R divides
the impurity region into a source region 261s and a drain region
261d, which are spaced apart from each other. In other words, a
depth of the recessed region R may be greater than thicknesses of
the impurity region, i.e., the source and drain regions 261s and
261d, and less than the total thickness of the active semiconductor
pattern 255a. Also, the recessed region R includes first and second
sidewalls SW1 and SW2, which are adjacent to the source and drain
regions 261s and 261d, respectively.
[0070] The recessed region R may contain an insulated gate
electrode 269g, which is insulated from the active semiconductor
pattern 255a by a gate insulating layer 267. That is, the gate
insulating layer 267 may be positioned between the insulated gate
electrode 269g and inner walls of the first and second sidewalls
SW1 and SW2 of the recessed region R. Furthermore, the insulated
gate electrode 269g may extend upwardly beyond the top surfaces of
the source and drain regions 261s and 261d. In this case, spacers
271 may be located on sidewalls of the projecting part of the
insulated gate electrode 269g. The active semiconductor pattern
255a, the insulated gate electrode 269g, the spacers 271 and the
buried insulating layer 253 may be covered with an insulating layer
277.
[0071] A first insulating region 277s of the insulating layer 277
may be located between the source region 261s and the bulk region
255c, and a second insulating region 277d may be located between
the drain region 261d and the bulk region 255c. The first and
second insulating regions 277s and 277d are in contact with bottom
surfaces of the source and drain regions 261s and 261d,
respectively. Also, the first and second insulating regions 277s
and 277d may be spaced a specific distance D from the first and
second sidewalls SW1 and SW2 of the recessed region R. Thus, a
first bulk region 255b' is located between the first insulating
region 277s and the first sidewall SW1, and a second bulk region
255b'' is located between the second insulating region 277d and the
second sidewall SW2. As a result, the first and second bulk regions
255b' and 255b'' may have the same width as the specific distance
D. When spacers 271 are included, the first and second bulk regions
255b' and 255b'' may be self-aligned with the spacers 271 to have
the same width as the spacers 271.
[0072] The first and second insulating regions 277s and 277d may be
empty spaces or insulating layer patterns. The bulk region 255c
includes the first and second bulk regions 255b' and 255b'', as
well as a base bulk region 259, which is located beneath the
insulating regions 277s and 277d, the first and second bulk regions
255b' and 255b'' and the recessed region R.
[0073] The bulk region 255c may include lower and upper bulk
regions. The lower bulk region may have a higher impurity
concentration than the upper bulk region. The first and second
insulating regions 277s and 277d may have the same thickness as the
upper bulk region. In this case, top surfaces of the insulating
regions 277s and 277d may be in contact with bottom surfaces of the
source and drain regions 261s and 261d, and bottom surfaces of the
insulating regions 277s and 277d may be in contact with a top
surface of the base bulk region 259. In other words, the lower bulk
region may correspond to the base bulk region 259 of FIG. 5, and
the upper bulk region may correspond to the first and second bulk
regions 255b' and 255b'' of FIG. 5. Alternatively, the first and
second insulating regions 277s and 277d may be thinner or thicker
than the upper bulk region. In any case, the top surfaces of the
first and second insulating regions 277s and 277d are in contact
with the bottom surfaces of the source and drain regions 261s and
261d.
[0074] A back gate interconnection 281b may be located on the
insulating layer 277. The back gate interconnection 281b may be
electrically connected to the semiconductor substrate 251 through a
back gate contact plug 279b, for example, filling a back gate
contact hole 278b passing through the insulating layer 277.
[0075] FIG. 6 is a cross-sectional view illustrating a program
operation which stores data "1" in the single transistor memory
cell illustrated with reference to FIGS. 4 and 5. The single
transistor memory cell illustrated in FIGS. 4 and 5 is assumed to
be an n-channel MOS transistor cell, merely for the convenience of
describing the operation, and it is thus understood that different
types of transistor cells may be incorporated into the depicted
embodiment without departing for the spirit and scope of the
present invention.
[0076] Referring to FIG. 6, the single transistor memory cell
according to the third exemplary embodiment of the present
invention may be programmed by several methods. For example, the
single transistor memory cell shown in FIGS. 4 and 5 may be
programmed by applying a source voltage V.sub.S of 0 volts to the
source region 261s, and a first drain voltage V.sub.D1 having a
positive pulse waveform to the drain region 261d. During
application of the first drain voltage V.sub.D1, a first gate
voltage V.sub.G1 may be applied to the gate electrode 269g, and a
back gate voltage V.sub.B having a negative voltage may be applied
to the semiconductor substrate 251.
[0077] The first gate voltage V.sub.G1 may be a voltage
corresponding to half of the drain voltage V.sub.D. In this case,
impact ionization occurs at a junction between the drain region
261d and the second bulk region 255b'', thereby generating a large
number of holes and electrons. The holes are stored in the bulk
region 255c to reduce a threshold voltage of the single transistor
memory cell of FIG. 6.
[0078] In particular, when the back gate voltage V.sub.B is applied
to the semiconductor substrate 251, most of the excess holes stored
in the bulk region 255c are stored in the lower region of the bulk
region 255c (i.e., the base bulk region 259) by an electric field
caused by the back gate voltage V.sub.B. Also, junction areas AS
and AD of the source and drain regions 261s and 261d may be
significantly less than the junction areas of the source and drain
regions 16s and 16d, for example, of the conventional single
transistor memory cell illustrated in FIG. 1, due to the presence
of the first and second insulating regions 277s and 277d.
Accordingly, even when the first drain voltage V.sub.D1 is changed
to 0 volts after the program operation, recombination paths between
the excess holes in the bulk region 255c and the electrons in the
source and drain regions 261s and 261d may be significantly
reduced, thereby increasing a holding time, i.e., a data retention
time of the excess holes in the bulk region 255c. As a result,
retention characteristics of data "1" improve in accordance with
the depicted embodiment.
[0079] In another embodiment, the first gate voltage V.sub.G1 may
be a negative voltage. In this case, holes may be induced into the
second bulk region 255b'' to cause band-to-band tunneling (BTBT)
between the drain region 261d and the second bulk region 255b''.
During BTBT, a large amount of excess holes are stored in the bulk
region 255c. Thus, the program operation may be performed.
[0080] Furthermore, when the bulk region 255c includes stacked,
lower and upper bulk regions, as described above, the retention
characteristics of data "1" are further improved. This is because
most excess holes stored in the bulk region 255c may be stably
stored in the lower bulk region having relatively larger volume and
higher impurity concentration than the upper bulk region without
application of the back gate voltage.
[0081] FIG. 7 is a cross-sectional view illustrating an erase
operation which stores data "0" in a single transistor memory cell
according to a third exemplary embodiment of the present invention.
Here, the single transistor memory cell is also assumed as an
n-channel MOS transistor cell for convenience of the
description.
[0082] Referring to FIG. 7, the single transistor memory cell
according to the third exemplary embodiment of the present
invention may be erased by applying a source voltage V.sub.S of 0
volts to the source region 261s, and a second drain voltage
V.sub.D2 having a negative pulse waveform to the drain region 261d.
The second drain voltage V.sub.D2 may have a negative voltage
during an erasing time T, and a voltage of 0 volts during an
initial state before the erasing time T and during a holding state
of data "0" after the erasing time T. Furthermore, a specific
voltage, for example, a second gate voltage V.sub.G2 of 0 volts,
may be applied to the gate electrode 269g during the erase
operation.
[0083] The holes in the bulk region 255c are injected into the
drain region 261d during the erasing time T, to increase a
threshold voltage of the single transistor memory cell of FIG. 7.
Thus, the single transistor memory cell may have data corresponding
to logic "0".
[0084] Subsequently, when the second drain voltage V.sub.D2 is
changed to 0 volts after the erasing time T, a surface potential of
the bulk region 255c, i.e., a channel region, may be changed. In
other words, when the channel region has a first surface potential
during the erasing time T, the channel region may have a second
surface potential different from the first surface potential after
the erasing time T. In this case, a difference between the first
and second surface potentials may change depending on magnitudes of
junction capacitances Cs' and Cd' in the source and drain regions
261s and 261d, respectively. Particularly, as the source and drain
junction capacitances Cs' and Cd' decrease, the difference between
the first and second surface potentials also decreases.
[0085] The source and drain junction capacitances Cs' and Cd' of
the single MOS transistor according to the present invention are
significantly smaller than the source and drain junction
capacitances Cs and Cd of the conventional single transistor memory
cell, for example, illustrated in FIG. 1, due to the presence of
the first and second insulating regions 277s and 277d. Accordingly,
after the conventional single transistor memory cell of FIG. 1 is
erased, e.g., using the method described above with respect to FIG.
7, the channel region of the conventional single transistor memory
cell may have a third surface potential higher than the second
surface potential. It is therefore understood that the difference
in threshold voltage before and after erasing the single transistor
memory cell further increases, as the surface potential of the
channel region after the erase operation decreases. As a result,
the difference between the threshold voltage before and after
erasing the single transistor memory cell according to the present
embodiment may be higher than that of the conventional single
transistor memory cell illustrated in FIG. 1. Thus, the single
transistor memory cell of the present embodiment shows a larger
sensing margin than the conventional single transistor memory cell
illustrated in FIG. 1.
[0086] Furthermore, after the erase operation, a specific voltage,
for example, 0 volts, may be continuously applied to the gate
electrode 269g. In this case, the second bulk region 255b'' may be
fully or partially depleted. Accordingly, even when a positive
voltage is applied to the drain regions 261d, the BTBT phenomenon
between the second bulk region 255b'' and the drain region 261d is
effectively prevented.
[0087] When the BTBT occurs at a junction of the drain region 261d
after the erase operation, excess holes are injected into the bulk
region 255c, and thus the single transistor memory cell of FIG. 7
may be programmed again. However, according to the embodiment, the
tunneling phenomenon of the erased single transistor memory cell is
suppressed, as described above, thus improving the data retention
characteristics of the erased single transistor memory cell.
Particularly, when the width of the second bulk region 255b'' is
decreased, the second bulk region 255b'' may be fully depleted. In
this case, the BTBT at a junction of the drain region 261d may be
further suppressed.
[0088] FIGS. 8 to 11 are cross-sectional views illustrating a
method of fabricating a single transistor floating-body DRAM
device, according to the first exemplary embodiment of the present
invention.
[0089] Referring to FIG. 8, a buried insulating layer 52 may be
formed on a semiconductor substrate 51, to cover the semiconductor
substrate 51. The buried insulating layer 52 may be formed from a
silicon oxide layer, for example. A floating body 55 and an
isolation layer 53 may be formed on the buried insulating layer
52.
[0090] The floating body 55 may be a semiconductor layer, for
example, formed from single crystal silicon. P- or n-type impurity
ions may be injected into the floating body 55. The impurity ions
may show a graded ion profile in the floating body 55. For example,
the p-type impurity ions may decrease in concentration toward a
surface of the floating body 55.
[0091] Assuming that the floating body 55 has the p-type impurity
ions, the fabrication method will be described below. The isolation
layer 53 may be formed to surround the floating body 55. The
isolation layer 53 may be an insulating layer, such as a silicon
oxide layer, a silicon nitride layer, a silicon oxynitride layer,
or a combination thereof. The isolation layer 53 may be formed by a
well-known shallow trench isolation (STI) method. Alternatively,
the floating body 55 and the isolation layer 53 may be formed using
a silicon on insulator (SOI) wafer.
[0092] Referring to FIG. 9, a gate dielectric layer 61 may be
formed to cover at least a portion of the floating body 55. The
gate dielectric layer 61 may be a silicon oxide layer or a high-k
dielectric layer, for example. In an embodiment, the gate
dielectric layer 61 may be formed to cover both the floating body
55 and at least a portion of the isolation layer 53.
[0093] A gate pattern crossing the floating body 55 may be formed
on the gate dielectric layer 61. The gate pattern may be formed
from a gate electrode 63, a pad oxide layer 65, and a mask nitride
layer 67, which are sequentially stacked on one another. The gate
electrode 63 may be a conductive layer, formed from a polysilicon
layer, a metal silicide layer, a metal later, or a combination
thereof, for example. The mask nitride layer 67 may be a nitride
layer, formed from a silicon nitride layer, for example. The pad
oxide layer 65 may be a silicon oxide layer, for example. When the
gate electrode 63 is formed from polysilicon, and the mask nitride
layer 67 is formed from silicon nitride, the pad oxide layer 65 may
serve to relieve stress caused by a difference in thermal expansion
coefficients between the polysilicon layer and the silicon nitride
layer.
[0094] Referring to FIG. 10, oxygen ions OX may be injected into
the floating body 55 using the gate pattern as an ion injection
mask, thereby forming temporary patterns 71. The injection of the
oxygen ions may be performed using various angles and energies.
[0095] The temporary patterns 71 may be formed at both sides of the
gate pattern, and arranged at outer sides of the gate pattern.
Furthermore, the temporary patterns 71 may be locally formed at a
predetermined depth in the floating body 55. That is, the floating
body 55 may be formed to remain beneath the temporary patterns 71.
One surface of the temporary patterns 71 may be formed to be in
contact with the isolation layer 53. The mask nitride layer 67 may
serve to prevent the injection of the oxygen ions into the gate
electrode 63.
[0096] Referring to FIG. 11, the semiconductor substrate 51 having
the temporary patterns 71 may be annealed to form leakage shielding
patterns 71'. While annealing the semiconductor substrate 51, the
oxygen ions in the temporary patterns 71 react with silicon, thus
forming a silicon oxide layer. Thus, the leakage shielding patterns
71' may be formed from the silicon oxide layer. As a result, the
leakage shielding patterns 71' are generally positioned the same as
the temporary patters 71, e.g., arranged at outer sides of the gate
pattern, and thus the gate electrode 63.
[0097] Impurity ions are injected into the floating body 55 using
the gate pattern as an ion injection mask, thereby forming source
and drain regions 73. When the floating body 55 has p-type impurity
ions, the source and drain regions 73 may be formed by injecting
n-type impurity ions.
[0098] The source and drain regions 73 may be formed at both sides
of the gate electrode 63, respectively. Also, the source and drain
regions 73 may be formed to face each other, over a distance or
separation. In this case, at least a portion of the floating body
55 may remain in the space between the source and drain regions 73.
The source and drain regions 73 and the floating body 55 may be in
contact with each other.
[0099] The source and drain regions 73 may be formed on the leakage
shielding patterns 71', and at least a portion of the floating body
55 may remain under the leakage shielding patterns 71'. As a
result, the floating body 55 may be positioned between the source
and drain regions 73, as well as beneath the leakage shielding
patterns 71'. The floating body 55 may be formed to have a larger
width than the gate electrode 63.
[0100] As described with reference to FIG. 2, an excess carrier
storage region 55S may be formed in the floating body 55. The
excess carrier storage region 55S may also have a larger width than
the gate electrode 63.
[0101] A contact surface of the source and drain regions 73 and the
floating body 55 may be minimized by the leakage shielding patterns
71'. That is, the leakage shielding patterns 71' serves to block a
leakage path of the excess holes. Accordingly, the leakage
shielding patterns 71' extends the time during which excess holes
are retained in the excess carrier storage region 55S.
[0102] Subsequently, a single transistor floating-body DRAM device
may be formed by known fabrication process of a semiconductor
device such as formation of a bit line and a plug.
[0103] FIGS. 12 to 15 are cross-sectional views illustrating a
method of fabricating a single transistor floating-body DRAM
device, according to the second exemplary embodiment of the present
invention.
[0104] Referring to FIG. 12, a buried insulating layer 152, a
floating body 155, and an isolation layer 153 may be formed on a
semiconductor substrate 151 by the same method as described above
with reference to FIG. 8. A gate dielectric layer 161 covering the
floating body 155 may be formed. A gate pattern crossing the
floating body 155 may be formed on the gate dielectric layer 161,
as described above with reference to FIG. 9. The gate pattern may
be formed from a gate electrode 163, a pad oxide layer 165, and a
mask nitride layer 167, which are sequentially stacked.
[0105] Germanium (Ge) ions are injected into the floating body 155
using the gate pattern as an ion injection mask, thereby forming
temporary patterns 171, as shown in FIG. 12. The injection of the
Ge ions may be performed using various angles and energy. The
temporary patterns 171 may be formed at both sides of the gate
pattern, in which case, the temporary patterns 171 may be arranged
at outer sides of the gate pattern. Furthermore, the temporary
patterns 171 may be locally formed at a predetermined depth in the
floating body 155, so that at least a portion of the floating body
155 may remain under the temporary patterns 171. One side of the
temporary patterns 171 may be formed to be in contact with the
isolation layer 153. The mask nitride layer 167 prevents the
injection of the Ge ions into the gate electrode 163.
[0106] Referring to FIG. 13, a mask pattern 175 is formed on the
semiconductor substrate 151. The mask pattern 175 may be formed to
have an opening 175H exposing the isolation layer 153. The mask
pattern 175 may be formed from a photoresist layer or a hard mask
layer, for example.
[0107] The isolation layer 153 may be etched until the temporary
patterns 171 are exposed using the mask pattern 175 as an etch
mask. As a result, the floating body 155 and the temporary patterns
171 may be exposed in the opening 175H.
[0108] Gaps 171G may be formed by removing the temporary patterns
171. The gaps 171G may be formed by an isotropic etching process
having an etch selectivity between the temporary patterns 171 and
the floating body 155.
[0109] As described above, in the temporary patterns 171, the state
that the Ge ions are injected into the single crystal silicon may
be maintained. In this case, the single crystal silicon into which
the Ge ions are injected may have an etch rate, for example, 100
times higher than the floating body 155. The mask pattern 175 may
then be removed.
[0110] Referring to FIG. 14, impurity ions may be injected into the
floating body 155 through the gaps 171G, using the gate pattern as
an ion injection mask, thereby forming the source and drain regions
173. When the floating body 155 has p-type impurity ions, the
source and drain regions 173 may be formed by injecting n-type
impurity ions. The source and drain regions 173 may be formed at
both sides of the gate electrode 163, respectively. Also, the
source and drain regions 173 may be formed to face each other over
a space or distance. In this case, the floating body 155 may remain
in the space between the source and drain regions 173, which face
each other. As stated above, the floating body 155 may remain under
the gaps 171G.
[0111] Referring to FIG. 15, an interlayer insulating layer 177 is
formed to cover the semiconductor substrate 151, filling the gaps
171G. The interlayer insulating layer 177 may be formed, for
example, from a silicon oxide layer, a silicon nitride layer, a
silicon oxynitride layer, or a combination thereof. A top surface
of the mask nitride layer 167 may be exposed by planarizing the
interlayer insulating layer 177. In this case, top surfaces of the
interlayer insulating layer 177 and the mask nitride layer 167 may
be exposed on substantially the same plane.
[0112] The interlayer insulating layer 177 filling the gaps 171G
serves as leakage shielding patterns 171'. That is, the leakage
shielding patterns 171' may be formed between the source and drain
regions 173 and the floating body 155.
[0113] Subsequently, the single transistor floating-body DRAM
device may be fabricated by known fabrication processes of the
semiconductor device, such as the formation of a bit line and a
plug.
[0114] As a result, the leakage shielding patterns 171' are formed
beneath the source and drain regions 173. The floating body 155 may
remain under the leakage shielding patterns 171'. The leakage
shielding patterns 171' may be arranged at outer sides of the gate
electrode 163. That is, the floating body 155 may be positioned
between the source and drain regions 173 facing each other, and
formed to extend beneath the leakage shielding patterns 171'. The
floating body 155 may have a larger width than the gate electrode
163.
[0115] As described with reference to FIG. 3, an excess carrier
storage region 155S may be formed in the floating body 155. The
excess carrier storage region 155S may also be formed to have a
larger width than the gate electrode 163.
[0116] A contact surface of the source and drain regions 173 and
the floating body 155 may be minimized by the leakage shielding
patterns 171'. In other words, the leakage shielding patterns 171'
serve to block a leakage path of the excess holes. Accordingly, the
leakage shielding patterns 171' extend the time for which the
excess holes are retained in the excess carrier storage region
155S.
[0117] FIGS. 16 to 24 are cross-sectional views taken along line
I-I' of FIG. 4 illustrating the method of fabricating a single
transistor memory cell, according to the third exemplary embodiment
of the present invention.
[0118] Referring to FIGS. 4 and 16, an SOI substrate 256 is
prepared. The SOI substrate 256 may include a supporting substrate
251, a buried insulating layer 253 formed on the supporting
substrate 251, and a semiconductor body layer 255 formed on the
buried insulating layer 253. The supporting substrate 251 may be a
semiconductor substrate, and the semiconductor body layer 255 may
be a silicon layer, for example.
[0119] Referring to FIGS. 4 and 17, an isolation layer 257 is
formed in a predetermined region of the semiconductor body layer
255, to form an active region 255r. The isolation layer 257 may be
formed to be in contact with the buried insulating layer 253. As a
result, the active region 255r may be electrically insulated from
the supporting substrate 251 by the isolation layer 257 and the
buried insulating layer 253.
[0120] Referring to FIGS. 4 and 18, first impurity ions are
injected into a surface of the active region 255r to form an
impurity region 261. The impurity region 261 may be formed to have
a different conductivity type from the active region 255r. For
example, when the active region 255r is a p-type, the impurity
region 261 may be an n-type. Second impurity ions are injected into
a lower portion of the active region 255r to form a bulk region
255c, which includes a lower bulk region 259 and an upper bulk
region 255b (between the lower bulk region 259 and the impurity
region 261) defined by the lower bulk region 259. The lower bulk
region 259 may be formed to have the same conductivity type as the
active region 255r. In this case, the lower bulk region 259 may
have a higher impurity concentration than the upper bulk region
255b. The bulk region 255c and the impurity region 261 constitute
an active semiconductor pattern 255a.
[0121] Alternatively, the active semiconductor pattern 255a may be
formed by a different method from that described above. For
example, the process of forming the lower bulk region 259 may be
omitted. Also, before forming the isolation layer 257, an impurity
layer and a lower bulk layer may be respectively formed on an upper
surface and in a lower region of the semiconductor body layer 255
to define an upper bulk layer between the impurity layer and the
lower bulk layer. Then, the isolation layer 257 may be formed in
the impurity layer, the upper bulk layer and the lower bulk layer
to define the active semiconductor pattern 255a.
[0122] Referring to FIGS. 4 and 19, a mask pattern 266 is formed on
the substrate having the active semiconductor pattern 255a. The
mask pattern 266 may be formed to define an opening 266a over the
active semiconductor pattern 255a. The mask pattern 266 may also
include at least two insulating layers. For example, the mask
pattern 266 may be formed to include a pad oxide layer pattern 263
and a pad nitride layer pattern 265, which are sequentially
stacked.
[0123] The active semiconductor pattern 255a is etched using the
mask pattern 266 as an etch mask, to form a recessed region R
passing through the impurity region 261. Accordingly, a recessed
channel region is formed along a bottom surface and sidewalls of
the recessed region R. The recessed region R may be formed deeper
than the thickness of the impurity region 261, and shallower than
the total thickness of the active semiconductor pattern 255a. As a
result, the recessed region R divides the impurity region 261 into
a source region 261s and a drain region 261d, which are spaced
apart or separated from each other. The recessed region R may
include a first sidewall SW1 adjacent to the source region 261s,
and a second sidewall SW2 adjacent to the drain region 261d.
[0124] Referring to FIGS. 4 and 20, a gate insulating layer 267 is
formed on an inner wall of the recessed region R. The gate
insulating layer 267 may be a thermal oxide layer, for example. A
gate conductive layer filling the recessed region R and the opening
266a is formed on the substrate having the gate insulating layer
267 and then planarized to expose a top surface of the mask pattern
266. As a result, a gate conductive layer pattern is formed in the
recessed region R and the opening 266a. The gate conductive layer
pattern may be further etched to form a recessed gate electrode
269g. The gate conductive layer may be formed from a doped
polysilicon layer, for example.
[0125] A gate capping insulating layer is formed on the substrate
having the recessed gate electrode 269g and then planarized to
expose a top surface of the mask pattern 266. As a result, a
capping insulating layer pattern 270 may be formed in the opening
266a above the recessed gate electrode 269g. The gate capping
insulating layer may be formed from a material having etch
selectivity with respect to the mask pattern 266, for example, a
silicon oxide layer. The gate electrode 269g and the capping
insulating layer pattern 270 constitute a gate pattern 270g.
[0126] Referring to FIGS. 4 and 21, at least a portion of the mask
pattern 266, e.g., the pad nitride layer pattern 265, may be
removed, thereby exposing upper sidewalls of the gate pattern 270g.
As a result, an upper region of the gate pattern 270g may project
above the isolation layer 257. Spacers 271 may be formed on
sidewalls of the projecting portion of the gate pattern 270g. The
spacers 271 may be formed of an insulating layer having an etch
selectivity with respect to the capping insulating layer pattern
270. For example, the spacers 271 may be formed from a silicon
nitride layer. During the formation of the spacers 271, the pad
oxide layer 263 (FIG. 20) on the source and drain regions 261s and
261d may be over-etched. In this case, the source and drain regions
261s and 261d may be exposed.
[0127] Referring to FIGS. 4 and 22, sacrificial impurity ions 273
are injected into the bulk region 255c using the gate pattern 270g
and the spacers 271 as ion injection masks, forming first and
second sacrificial impurity layers 273s and 273d beneath the source
and drain regions 261s and 261d, respectively. A top surface of the
first sacrificial impurity layer 273s is formed to contact a bottom
surface of the source region 261s, and a top surface of the second
sacrificial impurity layer 273d is formed to contact a bottom
surface of the drain region 261d. As a result, a first bulk region
255b' is defined between the first sacrificial impurity layer 273s
and the recessed region R, and a second bulk region 255b'' is
defined between the second sacrificial impurity layer 273d and the
recessed region R. In the embodiment, the first and second bulk
regions 255b' and 255b'' may have the same width D as the spacers
271 because they are self-aligned with the spacers 271. The
sacrificial impurity ions may be silicon germanium ions, for
example. In this case, the first and second sacrificial impurity
layers 273s and 273d may be formed from silicon germanium
layers.
[0128] In alternative embodiments, the first and second sacrificial
impurity layers 273s and 273d may be the same or different from the
upper bulk region 255b (FIG. 21) in thickness. For example, the
first and second sacrificial impurity layers 273s and 273d, as
illustrated in FIG. 22 are depicted as having the same thickness as
the upper bulk region 255b. Also, the recessed region R may be
formed to have the same depth as the total thickness of the source
and drain regions 261s and 261d and the upper bulk region 255b. In
this case, the first and second bulk regions 255b' and 255b'' may
be the upper bulk region 255b remaining under the spacers 271, and
the bulk region 255c may include the first and second bulk regions
255b' and 255b'' and the lower bulk region 259. However, in
alternative embodiments of the present invention, the thicknesses
of the first and second sacrificial impurity layers 273s and 273d
and the depth of the recessed region R may vary, without departing
for the spirit and scope of the present invention. For example, the
first and second sacrificial impurity layers 273s and 273d may be
formed thinner or thicker than the upper bulk region 255b, and the
recessed region R may be formed to have a bottom surface higher or
lower than the juncture between the lower bulk region 259 and the
upper bulk region 255b.
[0129] Referring to FIGS. 4 and 23, the isolation layer 257 (FIG.
22) is etched to expose the first and second sacrificial impurity
layers 273s and 273d. When the capping insulating layer pattern 270
(FIG. 22) is formed of the same material layer as the isolation
layer 257 (for example, a silicon oxide layer), the capping
insulating layer pattern 270 may be removed during the etching of
the isolation layer 257. Then, the exposed sacrificial impurity
layers 273s and 273d are likewise selectively removed. As a result,
a first undercut region 275s may be formed under the source region
261s, and a second undercut region 275d may be formed under the
drain region 261d.
[0130] Referring to FIGS. 4 and 24, an insulating layer 277 is
formed on the substrate having the undercut regions 275s and 275d.
In the process, the insulating layer 277 fills the undercut regions
275s and 275d to define first and second insulating regions 277s
and 277d under the source and drain regions 261s and 261d,
respectively. In other words, the first insulating region 277s may
be a first insulating layer pattern 277s in the first undercut
region 275s, and the second insulating region 277d may be a second
insulating layer pattern 277d in the second undercut region 275d.
Alternatively, the insulating layer 277 may be formed to leave
empty spaces in the undercut regions 275s and 275d, respectively.
In this case, the first insulating region 277s may be a first empty
space in the first undercut region 275s, and the second insulating
region 277d may be a second empty space in the second undercut
region 275d.
[0131] Subsequently, the insulating layer 277 and the buried
insulating layer 253 may be patterned to form a back gate contact
hole 278b, a source contact hole 278s, a gate contact hole 278g and
a drain contact hole 278d, which respectively expose the supporting
substrate 251, the source region 261s, the gate electrode 269g and
the drain region 261. A back gate contact plug 279b, a source
contact plug 279s, a gate contact plug 279g and a drain contact
plug 279d may be formed in the contact holes 278b, 278s, 278g and
278d, respectively. A conductive layer, such as a metal layer, is
formed on the substrate having the contact plugs 279b, 279s, 279g
and 279d. The conductive layer is patterned to form a back gate
interconnection 281b, a source interconnection 281s, a gate
interconnection 281g and a drain interconnection 281d, which are
electrically connected to the contact plugs 279b, 279s, 279g and
279d, respectively.
[0132] According to the embodiments described above, leakage
shielding patterns are located under source and drain regions. The
leakage shielding patterns may be arranged at outer sides of a gate
electrode. A floating body may be located between the source and
drain regions facing each other, and extend beneath the leakage
shielding patterns. An excess carrier storage region may be located
in the floating body.
[0133] Accordingly, a contact surface of the source and drain
regions and the floating body is minimized by the leakage shielding
patterns. The leakage shielding patterns block a leakage path of
the excess holes accumulated in the excess carrier storage region.
Therefore, the retention time of the excess holes accumulated in
the excess carrier storage region is significantly extended, as
compared to the conventional art.
[0134] Furthermore, insulating regions are located under source and
drain regions on sides of a recessed channel region. Accordingly, a
junction area of the source and drain regions significantly
decreases, thus improving retention characteristics (e.g., of data
"1") of excess electric charges (e.g., excess holes) stored in a
bulk region under the recessed channel region. Also, first and
second bulk regions having fine widths between the insulating
regions and the recessed channel region may be provided. Thus, when
a certain voltage is applied to a gate electrode located in the
recessed region, the first and second bulk regions may be fully or
partially depleted. As a result, band-to-band tunneling is
significantly suppressed at a junction between the source and drain
regions, thus improving the retention characteristics (e.g., of
data "0"). Also, the first and second bulk regions may be aligned
with spacers formed on sidewalls of the gate electrode.
[0135] While the present invention has been described with
reference to exemplary embodiments, it will be apparent to those
skilled in the art that various changes and modifications may be
made without departing from the spirit and scope of the present
invention. Therefore, it should be understood that the above
embodiments are not limiting, but illustrative.
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