U.S. patent application number 10/054957 was filed with the patent office on 2002-10-10 for asymmetrical devices for short gate length performance with disposable sidewall.
Invention is credited to Chatterjee, Amitava, Houston, Theodore W..
Application Number | 20020145154 10/054957 |
Document ID | / |
Family ID | 26790096 |
Filed Date | 2002-10-10 |
United States Patent
Application |
20020145154 |
Kind Code |
A1 |
Houston, Theodore W. ; et
al. |
October 10, 2002 |
Asymmetrical devices for short gate length performance with
disposable sidewall
Abstract
An asymmetrical channel implant from source to drain improves
short channel characteristics. The implant provides a relatively
high V.sub.T net dopant adjacent to the source region and a
relatively low V.sub.T net dopant in the remainder of the channel
region. One way to achieve this arrangement with disposable gate
processing is to add disposable sidewalls inside the gate opening
(after removing the disposable gate), patterning to selectively
remove the source or gate side sidewalls, implant the source and
drain regions and remove the remaining sidewall and the proceed.
According to a second embodiment, wherein the channel implant can
be symmetrical, a relatively low net V.sub.T implant is provided in
the central region of the channel and a relatively high net V.sub.T
implant is provided in the channel regions adjacent to the source
and drain regions.
Inventors: |
Houston, Theodore W.;
(Richardson, TX) ; Chatterjee, Amitava; (Plano,
TX) |
Correspondence
Address: |
TEXAS INSTRUMENTS INCORPORATED
P O BOX 655474, M/S 3999
DALLAS
TX
75265
|
Family ID: |
26790096 |
Appl. No.: |
10/054957 |
Filed: |
January 25, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10054957 |
Jan 25, 2002 |
|
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09368387 |
Aug 4, 1999 |
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60095327 |
Aug 4, 1998 |
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Current U.S.
Class: |
257/221 ;
257/213; 257/E21.62; 257/E21.624 |
Current CPC
Class: |
H01L 21/823425 20130101;
H01L 21/823456 20130101 |
Class at
Publication: |
257/221 ;
257/213 |
International
Class: |
H01L 029/76 |
Claims
1. A method of fabricating a transistor which comprises the steps
of: (a) providing a semiconductor substrate having a first
source/drain region and a second source/drain region therein; and
(b) providing a channel region between said first and second
source/drain regions in said substrate having a first dopant
profile in a first region substantially in said channel region and
a second dopant profile different from said first dopant profile in
a second region substantially between the first region and said
first source/drain region, the second dopant profile comprising an
implant substantially bounded between said first region and said
first source/drain region.
2. The method of claim 1 wherein said channel region is formed by
implanting a dopant providing a relatively high V.sub.T dopant
profile adjacent to said first source/drain region and implanting a
dopant providing a relatively low dopant profile adjacent to said
second source/drain region.
3. The method of claim 1 wherein said channel region is formed by
implanting a dopant providing a relatively low V.sub.T dopant
profile along said channel and then selectively implanting a dopant
providing a relatively high V.sub.T dopant profile adjacent to said
first source/drain region.
4. The method of claim 2 wherein said channel region is formed by
implanting a dopant providing a relatively high V.sub.T dopant
profile adjacent to said first source/drain region and implanting a
dopant providing a relatively low dopant profile adjacent to said
second source/drain region.
5. The method of claim 2 wherein said channel region is formed by
implanting a dopant along the entire channel and then selectively
implanting a dopant providing a relatively high V.sub.T dopant
profile of opposite conductivity type only adjacent to said first
source/drain region to provide a net relatively high V.sub.T region
only adjacent to said first source/drain region
6. A method of fabricating a transistor which comprises the steps
of: (a) providing a semiconductor substrate having source and drain
regions therein; and (b) providing a channel region between said
source and drain regions in said substrate having a dopant
providing a relatively low V.sub.T dopant profile central region
between said source and drain regions and regions in said channel
region between said source region and said central region and
between said drain region and said central region having a dopant
providing a relatively high V.sub.T dopant profile.
7. The method of claim 6 wherein said channel region is formed by
implanting a dopant having a relatively low V.sub.T dopant profile
intermediate said source and drain regions and implanting a dopant
having a relatively high V.sub.T dopant profile adjacent to said
source and drain regions.
8. The method of claim 6 wherein said channel region is formed by
implanting a dopant having a relatively low V.sub.T dopant profile
along said channel and then selectively implanting a dopant having
a relatively high V.sub.T dopant profile adjacent to said source
region.
9. A transistor which comprises: (a) a semiconductor substrate
having source and drain regions therein; and (b) a channel region
between said source and drain regions in said substrate having a
relatively low V.sub.T central region between said source and drain
regions and relatively high V.sub.T regions adjacent to said source
and drain regions.
10. The transistor of claim 9 wherein said channel region is an
implanted low V.sub.T dopant intermediate said source and drain
regions are an implanted high V.sub.T dopant adjacent said source
and drain regions.
11. The transistor of claim 9 wherein said channel region is an
implanted relatively low V.sub.T dopant along said channel and a
selectively implanted relatively high V.sub.T dopant adjacent to
said source region.
12. A method of fabricating a transistor which comprises the steps
of: (a) providing a substrate; (b) providing a first mask layer
over said substrate, said first layer having a trench extending
therethrough to said substrate; (c) forming sidewalls in said
trench with a material different from said first mask layer; and
(d) performing an implant into said substrate through said trench,
said layer providing a mask to said implant external of said
trench.
13. The method of claim 12 further including the step of removing a
portion of said sidewall prior to performing said implant.
14. A method of fabricating a transistor which comprises the steps
of: (a) providing a substrate; (b) providing a first mask layer
over said substrate, said first layer having a trench extending
therethrough to said substrate; (c) forming sidewalls on said
trench with a sidewall material different from said first mask
layer; (d) filling the space bordered by said sidewalls with a
filling material different from said sidewall material; (e)
removing at least a portion of said sidewalls while retaining at
least a portion of said filling material; and (f) performing an
implant into said substrate through said trench, said layer, said
filling material and any remaining sidewall providing a mask to
said implant external of said trench.
15. A transistor which comprises: (a) a semiconductor substrate
having a first source/drain region and a second source/drain region
therein; (b) a channel region between said first and second
source/drain regions; (c) a gate dielectric disposed outwardly from
said channel region; (d) a gate electrode disposed outwardly from
said gate dielectric; and (e) a dopant in said substrate comprising
an implant aligned with said gate contained substantially within a
projection of said gate electrode to said substrate, the projection
of the area of the implant to the surface of the substrate being
less than the projection of the gate.
16. A method of fabricating a transistor which comprises the steps
of: (a) providing a substrate; (b) providing a first layer of said
substrate, said first layer having a trench for formation of a
gate; (c) forming a sidewall within said trench; and (d) performing
a first implant into said substrate, said first implant being
partially masked by said sidewall.
17. The method of claim 16 wherein a portion of said sidewall is
removed prior to said first implant.
18. The method of claim 16 further including performing a second
implant into said substrate which is not masked by said
sidewall.
19. The method of claim 18 wherein said second implant is performed
prior to formation of said sidewall.
20. The method of claim 18 wherein said second implant is performed
after removal of said sidewall.
21. The method of claim 16 further including the steps of: (e)
filling any space adjacent said sidewall; (f) planarizing the
surface of the structure being fabricated; and (g) removing a
portion of said sidewall.
22. The transistor of claim 15 wherein one edge of said implant
region is substantially aligned with one edge of said gate
electrode and the opposite edge of said implant region is within
the projection of the gate electrode.
23. A transistor which includes: (a) a substrate; (b) a gate
electrode disposed over said substrate; and (c) a punch through
implant in said substrate contained within a projection of said
gate electrode.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a field effect transistor (FET)
with an implant within the gate length of the FET and a method of
making the FET.
[0003] 2. Brief Description of the Prior Art
[0004] It has been found that there are advantages for an FET to
have an implant in the channel region that does not extend from the
source to the drain and is aligned to the gate edge. For example, a
pocket implant can be used to reduce short channel length effects.
Yet, it can be undesirable to have a pocket implant extend into the
source/drain (S/D) area because of resulting increased junction
capacitance. It has also been found that there are advantages for
an FET to have a region of controlling V.sub.T on the source side
of the channel with a more conductive V.sub.T on the drain side.
The short effective channel length and low source resistance of
such an FET results in increased drive current and the reduced
influence of the drain provides superior short channel
characteristics. In the prior art, angled implants and lateral
diffusions have been used to obtain channel impurity profiles
self-aligned to the gate edge. However, these approaches have
disadvantages resulting from restrictions in the channel implant
profiles that can be obtained and from the interdependence of the
channel profile and the S/FD extension profiles. It is therefore
apparent that transistors having an independent channel implant
region that is self aligned to the S/D edge is highly
desirable.
SUMMARY OF THE INVENTION
[0005] In accordance with the present invention, there is provided
a technique for fabrication of FET devices having a well controlled
implant region between the two source/drain regions, self aligned
to the S/D edge as well as the FET itself. In one embodiment, this
implant region is used to create a controlling V.sub.T region of
shorter effective length than the S/D spacing. Preferably, the
controlling V.sub.T region is on the source side of the space
between source and drain. In a second embodiment, the implant
region is a pocket implant with limited extension into the S/D
region. In a third embodiment, the implant is a S/D extension
implant with extension into the channel region being independent of
the diffusion of the S/D implant. The term "controlling V.sub.T
region" as used herein refers to that region which is the least
conducting region and thus controls the current flow. For an
n-channel transistor, this will generally be the more positive
(higher) V.sub.T. The controlling region could have a positive
V.sub.T and the rest of the channel could have a negative V.sub.T,
or the controlling region could have a negative V.sub.T with the
rest of the channel having a more negative V.sub.T. The same
applies for p-channel transistors except for a polarity
reversal.
[0006] In accordance with the present invention, there is provided
a fabrication technique for fabrication of FET devices having a
well controlled sub-lithographic effective gate length, independent
of gate orientation. The shorter effective channel length provides
increased drive current since drive current is inversely
proportional to channel length. The shorter effective channel
length is achieved without requiring that the physical gate also be
as short as the effective channel length. This provides superior
short gate length device characteristics and avoids the problems
inherent in requiring the shorter gate length during fabrication of
the device.
[0007] One way to achieve the effective short channel is to use a
disposable gate process with masked/disposable sidewalls. This is
accomplished by providing a doped substrate having spaced apart
source/drain regions with a pad oxide and a patterned mask over the
source/drain regions, preferably of silicon nitride, leaving the
gate region exposed after disposal of the disposable gate. A
removable sidewall is formed on the mask, preferably with silicon
nitride, and the remaining gate region between the disposable
sidewalls is filled with silicon dioxide with the surface then
being planarized, preferably with a chemical mechanical polish
(CMP). The sidewall adjacent to the source region is then removed
by depositing and patterning a resist over the planarized surface
and etching. A high V.sub.T implant is then performed through the
space from which the sidewall was removed to dope the channel
region adjacent to the source region. The remaining sidewall is
then removed. This is followed by a low V.sub.T implant in the
channel region between the source and drain regions. It should be
understood that the high V.sub.T implant can be a counter doping of
the low V.sub.T implant with appropriate masking to perform the
implants in this manner. The result is an effective channel length,
L, which is independent of the original disposable gate length.
Fabrication then proceeds in standard manner to complete the
device.
[0008] As a second embodiment, the low V.sub.T channel implant can
be performed initially after removal of the disposable gate and
formation of disposable sidewalls on the mask as in the first
embodiment, the sidewall on the source side is retained and the
sidewall on the drain side is removed with appropriate deposition
and patterning of resist and etching. A low V.sub.T is then
provided into the exposed portion of the channel. The remaining
sidewall at the source end of the channel is then removed and a
high V.sub.T implant is then provided of opposite conductivity type
to the low V.sub.T implant to provide the same result as in the
first embodiment. Fabrication then proceeds in standard manner to
complete the device.
[0009] As a third embodiment, it is desirable to add a liner over
the mask surface, sidewalls and pad oxide prior to fabrication as
noted above with reference to the first and second embodiments. The
liner is preferably silicon nitride. The liner aids etch
selectivity and protects the surface of the silicon when performing
the implants.
[0010] As a fourth embodiment, the FET can be made symmetrical
rather than asymmetrical as described in the first and second
embodiments with a different implant in the center of the channel
region relative to the source and drain ends of the channel region.
An implant can be performed following sidewall formation. The
sidewalls would then be removed and the entire channel region would
be doped n- or p-type to provide either a less heavily net doped
region adjacent the source and drain regions if the same
conductivity type dopant is used or a more heavily net doped region
adjacent the source and drain if the opposite conductivity type
dopant is used. The resist pattern used to mask removal of selected
sidewalls can also distinguish n- and p-channel transistors for
different V.sub.T implants. Optionally, an implant can be performed
before formation of the sidewalls, followed by an implant after
formation of the sidewalls. For this option, the sidewall can be
left in place for formation of the actual gate.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] FIGS. 1A to 1D represent a process flow for formation of an
asymmetrical FET device for short gate length performance in
accordance with a first embodiment of the present invention;
[0012] FIGS. 2A to 2C represent a process flow for formation of an
asymmetrical FET device for short gate length performance in
accordance with a second embodiment of the present invention;
[0013] FIG. 3 shows a step in the process flow in accordance with a
third embodiment of the present invention;
[0014] FIGS. 4A and 4B show a process flow for formation of a
symmetrical FET device for short gate length performance in
accordance with a fourth embodiment of the present invention;
[0015] FIGS. 5A to 5C show a process flow for formation of an FET
in accordance with a fifth embodiment of the present invention;
and
[0016] FIGS. 6A and 6B show a process flow for formation of an FET
in accordance with a sixth embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
[0017] In accordance with a first embodiment of the invention, the
effective short channel is achieved by using a disposable gate
process with masked/disposable sidewalls as shown in FIGS. 1A to
1D. For an n-channel device (it is understood that all conductivity
types will be opposite for a p-channel device), there is provided a
p-doped substrate 1 having spaced apart source/drain regions 3, 5
with a pad oxide 7 over the substrate and a patterned silicon
nitride mask 9 over the source/drain regions. This arrangement is
provided, leaving the gate region exposed after prior disposal of
the disposable gate in standard manner as shown in FIG. 1A. A
removable polysilicon sidewall 11, 13 is formed on each side of the
nitride mask 9 and the remaining gate region between the disposable
sidewalls is filled with silicon dioxide 15 with the surface then
being planarized, preferably with a chemical mechanical polish
(CMP) as shown if FIG. 1B. The sidewall 11 adjacent what will
ultimately be the source region is then removed by depositing and
patterning a resist over the planarized surface and etching in
standard manner. A high V.sub.T implant is then performed through
the space from which the sidewall was removed to dope the channel
region 17 adjacent to the source region 3 as shown in FIG. 1C and
the silicon dioxide 15 and remaining sidewall 13 are then removed.
This is followed by a low V.sub.T implant in the channel region
between the source and drain regions to leave the high V.sub.T
region 17 adjacent to the source region 3 and a low V.sub.T region
19 adjacent to the drain region 5 as shown in FIG. 1D. Optionally,
the low V.sub.T implant can be performed before formation of the
sidewalls.
[0018] It should be understood that the V.sub.T of region 17 is a
result of the combination of what has been referred to as the high
V.sub.T and the low.sub.T implant. One can be a counter-doping of
the other. The result is an effective channel length, L, which is
independent of the original disposable gate length. Fabrication
then proceeds in standard manner to complete the device.
[0019] As a second embodiment, the low V.sub.T channel implant can
be performed prior to the high V.sub.T implant. After removal of
the disposable gate, disposable sidewalls are formed on the mask as
in FIG. 1B of the first embodiment and as shown in FIG. 2A wherein
like reference characters refer to the same or similar structure.
The sidewall on the source region side 11 is retained and the
sidewall on the drain region side 13 is removed with appropriate
deposition and patterning of resist and etching in standard manner.
A low V.sub.T implant is then provided into the exposed portion 21
of the channel as shown in FIG. 2B. The remaining sidewall 11 at
the source end of the channel is then removed and a high V.sub.T
implant is implanted into the channel region between the source and
drain regions. Preferably, the high V.sub.T implant is of opposite
conductivity type to the low V.sub.T implant to provide a high
V.sub.T region 23 and a low V.sub.T region 21. Fabrication then
proceeds in standard manner to complete the device. Optionally, the
high V.sub.T implant can be performed prior to formation of the
sidewalls.
[0020] As a third embodiment, it is desirable to add a liner 25
over the mask surface, sidewalls and pad oxide prior to fabrication
as noted above with reference to the first and/or second
embodiments and particularly over the structure as shown in FIG. 1A
and the structure as shown in FIG. 2A prior to formation of the
sidewalls 11, 13 with subsequent formation of the sidewalls 11, 13
over the liner as shown in FIG. 3. The liner 25 is preferably
silicon nitride. Fabrication then proceeds as in the prior
embodiments as discussed above.
[0021] As a fourth embodiment as shown if FIGS. 4A and 4B, the FET
can be made in a symmetrical rather than asymmetrical embodiment as
described in the first and second embodiments with a different
implant 27 in the center of the channel region as shown in FIG. 4A.
Optionally, implant 27 may include a punch-through implant. The
sidewalls 11, 13 are then removed and the entire channel region is
doped wither n- or p-type to provide either a less heavily net
doped region adjacent the source and drain regions 29 if the same
conductivity type dopant is used or a more heavily net doped region
adjacent the source and drain regions if the opposite conductivity
type dopant is used and the opposite characteristic in region 27 as
shown in FIG. 4B. Optionally, the implant in the full channel
region can be performed prior to sidewall formation. Fabrication
then proceeds in standard manner to complete the device.
[0022] As a fifth embodiment as shown in FIGS. 5A to 5C, the FET
can be made symmetrical with the implant in the channel regions
adjacent to the source and drain being performed prior to the
implant in the central region. As in the first embodiment,
sidewalls 11 and 13 are formed and the central region 15 is filled
and planarized as shown in FIG. 5A. Sidewalls 11 and 13 are then
removed and an implant is performed into regions 50. As depicted in
FIG. 5B, the implant regions 50 may be pocket implants. Optionally,
source/drain extension or V.sub.T adjust implants can be performed
at this time. Center masking material 15 is then removed and in
implant is performed into the channel region 52 between the source
and drain regions.
[0023] As a sixth embodiment, the sidewalls are left in place,
reducing the physical gate length, as shown in FIGS. 6A and 6B. An
implant is performed in the full channel region 60 prior to
formation of the sidewalls. A second implant is performed into
region 64 after formation of the sidewalls, resulting in a
different implant profile in regions 62 versus region 64. Sidewalls
11 and 13 can then be left in place as formation of the gate
proceeds in normal fashion. Optionally, sidewall 11 or sidewall 13
can be removed with appropriate pattern and etch steps prior to or
subsequent to the implant to region 64.
[0024] Optionally, the sidewall material can be of the same
material as the masking material and left in place or removed with
a timed isotropic etch prior to gate formation.
[0025] Though the invention has been described with reference to
specific preferred embodiments thereof, many variations and
modifications will immediately become apparent to those skilled in
the art. For example, the opening for the damascene gate can be
made by a trench etch into the masking material rather than by
removal of a disposable gate. Various materials or stacks of
materials can be used for the masking material, the sidewalls, and
the fill material. The source/drain implants can be performed after
the actual gate formation rather than with the disposable gate, and
the edge of the source/drain implant relative to the opening for
the damascene gate can vary from that shown, and a source/drain
extension may be used. The implants into the various regions
encompassed by the gate opening can include but are not restricted
to threshold adjustment implants, source/drain extension implants,
pocket implants and punch through implants. Multiple implants of
different species or energy can be performed in any of the regions
to obtain the desired doping profile. The depth of implants
illustrated is not intended to be restrictive. Different sequences
of sidewall formation, sidewall removal, and implantation,
including multiple sidewall formation can be used. Sidewalls can be
added after completion of the channel region implants to adjust the
physical gate length. It is therefore the intention that the
appended claims be interpreted as broadly as possible in view of
the prior art to include all such variations and modification.
* * * * *