U.S. patent application number 10/954121 was filed with the patent office on 2006-03-30 for method for forming a semiconductor device having a strained channel and a heterojunction source/drain.
Invention is credited to Alexander L. Barr, Venkat R. Kolagunta, Bich-Yen Nguyen, Mariam G. Sadaka, Voon-Yew Thean, Victor H. Vartanian, Ted R. White, Da Zhang.
Application Number | 20060068553 10/954121 |
Document ID | / |
Family ID | 36084578 |
Filed Date | 2006-03-30 |
United States Patent
Application |
20060068553 |
Kind Code |
A1 |
Thean; Voon-Yew ; et
al. |
March 30, 2006 |
METHOD FOR FORMING A SEMICONDUCTOR DEVICE HAVING A STRAINED CHANNEL
AND A HETEROJUNCTION SOURCE/DRAIN
Abstract
A semiconductor device (10) is formed by positioning a gate (22)
overlying a semiconductor layer (16) of preferably silicon. A
semiconductor material (26) of, for example only, SiGe or Ge, is
formed adjacent the gate over the semiconductor layer and over
source/drain regions. A thermal process diffuses the stressor
material into the semiconductor layer. Lateral diffusion occurs to
cause the formation of a strained channel (17) in which a stressor
material layer (30) is immediately adjacent the strained channel.
Extension implants create source and drain implants from a first
portion of the stressor material layer. A second portion of the
stressor material layer remains in the channel between the strained
channel and the source and drain implants. A heterojunction is
therefore formed in the strained channel. In another form,
oxidation of the stressor material occurs rather than extension
implants to form the strained channel.
Inventors: |
Thean; Voon-Yew; (Austin,
TX) ; Sadaka; Mariam G.; (Austin, TX) ; White;
Ted R.; (Austin, TX) ; Barr; Alexander L.;
(Crolles, FR) ; Kolagunta; Venkat R.; (Austin,
TX) ; Nguyen; Bich-Yen; (Austin, TX) ;
Vartanian; Victor H.; (Dripping Springs, TX) ; Zhang;
Da; (Austin, TX) |
Correspondence
Address: |
FREESCALE SEMICONDUCTOR, INC.;LAW DEPARTMENT
7700 WEST PARMER LANE MD:TX32/PL02
AUSTIN
TX
78729
US
|
Family ID: |
36084578 |
Appl. No.: |
10/954121 |
Filed: |
September 29, 2004 |
Current U.S.
Class: |
438/285 ;
257/E21.148; 257/E21.438; 257/E21.442; 257/E29.275 |
Current CPC
Class: |
H01L 29/7848 20130101;
H01L 29/665 20130101; H01L 21/2254 20130101; H01L 29/66787
20130101; H01L 29/78648 20130101; H01L 29/785 20130101 |
Class at
Publication: |
438/285 |
International
Class: |
H01L 21/336 20060101
H01L021/336 |
Claims
1. A method for forming a semiconductor device, comprising:
providing a semiconductor layer; providing a gate dielectric
overlying a first portion of the semiconductor layer; providing a
gate electrode overlying the gate dielectric; encapsulating the
gate electrode and the gate dielectric with an encapsulant;
selectively growing a semiconductor material overlying a second
portion of the semiconductor layer outside the first portion,
wherein the semiconductor material comprises a material different
from a material of the semiconductor layer; and using the
semiconductor material as a diffusion source by oxidizing the
semiconductor material to control an amount of diffusion of the
semiconductor material into a channel region of the semiconductor
layer, wherein the diffusion of the semiconductor material into the
channel region produces a strained channel.
2. The method of claim 1, wherein using the semiconductor material
as a diffusion source modifies the semiconductor layer underlying
the semiconductor material to become a stressor material layer.
3. The method of claim 2, wherein the stressor material layer
comprises silicon germanium.
4. The method of claim 3, further wherein the oxidizing of the
semiconductor material forms a dielectric which is removed and
replaced by source/drain extensions on respective sides of the gate
electrode.
5. The method of claim 1, wherein the semiconductor material
includes at least one selected from the group consisting of a
source of germanium, carbon doped silicon, boron, phosphorus and
arsenic.
6. The method of claim 4, further comprising forming a conductive
contact above each of the source/drain extensions and the gate
electrode.
7. The method of claim 1, wherein oxidizing the semiconductor
material further comprises performing an oxidation of the
semiconductor material to produce effects of condensation as one
component of the semiconductor material is preferentially consumed
and diffused by formation of an oxide and diffused by a
semiconductor material enrichment of the semiconductor layer; the
method further comprising: selectively removing the oxide with
respect to the encapsulant that encapsulates the gate electrode and
the gate dielectric; and selectively growing a layer of another
semiconductor material by selective epitaxy.
8. The method of claim 7, wherein the another semiconductor
material includes one selected from the group consisting of a same
initial semiconductor material and a different semiconductor
material.
9. The method of claim 7, further comprising: selectively growing a
layer of another semiconductor material and oxidizing the layer of
another semiconductor material to achieve a desired amount of
semiconductor material enrichment in the channel region of the
semiconductor layer.
10. The method of claim 7, wherein selectively removing includes
using a wet etch process.
11. The method of claim 7, wherein the semiconductor material
includes SiGe, and Si is preferentially consumed and Ge is
preferentially diffused and enriched by formation of the oxide
SiO.sub.2; the method further comprising: selectively removing the
oxide SiO.sub.2 with respect to the encapsulant that encapsulates
the gate electrode and the gate dielectric; and selectively growing
a layer of another semiconductor material by epitaxy.
12. The method of claim 1, further comprising: performing
source/drain extension region implants and a dopant activation
anneal; forming sidewall spacers adjacent the gate electrode;
performing source/drain region implants to create a source/drain;
and siliciding regions of the source/drain and the gate
electrode.
13. The method of claim 1, wherein providing the semiconductor
layer includes providing a semiconductor on insulator
substrate.
14. The method of claim 13, wherein the semiconductor on insulator
substrate includes a silicon on insulator substrate.
15. The method of claim 1, wherein encapsulating includes using an
encapsulant for minimizing oxygen diffusion into the gate electrode
and the gate dielectric.
16. The method of claim 1, wherein encapsulating the gate electrode
includes using an encapsulant selected from the group consisting of
nitride, oxide and a combination of nitride and oxide.
17. The method of claim 1, wherein the semiconductor material
includes silicon germanium (SiGe) with a predetermined Ge
concentration.
18. The method of claim 17, further wherein for a predetermined
thickness of the semiconductor material, the predetermined Ge
concentration is inversely proportional to the predetermined
thickness, and a function of an oxidation time, in order to
preserve a total amount of Ge to be diffused into the semiconductor
layer.
19. The method of claim 17, further wherein the predetermined Ge
concentration is greater than 15%.
20. The method of claim 1, wherein the semiconductor material
diffuses vertically into the semiconductor layer and further
diffuses laterally within the semiconductor layer to a
source/channel interface and a drain/channel interface to form
heterojunctions at respective interfaces of the channel region.
21. The method of claim 1, wherein the semiconductor device
includes one selected from the group consisting of a lateral double
gate device and a vertical double gate device.
22. The method of claim 1, wherein the semiconductor material
comprises one selected for obtaining a desired threshold voltage
for the semiconductor device.
23. A method for forming a transistor, comprising: providing a
semiconductor layer; providing a gate dielectric overlying a first
portion of the semiconductor layer; providing a gate electrode
overlying the gate dielectric; encapsulating the gate electrode and
the gate dielectric with a dielectric layer; selectively growing a
semiconductor material overlying a second portion of the
semiconductor layer outside the first portion, wherein the
semiconductor material comprises a material different from a
material of the semiconductor layer; oxidizing the semiconductor
material in an oxygen ambient to form a stressor layer from the
second portion of the semiconductor layer thereby forming a
strained channel within the first portion of the semiconductor
layer; and forming source and drain regions in the second portion
of the semiconductor layer and the semiconductor material.
24. A method for forming a transistor, comprising: providing a
semiconductor layer; providing a gate dielectric overlying a first
portion of the semiconductor layer; providing a gate electrode of
the transistor overlying the gate dielectric; encapsulating the
gate electrode and the gate dielectric; selectively growing a
semiconductor material containing germanium overlying a second
portion of the semiconductor layer outside the first portion,
wherein the semiconductor material comprises a material different
from a material of the semiconductor layer; heating the
semiconductor material to diffuse germanium into the second portion
of the semiconductor layer thereby forming a strained channel
underlying the gate dielectric; and forming source and drain
regions of the transistor in the second portion of the
semiconductor layer and the semiconductor material.
Description
CROSS REFERENCE TO RELATED APPLICATION
[0001] This application is related to our copending U.S. Patent
Application, (Attorney Docket No. SC-13377TP) entitled "Double Gate
Device Having A Heterojunction Source/Drain and a Strained
Channel", filed simultaneously herewith and assigned to the
assignee hereof.
FIELD OF THE INVENTION
[0002] This invention relates generally to semiconductor, and more
specifically, to making semiconductor devices having very small
dimensions.
BACKGROUND OF THE INVENTION
[0003] Semiconductor devices, such as transistor structures,
continue to be scaled to smaller dimensions as process lithography
improves. However, different challenges have been encountered in
the scaling of transistor structures much below 100 nm.
Additionally, when transistor dimensions on the order of 100 nm and
smaller are used, implants cannot be adequately controlled with
conventional semiconductor fabrication equipment. Channel dopant
fluctuations adversely affect device uniformity within circuits. To
control a conventional bulk transistor's threshold voltage which is
the voltage at which the transistor becomes conductive, dopants in
the channel are used. However, channel doping is not an efficient
method for ultra-thin devices due to the large amount of channel
impurities that are required. Therefore, highly doped ultra-thin
devices are even more susceptible to threshold voltage
fluctuations. Additionally, high channel doping concentrations
degrade both electron and hole mobility and promote source/gate and
drain/gate junction leakage.
[0004] A technique to improve bulk transistor performance is to
provide a bulk transistor having a strained channel. Such devices
are structured to place a strain on the transistor's channel. An
appropriately strained channel results in electron and hole
mobility enhancement that increases the conduction current which
provides a higher device drive performance.
[0005] One method to form a transistor having a strained channel is
to recess silicon material in those areas where the source and
drain are to be formed and re-grow a stressor material in the
recessed areas. However, when thin-body devices are being
implemented, the depth available for the stressor material is
insufficient to adequately strain the channel. Another issue with
this technique is that the silicon material is recessed with an
etch process. Stopping the etch process at a desired depth is a
challenge and subject to variation. Additionally, re-growth of the
stressor material on the remaining ultra-thin silicon is
problematic. Also, the ultra-thin silicon can agglomerate at
temperatures required for growing the stressor material.
Additionally, this method does not apply to the known FINFET
structures or any thin-body transistor devices.
[0006] Another known method to induce stress into a channel is the
use of a substrate as a stressor material. A shortcoming with this
approach is that when the stressor material is SiGe, the SiGe
causes degradation of the gate dielectric due to increased
interface states when the Ge diffuses to the dielectric
semiconductor interface. The material SiGe has a narrow bandgap.
Therefore, another issue with this approach is that the presence of
SiGe in the transistor's channel increases the transistor's
off-state current leakage. Additionally, this method does not apply
to the known FINFET structures or any vertical thin body double
gate transistor.
[0007] Yet another known method of stressing a transistor channel
is the use of overlying stress inducing layers over the active
regions of the transistor. However, the stressor material is
located far enough from the channel so that the influence of the
stressor material on the channel is diminished.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] The present invention is illustrated by way of example and
not limitation in the accompanying figures, in which like
references indicate similar elements.
[0009] FIGS. 1-5 illustrate in cross-sectional form a method of
forming an ultra-thin body transistor in accordance with one form
of the present invention;
[0010] FIGS. 6-9 illustrate in cross-sectional form a method of
forming an ultra-thin body transistor in accordance with another
form of the present invention;
[0011] FIGS. 10-15 illustrate in cross-sectional form an ultra-thin
double gate device in accordance with one form of the present
invention;
[0012] FIGS. 16-19 illustrate in cross-sectional form another
ultra-thin double gate device in accordance with another form of
the present invention; and
[0013] FIGS. 20 and 21 illustrate in cross-sectional form yet an
ultra-thin double gate planar transistor in accordance with another
form of the present invention.
[0014] Skilled artisans appreciate that elements in the figures are
illustrated for simplicity and clarity and have not necessarily
been drawn to scale. For example, the dimensions of some of the
elements in the figures may be exaggerated relative to other
elements to help improve the understanding of the embodiments of
the present invention.
DETAILED DESCRIPTION
[0015] Illustrated in FIG. 1 is a semiconductor device 10 in
accordance with the present invention. A substrate 12 is provided.
In one form substrate 12 is silicon. However, any semiconductor
material may be used. An overlying dielectric layer 14 is formed.
Wafer bonding or implantation of an oxygen species material may be
used to form the dielectric layer 14. In one form, the dielectric
layer 14 is an oxide. A semiconductor layer 16 is formed and
patterned overlying a portion of the dielectric layer 14. In one
form the semiconductor layer 16 is silicon and has a thickness such
that an aspect ratio that is equal to the gate length divided by
the thickness of semiconductor layer 16 is at least three.
Therefore, semiconductor layer 16 is relatively thin. Adjacent the
semiconductor layer 16 is isolation region 18. It should be
understood that isolation region 18 may either be a void of
material or any dielectric material. Therefore, in one form the
isolation region 18 is a void. A gate dielectric 20 is formed
overlying the semiconductor layer 16. A gate 22 is formed overlying
the gate dielectric 20. A dielectric layer 24, such as a nitride,
is formed around the gate 22 and the gate dielectric 20. The
composition of dielectric layer 24 is a material that minimizes
oxygen diffusion into gate dielectric 20 and is also thermally
stable. Other materials in addition to nitride may therefore be
used.
[0016] Illustrated in FIG. 2 is further processing of semiconductor
device 10. In FIG. 2 a semiconductor material 26 is selectively
deposited or epitaxially grown in the source/drain regions of
semiconductor device 10. In one form, the semiconductor material 26
is SiGe or even pure Ge and can be used as a material to induce
stress in a transistor channel. In yet other forms semiconductor
material 26 may be implemented as carbon doped silicon. If carbon
doped silicon is implemented, the resulting strain as described
below is tensile instead of compressive. Other semiconductor
materials may also be used for semiconductor material 26. Selection
of which semiconductor material will also directly affect the
semiconductor device 10 threshold voltage, Vt. It should be well
understood that other stressor materials may be used. In the
illustrated form, the semiconductor material 26 is selectively
grown. Various thicknesses may be used for semiconductor material
26.
[0017] Illustrated in FIG. 3 is further processing of semiconductor
device 10. In particular, a thermal process like RTA or a furnace
anneal, is used to cause the semiconductor material 26 to diffuse
into the underlying semiconductor layer 16. Semiconductor material
26 laterally diffuses to the source-to-channel interface and the
drain-to-channel interface. The thermal processing changes the
semiconductor material 26 into a diffusion source 28 of SiGe. The
diffusion source 28 has a reduced concentration of Ge as a result
of the diffusing from an original concentration of Ge in the
as-deposited or grown layer. The SiGe diffusion source 28 has a
lower Ge concentration. It should be noted that the diffusion from
the diffusion source 28 is into semiconductor layer 16 and spreads
toward and under the gate 22 as indicated by the arrows of FIG. 3.
As a result of the Ge diffusion, the semiconductor layer 16 becomes
a stressor material layer 30. A channel region underlies gate 22.
The diffusion of Ge into stressor material layer 30 causes the
formation of a strained channel 17. The vertical dashed line
boundary of the strained channel denotes the stressor material
layer 30-to-strained channel 17 boundary formed by the lateral
diffusion of Ge. It should be noted that the stressor material
layer 30 is immediately adjacent to the strained channel 17 and
therefore can exert significant influence on the channel.
[0018] In an alternative form, the diffusion of Ge into stressor
material layer 30 is continued with additional thermal processing
or oxidation according to the requirements of a particular
semiconductor device. In one embodiment the additional processing
is continued until a uniform material exists in both the stressor
material layer 30 and the strained channel 17. In this form, a
compressive material exists uniformly laterally in the source, the
channel and the drain. The substantially uniform compressive layer
that extends through the channel from the source and the drain is
desirable for P-channel conductivity transistors. The remaining
drawings will however illustrate a structure wherein such
additional processing is not implemented and will illustrate
different channel and source/drain materials.
[0019] Illustrated in FIG. 4 is further processing of semiconductor
device 10. In particular, extension implants are performed to
create source and drain implants. The extension implants result in
formation of dopants in the stressor material layer 30 and
diffusion source 28. The implanted dopants into diffusion source 28
and stressor material layer 30 are annealed to form source and
drain extensions in both of these layers.
[0020] Illustrated in FIG. 5 is further processing of semiconductor
device 10. In particular, a completed functional transistor is
formed. Sidewall spacers 34 are formed adjacent to dielectric layer
24 in a conventional manner. A thermal anneal process step is
performed to cause diffusion of the implanted dopants to form the
doped source/drain regions 36. A first portion of stressor material
layer 30 becomes doped forming the source/drain regions 36. A
second portion of stressor material layer 30 remains in the channel
between the vertical dashed lines defining the strained channel 17
and the edge of the doped source/drain regions 36. Two
heterojunctions as indicated by each of the two vertical dashed
lines in FIG. 5 are therefore formed between the strained channel
17 and each of a respective one of the doped source/drain regions
36. Silicided regions 38 are formed in a conventional manner
overlying the diffusion source 28 and the gate 22. At this point
there has been provided a transistor having a strained channel and
heterojunctions in the channel. Also, semiconductor device 10 has a
raised source/drain with respective elevated extensions that are
the portion of diffusion source 28 underlying the sidewall spacer
34. Because the stressor material layer 30 is in very close
proximity to the strained channel 17, improved electron and hole
mobility and transistor conduction current.
[0021] Another form of the present invention begins with processing
of the structure illustrated in FIG. 2. Illustrated in FIG. 6 is a
semiconductor device 40 resulting after oxidation of semiconductor
material 26 of the semiconductor device 10 of FIG. 2 in an oxygen
ambient. For ease of explanation, elements that are common between
FIGS. 1 and 2 and FIG. 6 are similarly numbered and the formation
of common structural elements will not be repeated. The oxidizing
ambient may contain H.sub.2O and/or HCl. As a result of the
oxidation processing, a portion of the semiconductor layer 16
becomes a silicon germanium (SiGe) stressor material layer 44. A
portion of the semiconductor layer 16 that does not comprise SiGe
remains as the strained channel 17. The semiconductor material 26
becomes an overlying SiO.sub.2 insulating layer 42. Both the
insulating layer 42 and the silicon germanium stressor material
layer 44 overlie dielectric layer 14. Again, the silicon germanium
stressor material layer 44 should have a thickness such that an
aspect ratio of the gate length ratioed to the thickness of silicon
germanium stressor material layer 44 is at least three. Therefore,
silicon germanium stressor material layer 44 is relatively thin.
Therefore, in this form the silicon germanium is formed in stressor
material layer 44 by an oxidizing ambient rather than by diffusion
alone. An advantage of this form is that the germanium content of
silicon germanium stressor material layer 44 can be higher than
when germanium is used in the stressor material layer 30 of FIG. 3.
It should be noted that other stressor materials besides SiGe may
be used.
[0022] Illustrated in FIG. 7 is further processing of semiconductor
device 40. In FIG. 7, the insulating layer 42 is removed from
semiconductor device 40. In one form, insulating layer 42 is
removed by an etch, either wet or dry. A strained channel 17 is
indicated by vertical dashed lines. It should be noted that the
stressor material layer 44 is immediately adjacent to the strained
channel 17 and therefore can exert significant influence on the
strained channel 17.
[0023] Illustrated in FIG. 8 is further processing of semiconductor
device 40. A raised source/drain extension is formed adjacent the
dielectric layer 24 by selective growth or deposition of a
semiconductor layer 46. Semiconductor layer 46 in one form is made
of silicon, but may be any semiconductor material such as silicon
germanium, etc. Extension implants are performed to create source
and drain implants. The extension implants result in formation of
dopants at an interface between stressor material layer 44 and
semiconductor layer 46. The implanted dopants into semiconductor
layer 46 and stressor material layer 44 are annealed to form source
and drain extensions in both of these layers. It should be noted
that the presence of semiconductor layer 46 functions to decrease
the sheet resistivity of the material to be used as the source and
drain of semiconductor device 40 since semiconductor device 40 is a
thin body device. A reason for the decreased resistivity is the
increased amount of semiconductor material that is present for the
source/drain elements.
[0024] Illustrated in FIG. 9 is further processing of semiconductor
device 40. In particular, a completed functional transistor is
formed. Sidewall spacers 50 are formed adjacent to dielectric layer
24 in a conventional manner. A thermal anneal process step is
performed to cause diffusion of the implanted dopants to form the
doped source/drain regions 52. A first portion of stressor material
layer 44 becomes doped forming the source/drain regions 52. A
second portion of stressor material layer 44 remains in the channel
between the vertical dashed lines defining the strained channel 17
and the edge of the doped source/drain regions. Two heterojunctions
as indicated by each of the two vertical dashed lines in FIG. 9 are
therefore formed between the strained channel 17 and each of a
respective one of the doped source/drain regions 52. Silicided
regions 54 are formed in a conventional manner overlying the
semiconductor layer 46 and the gate 22. At this point there has
been provided a transistor having a strained channel and
heterojunctions in the channel. Also, semiconductor device 40 has a
raised source/drain with respective elevated extensions that are
the portion of semiconductor layer 46 underlying the sidewall
spacer 50. Because the stressor material layer 44 is in very close
proximity to the strained channel 17, improved electron and hole
mobility and transistor conduction current.
[0025] Illustrated in FIG. 10 is a semiconductor device 60 that is
the initial formation of a double gate transistor. For convenience
of explanation, elements that are analogous in the initial
formation with FIGS. 1-9 are similarly numbered. In one form, the
double gate transistor described herein is implemented as a FINFET.
Other double gate transistors may be formed. Substrate 12 is
provided with overlying dielectric layer 14 as previously
described. Overlying the dielectric layer 14 is a silicon layer 62
that will function as a fin structure for the double gate
transistor. It should be understood that semiconductor materials
other than silicon may be used to implement silicon layer 62.
Portions of silicon layer 62 will subsequently function as the
source and drain of the double gate transistor. Overlying silicon
layer 62 is an oxide layer 64. The oxide layer 64 functions as: (1)
an oxidation barrier; and (2) a stress buffer between silicon layer
62 and a thicker insulator 66 that overlies the oxide layer 64.
Overlying the thicker insulator 66 is a gate 68. A surrounding
dielectric layer 70 is formed around the periphery of gate 68 to
function as a barrier.
[0026] Illustrated in FIG. 11 is a cross-sectional view taken along
line 11-11 of FIG. 10. In addition to the reference numbers
described above in connection with FIG. 10, there is also disclosed
a gate dielectric 67 that surrounds the vertical sidewalls of the
silicon layer 62.
[0027] Illustrated in FIG. 12 is a cross-sectional view of
semiconductor device 60 wherein a semiconductor material 72 is
selectively grown or deposited by overlying and surrounding silicon
layer 62. The semiconductor material 72 is silicon germanium (SiGe)
and may be of other materials such as silicon carbon, carbon and
other Group IV elements or alloys.
[0028] Illustrated in FIG. 13 is a cross-sectional view of
semiconductor device 60 wherein the germanium of semiconductor
material 72 is diffused by an anneal into silicon layer 62 forming
a depleted SiGe region 74. The arrows in FIG. 13 indicated that the
Ge is diffused both vertically and horizontally into a diffused
SiGe stressor layer 76. The channel region is now illustrated below
the gate electrode as indicated by vertical dashed lines. The
diffusion of Ge into the SiGe stressor layer 76 forms a strained
channel 63 in which Ge is located in close proximity to the
strained channel 63. The vertical dashed lines of strained channel
63 represent a heterojunction within the semiconductor device
60.
[0029] In an alternative form, the diffusion of Ge into stressor
layer 76 is continued with additional thermal processing or
oxidation according to the requirements of a particular
semiconductor device. In one embodiment the additional processing
is continued until a uniform material exists in both the stressor
layer 76 and the strained channel 63. In this form, a compressive
material exists uniformly laterally in the source, the channel and
the drain. The substantially uniform compressive layer that extends
through the channel from the source and the drain is desirable for
P-channel conductivity transistors. In such an embodiment, there
are no heterojunctions in the channel.
[0030] Illustrated in FIG. 14 is a cross-sectional view of
semiconductor device 60 wherein source/drain extension implants are
formed. The extension implants are performed to create source and
drain implants. The extension implants result in formation of
dopants at an interface between stressor layer 76 and SiGe region
74 as indicated by the dashed line separating stressor layer 76 and
SiGe region 74. The implanted dopants into diffusion SiGe region 74
and stressor layer 76 are annealed to form source and drain
extensions 77 in both of these layers. Two heterojunctions as
indicated by each of the two vertical dashed lines in FIG. 15 are
therefore formed between the strained channel 63 and each of a
respective one of the source and drain extensions 77. In one form
the strained channel is undoped silicon. A section 65 and a section
67 of the channel respectively exist between each heterojunction
and a junction between the channel and the source and drain
extensions 77, respectively. In one form, section 65 and section 67
are undoped silicon germanium and the source and drain extensions
77 are doped silicon germanium.
[0031] Illustrated in FIG. 15 is a cross-sectional view of
semiconductor device 60 wherein a completed functional transistor
is formed. Sidewall spacers 80 are formed adjacent to dielectric
layer 70 in a conventional manner. A thermal anneal process step is
performed to cause diffusion of the implanted dopants to form the
doped source/drain regions 82. A first portion of stressor layer 76
becomes doped forming the source/drain regions 82. A second portion
of stressor layer 76 remains adjacent the strained channel 63
between the vertical dashed lines defining the strained channel 63
and the edge of the doped source and drain extensions 77. Silicided
regions 84 are formed in a conventional manner overlying the
source/drain regions 82 and the gate 68. At this point there has
been provided a transistor having a strained channel and
heterojunctions in the channel. Also, semiconductor device 60 has a
raised source/drain with respective elevated extensions that are
the former portion of SiGe region 74 underlying the sidewall spacer
80. Because the stressor layer 76 is in very close proximity to the
strained channel 63, improved electron and hole mobility and
transistor conduction current. While a FINFET embodiment is herein
disclosed, it should be well understood that a strained channel
transistor may be implemented for any double gate structure using
the teachings provided herein.
[0032] Illustrated in FIG. 16 is a cross-sectional view of a
semiconductor device 90 wherein the initial formation of a double
gate transistor has been performed in accordance with the formation
of semiconductor device 60 of FIGS. 10-12. For purposes of
convenience, common element numbers between FIGS. 10-12 and FIG. 16
will be used and a repetition of the discussion of those common
elements will not be repeated. Illustrated in FIG. 16 is
semiconductor device 90 resulting after oxidation in an oxygen
ambient of semiconductor material 72 of the semiconductor device 60
of FIG. 12. The oxidizing ambient may contain H.sub.2O and/or HCl.
A silicon germanium (SiGe) stressor layer 94 and overlying
SiO.sub.2 insulating layer 92 both overlie dielectric layer 14.
Again, the silicon germanium stressor layer 94 should have a
thickness such that an aspect ratio of the gate length ratioed to
the thickness of silicon germanium stressor layer 94 is at least
three. Therefore, silicon germanium stressor layer 94 is relatively
thin. Therefore, in this form the silicon germanium is formed in
stressor layer 94 by an oxidizing ambient rather than by diffusion
alone. An advantage of this form is that the germanium content of
silicon germanium stressor layer 94 can be higher than when
germanium is used in the stressor layer 76 of FIG. 13. It should be
noted that other stressor materials besides SiGe may be used.
[0033] Illustrated in FIG. 17 is a cross-sectional view of
semiconductor device 90 wherein SiO.sub.2 insulating layer 92 is
removed from semiconductor device 90. Insulating layer 92 may be
removed by an etch, either wet or dry.
[0034] Illustrated in FIG. 18 is a cross-sectional view of
semiconductor device 90 wherein a raised source/drain 96 is
selectively grown or deposited by overlying and surrounding silicon
germanium stressor layer 94. The stressor layer 94 is silicon
germanium (SiGe) and may be of other materials such as silicon
carbon, carbon and other Group IV elements or alloys.
[0035] Illustrated in FIG. 19 is a cross-sectional view of
semiconductor device 90 wherein source/drain extension implants are
formed. The extension implants are performed to create source and
drain implants. The extension implants result in formation of
dopants at an interface between stressor layer 94 and the raised
source/drain 96. The implanted dopants into raised source/drain 96
and stressor layer 94 are annealed to form source and drain
extensions 98 in both of these layers. A completed functional
transistor is subsequently formed. Sidewall spacers 100 are formed
adjacent to dielectric layer 70 in a conventional manner. A thermal
anneal process step is performed to cause diffusion of the
implanted dopants to form the doped source/drain regions 102. A
first portion of stressor layer 94 becomes doped forming the
source/drain regions 102. A second portion of stressor layer 94
remains adjacent to the strained channel 63 between the vertical
dashed lines defining the strained channel 63 and the edge of the
doped source and drain extensions 98. Two heterojunctions as
indicated by each of the two vertical dashed lines in FIG. 19 are
therefore formed between the strained channel 63 and each of a
respective one of the source and drain extensions 98. Silicided
regions 104 are formed in a conventional manner overlying the
raised source/drain 96 and the gate 68. At this point there has
been provided a transistor having a strained channel and
heterojunctions in the channel. Also, semiconductor device 90 has a
raised source/drain 96 with respective elevated extensions that are
the portion of raised doped source and drain extensions 98
underlying the sidewall spacer 100. Because the stressor layer 94
is in very close proximity to the strained channel 63, improved
electron and hole mobility and transistor conduction current. While
a FINFET embodiment is herein disclosed, it should be well
understood that a strained channel transistor may be implemented
for any double gate structure using the teachings provided
herein.
[0036] Illustrated in FIGS. 20 and 21 are cross-sectional views of
a planar double gate transistor 110. For ease of discussion,
elements having reference numbers analogous to the elements
illustrated in FIGS. 1-5 and elsewhere are used. Therefore, a
complete discussion of the formation of all of the illustrated
elements in FIGS. 20 and 21 would be redundant. After dielectric
layer 14 is formed overlying substrate 12, a gate 112 is formed
within dielectric layer 14 by one of several conventional methods.
An overlying gate oxide 114 is formed and has a top surface that is
planar with a top surface of the dielectric layer 14. Overlying
semiconductor layer 16 is a gate oxide 116. Overlying the gate
oxide 116 is a gate 118. All surfaces of the gate oxide 116 and the
gate 118 are covered by dielectric layer 24. Transistor 110 of FIG.
20 is a planar double gate device.
[0037] Illustrated in FIG. 21 is a completed transistor 110 in
which the remaining elements of transistor 110 are formed by an
analogous method used to form the equivalent elements in the
embodiment of FIG. 5. Source/drain regions 36 are formed as
discussed above from the formation of stressor material layer 30
and diffusion source 28. Transistor 110 therefore is a double gate
device having a heterojunction between its source and channel and a
heterojunction between its drain and channel. Transistor 110 is a
planar device, as opposed to a FINFET, and has a strained channel
of selective characteristics, whether compressive or tensile.
[0038] By now it should be appreciated that there have been
provided methods for forming a semiconductor device having a
strained channel that provides improved transistor performance. A
heterojunction, raised source/drain regions and strained channel
combine to significantly improve transistor device performance.
Methods of formation are herein described in the context of forming
both a bulk transistor device and a double gate transistor device.
In one form there is provided a method for forming a semiconductor
device by providing a semiconductor layer and providing a gate
dielectric overlying a first portion of the semiconductor layer. A
gate electrode is provided overlying the gate dielectric. The gate
electrode and the gate dielectric are encapsulated. A stressor
source material is selectively grown overlying a second portion of
the semiconductor layer outside the first portion. The stressor
material is changed into a diffusion source, wherein changing the
stressor material into the diffusion source includes tailoring an
amount of diffusion of the stressor material into a channel region
of the underlying semiconductor layer. The channel becomes a
strained channel.
[0039] As the dimensions of thin body transistors continue to grow
smaller, a thinner channel region causes threshold voltage of the
transistor to increase. By using the materials described herein,
such as SiGe, etc., for the semiconductor layer from which the
channel is formed, the threshold voltage is lowered and thereby
offsets the increase in conventional thin body transistors when
scaled to a smaller dimension. Additionally, when mid-gap metals
are used as a gate material, a higher transistor threshold voltage.
To counteract the rising threshold voltage, the body of the
transistor can be doped. However, the doping degrades electrical
performance of the transistor and results in increased variability
in operating parameters. The use of a source material to provide
controlled diffusion into the source/drain and channel regions
allows low Vt transistors with mid-gap metal gates and without
doping the body.
[0040] In one form there is herein provided a method for forming a
semiconductor device wherein a semiconductor layer is provided. A
gate dielectric is provided overlying a first portion of the
semiconductor layer. A gate electrode is provided overlying the
gate dielectric. The gate electrode and the gate dielectric are
encapsulated with an encapsulant. A semiconductor material is
selectively grown overlying a second portion of the semiconductor
layer outside the first portion, wherein the semiconductor material
is a material different from a material of the semiconductor layer.
The semiconductor material is used as a diffusion source to control
an amount of diffusion of the semiconductor material into a channel
region of the semiconductor layer. The diffusion of the
semiconductor material into the channel region produces a strained
channel. In one form the semiconductor material is used as a
diffusion source and a thermal anneal process is used to control an
amount of diffusion source diffusion into the channel region of the
semiconductor layer. In another form the thermal anneal process
includes is a short duration high temperature anneal and a long
duration low temperature anneal, wherein short is less than long
and high is greater than low. In another form the thermal anneal
process further includes one of a hydrogen chloride (HCl) ambient
and an inert gas ambient of different temperature and time duration
to control an amount of semiconductor material diffusion into the
channel region and an amount of strain to be induced in the
strained channel of the semiconductor device. In another form the
semiconductor material includes at least one selected from the
group consisting of a source of germanium, carbon doped silicon,
boron, phosphorus and arsenic. In another form the inert gas
ambient includes at least one of nitrogen and argon. In one form
the semiconductor material is used as a diffusion source and an
oxidation of the semiconductor material is performed to produce
effects of condensation as one component of the semiconductor
material is preferentially consumed and diffused by formation of an
oxide and diffused by a semiconductor material enrichment of the
semiconductor layer. The oxide is selectively removed with respect
to the encapsulant that encapsulates the gate electrode and the
gate dielectric. In one form a layer of another semiconductor
material is selectively grown by selective epitaxy. In another form
the another semiconductor material includes one of a same initial
semiconductor material and a different semiconductor material. In
another form the semiconductor material is repeatedly used as a
diffusion source, the oxide is selectively removed, and the layer
of another semiconductor material is selectively grown to achieve a
desired amount of semiconductor material enrichment in the channel
region of the semiconductor layer. In another form the selectively
removing includes using a wet etch process. In one form the
semiconductor material includes SiGe, and Si is preferentially
consumed and Ge is preferentially diffused and enriched by
formation of the oxide SiO.sub.2. In another form the method
further includes selectively removing the oxide SiO.sub.2 with
respect to the encapsulant that encapsulates the gate electrode and
the gate dielectric. A layer of another semiconductor material is
selectively grown by epitaxy. In another form source/drain
extension region implants are formed and a dopant activation anneal
implemented. Sidewall spacers are formed adjacent the gate
electrode. Source/drain region implants are performed to create a
source/drain. Regions of the source/drain and the gate electrode
are silicided in one form. In one form the semiconductor layer
includes providing a semiconductor on insulator substrate. In
another form the semiconductor on insulator substrate includes a
silicon on insulator substrate. In another form the encapsulating
includes using an encapsulant for minimizing oxygen diffusion into
the gate electrode and the gate dielectric. In yet another form
encapsulating the gate electrode includes using an encapsulatant
selected from the group of nitride, oxide and a combination of
nitride and oxide. In another form the semiconductor material
includes silicon germanium (SiGe) with a predetermined Ge
concentration. In yet another form for a predetermined thickness of
the semiconductor material, the predetermined Ge concentration is
inversely proportional to the predetermined thickness, and a
function of an oxidation time, in order to preserve a total amount
of Ge to be diffused into the semiconductor layer. In yet another
form the predetermined Ge concentration is greater than 15%. In
another form the semiconductor material diffuses vertically into
the semiconductor layer and further diffuses laterally within the
semiconductor layer to a source/channel interface and a
drain/channel interface to form heterojunctions at respective
interfaces of the channel region. In one form the semiconductor
device includes one of a lateral device and a FinFET device. In yet
another form the semiconductor material is a material selected for
obtaining a desired threshold voltage for the semiconductor
device.
[0041] In yet another form there is provided a method for forming a
transistor. A semiconductor layer is provided. A gate dielectric
overlying a first portion of the semiconductor layer is provided. A
gate electrode overlying the gate dielectric is provided. The gate
electrode and the gate dielectric are encapsulated with a
dielectric layer. A semiconductor material is selectively grown
overlying a second portion of the semiconductor layer outside the
first portion, wherein the semiconductor material comprises a
material different from a material of the semiconductor layer. The
semiconductor material is oxidized in an oxygen ambient to form a
stressor layer from the second portion of the semiconductor layer
thereby forming a strained channel within the first portion of the
semiconductor layer. Source and drain regions are formed in the
second portion of the semiconductor layer and the semiconductor
material.
[0042] In a further form there is provided a method for forming a
transistor. A semiconductor layer is provided. A gate dielectric is
provided overlying a first portion of the semiconductor layer. A
gate electrode of the transistor is provided overlying the gate
dielectric. The gate electrode and the gate dielectric are
encapsulated. A semiconductor material containing germanium is
selectively grown overlying a second portion of the semiconductor
layer outside the first portion, wherein the semiconductor material
is a material different from a material of the semiconductor layer.
The semiconductor material is heated to diffuse germanium into the
second portion of the semiconductor layer thereby forming a
strained channel underlying the gate dielectric. Source and drain
regions of the transistor are formed in the second portion of the
semiconductor layer and the semiconductor material.
[0043] In the foregoing specification, the invention has been
described with reference to specific embodiments. However, one of
ordinary skill in the art appreciates that various modifications
and changes can be made without departing from the scope of the
present invention as set forth in the claims below. For example,
silicon carbon or any material that forms an alloy with silicon may
be used in lieu of silicon germanium. Various conductivities may be
used and differing doping concentrations may be used. Various
transistor structures may implement the strained channel methods
taught herein including various multiple gate structures, including
double gate structures. Accordingly, the specification and figures
are to be regarded in an illustrative rather than a restrictive
sense, and all such modifications are intended to be included
within the scope of the present invention.
[0044] Benefits, other advantages, and solutions to problems have
been described above with regard to specific embodiments. However,
the benefits, advantages, solutions to problems, and any element(s)
that may cause any benefit, advantage, or solution to occur or
become more pronounced are not to be construed as a critical,
required, or essential feature or element of any or all the claims.
As used herein, the terms "comprises," "comprising," or any other
variation thereof, are intended to cover a non-exclusive inclusion,
such that a process, method, article, or apparatus that comprises a
list of elements does not include only those elements but may
include other elements not expressly listed or inherent to such
process, method, article, or apparatus. The terms a or an, as used
herein, are defined as one or more than one. The term plurality, as
used herein, is defined as two or more than two. The term another,
as used herein, is defined as at least a second or more. The terms
including and/or having, as used herein, are defined as comprising
(i.e., open language). The term coupled, as used herein, is defined
as connected, although not necessarily directly, and not
necessarily mechanically.
* * * * *