U.S. patent application number 11/107141 was filed with the patent office on 2005-08-25 for semiconductor transistor having a stressed channel.
Invention is credited to Chau, Robert S., Ghani, Tahir, Mistry, Kaizad R., Murthy, Anand.
Application Number | 20050184311 11/107141 |
Document ID | / |
Family ID | 21700892 |
Filed Date | 2005-08-25 |
United States Patent
Application |
20050184311 |
Kind Code |
A1 |
Murthy, Anand ; et
al. |
August 25, 2005 |
Semiconductor transistor having a stressed channel
Abstract
A process is described for manufacturing an improved PMOS
semiconductor transistor. Recesses are etched into a layer of
epitaxial silicon. Source and drain films are deposited in the
recesses. The source and drain films are made of an alloy of
silicon and germanium. The alloy is epitaxially deposited on the
layer of silicon. The alloy thus has a lattice having the same
structure as the structure of the lattice of the layer of silicon.
However, due to the inclusion of the germanium, the lattice of the
alloy has a larger spacing than the spacing of the lattice of the
layer of silicon. The larger spacing creates a stress in a channel
of the transistor between the source and drain films. The stress
increases I.sub.DSAT and I.sub.DLIN of the transistor. An NMOS
transistor can be manufactured in a similar manner by including
carbon instead of germanium, thereby creating a tensile stress.
Inventors: |
Murthy, Anand; (Portland,
OR) ; Chau, Robert S.; (Beaverton, OR) ;
Ghani, Tahir; (Portland, OR) ; Mistry, Kaizad R.;
(Lake Oswego, OR) |
Correspondence
Address: |
BLAKELY SOKOLOFF TAYLOR & ZAFMAN
12400 WILSHIRE BOULEVARD
SEVENTH FLOOR
LOS ANGELES
CA
90025-1030
US
|
Family ID: |
21700892 |
Appl. No.: |
11/107141 |
Filed: |
April 14, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11107141 |
Apr 14, 2005 |
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10626257 |
Jul 23, 2003 |
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6885084 |
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10626257 |
Jul 23, 2003 |
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10002465 |
Nov 1, 2001 |
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6621131 |
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Current U.S.
Class: |
257/197 ;
257/E21.431; 257/E29.056; 257/E29.085; 257/E29.267 |
Current CPC
Class: |
H01L 29/161 20130101;
H01L 27/092 20130101; H01L 21/823814 20130101; H01L 29/6656
20130101; H01L 29/66636 20130101; H01L 29/7848 20130101; H01L
29/7834 20130101; H01L 29/165 20130101; H01L 29/66628 20130101;
H01L 29/7842 20130101; H01L 29/1054 20130101; H01L 29/6659
20130101; H01L 29/7833 20130101 |
Class at
Publication: |
257/197 |
International
Class: |
H01L 029/739 |
Claims
What is claimed:
1. A semiconductor transistor comprising: a layer having source and
drain recesses formed therein with a channel between the source and
drain recesses, and being made of a semiconductor material having a
first lattice with a first structure and a first spacing; a source
and a drain formed in the source and drain recesses respectively,
at least one of the source and the drain being made of a film
material which: (a) includes a dopant selected from one of a
p-dopant and an n-dopant; and (b) is formed epitaxially on the
semiconductor material so as to have a second lattice having a
second structure which is the same as the first structure, the
second lattice having a second spacing which differs from the first
spacing; a gate dielectric layer on the channel; and a conductive
gate electrode on the gate dielectric layer.
2. The semiconductor transistor of claim 1 wherein: (a) if the
dopant is a p-dopant, the second spacing is larger than the first
spacing; and (b) if the dopant is an n-dopant, the second spacing
is smaller than the first spacing.
3. The semiconductor transistor of claim 1 wherein the difference
between the first spacing and the second spacing creates a stress
in the channel.
4. The semiconductor transistor of claim 1 wherein the second
material includes the semiconductor material and an additive, the
difference between the first spacing and the second spacing being
due to the additive.
5. The semiconductor transistor of claim 4 wherein the
semiconductor material is silicon and the additive is selected from
one of germanium and carbon.
6. The semiconductor transistor of claim 5 wherein the additive is
germanium.
7. The semiconductor transistor of claim 6 wherein the germanium
comprises between 1 and 20 atomic percent of the silicon and the
germanium of the film material.
8. The semiconductor transistor of claim 7 wherein the germanium
comprises approximately 15 atomic percent of the silicon and the
germanium of the film material.
9. The semiconductor transistor of claim 4, further comprising: tip
regions formed between the source and the drain with the channel
between the tip regions, the tip regions being formed by implanting
of dopants and excluding the additive.
10. The semiconductor transistor of claim 9 wherein: (a) if the
dopant of the film material is a p-dopant, the dopants of the tip
regions are p-dopants; and (b) if the dopant of the film material
is an n-dopant, the dopants of the tip regions are n-dopants.
11. The semiconductor transistor of claim 1 wherein the dopant
comprises at least 0.5.times.10.sup.20/cm.sup.3 of the film
material.
12. The semiconductor transistor of claim 11 wherein the film
material has a resistivity of less than 1.1 mOhm-cm.
13. The semiconductor transistor of claim 1 wherein the source and
drain have a depth into the layer and are spaced by a width from
one another, a ratio of the depth to the width being at least
0.12.
14. The semiconductor transistor of claim 13 wherein the ratio is
at least 0.15.
15. The semiconductor transistor of claim 14 wherein the ratio is
at least 0.2.
16. The semiconductor transistor of claim 15 wherein the ratio is
at least 0.35.
17. The semiconductor transistor of claim 16 wherein the ratio is
approximately 92/215.
18. A semiconductor transistor comprising: a layer having source
and drain recesses formed therein with a channel between the source
and drain recesses and being made of a semiconductor material
having a first lattice with a first structure and a first spacing;
a source and a drain formed in the source and drain recesses
respectively, at least one of the source and the drain being made
of film material which: (a) includes a dopant selected from one of
a p-dopant and an n-dopant; and (b) is formed epitaxially on the
semiconductor material so as to have a second lattice having a
second structure which is the same as the first structure; and (i)
if the dopant is a p-dopant, the second lattice has a second
spacing which is larger than the first spacing, so that a
compressive stress is created between the source and the drain in
the channels; and (ii) if the dopant is an n-dopant, the second
lattice has a second spacing which is smaller than the first
spacing, so that a tensile stress is created between the source and
the drain in the channel; a gate dielectric layer on the channel;
and a conductive gate electrode on the gate dielectric layer.
19. The semiconductor transistor of claim 18 wherein the film
material includes the semiconductor material and an additive,
wherein: (a) if the dopant is a p-dopant, the second spacing is
larger than the first spacing due to the additive; and (b) if the
dopant is an n-dopant, the second spacing is smaller than the first
spacing due to the additive.
20. The semiconductor transistor of claim 19 wherein: (a) if the
dopant is a p-dopant, the additive is germanium; and (b) if the
dopant is an n-dopant, the additive is carbon.
21. A semiconductor transistor comprising: a layer having source
and drain recesses formed therein with a channel between the source
and drain recesses, the layer being made of a semiconductor
material; a source and a drain formed in the source and drain
recesses respectively, the source and the drain being made of a
film material which includes a dopant selected from one of a
p-dopant and an n-dopant, the source and the drain having a depth
into the layer and being spaced by a width from one another, a
ratio between the depth and the width being at least 0.12; a gate
dielectric layer on the channel; and a conductive gate electrode on
the gate dielectric layer.
22. The semiconductor transistor of claim 21 wherein the ratio is
at least 0.35.
23. The semiconductor transistor of claim 21 wherein the depth is
at least 80 nm.
24. The semiconductor transistor of claim 21 wherein the width is
less than 220 nm.
25. A method of forming a transistor comprising: forming a gate
dielectric layer on a layer of semiconductor material; forming a
gate electrode on the gate dielectric layer; implanting dopants
into the layer of semiconductor material to form doped tip regions
in the layer with a channel between the tip regions; etching the
layer to form source and drain recesses in the layer with the tip
regions between the recesses; and filling the source and drain
recesses with a source and a drain respectively.
26. The method of claim 25 wherein at least one of the source and
the drain is made of a film material which: (a) includes a dopant
selected from one of a p-dopant and an n-dopant; and (b) is formed
epitaxially on the semiconductor materials.
27. The method of claim 25 wherein the source and drain have a
depth into the layer and are spaced by a width from one another, a
ratio of the depth to the width being at least 0.12.
Description
BACKGROUND OF THE INVENTION
[0001] 1). Field of the Invention
[0002] This invention relates to the field of semiconductor
manufacturing, and more specifically to a semiconductor transistor
and its manufacture.
[0003] 2). Discussion of Related Art
[0004] Integrated circuits are often manufactured in and on silicon
and other semiconductor wafers. Such integrated circuits include
literally millions of metal oxide semiconductor (MOS) field effect
transistors, having gate lengths on the order of 0.05 microns. Such
MOS transistors may include p-channel MOS (PMOS) transistors, and
n-channel MOS (NMOS) transistors, depending on their dopant
conductivity types.
[0005] Wafers are obtained by drawing an ingot of silicon out of a
liquid silicon bath. The ingot is made of monocrystalline
(single-crystal) silicon, and is subsequently sawed into individual
wafers. A layer of silicon is then deposited over each wafer.
Because the wafer is made of monocrystalline silicon, the
deposition conditions can be controlled so that the layer of
silicon deposits "epitaxially" over the wafer. "Epitaxy" refers to
the manner in which the silicon layer deposits on the wafer--the
layer of silicon has a lattice which has a structure which follows
a structure of a lattice of the monocrystalline silicon of the
wafer. The layer of silicon is also substantially the same material
as the monocrystalline silicon of the wafer, so that the lattice of
the silicon layer also has substantially the same spacing as the
spacing of the lattice of the monocrystalline silicon of the
wafer.
[0006] A gate dielectric layer, a gate electrode, and spacers are
subsequently formed on the layer of silicon. Ions are also
implanted into the layer of silicon, which form source and drain
regions on opposing sides of the gate electrode. A voltage can be
applied over the source and drain regions. Current flows from the
source region to the drain region through a channel below the gate
dielectric layer when a voltage is applied to the gate
electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] The invention is described by way of example, with reference
to the accompanying drawings, wherein:
[0008] FIG. 1 is a cross-sectional side view of a partially
manufactured PMOS transistor, including a gate electrode and
lightly doped regions on opposing sides of the gate electrode;
[0009] FIG. 2 is a view similar to FIG. 1, after the formation of
spacers on opposing sides of the gate electrode;
[0010] FIG. 3 is a view similar to FIG. 2, after the formation of
deeper source and drain regions;
[0011] FIG. 4 is a view similar to FIG. 3, after diffusion of the
doped regions in a thermal step;
[0012] FIG. 5 is a view similar to FIG. 4, after a selective etch
to form recesses in the source and drain regions;
[0013] FIG. 6 is a view similar to FIG. 5, after depositing source
and drain films epitaxially in the recesses; and
[0014] FIG. 7 is an enlarged view of a portion of FIG. 6,
illustrating stresses that are created by the films.
DETAILED DESCRIPTION OF THE INVENTION
[0015] A process is described for manufacturing an improved PMOS
semiconductor transistor. Recesses are etched into a layer of
epitaxial silicon. Source and drain films are deposited in the
recesses. The source and drain films are made of an alloy of
silicon, germanium, and boron incorporated during deposition. By
incorporating boron during deposition, a higher active dopant
concentration can be obtained than with implantation techniques.
The alloy is epitaxially deposited on the layer of silicon. The
alloy thus has a lattice having the same structure as the structure
of the lattice of the layer of silicon. However, due to the
inclusion of the germanium, the lattice of the alloy has a larger
spacing than the spacing of the lattice of the layer of silicon.
The larger spacing creates a stress in a channel of the transistor
between the source and drain films. The stress, together with
reduced resistivity due to the higher active dopant concentration,
increases I.sub.DSAT and I.sub.DLIN of the transistor. An NMOS
transistor can be manufactured in a similar manner by including
carbon instead of germanium, thereby creating a tensile stress. The
present invention will be described with respect to the formation
of a PMOS transistor. One skilled in the art will appreciate that
an NMOS transistor may be manufactured in a similar manner, except
that doping conductivity types and lattice spacing will be
reversed.
[0016] FIG. 1 of the accompanying drawings illustrates an epitaxial
silicon layer 10 which is epitaxially formed on a monocrystalline
wafer substrate. Because the silicon layer 10 is epitaxially
formed, it follows the monocrystalline crystal structure of the
wafer substrate. The silicon of the layer 10 is thus also
monocrystalline. The silicon layer 10 includes an n-type dopant,
which can be formed by implanting phosphorous and arsenic ions to
produce an n-well, having an n-type dopant concentration of
approximately 5.0 times 10.sup.18/cm.sup.3. (An N+ film is thus
created.)
[0017] A plurality of field isolation regions 12 are formed in the
layer 10. The field isolation regions 12 isolate wells of different
conductivity types, and isolate adjacent transistors. The field
isolation regions 12 may, for example, be shallow trench isolation
(STI) regions formed by etching a trench into the layer 10, and
then filling the trench with deposited oxide.
[0018] A gate dielectric layer 14 is formed on a top surface 16 of
the layer 10. The gate dielectric layer 14 may be a nitrided oxide
layer formed to a thickness of between 5 and 30 .ANG., preferably
approximately 8 .ANG..
[0019] A gate electrode 18 is formed on the gate dielectric layer
14. The gate electrode 18 is preferably between 1,000 and 3,500
.ANG. thick. The gate electrode 18 may be formed by blanket
deposition of polysilicon, and patterning the polysilicon into the
gate electrode 18 utilizing known photolithographic techniques. In
the exemplary embodiment, the gate electrode 18 has a width 20 of
approximately 89 nm.
[0020] P-dopant ions are subsequently implanted from the top into
an exposed upper surface of the layer 10, and into an exposed upper
surface of the gate electrode 18. The dopant ions may, for example,
be boron ions. The ions form conductive p-doped regions 22A and
22B. The regions 22A and 22B are located on opposing sides of the
gate electrode 18, and are spaced from one another by the width 20.
A conductive p-doped region 24 is also formed in an upper portion
of the gate electrode 18.
[0021] FIG. 2 illustrates that spacers 26A and 26B are formed on
opposing sides of the gate electrode 18. The spacers 26A and 26B
cover sides of the gate electrode 18, and also cover portions of
the surface 16 adjacent and on opposing sides of the gate electrode
18. In the present example, the spacers 26A and 26B are L-shaped
spacers, the formation of which is known in the art.
[0022] As shown in FIG. 3, upper surfaces of the gate electrode 18
and the surface 16 are then again implanted with p-dopant ions,
typically boron ions as in the implantation step of FIG. 1. The
implantation energy is increased, compared to the implantation step
of FIG. 1, so that the boron ions implant deeper into the layer 10.
The spacers 26A and 26B form a mask which prevents implantation of
the ions into the layer 10 below the spacers 26A and 26B. P-doped
conductive regions 28A and 28B are formed by the ions in the layer
10 to a depth deeper than the regions 22A and 22B. However, a
shallow channel 30 is defined between inner edges of the doped
regions 22A and 22B resulting from the implantation step of FIG. 1.
The doped region 24 in the gate electrode 18 is also deeper after
the second implantation step.
[0023] A heat treatment or "annealing" step is subsequently carried
out, wherein the structure of FIG. 3 is heated. Heating causes
diffusion of the regions 22A, 22B, 28A, and 28B into the layer 10.
As shown in FIG. 4, inner tips 34A and 34B are then located below
the gate electrode 18. Lower edges of the regions 28A and 28B move
downward into the layer 10. The regions 22A and 22B are epitaxial
silicon with a p-dopant concentration of approximately
1.times.10.sup.19/cm.sup.3. (The regions 22A and 22B are thus doped
P-.) No other materials are present in the regions 22A and 22B,
except silicon, arsenic, phosphorous, and boron. The doped region
24 in the gate electrode 18 also diffuses down to the gate
dielectric layer 14.
[0024] FIG. 5 shows the structure of FIG. 4 after a selective etch
step. An anisotropic etchant is used which selectively removes
silicon over the other exposed materials of the structure of FIG.
4. Recesses 36A and 36B are thereby etched into the regions 28A and
28B. Inner edges of the recesses 36A and 36B are aligned with outer
edges of the spacers 26A and 26B. Outer edges of the recesses 36A
and 36B are at the field isolation regions 12. It should be noted
that surfaces 38 of the recesses 36A and 36B are monocrystalline
epitaxial silicon. Epitaxial silicon has a lattice with a known
structure and spacing. An upper portion of the gate electrode 18 is
also etched out.
[0025] As shown in FIG. 6, source and drain films 40A and 40B are
subsequently formed in the recesses 36A and 36B. The films 40A and
40B are epitaxially formed on the surfaces 38. The films 40A and
40B include silicon, germanium, and boron. The films can be formed
in a 200 mm chemical vapor deposition chamber with the following
processing conditions: dichlorosilane of 20 sccm, diborane of 70
sccm at 1% concentration, and germane of 50 sccm, at a temperature
of 740.degree. C.
[0026] The silicon and the germanium form an alloy having a lattice
which has the same structure as the structure of the lattice of the
epitaxial silicon of the surfaces 38. The lattice of the alloy of
silicon and germanium, however, has a larger spacing than the
spacing of the lattice of the epitaxial silicon of the surfaces 38,
at least in a relaxed state. Because the lattice of the alloy has
the same structure as the surfaces 38, the films 40A and 40B form
epitaxially on the surfaces 38. However, because of the larger
spacing of the lattice of the alloy, the films 40A and 40B create a
compressive stress in the channel 30. The germanium is present in
the combination of the silicon and the germanium in about 15 atomic
percent. It has been found that epitaxy can be maintained with a
germanium concentration of up to 20 atomic percent of the
combination of the silicon and germanium by volume. Epitaxy thus
tends to break down at an atomic percentage of germanium of above
20 percent. A further advantage of depositing the films 40A and 40B
is that a relatively large boron concentration can be included. The
boron concentration is preferably approximately
3.times.10.sup.20/cm.sup.3. (The films 40A and 40B are thus doped
P+.) The relatively large concentration of boron creates a
relatively low resistance of approximately 0.9 mOhm-cm. A
conductive p-doped film 42 also deposits on the etched-back gate
electrode 18. Suitable results can be obtained with dopant
concentrations of 0.5.times.10.sup.20/cm.sup.3 and above. The
resistivity is preferably less than 1.1 mOhm-cm.
[0027] FIG. 7 illustrates the direction of compressive stresses
created by the films 40A and 40B. The directions of the compressive
stresses are along the lines 50. A more dense spacing between the
lines 50 indicates a larger stress, and a larger spacing between
the lines 50 indicates a smaller stress. It can be seen that the
largest stress is created at or near the channel 30. The films 40A
and 40B extend to a depth 52 into the layer 10, and are spaced from
one another by a width 54. A smaller ratio between the depth 52 and
the width 54 will result in a smaller stress in the channel 30, and
a larger ratio between the depth 52 and the width 54 will result in
a larger stress in the channel 30. A ratio between the depth 52 and
the width 54 is preferably at least 0.12, more preferably 0.15,
more preferably 0.2, and more preferably 0.35. In the present
example, the depth 52 is 92 nm, and the width 54 is 215 nm.
[0028] The compressive stress reduces the effective mass in the
channel, which in turn increases hole mobility. It has been found
that a compressive stress in the channel 30 increases the
I.sub.DSAT of the PMOS transistor 60 by approximately 20 percent.
The I.sub.DLIN is increased by approximately 40 percent.
[0029] In the present example, the layer 10 is epitaxial silicon,
and the films 40A and 40B are silicon with a germanium additive. It
may be possible to create similar structures utilizing additives
other than germanium. The present example has also been described
with reference to a PMOS transistor. An NMOS transistor may be
manufactured in a similar manner. In an NMOS transistor, doping
conductivity types would be reversed. Furthermore, a tensile stress
will be created in the channel. A tensile stress can be created
utilizing source and drain films of silicon which includes carbon.
The silicon and carbon form an alloy which has a lattice with the
same structure as the structure of the lattice of the epitaxial
silicon, but with a smaller spacing. The source and drain films
will tend to contract, and create a tensile stress in the
channel.
[0030] While certain exemplary embodiments have been described and
shown in the accompanying drawings, it is to be understood that
such embodiments are merely illustrative and not restrictive of the
current invention, and that this invention is not restricted to the
specific constructions and arrangements shown and described since
modifications may occur to those ordinarily skilled in the art.
* * * * *