# Patent

## Title
Replacement metal gates to enhance transistor strain

## Abstract

Some embodiments of the present invention include apparatuses and methods relating to NMOS and PMOS transistor strain.

## Background
RELATED APPLICATION
This is a Continuation of application Ser. No. 14/882,434 filed Oct. 13, 2015 which is a Continuation of application Ser. No. 14/573,242 filed Dec. 17, 2014 now U.S. Pat. No. 9,159,566 which is a Continuation of application Ser. No. 13/909,792 filed Jun. 4, 2013 now U.S. Pat. No. 8,946,016 issued Oct. 13, 2015 which is a Continuation of application Ser. No. 13/233,297 filed on Sep. 15, 2011 now abandoned which is a Divisional of application Ser. No. 11/305,465 filed Dec. 16, 2005, now U.S. Pat. No. 8,101,485 issued Jan. 24, 2012.


BACKGROUND
In semiconductor processing, transistors may be formed on semiconductor wafers. The transistors may include a gate structure having a gate dielectric and a gate electrode, a source, a drain, and a channel region between the source and the drain. In CMOS (complimentary metal oxide semiconductor) technology, transistors may typically be of two types: NMOS (negative channel metal oxide semiconductor) or PMOS (positive channel metal oxide semiconductor) transistors. The transistors and other devices may be interconnected to form integrated circuits (ICs) which perform numerous useful functions.
The performance of the ICs may directly depend on the performance of the transistors. Transistor performance may be improved by providing a strain in the channel region. Specifically, NMOS transistor performance may be improved by providing a tensile strain in the channel region and PMOS transistor performance may be improved by providing a compressive strain in the channel region.



BRIEF DESCRIPTION OF THE DRAWINGS
The invention is illustrated by way of example and not by way of limitation in the figures of the accompanying drawings, in which the like references indicate similar elements and in which:
FIG. 1 is a cross-sectional view of a partially formed NMOS transistor and a partially formed PMOS transistor, and a layer over the transistor gate structures.
FIG. 2 is a view similar to FIG. 1 with a portion of the layer removed to expose the gate structures.
FIG. 3 is a view similar to FIG. 2 with the transistor gate structures removed to form trenches.
FIG. 4 is a view similar to FIG. 3 with a gate dielectric formed in the trenches.
FIG. 5 is a view similar to FIG. 4 with gate electrodes formed in the trenches.
FIG. 6 is a view similar to FIG. 4 with an n-type material formed in the trenches.
FIG. 7 is a view similar to FIG. 6 with a portion of the n-type material removed and a p-type material formed in the trenches.
FIG. 8 is a view similar to FIG. 7 with fill materials formed in the trenches.
FIG. 9 is a view similar to FIG. 2 with a portion of the gate structures removed to form trenches.
FIG. 10 is a view similar to FIG. 9 with gate electrodes formed in the trenches.



The following Detailed Description is provided with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number usually identifies the figure in which the reference number first appears. The use of the same reference numbers in different figures indicates similar or identical items.
DETAILED DESCRIPTION
In various embodiments, apparatuses and methods relating to strained transistors are described. However, various embodiments may be practiced without one or more of the specific details, or with other methods, materials, or components. In other instances, well-known structures, materials, or operations are not shown or described in detail to avoid obscuring aspects of various embodiments of the invention. Similarly, for purposes of explanation, specific numbers, materials, and configurations are set forth in order to provide a thorough understanding of the invention. Nevertheless, the invention may be practiced without specific details. Furthermore, it is understood that the various embodiments shown in the figures are illustrative representations and are not necessarily drawn to scale.
Transistor performance characteristics, such as transistor drive current, may be enhanced by providing strain to the transistor channels. For example, NMOS transistor performance may improve when the NMOS transistor channel is under lateral tensile stress. Also, PMOS transistor performance may improve when the PMOS transistor channel is under lateral compressive stress. Briefly, the present invention may provide for increased channel strain to enhance the performance of NMOS and PMOS transistors.
FIG. 1 illustrates an NMOS transistor 101 and a PMOS transistor 102 on a substrate 103. NMOS transistor 101 and PMOS transistor 102 may be on a continuous substrate 103. However, they are illustrated separately in FIGS. 1-10 for the sake of clarity. Substrate 103 may include any suitable semiconductor material. In an embodiment, substrate 103 may include monocrystalline silicon. Substrate 103 may also include isolation structures (not shown) to isolate the NMOS and PMOS transistors.
As shown, PMOS transistor 102 includes an n-well 111, a channel 112, source and drain films 113, a gate dielectric 114, a gate electrode 115 and spacers 116. PMOS transistor 102 may be formed by any suitable processing techniques.
N-well 111 may include any suitable n-type dopants, such as phosphorus and arsenic, and may be formed by any suitable technique. In an embodiment, n-well 111 may be formed by doping substrate 103 by ion implantation.
Channel 112 may be under a compressive strain from any stressor source or material. As discussed, a compressive strain may increase the performance of PMOS transistor 102. In an embodiment, source and drain films 113 may provide a compressive strain on channel 112. In such embodiments, source and drain films 113 may therefore be considered stressors. In an embodiment, source and drain films 113 may be epitaxial and may have a greater lattice spacing constant than n-well 111 and channel 112. Source and drain films 113 may transfer a compressive strain on channel 112 as they tend to, but are constrained from, expanding to their natural lattice spacing. In an embodiment, source and drain films 113 may include an alloy of materials. In an embodiment, source and drain films 113 may include an alloy of silicon and germanium. In an embodiment, source and drain films 113 may include a p-type dopant, such as boron. In an embodiment, source and drain films 113 may be formed in recesses of n-well 111.
As illustrated, channel 112 may be under a compressive strain from source and drain films 113. However, channel 112 may be under a compressive strain from any suitable stressor source or material. In an embodiment, a material may be formed over gate electrode 115 and spacers 116 to provide a compressive strain on channel 112.
Gate dielectric 114 may be any suitable material. In an embodiment, gate dielectric 114 may include silicon dioxide. In other embodiments, gate dielectric 114 may include a high-k gate dielectric. In general, a high-k gate dielectric may include any material having a dielectric constant, k, that is greater than about 3.9 (the dielectric constant of silicon dioxide). In an embodiment, gate dielectric 114 may include hafnium oxide. In other embodiments, gate dielectric 114 may include hafnium silicon oxide, lanthanum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide aluminum oxide, lead scandium tantalum oxide, or lead zinc niobate.
Gate electrode 115 may include any suitable material. In an embodiment, gate electrode 115 may include polysilicon. In another embodiment, gate electrode 115 may include polysilicon doped with a p-type dopant, such as boron.
Gate dielectric 114 and gate electrode 115 together may be considered a gate stack or a gate structure. In an embodiment, both gate dielectric 114 and gate electrode 115 may be a part of the gate structure. In other embodiments, the gate structure may be a single material. In other embodiments, the gate structure may include a hard mask or any number of additional materials. As is described further below, a portion or an entirety of the gate structure may be removed to enhance the strain in channel 112. Therefore, a portion or an entirety of the gate structure may be considered sacrificial.
Spacers 116 may include any suitable dielectric materials, such as a nitride or an oxide. Spacers 116 may be along the sidewalls of gate electrode 108 and may therefore be considered sidewall spacers.
As shown, NMOS transistor 101 includes a p-well 104, a channel 105, source and drain regions 106, a gate dielectric 107, a gate electrode 108, and spacers 109. NMOS transistor 101 may be formed by any suitable processing techniques. P-well 104 may include any suitable p-type dopants, such as boron and indium, and may be formed by any suitable technique. Source and drain regions 106 may include any suitable n-type dopants, such as phosphorus and arsenic, and may be formed by any suitable technique, such as ion implantation or epitaxial deposition. Channel 105 may be between source and drain regions 106.
Gate dielectric 107 may be any suitable material. In an embodiment, gate dielectric 107 may include silicon dioxide. In other embodiments, gate dielectric 107 may include a high-k gate dielectric. In an embodiment, gate dielectric 107 may include hafnium oxide. In other embodiments, gate dielectric 107 may include hafnium silicon oxide, lanthanum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide aluminum oxide, lead scandium tantalum oxide, or lead zinc niobate. In an embodiment, gate dielectric 107 and gate dielectric 114 may include the same material.
Gate electrode 108 may include any suitable material. In an embodiment, gate electrode 108 may include polysilicon. In another embodiment, gate electrode 108 may include polysilicon doped with an n-type dopant, such as phosphorus and arsenic.
Gate dielectric 107 and gate electrode 108 together may be considered a gate stack or a gate structure. In an embodiment, both gate dielectric 107 and gate electrode 108 may be a part of the gate structure. In other embodiments, the gate structure may be a single material. In other embodiments, the gate structure may include a hard mask or any number of additional materials. As is described further below, a portion or an entirety of the gate structure may be removed to enhance the strain in channel 105. Therefore, a portion or an entirety of the gate structure may be considered sacrificial.
Spacers 109 may include any suitable dielectric materials, such as a nitride or an oxide. Spacers 109 may be along the sidewalls of gate electrode 108 and may therefore be considered sidewall spacers.
Channel 105 may be under a tensile strain from any stressor source. As discussed, a tensile strain in channel 105 may improve the performance of NMOS transistor 101. In an embodiment, a layer 110 may cover NMOS transistor 101 and PMOS transistor 102 and layer 110 may provide a tensile stress on channel 105 and channel 112 and may therefore be considered a tensile layer or a stressor. In an embodiment, layer 110 may include a silicon nitride, such as Si3N4. Layer 110 may be formed by any suitable technique. In an embodiment, a tensile strain in channel 112 may diminish the performance of PMOS transistor 102; however, due to source and drain films 113, channel 112 may be under a net compressive strain.
As discussed, channel 105 may be under a tensile strain due to layer 110. However, channel 105 may be under a tensile strain from any suitable stressor material or source. In an embodiment, source and drain regions 106 may provide a tensile strain on channel 105. In an embodiment, source and drain regions 106 may be epitaxial and may have a smaller lattice spacing constant than p-well 104 and channel 105. Source and drain regions 106 may then be constrained from achieving their natural lattice spacing and may transfer a tensile strain on channel 105 as they tend to, but are constrained from, contracting to their natural lattice spacing. In an embodiment, source and drain regions 106 films may include carbon.
NMOS transistor 101 and PMOS transistor 102 may also include other features that are not shown for the sake of clarity, such as halo implants, tip implants, silicide regions, and the like.
As illustrated in FIG. 2, a portion of layer 110 may be removed to expose gate electrode 108 and gate electrode 115. In an embodiment, a portion of layer 110 may be removed by a planarization or chemical mechanical polishing (CMP) process. In an embodiment, a portion of gate electrodes 108, 115 may also be removed.
As illustrated in FIG. 3, gate electrodes 108, 115 and gate dielectrics 107, 114 may be removed to form trenches 121, 122. Gate electrodes 108, 115 and gate dielectrics 107, 114 may be removed by any suitable technique, such as a selective etch technique.
As discussed, channel 105 may be under a tensile strain and channel 112 may be under a compressive strain. Due to the removal of gate electrodes 108, 115 and gate dielectrics 107, 114, the strain on channels 105, 112 may be increased or enhanced. The increased strain may be due to the removal of material from trenches 121, 122, which may have been offsetting the desired stresses. In general, removing a portion or the entirety of a gate structure may remove an offsetting stress and allow the stressor to relax, which may increase the strain on the transistor channel. In other words, removing the gate structure may allow the stressor to enhance the desired strain on the channel.
In an embodiment, NMOS transistor 101 may include a stressor that is relaxed to increase a tensile strain on channel 105. In an embodiment, the stressor may include layer 110. In another embodiment, the stressor may include source and drain regions 106. In an embodiment, PMOS transistor 102 may include a stressor that is allowed to increase a compressive strain on channel 112. In an embodiment, the stressor may include source and drain films 113.
As illustrated in FIG. 4, a gate dielectric 131 may be formed in trenches 121, 122. Gate dielectric 131 may be any suitable material and may be formed by any suitable technique. In an embodiment, gate dielectric 131 may include a high-k gate dielectric, such as hafnium oxide. In various other embodiments, gate dielectric 131 may include hafnium silicon oxide, lanthanum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide aluminum oxide, lead scandium tantalum oxide, or lead zinc niobate. In another embodiment, gate dielectric 131 may include silicon dioxide.
In an embodiment, gate dielectric 131 may be formed by a deposition process, such as chemical vapor deposition (CVD). As illustrated in FIG. 4, gate dielectric 131 may be formed along the bottom of trenches 121, 122 and along the sidewalls of trenches 121, 122. In an embodiment, gate dielectric 131 may also be formed over layer 110 (not shown). In such embodiments, gate dielectric 131 may be subsequently removed from layer 110 by a CMP process such that gate dielectric 131 only remains in trenches 121, 122.
As illustrated in FIG. 5, a gate electrode 141 and a gate electrode 142 may be formed. Gate electrode 141 may include any suitable material or materials. In an embodiment, gate electrode 141 may include an n-doped polysilicon. In another embodiment, gate electrode 141 may include a metal with an n-type work function. In other embodiments, gate electrode 141 may include a stack or structure of materials with the material in contact with gate dielectric 131 including a material with an n-type work function. In various embodiments, the n-type work function material may include hafnium, zirconium, titanium, tantalum, aluminum, their alloys, or carbides of those metals.
In an embodiment, gate electrode 141 may include a material that provides an additional tensile strain on channel 105. Gate electrode 141 may provide an additional tensile strain on channel 105 by including a material with a coefficient of thermal expansion (CTE) less than the CTE of substrate 103 that is deposited at a temperature greater than room temperature and the operating temperature of NMOS transistor 101. Upon cooling, gate electrode 141 may contract more slowly than the surrounding materials and gate electrode 141 may then transfer a tensile strain to channel 105 via gate dielectric 131 or via spacers 109. In an embodiment, the material that provides the additional tensile strain may be the n-type work function metal, as listed above. In other embodiments, the material that provides the additional tensile strain may be another material in the gate structure. In an embodiment, the material that provides the additional tensile strain may include tungsten. In another embodiment, the material that provides the additional tensile strain may include titanium carbide.
Gate electrode 142 may include any suitable material or materials. In an embodiment, gate electrode 142 may include a p-doped polysilicon. In another embodiment, gate electrode 142 may include a metal with a p-type work function. In other embodiments, gate electrode 142 may include a stack or structure of various materials with the material in contact with gate dielectric 131 including a material with a p-type work function. In various embodiments, the p-type work function material may include ruthenium, palladium, platinum, cobalt, nickel, and their oxides.
In an embodiment, gate electrode 142 may include a metal that provides an additional compressive strain on channel 112. Gate electrode 142 may provide an additional compressive strain on channel 112 by including a material with a CTE greater than the CTE of substrate 103 that is deposited at a temperature greater than room temperature and the operating temperature of PMOS transistor 102. Upon cooling, gate electrode 142 may contract more quickly than the surrounding materials and gate electrode 142 may then transfer a compressive strain to channel 112 via gate dielectric 131 or via spacers 116. In an embodiment, the material that provides the additional compressive strain may be the p-type work function metal, as listed above. In other embodiments, the material that provides the additional compressive strain may be another material in the gate structure. In various embodiments, the material that provides the additional compressive strain may include boron carbide, tungsten, molybdenum, rhodium, vanadium, platinum, ruthenium, beryllium, palladium, cobalt, titanium, nickel, copper, tin, aluminum, lead, zinc, alloys of these metals, or their silicides.
Gate electrode 141 and gate electrode 142 may be formed by any suitable technique. In an embodiment, gate electrode 141 may be selectively formed and then gate electrode 142 may be selectively formed. In an embodiment, gate electrode 142 may be selectively formed and then gate electrode 141 may be selectively formed.
FIGS. 6-8 illustrate a method for forming gate electrodes with reference to the illustration of FIG. 4. As illustrated in FIG. 6, an n-type material 161 may be formed in trenches 121, 122. N-type material 161 may be any suitable n-type work function material as discussed above with reference to FIG. 5. In an embodiment, n-type material 161 may partially fill trenches 121, 122. In another embodiment, n-type material 161 may completely fill trenches 121, 122.
As illustrated in FIG. 7, n-type material 161 may be selectively removed from PMOS transistor 102 and a p-type material 162 may be formed. P-type material 162 may be any suitable p-type work function material as discussed above with reference to FIG. 5. In an embodiment, p-type material 162 may be formed such that an opening remains in either or both of the trenches. In another embodiment, p-type material 162 may complete the gate structure of PMOS transistor 102 or NMOS transistor 101.
As illustrated in FIG. 8, fill material 171 and fill material 172 may be selectively formed. Fill materials 171, 172 may be the same material or they may be different. In an embodiment, fill material 171 and fill material 172 may be chosen based on their conductive properties. In another embodiment, fill material 171 may be chosen to provide a tensile strain on NMOS transistor 101 as discussed with respect to FIG. 5. In various embodiments, fill material 171 may include tungsten or titanium carbide.
Fill material 172 may also include any suitable material. In an embodiment, fill material 172 may be chosen to provide a compressive strain on PMOS transistor 102 as discussed with respect to FIG. 5. In some embodiments, fill material 172 may include boron carbide, tungsten, molybdenum, rhodium, vanadium, platinum, ruthenium, beryllium, palladium, cobalt, titanium, nickel, copper, tin, aluminum, lead, zinc, alloys of these metals, or their silicides.
As illustrated in FIGS. 6-8, n-type material 161 may be formed and selectively removed, p-type material 162 may be formed, and optional fill materials may be formed. In another embodiment, the n-type and p-type materials may be formed in the opposite order. For example, the p-type material may be formed first. Then, the p-type material may be selectively removed from NMOS transistor 101 and a subsequent n-type material may then be formed. Lastly, optional fill materials may then be formed.
The illustrated methods of FIGS. 3-8 show trenches 121, 122 being formed and the stressors on channels 105, 112 being allowed to simultaneously increase the strain on channels 105, 112. However, in other embodiments, trenches 121, 122 may be formed and the strains increased independently. In an embodiment, trench 121 may be formed while gate dielectric 114 and gate electrode 115 remain. The strain on channel 105 of NMOS transistor 101 may then be independently increased. In another embodiment, trench 122 may be formed while gate dielectric 107 and gate electrode 108 remain. The strain on channel 112 of PMOS transistor 102 may then be independently increased.
The methods illustrated in FIGS. 3-8 show the entire gate structure being sacrificial. As illustrated in FIGS. 9-10, only a portion of the gate structure may be sacrificial.
In FIG. 9, gate electrodes 108, 115 may be removed from the structure of FIG. 2 to form trenches 151, 152. Gate electrodes 108, 115 may be removed by any suitable technique such as a selective etch technique.
Due to the removal of gate electrodes 108, 115 the strain on channels 105, 112 may be increased. The increased strain may be due to the removal of material from trenches 151, 152, which may have been an offsetting stress to the desired stresses as discussed above.
As illustrated in FIG. 10, gate electrodes 141, 142 may be formed. Gate electrodes 141, 142 may include any of the materials or attributes and may be formed by any of the methods as discussed above with respect to FIGS. 5-8.
Reference throughout this specification to “one embodiment” or “an embodiment” means that a particular feature, structure, material, or characteristic described in connection with the embodiment is included in at least one embodiment of the invention. Thus, the appearances of the phrases “in one embodiment” or “in an embodiment” in various places throughout this specification are not necessarily referring to the same embodiment of the invention. Furthermore, the particular features, structures, materials, or characteristics may be combined in any suitable manner in one or more embodiments.
It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of ordinary skill in the art upon reviewing the above description. The scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.

## Claims

1. A semiconductor structure, comprising:
an NMOS transistor and a PMOS transistor above a silicon substrate;
a first gate stack for the PMOS transistor, the first gate stack comprising a workfunction metal layer;
a second gate stack for the NMOS transistor, the second gate stack comprising a layer comprising titanium and aluminum, wherein the workfunction metal layer of the first gate stack is not included in the second gate stack, and wherein the layer comprising titanium and aluminum is over the workfunction metal layer in the first gate stack;
a source region and a drain region for the NMOS transistor;
a raised source region and a raised drain region for the PMOS transistor, the raised source and drain regions comprising a silicon germanium layer creating a stressed channel for the PMOS device, and the raised source and drain regions extending above a surface of the silicon substrate over which the first gate stack is formed; and
a nitride layer over the silicon germanium layer and co-planar with an uppermost surface of the first gate stack.

2. The semiconductor structure of claim 1, wherein the first and second gate stacks comprise a same fill metal.

3. The semiconductor structure of claim 1, wherein the first and second gate stacks comprise a first and second gate dielectric layer, respectively.

4. The semiconductor structure of claim 3, wherein the first and second gate dielectric layers comprise U-shaped gate dielectric layers.

5. The semiconductor structure of claim 3, wherein the first and second gate dielectric layers comprise hafnium oxide.

6. The semiconductor structure of claim 1, further comprising a silicide layer on the silicon germanium layer.