# Patent

## Title
Elevated source and drain elements for strained-channel heterojuntion field-effect transistors

## Abstract

A semiconductor structure having a surface layer disposed over a substrate, the surface layer including strained silicon. A contact layer is disposed over a portion of the surface layer, the contact layer including a metal-semiconductor alloy. A bottommost boundary of the contact layer is disposed above a bottommost boundary of the surface layer.

## Background
FIELD OF THE INVENTION
This invention generally pertains to semiconductor structures formed on strained layers.
BACKGROUND
Field-effect transistors (FETs) based on strained heterojunction channels have shown significant promise for increased drive current and higher frequency operation, in comparison to traditional FETs with relaxed silicon channels. Strained heterojunction channels are fabricated from materials such as silicon (Si), silicon-germanium (SiGe), germanium (Ge), etc. Forming high-quality electrical contacts is difficult when the source and drain regions of a FET include materials other than pure silicon. High quality contacts to silicon-based FETs are traditionally achieved through a silicide process, where a metal such as cobalt or titanium is alloyed with silicon to form the contact. This process is typically not as effective with semiconductor materials other than silicon. For example, a cobalt silicide contact formed to a source disposed in a strained Si layer and an underlying SiGe layer may form a high-resistivity compound in the SiGe layer, thereby compromising the functionality of the contact. More specifically, discrete regions of cobalt silicide may form in the SiGe layer, with Ge excluded from the cobalt silicide.
SUMMARY
An elevated source/drain process enables the formation of good contacts to source and drain regions of a FET formed in a strained channel, such as strained silicon. In an embodiment, a bottommost boundary of the metal-semiconductor alloy contact extends into the strained silicon portion of each of the source and drain regions. The metal-semiconductor alloy contact does not extend into an underlying relaxed layer that may contain elements not found in the strained silicon layer, such as Ge, thereby avoiding the formation of high-resistivity compounds. By extending the metal-semiconductor alloy only into the strained silicon portion, the area of the metal-semiconductor alloy interface with the strained silicon is increased, thereby reducing contact resistance.
In another embodiment, the bottommost boundary of the metal-semiconductor alloy is disposed in a raised semiconductor layer over the source and drain regions. The raised semiconductor layer is formed over a strained upper layer. If the raised semiconductor layer is silicon, it is therefore also at least partially strained. This strain increases carrier mobilities, thereby lowering the sheet resistance of the semiconductor layer. This strain also results in a reduced energy bandgap, thereby lowering the contact resistivity between the metal-semiconductor alloy and the raised semiconductor layer. The raised semiconductor layer may also be germanium, which has a smaller bandgap than relaxed or strained silicon. This material choice, therefore, promotes a low contact resistivity between the metal-semiconductor alloy and the raised semiconductor layer.
In an alternative embodiment, the metal-semiconductor alloy contact extends a minimal distance into an underlying relaxed layer. This minimal distance is such that the formation of structural inhomogeneities is avoided.
In an aspect, the invention features a semiconductor structure including a surface layer disposed over a substrate, the surface layer including strained silicon. A contact layer is disposed over at least a portion of the surface layer, with the contact layer including a metal-semiconductor alloy. A bottommost boundary of the contact layer is disposed above a bottommost boundary of the surface layer.
One or more of the following features may also be included. The bottommost boundary of the contact layer may share an interface with the surface layer. The interface may have a low contact resistivity, e.g., less than approximately 5×10−7 Ω-cm2. The surface layer may have a low resistivity, e.g., less than approximately 0.001 ohm-cm.
The semiconductor structure may also include a semiconductor layer disposed over a portion of the surface layer, with the bottommost boundary of the contact layer sharing an interface with the semiconductor layer. The semiconductor layer may include an epitaxial layer, and generally includes at least one of germanium and silicon. The interface between the contact layer and the semiconductor layer may have a low contact resistivity. The contact layer may have a low sheet resistance, e.g., less than approximately 10 Ω/□. The metal-semiconductor alloy may include a metal including at least one of cobalt, titanium, tungsten, nickel, and platinum. The metal-semiconductor alloy may include a semiconductor including at least one of silicon and germanium.
The semiconductor structure may also include a relaxed layer disposed under the surface layer. The relaxed layer may include at least one of germanium and silicon. The bottommost boundary of the contact layer may be disposed above the relaxed layer.
The semiconductor structure may also include a doped region in the surface layer, with the bottommost boundary of the contact layer being disposed above a bottommost boundary of the doped region. The doped region may include at least a portion of a transistor source and/or drain.
In another aspect, the invention features a semiconductor structure including a surface layer disposed over a substrate, the surface layer including strained silicon. A relaxed layer is disposed over the substrate, the relaxed layer being disposed underneath and proximate the surface layer. The relaxed layer and the surface layer generally share an interface. A contact layer is disposed over a portion of the surface layer, the contact layer including a metal-semiconductor alloy. A portion of the contact layer extends into the relaxed layer, a bottommost boundary of the contact layer shares an interface with the relaxed layer, and the interface between the contact layer and relaxed layer is a minimal distance from the interface between the surface layer and the relaxed layer, the minimal distance inhibiting formation of structural inhomogeneities. The minimal distance may be less than 50 Å.
In another aspect, the invention features a method for fabricating a semiconductor structure. The method includes providing a substrate having a surface layer disposed thereon, the surface layer including strained silicon; forming a semiconductor layer over a portion of the surface layer; depositing a metal over the semiconductor layer to form a metal layer; and heating the substrate to form an alloy including at least a portion of the semiconductor layer and at least a portion of the metal layer.
One or more of the following features may also be included. The semiconductor layer may include an epitaxial layer. The semiconductor layer may include at least one of silicon and germanium. The metal may include at least one of cobalt, titanium, tungsten, nickel, and platinum. The substrate may include a relaxed layer disposed underneath the surface layer, and the relaxed layer may include at least one of germanium and silicon.
A plurality of dopants may be introduced to form a doped region in the portion of the surface layer. The dopants may be introduced into the semiconductor layer. The doped region may include at least a portion of a transistor source and/or drain. The dopants may be introduced prior to depositing the metal.
A plurality of dopants may be introduced into the semiconductor layer. The dopants may be introduced prior to depositing the metal.
The substrate may include a sidewall spacer disposed on the surface layer and the semiconductor layer may be formed selectively with respect to the sidewall spacer. The substrate may include a gate electrode disposed on the surface layer and the semiconductor layer may be formed on a top surface of the gate electrode.



BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1-5 are schematic cross-sectional views of a semiconductor structure, illustrating a process for fabricating the structure; and
FIGS. 6-7 are schematic cross-sectional views of alternative embodiments of the semiconductor structure.



DETAILED DESCRIPTION
Referring to FIGS. 1a and 1b, a transistor 10 is formed on a substrate 12. Substrate 12 includes a relaxed layer 14, which has a lattice constant greater than that of silicon. Underlying layer 14 includes, for example, Si0.80Ge0.20 and has a lattice constant of 5.472 Å. Substrate 12 also has a surface layer 16, disposed over relaxed layer 14. Surface layer 16 includes strained silicon, has a thickness t1 of, e.g., 200 Å, and has a low resistivity of, e.g., less then approximately 0.001 ohm-cm. A suitable substrate 12 with relaxed layer 14 and surface layer 16 can be readily obtained from, e.g., IQE Silicon Compounds, Ltd., UK.
Transistor 10 includes a gate 20 disposed over a gate dielectric 22. Gate 20 can be formed by, for example, depositing and patterning polysilicon, and gate dielectric 22 can be formed by, for example, growth of a gate oxide, such as silicon dioxide. A first and a second sidewall spacer 24a, 24b are formed proximate gate 20. Sidewall spacers 24a, 24b are formed of, e.g., silicon dioxide, silicon nitride, or a stack of both or other suitable materials. A source 26 and a drain 28, as indicated by the boundaries Bs and BD, are formed proximate gate 20 in substrate 12 by, for example, ion implantation. In an embodiment, transistor 10 is an NMOS transistor, and source 26 and drain 28 are formed by the implantation of n-type ions, such as arsenic. Source 26 and drain 28 may extend through surface layer 16 into relaxed layer 14. Source 26 has a source extension region 26a disposed under first sidewall spacer 24a, and drain 28 has a drain extension region 28a disposed under second sidewall spacer 24b. Source and drain extension regions 26a, 28a have a depth d1 that is less than a depth d2 of source 26 and drain 28. Extension region depth d1 is, for example, 100-300 Å and source and drain depth d2 is, for example, 400-1000 Å. Isolation regions 30, 32 separate transistor 10 from adjacent devices. Isolation regions 30, 32 are, for example, trenches filled with a dielectric material.
Referring to FIG. 2, a semiconductor layer 40a-40c is formed selectively on exposed silicon surfaces, i.e., on top surface 42 of polysilicon gate 20, top surface 44 of source 26, and top surface 46 of drain 28. In an embodiment, semiconductor layer 40a-40c is an epitaxial layer, such as epitaxial silicon. No semiconductor layer is formed on non-silicon features, such as sidewall spacers 24a, 24b and isolation regions 30, 32. Semiconductor layer 40a-40c has a thickness t2 of, for example, approximately 100-300 Å. Semiconductor layer 40a-40c has a low resistivity of, e.g., 0.001 ohm-cm, that facilitates the formation of low-resistance contacts. To achieve this low resistivity, semiconductor layer 40a-40c is, for example, epitaxial silicon doped with, for example, arsenic to a concentration of 1×1020 cm−3. Semiconductor layer 40a-40c may be doped in situ, during deposition. In alternative embodiments, semiconductor layer 40a-40c may be doped after deposition by ion implantation or by gas-, plasma- or solid-source diffusion. In some embodiments, the doping of semiconductor layer 40a-40c and the formation of source 26 and drain 28 are performed simultaneously.
Referring to FIG. 3, a metal layer 50 is formed over transistor 10. Metal layer 50 is formed by, for example, sputter deposition. Metal layer 50 has a thickness t3 of, e.g., 50-200 Å and includes a metal such as cobalt, titanium, tungsten, nickel, or platinum. The metal is selected to react with semiconductor layer 40a-40c to form a low-resistance metal-semiconductor alloy when exposed to heat, as described below. The metal is also selected such that the metal-semiconductor alloy remains stable at temperatures typically required to complete transistor 10 fabrication, e.g., 400-700° C.
Referring also to FIG. 4, subsequent to deposition of metal layer 50, a first rapid thermal anneal is performed, e.g., at 550° C. for 60 seconds. This heating step initiates a reaction between metal layer 50 and semiconductor layers 40a-40c, forming a high resistivity phase of a metal-semiconductor alloy, e.g., cobalt silicide (CoSi). Portions of metal layer 50 are removed by a wet etch, such as sulfuric acid and hydrogen peroxide. In an alternative embodiment, the wet etch may be ammonium hydroxide, peroxide, and water. This wet etch removes portions of metal layer 50 disposed over dielectric material, such as over first and second sidewall spacers 24a, 24b and isolation regions 30, 32. Portions 55 of metal layer 50 disposed over semiconductor layer 40a-40c that have reacted to form the metal-semiconductor alloy remain in place after the anneal and wet etch.
Referring to FIG. 5, substrate 12, including transistor 10, is subjected to a second heat treatment. For example, in an embodiment in which metal layer 50 includes cobalt, substrate 12 undergoes a rapid thermal anneal at 800° C. for 60 seconds in a nitrogen ambient. This heating step initiates a reaction in the metal-semiconductor alloy layer which substantially lowers its resistivity, to form a substantially homogeneous contact layer 60a-60c. Contact layer 60a-60c includes a metal-semiconductor alloy, e.g., a metal silicide such as a low resistivity phase of cobalt silicide (CoSi2). Contact layer 60a-60c has a thickness t4 of, for example, 400 Å. Contact layer 60a-60c has a low sheet resistance, e.g., less than about 10 Ω/□, and enables a good quality contact to be made to source 26 and drain 28, as well as to gate 20.
During formation, contact layer 60a-60c consumes substantially all of semiconductor layer 40a-40c. Moreover, contact layer portions 60a, 60c, disposed over source 26 and drain 28, extend into surface layer 16. A bottommost boundary 62a of contact layer 60a, therefore, shares an interface 70a with surface layer 16 in source 26, and a bottommost boundary 62c of contact layer 60c, therefore, shares an interface 70c with surface layer 16 in drain 28. A bottommost boundary 62b of contact layer 60b shares an interface 70b with gate 20. Interfaces 70a, 70c between contact layer 60a, 60c and surface layer 16 are disposed within source 26 and drain 28, respectively, above bottommost boundaries 72a, 72c of surface layer 16, and above relaxed layer 14, i.e. contact layer 60a, 60c does not extend into relaxed layer 14. Interfaces 70a, 70c have a low contact resistivity, e.g., less than approximately 5×10−7 Ω-cm2. Because surface layer 16 includes strained silicon, carrier mobilities in surface layer 16 are enhanced, facilitating lower sheet resistances. This strain also results in a reduced energy bandgap, thereby lowering the contact resistivity between the metal-semiconductor alloy and the surface layer.
Referring to FIG. 6, in an alternative embodiment, formation of contact layer 60a′-60c′ consumes only a portion of semiconductor layer 40a′-40c′. Here, as described with reference to FIGS. 1a and 1b, a transistor 10′ is formed on substrate 12 having relaxed layer 14 and surface layer 16. Semiconductor layer 40a′-40c′ is formed over surface layer 16 and gate 20, as described above with reference to FIG. 2. Semiconductor layer 40a′-40c′ may be an epitaxial layer containing a semiconductor such as silicon, silicon germanium, or germanium. In this embodiment, semiconductor layer 40a′-40c′ has a thicker starting thickness, e.g., 300-500 Å. Here, too, dopants are introduced into semiconductor layer 40a′-40c′ prior to metal layer (not shown) formation to reduce the resistivity of semiconductor layer 40a′-40c′. In some embodiments, source 26 and drain 28 are formed simultaneously with the introduction of dopants into semiconductor layer 40a′-40c′. The metal layer is subsequently formed as described above with reference to FIG. 3. The metal layer may include cobalt, titanium, tungsten, nickel, or platinum. Contact layer 60a′-60c′ is formed by annealing the metal layer and performing a wet etch, as described above with reference to FIG. 4. Contact layer 60a′-60c′ includes a metal-semiconductor alloy. A bottommost boundary 62a′ of contact layer 60a′ shares an interface 80a with semiconductor layer 40a′ over source 26, a bottommost boundary 62b′ of contact layer 60b′ shares an interface 80b with semiconductor layer 40b′ over gate 20, and a bottommost boundary 62c′ of contact layer 60c′ shares an interface 80c with semiconductor layer 40c′ over drain 28.
Contact resistance between contact layer 60a′-60c′ and semiconductor layer 40a′-40c′ is directly proportional to the bandgap of semiconductor layer 40a′-40c′. The bandgap of semiconductor layer 40a′-40c′ depends on the material of which it is formed. Strained silicon has a lower bandgap than relaxed silicon, and pure germanium has an even lower bandgap. In an embodiment, semiconductor layer 40a′-40c′ has a relatively high Ge content, e.g., 90-100%. The contact resistance between contact layer 60a′-60c′ and semiconductor layer 40a′-40c′ is therefore relatively low because the presence of Ge reduces the bandgap of contact layer 60a′-60c′ in comparison to pure silicon. Further, the high concentration of germanium results in the contact layer 60a′-60c′ having a high germanocide content with a uniform homogeneity.
Moreover, the sheet resistance of semiconductor layer 40a′-40c′ is lowered if carrier mobilities in semiconductor layer 40a′-40c′ are increased. Strained silicon has higher carrier mobilities, both for holes and electrons. Further, pure Ge, regardless of whether or not it is strained, has higher mobilities for both holes and electrons than even strained silicon. In embodiments in which semiconductor layer 40a′-40c′ is an epitaxial layer including primarily Si, semiconductor layer 40a′-40c′ has a relatively low sheet resistance because it is formed on surface layer 16 that is under tensile strain. Semiconductor layer 40a′-40c′, therefore, is also under tensile strain. The tensile strain increases carrier mobilities, thereby lowering the sheet resistance of semiconductor layer 40a′-40c′. Furthermore, in embodiments in which the semiconductor layer 40a′-40c′ is an epitaxial layer including primarily Ge, semiconductor layer 40a′-40c′ has a relatively low sheet resistance because carrier mobilities are higher in Ge then in relaxed Si.
Referring to FIG. 7, contact layer 60a′′, 60c′′ extends through surface layer 16 and into underlying layer 14 within source 26 and drain 28, and contact layer 60b′′extends into gate 20. Here, as described above with reference to FIGS. 1a and 1b, transistor 10′′ is formed in substrate 12 having relaxed layer 14 and surface layer 16. A semiconductor layer (not shown) is formed over surface layer 16 and gate 20, as described above with reference to FIG. 2. The semiconductor layer may be an epitaxial layer containing a semiconductor such as silicon, silicon germanium, or germanium. In this embodiment, the semiconductor layer has a starting thickness of, e.g., 100-300 Å. Here, too, dopants are introduced into the semiconductor layer prior to metal layer (not shown) formation to reduce the resistivity of the semiconductor layer. In some embodiments, source 26 and drain 28 are formed simultaneously with the introduction of dopants into the semiconductor layer. The metal layer is subsequently formed as described above with reference to FIG. 3. The metal layer may include cobalt, titanium, tungsten, nickel, or platinum. The metal layer is of a sufficient thickness and/or the semiconductor layer is sufficiently thin such that the subsequently formed metal-semiconductor alloy layer extends into underlying layer 14. Contact layer 60a′′-60c′′ is formed by annealing the metal layer and performing a wet etch, as described above with reference to FIG. 4. Contact layer 60a′′-60c′′ includes a metal-semiconductor alloy. A bottommost boundary 62a′ of contact layer 60a′′ shares an interface 90a with underlying layer 14 in source 26, a bottommost boundary 62b′′ of contact layer 60b′ shares an interface 90b with gate 20, and a bottommost boundary 62c′′ of contact layer 60c′ shares an interface 90c with underlying layer 14 in drain 28. Contact layer 60a′′, 60c′′ extends a minimal distance d3 into relaxed underlying layer 14 beyond an interface 100 between relaxed underlying layer 14 and surface layer 16. Structural inhomogeneities may occur, for example, when a metal reacts with a mixture of silicon and germanium, i.e. a metal silicide may form along with discrete regions of high-resistivity germanium. This inhomogeneity may be inhibited by minimizing the depth d3 which contact layer 60a′′, 60c′′, formed of a metal-semiconductor alloy, extends into relaxed underlying layer 14 that may contain germanium. Mininimal distance d3 is desirably the largest distance compatible with the inhibition of the formation of structural inhomogeneities. It is noted that structural inhomogeneities tend to form at a metal-semiconductor alloy extension into relaxed underlying layer 14 of approximately 50 Å. Minimal distance d3 is, for example, less than 50 Å. The amount of metal that reacts with underlying layer 14 is thereby minimized and structural inhomogeneities are avoided.
In an alternative embodiment, the transistor is a PMOS transistor. Contact may made to a region other than a gate, a source, or a drain of a transistor, and the source and drain may be formed by introducing dopants through the alloy or prior to depositing the metal. The described structure may be formed on a semiconductor-on-insulator (SOI) substrate, with a buried insulator layer separating the surface layer from the semiconductor substrate. In some embodiments, the thickness of the surface semiconductor layer may be too thin to form a silicide. This feature, therefore, provides an additional motivation for elevating the source and drain prior to silicidation.
The epitaxial layer may be a semiconductor other then silicon. One possibility is to use germanium, which is easier to grow selectively then silicon, and may promote the formation of low-resistance contacts because of its relatively low bandgap. Another option is to use a stack of two or more semiconductor materials. For example, because germanium is known to be aggressively etched by various cleaning steps used in the semiconductor industry, the raised source/drain material could be a stack of germanium protected by a layer of silicon.
The invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. The foregoing embodiments are therefore to be considered in all respects illustrative rather than limiting on the invention described herein. Scope of the invention is thus indicated by the appended claims rather than by the foregoing description, and all changes which come within the meaning and range of equivalency of the claims are intended to be embraced therein.

## Claims

1. A semiconductor structure comprising:
a surface layer disposed over a substrate, the surface layer comprising strained silicon; and
a contact layer disposed over at least a portion of the surface layer, the contact layer comprising a metal-semiconductor alloy, the metal-semiconductor alloy consisting of a metal germanocide,
wherein a bottommost boundary of the contact layer is disposed above a bottommost boundary of the surface layer.