# Patent

## Title
High performance CMOS device design

## Abstract

A semiconductor device includes a gate, which comprises a gate electrode and a gate dielectric underlying the gate electrode, a spacer formed on a sidewall of the gate electrode and the gate dielectric, a buffer layer having a first portion underlying the gate dielectric and the spacer and a second portion adjacent the spacer wherein the top surface of the second portion of the buffer layer is recessed below the top surface of the first portion of the buffer layer, and a source/drain region substantially aligned with the spacer. The buffer layer preferably has a greater lattice constant than an underlying semiconductor substrate. The semiconductor device may further include a semiconductor-capping layer between the buffer layer and the gate dielectric, wherein the semiconductor-capping layer has a smaller lattice constant than the buffer layer.

## Background
This application is a divisional of U.S. patent application Ser. No. 11/115,484, filed Apr. 27, 2005, and entitled “High Performance CMOS Device Design,” which application claims priority to U.S. Provisional Patent Application Ser. No. 60/645,649, filed Jan. 21, 2005, and entitled “High Performance CMOS Device Design,” which applications are hereby incorporated herein by reference.


TECHNICAL FIELD
This invention relates generally to semiconductors, and more specifically to CMOS devices with strained channel regions.
BACKGROUND
The scaling of VLSI circuits is a constant effort. With circuits become smaller and faster, device driving current improvement becomes more important. Device current is closely related to gate length, gate capacitance, and carrier mobility. Shortening poly-gate length, increasing gate capacitance and increasing carrier mobility can improve the device current performance. Gate length reduction is an ongoing effort in order to shrink circuit size. Increasing gate capacitance has also been achieved by efforts such as reducing gate dielectric thickness, increasing gate dielectric constant, and the like. In order to further improve device current, enhancing carrier mobility has also been explored.
Among efforts made to enhance carrier mobility, forming a strained silicon channel is a known practice. Strain can enhance bulk electron and hole mobility. The performance of a MOS device can be enhanced through a strained-surface channel. This technique allows performance to be improved at a constant gate length, without adding complexity to circuit fabrication or design.
When silicon is placed under strain, the in-plane, room temperature electron mobility is dramatically increased. One way to develop strain is by using a graded SiGe epitaxy layer as a substrate on which a layer of relaxed SiGe is formed. A layer of silicon is formed on the relaxed SiGe layer. MOS devices are then formed on the silicon layer, which has inherent strain. Since the lattice constant of SiGe is larger than that of Si, the Si film is under biaxial tension and thus the carriers exhibit strain-enhanced mobility.
Strain in a device may have components in three directions; parallel to the MOS device channel length, parallel to the device channel width, and perpendicular to the channel plane. The strains parallel to the device channel length and width are called in-plane strains. Research has revealed that bi-axial, in-plane tensile strain can improve nMOS performance, and compressive strain parallel to the channel length direction can improve pMOS device performance.
What is needed, then, are methods for improving device performance by promoting tensile strain in nMOS devices and compressive strain in pMOS devices.
SUMMARY OF THE INVENTION
The preferred embodiment of the present invention provides a semiconductor device with a strained channel and a method of forming the same.
In accordance with one aspect of the present invention, the semiconductor device includes a gate, including a gate electrode and a gate dielectric underlying the gate electrode, a spacer formed on a sidewall of the gate electrode and the gate dielectric, a buffer layer on a semiconductor substrate, wherein the buffer layer has a first portion underlying the gate dielectric and the spacer and a second portion adjacent the spacer, wherein the top surface of the second portion of the buffer layer is recessed below the top surface of the first portion of the buffer layer, and a source/drain region substantially aligned with the spacer. The buffer layer preferably has a greater lattice constant than the underlying semiconductor substrate. The semiconductor device may further include a semiconductor-capping layer between the buffer layer and the gate dielectric, wherein the semiconductor-capping layer has a smaller lattice constant then the buffer layer.
In accordance with another aspect of the present invention, the second portion of the buffer layer is removed and a recess is formed in the semiconductor substrate under the second portion of the buffer layer.
In accordance with yet another aspect of the present invention, the semiconductor-capping layer for pMOS devices is thinner than the semiconductor-capping layer for nMOS devices in order to improve the device performance.
Due to the lattice constant difference, the semiconductor-capping layer has inherent tensile strain and the buffer layer has inherent compressive strain. Performance of both pMOS and nMOS devices is improved by hybrid strains of tensile strain and compressive strain.



BRIEF DESCRIPTION OF THE DRAWINGS
For a more complete understanding of the present invention, and the advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:
FIGS. 1 through 5D are cross-sectional views of intermediate stages in the manufacture of a preferred embodiment of the present invention;
FIGS. 6A through 6C are schematic views of the respective channels of an nMOS device and a pMOS device; and
FIG. 7 illustrates off-state leakage current as the function of drive current.



DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS
The making and using of the presently preferred embodiments are discussed in detail below. It should be appreciated, however, that the present invention provides many applicable inventive concepts that can be embodied in a wide variety of specific contexts. The specific embodiments discussed are merely illustrative of specific ways to make and use the invention, and do not limit the scope of the invention.
The intermediated stages of manufacturing a preferred embodiment of the present invention are illustrated. Throughout the various views and illustrative embodiments of the present invention, like reference numbers are used to designate like elements. Each figure number may be followed by a letter indicating variations of the same structure or process step.
FIG. 1 illustrates stacked layers 2, 4 and 6. Substrate 2 is preferably a semiconductor material, and more preferably, a silicon substrate. Substrate 2 can be in the form of bulk silicon or in a well-known silicon-on-insulator (SOI) structure, with a layer of silicon formed on an insulating layer such as a buried oxide layer (BOX).
A buffer layer 4 is formed, preferably epitaxially grown, on the substrate 2. The buffer layer 4 is preferably a semiconductor having a greater lattice constant than the substrate 2. In the preferred embodiment, the buffer layer 4 comprises silicon and germanium. For simplicity, it is equally referred to as the SiGe layer 4, although other materials can be used. In other embodiments, the buffer layer 4 comprises silicon, germanium and carbon. The preferred thickness of the buffer layer 4 is between about 2 nm and about 50 nm.
An optional semiconductor-capping layer 6 is formed, preferably epitaxially grown, on the buffer layer 4. The semiconductor-capping layer 6 preferably has a lattice constant smaller than the lattice constant of the buffer layer 4. In the preferred embodiment, it is formed of silicon. In other embodiments, it comprises materials such as silicon, germanium, and the like, and the germanium concentration is preferably less than in the buffer layer 4, so that its overall lattice constant is smaller than that of the buffer layer 4. The semiconductor-capping layer 6 is also referred to as silicon-capping layer 6 although it may comprise other materials. The thickness of the semiconductor-capping layer 6 for pMOS devices is preferably less than for nMOS devices. Different thicknesses can preferably be achieved by two methods: (1) forming the semiconductor-capping layer 6 for both pMOS and nMOS devices, and then reducing the thickness for pMOS devices; or (2) forming the semiconductor-capping layer 6 for both pMOS and nMOS devices to a first thickness, masking pMOS devices, and further forming the capping layer for the nMOS devices to a second thickness. In regions where pMOS devices are to be formed, the semiconductor-capping layer 6 preferably has a thickness of between about 0.5 nm and about 20 nm. In regions where nMOS devices are to be formed, the semiconductor-capping layer 6 preferably has a thickness of between about 0.6 nm and about 25 nm.
Gate dielectric 8 and gate electrode 10 are then formed, as illustrated in FIG. 2. A gate dielectric layer is formed on the capping layer 6, followed by a gate electrode layer. These layers are then patterned and etched to form gate dielectric 8 and gate electrode 10. In the preferred embodiment, gate dielectric 8 comprises SiO2, and may be referred to as gate oxide 8. In other embodiments, gate dielectric 8 is formed of oxynitride, nitride, high-k materials, and the like. Gate electrode 10 is preferably polysilicon, although it may be formed of other commonly used materials such as metal, metal silicide, and the like.
FIG. 3 illustrates a pair of spacers 12 formed along the sidewalls of the gate dielectric 8 and gate electrode 10. Spacers 12 serve as self-aligning masks for subsequent source/drain formation steps, as will be described hereinafter. The spacers 12 may be formed by well-known methods such as blanket depositing a dielectric layer over the entire region, then anisotropically etching to remove the dielectric from the horizontal surfaces and leaving spacers 12. It is to be noted that while patterning the gate dielectric 8 and gate electrode 10, over etching may occur due to process deviations, so that spacers 12 extend into the semiconductor-capping layer 6. As a result, the tensile strain in the semiconductor-capping layer 6 is also reduced and the device performance improvement is less than expected. Therefore, it is preferred that the over etching depth Dpoly in the semiconductor capping layer 6 is less than about 30 nm.
Recesses 9 are then formed along the gate spacers 12, as illustrated in FIGS. 4A and 4B. In the preferred embodiments, as illustrated in FIG. 4A, recesses 9 extend through the semiconductor-capping layer 6 and buffer layer 4 and into the substrate 2. The recess depth Dsubstrate in the substrate 2 is preferably less than about 50 nm. Free surfaces 5 and 7 are created on sidewalls of the buffer layer 4. When “free surface” is referred to, it should be interpreted that there is substantially no material covering it at the time of annealing, so that the material under the free surface is free to expand or contract to its greatest capacity, although in the final structure, the free surface is covered with materials such as inter-layer dielectrics. When the buffer layer 4 is annealed, the buffer layer 4 tends to relax and restore its lattice structure and thus will expand. The free surfaces 5 and 7 allow the buffer layer 4 to expand freely. In subsequent annealing processes, the buffer layer 4 can be expanded toward the left of the free surface 5 and the right of the free surface 7. Due to force balance effects and free surface effects, compressive strain will be generated in the buffer layer 4, and tensile strain will be generated in the semiconductor-capping layer 6. In the preferred embodiment, two free surfaces 5 and 7 are available for buffer layer 4 to expand. In alternative embodiments, recessing 9 is formed only on one side of the gate 8 and 10 and only one of the free surfaces 5 and 7 exists. When annealed, the buffer layer 4 will expand substantially in one direction.
FIGS. 4A and 4B illustrate a preferred embodiment where the recesses 9 extend from an outer edge of the spacer 12 to respective shallow trench isolations (STI) 11. In alternative embodiments, as shown in FIG. 4C, recesses 9 have a width W smaller than the distance between an outer edge of the spacers 12 and respective STI 11. Width W is preferably great enough for the buffer layer 4 to expand when annealed.
Compressive strain can also be generated even though sidewalls of the buffer layer 4 are not fully exposed. In alternative embodiments, as shown in FIG. 4B, the capping layer 6 is etched through while buffer layer 4 is partially recessed. Recesses 9 have a depth Dlayer in the buffer layer. For pMOS devices, the depth Dlayer is preferably between about 0 nm and about 50 nm, and more preferably between about 0 nm and about 20 nm. For nMOS devices, the recess depth Dlayer is between about 2 nm and 50 nm, and more preferably between about 2 nm and about 20 nm.
Due to process variations, it is possible that the recess formation is not uniform for nMOS devices and pMOS devices. While the nMOS devices have recesses extending into the buffer layer 4, pMOS devices may only have recesses extending into the semiconductor-capping layer 6 without further extending into the buffer layer 4.
Source and drain regions 14 are then formed, as illustrated in FIGS. 5A through 5D. In the preferred embodiment, source and drain regions 14 are formed by doping on either side of gate spacers 12. In other embodiments, source/drain regions may be formed by recessing exposed semiconductor materials using spacers 12 as masks, and epitaxially growing semiconductor material (along with desired impurities) in the recesses.
Through previously discussed processes, several variations of the preferred embodiment can be formed. In FIGS. 5A and 5B, there is no semiconductor-capping layer 6 formed, and recesses 9 may be formed entirely in the buffer layer 4, as shown in FIG. 5A, or extending into the substrate 2, as shown in FIG. 5B. As also shown in FIG. 5B, when no semiconductor-capping layer 6 is formed, the bottom surfaces of gate spacers 12 are at the level lower than the top surface and higher than the bottom surface of buffer layer 4. Preferably, embodiments shown in FIGS. 5A and 5B are suitable for pMOS devices wherein compressive stress is desired. In FIGS. 5C and 5D, the semiconductor-capping layer 6 is formed on the buffer layer 4. Similar to the embodiments referenced in FIGS. 5A and 5B, recesses 9 may be formed entirely in the semiconductor-capping layer 6 and the buffer layer 4, as shown in FIG. 5C, or extending into the substrate 2, as shown in FIG. 5D. For pMOS devices, recesses 9 are preferably formed in the semiconductor-capping layer 6 without extending into the buffer layer 4.
FIGS. 6A and 6B schematically illustrate strains 16 and 18 in the buffer layer 4 and semiconductor-capping layer 6, respectively. Buffer layer 4 has a compressive strain, as symbolized by arrows 16, mainly due to the smaller lattice constant of the substrate 2. The semiconductor-capping layer 6 has a tensile strain, as symbolized by arrows 18, due to the greater lattice constant of the buffer layer 4. FIGS. 6A and 6B also schematically illustrate the channels 20 and 22 of nMOS and pMOS devices, respectively. In an nMOS device, channel region 20 is substantially within the semiconductor-capping layer 6. Since the capping layer 6 has an inherent tensile strain, the performance of the nMOS devices is improved. For pMOS devices, the addition of germanium causes the valence band to be lower. For example, if germanium concentration is from about 10% to about 30%, the valence band will be about 100 mV to about 300 mV lower, thus the channel region 22 of a pMOS device tends to be in the SiGe layer 4 rather than in silicon capping layer 6. Therefore, channel regions of pMOS devices have compressive strains, and the device performance is improved. The nMOS device as shown in FIG. 6A and the pMOS device as shown in FIG. 6B may be formed on a same substrate 2, as shown in FIG. 6C.
In a pMOS device, the buffer layer preferably uses SiGe, however, if SiGe directly touches the gate oxide 8, there may be reliability issues. Therefore, a thin silicon-capping layer 6 is preferred between the SiGe layer 4 and gate oxide 8. If other materials are used as the gate dielectric, such as nitride or high-K dielectric materials, the thin silicon-capping layer 6 can be omitted and the resulting structure is shown in FIGS. 5A and 5B.
The preferred embodiments have hybrid strains, with tensile strain in the semiconductor-capping layer 6 and compressive strain in the buffer layer 4. With the hybrid strains, device performance is improved for both nMOS and pMOS devices. FIG. 7 illustrates the devices' leakage current in an off-state (off-current) as the function of drive current (on-current). The exemplary results shown are obtained on pMOS sample devices. Line 26 illustrates a result obtained from a preferred embodiment having both the semiconductor-capping layer and the buffer layer, while line 28 illustrates a result obtained from a pMOS device formed on bulk silicon. It is noted that at a same off-current, the preferred embodiment has an on-current of about 15 percent higher than the device formed on bulk silicon. The exemplary results from nMOS (not shown) also indicate more than about 10 percent improvement. Experiments also revealed that there is improvement in Ion-Ioff characteristics of the device, with no substantial degradation found in other characteristics.
Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.
Several advantageous method embodiments are within the scope of the present invention. These include but are not limited to the following: A method of forming a semiconductor device comprises forming on the substrate a buffer layer having a lattice constant mismatch with the substrate, forming a gate dielectric layer on the buffer layer, forming a gate electrode layer on the gate dielectric, patterning the gate dielectric layer and gate electrode layer to form a gate dielectric and a gate electrode, respectively, forming a spacer on a sidewall of the gate, recessing a portion of the buffer layer not underlying the gate and spacer and thus forming a recess, and forming a source/drain region substantially aligned with the spacer. The semiconductor device may be a pMOS device wherein the recess has a depth of less than about 50 nm. The semiconductor device may also be an nMOS device wherein the recess has a depth of between about 2 nm and about 50 nm.
The recess may extend into the semiconductor substrate, preferably below the gate dielectric for less than about 30 nm.
The method may further comprise forming a semiconductor-capping layer having a smaller lattice constant than the buffer layer between the buffer layer and the gate oxide. If the semiconductor device is a pMOS device, the semiconductor-capping layer preferably has a thickness of between about 0.5 nm and about 20 nm. If the semiconductor device is an nMOS device, the semiconductor-capping layer preferably has a thickness of between about 0.6 nm and 25 nm. Preferably, the semiconductor-capping layer for a pMOS device has a thickness of less than a thickness of a semiconductor-capping layer for an nMOS device.

## Claims

1. A semiconductor device comprising:
a semiconductor substrate;
a gate dielectric over the semiconductor substrate;
a gate electrode over the gate dielectric;
a buffer layer between and adjoining the semiconductor substrate and the gate dielectric, wherein the buffer layer and the semiconductor substrate comprise different materials;
a gate spacer on a sidewall of the gate electrode and on a side edge of the gate dielectric, wherein the gate spacer comprises a bottom surface at an intermediate level lower than a top surface of the buffer layer and higher than a bottom surface of the buffer layer;
a source/drain region adjacent to the gate dielectric; and
an insulation region in the semiconductor substrate, wherein the insulation region is spaced apart from the gate dielectric by the source/drain region, and wherein a portion of the top surface of the source/drain region adjoining the insulation region is lower than the bottom surface of the buffer layer.

2. The semiconductor device of claim 1, wherein substantially an entirety of the buffer layer has a greater lattice constant than the semiconductor substrate.

3. The semiconductor device of claim 1, wherein the buffer layer comprises a material selected from the group consisting essentially of silicon, germanium, carbon, and combinations thereof.

4. The semiconductor device of claim 1, wherein the buffer layer does not extend beyond an outer edge of the gate spacer.

5. The semiconductor device of claim 4, wherein an outer edge of the buffer layer is at an inner side of the outer edge of the gate spacer.

6. A semiconductor device comprising:
a semiconductor substrate;
a gate structure comprising:
a gate dielectric over the semiconductor substrate; and
a gate electrode over the gate dielectric;

a gate spacer on a sidewall of the gate structure;
a buffer layer vertically between and adjoining the semiconductor substrate and the gate dielectric, wherein the buffer layer and the semiconductor substrate comprise different materials, and wherein the gate spacer comprises a bottom surface at an intermediate level lower than a top surface of the buffer layer and higher than a bottom surface of the buffer layer;
an insulation region extending into the semiconductor substrate, wherein the insulation region is adjacent the buffer layer; and
a source/drain region laterally adjoining the insulation region wherein a portion of the source/drain region adjoining the insulation region has a top surface below a bottom surface of the buffer layer.

7. The semiconductor device of claim 6, wherein substantially an entirety of the buffer layer has a greater lattice constant than the semiconductor substrate.

8. The semiconductor device of claim 6, wherein the buffer layer comprises a material selected from the group consisting essentially of silicon, germanium, carbon, and combinations thereof.

9. The semiconductor device of claim 6, wherein the buffer layer comprises a portion extending under the gate spacer.

10. The semiconductor device of claim 6, wherein an outer edge of the buffer layer is at an inner side of an outer edge of the gate spacer.

11. The semiconductor device of claim 6, wherein the buffer layer has a thickness between about 2 nm and about 50 nm.

12. The semiconductor device of claim 1, wherein the source/drain region has a top surface lower than a bottom surface of the buffer layer.

13. A semiconductor device comprising:
a semiconductor substrate;
a gate dielectric over the semiconductor substrate;
a gate electrode over the gate dielectric;
a buffer layer between and adjoining the semiconductor substrate and the gate dielectric, wherein the buffer layer and the semiconductor substrate comprise different materials;
a gate spacer on a sidewall of the gate electrode and on a side edge of the gate dielectric, wherein the gate spacer comprises a bottom surface at an intermediate level lower than a top surface of the buffer layer and higher than a bottom surface of the buffer layer, and wherein the buffer layer does not extend beyond an outer edge of the gate spacer; and
a source/drain region adjacent to the gate dielectric.

14. The semiconductor device of claim 13, wherein an outer edge of the buffer layer is at an inner side of the outer edge of the gate spacer.

15. The semiconductor device of claim 13 further comprising an insulation region in the semiconductor substrate, wherein the insulation region is spaced apart from the gate dielectric by the source/drain region.

16. The semiconductor device of claim 15, wherein a portion of the top surface of the source/drain region adjoining the insulation region is lower than the bottom surface of the buffer layer.

17. The semiconductor device of claim 13, wherein substantially an entirety of the buffer layer has a greater lattice constant than the semiconductor substrate.

18. The semiconductor device of claim 13, wherein the buffer layer comprises a material selected from the group consisting essentially of silicon, germanium, carbon, and combinations thereof.