CMOS DEVICE COMPRISING MOS TRANSISTORS WITH RECESSED DRAIN AND SOURCE AREAS AND NON-CONFORMAL METAL SILICIDE REGIONS
A non-conformal metal silicide in a transistor of recessed drain and source configuration may provide enhanced efficiency with respect to strain-inducing mechanisms, drain/source resistance and the like. For this purpose, in some cases, an amorphizing implantation process may be performed prior to the silicidation process, while in other cases an anisotropic deposition of the refractory metal may be used.
1. Field of the Invention
Generally, the subject matter disclosed herein relates to integrated circuits, and, more particularly, to high performance transistors having recessed drain and source regions and strained channel regions by using stress sources, such as stressed overlayers, to enhance charge carrier mobility in the channel region of a MOS transistor.
2. Description of the Related Art
Generally, a plurality of process technologies are currently practiced in the field of semiconductor production, wherein, for complex circuitry, such as microprocessors, advanced storage chips and the like, CMOS technology is currently the most promising approach due to the superior characteristics in view of operating speed and/or power consumption and/or cost efficiency. During the fabrication of complex integrated circuits using CMOS technology, millions of transistors, i.e., N-channel transistors and P-channel transistors, are formed on a substrate including a crystalline semiconductor layer. A MOS transistor, irrespective of whether an N-channel transistor or a P-channel transistor is considered, comprises so-called PN junctions that are formed by an interface of highly doped drain and source regions with an inversely or weakly doped channel region disposed between the drain region and the source region. The conductivity of the channel region, i.e., the drive current capability of the conductive channel, is controlled by a gate electrode formed near the channel region and separated therefrom by a thin insulating layer. The conductivity of the channel region, upon formation of a conductive channel due to the application of an appropriate control voltage to the gate electrode, depends on, among other things, the dopant concentration, the mobility of the majority charge carriers and, for a given extension of the channel region in the transistor width direction, on the distance between the source and drain regions, which is also referred to as channel length. Hence, in combination with the capability of rapidly creating a conductive channel below the insulating layer upon application of the control voltage to the gate electrode, the overall conductivity of the channel region substantially determines the performance of the MOS transistors. Thus, the reduction of the channel length is a dominant design criterion for accomplishing an increase in the operating speed and packing density of the integrated circuits.
The continuing shrinkage of the transistor dimensions, however, involves a plurality of issues associated therewith that have to be addressed so as to not unduly offset the advantages obtained by steadily decreasing the channel length of MOS transistors. One major problem in this respect is providing low sheet and contact resistivity in drain and source regions and any contacts connected thereto and maintaining channel controllability. For example, reducing the channel length may necessitate an increase of the capacitive coupling between the gate electrode and the channel region, which may call for reduced thickness of the gate insulation layer. Presently, the thickness of silicon dioxide based gate insulation layers is in the range of 1-2 nm, wherein a further reduction may be less desirable in view of leakage currents which typically exponentially increase when reducing the gate dielectric thickness. For this reason, in advanced transistor designs, a high-k dielectric material may be used in the gate dielectric layer, possibly in combination with a metal in the gate electrode, to enhance channel controllability and reduce signal propagation delay caused by a high gate resistivity of conventional gate materials, such as polysilicon in combination with a metal silicide.
The continuous size reduction of the critical dimensions, i.e., the gate length of the transistors, necessitates the adaptation and possibly the new development of highly complex process techniques concerning the above-identified problems. It has, therefore, been proposed to improve transistor performance by enhancing the channel conductivity of the transistor elements by increasing the charge carrier mobility in the channel region for a given channel length, thereby offering the potential for achieving a performance improvement that is comparable with the advance to a future technology node while avoiding or at least postponing many of the above-mentioned problems, or achieving further increased performance when combined with other performance enhancing techniques, such as high-k gate dielectrics and the like. One efficient mechanism for increasing the charge carrier mobility is the modification of the lattice structure in the channel region, for instance by creating tensile or compressive stress in the vicinity of the channel region to produce a corresponding strain in the channel region, which results in a modified mobility for electrons and holes, respectively. For example, for standard silicon substrates, creating tensile strain in the channel region increases the mobility of electrons, which in turn may directly translate into a corresponding increase in the conductivity and thus drive current and operating speed. On the other hand, compressive strain in the channel region may increase the mobility of holes, thereby providing the potential for enhancing the performance of P-type transistors. The introduction of stress or strain engineering into integrated circuit fabrication is an extremely promising approach since, for example, strained silicon may be considered as a “new” type of semiconductor material, which may enable the fabrication of fast powerful semiconductor devices without requiring expensive semiconductor materials, while many of the well-established manufacturing techniques may still be used.
According to one promising approach for creating strain in the channel region of transistor elements, the dielectric material that is formed above the basic transistor structure may be provided in a highly stressed state so as to induce a desired type of strain at the transistor and in particular in the channel region thereof. For example, the transistor structures are typically embedded in an interlayer dielectric material, which may provide the desired mechanical and electrical integrity of the individual transistor structures and which may provide a platform for the formation of additional wiring layers, which are typically required for providing the electrical interconnections between the individual circuit elements. That is, a plurality of wiring levels or metallization layers may typically be provided which may include horizontal metal lines and vertical vias including appropriate conductive materials for establishing the electrical connections. Consequently, an appropriate contact structure has to be provided which connects the actual circuit elements, such as transistors, capacitors and the like, or respective portions thereof, with the very first metallization layer. For this purpose the interlayer dielectric material has to be appropriately patterned in order to provide respective openings connecting to the desired contact areas of the circuit elements, which may typically be accomplished by using an etch stop material in combination with the actual interlayer dielectric material.
For example, silicon dioxide is a well-established interlayer dielectric material, in combination with silicon nitride, which may act as an efficient etch stop material during the formation of the contact openings. Consequently, the etch stop material, i.e., the silicon nitride material, is in close contact with the basic transistor structure and thus may efficiently be used for inducing strain in the transistors, in particular as silicon nitride may be deposited on the basis of well-established plasma enhanced chemical vapor deposition (CVD) techniques with high internal stress. For instance, silicon nitride may be deposited with high internal compressive stress of up to 2 GPa and even higher by selecting appropriate deposition parameters. On the other hand, a moderately high internal tensile stress level may be created up to 1 GPa and higher by appropriately adjusting the process parameters, for instance, in particular, the degree of ion bombardment during the deposition of the silicon nitride material. Consequently, the magnitude of the strain created in the channel region of a transistor element may depend on the internal stress level of the dielectric etch stop material and the thickness of stressed dielectric material in combination with the effective offset of the highly stressed dielectric material with respect to the channel region. Consequently, in view of enhancing transistor performance, it may be desirable to increase the internal stress level and also provide increased amounts of highly stressed dielectric material in the vicinity of the transistor element, while also positioning the stressed dielectric material as closely as possible to the channel region.
It turns out, however, that the internal stress levels of silicon nitride material may be restricted by the overall deposition capabilities of presently available plasma enhanced CVD techniques, while also the effective layer thickness may be substantially determined by the basic transistor topography and the distance between neighboring circuit elements. Consequently, although providing significant advantages, the efficiency of the stress transfer mechanism may significantly depend on process and device specifics and may result in reduced performance gain for well-established standard transistor designs having gate lengths of 50 nm and less, since the given device topography and the gap fill capabilities of the respective deposition process, in combination with a moderately high offset of the highly stressed material from the channel region caused by sophisticated spacer structures, may reduce the finally obtained strain in the channel region.
For these reasons, it has been suggested to use a recessed transistor architecture, i.e., an architecture in which portions of the drain and source regions are recessed with respect to the channel region, at least a top surface thereof, when two-dimensional transistor configurations are considered in the vicinity of the interface between the channel region and the gate insulation layer, in order to enable the deposition of the highly stressed dielectric material at a lower height level compared to non-recessed configuration, thereby efficiently enhancing the lateral stress transfer mechanism into the channel region since the internal stress of the dielectric material may also be applied in a lateral direction and may not only be restricted to the surface area of the semiconductor material. Moreover, in addition to in principle enhancing the overall stress transfer mechanism, the recess formed in the drain and source regions may also provide an increased surface area that is available for a silicidation process, which is typically applied in order to reduce the overall series resistance within the drain and source regions. In advanced transistor elements, typically a plurality of strain-inducing mechanisms may be used, such as a strained semiconductor material that may be embedded in the drain and source regions of, for example, P-channel transistors in which, for instance, a silicon/germanium alloy may provide a strained state in the drain and source regions, which may also induce a compressive strain in the channel region. Furthermore, contact elements may be formed on the basis of deposition recipes in which a moderately high internal stress level may also be generated, which may also be advantageously used to enhance transistor performance. Consequently, a delicate combination of various influences obtained by strain-inducing mechanisms, the size and shape of the recesses in combination with the metal silicide may thus determine the overall drive current capability of the transistors. For example, although an increased amount of metal silicide may be advantageous in view of enhancing the conductivity of the basic silicon material, the quality of the metal silicide for a given material composition, such as cobalt silicide, nickel silicide, nickel platinum silicide and the like, and the distance of the metal silicide with respect to the channel region and the like may have to be taken into consideration in view of the mutual dependencies with other performance enhancing mechanisms when adjusting the overall transistor characteristics. For example, although a reduced distance of the metal silicide with respect to a channel region may per se be considered as being advantageous, in a strained channel configuration, an increased degree of strain may overcompensate for a slight local reduction of the series resistance, which may be achieved by reducing the distance between the metal silicide and the channel region. That is, by positioning the metal silicide closer to the channel region, a certain reduction of the strain transfer may also be observed, since the metal silicide may act as a buffer material and thus as a resistance with respect to stress propagation into the channel region. Consequently, a reduced electrical resistance may possibly be overcompensated for by an increased “stress transfer resistance,” thereby resulting in a reduced performance gain as would be expected when considering each of the performance enhancing mechanisms independently. In other cases, when, for instance, a silicon/germanium material may be incorporated in the drain and source regions, the metal silicide positioned close to the channel region may consume strained silicon/germanium material, thereby also reducing the efficiency thereof, which may also result in a less pronounced performance gain, thereby rendering respective manufacturing techniques, such as epitaxial growth techniques and the like, less efficient, while nevertheless requiring significant efforts in terms of cycle time and thus overall production costs.
The present disclosure is directed to various methods and devices that may avoid, or at least reduce, the effects of one or more of the problems identified above.SUMMARY OF THE INVENTION
The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
Generally, the present disclosure relates to techniques and semiconductor devices in which drain and/or source resistance in recessed transistor configurations may be enhanced by adapting a thickness profile of a metal silicide layer formed on exposed surface portions of a recessed drain region and/or source region. Contrary to conventional strategies for forming metal silicide regions, in which a substantially uniform layer thickness and thus a substantially conformal metal silicide layer may be provided in the drain and source regions, the present disclosure contemplates a non-conformal silicidation process, at least in one of the drain and source regions of at least one type of transistor, thereby enabling a reduction of the metal silicide thickness at substantially vertical surface portions within the recess, while nevertheless maintaining a desired increased thickness at substantially horizontal portions, i.e., at the bottom of the recesses. Consequently, an overall increased area for accepting charge carriers from the channel regions may still be provided, thereby reducing the resistance for “spreading” the drain/source current while, on the other hand, the thickness and thus the distance of metal silicide from the channel region in the vicinity of the gate insulation layer may be reduced. Consequently, a reduced influence of the metal silicide on other strain-inducing mechanisms, such as a stressed dielectric material formed within the recess, embedded semiconductor alloys and the like, may be achieved so that, in combination, an enhanced overall transistor performance may be accomplished. In some illustrative aspects disclosed herein, a non-conformal formation of metal silicide may be achieved by “non-conformally” modifying the crystalline structure of the recessed drain and/or source areas, for instance by performing an ion implantation process, thereby providing an enhanced diffusion of metal and silicon during the corresponding process, so that the reaction rate may be adjusted on the basis of implantation parameters, which may thus be controlled with high accuracy. Consequently, a high degree of flexibility may be provided in appropriately adapting the local thickness of the metal silicide layer within the recesses, which may thus enable a gradual adaptation of transistor characteristics. In other illustrative aspects disclosed herein, the non-conformal metal silicide may be obtained on the basis of the highly aniso-tropic deposition recipe for providing the refractory metal, which may then result in a corresponding non-conformal layer thickness.
One illustrative semiconductor device disclosed herein comprises a transistor comprising drain and source regions connecting to a channel region, wherein the drain region and/or the source region has a recessed configuration and comprises a metal silicide layer having a non-conformal thickness. The semiconductor device further comprises a stress-inducing dielectric layer that is in contact with the metal silicide layer.
One illustrative method disclosed herein comprises forming a recess in a drain region and/or a source region of a transistor. Moreover, a metal silicide layer is formed in a non-conformal manner in the recess and finally a strain-inducing layer is formed above the drain and source regions.
A still further illustrative method disclosed herein comprises forming a recess in a drain region and/or a source region of a transistor. Furthermore, an ion implantation process is performed to modify an exposed area of the drain region and/or source region in the recess in a non-conformal manner. The method further comprises depositing a metal above the drain and source regions and performing a heat treatment to create a metal silicide in a non-conformal manner within the recess.
The disclosure may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:
While the subject matter disclosed herein is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.DETAILED DESCRIPTION
Various illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
The present subject matter will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present disclosure with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present disclosure. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.
Generally, the present disclosure provides semiconductor devices and techniques for forming the same, in which a recessed configuration may be realized in MOS transistors, that is, in planar transistor configurations, a portion of the metal silicide formed in the drain and source regions may be positioned below a height level as defined by an interface between the gate insulation layer and the channel region of the transistor. In three-dimensional transistor configurations, such as FinFETs, tri-gate transistors and the like, a recessed transistor configuration is to be understood as a transistor architecture in which at least a portion of the metal silicide in the drain and source areas may be positioned below a height level of a top surface of a corresponding fin, in which the fully depleted channel builds up during transistor operation. Furthermore, in the context of the present disclosure, a recessed transistor configuration is to be understood as a transistor in which the drain region or the source region, or both, comprise a portion of metal silicide that is positioned below the interface of the gate insulation layer and the channel region or positioned below a top surface of a corresponding fin in a three-dimensional transistor architecture. According to the principles disclosed herein, enhanced transistor performance may be accomplished in a recessed configuration by forming a non-conformal metal silicide layer in at least one of the drain and source regions, thereby providing the potential for appropriately adjusting the overall transistor characteristics so that advantages of the recessed configuration with respect to reducing series resistance and generally enhancing overall stress transfer may be maintained, while at the same time a certain degree of mobility degradation, which may be associated with conventional recessed transistor configurations, may be reduced by appropriately adapting the thickness of the metal silicide immediately adjacent to the channel region. For example, in the recessed configuration, a substantially vertically extending surface area of the drain and/or source regions may receive a moderately thin metal silicide so as to not unduly reduce the efficiency of any other stress-inducing sources, such as a highly stressed dielectric material provided in the recess, a semiconductor alloy, the stress created by the contact elements and the like, whereas a moderately thick metal silicide may be provided at horizontal surface portions of the recess, thereby maintaining the overall series resistance at a low level. Consequently, due to the increased surface area including a metal silicide, the “spreading” resistance, i.e., the resistance for connecting to the channel region of the transistor, may be reduced due to the generally increased surface area, while nevertheless the “shielding” effect of the amount of metal silicide positioned in close proximity to the channel region may be reduced. Consequently, in N-channel transistors, generally, a reduced source/drain junction resistance may be accomplished wherein generally enhanced performance may be obtained by providing the non-conformal metal silicide thickness on the basis of an amorphizing ion implantation, which may generally result in enhanced quality of the resulting metal silicide. Moreover, enhanced electron mobility and thus drive current may be obtained compared to conventional recessed configurations due to the enhanced stress transfer of a tensilely stressed dielectric material formed in the recess and possibly in combination with a tensile stress created by contact elements. On the other hand, the efficiency of the stress transfer is enhanced, as explained above. Similarly, for P-channel transistors, the recessed configuration may also be advantageous in that metal silicide may be positioned more closely to the channel region compared to planar transistor configurations and also a dielectric cap layer may be formed within the recess while at the same time the enhanced stress transfer may be accomplished, as discussed above. In particular, in combination with an embedded silicon/germanium alloy and the like, a reduced degree of material consumption may be accomplished by providing the non-conformal metal silicide, while still reducing the spreading resistance in the drain and source regions.
The semiconductor device 100 as shown in
For example, for P-channel transistors, a silicon/germanium alloy with a germanium content of up approximately 30 atomic percent may provide a moderately high strain component, which may also be efficiently transferred into the channel region 153. In other cases, one or more of the anneal processes performed for re-crystallizing the drain and source areas may be performed in the presence of an appropriate rigid material, which may also be provided with a high internal stress level, if desired, so that the re-growth of the previously highly damaged drain and source areas, or other areas which may have previously been amorphized, may take place in a strained state, which may even be maintained, at least to a certain degree, after the partial or complete removal of the overlying cap layer. Respective techniques may also be referred to as stress memorization techniques and may enable a moderately high strained state, in particular in N-channel transistors, the effect of which may also be higher in combination with the non-conformal metal silicide creation, as will be described later on. Consequently, after providing the basic transistor configuration, the etch process 103 may be performed, for instance on the basis of well-established etch recipes, in which silicon material or silicon-containing material may be efficiently removed selectively with respect to the spacer structure 154. In the embodiment shown in
Consequently, a high degree of flexibility may be provided by the embodiment described above so as to appropriately position and shape the damaged zones 155D in order to control the actual silicidation process. Thus, a recessed configuration may be selectively provided in order to obtain the advantages associated with the recessed configuration while also reducing the shielding effect corresponding to the surface areas S1, while in other cases still a certain degree of stressed components, which may be induced by the metal silicide itself, for instance in the form of a compressive stress component, may be maintained at the portion corresponding to the surface area S2. Moreover, by appropriately selecting the process parameters for the implantation processes 104, 104A, the degree of non-conformal thickness of the metal silicide and any degree of asymmetry with respect to drain and source regions of the same transistor or with respect to different transistors may readily be obtained on the basis of corresponding masking regimes, as described above.
With reference to
The metal silicide 256 may be formed on the basis of process techniques as previously described, i.e., a heat treatment may be performed in which a difference in thickness of the layer 205 (
Hence, also in this case, the non-conformal metal silicide 256 may provide a reduced negative effect on any strain-inducing mechanisms, such as stress memorization techniques, embedded semiconductor alloys, highly stressed dielectric materials to be formed in the recess 255R and the like.
As a result, the present disclosure provides techniques and semiconductor devices in which at least one of the drain and source regions may have a non-conformal metal silicide region in order to enhance overall device performance, for instance by not reducing using the effect of strain-inducing mechanisms, such as dielectric cap layers, strained drain and source areas, which may be formed by stress memorization techniques, embedded semiconductor alloys and the like, stressed contact materials and the like. It should be appreciated that the principles disclosed herein may also be readily applied to three-dimensional transistor configurations, such as FinFETS, tri-gate transistors and the like, wherein a portion of the metal silicide may be recessed with respect to a top surface of a corresponding channel region wherein also in this case a reduced strain relaxing effect of at least a portion of the metal silicide may be accomplished.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. For example, the process steps set forth above may be performed in a different order. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
1. A semiconductor device, comprising:
- a transistor comprising drain and source regions connecting to a channel region, at least one of said drain and source region having a recessed configuration and comprising a metal silicide layer having a non-conformal thickness; and
- a strain-inducing dielectric layer that is in contact with said metal silicide layer.
2. The semiconductor device of claim 1, wherein said metal silicide region has a first surface area and a second surface area that are substantially perpendicular to each other.
3. The semiconductor device of claim 2, wherein a normal of said first surface area is oriented along a channel length direction.
4. The semiconductor device of claim 3, wherein the thickness of said metal silicide in said first surface area is less than the thickness of said metal silicide in said second surface area.
5. The semiconductor device of claim 1, wherein each of said drain and source regions comprises said recessed configuration and comprises said metal silicide with a non-conformal thickness.
6. The semiconductor device of claim 5, wherein said non-conformal thickness in said drain region varies non-symmetrically relative to said non-conformal thickness of the metal silicide in the source region.
7. The semiconductor device of claim 5, wherein each of said drain and source regions comprises said recessed configuration and wherein one of said drain and source regions comprises the metal silicide with a substantially conformal thickness.
8. The semiconductor device of claim 1, wherein one of the drain region and the source region has a non-recessed configuration.
9. The semiconductor device of claim 1, further comprising a second transistor comprising drain and source regions having a non-recessed configuration.
10. The semiconductor device of claim 1, further comprising a second transistor having drain and source regions, wherein said second transistor is of opposite conductivity type compared to said transistor and wherein the drain and source regions of said second transistor have a recessed configuration.
11. The semiconductor device of claim 10, wherein at least one of said drain and source regions of said second transistor comprises a metal silicide layer with a non-conformal thickness.
12. A method, comprising:
- forming a recess in at least one of a drain region and a source region of a transistor;
- forming a metal silicide layer in a non-conformal manner in said recess; and
- forming a strain-inducing layer above said drain and source regions.
13. The method of claim 12, wherein forming said metal silicide layer in a non-conformal manner comprises generating crystal damage in a non-conformal manner in said recess.
14. The method of claim 13, wherein generating crystal damage comprises performing an ion implantation process.
15. The method of claim 14, wherein said implantation process comprises an implantation using a non-zero tilt angle.
16. The method of claim 12, wherein forming said metal silicide in a non-conformal manner comprises anisotropically depositing a metal and initiating a chemical reaction between said metal and a silicon material of the drain and source regions.
17. The method of claim 12, wherein a recess is formed in said drain region and said source region and wherein said metal silicide is formed in a substantially conformal manner in one of said drain region and source region.
18. The method of claim 12, wherein a recess is formed in said drain region and said source region and wherein said metal silicide is formed in a non-conformal manner in said drain region and said source region and wherein a degree of non-conformality is different in said drain and source regions.
19. A method, comprising:
- forming a recess in at least one of a drain region and a source region of a transistor;
- performing an ion implantation process to modify an exposed area of said at least one of a drain region and a source region in said recess in a non-conformal manner;
- depositing a metal above said drain and source regions; and
- performing a heat treatment to create a metal silicide in a non-conformal manner within said recess.
20. The method of claim 19, further comprising forming a strain-inducing layer above said drain and source regions.
21. The method of claim 19, wherein performing an ion implantation process comprises using a non-zero tilt angle.
22. The method of claim 19, further comprising masking a second transistor prior to performing said implantation process.
23. The method of claim 22, wherein said second transistor comprises a second recess in at least one of a drain region and a source region of said second transistor.
24. The method of claim 19, wherein said recess is formed in the drain region and a further recess is formed in the source region and wherein said ion implantation is performed asymmetrically with respect to said drain and source regions.
25. The method of claim 22, wherein said second transistor is a P-channel transistor and said transistor is an N-channel transistor.
International Classification: H01L 27/092 (20060101); H01L 21/8238 (20060101);