Abstract:
The present invention relates to semiconductor integrated circuits. More particularly, but not exclusively, the invention relates to strained channel complimentary metal oxide semiconductor (CMOS) transistor structures and fabrication methods thereof. A strained channel CMOS transistor structure comprises a source stressor region comprising a source extension stressor region; and a drain stressor region comprising a drain extension stressor region; wherein a strained channel region is formed between the source extension stressor region and the drain extension stressor region, a width of said channel region being defined by adjacent ends of said extension stressor regions.

Description:
CROSS-REFERENCE TO RELATED APPLICATION(S) 
       [0001]    This is a continuation of co-pending U.S. patent application Ser. No. 11/383,951 filed May 17, 2006. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates to semiconductor integrated circuits. More particularly, but not exclusively, the invention relates to strained channel complimentary metal oxide semiconductor (CMOS) transistor structures and fabrication methods thereof. 
       DESCRIPTION OF THE RELATED ART 
       [0003]    Integrated circuits comprising many thousands of semiconductor devices play an important role in a number of technology areas. The continued development of devices with higher performance at reasonable cost is important to the future development of many of these technologies. Metal oxide semiconductor field effect transistors (MOSFETs) are commonly used in semiconductor integrated circuits. It has been shown that the performance of a MOSFET device may be enhanced by the application of mechanical stresses to portions of the device. 
         [0004]    A known MOSFET  10  is shown schematically in  FIG. 1  (PRIOR ART). A MOSFET  10  is typically fabricated on a semiconductor substrate  12  such as silicon and has a source region  15  (also known as a ‘deep’ source region) and a drain region  16  (also known as a ‘deep’ drain region) separated by a conduction channel  17 . 
         [0005]    A gate stack  18  is provided over the conduction channel  17  (hereinafter referred to as the ‘channel’). The gate stack  18  is formed from a gate dielectric layer  19  above the channel  17 , and a gate electrode  20  above the gate dielectric layer  19 . The application of a potential to the gate electrode  20  allows a flow of current through the channel  17  between the source  15  and the drain  16  to be controlled. 
         [0006]    The gate stack  18  is provided with spacer elements  21 ,  22  on a source side and a drain side, respectively, of the gate stack  18 . The purpose of the spacer elements  21 ,  22  is to define boundaries of the source and drain regions  15 ,  16  with respect to the channel  17 . For example, the source and drain regions  15 ,  16  may be made by implantation of the substrate  12  with dopant. 
         [0007]    The spacer elements  21 ,  22  may serve as an implantation mask during formation of the source and drain regions  15 ,  16  to define the boundary between the source and drain regions  15 ,  16  and the channel  17 . Alternatively, the source and drain regions  15 ,  16  may be made by etching a source recess and a drain recess, and filling the recesses with in-situ doped silicon. In this case, the spacer elements  21 ,  22  serve to protect the underlying substrate from the effects of the etching process. 
         [0008]    The source  15  and drain  16  also have shallow extension regions  25 ,  26 , respectively. The presence of shallow extension regions  25 ,  26  near the ends of the channel  17  helps to reduce short channel effects, thereby improving the performance of the device. 
         [0009]    The performance of a MOSFET device may be further improved by providing ‘halo’ regions  27 ,  28  between the substrate  12  and shallow source and drain extensions  25 ,  26 . The halo regions  27 ,  28  are formed by implantation of the substrate with a dopant of opposite conductivity type to that used to form the source and drain extensions  25 ,  26 . By way of example, in an n-type FET (nFET) formed on a silicon substrate, the source and drain extensions may be made by implanting a silicon substrate with an n-type dopant such as arsenic or phosphorous. Halo regions in this device would be formed by implanting the substrate with a p-type dopant such as boron. 
         [0010]    The purpose of forming the halo regions  27 ,  28  is to suppress ‘punchthrough’, one of several short channel effects that degrade the performance of the device. Punchthrough occurs when the channel length of the device is sufficiently short to allow the depletion regions at the ends of the source and drain extensions to overlap, leading to a breakdown condition. Although punchthrough is generally avoided by appropriate circuit design, the presence of the halo regions  27 ,  28  shortens the depletion regions at the ends of the source and drain extensions  25 ,  26 . This allows the fabrication of devices having shorter channel regions whilst still avoiding breakdown by punchthrough. 
         [0011]    A recent advance in the development of higher performance MOSFET devices has been the inclusion of strained silicon regions in the device. It has been found that the drive current of an nFET may be enhanced by applying a tensile stress along the length of the channel  17 . The performance of a p-type FET (pFET) may be enhanced by applying a compressive stress along the length of the channel  17  instead of a tensile stress. 
         [0012]    However, if a compressive stress is applied along the length of the channel  17  of an nFET the performance of the nFET is degraded. Similarly, if a tensile stress is applied along the length of the channel  17  of a pFET, the performance of the pFET is degraded. 
         [0013]    A number of methodologies have been developed for introducing a strain along the length of the channel  17 . These include the growth of an epitaxial layer (hereinafter referred to as an epilayer) of a semiconductor material having a first natural lattice constant on top of a substrate  12  having a second natural lattice constant different from the first. A biaxially strained epitaxial layer of the overlying semiconductor material may thereby be formed. By natural lattice constant is meant the lattice constant of the bulk, unstrained crystalline material. 
         [0014]    For example, the epitaxial layer may be formed from silicon, and the substrate may be formed from a silicon germanium alloy (hereinafter referred to as silicon germanium). Silicon has a natural lattice constant of approximately 5.43 Å. Silicon germanium has a natural lattice constant of between 5.43 Å and 5.66 Å, depending upon the concentration of germanium in the alloy. The higher the concentration of germanium, the larger the natural lattice constant of the alloy. Since the natural lattice constant of silicon germanium is higher than that of silicon, the entire silicon epilayer will be in a state of biaxial tensile stress. U.S. Pat. No. 6,867,428 (BESSER et al.) discloses a strained silicon nFET having a strained silicon channel formed in such an epilayer. 
         [0015]    An alternative approach to the formation of a strained channel region is to use an unstrained silicon substrate and introduce strain into the channel by forming stressor regions within each of the source and drain regions of the device. A stressor region is formed from an epitaxial material having a lattice constant different to that of the substrate. If the natural lattice constant of the stressor material is larger than that of the substrate, the stressor regions will exert a compressive stress on the channel. In the case of a silicon substrate, the inclusion of epitaxial silicon-germanium stressor regions will create a compressive stress in the channel. 
         [0016]    Silicon-carbon alloy (hereinafter referred to as silicon carbon) has a smaller natural lattice constant than silicon, and may also be grown epitaxially on silicon. The inclusion of an epitaxial silicon carbon stressor region within the source and drain regions produces a tensile stress along the length of the channel. 
         [0017]    In certain advanced device structures it is desirable to have the stressor regions as close to the channel as possible in order to further optimise the stress applied to the channel. Thus, stressor regions may be formed within the source and drain extension regions. The extension regions may undercut spacers formed on sidewalls of the gate stack. However, this has proved difficult to achieve in practice. 
         [0018]    The source and drain stressors may be formed by implantation of the substrate. For example, germanium may be implanted into a silicon substrate to form a silicon germanium alloy. Alternatively, source and drain recesses may be formed in the silicon substrate, and an epitaxial stressor material such as silicon germanium deposited in the recesses. 
         [0019]    In several known transistor structures having source/drain extension stressors the channel region is not defined by ends of the source/drain extension stressors. Rather, the source/drain extension stressors lie within a boundary of the source/drain extension region, such that a region of doped silicon is provided between the stressor region and respective ends of the source/drain extension regions. 
         [0020]    Methods of forming the source and drain extension stressors by etching of the substrate may be adapted to form source and drain stressor extension recesses by exploiting a feature of isotropic etching processes known as ‘undercut’. If the source and drain recesses are formed by an isotropic etching technique, removal of portions of the substrate underlying spacer elements  21 ,  22  will occur. This phenomenon is known as ‘undercut’. 
         [0021]    A disadvantage of isotropic etching processes, however, is that they are difficult to control. Isotropic etching processes may be affected by the presence of residual film on the surface of the device. In addition they may be affected by microloading effects arising from nearby protective layers. It is therefore difficult to ensure that source and drain stressor extension recesses formed by isotropic etching are of a reproducible morphology. By protective layers are meant layers that are used to protect areas of the substrate that are not to be etched to form a recess. 
         [0022]    Solutions to these problems have been long sought but prior developments have not taught or suggested any solutions and, thus, solutions to these problems have long eluded those skilled in the art. 
       SUMMARY OF THE INVENTION 
       [0023]    It is an object of the present invention to at least partly mitigate the above-mentioned problems. 
         [0024]    It is a further object of embodiments of the invention to provide an improved transistor structure with a strained channel region. 
         [0025]    Another object of embodiments of the present invention is to provide a fabrication method for a strained channel transistor structure. 
         [0026]    According to a first aspect of the present invention there is provided a strained channel transistor structure, comprising: a source stressor region comprising a source extension stressor region; and a drain stressor region comprising a drain extension stressor region; wherein a strained channel region is formed between the source extension stressor region and the drain extension stressor region, a width of said channel region being defined by adjacent ends of said extension stressor regions. 
         [0027]    According to a second aspect of the present invention there is provided a strained channel transistor structure, comprising: a source stressor region; and a drain stressor region; wherein each stressor region comprises a lower stressor region and an upper stressor region, said upper stressor region comprising a stressor extension region; and a width of at least one of said source and/or drain upper stressor regions is greater than a width of the lower stressor region. 
         [0028]    According to a third aspect of the present invention there is provided a method of forming a strained channel transistor structure on a substrate comprising a first semiconductor material, comprising the steps of: forming a source stressor region comprising a source extension stressor region; and forming a drain stressor region comprising a drain extension stressor region; whereby a channel region is formed between said source extension stressor region and said drain extension stressor region, said channel region having a width defined by adjacent ends of said extension stressor regions. 
         [0029]    According to a fourth aspect of the present invention there is provided a method of forming a strained channel transistor structure, comprising the steps of: forming a source stressor region comprising an upper source stressor region and a lower source stressor region, the upper source stressor region comprising a source extension stressor region; and forming a drain stressor region comprising an upper drain stressor region and a lower drain stressor region, the upper drain stressor region comprising a drain extension stressor region; whereby the upper stressor regions are formed to have a width greater than a width of the lower stressor region. 
         [0030]    Embodiments of the present invention provide a number of advantages over the prior art. In some embodiments of the present invention, the stressors are formed in the entire source/drain and source/drain extension regions. Thus, the width of the channel region is defined by adjacent ends of the extension stressor regions. This has the advantage that a level of strain in the channel may be made higher than in prior art devices where stressors are formed only in portions of the source/drain and source/drain extension regions. In other embodiments, the width of the channel region is not exactly defined by adjacent ends of the extension stressor regions, due to a blurring effect at the interface between the stressor and substrate caused by diffusion of dopants from the extension stressor regions toward the channel during a subsequent spike anneal step. Nevertheless, extension stressor regions are extremely close to the channel region. 
         [0031]    Furthermore, in embodiments of the present invention the source/drain and source/drain extension stressor regions are formed by means of a two-step etch and epitaxy process. An advantage of the two-step etch and epitaxy process is that boundaries of the source/drain stressor regions may be made highly abrupt. This enables highly abrupt junctions to be formed between the source/drain extension stressor regions and the strained channel region, allowing improved short channel behaviour to be attained. The two-step etch and epitaxy process also enables a source/drain extension recess to be formed having a depth that is less than that of the rest of the source/drain recess. This has the advantage of improving the stress along the channel by forming the stressor closer to the channel without causing an increase in short channel effects, as would be the case if the source/drain extension recess was made more deep. 
         [0032]    During the process of growing source/drain and source/drain extension stressor regions according to embodiments of the invention, the source/drain and source/drain extension stressor regions may be doped in-situ. An advantage of this feature is that the source and drain extension stressor regions may be made highly activated. If the source/drain and source/drain extension stressor regions are not doped in-situ, conventional ion implantation and annealing can be performed to form doped source/drain and source/drain extension stressor regions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0033]    Embodiments of the present invention will now be described hereinafter, by way of example only, with reference to the accompanying drawings, in which: 
           [0034]      FIG. 1  (PRIOR ART) is a cross-section illustrating a prior art MOSFET device. 
           [0035]      FIGS. 2A ,  2 B,  2 C,  2 D,  2 E,  2 F,  2 G,  2 H,  2 I,  2 J,  2 K, and  2 L show structures formed during a process of fabrication of a MOSFET in accordance with an embodiment of the invention. 
           [0036]      FIGS. 3A ,  3 B,  3 C,  3 D,  3 E,  3 F,  3 G and  3 H show structures formed during a process of fabrication of a MOSFET in accordance with a further embodiment of the invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0037]    The following embodiments are intended to illustrate the invention more fully without limiting their scope, since numerous modifications and variations will be apparent to those skilled in the art. 
         [0038]      FIGS. 2A to 2L  illustrate structures formed during fabrication of a strained channel transistor structure  200  ( FIG. 2L ) in accordance with a first embodiment of the present invention. The final transistor structure  200  is illustrated in  FIG. 2L . 
         [0039]      FIG. 2A  shows a structure having a gate structure  201  formed on a substrate  202  of a first semiconductor material which is silicon. Whilst the first semiconductor material is described here as silicon it will be understood that other materials such as germanium or GaAs or others could be used. The gate structure  201  has a gate insulating layer  201 A formed on the silicon substrate  202  and a gate electrode  201 B formed above the gate insulator layer  201 A. 
         [0040]    The gate insulator layer  201 A is silicon oxide but it will be appreciated that other materials could be used such as silicon nitride, aluminium oxide, hafnium oxide or others alone or in combination, for example a combination of layers of silicon oxide and silicon nitride. Silicon oxide may be grown by thermal oxidation of the silicon substrate. Nitrogen can be introduced into the silicon oxide by means of plasma nitridation or thermal nitridation. 
         [0041]    The gate electrode  201 B is typically polysilicon, but may alternatively be germanium, nickel silicide, copper, aluminium or others such as polysilicon implanted with a second material such as germanium. 
         [0042]    A bilayer hardmask structure  203  is formed above the gate structure  201 . The bilayer hardmask structure  203  has a lower hardmask layer  203 A and an upper hardmask layer  203 B. Advantageously, the lower hardmask layer  203 A is an oxide hardmask layer and is formed above the gate structure  201 . Advantageously, the upper hardmask layer  203 B is a nitride hardmask layer and is formed above the lower hard mask layer  203 A. 
         [0043]    Shallow trench isolation (STI) regions  205  are formed in the substrate  202  between transistor structures  200 . The STI regions  205  may be formed by forming trenches having tapered sidewalls in the substrate  202 , and filling the trenches with an oxide such as silicon oxide. 
         [0044]      FIG. 2B  shows the structure of  FIG. 2A  after the formation of a conformal oxide layer  206  over said structure. Sidewalls of the gate structure  201 , and both the sidewalls and a top surface of the bilayer hardmask structure  203 , are coated with oxide, in addition to an exposed surface  202 A of the substrate  202 , in this case an area of the substrate  202  not underlying the gate structure  201 . The purpose of forming the conformal oxide layer  206  is to enable the formation of offset spacer elements  207 ,  208  on sidewalls of the gate stack  201  and bilayer hardmask structure  203 . 
         [0045]    The conformal oxide layer  206  may be a low temperature oxide (LTO) deposited by plasma enhanced chemical vapor deposition (PECVD), or an oxide formed by low pressure chemical vapor deposition (LPCVD) or rapid thermal chemical vapor deposition (RTCVD). Advantageously the thickness of the oxide layer is from about 20 Angstroms to about 300 Angstroms. 
         [0046]      FIG. 2C  shows the structure of  FIG. 2B  after the conformal oxide layer  206  has been subjected to a step of reactive ion etching. The step of reactive ion etching is performed in order to remove portions of the conformal oxide layer  206  that were formed on surfaces other than those that are substantially perpendicular to the plane of the substrate  202 . The step of reactive ion etching results in the formation of offset spacer elements  207 ,  208  on the sidewalls of the gate stack  201  and bilayer hardmask structure  203 . 
         [0047]    Following formation of the offset spacer elements  207 ,  208 , an optional Halo implant may be performed. 
         [0048]      FIG. 2D  shows the structure of  FIG. 2C  following the deposition of a thin conformal oxide layer  209 , and the deposition of a conformal nitride layer  210  over the thin conformal oxide layer  209 . The thin conformal oxide layer  209  may be a low temperature oxide (LTO) deposited by plasma enhanced chemical vapor deposition (PECVD). Alternatively the thin conformal oxide layer  209  may be formed by low pressure chemical vapor deposition (LPCVD) or rapid thermal chemical vapor deposition (RTCVD). Advantageously the thin conformal oxide layer  209  is from about 20 Angstroms to about 200 Angstroms in thickness. 
         [0049]    The conformal nitride layer  210  may be formed by plasma enhanced chemical vapor deposition (PECVD), low pressure chemical vapor deposition (LPCVD) or rapid thermal chemical vapor deposition (RTCVD). Advantageously the conformal nitride layer  210  is from about 100 Angstroms to about 700 Angstroms in thickness. 
         [0050]      FIG. 2E  shows the structure of  FIG. 2D  following the step of reactive ion etching of the conformal nitride layer  210 , and removal of areas of the thin conformal oxide layer  209  exposed following reactive ion etching of the conformal nitride layer  210 . The areas of the thin conformal oxide layer  209  thereby exposed may be removed by a step of wet etching, a step of dry etching, or any other suitable step of etching. 
         [0051]    The step of reactive ion etching of the conformal nitride layer  210  is performed in order to form disposable nitride spacers  211 ,  212  on the sidewalls of the gate stack  201  and bilayer hardmask structure  203 . Portions  215 ,  216  of the thin conformal oxide layer  209  remain sandwiched between the nitride spacers  211 ,  212  and the substrate  202 . Portions  215 ,  216  of the thin conformal oxide layer  209  will hereafter be referred to as disposable oxide spacers  215 ,  216 . 
         [0052]      FIG. 2F  shows the structure of  FIG. 2E  following the step of etching of exposed areas of the substrate to form a lower source stressor recess  217  and a lower drain stressor recess  218 . The disposable nitride spacers  211 ,  212  and the upper hardmask layer  203 B protect the gate stack  201  from damage during this step of etching. Advantageously the lower source and drain stressor recesses  217 ,  218  are formed by a step of anisotropic reactive ion etching. Advantageously, the depths of the lower source stressor recess  217  and lower drain stressor recess  218  are from about 200 Angstroms to about 2000 Angstroms. 
         [0053]    Anisotropic reactive ion etching has the advantage that it is more stable than isotropic etching and does not result in undercut. In this case, undercut refers to the removal of portions of the substrate underlying the disposable nitride spacers  211 ,  212  and disposable oxide spacers  215 ,  216 . 
         [0054]    Formation of stressor regions by etching, followed by deposition of stressor material, is also advantageous over stressor formation by implantation (e.g., implantation of Ge into a silicon substrate). Stressor regions may be formed with superior uniformity when formed by the steps of etching and deposition as opposed to implantation, due to the need for careful control of implantation energies in the case of stressor formation by implantation. It will of course be appreciated that embodiments of the present invention form part of the stressor region by implantation. 
         [0055]      FIG. 2G  shows the structure of  FIG. 2F  following selective epitaxial growth of a second semiconductor material in the lower source and drain stressor recesses  217 ,  218 . Deposition of the in-situ doped second semiconductor material results in the formation of an intermediate source stressor region  219  and an intermediate drain stressor region  220  in the source and drain stressor recesses  217 ,  218  respectively. 
         [0056]    Advantageously the selective growth of the second semiconductor material is performed such that growth of second semiconductor material occurs only over exposed surfaces of substrate  202 . Advantageously, the intermediate stressor regions  219 ,  220  are formed such that an upper surface of the intermediate stressor regions  219 ,  220  is substantially coplanar with an upper surface  202 A of the substrate  202 . Furthermore it is advantageous that the second semiconductor material is doped in-situ with a dopant. 
         [0057]    An advantage of in-situ doping is that a separate dopant implantation step is not required. Furthermore, the uniformity of dopant concentration within the stressor regions may be more closely controlled than in the case of implantation of dopant. More highly activated source and drain stressor regions may also be formed by the step of in-situ doping. 
         [0058]    An epitaxy preclean of the exposed surfaces of the substrate is performed prior to selective epitaxial growth so that growth of high quality epitaxial material may take place. Advantageously, the epitaxy preclean is performed using hydrogen fluoride (HF), either in a gaseous or liquid solution form, or by a combination of steps and chemicals that include HF in a gaseous or liquid form. Prior to selective epitaxial growth, a pre-bake step may also be performed to ensure good quality epitaxial layers are formed. The pre-bake step may include the steps of heating to a temperature in the range of about 750° C. to about 1000° C. for a duration in the range of about 2 seconds to about 20 minutes. Advantageously the pre-bake step is performed in a temperature range of about 750° C. to about 850° C. for a duration in the range of about 10 seconds to about 2 minutes. 
         [0059]    The ambient atmosphere during the pre-bake step may include a hydrogen (H 2 ) atmosphere. Alternatively a nitrogen (N 2 ) atmosphere, an argon atmosphere, or another atmosphere may be used, such as a combination of both a hydrogen atmosphere and a nitrogen atmosphere. Advantageously the pre-bake step includes a hydrogen atmosphere. Advantageously the epitaxial growth is performed by chemical vapor deposition or molecular beam epitaxy. More advantageously the epitaxial growth is performed by rapid thermal chemical vapor deposition (RTCVD). 
         [0060]    Advantageously, in the case of the formation of pFET devices, the second semiconductor material contains a silicon-germanium alloy (hereinafter referred to as silicon germanium). Advantageously, the Ge composition in the silicon germanium is about 10 to about 45 atomic percent. The Ge atoms are advantageously positioned in the substitutional lattice sites in order to produce a strained channel. 
         [0061]    Advantageously, in the case of the formation of nFET devices, the second semiconductor material contains a silicon-carbon alloy (hereinafter referred to as silicon carbon). Advantageously, the C composition in the silicon carbon is about 0.1 to about 15 atomic percent. The C atoms are advantageously positioned in the substitutional lattice sites in order to produce a strained channel. 
         [0062]      FIG. 2H  shows the structure of  FIG. 2G  following removal of the upper hardmask layer  203 B and disposable nitride spacers  211 ,  212 . The upper hardmask layer  203 B and disposable nitride spacers  211 ,  212  may be removed by a step of wet etching. The step of wet etching may include a step of exposing the spacers to a hot phosphoric acid. Alternatively, upper hardmask layer  203 B and disposable nitride spacers  211 ,  212  may be removed by a step of isotropic dry etching. 
         [0063]      FIG. 2I  shows the structure of  FIG. 2H  following removal of disposable oxide spacers  215 ,  216 . The disposable oxide spacers  215 ,  216  may be removed by a step of etching. The step of etching may include a step of wet etching. The step of wet etching may include a step of dipping the structure in a HF liquid solution. Alternatively, the disposable oxide spacers  215 ,  216  may be removed by a step of dry oxide etching. 
         [0064]    The purpose of removing the disposable nitride spacers  211 ,  212  and the disposable oxide spacers  215 ,  216  is to expose an area of the substrate wherein an upper source stressor recess  221  and an upper drain stressor recess  222  are to be formed. 
         [0065]      FIG. 2J  shows the structure of  FIG. 2I  following a step of etching of exposed areas of the substrate to form the upper source stressor recess  221  and the upper drain stressor recess  222 . Upper portions of both the intermediate source stressor region  219  and the intermediate drain stressor region  220  are thereby removed. A lower source stressor region  227  and a lower drain stressor region  228  remain. Lower hardmask layer  203 A and offset spacers  207 ,  208  protect the gate structure  201  from damage during the step of etching. Advantageously the step of etching is performed by reactive ion etching. Advantageously, the depths of the upper source stressor recess  221  and the upper drain stressor recess  222  are from about 100 Angstroms to about 1000 Angstroms. 
         [0066]    Advantageously the upper source stressor recess  221  and the upper drain stressor recess  222  each have substantially planar basal surfaces  221 A,  222 A, respectively, following the step of reactive ion etching. Advantageously, the upper source stressor recess  221  and the upper drain stressor recess  222  are formed by a step of anisotropic reactive ion etching. In the case where isotropic reactive ion etching is used, it is advantageous to ensure that the undercutting does not extend too far beneath the spacers  207 ,  208 , in order to prevent severe short channel effects. 
         [0067]      FIG. 2K  shows the structure of  FIG. 2J  following epitaxial growth of an upper source stressor region  225  and an upper drain stressor region  226  in the upper source stressor recess  221  and the upper drain stressor recess  222 , respectively. Advantageously, the composition of the upper source stressor region  225  and the upper drain stressor region  226  is the same as that of the intermediate source stressor region  219  and the intermediate drain stressor region  220 . Thus, in the case of silicon germanium stressors, advantageously the Ge composition of the upper source stressor region  225  and the upper drain stressor region  226  is the same as that of the intermediate source stressor region  219  and the intermediate drain stressor region  220 . Advantageously, the upper stressor regions  225 ,  226  have the same dopant concentration as the intermediate stressor regions  219 ,  220 . 
         [0068]    Alternatively, the composition of the upper stressor regions  225 ,  226  may be different to that of the intermediate stressor regions  219 ,  220 . Thus, in the case of silicon germanium stressor regions, the Ge composition of the upper stressor regions  225 ,  226  may be different to that of the intermediate stressor regions  219 ,  220 . In a further alternative, the concentration of dopant in the upper stressor regions  225 ,  226  may be different to the dopant concentration in the intermediate stressor regions  219 ,  220 . 
         [0069]    An epitaxy preclean of the surface of the structure is performed prior to the epitaxial growth step so that growth of high quality epitaxial material may take place. Advantageously the epitaxial growth is performed by chemical vapor deposition, ultrahigh vacuum chemical vapor deposition, or molecular beam epitaxy. More advantageously, the epitaxial growth is performed by rapid thermal chemical vapor deposition. 
         [0070]    The upper source stressor region  225  has a source extension stressor region  225 A. The upper drain stressor region  226  has a drain extension stressor region  226 A. A conduction channel (hereinafter referred to as the ‘channel’  229 ) is defined by adjacent ends  225 B,  226 B of the source extension stressor region  225 A and drain extension stressor region  226 A. Thus, the source and drain extension stressor regions  225 A,  226 A extend to the boundary of the channel  229 . This feature has the advantage that a level of stress applied to the channel  229  is enhanced. Alternatively, the width of the channel  229  may be defined by adjacent ends  225 B,  226 B of the extension stressor regions  225 A,  226 A, together with a blurring effect due to diffusion of dopants from the extension stressor regions  225 A,  226 A toward the channel  229  during a subsequent spike anneal step. 
         [0071]    Advantageously, the source extension stressor region  225 A and drain extension stressor region  226 A are formed such that their upper surfaces are substantially coplanar with the surface  202 A of the substrate  202 . 
         [0072]    However, portions of the upper source stressor region  225  and the upper drain stressor region  226  that are directly above the lower source stressor region  227  and the lower drain stressor region  228 , respectively, are formed to protrude above a level of the substrate surface  202 A, resulting in the formation of a raised source structure  235  and a raised drain structure  236 . Raised source and drain structures have the advantage that device performance is enhanced by the increase in stress or decrease in sheet resistance. Another advantage is that the raised source and drain structure can be used to moderate the stress from a subsequent silicide layer or from a contact etch stop layer. 
         [0073]    Alternatively, the upper source and drain stressor regions  225 ,  226  may be formed such that upper surfaces of the upper source and drain stressor regions  225 ,  226  are below the level of the surface  202 A of the substrate  202 . 
         [0074]    In a further alternative, the upper source and drain stressor regions  225 ,  226  may be formed such that their upper surfaces are substantially at the same level as the surface  202 A of the substrate  202 . 
         [0075]    The reason for a difference in thickness within upper stressor regions  225 ,  226  arises due to a difference in epitaxial growth rates between the cases of homoepitaxial growth of a material and heteroepitaxial growth of that material on silicon. 
         [0076]    Heteroepitaxial growth refers to the epitaxial growth of a crystal A on a surface of a substrate of crystal B, in which the structure and chemical composition of crystal A is different from the structure and chemical composition of crystal B. The orientations of crystals A and B with respect to one another may also be different. 
         [0077]    Homoepitaxial growth refers to the epitaxial growth of a crystal A on a surface of a substrate of crystal B, where crystals A and B are of the same structure and composition. Furthermore, the orientations of crystals A and B with respect to one another are also the same. 
         [0078]    In the case of the growth of epitaxial silicon germanium on a silicon germanium substrate of the same composition, or the growth of epitaxial silicon carbon on a silicon carbon substrate of the same composition, the growth rate is found to be higher than the corresponding growth rates of silicon germanium on silicon, and silicon carbon on silicon, respectively, under otherwise identical conditions. 
         [0079]    The structure of  FIG. 2K  may be formed by terminating the epitaxial growth of second semiconductor material when upper surfaces of the extension stressor regions  225 A,  226 A are substantially coplanar with the surface  202 A of the substrate  202 . 
         [0080]    As discussed above, the formation of source and drain stressors by etching followed by deposition, has the advantage that more abrupt junction profiles may be produced. This enables improved short channel behaviour to be attained. Advantageously the second semiconductor material is doped in-situ. The step of in-situ doping has the advantage over implantation that a separate implantation step is not required, and enhanced uniformity of dopant concentration may be achieved. 
         [0081]      FIG. 2L  shows the structure of  FIG. 2K  after final spacers  231 ,  232  have been formed on sidewalls of the offset spacer elements  207 ,  208 . The final spacers  231 ,  232  may be formed from a nitride material as in the case of the disposable nitride spacers  211 ,  212 . Alternatively the final spacers  231 ,  232  may be formed from a different material to the disposable nitride spacers  211 ,  212 . 
         [0082]    Embodiments of the present invention have been hereinbefore described with reference to the use of hard mask layers. It will be understood that other types of masking layers such as soft mask layers or others could be used. 
         [0083]    At this stage, the structure may be subject to a spike anneal in order to form final junctions of the device. 
         [0084]      FIGS. 3A to 3H  illustrate structures formed during fabrication of a strained channel transistor structure  300  in accordance with a second embodiment of the present invention. The final transistor structure  300  is illustrated in  FIG. 3H . 
         [0085]      FIG. 3A  shows a device structure having a gate structure  301  formed on a substrate  302  of a first semiconductor material which is silicon. Whilst the first semiconductor material is described here as silicon it will be understood that other materials such as germanium or GaAs or others could be used. The gate structure  301  has a gate insulating layer  301 A formed on the silicon substrate  302  and a gate electrode  301 B formed above the gate insulator layer  301 A. 
         [0086]    The gate insulator layer  301 A is silicon oxide but it will be appreciated that other materials could be used such as silicon nitride, aluminium oxide, hafnium oxide or others alone or in combination, for example a combination of layers of silicon oxide and silicon nitride. Silicon oxide may be grown by thermal oxidation of the silicon substrate. Nitrogen can be introduced into the silicon oxide by means of plasma nitridation or thermal nitridation. 
         [0087]    The gate electrode  301 B is typically polysilicon, but may alternatively be germanium, nickel silicide, copper, aluminium or others such as polysilicon implanted with a second material such as germanium. 
         [0088]    A bilayer hardmask  303  is formed above the gate structure  301 , and offset spacers  307 ,  308  are formed on sidewalls of the gate structure  301  and bilayer hardmask  303 . The bilayer hardmask  303  has a lower hardmask layer  303 A and an upper hardmask layer  303 B. Advantageously, the lower hardmask layer  303 A is an oxide layer. Advantageously, the upper hardmask layer  303 B is a nitride layer. 
         [0089]    Shallow trench isolation (STI) regions  305  are also formed in the substrate  302 , as described in the context of the first embodiment of the invention. 
         [0090]      FIG. 3B  shows the structure of  FIG. 3A  following a step of anisotropic reactive ion etching of exposed areas of the substrate to form an upper source stressor recess  321  and an upper drain stressor recess  322 . In the case where isotropic reactive ion etching is used, it is advantageous to ensure that any undercutting associated with isotropic reactive ion etching does not extend too far beneath the spacers  307 ,  308  in order to prevent short channel effects. Upper hardmask layer  303 B and nitride offset spacers  307 ,  308  protect the gate structure from damage during the step of reactive ion etching. 
         [0091]    Anisotropic reactive ion etching has the advantage that it is more stable than isotropic etching and does not result in undercut, in this case the removal of portions of the substrate underlying the offset spacers  307 ,  308 . Advantageously the depths of the upper source stressor recess  321  and upper drain stressor recess  322  are from about 100 Angstroms to about 1000 Angstroms. 
         [0092]    Advantageously the upper source stressor recess  321  and the upper drain stressor recess  322  each have substantially planar basal surfaces  321 A,  322 A respectively. Advantageously, the upper source stressor recess  321  and the upper drain stressor recess  322  are formed by a step of anisotropic reactive ion etching. 
         [0093]      FIG. 3C  shows the structure of  FIG. 3B  following epitaxial growth of a second semiconductor material in the upper source stressor recess  321  and the upper drain stressor recess  322 . An epitaxy preclean of the exposed surfaces of the substrate is performed prior to selective epitaxial growth so that growth of high quality epitaxial material may take place. Advantageously the epitaxy preclean is performed using HF, either in a gaseous or liquid solution form, or by a combination of steps and chemicals that include HF in a gaseous or liquid form. Prior to selective epitaxial growth, a pre-bake step may also be performed to ensure good quality epitaxial layers are formed. The pre-bake step may include the steps of heating to a temperature in the range of about 750° C. to about 1000° C. for a duration in the range of about 2 seconds to about 20 minutes. Advantageously the pre-bake step is performed in a temperature range of about 750° C. to about 850° C. for a duration in the range of about 10 seconds to about 2 minutes. 
         [0094]    The ambient atmosphere during the pre-bake step may include a hydrogen (H 2 ) atmosphere. Alternatively a nitrogen (N 2 ) atmosphere, an argon atmosphere, or another atmosphere may be used, such as a combination of both a hydrogen atmosphere and a nitrogen atmosphere. Advantageously the pre-bake step includes a hydrogen atmosphere. Advantageously the epitaxial growth is performed by chemical vapor deposition or molecular beam epitaxy. More advantageously the epitaxial growth is performed by rapid thermal chemical vapor deposition (RTCVD). 
         [0095]    Deposition of the second semiconductor material results in the formation of an upper source stressor region  325  and an upper drain stressor region  326  in the upper source stressor recess  321  and upper drain stressor recess  322 , respectively. Advantageously, an upper surface of the upper source stressor region  325  and the upper drain stressor region  326  is substantially coplanar with an upper surface  302 A of the substrate  302 . 
         [0096]    The upper source stressor region  325  has a source extension stressor region  325 A, and the upper drain stressor region  326  has a drain extension stressor region  326 A. 
         [0097]    Advantageously the second semiconductor material is doped in-situ. The step of in-situ doping has the advantage over implantation that a separate implantation step is not required, and enhanced uniformity of dopant concentration may be achieved. In addition, more highly activated source and drain regions may be formed by in-situ doping. 
         [0098]    Formation of stressor regions by etching, followed by deposition of stressor material in a recess formed by etching, is also advantageous over stressor formation by implantation (e.g., implantation of Ge into a silicon substrate). Stressor regions may be formed with superior uniformity when formed by the steps of etching and deposition as opposed to implantation, due to the need for careful control of implantation energies in the case of stressor formation by implantation. It will of course be appreciated that embodiments of the present invention form part of the stressor region by implantation. 
         [0099]    A conduction channel (hereinafter referred to as the ‘channel’  329 ) is defined by adjacent ends  325 B,  326 B of the source extension stressor region  325 A and drain extension stressor region  326 A. Thus, the source and drain extension stressor regions  325 A,  326 A extend to the boundary of the channel  329 . This feature has the advantage that a level of stress applied to the channel  329  is enhanced. Alternatively, the width of the channel  329  may be defined by adjacent ends  325 B,  326 B of the extension stressor regions  325 A,  326 A, together with a blurring effect due to diffusion of dopants from the extension stressor regions  325 A,  326 A toward the channel  329  during a subsequent spike anneal step. 
         [0100]      FIG. 3D  shows the structure of  FIG. 3C  following the deposition of a thin conformal oxide layer  309  over the substrate, and the deposition of a conformal nitride layer  310  over the thin conformal oxide layer  309 . The thin conformal oxide layer  309  may be a low temperature oxide (LTO) deposited by plasma enhanced chemical vapor deposition (PECVD). Alternatively, the thin conformal oxide layer  309  may be formed by low pressure chemical vapor deposition (LPCVD) or rapid thermal chemical vapor deposition (RTCVD). Advantageously the thin conformal oxide layer  309  is about 20 Angstroms to about 200 Angstroms in thickness. 
         [0101]    The conformal nitride layer  310  may be formed by plasma enhanced chemical vapor deposition (PECVD) or low pressure chemical vapor deposition (LPCVD). Alternatively the conformal nitride layer  310  may be formed by rapid thermal chemical vapor deposition (RTCVD). Advantageously the conformal nitride layer  310  is from about 100 Angstroms to about 700 Angstroms in thickness. 
         [0102]      FIG. 3E  shows the structure of  FIG. 3D  following the step of reactive ion etching of the nitride layer  310 , and removal of areas of the thin conformal oxide layer  309  exposed by the nitride RIE step. The exposed areas of the thin conformal oxide layer  309  may be removed by a step of etching of these exposed areas. The step of etching of the exposed areas may include a step of dry etching of the exposed areas. Alternatively, the step of etching of the exposed areas may include a step of wet etching of the exposed areas. 
         [0103]    The step of reactive ion etching of the nitride layer  310  is performed in order to form nitride spacers  311 ,  312  on sidewalls of the gate stack structure  301  and bilayer hardmask structure  303 . The step of etching of the exposed areas of the thin conformal oxide layer  309  is performed in order to expose areas of the substrate  302 . Oxide spacers  315 ,  316  remain following etching of exposed areas of the thin conformal oxide layer  309 . Oxide spacers  315 ,  316  underlie nitride spacers  311 ,  312  respectively. 
         [0104]      FIG. 3F  shows the structure of  FIG. 3E  following the step of etching of exposed areas of the substrate  302  to form a lower source stressor recess  317  and a lower drain stressor recess  318 . The nitride spacers  311 ,  312  and upper hardmask layer  303 B protect the gate structure  301  from damage during this step. Advantageously the lower source stressor recess  317  and the lower drain stressor recess  318  are formed by a step of anisotropic reactive ion etching. Anisotropic reactive ion etching has the advantage over isotropic etching that it is more stable and does not result in undercut of the nitride spacers  311 ,  312  and oxide spacers  315 ,  316 . Advantageously the depths of the lower source stressor recess  317  and the lower drain stressor recess  318  are from about 200 Angstroms to about 2000 Angstroms. 
         [0105]    It may be seen from  FIG. 3F  that the only portions of the upper stressor regions  325 ,  326  remaining following this anisotropic etching process are the source extension stressor region  325 A and the drain extension stressor region  326 A. 
         [0106]      FIG. 3G  shows the structure of  FIG. 3F  following epitaxial growth of a deep source stressor region  319  and a deep drain stressor region  320 . Advantageously, the composition of the deep source stressor region  319  and the deep drain stressor region  320  is the same as that of the source extension stressor region  325 A and the drain extension stressor region  326 A. 
         [0107]    Alternatively, a composition of the deep source stressor region  319  and the deep drain stressor region  320  may be different to that of the source extension stressor region  325 A and the drain extension stressor region  326 A. 
         [0108]    Advantageously, the steps of epitaxial growth of the deep source stressor region  319  and the deep drain stressor region  320  includes the steps of in-situ doping of the deep source stressor region  319  and the deep drain stressor region  320 . If the deep source/drain stressors are undoped, conventional ion implantation and annealing can be performed to form doped deep source/drain stressor regions. 
         [0109]    An epitaxial preclean of the surface of the structure is again performed before the epitaxial growth of the stressor regions  319 ,  320  is performed. Advantageously, the epitaxial growth of the stressor regions  319 ,  320  is performed by rapid thermal chemical vapor deposition (RTCVD). Alternatively, the epitaxial growth of the stressor regions  319 ,  320  may be performed by chemical vapor deposition, ultra-high vacuum chemical vapor deposition or molecular beam epitaxy. 
         [0110]    The formation of source and drain stressor regions by etching followed by deposition has the advantage that more abrupt junction profiles may be produced. This enables improved short channel behaviour to be attained. 
         [0111]    From  FIG. 3G  it may be seen that the deep source stressor region  319  and deep drain stressor region  320  are formed such that their upper surfaces protrude above the level of the substrate surface  302 A. A raised source and drain structure is thereby formed. Raised source and drain structures have the advantage that device performance is enhanced. 
         [0112]    Alternatively, the upper source and drain stressor regions  325 ,  326  may be formed such that upper surfaces of the upper source and drain stressor regions  325 ,  326  are below the level of the surface  302 A of the substrate  302 . 
         [0113]    In a further alternative, the upper source and drain stressor regions  325 ,  326  may be formed such that their upper surfaces are substantially at the same level as the surface  302 A of the substrate  302 . 
         [0114]      FIG. 3H  shows the structure of  FIG. 3G  following removal of the hardmask bilayer  303  and upper portions of nitride spacers  311 ,  312 . A step of spike annealing may be performed at this stage to form final junctions of the structure. 
         [0115]    In both the first and second preferred embodiments of the invention the source stressor regions  235 ,  335  and the drain stressor regions  236 ,  336  are formed by in-situ doping during epitaxial growth of the stressor regions. Consequently the source extension stressor regions  225 A,  325 A and drain extension stressor regions  226 A,  326 A may be made highly activated. 
         [0116]    Furthermore, the profiles of junctions between the source stressor regions  235 ,  335  and the substrate  202 ,  302 , respectively, and between the drain stressor regions  236 ,  336  and the substrate  202 ,  302 , respectively, are determined by an etch profile. Consequently, highly abrupt junctions may be formed. This enables excellent short channel behaviour to be attained by the transistor structure. 
         [0117]    Embodiments of the present invention have been hereinbefore described with reference to the use of hard mask layers. It will be understood that other types of masking layers such as soft mask layers or others could be used. 
         [0118]    Throughout the description and claims of this specification, the words “comprise” and “contain” and variations of the words, for example “comprising” and “comprises”, means “including but not limited to”, and is not intended to (and does not) exclude other moieties, additives, components, integers or steps. 
         [0119]    Throughout the description and claims of this specification, the singular encompasses the plural unless the context otherwise requires. In particular, where the indefinite article is used, the specification is to be understood as contemplating plurality as well as singularity, unless the context requires otherwise. 
         [0120]    Features, integers, characteristics, compounds, chemical moieties or groups described in conjunction with a particular aspect, embodiment or example of the invention are to be understood to be applicable to any other aspect, embodiment or example described herein unless incompatible therewith. 
         [0121]    While the invention has been described in conjunction with a specific best mode, it is to be understood that many alternatives, modifications, and variations will be apparent to those skilled in the art in light of the aforegoing description. Accordingly, it is intended to embrace all such alternatives, modifications, and variations that fall within the scope of the included claims. All matters hithertofore set forth herein or shown in the accompanying drawings are to be interpreted in an illustrative and non-limiting sense.