Abstract:
A method for forming a semiconductor device includes providing a semiconductor substrate; forming a gate dielectric over the semiconductor substrate; forming a gate electrode over the gate dielectric; forming an insulating layer over a sidewall of the gate electrode; defining source and drain regions in the semiconductor substrate adjacent to the insulating layer; implanting a dopant in the source and drain regions of the semiconductor substrate to form doped source and drain regions; forming a sidewall spacer adjacent to the insulating layer; forming a recess in the semiconductor substrate in the source and drain regions, wherein the recess extends directly underneath the spacer a predetermined distance from a channel regions; and forming a stressor material in the recess. The method allows the stressor material to be formed closer to a channel region, thus improving carrier mobility in the channel while not degrading short channel effects.

Description:
BACKGROUND 
       [0001]    1. Field 
         [0002]    This disclosure relates generally to semiconductor devices, and more specifically, to semiconductor devices with embedded stressors. 
         [0003]    2. Related Art 
         [0004]    Embedded stressors have been found to be effective in increasing transistor performance by increasing carrier mobility. The typical process includes forming recesses in the source/drain regions and then filling the recesses with a semiconductor material that acts as a stressor to provide stress to the channel while also being effective as a source/drain. For the channel being silicon, the use of silicon germanium stressors has been found to be effective for the P channel transistors and silicon carbon has been found to be effective for N channel transistors. To increase stress and thereby increase transistor performance, the stressors are desirable adjacent to the channel. Thus a benefit of the stressors being immediately adjacent to the channel is maximizing the stress to the channel. The cost of this benefit can be increased leakage due to loss of short channel control if the stressors continue straight down from the lateral edge of the channel. Thus, a benefit is seen in attempting to provide a stressor that is shallow in the area immediately adjacent to the channel to and is deep in the area where the source/drain contact is formed. This then brings the stressor adjacent to the channel to increase transistor performance while not degrading current leakage. This can be achievable but has been found to add processing complexity. 
         [0005]    Accordingly there is a need to achieve the benefits of embedded stressors while avoiding or reducing the problems associated with making them. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0006]    The present invention is illustrated by way of example and is not limited by the accompanying figures, in which like references indicate similar elements. Elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. 
           [0007]      FIG. 1  is a cross section of a semiconductor device at a stage in processing according to one embodiment; 
           [0008]      FIG. 2  is a cross section of the semiconductor device of  FIG. 1  at a subsequent stage in processing; 
           [0009]      FIG. 3  is a cross section of the semiconductor device of  FIG. 2  at a subsequent stage in processing; 
           [0010]      FIG. 4  is a cross section of the semiconductor device of  FIG. 3  at a subsequent stage in processing; 
           [0011]      FIG. 5  is a cross section of the semiconductor device of  FIG. 4  at a subsequent stage in processing; 
           [0012]      FIG. 6  is a cross section of the semiconductor device of  FIG. 5  at a subsequent stage in processing; and 
           [0013]      FIG. 7  is a cross section of the semiconductor device of  FIG. 6  at a subsequent stage in processing. 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    In one aspect, a source/drain and extension region is formed by implanting into a semiconductor layer using a gate and an offset spacer as a mask. The implant has the affect of altering the etch characteristic of the portion of the semiconductor that receives the implant. The result is that the source/drain and extension region has a different etch characteristic from the semiconductor layer in which it is formed. A sidewall spacer is formed around the gate. A source/drain contact region is recessed with a first etchant using the sidewall spacer as a mask. The source/drain extension region is recessed using a second etchant that etches the extension region selectively over the semiconductor layer due to the different doping concentrations in the two regions. The second etch thus leaves a recess under the sidewall spacer. The recess is then filled with a stressor. This also partially fills the recess of the source/drain region. The recess is then completely filled with stressor material that can be of a different in-situ doping concentration from what filled the source/drain extension region. This is better understood by reference to the drawings and the following description. 
         [0015]    Shown in  FIG. 1  is a semiconductor device  10  comprising a substrate  12 , an insulating layer  14  over substrate  12 , a semiconductor layer  16  over insulating layer  14 , a gate dielectric  18  over a portion of semiconductor layer  16 , gate electrode  20  over gate dielectric  18 , an insulating layer  24  over gate electrode  20 , a sidewall spacer  22  around gate electrode  20 , and source/drain and extension regions  28  and  30  in semiconductor layer  16  formed by an implant  26  using gate electrode  20  as a mask. Substrate  12  is for providing physical support for semiconductor device  10  and may be silicon. Insulating layer  14  provides insulation between substrate  12  and semiconductor layer  16  and may be silicon oxide. Semiconductor layer  16  is for use in forming transistors and potentially other devices and may be silicon. Substrate  12 , insulating layer  14 , and semiconductor layer  16  together form a semiconductor-on-insulator (SOI) substrate. A bulk substrate may be used instead. Gate dielectric  18  is very thin, such as 20 Angstroms, compared to all of the other layers and may be silicon oxide. Gate electrode can be doped polysilicon but could be a stack of materials, especially one or more metal layers and may or may not further include polysilicon. Insulating layer  24  may be silicon nitride. Sidewall spacer  22 , which may be silicon oxide or silicon nitride, is relatively thin for a sidewall spacer, about 100 Angstroms, and is for protecting the sidewall of gate electrode  20  and providing a small offset of the source/drain and extensions  28  and  30  from the edge of gate electrode  20 . Implant  26  can be a conventional source/drain extension implant using arsenic for the case of an N channel transistor. Implant  26 , in this case, is for changing the etch characteristic for source/drain and extension regions  28  and  30  from that of semiconductor layer  16 . That a conventional source/drain extension implant can be used to achieve this is a benefit in that standard processing can be used. In this example of semiconductor layer  16  being silicon, arsenic doping changes the etch characteristic so that arsenic-doped silicon, with the properly selected etchant, can be etched selective to silicon that has not been arsenic doped. Source/drain extensions  28  and  30  are only doped to a depth of about a third of the width of gate electrode  20  which corresponds to the channel length but can be adjusted to optimize the extension recess depth. 
         [0016]    Shown in  FIG. 2  is semiconductor device  10  after forming an insulating layer  32  over gate electrode  20  and source/drain extension regions  26  and  28 . In a typical integrated circuit, transistor of both P and N type would be formed. This example is for N-type but is applicable to P channel transistors as well. Insulating layer  32  would be formed over the P channel transistors as well as shown for the N channel transistor. 
         [0017]    Shown in  FIG. 3  is semiconductor device  10  after forming a sidewall spacer  34  around gate electrode  20 . Sidewall spacer  34 , at the widest point, is about the same as the width of gate electrode  20  but can be thicker or thinner to adjust the final length of a stressor yet to be formed in the source/drain extension portion of regions  28  and  30 . Sidewall spacer may be silicon nitride. 
         [0018]    Shown in  FIG. 4  is semiconductor device  10  after removing insulating layer  32  in the areas not covered by sidewall spacer  34 . For the other type of transistors, which are not shown, insulating layer  32  would not be removed. 
         [0019]    Shown in  FIG. 5  is semiconductor device  10  after etching semiconductor layer  16  using sidewall spacer  34  and insulating layer  24  as a mask to leave a recess  36  for one source/drain contact region and a recess  38  for another source/drain contact region on an opposing side of gate electrode  20  from that of recess  36 . An effective etchant for etching through arsenic-doped silicon and undoped silicon is sulfur hexafluoride with parameters that cause the etch to be anisotropic. For this example of sulfur hexafluoride, the etch is a dry plasma etch using a bias source. Other anisotropic etches may also be used. Nitrogen trifluoride in the presence of chlorine is one example. Another example is hydrogen bromide in the presence of chlorine. Oxygen may also be an effective component of the etch chemistry. In this case undoped silicon means a doping level below 1 e 18 (one times ten to the eighteenth) atoms per cubic centimeter. Doped means in excess of a doping level of greater than 1 e 19 atoms per cubic centimeter. Carrier gases include argon and helium and others may also be effective. It is preferable that the difference between doped and undoped concentrations be at least a factor of about one hundred. Remaining portions of source/drain extension regions  28  and  30  remain under sidewall spacer  34 . 
         [0020]    Shown in  FIG. 6  is semiconductor device  10  after changing the etch conditions to remove the remaining portions of source/drain extension regions  28  and  30  to leave recesses  40  and  42 , respectively. Disabling the bias source is effective in converting the etch to an isotropic etch. With the etch being isotropic, the remaining portions of source/drain extension regions  28  and  30  etch much faster than semiconductor layer  16 . This etch can be more tightly controlled and the selectivity can be somewhat tuned by controlling the concentration of the sulfur hexafluoride. Thus the selective etch of the remaining portions of source/drain extension regions  28  and  30  can be achieved with a change in etch conditions of turning off the bias power and diluting the sulfur hexafluoride with an increase in the inert carrier gas. The etch that forms recesses  36  and  38  is different than the etch that forms recesses  40  and  42  but it can also be viewed as a continuous etch because the two different conditions for the two etches do not require any delay or removal of semiconductor device  10  from the etch chamber. Even a change in the active reactant may not require any delay in the etching. Such a change may not require a change in the etch equipment. 
         [0021]    An alternative is for the second etch to be performed in a tool in which epitaxial growth can be performed. This can be done with minimal change, if any, to currently available equipment because the second etch does not require a bias power. Thus, the isotropic etch can be performed in a chamber that is also capable of performing epitaxial growth. In such case, the subsequent epitaxial growth can be done in situ following the second etch. 
         [0022]    Shown in  FIG. 7  is semiconductor device  10  after epitaxially growing source/drain region  44  in recesses  36  and  40  and source/drain region  46  in recesses  42  and  46 . Source/drain regions  44  and  46  also are stressors that are, for this described case of N-type, formed from silicon carbon that are in situ doped with phosphorus in that they are doped as they are grown. Being silicon carbon, they provide a tensile stress to the channel, the region between source/drain regions  44  and  46  immediately under gate  20 . Semiconductor device  10  of  FIG. 7  is a functional transistor. Subsequent processing would typically include siliciding source/drain regions  44  and  46  and forming overlying interconnect layers. Other fills for different stresses can be silicon germanium (SiGe) or even silicon, germanium, and carbon (SiGeC). 
         [0023]    As an option, the phosphorus doping concentration of source/drain regions  44  and  46  can be changed after the extension recesses  40  and  42  have been filled. In such case the region of recesses  40  and  42  can have a doping concentration selected for the purpose of being source/drain extensions and the region for making contact to source/drain regions  44  and  46  can have a doping concentration selected for the purpose of siliciding and making electrical contact. This staged doping is also applicable for the reverse case of forming a P channel transistor with silicon germanium stressors that are compressive as source/drain regions  44  and  46 . In the case of P channel transistors, source/drain regions  44  and  46  would be in situ boron doped instead of phosphorus. Also for the N channel case, arsenic may be used as the dopant. 
         [0024]    The resulting structure, which requires minimal increased processing complexity, has the desired stressor immediately adjacent to the channel but spaced back at the lower depth. Thus, the current leakage issues that arise from the source/drain being to close at the lower depths does not arise. 
         [0025]    By now it should be appreciated that there has been provided a method for forming a semiconductor device that includes providing a semiconductor substrate, forming a gate dielectric, forming a gate electrode, forming an insulating layer, defining source and drain regions, implanting a dopant, forming a sidewall spacer, forming a recess, and forming a stressor material. The gate dielectric is formed over the semiconductor substrate. The gate electrode is over the gate dielectric. The insulating layer is over the sidewall of the gate electrode. The source and drain regions are in the semiconductor substrate adjacent to the insulating layer. The dopant is in the source and drain regions of the semiconductor substrate to form doped source and drain regions. The sidewall spacer is adjacent to the insulating layer. The recess is in the semiconductor substrate in the source and drain regions. The recess extends directly underneath the spacer a predetermined distance from a channel region. The stressor material is in the recess. The implanting the dopant in the source and drain regions may further comprise implanting one of arsenic, phosphorus, boron, oxygen, carbon, nitrogen, or boron difluoride in the source and drain regions. The forming the recess in the semiconductor substrate may further comprise isotropically etching the doped source and drain regions. The forming the stressor material in the recess may further comprise epitaxially growing silicon carbon (SiC), silicon germanium (SiGe), or a combination of silicon, carbon, or germanium (SiGeC) in the recess. The forming the stressor material in the recess may comprise growing a first stressor layer in the recess and growing a second stressor layer over the first stressor layer. The forming the recess may further comprises performing first and second etch processes. The first etch process removes a first portion of the recess adjacent to the sidewall spacer. The second etch process removes a second portion of the recess directly underneath the sidewall spacer. The second etch process may be performed in-situ with forming the stressor material. The second etch process may be performed using process gases selected from a group comprising HBr, SF 6 , NF 3 , Cl 2 , O 2 , Ar, and He. A thickness of the insulating layer may determine the predetermined distance. The forming the stressor material may further comprise providing one of either a tensile stress or a compressive stress on the channel region of the semiconductor device. The forming the stressor material in the recess may comprise growing first and second stressor layers. The first stressor layer is formed with a first in-situ doping concentration in the recess. The second stressor layer is formed with a second in-situ doping concentration over the first stressor layer. The forming a stressor material may further comprise growing the stressor material on exposed silicon in the recess. The forming a stressor material may further comprise growing the stressor material over the semiconductor device, selectively etching the stressor material from the gate electrode and the spacer, and repeating the growing and etching a predetermined number of times. 
         [0026]    Also there is described a method for forming a semiconductor device. A semiconductor substrate is provided. A gate dielectric is formed over the semiconductor substrate. A gate electrode is formed over the gate dielectric. A capping layer is formed over the gate electrode. An offset spacer is formed over a sidewall of the gate electrode and the capping layer. A dopant in source and drain regions of the semiconductor substrate is implanted adjacent to the offset spacer to form doped source and drain regions. A sidewall spacer is formed adjacent to the offset spacer. A recess is formed in the semiconductor substrate in the source and drain regions. The recess extends directly underneath the sidewall spacer a predetermined distance from a channel region, the predetermined distance determined by a thickness of the offset spacer. A stressor material is formed in the recess. The forming the stressor material may further comprise forming one of either a first stressor material or a second stressor material in the recess, the first stressor material comprising silicon carbon (SiC) for providing tensile stress for the channel region being of an N-channel transistor, and the second stressor material comprising silicon germanium (SiGe) for providing compressive stress for the channel region being of a P-channel transistor. The forming the recess may further comprise performing a first etch process to remove a first portion of the recess adjacent to the sidewall spacer and performing a second etch process to remove a second portion of the recess directly underneath the sidewall spacer. The second etch may be performed in-situ with forming the stressor material. The second etch may be performed using process gases selected from a group comprising HBr, SF 6 , NF 3 , Cl 2 , O 2 , Ar, and He. 
         [0027]    Yet also described is a method for forming a semiconductor device. A semiconductor substrate is provided. A gate dielectric is formed over the semiconductor substrate. A gate electrode is formed over the gate dielectric. A capping layer is formed over the gate electrode. An offset spacer is formed over a sidewall of the gate electrode and the capping layer. A dopant is implanted in source and drain regions of the semiconductor substrate adjacent to the offset spacer to form doped source and drain regions. A sidewall spacer is formed adjacent to the offset spacer. A recess is formed in the semiconductor substrate in the source and drain regions using a first etch process and a second etch process. The recess extends directly underneath the sidewall spacer a predetermined distance from a channel region. The predetermined distance is determined by a thickness of the offset spacer. The first etch process is for removing a first portion of the recess adjacent to the sidewall spacer and the second etch process is for removing a second portion of the recess directly underneath the sidewall spacer. The forming the stressor material may further comprise providing one of either a tensile stress or a compressive stress on a channel region of the semiconductor device. The second etch process may be performed in-situ with forming the stressor material. The second etch process may be performed using process gases selected from a group comprising HBr, SF 6 , NF 3 , Cl 2 , O 2 , Ar, and He. 
         [0028]    Moreover, the terms “front,” “back,” “top,” “bottom,” “over,” “under” and the like in the description and in the claims, if any, are used for descriptive purposes and not necessarily for describing permanent relative positions. It is understood that the terms so used are interchangeable under appropriate circumstances such that the embodiments of the invention described herein are, for example, capable of operation in other orientations than those illustrated or otherwise described herein. 
         [0029]    Although the invention is described herein with reference to specific embodiments, various modifications and changes can be made without departing from the scope of the present invention as set forth in the claims below. For example, the implant for changing the etch characteristic is described as being also useful as a source/drain dopant that is not necessarily required. Because the area receiving the implant is subsequently removed, it may be preferably to implant with a different element that causes a change in the etch characteristic. Other examples to consider for this include but are not limited to phosphorus, boron, oxygen, carbon, nitrogen, and boron difluoride. It may also be preferable for the purpose of etch optimization for the implant to be self-amorphizing and to then leave the implanted region in an amorphous state prior to performing the recess etch. Accordingly, the specification and figures are to be regarded in an illustrative rather than a restrictive sense, and all such modifications are intended to be included within the scope of the present invention. Any benefits, advantages, or solutions to problems that are described herein with regard to specific embodiments are not intended to be construed as a critical, required, or essential feature or element of any or all the claims. 
         [0030]    Furthermore, the terms “a” or “an,” as used herein, are defined as one or more than one. Also, the use of introductory phrases such as “at least one” and “one or more” in the claims should not be construed to imply that the introduction of another claim element by the indefinite articles “a” or “an” limits any particular claim containing such introduced claim element to inventions containing only one such element, even when the same claim includes the introductory phrases “one or more” or “at least one” and indefinite articles such as “a” or “an.” The same holds true for the use of definite articles. 
         [0031]    Unless stated otherwise, terms such as “first” and “second” are used to arbitrarily distinguish between the elements such terms describe. Thus, these terms are not necessarily intended to indicate temporal or other prioritization of such elements.