Abstract:
The present invention is directed to a transistor with an asymmetric silicon germanium source region, and various methods of making same. In one illustrative embodiment, the transistor includes a gate electrode formed above a semiconducting substrate comprised of silicon, a doped source region comprising a region of epitaxially grown silicon that is doped with germanium formed in the semiconducting substrate and a doped drain region formed in the semiconducting substrate.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This is a divisional of application Ser. No. 11/278,618, filed Apr. 4, 2006 now U.S. Pat. No. 8,035,098. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention is generally directed to the field of integrated circuit devices, and, more particularly, to a transistor with an asymmetric silicon germanium source region, and various methods of making same. 
     2. Description of the Related Art 
     The manufacturing of semiconductor devices may involve many process steps. For example, semiconductor fabrication typically involves processes such as deposition processes, etching processes, thermal growth processes, various heat treatment processes, ion implantation, photolithography, etc. Such processes may be performed in any of a variety of different combinations to produce semiconductor devices that are useful in a wide variety of applications. 
     In general, there is a constant drive within the semiconductor industry to increase the operating speed and efficiency of various integrated circuit devices, e.g., microprocessors, memory devices, and the like. This drive is fueled by consumer demands for computers and electronic devices that operate at increasingly greater speeds and efficiency. This demand for increased speed and efficiency has resulted in a continual reduction in the size of semiconductor devices, e.g., transistors, as well as the packing density of such devices on an integrated circuit device. That is, many components of a typical field effect transistor (FET), e.g., channel length, junction depths, gate insulation thickness, and the like, are reduced. For example, all other things being equal, the smaller the channel length of the transistor or the thinner the gate insulation layer, the faster the transistor will operate. Thus, there is a constant drive to reduce the size, or scale, of the components of a typical transistor to increase the overall speed of the transistor, as well as integrated circuit devices incorporating such transistors. 
     Modern field effect transistors comprise a gate electrode, a gate insulation layer, a source region and a drain region. When an appropriate voltage is applied to the gate electrode, a channel region is formed between the source region and the drain region and electrons (or holes) flow between the source region and drain region. The source and drain regions of such transistors are normally the same. For example, for an NMOS transistor, both the source and drain regions are formed by introducing an N-type dopant material, e.g., arsenic, into the semiconductor material. For a PMOS transistor, the source and drain regions are formed by introducing a P-type dopant material, e.g., boron, into the semiconductor material. In some cases, there may be a difference in the dopant concentration of the source and drain regions. 
     Such transistors with symmetric source and drain regions are sufficient for many applications, as evidenced by their widespread use within the industry. However, in some applications, it may be desirable to make transistors that depart from this traditional structure to enhance the performance of the device. 
     The present invention is directed to various methods and systems that may solve, or at least reduce, some or all of the aforementioned problems. 
     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 invention is directed to a transistor with an asymmetric silicon germanium source region, and various methods of making same. In one illustrative embodiment, the transistor includes a gate electrode formed above a semiconducting substrate comprised of silicon, a doped source region comprising a region of epitaxially grown silicon that is doped with germanium formed in the semiconducting substrate and a doped drain region formed in the semiconducting substrate. 
     In another illustrative embodiment, the transistor comprises a gate electrode formed above a semiconducting substrate comprised of silicon, a doped source region comprising a region of epitaxially grown silicon that comprises approximately 10-25% germanium formed in the semiconducting substrate and a doped drain region formed in the semiconducting substrate. 
     In one illustrative embodiment, the method comprises forming a gate electrode above a semiconducting substrate, etching a trench into the semiconducting substrate between one side of the gate electrode and an isolation structure, forming a silicon germanium structure in the trench and forming a doped source region and a doped drain region, the doped source region being formed at least partially in the silicon germanium structure, the doped drain region being formed in the semiconducting substrate. 
     In another illustrative embodiment, the method comprises forming a gate electrode above a semiconducting substrate, etching a trench into the semiconducting substrate between one side of the gate electrode and an isolation structure, epitaxially growing a layer of silicon in the trench while introducing germanium during the epitaxial growth process to thereby form a silicon germanium structure in the trench, and forming a doped source region and a doped drain region, the doped source region being formed at least partially in the silicon germanium structure, the doped drain region being formed in the semiconducting substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention 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: 
         FIG. 1  is a cross-sectional side view of an illustrative embodiment of a transistor in accordance with one aspect of the present invention; 
         FIG. 2  is a cross-sectional side view depicting an early stage of manufacture of an illustrative transistor in accordance with one illustrative embodiment of the present invention; 
         FIG. 3  is a view of the device depicted in  FIG. 2  having a trench formed therein for the source region of the illustrative transistor depicted herein; 
         FIG. 4  is a view of the device depicted in  FIG. 3  after a layer of epitaxially grown silicon is formed in the trench for the source region; 
         FIG. 5  is a view of the device depicted in  FIG. 4  wherein dopant materials are implanted to form the LDD regions on the source and drain regions of the illustrative transistor depicted herein; 
         FIG. 6  is a view of the device depicted in  FIG. 5  wherein a source/drain implant procedure is performed to form the source and drain regions of the illustrative transistor depicted herein; and 
         FIG. 7  is an illustrative cross-sectional view of the transistor described herein. 
     
    
    
     While the invention 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 OF THE INVENTION 
     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&#39; 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 invention will now be described with reference to the attached figures. Various structures and regions are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. The relative sizes of the various structures and regions depicted in the drawings may be exaggerated for purposes of explanation. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. 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. 
       FIG. 1  depicts one illustrative embodiment of the transistor  10  disclosed herein. As shown therein, the transistor  10  comprises a gate insulation layer  11 , a gate electrode  15 , at least one sidewall spacer  19 , a source region  12  and a drain region  14 . In the illustrative embodiment depicted in  FIG. 1 , the transistor  10  is formed in a silicon-on-insulator (SOI) substrate  16  comprised of a bulk substrate  16 A, a buried insulation layer  16 B (sometimes referred to as a “buried oxide layer” or “BOX” layer), and an active layer  16 C. A trench isolation structure  18  may be employed to electrically isolate the transistor  10  from other semiconductor devices. Also depicted in  FIG. 1  are illustrative metal silicide regions  17  and illustrative conductive contacts  20  that are positioned in a layer of insulating material  22 . The conductive contacts  20  are conductively coupled to the source region  12  and the drain region  14 , as depicted in  FIG. 1 . A conductive contact is also formed to the gate electrode  15 , although that conductive contact is not depicted in  FIG. 1 . 
     The gate insulation layer  11 , the gate electrode  15 , the sidewall spacers  19 , and the metal silicide regions  17  may all be formed using known techniques and materials. For example, the gate insulation layer  11  may be a thermally grown layer of silicon dioxide. The gate electrode  15  may be a doped layer of polysilicon that is formed by traditional deposition, doping and etching processes. Similarly, the sidewall spacer  19  may comprise a material such as, for example, silicon nitride, and it may be formed by conformally depositing a layer of spacer material and performing an anisotropic etching process. The metal silicide regions  17  may be comprised of, for example, cobalt silicide, and they may be formed using traditional techniques. 
     In one illustrative embodiment, the source region  12  of the transistor  10  is comprised of an epitaxially grown layer of silicon having a concentration of germanium ranging from approximately 10-25%. The germanium may be introduced into the layer of epitaxially grown silicon by performing an in situ doping process that is performed as the layer of epitaxial silicon is grown. After a complete reading of the present application, those skilled in the art will appreciate that the present invention has broad application. For example, the present invention may be employed in connection with the formation of NMOS or PMOS transistors. For convenience, the present invention will be disclosed in the context where an illustrative NMOS transistor  10  is formed. However, the present invention is not limited to the formation of such illustrative devices. 
       FIGS. 2-7  depict one illustrative process flow for forming the illustrative transistor  10  depicted herein. As shown in  FIG. 2 , the isolation region  18  may be formed in the active layer  16 C by performing known etching and deposition techniques.  FIG. 2  depicts the transistor  10  at the point of manufacture wherein the gate insulation layer  11  and the gate electrode  15  have been formed in accordance with known techniques. The sidewall spacer  19  may be comprised of a variety of materials and may be formed using a variety of known techniques. For example, the spacer  19  may be formed by conformally depositing a layer of spacer material, e.g., silicon dioxide, silicon nitride, and thereafter performing an anisotropic etching process. In one illustrative process flow, the sidewall spacer  19  is employed to protect the gate electrode  11  during a subsequent etching process performed in forming the source region  12 , as described more fully below. The sidewall spacer  19  may be sacrificial or permanent as described more fully below. 
       FIG. 3  depicts the device shown in  FIG. 2  after an etching process  29  is performed to form a trench  30  in the active layer  16 C between the isolation structure  18  and the sidewall spacer  19 . In some cases, the spacer  19  may or may not be present. Thus, when it is stated that the trench  30  is formed between the isolation structure  18  and the gate electrode  15 , it is to be understood that the gate electrode structure may or may not have the spacer  19  formed adjacent thereto. A masking layer  31 , e.g., photoresist, is employed during the etching process  29  to protect the remainder of the substrate  16 . The sidewall spacer  19  protects the gate electrode  11  during the etching process  29 . In the particular embodiment depicted herein, the trench  30  is self-aligned with respect to the sidewall spacer  19 . In one illustrative embodiment, the trench  30  does not extend all the way to the buried insulation layer  16 B so that the remaining portions of the active layer  16 C in the trench  30  can serve as a seed layer for the subsequent epitaxial growth of silicon in the trench  30 , as described more fully below. In some cases, the trench  30  may have a depth or thickness of approximately 200-800 Å. 
     Next, as shown in  FIG. 4 , in one illustrative embodiment, a layer of germanium-doped epitaxial silicon  32  (“eSi-Ge”) is grown in the trench  30 . The epitaxial silicon  32  may be grown using known processing techniques and known epi-deposition tools. A hard mask layer  37  is formed above the substrate  16  during the epitaxial growth process. The hard mask material may be comprised of the same materials as the spacer  19 . The material selected for the hard mask layer  37  must be able to withstand the processing conditions during the growth of the epitaxial layer of silicon  32  and still perform the necessary masking function. 
     In accordance with one aspect of the present invention, germanium is introduced into the epitaxial layer of silicon  32  by introducing germanium during the epitaxial growth process. The concentration of the germanium may vary depending upon the particular application. For example, the concentration of germanium in the final source region  12  may comprise approximately 10-25%. The germanium in the layer  32  may act to reduce the effective bandgap of the silicon, thereby improving device performance. 
     After the silicon-germanium epitaxial layer of silicon is formed, standard processing techniques are employed to complete the manufacture of the transistor  10 , e.g., LDD and source/drain implants may be performed to complete the formation of the source region  12  and drain region  14  of the transistor  10 . As shown in  FIG. 5 , a new masking layer  37  may be formed above the substrate  16  and thereby exposes the area where the source region  12  and drain region  14  are to be formed. An LDD ion implant process  33  is performed to introduce an N-type dopant material, e.g., arsenic, into the silicon germanium layer  32  and the portion  42  of the active layer  16 C wherein the drain region  14  will be formed. This LDD implant  33  results in the formation of LDD regions  45  that are self-aligned with respect to the gate electrode  15 . Illustrative N-type dopant materials that are introduced in the LDD implant process  33  include, for example, arsenic, phosphorus, etc. The LDD implant process  33  may be performed at a dopant dose and an energy level appropriate for the device under construction. Next, as shown in  FIG. 6 , a so-called source/drain ion implant process  35  is performed to introduce a relatively high concentration of an N-type dopant material, such as, e.g., arsenic or phosphorous, at a dopant dose and energy level sufficient for the intended application. In the embodiment shown in  FIG. 6 , a new spacer  41  has been added prior to performing the source/drain implant process  35 . 
     Prior to performing the LDD implant process  33 , the spacer  19  may or may not be removed. If it is removed, at least one new spacer (not shown) may be formed adjacent the gate electrode  15 . For example, such a spacer may be employed prior to performing the LDD implant  33  on a PMOS transistor. It should be understood that one or more spacers may be formed prior to or during the various ion implant processes performed to form the source region  12  and the drain region  14 . 
     Thereafter, known processing techniques may be employed to complete the formation of the transistor  10 , as shown in  FIG. 7 . For example, one or more heat treatment processes may be performed to activate the implanted dopant material and repair any damage to the lattice structure. If desired, metal silicide regions  17  may be formed on the source region  12 , the drain region  14  and the gate electrode  15  as shown in  FIG. 1  using known techniques. It should be noted that the transistor depicted in  FIG. 8  is depicted after various heat treatments have been performed to thereby cause the implanted dopant material to migrate somewhat under the gate electrode  15 . 
     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.