Patent Publication Number: US-2019172920-A1

Title: Junctionless transistor device and method for preparing the same

Description:
PRIORITY CLAIM AND CROSS-REFERENCE 
     This application claims the benefit of provisional application Ser. 62/595,748 filed on Dec. 6, 2017, entitled “JUNCTIONLESS TRANSISTOR DEVICE AND METHOD FOR PREPARING THE SAME” the disclosure of which is hereby incorporated by reference in its entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to a junctionless transistor device and method for preparing the same, and more particularly, to a gate-all-around (GAA) junctionless transistor device with a vertical channel and method for preparing the same. 
     DISCUSSION OF THE BACKGROUND 
     A conventional Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) transistor device is a four terminal device, which includes a gate terminal, a source terminal, a drain terminal and a bulk (substrate) terminal. The source/drain (S/D) and the channel of the MOSFET transistor device have opposite doping types. Accordingly, a depletion region can generate in the interface between the S/D and the channel. When the MOSFET transistor device is scaled down, the depletion region will “punch,” leading to high leakage current, worse subthreshold swing, and Drain Induced Barrier Lowering (DIBL) effect. In other words, short channel effect (SCE) becomes severe. In addition, another depletion region may also exist in the interface between the S/D and the bulk. 
     In a conventional fabrication process, an annealing process is required to repair the defect that appears subsequent to a high-energy implantation process for forming the S/D. The junction depth and the doping concentration gradient, however, are affected by the annealing process. This may deteriorate the diode (between the S/D and the channel or between the S/D and the bulk) properties, which may increase leakage current. 
     This Discussion of the Background section is for background information only. The statements in this Discussion of the Background are not an admission that the subject matter disclosed in this section constitutes a prior art to the present disclosure, and no structure of this section may be used as an admission that any structure of this application, including this Discussion of the Background section, constitutes prior art to the present disclosure. 
     SUMMARY 
     One aspect of the present disclosure provides a junctionless transistor device including a semiconductor substrate, a channel, a first source/drain, a second source/drain, a gate and a gate dielectric layer. The semiconductor substrate has a surface. The channel is disposed over the semiconductor substrate. The channel includes a first channel extending in a lateral direction substantially parallel to the surface of the semiconductor substrate, and a second channel extending in a vertical direction substantially perpendicular to the surface of the semiconductor substrate. The first channel and the second channel are in contact at one end, and the channel has a first doping type. The first source/drain is disposed over the semiconductor substrate, and is in contact with the first channel, wherein the first source/drain has the first doping type. The second source/drain is disposed over the second channel, and is in contact with the second channel. The second source/drain has the first doping type. The gate is disposed over, an upper surface of the first channel and side surfaces of the second channel, and the gate has a second doping type opposite to the first doping type. The gate dielectric layer is disposed between the gate and the channel. 
     In some embodiments, a doping concentration of the first source/drain, a doping concentration of the second source/drain and a to doping concentration of the channel are substantially the same. 
     In some embodiments, a doping concentration of the gate is higher than a doping concentration of the channel. 
     In some embodiments, the semiconductor substrate includes a doped well under the channel, and the doped well has the second doping type. 
     In some embodiments, a doping concentration of the doped well is lower than a doping concentration of the Channel. 
     In some embodiments, the junctionless transistor device further includes a first electrical contact electrically connected to the first source/drain, and a second electrical contact electrically connected to the second source/drain. 
     In some embodiments, the gate surrounds a plurality of side surfaces of the second channel. 
     Another aspect of the present disclosure provides a method for preparing a junctionless transistor device. The method includes the following steps. A semiconductor substrate is provided. A semiconductor doped structure is formed over the semiconductor substrate. The semiconductor doped structure has a first doping type. The semiconductor doped structure includes a first doped structure extending in a lateral direction substantially parallel to a surface of the semiconductor substrate, and a second doped structure extending in a vertical direction substantially perpendicular to the surface of the semiconductor substrate. A gate dielectric layer and a gate are formed over the semiconductor doped structure. The gate has a second doping type opposite to the first doping type. 
     In some embodiments, the forming the semiconductor doped structure over the semiconductor substrate includes forming a doped region having the first doping type in the semiconductor substrate, and patterning the doped region to form the semiconductor doped structure. 
     In some embodiments, the gate dielectric layer and the gate cover an upper surface of the first doped structure and side surfaces of the second doped structure, and expose an upper surface of the second doped structure. 
     In some embodiments, the forming the gate dielectric layer and the gate over the semiconductor doped structure includes forming the gate dielectric layer over the semiconductor doped structure; forming the gate over the gate dielectric layer; doping the gate; and partially removing the gate and the gate dielectric layer to expose the upper surface of the second doped structure. 
     In some embodiments, the forming the gate dielectric layer over the semiconductor doped structure comprises thermally growing a thermal oxide layer over the semiconductor doped structure. 
     In some embodiments, a doping concentration of the first doped structure and a doping concentration of the second doped structure are substantially the same. 
     In some embodiments, a doping concentration of the gate is higher than a doping concentration of the semiconductor doped structure. 
     In some embodiments, an end of the first doped structure is configured as a first source/drain, an end of the second doped structure is configured as a second source/drain, and the first doped structure and the second doped structure are configured as a channel. 
     In some embodiments of the present disclosure, the junctionless transistor device is a gate-all-around junctionless field effect transistor (GAAJLFET) device with a vertical channel structure. The GAAJLFET device is advantageous for its lower thermal budget, lower leakage current, greater on/off ratio and greater swing. The vertical channel structure can conserve layout area, and can be integrated into various electronic device fabrications such as DRAM fabrication. 
     In contrast, an FET transistor device with PN junction faces short channel effect (SCE) when scaling down, and the SCE may result in high leakage current, worse subthreshold swing and Drain Induced Barrier Lowering (DIBL) effect. 
     The foregoing has outlined rather broadly the features and technical advantages of the present disclosure in order that the detailed description of the disclosure that follows may be better understood. Additional features and technical advantages of the disclosure are described hereinafter, and form the subject of the claims of the disclosure. It should be appreciated by those skilled in the art that the concepts and specific embodiments disclosed may be utilized as a basis for modifying or designing other structures, or processes, for carrying out the purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit or scope of the disclosure as set forth in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present disclosure may be derived by referring to the detailed description and claims. The disclosure should also be understood to be connected to the figures&#39; reference numbers, which refer to similar elements throughout the description, and: 
         FIG. 1  is a flow diagram illustrating a method for preparing a junctionless transistor device, in accordance with some embodiments is of the present disclosure; 
         FIG. 2A ,  FIG. 2B ,  FIG. 2C ,  FIG. 2D ,  FIG. 2E  and  FIG. 2F  are schematic diagrams at one or more of various steps of preparing a junctionless transistor device, in accordance with some embodiments of the present disclosure; 
         FIG. 3A  is a perspective view of a junctionless transistor device, in accordance with some embodiments of the present disclosure; 
         FIG. 3B  is a cross-sectional view of a junctionless transistor device along a line A-A of  FIG. 3A , in accordance with some embodiments of the present disclosure; 
         FIG. 3C  is a top view of a junctionless transistor device, in accordance with some embodiments of the present disclosure; and 
         FIG. 4  is a plot illustrating an Id-Vg curve of a functionless transistor device when Vd=1V, in accordance with some embodiments of the present disclosure, 
     
    
    
     DETAILED DESCRIPTION 
     Embodiments, or examples, of the disclosure illustrated in the drawings are now described using specific language. It shall be understood that no limitation of the scope of the disclosure is hereby intended. Any alteration or modification of the described embodiments, and any further applications of principles described in this document, are to be considered as normally occurring to one of ordinary skill in the art to which the disclosure relates. Reference numerals may be repeated throughout the embodiments, but this does not necessarily mean that feature(s) of one embodiment apply to another embodiment, is even if they share the same reference numeral. 
     It shall be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers or sections, these elements, components, regions, layers or sections are not limited by these terms. Rather, these terms are merely used to distinguish one element, component, region, layer or section from another region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present inventive concept. 
     The terminology used herein is for the purpose of describing particular example embodiments only and is not intended to be limited to the present inventive concept. As used herein, the singular forms “a,” “an” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It shall be further understood that the terms “comprises” and “comprising,” when used in this specification, point out the presence of stated features, integers, steps, operations, elements, or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, or groups thereof. 
     As used herein, the term “junctionless transistor device” refers to a transistor device in which the source/drain and the channel have the same doping type. 
     As used herein, the terms “patterning” and “patterned” are used in the present disclosure to describe an operation of forming a predetermined pattern on a surface. The patterning operation includes various steps and processes and varies in accordance with different embodiments. In some embodiments, a patterning process is adopted to pattern an existing film or layer. The patterning process includes forming a mask on the existing film or layer and removing the unmasked film or layer with an etching or other removal process. The mask can be a photoresist, or a hard mask. In some embodiments, a patterning process is adopted to form a patterned layer directly on a surface. The patterning process includes forming a photosensitive film on the surface, conducting a photolithography process including an exposure process and a developing process, and performing an etching process. 
       FIG. 1  is a flow diagram illustrating a method for preparing a junctionless transistor device, in accordance with some embodiments of the present disclosure. As shown in  FIG. 1 , the method  100  for preparing a junctionless transistor device begins with a step  110  in which a semiconductor substrate is provided. The method  100  for preparing a junctionless transistor device proceeds with a step  120  in which a semiconductor doped structure is formed over the semiconductor substrate. The semiconductor doped structure has a first doping type, the semiconductor doped structure includes a first doped structure extending in a lateral direction substantially parallel to a surface of the semiconductor substrate, and a second doped structure extending in a vertical direction substantially perpendicular to the surface of the semiconductor substrate. The method  100  for preparing a junctionless transistor device continues with a step  130  in which a gate dielectric layer and a gate are formed over the semiconductor doped structure. The gate has a second doping type opposite to the first doping type. 
     The method  100  is merely an example, and is not intended to limit the present disclosure beyond what is explicitly recited in the claims. Additional steps can be provided before, during, and after the method  100 , and some operations described can be replaced, eliminated, or moved around for additional embodiments of the method. 
       FIG. 2A ,  FIG. 2B ,  FIG. 2C ,  FIG. 2D ,  FIG. 2E  and  FIG. 2F  are schematic diagrams at one or more of various steps of preparing a junctionless transistor device, in accordance with some embodiments of the present disclosure. As shown in  FIG. 2A , a substrate  10  is provided. The substrate  10  may include a semiconductor substrate. By way of examples, the material of the substrate  10  may include elementary semiconductor such as silicon or germanium; compound semiconductor such as silicon germanium, silicon carbide, gallium arsenic, gallium phosphide, indium phosphide or indium arsenide; combinations thereof; or other suitable material. 
     As shown in  FIG. 2B , a doped region  20  is formed in the semiconductor substrate  10 . In some embodiments, the doped region  20  has a first doping type, while the semiconductor substrate  10  has a second doping type opposite to the first doping type. By way of example, the first doping type may be N type, while the second doping type is P type. In some embodiments, the doped region  20  may be formed by an implantation process, or other suitable doping technique. In some embodiments, the depth of the doped region  20  may be controlled by adjusting the doping energy of the implantation process. In some embodiments, a portion of the semiconductor substrate  10  may be a doped well (not shown) having the second doping type, and the doped well may be in contact with the doped region  20 . 
     As shown in  FIG. 2G , the doped region  20  is patterned to form a semiconductor doped structure  30 . In some embodiments, the doped region  20  is patterned by photolithography and etching techniques. For example, a sacrificial layer  22  is formed over the doped region  20 . The sacrificial layer  22  may include a hard mask layer such as a silicon nitride layer, which partially exposes the doped region  20 . Next, the doped region  20  exposed through the sacrificial layer  22  is etched to form the semiconductor doped structure  30 . 
     In some embodiments, the semiconductor doped structure  30  includes a first doped structure  32  and a second doped structure  34  connected to each other. The first doped structure  32  is lying over the semiconductor substrate  10 , and extends in a lateral direction L 1  substantially parallel to a surface  10 S of the semiconductor substrate  10 . The second doped structure  34  extends in a vertical direction L 2  substantially perpendicular to the surface  10 S of the semiconductor substrate  10 . By way of example, the semiconductor doped structure  30  is an L-shaped structure, or a T-shaped structure. 
     In some embodiments, the first doped structure  32  and the second doped structure  34  are in contact with each other. An end of the first doped structure  32  may be configured as a first source/drain, and an end of the second doped structure  34  may be configured as a second source/drain. The first doped structure  32  and the second doped in structure  34  may be configured as a channel. Since the first doped structure  32  and the second doped structure  34  are formed from the doped region  20 , the doping concentration of the first doped structure  32  and the doping concentration of the second doped structure  34  are substantially the same. Accordingly, the interface between the channel and the first source/drain and the interface between the channel and the second source/drain are functionless. In addition, an extra implantation process and an annealing process for doping the first source/drain and the second source/drain may be omitted. Accordingly, thermal budget can be reduced. After the semiconductor doped structure  30  is formed, the sacrificial layer  22  is removed. 
     A gate dielectric layer and a gate are formed over the semiconductor doped structure. In some embodiments, the gate dielectric layer and the gate may be formed by the following operations. As shown in  FIG. 2D , a gate dielectric layer  40  is formed over the semiconductor doped structure  30 . The gate dielectric layer  40  may include a. thermal oxide layer such as a silicon oxide layer formed by thermal growing. In some embodiments, the gate dielectric layer  40  may be formed by thermal oxidization, but the disclosure is not limited thereto. 
     As shown in  FIG. 2E , a gate  50  is formed over the gate dielectric layer  40 . In some embodiments, the material of the gate  50  may include a semiconductor material such as polycrystalline silicon. The gate  50  has a second doping type opposite to the first doping type of the semiconductor structure  30 . In some embodiments, the gate  50  is doped by an implantation process. In some embodiments, the doping concentration of the gate  50  is higher than the doping concentration of the semiconductor doped structure  30 . 
     The gate  50  and the gate dielectric layer  40  are then partially removed to expose an upper surface  34 U of the second doped structure  34 , while side surfaces  34 S of the second doped structure  34  and an upper surface  32 U of the first doped structure  32  are covered by the gate dielectric layer  40  and the gate  50 . In some embodiments, the gate  50  and the gate dielectric layer  40  may cover two surfaces  34 S of the second doped structure  34 , or may surround the surfaces  34 S of the second doped structure  34 . In some embodiments, the gate  50  and the gate dielectric layer  40  may be partially removed by photolithography and etching techniques. 
     As shown in  FIG. 2F , at least one dielectric layer  60  may be formed over the semiconductor substrate  10 . In some embodiments, a first electrical contact  62  and a second electrical contact  64  may be formed to fabricate a junctionless transistor device  1  of some embodiments of the present disclosure. The first electrical contact  62  may penetrate through the at least one dielectric layer  60  and be electrically connected to a portion of the first doped structure  32  (i.e., the first source/drain). The second electrical contact  64  may penetrate through the at least one dielectric layer  60  and be electrically connected to a portion of the second doped structure  34  (i.e., the second source/drain). 
       FIG. 3A ,  FIG. 3B  and  FIG. 3C  are schematic diagrams of a junctionless transistor device, in accordance with some embodiments of the present disclosure, wherein  FIG. 3A  is a perspective view of a junctionless transistor device,  FIG. 3B  is a cross-sectional view of a junctionless transistor device along a line A-A of  FIG. 3A , and  FIG. 3C  is a top view of a junctionless transistor device. As shown in  FIG. 3A ,  FIG. 3B  and  FIG. 3C , the junctionless transistor device  2  includes a semiconductor substrate  10 , a channel  70 , a first source/drain  76 , a second source/drain  78 , a gate  50  and a gate dielectric layer  40 . In some embodiments, the semiconductor substrate  10  my have a second doping type such as P type. In some embodiments, the semiconductor is substrate  10  may further include a doped well  12  under the channel  70 . In some embodiments, the doped well  12  may have the second doping type such as P type. In some embodiments, the doping concentration of the doped well  12  may be higher than the doping concentration of the semiconductor substrate  10 , and lower than the doping concentration of the channel  70 . In some exemplary embodiments, the doping concentration of the doped well  12  is about 1.375*10 17  atoms/cm 3 , but the present disclosure is not limited thereto. In some embodiments, the junctionless transistor device  2  may further include an isolation structure  14  such as a shallow trench isolation (STI) in the semiconductor substrate  10 . 
     The channel  70  is disposed over the semiconductor substrate  10 . In some embodiments, the channel  70  includes a first channel  72  and a second channel  74 . The first channel  72  may extend in a lateral direction L 1  substantially parallel to a surface  10 S of the semiconductor substrate  10 . The second channel  74  may extend in a vertical direction L 2  substantially perpendicular to the surface  10 S of the semiconductor substrate  10 . The first channel  72  and the second channel  74  are in contact at one end, and the channel  70  has a first doping type such as N type. In some exemplary embodiments, the thickness of the channel  70 , e.g., the thickness of the second channel  74 , is about 5 nm, but the present disclosure is not limited thereto. In some exemplary embodiments, the length of the channel  70 , e.g., the length of the first channel  72  plus the length of the second channel  74 , is about 100 nm, but the present disclosure is not limited thereto. 
     In some embodiments, the first source/drain  76  is disposed over the semiconductor substrate, and is in contact with the first channel  72 . In some embodiments, the second source/drain  78  is disposed over the second channel  74 , and is in contact with the second channel  74 . The first source/drain  76  and the second source/drain  78  have the same first doping type as the channel  70  such that a functionless interface is present between the first source/drain  76  and the first channel  72  and between the second source/drain  78  and the second channel  74 . In some embodiments, the doping concentration of the first source/drain  76 , the doping concentration of the second source/drain  78  and the doping concentration of the channel  70  are substantially the same. In some exemplary embodiments, the doping concentration of the first source/drain  76 , the second source/drain  78  and the channel  70  is about 1*10 19  atoms/cm 3 , but the present disclosure is not limited thereto. 
     The gate  50  is disposed over an upper surface  72 U of the first channel  72  and side surfaces  74 S of the second channel  74 . The gate  50  may expose an upper surface  74 U of the second channel  74 . In some embodiments, the gate  50  has the second doping type such as P type opposite to the first doping type. In some embodiments, the doping concentration of the gate  50  is higher than the doping concentration of the channel  70 . In some exemplary embodiments, the doping concentration of the gate  50  is about 1*10 20  atoms/cm 3 , but the present disclosure is not limited thereto. 
     The gate dielectric layer  40  is disposed between the gate  50  and the channel  70 . In some embodiments, the gate dielectric layer  40  may include an oxide layer such as a silicon oxide layer. In some exemplary embodiments, the thickness of the gate dielectric layer  40  may be about 1 nm, but the present disclosure is not limited thereto. The doping concentrations of the gate  50 , the channel  70 , the first source/drain  76  and the second source/drain  78 , the thickness of the gate dielectric layer  40 , and the thickness of the channel  70  may be configured to adjust the location of the depletion region, such that the junctionless transistor device  2  is in a “normally off” state, i.e., the threshold voltage (Vt) is a positive value. 
     In some embodiments, the junctionless transistor device  2  may further include a first electrical contact  62  electrically connected to the first source/drain  76 , and a second electrical contact  64  electrically connected to the second source/drain  78  for applying or receiving voltage. In some embodiments, the junctionless transistor device  2  may further include another electrical contact (not shown) electrically connected to the gate  50  for applying voltage to the gate  50 . 
     In some embodiments, the gate  50  may surround the side surfaces  74 S of the second channel  74  to form a gate-all-around (GA) structure. Accordingly, a channel width of the channel  70  is substantially equal to the perimeter of the second channel  74 . For example, the second channel  74  may have a cuboid structure, and all four side surfaces  74 S may be surrounded by the gate  50  as shown in  FIG. 3C . 
       FIG. 4  is a plot illustrating an Id-Vg curve of a junctionless transistor device when Vd=1V, in accordance with some embodiments of the present disclosure. As shown in  FIG. 4 , the drain current (Id) of the junctionless transistor device is greater when a lower gate voltage (Vg=1.6V) is applied. The high drain current shows that the junctionless transistor device has greater carrier mobility. 
     In some embodiments of the present disclosure, the junctionless transistor device is a gate-all-around junctionless field effect transistor (GAAJLFET) device with a vertical channel structure. The GAAJLFET device is advantageous for its lower thermal budget, lower leakage current, greater on/off ratio and greater swing. The vertical channel structure can conserve layout area, and can be integrated into various electronic device fabrications such as DRAM fabrication. 
     In contrast, an FET transistor device with PN junction faces short channel effect (SCE) when scaling down, and the SCE may result in high leakage current, worse subthreshold swing and Drain Induced Barrier Lowering (DIBL) effect. 
     One aspect of the present disclosure provides a junctionless transistor device including a semiconductor substrate, a channel, a first source/drain, a second source/drain, a gate and a gate dielectric layer. The semiconductor substrate has a surface. The channel is disposed over the semiconductor substrate. The channel includes a first channel extending in a lateral direction substantially parallel to the surface of the semiconductor substrate, and a second channel extending in a vertical direction substantially perpendicular to the surface of the semiconductor substrate. The first channel and the second channel are in contact at one end, and the channel has a first doping type. The first source/drain is disposed over the semiconductor substrate, and is in contact with the first channel, wherein the first source/drain has the first doping type. The second source/drain is disposed over the second channel, and is in contact with the second channel. The second source/drain has the first doping type. The gate is disposed over an upper surface of the first channel and side surfaces of the second channel, and the gate has a second doping type opposite to the first doping type. The gate dielectric layer is disposed between the gate and the channel. 
     Another aspect of the present disclosure provides a method for preparing a functionless transistor device. The method includes the following steps. A semiconductor substrate is provided. A semiconductor doped structure is formed over the semiconductor substrate. The semiconductor doped structure has a first doping type. The semiconductor doped structure includes a first doped structure extending in a lateral direction substantially parallel to a surface of the semiconductor substrate, and a second doped structure extending in a vertical direction substantially perpendicular to the surface of the semiconductor substrate. A gate dielectric layer and a gate are formed over the semiconductor doped structure. The gate has a second doping type opposite to the first doping type. 
     Although the present disclosure and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made herein without departing from the spirit and scope of the disclosure as defined by the appended claims. For example, many of the processes discussed above can be implemented in different methodologies and replaced by other processes, or a combination thereof. 
     Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, and composition of matter, means, methods and steps. in described in the specification. As one of ordinary skill in the art will readily appreciate from the present disclosure, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed, that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present disclosure. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.