Abstract:
A method for fabricating a semiconductor device comprises: forming a gate pattern over a silicon active region and an insulating layer, which form a semiconductor substrate; removing the silicon active region exposed between the gate patterns; and filling a space between the gate patterns to form a plug.

Description:
CROSS-REFERENCES TO RELATED APPLICATIONS 
       [0001]    Priority to Korean patent application number 10-2008-0038345, filed on Apr. 24, 2008, which is incorporated by reference in its entirety, is claimed. 
       BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to a method for fabricating a semiconductor device, and more specifically, to a method of forming a floating body transistor used in a highly-integrated semiconductor device using a silicon-on-insulator (SOI) substrate. 
         [0003]    In a system containing a plurality of semiconductor devices, a semiconductor memory apparatus is configured to store data generated or processed therein. For example, if a request from a data processor such as a central processing unit (CPU) is received, the semiconductor memory apparatus outputs data to the data processor from unit cells therein or stores data processed by the data processor to the unit cells, according to an address transmitted with the request. 
         [0004]    Recently, data storage capacity of the semiconductor memory apparatus has increased, but the size of the semiconductor memory apparatus has not increased proportionally. Thus, the size of each of the multiple unit cells included in the semiconductor memory apparatus has dwindled, and the sizes of various components and elements for read or write operations has been reduced. Accordingly, components and elements duplicated unnecessarily in the semiconductor memory apparatus, such as transistors or wires, are combined or merged to decrease the area occupied by each component. Particularly, the reduction of the size of unit cells included in the semiconductor memory apparatus affects improvement of integration because the unit cells occupy one of the largest areas therein. 
         [0005]    An example of a semiconductor memory apparatus is Dynamic Random Access Memory (DRAM), which is a type of a volatile memory device configured to retain data while a power source is supplied. The unit cell comprises a transistor and a capacitor. In the case of the unit cell having a capacitor, after the data “1” is delivered to the capacitor, charges that are temporarily stored in the storage node are dissipated, i.e., the number of charges are reduced, because of both leakage currents generated at junction of the storage nodes and inherent characteristics of the capacitor which actually lost charges while time passes. As a result, a refresh operation is periodically required on the unit cells so that data of the DRAM cannot be destroyed. 
         [0006]    To prevent the reduction of charges, numerous methods for increasing capacitance (Cs) of the capacitor included in the unit cell have been suggested so that more charges may be stored in the storage node. For example, a prior insulating film of the capacitor, for example, an oxide film, is replaced with an advanced insulating film that has a larger dielectric constant, such as a nitrified oxide film, and a high dielectric film. Also, the capacitor having a two-dimensional structure is changed to have a three-dimensional cylinder structure or a trench structure, thereby increasing the surface of both electrodes of the capacitor. 
         [0007]    As the design rule is reduced, the plane area where a capacitor can be formed is reduced, and it is difficult to develop materials for constituting an insulating film in the capacitor. As a result, the junction resistance value of the storage node (SN) and the turn-on resistance value of the transistor in the unit cell are greater, it is difficult to perform normal read and write operations, and refresh characteristics are deteriorated. 
         [0008]    To improve the above-described shortcomings, the unit cell includes a transistor having a floating body. That is, the unit cell of the semiconductor memory apparatus does not include a capacitor used for storing data, but stores data in a floating body of the transistor included in the unit cell. To store data in the floating body, the voltage level supplied on the word line is reduced by ½ or ⅓ of the voltage level applied to the bit line connected to one active region of the transistor, thereby generating hot carriers. When the data “1” is delivered, a large amount of hot carriers are generated in a junction region of the bit lines BL. Then, electrons are slipped out into the bit line BL and holes remain in the floating body. When the data “0” is transmitted, the hot carriers are not generated in the junction region, and no holes remain in the floating body FB. The holes kept in the floating body lower the threshold voltage of the transistor of the unit cell; consequently the amount of current flowing through the transistor increases. That is, the amount of the current flowing when the holes are stored in the floating body of the transistor is greater than that flowing when no holes are stored. As a result, it is possible to distinguish whether the data “1” or “0” is stored in the unit cell. 
         [0009]    The semiconductor memory apparatus that includes the floating body transistor does not include a capacitor, thereby improving the integration. However, it is difficult to prevent reduction of the number of holes that the floating body of the transistor stores because of the leakage current that occurs at the source line junction or the bit line junction. Generally, the active region of the transistor, which is connected to the bit line or the source line, includes impurities of high concentration to reduce resistance resulting from junction with metal layers. However, if the active region (e.g., source or drain region) of the transistor contains impurities of high concentration, the amount of leakage current between the active region and the floating body may increase. As a result, the number of holes stored in the floating body dissipates as time goes by. Also, since the amount of the leakage current increases in proportion to temperature, data stored in the transistor of the unit cell is easily deleted (lost) under a high temperature. 
         [0010]      FIGS. 1   a  to  1   e  are cross-sectional diagrams illustrating a method for manufacturing a floating body transistor in a general semiconductor memory apparatus. 
         [0011]    Referring to  FIG. 1   a,  a gate pattern  103  including a gate electrode and a hard mask for protecting the gate electrode is formed over a silicon active region  101 . The semiconductor memory apparatus including the floating body transistor is fabricated with a silicon-on-insulator (SOI) wafer including a lower insulating oxide layer  102  and the silicon active region  101  formed over the lower insulating oxide layer  102 . The SOI wafer includes an insulating layer that is artificially formed between the top surface (silicon active region) and a silicon substrate to remove the impact from the silicon substrate, thereby improving the process, efficiency and characteristics of a high-pure silicon layer formed over the insulating layer. Since the SOI wafer provides a zero-defective thin silicon layer isolated with an insulating material (thermal oxide film), an insulating wall or a well-forming process is not required, so that the product developing and producing time and cost are reduced. Also, there is no burden on additional equipment investment because the equipment for general silicon wafer products is also available for SOI wafer products. 
         [0012]    Referring to  FIG. 1   b,  an interlayer dielectric (ILD) oxide film  104  is formed over the resulting structure including the gate pattern  103 . 
         [0013]    Referring to  FIG. 1   c,  the ILD oxide film  104  is etched to form a self-aligned contact (SAC), thereby obtaining a contact hole  105  and an ILD oxide film  104   a  that remains on a spot having no contact hole  105 . A part of the upper portion of the silicon active region  101  exposed between gate patterns  103  is etched. 
         [0014]    Referring to  FIG. 1   d,  an ion-implanting process is performed on the exposed contact hole  105  to form a landing plug contact (LPC)  108 . 
         [0015]    Referring to  FIG. 1   e,  a polysilicon film  109  of high concentration is deposited in the exposed contact hole  105 . A diffusion process is performed at a high temperature after the polysilicon film  109  is filled in the exposed contact hole  105 . When the diffusion process is performed at a high temperature, the landing plug contact  108  formed in the silicon active region  101  by ion-implantation diffuses to the lower insulating oxide layer  102 , thereby isolating floating bodies of each transistor. 
         [0016]    Referring to  FIG. 1   e,  when the diffusion process is performed at a high temperature after the polysilicon film  109  of high concentration ion-impurities are diffused vertically as well as horizontally, so that a diffusion plug  110  is formed, the horizontal diffusion may reduce the volume of the body of each transistor, and a punch-through phenomenon, whereby may occur in the upper or lower portion of the floating body formed in the silicon active region  101 . Particularly, the punch-through phenomenon frequently occurs in the lower portion of the floating body having a lower concentration of impurities rather than in the upper portion of the floating body where the concentration of impurities increases by channel doping when the gate pattern  103  is formed. 
         [0017]    In the floating body transistor fabricated over the SOI wafer, it is advantageous to isolate cells in a single active region rather than to isolate unit cells through a device isolation film by the (STI) Shallow Trench Isolation process to maximize the cell packing density. The cell packing density is the number of unit cells per length, area or volume of the memory apparatus. When the cell packing density is maximized, a unit cell having an area of 4F 2  or 6F 2  is formed. Herein, F is the minimum distance between fine patterns, according to a design rule. 
         [0018]    While the size of each unit cell is reduced, the volume of the floating body is maximized so that more holes may be stored in the floating body corresponding to the data. To maximize the volume of the body of the transistor, a three-dimensional transistor having a fin region or a transistor having a plane channel region is preferably used rather than a three-dimensional transistor having a recess gate. However, when the full size of the transistor is reduced, the distance between the source and drain regions of the transistor having a plane channel region is reduced, enabling a punch-through phenomenon that is difficult to prevent. 
         [0019]    Although the punch-through phenomenon does not occur after the diffusion process is performed at a high temperature, the horizontal diffusion reduces the volume of the floating body. If the volume of the floating body is reduced, the number of holes accumulated in the floating body that are able to correspond to data is reduced. The reduction of the floating body effect means a decrease in the capacity of storing data in unit cells for a long duration and in a sensing margin of data outputted from unit cells. That is, the operating margin of unit cells in the semiconductor memory apparatus is reduced. 
       BRIEF SUMMARY OF THE INVENTION 
       [0020]    Various embodiments of the present invention are directed to providing a semiconductor device and a method for fabricating the same. In the method, the upper portion of a landing plug is preferably formed to be broader than the lower portion of the landing plug in the manufacturing of a floating body transistor so as to prevent the volume of the floating body from being reduced. Also, the ion-implanting process is preferably performed on the lower portion of the floating body to increase the concentration to prevent a punch-through phenomenon. 
         [0021]    According to an embodiment of the present invention, a method for fabricating a semiconductor device includes: forming a gate pattern over a silicon active region and an insulating layer, which form a semiconductor substrate; removing the silicon active region exposed between the gate patterns; and filling a space between the gate patterns to form a plug. 
         [0022]    Preferably, removing-the-silicon-active-region includes: etching a part of the silicon active region; forming a spacer on sidewalls of the gate pattern and the silicon active region; and etching the silicon layer between the spacers to expose the insulating layer. 
         [0023]    Preferably, forming-a-spacer includes: forming an oxide film over the gate pattern and the silicon active region; and blanket-etching the oxide film so that the oxide film remains on sidewalls of the gate pattern and the silicon active region. 
         [0024]    Preferably, the silicon active region, which remains below the gate pattern, has a lower portion that is broader than an upper portion by more than the thickness of the spacer. 
         [0025]    Preferably, etching-a-part-of-the-silicon-active-region includes forming an interlayer insulating film over the resulting structure including the gate pattern and etching the interlayer insulating film between the gate patterns with a landing contact mask to expose the silicon layer. 
         [0026]    The method further comprises implanting impurities into a lower portion of the silicon active region by a tilt ion-implantation method. 
         [0027]    According to an embodiment, a method for fabricating a semiconductor device includes: etching a portion of a substrate to form a trench; forming a protective film on sidewalls of the trench; and forming a floating body of a transistor by etching a lower portion of the trench. 
         [0028]    Preferably, the substrate is a silicon-on-insulator (SOI) wafer including an insulating layer and a silicon active region. 
         [0029]    The method further includes forming a gate pattern over the substrate. 
         [0030]    Preferably, forming-a-floating-body-of-a-transistor includes: etching the silicon active region between the gate patterns; forming a protective film over the resulting structure including the gate pattern and the silicon active region; etching the protective film so that the protective film remains on sidewalls of the gate pattern and the silicon active region; and removing the exposed silicon active region. 
         [0031]    Preferably, the protective film includes an oxide film. 
         [0032]    The method can further include: implanting impurities into the floating body through a tilt ion-implantation method; and forming a landing plug between the floating bodies. 
         [0033]    Preferably, the landing plug includes a conductive material. 
         [0034]    According to an embodiment, a semiconductor device includes a floating body transistor that includes a floating body where a lower portion is broader than an upper portion at the bottom of a gate pattern as a cell transistor. 
         [0035]    Preferably, the floating body includes a local doping region at lower sidewalls, which has a higher ion-concentration than a central region of the floating body. 
         [0036]    Preferably, the floating body is formed in a silicon active region over a silicon-on-insulator (SOI) wafer that includes an insulating layer and the silicon active region. 
         [0037]    Preferably, both sides of the floating body contact a conductive material. 
         [0038]    Preferably, the conductive material is connected to a bit line and a source line. 
         [0039]    Preferably, the floating body transistor stores holes generated by hot carriers when the delivered data is “1”. 
         [0040]    Preferably, the floating body has a sloped side surface. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0041]    For a more complete understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings. 
           [0042]      FIGS. 1   a  to  1   e  are cross-sectional diagrams illustrating a method for manufacturing a floating body transistor in a general semiconductor memory apparatus. 
           [0043]      FIG. 2   a  to  2   g  are cross-sectional diagrams illustrating a method for manufacturing a floating body transistor in a semiconductor memory apparatus according to an embodiment of the present invention. 
       
    
    
       [0044]    While the disclosed method is susceptible of embodiments in various forms, specific embodiments are illustrated in the drawings (and will hereafter be described), with the understanding that the disclosure is intended to be illustrative, and is not intended to limit the invention to the specific embodiments described and illustrated herein. 
       DESCRIPTION OF EMBODIMENTS 
       [0045]      FIG. 2   a  to  2   g  are cross-sectional diagrams illustrating a method for manufacturing a floating body transistor in a semiconductor memory apparatus according to an embodiment of the present invention. 
         [0046]    Referring to  FIG. 2   a,  a gate pattern  203  including a gate electrode and a hard mask for protecting the gate electrode is formed over a silicon active region  201 . Impurities are doped between the gate patterns  203  to form a lightly doped drain (LDD) (not shown). According to an embodiment, a semiconductor memory apparatus including a floating body transistor is preferably fabricated with a silicon-on-insulator (SOI) wafer including a lower insulating oxide layer  202  and the silicon active region  201  formed over the lower insulating oxide layer  202 . 
         [0047]    Referring to  FIG. 2   b,  an interlayer dielectric (ILD) oxide film  204  is formed over the resulting structure including the gate pattern  203 . 
         [0048]    Referring to  FIG. 2   c,  to form a self-aligned contact (SAC), the ILD oxide film  204  is preferably etched with a landing plug contact mask, thereby obtaining a contact hole  205 . An ILD oxide film  204   a  preferably remains in a place having no contact hole  205 . A partial upper portion of the silicon active region  201  is exposed between gate patterns  203 . The silicon active region  201  is preferably etched deeper than in conventional art. 
         [0049]    Referring to  FIG. 2   d,  an oxide film  206  is formed on the gate pattern exposed by the contact hole  205 , the silicon active region  201 , and the residual ILD oxide film  204   a.    
         [0050]    Referring to  FIG. 2   e,  the oxide film  206  is preferably blanket-etched like an etching process for forming a spacer at sidewalls of a conventional gate pattern, thereby removing the oxide film  206  formed over the ILD oxide film  204   a,  the gate pattern  203 , and the silicon active region  201 . After the blanket-etch process, the oxide film  206  considered as a spacer, remains on sidewalls of the ILD oxide film  204   a,  the gate pattern  203 , and the silicon active region  201 . The silicon active region  201  exposed between the oxide films  206  that remain on the sidewalls of the gate pattern  203  is preferably etched until the lower insulating oxide layer  202  is exposed, thereby obtaining a landing plug forming region  207 . The silicon active region  201  that remains in the bottom of the gate pattern  203  is formed in the shape of a trapezoid. That is, the lower portion is broader than the upper portion in the silicon active region  201 . This shape of the silicon active region  201  is obtained generally when a material deposited between conventional fine patterns is deeply etched. Thus, in the invention, it is not necessary to perform an additional etch process to prevent sidewalls of the silicon active region  201  from being tilted. The lower insulating oxide layer  202  is preferably exposed so that the silicon active region  201  that remains in the lower portion of each gate pattern  203  is separated. If the sidewall inclination of the silicon active region  201  is reduced, the volume of the floating body becomes larger. 
         [0051]    Referring to  FIG. 2   f,  after the oxide film  206  that remains on the gate pattern  203 , the silicon active region  201 , and the ILD film  204   a  is removed, an ion-implanting process is performed on the lower portion of the silicon active region  201  corresponding to the lower region of the floating body, thereby obtaining a local doping region  208 . Ion-impurities are halo-doped on the lower portion of the silicon active region  201  by tilt ion-implantation or slant implanting. The halo-implant area can be described as a region doped with impurities in source and drain regions of the transistor to prevent a punch-through phenomenon that may occur when a distance between a source and a drain becomes shorter as the size of the transistor becomes smaller. 
         [0052]    Through this process, the silicon active region  201  formed as a floating body in the lower portion of the gate pattern  203  preferably has an a lower portion formed to be broader than its upper portion, thereby preventing the punch-through phenomenon that occurs in the lower region of the silicon active region  201  having a low impurity concentration. Furthermore, ions are implanted into the lower portion of the silicon active region  201  by tilt ion-implantation, thereby preventing the punch-through phenomenon. 
         [0053]    Referring to  FIG. 2   g,  the landing plug forming region  207  is filled with a conductive material, preferably with polysilicon, thereby forming a landing plug  209 . The polysilicon in the process of forming the landing plug  209  is diffused horizontally into the silicon active region  201 , thereby obtaining a diffusion plug  210 . 
         [0054]    In the conventional art, referring to  FIG. 1   e,  the polysilicon  109  of high concentration is deposited over the landing plug  108  formed in the silicon active region  101 . The landing plug  108  is diffused into the lower insulating oxide layer  102  through thermal treatment performed at a high temperature, thereby separating the floating bodies of the transistor from each other through the diffusion plug  110 . That is, the diffusion plug  110  is formed as the polysilicon  109  of high concentration and the landing plug  108  is subjected to a thermal treatment performed at a high temperature. However, in the embodiment of the present invention, because the floating bodies of the transistor are separated previously, the process for forming the landing plug  108  through ion-implantation is not required. Moreover, the polysilicon for forming the landing plug  209  is not required to have a high concentration. Also, except for a thermal treatment for depositing the polysilicon, it is not necessary to perform a diffusion process, for example, a thermal treatment performed at a high temperature, for diffusion of the landing plug  209 . 
         [0055]    As described above, in an embodiment, to form a landing plug, the silicon active region  201  is etched in the two steps, so that the lower portion of the silicon active region  201  used as a floating body is preferably formed to be broader than the upper portion of the silicon active region  201 , thereby preventing the punch-through phenomenon that occurs in the floating body. The silicon active region  201  is preferably etched until the lower insulating oxide layer  202  is exposed, so that it is possible to separate the floating bodies between each transistor without reducing the thickness of the silicon active region  201 . 
         [0056]    Furthermore, the landing plug  209  is formed deep to the lower insulating oxide layer  202 , and the silicon active region  201  in the landing plug region  207  is removed, so that the ion-implanting process for forming the landing plug  209  is not required. The junction area between the silicon active region  201 , which is a floating body, and the landing plug  209  increases because the landing plug  209  is formed in contact with the lower insulating oxide layer  202 , thereby reducing junction resistance. 
         [0057]    The inclination of the side surface of the trapezoidal silicon active region  201  located in the lower portion of the gate pattern  103  increases the effect of the tilt implantation. Although there is a limit in the tilt angle of the ion-implantation process due to the gate pattern  103  having a narrow interval, the limit of the tilt angle of the ion-implantation process increases corresponding to the inclination degree of the side surface of the silicon active region  201 . 
         [0058]    As a result, the floating transistor in the semiconductor memory apparatus fabricated according to the embodiment of the present invention to prevent the punch-through phenomenon can be formed to have a smaller size. That is, as the size of the floating body transistor is reduced, the increased possibility of the punch-through phenomenon occurrence can be decreased, so that it is possible to manufacture a smaller-sized floating body transistor. 
         [0059]    A semiconductor memory apparatus fabricated by the above-described method includes a floating body transistor that has a floating body, where the lower portion is broader than the upper portion, under a gate pattern as a cell transistor. Although a unit cell in a conventional semiconductor memory apparatus includes a transistor and a capacitor, a unit cell of the semiconductor memory apparatus includes a floating body transistor without a capacitor. When the delivered data is “1”, the floating body transistor stores holes generated by hot carriers, and amplifies data using a difference in the amount of charges flowing through a channel of the transistor depending on the holes stored in the floating body when the data is “0” and “1”. 
         [0060]    Specifically, the local doping region  208  having a high implanted ion impurity concentration is formed in the lower portion of the floating body in the semiconductor memory apparatus. The floating body is formed in the silicon active region  201  of the SOI wafer including the lower insulating oxide layer  202  and the silicon active region  201 . Both sides of the floating body preferably contact conductive materials, which are connected to a bit line and a source line. 
         [0061]    Particularly, the side surface of the floating body in the semiconductor memory apparatus obtained through the twice etching process is sloped. Moreover, the lower portion of the floating body is formed to be broader than the upper portion, thereby preventing the punch-through phenomenon between source and drain regions located at both sides of the floating body. 
         [0062]    The above embodiments of the present invention are illustrative and not limiting. Various alternatives and equivalents are possible. The invention is not limited by the type of deposition, etching polishing, and patterning steps described herein. Nor is the invention limited to any specific type of semiconductor device. For example, the present invention may be implemented in a dynamic random access memory (DRAM) device or a non-volatile memory device. While the present invention has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the invention as defined in the following claims.