Abstract:
A method for forming a semiconductor device is disclosed. In the semiconductor device, a gate is formed to enclose a fin structure in a 6F 2  saddle fin gate structure transistor, so that the size of a channel region increases. In accordance with an aspect of the present invention, a method for forming a semiconductor device includes: defining an active region by forming a device isolation film over a semiconductor substrate; forming a first recess extending to a first level in the active region; forming a sacrificial film at a lower portion of the first recess; forming a fin structure over the sacrificial film; separating the fin structure from the semiconductor substrate in the active region by removing the sacrificial film and forming a hole between the fin structure and the active region; and forming a gate to enclose the fin structure.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The priority of Korean patent application No. 10-2012-0056911 filed on 29 May 2012, the disclosure of which is hereby incorporated by reference in its entirety, is claimed. 
       BACKGROUND 
       [0002]    Embodiments relate to a method for forming a semiconductor device, and more particularly to a technology in which a gate has a fin structure with an increased channel length. More specifically, particular embodiments relate to a 6F 2  saddle fin gate structure with an increased channel region. 
         [0003]    Generally, a semiconductor is a material that falls within an intermediate region between a conductor and a nonconductor from among materials affected by electrical conductivity. Although a semiconductor is similar to a nonconductor in a pure state, electrical conductivity of the semiconductor device is increased by impurity implantation or other manipulation. The semiconductor is used to form a semiconductor device such as a transistor through impurity implantation and conductor connection. A device that has various functions simultaneously while being formed of a semiconductor element is referred to as a semiconductor device. A representative example of the semiconductor device is a semiconductor memory device. 
         [0004]    A semiconductor memory device includes a plurality of unit cells each having a capacitor and a transistor. The capacitor is used to temporarily store data, and the transistor is used to transfer data between a bit line and the capacitor in response to a control signal (word line). The data transfer occurs using a semiconductor property whereby an electrical conductivity changes depending on environment. The transistor has three regions, i.e., a gate, a source, and a drain. Electric charges are moved between the source and the drain according to a control signal input to the gate of the transistor. The charges between the source and the drain move through a channel region in accordance with the properties and operation of the semiconductor device. 
         [0005]    In the case where a general transistor is formed over a semiconductor substrate, a method of forming a gate over the semiconductor substrate and doping impurities into both sides of the gate so as to form a source and a drain has been used. As the data storage capacity of a semiconductor memory device has increased and integration degree has increased, the size of each unit cell must be gradually decreased. 
         [0006]    That is, the design rule of the capacitor and the transistor included in the unit cell has been reduced. Thus, as the channel length of a cell transistor is gradually decreased, the short channel effect, Drain Induced Barrier Lowering (DIBL), etc. occur in the general transistor and thus operational reliability is decreased. By maintaining a threshold voltage such that the cell transistor performs a normal operation, it is possible to solve the phenomena generated due to decrease in channel length. In general, as the channel length of the transistor shortens, the concentration of impurities doped into a region in which the channel is formed has been increased. 
         [0007]    However, if the concentration of the impurities doped into the channel region is increased while the design rule is reduced to 100 nm or less, the electric field of a Storage Node (SN) junction is increased, thereby deteriorating the refresh characteristics of a semiconductor memory device. In order to solve this problem, a recessed gate and a saddle fin structure gate have been developed to physically increase a channel length. 
         [0008]    Specifically, although a horizontal channel width of the saddle fin structure transistor is short, a vertical channel length thereof can be guaranteed so that doping concentration can be decreased, and refresh characteristics are prevented from being deteriorated. 
         [0009]    However, a fin width of the saddle fin structure transistor is extremely reduced due to size reduction of a very small-sized cell of 30 nm or less, so that the effect of channel increase is unavoidably deteriorated. 
       BRIEF SUMMARY OF THE INVENTION 
       [0010]    Various embodiments of the present invention are directed to providing a method for forming a semiconductor device that substantially obviates one or more problems due to limitations and disadvantages of the related art. 
         [0011]    An embodiment of the present invention relates to a method for forming a semiconductor device in which a gate is formed to enclose a fin structure in a 6F 2  saddle fin gate structure transistor, so that the size of a channel region increases. 
         [0012]    An embodiment of the present invention relates to a method for forming a semiconductor device, so that an operation current of a cell transistor increases and the operation current and gate control performance of the cell transistor can be improved. 
         [0013]    In accordance with an aspect of the present invention, a method for forming a semiconductor device includes: defining an active region by forming a device isolation film over a semiconductor substrate; forming a first recess extending to a first level in the active region; forming a sacrificial film at a lower portion of the first recess; forming a fin structure over the sacrificial film; separating the fin structure from the semiconductor substrate in the active region by removing the sacrificial film and forming a hole between the fin structure and the active region; and forming a gate to enclose the fin structure. 
         [0014]    The forming the fin structure includes: forming an epitaxial growth layer over the sacrificial film; and forming a second recess by partially etching the silicon epitaxial growth layer and the device isolation film, and forming the remaining silicon epitaxial growth layer as the fin structure. 
         [0015]    The forming the gate to enclose the fin structure includes: depositing a polysilicon material to fill the hole and the second recess; and forming the gate by etching back the polysilicon material. 
         [0016]    The forming the first recess includes: forming a first hard mask pattern over the device isolation film and the active region; and etching the active region using the first hard mask pattern as a mask to form the first recess at the lower portion of the active region. 
         [0017]    The method may further include further etching the first hard mask pattern formed over the device isolation film to form a second hard mask pattern to define a region where the gate is formed. 
         [0018]    The step of forming the second recess includes: forming the second recess in the device isolation film and in the active region using the second hard mask pattern as a mask. 
         [0019]    The first hard mask pattern and the second hard mask pattern include a polysilicon material. The first recess is formed to a depth of between about 1000 Å to 1500 Å. 
         [0020]    The sacrificial film includes a silicon germanium (SiGe) material or a silicon nitride (SiN) material. The sacrificial film is formed to have a thickness of between about 100 Å to 300 Å. 
         [0021]    The removing of the sacrificial film includes wet-etching the sacrificial film using a solution in which the ratio of hydrogen fluoride (HF), acetic acid (CH 3 COOH), and hydrogen peroxide (H 2 O 2 ) is 1:50:50. 
         [0022]    It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are intended to provide further explanation. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]      FIG. 1  is a plan view illustrating a semiconductor device. 
           [0024]      FIGS. 2   a ( i ) to  2   f ( ii ) are cross-sectional views illustrating a method for forming a semiconductor device according to an embodiment of the present invention. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0025]    Reference will now be made in detail to embodiments, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In the following description of the present invention, a detailed description of related known configurations or functions incorporated herein will be omitted when it may make the subject matter unclear. 
         [0026]    A method for forming a semiconductor device according to embodiments will hereinafter be described with reference to  FIGS. 1 to 3 . 
         [0027]      FIG. 1  is a plan view illustrating a 6F 2 -type semiconductor device according to an embodiment. 
         [0028]    Referring to  FIG. 1 , an active region  100  is defined by a device isolation film  200  over a semiconductor substrate, and the active region  100  is formed to cross two word line (WL) gates. 
         [0029]    A semiconductor memory cell divided into a plurality of sections by a plurality of columns includes one or more active regions  100  and a bit line contact (not shown). The active regions  100  are arranged at intervals of a predetermined distance in a manner that column directions of the individual columns are angled against longitudinal directions thereof and include a storage node contact region formed at both ends of each active region  100 , and a bit line contact region formed at the center of each active region  100 . The bit line contact (not shown) is coupled to the bit line contact region over the active region  100 , overlaps with at least some portion of the bit line contact region and is extended to some parts of the next contiguous column. 
         [0030]      FIGS. 2   a  to  2   f  are cross-sectional views illustrating a method for forming a semiconductor device according to an embodiment. In each of  FIGS. 2   a  to  2   g , (i) is a cross-sectional view illustrating a semiconductor device taken along the line of a Y-axis, and (ii) is a cross-sectional view illustrating a semiconductor device taken along the line of an X-axis. 
         [0031]    Referring to  FIG. 2   a , a hard mask pattern  105   a  for forming a recess over an active region  101  defined by a device isolation film  103 , is formed over a semiconductor substrate. That is, an upper part of the device isolation film  103  is covered with the hard mask pattern  105   a,  and some parts (i.e., a region for recess formation) of the upper part of the active region  101  are opened. The hard mask pattern  105   a  may be formed of a polysilicon material in such a manner that the hard mask pattern formed over the device isolation film  103  is etched in a subsequent etching process. 
         [0032]    Subsequently, if the etching process is performed, a recess  107  is formed in the active region  101 , and some parts of the hard mask pattern  105   a  formed over the device isolation film  103  are etched, so that a hard mask pattern  105   b  for a subsequent etching process is formed as shown in  FIG. 2   b . The device isolation film  103  is not etched but the hard mask pattern  105   a  is etched so that an upper surface of the device isolation film  103  is exposed. In this case, the recess  107  may be formed more deeply than a recess for forming a general buried gate. The recess  107  may have a depth of about 1000 Å to 1500 Å. 
         [0033]    Referring to  FIG. 2   b , a silicon germanium (SiGe) material or a silicon nitride (SiN) material is deposited at the bottom of the recess  107  of the active region  101 , and is then etched so that a sacrificial film  109  is formed to have a predetermined thickness. Preferably, the sacrificial film  109  may have a thickness of about 100 Å to 300 Å. 
         [0034]    Subsequently, as shown in  FIG. 2   c , a silicon epitaxial growth layer  111  is formed over the sacrificial film  109  located at the bottom in the recess  107 . In this case, the silicon epitaxial growth layer  111  may be formed by depositing/growing a monocrystalline silicon film. 
         [0035]    Referring to  FIG. 2   d , when an etching process is performed using the hard mask pattern  105   b  of  FIG. 2   a  as a mask, a fin structure  113  having a predetermined thickness is formed by etching the device isolation film  103  and the epitaxial growth layer  111 , and recesses ( 115   a,    115   b ) for forming a buried gate are formed. In this case, the recess  115   a  and  115   b  may have the same or similar depth, and each of the recesses  115   a  and  115   b  may have a thickness of about 1000 Å. 
         [0036]    Referring to  FIG. 2   e , the sacrificial film  109  of  FIG. 2   b  is wet-etched so that a hole  117  is formed at the position. The recess  115   b  of  FIG. 2   e  is also etched while the sacrificial film  109  is removed to form a recess  119 . That is, as can be seen from the cross-sectional view (ii), the device isolation film  103  is etched to expose a sidewall of the sacrificial film  109 , and the exposed sacrificial film  109  is wet-etched, so that the fin structure  113  is separated from the active region  101 . When the sacrificial film  109  is wet-etched, a solution can be used in which a mixture ratio of hydrogen fluoride (HF), acetic acid (CH 3 COOH), and hydrogen peroxide (H 2 O 2 ) is about 1:50:50. 
         [0037]    Referring to  FIG. 2   f , a gate oxide film  123  having a predetermined thickness is formed over the entire inner surface of each recess ( 115   a,    115   b ). The gate oxide film  123  also formed over an inner surface of the recesses  115   a  and  115   b  and the hole  117 . Polysilicon material is deposited over the gate oxide film  123  and is then planarized so that buried gates ( 121   a,    121   b ) are formed. As a result, the gate  121   a  is formed in the hole  117  to enclose a fin structure  113  acting as a channel. Accordingly, a channel is formed in such a manner of being surrounded by the gate  121   a.  In this case, chemical mechanical polishing (CMP) or etch-back process may be used for such planarization. 
         [0038]    Thereafter, a nitride film (not shown) is formed over the buried gates ( 121   a,    121   b ). A bit line contact plug (not shown) is formed to be coupled to the gate  121   a.  Then, a bit line (not shown) is formed to be coupled to the bit line contact plug. In addition, after a storage node contact plug (not shown) is formed to be coupled to the gate  121   a,  a capacitor (not shown) is formed, so that a surround-gate type semiconductor device in which the fin structure is surrounded by the gate  121   a  is formed. 
         [0039]    The saddle fin structure gate formed by the above-mentioned methods is formed in such a manner that the fin structure  113  is surrounded by the gate  121   a,  so that the size of a channel region can be elongated. 
         [0040]    As described above, the recess  107  for forming the buried gate in the active region is formed more deeply than a general recess, the sacrificial film  109  having a predetermined thickness is formed at the bottom of the recess  107 , and the fin structure  113  is formed over the sacrificial film  109 . Thereafter, the sacrificial film  109  is removed, a polysilicon material fills a space where the sacrificial film  109  was removed, a channel (fin structure) is formed at a specific position which is higher than a bottom of the device isolation film  103 , and the gate of the semiconductor device is formed to enclose the fin structure  113 . The fin structure  113  be surrounded by the gate  121   a,  so that a channel region can be elongated. 
         [0041]    As is apparent from the above description, the semiconductor device according to forms a surrounding-type channel under a 6F 2  buried saddle fin gate structure, so that a length of a channel region can increase. 
         [0042]    In addition, the semiconductor device is configured to increase the size of channel region even in a very small-sized cell of whose minimum pattern size is about 30 nm or less. Since the channel length is elongated, operation current of a cell transistor and gate controllability thereof can be improved. 
         [0043]    Those skilled in the art will appreciate that embodiments may be carried out in other specific ways than those set forth herein without departing from the spirit and essential characteristics of the present invention. The above exemplary embodiments are therefore to be construed in all aspects as illustrative and not restrictive. The scope of the subject matter should be determined by the appended claims and their legal equivalents, not by the above description, and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein. Claims that are not explicitly cited in each other in the appended claims may be presented in combination as an exemplary embodiment or included as a new claim by a subsequent amendment after the application is filed. 
         [0044]    The above embodiments are illustrative and not limitative. Various alternatives and equivalents are possible. Embodiments are not limited by the type of deposition, etching polishing, and patterning steps described herein. Nor are embodiments limited to any specific type of semiconductor device. For example, embodiments may be implemented in a dynamic random access memory (DRAM) device or non volatile memory device. Other additions, subtractions, or modifications are obvious in view of the present disclosure and are intended to fall within the scope of the appended claims.