Abstract:
A method for fabricating a quantum dot, which can be used to fabricate a single electron memory device. The method includes forming a first insulation layer on a semiconductor layer, then forming a second insulation layer on the first insulation layer. Next, the second insulation layer is patterned to form an opening to partially expose the upper surface of the first insulation layer. Using the opening in the second insulation layer, a silicon ion is then implanted into the first insulation layer through the opening by using a tilt angle ion implantation method. Finally, the semiconductor layer is treated to re-crystallize the silicon ion implanted into the first insulation layer.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a semiconductor device and a method for fabricating thereof, and more particularly, to a single electron semiconductor device and method thereof.  
           [0003]    2. Background of the Related Art  
           [0004]    In response to the semiconductor industry&#39;s desire to further integrate semiconductor devices, a single electron memory device has been developed which is programmable and erasable by using just a single or a few electrons.  
           [0005]    [0005]FIG. 1 shows a structure of a single electron memory device in accordance with the related art where a semiconductor layer  100  made of silicon or gallium-arsenic (GaAs) is formed with a tunneling insulation film  102  on the upper surface of the semiconductor layer  100  The tunneling insulation film  102  is formed by a silicon oxide film having a thickness of 2-3 nm. Next, a quantum dot  104  is formed on the upper surface of the tunneling insulation film  102  of a fine-sized polycrystalline silicon pattern having a width of about 50 nm and a height of about 50 nm. The size of the quantum dot  104  is preferably such that a single electron or several electrons at the most can tunnel to generate a Coulomb Blockade phenomenon.  
           [0006]    A control insulation film  106  is formed on the upper surface of the quantum dot  104 . The control insulation film  106  is a silicon oxide film or a silicon nitride film formed with a thickness of about 2-3 nm. Next, a control gate electrode  108  is formed on the upper surface of the control insulation film  106 .  
           [0007]    An n-type or a p-type of impurity ion-implanted source region  110  and a drain region  112  are formed in the semiconductor layer  100  at the both sides of the control gate electrode  108 . Then, an interlayer insulation film  114  is formed at the upper surface and side surface of the control gate electrode  108 , and a planarization layer  116  is formed on the upper surface of the interlayer insulation film  114 . A contact hole  117  is then formed on the upper surface of the source region  110  and the drain region  112  and a conductive plug  118  is formed through the contact hole  117 , where the conductive plug is connected with a metal wiring layer  120 .  
           [0008]    The operational principle of a single electron memory having the construction of FIG. 1 is the same as that of an EEPROM (Electrically Erasable Programmable Read Only Memory) of the related art. But, unlike an EEPROM of the related art, the single electron memory can vary a threshold voltage with merely single electron or several electrons at the most and is operable at a lower voltage than a EEPROM of the related art because when a write voltage higher than the threshold voltage is applied to the control gate electrode, an inversion layer is formed in a channel region and an electron from the source region is induced into the channel, reducing the channel conductance.  
           [0009]    This occurs because one or several electrons when in the inversion layer of the channel region, tunnel into the quantum dot (which becomes a floating gate) and one by one the electrons tunnel through a thin tunneling insulation layer at room temperature. As the electrons tunnel into the floating quantum dot, the threshold voltage changes.  
           [0010]    Ideally, it is preferred that a single electron tunnels for programming. However, since it is difficult to detect the change in the size of the threshold voltage, three or four electrons are often used to change the threshold voltage by about 1V to program the memory.  
           [0011]    [0011]FIGS. 2A through 2H show a series of processes of the method for fabricating a single electron memory device in accordance with the related art.  
           [0012]    As shown in FIG. 2A, a plurality of device isolation regions  201  are formed at predetermined portions of a semiconductor layer  200 . The device isolation regions  201  are called field regions and the other regions which are not the device isolation regions  201  are called active regions. Next, a tunneling insulation layer  202  is formed on an upper surface of the semiconductor layer  200  including the field region  201 , then a polysilicon layer  203  is formed on the upper surface of the tunneling insulation layer  202 .  
           [0013]    As shown in FIGS. 2B and 2C, the polysilicon layer  203  is patterned to form a polysilicon layer pattern  203   a , the surface of the polysilicon layer pattern  203   a  is oxidized to form a silicon oxide film  204  on the surface of the polysilicon layer pattern  203   a  as illustrated in FIG. 2C. Thereafter, as shown in FIG. 2D, the silicon oxide film  204  is selectively etched using a buffered HF solution to reduce the polysilicon layer pattern  203   a  to a smaller size polysilicon layer pattern  203 B.  
           [0014]    The processes of FIGS. 2C and 2D are repeatedly performed until, as shown in FIG. 2E, a quantum dot  203   c  is formed having a length that is at most 50 nm. Next, as shown in FIG. 2F, a control insulation film  205  is formed on the upper surface of the polysilicon layer pattern  203   c , the tunneling insulation layer  202  and the isolation regions  201 , and then a polysilicon layer  206  is deposited on the upper surface of the control insulation film  205 .  
           [0015]    Next, as shown in FIG. 2G, the polysilicon layer  206  and the control insulation film  205  are patterned to form a control gate electrode  206   a , source  207  and drain regions  208  are then formed on both sides of the control gate electrode  206   a  by implanting an impurity ion into the semiconductor layer  200 , and an interlayer insulation film  209  is formed on the entire upper surface of the structure formed on the semiconductor layer  200 . Then, a planarization layer  210  is formed on the upper surface of the interlayer insulation film  209 , a contact hole is then formed on both the source  207  and drain regions  208  and each contact hole is filled with a conductive material to form a conductive plug  211  as shown in FIG. 2H. Finally, a metal wiring layer  212  is formed on the upper surface of the conductive plug  211 , thereby completing the fabricating of a single electron dot memory device.  
           [0016]    However, the above method for fabricating a single electron memory device has various problems. For example, a very fine pattern must formed to form the quantum dot, but the smallest line feature that can currently be formed by using the currently available photolithography processes are about 0.1 μm. Accordingly, it is difficult to fabricate a quantum dot having a size less than 50 nm by using the currently available photolithography and an etching process which starts with pattern line features of about 0.1 μm.  
           [0017]    Further, as mentioned above in the related art method, a comparatively large polysilicon layer pattern is formed, and then the size of the polysilicon layer pattern is reduced by using iterations of oxidation and wet etching. Accordingly, this method has a problem with the evenness of the size of the quantum dot because of the inexactness of the oxidation and wet etching and a problem with the reproduction of the process because of the iterations of oxidation and wet etching required to reduce the size of the quantum dot.  
           [0018]    The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.  
         SUMMARY OF THE INVENTION  
         [0019]    An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter.  
           [0020]    Another object of the present invention is to provide a quantum dot having a more consistent size.  
           [0021]    Another object of the present invention is to provide a quantum dot having an improved reproductiveness of the process.  
           [0022]    A further object of the present invention is to provide a single electron memory device.  
           [0023]    To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for fabricating a quantum dot including the steps of forming a first insulation layer on a semiconductor layer, forming a second insulation layer on the first insulation layer, patterning the second insulation layer to form an opening of ‘T’-shape and partially exposing the upper surface of the first insulation layer, implanting a silicon ion into the first insulation layer through the opening by using a tilt angle ion implantation method, thermally treating the semiconductor layer to re-crystallize the silicon ion implanted into the first insulation film.  
           [0024]    In order to achieve the above objects, in the above method for fabricating a quantum dot, the re-crystallizing refers to a thermal treatment of the semiconductor layer at a temperature of about 600˜700° C.  
           [0025]    In order to achieve the above objects, in the above method for fabricating a quantum dot, the first insulation film is a silicon oxide film.  
           [0026]    In order to achieve the above objects, in the above method for fabricating a quantum dot, in the step of forming the first insulation film, the first insulation film has the thickness of about 30 nm.  
           [0027]    In order to achieve the above objects, in the above method for fabricating a quantum dot, in the step of implanting the silicon ion, the silicon ion is implanted with the depth of about 5 nm from the upper surface of the silicon oxide film.  
           [0028]    In order to achieve the above objects, in the above method for fabricating a quantum dot, the second insulation film is a nitride film.  
           [0029]    In order to achieve the above objects, in the above method for fabricating a quantum dot, the nitride film has the thickness of about 30 nm.  
           [0030]    In order to achieve the above objects, there is also provided a method for fabricating a single electron memory device including the steps of forming a first insulation layer on a semiconductor layer, forming a second insulation layer on the first insulation layer, patterning the second insulation layer to form an opening of ‘T’-shape and partially exposing the upper surface of the first insulation layer, implanting a silicon ion into the first insulation layer through the opening by using a tilt angle ion implantation method, thermally treating the semiconductor layer to re-crystallize the silicon ion implanted into the first insulation film and forming a quantum dot, removing the second insulation film, forming a control gate electrode of polysilicon layer pattern on the upper surface of the first insulation film, patterning the first insulation film to have the same size of the control gate electrode, and forming a source and a drain regions in the semiconductor layer at both sides of the control gate electrode.  
           [0031]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, the re-crystallizing refers to a thermal treatment of the semiconductor layer at a temperature of about 600˜700° C.  
           [0032]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, the first insulation film is a silicon oxide film.  
           [0033]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, in the step of forming the first insulation film, the first insulation film has the thickness of about 30 nm.  
           [0034]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, in the step of implanting the silicon ion, the silicon ion is implanted with the depth of about 5 nm from the upper surface of the silicon oxide film.  
           [0035]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, the second insulation film is a nitride film  
           [0036]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, the nitride film has the thickness of about 30 nm.  
           [0037]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, in the step of removing the nitride film, the nitride film is removed by using a hot phosphoric acid solution by wet etching.  
           [0038]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, in the step of implanting the silicon ion, the silicon ion is implanted into the first insulation film so that the concentration of the silicon ion is 10 21  atoms/cm 3 .  
           [0039]    In order to achieve the above objects, in the above method for fabricating a single electron memory device, the quantum dot has the diameter of about 10 nm.  
           [0040]    To further achieve the above objects, a method for fabricating a quantum dot includes forming a first insulation layer on an upper surface of a semiconductor layer, forming a second insulation layer on an upper surface of the first insulation layer, patterning the second insulation layer to form an opening to partially expose the upper surface of the first insulation layer, implanting an ion into the first insulation layer through the opening by using a tilt angle ion implantation method, and recrystallizing the implanted ion in the first insulation layer.  
           [0041]    To further achieve the above objects, a method for fabricating a single electron memory device includes forming a first insulation layer on an upper surface of a semiconductor layer, forming a second insulation layer on an upper surface of the first insulation layer, patterning the second insulation layer to form an opening to partially expose the upper surface of the first insulation layer, implanting an ion into the first insulation layer through the opening by using a tilt angle ion implantation method, recrystallizing the ion implanted into the first insulation layer and forming a quantum dot, removing the second insulation layer, forming a control gate electrode on the upper surface of the first insulation layer, patterning the first insulation layer to the same width as the control gate electrode, and forming source and drain regions in the semiconductor layer at both sides of the control gate electrode.  
           [0042]    To further achieve the above objects, a single electron memory device includes a semiconductor layer, a first insulation layer on an upper surface of the semiconductor layer, a second insulation layer on an upper surface of the first insulation layer, and a recrystallized implanted ion in the first insulation layer.  
           [0043]    Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0044]    The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein:  
         [0045]    [0045]FIG. 1 shows a structure of a single electron memory device in accordance with a conventional art;  
         [0046]    [0046]FIGS. 2A through 2H show a series of processes of the method for fabricating a single electron memory device in accordance with the conventional art; and  
         [0047]    [0047]FIGS. 3A through 3F show a series of processes of a method for fabricating a single electron memory device and of a method for fabricating a quantum dot in accordance with the present invention. 
     
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       [0048]    Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings.  
         [0049]    [0049]FIGS. 3A through 3F illustrate a preferred embodiment method of the present invention for fabricating a single electron memory device and a preferred embodiment method for fabricating a quantum dot.  
         [0050]    In FIG. 3A, a device isolation region  300   a  is formed in a semiconductor layer  300 , then a silicon oxide film  301  is formed on the upper surface of the semiconductor layer  300 , where the silicon oxide film  301  has a thickness of about 30 nm. Next, as illustrated in FIG. 3B, a nitride film  302  having the thickness of more than 300 nm is formed on the silicon oxide film  301 , then an opening  303  is formed by partially etching the nitride film  302 , which exposes the upper surface of the silicon oxide film  301 . The opening  303  is preferably formed in a ‘T’ shape with a long axis pattern  303   a , and a short axis pattern  303   b  where the short axis pattern is formed protrusively from the central portion of the long axis pattern  303   a . The opening can be any shape which meets the requirements of a tilt angle ion implantation such as “L”, “E”, etc.  
         [0051]    Referring to FIG. 4B, the thickness of the nitride film pattern is denoted by ‘a’. Preferably, the longest portion b 1  of the pattern of the opening  303  is equal to the value obtained by adding the width of the long axis pattern  303   a  and the length of the short axis pattern  303   b , and the short length b 2  is the same as the width of the long axis pattern  303   a . The length of each axis pattern being the relatively longer side, while the width of each axis pattern being the relatively shorter side.  
         [0052]    [0052]FIG. 3C is a sectional view taken along line of IIIc-IIIc of FIG. 3B. As shown in FIG. 3C, silicon ions are implanted into the silicon oxide film  301  through the opening  303  by using a tilt angle ion implantation method having a tilt angle of θ to form a silicon ion implantation region  304 . Any suitable ion for a quantum dot may also be adequate. The silicon ion implantation region  304  is formed only in the silicon oxide film  301  which is related to the short axis pattern  303   b . That is, ion implantation shadowing occurs in the portion of the long axis pattern  303   a  of the opening  303 , due to the tilt angle ion implantation such that the shadowed silicon ions, or the ions not implanted through the opening  303 , are implanted into the silicon nitride film  302 , rather than into the silicon oxide film  301 .  
         [0053]    For the convenience of understanding a more detailed description thereon will now be given with reference to FIGS. 4A and 4B.  
         [0054]    [0054]FIG. 4A is an enlarged view of the portion indicated by a circle ‘A’ of FIG. 3C. An ion implantation shadowing region length B is determined by the thickness a of the nitride film  302  and the ion implantation tilt angle θ, as expressed by B=a*tan(θ). Accordingly, the silicon ion can be implanted into the silicon oxide film only when the lengths (b 1 ) of the opening are greater than the length of the ion implantation shadowing region B.  
         [0055]    As shown in FIG. 4A, since the length of the opening b 1  is greater than the length of the ion implanting region B or a*tan(θ), the silicon ions are implanted into the silicon oxide film  301 , thus forming a quantum dot. As shown in FIG. 4B, which is a vertical-sectional view taken along line of IVb-IVb of FIG. 3B, the length of the opening b 2  is not greater than the length of the ion implanted shadowing region B or a*tan(θ), therefore the silicon ions are implanted on the nitride film  302  and not implanted into the silicon oxide film  301  and do not form a quantum dot. Therefore, in FIG. 4A, ions are implanted into the silicon oxide film  301  to form a quantum dot, but in FIG. 4B are not implanted into the silicon oxide film  301  and therefore a quantum dot is not formed.  
         [0056]    In a preferred embodiment of the present invention the size of silicon ion implanted region B, the length of the opening b 1 , b 2 , the tilt angle θ in ion implanting and the thickness a of the nitride film are determined to coincide with a nitride film which has a thickness of about 30 nm for a single electron memory device. Also preferably, the strength of the ion implantation energy is set so that the silicon ion implanted region  304  is formed at a depth of about 5 nm from the upper surface of the silicon oxide film  301 , since the silicon oxide film  301  at the upper portion of the silicon ion  304  becomes a tunneling oxide film and the thickness of the tunneling oxide film is set depending on the depth at which the silicon ion is implanted.  
         [0057]    In the present invention, in order for the tunneling oxide film to have the thickness of 5 nm, ion implantation is carried out so that the silicon ion can be distributed at the depth of about 5 nm from the upper surface of the silicon oxide film  301 . Also, the concentration of the silicon ion in the silicon ion implanted region  304  is preferably about on the order of 10 21  atoms/cm 3 .  
         [0058]    Next, as shown in FIG. 3D, the nitride film  302  is preferably removed by using a hot phosphoric acid solution, then the semiconductor layer  300  is preferably subjected to a thermal treatment, which causes the silicon ions in the silicon ion implanted region  304  to recrystallize where the temperature for the thermal treatment is preferably about 700˜800° C. After undergoing the recrystallizing process, the silicon ions are recrystallized, forming a silicon quantum dot having the diameter of less than about 10 nm. Next, as shown in FIG. 3E, a polysilicon layer  306  is formed on the upper surface of the silicon oxide film  301 .  
         [0059]    Then, as shown in FIG. 3F, the polysilicon layer  306  is patterned to form a control gate electrode  306   a , and the silicon oxide film  301  is patterned to form a tunneling insulation film  301   a . Next, an impurity ion is implanted into the semiconductor layer  300  using the control gate electrode  306   a  as a mask, thereby forming a source region  307  and a drain region  308  on both sides of the control gate electrode  306   a.    
         [0060]    Thereafter, an interlayer insulation film  309 , which is preferably a silicon oxide film, is formed on the upper surface of the source  307  and the drain regions  308  and the upper surface of the control gate electrode  306   a  preferably by a vapor deposition method. Next, a planarization layer  310  is formed on the upper surface of the interlayer insulation film  309 .  
         [0061]    Next, the planarization layer  310  and the interlayer insulation film  309  are selectively etched to form contact holes at the upper surface of the source  307  and the drain regions  308  and then the contact holes are filled with a conductive material thereby forming conductive plugs  311 . Finally, a metal wiring  312  is formed on the upper surface of the conductive plug  311  and the upper surface of the planarization layer  310 , thereby completing the fabrication of a single electron device.  
         [0062]    The method for fabricating a quantum dot and a device thereof, as described above, lead to a consistent quantum dot size and an improved reproductiveness for fabricating a quantum dot. This in turn leads to improved reliability of a single electron memory device. Further, since a quantum dot having a size of less than 10 nm can be fabricated, a stable Coulomb Blockade phenomenon can occur even at a room temperature, so that an ultra-highly integrated memory of more than 4 Gbit can be fabricated by adopting the present invention. Moreover, since the quantum dot has a very small size and the tunneling insulation film is very thin, the single electron memory device is operable even at a very low voltage and is quick about programming and erasing  
         [0063]    The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures.