Abstract:
A method of manufacturing a floating gate provides an enhancement for the efficiencies of electron discharge and injection. First, a conductive pattern, constituting the arefloating gateforeon, is formed on a substrate. A first insulation layer is formed on a sidewall of the conductive pattern, and then a second insulation layer is formed at an upper portion of the conductive pattern in ways that each increase the sharpness of an edge portion where the sidewall and upper portions of the conductive pattern meet. Therefore, electron transference from the floating gate to a control gate is facilitated.

Description:
BACKGROUND OF THE PRESENT INVENTION  
         [0001]    1. Field of the Present Invention  
           [0002]    The present invention relates to a method of forming a floating gate and to a method of manufacturing a non-volatile semiconductor memory device comprising a floating gate.  
           [0003]    2. Description of the Related Art  
           [0004]    Semiconductor memory devices are generally divided into volatile semiconductor memory devices and non-volatile semiconductor memory devices. In the volatile semiconductor memory device, data stored in the cell is dissipated when power is not applied. However, in the non-volatile semiconductor memory, stored data in the cell is retained even when power is not applied thereto. Because non-volatile semiconductor memory devices can store data for long periods of time, they are used to meet the current high demand for flash semiconductor memory devices such as EEPROMs (Electrically Erasable and Programmable Read Only Memories).  
           [0005]    Meanwhile, flash semiconductor memory devices can be generally categorized as stacked flash semiconductor memory devices and split gate flash semiconductor memory devices. The split gate type of flash semiconductor memory device has a structure wherein a floating gate and a control gate are separated from each other, and the floating gate is electrically insulated from the outside. Information is stored in a memory cell of the split gate type of flash semiconductor memory device using the yprinciple that current in a memory cell changes depending on electron injection (programming)/electron discharge (erasing) into/from the floating gate. In the electron injection, hot electrons are injected into the floating gate by a channel hot electron injection (CHEI) mechanism. The electron discharge is accomplished by Fowler-Nordheim (F-N) tunneling through a tunnel insulation layer between the floating gate and the control gate of the split gate type of flash semiconductor memory device. In connection with the electron injection (programming) and electron discharge (erasing), a voltage distribution may be explained as an equivalent capacitor model. Recently, the split gate type of flash semiconductor memory device has been widely used for the purpose of storing data.  
           [0006]    The efficiency of the split gate type of flash semiconductor memory deviceeasy transferenceis required depends on the ease in which electrons can be transferred from the floating gate to the control gate. Therefore, various research into the structure of the floating gate aims at improving the efficiency of electron transference in the hope of realizing a floating gate having a small cell and, in turn, a non-volatile semiconductor memory device having lower power consumption, and an excellent ability to be integrated with a logic device.  
           [0007]    For example, U.S. Pat. No. 5,029,130 discloses a method of manufacturing a floating gate capable of promoting the transference of electrons from the floating gate to a control gate. The method entails oxidizing an upper portion of the floating gate to increase the sharpness of the edge of the floating gate. However, the sharpness is increased only at the upper portion of the floating gate. Accordingly, the speed at which electrons can be transferred from the floating gate to the control gate is still rather limited.  
           [0008]    Korean Laid-Open Patent Publication No. 2001-91532 discloses a method of manufacturing a split gate type of flash semiconductor memory device in which a gate oxide is formed on a silicon substrate, and then a polysilicon layer and a nitride layer are sequentially formed on the substrate including over the gate oxide. The nitride layer is selectively etched by a photolithographic process to form a nitride mask pattern. Then, an oxide layer is formed on the polysilicon layer. The polysilicon layer and the nitride mask pattern are removed by etching to leave a portion of the polysilicon layer beneath the oxide layer. After an interpoly tunnel insulation layer is formed, a control gate is formed on the oxide layer, the interpoly tunnel insulation layer and the gate oxide. Impurities are implanted between the polysilicon layer and the oxide layer to form source/drain regions, whereby the split gate type flash semiconductor memory device is completed. According to the above-mentioned publication, the split gate flash semiconductor memory device has enhanced programming and erasing efficiencies and improved endurance in terms of its programmability and erasability.  
           [0009]    Meanwhile, Japanese Laid-Open Patent Publication No. 1999-26616 discloses a split gate type of memory device including an insulation layer for a floating gate formed on a semiconductor substrate, an insulation layer on the floating gate, a sidewall silicon oxide layer covering the sidewall of the floating gate, and a control gate insulated from the floating gate by the insulation layer and the sidewall silicon oxide layer. In this split gate type of memory device, the floating gate comprises polysilicon, and a silicon oxide layer is substituted for at least a portion of the polysilicon near the sidewall of the floating gate electrode. According to the publication, data writing and holding characteristics of the split gate type of memory device are improved without causing variations in the threshold voltage at the control gate, and data from being excessively erased.  
           [0010]    However, in the methods described above, additional processes are required for forming the insulation layer between the floating gate and the control gate, and the gates may not be precisely aligned. Therefore, problems still remain, such as excessive cell size and the difficulty of integrating the memory device with a logic device.  
         SUMMARY OF THE INVENTION  
         [0011]    An feature of the present invention is to provide a method of forming a floating gate having enhanced electron discharging and injecting efficiencies.  
           [0012]    Another feature of the present invention is to provide a method of manufacturing a non-volatile semiconductor memory device having a floating gate that is accurately aligned with a control gate.  
           [0013]    In accordance with one aspect of the present invention, an edge portion of a conductive pattern, constituting a floating gate, is provided with a high degree of sharpness. To this end, first, a conductive layer is formed on a semiconductor substrate. Next, the conductive layer is patterned using a photolithographic process to form a conductive a pattern on the semiconductor substrate. Then, a first insulation layer is formed on a sidewall of the conductive pattern in such a way that the sharpness of the edge portion of the conductive pattern is increased. Subsequently, a second insulation layer is formed at the upper portion of the conductive pattern so that the edge portion of the conductive pattern is even further increased.  
           [0014]    In accordance with another aspect of the present invention, an underlying structure including a first conductive pattern is formed on a semiconductor substrate. A first insulation layer is formed on a sidewall of the first conductive pattern. Subsequently, a second conductive pattern that will serve as a control gate is formed on the first insulation layer. Preferably, the second conductive pattern is formed by etching conductive material using a dry etching process. Then, a second insulation layer is formed on the second conductive pattern.  
           [0015]    In accordance with still another aspect of the present invention, a first insulation layer and a first conductive layer are sequentially formed on a semiconductor substrate. Then, the first conductive layer is etched to pattern the same in a first direction. A second insulation layer is formed on the etched first conductive layer. The first insulation layer and the etched first conductive layer are etched to pattern the same in a second direction and thereby form a first conductive pattern on the first insulation layer. A sidewall of the first conductive pattern is oxidized to form a first oxide layer on the sidewall of the conductive layer. A second conductive layer is formed on the semiconductor substrate including over the first conductive pattern and the second insulation layer. The second conductive layer is then patterned to form a second conductive pattern. Preferably, the patterning of the second conductive layer is performed using a dry etching process. Subsequently, a source region is formed in the semiconductor substrate adjacent the first conductive pattern. Then, the second insulation layer is etched away. An upper portion of the first conductive pattern is oxidized to form a second oxide layer. Finally, a drain region is formed in the semiconductor substrate adjacent the second conductive pattern.  
           [0016]    According to the present invention, the sharp edge portion of the floating gate enhances the efficiencies of electron discharging and injecting. In addition, the alignment between the floating gate and the control gate is ensured by forming the control gate using a dry etching process. Furthermore, the split gate type of flash semiconductor memory device of the present invention can be produced with a, a higher degree of integration than a conventional flash semiconductor memory device resulted whose cell size is 2-Tr. Still further, the split gate type of flash semiconductor memory device of the present invention, when used in a logic circuit, facilitates a high speed reading and writing of data without consuming a large amount of power.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0017]    These and other objects, features and advantages of the present invention will become more readily apparent by referring to the following detailed description thereof made in conjunction with the accompanying drawings.  
         [0018]    [0018]FIG. 1 is a flowchart illustrating one embodiment of a method of manufacturing a floating gate according to the present invention;  
         [0019]    [0019]FIGS. 2A to  2 C are cross-sectional views of a substrate, illustrating the method of forming the floating gate of a non-volatile semiconductor memory device according to the present invention as outlined in FIG. 1;  
         [0020]    [0020]FIG. 3 is a plan view of a non-volatile semiconductor memory device according to the present invention;  
         [0021]    [0021]FIG. 4 is a flowchart illustrating another embodiment of a method of manufacturing a non-volatile semiconductor memory device according to the present invention;  
         [0022]    [0022]FIGS. 5A to  5 C are cross-sectional views of a substrate, illustrating the method of manufacturing the non-volatile semiconductor memory device according to the embodiment outlined in FIG. 4;  
         [0023]    [0023]FIG. 6 is a flowchart illustrating still another embodiment of a method of manufacturing a non-volatile semiconductor memory device according to the present invention; and  
         [0024]    [0024]FIGS. 7A to  7 K are cross-sectional views of a substrate, illustrating still another embodiment of a method of forming a non-volatile semiconductor memory device according to present invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0025]    The present invention now will be described more fully hereinafter with reference to the accompanying drawings. Note, like reference numbers designate like elements throughout the drawings. Also, the relative thickness of layers may be exaggerated in the drawings for clarity in illustrating the present invention.  
         [0026]    [0026]FIG. 1 outlines a first embodiment of a method of manufacturing a floating gate according to the present invention.  
         [0027]    Referring to FIG. 1, a conductive layer of silicon (Si), e.g., amorphous silicon, polysilicon, or silicon doped with impurities, is formed on a semiconductor substrate (step S 11 ). Then, the conductive layer is patterned using a photolithographic process to form a conductive pattern on the semiconductor substrate. Alternatively, the conductive layer may comprise a metal such as copper (Cu), tungsten (W), aluminum (Al), titanium (Ti), and the like.  
         [0028]    Next, a first insulation layer is formed on a sidewall of the conductive pattern so that an edge portion of the conductive pattern has a first sharpness (step S 12 ). More specifically, the first insulation layer is formed by oxidizing the sidewall of the conductive pattern.  
         [0029]    Then, a second insulation layer is formed at an upper portion the conductive pattern so that the edge portion of the conductive pattern becomes even sharper (step S 13 ). The second insulation layer is also formed by oxidizing the conductive pattern but this time at the upper portion thereof.  
         [0030]    [0030]FIGS. 2A to  2 C illustrate, in detail, the first embodiment of a method of forming a floating gate of a non-volatile semiconductor memory device according to the present invention.  
         [0031]    Referring to FIG. 2A, a conductive layer is formed on a semiconductor substrate  10 . The conductive layer is patterned by a photolithographic process to form a conductive pattern  20  on the semiconductor substrate  10 . The conductive pattern  20  constitutes an underlying structure such as an electrode, a plug, a bit line or a word line. Preferably, however, the conductive pattern  20  constitutes a floating gate of a flash semiconductor memory device.  
         [0032]    When the conductive pattern  20  constitutes the floating gate of a flash semiconductor memory device, the conductive pattern  20  is formed of amorphous silicon, polysilicon or silicon doped with impurities by a low pressure chemical vapor deposition (LPCVD) process. When the conductive pattern  20  is formed using polysilicon, the conductive pattern  20  may have a polycide structure in which a metal silicide film is formed on a polysilicon film. Alternatively, when the conductive pattern  20  is formed using polysilicon or amorphous silicon, the conductive pattern  20  may be doped with impurities by a PCl 3  diffusion process, an ion implantation process, or an in-situ doping process.  
         [0033]    Also, an insulation layer comprising an oxide or nitride layer may be formed on the semiconductor substrate  10  before the conductive pattern  20  is formed. Then, the conductive layer is formed on the insulation layer. In this case, the conductive layer is formed on the semiconductor substrate  10  by a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) process or a sputtering process. The conductive layer is patterned to form the conductive pattern  20  on the insulation layer.  
         [0034]    Referring to FIG. 2B, a first insulation layer  22  is formed on a sidewall of the conductive pattern  20  by oxidizing only the sidewall of the conductive pattern  20 . As a result, the edge portion  23  of the conductive pattern  20  becomes cuspate, subtending a first angle Θ 1  with a vertical axis (an axis perpendicular to substrate  10 ). That is, the sidewall of the conductive pattern  20  becomes concave, whereby the edge portion  23  of the conductive pattern  20  becomes more pointed, in accordance with the formation of the first insulation layer  22  on the sidewall of the conductive pattern  20 .  
         [0035]    In the present embodiment, the first insulation layer  22  is formed on the sidewall of the conductive pattern  20  by merely oxidizing the sidewall of the conductive pattern  20 , i.e., without an additional process such as a CVD process or a PVD process. More specifically, the first insulation layer  22  is formed by a simple thermal oxidation process or a local oxidation of silicon (LOCOS) process. As a result, the first insulation layer  22  is rounded to thereby form the cuspate edge portion  23  of the conductive pattern  20 . The edge portion  23  thus has a first sharpness corresponding to the first angle Θ 1 .  
         [0036]    Note, also, a nitride layer may be formed at an upper portion of the conductive pattern  20  as an oxidation blocking layer before the first insulation layer  22  is formed on the sidewall of the conductive pattern  20 .  
         [0037]    Referring to FIG. 2C, a second insulation layer  26  is formed at the upper portion of the conductive pattern  20  by oxidizing the upper portion of the conductive pattern  20 . The oxidizing process forms a concavity in the upper portion of the conductive pattern  20  such that the second insulation layer  26  has a slightly rounded shape. As a result, the edge portion of the conductive pattern  20  becomes even more cuspate, as designated by reference numeral  27 , subtending a second angle Θ 2  with a horizontal axis (parallel to the substrate  10 ). That is, the edge portion  27  of the conductive pattern  20  attains a second sharpness greater than that of the first sharpness.  
         [0038]    In the present embodiment, the second insulation layer  26  is formed at the upper portion of the conductive pattern  20  by a thermal oxidation process or an LOCOS process, i.e., without a complex additional process such as a CVD process or a PVD process.  
         [0039]    Referring to FIG. 3, a non-volatile semiconductor memory device of the present invention includes an insulation region and active regions, and a floating gate  220  and a control gate  320  disposed on both sides of the insulation region. The direction A-A′ in FIG. 3 across the floating gate  220  will hereinafter be referred to as the “first direction” while the direction B-B′ across the active regions and the insulation region will be hereinafter referred to as the “second direction”.  
         [0040]    According to the present embodiment, both the upper portion and the sidewall of the floating gate are oxidized to form a sharp edge portion. Thus, the edge portion of the floating gate is much sharper than that of the conventional floating gate in which only the sidewall is oxidized. The sharper edge portion allows electrons to move more efficiently from the floating to a control gate. Accordingly, the characteristics of a non-volatile semiconductor memory device employing the floating gate of the present invention are enhanced.  
         [0041]    [0041]FIG. 4 outlines a method of manufacturing a non-volatile semiconductor memory device according the present invention.  
         [0042]    Referring to FIG. 4, an underlying structure, including a first conductive pattern, is formed on a semiconductor substrate (step S 21 ). Then, a first insulation layer is formed on a sidewall of the conductive pattern (step S 22 ).  
         [0043]    Next, a second conductive pattern serving as a control gate is formed on the first insulation layer (step S 23 ). Then, a second insulation layer is formed on the second conductive pattern (step S 24 ).  
         [0044]    [0044]FIGS. 5A to  5 D illustrate in detail the method of manufacturing the non-volatile semiconductor memory device according to the present invention. FIGS. 5A to  5 D are each a sectional view as taken along line B-B′ in FIG. 3.  
         [0045]    Referring to FIGS. 3 and 5A, an underlying structure  40  including a first conductive pattern  50 , which corresponds to a floating gate of the non-volatile semiconductor memory device, is formed on a substrate  30 . Note, also, an underlying insulation layer  45  comprising an oxide or nitride layer may be formed between the substrate  30  and the first conductive pattern  50 . In that case, the underlying insulation layer  45  is formed by a CVD process, a PVD process, or a sputtering process using an oxide, a nitride or oxynitride.  
         [0046]    The first conductive pattern  50  is formed of amorphous silicon, polysilicon, silicon doped with impurities or polysilicon having a metal silicide formed thereon by an LPCVD process. The first conductive pattern  50  is produced by forming a first conductive layer (not shown) on the substrate  30 , primarily etching the first conductive layer to pattern the same in the first direction, and by then secondarily etching first conductive layer to pattern the same in the second direction.  
         [0047]    More specifically, after the first conductive layer is formed on the substrate  30 , an oxide layer (not shown) or a nitride layer (not shown) is formed on the first conductive layer to define the active region. A photoresist pattern is formed on the oxide layer or the nitride layer, and then the first conductive layer is patterned in the first direction using a photolithographic process. The patterning of the first conductive layer in the first direction prevents the active region and the first conductive pattern corresponding to the floating gate from being misaligned. Subsequently, an insulation layer (not shown) is formed on the etched first conductive layer. Preferably, the insulation layer comprises nitride. After a photoresist pattern is formed on the insulation layer, the etched first conductive layer is patterned in the second direction using the photoresist pattern as an etching mask.  
         [0048]    Referring to FIG. 5B, a first insulation layer  52  is formed on a sidewall of the first conductive pattern  50 . The first insulation layer  52  is an oxide layer formed by oxidizing the sidewall of the first conductive pattern  50 , i.e., without an additional process such as a CVD process or a PVD process. More specifically, the first insulation layer  52  is formed from the first conductive pattern  50  by a simple thermal oxidation process or a local oxidation of silicon (LOCOS) process. As a result, the first insulation layer  52  is rounded, i.e., is crescent-shaped. Accordingly, the sharpness of the edge portion of the first conductive pattern  50  is increased.  
         [0049]    Referring to FIG. 5C, a second conductive pattern  60  corresponding to a control gate of the non-volatile semiconductor memory device is formed on the first insulation layer  52 . The second conductive pattern  60  is formed of a layer of amorphous silicon, polysilicon, silicon doped with impurities or polysilicon having a metal silicide thereon. The layer is then anisotropically etched (dry etched) to pattern the same and thereby complete the forming of the second conductive pattern.  
         [0050]    According to the present embodiment, the second conductive pattern  60  is formed in the shape of a spacer by an anisotropic dry etching process. To this end, a plasma etching process or a reactive ion etching (RIE) process may be used. Unlike a photolithographic process whose degree of resolution is limited, the dry etching process prevents a misalignment between the first conductive pattern  50  and the second conductive pattern  60 , thereby ensuring that the non-volatile semiconductor memory deviceperforms well.  
         [0051]    Referring to FIG. 5D, a second insulation layer  54  is formed on the first conductive pattern  50 . The second insulation layer  54  is an oxide layer formed by oxidizing an upper portion of the first conductive pattern  50 . In particular, the second insulation layer  54  is formed at the upper portion of the first conductive pattern  50  by a thermal oxidation process or an LOCOS process without a complex extra process such as a CVD process or a PVD process. Accordingly, the second insulation layer  54  is interposed between the first conductive pattern  50  and the second conductive pattern  60 . As a result, the edge portion of the first conductive pattern  50  becomes even sharper.  
         [0052]    In this embodiment as well, both the upper portion and the sidewall of the first conductive pattern  50  are oxidized to increase the sharpness at the edge portion thereof. When the non-volatile semiconductor memory device employs the first conductive pattern  50  as a floating gate, electrons migrate efficiently from the floating gate to the control gate.  
         [0053]    In the embodimentas described above, the second insulation layer  54  is formed at the upper portion of the first conductive pattern  50  by oxidizing the upper portion of the first conductive pattern  50 , after the second conductive pattern  60  is formed. Alternatively, the second insulation layer  54  may be formed before the second conductive pattern  60  is formed on the first conductive pattern  50 .  
         [0054]    [0054]FIG. 6 outlines another method of manufacturing a non-volatile semiconductor memory device according to the present invention.  
         [0055]    Referring to FIG. 6, a first insulation layer and a first conductive layer are formed sequentially on a semiconductor device (step S 30 ). Then, the first conductive layer is etched so as to be patterned in the first direction (step S 31 ). Subsequently, a second insulation layer is formed on the first conductive layer (step S 32 ).  
         [0056]    Next, the first insulation layer and the first conductive layer are etched so as to be patterned in the second direction to form a first conductive pattern on the first insulation layer (step S 33 ). A sidewall of the first conductive pattern is oxidized to form a first oxide layer on the sidewall of the first conductive layer (step S 34 ).  
         [0057]    A second conductive layer is then formed on the semiconductor substrate including over the first conductive pattern and the second insulation layer (step S 35 ). The second conductive layer is patterned to form a second conductive pattern (step S 36 ). After the second conductive pattern is formed, a source region is formed in the semiconductor substrate adjacent the first conductive pattern (step S 37 ).  
         [0058]    Next, the second insulation layer is etched (step S 38 ). Then, the upper portion of the first conductive pattern is oxidized to form a second oxide layer at the upper portion of the first conductive pattern (step S 39 ). Subsequently a drain region is formed in the semiconductor substrate adjacent the second conductive pattern (step S 40 ).  
         [0059]    [0059]FIGS. 7A to  7 K illustrate in more detail the method of forming the non-volatile semiconductor memory device as outlined above. More specifically, FIGS. 7A to  7 C are cross-sectional views of the substrate taken in the first direction in FIG. 3, and FIGS. 7D to  7 K are cross-sectional views of the substrate taken in the second direction in FIG. 3.  
         [0060]    Referring to FIG. 7A, a first insulation  110  and a first conductive layer  200  are sequentially formed on a semiconductor substrate  100 . The first insulation layer  110  is an oxide, nitride, or oxynitride layer, and the first conductive layer  200  comprises polysilicon, amorphous silicon, silicon doped with impurities, or polysilicon having a metal silicide thereon. The first conductive layer  200  is formed by a CVD process, a PVD process or a sputtering process as  
         [0061]    Referring to FIG. 7B, the first conductive layer  200  is etched so as to be patterned in the first direction. In particular, an oxide layer (not shown) or a nitride layer (not shown) is formed on the first conductive layer  200 . After a photoresist pattern is formed on the oxide layer or the nitride layer, the first conductive layer  200  is patterned in the first direction by a photolithographic process. This prevents misalignment in the first direction between the active region and the first conductive pattern  201  constituting the floating gate.  
         [0062]    Referring to FIG. 3 and FIG. 7C, a second insulation layer  220  is formed on the first conductive pattern  201 . Preferably, the second insulation layer  220  comprises a nitride, e.g., Si 3 N 4 , SiN x , SiONI. In particular, the second insulation layer  220  is formed by a CVD process, a plasma enhanced chemical vapor deposition (PECVD) process, a PVD process or a sputtering process.  
         [0063]    Referring to FIG. 7D, the first insulation layer  110  and the first conductive pattern  201  are secondarily etched so as to be patterned in the second direction. In particular, a photoresist pattern is formed on the second insulation layer  220 , and the first conductive pattern  201  and second insulation layer  220  are etched using the photoresist pattern as a mask.  
         [0064]    Referring to FIG. 7E, a sidewall of the first conductive pattern  201  is oxidized to form a first oxide layer  240  on the sidewall of the first conductive pattern  201 . Accordingly, an edge portion of the first conductive pattern  201  has a slightly rounded shape at the sidewall of the first conductive pattern  201 , whereby the sharpness of the edge portion is increased. And, as with the embodiments described above, the first oxide layer  240  is formed by merely oxidizing the sidewall of the first conductive pattern  201 , i.e., without a complex additional process such as a CVD process or a PVD process. That is, the first oxide layer  240  is formed by a simple thermal oxidation process or an LOCOS process. Furthermore, the second insulation layer  220  functions as an anti-oxidant layer during the forming of the first oxide layer  240 , preventing the upper portion of the first conductive pattern  201  from oxidizing while allowing only the sidewall of the first conductive pattern  201  to be oxidized.  
         [0065]    Referring to FIG. 7F, a second conductive layer  300  is formed on the semiconductor substrate  100  including over the first conductive pattern  201  and the second insulation layer  220 . The second conductive layer  300  is advantageously formed by an LPCVD process and comprises polysilicon, amorphous silicon, silicon doped with impurities or polysilicon having a metal silicide thereon. The second conductive layer  300  is etched to form a control gate of the flash semiconductor memory device.  
         [0066]    Referring to FIG. 7G, a second conductive pattern  320  is formed in the shape of a spacer by etching the second conductive layer  300 . In particular, the second conductive pattern  320  is formed by an anisotropic dry etching process. To this end, a plasma etching process, a reactive ion etching (RIE) process, may be used for forming the second conductive pattern  320 . A misalignment between the first conductive pattern  201  and the second conductive pattern  320  is prevented because the second conductive pattern  320  is formed by an anisotropic dry etching process, and not by a photolithographic process whose degree of resolution is rather limited. In addition, the anisotropic dry etching process removes part of the second conductive layer  300  to facilitate the forming of a source region.  
         [0067]    Referring to FIG. 7H, the source region  400  is formed in the semiconductor substrate  100  adjacent the first conductive pattern  201 . The source region  400  is formed by doping impurities into the semiconductor substrate  100  using an ion implantation process.  
         [0068]    Referring to FIG. 7I, the second insulation layer  220  on the first conductive pattern  201  is removed. To this end, the second insulation layer  220  is treated with an etching solution containing phosphoric acid (H 3 PO 4 ) at a temperature of about 180° C.  
         [0069]    Referring to FIG. 7J, a second oxide layer  260  is formed by oxidizing the upper portion of the first conductive pattern  201 . Accordingly, the edge portion of the first conductive pattern  201  becomes slightly rounded at the upper portion of the first conductive pattern  201 , i.e., relative a horizontal direction or a direction parallel to the semiconductor substrate  100 .  
         [0070]    According to the present embodiment, both of the upper portion and the sidewall of the first conductive pattern  201  are oxidized to form a sharp edge portion where the sidewall and the upper portion of the first conductive pattern  210  meet. That is, the edge portion of the first conductive pattern  201  will be much sharper than that of the conventional floating gate. As a result, electrons will move more efficiently from the floating to the control gate, thereby significantly enhancing the characteristics of the non-volatile semiconductor memory device.  
         [0071]    Referring to FIG. 7K, a drain region  420  is formed in the semiconductor substrate  100  adjacent the second conductive pattern  320 . The drain region  420  is formed by implanting impurities into the semiconductor substrate  100  using an ion implantation process.  
         [0072]    A wiring (not shown) and a drain contact (not shown) are formed at the upper portion of and adjacent the second conductive pattern  320  by a silicidation process and a metallization process to complete the flash semiconductor memory device.  
         [0073]    Compared to the conventional non-volatile semiconductor memory device, such as a stacked type of flash semiconductor memory device, the split gate type of flash semiconductor memory device manufactured according to the method of the present invention is less prone to problems such as over-erasing and high power consumption in a data writing mode. Also, the split gate type of flash semiconductor memory device can be manufactured according to the present invention to have a lower integration density than a conventional flash semiconductor memory device having a cell size of 2-Tr. Furthermore, the split gate type of flash semiconductor memory device of the present invention can be readily integrated with a logic circuit to produce a device that can read and write of data, at a high speed and yet consume a relatively low amount of power in doing so.  
         [0074]    Finally, although the present invention has been described in connection with the preferred embodiments thereof, the present invention is not so limited. Rather, various changes to and modifications of the preferred embodiments are seen to be within the true spirit and scope of the present invention as hereinafter claimed.