Abstract:
An electrical erasable programmable read-only memory (EEPROM) including a floating transistor formed on a semiconductor substrate and a tunneling transistor formed on a semiconductor substrate and configured to erase electrons trapped in the floating transistor. The tunneling transistor has a source junction region and a drain junction region that are integrally joined by lateral diffusion. The EPROM maintains a small cell size without any additional mask process, and is useable as an MTP EEPROM because electrical erasure is enabled. In addition, the adjustment of the width of a gate constituting the tunneling transistor ensures an improved degree of freedom to adjust an erasure voltage can be enhanced.

Description:
[0001]    The present application claims priority under 35 U.S.C. 119 and 35 U.S.C. 365 to Korean. Patent Application No. 10-2011-0105445 (filed on Oct. 14, 2011), which is hereby incorporated by reference in its entirety. 
       BACKGROUND 
       [0002]    Non-volatile memory devices such as EEPROMs or similar devices may be capable of storing information even when the supply of external power is stopped. EPROMs may include an EPROM having a stacked-gate structure in which two polycrystalline silicon layers acting as a gate are vertically stacked, an EPROM having a single gate structure using a single polycrystalline silicon layer, and similar configurations. 
         [0003]    Stacked-gate structure EPROMs may be advantageous for high integration of devices, but stacked-gate structure EPROMs may have the drawback of requiring a relatively complicated manufacturing process that includes manufacturing together with logic devices (e.g. metal oxide semiconductor field effect transistors (MOSFET) or complementary MOSFET (CMOSFET) that use a single gate process in a single-layered structure). However, single gate structure EPROMs have a relatively simple standard process, even though single gate structure EPROMs may have drawbacks compared to stacked-gate structure EPROMs in terms of cell integration and performance. 
         [0004]    Accordingly, single gate structure EEPROMs may often be embedded in CMOS logic and mixed-signal circuits and usefully applied as low-priced, low-density devices. A single gate structure EPROM may be compatible with a standard logic process and therefore memory cell functions may be added without significant additional processes or cost. Accordingly, single gate structure EPROMs may be easily mounted in a logic device product. 
         [0005]      FIG. 1  is a top plan view of an EPROM having a single gate structure and  FIG. 2  is a cross-sectional view of the EPROM shown in  FIG. 1 , in accordance with the related art. An. EPROM may include P-type well  12  and N-type well  13  formed in parallel on/over a semiconductor substrate  11 . P-type well  12  and N-type well  13  may be isolated from each other by swallow trench isolation (STI) region  14 . First gate insulation film  15  may be formed on/over semiconductor substrate  11  where P-type well  12  is formed. First gate  16  or a select gate (SG) may be formed on/over first gate insulation film  15 . 
         [0006]    An N-type source junction region may be formed in an upper portion of P-type well  12  at one side of first gate  16 . An N-type drain junction region may be formed in an upper portion of P-type well  12  at the other side of first gate  16 . Accordingly, select NMOS transistor  1  may be formed by first gate  16  and source/drain junction regions  18 . Similarly, first gate insulation film  15  may be formed on/over semiconductor substrate  11  where N-type well  13  is formed. Second gate  17  or floating gate (FG) may be formed on/over first gate insulation film  15 . 
         [0007]    A P-type source junction region may be formed in an upper portion of N-type well  13  at one side of second gate  17 . A P-type drain junction region may be formed in an upper portion of N-type well  13  at the other side of second gate  17 . Floating PMOS transistor  2  may be formed by second gate  17  and source/drain junction regions  19 . Salicide blocking layer  20  may be formed on/over second gate  17 , thereby preventing salicide from being formed on second gate  17 . 
         [0008]    However, EPROMs according to the related art may have a problem that electrical erasure is impossible or unreliable even with the advantage of a simplified standard manufacturing process. 
       SUMMARY 
       [0009]    Embodiments relate to an electrical erasable programmable read-only memory (EEPROM) which is useable as multiple time programmable (MTP) EEPROM with by enabling electrical erasure while maintaining a small cell size through the change of the structure of a single gate EPROM, and a manufacturing method thereof. 
         [0010]    Embodiments relate to an EEPROM including at least one of: (1) A floating transistor formed on/over a semiconductor substrate. (2) A tunneling transistor formed on/over a semiconductor substrate and configured to erase electrons trapped in the floating transistor, wherein the tunneling transistor has a source junction region and a drain junction region that are integrally joined by lateral diffusion. 
         [0011]    In embodiments, the width of the gate of the tunneling transistor is narrower than the width of the gate of the floating transistor. In embodiments, the gate of the floating transistor has a width ranging from about 0.5 μm to 0.6 μm. In embodiments, the gate of the tunneling transistor has a width ranging from. about 0.16 μm to 0.2 μm. In embodiments, the gate of the tunneling transistor has an end portion intersecting edge portions of the source junction region and the drain junction region at a preset length. In embodiments, the preset length ranges from about 0.16 μm to 0.2 μm. 
         [0012]    Embodiments relate to a method for manufacturing an EEPROM including at least one of: (1) Forming a floating gate and a tunneling gate on/over a semiconductor substrate. (2) Forming a floating transistor by forming a source junction region and a drain junction region in contact with the floating gate. (3) Forming a tunneling transistor by forming a source junction region and a drain junction region integrally joined by lateral diffusion in contact with the tunneling gate. 
         [0013]    In embodiments, the width of the tunneling gate is formed to be narrower than of the width of the floating gate. In embodiments, an end portion of the tunneling gate is formed to intersect edge portions of the integrally connected source junction region and drain junction region at a preset length. In embodiments, the integrally joined source junction region and drain junction region are formed by an ion implantation process with a tilt of about 25° to 45°. In embodiments, the ion implantation process is performed at an ion implantation energy ranging from about 65 KeV to 100 KeV is used and a dose amount ranges from about 5E12/cm 2  to 1E13/cm 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    The above and other objects and features of embodiments will become apparent from the following description, given in conjunction with the accompanying drawings, in which: 
           [0015]      FIG. 1  is a top plan view of an EPROM having a single gate structure, in accordance with the related art. 
           [0016]      FIG. 2  is a cross-sectional view of the EPROM illustrated in  FIG. 1 , in accordance with the related art. 
           [0017]      FIG. 3  is a top plan view of an EEPROM, in accordance with embodiments. 
           [0018]      FIG. 4  is a cross-sectional view of the EEPROM illustrated in  FIG. 3 , in accordance with embodiments. 
           [0019]      FIG. 5  is a top plan view of an EEPROM, in accordance with embodiments. 
           [0020]      FIG. 6  is a cross-sectional view of the EEPROM, in accordance with embodiments. 
           [0021]      FIG. 7  is a top plan view of an EEPROM, in accordance with embodiments. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    The advantages and features of embodiments and methods of accomplishing these will be clearly understood from the following description taken in conjunction with the accompanying drawings. However, embodiments are not limited to those embodiments described, as embodiments may be implemented in various forms. It should be noted that the present embodiments are provided to make a full disclosure and also to allow those skilled in the art to know the full range of the embodiments. Therefore, the embodiments are to be defined only by the scope of the appended claims. 
         [0023]      FIG. 3  is a top plan view of an .EEPROM in accordance with embodiments.  FIG. 4  is a cross-sectional view of the EEPROM illustrated in  FIG. 3 , in accordance with embodiments. A cross-sectional view taken along the line IV-IV of  FIG. 3  is illustrated on the left in  FIG. 4  and a cross-sectional view taken along the line IV′-IV′ of  FIG. 3  is illustrated on the right in  FIG. 4 .  FIGS. 3 and 4  illustrate only a unit cell region of the EEPROM, in accordance with embodiments. 
         [0024]    Referring to  FIGS. 3 and 4 , an EEPROM in accordance with embodiments includes at least one of (1) Semiconductor substrate  101 . (2) First well.  111  of a first conductivity. (3) Second well  131  of the first conductivity. (3) Well  121  of a second conductivity. (4) STI region  103 . (5) Source/drain junction regions  127  of the first conductivity. (6) First source/drain junction regions  117  of the second conductivity. (7) Second source/drain junction regions  137  of the second conductivity. (8) First gate insulation film  113 . (9) Second gate insulation film  123 . (10) Third gate insulation film  133 . (11) First gate  115 . (12) Second gate  125 . (13) Third gate  135 . (14) First salicide blocking layer  129  or none-salicide (NSAL) layer or similar material. (15) Second salicide blocking layer  139  or none-salicide (NSAL) layer or similar material. 
         [0025]    Hereinafter, for convenience of explanation, first well  111  of the first conductivity, second well  131  of the first conductivity, well  121 , source/drain junction regions  127  of the first conductivity, first source/drain junction regions  117  of the second conductivity, and second source/drain junction regions  137  of the second conductivity are designated as “P-type first well  111 ”, “P-type second well  131 ”, “N-type well  121 ”, “P-type source/drain junction regions  127 ”, “N-type first source/drain junction regions  117 ”, and “N-type second source/drain junction regions  137 ”, respectively. As such, while the following description will be made on the assumption that the first conductivity means the P-type and the second conductivity means the N-type, they may be interchangeable, in accordance with embodiments. 
         [0026]    In embodiments, an EEPROM may be divided into select transistor  110 , floating transistor  120 , and tunneling transistor  130 . A method of manufacturing the EEPROM in accordance with embodiments is illustrated in  FIGS. 3 and 4 . 
         [0027]    First, P-type first well (PW)  111  may be formed on one side of the top of semiconductor substrate  101 . N-type well (NW)  121  may be formed at the front part of the other side of the top of the semiconductor substrate  101 . P-type second well (PW)  131  may be formed at the rear part of the other side of the top of the semiconductor substrate  101 . In embodiments, P-type first well (PW)  111 , N-type well (NW)  121 , and P-type second well (PW) may be formed next to each other in/on/over semiconductor substrate  110 , in that respective order. In embodiments, P-type first well  111 , N-type well  121 , and P-type second well  131  may be formed at the same depth. 
         [0028]    STI region  103  may be formed in an upper portion of the semiconductor substrate  101  to define an active region and an inactive region. P-type first well  111 , N-type well  121 , and P-type second well  131  may be isolated from each other by STI region  103 . In embodiments, as illustrated in  FIG. 4 , only upper portions of P-type first well  111 , N-type well  121 , and P-type second well  131  may be isolated by STI regions  103 . In embodiments, P-type first well  111 , the N-type well  121 , and the P-type second well  131  may be fully separated and/or isolated from each other. P-type first well  111  may serve as a base layer for select transistor  110 . N-type well  121  may serves as a base layer for floating transistor  120 . P-type second well  131  may serves as a base layer for tunneling transistor  130 . 
         [0029]    In accordance with embodiments, an insulation film and a polysilicon film may be sequentially formed on the top surface of semiconductor substrate  101  and then patterned, thereby forming first gate insulation film  113 , second gate instillation film  123 , and third gate insulation film  133 . First gate (SG)  115  may be formed on/over first gate insulation film  113 , which is on/over P-type first well  111 , in accordance with embodiments. Second gate  125  may be formed on/over second gate insulation film  123 , which is on/over N-type well  121 , in accordance with embodiments. Third gate  135  may be formed on/over third gate insulation film  133 , which is on/over P-type second well  131 , in accordance with embodiments. In embodiments, during these processes, first gate  115 , second gate  125 , and third gate  135  may be simultaneously formed by the same process, or sequentially formed by separate processes. 
         [0030]    In embodiments, a salicide reaction preventing film may be formed before patterning insulation films and a polysilicon film. The salicide reaction preventing film, insulation films, and the polysilicon film may be patterned together to form first salicide blocking layer  129  on/over second gate  125  and second salicide blocking layer  139  on/over third gate  135 , in accordance with embodiments. First salicide blocking layer  129  and second salicide blocking layer  139  may prevent salicide from being formed on second gate  125  and third gate  135 , in accordance with embodiments. 
         [0031]    In embodiments, an N-type source junction region may be formed by ion implantation in an upper portion of P-type first well  111  at one side of the first gate  115  and an N-type drain junction region may be formed in an upper portion of P-type first well  111  at the other side of first gate  115 , thereby forming N-type first source/drain junction regions  117  in contact with the first gate  115 , in accordance with embodiments. Similarly, a P-type source junction region may be formed by ion implantation in an upper portion of N-type well  121  at one side of second gate  125  and a P-type drain junction region is formed in an upper portion of N-type well  121  at the other side of second gate  125 , thereby forming P-type second source/drain junction regions  127  in contact with second gate  125 , in accordance with embodiments. Similarly, an N-type source junction region may be formed by ion implantation in an upper portion of P-type second well  131  on one side of third gate  135  and an N-type drain junction region may be formed in an upper portion of P-type second well  131  on the other side of third gate  135 , thereby forming N-type second source/drain junction regions  137  in contact with third gate  135 , in accordance with embodiments. In embodiments, N-type first source/drain junction regions  117  and N-type second source/drain junction regions  137  may be simultaneously formed by the same process or sequentially formed by separate processes. 
         [0032]    In embodiments, select transistor  110  including first gate  115  and first source/drain junction regions  117  may be formed in the region of P-type first well  111 . In embodiments, floating transistor  120  including second gate  125  and. P-type source/drain junction regions  127  may be formed in the region of N-type well  121 . In embodiments, tunneling transistor  130  including third gate  135  and N-type second source/drain junction regions  137  may be formed in the region of P-type second well  131 . Though not shown in detail, structures such as a lightly doped drain (LDD) region, a sidewall, a spacer, and the like may be further be selectively included in each transistor region, in accordance with embodiments. 
         [0033]    In embodiments, as illustrated in  FIG. 4 , Fowler-Nordheim (F-N) tunneling may occur in the source junction region and the drain junction region, separately, as indicated by the “arrows”. In embodiments, a single gate EPROM may be modified to include tunneling transistor  130  in its structure, while maintaining a relatively small cell size without additional mask processes. In operation, an erasure voltage is applied to a single gate EPROM using tunneling transistor  130 , thereby enabling the erasing of electrons trapped in floating transistor  120 , in accordance with embodiments. 
         [0034]      FIG. 5  is a top plan view of an EEPROM in accordance with embodiments and  FIG. 6  is a cross-sectional view of the. EEPROM illustrated in  FIG. 5 . A cross-section of line VI-VI of  FIG. 5  is illustrated on the left in  FIG. 6  and a cross-section of line VI′-VI′ of  FIG. 5  is illustrated on the right in  FIG. 6 .  FIGS. 5 and 6  illustrate only a unit cell region of an EEPROM, in accordance with embodiments. 
         [0035]    Embodiments illustrated in  FIGS. 5 and 6  have several similarities with the embodiments illustrated in  FIGS. 3 and 4 , corresponding with like reference numbers. A notable difference between the embodiments illustrated in  FIGS. 5 and 6  is that tunneling transistor  130   a  may be modified based on modifications of third gate insulation film  133   a , a third gate  135   a,  N-type second source/drain junction regions  137   a,  and a second salicide blocking layer  139   a  or none-salicide (NSAL) layer. 
         [0036]    In embodiments, in the patterning process for the formation of third gate  135   a,  the width W 2  of third gate  135   a  may be patterned to be narrower than the width W 1  of second gate  125 . For example, in embodiments, the width W 1  of second gate  125  may be patterned to be between about 0.5 μm and 0.6 μm and the width W 2  of third gate  135   a  may be patterned to be between about 0.16 μm and 0.2 μm. 
         [0037]    In embodiments, when the width W 2  of third gate  135   a  is patterned to be narrower than the width W 1  of second gate  125 , N-type second source/drain junction regions  137   a  may be formed to be integrally joined by lateral diffusion. The width W 2  of third gate  135   a  may be formed to be narrow so that lateral diffusion occurs during an ion implantation process for formation of N-type second source/drain junction regions  137   a,  in accordance with embodiments. For example, in embodiments, an ion implantation process for formation of N-type second source/drain junction regions  137   a  may be performed under the condition that an ion implantation angle with a tilt of about 25° to 45° is used, an ion implantation energy ranging from about 65 KeV to 100 KeV is used, and a dose amount ranges from about 5E12/cm 2  to 1E13/cm 2 . 
         [0038]    In accordance with embodiments,  FIG. 6  illustrates F-N tunneling occurring at the connecting portion of source/drain junction regions, as indicated by the “arrow”. Embodiments illustrated in  FIG. 6  may be distinguished from embodiments illustrated in  FIG. 4 , as  FIG. 4  illustrates Fowler-Nordheim (F-N) tunneling occurring in the source junction region and the drain junction region separately, as indicated by the “arrow”. This may contribute to increases in the junction breakdown voltage and may reduce an erase time using a high bias. 
         [0039]      FIG. 7  is a top plan view of an EEPROM, in accordance with embodiments. Embodiments illustrated in  FIG. 7  have several similarities with the embodiments illustrated in  FIG. 5 , corresponding with like reference numbers. A notable difference between the embodiments illustrated in  FIG. 7  is that tunneling transistor  130   b  is modified, in accordance with embodiments. 
         [0040]    Tunneling transistor  130   b  may formed by adjusting the length at which an end portion of third gate ( 135   a  of  FIG. 6 ) intersects edge portions of N-type second source/drain junction regions ( 137   a  of  FIG. 6 ), so as to be reduced by a preset length L, in accordance with embodiments. For example, in embodiments, preset length L may be between about 0.16 μm and 0.2 μm. In embodiments, preset length L may be made substantially equal to the width of third gate  135   a  of  FIG. 6 . In embodiments, by adjusting the length at which an end portion of third gate ( 135   a  of  FIG. 6 ) intersects edge portions of N-type second source/drain junction regions ( 137   a  of  FIG. 6 ) to be reduced, N-type second source/drain junction regions ( 137   a  of  FIG. 6 ) may be integrally joined by lateral diffusion and may be formed more easily. 
         [0041]    In embodiments, the single gate EPROM may maintain a relatively small cell size by changing its structure without additional mask processes and include a tunneling transistor. Single gate EPROM may be useable as a MTP EEPROM because electrical erasing may be enabled. In embodiments, a single gate structure EEPROM may have a relatively small cell size. In embodiments, a degree of freedom for adjusting an erasure voltage may be enhanced by adjusting the width of a gate constituting the tunneling transistor. 
         [0042]    While embodiments have been shown and described, it will be understood by those skilled in the art that various changes and modification may be made without departing the scope of the embodiments as defined the following claims.