Patent Publication Number: US-9431461-B2

Title: Transistor having a vertical channel

Description:
CROSS-REFERENCES TO RELATED APPLICATION 
     This application is a division of U.S. patent application Ser. No. 14/634,121 filed on Feb. 27, 2015, which is a division of U.S. patent application Ser. No. 13/948,490 filed on Jul. 23, 2013, now abandoned, which claims priority under 35 U.S.C. 119(a) to Korean application number 10-2013-0021164, filed on Feb. 27, 2013, in the Korean Intellectual Property Office. The disclosures of each of the foregoing applications are incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     1. Technical Field 
     Exemplary embodiments of the present invention relate to a semiconductor integrated circuit device, and more particularly, to a transistor, and a resistance variable memory device including the same, and a manufacturing method thereof. 
     2. Related Art 
     With the rapid development of mobile and digital information communication and consumer-electronic industry, studies on existing electronic charge controlled-devices are expected to encounter the limitation. Thus, new functional memory devices of new concept other than the existing electronic charge devices need to be developed. In particular, next-generation memory devices with large capacity, ultra-high speed, and ultra-low power are in demand. 
     Currently, resistive memory devices using a resistance device as a memory medium have been suggested as the next-generation memory devices, and some of the examples are phase-change random access memories (PCRAM5), resistance RAMS (ReRAMs), and magentoresistive RAMs (MRAMs). 
     The resistive memory devices may be basically configured of a switching device and a resistance device and store data “0” or “1” according to a state of the resistance device. 
     Even in the resistive memory devices, the first priority is to improve an integration density and to integrate most memory cells in a narrow area. 
     To meet these demands, the resistive memory devices have also adopted three-dimensional (3D) vertical transistor structures. 
     However, even in the 3D vertical transistors, thin gate insulating layers may be required. Thus, when a high voltage is supplied to a gate, a high electric field is applied to a lightly doped drain (LDD) region and gate induced drain leakage (GIDL) may be caused. 
     SUMMARY 
     According to one aspect of an exemplary embodiment of the present invention, a transistor may include an active pillar including a channel region, a source formed in one end of the channel region, and a lightly doped drain (LDD) region and a drain formed in the other end of the channel region, a first gate electrode formed to surround a periphery of the LDD region and having a first work function, and a second gate electrode formed to be connected to the first gate electrode and to surround the channel region, and having a second work function that is higher than the first work function. 
     According to another aspect of an exemplary embodiment of the present invention, a resistance variable memory device may include a vertical transistor including an active pillar including a channel region, a source formed in one end of the channel region, and a lightly doped drain (LDD) region and a drain formed in the other end of the channel region, a first gate electrode formed to surround a periphery of the LDD region and having a first work function, and a second gate electrode formed to be connected to the first gate electrode and to surround the channel region and having a second work function higher than the first work function, and a resistive memory structure connected to the drain of the vertical transistor. 
     According to still another aspect of an exemplary embodiment of the present invention, a method of manufacturing a resistance variable semiconductor device may include forming a source region in a semiconductor substrate, forming a semiconductor layer on the source region, patterning the semiconductor layer to form an active pillar, forming a first gate electrode to surround the active pillar, surrounding an upper region of the first gate electrode with an insulating layer while exposing a lower region of the first gate electrode, and forming a second gate electrode by increasing a work function of the exposed first gate electrode. 
     These and other features, aspects, and embodiments of the present invention are described below in the section entitled “DETAILED DESCRIPTION”. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other aspects, features and other advantages of the subject matter of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a schematic cross-sectional view illustrating a resistance variable memory device including a vertical transistor according to an exemplary implementation of the inventive concept; 
         FIGS. 2 to 5  are cross-sectional views sequentially illustrating a process of manufacturing a vertical transistor of a resistance variable memory device according to an exemplary implementation of the inventive concept; 
         FIG. 6  is a schematic cross-sectional view illustrating a resistance variable memory device including a vertical transistor according to another exemplary implementation of the inventive concept; 
         FIGS. 7 and 8  are cross-sectional views sequentially illustrating a process of manufacturing a vertical transistor of  FIG. 6 ; 
         FIG. 9  is a schematic diagram illustrating a vertical transistor according to another exemplary implementation of the inventive concept; and 
         FIG. 10  is a schematic cross-sectional view illustrating a vertical transistor according to another exemplary implementation of the inventive concept. 
     
    
    
     DETAILED DESCRIPTION 
     Hereinafter, various exemplary embodiments of the present invention will be described in greater detail with reference to the accompanying drawings. 
     Exemplary embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of exemplary embodiments (and intermediate structures). As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, exemplary embodiments should not be construed as limited to the particular shapes of regions illustrated herein but may include deviations in shapes that result, for example, from manufacturing. In the drawings, lengths and sizes of layers and regions may be exaggerated for clarity. Like reference numerals in the drawings denote like elements. 
     It should be readily understood that the meaning of “on” and “over” in the present disclosure should be interpreted in the broadest manner such that “on” means not only “directly on” but also “on” something with an intermediate feature(s) or a layer(s) therebetween, and that “over” means not only directly on top but also on top of something with an intermediate feature(s) or a layer(s) therebetween. It is also noted that in this specification, “connected/coupled” refers to one component not only directly coupling another component but also indirectly coupling another component through an intermediate component. In addition, a singular form may include a plural form as long as it is not specifically mentioned in a sentence. 
     Referring to  FIG. 1 , a resistance variable memory device  100  according to an exemplary embodiment may include a vertical transistor  101  and a resistive memory structure  185 . 
     The vertical transistor  101  may include an active pillar  120 , a first gate electrode  140 , and a second gate electrode  160 . 
     A source S may be provided below the active pillar  120  and a drain D is provided on the active pillar  120 . The active pillar  120  between the source S and the drain D serves as a channel region of the vertical transistor  101 . At this time, the active pillar  120  may be interpreted as a structure including the source S, or the active pillar  120  may have a structure separately formed on the source S. The source S and the active pillar  120  may be semiconductor layers. Further, a lightly doped drain (LDD) region LDD, which is a low concentration impurity region, may be formed in the active pillar  120  between a portion of the active pillar  120  serving as the channel region and the drain D, and thus a short channel effect may be alleviated. 
     The first gate electrode  140  may be formed to surround around an upper portion of the active pillar  120  in which the LDD region LDD is formed. The first gate electrode  140  may partially overlap a portion of the drain D, but the first gate electrode  140  may be formed substantially on a location of the active pillar  120  corresponding to the LDD region LDD. 
     The second gate electrode  160  may be connected to the first gate electrode  140  and surround the channel region of the active pillar  120 . For example, the second gate electrode  160  may be in contact with the first gate electrode  140  and located below the first gate electrode  140 . At this time, the first gate electrode  140  may include a material having a work function lower than that of the second gate electrode  160 . That is, when the work function of the first gate electrode  140  overlapping the LDD region LDD is lowered, high electric field characteristics causing GIDL may be alleviated, and thus the GIDL characteristics of the LDD region LDD and the drain D adjacent to the LDD region LDD may be improved. 
     At this time, a gate insulating layer  135  may be interposed between the first and second gate electrodes  140  and  160  and the active pillar  120 . Various insulating layers such a metal oxide layer and a silicon oxide layer may be used as the gate insulating layer  135 . 
     The resistive memory structure  185  may be configured of a lower electrode  170  and a resistive memory layer  180 . The lower electrode  170  may be a conductive layer formed on the drain D and provide a current and a voltage to the resistive memory layer  180 . Although not illustrated in  FIG. 1 , an ohmic layer may be interposed between the lower electrode  170  and the drain D depending on material properties of the lower electrode  170 . The resistive memory layer  180  may be a layer of which a resistance value is changed according to the voltage and current provided from the lower electrode  170 . As the resistive memory layer  180 , a PCMO layer that is a material for a Re RAM, a chalcogenide layer that is a material for a PCRAM, a magnetic layer that is a material for a MRAM, a magnetization reversal device layer that is a material for a spin-transfer torque magnetoresistive RAM (STTMRAM), a polymer layer that is a material for a polymer RAM (PoRAM), or the like, may be variously used. 
     In the vertical transistor according to the exemplary embodiment, the gate electrode is formed of a material having a relatively lower work function in the LDD region LDD, which has a lower GIDL barrier and a high electric field is applied to, than in the channel region. 
     As described above, the gate electrode having a relatively low work function is disposed around the LDD region LDD to compensate the low GIDL barrier according to application of the high electric field, and thus leakage current may be reduced. 
     A method of manufacturing a resistance variable memory device including a vertical transistor will be described in detail with reference to  FIGS. 2 to 5 . 
     Referring to  FIG. 2 , a source  110  is formed in a semiconductor substrate  105  by implementing impurities into an upper portion of the semiconductor substrate  105 . A semiconductor layer is formed on the semiconductor substrate  105  in which the source  110  is formed. For example, the semiconductor layer may be an impurity-doped polysilicon layer or a layer that epitaxially grows the semiconductor substrate  105  in which the source is formed. A hard mask layer  130 , for example, a silicon nitride layer is deposited on the semiconductor layer. Predetermined portions of the hard mask layer  130  and the semiconductor layer are patterned to form a plurality of active pillars  120 . A gate insulating layer  135  is formed on surfaces of the plurality of active pillars  120  and the semiconductor substrate  105 . As the gate insulating layer  135 , a layer in which a conductive material such as silicon (Si), tantalum (Ta), titanium (Ti), barium titanium (Bari), barium zirconium (BaZr), zirconium (Zr), hafnium (Hf), lanthanum (La), aluminum (Al), yttrium (Y), or zirconium suicide (ZrSi) is oxidized, may be used. A first conductive layer is deposited on the semiconductor substrate  105  including the gate insulating layer  135 , and anisotropically etched to surround the active pillar  120 . Therefore, a first gate electrode  140  is formed over an outer circumference of each of the active pillar  120  covered with the gate insulating layer  135 . At this time, by anisotropic over-etching, the first gate electrode  140  may be formed to have a height lower than that of the active pillar  120 . For example, as the first gate electrode  140 , a transition metal layer including a metal such as Ti, Ta, cobalt (co), or platinum (Pt) may be used. 
     As illustrated in  FIG. 3 , a first insulating layer  145  is formed to fill a space between the active pillars  120 . Next, the first insulating layer  145  is recessed to expose an upper region of the first gate electrode  140 . At this time, an upper surface of the first insulating layer  145  may be located to correspond to a channel formation region of the active pillar  120 . A second insulating layer  150  is formed to cover the exposed upper region of the first gate electrode  140 . The second insulating layer  150  may be formed of a material having an etch selectivity different from that of the first insulating layer  145 . 
     Referring to  FIG. 4 , the first insulating layer  145  is selectively removed to expose a lower region of the first gate electrode  140 . Next, nitrogen ions are implanted into the exposed first gate electrode  140  to form a second gate electrode  160  formed of a metal nitride layer, for example, a titanium nitride (TiN) layer as illustrated in  FIG. 5 . As known, a refractory metal layer such as a Ti layer has a work function lower than that of a metal nitride layer such as a TiN layer. Therefore, a portion of the gate electrode corresponding to an LDD region LDD is formed of a material having a relatively low work function, and thus leakage current due to GIDL may be reduced. 
     Next, referring back to  FIG. 1 , the hard mask layer  130  on the active pillar  120  is removed, and the LDD region LDD is formed by implanting impurities having a law concentration into the active pillar  120 . Subsequently, impurities having a high concentration are implanted in the active pillar  120  in which the LDD region LDD is formed to define a drain D. 
     A lower electrode  170  and a resistive memory layer  180  are sequentially formed on the drain D to fabricate the resistance variable memory device. 
     A metal silicide layer other than the the a nitride layer may be used as the second gate electrode  160 . 
     That is, as illustrated in  FIG. 6 , a first gate electrode  140  surrounding the LDD region LDD may be formed of a transition metal layer like examples in the above-described exemplary embodiment, and a second gate electrode  165  may be formed of a transition metal silicide layer located below the first gate electrode  140  and having a work function higher than that of the first gate electrode  140 . At this time, a thickness b of the second gate electrode  165  may be larger than a thickness a of the first gate electrode  140 . 
     Since the transition metal layer also has a work function lower than that of the transition metal silicide layer, leakage current around the LDD region LDD having weak GIDL characteristic may be reduced. 
     A method of manufacturing the vertical transistor illustrated in  FIG. 6  will be described in detail with reference to  FIGS. 7 and 8 . Here, some of the manufacturing method of the resistance variable memory device in the exemplary embodiment are substantially the same as the processes of  FIGS. 1 to 3  in the manufacturing method of the resistance variable memory device in the above-described exemplary embodiment, and thus processes subsequent to the process of  FIG. 3  will be described. 
     Referring to  FIG. 7 , the first insulating layer ( 145  of  FIG. 3 ) is selectively removed to expose a sidewall of the first gate electrode  140 . A silicon layer  163  is deposited on an exposed surface of the first gate electrode  140  to a predetermined thickness. The silicon layer  163  may be formed to be located below a second insulating layer  150 . 
     Referring to  FIG. 8 , a heat treatment is performed on the semiconductor substrate  105  so that the first gate electrode  140  is reacted with the silicon layer  163  in contact with the first gate electrode  140  to form the second gate electrode  165  formed of a transition metal silicide layer. At this time, since the second gate electrode  165  is a layer formed through the heat reaction of the first gate electrode  140  with the silicon layer  163 , a thickness of the silicon layer  163  may be provided as a thickness of the second gate electrode  165 . Therefore, the second gate electrode  165  may have a thickness larger than that of the first gate electrode  140 . 
     As illustrated in  FIG. 9 , first and second gate electrodes  142  and  167  may be sequentially formed to surround an active pillar  120 . 
     That is, the first gate electrode  142  is formed to surround an outer circumference of the active pillar  120  covered with the gate insulating layer  135 . At this time, it is important that the first gate electrode  142  is formed not to overlap an LDD region LDD. 
     Next, the second gate electrode  167  is formed to surround an outer circumference of the gate electrode  142 . At this time, the second gate electrode  167  may extend by a predetermined length c more than the first gate electrode  142  so that the second gate electrode  167  overlap a portion of the LDD region LDD. Therefore, for example, only a portion of the insulating layer  135  is present between the LDD region LDD and the second gate electrode  167  without interposing of the first gate electrode  142 . Here, the second gate electrode  167  may have a work function higher than that of the first gate electrode  142 . However, in some cases, the second gate electrode  167  may be formed of a material having a work function similar to or lower than that of the first gate electrode  142 . 
     In the vertical transistor having the above-described structure, since the first gate electrode  142  is formed to have a relatively low work function and a distance between the LDD region LDD and the second gate electrode  167  overlapping the LDD region LDD is increased, a high electric field applied to the LDD region LDD may be alleviated and leakage current due to low GIDL may be reduced. 
     In addition to the vertical transistor structure, the dual gate electrode structure may be applied to a buried gate electrode structure. 
     That is, as illustrated in  FIG. 10 , a trench  210  is formed in a semiconductor substrate  200 . A source S and a drain D are formed in the semiconductor substrate  200  at both sides of the trench  210 . 
     A first electrode  220  and a second electrode  230  may be formed in the trench  210  in which a gate insulating layer  215  is formed. The first gate electrode  220  may be formed on an inner surface of the trench  210 . The first gate electrode  220  may be formed to be located substantially in a lower portion of the trench  210  so that the first gate electrode  220  may not overlap the source S and the drain D. 
     The second gate electrode  230  may be formed to fill the inside of the trench  210  surrounded with the first gate electrode  220 . At this time, the second gate electrode  230  may be formed to have a height longer than that of the first gate electrode  220  so that the second gate electrode  230  may overlap portions of the source S and the drain D. 
     Although not shown in  FIG. 10 , it would have been obvious to a person having ordinary skill in the art that the resistive memory structure  185  illustrated in  FIGS. 2 and 6  may be additionally formed. The reference numerals  240  and  250  denote insulating layers. 
     Therefore, a region around the drain D overlaps the second gate electrode  230  without interposing of the first gate electrode  220 . Accordingly, a distance of the region around the drain affected by the high electric field to the gate electrode is substantially increased so that the GIDL effect may be reduced. 
     Further, since the first gate electrode  220  is formed of a material having a work function lower than that of the second gate electrode  230 , an effect of the electric field on the region around the drain D, that is, a region corresponding to the LDD region LDD may be further alleviated. 
     Further, the second gate electrode  230  may be formed to fill the inside of the trench  210  surrounded with the first gate electrode  220 . 
     As specifically described above, according to the exemplary embodiments, since the gate electrode having a relatively low work function is formed around the LDD region LDD, the low GIDL barrier due to application of a high electric field may be compensated and the leakage current may be reduced. 
     The above embodiments of the present invention are illustrative, and the invention is not limited by the embodiments described above. Various alternatives and equivalents are possible, and the invention is not limited to any specific type of semiconductor device. Other additions, subtractions, or modifications may be made in view of the present disclosure and are intended to fall within the scope of the following claims.