Abstract:
A method for forming a transistor comprising the steps of: forming a trench in a substrate; filling an insulating layer in the lower portion of said trench except for the upper portion thereof; filling a conductive layer in the upper portion of said trench and on said insulating layer for a channel of the said transistor; forming a gate oxide layer on the resulting structure; and forming a gate electrode on said gate oxide layer; and implanting impurity ions into said substrate to form a source/drain region.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a transistor having an improved electric feature and a method for forming the same. 
     In general, a transistor structure includes the steps of forming an oxide layer on a substrate and forming a transistor on a silicon substrate layer is formed under. This transistor structure improves the characteristics of the device in that the capacitance in the source/drain junction layers is minimized. 
     On the other hand, since the short channel effect and the hot carrier effect are increased, according to the scaled-down transistors in size thereof, the transistors are designed in order to improve these effects by the theory of the transistor scaling and the LDD(Lightly Doped Drain) structure. 
     Although, in deep submicron transistors, these betterments of transistors have been developed, operating features of these transistors are deteriorated by the short channel effects such as punchthrough. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide a transistor and a method for forming the same capable of preventing the short channel effects (particularly, punchthrough of PMOS transistors) from being generated and improving the operating feature thereof. 
     In accordance with the present invention, a method for forming a transistor comprises the steps of: forming a trench in a substrate; filling an insulating layer in the lower portion of said trench except for the upper portion thereof; filling a conductive layer in the upper portion of said trench and on said insulating layer for a channel of the said transistor; forming a gate oxide layer on the resulting structure; forming a gate electrode on said gate oxide layer; and implanting impurity ions into said substrate to form a source/drain region. 
     In accordance with the present invention, a method for forming a transistor comprises the steps of: forming a lightly doped layer on a substrate; forming a first insulating layer on said lightly doped layer and patterning said first insulating layer; forming a trench in a substrate by etching said lightly doped layer and said substrate, using said patterned first insulating layer as an etching mask; filling a second insulating layer in the lower portion of said trench except for the upper portion thereof, wherein a selective etching rate of said second insulating layer is much lower than that of said first insulating layer; filling a conductive layer in the upper portion of said trench and on said second insulating layer for a channel of the said transistor; removing said first insulating layer; forming a gate oxide layer on the resulting structure; forming a gate electrode on said gate oxide layer; and implanting impurity ions into said substrate to form a source/drain region. 
     In accordance with the present invention, a method for forming a transistor comprises the steps of: forming a first and second insulating layer, in turn, on a substrate; patterning said first and second insulating layer to expose said substrate; forming a trench in said substrate, using said patterned second insulating layer as an etching mask; filling an oxide layer in said trench by oxidizing said exposed substrate; removing said second insulating layer; etching back said oxide layer, leaving the residual thereof in the lower portion of said trench; filling a conductive layer in the upper portion of said trench and on said oxide layer for a channel of the said transistor; removing said first insulating layer; forming a gate oxide layer on the resulting structure; forming a gate electrode on said gate oxide layer; and implanting impurity ions into said substrate to form a source/drain region. 
     In accordance with the present invention, a transistor having a gate insulating on a substrate, a gate electrode, and a source/drain region comprises: a trench formed in said substrate between said source region and said drain region; an insulating layer filled in the lower portion of said trench; and a silicon layer layer filled up the upper portion of said trench and on said insulating layer for forming a channel of the said transistor; whereby said insulating layer is formed under said transistor&#39;s channel to improve a punchthrough feature thereof. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention may be better understood and its numerous objects and advantages will become apparent to those skilled in the art by reference to the accompanying drawings as follow: 
     FIGS. 1A through 1J are cross-sectional views illustrating a method for forming a transistor in accordance with an embodiment of the present invention. 
     FIGS. 2A through 2I are cross-sectional views illustrating a method for forming a transistor in accordance with another embodiment of the present invention. 
     FIGS. 3A through 3H are cross-sectional views illustrating a method for forming a transistor in accordance with further another embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, the present invention will be described referring to the accompanying drawings. 
     First, an embodiment of the present invention will be described referring to FIGS. 1A through 1J. 
     As shown in FIG. 1A, an oxide layer 102 is deposited on a silicon substrate 101, and a portion of the oxide layer 102 is etched by a mask process. An etching process to form a trench in the silicon substrate 101 is performed. 
     As shown in FIG. 1B, an oxide spacer layers 103 are formed on the sidewall of the oxide layer 102 to form the trench which has a narrower width than that of the patterned oxide layer by the mask process in FIG. 1A, and a trench 104 is formed by etching the exposed silicon substrate 101, using the oxide layer 102 and the oxide spacer layers 103 as an etching mask. Of course, since oxide spacer layers 103 are used for decreasing the width of the trench 104, the step of forming the oxide spacer layers 103 can be skipped according to manufacturing conditions. 
     As shown in FIG. 1C, the residual oxide layers 102 and 103 on the silicon substrate 101 are removed, and an oxide layer 105 is formed on the resulting structure, thereby filling in the trench 104. 
     Next, as shown in FIG. 1D, the oxide layer 105 is etched back except for the oxide layer 105&#39; in the trench 104. Also, the etching rate is controled in order that the depth &#34;d&#34; is formed from the surface of the silicon substrate 101 to the surface of the oxide layer 105&#39; in the trench 104. The depth &#34;d&#34; is needed for forming a channel of the MOS transistor which will be described by following process. 
     Referring now to FIG. 1E, a polysilicon layer 106 is formed on the resulting structure, filling in the trench 104 which is not still filled with the oxide layer 105&#39;. The polysilicon layer 106 can be substituted for a silicon layer. 
     As shown in FIG. 1F, the polysilicon layer 106 over the surface of the silicon substrate 101 is oxidized by an oxidation process which is used in the conventional semiconductor process. Then, an oxide layer 107 is formed by the the oxidation process and a polysilicon layer 106&#39; remains from the surface of the silicon substrate 101 to the surface of the oxide layer 105&#39; in the trench 104. At this time, the polysilicon layer 106&#39; serves as a substrate. 
     The oxide layer 107 is removed and an ion implantation is performed to adjust the threshold voltage of the transistor, as shown in FIG. 1G. 
     FIG. 1H illustrates the step of forming, in turn, an oxide layer 108 and a polysilicon layer 109 for the gate electrode. 
     FIG. 1I illustrates the step of patterning the oxide layer 108 and the polysilicon layer 109 in a predetermined size, and then, forming a gate oxide layer 108&#39; and a gate electrode 109&#39;. 
     Finally, as shown in FIG. 1J, a source/drain region 110 is formed by an ion implantation. 
     As illustrated above, the transistor, in accordance with an embodiment of the present invention, differs from the conventional transistor in the formation of the oxide layer 105&#39; and the polysilicon layer 106&#39; in the silicon substrate 101, in order to improve the feature of the punchthrough of the transistor. 
     Now, another embodiment of the present invention will be described referring to FIGS. 2A through 2I. 
     As shown in FIG. 2A, low concentration ions (n- or p-), whose impurity type is different from that of a substrate 201, are implanted into a silicon substrate 201, and then a doped region 202 is formed on the silicon substrate 201. 
     As shown in FIG. 2B, a nitride layer 203 is formed on the doped region 202, and the nitride layer 203 is patterned in order to expose a portion of the silicon substrate 201 using a mask process and an etching process. 
     As shown in FIG. 2C, a trench 204 is formed by etching the doped region 202 and the exposed silicon substrate 201, using the nitride layer 203 as an etching mask. Of course, oxide spacer layers as illustrated in FIG. 1B can be used for decreasing the width of the trench 204. After forming the trench 204, an oxide layer 205 is formed on the resulting structure, thereby filling in the trench 204. 
     Next, as shown in FIG. 2D, the oxide layer 205 is etched back except for the oxide layer 205&#39; in the trench 204. Also, the etching rate must be controled in order that the depth &#34;d&#34; is formed from the surface of the silicon substrate 201 to the surface of the oxide layer 205&#39; in the trench 204. The depth &#34;d&#34; is needed for forming a channel of the MOS transistor which will be described by the following process. 
     Referring now to FIG. 2E, a polysilicon layer 206 is formed on the resulting structure, filling in the trench 204 which is not still filled with the oxide layer 205&#39;. 
     As shown in FIG. 2F, the polysilicon layer 206 is etched back such that a polysilicon layer 206&#39;, which serves as a substrate, is formed only in the trench 204, and an ion implantation is performed to adjust the threshold voltage of the transistor. 
     FIG. 2G illustrates the step of forming, in turn, an oxide layer 207 and a polysilicon layer 208 for the gate electrode, and FIG. 2H illustrates the step of patterning the gate oxide layer 207&#39; and a gate electrode 208&#39; in a predetermined size. 
     Finally, as shown in FIG. 2I, a source/drain region 209, which is highly doped region, is formed by an ion implantation. 
     As illustrated above, this embodiment shows that transistors having the LDD structure in FIG. 1J are accomplished with the doped region 202. 
     Furthermore, another embodiment of the present invention will be described referring to FIGS. 3A through 3H. 
     As shown in FIG. 3A, an oxide layer 302 and a nitride layer 303 is, in turn, deposited on a silicon substrate 301, and a portion of the oxide layer 302 and the nitride layer 303 are etched by a mask process and an etching process in order to form a trench in the silicon substrate 301. 
     As shown in FIG. 3B, a nitride spacer layer 304 are formed on the sidewall of the oxide layer 302 and the nitride layer 303 to form the trench, which has a narrower width than that of the patterned oxide and nitride layer, by the mask process in FIG. 3A. 
     As shown in FIG. 3C, a trench 305 is formed by etching the exposed silicon substrate, using the nitride layer 303 and the nitride spacer layers 304 as an etching mask. Of course, since the nitride spacer layer 304 is used for decreasing the width of the trench 305, the step of forming the nitride spacer layers 304 can be skipped according to manufacturing conditions as illustrated in FIG. 1B. 
     Next, as shown in FIG. 3D, the exposed substrate 301 is oxidized using the nitride layer 303 and the nitride spacer layer 304 as an oxidation mask so that an oxide layer 306 is formed in the trench 305. 
     After the nitride layer 303 and the nitride spacer layer 304 are removed, the oxide layer 306 is etched back except for the oxide layer 306&#39; in the trench 305, as shown in FIG. 3E. At this time, the etching rate is controled in order that the depth &#34;d&#34; is formed from the surface of the silicon substrate 301 to the surface of the oxide layer 306&#39; in the trench 305. The depth &#34;d&#34; is needed for forming a channel of the MOS transistor which will be described by the following process. 
     Referring now to FIG. 3F, an epitaxy process is applied to the exposed silicon substrate 301 such that a silicon layer 307 is filled in the trench 305 which is not still filled with the oxide layer 306&#39;. After forming the silicon layer 307, impurity ions to control the threshold voltage is implanted into the silicon layer 307 using the oxide layer 302 as an ion implanting mask 
     As shown in FIG. 3G, a gate oxide layer 308 is formed on the polysilicon layer 307 grown by epitaxy process and a polysilicon layer 309 for the gate electrode is formed on the resulting structure. 
     Referring to FIG. 3H, a polysilicon layer 309&#39; and an oxide layer 302&#39; is patterned in a predetermined size, and a source/drain region 310 is formed by an ion implantation. 
     As stated above, the present invention can effectively improve punchthrough feature PMOS transistors having a short channel and forming an insulating layer under the channel, therefore, having an effect on stability of transistors&#39; operation. 
     In compliance with statute, the invention has been described in language more or less specific as to structural and methodical features. It is to be understood, however, that the invention is not limited to the specific features shown and described, since the means herein disclosed comprise preferred forms of putting the invention into effect. The invention is, therefore, claimed in any of its forms or modifications within the proper scope of the appended claims appropriately interpreted in accordance with the doctrine of equivalents.