Abstract:
A semiconductor device including fin-FETs capable of suppressing both OFF-current resulting from the short channel effect and junction leakage, and a manufacturing method thereof are provided. A semiconductor device comprises: an active region defined to have a crank shape by an STI region formed on a semiconductor substrate, the active region having an upper surface higher than an upper surface of the STI region; a source region and a drain region formed on both ends of the active region, respectively; a channel region formed between the source region and the drain region in the active region; and a gate electrode covering an upper surface and side surfaces of a central portion of the active region including the channel region.

Description:
TECHNICAL FIELD 
     The present invention relates to a semiconductor device and a semiconductor device manufacturing method, and, more particularly to a semiconductor device including fin field effect transistors and a manufacturing method thereof. 
     BACKGROUND OF THE INVENTION 
     In recent years, following downsizing of a memory cell in a DRAM (Dynamic Random Access Memory), a gate length of a memory cell transistor is inevitably reduced. However, if the gate length is smaller, then the short channel effect of the transistor disadvantageously becomes more conspicuous, and sub-threshold current is disadvantageously increased. Furthermore, if substrate concentration is increased to suppress the short channel effect and the increase of the sub-threshold current, junction leakage increases. Due to this, the DRAM is confronted with a serious problem of deterioration in refresh characteristics. 
     As a technique for avoiding the above-stated problem, attention is paid to a fin field effect transistor (hereinafter, “fin-FET”) structured so that channel regions are formed to be thin each in the form of a fin in a perpendicular direction to a semiconductor substrate and so that gate electrodes are arranged around the channel regions (see Japanese Patent Application Laid-open No. 2006-100600). The fin-FET is expected to be able to realize acceleration of operating rate, increase in ON-current, reduction in power consumption and the like, as compared with a planer transistor. 
     A structure of a conventional fin-FET will be described below with reference to  FIGS. 14A and 14B . 
       FIG. 14A  is a general perspective view showing the structure of the conventional fin-FET, and  FIG. 14B  is a general cross-sectional view taken along a line A-A of  FIG. 14A . 
     As shown in  FIG. 14A , a trench  201   t  for STI (Shallow Trench Isolation) (hereinafter, “STI trench  201   t ”) is formed in a semiconductor substrate  200  and an element isolation film  2011  is buried in the STI trench  201   t  by a predetermined depth from a bottom of the STI trench  201   t . A part of the semiconductor substrate  200  surrounded by the STI trench  201   t  and located above the element isolation insulating film  2011  becomes a fin-shaped active region  202 . The active region  202  includes a central portion  202   a  and portions  202   b  and  202   c  located on both sides of the central portion  202   a , respectively. A gate electrode  203  is formed to cover an upper surface and both side surfaces of the fin-shaped active region  202  in the central portion  202   a  of the fin-shaped active region  202 . 
     By performing ion implantation into the active region  202  of the semiconductor substrate  200  with the gate electrode  203  used as a mask, a source region  202   s  and a drain region  202   d  are formed in the active region  202  and a channel region  202   n  is formed between the source region  202   s  and the drain region  202   d  as shown in  FIG. 14B . 
     In the fin-FET structured as shown in  FIGS. 14A and 14B , it is preferable to form the source region  202   s  and the drain region  202   d  to be deep so that three surfaces, i.e., an upper surface and two side surfaces of the central portion  202   a  of the active region  202  function as channels. Due to this, the ion implantation needs to be performed with high energy to form the source and drain regions  202   s  and  202   d.    
     However, if the ion implantation is performed with high energy so as to form the source and drain regions  202   s  and  202   d , the source and drain regions  202   s  and  202   d  are diffused into the channel region  202   n  covered with the gate electrode  203  as shown in  FIG. 14B . In such a fin-FET, if a gate length is smaller, a distance between the source and drain regions  202   s  and  202   d  shown as two-headed arrow in  FIG. 14B  is narrower, with the result that the short channel effect cannot be ignored. 
     To suppress OFF-current (Ioff) resulting from the short channel effect, a concentration of the channel region  202   n  may be increased. However, in a device, e.g., a DRAM memory cell, in which it is necessary to suppress junction leakage, if the concentration of the channel region is increased, junction electric field is deteriorated. The deterioration in junction electric field disadvantageously increases the junction leakage, resulting in deterioration in data holding characteristics. 
     SUMMARY OF THE INVENTION 
     It is, therefore, an object of the present invention to provide a semiconductor device including fin-FETs capable of suppressing both OFF-current resulting from the short channel effect and junction leakage, and a manufacturing method thereof. 
     According to the present invention, there is provided a semiconductor device comprising: an active region defined to have a crank shape by an STI region formed on a semiconductor substrate, the active region having an upper surface higher than an upper surface of the STI region; a source region and a drain region formed on both ends of the active region, respectively; a channel region formed between the source region and the drain region in the active region; and a gate electrode covering an upper surface and side surfaces of a central portion of the active region including the channel region. 
     According to the present invention, there is provided a semiconductor device comprising: a fin-shaped active region formed by a part of a semiconductor substrate; and a gate electrode extending in an X direction to cross the active region, and covering an upper surface and side surfaces of an almost central portion of the active region, wherein the active region has a channel region covered by the gate electrode and a source region and a drain region formed on both ends of the active region, respectively, wherein the source region and the drain region are offset each other in the X direction. 
     According to the present invention, there is provided a semiconductor device comprising: a fin-shaped active region; a gate electrode covering a first portion of the active region; and a source region and a drain region provided on a second portion and a third portion opposed to each other across the first portion of the active region, respectively, wherein the first portion includes at least one bent portion. 
     According to the present invention, the at least one bent portion includes a first bent portion and a second bent portion, and a region between the first bent portion and the second bent portion in the active region extends in a direction substantially identical to an extension direction of the gate electrode. 
     According to the present invention, there is provided a semiconductor device comprising: a fin-shaped active region including a first portion extending in a first direction, a second portion connected to one end of the first portion and extending in a second direction, and a third portion connected to other end of the first portion and extending in the second direction; a gate electrode extending in the first direction and covering the first portion, a part of the second portion, and a part of the third portion; a source region provided on another part of the second portion; and a drain region provided on another part of the third portion. 
     According to the present invention, there is provided a method of manufacturing a semiconductor device comprising: a step of forming a mask layer on a semiconductor substrate, the mask layer having a crank shape in a plan view; a step of etching the semiconductor substrate using the mask layer having the crank shape to form a trench in the semiconductor substrate; a step of burying an element isolation insulating film halfway in the trench to form a fin-shaped active region protruding from an upper surface of the element isolation insulating film, the fin-shaped active region having the crank shape in the plane view, a step of removing the mask layer; a step of forming a gate electrode extending in a direction perpendicular to a major axis direction of the fin-shaped active region, and covering an upper surface and side surfaces of an almost central portion of the active region; and a step of performing ion implantation with the gate electrode used as a mask to form a source region and a drain region on both ends of the fin-shaped active region, respectively. 
     According to the present invention, by using the fin-shaped active region, it is expected to realize an improvement in operating rate, an improvement in ON-current, a reduction in power consumption, and the like. Furthermore, a channel region is arranged between a source region and a drain region and the source region and the drain region are not in alignment. Due to this, even if the source region and the drain region are diffused into a region under a gate electrode by performing ion implantation for forming the source region and the drain region with high energy, the short channel effect can be suppressed without increasing a concentration of the channel region. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, features and advantages of this invention will become more apparent by reference to the following detailed description of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a general perspective view for explaining an outline of a semiconductor device according to an embodiment of the present invention; 
         FIG. 2  is a cross-sectional view and a plane view showing a step (formation of a silicon nitride film  11  and a silicon oxide film  12 ) in a method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 3  is a cross-sectional view and a plane view showing a step (formation of a photoresist  13 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 4  is a cross-sectional view and a plane view showing a step (patterning of the silicon nitride film  11  and the silicon oxide film  12 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 5  is a cross-sectional view and a plane view showing a step (removal of the silicon oxide film  12 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 6  is a cross-sectional view and a plane view showing a step (formation of a trench  15   t ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 7  is a cross-sectional view and a plane view showing a step (formation of an element isolation insulating film  15 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 8  is a cross-sectional view and a plane view showing a step (formation of a gate insulating film  16 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 9  is a cross-sectional view and a plane view showing a step (formation of a gate electrode  17  and a cap insulating film  18  (gates  19 )) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 10  is a cross-sectional view and a plane view showing a step (formation of source and drain regions  20 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 11  is a cross-sectional view and a plane view showing a step (removal of the gate insulating film  16  on the source and drain regions  20 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 12  is a cross-sectional view and a plane view showing a step (formation of silicon epitaxial layers  22 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 13  is a cross-sectional view and a plane view showing a step (formation of an interlayer insulating film  23  and contact plugs  24 ) in the method of manufacturing the semiconductor device according to the embodiment of the present invention; 
         FIG. 14A  is a general perspective view showing a structure of a conventional fin field effect transistor; and 
         FIG. 14B  is a general cross-sectional view taken along a line A-A of  FIG. 14A . 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Preferred embodiments of the present invention will be explained below with reference to the accompanying drawings. 
     With reference to the general perspective view of  FIG. 1 , an outline of a fin-FET  10  according to a preferred embodiment of the present invention is described. 
     As shown in  FIG. 1 , an element isolation insulating film  2  is buried in a trench  2   t  formed in a semiconductor substrate  1  by a height halfway along a depth of the trench  2   t . An active region  4  is thereby formed to be surrounded by the trench  2   t  and to protrude from an upper surface of the element isolation insulating film  2 . As shown in  FIG. 1 , the active region  4  is crank-shaped and includes a central portion  4   a , a portion  4   b  extending from one end of the central portion  4   a  in a Y direction, and a portion  4   c  extending from the other end of the central portion  4   a  in the Y direction. An upper surface and side surfaces of each of the central portion  4   a , a part of the portion  4   b , and a part of the portion  4   c  are covered with a gate electrode  3 . 
     Although not shown in  FIG. 1 , a source region and a drain region are formed in parts of the both side portions  4   b  and  4   c  of the active region  4 , which parts are not covered with the gate electrode  3 , respectively, by performing ion implantation with the gate electrode  3  used as a mask. At the time of ion implantation, impurities used in the ion implantation are also diffused into parts of the portions  4   b  and  4   c  which parts (parts mentioned above) are covered with the gate electrode  4  (implantation lowering). 
     However, in the fin-FET  10  according to the embodiment, the both side portions  4   b  and  4   c  of the active region  4  are connected to the central portion  4   a  thereof formed in an X direction in which the gate electrode  3  extends at positions offset to each other in the X direction, respectively. Due to this, the source region and the drain region formed in the respective portions  4   b  and  4   c  can be distanced from each other, thereby making it possible to suppress the short channel effect. Namely, an effective channel length of the fin-FET  10  is a sum of a width of the gate electrode  3  and an offset width between the portions  4   b  and  4   c  in the X direction. Therefore, by increasing this offset width, the short channel effect can be sufficiently suppressed accordingly. 
     With reference to  FIGS. 2A to 13B , a method of manufacturing a fin-FET according to the embodiment is described next in detail.  FIG. 2B  is a cross-sectional view taken along a line A-A of  FIG. 2A . The same shall apply to  FIGS. 3A and 3B  to  12 A to  12 B. 
     As shown in  FIGS. 2A and 2B , a silicon nitride film  11  is formed on a semiconductor substrate  100 . The silicon nitride film  11  is patterned with a photomask (not shown) used as a mask, thereby leaving the silicon nitride film  11  in the form of a plurality of lands as shown in  FIG. 1B . 
     Next, a silicon oxide film  12  is formed on an entire surface of the semiconductor substrate  100  including portions among and around the lands of the silicon nitride film  11 . Then using the silicon nitride film  11  as a stopper, the silicon oxide film  12  is polished by CMP (Chemical Mechanical Polishing). As a result, the silicon nitride film  11  and the silicon oxide film  12  are flattened so that an upper surface of the silicon nitride film  11  is almost flush with that of the silicon oxide film  12 . 
     As shown in  FIGS. 3A and 3B , a photoresist  13  including a plurality of openings  14  is formed. As shown in  FIG. 3B , each of the openings  14  is formed to partially expose the silicon nitride film  11  and the silicon oxide film  12 . 
     Using the photoresist  13  as a mask, the silicon nitride film  11  and the silicon oxide film  12  are dry etched. As a result, as shown in  FIGS. 4A and 4B , the patterned silicon nitride film  11  and the patterned silicon oxide film  12  are left on the semiconductor substrate  100 . 
     The silicon oxide film  12  is entirely removed by wet etching. As a result, as shown in  FIG. 5A , only a plurality of land patterns each made of the silicon nitride film  11  is left on the semiconductor substrate  100 . As shown in  FIG. 5B , each of the land patterns is crank-shaped in a plane view. 
     Using the crank-shaped silicon nitride film  11  as a mask, the semiconductor substrate  100  is dry etched. As a result, as shown in  FIG. 6A , a plurality of fin-shaped parts  100   f  each defined by trenche  15   t  are formed. 
     Next, a silicon oxide film is formed on an entire surface of the semiconductor substrate  100  including interior of the trench  15   t  as an element isolation insulating film. After performing the CMP with the silicon nitride film  11  as a stopper, the element isolation insulating film is wet etched so that a height of the element isolation insulating film is, for example, about 100 nanometers (nm) from the surface of the semiconductor substrate  100 . Thereafter, the silicon nitride film  11  is removed. 
     As a result, as shown in  FIGS. 7A and 7B , an element isolation insulating film  15  is formed in trench  15   t  by a predetermined height. Upper portions of the fin-shaped parts  100   f  protrude from an upper surface of the element isolation insulating film  15 . These upper portions serve as active regions  100   a , respectively. A pattern of the patterned silicon nitride film  11  (see  FIG. 6B ) is transferred onto each of the active regions  100   a . Due to this, as shown in  FIG. 7B , each of the active regions  100   a  is crank-shaped in a plane view. In this manner, the fin-shaped active regions  100   a  surrounded by the element isolation insulating film  15  and crank-shaped in a plane view are formed. 
     As shown in  FIG. 7B , each of the active regions  10   a  includes a central portion fa, a portion fb extending from one end of the central portion fa in the Y direction, and a portion fc extending from the other end thereof in the Y direction. The portions fb and fc are arranged to be offset each other in the X direction. 
     As shown in  FIGS. 8A and 8B , a gate insulating film  16  is formed on a surface of each of the active regions  100   a  by performing thermal oxidation. 
     Next, a gate electrode film and a silicon nitride film are formed on the entire surface of the semiconductor substrate  100 , and the gate electrode film and the silicon nitride film are patterned using a photoresist (not shown) having a gate electrode shape. As a result, as shown in  FIG. 9A , gates  19  each including a gate electrode  17  and a cap insulating film  18  are formed. 
     As shown in  FIG. 9B , each of the gates  19  is formed so as to cover the central portion fa of each of the crank-shaped active regions  100   a  covered with the gate insulating film  16  and to cover parts of the portions fb and fc on the respective both sides of the central portion fa (which parts are connected to the central portion fa). 
     In  FIG. 9B , each of the gates  19  (the cap insulating film  18  and the gate electrode  17 ) and the gate insulating film  16  are not hatched so as to show states of the active regions  100   a  present under the respective gates  19 . 
     As indicated by arrows in  FIG. 1A , ion implantation is performed on the entire surface while using the gates  19  as a mask, thereby forming source and drain regions  20 . At this time, the ion implantation is performed with high energy so as to implant impurity ions deeply. As a result, the source and drain regions  20  are formed to be diffused even into regions under the gates  19  serving as the mask (implantation lowering). 
     As shown in  FIG. 10B , the source/drain regions  20  formed on both sides of each of the gates  19  are diffused into the region under each gate  19  in each of the active regions  100   a . Nevertheless, because of the crank-shaped active regions  100   a , the source region  20  and the drain region  20  formed to put the central portion fa of each of the active regions  100   a  between the source region  20  and the drain region  20  are located offset each other in the X direction in which the gates  19  extend. Namely, the portions fb and fc on the both sides of each active region  100   a  that portions serve as the source/drain regions  20  are arranged on +X side and −X side in the X direction in which the gates  19  extend, respectively. Due to this, even if the two source/drain regions  20  formed on the both sides of the central portion fa are diffused toward the central portion fa, it is possible to prevent the source/drain regions  20  from being close to each other by the distance that makes the short channel effect conspicuous. In other words, an effective channel length of the fin-FET formed in the embodiment is about a sum of an offset width between the portions fb and fc on the both sides of each active region  100   a  and a width of each gate  19 . By making this offset width large, it is possible to sufficiently suppress the short channel effect. 
     In  FIG. 10B , similarly to  FIG. 9B , each of the gates  19  (the cap insulating film  18  and the gate electrode  17 ) and the gate insulating film  16  are not hatched so as to show states of the active regions  100   a  present under the respective gates  19 . 
     As shown in  FIGS. 11A and 11B , sidewall insulating films  21  are formed on side surfaces of each of the gates  19 , respectively. The sidewall insulating films  21  are formed by forming an insulating film for sidewalls on the entire surface of the semiconductor substrate  100  and dry etching (anisotropically etching) the insulating film for sidewalls. Therefore, as shown in  FIGS. 11A and 11B , the sidewall insulating films  21  are also formed on sidewalls of portions, which are not covered with the gates  19  (in which portions the source and drain regions  20  are formed), of the active regions  100   a.    
     Subsequently, portions, which are not covered with the gates  19 , of the gate insulating film  16  on the source/drain regions  20  are selectively removed, thereby exposing surfaces of the source/drain regions  20  as shown in  FIGS. 11A and 11B . 
     Silicon selective epitaxial growth is then performed. In the silicon selective epitaxial growth, silicon is grown only in portions in which silicon is exposed. Due to this, as shown in  FIGS. 12A and 12B , silicon epitaxial layers  22  are grown on the exposed portions of the source/drain regions  20  that are a part of the semiconductor substrate  100 . The silicon epitaxial layers  22  are doped with impurities contained in the source/drain regions  20  during the epitaxial growth. Due to this, the silicon epitaxial layers  22  become conductive layers containing the same impurities as those contained in the source/drain regions  20 . 
     As shown in  FIG. 12B , the silicon epitaxial layers  22  are formed to be wider than the source/drain regions  20  and to run off edges of the active regions  100   a.    
     As shown in  FIGS. 13A and 13B , an interlayer insulating film  23  is formed on the entire surface of the semiconductor substrate  100 , and contact plugs  24  connected to the respective silicon epitaxial layers  22  are then formed. At this time, since the silicon epitaxial layers  22  are formed wide as stated above, it is possible to secure large positioning margins for the contact plugs  24 . 
     Although subsequent steps are not shown in the drawings, necessary interconnects and the like are formed. As a consequence, a fin-FET is completed. 
     As explained above, according to the embodiment of the present invention, each of the active regions  100   a  has a fin structure and each of the gates  19  (gate electrode  17 ) covers the upper and side surfaces of each active region  10   a . Due to this, not only the upper surface but also the side surfaces of each active region  100   a  become a channel region, thereby making it possible to ensure a large amount of current. Besides, the portions, which are covered with the gates  19 , of the active regions  100   a  include bent portions (a part of each of the portions fb and fc covered with the gates  19 ). Due to this, even if the source/drain regions  20  formed on the both sides of each active region  100   a  are diffused toward the central portion fa of the active region  100   a , it is possible to keep the distance between the source/drain regions  20  sufficiently wide. Therefore, even if the gate length is smaller, the short channel effect can be sufficiently suppressed. 
     While a preferred embodiment of the present invention has been described hereinbefore, the present invention is not limited to the aforementioned embodiment and various modifications can be made without departing from the spirit of the present invention. It goes without saying that such modifications are included in the scope of the present invention. 
     For example, the case where the active regions of the fin-FET are crank-shaped in a plane view has been described in the embodiment. However, the plane shape of each active region may be a shape other than the crank shape as long as the shape can suppress the short channel effect.