Abstract:
Integrated circuit memory devices having highly integrated SOI memory cells therein include an SOI substrate having a semiconductor active layer therein. A first trench isolation region is also provided. The first trench isolation region extends into and partitions the semiconductor active layer into first and second active regions. These first and second active regions are preferably electrically isolated from each other by the first trench isolation region. First and second access transistors are provided in the first and second active regions, respectively, and a first electrically insulating layer is provided on the SOI substrate. A first bit line is also provided at a first level. The first bit line is electrically connected to a first source/drain region of the first access transistor by a first bit line contact. This first bit line contact extends through the first electrically insulating layer and contacts the first source/drain region of the first access transistor. A second electrically insulating layer is also provided on the first bit line, opposite said first electrically insulating layer and a second bit line is provided on the second electrically insulating layer at a second level above the first level. The second bit line is electrically connected to a first source/drain region of the second access transistor by a second bit line contact which extends through the first and second electrically insulating layers and contacts the first source/drain region of the second access transistor. Higher integration densities can be achieved by dividing the active layer into electrically isolated active regions and then forming bit lines at different levels which are electrically connected to access transistors within these isolated active regions.

Description:
RELATED APPLICATION 
     This application is related to Korean Application No. 98-9247, filed Mar. 18, 1998, the disclosure of which is hereby incorporated herein by reference. 
     FIELD OF THE INVENTION 
     This invention relates to integrated circuit devices and methods of forming integrated circuit devices, and more particularly to integrated circuit memory devices and methods of forming integrated circuit memory devices. 
     BACKGROUND OF THE INVENTION 
     A semiconductor memory device such as a DRAM consists of a memory cell array where a plurality of memory cells are regularly arranged as a two-dimensional array, and a peripheral circuit for controlling the memory cells. Each memory cell is selected by selecting both a column signal line called a word line and a row signal line called a bit line. 
     Further, the DRAM device has a row decoder and a column decoder, each having a plurality of input and output terminals and a sense amplifier connected to each bit line, for amplifying a signal read from the memory cell. Here, with the high capacity and high integration of DRAM devices, a folded bit line type sense amplifier is used between a pair of bit lines BL and BL to amplify the potential difference between these two bit lines. When using such a sense amplifier, word lines equally intersect a pair of bit lines. The word lines may be arranged on top of the active region and opposite the high level bit line, or arranged on top of the field region and opposite the low level bit line. With this type of layout, the area of a unit cell may become 8F 2 , where F is a design rule. 
     In the meanwhile, with the higher integration of the DRAM device, reduction in the area of a unit cell has been demanded. However, it becomes more difficult to reduce the design rule of a unit cell due to the limit of photolithography processes and the deterioration of electric characteristics of elements. Hence, attempts have been made to reduce the area of a unit cell with the same design rule by changing the layout of a cell or the sensing method. A representative example of these attempts is an open bit line structure where a reference bit line is fixed to the edge of the cell block without being paired with a signal bit line. This can reduce the area of a unit cell down to 6F 2 , but has a problem of decreasing the sensing margin due to an increase in noise. 
     Recently, a structure where elements are arranged at both sides of an SOI substrate is being widely used to reduce the area of a unit cell. The SOI technique isolates elements from one another by forming the active elements in each silicon island on an insulating substrate. Thus, the SOI structure can provide higher integration and a reduction of processing steps, as compared to the bulk silicon structure. The active elements formed on the SOI substrate are called SOI elements. An SOI element can achieve high operating speed and low power consumption because of a reduction in parasitic capacitance. 
     A description of a conventional DRAM device having an SOI structure which can maximize the cell size by burying a capacitor under the silicon layer is disclosed in U.S. Pat. No. 5,102,819. This device will now be described with respect to FIGS. 1-2. 
     FIG. 1 shows a cell layout in a conventional DRAM device using an SOI structure. In the figure, reference numeral  20  (region shown in dotted line) denotes a unit cell consisting of an access transistor and an information storage capacitor. Reference numeral  10  denotes a storage node of the capacitor, reference numeral  5  denotes a semiconductor layer provided as an active region, reference numeral  8  denotes a storage node contact for connecting the source region of the transistor to the storage node of the capacitor, reference numeral  15  denotes a bit line contact for connecting the drain region of the transistor to the bit line, reference numeral  12  denotes a word line provided as a gate of the transistor and reference numeral “F” denotes a design rule. 
     FIG. 2 is a cross sectional view, taken along line  2 - 2 ′ of FIG.  1 . Referring to FIG. 2, a conventional DRAM cell has a semiconductor substrate  1 , a second polysilicon layer  2  for the plate electrode of the capacitor formed on top of the semiconductor substrate  1 , a first insulating layer  3  formed on the surface of the plate electrode  2 , a recess  4  formed by etching the first insulating layer  3 , and a semiconductor layer  5  formed in the recess  4 . The semiconductor substrate  1 , the second polysilicon layer  2 , the first insulating layer  3  and the semiconductor layer  5  constitute the SOI structure. The semiconductor layer  5  is formed of a separate semiconductor substrate different from the semiconductor substrate  1 . 
     Source/drain regions  7  and  6  of the access transistor are formed in the semiconductor layer  5 . The drain region  6  is connected to the bit line  16  via a bit line contact  15  formed at the second insulating layer  14 , and the source region  7  is connected to a storage node  10  of the capacitor via a storage node contact  8  formed at the first insulating layer  3 . Here, reference numeral  12  denotes a gate oxide film and reference numeral  13  denotes a gate. 
     Each capacitor is formed at the lower portion of a corresponding access transistor. That is, the storage node  10  composed of the first polysilicon layer is formed at the lower portion of the source region  7  and is connected to the source region  7  via the storage node contact  8 . A dielectric layer  11  of the capacitor is formed between the storage node  10  and the second polysilicon layer  2 . Thus, the semiconductor substrate  1 , the second polysilicon layer  2 , the dielectric layer  11  and the storage node  10  constitute the information storage capacitor. The semiconductor substrate  1  and the second polysilicon layer  2  serve as a plate electrode of the capacitor. 
     In the event that a folded bit lines type sense amplifier structure is applied to such conventional DRAM devices, the word line  12  will equally intersect a pair of bit lines, will be placed on the active region  5  in the high level bit line, and will be placed on the field region  18  in the low level bit line. In addition, the bit line  16  is extended over the second insulating layer  14  in the same direction as the active region  5 . Here, since a single layer bit line structure is used, a pair of bit lines of a folded bit line structure are arranged to be adjacent and at an identical height (i.e., height corresponding to the thickness of the second insulating layer  14 ). Therefore, to properly amplify the potential difference between these two bit lines, a sufficient distance “a” should be maintained between adjacent active regions  5  as shown in FIG. 1, and the area of a unit cell (reference numeral  20  in FIG. 1) of a conventional DRAM device with this structure becomes 8F 2 . 
     SUMMARY OF THE INVENTION 
     Accordingly, an object of the present invention is to provide a semiconductor memory device using an SOI structure which can increase integration by reducing the area of a unit cell having a buried capacitor and a double-layer bit line structure. 
     Another object of the present invention is to provide a method for manufacturing a semiconductor memory device using the SOI structure. 
     To achieve the above object of the present invention, there is provided a semiconductor memory device using a silicon-on-insulator (SOI) structure and a manufacturing method thereof. The semiconductor memory device has a semiconductor layer formed on top of a semiconductor substrate with interposition of a first insulating layer therebetween and provided as an active region, an element isolation film formed on top of the first insulating layer and diagonally arranged to isolate adjacent active regions which cross each other in a length direction and a transistor which is formed on the semiconductor layer and has a gate and source/drain regions. A capacitor is also provided having a first electrode, which is formed at the lower portion of the transistor (with interposition of a second insulating layer between the substrate and the first electrode) and a second electrode which is formed to face the first electrode (with interposition of a dielectric layer between the first electrode and the second electrode) and is connected to the source region of the transistor via a storage node contact formed at the first insulating layer. A first bit line is also formed on top of the semiconductor layer having the transistor formed thereon and connected to the drain region of corresponding transistor (while skipping every other active region). In addition, a second bit line is provided and connected to the drain region of the active region which is not connected to the first bit line at different heights from the first bit line. It is preferred that the element isolation film be arranged to isolate adjacent active regions (which cross each other by a pitch of the gate in a length direction). It is also preferred that the semiconductor memory device have a folded bit line type sense amplifier structure where adjacent first bit lines and adjacent second bit lines are sensed as a pair of bit lines, respectively. 
     The semiconductor memory device further comprises a third insulating layer which is formed between the first bit line and the semiconductor layer having the transistor and has a first bit line contact for exposing the drain region of a corresponding transistor while skipping every other active region. A fourth insulating layer is also formed between the first and second bit lines and has a second bit line contact for exposing the drain region of the active region where the first bit line contact is not formed. 
     A memory device according to another embodiment of the present invention includes an SOI substrate having a semiconductor active layer therein. A first trench isolation region is also provided. The first trench isolation region extends into and partitions the semiconductor active layer into first and second active regions. These first and second active regions are preferably electrically isolated from each other by the first trench isolation region. First and second access transistors are provided in the first and second active regions, respectively, and a first electrically insulating layer is provided on the SOI substrate. A first bit line is also provided. The first bit line is electrically connected to a first source/drain region of the first access transistor by a first bit line contact. This first bit line contact extends through the first electrically insulating layer and contacts the first source/drain region of the first access transistor. A second electrically insulating layer is also provided on the first bit line, opposite said first electrically insulating layer and a second bit line is provided on the second electrically insulating layer. The second bit line is electrically connected to a first source/drain region of the second access transistor by a second bit line contact which extends through the first and second electrically insulating layers and contacts the first source/drain region of the second access transistor. Accordingly, based on this embodiment, higher integration densities can be achieved by dividing the active layer into electrically isolated active regions and then forming bit lines at different levels which are electrically connected to access transistors within these isolated active regions. 
     To achieve another object of the present invention, there is provided a method for manufacturing a semiconductor memory device. The method includes the steps of forming an element isolation film diagonally on top of a first semiconductor substrate to isolate adjacent active regions to be crossed each other in a length direction. A storage node contact for exposing a predetermined portion of the first semiconductor substrate is also formed by depositing a first insulating layer on top of the resultant structure and etching the first insulating layer. A capacitor is formed by sequentially depositing a storage node of the capacitor, a dielectric layer and a plate electrode on top of the first insulating layer, and forming a second insulating layer on the plate electrode and bonding a second semiconductor substrate on the second insulating layer. The bonded structure is then reversed and the rear side of the first semiconductor substrate is polished. A transistor having a gate, a drain region and a source region connected to the storage node of the capacitor via the storage node contact is then formed on the semiconductor layer. A first bit line connected to the drain region of corresponding transistor with skipping every other active region, is formed on top of the resultant structure. A second bit line connected to the drain region of the active region which is not connected to the first bit line, is then formed on top of the first bit line. 
     As described above, the present invention can reduce the area of unit memory cell by applying a buried capacitor structure in which the capacitor is formed at the lower portion of the transistor by using a semiconductor substrate of SOI structure, and can secure sufficient distance between adjacent active regions for a pair of bit lines of folded bit line structure by forming double-layer bit lines which are formed at different heights from each other while skipping every other active region. Further, it is possible to achieve an active pitch corresponding to the half of the photolithography pitch using a line type trench element isolation region extending along the bit line. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a layout view of a prior art memory device. 
     FIG. 2 is a cross-sectional view of the memory device of FIG. 1, taken along line  2 - 2 ′. 
     FIG. 3 is a layout view of a memory device according to a first embodiment of the present invention. 
     FIG. 4 is a cross-sectional view of the memory device of FIG. 3, taken along line  4 - 4 ′. 
     FIG. 5 is a cross-sectional view of the memory device of FIG. 3, taken along line  5 - 5 ′. 
     FIGS. 6-10 and  17 - 18  are cross-sectional views of intermediate structures which illustrate a preferred method of forming the memory device of FIG.  3 . 
     FIGS. 11-16 are cross-sectional views of intermediate structures which illustrate preferred techniques to form electrically isolated active regions within an active layer of an SOI substrate. 
     FIG. 19 is a layout view of a memory device according to a second embodiment of the present invention. 
     FIG. 20 is a cross-sectional view of the memory device of FIG. 19, taken along line  20 - 20 ′. 
     FIG. 21 is a cross-sectional view of the memory device of FIG. 19, taken along line  21 - 21 ′. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. This invention may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. In the drawings, the thickness of layers and regions are exaggerated for clarity. It will also be understood that when a layer is referred to as being “on” another layer or substrate, it can be directly on the other layer or substrate, or intervening layers may also be present. Like numbers refer to like elements throughout. 
     Referring now to FIG. 3, a layout diagram of a cell in a DRAM device using an SOI structure according to a preferred embodiment of the present invention is illustrated. FIGS. 4 and 5 also represent cross-sectional views of the device of FIG. 3 taken along lines  4 - 4 ′ and  5 - 5 ′, respectively. In FIG. 3, reference numeral  102  denotes an element isolation film, reference numeral  106  denotes a storage node contact for connecting a source region of an access transistor to a storage node of a capacitor, reference numeral  108  denotes the storage node of the capacitor, reference numeral  118  denotes a line type trench element isolation region, reference numeral  122  denotes a word line provided as a gate of an access transistor. Reference numeral  132  as shown in FIG. 4 denotes a first bit line contact for connecting a drain region of the transistor to a first bit line. Reference numeral  134  as shown in FIG. 5 denotes the first bit line, reference numeral  138  denotes a second bit line contact for connecting the drain region of the transistor (which is not connected to the first bit line), to a second bit line, and reference numeral  140  denotes the second bit line. 
     As shown in FIG. 3, the element isolation films  102  for isolating the active regions (which cross one another by the pitch of the gate of the transistor in the length direction) are diagonally arranged. Hence, the storage node contact  106  for connecting the source region of the transistor to the storage node of the capacitor can be formed to properly extend across the element isolation film  102  and the active region, and this makes it possible to secure sufficient distance between the storage node contact  106  and the gate  122 . 
     Further, in the DRAM cell of the present invention, a folded bit line type sense amplifier is structured by using double-layer bit lines which are formed at different heights from each other. That is, after forming the first bit line  134  which is connected to the drain region of an active region of a corresponding transistor (skipping every other active region), an insulating layer (not shown) is deposited on top of the first bit line  134 . A second bit line  140  is formed on the insulating layer. The second bit line  140  is connected to the drain region of the active regions where the first bit line  134  is not formed. Thus, the first and second bit line contacts are alternately arranged. Since the DRAM device of the present invention uses a folded bit line type sense amplifier, the two adjacent first bit lines  134  are BL 1  and BL 1  and the two adjacent second bit lines  140  are BL 2  and BL 2 . 
     Accordingly, first and second bit lines  134  and  140  extend in the same direction as the active region, but skip every other active region and are at different heights from each other. This makes it possible to secure sufficient distance between adjacent active regions under the same cell area, for a pair of bit lines (BL 1  and BL 1  or BL 2  and BL 2 ). Further, it is possible to achieve an active pitch corresponding to half the photolithography process pitch by isolating the adjacent active regions using a line type trench element isolation region  118  which is extended along the bit line. Using this technique, the distance “b” between adjacent active regions can be remarkably reduced, as compared to a conventional method (see “a” in FIG.  1 ), and thus the area of a unit cell can be decreased down to 4(1+σ)F 2 , where σ is defined as a ratio of the difference between the actual active pitch and the design rule(D/R) to the design rule(D/R): 
     
       
         σ=( W   active region   +W   element isolation region   −D/R ) /D/R   (1) 
       
     
     Referring now to FIGS. 4 and 5, the DRAM cell of the present invention has a semiconductor substrate  114 , a second insulating layer  113  on the substrate  114 , a plate electrode  112  of a capacitor on the second insulating layer  113 , a first insulating layer  104  formed on the surface of the plate electrode  112 , an element isolation film  102  formed on top of the first insulating layer  104  and a semiconductor layer  116  provided as an active region. The semiconductor substrate  114 , the second insulating layer  113 , the first insulating layer  104  and the semiconductor layer  116  constitute the SOI structure. The element isolation film  102  isolates the active regions to be crossed each other by the pitch of the gate of the transistor in the length direction, and also serves as a polish stopping layer. 
     The source/drain regions  124  and  126  of the transistor are formed in the semiconductor layer  116 . The drain region  126  formed in one active region is connected to the first bit line  134  via the first bit line contact  132  formed at the third insulating layer  130 , while the drain region of another active region is connected to the second bit line  140  via the second bit line contact  138  formed in the third and fourth insulating layers  130  and  136 , as illustrated by FIG. 5 
     Referring still to FIG. 5, the source region  124  is connected to the storage node  108  of the capacitor via the storage node contact  106  formed in the first insulating layer  104 . The storage node contact  106  is formed to properly extend across the lower portions of the element isolation film  102  and semiconductor layer  116 , thus securing sufficient distance from the gate  122  of the transistor. Each capacitor is formed at the lower portion of corresponding access transistor. That is, the storage node  108  of the capacitor is formed at the lower portion of the source region  124  and is connected to the source region  124  via the storage node contact  106 . The dielectric layer  110  of the capacitor is formed on the surface of the storage node  108 , and the plate electrode  112  of the capacitor is formed at the lower portions of the dielectric layer  110  and first insulating layer  104 , as illustrated. 
     The third insulating layer  130  having the first bit line contact  132  for exposing the drain region  126  of a corresponding transistor is formed on top of the semiconductor layer  116  where the access transistor is formed, as illustrated by FIG.  4 . The first bit line  134  formed on top of the third insulating layer  130  is connected to corresponding drain region  126  via the first bit line contact  132 . In addition, the fourth insulating layer  136  (having the second bit line contact  138  for exposing the drain region  126  of the active region where the first bit line contact  132  is not formed) is formed on top of the first bit line  134 . The second bit line contact  138  is formed in the fourth and third insulating layers  136  and  130 . The second bit line  140  on top of the fourth insulating layer  136  is connected to the corresponding drain region  126  in the second bit line contact  138 . Thus, the first and second bit lines  134  and  140  extend in the same direction as the active region, but are at different heights from each other. Further, as shown by FIGS. 3 and 5, the line type trench element isolation region  118  for isolating adjacent active regions  116  is extended along the bit line direction. 
     Referring now to FIGS. 6-18, preferred methods of forming DRAM device according to preferred embodiments of the present invention will be described. Like FIG. 4, the cross-sectional views of FIGS. 6-10 and  17 - 18  are taken along line  4 - 4 ′ of FIG.  3 . FIG. 6 illustrates the step of forming the element isolation film  102 . To form thin film  102 , a mask layer (not shown) is formed by depositing a CVD (chemical vapor deposition) oxide or a high temperature oxide on top of a P type first semiconductor substrate  100 . The mask layer is then patterned using a conventional photolithography technique. Subsequently, the first semiconductor substrate  100  is then etched to a predetermined depth by using the patterned mask layer as an etching mask, to form a trench  101 . In this case, the trench  101  is formed so that adjacent active regions (to be crossed each other by the pitch of the gate of the transistor in the length direction) can be isolated from each other. 
     After removing the mask layer, an insulating material, e.g., an oxide is deposited on the whole surface of the resultant structure and then is etched-back to fill in the trench  101  with the insulating material, thus forming the element isolation film  102 . The element isolation films  102  are diagonally arranged as shown in FIG. 3, thus serving to isolate the active regions from each other in the length direction and serving as a polish stopping layer when polishing the rear side of the first semiconductor substrate  100  in a subsequent process. 
     FIG. 7 illustrates the step of forming the first insulating layer  104  and the storage node contact hole  106 . After forming the element isolation film  102  as described above, an insulating material (e.g., oxide) is deposited on the whole surface of the resultant structure, to form the first insulating layer  104 . Subsequently, the first insulating layer  104  is then etched using a masked photolithography step to form a storage node contact hole  106 . In this case, the storage node contact hole  106  is formed to expose portions of the element isolation film  102  and active region. The placement of the contact hole  106  takes into account the diffusion of impurities in the storage node during a subsequent thermal process, so that a sufficient distance from the gate of the subsequently formed transistor can be achieved. 
     FIG. 8 illustrates the steps of forming the capacitor. After forming the storage node contact hole  106  as described above, a first conductive layer (e.g., a first polysilicon layer doped with impurity) is deposited on the whole surface of the resultant structure. This first conductive layer is then photolithographically patterned to define a plurality of storage nodes  108  which extend into the contact holes  106 . Next, an electrically insulating material such as an oxide or ONO (oxide/nitride/oxide) is deposited on the whole surface of the storage node  108 , to form the dielectric layer  110  of the capacitor. Next, a second conductive layer (e.g., a second polysilicon layer doped with impurity) is deposited on the dielectric layer  110 , to form the plate electrode  112  of the capacitor. As a result, the capacitor for storing information consisting of the storage node  108 , the dielectric layer  110  and the plate electrode  112  is completed. 
     FIG. 9 shows the steps of forming the second insulating layer  113  and the second semiconductor substrate  114 . After forming the capacitor as described above, an insulating material (e.g., an oxide) is deposited on top of the plate electrode  112  to form a second insulating layer  113 . The surface of the second insulating layer  113  is then planarized using an etch-back or a chemical mechanical polishing (CMP) technique. Next, a new wafer is bonded on top of the planarized second insulating layer  113 . This banded wafer acts as a second semiconductor substrate  114 . As illustrated by FIG. 10, the second semiconductor substrate  114  serves as a support for all elements formed on the first semiconductor substrate  100 . 
     FIG. 10 shows the step of forming the semiconductor layer  116 . After bonding the second semiconductor substrate  114  onto the first semiconductor substrate  100 , the resultant structure is reversed. Next, the rear side of the first semiconductor substrate  100  is polished by CMP. This polishing is processed until the surface of the element isolation film  102  is exposed. As a result, the semiconductor layer  116  is formed as an active region. Here, the second semiconductor substrate  114 , the second and first insulating layers  113  and  104  and the semiconductor layer  116  constitute an SOI substrate. 
     FIGS. 11 to  16  illustrate the steps of forming the line type element isolation regions  118  illustrated by FIG.  3 . Referring to FIG. 11, after forming the semiconductor layer  116  as described above, a first oxide film  141 , a polysilicon layer  142 , a second oxide film  144  and a nitride film  146   a  are sequentially deposited on top of the semiconductor layer  116 . After patterning the nitride film  146   a,  a third oxide film is deposited on the whole surface of the resultant structure and then is etched-back, for form first spacers  148   a  at the side walls of the patterned nitride film  146   a . Referring to FIG. 12, the second oxide film  144 , the polysilicon layer  142 , the first oxide film  141  and the semiconductor layer  116  are then sequentially etched using the first spacer  148   a  as an etching mask, for form first trenches  150   a.  Referring to FIG. 13, a fourth oxide film is deposited on the whole surface of the resultant structure to a predetermined depth such that the first trenches  150   a  can be sufficiently filled. The first spacers  148   a  and the fourth oxide film are then etched back, to form the first oxide film patterns  152  in the first trenches  150   a . Next, the nitride film  146   a  is removed by a wet etching step using phosphoric acid (H 3 PO 4 ) solution. 
     Referring to FIG. 14, a fifth oxide film is deposited on the whole surface of the resultant structure having the first oxide film pattern  152  formed therein. The fifth oxide film is then etched-back, to form a second spacer  154  at the side walls of the first oxide film patterns  152 . Next, the second oxide film  144 , the polysilicon layer  142 , the first oxide film  141  and the semiconductor layer  116  are sequentially etched using the second spacer  154  as an etching mask, to form a second trench  150   b . Referring to FIG. 15, a sixth oxide film is deposited on the whole surface of the resultant structure to a predetermined depth such that the second trench  150   b  can be sufficiently filled and then the first oxide film pattern  152  and the sixth oxide film are etched-back, to form a second oxide film pattern  156  for filling the first and second trenches  150   a  and  150   b.    
     Referring to FIG. 16, after forming the second oxide film patterns  156  as described above, the polysilicon layer  142  is etched-back. Next, a seventh oxide film is deposited on the whole surface of the resultant structure and then is etched-back, thus forming a third spacer consisting of the seventh oxide film at the side walls of the second oxide film pattern  156 . As a result, the line type trench element isolation regions  118  for isolating adjacent active regions is formed. The line type trench element isolation regions  118  extend in the bit line direction as shown in FIG.  3 . In the above described method for forming the line type trench element isolation regions  118 , the width ratio of the active region to the element isolation region can be maximized by controlling the width of the spacers. 
     FIG. 17 illustrates the step of forming access transistors in the semiconductor layer  116 . After forming the line type trench element isolation regions  118  in the semiconductor layer  116  as described above, a gate oxide film  120  is formed on the surface of the semiconductor layer  116  using thermal oxidation techniques. Subsequently, a conductive layer (e.g., a polysilicon layer doped with impurity or a composite of a polysilicon layer doped with impurity and a metal silicide layer) is deposited on the whole surface of the resultant structure having the gate oxide film  120  formed thereon. This conductive layer is then patterned by photolithography, for form a plurality of gate electrodes  122 . Next, an n +  type impurity is ion-implanted by using the gate  122  as an ion-implantation mask, to form n +  source and drain regions  124  and  126  on the surface of the semiconductor layer  116  at both sides of the gate  122 . As a result, the access transistor is formed on the semiconductor layer  116 . Thereafter, the third insulating layer  130  is formed on the whole surface of the semiconductor layer  116  where the transistor is formed. 
     FIG. 18 illustrates the step of forming the first bit line  134 . After forming the third insulating layer  130  as described above, the third insulating layer  130  is etched to expose the drain region  126  of a corresponding access transistor (skipping every other active region) and form the first bit line contact  132 . Next, a conductive layer is deposited on top of the resultant structure having the first bit line contact  132  formed therein and then is patterned to form the first bit line  134  connected to the drain region  126  of the transistor via the first bit line contact  132 . Hence, the first bit line  134  is connected to the drain region  126  of a corresponding access transistor. 
     Referring again to FIG. 5, the fourth insulating layer  136  is formed on the whole surface of the resultant where the first bit line  134  is formed and then is etched to expose the drain region  126  of the active region where the first bit line contact  132  is not formed, thus forming the second bit line contact  138 . Next, a conductive layer is deposited on the top of the resultant structure where the second bit line contact  138  is formed and is patterned to define the second bit line  140  connected to the drain region  126  of the transistor via the second bit line contact  138 . Hence, the second bit line  140  is connected to the drain region  126  of every other transistor. Thereafter, though not shown, the fifth insulating layer is formed on the whole surface of the resultant structure where the second bit line  140  is formed, and then a metal wiring layer is formed thereon, to complete the DRAM cell. 
     FIG. 19 is a layout diagram showing a DRAM device using an SOI structure according to another preferred embodiment of the present invention, and FIGS. 20 and 21 are cross sectional views, taken along lines  20 - 20 ′ and  21 - 21 ′ of FIG. 19, respectively. According to this preferred embodiment of the present invention, if the distance between the storage node contact  106  and the gate  122  is not sufficient due to the small design rule of the DRAM cell (even though the storage node contact  106  is formed to properly extend across the lower portions of the element isolation film  102  and semiconductor layer  116 ), the storage node contact  106  can be formed to extend from the lower portions of the semiconductor layer  116  and element isolation film  102  to the lower portion of the passing transistor formed on top of the element isolation film  102  as shown in FIG.  20 , to secure sufficient distance between the storage node contact  106  and the gate  122 . 
     In the event the storage node contact  106  is formed to extend to the lower portion of the passing transistor as described above, it becomes difficult to pattern the storage node  108  in parallel or perpendicular to the existing layers. To solve this problem and secure maximum capacitor area, the storage node  108  is diagonally arranged identical to the angle of the element isolation film  102 , as shown in FIG.  19 . 
     As described above, the present invention can reduce the area of a unit cell by applying a buried capacitor structure in which the capacitor is formed at the lower portion of the transistor by using a semiconductor substrate of SOI structure, and can secure sufficient distance between adjacent active regions for a pair of bit lines of folded bit line structure by forming double-layer bit lines at different heights from each other and skipping every other active region. Further, it is possible to achieve an active pitch corresponding to half of the photolithography pitch using the line type trench element isolation region extending along the bit line. Therefore, the area of the unit cell can be reduced to 4(1+σ)F 2 . 
     In the drawings and specification, there have been disclosed typical preferred embodiments of the invention and, although specific terms are employed, they are used in a generic and descriptive sense only and not for purposes of limitation, the scope of the invention being set forth in the following claims.