Abstract:
Static random access memory (SRAM) cell is disclosed, which is suitable for high packing density and cell stabilization, including a substrate, a wordline formed over the substrate, including two parallel legs having gates of first and second access transistors, respectively, gates of first and second drive transistors formed between the two parallel legs, and an active area defined in a surface of the substrate under the gates of the first and second access transistors and gates of the first and second drive transistors.

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates to a semiconductor device, and more particularly, to a static random access memory (SRAM) cell and method of manufacturing the same, which are suitable for high packing density and cell stabilization. 
     FIG. 1A is a plan view of a conventional SRAM. FIG. 1B is an equivalent circuit diagram of FIG. 1A. 
     Referring to FIGS. 1A and 1B, in the conventional SRAM cell, a wordline W/L acting a, the gate of first and second access transistors TA1 and TA2, and bitlines B/L1 and B/L2 connected to each other through drain area D of access transistors TA1 and TA2 and a contact I4. 
     Gate G of first and second drive transistors TD1 and TD2 is connected to active area B, that is, source area S, of access transistors TA1 and TA2 via contacts I1 and I2, and simultaneously to a load resistor R through which a power voltage Vcc is supplied. Drain area D of first and second drive transistors TD1 and TD2 is coupled to source area S of first and second access transistors TA1 and TA2 via N+ junction. Source area S of first and second drive transistors TD1 and TD2 is coupled to a conductive line G via contact I3. 
     In the method of manufacturing the conventional SRAM cell, the gate of transistor made of a first polysilicon layer is formed on a substrate which is divided into a field area A and active area B. Gate G of first and second drive transistors TD1 and TD2 is floated to be in contact with active area B through first contact I1. Gate G of access transistors TA1 and TA2 becomes wordline W/L. 
     After a first CVD oxide layer is formed on the overall surface of the substrate, a second contact I2 is formed. Then, a second polysilicon layer is deposited and patterned in a form to thereby form conductive line G. A second CVD oxide layer is deposited on the overall surface of the substrate and selectively etched to thereby form a third contact I3. Thereafter, a third polysilicon layer is deposited and patterned to form load resistor R. This load resistor is connected to floating gate G of drive transistors TD1 and TD2 through third contact I3. 
     A third CVD oxide layer is deposited on the overall surface of the substrate and selectively etched to form a fourth contact I4 on the drain area of access transistors TA1 and TA2. A bitline of metal is formed to be connected to the drain area of access transistors TA1 and TA2 through fourth contact I4. By doing so, a conventional SRAM cell is fabricated. 
     In the above structure of SRAM cell, the cell is unstable due to asymmetricity and there is a limit in controlling the load resistor. This involves difficulty in controlling cell standby current and in high integration because the cell is large. 
     SUMMARY OF THE INVENTION 
     Therefore, it is an object of the present invention to provide an SRAM cell suitable for high integration and cell stabilization, and a method of fabricating the same. 
     To accomplish the object of the present invention, there is provided a semiconductor device comprising: a substrate divided into active and field areas; a first contact formed in the active area of the substrate; a substrate contact portion formed in the first contact; a first conductive line made up of a body spaced from the substrate contact portion and a pair of legs elongated in parallel from both edges of the body between the substrate contact portion; first and second electrodes formed above the pair of legs of the first conductive line and above the active area between the legs, while being spaced from the substrate contact portion; a pair of first high-concentration impurity areas formed in the active area of the first leg of the pair of legs; a pair of second high-concentration impurity areas formed in the active area of the second leg of the pair of legs; a pair of third high-concentration impurity areas formed in the active area between the respective electrodes and substrate contact portion; a second contact formed in one of the pair of first high-concentration impurity areas; a third contact formed in one of the pair of second high-concentration impurity areas; a fourth contact formed on the first electrode above the first leg; a fifth contact formed on the second electrode above the second leg; a second conductive line elongated from the second contact to be intersected with the first leg of the first conductive line, the second conductive line being elongated from the second contact to the fifth contact above the second leg to thereby come into contact with the second electrode; a third conductive line elongated to be intersected with the second leg of the first conductive line through the third contact, the third conductive line being elongated from the third contact to the fourth contact above the first leg to thereby come into contact with the first electrode; a sixth contact formed on the other portion of the pair of first highconcentration impurity area; a seventh contact formed on the other portion of the pair of second high-concentration impurity area; a fourth conductive line coming into contact with the other first high-concentration impurity area through the sixth contact, the fourth conductive line being formed long above the first electrode to be intersected with the first and second legs; and a fifth conductive line coming into contact with the other second high-concentration impurity area through the seventh contact, the fifth conductive line being formed long above the second electrode to be intersected with the first and second legs. 
     To accomplish the object of the present invention, there is further provided a method of fabricating a semiconductor device comprising the steps of: forming a first contact in an active area of a substrate; depositing a first conductive material on the overall surface of the substrate including the first contact; patterning the first conductive material to form a substrate contact portion in the contact, and forming a first conductive line made up of a body spaced from the substrate contact portion and a pair of legs elongated in parallel from both edges of the body between the substrate contact portion; depositing a second conductive material on the substrate including the first conductive line; patterning the second conductive material to form first and second electrodes above the pair of legs of the first conductive line and above the active area between the legs, while being spaced from the substrate contact portion; implanting an impurity into the active area, while taking the first conductive line and first and second electrodes as masks, and forming a pair of first high-concentration impurity areas in the active area of the first leg of the first conductive line adjacent to the first electrode, a pair of second high-concentration impurity areas in the active area of the second leg adjacent to the second electrode, and a pair of third high-concentration impurity areas in the active area between the respective electrodes and substrate contact portion; forming a first insulating layer on the overall surface of the substrate; selectively etching the first insulating layer to form a second contact and third contact in one of the pair of first high-concentration impurity areas and in one of the pair of second high-concentration impurity areas, respectively, and a fourth contact and fifth contact on the first electrode and on the second electrode, respectively; forming a third conductive material on the overall surface of the substrate; patterning the third conductive layer to form a second conductive line elongated from the second contact to be intersected with the first leg of the first conductive line, the second conductive line being elongated from the second contact to the fifth contact above the second leg to thereby come into contact with the second electrode, and to form a third conductive line elongated to be intersected with the second leg of the first conductive line through the third contact, the third conductive line being elongated from the third contact to the fourth contact above the first leg to thereby come into contact with the first electrode; forming a second insulating layer on the overall surface of the substrate; selectively etching the second and first insulating layers to form a sixth contact and seven contact on the other portion of the pair of first high-concentration impurity area and on the other portion of the pair of second high-concentration impurity area; depositing a fourth conductive material on the overall surface of the substrate; and patterning the fourth conductive material to form a fourth conductive line coming into contact with the other first high-concentration impurity area through the sixth contact, the fourth conductive line being formed long above the first electrode to be intersected with the first and second legs, and to form a fifth conductive line coming into contact with the other second high-concentration impurity area through the seventh contact, the fifth conductive line being formed long above the second electrode to be intersected with the first and second legs. 
    
    
     BRIEF DESCRIPTION OF THE ATTACHED DRAWINGS 
     FIG. 1A is a plan view of a conventional SRAM cell; 
     FIG. 1B is an equivalent circuit diagram of the SRAM cell of FIG. 1A; 
     FIG. 2A is a plan view of one embodiment of the SRAM cell of the present invention; 
     FIG. 2B is an equivalent circuit diagram of the SRAM cell of FIG. 2A; 
     FIGS. 3A-3D are cross-sectional views of the SRAM cell cut along lines A-A&#39;, B-B&#39;, C-C&#39; and D-D&#39; of FIG. 2A; 
     FIGS. 4A-4I show the sequence of manufacturing the SRAM cell cut along line A-A&#39; of FIG. 2A; 
     FIGS. 5A-5I show the sequence of manufacturing the SRAM cell cut along line B-B&#39; of FIG. 2A; 
     FIGS. 6A-6I show the sequence of manufacturing the SRAM cell cut along line C-C&#39; of FIG. 2A; 
     FIGS. 7A-7I show the sequence of manufacturing the SRAM cell cut along line D-D&#39; of FIG. 2A; 
     FIGS. 8A-8H are plan views of the respective layers&#39; patterns of the SRAM cell array of FIGS. 2A and 2B; and 
     FIG. 9 is a layout of the SRAM cell array of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Hereinafter, a preferred embodiment of the present invention will be described with reference to the attached drawings. 
     Referring to FIGS. 2A-3D, in an SRAM cell of the present invention, a substrate 30 has a active area 34, which is symmetric centering on a contact C1. A wordline W/L made up of a polysilicon layer is formed to be symmetric centering on the contact C1. 
     The wordline W/L comprises a body W/L-C spaced apart from the contact C1 by a predetermined distance, and a pair of legs W/L-A and W/L-B elongated in parallel from both edges of body W/L-C centering on the contact C1. 
     First and second access transistors TA1 and TA2 taking the wordline W/L as the gate are formed symmetric centering on the contact C1. First and second drive transistors TD1 and TD2 are formed symmetric on the substrate between the contact C1 and the first and second access transistors TA1 and TA2 respectively. 
     Gates GD1 and GD2 of first and second drive transistors TD1 and TD2 are made of a polysilicon layer and formed above the pair of legs W/L-A and W/L-B and above active area 34 placed therebetween. 
     A pair of bitlines B/L1 and B/L2 are made of metal and formed long to be intersected with the respective legs W/L-A and W/L-B of wordline W/L above gates GD1 and GD2 of first and second drive transistors TD1 and TD2. Of the bitlines, bitline B/L1 comes into contact with drain area 34E of first access transistor TA1 through a contact C7. Bitline B/L2 is in contact with the drain area 34A of second access transistor TA2 through a contact C9. 
     First load resistor R1 of the SRAM cell is intersected with first leg W/L-A of wordline W/L, and comes into contact with source area 34D of first access transistor TA1 (or drain area of first drive transistor TD1) through a contact C3. At the same time, a first conductive line L1 which is formed same layer of the first load resistor R1 is elongated from the contact C3 on second leg W/L-B, and comes into contact with the gate GD2 of second drive transistor TD2 through a contact C13. 
     Second load resistor R2 is intersected with first leg W/L-B of wordline W/L, and comes into contact with source area 34B of second access transistor TA2 (or drain area of second drive transistor TD2) through a contact C5. At the same time, the second conductive line L2 which is formed same layer of the first load resistor R2 is elongated from the contact C5 on first leg W/L-A, and comes into contact with gate GD1 of first drive transistor TD1 through a contact C11. 
     First and second load resistors R1 and R2 are made up of a resistor layer which is usually used as polysilicon. Of the patterned resistor layer, the portion between the contact C3 and contact C13 is doped with an impurity so as to serve as said conductive line L1 for connecting them, and the portion between the contact C5 and contact C11 is doped with an impurity so as to serve as said conductive line L2 for connecting them and VCC line portion is doped with an impurity so as to serve as a conductive line for being used as VCC line. 
     In the contact C1, a substrate contact portion C10 of polysilicon layer is formed. The source 34C of first and second drive transistors TD1 and TD2 is coupled to the substrate through substrate contact portion C1. 
     The SRAM cell of the present invention further comprises a first insulating layer 17 formed on the active area 34 under gate portions GA1 and GA2 of first and second access transistors TA1 and TA2 of W/L and under gates GD1 and GD2 of first and second drive transistors TD1 and TD2, a second insulating layer 27 for insulating wordline W/L from gates GD1 and GD2 of first and second drive transistors TD1 and TD2, a third insulating layer 37 for insulating worldline W/L from first and second resistors R1 and R2, a fourth insulating layer 47 formed on third insulating layer 37 including first and second resistors R1 and R2, and a planarization layer 57 formed on fourth insulating layer 47. 
     A method of fabricating the SRAM cell of the present invention will be described with reference to FIGS. 4A-8H. 
     Referring to FIGS. 4A, 5A, 6A and 7A, an insulating layer 32 and epitaxial layer 33 are sequentially formed on silicon substrate 31 to thereby form a substrate 30 of first silicon-insulator-second silicon structure. Here, for silicon substrate 31, a substrate where a high-concentration impurity is doped is used. 
     Subsequently, substrate 30 is divided into a field and active areas 35 and 34 (referring to FIG. 8A). A device isolating field oxide layer 36 is formed on field area 35 of substrate 30 through a general field oxidation process. 
     Referring to FIGS. 4B, 5B, 6B and 7B, a thin-film oxide layer is formed over the substrate as the first insulating layer. The oxide layer 17 of the active, epitaxial layer 33, insulating layer 32 and s portion of silicon substrate 31 are etched to form the contact C1 by exposing the silicon substrate 31. The oxide layer serves as the gate insulating layer. 
     As shown in FIGS. 4C, 5C, 6C and 7C, first polysilicon layer 39 is deposited on the overall surface of the substrate including internal surface of the contact C1. Referring to FIGS. 4D, 5D, 6D and 7D, first polysilicon layer 39 is patterned so that a substrate contact portion C10 is formed in the contact C1 and that the wordline W/L is formed on the oxide layer 17 (see FIG. 8C). Here, wordline W/L is formed to elongate a pair of legs W/L-A and W/L-B in parallel, centering on the contact C1. Of the wordline W/L, each portion GA1 and GA2 above active area 34 respectively act as the gate of first and second access transistors TA1 and TA2, as shown in FIGS. 6D and 7D. 
     Here, in case that a general monocrystal silicon substrate not the SOI substrate is used, the substrate contact portion C10 may not be formed. Instead, an impurity is implanted into the substrate to form a substrate contact portion. 
     Referring to FIGS. 4E, 5E, 6E and 7E, a second insulating layer 27 of CVD oxide layer is formed to embrace all the exposed surface of wordline W/L. The second polysilicon layer is deposited on the overall surface of the substrate and patterned to be left above legs W/L-A and W/L-B of the wordline W/L and the active area 34 therebetween. By doing so, gates GD1 and GD2 of drive transistors TD1 and TD2 are formed spaced apart from the substrate contact portion C10 (see FIG. 8D). 
     Referring to FIGS. 4F, 5F, 6F and 7F, using, as masks, gates GD1 and GD2 of drive transistors TD1 and TD2 and the wordline W/L, a high-concentration n+ type impurity is implanted into the active area 34 to thereby form impurity-areas 34A, 34B, 34C, 34D and 34E. In other words, source areas 34E and 34A and drain areas 34D and 34B of access transistors TA1 and TA2 and source area 34C and drain areas 34D and 34B of drive transistors TD1 and TD2 are formed, respectively. 
     Here, as shown in FIGS. 2A and 2B, drain area 34B of second drive transistor TD2 and drain area 34B of second access transistor TA2 are commonly formed. Drain area 34D of first drive transistor TD1 and drain area 34D of first accress transistor TA1 are commonly formed. As shown in FIG. 4F, the contact portion C10 is formed in a center portion of area 34C of drive transistors TD1 and TD2 so that they are in contact with the substrate through the substrate contact portion C10. They are grounded. 
     Referring to FIGS. 4G, 5G, 6G and 7G, gates GD1 and GD2 of drive transistors TD1 and TD2 are formed, and sidewall spacer 46 is formed on both sides of gates GD1 and GD2. A CVD oxide layer is formed as third insulating layer 37 on the overall surface of the substrate and selectively etched to form a contact C3 and contact C13 for connecting L1, and a contact C5 and contact C11 for connecting L2 (see FIG. 8E). Here, the contact C3 is formed on drain area 34D of first drive transistor TD1 (or source area of first access transistor TA1). The contact C5 is formed on drain area 34B of second drive transistor TD2 (or source area of second access transistor TA2). The contact C11 is formed on gate GD1 of first drive transistor TD1. The contact C13 is formed on gate of second drive transistor TD2. 
     Referring to FIGS. 4H, 5H, 6H and 7H, a third polysilicon layer is deposited on the overall surface of the substrate as resistor layer and patterned to form first and second load resistors R1 and R2, and L1 and L2 (see FIG. 8F). First load resistor R1 is located between L1 and VCC line. Second load resistor R2 is located between L2 and VCC line. 
     A high-concentration impurity is implanted into the L1, L2 and VCC portion of patterned resistor layer. By doing so, there are formed a low-resistance first wire line L1 for connecting source area 34D of first access transistor TA1 and gate GD2 of second drive transistor TD2, and a low-resistance second wire line L2, for connecting source area 34B of second access transistor TA2 and gate GD1 of first drive transistor TD1, and VCC line. 
     Referring to FIGS. 4I, 5I, 6I and 7I, an oxide layer 47 is formed on the overall surface of the substrate as the fourth insulating layer. A planarization layer 57 is formed thereon. Planarization layer 57, fourth insulating layer 47, and third insulating layer 37 are selectively etched to form a contact C7 and a contact C9 on source areas 34E and 34A of first and second access transistors TA1 and TA2. 
     Conductive layer is formed on the overall surface of the substrate including the contact C7 and C9, fabricating the embodiment of the SRAM cell of the present invention. 
     As described above, the SRAM cell of the present invention is so small as to be advantageous in high integration, and reduces resistance by being grounded through the highly doped substrate, stabilizing the cell more. In addition, the load resistor is easily controlled to sharply reduce resistance. 
     Furthermore, according to the present invention, the cell is symmetric to contribute to cell stabilization, and step coverage is improved to increase yield. This invention can be employed to all the devices using ground because it is grounded through the substrate besides the SRAM cell.