Abstract:
A semiconductor device at least has a memory cell array. The memory cell array has active regions extending linearly in parallel with one another at predetermined intervals and each containing alternating source and drain regions, gate electrodes orthogonally crossing the active regions between the source and drain regions and each having a floating gate and a control gate laid one upon another, first conductors extending linearly in parallel with the gate electrodes and connected to corresponding ones of the source regions through source contacts, and second conductors connected to corresponding ones of the drain regions through drain contacts.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims benefit of priority under 35 USC 119 on Japanese Patent Application No. P2000-289129 filed on Sep. 22, 2000, the entire contents of which are incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a semiconductor memory having a nonvolatile memory cell array, and particularly, to the structure of source wiring in the memory cell array. 
   2. Description of the Related Art 
   Flash memories are roughly classified into NOR flash memories and NAND flash memories based on cell array structures. The NOR flash memories are randomly accessible and are appropriate for high-speed read applications.  FIG. 1  is a plan view showing a memory cell array in a NOR flash memory according to a first prior art. This prior art employs a SAS (self-aligned source) structure to reduce cell size. The SAS structure is formed by forming word lines (control gates  100  in  FIG. 1 ), removing an element isolation oxide film in a self-aligning manner with respect to the word lines, and forming a diffusion layer on the part where the oxide film has been removed. The diffusion layer forms drain regions  102  and source regions  104 .  FIGS. 2A and 2B  are sectional views taken along a line I—I of  FIG. 1 , in which  FIG. 2A  is before the removal of the element isolation oxide film  112  and  FIG. 2B  is after the removal of the same. The oxide film  112  is formed by LOCOS (localized oxidation of silicon) method. After the removal of the oxide film  112 , ions are implanted to form the diffusion layer  114  of n + -type. 
   A recently used element isolation method is STI (shallow trench isolation) method, which is effective to reduce a memory cell array. Applying the STI method to the memory cell array of  FIG. 1  causes a problem.  FIGS. 3A and 3B  are sectional views taken along the line I—I of  FIG. 1  assuming that the memory cell array of  FIG. 1  is formed by the STI method. Trenches are formed in a substrate  110 , and an insulating film is formed over the trenches. An element isolation region formed by the STI method is steeper than that formed with the element isolation oxide film  112  of  FIG. 2A  of the LOCOS method. In  FIG. 3A , ions are vertically implanted into the substrate  110 , and no diffusion layer is formed on each side face of each trench from which an element isolation oxide film (not shown) has been removed. In  FIG. 3B , ions are obliquely implanted into the substrate  110 . This may form an n + -type diffusion layer  116  on each trench side face, too. The structure of  FIG. 3B , however, causes a problem shown in  FIG. 4B . Compared with a normal memory cell transistor of  FIG. 4A , the structure of  FIG. 4B  involves a source region  104  having a very deep depth “d” due to horizontally diffused impurities. This results in increasing the resistance of the source region  104  and deteriorating read/write performance. To avoid the problem, the prior art must form metal source wiring connected to source regions  104  at predetermined intervals in parallel with bit lines. This source wiring increases a memory cell array area. 
     FIG. 5  shows a second prior art that employs an extended source region  104 , instead of the SAS structure. However, the extended source region  104  of  FIG. 5  increases a cell size. Namely, a distance L 1  shown in  FIG. 6  must be long enough to absorb positioning errors in a lithography process, prevent variations in the shapes of channel regions, and avoid unstableness in memory cell characteristics, in particular, between vertically formed memory cells. A distance L 2  shown in  FIG. 6  must also be large enough to prevent a floating gate isolation region  128  from overlapping the source region  104  in the lithography process. If the overlapping happens, the source region  104  will be etched off in an etching process that follows the lithography process. It is difficult to maintain the shape of the The island-shaped floating gate isolation region  128 . If there are positioning errors between the floating gate isolation regions  128  and control gates  100  to vary the shapes of the control gates  100  between even and odd lines, there will be large cell characteristic differences between the even and odd lines. 
   BRIEF SUMMARY OF THE INVENTION 
   An aspect of the present invention provides a nonvolatile semiconductor memory having a memory cell array. The memory cell array includes active regions extending linearly in parallel with one another and containing impurity diffused source and drain regions, element isolation regions extending linearly and isolating the active regions from one another, gate electrodes orthogonally crossing the active regions and element isolation regions and each having a floating gate and a control gate laid one upon another, and first metal conductors extending linearly in parallel with the gate electrodes and each connecting the source regions to one another between adjacent ones of the gate electrodes. The drain regions are connected via the first metal conductors or direct to a second metal conductors. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a plan view showing a memory cell array of a NOR flash memory according to the first prior art; 
       FIG. 2A  is a sectional view taken along a line I—I of  FIG. 1  showing element isolation regions formed by the LOCOS method; 
       FIG. 2B  is a sectional view taken along the line I—I of  FIG. 1  showing a SAS structure formed from the structure of  FIG. 2A ; 
       FIG. 3A  is a sectional view taken along the line I—I of  FIG. 1  showing a SAS structure formed from a structure having element isolation regions made by the STI method, the SAS structure having vertically implanted ions; 
       FIG. 3B  is a sectional view taken along the line I—I of  FIG. 1  showing a similar SAS structure but with obliquely implanted ions; 
       FIGS. 4A and 4B  are sectional views explaining the problem of the SAS structure with obliquely implanted ions, in which  FIG. 4A  shows a normal state and  FIG. 4B  an abnormal state; 
       FIG. 5  is a plan view partly showing a memory cell array in a NOR flash memory according to the second prior art; 
       FIG. 6  is a plan view explaining the structural problem of the memory cell array of  FIG. 5 ; 
       FIG. 7  is a sectional view partly showing a memory cell array in a NOR flash memory according to the reference of the present invention; 
       FIG. 8  is an equivalent circuit diagram showing the memory cell array in a NOR flash memory with metal wiring connecting source diffusion layer according to the reference of the present invention; 
       FIG. 9  is a plan view partly showing a memory cell array in a NOR flash memory according to an embodiment of the present invention; 
       FIG. 10A  is a sectional view taken along a line II—II of  FIG. 9 ; 
       FIG. 10B  is a sectional view taken along a line III—III of  FIG. 9 ; 
       FIG. 11  is an equivalent circuit diagram showing the memory cell array of  FIG. 9 ; 
       FIGS. 12A ,  12 B, and  12 C are sectional views showing write, erase, and read operations, respectively, in a memory cell of the memory cell array of  FIG. 9 ; 
       FIG. 13A  is a sectional view partly showing a memory cell array according to an embodiment of the present invention installed with a logic device having a fourth wiring layer, this view corresponding to the sectional view taken along the line II—II of  FIG. 9 ; 
       FIG. 13B  is a sectional view partly showing the memory cell array of  FIG. 13A , corresponding to the sectional view taken along the line III—III of  FIG. 9 ; 
       FIG. 14  is a sectional view partly showing a memory cell array in a NOR flash memory according to an embodiment of the present invention without conductors dedicated to drain regions; 
       FIG. 15A  shows a design pattern of the drain conductor  20   b  of  FIG. 9  and a resist pattern formed from the design pattern; and 
       FIG. 15B  shows a corrected design pattern based on the design pattern of  FIG. 15A  and a resist pattern formed from the corrected design pattern. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Various embodiments of the present invention will be described with reference to the accompanying drawings. It is to be noted that the same or similar reference numerals are applied to the same or similar parts and elements throughout the drawings, and the description of the same or similar parts and elements will be omitted or simplified. 
   (Reference) 
     FIG. 7  shows a reference of the present invention that forms local wiring serving exclusively for source regions. In  FIG. 7 , stripe-shaped source regions  104  are connected to each other through high-melting point metal wiring  118  made of, typically, tungsten (W). To form the metal wiring  118 , an additional manufacturing process is needed. In addition, the high-melting point metal wiring  118  requires a drain contact  120  to be extended through interlayer insulating films  122  and  123 , to connect a drain region  102  to metal wiring  124  serving as a bit line. This results in increasing an aspect ratio, insufficiently filling the contacts  120  with metal, and increasing the resistance of the contacts  120 . The resistance drops further than connecting through the diffusion layer, when the source regions are connected to each other through the high-melting point metal wiring  118 . But a wiring resistance of the high-melting point metal wiring  118  is higher than a wiring resistance of a metal wiring. There are cases in which many memory cells exist in the array and in which it connects a large number of source diffusion regions and the case in which the resistance value becomes a problem. The performance is lowered, when the resistance value is not enough. It is necessary for the resistance to be reduced by connecting a source diffusion layer through metal wiring  45  for the source parallel placed with a bit line in every some cells, as shown in  FIG. 8 , in order to prevent the lowering of the performance. 
   (Embodiment) 
     FIG. 9  is a plan view partly showing a memory cell array in a NOR flash memory according to an embodiment of the present invention. The memory cell array includes drain regions  10 , source regions  12 , element isolation regions  14 , floating gate isolation regions  16 , and control gates  18  serving as word lines. The control gates  18  orthogonally cross the regions  10 ,  12 ,  14 , and  16 . The regions  10 ,  12 ,  14 , and  16  are basically linear and are arranged like stripes. 
   Each memory cell in the memory cell array contains a drain region  10  and a source region  12 . The source regions  12  between the adjacent control gates  18  are connected to one another through a conductor  20   a . Each drain region  10  is connected to a discrete island-shaped conductor  20   b , which is connected to a bit line formed in an upper metal wiring layer (not shown). The conductors  20   a  and  20   b  are formed from a single metal wiring layer. 
     FIG. 10A  is a sectional view taken along a line II—II of  FIG. 9 ,  FIG. 10B  is a sectional view taken along a line III—III of  FIG. 9 , and  FIG. 11  is an equivalent circuit diagram showing the memory cell array of  FIG. 9 . In  FIG. 10A , the source region  12  in each memory cell is connected to a source contact  28  formed through an interlayer insulating film  26 . The source contact  28  is connected to the conductor  20   a . In  FIG. 10B , the drain region  10  in each memory cell is connected to a drain contact  34  formed through the interlayer insulating film  26 . The drain contact  34  is connected to the conductor  20   b , which is connected to a via  36  formed through an interlayer insulating film  30 . The via  36  is connected to the bit line  32 . 
   In  FIG. 11 , the memory cell array contains a matrix of memory cells each having a floating gate and a control gate. A word line WL is provided for each row of memory cells and is connected to the control gates of the memory cells in the row. A bit line BL is provided for each column of memory cells and is connected to the drain regions of the memory cells in the column. A source line SL is provided for a pair of adjacent memory-cell rows and is connected to the source regions of the memory cells in the paired rows. 
     FIG. 12A  shows a write operation,  FIG. 12B  an erase operation, and  FIG. 12C  a read operation in any one of the memory cells in the memory cell array of  FIG. 9 . The write operation is carried out by applying, for example, 10 V to the control gate  18 , 5V to the drain region  10 , and a ground voltage Vss to the source region  12 . As a result, electrons flow from the drain region  10  to the source region  12 . The electrons are accelerated due to a potential difference between the drain region  10  and the source region  12 . Part of the electrons become hot electrons, which are captured by the floating gate  22 . The erase operation is carried out by applying, for example, −8 V to the control gate  18  and 10 V to a substrate  24 . As a result, an FN (Fowler-Nordheim) current draws the electrons from the floating gate  22  to the substrate  24 . The erase operation is carried out on the memory cells block by block. The read operation is carried out by applying, for example, 5V to the control gate  18  and 0.8 V to the drain region  10  and by testing whether or not the memory cell is conductive. Any memory cell having electrons captured in the floating gate  22  is nonconductive, and any memory cell having no electrons in the floating gate  22  is conductive. 
   The structure of the embodiment is effective to reduce a cell size. In the case of the second prior art of  FIG. 5 , a lateral cell size is 2.2 F and a longitudinal cell size is 3.8 F at a minimum due to the margins mentioned above. As a result, the total cell size of the related art is 8.36 F 2 . Here, “F” is a minimum design rule. On the other hand, the embodiment of  FIG. 9  involves a lateral cell size of 2.2 F and a longitudinal cell size of 3 F. This means that the total cell size of the embodiment is 6.6 F 2 , which is 25% smaller than that of the related art. 
   According to the embodiment, the source side and drain side in each memory cell are symmetrical with each other below the contacts  28  and  34 . This symmetry reduces stress during manufacturing, thereby reducing crystal defects. Since the drain region  10  and source region  12  in each memory cell have an identical impurity distribution, an erase operation using an FN current will effectively be carried out. 
   The embodiment is effective when applied to a semiconductor device provided with a memory and a logic device. When installing a memory and a logic device on a single chip, efforts must be made to minimize additional processes and reduce a cell size in the memory. A standard logic device involves two or more wiring layers. Without adding processes to the process of forming the wiring layers, the embodiment is capable of installing the standard logic device and a flash memory of small cells and minimum characteristic variations on a single chip. 
   The embodiment is particularly effective when applied to NOR flash memories. A standard NOR flash memory passes a cell current of 100 to 300 μA for a write operation carried out by channel hot electron injection, and 10 μA or higher for a high-speed random access read operation. To achieve these operations, source lines must have low resistance and must be fixed at a reference voltage of, for example, 0 V. If the source line voltage fluctuates, a voltage at each source region fluctuates. This fluctuation deteriorates the write and read performance of each memory cell. For example, the second prior art of  FIG. 5  extends the source region  104  to form source wiring. The source region  104 , i.e., a diffusion region generally has a sheet resistance of several tens of ohms. Even if the second prior art arranges a source shunting conductor for every 32 memory cells, the source region of a cell farthest from the source shunting conductor will have a parasitic resistance of about 1 kΩ. In this case, the source voltage will fluctuate by 100 mV at a minimum in a write operation and 10 mV at a minimum in a read operation. 
   On the other hand, the embodiment connects the source regions  12  to one another through the conductor  20   a  whose sheet resistance is 1/1000 or below that of the diffusion layer in which the source regions  12  are formed. This eliminates the source shunting conductor of the related art and greatly reduces source resistance, thereby reducing a cell size and improving the write and read performance of each memory cell. 
   The embodiment is also advantageous in an erase operation. When drawing electrons from an entire channel of each memory cell for an erase operation, there will be erase speed differences among memory cells if the memory cells involve coupling ratio differences. If such erase speed differences occur, the threshold values of the memory cells will show a distribution range. To cope with this problem, the present invention forms the diffusion regions  10  and  12 , floating gate isolation regions  16 , etc., in stripes, to minimize shape variations and cell coupling ratio differences, thereby minimizing a distribution range of threshold values. 
     FIG. 13A  is a sectional view partly showing a memory cell array according to an embodiment of the present invention installed with a logic device having a fourth wiring layer, this view corresponding to the sectional view taken along the line II—II of  FIG. 9 .  FIG. 13B  is a sectional view showing the memory cell array of  FIG. 13A , corresponding to the sectional view taken along the line III—III of  FIG. 9 . Conductors  40  formed from a third metal wiring layer are used as shunt lines for word lines  18 , and conductors  44  formed from a fourth metal wiring layer are used as sub-bit lines extending in parallel with bit lines  32 . 
   In  FIGS. 13A and 13B , each memory cell has a drain region  10 , which is connected to the bit line  32  through a conductor  20   b . The drain conductors  20   b  may be omitted as shown in  FIG. 14 . In  FIG. 14 , a drain region  10  of each memory cell is connected to a drain contact  34  formed through interlayer insulating films  26  and  30 , and the drain contact  34  is connected to a bit line  32 . 
   In the example of  FIG. 14 , a source contact  28  on each source region  12  and the drain contact  34  on each drain region  10  must be formed in separate processes. This is a disadvantage in terms of the number of processes, compared with the reference of  FIG. 7  that employs the conductors  118  for the source regions  104 . The conductors  118 , however, are usually made of tungsten. On the other hand, the conductors  20   a  of the present invention of  FIG. 14  are usually made of aluminum or aluminum-based alloy of low resistance. Namely, the resistance of the conductors  20   a  of  FIG. 14  is lower than that of  FIG. 7 . In addition, the use of the tungsten conductors  118  is limited to memory cells and is not versatile. On the other hand, the conductors  20   a  for the source regions  12  are versatile because of their material. For example, they may be applicable to the wiring of peripheral circuits, to improve the concentration of the peripheral circuits. 
   In  FIG. 14 , the first wiring layer to form the conductors  20   a  is preferable to be thin. If the first wiring layer is thick, it increases the aspect ratios of the second wiring layer from which the bit lines  32  are formed and the drain contacts  34  connected to the drain regions  10 , respectively. When using the wiring layers for peripheral circuits, the first wiring layer is used for short wiring in which parasitic resistance causes little problem, and the second wiring layer formed above the first wiring layer is used for long wiring that must have low resistance. In an area where the peripheral circuits are formed, the conductors formed from the first and second wiring layers may freely cross each other, to easily form an optional pattern layout. The first wiring layer helps increase a pattern concentration compared with the reference of  FIG. 7  having no first wiring layer. 
   The conductor  20   b  of  FIG. 9  connected to a drain region is a discrete island-shaped fine pattern. If this pattern is designed to be square on a photomask as shown in  FIG. 15A , it will be circular with four corners being rounded when transferred onto a resist. In this case, the via  36  of  FIG. 10B  will be out of the conductor  20   b  even with a slight positioning error in the via  36 . To avoid this, the conductor  20   b  may be designed to have a pattern shown in  FIG. 15B . This pattern will become square when transferred onto a photoresist. 
   As explained above, the nonvolatile semiconductor memory according to the embodiment is producible without greatly increasing the number of manufacturing processes and is capable of reducing the resistance of source regions in the memory cell array in the nonvolatile semiconductor memory. 
   The metal conductor has been exemplified material including aluminum as representative things. But it is effective to material, for example, the copper having resistance lower than aluminum. 
   Although the present invention has been explained in connection with several embodiments, it must be understood that the present invention is not limited by the embodiments or the accompanying drawings. 
   Additional advantages and modifications of the present invention will readily occur to those skilled in the art. Therefore, the present invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit or scope of the general inventive concept as defined by the appended claims and their equivalents.