Abstract:
A new memory cell combination is disclosed. It includes a static random access memory (SRAM) unit and a mask read only memory (MROM) unit. The prior art separates the two memory units in different areas on a chip so that the circuit layout is not optimized. The disclosed cell combines them in the same area, saving more than 20% of the area.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of Invention  
           [0002]    The invention relates to a non-volatile static random access memory (SRAM) cell and, in particular, to a non-volatile SRAM cell that makes use of mask read only memory (MROM).  
           [0003]    2. Related Art  
           [0004]    There are two main type of memory: the volatile memory and the non-volatile memory. In the development history of non-volatile memory, the first proposed mask read only memory (MROM) first load programs or data into the photo mask used during the manufacturing process. Thus, they can permanently store data. The structure of its cell does not need to be manufactured using any special process. Moreover, there is no writing involved. Therefore, it is very economical and simple. Nevertheless, there are the following problems to be solved. For instance, the time it takes for the memory to be made in the wafer fab and to be delivered to the customer is longer than other types of ROM (such as EPROM or EEPROM). This is mainly because the MROM is a configure-to-order (CTO) product. The fab starts preparing production only after receiving the memory codes sent from the customer.  
           [0005]    On the other hand, the photo mask has to be manufactured according to the ROM asked by the customer. It is not finished until a certain chip process is done. Therefore, its cost for small-quantity production tends to be higher. Once the manufacturing is over, there is no way to modify its internal memory data. As a result, there are some risks for both the customer and the manufacturer.  
           [0006]    Although there are the above-mentioned problems, the MROM still has its practical values. Driven by the needs for consuming electronics, memory of different specifications and types exist on the market. One of them is the non-volatile SRAM. The proposal of this type of technology is because the SRAM is volatile memory. Once if the power is turned off, the data stored there in disappear immediately. To compensate for this drawback, the non-volatile SRAM with memory properties becomes the modern trend.  
           [0007]    Since electronic products such as PDA&#39;s and game machines are getting more compact, the chip area plays an important role. However, the prior art has the SRAM unit and the MROM cell on separate regions of a chip. Therefore, it is very inefficient. Thus, it is of great importance to provide a non-volatile SRAM cell with a smaller area.  
         SUMMARY OF THE INVENTION  
         [0008]    An objective of the invention is to provide a new memory cell combination, including a SRAM unit and a MROM unit, for selectively being used as SRAM and ROM.  
           [0009]    Another objective of the invention is to provide a SRAM cell with a smaller area. The active area where the source contact (Vss contact) is located extends to have a cross intersection area with the polysilicon area where access transistors are formed, constituting a MROM unit. Such a layout enables the word line to be shared between the Vss contact of the SRAM unit and the MROM. They can also be shared in the X-decoder circuit. One does not need to design the two unit circuits in two separate areas, saving the chip area by about 20%. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]    The invention will become more fully understood from the detailed description given hereinbelow illustration only, and thus are not limitative of the present invention, and wherein:  
         [0011]    [0011]FIG. 1 is a circuit diagram of the non-volatile SRAM cell of the invention;  
         [0012]    [0012]FIG. 2 is a schematic circuit layout of a conventional SRAM cell; and  
         [0013]    [0013]FIG. 3 is a schematic circuit layout of the disclosed non-volatile SRAM cell. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0014]    The memory cell disclosed by the invention is shown in FIG. 1. It includes a static random access unit (SRAM) unit  10  (hereinafter as SRAM cell) and a mask read only memory (MROM) unit  20  (hereinafter as MROM cell).  
         [0015]    The SRAM cell  10  is a bit data memory structure for temporarily holding one bit of data. It provides the information to an operation environment at a later time according to the central processing unit (CPU) command.  
         [0016]    As shown in the drawing, the SRAM cell  10  contains six transistors, the first transistor Q 1 , the second transistor Q 2 , the third transistor Q 3 , the fourth transistor Q 4 , the fifth transistor Q 5 , and the sixth transistor Q 6 , forming a six-transistor one-bit memory unit. A pair of CMOS inverters is connected into a flip flop. The memory nodes N 1 , N 2  are connected to a pair of access transistors Q 5 , Q 6 , respectively, as the transmission gate. The gates of the fifth transistor Q 5  and the sixth transistor Q 6  are connected to a word line. Through the first transistor Q 1 , the second transistor Q 2 , the word line and a bit line perform data reading and writing. The third transistor Q 3  and the fourth transistor Q 4  are p-channel metal oxide semiconductor field effect transistors (pMOSFET&#39;s); while the first transistor Q 1  and the second transistor Q 2  are n-channel metal oxide semiconductor field effect transistors (nMOSFET&#39;s). The first transistor Q 1  and the third transistor Q 3 , the second transistor Q 2  and the fourth transistor Q 4  form CMOS inverters, respectively.  
         [0017]    The gates of the first transistor Q 1  and the third transistor Q 3  are connected to the drains of the second transistor Q 2  and the fifth transistor Q 5 . The gates of the second transistor Q 2  and the fourth transistor Q 4  are connected to the drains of the first transistor Q 1  and the third transistor Q 3 . The sources of the third transistor Q 3  and the fourth transistor Q 4  are connected to a power supply Vcc. The sources of the first transistor Q 1  and the second transistor Q 2  are connected to the ground Vss. When data  14  are latched in the SRAM cell  10 , the second transistor Q 2  is ON, the first transistor Q 2  is OFF, the memory node N 1  has the voltage Vcc, the memory node N 2  has the voltage 0. When the first transistor Q 1  is OFF while the second transistor is ON, it represents that there are data  1  stored in the SRAM cell  10 .  
         [0018]    The gates of the fifth transistor Q 5  and the sixth transistor Q 6  of the transmission gate are connected to the word line. Their drains are connected to the memory nodes N 1  and N 2 . Their sources are connected to a first bit line BLQ 5  and a first anti-bit line BLQ 6 . The fifth transistor Q 5  and the sixth transistor Q 6  function as switches. When they are ON, data can be sent out via the first bit line BLQ 5  and the first anti-bit line BLQ 6 . Its state as ON and OFF is determined by the voltage signal on the word line WL. When the word line voltage is raised, the fifth transistor Q 5  and the sixth transistor Q 6  are opened. One bit of data is stored or sent out through the first bit line BLQ 5  and the first anti-bit line BLQ 6 .  
         [0019]    The MROM cell  20  contains a seventh transistor Q 7 , which is an nMOSFET with its gate connecting to the word line WL. Its drain is connected to a third bit line BLROM. The word line WL is shared by the fifth, sixth and seventh transistors Q 5 , Q 6 , Q 7 . When the voltage on the word line WL is raised, the SRAM cell  10  and the MROM cell  20  are selected. Using the bit lines, the combination can be selectively used as an SRAM cell or MROM cell for the CPU.  
         [0020]    A primary objective of the invention is to combine the SRAM cell  10  and the MROM cell  20  into a single memory unit without increasing the cell area. As described above, the SRAM cell  10  and the MROM cell  20  share the same word line WL, saving the layout space of the seventh transistor Q 7 . We describe in the following how to integrate the MROM cell  20  together with the SRAM cell  10  without increasing the area.  
         [0021]    [0021]FIG. 2 shows the circuit layout of a conventional SRAM cell. There is a first active area A 1  in the shape of an inversed T, a second active area A 2  comprised of two E-shape regions, and a third active area A 3  also in the shape of an inversed T. It further has a first polysilicon area P 1 , a second polysilicon area P 3 , and a third polysilicon area P 3 . The first polysilicon area P 1 , the second polysilicon area P 2 , the first active area A 1 , and a second active area A 2  form four cross intersection areas for four transistors. They are the first transistor Q 1 , the second transistor Q 2 , the third transistor Q 3 , and the fourth transistor Q 4  in FIG. 1. In particular, the first transistor QI and the second transistor Q 2  are nMOSFET&#39;s, the third transistor Q 3  and the fourth transistor Q 4  are pMOSFET&#39;s.  
         [0022]    The protruding part of the T-shape first active area A 1  has a first Vcc contact CVcc. The protruding part of the E-shape second active area A 2  has a second Vss contact CVss. The third contact CPH 1  and the fourth contact CPH 2  are up-pulling contacts on both sides of the T area. The fifth contact CPD 1  and the sixth contact CPD 2  are down-pulling contacts on both sides of the E area. The second active area A 2  and the third polysilicon area P 3  overlap to form two cross intersection areas; they are the fifth transistor Q 5  and the sixth transistor Q 6  in FIG. 1.  
         [0023]    The lower part of FIG. 2 has a fourth polysilicon area P 4 , a fifth polysilicon area P 5 , and a sixth polysilicon area P 6 . They form six cross intersection areas with the second active area A 2  and the third active area A 3 . The six cross intersection areas are six transistors for another SRAM cell. The third polysilicon area P 3  and the sixth polysilicon area P 6  cross the second active area A 2 . The third polysilicon area P 3  and the sixth polysilicon area P 6  do not overlap and leave a blank gap. There is a first bit line contact CBL 1  and a second bit line contact CBL 2  in the layout. This configuration forms two basic SRAM units.  
         [0024]    Taking a closer look at FIG. 2, one can discover that the third polysilicon area P 3  and the sixth polysilicon area P 6  are not connected at the places close to the second contact CVss, leaving a blank area A 4 . The invention utilizes this blank area to put another ROM unit.  
         [0025]    The invention extends the active area at the second contact CVss to the third polysilicon area P 3  and the sixth polysilicon area P 6  and makes them cross to form two transistors, as shown in FIG. 3. The extending part at the second contact CVss of the third polysilicon area P 3  and the second active area A 2  constitutes a ROM unit. The sixth polysilicon area P 6  and the second active area A 2  also form another ROM unit. The bit line contact of the ROM units are located at the intersection of the protruding parts of the second contacts CVss, the bit line contact CBLR in FIG. 3.  
         [0026]    As described above, such a layout can embed ROM units inside the SRAM unit, effectively using the available area inside the SRAM. The word line can be shared to reduce the chip area by at least 20% than the prior art.  
         [0027]    Certain variations would be apparent to those skilled in the art, which variations are considered within the spirit and scope of the claimed invention.