Abstract:
A read only memory comprises a P-type substrate and a plurality of N +  -type diffusion layers arranged checkerwise on one major surface of the substrate in which four N +  -type diffusion layers having contacts are located at corners of an imaginary rectangle, and a fifth N +  -type region having a contact is formed substantially at the center of the imaginary rectangle. Between the fifth N +  -type diffusion layer and the first to fourth N +  -type diffusion layers four MOS transistors are formed for the single contact. Gate lines are provided, each extending between adjacent two N +  -type diffusion layers without overlapping them. Each of the four N +  -type diffusion layers also acts as a central N +  -type region of another imaginary rectangle adjacent to the first mentioned imaginary rectangle.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a read only memory (ROM) using MOS transistors as memory cells. 
     A read only memory using MOS transistors can be proposed as is shown in FIG. 1A. The MOS transistors arranged in such pattern constitute a so-called &#34;silicon gate type MOS IC&#34;. The cross sections of the MOS IC taken along line B--B, line C--C and line D--D are shown in FIGS. 1B, 1C and 1D, respectively. 
     The read only memory of FIGS. 1A to 1D comprises gate lines 1a, 1b, 1c, . . . , ROM output lines 2a, 2b, 2c, . . . , ground lines 3a, 3b, . . . , MOS transistors 4 11 , 4 12 , 4 13 , . . . , 4 33 , a P-type silicon substrate 5, N +  -type diffusion layers 6a, 6b, 6c, . . . , contacts 7a, 7b, 7c, . . . provided on the ROM output lines 2a, 2b, . . . , gate oxide films 8 1a , 8 1b , 8 1c , . . . field oxide films 8 2a , 8 2b , 8 2c , . . . On one major surface of the P-type silicon substrate 5 each ground line is juxtaposed with two gate lines. For example, the ground line 3a is juxtaposed with the gate lines 1b and 1c, and the ground line 3 b with the gate lines 1d and 1e. The ROM output lines 2a, 2b, . . . intersect with the gate lines 1a, 1b, . . . and the ground lines 3a, 3b, . . . The ground lines 3a, 3b, . . . are formed by N +  impurity diffusion. Thus the MOS transistors 4 11 , 4 12 , . . . are formed in the hatched areas in FIG. 1A, each constituted by adjacent ground line, N +  -type diffusion layer and gate line. The ground lines 3a, 3b, . . . constitute the sources of the MOS transistors 4 11  to 4 33 , and the N +  -type diffusion layers 6a, 6b, . . . the drains thereof. 
     The ROM of FIGS. 1A to 1D is disadvantageous in that its 1-bit memory cell indicated by double dot-dash line occupies a relatively large areas. As commonly known, the contacts 7a, 7b, . . . must occupy an area 1a larger than necessary. Otherwise the yield of the IC products cannot be maintained above a certain level. It is also known that the IC density of ROMs of this type depends largely on the size of contacts. In the ROM of FIGS. 1A to 1D the contacts 7a, 7b, . . . arranged parallel to one another at regular intervals are extending in the lateral direction of the MOS IC. The MOS IC is therefore elongated in the lateral direction. Moreover, the contacts 7a, 7b, . . . are not so frequently used since each contact is located between only two MOS transistors which extend in the longitudinal direction of the MOS IC along the ROM output lines 2a, 2b, . . . For these reasons, the area S occupied by one 1-bit memory cell is: 
     
         S=l×m=20 μm×17 μm=340 μm.sup.2. 
    
     340 μm 2  is an extremely large area for a 1-bit memory cell, and the MOS IC cannot have a large IC density. 
     The ROM of FIGS. 1A to 1D has such a circuit structure as illustrated in FIG. 2, wherein the same parts are denoted by the same reference numerals as used in FIGS. 1A to 1D. 
     Another ROM using MOS transistors can be proposed as is illustrated in FIGS. 3A, 3B and 3C. In this ROM the output lines act also as ground lines. To read out data from a memory cell (i.e. MOS transistor) via a selected one of the output lines, a switching circuit is so operated as to connect to the ground the output line which is adjacent to the selected output line. The ROM comprises gate lines 1a, 1b, 1c, . . . , output lines 2a, 2b, 2c, . . . , memory cells 4 11  to 4 32 , a P-type silicon substrate 5, gate oxide films 8 1  and field oxide films 8 2  . . . The ROM has such a circuit structure as shown in FIG. 4A. As illustrated in FIG. 4A, the switching among the output lines 2a, 2b, 2c, . . . is carried out by a switching circuit 11. 
     Another known ROM using MOS transistors is illustrated in FIG. 4B. This ROM is identical with the ROM shown in FIG. 3A with respect to ROM section 4. It is characterized in that four output lines 2a, 2b, 2c and 2d form one group and are so connected to the ROM section 4 as to obtain the outputs of the ROM section. Suppose the output line 2a connected to an MOS transistor 12a is selected. Then, an MOS transistor 12e connected to the output line 2b adjacent to the selected line 12a is driven at once by an output of a switching circuit such as one 11 shown in FIG. 4A. In this case the output line 2b acts as a ground line. When the output line 2d connected to an MOS transistor 12d, for example, is selected, an MOS transistor 12h connected to an output line 2e of the next group is driven immediately by an output of the switching circuit. In this case the output line 2e acts as a ground line. Since only one of the output lines of each group is selected at a time, one of the remaining output lines is used as a ground line. 
     Indeed the ROMs of FIGS. 4A and 4B have a little higher IC density than the ROM of FIGS. 1A to 1D. But their cost is inevitably high for their structures. That is, the gate lines 1a, 1b, 1c, . . . made by polysilicon extend in the lateral direction of the MOS IC, while the output lines (or diffusion layers) 2a, 2b, 2c, . . . extend in the longitudinal direction of the MOS IC. Between any two adjacent output lines a plurality of MOS transistors are formed. Apparently, the gate lines 1a, 1b, 1c, . . . which intersect with the N +  -type diffusion layers 2a, 2b, 2c, . . . make masks for forming the N +  -type diffusion layers 2a, 2b, 2c, . . . As a result, it becomes impossible to simultaneously achieve N +  impurity diffusion for forming the MOS transistors 4 11  to 4 33  and N +  impurity diffusion for forming the output lines 2a, 2b, 2c, . . . Thus, N +  impurity must diffused two times, first to form the MOS transistors and then to form the output lines. This increase of impurity diffusion processes is one of the causes for cost hike. 
     SUMMARY OF THE INVENTION 
     An object of this invention is to provide a read only memory which has an elevated IC density and which is manufactured by less impurity diffusion processes and thus at a reduced cost. 
     This invention aims to improve such an ROM as shown in FIG. 4A wherein one of the output lines is selected by a switching circuit so as to read data and the other output line adjacent to the selected one acts as a ground line. 
     A read only memory according to this invention comprises a semiconductor substrate of one conductivity type and a plurality of MOS transistor blocks formed on one major surface of the substrate. Each MOS transistor block comprises first, second, third and fourth regions of the opposite conductivity type which are formed at four corners of an imaginary rectangle; a fifth region of the opposite conductivity type which is formed substantially at the center of the rectangle and spaced from the first, second, third and fourth regions; a first gate conductor which is laid on a gate insulation layer between the fifth region and the first and second regions alinged in the lateral direction of the substrate; a second gate conductor which is laid on the gate insulation layer and extends between the fifth region and the third and fourth regions aligned in the lateral direction of the substrate; a group of output-ground lines laid on one insulation layer and intersecting with the first and second gate conductors, said group including a first output-ground line having first and third contacts connected to the first and third regions, respectively, a second output-ground line having second and fourth contacts connected to the second and fourth regions, respectively and a third output-ground line having a fifth contact connected to the fifth region; and first, second, third and fourth MOS transistors formed between the fifth region and the first, second, third and fourth regions and having each a predetermined memory capacity of digital one bit. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIGS. 1A to 1D schmatically show a read only memory in which one ground line is used commonly for two gate lines; 
     FIG. 2 is a circuit diagram of the read only memory shown in FIGS. 1A to 1D; 
     FIGS. 3A to 3C schmatically show a read only memory in which one of output lines is selected by a switching circuit and the output line adjacent to the selected output line is used as a ground line; 
     FIG. 4A is a circuit diagram of the read only memory shown in FIGS. 3A to 3C; 
     FIG. 4B is a circuit diagram of a read only memory which comprises an ROM section identical with that of FIG. 4A and a group of four output lines so connected as to read outputs of the ROM section; 
     FIG. 5A is an IC pattern of one embodied read only memory according to this invention; 
     FIGS. 5B and 5C are cross sectional views of the read only memory shown in FIG. 5A, taken along line B--B and line C--C in FIG. 5A, respectively; 
     FIG. 6A is an IC pattern of another embodiment of this invention; 
     FIGS. 6B to 6D are cross sectional views of the read only memory shown in FIG. 6A, taken along line B--B, line C--C and line D--D in FIG. 6A, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     As shown in FIGS. 5A to 5C, a read only memory according to this invention comprises a P-type silicon semiconductor substrate 21 and a plurality of rectangular N +  -type diffusion layers 22 11  to 22 33  formed on one major surface of the substrate, parallel to one another with a predetermined space between them. The N +  -type diffusion layers 22 11 , 22 12 , 22 31  and 22 32  are positioned at four corners of an imaginary rectangle T 1 . At the center of the rectangle T 1  the N +  -type diffusion layer 22 21  is located. Further, the N +  -type diffusion layers 22 12 , 22 13 , 22 32  and 22 33  are positioned at four corners of another imaginary rectangle T 2 . At the center of the rectangle T 2  the N +  -type diffusion layer 22 22  is located. The N +  -type diffusion layers 22 21  and 22 22  are positioned at upper two corners of another imaginary rectangle T 3 . The N +  -type diffusion layer 22 32  at the lower-right corner of the rectangle T 1  is located also at the lower-left corner of the rectangle T 2  and at the center of the rectangle T 3 . 
     In the read only memory of FIGS. 5A to 5C, four N +  -type diffusion layers located at four corners of an imaginary rectangle and an N +  -type diffusion layer located at the center of the imaginary rectangle constitute an MOS transistor block. Any adjacent MOS transistor blocks share some N +  -type diffusion layers. The MOS transistor blocks are arranged in a regular pattern. 
     The surface of the P-type silicon substrate 21 is covered with thin gate oxide films 23 1 , field oxide films 23 2  and thick gate oxide films 23 3 . These films 23 1 , 23 2  and 23 3  are made of insulative materials. On these films there are formed gate lines 24 1 , 24 2 , 24 3  and 24 4  made of polysilicon. The gate line 24 1  extends along the upper sides of the N +  -type diffusion layers 22 11 , 22 12  and 22 13  which are aligned with one another in the lateral direction of the substrate 21. The gate line 24 2  extends along the lower sides of the layers 22 11 , 22 12  and 22 13  and along the upper sides of the N +  -type diffusion layers 22 21  and 22 22  which are alinged with each other in the lateral direction of the substrate 21. The other gate lines 24 3  and 24 4  extend similarly along the sides of the N +  -type diffusion layers and thus in the lateral direction of the substrate 21. 
     More precisely, for example, the gate line 24 1  is formed on a thin gate oxide film 23 1  which is located between the N +  -type diffusion layers 22 12  and 22 22 , and the gate line 24 3  is formed on a thick gate oxide film 23 3  which is located between the N +  -type diffusion layers 22 22  and 22 32 . The N +  -type diffusion layers 22 12  and 22 22 , the thin gate oxide film 23 1  and the gate line 24 1  constitute one MOS transistor 4 13 . Similarly, the N +  -type diffusion layers 22 22  and 22 32 , the thick gate oxide film 23 3  and the gate line 24 3  constitute an MOS transistor 4 23 . The gate oxide film 23 1  is as thick as an ordinary active MOS transistor, and the gate oxide film 23 3  is far thickner than an ordinary MOS transistor. Thus the MOS transistor 4 13  functions as a memory cell for storing a &#34;1&#34;  bit, and the MOS transistor 4 23  is a so-called &#34;inactive transistor&#34; and acts as a memory cell for storing a &#34;0&#34; bit. 
     Thus, once the gate lines 24 1  to 24 4  have been formed on the gate oxide films 23 1  and 23 3  and field oxide films 23 2 , the contents of the MOS transistors (i.e. memory cells) are determined. Then, the major surface of the P-type silicon substrate 21 is covered entirely with a protective oxide film 27. On the protective oxide film 27 there are arranged output lines 25 1  to 25 5  at regular intervals to intersect with the gate lines 24 1  to 24 4 . Each of the output lines 25 1  to 25 5  has contacts 26 11  to 26 33  which are connected to the N +  -type diffusion layers 22 11  to 22 33 , respectively. 
     As a result, between the N +  -type diffusion layer 22 21  located at the center of the imaginary rectangle T 1  and the N +  -type diffusion layers 22 11 , 22 12 , 22 31  and 22 32  located at the corners of the rectangle T 1  there are formed four MOS transistors 4 11 , 4 12 , 4 21  and 4 22  each storing a &#34;1&#34; or &#34;0&#34; bit, the transistors 4 11  and 4 12  beneath the gate line 24 2  and the transistors 4 21  and 4 22  beneath the gate line 24 3 . Similarly, between the N +  -type diffusion layer 22 22  located at the center of the imaginary rectangle T 2  and the N +  -type diffusion layers 22 12 , 22 13 , 22 32  and 22 33  located at the corners of the rectangle T 2  there are formed four MOS transistors 4 13 , 4 14 , 4 23  and 4 24  each storing a &#34;1&#34; or &#34;0&#34; bit. 
     That surface area S of the substrate 21 indicated by double dot-dash line in FIG. 5A which is necessary for storing one bit is, for example: 
     
         S=l×m=14 μm×20 μm=280 μm.sup.2. 
    
     Apparently, the area S occupied by one memory cell is much smaller than the area S in the ROM of FIG. 1A which is 340 μm 2 . This is because the surface area of the substrate 21 is very effectively utilized. That is, the N +  -type diffusion layers 22 11  to 22 33  which constitute the sources and drains of the MOS transistors 4 11  to 4 24  are displaced from one another for half pitch in both the lateral and longitudinal directions of the substrate 21 so that four MOS transistors are formed around one N +  -type diffusion layer. 
     Further, the N +  -type diffusion layers and the gate lines can be formed by a single impurity diffusion process since, as shown in FIG. 5A, the layers 22 11  to 22 33  are on one plane, and the gate lines 24 1  to 24 4  on a different plane. Only one impurity diffusion being required instead of two as in manufacturing the ROMs of FIGS. 1A to 1D, 3A to 3C and 4B, the ROM shown in FIGS. 5A to 5C is manufactured at a low cost. 
     Another read only memory according to this invention is illustrated in FIGS. 6A to 6D, wherein the same or similar parts are denoted by the same reference numerals as used in the FIGS. 5A to 5C. In short, the ROM of FIGS. 6A to 6D differs from that of FIGS. 5A to 5C in that each N +  -type diffusion layers is octagonal as shown in FIG. 6A. That is, the rectangular N +  -type diffusion layers of the ROM shown in FIGS. 5A to 5C have their four corners cut off to become octagonal. As a result, the N +  -type diffusion layers 22 11  to 22 33  can be positioned more close to one another, and gate lines 24 1  to 24 4  assume a meandering shape. Further, MOS transistors 4 11  to 4 24  are positioned slantwise with respect to output 25 2  to 25 5 , at contacts 26 12  to 26 33 , respectively. 
     FIGS. 6B, 6C and 6D show the cross sections of the ROM of FIG. 6A, taken along line B--B, line C--C and line D--D in FIG. 6A. As shown in FIG. 6C, a gate oxide film 23 1  is formed thin beneath a gate line 24 2 , thereby forming an active transistor 4 14 . In this respect the ROM is similar to the ROM shown in FIG. 5B. But it differs in that a P-type layer 30 is formed by, for example, ion-implantation between the N +  -type diffusion layers 22 22  and 22 32 , though a gate oxide film 23 1  of an MOS transistor 4 23  is as thick as the active transistor 4 14 . Since the P-type layer 30 is formed in such position, the MOS transistor 4 23  becomes an inactive transistor. That is, the P-type layer 30 is formed on the channel portion of an MOS transistor so as to make the MOS transistor store a &#34;0&#34; bit permanently. Ion-implantation for forming the P-type layer 30 can be carried out much later than the forming of the thick gate oxide films 23 3  in the ROM of FIGS. 5A to 5C. Thus almost all the ROM manufacture processes but ion-implantation may be carried out, and then the ion-implantation is carried out to cause desired MOS transistors to store &#34;0&#34; bits in accordance with the customers&#39; instructions. In this way, the ROMs can be deliverted to the customers in a shorter time after they have given instructions than the ROMs shown in FIGS. 5A to 5C. 
     Having such an IC pattern as shown in FIG. 6A, the ROM of FIGS. 6A to 6D has a higher IC density than the ROM shown in FIGS. 5A to 5C. The surface area S of the substrate 21 indicated by double dot-dash line occupied by each 1-bit memory cell is, for example: 
     
         S=l×m=14 μm×18 μm=252 μm.sup.2. 
    
     This invention is not limited to the above-described embodiments. For example, an N-type semiconductor substrate may be used instead of a P-type semiconductor substrate. Further, the gate lines and the output lines may be made of metals, not polysilicon. If these alternatives are made, the read only memory according to this invention remains advantageous in that the surface area of the substrate is very effectively utilized and that only one impurity diffusion process is required. 
     As described above, according to this invention the surface area of a semiconductor substrate is so effectively utilized as to elevate the IC density of a read only memory, and only one impurity diffusion process suffices to form diffusion layers and gate lines of polysilicon, thereby reducing the cost of the read only memory.