Abstract:
Disclosed is a device and method for manufacturing a mask read only memory (ROM) using a spacing film and NAND logic to attain high integration density. Side wall layers are separated between connection regions and reference regions with gate oxide layers formed on a surface of the substrate and gate electrode formed over side walls of the side wall layers placed between each of the gate oxide layers and adjacent memory cells. Channel regions having a first conductivity type of ion are formed under memory cells adjacent to the connection region and oxide layers of selected memory cells. Channel regions exhibiting a second conductivity type of ion are formed under the oxide layers of the non-selected memory cells and the side wall layers.

Description:
This is a continuation of application Ser. No. 07/359,125 filed in the U.S. Patent &amp; Trademark Office on May 31, 1989 now abandoned. Priority is claimed under 35 U.S.C. §119 and section 120 based upon an application filed in the Republic of Korea on May 8, 1989 and assigned Application No. 1989/6137 now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to a mask read only memory (ROM), and more particularly to a structure and method for manufacturing a read only memory with a spacer film. 
     A mask programmable ROM is occasionally called a mask ROM. In data processing systems, a mask ROM is used for memorizing game contents in a game chip, or for storing control logic such as a microprogram. It is desirable that the control logic be integrated with high density and occupy smaller area. There is disclosed a ROM structure having a series-connected NAND logic as a ROM structure used for memorizing relatively by far more information of control logic in a fixed area. The art is disclosed in U.S. Pat. No. 4,142,176. The prior art is a ROM structure in which a plurality of depletion-mode and enhancement-mode transistors are arranged in the form of a series-connected NAND logic matrix. Gates of the transistors on each row share one row line, with sources and drains of adjacent transistors placed in a string of each column are connected in series in an electrical manner on each column. 
     Therefore, in such a ROM structure of NAND logic type, an insulating layer on the sources and drains of adjacent transistors in a string has been used as an insulator for separating row lines therebetween. However, in a ROM which includes a plurality of series-connected transistors arranged in an array of rows and columns, a ROM structure having the sources and drains, and an insulating layer thereon can not provide a higher integrated-density in a fixed area. 
     SUMMARY OF THE INVENTION 
     It is according an object of the present invention to provide a method for manufacturing a read only memory (ROM) having a high-integration density. 
     It is another object of the present invention to provide a method for producing a read only memory (ROM) in a structure of NAND logic having a high-integrated density. 
     To attain the above-mentioned objects, a mask read only memory having a plurality of memory strings, in which a pluality of memory cells are connected between each of the contact regions in a second conductive type on a semiconductor substrate in a first conductive type and a reference region in a second conductive type, includes: side wall layers separated by a given distance between a connection region and reference region in order to isolate the memory cells of each of the memory strings; gate oxide layers formed on the surface of said substrate on which said memory cells are formed; a gate electrode formed over side walls of side wall layers placed between each of said gate oxide layers and adjacent memory cells; channel regions having the first conductivity type of ion under a memory cell adjacent to said connective region and oxide layer or layers of a selected memory cell or cells; and channel regions having the second conductivity type of ion under oxide layer or layers of a non-selected memory cell or cells and said side wall layers. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a better understanding of the invention and to show how the same may be carried into effect, reference will now be made, by way of example, to the accompanying diagrammatic drawings, in which: 
     FIG. 1 is a top view of a portion of a ROM in the structure of NAND logic, according to the present invention; 
     FIG. 2 is a cross-sectional view taken along to the line a--a&#39; in FIG. 1; 
     FIG. 3 is a specific diagram illustrating an equivalent circuit of a memory string connected between a row line and a ground in FIG. 1; and 
     FIG. 4(A) to 4(G) are cross-sectional views showing a manufacturing process taken along line a--a&#39; in FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The above and other objects, effects and features of the present invention will become more apparent from the following description of preferred embodiments thereof taken in conjunction with the accompanying drawings. Same reference numerals are used to designate similar parts throughout the figures. 
     FIG. 1 is a top view of a portion of a ROM in the structure of NAND logic type, according to the invention, showing only two memory strings for the convenience of explanation. It is herein defined that a memory string 14 is a group of series-connected cells 20 to 28 between each column line 10 and a region applying a reference voltage. 
     Referring to FIG. 1 and FIG. 2, the ROM structure with respect to the invention will now be explained. On a p-type substrate surface are formed a N+ connection region 16 connected to a conductive column line 10 through a open hole 12 and a field oxide layer 40 for isolating an N+ reference region 18 connected to a reference voltage identical with the ground voltage from memory strings 14. Also, series-connected cells 20 to 28 are formed on the surface of the substrate 8 under the column line 10 between the N+ connection region 16 and N+ standard region. The cells include a string select cell 20 and eight ROM cells 21 to 28. It is well known to those skilled in this field of art that ROM cells cannot be specified by only eight cells. The individual cells 20 to 28 have a thin gate oxide layer 42 such as SiO 2  formed on the surface of the substrate and a conductive gate electrode 44 on the gate oxide layer 42. The gate electrode 44 may be N+ doped polycrystalline silicon. Further, between the cells, side walls 46 of an insulating material such as SiO 2  are formed to isolate the gate electrodes 44 from each other. And the gate electrodes 44 of adjacent cells are formed on both side walls of the side wall layer 46. Besides, a channel region 48 under the gate electrode 44 of each ROM cell 21 to 28 can be programmed by N or P-type ion implantation. P-type ion implantation is employed on the channel region 48 corresponding to string select cell 20. A cell in which p-type ions are implanted into the channel region 48 of the p-type substrate operates in the same manner as in a conventional enhancement-mode MOSFET transistor, and a cell in which N-type ions are implanted operates in the same manner of a depletion-mode MOSFET transistor. 
     Therefore, in the present invention, a program means that ROM cells are manufactured in enhancement-mode or depletion-mode by ion implantation technique. For example, it is assumed that a ROM cell 22 is programmed with enhancement-mode, a positive voltage to the gate electrode 44 of the ROM cell 22 enables the channel region 48 of the ROM cell 22 to be in a conductive state and a application of a ground potential enables the cell to be in a non-conductive state. On the other hand, when it is assumed that a ROM cell 22 is programmed by depletion-mode, in spite of ground state, the channel region 48 of the ROM cell 22 will be in conductive state. 
     An insulating layer 50 is formed on the gate electrode 44, and the column line 10 is formed on the insulating layer 50 and connected with the connection region 16 through the open hole 12. Meanwhile, the gate electrodes 44 of the cells 20 to 28 were coupled to row lines 30 to 38 which are extended perpendicularly to the column line 10. The material of the column lines 30 to 38 may be a doped-polycrystalline silicon which is the same material as that of the gate electrodes 44. 
     As shown in FIG. 3, it is assumed that ROM cells 21 and 23 are programmed by depletion-mode, and the remaining ROM cells are programmed by enhancement-mode. Then, the readout operation of the ROM cell 23 will be explained in detail. In order to read logic information stored in the ROM cell 23, about 2 V is applied to the selected column line 10 and ground voltage is applied to select the row line 33. At the same time, power supply Vcc (5 V) is applied to a string select row line 30 and non-selected row lines 31, 32 and 34 to 38. If so, because all the cells including the ROM cell 23 are conductive, the selected column line 10 is grounded, and reads are a logic &#34;0&#34;. But, if assumed that the ROM cell 23 is programmed by enhancement-mode, because the ROM cell 23 is nonconductive, the selected column line 10 maintains 2 V, which reads a logic &#34;1&#34;. 
     Referring to FIG. 4(A), the p-type substrate 8 is a wafer oriented to &lt;100&gt; and with resistance of 5 to 50 Ω-cm. The p-type substrate may be of p-type well. On the surface of the substrate 8 is formed the field oxide layer 40 of about 5000 Å and a P+ channel stopper layer 52 under a pad oxide layer 51 of about 380 Å and the field oxide layer 40 by well known LOCOS (Local Oxidation of Silicon) in order to define an active region formed with the memory strings. After this processing, in order to turn the channel regions 48 into depletion, arsenic (As) ions about 100 KeV are implanted with the dose of about 2.0×10 12  ions/cm 2 . 
     As shown in FIG. 4(B), a first silicon nitride layer 54 having a thickness of about 500 Å is formed on the oxide layers 51, 40 and a silicon oxide layer having a thickness of 7000 Å is deposited over the first silicon nitride layer 54 by means of a conventional CVD (Chemical Vapor Deposition) process. A silicon oxide layer 56 is etched by a photolithography method in order to form insulating side wall layers for isolating the cells therebetween. It should be noted that the height of the oxide layer 56 from the substrate surface 8 is higher than that of the field oxide layer 40 from the substrate surface. 
     Thereafter, as shown in FIG. 4(C), after a silicon nitride layer having a thickness of about 1000 Å is deposited on the entire surface of the structure in shown in FIG. 4(B), an side wall layer of a second silicon nitride layer 46 is formed on the side wall of the silicon oxide layer 56 by a etch-back process. After this processing, the silicon oxide layer 56 is removed by a conventional wet-etching process, and an exposed portion of the first silicon nitride layer 54 and the pad oxide layer 51 under the exposed portion are removed for exposing the surface of the substrate 8 by a conventional dry-etching or wet-etching process. Then a gate oxide layer 42 having a thickness of about 250 Å is grown on the exposed substrate surface, and a polycrystalline silicon layer having a thickness of about 1500 Å is deposited on all the surface by a conventional LPCVD (Low Pressure Chemical Vapor Deposition). Thereafter the polycrystalline silicon 58 is converted into a layer of conductivity type by doping N-type impurity such as POC1 3 , but N+ doped polycrystalline silicon 58 can be deposited. 
     Thereafter, as shown in FIG. 4(E), after forming the doped polycrystalline silicon 58 layer, the polycrystalline silicon 58 is covered with a coating of photoresist 60 and removed by an etch-back process until the upper surface of the side wall layer 46 is exposed. Therefore, because of the etch back process, it is to be understood that the height of the isolated layer 46 from the substrate surface 8 should be higher than the sum of a height of the field oxide layer from the substrate surface 8 and a thickness of the polycrystalline silicon layer 58. And the photoresistor 60 is removed. Then, the gate electrodes 44 of the ROM cells 21 to 27 are insulated with the gate electrodes of adjacent cells by the side wall layers 46. 
     As shown in FIG. 4(F), gate electrodes (not illustrated) of the string select cell 20 and ROM cell 28 are specified by depositing a photoresist layer 62 and etching polycrystalline silicon. 
     As shown in FIG. 4(G), a photo resistor 64 is formed in order to expose the string select cell 20 and gate electrodes 44 of the selected ROM cell 22 to be programmed. Then boron ions are implanted with an energy of 75 KeV and a dose of about 3×10 12  ions/cm 2 . By means of ion implantation with boron the string select cell 20 and the ROM cell 22 are programmed into the enhancement mode. 
     After removing the photoresist 64, as shown in FIG. 2, and forming the N+ connection region 16 and N+ reference region 18 by means of an ion implantation process, insulating layer 50 such as SiO2, PSG or BPSG is deposited on the entire surface. The open hole 12 is formed for exposing a portion of the N+ connection region 16, and a column line 10 is formed by coating a metal such as Al and by patterning technology. 
     A mask ROM according to the present invention described above can relatively have the geometric structure of high-density because a width of the side wall layer can be minimized. Furthermore, it has an advantage that channel resistance is reduced by the extension of the channel region due to eliminating sources and drains of the conventional MOSFET transistors. It is another advantage of the invention that can prevent the ion implantation of undesired portions by means of the polycrystalline silicon formed on the side wall of the isolation layer for isolating the gates while executing the photolithography process and ion implantation for programming, after forming the polycrystalline silicon gate of the cell. 
     The foregoing description shows only a preferred embodiment of the present invention. Various modifications are apparent to those skilled in the art without departing from the scope of the present invention which is only limited by the appended claims. For example, a mask read only memory may be constructed according to the foregoing according to the foregoing principles with side wall layers made of silicon nitride (Si 3  N 4 ) layers or a multi-layer structure may be constructed using silicon nitride (Si 3  N 4 ). Therefore, the embodiment shown and described is only illustrative, not restrictive.