Semiconductor memory device and a method of manufacturing the same
This is a Division of application Ser. No. 241,539, filed March 9, 1981, which is a continuation of application Ser. No. 891,030, filed March 28, 1978, now abandoned. 
    
    
     LIST OF PRIOR ART (37CFR 1.56(a)) 
     The following references are cited to show the state of the prior art: 
     (1) J. Barnes et al., &#34;Operation and Characterization of N-channel EPROM cells&#34;, Technical Digest 1976 International Electron Device Meeting, December 6, 7 and 8, 1976, pp. 173 to 176. 
     (2) V. L. Rideout et al. &#34;MOSFETs with Polysilicon Gates Self-aligned to the Field Isolation and to the Source/Drain Regions&#34;, Technical Digest 1976 International Electron Devices Meeting, December 6, 7, and 8, 1976, pp. 593 to 596. 
     (3) Frohman-Bentchkowsky, U.S. Pat. No. 3,660,819, May 1972. 
     (4) Dawson Kahng, U.S. Pat. No. 3,500,142, Mar. 1970. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to an insulated gate type field effect transistor and, more particularly, to a non-volatile semiconductive memory device having a floating polysilicon layer, as well as to a method of manufacturing the device. 
     In the past, various types of electrically programable read only memory device making use of MOS (metal oxide semiconductor) elements have been proposed. 
     Among these proposed read only memory devices, a device called EPROM (Electrically Programmable Read Only Memory) of nMOS type, manufactured by a double polysilicon layer technique and having a floating type first (underlying) polysilicon layer acting as an electric-charge accumulation layer, is most popular. 
     As a result of current demand for increased numbers of circuits elements integrated on a single semiconductive substrate, it is becoming a matter of significance to make each memory cell, which constitutes the unit of memory function in semiconductive chips having the ROM of the kind described, as small as possible. 
     At the same time, it is to be pointed out that the process for manufacturing these semiconductive devices is inevitably rendered complicated, as the density of the integration is increased. 
     BRIEF SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide an improved insulated gate type field effect transistor. 
     It is another object of the invention to provide an electrically programmable read only memory of reduced size of cells. 
     It is still another object of the invention to provide a simple construction of double polysilicon layer type electrically programmable read only memory having a good reproducibility. 
     It is a further object of the invention to provide an improved technique suitable for manufacturing electrically programmable read only memories of increased density of integration. 
     It is still another object of the invention to provide a method of manufacturing memory cells of electrically programmable read only memory, consisting of a simplified manufacturing process. 
     To these ends, according to the invention, there is provided a semiconductor device having a substrate including a source and a drain region, a first floating gate formed on said substrate through a first insulating film, a second control gate formed on the first floating gate through a second insulating film, and a field insulator formed on the field region of the substrate, wherein the floating gate and the control gate do not substantially overlap the source and drain regions, and the floating gate does not overlap the field insulator formed on the field region. 
     These and other objects, as well as advantageous features of the invention will become clear from the following description of the preferred embodiments taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a floating type memory cell, drafted for an easier understanding of the principle and basic construction of the same, 
     FIG. 2 is a sectional view taken along the line A--A of FIG. 1, 
     FIG. 3 is a sectional view taken along the line B--B of FIG. 1, 
     FIG. 4 is a plan view showing the basic construction of a floating memory section of a memory device in accordance with the invention, 
     FIG. 5 is a sectional view taken along the line A--A of FIG. 4, 
     FIG. 6 is a sectional view taken along the line B--B of FIG. 4, 
     FIGS. 7 to 10 are sectional views corresponding to that taken along the line A--A of FIG. 4, in respective steps of process in accordance with the invention for manufacturing the floating type memory cell, 
     FIGS. 11 to 16 are sectional views corresponding to that taken along the line B--B of FIG. 4, in respective steps of process in accordance with the invention for manufacturing the floating type memory cell, 
     FIGS. 17 and 18 are sectional views of essential parts of different embodiments in accordance with the invention, 
     FIG. 19 is a schematic illustration of planar pattern of a semiconductor chip for an electrically programmable read only memory device in accordance with the invention, 
     FIG. 20 is a plan view of a metal-oxide semiconductor field effect transistor which constitutes the peripheral circuit portion other than the memory cell mats, formed in the chip as shown in FIG. 19, 
     FIG. 21 is a sectional view taken along the line A--A of FIG. 20, 
     FIG. 22 is a sectional view taken along the line B--B of FIG. 20, and 
     FIGS. 23 to 28 and FIGS. 29 to 34 are sectional views of elements corresponding to those of FIG. 21 and FIG. 22, in respective steps of process for manufacturing the element. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before turning to the description of the preferred embodiments, an explanation will be made as to the theoretical construction of floating type memory cell, with specific reference to FIGS. 1 to 3. 
     Referring first to FIG. 1 showing the plan of the memory cell and to FIG. 2 showing the section taken along the line A--A of FIG. 1, reference numeral 1 denotes a P type silicon substrate, while reference numeral 2 designates a field oxidized film obtained by a selective oxidation. Reference numerals 3, 4 and 5 denote, respectively, a first insulated gate film (SiO 2 ), first floating polysilicon film and a second insulated gate film. A second polysilicon film which constitutes a control gate, and an intermediate insulating layer such as PSG are designated at numerals 6 and 7, respectively. Reference numeral 8 denotes an aluminum wiring layer. 
     Referring now to FIG. 3 showing the section of the cell taken along the line B--B of FIG. 1, reference numerals 9 and 10 denotes n +  type diffusion layers constituting a drain and a source region, respectively, while a contact area of the wiring layer is designated at numeral 11. In the above stated memory cell, the field oxidized film 2 is formed by a selective oxidation employing a mask of Si 3  N 4  selectively covering the active region. 
     As will be seen from FIGS. 1 through 3, the first polysilicon layer 4, which is a floating gate, overlaps the field oxidized film 2, while the second polysilicon layer 6 overlaps the first polysilicon layer 4. These overlaps are formed by means of a photoresist mask which, for effecting these overlaps, has to have dimensional margin of 2 to 5 microns, inconveniently resulting in an enlarged size of the cell. 
     It is also to be pointed out that the FAMOS (Floating Avalanche MOS) memory cell of the kind described can be manufactured only through an extremely complicated manufacturing process. 
     More specifically, according to the most popular way of manufacturing, the first gate oxidized film 3 is formed on the active region of the Si substrate, after forming the field oxidized film 2 on a part of the surface of the substrate 1. Then, the first polysilicon film 4, which constitutes the floating gate is formed on the first gate oxidized film 3. Subsequently, the floating gate pattern is formed by means of a photoetching. Then, the Si substrate and the polysilicon layer are doped with impurities, by means of an ion driving or heat diffusion technique making use of a mask of polysilicon, so as to form the second gate oxidized film 5. Then, the second polysilicon layer is formed by CVD (Chemical Vapor Deposition) method. Subsequently, a photoetching is carried out with such a pattern that the first polysilicon layer is completely covered with the second polysilicon layer, and the doping with the impurities is carried out again. 
     As a result of above stated complicated process having a number of steps, the cost of manufacture of the memory cell is rendered considerably high. 
     Under these circumstances, the present inventors have succeeded in obtaining an improved construction of memory cell as shown in FIGS. 4 thru 6, making a positive use of self-aligning formation of the first polysilicon layer which constitutes the floating gate with the second polysilicon layer which constitutes the control gate and the wiring layer, as well as a self-aligning formation of the first polysilicon layer which constitutes the floating gate with the field SiO 2  film. 
     Hereinafter, a preferred embodiment of the invention will be described with specific reference to FIGS. 4 thru 6. As will be seen from these Figures, in the memory cell and the electrically programmable read only memory device of this embodiment, a plurality of insulated gate type field effect transistors, each having an N type source region 30 and a drain region 29, are formed on a P type semiconductor substrate 21, so as to form respective memory cells. The opposite side surfaces of a channel region between the source and the drain regions 30, 29 are terminated by a thick isolation 22 such as, for example, a field oxide film made of silicon oxide. At the same time, a metallic layer which constitutes the first floating gate electrode, such as a polysilicon layer 24 is formed only just above the channel area defined as above, with a thin gate insulating film 23 interposed therebetween, so as to have an area substantially equal to that of the channel region. Thus, the layer 24 is not extended to lie over the thick field oxide film 22. 
     A second metallic layer such as a polysilicon layer 26 is formed just above the first polysilicon layer 24 constituting the aforementioned floating gate, with an interlayer insulating film 25 such as a silicon nitride film interposed therebetween. The second polysilicon layer 26 is extended to lie partially over the field oxide film 22 located at a side of the channel area. This second polysilicon layer 26 also self-aligns with the channel area between the source and the drain regions 30,29, so as to extend just above the channel area transversely of the same. 
     A wiring layer 28 made of a metallic material such as aluminum is formed across the second polysilicon layer 26, with a second insulating film 27 such as of phosphosilicate-glass interposed therebetween, and is connected to the drain region 30 through a window 31 formed in the insulating film 27. 
     Thus, in the electrically programmable read only memory device of the invention, the first floating polysilicon gate 24 is disposed on the channel region sandwiched between the source and drain regions and having side surfaces terminated by relatively thick field oxide film, so as to have substantially the same dimension as the channel area. At the same time, a second polysilicon layer 26 is disposed to have a width substantially equal to the length of the channel area and to extend at a right angle to the channel just above and across the same. In other words, in the memory cell of the embodiment under description, the boundaries of the first floating gate electrode 24, the first inter-layer insulating film 25, the second gate electrode 26 and the source and drain regions defining the channel area are self-aligned to one another, while the field oxide film 22 is self-aligned to the first floating gate electrode 24 and to the source and drain regions. 
     In FIGS. 4 thru 6 inclusive, reference numeral 33 denotes a semiconductive region of the same conductivity type as the semiconductive substrate 21 and formed on the latter to lie under the thick field oxide film 22. This semiconductive region 33 is provided to prevent a generation of a parasitic channel on the surface of the substrate attributable to an application of voltage to the second polysilicon layer (control gate line) 26. At the same time, as will be seen also from these Figures, this semiconductive layer 33 is also formed just beneath the field oxide film 22 in self-alignment with the latter. 
     A so-called EPROM (electrically programmable read only memory) is constituted by a plurality of memory cells in accordance with the invention regularly arrayed in two orthogonal directions as a matrix. FIG. 4 shows a part of the electrically programmable read only memory in accordance with the invention. Only four memory cells are shown for clarifying the drawings. 
     As will be understood from the following detailed description of process for manufacturing the memory cell and the electrically programable read only memory in accordance with the invention, as well as from FIGS. 4 to 6 inclusive, the first polysilicon layer (floating gate) 24 does not overlap the thick field oxide film 22, because they are formed in self-alignment. At the same time, as will be most clearly seen from FIG. 6, the edges of the first polysilicon layer 24 and the second polysilicon layer 26 are formed through a photoetching by means of the same photomask, so that no margin of mask alignment is required. Consequently, the size of cells can conveniently be reduced by 40%, as compared with that as shown in FIGS. 1 to 3, provided that the working precisions are equal. 
     FIGS. 7 to 16 show major steps of a process for manufacturing a FAMOS memory in accordance with the invention. Although the following description is made exemplarily as to the manufacture of n-channel type FAMOS, it is to be noted that the invention is never restricted to the n-channel type memory. 
     More specifically, FIGS. 7 to 10 show the steps for obtaining the construction as shown in FIG. 5, in section taken along the line A--A, while FIGS. 11 to 16 show the steps for obtaining the construction as shown in FIG. 6, in section taken along the line B--B of FIG. 4. 
     (1) At first, an Si monocrystal wafer (substrate) 21 of a specific resistance of 5 to 8 Ωcm and having a (100) crystal surface of P conductive type is prepared. After suitably cleaning the wafer 21, the first gate oxide film (SiO 2 ) is formed to have a thickness of, for example, 1000 Å, as shown in FIGS. 7 and 11. This can be performed by, for example, heat-treating the wafer 21 at a temperature of 1000° C. for 165 minutes, in the atmosphere of dry O 2 . Immediately after the formation of this film 23, the first polysilicon film 24, which constitutes the floating gate, is formed on the oxide film 23. 
     In conventional Si gate process, the polysilicon layer functions not only as the gate electrode but as the wiring layer as well. Therefore, it has been necessary to make the polysilicon layer have a relatively large thickness of 0.3 to 0.6 microns, and to effect the doping with impurities at a high concentration, so as to reduce the surface specific resistance as much as possible. 
     However, in the FAMOS memory in accordance with the invention, the floating gate is intended for spreading and holding the injected charges, so that the lowered specific surface resistance is not always essential. Thus, the thickness of the first polysilicon layer 24 may be as small as 0.1 to 0.4 micron, preferably 0.1 to 0.2 micron. At the same time, the doping with the impurities may not be performed at a high concentration, and such a low surface specific resistance as 100 Ω to 100 KΩ/cm 2  as could be obtained by a simple doped poly Si method is sufficient. Alternatively, the doping in this step can completely be omitted from this step of process, because a sufficient amount of impurities are doped to ensure the required memory characteristic of FAMOS, during the later-performed diffusion step for forming the source/drain and second gate, due to a side diffusion. 
     After the formation of the first polysilicon layer 24, an Si 3  N 4  film 25, which functions as the mask for the selective oxidation and as the insulating inter-layer between the first and the second polysilicon layers, is formed to have a thickness of 0.07 to 2.20 micron, preferably 0.10 to 0.15 micron. 
     (2) Subsequently, the active region consisting of the channel, source and drain regions is covered with a photoresist film 34, as shown in FIGS. 8 and 12, and the portions of the Si 3  N 4  film and of the first polysilicon layer 24 are removed through a plasma etching making use of, for example, CF 4  +O 2 . Then, for the purpose of enhancing the threshold of the field SiO 2  section, Boron ions are driven through the oxide film 23 exposed in the surface of the substrate, at a rate of, for example, 5×10 13  cm -2  at 75 KeV. The photoresist film 34 is suitably removed after the ion driving. 
     (3) Then, as shown in FIG. 9 and FIG. 13, a field SiO 2  film 22 of a thickness of 1.0 to 1.5 micron is formed on the portion of the substrate surface where the Si 3  N 4  film 25 and the first polysilicon layer 24 have been removed, by a heat oxidation method making use of the Si 3  N 4  film as a mask. This can be performed by heat treating the substrate in the atmosphere of wet O 2  or steam for 6 to 18 hours. During this heat treating, the Si 3  N 4  film is oxidized to the depth of 200 to 500 Å. However, this depth is negligibly small as compared with the whole thickness of the field SiO 2  film 22 and, therefore, has been neglected from the drawings, for the clarification of the latter. As a result of the above stated heat treatment, the boron ions driven into the region under the SiO 2  film 22 are diffused through the Si substrate to form a P +   diffusion layer 33. 
     (4) As a subsequent step, as shown in FIGS. 10 and 14, the second polysilicon layer 26 is deposited over the entire surface, without removing the Si 3  N 4  film 25 on the first polysilicon layer 24 and the SiO 2  film (not shown) on the Si 3  N 4  film 25. 
     (5) Then, the control gate and the polysilicon wiring layer are formed by a photoetching. As will be seen from FIG. 15, the etching is effected on the second polysilicon layer 26, insulating interlayer 25, first polysilicon layer 24 and the first insulating layer 23, by means of the same mask. Since three or four layers are photoetched by the same mask, the control gate 26 and the floating gate 24 are formed in self-alignment, as shown in FIG. 15. 
     (6) After the completion of the photoetching, the exposed substrate surface and the polysilicon layer 26 are doped with impurities of the opposite conductive type to the substrate, e.g. phosphorus (P) and arsenic (As), by means of, for example, a heat diffusion method. As a result of this doping, N-type source region 30 and drain region 29 are formed as shown in FIG. 16. 
     (7) Subsequent steps are same as those of the conventional process for manufacturing the Si gate MOSIC. Namely, a PSG (phosphorus silicate glass) film 27, which constitutes an insulation layer between the Al wiring 28 and the polysilicon layer 26, is formed by CVD (chemical vapor deposition) method, as shown in FIGS. 5 and 6. Then, a photoetching is effected to form the contact window 31. Finally, the Al wiring 28 is formed by means of Al evaporation film. 
     The aforementioned objects of the invention can be achieved by the above described embodiment, as will be understood from the following description. 
     (1) The first polysilicon layer 24 which constitutes the floating gate and the field SiO 2  film 22 are formed in self-alignment, because the field SiO 2  film 22 is formed making the Si 3  N 4  film on the first polysilicon layer 24 as the mask. 
     (2) The first and the second polysilicon layers 24,26 are formed in complete self-alignment, because the first layer 24, the insulating-interlayer 25 and the second layer 26 of the memory cell section are formed by a simultaneous etching. 
     (3) The constituents of the memory section are completely self-aligned by a relatively simple process, so as to provide a construction of minimized size. 
     (4) A relatively thick field oxide film is formed on the surface of the semiconductive substrate surface, making use of the silicon nitride film as a mask. Consequently, the multi-layered wiring in the memory cell section is rather flattened. 
     In the foregoing embodiment, the insulating interlayer 25 between the first polysilicon layer 24 and the second polysilicon layer 26 is constituted by the silicon nitride film. However, in order to minimize the strain caused in the first polysilicon layer by the difference of coefficients of thermal expansion, the first step (1) of the foregoing embodiment is preferably modified such that the Si 3  N 4  film 25 is formed on an extremely thin heat-oxidized film (SiO 2 ) film of a thickness of 0.01 to 0.1 micron, after forming the extremely thin film on the surface of the first polysilicon layer 24. 
     As stated before in relation with the step (3) of the process of the embodiment, a thin silicon oxide film of thickness of 200 to 500 Å is formed on the surface of the Si 3  N 4  film, during the selective oxidation of the substrate employing the mask of Si 3  N 4 . This silicon oxide film may be positively utilized to form a memory cell construction as shown in FIGS. 17 and 18. FIGS. 17 and 18 correspond, respectively, to the sectional views as shown in FIGS. 5 and 6. In these Figures, reference numerals 35 and 37 denote, respectively, the silicon oxide film of 100 to 1000 Å and the silicon oxide film of 200 to 500 Å. The silicon nitride film sandwiched by these silicon oxide films is denoted by reference numeral 36. By adopting this construction to avoid the direct contact of the silicon nitride film with the gate electrode, i.e. when the silicon nitride film is formed through a medium of the silicon oxide film, it is conveniently avoided that the carrier charged to the gate electrode in accordance with the wiring operation is trapped by the silicon nitride film, so that the lowering of the potential of the gate electrode can be diminished. Namely, the above mentioned two silicon oxide films function as stoppers against the trapping of the carrier, so as to provide a desirable electric characteristics in memory devices such as the non-volatile semiconductive memory in accordance with the invention. 
     At the same time, instead of forming the insulating interlayer 25 of silicon nitride film, the step (4) of the process of the foregoing embodiment may be modified such that the Si 3  N 4  film and the SiO 2  film on the first polysilicon film 24 are removed and a heat oxidized (SiO 2 ) film of 1000 to 2000 Å is formed instead. The second polysilicon layer 26 is formed on this newly formed SiO 2  film. However, in this case, the thickness of the SiO 2  film must be as small as possible, because the electrostatic capacitance between two gate electrodes is made small due to the dielectric constant of SiO 2  smaller than that of Si 3  N 4 . 
     The so-called memory cell matrix or mat as shown in FIG. 4 having a number of memory cells arrayed in columns and rows is produced by the above described process. 
     In practically mounting an electrically programmable read only memory on a single silicon semiconductor chip, as shown, for example, in FIG. 19 schematically illustrating the pattern on the chip, the aforementioned memory cell mats 41, 42, 43, 44 are mounted on the semiconductor chip 40. At the same time, peripheral circuits electrically connected to these mats, e.g. sense-amp. circuit sections 45, 46; X decoder circuit sections 47, 48; Y decoder circuit section 49; input buffer circuit sections 50, 51; input/output circuit sections 52, 53 and so forth are arranged around these memory cell mats. 
     These peripheral circuits are constituted by insulating gate type semiconductor elements, similarly to the formation of the memory mats. Thus, according to the invention, these peripheral circuits can conveniently be mounted on the same semiconductor substrate as the memory mats, simultaneously with the formation of the latter, by making use of the above described process without substantially changing the same. 
     By way of example, a reference is made here to FIG. 20 which shows a plan view of a typical insulated gate type field effect transistor constituting the above stated peripheral circuits, as well as to FIGS. 21 and 22 showing sections taken along the line A--A and line B--B of FIG. 20, respectively. In these Figures, the members and parts similar to those of the foregoing embodiment are denoted by the same reference numerals for an easier understanding. 
     In this regard, each of 31&#39;, 31&#34; and 31&#39;&#34; are contact windows, in PSG film 27, for connections to drain and source regions 29&#39; and 30&#39;, and to second polysilicon layer 26&#39;, respectively; 28&#39;&#34; is a wiring layer for connecting through window 31&#39;&#34; to second polysilicon layer 26&#39;. 
     As will be seen from these Figures, as well as from the later-mentioned detailed illustration of process, the first gate electrode of a MOS transistor constituting a part of the peripheral circuits other than the memory mats is inconveniently formed in a floating state, if the MOS transistor is formed in the same process as the memory cells. Therefore, in order to ensure the direct contact of the second polysilicon layer 26 with the first polysilicon layer 24, the desired portion of the insulating interlayer 25 has been selectively removed immediately before the formation of the second polysilicon layer 26. The polysilicon layer 26&#34; found on the thick field oxide layer 22 in FIGS. 20 and 21 is formed during the formation of the second polysilicon layer 26, and can be made use of as a wiring layer, constituting a so-called cross wiring which is separated from other Al wiring layers 28&#34;. 
     Sections taken along the lines A--A and B--B of FIG. 20 are shown by FIGS. 23 to 28 and FIGS. 29 to 34, respectively, in each step of the manufacturing process. These steps are not detailed here, because they are obviously understood from the description of the foregoing embodiment taken in relation with FIGS. 4 thru 16, except for the following characteristic features. 
     Namely, as will be seen from FIGS. 25, 26 and FIGS. 31, 32, the thick field oxide layer 22 is formed by oxidizing the substrate surface making use of the insulating layer 25 of silicon nitride film as the mask, as in the foregoing embodiment. For removing the parts of the insulating layer 25 corresponding to the peripheral circuits an etching is performed making use of a mask covering the area occupied by the memory cell mats 41, 42, 43, 44, whose pattern is shown in FIG. 19 in plan, so that the first polysilicon layer 24 may be exposed at its portions corresponding to the area on which the peripheral circuits are to be formed. Then, after removing the mask, the second polysilicon layer 26 is deposited over the entire area including the insulating layer 25 left on the memory cell mats, exposed portion 24&#39; of the first polysilicon layer 24 and the field oxide layer 22. 
     It will be seen that the first polysilicon layer of the MOS transistors in the peripheral circuits can be connected to the second polysilicon layer, by simply adding a step of removing the other portion of the insulating layer 25 than that covering the memory cell mats. 
     In addition, as will be seen from FIG. 27, the drain and the source regions 29&#39; and 30&#39; of the transistors of the peripheral circuits are formed, as is in the foregoing embodiment, in self-alignment with the second and the first polysilicon layers 26&#39;, 24&#39;. Therefore, it is possible to form the transistors of the peripheral circuits simultaneously with the transistors of the memory cells. 
     It will be understood from the foregoing description that not only the area occupied by the memory cell mat, but the area occupied by the peripheral circuits as well can be remarkably diminished, according to the construction of transistor and the method of producing the same in accordance with the present invention. 
     Having described the invention through preferred embodiments, it is to be noted here that these embodiments are not exclusive, and various changes and modifications may be imparted thereto without departing from the scope of the invention.