Non-volatile semiconductor memory device

Abstract: In a nonvolatile semiconductor memory device having memory cells of a structure in which a control gate is stacked on a floating gate through an insulating film, a phospho-silicate glass film is formed on side walls of the floating gate and the control gate and a silicon nitride film and an arseno-silicate glass film or boro-phospho-silicide glass film are sequentially formed so as to cover the control gate. A method of manufacturing a nonvolatile semiconductor memory device of a structure in which a control gate is stacked on a floating gate through an insulating film, comprises the steps of: forming the floating gate made of a semiconductor layer and the control gate made of a layer containing a refractory metal which is stacked on the floating gate; covering the floating gate and the control gate by a film which prevents a diffusion of metal atoms and transmits oxygen; and forming a semiconductor oxide film on side walls of at least the floating gate by executing a thermal oxidation.

Description: BACKGROUND OF THE INVENTION Field of the Invention The present invention relates to a nonvolatile semiconductor memory device. Description of Related Art Prior art document JP-A-1 119 070 (pub. 11.05.89), & Patent Abstracts of Japan, vol. 13, no. 359 (E-804), August 10, 1990, discloses a nonvolatile semiconductor memory device having memory cells in which a control gate is stacked on a floating gate through an insulating film, comprising a phosphosilicate glass film which is formed on the side walls of said floating gate and said control gate. Further, document JP-A-2-79477, which is not part of the prior art, discloses a nonvolatile semiconductor memory device and a manufacturing method in which a phospho-silicate glass (PSG) film containing phosphorus of 1 to 8 weight %, a silicon nitride (Si 3 N 4 ) film, and an arseno-silicate glass (AsSG) film or a borophospho-silicate glass (BPSG) film are sequentially formed on a floating gate, and by using such a structure, the data retention characteristics of an EPROM (Erasable and Programmable Read Only Memory) can be improved. As mentioned above, by using the three layer structure comprising PSG/Si 3 N 4 /AsSG or PSG/Si 3 N 4 /BPSG as the interlayer insulating film, the data retention characteristics can be improved. However, when the integration density increases and contact holes become small, the following problems result. These problems will be described in detail hereinbelow with reference to FIGS. 1A and 1B. FIGS. 1A and 1B show a manufacturing method of an EPROM. As shown in FIG. 1A, a field insulating film 102, a gate insulating film 103, a floating gate FG", a control gate CG", insulating films 104 and 105, and a source region 106 and a drain region 107 of, for instance, an N + -type are formed on a p-type silicon (Si) substrate 101, for example. Then a PSG film 108, an Si 3 N 4 film 109, and an AsSG film 110 are formed as interlayer insulating films on the whole surface. As already mentioned above, a BPSG film can be also used in place of the AsSG film 110. Then, a resist pattern (not shown) in which portions corresponding to contact holes to be formed is formed on the AsSG film 110. Then the AsSG film 110, Si 3 N 4 film 109, PSG film 108, and gate insulating film 103 are sequentially etched by using the resist pattern as mask. Thus, as shown in FIG. 1B, contact holes C 1 ' and C 2 ' are formed. Then the resist pattern is removed. A reflow of the AsSG film 110 is then executed by performing a thermal treatment. Thus, the shoulder portions of the contact holes C 1 ' and C 2 ' are set into a smooth round shape as shown by alternate long and short dash lines in FIG. 1B. Then, an Al film and An Al-Si alloy film to form wirings are formed by a sputtering method or an evaporation method. Prior to this as a pre-processing, light etching is first executed by using an etchant of the hydrofluoroic acid (HF) system. Then, after the Al film or Al-Si alloy film has been formed, this film is patterned into a predetermined shape by etching, thereby forming wirings (not shown) which respectively come into contact with the source region 106 and drain region 107 through the contact holes C 1 ' and C 2 '. However, during the above described light etching, the PSG film 108 which is exposed on the inside of the contact holes C 1 ' and C 2 ' is also etched into a shape as shown bv broken lines in FIG. 1B. Thus, there is a problem such that the shapes of the contact holes C 1 ' and C 2 ' after completion of the light etching deteriorate and there is the danger that the shape deterioration results in a cause of a defective contact. OBJECTS AND SUMMARY OF THE INVENTION It is an object of the invention to provide a nonvolatile semiconductor memory device which has a good data retention characteristics and which allows high integration density. To solve this object the present invention provides a nonvolatile semiconductor memory device as specified in claim 1. A preferred embodiment of the invention is described in the subclaim. The above, and other, objects, features and advantages of the present invention will become readily apparent from the following detailed description thereof which is to be read in connection with the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS FIGS. 1A and 1B are cross sectional views for explaining a conventional manufacturing method of an EPROM; FIG. 2 is a cross sectional view showing an EPROM according to an embodiment of the invention; FIGS. 3A to 3C are cross sectional views for explaining an example of a method of manufacturing the EPROM shown in FIG. 2. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS An embodiment of the invention will be described hereinbelow with reference to the drawings. FIG. 2 shows an EPROM according to an embodiment of the invention. As shown in FIG. 2, in the EPROM according to the embodiment, a field insulating film 22 such as an SiO 2 film is formed on the surface of a p-type Si substrate 21, for instance, so as to thereby accomplish isolation between devices. A gate insulating film 23 such as an SiO 2 film is formed on the surface of an active region surrounded by the field insulating film 22. A floating gate FG made of an N + -type polycrystalline Si film in which impurities such as P are doped is formed on the gate insulating film 23. A control gate CG is stacked over the floating gate FG through an insulating film 24 such as an SiO 2 film, an ONO film, or an NO film. The control gate CG can be formed by, for instance, an n + -type polycrystalline Si film or a polycide film. Further, an insulating film 25 such as an SiO 2 film is formed on the upper surface of the control gate CG and on the side walls of the floating gate FG and control gate CG. In the embodiment, a PSG film 25 is formed on the side walls of the floating gate FG and control gate CG. Here, a concentration of phosphorus in the PSG film 26 is preferably set to 1 to 8 weight %. On the other hand, the thickness of PSG film 26 is set to, for instance, about 200 nm (2000 Å). A source region 27 and a drain region 28 of, for example, an n + -type are formed in the p-type Si substrate 21 in a self-aligned manner with respect to the floating gate FG and the control gate CG. A memory transistor is formed by the floating gate FG, the control gate CG, the source region 27, and the drain region 28. Reference numeral 29 denotes an Si 3 N 4 film having a thickness of, for instance, about 50 nm (500Å). Reference numeral 30 indicates an AsSG film having a thickness of, e.g., about 600 nm (6000Å). An interlayer insulating film is constructed by the PSG film 26, Si 3 N 4 film 29, and AsSG film 30. C 1 and C 2 represent contact holes. Wirings 31 and 32 made of, for instance, Al or an Al-Si alloy are in contact with the source region 27 and drain region 28 through the contact holes C 1 and C 2 , respectively. An example of a method of manufacturing the EPROM according to the embodiment with the above construction will now be described with reference to FIGS. 3A to 3C. As shown in FIG. 3A, after the isolation between devices is executed by forming the field insulating film 22 by selectively thermally oxidizing the surface of the p-type Si substrate 21, the gate insulating film 23 is formed on the surface of the active region surrounded by the field insulating film 22. Then, after a polycrystalline Si film which forms the floating gate is formed on the whole surface by, for instance, a CVD method, n-type impurities such as P are doped into the polycrystalline Si film at a high concentration, thereby forming an n + -type polycrystalline Si film. Subsequently, the n + -type polycrystalline Si film is patterned by etching so that it has a predetermined width in the direction perpendicular to the cross section shown in FIG. 3A. Then, the insulating film 24 such as an SiO 2 film is formed on the n + -type polycrystalline Si film by e.g., a thermal oxidation method. A polycrystalline Si film to form the control gate is formed on the whole surface by, for instance, the CVD method. Then n-type impurities such as P are doped into the polycrystalline Si film at a high concentration, thereby forming an n + -type polycrystalline Si film. Next, a resist pattern (not shown) having a shape corresponding to the control gate CG to be formed is formed by a lithography onto the n + -type polycrystalline Si film to form the control gate. In accordance with the double self-aligned method, by using the resist pattern as a mask, the n + -type polycrystalline Si film to form the control gate and the n + -type polycrystalline Si film to form the floating gate are sequentially anisotropically etched in the direction perpendicular to the surface of the substrate by, for example, the RIE method, thereby simultaneously forming the floating gate FG and the control gate CG. Due to this, the floating gate FG and the control gate CG are formed in a self-aligned manner. Then, for instance, by using the resist pattern as a mask, n-type impurities such as arsenic As are ion implanted into the p-type Si substrate 21 at a high concentration, thereby forming the source region 27 and drain region 28 of, for example, the n + -type in a self-aligned manner with respect to the floating gate FG and control gate CG. After that, the resist pattern is removed. The source region 27 and drain region 28 can be also formed by executing the ion implantation by using the control gate CG and floating gate FG as masks after the resist pattern is removed. The insulating film 25 is then formed on the upper surface of the control gate CG and on the side walls of the floating gate FG and the control gate CG by, e.g., a thermal oxidation method. After that, the PSG film 26 is formed on the whole surface by, e.g., the CVD method. Then, by anisotropically etching the PSG film 26 in the direction perpendicular to the substrate surface by, e.g., the RIE method, as shown in FIG. 3B, the PSG film 26 is left only on the side walls of the floating gate FG and the control gate CG. Then, the Si 3 N 4 film 29 and the AsSG film 30 are formed on the whole surface by, e.g., the CVD method. Then, a resist pattern (not shown) in which the portions corresponding to the contract holes C 1 and C 2 to be formed are opened is formed on the AsSG film 30. The AsSG film 30, the Si 3 N 4 film 29, and the gate insulating film 23 are sequentially etched by using the resist pattern as a mask. Thus, as shown in FIG. 3C, the contact holes C 1 and C 2 are formed on the source region 27 and drain region 28 at locations away from the PSG film 25, respectively. Then, the reflow of the AsSG film 30 is performed by executing a thermal treatment at a temperature of, e.g., about 850°C. Due to this, the shoulder portions of the contact holes C 1 and C 2 are set into a round smooth shape (refer to FIG. 2). Then, light etching is executed by an etchant of the HF system as a pre-processing of formation of the Al film or Al-Si alloy film to form the wirings. In this case, as will be obvious from FIG. 3C, since the PSG film 26 is never exposed to the inner portions of the contact holes C 1 and C 2 , the PSG film 26 will not be etched by light etching. Subsequently, for instance, after the Al film or Al-Si alloy film is formed on the whole surface by, e.g., a sputtering method or an evaporation method, the film is patterned into a predetermined shape by etching, thereby forming the wirings 31 and 32 as shown in FIG. 2 and completing an objective EPROM. According to the embodiment, since the PSG film 26 is formed only on the side walls of the floating gate FG and the control gate CG and the PSG film 26 is never exposed on the inner portions of the contact holes C 1 and C 2 , a problem is eliminated in that the PSG film 26 is not etched during light etching by the HF system etchant which is executed before the formation of the Al film or the Al-Si alloy film, after the contact holes C 1 and C 2 are formed. Therefore, even if the contact holes C 1 and C 2 become small in association with the realization of a high integration density of the EPROM, there is no danger that the shapes of the contact holes C 1 and C 2 will deteriorate due to light etching. On the other hand, when seen from the side of the floating gate FG, since the interlayer insulating film has a three-layer structure comprising the PSG film 25, the Si 3 N 4 film 29, and the AsSG film 30, good data retention characteristics can be obtained as already mentioned above. Accordingly, it is possible to realize an EPROM which has good data retention characteristics and can also have a high integration density. The EPROM according to the embodiment can be applied to, for example, an OTP (One Time Programmable) one-chip microcomputer or the like. In the embodiment, a BPSG film can be also used in place of, e.g., the AsSG film 30. The memory transistor can be also set to what is called an LDD (Lightly Doped Drain) structure. Further, it is also possible to use a structure in which another film such as an SiO 2 film is interposed between the PSG film 26 and the Si 3 N 4 film 29 and between the Si 3 N 4 film 29 and the AsSG film 30. Having described a specific preferred embodiment of the present invention with reference to the accompanying drawings, it is to be understood that the invention is not limited to this precise embodiment, and that various changes and modifications may be effected therein by one skilled in the art without departing from the scope of the invention as defined in the appended claims. For instance, although the above embodiments have been described with respect to the case where the invention was applied to an EPROM, the invention can be applied to an EEPROM and other various kinds of floating gate memory devices. According to the invention, the phospho-silicate glass film is formed on an insulating layer formed on the side walls of the floating gate and control gate and the silicon nitride film and the arseno-silicate glass film or boro-phospho-silicate glass film are sequentially formed so as to cover the control gate and said phospho-silicate glass film. Therefore, the shapes of the contact holes are not deteriorated by the light etching which is executed as a preprocessing of the formation of the film to form the wirings. Thus, it is possible to realize a nonvolatile semiconductor memory device which has good data retention characteristics and can also cope with the realization of a high integration density.

Claims: A nonvolatile semiconductor memory device having memory cells wherein each memory cell comprises a field-effect transistor in which a control gate is stacked on a floating gate through an insulating film (24) and a phospho-silicate glass film (26) is formed on an insulating layer (25) formed on the side walls of said floating gate (FG) and said control gate (CG), characterised in that a silicon nitride film (29) and a film (30) of arseno-silicate glass or boro-phospho-silicate glass are sequentially formed so as to cover the control gates (CG) of said field-effect transistors, and said phospho-silicate glass film (26). A nonvolatile semiconductor memory device according to claim 1, wherein each of said memory cell field-effect transistors has contact holes (C 1 , C 2 ) for a source region (27) and a drain region (28) formed distant from said phospho-silicate glass film (26).

Inventors: NAKAMURA AKIHIRO, NODA MASANORI, NAKAMURA, AKIHIRO, NODA, MASANORI, NAKAMURA, AKIHIRO, C/O SONY CORPORATION, NODA, MASANORI, C/O SONY CORPORATION
Applicants: SONY CORP, SONY CORPORATION
Application year: 1990
Main classification: H01L27
IPC codes: H01L27, H01L21