Patent Publication Number: US-2002011624-A1

Title: Nonvolatile semiconductor memory device and fabrication method

Description:
BACKGROUND OF THE INVENTION  
       [0001] 1. Field of the Invention  
       [0002] The present invention relates to a nonvolatile semiconductor memory device and a method of fabricating such a device, and particularly to a nonvolatile semiconductor memory device in which a dummy pattern is formed in a boundary area between a memory cell area and a peripheral circuit area, and a method of fabricating such a device.  
       [0003] 2. Description of the Related Art  
       [0004] A nonvolatile memory having a floating gate comprises a memory cell area, a peripheral circuit area containing transistors, and a boundary area that forms the boundary between the memory cell area and the peripheral circuit area.  
       [0005] In the prior art, such a nonvolatile memory was fabricated by processes such as shown in FIGS.  1 - 8 . The fabrication method of the prior art is disclosed in Japanese Patent Laid-open No. 151580/94.  
       [0006] In this fabrication method, as shown in FIG. 1, element isolating area  14  and gate insulating film  12  are first formed on, for example, p-type silicon substrate  10 . Polysilicon  16  is next formed over the entire surface, and polysilicon  16  of the memory cell area is then patterned using resist pattern  18  to form floating gate electrodes  20 . At this time, the polysilicon of the peripheral circuit area is not removed and remains so as to prevent implantation of ions into the peripheral circuit area when ions for channel stopping are implanted in the next process.  
       [0007] Next, as shown in FIG. 2, ions are implanted for a channel stopper. In this case, silicon substrate  10  is p-type, and boron (B) ions, which can form a p-type area, are therefore implanted. The purpose of this ion implantation is as follows:  
       [0008] The width of element isolating areas  14  that are to be formed between floating gate electrodes shrinks with the miniaturization of memory cells. A decrease in the width of element isolating area  14  means that the element isolating areas are also thinner than in a case in which element isolating areas are sufficiently large, and this leads to a drop in element isolating capability and the occurrence of adverse effects such as the flow of current between adjacent channels. To prevent such adverse effects, a channel stopper, which is a p-type area of higher concentration than silicon substrate  10 , is formed inside and on the lower surface of element isolating areas.  
       [0009] After removing resist pattern  18 , resist pattern  22  is provided on the memory cell area and polysilicon  16  of the peripheral circuit area is removed by dry etching, as shown in FIG. 3. At this time, gate insulating film  12  below polysilicon  16  is also partially removed.  
       [0010] Next, resist pattern  22  is removed, and after removing gate insulating film  12  of the peripheral circuit area by wet etching, an ONO film (a three-layer structure of silicon oxide film-silicon nitride film-silicon oxide film) is formed over the entire surface as shown in FIG. 4. This ONO film  24  is an insulating film for preventing the loss of the charge held by floating gate electrodes  20 . Although this ONO film  24  has ideal functions if formed as a film above floating gate electrodes  20 , it is not appropriate as a gate insulating film of transistors of the peripheral circuit area, and ONO film  24  of the peripheral circuit area is therefore eliminated using resist pattern  25  as shown in FIG. 5. Methods of removing the ONO film include dry etching and wet etching.  
       [0011] Next, as shown in FIG. 6, gate oxidation is carried out and gate insulating film  26  is formed in the peripheral circuit area.  
       [0012] Polysilicon  30  is next formed over the entire surface as shown in FIG. 7, and control gate electrodes  30  are patterned in the memory cell area using resist pattern  28 . The removal of polysilicon  30  by patterning also encroaches into element isolating area  14  below polysilicon  30  in area  38 .  
       [0013] Finally, resist pattern  28  is eliminated, and, as shown in FIG. 8, the polysilicon of the peripheral circuit area is patterned using resist pattern  32  to form gate electrodes  34 . At this time, resist pattern  32  is formed to cover area  38  so that the film thickness of element isolating area  14  below area  38  is not etched and diminished as an element isolation film. As a result, dummy pattern  36  composed of polysilicon is left in the boundary area between the memory cell area and the peripheral circuit area. This dummy pattern  36  surrounds the memory cell area and connects to ground when the memory cell is used.  
       [0014] The dry etching or wet etching that is used when removing the ONO film from the peripheral circuit area in the prior-art fabrication method described hereinabove has the following problems:  
       [0015] When dry etching is used, there is the disadvantage that over-etching may encroach into the silicon substrate in the formation area of the gate insulating film. This problem occurs because the etching rate of nitride film and the etching rate of oxide film in the ONO film are substantially equal, and it is therefore difficult to carry out etching such that only the oxide film of the gate insulating film formation area remains. Etching into the silicon substrate in the formation area of the gate insulating film causes problems such as deterioration of the withstand voltage of the gates, leakage of current at the field end, and a decrease in the ON current of transistors.  
       [0016] In the case of wet etching, the use of nitride film etching liquid can introduce a difference between the etching rate of the nitride film and the etching rate of the oxide film, but an oxide film must be formed as a mask instead of a resist because a resist cannot serve as a mask against the wet etching liquid of the nitride film. When this oxide film for masking is removed, however, there is the problem that the uppermost oxide film layer of the ONO film in the memory cell area is also removed.  
       [0017] Removal of the uppermost oxide layer of the ONO film in the memory cell area causes such problems as variation in the nonvolatile memory operating properties and a reduction in the product yield.  
       SUMMARY OF THE INVENTION  
       [0018] It is an object of the present invention to provide a construction of a nonvolatile semiconductor memory device that does not cause the above-described problems during fabrication and a method of fabricating a nonvolatile semiconductor memory device of this construction.  
       [0019] The nonvolatile semiconductor memory device of this invention is provided with: a memory cell area including nonvolatile memory, a peripheral circuit area including circuits for controlling this nonvolatile memory, and dummy pattern provided in the boundary area between the memory cell area and the peripheral circuit area.  
       [0020] In the first embodiment, this dummy pattern is made up of: a conductive material that constitutes the control gate electrodes of the nonvolatile memory, and residue of an insulative material that constitutes an insulating film between the floating gate electrodes and control gate electrodes that remain without being removed in the fabrication process; this residue being covered by the conductive material.  
       [0021] In the second embodiment, the dummy pattern is made up of: a first conductive material that constitutes the floating gate electrodes of the nonvolatile memory, a second conductive material that constitutes the control gate electrodes of the nonvolatile memory, and an insulative material that makes up the insulating film between the floating gate electrodes and the control gate electrodes.  
       [0022] The method of fabricating the nonvolatile semiconductor memory device of this invention is a method of fabricating a nonvolatile semiconductor memory device provided with: a memory cell area including nonvolatile memory, a peripheral circuit area including circuits for controlling this nonvolatile memory, and a dummy pattern provided in the boundary area between the memory cell area and the peripheral circuit area.  
       [0023] The first embodiment of the fabrication method includes a step of providing, in a boundary area between the memory cell area and peripheral circuit area, a dummy pattern that is made up of: residue that is the insulative material that constitutes the insulating film between the floating gate electrodes and the control gate electrodes of the nonvolatile memory that remains without being eliminated in the fabrication process, and a conductive material that constitutes the floating gate electrodes of the nonvolatile memory that covers this residue.  
       [0024] The second embodiment of the fabrication method includes a step of providing, in a boundary area between the memory cell area and the peripheral circuit area, a dummy pattern that is made up of: a first conductive material that constitutes the floating gate electrodes of the nonvolatile memory, a second conductive material that constitutes the control gate electrodes of the nonvolatile memory, and an insulative material that constitutes the insulating film between the floating gate electrodes and the control gate electrodes. 
     
    
    
     [0025] The above and other objects, features, and advantages of the present invention will become apparent from the following descriptions based on the accompanying drawings which illustrate examples of preferred embodiments of the present invention.  
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0026] FIGS.  1 - 8  are partial sectional views showing each fabrication step of a nonvolatile semiconductor memory device of the prior art;  
     [0027]FIG. 9 is a partial plan view of the nonvolatile semiconductor memory device according to the first embodiment of the present invention;  
     [0028] FIGS.  10 A- 10 D are partial sectional views taken at line  10 - 10  of FIG. 9 of the nonvolatile semiconductor memory device in each fabrication step of the first embodiment;  
     [0029] FIGS.  11 A- 11 D are partial sectional views taken at line  11 - 11  of FIG. 9 of the nonvolatile semiconductor memory device in each fabrication step of the first embodiment;  
     [0030] FIGS.  12 A- 12 D are partial sectional views taken at line  12 - 12  of FIG. 9 of the nonvolatile semiconductor memory device in each fabrication step of the first embodiment;  
     [0031]FIG. 13 is a partial plan view of the nonvolatile semiconductor memory device according to the second embodiment of the present invention;  
     [0032] FIGS.  14 A- 14 D are partial sectional views taken at line  14 - 14  of FIG. 13 of the nonvolatile semiconductor memory device in each fabrication step of the second embodiment;  
     [0033] FIGS.  15 A- 15 D are partial sectional views taken at line  15 - 15  of FIG. 13 of the nonvolatile semiconductor memory device in each fabrication step of the second embodiment; and  
     [0034] FIGS.  16 A- 16 D are partial sectional views taken at line  16 - 16  of FIG. 13 of the nonvolatile semiconductor memory device in each fabrication step of the second embodiment. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
     [0035] The best modes of embodiments of the present invention are next explained based on the first and second embodiments.  
     [0036] Explanation is presented regarding both the fabrication method and structure of the nonvolatile semiconductor memory device of the first embodiment of the present invention, a partial plan view of which is shown in FIG. 9.  
     [0037] As shown in FIG. 10A, FIG. 11A, and FIG. 12A, on a p-type semiconductor substrate such as p-type silicon substrate  10 , an oxide film (not shown in the figure) is formed to a thickness of 30-200 Å and a field nitride film (not shown) is formed to a thickness of 1000-2000 Å, following which portions other than the element area are patterned, whereby element isolating area  14  composed of a SiO 2  film is formed by forming a field insulating film (SiO 2 ). In addition, gate insulating film  12  is formed to a thickness of 50-150 Å by a thermal oxidation method in element areas other than element isolating area  14 .  
     [0038] Next, polysilicon containing impurities such as phosphorus is formed over the entire surface to a thickness of 500-1500 Å, and the memory cell area alone is patterned using resist pattern  18  to form floating gate electrodes  20 .  
     [0039] Width T of floating gate electrode  20  shown in FIG. 10A is, for example, 0.3-0.6 μm, and distance L between electrodes is, for example, 0.15-0.4 μm.  
     [0040] When polysilicon  16  is patterned, polysilicon  16  outside the memory cell area is not removed and is left so as to prevent implantation of ions in these areas when ions for use as a channel stopper are implanted in the next process. The channel stopper ions are then implanted. In this case, silicon substrate  10  is p-type, and boron (B) ions that can form a p-type area therefore are implanted to form p-type areas having a still higher concentration than silicon substrate  10  in the interior and lower portion of element isolating area  14 , these areas serving as channel stoppers.  
     [0041] After resist pattern  18  is removed, insulating film  24  made up of ONO (silicon oxide film-silicon nitride film-silicon oxide film) is formed over the entire surface to a thickness of 100-200 Å by thermal oxidation or chemical vapor deposition as shown in FIG. 10B, FIG. 11B, and FIG. 12B, following which ONO film  24  in areas other than the memory cell area is removed by dry etching using resist pattern  22  formed in the memory cell area.  
     [0042] ONO film  24  is removed with polysilicon  16  underlying the film, thereby avoiding the problem described in the above-described method of the prior art in which the silicon substrate in the gate insulating film formation areas is reduced during ONO film removal. As a result, the ONO film can be removed by dry etching.  
     [0043] When ONO film  24  is removed in this way, ONO film  25  formed in the side walls of polysilicon  16  in the boundary area has a height, and as a result, the ONO film is not entirely removed by dry etching and remains in the side wall portions. Next, polysilicon  16  in areas other than the memory cell area is removed by dry etching. A portion of gate insulating film  12  is also removed at this time, planing down this surface and rendering gate insulating film  12  unusable as a gate insulating film. This gate insulating film  12  is therefore removed by wet etching, and gate insulating film  13  is newly formed to a thickness of 50-200 Å.  
     [0044] Next, after removing resist pattern  22 , control gate electrodes  30  are formed by forming over the entire surface film  27  of a two-layer structure made up of a lower layer of polysilicon containing impurities such as phosphorus and having a thickness of 500-1500 Å and an upper layer of a refractory metal silicide having a thickness of 500-1500 Å, and then patterning only the memory cell area using resist pattern  28 , as shown in FIG. 10C, FIG. 11C, and FIG. 12C. As shown in each figure, ONO film  31  that remains in the side wall portion as a thin wedge shape in cross-section after wet etching is buried in film  27  of two-layer structure composed of a lower layer of polysilicon and an upper layer of a high refractory metal silicide.  
     [0045] Next, after removing resist pattern  28 , resist pattern  32  is formed, and film  27 , which is of a two-layer structure composed of a lower layer of polysilicon and an upper layer of refractory metal silicide, that remains in the peripheral circuit area is patterned to form gate electrodes  34 , and dummy pattern  37  in the boundary area is simultaneously patterned to surround the memory cell area, as shown in FIG. 10D, FIG. 11D, and FIG. 12D. As shown in the figures, remaining ONO film  31  is buried inside dummy pattern  37 , and this dummy pattern  37  is connected to GND when the memory device is used.  
     [0046] In this embodiment, remaining ONO film  31  that is not eliminated by etching is buried inside dummy pattern  37 , but there is a danger of manufacturing defective products if this remaining ONO film  31  should peel off during the wet etching of gate insulating film  12 , remain suspended in the etching solution as residue, and then adhere to an element.  
     [0047] In the next embodiment, a fabrication method is disclosed in which this type of problem does not occur.  
     [0048] Explanation is next presented regarding both the structure and method of fabrication of the nonvolatile semiconductor memory device of the second embodiment of the present invention, a partial plan view of which is shown in FIG. 13.  
     [0049] First, as shown in FIG. 14A, FIG. 15A, and FIG.  16 A, an oxide film (not shown in the figures) is formed on p-type silicon substrate  10  to a thickness of 30-200 Å and a field nitride film (not shown) is formed to a thickness of 1000-2000 Å, following which portions other than the element area are patterned and element isolating area  14  composed of a SiO 2  film is formed by forming a field insulating film (SiO 2  film). Gate insulating film  12  is then formed to a thickness of 50-150 Å by thermal oxidation on element areas other than element isolating area  14 .  
     [0050] Next, floating gate electrodes  20  are formed by forming polysilicon  16  containing phosphorus to a thickness of 500-1500 Å over the entire surface and patterning only the memory cell area of polysilicon  16  using resist pattern  18 . At this time, polysilicon  16  of the peripheral circuit area is not removed and remains. Next, ions are implanted for use as a channel stopper. Since silicon substrate  10  is p-type in this case, boron (B) ions that can form a p-type area are implanted to form a p-type area having a higher concentration than silicon substrate  10  inside and on the bottom of element isolating area  14 , and this area becomes a channel stopper. The above-described processes are the same as in the first embodiment.  
     [0051] Resist pattern  18  is then eliminated, and as shown in FIG. 14B, FIG. 15B, and FIG. 16B, ONO insulating film  24  is formed to a thickness of 100-200 Å by thermal oxidation or chemical vapor deposition. ONO film  24  and polysilicon  16  of a portion of the boundary area and the peripheral circuit area are then removed by dry etching using resist pattern  21  formed on a portion of the boundary and memory cell area, following which gate insulating film  12  is removed by wet etching. Resist pattern  21  is formed so as to cover a portion of polysilicon  17 , which is a portion of polysilicon  16 , ONO film  25  having been formed above and on the side walls of polysilicon  16  as shown in the figures. Accordingly, polysilicon  17  having ONO film  24  above as well as on the side walls remains in the boundary area. The width L 1  of this polysilicon  17  is, for example, 0.3-0.6 μm.  
     [0052] Next, resist pattern  21  is removed, and, as shown in FIG. 14C, FIG. 15C, and FIG. 16C, control gate electrodes  30  are formed by forming over the entire surface film  27 , which has a two-layer structure in which the lower layer is polysilicon containing phosphorus and having a thickness of 500-1500 Å and the upper layer is refractory metal silicide having a thickness of 500-1500 Å, and then patterning only memory cell area using resist pattern  28 .  
     [0053] Next, resist pattern  28  is removed, and as shown in FIG. 14D, FIG. 15D, and FIG. 16D, gate electrodes  34  are formed by forming resist pattern  32  and patterning film  27  that is formed in the peripheral circuit area and that has a two-layer structure in which the lower layer is polysilicon and the upper layer is refractory metal silicide. At the same time, dummy pattern  38  that covers the portion of polysilicon  17  is patterned so as to surround the memory cell area in the boundary area. The widths L 2  and L 3  of the two sides of this dummy pattern  38  that enclose polysilicon  17  are each, for example, 0.3-0.6 μm. This dummy pattern is connected to GND when the memory device is used.  
     [0054] According to this second embodiment, ONO film  25  that is formed on the side walls of polysilicon  17  is buried inside dummy pattern  38  without being subjected to the etching removal process, and the problem of the first embodiment that is caused by residue of ONO film  25  in suspension therefore does not occur.  
     [0055] In addition, the control gate electrodes in the first embodiment and second embodiment may also be formed by polysilicon as in the fabrication method of the prior art.  
     [0056] According to the present invention, dry etching can be used when removing portions outside the memory cell area of the insulating film, which was formed over the entire surface to form the insulating film between the floating gate and control gate, because polysilicon is present at the lower surface of the insulating film. The presence of the polysilicon at the lower surface also eliminates the danger of etching the silicon substrate in the gate oxide film formation area even in the event of over-etching. The present invention therefore enables an improvement in product yield.  
     [0057] Further, the second embodiment can eliminate the occurrence of extraneous material due to residue of the ONO film.  
     [0058] It is to be understood, however, that although the characteristics of the present invention have been set forth in the foregoing description, the disclosure is illustrative only, and changes may be made in the arrangement of the parts within the scope of the appended claims.