Abstract:
A semiconductor memory device capable of electrically writing and erasing data has a plurality of cell transistors for storing data, each of the cell transistors having a floating gate electrode and a control gate electrode, and a plurality of select transistors for controlling and selecting the cell transistors. Before the control gate electrodes of the cell transistors are formed, the surface of a substrate directly above channel regions of the select transistors fabricated in the same process as the cell transistors is exposed, and gate insulating films of the select transistors are formed on the exposed surface of the substrate. The control gate electrodes of the cell transistors are formed, and gate electrodes of the select transistors are formed on the gate insulating films.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a method of manufacturing a semiconductor memory device, and more particularly to a method of manufacturing a semiconductor memory device such as a flash EEPROM (Electrical erasable programable read only memory) capable of electrically writing and erasing data, and such a semiconductor memory device. 
     2. Description of the Related Art 
     Flash EEPROMs, or nonvolatile semiconductor memory devices capable of electrically writing and erasing data, have a plurality of cell transistors disposed in a memory cell assembly for storing data and having floating gate electrodes and control gate electrodes, and a plurality of select transistors disposed in the memory cell assembly for selecting the cell transistors. In more practical flash EEPROMs, the transistors of a peripheral circuit such as a logic operation circuit are disposed on the same substrate as the cell and select transistors. 
     One known flash EEPROM comprises parallel arrays of cell transistors connected to respective data lines for writing and reading data, and a plurality of select transistors connected in series with the respective arrays of cell transistors, as shown in FIG. 1 of the accompanying drawings. The flash EEPROM shown in FIG. 1 has a small device area and can be manufactured in a relatively small number of fabrication steps though the speed at which memory cells are accessed is not significantly high. Therefore, the flash EEPROM shown in FIG. 1 is used as memories for IC cards, for example, which need to be highly integrated and low in cost. 
     The flash EEPROM shown in FIG. 1 as an example of a conventional semiconductor memory device and a method of manufacturing same will be described below. 
     The flash EEPROM shown in FIG. 1 has a memory cell assembly comprising a plurality of cell transistors M 101 -M 164 , M 201 -M 264 , M 301 -M 364 , M 401 -M 464  arranged in a grid pattern. The memory cell assembly is divided into a plurality of blocks each comprising a predetermined number of ( 64  in FIG. 1) cell transistors, parallel to data lines D 1 -D 4 . In each of the blocks, the cell transistors have respective sources connected in common and respective drains connected in common. The cell transistors are also arranged in transverse arrays across the data lines, and the cell transistors in those transverse arrays have respective control gate electrodes connected in common to word lines W 1 -W 64  for selecting positions to storing data. 
     Each of the blocks has two select transistors for selecting a predetermined number of parallel-connected cell transistors. First select transistors Q 11 , Q 21 , Q 31 , Q 41  shown in FIG. 1 are inserted between the drains of the cell transistors and respective contacts J 1 -J 4  on the data lines, and second select transistors Q 12 , Q 22 , Q 32 , Q 42  shown in FIG. 1 are inserted between the sources of the cell transistors and common source CS kept at ground potential. The first select transistors in the respective blocks have gate electrodes connected in common to first selection gate line SG 1 , and the second select transistors in the respective blocks have gate electrodes connected in common to second selection gate line SG 2 . 
     As shown in FIG. 2 of the accompanying drawings, the blocks of the memory cell assembly are separated by a field oxide film. The cell transistors, represented by M 101 -M 164 , M 201 -M 264  in FIG. 2, have source regions S and drain regions D formed in common in the respective blocks. Those common source regions S and drain regions D are also used as interconnections connecting the cell transistors parallel to each other in each of the blocks. 
     Floating gate electrodes (not shown) and control gate electrodes are disposed at spaced intervals on and across source regions S and drain regions D. The control gate electrodes are connected in common in respective transverse arrays in FIG. 2, and also used as the word lines. Regions directly below the control gate electrodes sandwiched between the source regions and the drain regions serve as channel regions where currents flow. 
     The drain regions of the cell transistors have ends connected to source regions S of the first select transistors, which are represented by Q 11 , Q 21  in FIG.  2 . The first select transistors have respective drain regions D that are positioned across first selection gate line SG 1  from source regions S of the first select transistors. The contacts, which are represented by J 1 , J 2  in FIG. 2, for connection to the data lines are disposed on drain regions D of the first select transistors. Though the second select transistors are omitted from the illustration in FIG. 2, the second select transistors are arranged in the same manner as the first select transistors and connected to ends of the source regions of the cell transistors. 
     For writing or erasing data, a cell transistor is selected at the point of intersection of a data line and a word line to which certain voltages are applied. The data is stored in or erased from the selected cell transistor by a charge introduced into or removed from the floating gate electrode of the selected cell transistor. The data stored in the cell transistor is read by detecting a change in a threshold voltage that is caused by introducing a charge into the floating gate electrode of the selected cell transistor. 
     A conventional method of manufacturing the flash EEPROM shown in FIGS. 1 and 2 will be described below with reference to FIGS. 3 through 10 of the accompanying drawings. The structure of a select transistor shown in FIGS. 3 through 10 is taken along line A-A′ of FIG. 2, and the structure of a cell transistor shown in FIGS. 3 through 10 is taken along line B-B′ of FIG.  2 . 
     First, a thin SiO 2  film and a silicon nitride (Si 3 N 4 ) film is formed on substrate  101  of p-type semiconductor and patterned to a predetermined shape, and its openings are selectively oxidized to form field oxide film  106  as an inactive region for separating components. Then, gate insulating film  102   a  of the select transistor and tunneling oxide film  102   b  of the cell transistor are grown on the surface of substrate  101  by thermal oxidization. At this time, since the select transistor requires a high withstand voltage, the following multi-oxidizing process is performed: First, the surface of substrate  101  is thermally oxidized in order to form an oxide film thinner than a desired film thickness. The thickness of the oxide film is smaller than the desired film thickness by a thickness which will be added when tunneling oxide film  102   b  is subsequently to be formed. 
     Then, a photoresist is formed in the select transistor area, and the oxide film in the cell transistor area is etched away. Thereafter, the photoresist is removed, and the assembly is thermally oxidized until the oxide film in the cell transistor area gains a desired film thickness, growing the gate insulating film  102   a  of the select transistor and tunneling oxide film  102   b  of the cell transistor to respective desired film thicknesses (see FIG.  3 ). 
     Then, first N-type polysilicon film  103 , which serves as the floating gate electrode of the cell transistor, is grown on the surface formed so far. Pad oxide film  104  is grown on first N-type polysilicon film  103  by CVD (Chemical Vapor Deposition), and second N-type polysilicon film  105  is grown on pad oxide film  104 . Second N-type polysilicon film  105  will be used only as a mask in a subsequent ion implantation step. Therefore, second N-type polysilicon film  105  may be replaced with an amorphous silicon film or a silicon nitride film. 
     Then, first N-type polysilicon film  103 , pad oxide film  104 , and second N-type polysilicon film  105  are patterned to respective shapes. The width of first N-type polysilicon film  103  which is formed at this time determines the channel widths of the cell transistor and the select transistor (see FIG.  4 ). 
     Using second N-type polysilicon film  105  as a mask, an impurity of arsenic (As), for example, is introduced into substrate  101  by way of ion implantation, and thermally diffused to form source region  107  and drain region  108  of the cell transistor and the select transistor (see FIG.  5 ). 
     Then, over-the-diffused-layer oxide film  109  (see FIG. 6) in the form of a silicon oxide (SiO 2 ) film is grown by plasma CVD so as to fill up regions alongside of first N-type polysilicon film  103 , pad oxide film  104 , and second N-type polysilicon film  105 . 
     Thereafter, the upper surface of over-the-diffused-layer oxide film  109  is planarized by a CMP (Chemical Mechanical Polishing) process and an etchback process, exposing second N-type polysilicon film  105 . Depending on how over-the-diffused-layer oxide film  109  fills up the above regions, the upper surface of over-the-diffused-layer oxide film  109  may only be etched back without the CMP process. Second N-type polysilicon film  105  and pad oxide film  104  are etched away, exposing the surface of first N-type polysilicon film  103  (see FIG.  7 ). Prior to this step, over-the-diffused-layer oxide film  109  alongside of first N-type polysilicon film  103  may be etched to adjust its height. 
     Then, third N-type polysilicon film  110  which serves as an upper portion of the floating gate electrode of the cell transistor is grown on first N-type polysilicon film  103 . A photoresist is formed in the cell transistor area, and an impurity of phosphorus (P) or the like is introduced into first N-type polysilicon film  103  and third N-type polysilicon film  110  in the cell transistor area by way of ion implantation. In order to increase the capacitance between the floating gate electrode and a control gate electrode which will subsequently be formed, third N-type polysilicon film  110  in the cell transistor area is patterned to a wing shape, and ONO (Oxide Nitride Oxide) film  111  is grown on the patterned third N-type polysilicon film  110  by CVD (see FIG.  8 ). 
     Then, opening  114  is formed in ONO film  111  in the cell transistor area (see FIG.  9 ), and fourth N-type polysilicon film  112  and metal silicide film  113  of WSi, for example, which will serve as the gate electrode of the select transistor and the control gate electrode of the cell transistor, are grown on ONO film  111 . First N-type polysilicon film  103  and fourth N-type polysilicon film  112  are now short-circuited to each other via opening  114 . 
     The cell transistor area and a peripheral circuit area are covered with respective photoresists, and a control gate, ONO film  111 , and a floating gate of the cell transistor are patterned simultaneously. 
     Finally, the cell transistor area is covered with a photoresist, and gate electrodes (fourth N-type polysilicon film  112  and metal silicide film  113 ) of the select transistor and the transistors of the peripheral circuit are patterned (see FIG.  10 ). Metal silicide film  113  may not necessarily be formed, but only fourth N-type polysilicon film  112  may be grown. In FIGS. 9 and 10, opening  114  is defined in ONO film  111  directly above the channel region of the cell transistor area. Actually, however, opening  114  is defined in ONO film  111  over field oxide film  106 . 
     With the above conventional semiconductor memory device, since the select transistor and the cell transistor are of an identical structure, it is necessary for the fabrication process to have the step of changing the film thicknesses of the tunneling oxide film of the cell transistor and the gate insulating film of the select transistor which have different withstand voltages, and the step of short-circuiting the first N-type polysilicon film of the select transistor (corresponding to the floating gate of the cell transistor) and the fourth N-type polysilicon film of the select transistor (corresponding to the control gate of the cell transistor). Therefore, the cost of the conventional semiconductor memory device is increased because of the increased number of steps of fabrication process. 
     The impurity in the polysilicon film used as the gate electrode of the select transistor should preferably be of a high concentration in order to reduce the resistance for high-speed operation, and the impurity in the polysilicon film used as the floating gate electrode of the cell transistor should preferably be of a low concentration in order to prevent a data erasure failure and improve a data retention capability. It is thus necessary for the fabrication process to have the step of changing these impurity concentrations. 
     One solution is to fabricate the select transistor simultaneously with the transistors of the peripheral circuit, but not with the cell transistor. 
     However, since the channel width of the transistors of the peripheral circuit generally needs to be managed more accurately than the select transistor, after a gate electrode of the transistors of the peripheral circuit is formed, an impurity is introduced by way of ion implantation using the gate electrode as a mask, forming diffused layers which serve as source and drain regions. Because the diffused layer of the select transistor and the diffused layer of the cell transistor cannot be formed simultaneously, an impurity may be introduced twice into the junction between the diffused layers of the select transistor and the cell transistor, which is also used as an interconnection, resulting in a reduction in the withstand voltage, as shown in FIG. 11A of the accompanying drawings, or an area may be developed where no impurity is introduced, resulting in a disconnection, as shown in FIG. 11B of the accompanying drawings. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a method of manufacturing a semiconductor memory device to increase the film thickness of a gate insulating film of a select transistor and increase the concentration of an impurity in a polysilicon film used as a gate electrode of a select transistor with a reduced number of fabrication steps. 
     In a method of manufacturing a semiconductor memory device according to the present invention, before the control gate electrodes of cell transistors are formed, a polysilicon film and an oxide film directly above channel regions of select transistors are removed to expose the surface of a substrate. Thereafter, gate insulating films of the select transistors are formed on the exposed surface of the substrate. Gate electrodes of the select transistors are formed on the gate insulating films simultaneously with control gate electrodes of the cell transistors. In this manner, the control gate electrodes of the cell transistors and the gate electrodes of the select transistors are formed of a single-layer polysilicon film. Consequently, the method is free of the step of short-circuiting two polysilicon films and the step of introducing an impurity in order to reduce the resistance of the polysilicon film which corresponds to floating gate electrodes of the cell transistors. Therefore, the film thickness of the gate insulating films of the select transistors can be increased and the concentration of the impurity in the polysilicon film used as the gate electrodes of the select transistors can be increased with a reduced number of fabrication steps. 
     A first diffused layer serving as source and drain regions of the cell transistors and a second diffused layer serving as source and drain regions of the select transistors are formed simultaneously. As a result, the junction between the select transistors and the cell transistors is prevented from being cut off, and the impurity is prevented from being introduced twice. 
     Furthermore, gate insulating films of transistors of a peripheral circuit are formed simultaneously with the gate insulating films of the select transistors, and gate electrodes of the transistors of the peripheral circuit are formed simultaneously with the gate electrodes of the select transistors. Thus, the number of fabrication steps of the method of manufacturing the semiconductor memory device is reduced. 
     The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a circuit diagram of a memory cell assembly of a flash EEPROM; 
     FIG. 2 is a plan view of a structure of the flash EEPROM shown in FIG. 1; 
     FIGS. 3 through 10 are cross-sectional views showing the sequence of a conventional method of manufacturing the semiconductor memory device shown in FIGS. 1 and 2; 
     FIGS. 11A and 11B are cross-sectional views showing junctions in the case where a cell transistor and a select transistor of the semiconductor memory device shown in FIGS. 1 and 2 are not fabricated simultaneously; 
     FIGS. 12 through 19 are cross-sectional views showing a process of fabricating a cell transistor and a select transistor, of a method of manufacturing a semiconductor memory device according to the present invention; and 
     FIGS. 20 through 25 are cross-sectional views showing a process of fabricating transistors of a peripheral circuit, of the method of manufacturing a semiconductor memory device according to the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A method of manufacturing a semiconductor memory device according to the present invention will be described below with reference to FIGS. 12 through 25. A circuit arrangement and a structure, as viewed in plan, of a flash EEPROM to which the method according to the present invention is applied are identical to those of conventional flash EEPROMs, and will not be described below. 
     As shown in FIG. 12, a thin SiO 2  film and a silicon nitride film is formed on substrate  1  of p-type semiconductor and patterned to a predetermined shape, and its openings are selectively oxidized to form field oxide film  6  as an inactive region for separating components. The inactive region for separating components may be a region of shallow trench isolation (STI). 
     Then, a silicon oxide (SiO 2 ) film is grown on the surface of substrate  1  by thermal oxidization. Silicon oxide film  2   a  formed in a select transistor area will be removed in a subsequent step. A silicon oxide film formed in a cell transistor area serves as tunneling oxide film  2   b.    
     First N-type polysilicon film  3 , which will serve as the floating gate electrode of a cell transistor, is grown on silicon oxide film  2   a  and tunneling oxide film  2   b . Pad oxide film  4  is grown on first N-type polysilicon film  3  by CVD, and second N-type polysilicon film  5  is grown on pad oxide film  4 . Second N-type polysilicon film  5  will be used only as a mask in a subsequent ion implantation step. Therefore, second N-type polysilicon film  5  may be replaced with an amorphous silicon film or a silicon nitride film. 
     Then, first N-type polysilicon film  3 , pad oxide film  4 , and second N-type polysilicon film  5  are patterned to respective shapes. The width of first N-type polysilicon film  3  which is formed at this time determines the channel widths of the cell transistor and the select transistor. 
     As shown in FIG. 13, using second N-type polysilicon film  5  as a mask, an impurity of arsenic (As), for example, is introduced into substrate  1  by way of ion implantation, and thermally diffused to form source region  7  and drain region  8  of the cell transistor and the select transistor. At this time, side walls may be formed of insulating films on the sides of first N-type polysilicon film  3 , pad oxide film  4 , and second N-type polysilicon film  5 , so that source region  7  and drain region  8 , each comprising an N-type diffused layer, are of an LDD (Lightly Doped Drain) structure. 
     Then, as shown in FIG. 14, over-the-diffused-layer oxide film  9  in the form of a silicon oxide film is grown by plasma CVD so as to fill up regions alongside of first N-type polysilicon film  3 , pad oxide film  4 , and second N-type polysilicon film  5 . Prior to this step, the sides of first N-type polysilicon film  3 , pad oxide film  4 , and second N-type polysilicon film  5  of the cell transistor are covered with highly reliable thermal oxide films. If side walls are to be formed, then the sides are covered with similar thermal oxide films before the side walls are formed. The sides are covered with such thermal oxide films in order to prevent charges from leaking from first N-type polysilicon film  3  which is the floating gate electrode of the cell transistor into over-the-diffused-layer oxide film  9  and hence to prevent the data retaining capability from being reduced. 
     Thereafter, the upper surface of over-the-diffused-layer oxide film  9  is planarized by a CMP process and an etchback process, exposing second N-type polysilicon film  5  (see FIG.  14 ). Depending on how over-the-diffused-layer oxide film  9  fills up the above regions, the upper surface of over-the-diffused-layer oxide film  9  may only be etched back without the CMP process. 
     Second N-type polysilicon film  5  and pad oxide film  4  are etched away, exposing the surface of first N-type polysilicon film  3  (see FIG.  15 ). Prior to this step, over-the-diffused-layer oxide film  9  alongside of first N-type polysilicon film  3  may be etched to adjust its height. 
     Then, as shown in FIG. 16, third N-type polysilicon film  10  which serves as an upper portion of the floating gate electrode of the cell transistor is grown on first N-type polysilicon film  3 . In order to increase the capacitance between the floating gate electrode and a control gate electrode which will subsequently be formed, third N-type polysilicon film  10  in the cell transistor area is patterned to a wing shape, and ONO film  11  is grown on patterned third N-type polysilicon film  10  by CVD. 
     Then, as shown in FIG. 17, ONO film  11 , third N-type polysilicon film  10 , first N-type polysilicon film  3 , and silicon oxide film  2  in the select transistor area are etched away, exposing the surface of substrate  1 . 
     Then, the surface of substrate  1  in the select transistor area is thermally oxidized to form gate insulating film  14  (see FIG.  18 ). As shown in FIG. 19, fourth N-type polysilicon film  12 , which will serve as the gate electrode of the select transistor and the control gate electrode of the cell transistor, is grown on gate insulating film  14  and ONO film  11 . Metal silicide film  13  of WSi, for example, is grown on fourth N-type polysilicon film  12 . 
     The cell transistor area and a peripheral circuit area are covered with respective photoresists, and a control gate, ONO film  111 , and a floating gate of the cell transistor are patterned simultaneously. Finally, the cell transistor area is covered with a photoresist, and gate electrodes (fourth N-type polysilicon film  12  and metal silicide film  13 ) of the select transistor and the transistors of the peripheral circuit are patterned, as shown in FIG.  19 . Metal silicide film  13  may not necessarily be formed, but only fourth N-type polysilicon film  12  may be grown. 
     In the above method of manufacturing a semiconductor memory device, since the N-type diffused layer which serves as the source and drain regions of the select transistor and the N-type diffused layer which serves as the source and drain regions of the cell transistor are formed simultaneously, the junction between the select transistor and the cell transistor is prevented from being cut off, and the impurity is prevented from being introduced twice. Because the gate electrode of the select transistor does not have a double-layer N-type polysilicon film unlike the conventional semiconductor memory device, the method according to the present invention is free of the step of short-circuiting two N-type polysilicon films and the step of introducing an impurity in order to reduce the resistance of the first N-type polysilicon film which corresponds to the floating gate electrode of the cell transistor. 
     Consequently, the film thickness of the gate insulating film of the select transistor can be increased and the concentration of the impurity in the polysilicon film used as the gate electrode of the select transistor can be increased with a reduced number of fabrication steps. 
     If the transistors of the peripheral circuit are to be fabricated simultaneously with the cell transistor and the select transistor, then transistors of the peripheral circuit are fabricated as follows: 
     At the same time as the step of fabricating the cell transistor and the select transistor as shown in FIG. 12, a silicon nitride film is formed on a substrate  1  of p-type semiconductor and patterned to a predetermined shape, and its openings are selectively oxidized to form field oxide film  6  as an inactive region for separating components, as shown in FIG.  20 . 
     Then, silicon oxide (SiO 2 ) film  2   a  and first N-type polysilicon film  3  are grown on substrate  1  by thermal oxidization. Pad oxide film  4  is deposited on N-type polysilicon film  3  by CVD, and second N-type polysilicon film  5  is grown on pad oxide film  4 . 
     Then, at the same time as the step of planarizing over-the-diffused-layer oxide film  9  as shown in FIG. 15, second N-type polysilicon film  5  and pad oxide film  4  are etched away, exposing the surface of first N-type polysilicon film  3 , as shown in FIG.  21 . 
     At the same time as the step shown in FIG. 16, third N-type polysilicon film  10  is deposited on first N-type polysilicon film  3 , and ONO film  11  is grown on third N-type polysilicon film  10  by CVD, as shown in FIG.  22 . 
     Then, at the same time as the step shown in FIG. 17, ONO film  11 , third N-type polysilicon film  10 , first N-type polysilicon film  3 , and silicon oxide film  2  are etched away, exposing the surface of substrate  1 , as shown in FIG.  23 . 
     Then, at the same time as the step shown in FIG. 18, the surface of substrate  1  is thermally oxidized, forming gate insulating films of the transistors of the peripheral circuit. Of the transistors of the peripheral circuit, a high-withstand-voltage transistor which requires a high withstand voltage has gate insulating film  16  (see FIG. 24) whose thickness is increased according to a multi-oxidizing process described below. A Vcc-driven transistor which operates under a power supply voltage Vcc that is supplied from an external source does not need to increase the thickness of gate insulating film  15 . According to the multi-oxidizing process, the surface of substrate  1  is thermally oxidized to form an oxide film whose thickness is smaller than a desired film thickness. The thickness of the oxide film is smaller than the desired film thickness by a thickness which will be added when gate insulating film  15  of the Vcc-driven transistor is subsequently to be formed. Then, a photoresist is formed in a high-withstand-voltage transistor area, and the oxide film in the Vcc-driven transistor area is etched away. Then, the photoresist is removed, and the assembly is thermally oxidized until the film thickness of the oxide film of the Vcc-driven transistor reaches a desired film thickness, growing gate insulating film  15  of the Vcc-driven transistor and gate insulating film  16  of the high-withstand-voltage transistor to respective desired film thicknesses (see FIG.  24 ). The film thickness of gate insulating film  12  of the select transistor is increased in this step in the same manner as with the high-withstand-voltage transistor. 
     Then, fourth N-type polysilicon film  12  and metal silicide film  13  which will serve as the gate electrodes of the transistors of the peripheral circuit are grown respectively on gate insulating films  15 ,  16 , and then patterned. Finally, as shown in FIG. 25, using the patterned films as a mask, an impurity of arsenic (As), for example, is introduced into substrate  1  by way of ion implantation, and thermally diffused to form source region  17  and drain region  18  of the transistors of the peripheral circuit. 
     As described above, inasmuch as the process of forming the gate insulating films and gate electrodes of the transistors of the peripheral circuit is carried out simultaneously with the process of fabricating the cell transistor and the select transistor as shown in FIGS. 12 through 19, the number of steps of the method of manufacturing the flash EEPROM can be reduced. 
     While in the illustrated embodiment the method according to the present invention has been described with respect to N-channel transistors of the semiconductor memory device, the principles of the present invention are also applicable to the fabrication of P-channel transistors by changing the impurity. 
     While preferred embodiment of the present invention has been described using specific terms, such description is for illustrative purposes only, and it is to be understood that changes and variations may be made without departing from the spirit or scope of the following claims.