Semiconductor memory device and method of manufacturing the same

Abstract: An improved semiconductor memory device having a floating gate (4) and a control gate (7) and a method of manufacturing the same are disclosed. To obtain good write characteristics, the device has an impurity region (31) of the opposite conductivity type as that used for forming the channel in the drain side of the channel. This region can be made self aligned with the floating gate (4).

Description: BACKGROUND OF THE INVENTION This invention relates to a semiconductor memory device and a method of manufacturing the same. In particular, it relates to a semiconductor memory device provided with a control gate on top of a floating gate or a channel that is on a drain side, and a method of manufacturing such a semiconductor memory device. A conventional flash EEPROM type of semiconductor device has an n-type source and an n-type drain formed in the surface of a p-type semiconductor substrate, and a floating gate is formed in the proximity of the drain on top of an oxide film on a channel between the drain and source. A floating gate and a control gate are formed above an offset portion (F) between the floating gate end and the source end, with an oxide film thereunder. In addition, a p-type impurity region of an impurity concentration greater than that of the substrate is formed uniformly in the channel between the drain and source. However, this conventional semiconductor memory device has the following problem. In order to improve the memory cell write characteristics of such a semiconductor memory device wherein the impurity concentration of an impurity region formed in a channel is uniform over the entire area from a portion in contact with the drain to a portion in contact with the source, it is preferable to increase the impurity concentration of the impurity region. The electric field can easily focus in this region, the generation probability of electrons injected into the floating gate is increased, and the memory cell write characteristics are improved. However, if the concentration of the impurity in the impurity region is increased, the threshold voltage V th of the offset transistor (hereinafter the transistor in the offset portion (F) will be called the offset transistor) also increases, making it difficult for current to flow in the memory cell and hence increasing access times. Conversely, if the concentration of the impurity in the impurity region is reduced, the threshold voltage V th of the offset transistor will drop, improving the memory cell's access times but making the memory cell write characteristics worse. Because of these conflicting problems, it is difficult to satisfy demands for both improved memory cell write characteristics and better access times in the conventional semiconductor memory device. SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide a semiconductor memory device that has good memory cell write characteristics and offset transistors with desired threshold voltages. Another object of the present invention is to provide a manufacturing method that can stably manufacture such a semiconductor memory device. According to the present invention, there is provided a semiconductor memory device comprising: a semiconductor substrate of a first conductivity type; a source and a drain of a second conductivity type, the source and the drain being provided separately in the surface of the semiconductor substrate; a channel between the source and the drain; a first impurity region of the first conductivity type, the first impurity region having an impurity concentration greater than that of the semiconductor substrate, and being provided in the drain side of the channel; a second impurity region of the first conductivity type, the second impurity region having an impurity concentration greater than that of the semiconductor substrate but less than that of the first impurity region, and being provided in the source side of the channel; a floating gate provided on top of the first impurity region, with an isolation film thereunder; and a control gate provided on top of the floating gate and the first impurity region, with an isolation film thereunder. In the semiconductor memory device of the present invention, since the first impurity region with a concentration of an impurity of the first conductivity type that is higher than that of the semiconductor substrate is provided in the surface of an offset portion (F) of the channel between the drain and source, the threshold voltage of the offset transistor can be set to any desired value. In addition, since the second impurity region with a concentration of an impurity of the first conductivity type that is greater than that of the first impurity region is provided in the channel under the floating gate, the number of electrons implanted into the floating gate can be increased, improving the memory cell's write characteristics. According to the present invention there is also provided a method of manufacturing a semiconductor memory device comprising the steps of forming a first impurity region in a specific region of the surface of a semiconductor substrate of a first conductivity type, the first impurity region having an impurity concentration greater than that of the semiconductor substrate; forming a floating gate corresponding to a high-impurity-concentration region above an isolation film formed over the semiconductor substrate of the first conductivity type; forming a second impurity region of the first conductivity type adjacent to the first impurity region, the second impurity region having an impurity concentration greater than that of the semiconductor substrate but less than that of the first impurity region, and being formed by ion implantation using as a mask the floating gate and a layer of resist patterned so as to define a channel width; forming a drain region in the first impurity region, on the side thereof opposite to the second impurity region, the drain region being a high-impurity-concentration region of a second conductivity type opposite to the first conductivity type; forming an oxide film surrounding the floating gate; forming a control gate over the drain region, the floating gate, and the second impurity region; and forming a source region adjacent to the second impurity region, the source region being a high-impurity-concentration region of the second conductivity type, and being formed by ion implantation using as a mask the control gate. The above method is capable of providing in a simple manner the semiconductor memory device of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS In the accompanying drawings, Fig. 1 is a plan view of the structure of a first embodiment of the semiconductor memory device according to the present invention; Fig. 2 is a cross-sectional view taken along the line A-A′ of Fig. 1; Fig. 3 is a cross-sectional view taken along the line B-B′ of Fig. 1; Figs. 4A to 4D are process diagrams of the method of manufacturing the first embodiment; Fig. 5 is a cross-sectional view through a second embodiment of the present invention; Figs. 6A to 6D are process diagrams of the method of manufacturing the second embodiment; Figs. 7A to 7E are process diagrams of the method of manufacturing a third embodiment of the present invention; Fig. 8 is a cross-sectional view through a fourth embodiment of the present invention; Fig. 9 is a cross-sectional view through a fifth embodiment of the present invention; and Fig. 10 is a graph of impurity density against depth from the surface of a channel on the source side of a semiconductor memory device according to the third embodiment. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a plan view of an example of a memory cell of a flash EEPROM (batch-erasable non-volatile memory) of a first embodiment of the present invention, and Figs. 2 and 3 are cross sectional views of the memory cell taken respectively along the lines A-A′ and B-B′ of Fig. 1. As shown in Fig. 2, an n-type drain 6 and an n-type source 14 are formed separately on the surface of a p-type semiconductor substrate 1 on which an oxide film (SiO₂) 2 has been formed as an insulation film. A floating gate 4 of polysilicon is formed on top of the oxide film 2 on a channel between the drain 6 and the source 14, in such a manner that it is disposed above the drain 6. A control gate 7 of polysilicon is formed over the floating gate 4 with an oxide film of (of SiO₂) 63 therebetween and a channel between the floating gate 4 and the source 14 with the oxide film of (of SiO₂) 2 therebetween. An oxide film (of SiO₂) 64 is then formed over the control gate 7. A first impurity region 31 of an impurity concentration much greater than that of the semiconductor substrate 1 is provided below the floating gate 4, in contact with the drain 6, and a second impurity region 33 of an impurity concentration greater than that of the semiconductor substrate 1 but less than that of the first impurity region 31 is formed in the channel. To give one example of the impurity concentrations of the above structure: the impurity concentration of the semiconductor substrate 1 is approximately 2 × 10¹⁵ cm⁻³, that of the drain 6 is 10²⁰ cm⁻³ (at a position close to the surface), that of the source 14 is 10²⁰ cm⁻³ (at a position close to the surface), that of the first impurity region 31 is 1 × 10¹⁷ cm⁻³ (at a position close to the surface), and that of the second impurity region 33 is 4 × 10¹⁶ cm⁻³ (at a position close to the surface). As can be clearly seen in Figs. 1 and 3, an erase gate 20 that erases electrons implanted into the floating gate 4 is formed on this memory structure. The method of manufacturing the semiconductor memory device of this embodiment will now be described with reference to Figs. 4A to 4D. As shown in Fig. 4A, a layer of resist is deposited on the oxide film 2 over the semiconductor substrate 1, using a spin coating method, and it is exposed to exposure light to form a patterned resist film 101. The resist film 101 is then used as a mask boron ions 111 are implanted into the semiconductor substrate 1 to form the first impurity region 31. After the resist film 101 is removed, the floating gate 4 made of polysilicon is selectively formed over the first impurity region 31, as shown in Fig. 4B. A resist film 102 is then deposited by spin coating and is patterned by exposure to light. The floating gate 4 and the resist film 102 are used as a mask for the implantation of boron ions 112 into the semiconductor substrate 1 to form the second impurity region 33 adjacent to the first impurity region 31. The dose at which the boron ions are implanted is less than that used during the implantation of the first impurity region 31, so the first impurity region 31 has a higher concentration of the boron impurity than the second impurity region 33. After the resist film 102 is removed, a resist film 103 is deposited by spin coating and is patterned by exposure to light, and the floating gate 4 and the resist film 103 are used as a mask for the implantation of arsenic ions 113 to form the drain 6, as shown in Fig. 4C. The arsenic ions are implanted at a dose of 5 × 10¹⁵ cm⁻² with an accelerating energy of 50 keV. After the resist film 103 is removed, the oxide film 63 having a thickness of e.g. 500 Å (10 Å = 1 nm) is formed above the floating gate 4, using a method such as thermal oxidation, for example oxidation in an atmosphere of dry oxygen at approx. 1000°C, as shown in Fig. 4D. The control gate 7 is then formed over the floating gate 4 and the first impurity region 31, with the oxide films 63 and 2 thereunder. Arsenic ions 114 are implanted into the entire surface at a dose of 5 × 10¹⁵ cm⁻² with energy of 50 keV, using the control gate 7 as a mask, to form the source 14. Finally, the oxide film 64 is formed over the control gate 7 by a method such as thermal oxidation, to obtain the semiconductor memory device of the embodiment of the present invention shown in Fig. 2. In this embodiment, since the first impurity region 31 with its high concentration of p-type impurity is adjacent to the n-type drain 6, the electric field can easily focus. Since this increases the generation probability of electrons injected into the floating gate 4, the memory cell write characteristics are improved. In addition, since the second impurity region 33 with its required impurity concentration that is lower than that of the first impurity region 31 is provided in the channel below the control gate 7, the threshold voltage V th of the memory cell's transistor can be set to any desired value. Therefore, current flows easily in the memory cell, and thus access times can be increased. A second embodiment of the present invention will now be described with reference to Fig. 5. In this embodiment, the drain 6 and source 14 are formed separately on the surface of the p-type semiconductor substrate 1 on which the oxide film 2 that acts as an insulation film has been formed, and the first impurity region 31 is formed so as to surround the drain 6, of a p-type impurity of a higher concentration than that of the semiconductor substrate 1. The p-type second impurity region 33 is formed with an impurity concentration that is higher than that of the semiconductor substrate 1, but lower than that of the first impurity region 31, between the first impurity region 31 and the source 14. The floating gate 4 is formed above the channel, in such a way that it is in contact with the drain 6, with the oxide film 2 therebetween. The control gate 7 is formed above the floating gate 4 (with the oxide film 63 therebetween) and an offset portion (F) (with the oxide film 2 therebetween), then the oxide film 64 is formed over the surface of the floating gate 4. The method of manufacturing the semiconductor memory device of the second embodiment will now be described with reference to Figs. 6A to 6D. As shown in Fig. 6A, boron ions 121 are implanted by an ion implantation method at a dose of 4 × 10¹² cm⁻² with energy of 60 keV into the surface of the p-type semiconductor substrate 1 on which the oxide film 2 has been formed, to form the second impurity region 33 that has a higher impurity concentration than that of the semiconductor substrate 1. Next, as shown in Fig. 6B, the floating gate 4 is formed at a desired position on the semiconductor substrate 1 by depositing a polysilicon film thereon by a CVD method then patterning it. A layer of resist is deposited and patterned to form a resist film 105. The resist film 105 and the floating gate 4 are used as a mask for the implantation of boron ions 122 to form the first impurity region 31. The boron ions 122 are implanted at a dose of 2 × 10¹³ cm⁻² with an accelerating voltage of 100 keV that is higher than that used during the ion implantation that formed the second impurity region 33, and, after the ion implantation, a thermal process (annealing at approx. 1000°C in inert gas (N₂) atmosphere for 20 minutes) is employed to diffuse the first impurity region 31 into the semiconductor substrate 1. Next, the resist film 105 and the floating gate 4 are again used as a mask for the implantation of arsenic ions 123 at a dose of 5 × 10¹⁵ cm⁻² using an acceleration energy of 50 keV to form the drain 6 in a self-aligning manner, as shown in Fig. 6C. After the resist film 105 is removed, the oxide (SiO₂) film 63 having a thickness of 500 Å is formed above the floating gate 4 by a thermal oxidation method, and the control gate 7 is formed above the floating gate 4 (with the oxide film 63 therebetween) and an offset portion (F) (with the oxide film 2 therebetween). The control gate 7 is then used as a mask for the implantation of arsenic ions 124 at a dose of 5 × 10¹⁵ cm⁻² with energy of 50 keV to form the source 14. Finally, the oxide film 64 is formed over the control gate 7 by a thermal oxidation method, to obtain the semiconductor memory device of the second embodiment. In the structure of the semiconductor memory device of this embodiment, the mask used for the formation of the first impurity region 31 can readily be used for the formation of the drain 6. Since the drain 6 and the first impurity region 31 are adjacent to each other, the electric field can easily focus. This increases the generation probability of electrons, accelerating the electrons injected in the floating gate 4 and improving the memory cell write characteristics. In addition, the impurity concentration of the second impurity region 33, which is higher than that of the semiconductor substrate 1, can be adjusted to a required concentration such that the threshold voltage V th of the memory cell transistor can be set to any desired value. Therefore, current flows easily in the memory cell, and thus access times can be increased. The method of manufacturing the semiconductor memory device of a third embodiment of the present invention will now be described with reference to Figs. 7A to 7E. As shown in Fig. 7A, after a silicon oxide film 2 is formed as an insulation film over the semiconductor substrate 1 made of p-type polysilicon, ions are implanted into the entire surface to form a high-impurity-concentration region 11. The implantation could be, for example, of boron ions using an accelerating energy of about 60 keV. The conventional dose for such implantation is 3 × 10¹² cm⁻², but in this embodiment it is increased to a higher dose of 6 × 10¹² cm⁻². Polysilicon is deposited on the oxide film 2 and is patterned to form the floating gate 4 in a desired position, then photoresist 51 is selectively left in the position shown in Fig. 7B. The floating gate 4 and the photoresist 51 are used as a mask for the implantation of arsenic ions with energy of 50 keV, for example, to form the drain 6 in a self-aligning manner. Subsequently, the dose is set to 5 × 10¹⁵ cm⁻². After the photoresist 51 is removed, more photoresist 52 is selectively left over the drain 6, and this photoresist 52 and the floating gate 4 are used as a mask for the shallow implantation of arsenic ions into the surface of the semiconductor substrate 1, to form a shallow region 13 (Fig. 7C). In this case, the accelerating energy is about 80 to 170 keV, and the dose is 1 to 3 × 10¹² cm⁻². After the photoresist 52 is removed, the oxide film 63 having a thickness of 400 Å is formed on the floating gate 4 by a method such as thermal oxidation for example, heating at 1000°C in dry oxygen atmosphere, and the control gate 7 is formed of polysilicon over the oxide film 63. The source 14 is then formed by the implantation of ions at a dose of 5 × 10¹⁵ cm⁻² with energy of 50 keV, for example arsenic ions, into the entire surface, using the control gate 7 as a mask (Fig. 7D). Next, the oxide film 64 is formed by thermal oxidation over the control gate 7. At the same time, the source 14 is diffused by a method such as thermal oxidation for example, annealing at 1000°C in dry oxygen atmosphere for 10 minutes, to obtain the semiconductor memory device of the third embodiment. Results of measuring impurity concentration with respect to distance from the surface of the substrate of the semiconductor memory device of the third embodiment are shown in Fig. 10. Curve 101 shows the concentration of the p-type impurity boron, curve 102 shows the concentration of the n-type impurity arsenic, and curve 103 shows the concentration of a p-type impurity which is a combination of boron and arsenic. It is clear from curve 103 that the concentration of p-type impurity near the surface of the semiconductor substrate 1 is controlled and reduced by arsenic. As described above, since the semiconductor memory device of Fig. 7E is provided with a shallow region 13 that has a desired concentration of p-type impurity that is less than that of the source 14, the threshold voltage V th of the offset transistor can be set to any desired value, facilitating current flow in the memory cell and improving access times. In addition, since the n-type drain 6 and the p-type high-impurity-concentration region 11 are in contact with each other, the generation probability of electrons implanted into the floating gate 4 is increased and the memory cell write characteristics are improved. Further, since this semiconductor memory device according to the third embodiment of the present invention can be manufactured to a higher precision than those of the first and second embodiments, it can provide an even smaller semiconductor memory device. Other embodiments of the present invention can be conceived, such as a fourth embodiment of the present invention, shown in Fig. 8, which is the same as the third embodiment except that a portion of the isolation film 40 under the floating gate 4 and the drain 6 is formed to be thinner than the rest of the isolation film, which improves the memory cell write characteristics, and a fifth embodiment of the present invention, shown in Fig. 9, which is the same as the third embodiment except that the control gate 7 and the floating gate 4 are formed in a self-aligning manner on the drain 6 side. These and other embodiments of the present invention provide a semiconductor memory device having good memory cell write characteristics and offset transistors with desired threshold voltages V th .

Claims: A semiconductor memory device comprising: a semiconductor substrate (1) of a first conductivity type; a source (14) and a drain (6) of a second conductivity type, said source and said drain being provided separately in the surface of said semiconductor substrate; a channel between said source and said drain; a first impurity region (11,31) of said first conductivity type, said first impurity region having an impurity concentration greater than that of said semiconductor substrate, and being provided in the drain side of said channel; a second impurity region (13,33) of said first conductivity type, said second impurity region having an impurity concentration greater than that of said semiconductor substrate but less than that of said first impurity region, and being provided in the source side of said channel; a floating gate (4) provided on top of said first impurity region, with an isolation film (2) thereunder; and a control gate (7)provided on top of said floating gate and said first impurity region, with an isolation film (63) thereunder. The semiconductor memory device according to claim 1, wherein said first impurity region (31) is provided surrounding said drain. The semiconductor memory device according to claim 1, wherein said second impurity region (13) is provided on the source side of the surface of said first impurity region. The semiconductor memory device according to claim 1, wherein the thickness of said isolation film under said floating gate is less than the thickness of other portions thereof. The semiconductor memory device according to claim 1, wherein an end surface of said control gate on said drain side is formed to be self-aligning with respect to an end surface of said floating gate. A method of manufacturing a semiconductor memory device according to claims 1,4 or 5 comprising the steps of: forming a first impurity region (31) in a specific region of the surface of a semiconductor substrate (1) of a first conductivity type, said first impurity region having an impurity concentration greater than that of said semiconductor substrate; forming a floating gate (4) corresponding to said first impurity region above an isolation film (2) formed over said semiconductor substrate of said first conductivity type; forming a second impurity region (33) of said first conductivity type adjacent to said first impurity region, said second impurity region having an impurity concentration greater than that of said semiconductor substrate but less than that of said first impurity region, and being formed by ion implantation using as a mask said floating gate and a layer of resist (102) patterned so as to define a channel width; forming a drain region (6) in said first impurity region, on the side thereof opposite to said second impurity region, said drain region being a high-impurity-concentration region of a second conductivity type opposite to said first conductivity type; forming an oxide film (63) surrounding said floating gate; forming a control gate (7) over said drain region, said floating gate, and said second impurity region; and forming a source region (14) adjacent to said second impurity region, said source region being a high-impurity-concentration region of said second conductivity type, and being formed by ion implantation using as a mask said control gate. The method of manufacturing a semiconductor memory device according to claim 6, wherein said oxide film surrounding said floating gate is formed by a thermal oxidation method. A method of manufacturing a semiconductor memory device according to claims 1,2,4 or 5 comprising the steps of: forming a second impurity layer (33) by ion implantation into a semiconductor substrate (1) of a first conductivity type and having an insulation film (2) on the surface thereof, said second impurity layer having an impurity concentration greater than that of said semiconductor substrate; forming a floating gate (4) on said isolation film at a specific position thereon; forming a first impurity layer (31) of said first conductivity type, said first impurity layer having an even greater impurity concentration than said second impurity layer and being deeper than said second impurity layer, and being formed by ion implantation using as a mask a layer of resist (105) selectively formed over said floating gate and a region destined to become a channel; forming an oxide film (63) surrounding said floating gate; forming a drain region (6) in said first impurity region, said drain region being a high-impurity-concentration diffusion region formed in a self-aligning manner of an impurity of a second conductivity type opposite to said first conductivity type, and being formed by ion implantation using as a mask said floating gate and said resist; forming a control gate (7) above said insulation film and said floating gate, said control gate being of a length such as to determine the width of said channel; and forming a source region (14), said source region being a high-impurity-concentration diffusion region formed in a self-aligning manner of an impurity of said second conductivity type, and being formed by ion implantation using as a mask said control gate. A method of manufacturing a semiconductor memory device according to claims 1,3,4 or 5 comprising the steps of forming a first impurity layer (11) by ion implantation into a semiconductor substrate (1) of a first conductivity type and having an insulation film (2) on the surface thereof, said first impurity layer having an impurity concentration greater than that of said semiconductor substrate; forming a floating gate (4) on said isolation film at a specific position thereon; forming a high-concentration layer (6) of a second conductivity type opposite to said first conductivity type, within said first impurity layer, said high-concentration layer being formed by ion implantation using as a mask a layer of resist (51) selectively formed over said floating gate and a region destined to become a channel; forming a second impurity layer (13) of said first impurity type, said second impurity layer having a concentration less than that of said first impurity layer, and being formed in a self-aligning manner with respect to said floating gate in a region destined to be a channel region by shallow implantation of ions of said second conductivity type, using as a mask said floating gate and a layer of resist (52) formed on said first impurity region; forming by oxidation an oxide film (63) surrounding said floating gate and forming by dispersing said high-concentration region (6) a drain region having a dispersion depth deeper than that of said first impurity layer; selectively forming a control gate (7) above said drain region, said floating gate, and said channel region; and forming a source region (14) in a self-aligning manner with respect to said control gate, said source region being formed by implantation and dispersion of ions of said second conductivity type within said second impurity layer, using as a mask said control gate.

Inventors: KANEBAKO KAZUNORI, NODA MICHIAKI, TANAKA SHINICHI, KANEBAKO, KAZUNORI, NODA, MICHIAKI, TANAKA, SHINICHI
Applicants: TOSHIBA KK, KABUSHIKI KAISHA TOSHIBA
Application year: 1990
Main classification: H01L29
IPC codes: H01L29, H01L21, H01L27