JP2012114475A - Nonvolatile semiconductor memory device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cell structure capable of ensuring the breakdown voltage of a memory cell and capable of controlling accurate threshold distribution even in a microfabricated memory cell.SOLUTION: A nonvolatile semiconductor memory device comprises: a plurality of element isolation insulating films DI formed in a surface layer of a semiconductor substrate S; a plurality of element regions AA defined by the element isolation insulating films DI; a plurality of gate structures each including a charge storage layer FG formed on the semiconductor substrate S via a tunnel oxide film 10 and a control gate CG formed on the charge storage layer FG via a gate insulating film 20; a plurality of impurity diffusion layers IDL formed in the element regions AA so as to sandwich the surface layer of the semiconductor substrate S directly under the gate structures therebetween; insulating films 60 formed from oxide silicon so as to fill the gaps between the gate structures; and an insulating film 40 formed from nitride silicon so as to contact the side walls of the gate structures. The bottoms of the insulating film 40 are spaced apart from the surface of the semiconductor substrate S by at least a half or more of the height of the charge storage layers FG.

Description

  The present invention relates to a nonvolatile semiconductor memory device.

  A nonvolatile semiconductor memory device includes a charge storage layer (floating gate) formed on a substrate via a first gate insulating film, and a control formed on the charge storage layer via a second gate insulating film. When writing data, for example, by applying a positive voltage to the control gate and drain region while grounding the substrate and source region, electrons are injected into the charge storage layer to change the threshold value of the control gate Thus, the division of “0” or “1” is performed. A representative example of a large-capacity, high-density nonvolatile semiconductor memory is a NAND-type EEPROM having a self-aligned STI (Shallow Trench Isolation) memory structure that can be integrated at the highest density.

  However, in the conventional self-aligned STI cell structure, when the element region width, the element isolation width, the gate width, and the space between the gates are reduced in order to increase the density and capacity, the breakdown voltage deteriorates between adjacent gates. In addition to an increase in parasitic capacitance (see, for example, Patent Document 1), memory cell characteristics are degraded due to cell channel current degradation due to charge trapping on the surface of the n-type diffusion layer, and threshold distribution is degraded. There was a problem.

International Publication No. 2005/081318

  An object of the present invention is to realize a cell structure capable of securing a dielectric breakdown voltage of a memory cell and controlling a precise threshold distribution even when the memory cell is miniaturized.

According to a first aspect of the invention,
A plurality of element regions provided extending in a first direction on the semiconductor substrate; and a first gate insulation provided extending in a second direction intersecting the first direction on the semiconductor substrate. A charge storage layer formed on the semiconductor substrate through a film; a second gate insulating film formed on the charge storage layer; a control gate formed on the second gate insulating film; A plurality of gate structures each including an element isolation insulating film that is formed of an insulating film selectively formed on a surface layer of the semiconductor substrate, demarcates the element region and electrically isolates the element region; An impurity diffusion layer formed in the element region so as to sandwich a surface layer of the semiconductor substrate directly under the gate structure, and a third gate insulating film formed of a first insulating material between the gate structures; The sidewalls of the gate structure and the gate And a fourth gate insulating film formed of a second insulating material having a dielectric constant lower than that of the first insulating material so as to be in contact with the semiconductor substrate between the first and second structures. An apparatus is provided.

According to the second aspect of the present invention,
A plurality of element regions provided extending in a first direction on the semiconductor substrate; and a first gate insulation provided extending in a second direction intersecting the first direction on the semiconductor substrate. A charge storage layer formed on the semiconductor substrate through a film; a second gate insulating film formed on the charge storage layer; a control gate formed on the second gate insulating film; A plurality of gate structures each including an element isolation insulating film that is formed of an insulating film selectively formed on a surface layer of the semiconductor substrate, demarcates the element region and electrically isolates the element region; An impurity diffusion layer formed in the element region so as to sandwich a surface layer of the semiconductor substrate immediately below the gate structure, and a third gate formed of a first insulating material so as to embed between the gate structure An insulating film and the gate structure; A fourth gate insulating film formed of a second insulating material different from the first insulating material so as to be in contact with the side wall, and a third gate insulating film different from the first insulating material in the third gate insulating film And a fifth gate insulating film formed of an insulating material, wherein a dielectric constant of the second and third insulating materials is higher than a dielectric constant of the first insulating material. A storage device is provided.

  According to the present invention, even when a memory cell is miniaturized, it is possible to realize a cell structure that can secure a dielectric breakdown voltage of a memory cell and can control a precise threshold distribution.

The figure which shows typically the solution which relieves the electric field between gates and suppresses insulation degradation. The graph which shows an example of the relationship between the film thickness of the insulating material of the insulating film between gates which touches a gate structure, and the voltage which can be applied to a control gate at the time of writing. 1 is a schematic plan view showing a memory structure of a nonvolatile semiconductor memory device according to a first embodiment of the present invention. 1 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a first embodiment of the present invention. 1 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a first embodiment of the present invention. 1 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a first embodiment of the present invention. 1 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a first embodiment of the present invention. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 1st comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 1st comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 1st comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 1st comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 2nd comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 2nd comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 2nd comparative example. Sectional drawing which shows an example of the non-volatile semiconductor memory device by a 2nd comparative example. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a second embodiment of the present invention. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a second embodiment of the present invention. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a second embodiment of the present invention. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a second embodiment of the present invention. FIG. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a third embodiment of the invention. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a third embodiment of the invention. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a third embodiment of the invention. FIG. 6 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a third embodiment of the invention. 10 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fourth embodiment of the invention. 10 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fourth embodiment of the invention. 10 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fourth embodiment of the invention. 10 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fourth embodiment of the invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fifth embodiment of the invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fifth embodiment of the invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fifth embodiment of the invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a fifth embodiment of the invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a sixth embodiment of the present invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a sixth embodiment of the present invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a sixth embodiment of the present invention. FIG. 9 is a schematic cross-sectional view showing a memory structure of a nonvolatile semiconductor memory device according to a sixth embodiment of the present invention. FIGS. 11A to 11D schematically show a solution method for suppressing insulation deterioration by relaxing an electric field between gates in the memory structure shown in FIGS.

  Hereinafter, some embodiments of the present invention will be described in detail with reference to the drawings. In the drawings, the same parts are denoted by the same reference numerals, and redundant description will be given only when necessary.

(1) Method for suppressing insulation deterioration FIG. 1 is a diagram schematically showing a solution method for suppressing insulation deterioration by relaxing an inter-gate electric field. This figure shows an electric field relationship in a cell structure of an example of a NAND type EEPROM having a self-aligned STI memory structure. Laminated gate structures formed in the form of parallel plates are arranged on the left and right sides of the drawing. Between the control gates CG respectively formed on the charge storage layer, for example, inter-gate insulating films IF1a, IF2 and IF1b having a three-layer structure made of two kinds of insulating materials are arranged. In the cell structure of FIG. 1, for example, the control gate CG to which the write voltage (Vpgm) is applied is shown on the right side of the paper, and the charge storage layer FG directly under the control gate to which the write prevention intermediate voltage (Vpass) is applied to the left side of the paper. It is shown. The insulating materials (referred to as “first insulating material A”) of the intergate insulating films IF1a and IF1b that are in contact with the control gate CG and the charge storage layer FG immediately below the control gate CG are the same type, and have a relative dielectric constant of eps_1 and a thickness of d_1. The applied electric field strength is E_1. The inter-gate insulating film IF2 inserted between the inter-gate insulating films IF1a and IF1b is formed of an insulating material B different from the insulating material A, and has a relative dielectric constant of eps_2 and a film thickness of d_2. The intensity is E_2.

The electric field strength E_i (i = 1, 2) in each insulating film is expressed by [Equation 1] E_i = K * V / eps_i
It can be expressed. here,
[Formula 2] K = 1 / (2 * d_1 / eps_1 + d_2 / eps_2)
It is.

  In general, a current flowing through a relatively thick insulating film is expressed by a Fowler-Nordheim type or a Pool-Frankel type. Therefore, it is important to lower an electric field applied to the insulating film. In other words, since it is considered that the leakage current can be suppressed by relaxing the electric field in the portion that emits electrons, the structure shown in FIG. 1 needs to satisfy E_1 <E_2. As a condition for satisfying E_1 <E_2 from (Equation 1), eps_1> eps_2 may be satisfied. Note that since the potential relationship between adjacent cells is switched with writing, the actual memory cell structure has a symmetrical structure.

FIG. 2 is a graph showing an example of the relationship between the film thickness of the insulating material A and the voltage (Vpgm) that can be applied to the control gate during writing. The distance between adjacent cells was 10 nm, and the potential V _FG of the charge storage layer FG immediately below the control gate to which the write preventing intermediate voltage Vpass was applied was 4V. In addition, the withstand voltage is assumed to be 10 MV / cm for convenience.

  Here, the first-layer insulating film IF1a is, for example, a silicon nitride film (eps_1 = 7.5, d_1 = 2 nm), and the second-layer insulating film IF2 is, for example, a silicon oxide film (eps_2 = 2.3.9, d_2). = 6 nm), the maximum voltage that can be applied to the first silicon nitride film is 19.5V. It can be seen that when the film thickness of the silicon nitride film of the first layer is reduced under this setting, the voltage that can be applied increases due to the electric field relaxation effect. When the first silicon nitride film having a high dielectric constant is thickened, the parasitic capacitance between the charge storage layers of adjacent cells increases, so that the spread of the threshold distribution due to the interference between adjacent cells deteriorates and a precise threshold value is obtained. There is also an adverse effect that control becomes difficult. Therefore, a device for reducing the thickness of the first silicon nitride film is required. Specifically, it is desirable that the thickness be less than or equal to the thickness of the inter-gate insulating film between the control gate and the charge storage layer of the memory cell.

(2) First Embodiment A first embodiment of the present invention will be described with reference to FIGS. 3 to 6D.

  FIG. 3 is a schematic plan view showing the memory structure of the nonvolatile semiconductor memory device of this embodiment, and FIGS. 4A to 4D are schematic cross-sectional views thereof. FIG. 3 shows only the layout of the element isolation insulating film DI region, element region AA, charge storage layer FG, and control gate CG in the memory structure. 4A to 4D are cross-sectional views taken along line AA, line BB, line CC, and line DD in FIG. 3, respectively. The relationship between these cutting lines and cross-sectional views is the same for the following FIGS. 5A to 11D.

  First, the memory structure of the nonvolatile semiconductor memory device of this embodiment will be described with reference to FIGS. 3 to 4D.

  An element isolation trench (Shallow Trench) ST is formed in an element isolation region on the surface of the silicon substrate S, and an element isolation insulating material, for example, a silicon dioxide material is embedded in the trench ST to form an element isolation insulating film DI. The device region AA is defined. A thin tunnel insulating film 10 through which a tunnel current can flow is formed on the entire surface of the element region AA that has been element-isolated by such an element isolation insulating film DI. A charge storage layer FG is formed in a region intersecting the gate structure in the element region AA on the tunnel insulating film 10. The side edge of the charge storage layer FG is aligned with the boundary of the element isolation region. A part of the element isolation insulating film DI is in contact with the charge storage layer FG (see FIG. 4C). The top surface of the charge storage layer FG faces the control gate CG via the inter-gate insulating film 20, and thereby, a device for increasing the capacitance between the charge storage layer FG and the control gate CG is devised. A laminated gate cap material 30 is formed on the top surface of the control gate CG. The stacked gate cap material 30, the control gate CG, the intergate insulating film 20, and the charge storage layer FG form a stacked gate structure. The control gate CG and the charge storage layer FG are vertically processed in a self-aligning manner so that the side walls are aligned. In the surface layer of the semiconductor substrate S, an n-type impurity diffusion layer IDL is formed so as to sandwich the surface layer immediately below the control gate CG, thereby forming source / drain regions having the surface layer immediately below the control gate CG as a channel region. Yes.

  As shown in FIG. 4A, a silicon nitride insulating film 40 is formed in contact with the stacked gate structure so as to cover each stacked gate structure. As a result, impurities such as hydrogen and metal elements can be prevented from entering from the outside. Further, between the adjacent control gates CG and between the charge storage layers FG, a silicon oxide insulating material is embedded via a silicon nitride insulating film 40 to form a silicon oxide insulating film 60, whereby the adjacent control The gates CG and the charge storage layer FG are electrically isolated from other elements.

  In the present embodiment, the Y direction shown in FIG. 3 corresponds to, for example, the first direction, and the X direction corresponds to, for example, the second direction. The tunnel insulating film 10 corresponds to, for example, a first gate insulating film, and the inter-gate insulating film 20 corresponds to, for example, a second gate insulating film. Further, silicon nitride and silicon oxide correspond to, for example, the first and second insulating materials, respectively. The silicon nitride insulating film 40 and the silicon oxide insulating material 60 include, for example, the fourth gate insulating film and the third gate insulating, respectively. Corresponds to the membrane.

  Here, one of the features of the present embodiment is that the silicon nitride insulating film 40 is not in contact with the semiconductor substrate S and is separated at least up to half the height of the charge storage layer FG. The silicon oxide insulating material 60 is embedded. The thickness of the silicon nitride insulating film 40 is equal to or less than the thickness of the inter-gate insulating film 20. In the present embodiment, the silicon oxide insulating film 60 is formed so as to cover the top surface of the stacked gate FG.

  Here, the effect of the semiconductor memory device of this embodiment will be described with reference to a comparative example. The semiconductor memory device illustrated in FIGS. 5A to 5D is an example of a nonvolatile semiconductor memory device according to a first comparative example.

  In each of the semiconductor memory devices shown in FIGS. 5A to 5D, each stacked gate structure is embedded with a silicon oxide-based insulating material so that each stacked gate is electrically isolated. Further, a silicon nitride insulating film 140 is formed on the upper part of the laminated gate structure, thereby preventing impurities such as hydrogen and metal elements from entering from the outside.

  However, the cell structure shown in FIGS. 5A to 5D has the following problems when the element region AA and the element isolation region between the element areas AA are reduced for further miniaturization.

  First, when the space between the control gates CG is reduced, if a high voltage (for example, 20V) is applied to the control gate CG during charge transfer, the potential of the adjacent charge storage layer FG is about 4V. A high electric field is applied between the CG and the charge storage layer FG adjacent thereto. Since the distance between them is several tens of nanometers, the electric field reaches the 8 MV / cm level. Since there is an inter-gate insulating film 20 between the control gate CG and the charge storage layer FG immediately below the control gate CG, insulation deterioration due to application of a high electric field is suppressed. However, since there is only a silicon oxide-based insulating film that insulates between the charge storage layers FG immediately below the adjacent stacked gates, it is very vulnerable to insulation deterioration, and a high voltage cannot be applied to the control gate. There is a danger that occurs.

  Also, in recent large-capacity nonvolatile memories, super multi-value technology is used that increases the effective data amount in units of bits by providing 8-value information or 16-value information in one memory cell. In this case, each data needs to have the threshold voltage of the memory cell separately by the amount of information. For this reason, it is necessary to apply a higher voltage (for example, 24 V) to the control gate CG and to perform cell writing to a higher threshold voltage as the number of super multi-values increases. However, in the cell structure shown in FIGS. 5A to 5D, only the silicon oxide insulating film 140 that insulates between the control gates CG is formed between the charge storage layers FG immediately below the adjacent stacked gates. There is a problem that high voltage may not be applied due to deterioration.

  On the other hand, in the cell structure according to the first embodiment described above, the silicon nitride system formed of silicon nitride having a dielectric constant higher than the dielectric constant of the silicon oxide-based insulating material is in contact with the stacked gate structure. Since the insulating film 40 exists, when a high voltage (for example, 20 V) is applied to the control gate CG, due to the electric field relaxation effect in the silicon nitride insulating film 40, the charge storage layer FG directly below the adjacent control gate CG It has a structure that does not cause dielectric breakdown.

  6A to 6D show an example of a nonvolatile semiconductor memory device according to a second comparative example. A silicon nitride insulating film 150 is formed not only on the side surface of each stacked gate structure but also on the substrate surface between these stacked gate structures so as to cover them. Impurity entry is prevented. Further, between the adjacent control gates CG and between the charge storage layers FG are embedded with a silicon oxide insulating material through the silicon nitride insulating film 150 to isolate elements.

  The cell structure of this example is characterized in that a silicon nitride insulating film 150 that easily captures electrons is inserted in the side wall of the stacked gate, so that the selected control gate CG and the control gate CG immediately below it are directly below. Even if a high voltage is applied between the charge storage layer FG and the silicon oxide insulating film 60 is insulated and deteriorates to cause a leakage current, the silicon nitride insulating film 150 suppresses it, so that a high voltage is applied. There is in point that can.

  However, in the cell structure of this example, the silicon nitride insulating film 150 that easily captures electrons is in contact with the space between the charge storage layers FG immediately above the semiconductor substrate. Therefore, when charge transfer is repeated between the charge storage layer FG and the semiconductor substrate S, a part of the charge is transferred to the interface of the silicon nitride insulating film 150 or in the film on the n-type impurity diffusion layer IDL at the stacked gate end. There is a risk that a trap level is trapped, whereby the electron density on the surface of the n-type impurity diffusion layer IDL is remarkably lowered, resulting in a problem that the cell channel current is greatly deteriorated. Further, when the gate width and the gate space are reduced in order to increase the memory density and capacity, there is a silicon nitride insulating film 150 having a high relative dielectric constant between the adjacent charge storage layers FG. The parasitic capacitance formed between the layers FG cannot be ignored, and there is a possibility that precise threshold control cannot be performed due to interference between adjacent cells.

  On the other hand, in the cell structure according to the first embodiment described above, the silicon nitride insulating film 40 exists immediately above the semiconductor substrate in the space portion between the charge storage layers FG, but is in contact with the semiconductor substrate S. Well separated. For this reason, even if charge transfer is repeatedly performed between the charge storage layer FG and the semiconductor substrate S, the charge reaches the interface of the silicon nitride insulating film 40 or the trap level in the film on the n-type impurity diffusion layer IDL at the end of the stacked gate. The situation of being captured does not occur. Therefore, there is no problem that the electron channel density on the surface of the n-type impurity diffusion layer IDL is lowered and the cell channel current is deteriorated. Further, the silicon nitride insulating film 40 is buried only up to half of the height of the charge storage layer FG, and its thickness is very thin, less than or equal to the film thickness of the inter-gate insulating film 20, so that the adjacent charge storage layer FG The parasitic capacitance between them can also be made very small. As a result, the spread of the threshold distribution due to the interference between adjacent cells can be sufficiently suppressed, and precise threshold control is possible.

(3) Second Embodiment FIGS. 7A to 7D are schematic plan views showing a memory structure of a nonvolatile semiconductor memory device according to a second embodiment of the present invention. In this embodiment, the cell structure is modified by changing the arrangement of the silicon nitride insulating film in the first embodiment described above. More specifically, as is clear from comparison with FIGS. 4A to 4D, the shape of the silicon nitride insulating film 42 is the same as that of the control gate CG in the silicon nitride insulating film 40 in the first embodiment. This corresponds to a structure in which the top surface is removed and the top surface of the control gate CG is exposed. Such a shape can be realized by forming a silicon nitride insulating film and then planarizing the upper portion of the control gate by a planarization process, for example, CMP (Chemical Mechanical Polishing) technique. The other memory structure of this embodiment is substantially the same as that of the first embodiment described above. In the present embodiment, the silicon nitride insulating film 42 corresponds to, for example, a fourth gate insulating film.

  The memory structure of the present embodiment is suitable when, for example, a salicide material is formed on the top surface of the control gate CG.

(4) Third Embodiment FIGS. 8A to 8D are schematic plan views showing a memory structure of a nonvolatile semiconductor memory device according to a third embodiment of the present invention. The feature of this embodiment is that a silicon nitride insulating film 44 is formed only on the surface of the control gate CG. Such a shape can be realized by a selective growth technique, a thermal nitriding technique, or the like when the material of the control gate CG is, for example, polycrystalline silicon. The other memory structure of this embodiment is substantially the same as that of the first embodiment described above. In the present embodiment, the silicon nitride insulating film 44 corresponds to, for example, a fourth gate insulating film.

  Regardless of the material of the control gate CG, by modifying the arrangement of the silicon nitride insulating film in this way, the same effects as those of the first embodiment described above can be obtained.

(5) Fourth Embodiment FIGS. 9A to 9D are schematic plan views showing a memory structure of a nonvolatile semiconductor memory device according to a fourth embodiment of the present invention. The first feature of the present embodiment is that the silicon nitride insulating film 46 is formed only on the side wall of the control gate CG, and no silicon nitride insulating material is embedded between the charge storage layers FG. Therefore, each stacked gate structure is embedded with a silicon oxide insulating material. Further, the second feature of the present embodiment is that the charge storage layer FG is vertically processed in a self-aligned manner so that the side wall of the charge storage layer FG immediately below the control gate CG is aligned with the side wall of the silicon nitride insulating film 46. There is in point. Except for these two points and the point that the laminated gate cap material 30 and the control gate CG are thinned by an amount corresponding to the thickness of the silicon nitride insulating film 46, the other memory structure of this embodiment is the same as that described above. This is substantially the same as the third embodiment. In the present embodiment, the silicon nitride insulating film 46 corresponds to, for example, a fourth gate insulating film.

  According to the structure of this embodiment, since the silicon nitride insulating film 46 exists on the side wall of the control gate CG, the same effect as that of the first embodiment can be obtained. Other excellent effects described in detail below can also be achieved.

  That is, generally, when the element region is reduced, the gate capacitance of the memory cell itself is reduced, while the space between the elements is reduced, so that the parasitic capacitance between the adjacent charge storage layers FG is increased. As a result, interference between adjacent cells increases and threshold control becomes very difficult. As a result, the threshold distribution after data writing is widened, so that there arises a problem that the multi-value and super-multi-value storage bits such as 4-value, 8-value, and 16-value are prevented. In order to solve this problem, it is necessary to increase the gate capacitance. For this purpose, the charge storage layer FG is formed thick, the height of the side wall of the charge storage layer FG is increased, and the space between the charge storage layer FG and the control gate CG is increased. It is important to increase the gate capacity. However, if the charge storage layer FG is thickened, the processing margin of the stacked gate structure is significantly reduced when the gate width and gate space are reduced. For this reason, it can be said that it is generally a good idea to reduce the height of the element isolation insulating film DI and widen the facing area between the charge storage layer FG and the control gate CG. However, if the height of the element isolation insulating film DI is lowered, the control gate CG on the element isolation insulating film DI and the element region at the side end thereof approach each other, so that a high voltage (for example, 20 V) is applied to the control gate CG during charge transfer. Is applied, a high electric field is applied between them. Since the distance between them is several tens of nanometers, the electric field reaches the 10 MV / cm level. Since there is an inter-gate insulating film 20 between the control gate CG and the element region at the end of the element isolation region immediately below the control gate CG, insulation deterioration due to application of a high electric field and charge trapping at the interface are suppressed. On the other hand, on the side of the control gate of a general memory cell structure, only the silicon oxide insulating film that insulates between the gates is formed on the element region. It was very weak, and there was a risk that cell characteristics would deteriorate if charge transfer was repeated.

  On the other hand, according to the memory cell structure according to the present embodiment, the silicon nitride insulating film 46 formed on the side wall of the control gate CG suppresses the insulation deterioration between the control gate CG and the element region AA. Can be suppressed. The memory structure shown in FIGS. 9A to 9D can also be formed by partially changing the vertical gate processing procedure. For example, after the control gate CG and the intergate insulating film 20 are vertically processed, a silicon nitride insulating film having a thickness equal to or less than the thickness of the intergate insulating film 20 is formed, and the silicon nitride insulating film is formed by dry etching technology. Are removed from the top surface of the control gate CG, and a silicon nitride insulating film 46 is formed only on the side wall of the control gate CG. Thereafter, a desired stacked gate structure can be formed by vertically processing the charge storage layer FG directly below, for example, a polycrystalline silicon material, using a gas having a high selectivity with respect to the silicon nitride material. .

(6) Fifth Embodiment FIGS. 10A to 10D are schematic plan views showing a memory structure of a nonvolatile semiconductor memory device according to a fifth embodiment of the present invention. The feature of this embodiment is that the silicon nitride insulating film 48 is not in contact with the control gate CG but is arranged near the center of the space between the adjacent control gates CG. Such a structure can be realized, for example, by first filling the stacked gate with a silicon oxide insulating material to form the silicon oxide insulating film 60 and then filling the gap with a silicon nitride insulating material. In the structures shown in FIGS. 10A to 10D, the silicon nitride insulating film can be removed from the upper portion of the control gate CG by a planarization process, as in the second embodiment described above. In the present embodiment, silicon nitride and silicon oxide correspond to, for example, the first and second insulating materials, respectively. The silicon nitride insulating film 48 and the silicon oxide insulating material 60 include, for example, the third gate insulating film and the second insulating material, respectively. Corresponds to 4 gate insulating film.

  In this embodiment, since the silicon nitride insulating film 48 is inserted not in the gate structure side but in the middle portion of the space between the gates, the electric field relaxation effect according to the above-described embodiment cannot be expected. On the other hand, since the silicon nitride insulating film also has a characteristic that electrons are easily trapped in the film, an effect of relaxing the leakage current injected from the control gate CG and suppressing insulation deterioration can be expected. Note that the effectiveness of either the method described above with reference to FIG. 1 to suppress insulation degradation by relaxing the inter-gate electric field or the method of this embodiment depends on the laminated gate structure, the type of insulating film, and the like. Note that it is not possible to compare the effects of both.

(7) Sixth Embodiment FIGS. 11A to 11D are schematic plan views showing a memory structure of a nonvolatile semiconductor memory device according to a sixth embodiment of the present invention. The feature of this embodiment is that both the electric field relaxation effect according to the first to fourth embodiments and the electron trapping effect according to the fifth embodiment described above can be obtained simultaneously. When attention is paid to the cutting line FF in FIG. 11A, it can be seen that the memory structure of this embodiment has an inter-gate insulating film structure composed of five layers. More specifically, a silicon nitride insulating film 40 that is in contact with adjacent control gates CG, and a silicon nitride insulating material 48 are disposed in the middle between the control gates CG, and a silicon oxide-based insulating material is interposed between them. An insulating film 60 is inserted. Accordingly, it can be expected that both the electric field relaxation effect on the gate structure side and the leakage current relaxation effect on the intermediate portion between the gates can be obtained simultaneously. In the present embodiment, silicon nitride corresponds to, for example, the first and third insulating materials, and silicon oxide corresponds to, for example, the second insulating material. The silicon oxide insulating material 60 corresponds to, for example, a third gate insulating film, and the silicon nitride insulating films 40 and 48 correspond to, for example, a fourth gate insulating film and a fifth gate insulating film, respectively.

  FIG. 12 schematically shows a solution for suppressing insulation degradation by relaxing the inter-gate electric field in the memory structure shown in FIGS. 11A to 11D. More specifically, stacked gate structures adjacent to each other are arranged on the right side and the left side of the drawing, and a five-layer intergate insulating film made of, for example, three kinds of insulating materials is arranged between the gates. The electric field relationship in each film in the structure is shown under the assumption that adjacent control gates CG are parallel plates. In FIG. 12, the control gate CG to which the write voltage (Vpgm) is applied is shown on the right side of the paper surface, and the charge storage layer FG just below the control gate CG to which the write prevention intermediate voltage (Vpass) is applied is shown on the left side of the paper surface. Has been. The insulating materials A of the intergate insulating films IF1a and IF1b in contact with the control gate CG and the charge storage layer FG are of the same type, the relative dielectric constant is eps_1, the film thickness is d_1, and the applied electric field strengths are E_5 and E_1, respectively. And Further, inter-gate insulating films IF2a and IF2b made of an insulating material B are inserted between the inter-gate insulating films IF1a and IF1, the relative dielectric constant is eps_2, the film thickness is d_2, and the applied electric field strength is Let them be E_4 and E_2, respectively. Further, an inter-gate insulating film IF3 made of an insulating material C is inserted between the inter-gate insulating films IF2a and IF2b. The relative dielectric constant is eps_3, the film thickness is d_3, and the applied electric field strength is E_3. Let the dielectric constant of the vacuum be eps_0. It is assumed that a negative charge having a volume density r is captured at the position x in the inter-gate insulating film IF3.

The electric field strength E_i (i = 1, 5) in each film is expressed by [Equation 3] E_1 = s / eps_1.
[Formula 4] E_2 = s / eps_2
[Formula 5] E_3 = s / eps_3 + ((ρ / eps_0) / (ρ / eps_3) (x−d_1−d_2)
[Expression 6] E_4 = (s + (ρ / eps_0) * d_3) / eps_2
[Expression 7] E_5 = (s + (ρ / eps_0) * d_3) / eps_1
It can be expressed. here,
[Equation 8] s = K * [V- (r / eps_0) * d_3 * (0.5 * d_3 / eps_3 + d_2 / eps_2 + d_1 / eps_1)]
[Expression 9] K = 1 / (2 * d_1 / eps_1 + 1 + 2 * d_2 / eps_2 + d_3 / eps_3)
It is. When negative charges are trapped in the inter-gate insulating film IF3, the electric field between the inter-gate insulating film IF3 and the charge storage layer FG immediately below the control gate CG to which the write preventing intermediate voltage Vpass is applied can be relaxed. . Even when a leakage current flows between adjacent gate structures, the electrons of the leakage current are captured by the inter-gate insulating film IF3 and the electric field is further relaxed. As a result, negative feedback that suppresses the leakage current is applied. This makes it possible to suppress insulation deterioration. Here, the first-layer intergate insulating films IF1a and IF1b are, for example, silicon nitride films (eps_1 = 7.5, d_1 = 2 nm), and the second-layer intergate insulating films IF2a, IF2b are, for example, silicon oxide films (Eps_2 = 2.3.9, d_2 = 2 nm), the third-layer intergate insulating film IF3 is, for example, a silicon nitride film (eps_3 = 7.5, d_1 = 2 nm), and an intermediate voltage Vpass for write prevention is applied. Assuming that the potential V _FG of the charge storage layer FG immediately below the control gate CG is 4 V and the withstand voltage is 10 MV / cm for convenience, the maximum voltage that can be applied between adjacent gate structures is 17.7 V. When a negative charge of 1 × 10 ²⁰ cm ^{−3 is} arranged in the three layers, the voltage that can be applied is improved to 20.0V.

(8) Others While some of the embodiments of the present invention have been described above, the present invention is by no means limited to the above embodiments, and it goes without saying that various modifications can be applied within the technical scope thereof. For example, in the above-described embodiment, the case where only the single layer of the silicon nitride films 40, 42, 44, and 46 is taken up as the fourth gate insulating film formed so as to be in contact with the gate structure is limited. Alternatively, the fourth gate insulating film may be formed of a multilayer insulating film including an insulating film other than the silicon nitride film.

10: Tunnel oxide film (first gate insulating film)
20: Inter-gate insulating film (second gate insulating film)
40, 42, 44, 46: fourth gate insulating film 48: fifth gate insulating film 60: third gate insulating film AA: element region CG: control gate DI: element isolation insulating film FG: charge storage layer IDL: n-type Impurity diffusion layer S: Semiconductor substrate

Claims (4)

A plurality of element regions provided extending in a first direction on the semiconductor substrate;
A charge storage layer provided on the semiconductor substrate so as to extend in a second direction intersecting the first direction and formed on the semiconductor substrate via a first gate insulating film; and the charge storage layer A plurality of gate structures each including a second gate insulating film formed on the first gate insulating film and a control gate formed on the second gate insulating film;
An insulating film formed selectively on a surface layer of the semiconductor substrate, defining an element region and electrically isolating the element region; and
An impurity diffusion layer formed in the element region so as to sandwich a surface layer of the semiconductor substrate directly under the gate structure;
A third gate insulating film formed of a first insulating material between the gate structures;
A fourth gate insulating film formed of a second insulating material having a dielectric constant lower than that of the first insulating material so as to contact a sidewall of the gate structure and the semiconductor substrate between the gate structures; ,
A non-volatile semiconductor memory device.   The nonvolatile semiconductor memory device according to claim 1, wherein the first insulating material is silicon nitride, and the second insulating material is silicon oxide. A plurality of element regions provided extending in a first direction on the semiconductor substrate;
A charge storage layer provided on the semiconductor substrate so as to extend in a second direction intersecting the first direction and formed on the semiconductor substrate via a first gate insulating film; and the charge storage layer A plurality of gate structures each including a second gate insulating film formed on the first gate insulating film and a control gate formed on the second gate insulating film;
An insulating film formed selectively on a surface layer of the semiconductor substrate, defining an element region and electrically isolating the element region; and
An impurity diffusion layer formed in the element region so as to sandwich a surface layer of the semiconductor substrate directly under the gate structure;
A third gate insulating film formed of a first insulating material so as to be embedded between the gate structures;
A fourth gate insulating film formed of a second insulating material different from the first insulating material so as to be in contact with the side wall of the gate structure;
A fifth gate insulating film formed of a third insulating material different from the first insulating material in the third gate insulating film;
With
The nonvolatile semiconductor memory device, wherein the second and third insulating materials have a dielectric constant higher than that of the first insulating material.   Both sides of the fifth gate insulating film are in contact with the third gate insulating film, the second and third insulating materials are silicon nitride, and the first insulating material is silicon oxide. The nonvolatile semiconductor memory device according to claim 3.

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