MANUFACTURING METHOD OF SEMICONDUCTOR MEMORY DEVICE

Abstract

A manufacturing method of a semiconductor memory device is provided. The semiconductor memory device can suppress current leakage generated during a programming action so that the programming action can be executed with high reliability. A flash memory of this invention has a memory array in which NAND type strings are formed. Gates of memory cells in row direction of strings are commonly connected to a word line. Gates of bit line select transistors are commonly connected to a select gate line (SGD). Gates of source line select transistors are commonly connected to a select gate line (SGS). An interval (S4) of the select gate line (SGS) and a gate of a word line (WL0) adjacent to the select gate line (SGS) is larger than an interval (S1) of the select gate line (SGD) and a gate of a word line (WL7) adjacent to the select gate line (SGD).

Claims

1. A manufacturing method of a semiconductor memory device, wherein the semiconductor memory device comprises a memory array having a NAND type string, and the string comprises a plurality of memory cells connected in series, the manufacturing method of the semiconductor memory device comprises: a step of forming a plurality of photoresist patterns on a processed film constituting a gate of the memory cell; a step of forming an insulation layer on an entire surface containing the photoresist patterns; a step of forming a photoresist layer on the insulation layer; a step of etching the photoresist layer in a manner of remaining the photoresist layer on sidewalls at two sides of the insulation layer; a step of removing the insulation layer; a step of using the photoresist patterns and the remained photoresist layer as a mask to etch the processed film to form the plurality of gates; and a step of removing the photoresist patterns and the remained photoresist layer, wherein the memory array has an asymmetric structure in which a first interval is greater than a second interval, the first interval is an interval between a gate of a source line select transistor and the gate of the memory cell adjacent thereto, and the second interval is an interval between a gate of a bit line select transistor and the gate of the memory cell adjacent thereto.

2. The manufacturing method of the semiconductor memory device according to claim 1, further comprising a step of removing a part of the gates.

3. The manufacturing method of the semiconductor memory device according to claim 1, wherein the photoresist layer is formed on the entire surface with a fixed film thickness, and the photoresist layer is then etched back for flattening the photoresist layer remained on sidewalls at two sides of the insulation layer.

4. The manufacturing method of the semiconductor memory device according to claim 1, wherein the insulation layer is removed by an anisotropic etching with selectivities.

5. The manufacturing method of the semiconductor memory device according to claim 1, wherein the first interval is greater than a third interval between the adjacent two gates of the memory cells in the same string.

6. The manufacturing method of the semiconductor memory device according to claim 5, wherein the first interval is a double of the second interval, and the second interval is equal to the third interval.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0019] FIG. 1 is a schematic circuit diagram illustrating a structure of NAND strings of a flash memory.

[0020] FIG. 2 is a two-dimensional diagram illustrating a schematic layout of a memory array of the flash memory in conventional art.

[0021] FIG. 3 is a schematic cross-sectional view of FIG. 2 along a line A-A.

[0022] FIG. 4 is a diagram illustrating current leakage of the non-selected memory cell during a programming action of the flash memory in conventional art.

[0023] FIG. 5 is a block diagram illustrating a structure of a flash memory according to an exemplary embodiment of the invention.

[0024] FIG. 6 is a two-dimensional diagram illustrating a schematic layout of a memory array of the flash memory according to an embodiment of the invention.

[0025] FIG. 7 is a schematic cross-sectional view of FIG. 6 along a line A-A.

[0026] FIG. 8 is a flowchart illustrating the programming action according to a second embodiment of the invention.

[0027] FIG. 9A to FIG. 9E are diagrams illustrating a manufacturing method of an asymmetric memory array according to a third embodiment of the invention.

[0028] FIG. 10A to FIG. 10E are diagrams illustrating a manufacturing method of an asymmetric memory array according to a fourth embodiment of the invention.

[0029] FIG. 11A to FIG. 11D are diagrams illustrating another manufacturing method of the asymmetric memory array according to the fourth embodiment of the invention.

DESCRIPTION OF THE EMBODIMENTS

[0030] Embodiments of the invention are described in detail below by reference with accompanied figures. In addition, it is worth to note that parts of the accompanied figures are exaggerated to emphasize on certain features to make it more apparent for understanding, and the proportion thereof is different from the real devices.

[0031] FIG. 5 is a block diagram illustrating a structure of a NAND type flash memory according to an exemplary embodiment of the invention. A flash memory 100 of the present embodiment includes: a memory array 110 in which a plurality of memory cells arranged in rows and columns are formed; an input/output buffer 120, connected to an external input/output terminal I/O, and retaining input/output data; an address register 130, receiving address data from the input/output buffer 120; a data register 140, retaining input/output data; a controller 150, providing control signals C1, C2 and C3, and the control signals C1, C2 and C3 controlling each part based on command data from the input/output buffer 120 and an external control signal (e.g., a chip enable or an address latch enable, which are not illustrated); a word line select circuit 160, performing a decoding operation on row address information Ax from the address register 130, and performing a block selection and a word line selection based on a decoding result; a page buffer/sense circuit 170, retaining data read through the bit line, or retaining programming data through the bit line; a column select circuit 180, performing a decoding operation on column address information Ay from the address register 130, and performing a bit line selection based on a decoding result; and an internal voltage generating circuit 190, generating voltages required for reading, programming and erasing the data (e.g., a programming voltage Vprog, a pass voltage Vpass, a reading voltage Vread, an erasing voltage Vers, etc.).

[0032] The memory array 110 has a plurality of blocks BLK(0), BLK(1), . . . , BLK(m) which are disposed along column direction. The page buffer/sense circuit 170 is disposed on one end portion of the blocks. Nonetheless, the page buffer/sense circuit 170 may also be disposed on another end portion of the blocks, or on both end portions at two sides. In one memory block, as shown in the FIG. 1, a plurality of NAND strings NU is formed. The memory cell typically includes a MOS (Metal Oxide Semiconductor) structure, and the MOS structure includes: source/drain, served as a N-type diffusion region and formed inside a P-well; a tunneling oxide film, formed on a channel between the source/drain; a floating gate (a charge accumulation layer), formed on the tunneling oxide film; and a control gate, formed on the floating gate with a dielectric film interposed therebetween. When the charges are not accumulated in the floating gate (i.e., writing data 1), a threshold is in a negative state, and the memory cells are normally on. When the electrons are accumulated in the floating gate (i.e., writing data 0), the threshold shifts to a positive state, and the memory cells are normally off.

[0033] FIG. 6 is a schematic layout of a memory array according to the present embodiment, and FIG. 7 is a cross-sectional view of FIG. 6 along a line A-A. Voltages applied to each part during operations of the flash memory according to an embodiment of the invention are listed in Table 1 below.

TABLE-US-00001 TABLE 1 Erasing Writing Reading Selected W/L 0 15~20 V Vx Non-selected W/L F .sup.10 V 4.5 SGD F Vcc 4.5 SGS F 0 4.5 SL F Vcc~ 0 P-well 21 0 0

[0034] The memory array of the present embodiment is as shown in FIG. 6, and the word line WL0 adjacent to the select gate line SGS of the source line select transistor is substantially far away from the select gate line SGS. That is, an interval S4 between a gate of the source line select transistor and the floating gate of the memory cell MC0 connected to the word line WL0 is greater than the interval S2 shown in FIG. 2 of conventional art (S4>S2). It is preferable that S4=2S1 and S1=S3.

[0035] During a programming action, when the word line WL0 adjacent to the select gate line SGS is the selected page, the programming voltage is applied to the word line WL0. A situation of the non-selected memory cell MC0 at the time is shown in FIG. 7. 0 V is provided to the selected bit line and 1.5 V is provided to the non-selected bit line for example, and a voltage being, for example, 1.5 V or greater than 1.5 V is applied to the select gate line SGD of the bit line select transistor. Accordingly, the channel potential of the selected memory cell is 0 V, and the potential of the channel 10 of the non-selected memory cell MC0 boosts to 7 V to 8 V.

[0036] Because the potential of the N+ diffusion region 16 also boosts together with the channel 10, the depletion region(s) is/are formed near the N+ diffusion region 16. 0 V is applied to the select gate line SGS, so that almost none of the depletion regions is formed in the channel 20 under the select gate line SGS. Therefore, the electrons passed through the channel 20 enters the diffusion region 16 having high voltage. However, because the width of the diffusion region 16 (i.e., the interval S4 of the select gated line SGS and the floating gate of the memory cell MC0) is sufficiently large, the electrons entered the diffusion region 16 may lose energy during a traveling process in the diffusion region 16, and thus the electrons cannot easily penetrate the gate insulation film.

[0037] As described above, according to the present embodiment, by making the memory cell adjacent to the source line select transistor far away from the source line select transistor, an injection of the electrons towards the floating gate 12 of the non-selected memory cell MC0 caused by GIDL during the programming action may be suppressed, such that the undesired variation on the threshold of the non-selected memory cell MC0 may also be suppressed.

[0038] Next, a second embodiment of the invention is described below. In the first embodiment, a method for suppressing the variation on the threshold of the memory cell at the source select transistor side caused by GIDL is illustrated. On the other hand, the second embodiment relates to a method for suppressing the variation on the threshold of the memory cell at the bit line select transistor side caused by GIDL.

[0039] It is required that a voltage VSGD applied to the select gate line SGD is large enough to turn on the bit line select transistor. For example, when a voltage provided to the non-selected bit line is Vb, the voltage VSGD is a value greater than Vb+Vth (Vth is a threshold of the bit line select transistor).

[0040] In the present embodiment, by making the voltage VSGD applied to the select gate line SGD of the bit line select transistor greater than the previous value, the inversion layer and the depletion region may be sufficiently formed at the channel 30 under the select gate line SGD. Accordingly, it is difficult for the electrons in a P-well region 22 to pass through the channel 30 and enter into a diffusion region 32. As a result, the variation occurred on the threshold of the non-selected memory cell MC7 is suppressed.

[0041] In a preferred implementation of the second embodiment, when the word line WL7 adjacent to the select gate line SGD is the selected page, the voltage VSGD of the select gate line SGD may also be enlarged. FIG. 8 illustrates a flowchart for performing aforesaid action. When a programming command is inputted from an external host (S100), the controller 150 starts a sequence of initiating the programming action. Subsequently, when programming data and address data are inputted from the external host (S102), the controller 150 determines whether a page to be programmed is equivalent to a page adjacent to the bit line select transistor (the word line WL7 in the example of FIG. 7) (S104). If so, the word line select transistor 160 is controlled in a manner of the voltage of the select gate line SGD (VSGD)=V1 (S106); and if not, the word line select transistor 160 is controlled in a manner of the voltage VSGD=V2 (S108). Herein, a relation of the voltages is V1>V2 (e.g., V1=3.0 V and V2=1.5 V).

[0042] Accordingly, the undesired variation on the threshold of the non-selected memory cell at the bit line select transistor side caused by GIDL can be suppressed.

[0043] Next, a manufacturing method of a memory array with an asymmetric structure according to a third embodiment of the invention is illustrated in FIG. 9A to FIG. 9E. These cross-sectional views are schematic cross-sectional views in a bit line direction shown in FIG. 6 (the NAND strings direction). In a preferred implementation, the memory array of the present embodiment is manufactured by using a double patterning technique.

[0044] First, as shown in FIG. 9A, a hard mask 210 and an underlayer film 220 is formed on a processed layer 200, and photoresist patterns PR are formed on the underlayer film 220. The photoresist patterns PR have a width W.sub.PR and are formed with a fixed interval L. Herein, a preferable condition is that, for example, a ratio of the width W.sub.PR to the interval L is 1:3. The hard mask 210 and the underlayer film 220 are constituted by, for example, insulation films such as SiO.sub.2 or Si.sub.3N.sub.4. In other implementations, the underlayer film 220 may be constituted by a material identical to the hard mask 210. Herein, the processed layer 200 constitutes the gates (the floating gate and the control gate) of the memory cells, and is, for example, constituted by a ploy-silicon layer. In addition, although it is not illustrated, the gate of the memory cell may be manufactured together with gates of the source line select transistor and a drain line select transistor at the same time.

[0045] Next, after an insulation film (e.g., SiO.sub.2 with a fixed film thickness) is formed on the underlayer film 220 and the photoresist patterns PR, the insulation film is flattened by an etch back process. Accordingly, as shown in FIG. 9B, spacer layers SP are formed on sidewalls at two sides of the photoresist patterns PR. Specifically, after the insulation film (e.g., SiO.sub.2 with the fixed film thickness) is formed on the underlayer film 220 and the photoresist patterns PR, the insulation film is flattened by using, for example, an anisotropic etching method such as a dry etching (e.g., an etching method in which an etching rate in an up and down direction of FIG. 9A is greater than an etching rate in a left and right direction) to perform an etch back process, and then the insulation film formed on the sidewalls of the photoresist patterns PR are remained to form the spacer layers SP. A width W.sub.SP of the spacer layer SP may be properly selected according to etching conditions. Herein, a preferable condition is, for example, a ratio of the width W.sub.PR to the width W.sub.SP is 1:1.

[0046] Next, as shown in FIG. 9C, after the photoresist patterns PR are removed through methods such as ashing, the spacers SP may then be used as a mask to etch the underlayer film 220 and the hard mask 210. Because the spacer layers SP are also partially etched while etching the underlayer film 220 and the hard mask 210, a width W.sub.SP of the etched spacer layer SP is less than the original width W.sub.SP. Similarly, after using the spacer layers SP as the mask for etching the underlayer film 220 and the hard mask 210, widths of the etched underlayer film 220 and the etched hard mask 210 are correspondingly less than the width W.sub.SP of the original spacer layer and approximately equal to the width W.sub.SP of the etched spacer layer.

[0047] Subsequently, as shown in 9(D), an etchant that has poor etching rate for the hard mask 210 and the processed layer 200 is used to remove the spacer layers SP and the underlayer film 220, so as to obtain the patterned hard mask 210, and the hard mask 210 has a width W.sub.HM. In the present embodiment, the width W.sub.HM equal to the width W.sub.SP is slightly less than the width W.sub.SP, and an interval L between the hard masks 210 is slightly greater than the interval L between the spacer layers shown in FIG. 9B. In other words, by using the etching conditions to adjust the width W.sub.PR of the photoresist patterns PR, the intervals L and L, the widths W.sub.SP and W.sub.SP of the spacer layers SP and the like, the width W.sub.HM of the hard mask 210 may be set to, for example, W.sub.HM=W.sub.PR or any other values.

[0048] Finally, as shown in FIG. 9E, the hard mask 210 is removed to obtain the patterned processed layer 200. The processed layer 200 has a width W. In the present embodiment, the width W equal to the width W.sub.HM, such that the width W is slightly less than the width W.sub.SP, and an interval L between the hard masks 210 is slightly greater than the interval L between the spacer layers shown in FIG. 9B.

[0049] As described above, according to the present embodiment, a gate structure of the memory cell with short channel may be obtained by using the double patterning technique, such that the memory cell with a line width that exceeds a resolution limit in a photolithography step may be formed. Therefore, the memory array integrated in high density may be formed and an area occupied by the memory array may be reduced.

[0050] In an embodiment, the spacer layers SP and the underlayer film 220 are removed to expose the hard mask 210 after the step shown in FIG. 9C, but the invention is not limited thereto. In the state of FIG. 9C, the spacer layers SP may also be used together as the mask for etching the processed layer 200.

[0051] Further, as described above, the memory array of the present embodiment has the asymmetric structure. The following methods may be used when forming the asymmetric structure by using the double patterning technique. In the double patterning technique, the spacer layers SP formed on sidewalls at two sides of photoresist patterns may be used to form the hard mask. Therefore, an amount of the hard mask eventually formed is a multiple of 2. Accordingly, as shown in FIG. 6, in the circumstance where one NAND string includes eight memory cells (eight word lines WL0 to WL7), nine hard masks may be formed by using the double patterning technique. Then, one hard mask at one end portion side is removed. As such, the interval S4 of the word line WL0 and the select gate line SGS may be sufficiently larger than the interval S1 of the word line WL7 and the select gate line SGD.

[0052] Further, in other methods which are different from the above, eight hard masks may be formed by using the double patterning technique, but a positioning is further performed in a manner of the interval S4 of the hard mask formed on one end portion and the select gate line SGS being greater than the interval S1 of the hard mask formed on another end portion and the select gate line SGD.

[0053] Next, a manufacturing method of a memory array with an asymmetric structure according to a fourth embodiment of the invention is illustrated in FIG. 10A to FIG. 10E. As shown in FIG. 10A, a processed layer 310 is formed on an underlayer film 300, and photoresist patterns PR are formed on the processed layer 310 through the photolithography step. Next, after an insulation film 320 with a fixed film thickness is formed on the entire surface as shown in FIG. 10B, the insulation film 320 is proceeded with an etch back process to form the spacer layers SP on sidewalls at two sides of the photoresist patterns PR as shown in FIG. 10C. Next, as shown in FIG. 10D, the photoresist patterns PR are removed through methods such as ashing so that the spacer layers SP are formed. Next, as shown in FIG. 10E, the spacer layers SP are used as a mask to etch the processed layer 310.

[0054] As such, according to the present embodiment, mask patterns for the spacer layers SP may be formed in self-alignment by using one photolithography step. Accordingly, by properly adjusting the width W.sub.PR and the interval L of the photoresist patterns PR and the width W of the spacer layers SP (which can be adjusted through adjustment for a growing film thickness of the insulation film), the mask pattern for the spacer layer SP having an interval smaller than the interval (spacing) of the photoresist patterns PR (e.g., the interval) may be formed accordingly.

[0055] Next, a modification of the fourth embodiment is illustrated in FIG. 11A to FIG. 11D. After the steps of FIG. 10A and FIG. 10B are performed, photoresist patterns 330 with a fixed film thickness are formed on the entire surface as shown in FIG. 11A. Next, as shown in FIG. 11B, the photoresist patterns 330 are etched back for flattening the photoresist patterns 330 remained on sidewalls at two sides of the insulation film 320. Next, as shown in FIG. 11C, the exposed insulation film 320 is removed by an anisotropic etching with selectivities. Accordingly, the photoresist patterns 330 are formed between the photoresist patterns PR. Next, as shown in FIG. 11D, the photoresist patterns PR, 330 are used for etching the processed layer 310. Thereafter, the photoresist patterns PR, 330 are removed by method such as ashing. As a result, a pattern inverted from the pattern of FIG. 10E is formed.

[0056] In the fourth embodiments, the memory cell array with narrow interval and short channel as similar to that of the third embodiment may also be formed. Furthermore, as similar to the third embodiment, the interval S4 of the word line WL0 and the select gate line SGS is made to be greater than the interval S1 of the word line WL7 and the select gate line SGD. As such, one hard mask needlessly formed on one end portion may be removed, or the positioning may be performed in a manner of a location where the hard mask is formed on one end portion being away from the select gate line SGS by an interval of S4.

[0057] The preferable embodiments of the invention had been described in detail above, but the invention is not limited to a specific embodiment. It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention.

[0058] Although the embodiments illustrate the relation of the intervals being S1=S3 and S4=2S1, such example is only illustrative and the invention is not limited to such relation. Moreover, although the embodiments illustrate that the example of eight NAND strings in one block and one string having eight memory cells, such example is only illustrative and the invention is not limited thereto.