US20090250746A1

US20090250746A1 - NOR-Type Flash Memory Cell Array and Method for Manufacturing the Same

Info

Publication number: US20090250746A1
Application number: US12/487,566
Authority: US
Inventors: Heong Jin Kim
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-12-26
Filing date: 2009-06-18
Publication date: 2009-10-08
Also published as: US7563676B2; CN100499081C; CN1992235A; KR100654559B1; US20070148830A1

Abstract

Disclosed is a non-volatile (e.g., NOR type flash) memory cell array and a method for manufacturing the same. The memory cell array includes a plurality of isolation layers on a semiconductor substrate, parallel to a bit line and defining an active device area, a plurality of common source areas in the semiconductor substrate, separated from each other by the isolation layers such that the common source areas connect memory cells adjacent to each other in a bit line direction, a common source line on the semiconductor substrate, connected to each source area and extending in a word-line direction, an insulating spacer along a first sidewall of the common source line, a gate at a second sidewall of the insulating spacer including a tunnel oxide layer, a first electrode, an inter-electrode dielectric layer, and a second electrode, and a drain area in the semiconductor substrate on an opposite side of the gate from the common source area.

Description

This application is a divisional of U.S. patent application Ser. No. 11/646,088 (Attorney Docket No. OPP-GZ-2007-0437-US-00), filed on Dec. 26, 2006, which claims the benefit of Korean Application No. 10-2005-0129789, filed on Dec. 26, 2005, each of which is incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to a nonvolatile (e.g., flash) memory device. More specifically, the present invention relates to a NOR-type flash memory cell array structure and a method for manufacturing the same.
2. Description of the Related Art
A flash memory is a kind of PROM (programmable ROM) capable of electrically re-writing data. The flash memory can perform a program input scheme of an erasable PROM (EPROM) and an erase scheme of an electrically erasable PROM (EEPROM) using one transistor by combining the advantages of an EPROM, in which a memory cell includes one transistor so that a cell area is small, but data must be erased at a time by UV rays, and the EEPROM, in which data can be electrically erased, but the cell may include two transistors so that a cell area becomes large. Flash memory may also be known as flash EEPROM. Such a flash memory is referred to as a nonvolatile memory since stored information is not erased even though power is turned off, which is different from a dynamic RAM (DRAM) or a static RAM (SRAM).
Flash memory may be classified into a NOR-type structure in which cells are arranged in parallel between a bit line and a ground and a NAND-type structure in which cells are serially arranged between the bit line and the ground. Since the NOR-type flash memory having the parallel structure can perform high-speed random access when a reading operation is performed, the NOR-type flash memory is widely used for booting a mobile telephone. The NAND-type flash memory having the serial structure has low reading speed but high writing speed so that the NAND-type flash memory is suitable for storing data and is advantageous for miniaturization.
In addition, the flash memory can be classified into a stack gate type and a split gate type in accordance with the structure of a unit cell, and can be similarly classified into a floating gate device and a silicon-oxide-nitride-oxide-silicon (SONOS) device in accordance with the shape and/or materials of a charge storage layer. Among them, the floating gate device includes floating gates having polycrystalline silicon and being surrounded by an insulating substance. Charges are implanted into or discharged from the floating gates by channel hot carrier injection or Fowler-Nordheim (F-N) tunneling so that data can be stored and erased.
Meanwhile, in the procedure of manufacturing the NOR type flash memory device, a cell threshold voltage is adjusted, and a stack gate including a floating gate, an inter-gate insulating layer (e.g., Oxide-Nitride-Oxide) and a control gate is formed. In addition, a common source line is formed through a self-aligned source (SAS) process. The SAS technique is used for reducing a cell size in a word-line direction. According to SAS technique, a common source line is formed through a dopant implantation process after etching a field oxide layer on the basis of etching selectivity among a polysilicon layer for a gate electrode, a silicon substrate, and a field oxide layer.
Recently, as the high integration and the high speed of a device are accelerated, it is necessary to design a smaller memory cell and a resistor having lower resistance. However, a manufacturing process of a conventional NOR-type cell structure may have some problems. For example, when an SAS process is performed in order to form a common source line, the unevenness or topology of an SAS line can cause an increase in resistance. This becomes a factor of reducing an operational speed of the device. In addition, the limitation of a photo process makes it difficult to control a line width of a control gate, so voids may be created due to the lack of a gap-fill margin when a polysilicon metal dielectric (PMD) layer is deposited on a drain area. Accordingly, when a tungsten plug is formed in the drain area, there may occur a short between a control gate and a bit line.

SUMMARY OF THE INVENTION

The present invention has been made to solve the above problem(s), and therefore, it is an object of the present invention to provide a method for manufacturing a highly integrated nonvolatile (e.g., NOR type flash) memory device without employing an SAS technique.
It is another object of the present invention to provide a nonvolatile (e.g., NOR type flash) memory device and a method for manufacturing the same, in which a control gate forming a stacked (or control) gate is self-aligned with a floating gate.
In order to accomplish the objects according to the present invention, there is provided a method for manufacturing a nonvolatile (e.g., NOR type flash) memory cell array, the method comprising the steps of: (a) forming a plurality of isolation layers, separated from each other by a predetermined distance and parallel to each other, on a semiconductor substrate, (b) forming a plurality of lines (or line patterns) orthogonal to the isolation layers and separated from each other by a predetermined distance, in which the lines include a tunnel oxide layer, a first electrode layer, an inter-electrode dielectric layer, and a first capping layer, (c) forming an insulating spacer on a sidewall of each line, (d) forming a source area by implanting a first dopant into an upper part of the semiconductor substrate between the lines, (e) selectively removing the first capping layer from the lines, (f) forming a common source line on the source area and between neighboring insulating spacers while forming a plurality of second electrodes separated from each other by a predetermined distance on the inter-electrode dielectric layers, (g) forming a plurality of gates opposite to each other by removing a portion of the line exposed between the second electrodes, and (h) forming a drain area by implanting a second dopant into an upper part of the semiconductor substrate between the gates.
According to another aspect of the present invention, there is provided a non-volatile (e.g., NOR type flash) memory cell array including a plurality of isolation layers on a semiconductor substrate and parallel to a bit line and defining an active device area, a plurality of common source areas in the semiconductor substrate and separated from each other by the isolation layers such that the common source areas connect memory cells adjacent to each other in a bit line direction, a common source line on the semiconductor substrate, connected to each source area and extending in a word-line direction, an insulating spacer along a first sidewall of the common source line, a gate at a second sidewall of the insulating spacer and comprising a tunnel oxide layer, a first electrode, an inter-electrode dielectric layer, and a second electrode, and a drain area in the semiconductor substrate on an opposite side or the gate from the common source area.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1 to 8 are sectional views showing an active device area vertical to a word line to illustrate the manufacturing process for a NOR-type flash memory cell array in the order of process sequence according to the present invention; and

FIG. 9 is a sectional view showing an active device area vertical to a word line to illustrate the structure of a NOR-type flash memory cell according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, a NOR type flash memory cell array and a method for manufacturing the same according to a preferred embodiment of the present invention will be described with reference to FIGS. 1 to 9.
The method for manufacturing a NOR-type flash memory cell array according to the present invention will be described in detail with reference to FIGS. 1 to 8. FIGS. 1 to 8 show sections of an active device area vertical to a word line.
First, a plurality of isolation layers (not shown), which are separated from each other by a predetermined distance (e.g., a predetermined distance apart), are formed on a semiconductor substrate 10 by employing a shallow trench isolation (STI) technique. The isolation layers are parallel to each other in a bit-line direction, and define an active device area. Then, a well is formed in the active device area of the semiconductor substrate. For example, in a case of a P type substrate, a deep N well is formed, and then a pocket P well is formed. Thereafter, a cell threshold voltage is determined, adjusted or set through an implant process.
Then, as shown in FIG. 1, a tunnel oxide layer 12 and a first electrode layer 14 are formed in the active device area of the substrate 10. The first electrode layer 14 includes polysilicon doped with impurities. Next, a portion of the first electrode layer 14 formed on the isolation layer is removed, thereby forming a pattern of lines or structures parallel to the isolation layer. The first electrode layer pattern structures are spaced apart from each other in a word-line direction by a predetermined distance and are arranged in parallel to each other in a bit-line direction.
Next, an inter-electrode dielectric layer 16 and a first capping layer 18 are sequentially formed on the entire surface of the substrate 10. The inter-electrode dielectric layer 16 may comprise a single oxide layer or an oxide-nitride-oxide (ONO) dielectric layer, and the first capping layer 18 includes a silicon oxide layer (which may be the same as or different from the single oxide layer when the inter-electrode dielectric layer 16 includes only a single oxide layer). Thereafter, dopants (e.g., As) are implanted into the entire surface of the first capping layer 18 through an implant process, at least when first capping layer 18 comprises an oxide having a similar etch rate as an (upper) oxide layer of the inter-electrode dielectric layer 16. Since the first capping layer into which the dopants are implanted has an etching rate higher than that of other oxide layers formed on the substrate 10, one can selectively remove the first capping layer.
Then, as shown in FIG. 2, a tunnel oxide layer 12, the first electrode layer 14, the inter-electrode dielectric layer 16, and the capping layer 18 are partially removed by a predetermined width in a direction vertical to the isolation layer (that is, in a word-line direction). Through this patterning process, the first electrode layer 14 is divided into a plurality of patterns or lines separated from each other by a predetermined distance in a word-line direction, and the divided pieces of the first electrode layer may be patterned again through a following process, thereby finally forming a floating gate. When the patterning process as shown in FIG. 2 has been completed, a plurality of stacks including the tunnel oxide layer 12, the first electrode layer 14, the inter-electrode dielectric layer 16, and the first capping layer 18 are formed. Hereinafter, these stacks may be referred to as “line patterns”.
After forming the line pattern, an insulating layer is formed on the entire surface of the substrate 10, and then an etch back or anisotropic etch process is performed, thereby forming an insulating spacer 20 a. The insulating spacer 20 a is thereby formed at sidewalls of the line patterns. In addition, if the first capping layer includes a silicon oxide layer, it is preferred that the insulating spacer 20 a includes a silicon nitride layer.
Thereafter, as shown in FIG. 3, dopants are implanted into an upper part of the substrate 10 between the line patterns, that is, between the insulating spacers 20 a, thereby forming a diffusion area. This diffusion area serves as a common source (S).
Next, referring to FIG. 4, the first capping layer 18 formed on an upper part of the line pattern is selectively removed. In this case, since the first capping layer 18 has a relatively high etching rate (e.g., as a result of the dopant implantation process described above or an ion implantation process for forming the source area S), the first capping layer is etched prior to other oxide layers. Accordingly, if an etching time is adjusted, only the first capping layer 18 may be selectively removed.
As shown in FIG. 5, a conductive material 22 is formed on the upper parts of the line patterns (that is, the upper parts of the inter-electrode dielectric layers), in which the first capping layer 18 is selectively removed. At the same time, a gap formed between two neighboring insulating spacers 20 a is filled with the conductive material 22. It is preferred that the conductive material 22 includes polysilicon doped with impurities. Thereafter, an etch back or anisotropic etch process is performed with respect to the conductive material 22, thereby forming a second electrode layer 22 a having a spacer shape and a common source line 22 b shown in FIG. 6. In this case, the etch back process is performed until an upper part of the insulating spacer 20 a is exposed between the common source line 22 b and two second electrode layers 22 a adjacent to the common source line 22 b. Therefore, the second electrode layer 22 a is electrically insulated from the common source line 22 b by the exposed spacer 20 a.
Next, a second capping layer 24 a and 24 b may be formed on upper parts of a pair of two electrode layers 22 a opposite to each other and an upper part of the common source line 22 b, respectively. The second capping layers 24 a and 24 b may be formed by thermally oxidizing the polysilicon of second electrodes 22 a and common source line 22 b. Thereafter, if a portion of both the inter-electrode dielectric layer 16 and the first electrode layer 14 between the second electrode layers 22 a formed within the spacers 20 a is removed through a photolithography process and an etching process, a pair of stacked gates opposite to each other is formed as shown in FIG. 7. These gates include a tunnel oxide layer 12 a, a first electrode layer 14 a, an inter-electrode dielectric layer 16 a, and the second electrode layer 22 a, in sequence from the substrate. The first electrode 14 a and the second electrode 22 a serve as a floating gate and a control gate, respectively.
Then, as shown in FIG. 8, a sidewall insulating layer 26 for insulating the first electrode 14 a is formed at sidewalls of the gates. In addition, if dopants are implanted into an upper part of the substrate 10 exposed between a pair of the gates, so a drain area D is formed, a flash memory cell array is completely formed.
FIG. 9 is a sectional view showing a device having a dielectric layer (PMD) 28, a drain contact 30, and a metal interconnection 32 formed on the flash memory cell array structure according to the present invention.
Referring to FIG. 9, a common source area S, which connects neighboring memory cells in a row, is linked to the common source lines 22 b formed on the substrate 10. The common source line 22 b has sidewalls formed with the insulating spacer 20 a while extending in a word-line direction. In addition, an opposite sidewall of the insulating spacer 20 a, which does not make contact with the common source line 22 b, is provided with a stacked gate in which the tunnel oxide layer 12 a, the first electrode 14 a, the inter-electrode dielectric layer 16 a, and the second electrode 22 a are sequentially formed from the substrate.
In this case, a sidewall of the second electrode 22 a aligned in a bit-line direction is self-aligned with a sidewall of the first electrode 14 a. Accordingly, it is possible to minimize the variation of a line width of a control gate. In particular, since the second electrode 22 a has the shape of a spacer, it is possible to prevent voids from being created when the dielectric layer 28 is formed, and it is possible to effectively prevent a bit line bridge from occurring between the control gate and a drain contact. In addition, since the margin space for forming the drain contact 30 can be sufficiently ensured within a given area, high integration of the cell can be advantageously achieved.
Meanwhile, different from a common source line formed through a conventional SAS process, the common source line 22 b formed together with the second electrode 22 a is provided on the substrate 10, so it is easy to control the profile of the common source line 22 b. In addition, since the common source line 22 b includes a conductive material having a relatively high thickness and a height identical to that of the second electrode 22 a, electrical resistance becomes reduced, so it is possible to realize a high-speed device.
As described above, according to the present invention, not only is a common source line formed without employing an SAS technique, but also the common source line is formed by using a material identical to the material forming the control electrode, so it is possible to provide a high-integrated and high-speed nonvolatile (e.g., NOR type flash) memory device.
Further, in a flash memory device according to the present invention, the control gate is self-aligned with the floating gate, so it is possible to sufficiently ensure a process margin for a drain contact.
While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.

Claims

1. A non-volatile memory cell array comprising:

a plurality of isolation layers on a semiconductor substrate and parallel to a bit line, the isolation layers defining an active device area;

a plurality of common source areas in the semiconductor substrate and separated from each other by the isolation layers such that the common source areas connect two memory cells adjacent to each other in a bit line direction;

a common source line on the semiconductor substrate, connected to each source area and extending in a word-line direction;

an insulating spacer along a sidewall of the common source line;

a gate at a sidewall of the insulating spacer, comprising a tunnel oxide layer, a first electrode, an inter-electrode dielectric layer, and a second electrode; and

a drain area in the semiconductor substrate on an opposite side of the stack gate from the common source area.

2. The cell array as claimed in claim 1, wherein the first electrode is separated from a memory cell adjacent to the first electrode in a word-line direction by the isolation layer.

3. The cell array as claimed in claim 1, wherein the second electrode extends in a word-line direction and a sidewall of the second electrode self-aligned with a sidewall of the first electrode in a bit-line direction.

4. The cell array as claimed in claim 1, wherein the insulating spacer electrically insulates the common source line from the gate.

5. The cell array as claimed in claim 1, wherein the common source line and the second electrode include a same conductive material.

6. The cell array as claimed in claim 1, wherein the second electrode has a spacer shape.

7. The cell array as claimed in claim 1, wherein the semiconductor substrate includes a P-type dopant.

8. The cell array as claimed in claim 1, wherein the inter-electrode dielectric layer comprises an oxide layer.

9. The cell array as claimed in claim 8, wherein the inter-electrode dielectric layer consists essentially of the oxide layer.

10. The cell array as claimed in claim 1, wherein the inter-electrode dielectric layer comprises an oxide-nitride-oxide dielectric layer.

11. The cell array as claimed in claim 1, wherein the common source line comprises polysilicon.

12. The cell array as claimed in claim 11, wherein the polysilicon is doped with at least one impurity.

13. The cell array as claimed in claim 1, wherein the first electrode is a floating gate and the second electrode is a control gate.

14. The cell array as claimed in claim 1, wherein the common source line comprises a conductive material having a height equal to a height of the second electrode.

15. The cell array as claimed in claim 1, further comprising a first capping layer on upper parts of the second electrode and a second capping layer on an upper part of the common source line.

16. The cell array as claimed in claim 15, further comprising a dielectric layer on the substrate.

17. The cell array as claimed in claim 16, further comprising a drain contact in the dielectric layer, wherein the drain contact is connected to the drain area.

18. The cell array as claimed in claim 17, further comprising a metal interconnection on the dielectric layer, wherein the metal interconnection is connected to the drain contact.