METHOD OF FABRICATING SEMICONDUCTOR DEVICE
A method of fabricating a semiconductor device includes forming a gate insulating film on a semiconductor substrate, forming a first conductive layer on the gate insulating film, forming an intergate insulating film on the first conductive layer, forming a second conductive layer on the intergate insulating film, dividing the conductive layers and the intergate insulating film with a mask pattern formed on the second conductive layer, thereby forming a plurality of gate electrodes, forming a first recess on a first side wall of the first conductive layer and a second recess on a second side wall of the second conductive layer with side surfaces of the gate electrodes being exposed, and burying insulating films between the gate electrodes respectively and forming air gap portions respectively in portions of the buried insulating films corresponding to the recesses.
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This application is a divisional of and claims the benefit of priority from U.S. Ser. No. 12/405,457, filed on Mar. 17, 2009, which claims the benefit of priority from the prior Japanese Patent Application No. 2008-151625, filed on Jun. 10, 2008, the entire contents of each of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention relates to a method of fabricating a semiconductor device which is configured to have an air gap defined between adjacent memory cells, and the semiconductor device fabricated by the method.
2. Description of the Related Art
A flash memory can maintain stored data even without power supply thereto and have accordingly been widely used as a memory element for a multimedia card. The memory capacity of the flash memory has recently been desired to be increased. For this purpose, high integration of memory cells is further necessitated. Technical problems posed by high integration of memory cells include an increase in a capacitance between cells adjacent to each other. A silicon oxide is buried between gate electrodes of adjacent memory cells as an insulator in an ordinary structure of the memory element. The silicon oxide can be formed easily and has a dielectric constant that is about half of that of a silicon nitride, so that the silicon oxide can reduce a capacitance between adjacent memory cells. However, a capacitance between adjacent memory cells is increased in inverse proportion to a distance therebetween as an interval of memory cells is reduced. As a result, an operating speed of the memory element is reduced and a malfunction (in data write and/or readout) occurs.
In view of the above-described problem, the conventional art has suggested a configuration in which nothing is formed between gate electrodes of adjacent memory cells, that is, a configuration in which an air gap with a minimum dielectric constant is defined, in order that the capacity between adjacent memory cells may be reduced (see Japanese patent application publication JP-A-2007-157927). An air gap has conventionally been formed by covering an upper part of each gate electrode when a silicon oxide film is buried between the gate electrodes. However, it has been difficult to form an air gap controllably by the conventional method.
BRIEF SUMMARY OF THE INVENTIONAccording to one aspect of the present invention, there is provided a method of fabricating a semiconductor device, comprising forming a gate insulating film on a semiconductor substrate, forming a first conductive layer, as a floating gate electrode, on the gate insulating film so that a vertically intermediate portion of the first conductive layer has a higher dopant concentration than vertically upper and lower portions of the first conductive layer, forming an intergate insulating film on the first conductive layer, forming a second conductive layer, as a control gate electrode, on the intergate insulating film so that a vertically intermediate portion of the second conductive layer has a higher dopant concentration than vertically upper and lower portions of the second conductive layer, dividing the first and second conductive layers and the intergate insulating film with a mask pattern formed on the second conductive layer, thereby forming a plurality of gate electrodes, forming a first recess on a first side wall of the first conductive layer and a second recess on a second side wall of the second conductive layer with side surfaces of the gate electrodes being exposed, and burying insulating films between the gate electrodes respectively and forming air gap portions respectively in portions of the buried insulating films corresponding to the first and the second recesses respectively.
According to another aspect of the present invention, there is provided a method of fabricating a semiconductor device, comprising forming a gate insulating film on a semiconductor substrate, forming a first conductive layer on the gate insulating film by forming a first undoped amorphous silicon layer on the gate insulating film, forming a first amorphous silicon layer doped with phosphor on the first undoped amorphous silicon layer and forming a second undoped amorphous silicon layer on the first amorphous silicon layer, forming an intergate insulating film on the first conductive layer, forming a second conductive layer on the intergate insulating film by forming a third undoped amorphous silicon layer on the intergate insulating film, forming a second amorphous silicon layer doped with phosphor on the third undoped amorphous silicon layer and forming a fourth undoped amorphous silicon layer on the second amorphous silicon layer, dividing the first and second conductive layers and the intergate insulating film with a mask pattern formed on the second conductive layer, thereby forming a plurality of gate electrodes, forming a first recess on a first side wall of the first conductive layer and a second recess on a second side wall of the second conductive layer with side surfaces of the gate electrodes being exposed, and burying insulating films between the gate electrodes respectively and forming air gap portions respectively in portions of the buried insulating films corresponding to the first and the second recesses.
In the accompanying drawings,
One embodiment of the present invention will be described with reference to the accompanying drawings. The invention is applied to a NAND flash memory in the embodiment. In the following description, identical or similar parts are labeled by the same reference numerals. The drawings typically illustrate the invention, and the relationship between a thickness and plane dimension, layer thickness ratio and the like differ from respective natural dimensions.
Firstly, an electrical arrangement of the NAND flash memory 1 will be described.
Bit line contacts CB are connected to drain regions of the selective gate transistors Trs1. The bit line contacts CB are connected to bit lines BL extending in the Y direction (corresponding to a direction of gate length or the bit lines) perpendicular to the X direction in
A plurality of word lines WL are formed so as to intersect the element regions Sa and so as to be spaced from each other in the Y direction. Furthermore, a pair of selective gate lines SGL1 of the selective gate transistors are formed so as to extend in the X direction in
Each intergate insulating film 5 serves both as an insulating film between the floating gate electrode FG and the control gate electrode CG and as an inter-conductive layer of the polycrystalline silicon layer 4 and the metal silicide layer 6. Each intergate insulating film 5 comprises, for example, a deposition of silicon oxide film-silicon nitride film-silicon oxide film (ONO). Alternatively, each intergate insulating film 5 may comprise a high dielectric film such as alumina. Furthermore, a low-concentrated impurity diffusion layer (not shown) is formed as a source/drain region in a part of a surface layer located between the gate electrodes MG of each memory cell transistor Trm. A silicon oxide film 7 is formed as an interelectrode insulating film on the part of the silicon substrate 2 located between the gate electrodes MG. Furthermore, a cavity or void where nothing is formed, or an air gap is formed in a part of the silicon oxide film 7 located between the gate electrodes MG. An interlayer insulating film (not shown) is formed on the aforesaid silicon oxide film 7. A silicon nitride film (not shown) is formed as a barrier film on the interlayer insulating film. Furthermore, another interlayer insulating film (not shown) is formed on the aforesaid silicon nitride film.
A method of fabricating the foregoing configuration will now be described with reference to
Firstly, a thermal oxidation process is applied to the silicon substrate 2 so that the silicon oxide film 3 is formed, as shown in
Each of the polycrystalline silicon layers 4 and 9 comprises the undoped amorphous silicon layer interposed between the upper and lower doped amorphous silicon layers. In other words, each polycrystalline silicon layer is composed of upper and lower layers each of which comprises the undoped amorphous silicon layer and an intermediate layer comprising the P-doped amorphous silicon layer. The amorphous silicon layers 9a, 9b and 9c may be formed continuously by the use of the same processing equipment or with suspension for every layer. Furthermore, the amorphous silicon layers 9a, 9b and 9c may be formed by different equipments. When the forming of each layer is suspended or carried out by a plurality of different equipments, a native oxide can be formed in a boundary between adjacent layers. As a result, uniform diffusion of phosphor in the three layers can be suppressed when subsequent steps include a high-temperature process. The film thicknesses of the layers can be adjusted to respective desired values. The polycrystalline silicon layer 4 for the floating gate electrodes FG also comprises an undoped amorphous silicon layer, a P-doped amorphous silicon layer and an undoped amorphous silicon layer in the same manner as the polycrystalline silicon layer 9.
Subsequently, a silicon nitride film 10 is formed on the polycrystalline silicon layer 9 (amorphous silicon layers 9a, 9b and 9c). The silicon nitride film 10 serves as a hard mask for forming the polycrystalline silicon layer 4 (see
Subsequently, a resist (not shown) is applied to the silicon nitride film 10 to be patterned by the photolithography process. The silicon nitride film 10 is then processed by a dry etching process (a reactive ion etching (RIE) method) as shown in
Subsequently, a thermal oxidation process is executed while both sides of each gate electrode MG (sides of each word line) are exposed. More specifically, the thermal oxidation process is executed at a temperature ranging from 950 to 1100° C. for a time period ranging 20 to 60 seconds under the atmosphere of oxygen. The amorphous silicon layers 4 and 9 are polycrystallized in the aforesaid process. Furthermore, the sides of the amorphous silicon layers 4 and 9 are oxidated in the aforesaid process. A degree of oxidation progress differs depending upon difference in the dopant (P) concentration in the oxidation. The amorphous silicon layers 4 and 9 include vertically intermediate portions having the predetermined dopant (P) concentrations respectively. The intermediate portions (regions designated by reference symbols 4d and 9d in
Subsequently, the silicon oxide films formed on the sidewalls of the floating and control gate electrodes FG and CG are selectively removed by an etching process with the use of the chemical solution, as shown in
Subsequently, the silicon oxide films 7 are formed between the adjacent gate electrodes MG (that is, the word lines) by the LPCVD method as shown in
Subsequently, as shown in
According to the foregoing embodiment, the forming of the recesses 11 and 12 on the sides of the adjacent gate electrodes MG reliably results in the void in which no silicon oxide film 7 is present between the adjacent gate electrodes MG or the air gap in which no silicon oxide film 7 is buried. Consequently, the capacity between the adjacent memory cells between the gate electrodes MG can effectively be suppressed. Accordingly, a failure in data write or data readout can be prevented when data is written onto or read out of the memory cell. Particularly in the foregoing embodiment, the air gaps 8 having different depths, horizontal size and the like can be formed by suitably adjusting the P concentration profiles of the amorphous silicon layers 9 and 4.
The present invention should not be limited by the foregoing embodiment. The embodiment may be modified or expanded as follows. In the foregoing embodiment, the difference in the phosphor (P) concentration is provided between the amorphous silicon layers 4 and 9 when the recesses 11 and 12 are formed in the sides of each word line WL. The formed silicon oxide film is removed by the etching process with the use of the chemical solution while the difference in the oxidation speeds based on the difference in phosphor concentration is utilized, whereupon the recesses 11 and 12 are formed. However, the phosphor concentration difference may be provided between the amorphous silicon layers 4 and 9, and the recesses 11 and 12 may be formed by utilizing the difference in dry etching speeds based on the phosphor concentration difference.
More specifically, the steps up to the provision of the P concentration difference as shown in
Additionally, the invention should not be limited to the NAND flash memory but may be applied to NOR, AND and other types of nonvolatile semiconductor devices.
The foregoing description and drawings are merely illustrative of the principles of the present invention and are not to be construed in a limiting sense. Various changes and modifications will become apparent to those of ordinary skill in the art. All such changes and modifications are seen to fall within the scope of the invention as defined by the appended claims.
Claims
1. A method of fabricating a semiconductor device, comprising:
- forming a gate insulating film on a semiconductor substrate;
- forming a first conductive layer, as a floating gate electrode, on the gate insulating film so that a vertically intermediate portion of the first conductive layer has a higher dopant concentration than vertically upper and lower portions of the first conductive layer;
- forming an intergate insulating film on the first conductive layer;
- forming a second conductive layer, as a control gate electrode, on the intergate insulating film so that a vertically intermediate portion of the second conductive layer has a higher dopant concentration than vertically upper and lower portions of the second conductive layer;
- dividing the first and second conductive layers and the intergate insulating film with a mask pattern formed on the second conductive layer, thereby forming a plurality of gate electrodes;
- forming a first recess on a first side wall of the first conductive layer and a second recess on a second side wall of the second conductive layer with side surfaces of the gate electrodes being exposed; and
- burying insulating films between the gate electrodes respectively and forming air gap portions respectively in portions of the buried insulating films corresponding to the first and the second recesses.
2. The method according to claim 1, wherein the first conductive layer includes a first polycrystalline silicon layer and the second conductive layer includes a second polycrystalline silicon layer.
3. The method according to claim 2, wherein the dopant introduced into the first and second polycrystalline silicon layers is phosphor (P).
4. The method according to claim 1, wherein the first recess is formed from an upper edge portion to a lower edge portion of the first side wall and the second recess is formed from an upper edge portion to a lower edge portion of the second side wall.
5. The method according to claim 1, wherein forming the first and the second recesses includes forming a first oxide film on the first side wall and a second oxide film on the second side wall by a thermal oxidation treatment, and selectively removing the first and the second oxide films with use of a chemical solution.
6. The method according to claim 5, wherein the first oxide film is formed from an upper edge portion to a lower edge portion of the first side wall and the second oxide film is formed from an upper edge portion to a lower edge portion of the second side wall.
7. The method according to claim 6, wherein a thickness of a middle portion between the upper and the lower edge portions of the first oxide film is larger than a thickness of the upper and the lower edge portions of the first oxide film and a thickness of a middle portion between the upper and the lower edge portions of the second oxide film is larger than a thickness of the upper and the lower edge portions of the second oxide film.
8. The method according to claim 1, wherein forming the first and the second recesses includes etching the first and the second side walls by reactive ion etching (RIE) method.
9. A method of fabricating a semiconductor device, comprising:
- forming a gate insulating film on a semiconductor substrate;
- forming a first conductive layer on the gate insulating film by forming a first undoped amorphous silicon layer on the gate insulating film, forming a first amorphous silicon layer doped with phosphor on the first undoped amorphous silicon layer and forming a second undoped amorphous silicon layer on the first amorphous silicon layer;
- forming an intergate insulating film on the first conductive layer;
- forming a second conductive layer on the intergate insulating film by forming a third undoped amorphous silicon layer on the intergate insulating film, forming a second amorphous silicon layer doped with phosphor on the third undoped amorphous silicon layer and forming a fourth undoped amorphous silicon layer on the second amorphous silicon layer;
- dividing the first and second conductive layers and the intergate insulating film with a mask pattern formed on the second conductive layer, thereby forming a plurality of gate electrodes;
- forming a first recess on a first side wall of the first conductive layer and a second recess on a second side wall of the second conductive layer with side surfaces of the gate electrodes being exposed; and
- burying insulating films between the gate electrodes respectively and forming air gap portions respectively in portions of the buried insulating films corresponding to the first and the second recesses.
10. The method according to claim 9, wherein the first recess is formed from an upper edge portion to a lower edge portion of the first side wall and the second recess is formed from an upper edge portion to a lower edge portion of the second side wall.
11. The method according to claim 9, wherein forming the first and the second recesses includes forming a first oxide film on the first side wall and a second oxide film on the second side wall by a thermal oxidation treatment, and selectively removing the first and the second oxide films with use of a chemical solution.
12. The method according to claim 11, wherein the first oxide film is formed from an upper edge portion to a lower edge portion of the first side wall and the second oxide film is formed from an upper edge portion to a lower edge portion of the second side wall.
13. The method according to claim 12, wherein a thickness of a middle portion between the upper and the lower edge portions of the first oxide film is larger than a thickness of the upper and the lower edge portions of the first oxide film and a thickness of a middle portion between the upper and the lower edge portions of the second oxide film is larger than a thickness of the upper and the lower edge portions of the second oxide film.
14. The method according to claim 9, wherein forming the first and the second recesses includes etching the first and the second side walls by reactive ion etching (RIE) method.
Type: Application
Filed: Jan 6, 2011
Publication Date: May 5, 2011
Applicant: KABUSHIKI KAISHA TOSHIBA (Tokyo)
Inventor: Hajime NAGANO (Yokkaichi)
Application Number: 12/985,771
International Classification: H01L 21/336 (20060101);