Source: http://www.google.com/patents/US6429072?dq=6,304,975
Timestamp: 2016-07-29 12:38:58
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Patent US6429072 - Method of forming a floating gate memory cell structure - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign inPatentsThe present invention provides a method of forming a floating gate memory cell structure. The method comprising the following steps. A dummy pattern is selectively formed on a predetermined region of a semiconductor substrate. Source and drain regions are selectively formed by use of a self-alignment...http://www.google.com/patents/US6429072?utm_source=gb-gplus-sharePatent US6429072 - Method of forming a floating gate memory cell structureAdvanced Patent SearchPublication numberUS6429072 B1Publication typeGrantApplication numberUS 09/332,109Publication dateAug 6, 2002Filing dateJun 14, 1999Priority dateJun 12, 1998Fee statusLapsedAlso published asCN1111905C, CN1239325A, US20020063276Publication number09332109, 332109, US 6429072 B1, US 6429072B1, US-B1-6429072, US6429072 B1, US6429072B1InventorsMasaru TsukijiOriginal AssigneeNec CorporationExport CitationBiBTeX, EndNote, RefManPatent Citations (6), Referenced by (11), Classifications (14), Legal Events (8) External Links: USPTO, USPTO Assignment, EspacenetMethod of forming a floating gate memory cell structure
US 6429072 B1Abstract
The present invention provides a method of forming a floating gate memory cell structure. The method comprising the following steps. A dummy pattern is selectively formed on a predetermined region of a semiconductor substrate. Source and drain regions are selectively formed by use of a self-alignment technique using the dummy pattern as a mask. Conductive films are selectively formed on the source and drain regions so that the conductive films sandwich the dummy pattern in a lateral direction. The dummy pattern is removed so that a channel region defined between the source and drain regions is shown. A first single insulation film is unitary formed, which extends on the channel region and also on inside walls and top surfaces of the conductive films. A single floating gate electrode film is unitary formed on the first single insulation film, thereby laminating a single pair of the first single insulation film unitary formed and the single floating gate electrode film unitary formed. A second insulation film is formed, which extends on side walls and a top surface of the single floating gate electrode film. A control electrode is formed, which extends on the second insulation film so that the control electrode is separated by the second insulation film from the single floating gate electrode film.
What is claimed is: 1. A method of forming a floating gate electrode structure, said method comprising the steps of:
selectively forming a dummy pattern on a predetermined region of a semiconductor substrate; selectively forming source and drain regions by use of a self-alignment technique using said dummy pattern as a mask; selectively forming conductive films on said source and drain regions so that said conductive films sandwich said dummy pattern in a lateral direction; removing said dummy pattern to expose a channel region defined in said substrate between said source and drain regions; unitarily forming a first single insulation film directly on said channel region and also on said conductive films; and unitarily forming a single floating gate electrode film on said single insulation film, thereby laminating a single pair of said first single insulation film unitarily formed and said single floating gate electrode film unitarily formed. 2. The method as claimed in claim 1, wherein said first single insulation film is made by a single dry oxidation process to both said channel region and said conductive films.
3. The method as claimed in claim 1, further comprising the steps of forming a silicon oxide thin film on said semiconductor substrate before said dummy pattern is selectively formed on said silicon oxide thin film, and forming said dummy pattern of silicon nitride.
4. A method of forming a floating gate memory cell structure, said method comprising the steps of:
selectively forming a dummy pattern on a predetermined region of a semiconductor substrate; selectively forming source and drain regions by use of a self-alignment technique using said dummy pattern as a mask; selectively forming conductive films on said source and drain regions so that said conductive films sandwich said dummy pattern in a lateral direction; removing said dummy pattern to expose a channel region defined in said substrate between said source and drain regions; unitarily forming a first single insulation film directly on said channel region and also on said conductive films; unitarily forming a single floating gate electrode film on said first single insulation film, thereby laminating a single pair of said first single insulation film unitarily formed and said single floating gate electrode film unitarily formed; forming a second insulation film extending on side walls and a top surface of said single floating gate electrode film; and forming a control electrode extending on said second insulation film so that said control electrode is separated by said second insulation film from said single floating gate electrode film. 5. The method as claimed in claim 4, wherein said first single insulation film is made by a single dry oxidation process to both said channel region and said conductive films.
6. The method as claimed in claim 4, further comprising the steps of forming a silicon oxide thin film on said semiconductor substrate before said dummy pattern is selectively formed on said silicon oxide thin film, and forming said dummy pattern of silicon nitride.
7. The method of claim 1, wherein said first single insulation film has a generally uniform thickness.
8. The method of claim 4, wherein said first single insulation film has a generally uniform thickness.
9. The method of claim 1, wherein said dummy pattern is completely removed from the channel region during the removing step.
10. The method of claim 4, wherein said dummy pattern is completely removed from the channel region during the removing step.
The present invention relates to a method of forming a semiconductor memory cell structure having a floating gate.
A conventional structure of a flash memory will be described. FIG. 1A is a fragmentary plan view illustrative of a conventional flash memory. FIG. 1B is a fragmentary cross sectional elevation view illustrative of a conventional flash memory taken along an A—A′ line of FIG. 1A. The flash memory has an alignment of memory cells 241 which has the following structure. A floating gate 203 is formed over a gate insulation film 202 over a semiconductor substrate 201. The floating gate 203 is T-shaped which comprises an upper portion and a lower portion, wherein the upper portion laterally extends from the opposite edges of a lower portion to form the T-shaped floating gate 203. The structure of the memory cell 241 will be described in detail with reference to FIG. 1B.
Field oxide films 206 are selectively formed on a semiconductor substrate 201 to define an active region or a device region of the semiconductor substrate 201. Source and drain regions 204 and 205 are selectively formed in the active region or the device region of the semiconductor substrate 201, whereby a channel region is defined between the source and drain regions 204 and 205. A gate insulation film 202 is formed on the channel region of the semiconductor substrate 201. Source side and drain side interconnections 204 a and 205 a are formed on the source and drain regions 204 and 205, wherein the source side and drain side interconnections 204 a and 205 a are made of polysilicon films doped with an impurity. In the memory cell 241, the source and drain 204 and 205 serve as parts of bit lines. The formation of the source side and drain side interconnections 204 a and 205 a on and in contact with the source and drain regions 204 and 205 results in reduction in resistance of the source and drain 204 and 205. A floating gate 203 is formed which comprises an lower part and an upper part. The upper part extends laterally from the opposite edges of the lower part so that the floating gate 203 is T-shaped. The lower part of the floating gate 203 is positioned on the gate insulation film 202. Side wall oxide films 213 are formed on opposite side walls of the lower part of the floating gate 203. Insulation films 216 are formed over the field oxide films 206 and the source side and drain side interconnections 204 a and 205 a as well as over the side wall oxide films 213. The upper portion of the floating gate 203 extends over the lower part thereof and the insulation films 216, so that opposite edges of the upper portion of the floating gate 203 are almost aligned to the edges of the source side and drain side interconnections 204 a and 205 a in plane view. An insulation film 207 made of ONO is formed on the upper portion of the floating gate 203 and on the insulation films 216. A control gate 208 is formed on the insulation film 207 so that the floating gate 203 is separated and electrically floated from the control gate 208 and whereby the floating gate 203 is completely surrounded by the insulation materials to be floated in the memory cell. The control gate 208 serves as a part of a word line in the memory. The above T-shape of the floating gate increases a capacitance thereof.
The source and drain 204 and 205 are commonly used for a plurality of the memory cells 241. The drain 05 is used as a part of the bit line so that a single bit contact is formed for the plural memory cells 241 so as to allow narrowing the distance between the memory cells, thereby reducing the cell size.
The following descriptions will focus onto the fabrication method of the above flash memory. FIGS. 2A through 2H are fragmentary cross sectional elevation views illustrative of a conventional method of forming a memory cell structure over a semiconductor substrate of a flash memory shown in FIGS. 1A and 1B.
With reference to FIG. 2A, a thin insulation film 202 a is formed on a surface of a semiconductor substrate 201. A bottom floating gate part 203 a is selectively formed on a predetermined region of the thin insulation film 202 a, wherein a film of a electrode material such as an impurity doped polysilicon is entirely formed on the surface of the semiconductor substrate 201 before patterning the same to form the bottom floating gate part 203 a. The bottom floating gate part 203 a is used as a mask for carrying out an ion-implantation into selected upper regions of the semiconductor substrate 201, whereby source and drain regions 204 and 205 are formed with a self-alignment technique in the selected upper regions of the semiconductor substrate 201.
With reference to FIG. 2B, the thin insulation film 202 a is removed except for its underlying part which underlies the bottom floating gate part 203 a, whereby the remaining underlying part of the thin insulation film 202 a serves as a gate insulation film 202. An insulation film is entirely formed on the source and drain regions 204 and 205 as well as on opposite side walls and a top surface of the bottom floating gate part 203 a before an etch back process using a dry etching technique so that the insulation film remain only on the opposite side walls of the bottom floating gate part 203 a, whereby the side wall insulation films 213 are formed on the opposite side walls of the bottom floating gate part 203 a. With reference to FIG. 2C, an impurity doped polysilicon film is entirely deposited over the source and drain regions 204 and 205 and the side wall insulation films 213 and the bottom floating gate part 203 a. An etch back process is carried out to the impurity doped polysilicon film until the top of the bottom floating gate part 203 a is shown, whereby conductive films 214 are formed on the source and drain regions 204 and 205 and positioned outside the side wall insulation films 213.
With reference to FIG. 2D, parts of the conductive films 214 on isolation regions are selectively removed. Grooves are further formed on the isolation region in the semiconductor substrate 201. An insulation material such as silicon oxide is buried within the grooves, whereby field oxide films 206 are then formed on the isolation regions. As a result, outside edges of the conductive films 214 are defined by inside boundaries of the field oxide films 206, whereby source side and drain side interconnections 204 a and 205 a are formed.
With reference to FIG. 2E, an insulation film 216 is entirely formed over the field oxide films 206, the source side and drain side interconnections 204 a and 205 a and the bottom floating gate part 203 a. With reference to FIG. 2F, an overlying part of the insulation film 216 overlying the bottom floating gate part 203 a is selectively removed by patterning the insulation film 216 with use of a photo-lithography and a subsequent dry etching technique, whereby the top of the bottom floating gate part 203 a is shown.
With reference to FIG. 2G, an impurity doped polysilicon film is entirely formed which extends over the insulation films 216 and the bottom floating gate part 203 a. The impurity doped polysilicon film is then patterned with a photo-lithography and a subsequent dry etching technique, whereby a top floating gate part 203 b is formed which extends over the top of the bottom floating gate part 203 a and inside parts of the insulation films 216 so that opposite edges of the top floating gate part 203 b are aligned to outside edges of the source side and drain side interconnections 204 a and 205 a. As a result, a combination of the top floating gate part 203 b and the bottom floating gate part 203 a forms a T-shaped floating gate 203.
With reference to FIG. 2H, an insulation film 207 made of ONO is formed on the top floating gate part 203 b of the floating gate 203 and on the insulation films 216. A control gate 208 is then formed on the insulation film 207 so that the floating gate 203 is separated and electrically floated from the control gate 208 and whereby the floating gate 203 is completely surrounded by the insulation materials to be floated in the memory cell.
In accordance with the above conventional fabrication processes, the floating gate comprises two parts, for example, the top floating gate part 203 b and the bottom floating gate part 203 a which are formed in the different steps, whereby the number of the fabrication processes are increased.
In the above circumstances, it had been required to develop a novel method of fabricating a T-shaped floating gate electrode of a memory cell in a flash memory with reduced number of the fabrication process for the T-shaped floating gate electrode.
Accordingly, it is an object of the present invention to provide a novel method of fabricating a floating gate electrode of a memory cell in a flash memory with reduced number of the fabrication process for the T-shaped floating gate electrode.
It is another object of the present invention to provide a novel floating gate electrode of a memory cell in a flash memory.
The present invention provides a floating gate memory cell structure comprising: a single pair of a first single insulation film unitary formed and a single floating gate electrode film unitary formed and laminated on the first single insulation film, the first single insulation film extending on a channel region of a semiconductor substrate and also on inside walls and top surfaces of conductive films over source and drain regions in the semiconductor substrate; a second insulation film extending on side walls and a top surface of the single floating gate electrode film; and a control electrode extending on the second insulation film so that the control electrode is separated by the second insulation film from the single floating gate electrode film.
The present invention also provides a method of forming a floating gate memory cell structure. The method comprising the following steps. A dummy pattern is selectively formed on a predetermined region of a semiconductor substrate. Source and drain regions are selectively formed by use of a self-alignment technique using the dummy pattern as a mask. Conductive films are selectively formed on the source and drain regions so that the conductive films sandwich the dummy pattern in a lateral direction. The dummy pattern is removed so that a channel region defined between the source and drain regions is shown. A first single insulation film is unitary formed, which extends on the channel region and also on inside walls and top surfaces of the conductive films. A single floating gate electrode film is unitary formed on the first single insulation film, thereby laminating a single pair of the first single insulation film unitary formed and the single floating gate electrode film unitary formed. A second insulation film is formed, which extends on side walls and a top surface of the single floating gate electrode film. A control electrode is formed, which extends on the second insulation film so that the control electrode is separated by the second insulation film from the single floating gate electrode film.
FIG. 1A is a fragmentary plan view illustrative of a conventional flash memory.
FIG. 1B is a fragmentary cross sectional elevation view illustrative of a conventional flash memory taken along an A—A′ line of FIG. 1A.
FIGS. 2A through 2H are fragmentary cross sectional elevation views illustrative of a conventional method of forming a memory cell structure over a semiconductor substrate of a flash memory shown in FIGS. 1A and 1B.
FIG. 3A is a fragmentary plan view illustrative of a flash memory in a first embodiment in accordance with the present invention.
FIG. 3B is a fragmentary cross sectional elevation view illustrative of the flash memory taken along an A—A′ line of FIG. 3A in a first embodiment in accordance with the present invention.
FIGS. 4A through 4H are fragmentary cross sectional elevation views illustrative of a novel method of forming a memory cell structure over a semiconductor substrate of a flash memory shown in FIGS. 3A and 3B.
FIGS. 5A through 5G are fragmentary cross sectional elevation views illustrative of another novel method of forming a memory cell structure over a semiconductor substrate of a f lash memory shown in FIGS. 3A and 3B in a fourth embodiment in accordance with the present invention.
The first aspect of the present invention provides a floating gate electrode structure comprising: a single floating gate electrode film unitary formed and laminated on a first single insulation film being unitary formed and extending on a channel region of a semiconductor substrate and also on inside walls and top surfaces of conductive films over source and drain regions in the semiconductor substrate.
It is preferable that the first single insulation film has a thin portion and thick portions united with the thin portion, and the thin portion is positioned on the channel region and the thick portions are positioned on the inside walls and the top surfaces of the conductive films
It is also preferable that the single floating gate electrode film has an almost uniform thickness.
The second aspect of the present invention provides a floating gate memory cell structure comprising: a single pair of a first single insulation film unitary formed and a single floating gate electrode film unitary formed and laminated on the first single insulation film, the first single insulation film extending on a channel region of a semiconductor substrate and also on inside walls and top surfaces of conductive films over source and drain regions in the semiconductor substrate; a second insulation film extending on side walls and a top surface of the single floating gate electrode film; and a control electrode extending on the second insulation film so that the control electrode is separated by the second insulation film from the single floating gate electrode film.
It is preferable that the first single insulation film has a thin portion and thick portions united with the thin portion, and the thin portion is positioned on the channel region and the thick portions are positioned on the inside walls and the top surfaces of the conductive films.
The third aspect of the present invention provides a method of forming a floating gate memory cell structure. The method comprising the following steps. A dummy pattern is selectively formed on a predetermined region of a semiconductor substrate. Source and drain regions are selectively formed by use of a self-alignment technique using the dummy pattern as a mask. Conductive films are selectively formed on the source and drain regions so that the conductive films sandwich the dummy pattern in a lateral direction. The dummy pattern is removed so that a channel region defined between the source and drain regions is shown. A first single insulation film is unitary formed, which extends on the channel region and also on inside walls and top surfaces of the conductive films. A single floating gate electrode film is unitary formed on the first single insulation film, thereby laminating a single pair of the first single insulation film unitary formed and the single floating gate electrode film unitary formed.
It is preferable that the first single insulation film is made by a single dry oxidation process to both the channel region and the conductive films.
It is preferable to further comprise a silicon oxide thin film on the semiconductor substrate before the dummy pattern is selectively formed on the silicon oxide thin film, provided that the dummy pattern is made of silicon nitride.
The fourth aspect of the present invention provides a method of forming a floating gate memory cell structure. The method comprising the following steps. A dummy pattern is selectively formed on a predetermined region of a semiconductor substrate. Source and drain regions are selectively formed by use of a self-alignment technique using the dummy pattern as a mask. Conductive films are selectively formed on the source and drain regions so that the conductive films sandwich the dummy pattern in a lateral direction. The dummy pattern is removed so that a channel region defined between the source and drain regions is shown. A first single insulation film is unitary formed, which extends on the channel region and also on inside walls and top surfaces of the conductive films. A single floating gate electrode film is unitary formed on the first single insulation film, thereby laminating a single pair of the first single insulation film unitary formed and the single floating gate electrode film unitary formed. A second insulation film is formed, which extends on side walls and a top surface of the single floating gate electrode film. A control electrode is formed, which extends on the second insulation film so that the control electrode is separated by the second insulation film from the single floating gate electrode film.
A first embodiment according to the present invention will be described in detail with reference to the drawings. The structure of the memory cell of the flash memory and the fabrication method thereof in the first embodiment in accordance with the present invention are different from the conventional ones shown FIGS. 1A and 1B as well as FIGS. 2A through 2H. FIG. 3A is a fragmentary plan view illustrative of the flash memory in a first embodiment in accordance with the present invention. FIG. 3B is a fragmentary cross sectional elevation view illustrative of the flash memory taken along an A—A′ line of FIG. 3A in a first embodiment in accordance with the present invention. The flash memory has an alignment of memory cells 141 which has the following structure. A floating gate 103 is formed over a gate insulation film 102 over a semiconductor substrate 101. The floating gate 103 is T-shaped which comprises an upper portion and a lower portion, wherein the upper portion laterally extends from the opposite edges of a lower portion to form the T-shaped floating gate 103. The structure of the memory cell 141 will be described in detail with reference to FIG. 3B.
Field oxide films 106 are selectively formed on a semiconductor substrate 101 to define an active region or a device region of the semiconductor substrate 101. Source and drain regions 104 and 105 are selectively formed in the active region or the device region of the semiconductor substrate 101, whereby a channel region is defined between the source and drain regions 104 and 105. A gate insulation film 102 is formed on the channel region of the semiconductor substrate 101. Source side and drain side interconnections 104 a and 105 a are formed on the source and drain regions 104 and 105, wherein the source side and drain side interconnections 104 a and 105 a are made of polysilicon films doped with an impurity. In the memory cell 141, the source and drain 104 and 105 serve as parts of bit lines. The formation of the source side and drain side interconnections 104 a and 105 a on and in contact with the source and drain regions 104 and 105 results in reduction in resistance of the source and drain 104 and 105. A floating gate 103 is formed which laterally extending upper portions which extends over source and drain regions 104 and 105. The lower part of the floating gate 103 is positioned on the gate insulation film 102. Insulation films 122 are united with the gate insulation film 102, wherein the insulation films 122 extend on inside walls and top surfaces of the source side and drain side interconnections 104 a and 105 a. The laterally extending upper parts of the floating gate 103 extend over the insulation films 122, so that opposite edges of the laterally extending upper parts of the floating gate 103 are almost aligned to the edges of the source side and drain side interconnections 104 a and 105 a in plane view. An insulation film 107 made of ONO is formed on the floating gate 103. A control gate 108 is formed on the insulation film 107 so that the floating gate 103 is separated and electrically floated from the control gate 108 and whereby the floating gate 103 is completely surrounded by the insulation materials to be floated in the memory cell. The control gate 108 serves as a part of a word line in the memory.
The source and drain 104 and 105 are commonly used for a plurality of the memory cells 141. The drain 105 is used as a part of the bit line so that a single bit contact is formed for the plural memory cells 141 so as to allow narrowing the distance between the memory cells, thereby reducing the cell size.
The following descriptions will focus onto the novel fabrication method of the above flash memory. FIGS. 4A through 4H are fragmentary cross sectional elevation views illustrative of a novel method of forming a memory cell structure over a semiconductor substrate of a flash memory shown in FIGS. 3A and 3B.
With reference to FIG. 4A, a thin insulation film 121 is formed on a surface of a semiconductor substrate 101. A dummy pattern 103 a is selectively formed on a predetermined region of the thin insulation film 121, wherein the thin insulation film 121 may be made of silicon oxide and the dummy pattern 103 a may be made of silicon nitride. The dummy pattern 103 a may be formed by forming a silicon nitride film and subsequent patterning the same with use of a photo-lithography and a subsequent dry etching process. The insulation film 121 made of silicon oxide absorbs a stress generated by forming the dummy pattern 103 a made of silicon nitride. The dummy pattern 103 a is used as a mask for carrying out an ion-implantation into selected upper regions of the semiconductor substrate 101, whereby source and drain regions 104 and 105 are formed with a self-alignment technique in the selected upper regions of the semiconductor substrate 101.
With reference to FIG. 4B, the thin insulation film 121 is removed except for its underlying part which underlies the dummy pattern 103 a, whereby the remaining underlying part of the thin insulation film 121 serves as a gate insulation film 121.
With reference to FIG. 4C, an impurity doped polysilicon film is entirely deposited over the source and drain regions 104 and 105 and the dummy pattern 103 a. An etch back process is carried out to the impurity doped polysilicon film until the top of the dummy pattern 103 a is shown, whereby conductive films 114 are formed on the source and drain regions 104 and 105 and positioned opposite side walls of the dummy pattern 103 a. With reference to FIG. 4D, parts of the conductive films 114 on isolation regions are selectively removed. Grooves are further formed on the isolation region in the semiconductor substrate 101. An insulation material such as silicon oxide is buried within the grooves, whereby field oxide films 106 are then formed on the isolation regions. As a result, outside edges of the conductive films 114 as well as outside edges of the source and drain regions 104 and 105 are defined by inside boundaries of the field oxide films 106, whereby source side and drain side interconnections 104 a and 105 a are formed.
With reference to FIG. 4E, the dummy pattern 103 a and the thin insulation film 121 are removed by selective wet etching processes. The dummy pattern 103 a made of silicon nitride may be removed by a selective wet etching which uses a phosphoric acid solution as an etchant. The thin insulation film 121 made of silicon oxide may be removed by a subsequent selective wet etching which uses a hydrofluoric acid solution as an etchant. As a result, a top surface of the channel region of the semiconductor substrate 101 defined between the source and drain regions 104 and 105 is shown.
With reference to FIG. 4F, a dry oxidation process is carried out at a temperature of about 850� C. in a dry oxygen atmosphere, thereby concurrently forming a gate oxide film 102 on the top surface of the channel region of the semiconductor substrate 101 and also forming inter-layer insulators 122 on side walls and top surfaces of the source side and drain side interconnections 104 a and 105 a. Since the channel region is made is a single crystal silicon whilst the source side and drain side interconnections 104 a and 105 a are made of polysilicon, the same dry oxidation process results in formation of the thicker inter-layer insulators 122 by three or four times than the gate insulation film 102.
With reference to FIG. 4G, an impurity doped polysilicon film is entirely formed which extends over the inter-layer insulators 122 and the gate insulation film 102 as well as over the field oxide films 106. The impurity doped polysilicon film is then patterned with a photo-lithography and a subsequent dry etching technique, whereby a floating gate electrode 103 is formed in a single pair of the film deposition and subsequent patterning processes, wherein opposite side edges of the floating gate electrode 103 are aligned to he opposite side edges of the inter-layer insulators 122. This single pair of the deposition and subsequent patterning processes for the floating gate electrode 103 reduces the number of the fabrication processes.
With reference to FIG. 4H, an insulation film 107 made of ONO is formed on the floating gate 103 and on the insulation films 106. A control gate 108 is then formed on the insulation film 107 so that the floating gate 103 is separated and electrically floated from the control gate 108 and whereby the floating gate 103 is completely surrounded by the insulation materials to be floated in the memory cell.
In accordance with the above novel fabrication processes, the floating gate comprises a single part which has laterally extending upper parts overlying the interconnections 104 a and 105 a over the source and drain regions 104 and 105. The floating gate is formed by the single pair of the film deposition and subsequent patterning processes, whereby the number of the fabrication processes is reduced.
A second embodiment according to the present invention will be described in detail with reference to the drawings. A difference of the second embodiment from the first embodiment is only in the fabrication processes, for which reason the descriptions to the structure of the memory cell of this embodiment will be omitted. FIGS. 5A through 5H are fragmentary cross sectional elevation views illustrative of another novel method of forming a memory cell structure over a semiconductor substrate of a flash memory shown in FIGS. 3A and 3B.
With reference to FIG. 5A, A dummy pattern 103 a is selectively formed on a predetermined region of a semiconductor substrate 101, wherein the dummy pattern 103 a may be made of silicon oxide. The dummy pattern 103 a may be formed by forming a silicon oxide film and subsequent patterning the same with use of a photo-lithography and a subsequent dry etching process. The dummy pattern 103 a is used as a mask for carrying out an ion-implantation into selected upper regions of the semiconductor substrate 101, whereby source and drain regions 104 and 105 are formed with a self-alignment technique in the selected upper regions of the semiconductor substrate 101.
With reference to FIG. 5B, an impurity doped polysilicon film is entirely deposited over the source and drain regions 104 and 105 and the dummy pattern 103 a. An etch back process is carried out to the impurity doped polysilicon film until the top of the dummy pattern 103 a is shown, whereby conductive films 114 are formed on the source and drain regions 104 and 105 and positioned opposite side walls of the dummy pattern 103 a. With reference to FIG. 5C, parts of the conductive films 114 on isolation regions are selectively removed. Grooves are further formed on the isolation region in the semiconductor substrate 101. An insulation material such as silicon oxide is buried within the grooves, whereby field oxide films 106 are then formed on the isolation regions. As a result, outside edges of the conductive films 114 as well as outside edges of the source and drain regions 104 and 105 are defined by inside boundaries of the field oxide films 106, whereby source side and drain side interconnections 104 a and 105 a are formed.
With reference to FIG. 5D, the dummy pattern 103 a is removed by a selective wet etching process. The dummy pattern 103 a made of silicon oxide may be removed by a selective wet etching which uses a hydrofluoric acid solution as an etchant. As a result, a top surface of the channel region of the semiconductor substrate 101 defined between the source and drain regions 104 and 105 is shown.
With reference to FIG. 5E, a dry oxidation process is carried out at a temperature of about 850� C. in a dry oxygen atmosphere, thereby concurrently forming a gate oxide film 102 on the top surface of the channel region of the semiconductor substrate 101 and also forming inter-layer insulators 122 on side walls and top surfaces of the source side and drain side interconnections 104 a and 105 a. Since the channel region is made is a single crystal silicon whilst the source side and drain side interconnections 104 a and 105 a are made of polysilicon, the same dry oxidation process results in formation of the thicker inter-layer insulators 122 by three or four times than the gate insulation film 102.
With reference to FIG. 5F, an impurity doped polysilicon film is entirely formed which extends over the inter-layer insulators 122 and the gate insulation film 102 as well as over the field oxide films 106. The impurity doped polysilicon film is then patterned with a photo-lithography and a subsequent dry etching technique, whereby a floating gate electrode 103 is formed in a single pair of the film deposition and subsequent patterning processes, wherein opposite side edges of the floating gate electrode 103 are aligned to he opposite side edges of the inter-layer insulators 122. This single pair of the deposition and subsequent patterning processes for the floating gate electrode 103 reduces the number of the fabrication processes.
With reference to FIG. 5G, an insulation film 107 made of ONO is formed on the floating gate 103 and on the insulation films 106. A control gate 108 is then formed on the insulation film 107 so that the floating gate 103 is separated and electrically floated from the control gate 108 and whereby the floating gate 103 is completely surrounded by the insulation materials to be floated in the memory cell.
As a modification to the foregoing embodiments of the present invention, the dummy pattern may be made of other materials than silicon nitride and silicon oxide, provided that the dummy pattern material should be selected to have such an etching selectivity that an etching rate of the dummy pattern is higher than that of the interconnections 104 a and 105 a over the source and drain regions 104 and 105.
Etch back process for forming the conductive films 114 may be carried out by a chemical mechanical polishing method. In this case, the dummy pattern 103 a may preferably be made of silicon nitride so that the dummy pattern 103 a may serve as an etching stopper to the chemical mechanical polishing.
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