# Patent

## Title
Surrounding stacked gate multi-gate FET structure nonvolatile memory device

## Abstract

Nonvolatile memory devices having a low off state leakage current and an excellent data retention time characteristics. The present invention provides a surrounding stacked gate fin field effect transistor nonvolatile memory structure comprising a silicon-on-insulator substrate of a first conductivity type and a fin active region projecting from an upper surface of the insulator. The structure further includes a tunnel oxide layer formed on the fin active region and a first gate electrode disposed on the tunnel oxide layer and upper surface of the insulator. Additionally, the structure includes an oxide/nitride/oxide (ONO) composite layer formed on the first gate electrode, a second gate electrode formed on the ONO composite layer and patterned so as to define a predetermined area of the ONO composite layer. The structure further includes a dielectric spacer formed on a sidewall of the second gate electrode and source/drain regions formed in the fin active region on both sides of the second gate electrode.

## Background
CROSS-REFERENCES TO RELATED APPLICATIONS
This application claims priority to Chinese Patent Application No. 200910057968.7, filed Sep. 28, 2009 by inventors Deyuan Xiao et al., commonly assigned and incorporated in its entirety by reference herein for all purposes.
BACKGROUND OF THE INVENTION
The present invention is generally related to integrated circuit devices and their processing for the manufacture of semiconductor devices. More particularly, the present invention provides a method for forming a surrounding stacked gate fin field effect transistor (fin FET) structure nonvolatile memory device.
As semiconductor memory devices become increasingly integrated, the feature size including the channel length of individual devices is gradually reduced. This causes a short channel effect and also increases the junction leakage current. Leakage current was not a significant problem in the past, but it has become more serious concern now that transistor gates and other chip components measure only a few atoms thick. In a notebook computer, leakage current means short battery life and in a server computer, it means higher power bills. Also, in a nonvolatile memory device, the leakage current leads to degradation of data retention time and other electrical characteristics as the device feature size is reduced.
To solve these problems, fin field effect transistors (fin FETs) that have fin-shaped silicon active regions have been introduced. Among other things, the fin-shaped silicon active region typically enables multi-gate FET devices. For example, a conducting region (e.g., gate region) wraps around the fin-shaped silicon structure. Since a gate electrode is formed on the fin active region, a fin FET may use an entire face of a projected-portion as a channel. Thus, the channel length is often sufficient to prevent or substantially reduce the short channel effect. Such a fin FET can effectively control a leakage current generated in a channel, hence, substantially improve swing characteristics of the transistor and decrease the leakage current.
However, depending on the dimension of the fin active region, the threshold voltage is often difficult to control in fin FETs compared to a planar-type transistor. Furthermore, a leakage current is generated in an off state of the fin FET, and a gate induced drain leakage (GIDL) current and a junction leakage current are not distinctly improved compared to the planar type transistor and the recess type transistor.
Therefore, there is a need for nonvolatile memory devices that have a low off state leakage current and reduced short channel effect. In addition, there is a demand for a nonvolatile memory device having an excellent data retention time characteristics.
BRIEF SUMMARY OF THE INVENTION
The present invention is generally related to integrated circuit devices and methods of manufacturing semiconductor devices. More particularly, the present invention provides a method and structure for forming a surrounding stacked gate fin field effect transistor (fin FET) nonvolatile memory device.
In a specific embodiment, the invention provides a fin field effect transistor nonvolatile memory structure comprising a silicon-on-insulator (SOI) substrate of a first conductivity type and a fin active region projecting from an upper surface of the insulator. The structure further includes a tunnel oxide layer formed on the fin active region and a first gate electrode disposed on the tunnel oxide layer and upper surface of the insulator. Additionally, the structure includes an oxide/nitride/oxide (ONO) composite layer formed on the first gate electrode, a second gate electrode formed on the ONO composite layer and patterned so as to define a predetermined area of the ONO composite layer. The structure further includes a dielectric spacer formed on a sidewall of the second gate electrode and source/drain regions formed in the fin active region on both sides of the second gate electrode.
In another specific embodiment, a method of forming a fin field effect transistor nonvolatile memory structure is provided. The method includes providing a silicon-on-insulator substrate and patterning a fin active region on a predetermined region of the silicon-on-insulator substrate, the fin active region projecting from a surface of the insulator. The method further includes forming a tunnel oxide layer on the fin active region and depositing a first gate electrode on the tunnel oxide layer and on the upper surface of the insulator. In addition, the method includes forming a dielectric composite layer on the first gate electrode and depositing a second gate electrode on the dielectric composite layer. The method additionally includes patterning the first and second gate electrodes so as to define the area of the dielectric composite layer and forming a spacer layer on a sidewall of the second gate electrode. The method further includes forming source/drain regions in the fin active region on both sides of the second gate electrode.
Many benefits are achieved by way of the present invention over conventional techniques. The present invention offers significant unobvious advantages in the fabrication of nonvolatile memory devices having a reduced short-channel effect and a low off-state leakage current. In some embodiments, the present invention provides an increased oxide/nitride/oxide (ONO) dielectric contact area, enlarged floating gate coupling ratio, improved program/erasure speed, and enlarged data retention time, which are not obtainable with conventional process technology or structure. Depending upon the embodiment, one or more of these benefits may be achieved.
Various additional embodiments, features, and advantages of the present invention, as well as the structure and operation of the various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1A is a simplified top view of a surrounding stacked gate fin FET structure non-volatile memory (NVM) device according to an embodiment of the present invention;
FIG. 1B is a simplified perspective sectional view taken along the line A-A′ of FIG. 1A;
FIG. 2 is an exemplary simplified process flows showing processes used for formation of a surrounding stacked gate fin FET structure NVM device according to embodiments of the present invention.
FIGS. 3A-3K are simplified sectional diagrams illustrating exemplary sequential processes for forming the fin FET of FIG. 1, taken along the line A-A′ of FIG. 1.
FIG. 4A is a simplified perspective view of the surrounding stacked gate fin FET nonvolatile memory structure according to an exemplary embodiment of the invention.
FIG. 4B is a graph illustrating the drain current characteristics of the nonvolatile memory structure of FIG. 4A according to an exemplary embodiment of the invention.



It will be appreciated that for simplicity and clarity of illustration, elements shown in the Figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to each other for clarity. Further, where considered appropriate, reference numerals have been repeated among the Figures to indicate corresponding elements.
DETAILED DESCRIPTION OF THE EMBODIMENTS
The present invention generally relates to integrated circuit devices and their processing for the manufacture of semiconductor devices. More particularly, the present invention provides a method for forming a surrounding stacked gate fin field effect transistor (fin FET) structure nonvolatile memory device.
Exemplary embodiments of the invention are more fully described in detail with reference to the accompanied drawings. The invention may be embodied in many different forms and should not be construed as being limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure is thorough and complete, and to convey the concepts of the invention to those skilled in the art.
FIG. 1A is a simplified top view of a surrounding stacked gate fin FET structure non-volatile memory (NVM) device according to an embodiment of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The fin FET nonvolatile memory device may employ a silicon-on-insulator (SOI) substrate. The SOI substrate is composed of silicon film/buried oxide (insulator)/and silicon substrate. Compared with bulk-silicon substrate, SOI field effect transistors show many outstanding features including better short channel effect and reduced junction leakage. Also, a three dimensional integration is possible. A surrounding stacked gate fin FET structure nonvolatile memory device employs three dimensional fin type stacked gates composed of floating gate and control gate.
FIG. 1B is a simplified perspective sectional view taken along the line A-A′ of FIG. 1A. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in FIG. 1B, silicon fin 102 is defined by photolithographic masking step followed by silicon active area etch step. Since the silicon fin 102 constitutes a channel region of the device, the dimension of the silicon fin 102 is predetermined in view of the required minimum channel width and other design factors including the gate dielectric contact area, which is related to the data retention time. A tunnel oxide 104 is formed on the surface of the silicon fin 102 to be used as a gate oxide of the floating gate 106.
As known to those in the art, nonvolatile memory's data retention time is closely related to the integrity and dimension of the gate dielectric layer 108 to be formed between the floating gate 106 and control gate 110. Source and drain regions are formed in areas 112 located on both sides of the control gate 110. In the present embodiment, a nonvolatile memory device 100 having a channel length of 20 nm, silicon fin thickness of 6 nm, and silicon fin width of 10 nm was fabricated.
FIG. 2 is an exemplary simplified process flows showing processes used for formation of a surrounding stacked gate fin FET structure nonvolatile memory device according to embodiments of the present invention. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications.
Referring to FIG. 2, a method 200 for forming a surrounding stacked gate fin FET structure nonvolatile memory device according to embodiments of the present invention can be outlined as follows:
1. Provide a semiconductor substrate, e.g., a silicon on insulator (SOI) wafer substrate;
2. Define (step 1) a silicon fin using lithography and silicon active area etching;
3. Anneal (step 2) the silicon fin in a hydrogen atmosphere;
4. Grow (step 3) a tunnel gate oxide of a thickness of about 20 Å to 80 Å on the surface of the silicon fin using a thermal method;
5. Adjust (step 4) the threshold voltage Vt of the nonvolatile memory device by implantation;
6. Deposit (step 5) a floating gate (FG) to a thickness of about 100 Å to 500 Å on the tunnel gate oxide by in-situ n+ doped polysilicon technique;
7. Grow FG isolation layer (step 6) such as oxide/nitride/oxide (ONO) dielectric layer having a thickness of about 20 Å to 100 Å;
8. Deposit (step 7) a control gate (CG) material such as a polysilicon having a thickness of about 500 Å to 4000 Å on the ONO dielectric layer;
9. Planarize (step 8) the control gate surface using a chemical mechanical polishing (CMP) process;
10. Remove (e.g., etch) a portion of the control gate, ONO dielectric layer, floating gate, tunnel oxide layer and insulation layer for obtaining a surrounding stacked gate fin structure (step 9);
11. Form (step 10) a gate spacer on the sidewalls of the stacked gates including the floating gate and the control gate;
12. Implant n-type impurity to form an n+ type source/drain region in the fin active region of both sides of the gate electrode using the gate spacer as a mask and anneal the source/drain region at 1000° C. to 1200° C. to obtain a lightly doped (LDD) structure (step 11);
13. Form (step 12) an elevated source/drain region;
14. Form contacts and interconnects; and
15. Perform other processes.
The process flows shown in FIG. 2 are further illustrated in FIG. 3A-3K where exemplary sequential processes for forming the fin FET of FIG. 1, taken along the line A-A′ of FIG. 1, are illustrated.
FIG. 3A is a cross-sectional diagram illustrating an SOI substrate composed of a pre-defined active fin silicon 306, insulator 304, and silicon substrate 302. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. In a specific embodiment, the active fin silicon 306 is formed to a height that is in the range of about 1000 Å to 3000 Å from the upper surface of the insulator 304 (step 1 of FIG. 2). Referring to step 2 of FIG. 2 and FIG. 3B, a defined fin-shaped silicon 306 is annealed in a hydrogen atmosphere at a temperature in the range of about 800° C. to 1000° C. This hydrogen annealing step is necessary to improve the surface characteristics of the silicon fin active area by annealing out dangling silicon bonds and other interface states. It is to be understood that other types of annealing processes may be used as well.
Referring to FIG. 3C, a tunnel gate oxide forming step is illustrated. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The tunnel gate oxide 308 is grown on the surface of the silicon fin area 306 by a thermal method to a thickness that is in the range of about 20 Å to 80 Å (step 3 of FIG. 2). For example, the thermal method for growing tunnel gate oxide 308 may be similar conventional techniques. Depending on the resistivity or doping level of the silicon fin area 306, a threshold voltage (Vt) implantation step (step 4 in FIG. 2) is performed after the tunnel gate oxide is formed in order to adjust the threshold voltage of the nonvolatile memory device.
Referring to FIGS. 3D-3F, a floating gate 310 is deposited before a gate dielectric layer 312 is formed on the floating gate (step 5). This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The floating gate 310 is deposited to a thickness of about 100 Å to 500 Å by in-situ n+ doped polysilicon technique according to one embodiment of the present invention. It can also be formed by other conventional polysilicon deposition method, which is followed by n+ doping.
The gate dielectric layer 312 is composed of, among other things, oxide/nitride/oxide (ONO) composite layer having a thickness of about 20 Å to 100 Å (step 6). ONO layers are sequentially formed before a control gate 314 is deposited on them. For example, the control gate is about 500 Å to 4000 Å thick (step 7). Since the integrity and thickness of the ONO layer can affect the data retention characteristics of the nonvolatile memory device, the thickness of ONO layers is subject to specific design targets.
Referring to FIG. 3G, a chemical mechanical polishing (CMP) step is performed to even out the irregular topography formed by the control gate 314 and other underlying layers including the fin silicon area 306 and the floating gate 310 (step 8). This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. The CMP process uses an abrasive and corrosive chemical slurry in conjunction with a polishing pad and retaining ring, typically of a greater diameter than the wafer. The pad and wafer are pressed together by a dynamic polishing head and held in place by a plastic retaining ring. The dynamic polishing head is rotated at different rates, with different axes of rotation. This removes material and tends to even out irregular topography, making the wafer flat or planar. This may be necessary in order to bring the entire surface within the depth of field of a photolithography system.
FIG. 3H illustrates a simplified plan view after the gate etch step (step 9) is done (top). A cross-sectional view taken along the line A-A′ is shown in the middle and another cross-sectional view taken along the line B-B′ is shown in the bottom side of FIG. 3H. This diagram is merely an example, which should not unduly limit the scope of the claims. One of ordinary skill in the art would recognize many variations, alternatives, and modifications. As shown in the cross-sectional views, exposed portions of the control gate, ONO layer, floating gate, tunnel gate oxide, and insulator 304 are sequentially etched during the gate etch step.
Referring to FIGS. 3I-3J, a gate spacer 316 composed of oxide/nitride composite layer is formed on a sidewall of the stacked gates including the floating gate 310 and control gate 314 (step 10). For example, the gate spacer 316 functions in at least two ways: (1) function as the LDD spacer; (2) functions as isolation layer for forming an elevated source/drain regions.
After forming the gate spacer 316, n-type impurity ions such as P (phosphorous) or As (Arsenic), etc., are ion implanted at an energy of about 10 KeV to 20 KeV and to a density of about 1.0 E15 to 3.0 E15 ion atoms/cm3. Here, the gate spacer 316 is used as an ion implantation mask to form an n+ type source/drain region 318 of a high density in the fin active region of both sides of the gate electrode. Also, before forming the gate spacer 316, n-type impurities are ion-implanted at a relatively low energy and to a relatively low density compared to the source/drain regions 318, thus, forming a low density n-type source/drain region. Next, a n+ type source/drain region having a density higher than the low density n-type source/drain region is formed on a portion of the low density n-type source/drain region. Thus, a source/drain region having a LDD (Lightly Doped Drain) structure is finally obtained after an annealing step at 1000° C. to 1200° C. in an inert ambient is carried out to activate the impurity ions (step 11 of FIG. 2).
As shown in FIG. 3K, an elevated source/drain region is formed (step 12). The elevated or raised source/drain structures 320 may be fabricated by various methods. For example, methods include growing a selective epitaxial silicon layer in the source and drain regions of the FET device after sidewall spacer creation and after LDD implantation. The elevated source/drain layers are grown in a low pressure LPCVD epitaxy reactor with a mixture of silane and dichlorosilane or any other gas mixture. Since series resistances can be minimized by employing the elevated source/drain structure, drive current characteristics are improved.
FIG. 4A is a simplified perspective view of the surrounding stacked gate fin FET nonvolatile memory structure and FIG. 4B is a graph illustrating the drain current vs. gate voltage characteristics of the nonvolatile memory structure of FIG. 4A according to an exemplary embodiment of the invention. Referring to FIG. 4B, better subthreshold slope (mV/dec) is obtained due to the excellent floating gate ratio and improved off state leakage current characteristics. Also, it has been observed that other characteristics of the nonvolatile memory device including a data retention time and program/erase speed are improved by adopting the structure mentioned above.
While various embodiments of the present invention have been described above, it should be understood that they have been presented by way of example only, and not limitation. It will be apparent to persons skilled in the relevant art that various changes in form and detail can be made therein without departing from the spirit and scope of the invention. Thus, the breadth and scope of the present invention should not be limited by any of the above-described exemplary embodiments, but should be defined only in accordance with the following claims and their equivalents.

## Claims

1. A nonvolatile memory device having a surrounding stacked gate fin FET, the device comprising:
a silicon-on-insulator (SOI) substrate being characterized by a first conductivity type, the substrate including an upper surface;
a fin active region projecting from the upper surface of the substrate;
a tunnel oxide layer overlying the fin active region;
a first gate electrode disposed on the tunnel oxide layer and upper surface of the substrate;
a dielectric composite layer formed on the surface of the first gate electrode;
a second gate electrode formed on a top surface and side surfaces of the dielectric composite layer and patterned so as to define a stacked gate structure including the second gate electrode, the dielectric composite layer, the first gate electrode, and the tunnel oxide layer;
a dielectric spacer formed on sidewalls of the stacked gate electrodes structure;
source/drain regions formed in the fin active region on both sides of the second gate electrode; and
elevated source/drain regions overlying the respective source/drain regions, wherein the elevated source/drain regions have a top surface that is substantially coplanar with a top surface of the second gate electrode.

2. The device of claim 1, wherein the first gate electrode is formed by an in-situ doped polysilicon material doped with a second conductive impurity.

3. The device of claim 1, wherein the tunnel oxide layer has a thickness in the range from 20 Å to 80 Å.

4. The device of claim 1, wherein the dielectric composite layer comprises oxide/nitride/oxide and has a thickness in the range of about from 20Å to 100Å.

5. The device of claim 1, wherein the first gate electrode is structured to function as a floating gate configured to store data by trapping electrons when a program voltage is applied to the second gate electrode.

6. The device of claim 1, wherein the second gate electrode is structured to function as a control gate configured to turn on a channel of the fin FET when a control voltage is applied to the second gate electrode.

7. The device of claim 5, wherein the program voltage is higher than a voltage applied to a drain region.

8. The device of claim 1, wherein the dielectric spacer is composed of an oxide/nitride composite layer.

9. The device of claim 1, wherein the source/drain regions are doped with phosphorus ions.

10. The device of claim 1, wherein the fin active region has a height that is about 1000 Å to about 3000 Å from the upper surface of the substrate.

11. The device of claim 1, wherein the first gate electrode has a thickness in the range from 100 Å to 500 Å.

12. The device of claim 1, wherein the second gate electrode has a thickness in the range from 500 Å to 4000 Å.