The invention relates to a method of fabricating a semiconductor structure, and more particularly to a method of fabricating a protruding floating gate of flash memory.
Flash memory cells are often bulky and difficult to fabricate in a desirable space due to complex geometries of the multiple gate layers used to form the control and floating gates. Accordingly, flash memory cells cannot generally be integrated as compact as other types of memory devices.
It is the general trend in the semiconductor market to manufacture devices with miniaturized dimensions, lower operation voltage, lower cost, higher speed, etc. The gate coupling ratio (GCR) between the floating gate and the control gate affects the operating voltage and device speed. The read/write method of a flash memory is affected by means of electrons transferring between the floating gate and the source/drain gate. Generally, the larger a gate coupling ratio between the floating gate and the control gate is, the lower required working voltage for operation will be. If floating gate-floating gate capacitance can be decreased, the interference issue will be suppressed. However, it is difficult to increase the overlapped area between the control gate and the floating gate in the conventional stacked gate structure. Accordingly the invention provides a method to fabricate a protruding floating gate, and the overlapped area between the control gate and floating gate will be increased.
According to one aspect of the present invention, a method for improving gate coupling ratio of a flash memory device is provided, and a protruding floating gate is formed. First, a substrate having a plurality of isolation structures is formed. Then, a first conductive layer is formed overlaying the substrate. A chemical-mechanical polishing process is performed to planarize the first conductive layer. After that, a portion of the isolation structures is removed, and a second conductive layer is formed overlaying the first conductive layer and the isolation structures. Finally, a lithography process with a photomask can be used to define a mask that covers the first conductive layer and the second conductive layer, and then an insulating layer is deposited overlaying the substrate, so that a third conductive layer is formed overlaying the insulating layer.
In one embodiment of the present invention, the isolation structures are shallow trench isolation (STI) structures.
In one embodiment of the present invention, the step of forming a first conductive layer overlaying the substrate comprises performing a physical vapor deposition (PVD) process or a chemical vapor deposition process (CVD)
In one embodiment of the present invention, the step of removing the portion of the isolation structures comprises performing a wet etching process or a dry etching process.
In one embodiment of the present invention, the step of removing the portion of the isolation structures forms a remained isolation structure having an upper surface lower than an upper surface of the first conductive layer.
In one embodiment of the present invention, the isolation structures comprise insulating materials.
In one embodiment of the present invention, the insulating materials are selected from the group consisting of boro phosphosilicate glass (BPSG), phosophosilicate glass (PSG), CVD oxide, fluorinated glass (FSG) or the like.
In one embodiment of the present invention, the first conductive layer is made of a doped polysilicon layer.
In one embodiment of the present invention, the second conductive layer is made of a doped polysilicon layer.
In one embodiment of the present invention, the step of patterning the second conductive layer further comprises etching and lithography processes.
In one embodiment of the present invention, the insulating layer is an inter-poly dielectric layer.
In one embodiment of the present invention, the first conductive layer and the second conductive layer are collectively formed a floating gate.
In one embodiment of the present invention, the third conductive layer is made of a doped polysilicon layer.
In one embodiment of the present invention, the third conductive layer is a control gate.
The above objects and advantages of the present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which:
The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of preferred embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed.
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Referring to
A planarizing step is performed to remove protruding portions of the first conductive layer 14, as shown in
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Referring to
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The gate coupling ratio is increased in this invention, and the interference from one floating gate to the other floating gate can be suppressed. Specifically, there is no need to add any extra lithograph processes.
While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.
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6294812 | Ding | Sep 2001 | B1 |
6326263 | Hsieh | Dec 2001 | B1 |
6562681 | Tuan | May 2003 | B2 |
6713336 | Shin | Mar 2004 | B2 |
6897116 | Lee | May 2005 | B2 |
8741754 | Lee | Jun 2014 | B2 |
20040182815 | Lee | Sep 2004 | A1 |
20080090353 | Park | Apr 2008 | A1 |
20080237680 | Pangal | Oct 2008 | A1 |
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