JPH02128399A - Eprom type nonvolatile memory - Google Patents

Abstract

PURPOSE:To improve the erasing efficiency of the title memory by making a floating gate easily accessible by ultraviolet rays by either exposing the floating gate by excluding part of a control gate or forming the control gate of a transparent conductive film. CONSTITUTION:A field oxide film 2 is formed on the main surface of a silicon substrate 1 and a gate oxide film 3 and floating gate 4 are successively formed on the area of the film 2 where a memory element is formed. Then after forming another gate oxide film 5 on the gate 4, a control gate 6 is formed so that the floating gate 4 can be exposed. In other words, the erase controlling gate 6 is not formed to cover the whole area of the floating gate 4. Therefore, electric charges are easily discharged from the floating gate and the erasing efficiency can be improved, since the floating gate 4 becomes easily accessible by ultraviolet rays for erasure through the thin gate oxide film 5.

Description

[Detailed description of the invention] [Industrial application field] The present invention relates to an EPROM type non-volatile memory.

[Conventional technology]

Conventionally, in a so-called EPROM type nonvolatile memory integrated circuit (hereinafter referred to as EPROM) in which program data can be electrically written and optically erased, a floating gate that stores charge is irradiated with light, and the floating gate is activated by light excitation. The stored charge was released and the program data was erased.

FIG. 3 is a sectional view of an example of a conventional EPROM.

A relatively thick layer of 0.5 to 1.0 mm is formed on the main surface of the silicon substrate 1.
A field oxide film 2 with a thickness of about μm is formed, and a relatively thin first gate oxide film 3 of about 10 nm is formed in a region where a memory element is to be formed.

A floating gate 4 made of a polycrystalline silicon layer is formed thereon. Next, a second gate oxide film 5 is formed, and then a control gate 8 made of a polycrystalline silicon layer is formed.

Erasing data is performed by irradiating ultraviolet rays from above, photo-exciting the charges accumulated in the floating gate 3, and releasing the charges to the silicon substrate 1 and the control gate 8.

[Problem to be solved by the invention]

However, in the conventional structure, even when light is irradiated to erase program data, the light does not directly enter the floating gate.
Since the method uses reflected light from the silicon substrate, there is a problem in that the erasing efficiency is not good.

[Means to solve the problem]

The EPROM type nonvolatile memory of the present invention includes a source region and a drain region provided in a single portion of a semiconductor substrate;
A floating gate provided on the surface of the semiconductor substrate between the source region and the drain region via a first gate insulating film, and a control provided on the floating gate via a second gate insulating film. The EPROM type nonvolatile memory having a gate is characterized in that the control gate is provided with an opening for allowing ultraviolet light to enter the floating gate, or the control gate is made of a conductive material that transmits ultraviolet light.

〔Example〕

Next, embodiments of the present invention will be described with reference to the drawings.

FIG. 1 is a sectional view of an embodiment of the present invention.

A relatively thick layer of 0.5 to 1.0 mm is formed on the main surface of the silicon substrate 1.
A field oxide film 2 with a thickness of about 10 nm is formed, and a relatively thin first gate oxide film 3 of about 10 nm is formed in a region where a memory element is to be formed.

A floating gate 4 made of a polycrystalline silicon layer is formed on this. Next, after forming the second gate oxide film 5, a control gate 6 made of a polycrystalline silicon layer is formed so that the floating gate 4 is exposed. In other words, the erase control gate 6 is made not to cover the entire surface of the floating gate 4.

With this structure, the ultraviolet rays for erasing enter the floating gate 4 through the thin second gate oxide JL%5, making it easier to release charges from the floating gate.
Erasing efficiency can be significantly improved.

FIG. 2 is a sectional view of a second embodiment of the invention.

In this embodiment, the control gate 7 is formed of a transparent conductive film so that ultraviolet rays can easily reach the floating gate 4. Examples of transparent conductive materials include In2O3 and S
A mixture with nO□ is chosen.

When the control gate is formed of a transparent conductive material in this manner, ultraviolet rays reach the floating gate, making it easier to release charges from the floating gate, and erasing efficiency can be greatly improved. Therefore, the control gate 7 can be provided so as to cover the entire surface of the floating gate 4, and there is no need to remove a part of the control gate to expose the floating gate as in the first embodiment.

〔Effect of the invention〕

As explained above, the present invention makes it easier for ultraviolet rays to reach the floating gate by either removing a part of the control gate to expose the floating gate or forming the control gate with a transparent conductive film. Therefore, the discharge of charges from the floating gate becomes easy, and the erase light rate is improved.

[Brief explanation of the drawing]

FIG. 1 is a sectional view of a first embodiment of the present invention, FIG. 2 is a sectional view of a second embodiment of the present invention, and FIG. 3 is a conventional EPROM.
It is a sectional view of an example.

Claims (1)

[Claims] a source region and a drain region provided on one main surface of a semiconductor substrate; a floating gate provided on the surface of the semiconductor substrate between the source region and the drain region via a first gate insulating film; EPROM having a control gate provided above a floating gate via a second gate insulating film
type nonvolatile memory, the control gate is provided with an opening for allowing ultraviolet rays to enter the floating gate;
Alternatively, an EPROM type nonvolatile memory characterized in that the control gate is made of an ultraviolet-transparent conductive material.