JPS60226181A - Non-volatile semiconductor memory device - Google Patents

Abstract

PURPOSE:To improve the dielectric withstanding capability of an insulating film and to obtain excellent non-volatile semiconductor memory device leading to the production of high-reliability devices with a high acceptable rate by a method wherein the thin gate insulating film region just under a floating gate is provided not adjoining a field insulating film region. CONSTITUTION:On the surface of a P type semiconductor substrate 11, a field oxide film 12 and a thin oxide film 13 are formed and, through the latter, As ions are implanted for the formation of an N type buried layer 14. Next, the oxide film 13 is removed by using F or the like. A process follows wherein thermal oxidation, patterning of a resist, partial removal of the oxide films, and second thermal oxidation are accomplished for the formation of a relatively thin first insulating film 15 and a relatively thick second insulating film 16. The first, insulating film 15 is not in contact with the field oxide film 12. Next, a floating gate 17, gate oxide film 18, control gate 19 are built for the completion of the required memory device. In a device wherein the gate is a lamination of the two insulating films 15, 16 greatly different from each other in thickness, a current flows in the thinner insulating film 15 generating a stronger electric field. This results in a memory device with its insulation not to be easily broken down due to pinholes and other defects.

Description

DETAILED DESCRIPTION OF THE INVENTION (Technical Field) The present invention relates to a nonvolatile semiconductor memory device, and more particularly to a nonvolatile semiconductor memory device having means for injecting and extracting electrons into and out of a conductive layer through an insulating film.

(Prior Art) Conventional nonvolatile semiconductor memory devices erase and write information by providing means for injecting and extracting electrons into and out of a conductive layer through an insulating film.

FIG. 1 is a cross-sectional view of an example of a conventional nonvolatile semiconductor memory device. A field insulating film 7 is formed on the p-type semiconductor substrate 1 using the LOC08 method. A thin gate insulating film 3 is provided. An n-type buried layer 2 is provided under the gate insulating film 3. Gate insulating film 3
floating gate 4° gate insulating film 5. The control gates 6 are formed by sequentially overlapping each other.

A voltage is applied between the buried layer 2 and the control gate 6 of the memory device having such a structure, and capacitive coupling between the electrodes (2-4-6) causes the thin gate insulating film 3 (for example, 3iQ2 film) to be A high electric field is applied and the Fowler-Nordhei A (Fowl)
By generating a channel current, electrons are injected into the floating gate 4 or electrons are extracted from the floating gate 4 to erase or write information.

However, such a thin insulating film (5iOz film has a thickness of 10
0 to 150 A), there is not enough margin for dielectric breakdown voltage against the high electric field applied during writing and erasing, and dielectric breakdown due to pinholes, defects, etc. also occurs, causing the entire device to short-circuit and reducing the yield rate. This has the drawback of causing a decline in product quality or turning good products into defective ones.

(Object of the Invention) An object of the present invention is to eliminate the above-mentioned drawbacks and to improve the dielectric breakdown resistance of the insulating film by providing at least a thin gate insulating film region under the floating gate so as not to be adjacent to the field insulating film region. The object of the present invention is to provide a nonvolatile semiconductor memory device that can provide a highly reliable product with a high yield rate.

(Structure of the Invention) A nonvolatile semiconductor memory device of the present invention includes a first insulating film provided on the main surface of a semiconductor substrate, and a first insulating film that is adjacent to at least a portion of the first insulating film and that is located in the adjacent portion. a second insulating film which is thicker than the first insulating film, and which is thinner than the adjacent film in an adjacent portion adjacent to a film other than the first insulating film; a conductive film provided covering at least a portion of the insulating film and at least a portion of the first insulating film; and a diffusion region provided in the semiconductor substrate under the first insulating film. It consists of:

(Example) Next, an example of the present invention will be described using the drawings.

FIGS. 2(a) to 2(C) are cross-sectional views showing an embodiment of the present invention and a manufacturing method thereof in order of steps.

First, as shown in FIG. 2 (al), a p-type semiconductor substrate 11
A field oxide film 12 and a thin oxide film 13 of, for example, about 100^ are formed on the surface using the usual LOCO8 method. Through this oxide film 13, arsenic is ion-implanted at about 1.0XIOcm at 50KeV to form an n-type buried layer 1.
form 4.

Next, as shown in FIG. 2 [b], the oxide film 13 is removed using hydrofluoric acid, etc., and then thermal oxidation, patterning of the resist, removal of a portion of the oxide film, and thermal oxidation are performed. A first insulating film 15 having a thin thickness (for example, about 120 Å) and a second insulating film 16 having a thick thickness (for example, about 600 Å) are formed. Here, it is important that the first insulating film 15 is not adjacent to the field oxide film 12.

Next, as shown in FIG. 2 (C1), a floating gate 17, a gate oxide film 18, and a control gate 19 are formed in the same way as in the conventional floating gate structure to complete a nonvolatile semiconductor memory device according to an embodiment of the present invention. The Fowler-Nordheim type tunneling current I is expressed by the electric field E as follows.

I=S-Kl -E -exp(-2/E) (S
:area.

(Kl, 2: constant) The tunnel current strongly depends on the electric field strength, and as shown in FIG. 2(C), two types of insulating films 15. When '16 exists, current flows through the thinner insulating film 15 where the electric field is stronger.

The inventor of the present application used a technique similar to the above-described embodiment to increase the thickness of the insulating film in the region between the thin gate oxide film 15 and the field oxide film 12 to be thicker than the gate oxide film 15, that is, the second As a result of comparing the insulating film 16 with the conventional insulating film 16 with a uniform thickness, it was found that the insulating film 16 has high reliability and high yield, with extremely low dielectric breakdown caused by pinholes, defects, etc. The result was that a memory element was obtained.

In order to investigate the cause of this, the inventors of the present invention fabricated a prototype MOS diode having a thin oxide film, and investigated its dielectric breakdown voltage.

That is, FIG. 3fa) shows a diode with a normal LOCO8 structure, 31 is a semiconductor substrate, 32 is a field oxide film,
A thin oxide film 33 corresponds to a gate oxide film and is in contact with the thick field oxide film 32. Further, 37 is an electrode.

Further, FIG. 3(b) shows a diode having a LOCO8 structure having the same structure as the gate oxide film of one embodiment of the present invention. That is, in this structure, the thin oxide film 35 is in contact with the edge of the LOCO8 structure so that the thin oxide film 35 is not in contact with the edge of the LOCO8 structure.
A thicker oxide layer 36 is formed, and the thicker oxide layer 36 contacts a much thicker field oxide layer at the other end.

All MOS diodes were formed to have the same film thickness and area of the thin oxide film portion.

Voltage was applied to these two types of diodes, their dielectric breakdown voltages were measured, and the breakdown voltage distribution is shown in Figure 4 (a).

(shown in bl.

Figure 4 fat is the conventional LOCO8 shown in Figure 3 fa)
This is a breakdown voltage distribution diagram of a MOS diode created according to the structure.
Further, FIG. 4(b) is a breakdown voltage distribution diagram of a MOS diode having a gate oxide film structure according to an embodiment of the present invention shown in FIG. 3(b). As is clear from FIGS. 4(a) and 4(b), many of the conventional structures have low dielectric breakdown voltages, and their distribution is wide. On the other hand, in the structure of the present invention, there is almost no decrease in dielectric breakdown voltage, and the distribution is concentrated in a narrow range.

From this result, it can be seen that the field edges of the LOCO8 structure are prone to defects, and should be avoided in terms of reliability and yield for devices that use strong electric field sounds such as tunnel currents. .

In the above embodiments, the case of an n-channel/channel floating gate type was explained, but the present invention is not limited to an n-channel type, nor is it limited to a floating gate structure.

(Effects of the Invention) As explained above, according to the present invention, by increasing the thickness of the gate oxide film in the portion adjacent to the field oxide film, which has many defects, the gate oxide film has a high dielectric breakdown voltage. structure and reliability of the device.

Durability can be increased.

[Brief explanation of drawings]

FIG. 1 is a sectional view of an example of a conventional nonvolatile semiconductor memory device, and FIGS. 2(a) to 2(C) are sectional views shown in order of steps to explain an embodiment of the present invention and its manufacturing method. 3(a), (bl is a cross-sectional view of a conventional example and a MOS diode of the present invention structure prototyped to investigate the dielectric breakdown voltage of the gate insulating film, FIG. a)
, is a dielectric breakdown voltage distribution diagram of the MOS diode in (b) - 1... Semiconductor substrate, 2... Buried layer, 3...
...Gate insulating film, 4...Floating gate, 5
...Gate insulating film, 6...Control gate, 7...Field insulating film, 11...Semiconductor substrate, 12...Field oxide film, 13...
...Oxide film, 14...Buried layer, 15...First insulating film, 16...Second insulating film, 17...
...Floating gate, 18...Gate oxide film, 19.
... Control gate, 31 ... Semiconductor substrate, 32.
・Field insulating film, 33...gate oxide film,
35...First insulating film, 36...Second insulating film, 37.38 - Electrode. Engineering・, −τ ゛・,. Agent Patent Attorney Uchihara ・(、・、・. Daiichi Rikimoto? Kuni 3rd 7) 4 Kuni

Claims (1)

[Claims] a first insulating film provided on the main surface of the semiconductor substrate; the first insulating film is adjacent to at least a portion of the first insulating film, and the film thickness at the adjacent portion is thicker than the first insulating film; In an adjacent portion adjacent to a film other than the first insulating film, there is a second insulating film that is thinner than the adjacent film, at least a portion of the second insulating film, and the first insulating film. A nonvolatile semiconductor memory device comprising: a conductive film provided to cover at least a portion of the first insulating film; and a diffusion region provided in the semiconductor substrate under the first insulating film.