JPH023981A - Method of rewriting nonvolatile memory - Google Patents

Abstract

PURPOSE:To substantially increase a number of possible rewriting operations in an EEPROM having floating gate structure, by baking the EEPROM always after repeating a certain number of rewriting operations. CONSTITUTION:A floating-gate-type EEPROM assembled into a package is subjected to up to 10<5> rewriting operations by a conventional rewriting process. The EEPROM in the packaged state is then introduced into a baking furnace at a temperature higher than 200 deg.C and left therein for about 5 hours. The EEPROM is taken out of the furnace and subjected to 10<5> rewriting operations by an ordinary rewriting process. Then, the EEPROM is baked again. By repeating this cycle, a number of possible rewriting operations can be increased substantially.

Description

DETAILED DESCRIPTION OF THE INVENTION (Field of Industrial Application) The present invention relates to a method for repeatedly rewriting a nonvolatile memory having a floating gate structure.

(Prior art) Conventionally, electrically programmable and erasable read-only memory (
Electrically Erasable and
Programmable ROM) (hereinafter referred to as EE F
As one type of ROM (ROM), a nonvolatile memory having a floating gate structure in which writing and erasing are performed by tunneling injection is well known.

FIG. 3 is a structural cross-sectional view illustrating a typical floating gate structure nonvolatile memory. In Figure 3,
A source 2 and a drain 3 made of N-type diffusion layers are formed in a P-type silicon substrate 1, and a silicon oxide film 4 is formed spanning the source 2 and drain 3, and only a portion of this silicon oxide film 4 is formed. A hole was opened, a thin silicon oxide film 5 serving as a tunneling medium was formed in the hole, and a floating gate electrode 6, a silicon oxide film 7, and a control gate electrode 8 were sequentially laminated on the silicon oxide films 4 and 5. It is a structure. A nonvolatile memory having such a floating gate structure performs charge tunneling through the thin silicon oxide film 5 on the drain 3, and stores charges in the -L floating gate electrode 6 of the thin silicon oxide film 5. The principle is to store information by changing the threshold voltage of the transistor. Therefore, as a method for rewriting a non-volatile memory as shown in FIG. This is done by applying a pulse (usually 15V to 25V),
On the other hand, when writing data, the P-type silicon substrate 1 and drain 3 are set to Ov, the source 2 is opened, and a pulse of high voltage (usually 15 V to 25 V) is applied to the control gate electrode 8. Rewriting is performed by alternately repeating and writing.

FIG. 4 is a characteristic diagram showing the deterioration of memory characteristics caused by the conventional rewriting method. When a semiconductor memory with a floating gate structure is repeatedly erased and written, as shown in FIG. There is a phenomenon in which it increases up to 10'' times and decreases when it becomes 1- after 102 times.Furthermore, when the number of repeated rewrites is increased, a destructive phenomenon occurs at about 10' to 10' times.

(Problems to be Solved by the Invention) As stated in Section 1, in the semiconductor memory with a floating gate structure, the conventional rewriting method described above has a deterioration phenomenon when the number of repeated rewrites is increased, and the number of repeated rewrites increases. When the number of times reaches approximately 104 to 1Os, a fatal destructive phenomenon occurs, which causes reliability problems.

The present invention solves the above-mentioned conventional problems, and an object of the present invention is to provide a rewriting method for a semiconductor memory having a floating gate structure that can easily increase the number of rewrites.

(Means for Solving the Problems) In order to achieve the above object, the present invention provides a method for rewriting a non-volatile memory having a floating game structure, in which baking is always performed after repeatedly rewriting a certain number of times. It is.

(Function) According to the study, holes are generated in the tunnel oxide film until the number of repeated rewrites is about 102, and these holes increase the effective tunnel electric field, so the memory window width (ΔV
th) is thought to increase, and when the number of repeated rewrites exceeds 102, electrons will become trapped in the tunnel oxide film, which will reduce the memory window width and eventually It is presumed that intrinsic destruction of the silicon oxide film occurs locally due to the charge of the trapped electrons. It was also found that electrons trapped in the tunnel oxide film can be easily released by baking at a temperature of 100° C. or higher. Therefore, by performing baking before the tunnel oxide film undergoes intrinsic destruction due to electron trapping, it has the effect of releasing the trapped electrons and increasing the number of repeated rewrites.

(Embodiment) FIG. 1 is a diagram showing a method for rewriting a nonvolatile memory according to an embodiment of the present invention. The rewriting method will be explained with reference to FIG. With the floating gate type non-volatile memory as shown in Figure 3 assembled into a package, the normal rewriting method (the conventional rewriting method described above) is performed.
Rewriting is performed up to ``'' times.Next, the package is left in a baking oven at 200°C for about 5 hours.Then, it is removed from the baking oven and rewritten in the usual way for 10'.
Rewriting is performed repeatedly, such as rewriting once and then baking again.

FIG. 2 shows the deterioration characteristics of the memory according to the rewriting method shown in FIG. 1 above. In Figure 2, the vertical axis shows the threshold voltage, and the horizontal axis shows the number of repeated rewrites. By performing baking every 10' rewrites, the voltage is restored to the initial threshold voltage, so the 10' rewrites are performed. It can be seen that even if this is done, the memory will not be destroyed and the number of rewrites can be significantly increased.

Regarding the baking temperature at which the effects of the present invention can be sufficiently obtained, a remarkable effect can be obtained at a temperature of 100°C or higher, and regarding the baking time, a sufficient effect can be obtained in 3 to 5 hours at a temperature of about 200°C. It will be done. On the other hand, the number of times of rewriting between baking is desirably 10 seconds or less because it is necessary to keep the number of times to a level that does not cause true destruction.

In this example, a method using a normal baking oven was shown as the baking method, but it goes without saying that any method that can heat the memory chip may be used, such as a method of embedding a heater in the package to heat it. stomach.

(Effects of the Invention) As is clear from the above embodiments, the present invention can easily realize a significant increase in the number of rewrites and has a great effect on improving the reliability of floating gate structure semiconductor memories.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing a processing method according to an embodiment of the present invention, Fig. 2 is a deterioration characteristic diagram showing the effect of an embodiment of the present invention, and Fig. 3 is a diagram showing a nonvolatile memory having a floating gate 1 structure. FIG. 4, which is a schematic cross-sectional view of the structure, is a diagram showing the deterioration characteristics of a memory according to a conventional rewriting method. 1...P-type silicon substrate, 2...source, 3
... Drain, 4, 7 ... Silicon oxide film, 5 ... Thin silicon oxide film serving as a tunneling medium, 6 ... Floating gate electrode,
8...Control gate electrode. Patent applicant: Matsushita Electronics Co., Ltd. Figure 3 Figure 2 ° P type - / 1 nofun difference 4 anti 2 Source 3... A list of doshin hyalurized silicon tonkis and rods. shi・）tsugae good free angle date

Claims (3)

[Claims] (1) A method for rewriting a nonvolatile memory having a floating gate structure, which comprises always performing baking after repeatedly rewriting a certain number of times. (2) The nonvolatile memory rewriting method according to claim (1), wherein the baking temperature is 100° C. or higher. (3) The method of rewriting a nonvolatile memory according to claim 1 or 2, wherein the number of times of rewriting the fixed number of times is 10^S times or less.