JPH02117174A - Screening method for nonvolatile memory - Google Patents

Abstract

PURPOSE:To make it possible to increase the number of rewriting times for screening more than before, and besides to ship products in a state of hardly having deteriorated, by performing baking after removing inferior products by a constant number of rewriting times. CONSTITUTION:First, floating gate-type nonvolatile memories are separated into good products and inferior products by rewriting 10<4> times in a state that the memories are assembled in a package. Then, nonvolatile memories judged good are put in a baking furnace of 180 deg.C in a state of being assembled in the package as they were, and they are left there for about 10 hours. Afterwards, they are taken out from the baking furnace and undergo the final test before the shipment. As the threshold voltage almost restores their original value by performing baking for 10 hours under 180 deg.C, they show almost the same deterioration characteristics as those from their not having deteriorated state even if rewritten 10<4> times after that.

Description

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

(Prior Art) Conventionally, electrically programmable and erasable EEPROM (E), electrically
Erasable and programmable
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.

A typical example of this floating gate type nonvolatile memory will be explained with reference to FIG. In this process, sources and drains 2 and 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 sources and drains 2 and 3. lI! A hole is formed only in a portion of I4, and a thin silicon oxide film 5 that can serve as a tunneling medium is formed in this hole, and a floating gate electrode 6, a silicon oxide film 7, and a control gate electrode 8 are formed on the silicon oxide films 4 and 5. It has a structure in which these are sequentially laminated.

A nonvolatile memory having such a floating gate structure tunnels charges by applying a high electric field to the thin silicon oxide film 5 on the drain 3, and transfers charges to the floating gate electrode 6 on the thin silicon oxide film 5. The principle is to store information by accumulating it and changing the threshold voltage of the transistor. Therefore, in a floating gate non-volatile memory as shown in Figure 3, a high electric field is applied to the tunnel oxidation [5] during rewriting, so the most important point in ensuring the reliability of the floating gate non-volatile memory is This is the number of times data can be rewritten. Conventionally, in order to guarantee a certain number of rewrites, all data must be rewritten approximately 1/10 to 1/1.00 times of the guaranteed number of times. A method was used to screen for early failures.

(Problem to be Solved by the Invention) However, since the conventional screening method as described above is a type of destructive test, it is not possible to rewrite the screening very many times, and at the same time, the product cannot be used in a deteriorated state to some extent. There was a problem with the product being shipped.

In view of these problems, an object of the present invention is to provide a screening method for non-volatile memory having a floating gate structure, which allows the screening to be rewritten more times than before, and which can be shipped in a state with almost no deterioration. There are two things to offer.

(Means for Solving the Problems) In order to achieve the above object, the present invention is characterized in that after defective products are removed by rewriting a certain number of times, baking is performed.

(Function) According to the inventor's study, when writing and erasing are repeatedly performed on a nonvolatile memory with a floating gate structure,
The memory window width ΔVih (the difference in threshold voltage between the written state and the erased state) increases until the number of rewrites reaches ~102 times, decreases when the number of rewrites exceeds 102 times, and becomes smaller when the number of rewrites exceeds 102 times.
It was found that fatal damage to the memory occurs after approximately 10 to 10 times.The reason for this is that holes are generated in the tunnel oxide film until the number of rewrites is approximately 102 times. It is thought that the memory window width ΔV < h increases because the effective tunnel electric field increases, and furthermore, when the number of repeated rewrites exceeds 103, electrons become trapped in the tunnel oxide film. It is presumed that the memory window width Δv0 decreases and eventually the trapped electron charges cause local intrinsic destruction of the silicon oxide film.

It has also been found that electrons trapped in the tunnel oxide film due to repeated rewriting can be easily released by baking at a temperature of 100° C. or higher, as shown in FIG.

The present invention was made based on the above fact, and the electrons trapped in the tunnel oxide film during selection of defective products by a certain number of rewrites are released by baking after one selection, and the initial undegraded We are trying to restore the product to its original condition before shipping it.

(Example) Specific embodiments of the present invention will be described with reference to the drawings. .

The first @ is a diagram showing an embodiment of the screening method of the present invention. First, a floating gate type non-volatile memory as shown in FIG. 3 is assembled into a package and rewritten 104 times to select defective products from non-defective products. Next, the nonvolatile memory of good quality is placed in a baking oven at 180° C. in its packaged state and left for about 10 hours. After that, it is taken out of the baking oven, subjected to a final inspection, and shipped.

Perform the screening method of the present invention as described above.

Figure 2 shows the memory deterioration and recovery characteristics when
As shown in Figure 2, after screening by rewriting 10' times, baking at 180°C for 10 hours restores the threshold voltage to the initial threshold voltage, and then rewriting 104 times. However, the deterioration characteristics are almost the same as the deterioration characteristics from the undegraded state.

(Effects of the Invention) As is clear from the above explanation, according to the present invention, even if defective products are sorted out by rewriting a certain number of times, products can be shipped in a state with almost no deterioration. As a result, it is possible to recover almost to the initial state, making it possible to increase the number of screening rewrites than before, which greatly contributes to ensuring the reliability of nonvolatile memories with a floating gate structure.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the operation flow of an embodiment of the present invention, Fig. 2 is a threshold voltage characteristic diagram due to a certain number of rewrites and baking, and Fig. 3 is a cross section of a floating gate structure nonvolatile memory. FIG. 4 is a diagram showing the amount of change in window width with respect to baking temperature. 1...P-type silicon substrate, 2,3...source,
Drain, 4... silicon oxide film, 5...
tunnel oxide film, 6... floating gate electrode, 7... silicon oxide film, 8... control gate electrode. Patent aJI applicant Matsushita Electronics Co., Ltd. Fig. 3 1°-P-type silicon base 1 anti-2.3-... Source, Doi 4°°° Tempered Siscon film 5... Tonsei-sealed film 6...・Floating gate electric current 27...Iodized Siscon film 8...Zuntro I rev'=T to extremely low speed 7 linear link torsion excellence ξ and return ゛-Pinching AL(0C)

Claims (1)

[Claims] A method for screening a non-volatile memory having a floating gate structure, the method comprising: removing defective products by rewriting a certain number of times, and then baking the memory.