JPH0738429B2 - Method of manufacturing memory device - Google Patents

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method of manufacturing a memory device having resistance to alpha rays.

2. Description of the Related Art When a semiconductor memory device is irradiated with radiation, for example, α-rays, electron-hole pairs are generated in the semiconductor substrate due to ionization by the α-rays.
In the case of a MOS transistor, holes flow down to the substrate electrode, electrons are collected in the active region, and inversion of stored information occurs. This phenomenon is called "soft error",
Trace amounts of uranium U and thorium contained in packaging materials
It is caused by alpha rays generated when Th decays. As a measure to prevent this soft error, "the package material is highly purified to reduce the amount of U: Th. The element surface is coated with a high-purity α-ray shielding material. The element itself has a structure that is strong against α-rays. There is a means such as “Yes.”, But the easiest and most effective method is widely adopted.

Problems to be Solved by the Invention Generally, in the α ray shielding means, it is a mainstream to drop a high-purity polyimide resin or silicone resin on a chip.
However, since the α-ray resistance varies depending on the manufactured memory device, the coating film thickness is changed by changing the coating drop amount experimentally, and the device effect at each film thickness is softened by forced irradiation with artificial α-rays. An error acceleration test was performed and evaluated, and the coating film thickness was determined from the obtained information. However, the energy of α rays emitted from uranium and thorium in the actual packaging material is up to 9M.
It is distributed up to near ev and is an artificial α used for accelerated tests.
Since the energy distribution of the radiation source is concentrated around 5.5 Mev, the optimum coating thickness obtained from the experimental value is 5.5 M
There is a problem that it cannot shield α rays having energy of ev or more. Further, due to the concern about this problem, in the resin-sealed semiconductor memory device, the film thickness of the sealing resin on the chip is significantly reduced in the resin-sealed semiconductor memory device by dropping more coating material than necessary, which causes a decrease in mechanical strength. was there.

Means for Solving the Problems According to the present invention, in order to solve the above-described problems, a first step of calculating a distance from a radiation source of α-rays and the magnitude of incident energy at that distance, and a memory element Between the radiation source and the memory element in which the memory element generates a soft error by performing the writing / reading operation while changing the distance between the radiation source and the memory element while irradiating the α-ray with A second step of determining the critical incident energy at that time based on the critical distance and the first step, and irradiating the α-ray to the coating material to be capped on the memory element to obtain the α-ray of the predetermined film thickness. And a third step of obtaining energy, the critical incident energy and the third step
Based on the above process, the optimum film thickness of the coating material is derived and the coating material on the memory element is capped.

Action According to the method described above, the critical energy at which a soft error occurs is obtained from the result of the acceleration test by the α-ray forced irradiation, and by comparing this critical energy with the detection result of the α-ray energy blocking effect of the coating resin,
It becomes possible to determine the optimum coating film thickness for any of the memory elements having different α-ray tolerance.

Examples Examples of the present invention will be described below. FIG. 1 is a flow chart for carrying out the present invention. First, as the first step,
The relationship between the distance from the α-ray source and the magnitude of the α-ray incident energy at that distance is calculated by theoretical calculation or experiment. As an α-ray source, about the energy emitted from the same source
Artificial 241Am (Americium 241), where 90% is concentrated around 5.5 Mev, is suitable. Next, as a second step, the distance between the α-ray source and the memory element when the soft error occurs in the memory element is performed by performing the write / read operation on the memory element while irradiating the memory element with this α-ray. . At this time, the memory element is not covered with a coating material but only has air interposed. By this, the relationship between the distance from the α-ray source and the soft error rate can be obtained. Also,
When the distance from the α-ray source is obtained, the incident energy of the α-ray at that time is found from the relationship obtained in the first step. At the same time, the critical incident energy at which soft error does not occur or can be ignored can be obtained. Next, in the third step, the material used for the coating material is irradiated with α-rays, and the magnitude of incident energy at a predetermined film thickness is obtained. The optimum coating film thickness is obtained from the above first to third steps.

FIG. 2 shows the relationship between the source distance obtained by theoretical calculation or actual measurement obtained in the first step and the incident energy of α rays at that distance. The one shown in FIG. 2 shows that the size of the α ray at the zero source is about 5.5 Mev and the incident energy is about 1.0 Mev when the source distance is 3.0 cm.

FIG. 3 corresponds to the second step. The measured values of the relationship between the distance from the radiation source of the α ray and the soft error are shown when the uncoated memory element is irradiated with the α ray. For example, when the incident energy of α rays is 5.5 Mev (that is, the energy of the source), the soft error rate is about 0.45. That is, 45% of the total indicates that a soft error has occurred. We also show that the soft error rate was zero when the incident energy was 1.0 Mev. Therefore, in order to completely eliminate the soft error, the incident energy of α-rays arriving at the memory element is
It suggests that the thickness should be selected so that the energy is attenuated to 1.0 Mev or less. This 1.0 Mev is the critical energy. However, the incident energy at which the soft error becomes zero does not necessarily have to be the critical energy. You may choose a soft error rate that does not hinder practical use. Further, since the characteristics shown in FIG. 3 vary depending on the capacity and size of the memory or the memory element structure, it is necessary to prepare some characteristic diagrams corresponding to these.

FIG. 4 corresponds to the third step, and shows incident (residual) energy of α-rays at a certain film thickness, for example, by irradiating the polyimide resin with α-rays. For example, it shows that when the coating thickness is zero (that is, at the point of contact with the radiation source), the incident energy is about 8.8 Mev. Also, the coating thickness is 20
At μm, it indicates that the energy was about 5.0 Mev. Further, it is shown that the incident energy of α ray is almost zero when the film thickness is about 70 μm.

Now, in the present invention, the film thickness of the resin coating the memory element is determined in the next step. First, the critical incident energy is determined from the actually measured value of FIG. 3 obtained in the second step. Third
The critical energy value found in the figure is 1.0 Mev.
The value of 1.0 Mev is adjusted to the scale of the "incident energy of α rays" (vertical axis) in Fig. 4 obtained in the third step, and the scale of the "coating film thickness" (horizontal axis) is read. The value is about 55 μm. Therefore, in order to suppress the occurrence of a soft error due to α rays, it is sufficient to coat at least 55 μm or more of polyimide resin. The resin is not limited to the polyimide resin and may be any material that absorbs and attenuates α rays. Also, for several types of resins,
If the characteristic diagram shown in FIG. 4 is prepared, the resin can be selected according to the critical incident energy of each memory element, so that it is not necessary to use a resin which is more expensive than necessary. There is no need to excessively increase the film thickness.

EFFECTS OF THE INVENTION As described above, according to the present invention, α-ray resistance characteristics due to differences in memory capacity, memory element size, memory element structure, package, etc. and α-ray resistance characteristics of the coating material to be capped are separately set. By preparing and combining these, it is possible to determine the optimum material and its film thickness in the memory integrated circuit device in the finished product in the state before the finished product. As a result, the optimum coating material and its film thickness can be applied to the finished product of the memory integrated circuit device without excess or deficiency. Therefore, it is possible to increase the thickness of the resin of the package on the memory element without covering the coating material with an unnecessarily thick coating material, and it is also possible to prevent a decrease in mechanical strength.

[Brief description of drawings]

FIG. 1 is a flow chart for carrying out the present invention, FIG. 2 is a characteristic diagram showing the relationship between the radiation source distance to the memory element and the incident energy of α rays to the element, and FIG. 3 is the incident energy of α rays ( FIG. 4 is a characteristic diagram showing the relationship between the radiation source distance) and the soft error rate, and FIG. 4 is a characteristic diagram showing the relationship between the film thickness of the coating resin of the present invention and the incident energy of α rays to the memory element.

Claims (1)

[Claims] 1. A first step of calculating the distance of an α-ray from a radiation source and the magnitude of incident energy at that distance, and writing / reading operation while irradiating the α-ray to a memory element, And changing the distance between the radiation source and the memory element to obtain a critical distance between the radiation source and the memory element at which the memory element causes a soft error and the critical incident energy at that time from the first step. The second step, and the third step of irradiating the coating material covering the memory element with the α-rays to obtain the energy of the α-rays in a predetermined film thickness. A method of manufacturing a memory device, comprising: deriving an optimum film thickness of the coating material based on the steps and applying a coating material onto the memory device.