Active solid-state devices (e.g. – transistors – solid-state diode – With means to control surface effects – Insulating coating
Reexamination Certificate
1994-12-22
2004-06-08
Ngô, Ngân V. (Department: 2814)
Active solid-state devices (e.g., transistors, solid-state diode
With means to control surface effects
Insulating coating
C257S640000, C257S684000, C257S698000, C257S773000, C257S774000
Reexamination Certificate
active
06747339
ABSTRACT:
BACKGROUND OF THE INVENTION
i) Field of the Invention
This invention relates to a semiconductor device of very excellent reliability and a method for producing the same, and more particularly to a semiconductor device free of any trouble due to alpha-rays from a package and a method for producing the same.
ii) Brief Description of the Prior Art
In general, semiconductor devices are sealed usually by the ceramic package (including also a method—cerdip—in which the ceramic package is sealed with glass), the plastic package, or the like. Especially in the ceramic package among these packages, ceramic materials contain uranium, thorium etc. on the order of several ppm. As stated in, for example, the “16th Proceedings of Reliability Physics (1978)”, at page 33, it has been known that these impurities emit alpha-rays and therefore cause failures (called “soft errors”) in memory devices etc. For this reason, the reliability of the semiconductor devices may lower conspicuously.
On the other hand, when powder, such as alumina, called filler is used in plastic package materials, the impurities are also contained in the filler. The filler is less influential on the integrated circuit devices than in the case of the ceramic encapsulation because it is surrounded with plastic materials such as epoxy resin and the alpha-rays emitted from the impurities are absorbed by the plastics. These plastic materials, however, have the disadvantages of comparatively low reliability in moisture resistance and heat resistance on account of the fact that the moisture absorbability is high, the fact that the contents of alkali impurities typified by Na are high and the fact that the heat resistance is as low as at most about 150° C. It is therefore difficult to employ plastic-encapsulated integrated circuit devices in systems of high reliability. Accordingly, integrated circuit devices for use in the high-reliability systems need to be subjected to the ceramic type encapsulation.
For these reasons it has been strongly desired to prevent soft error of the semiconductor memory device caused by alpha-particles coming from uranium and thorium contained in the ceramic package as impurities.
In
Electronics,
Jun. 8, 1978, pages 42-43, it is shown that this problem is attacking from several different directions, for instance, by devising new packaging material process or applying protective coating to the upper surface of the chip.
However, this article does not disclose the most favorable materials or necessary characteristics of the protective coating in order to prevent soft error caused by alpha particles.
IEEE Journal of Solid State Circuits,
vol. SC-13, No. 4, August, 1978, pages 462-467 shows planar multilevel interconnection technology employing a polyimide resin.
However, in this article, the polyimide films having a thickness of 2.5 &mgr;m are employed for interlevel dielectrics and final passivation. It must be recognized, however, that such thin films of polyimide cannot prevent penetration of alpha particles coming from outside of the film such as the ceramic package.
The use of the polyimide film as the protective film or coating to prevent soft error of the semiconductor memory device caused by alpha particles is not disclosed in this article.
Also, some prior art references show the use of polyimide resin in the field of the semiconductor devices, however, none of these references show the use of polyimide as a protective film or coating to prevent soft error of a semiconductor memory device caused by alpha particles.
For instance, U.S. Pat. No. 4,017,886 provides a polyimide layer between an SiO
2
layer and a metal layer to bond a wire with the electrode very easily by forming a flat upper surface on which the metal layer is formed.
Furthermore, Japanese Patent publications No. 47-12609 and No. 52-26989 show the uses of polyimide for insulation and final passivation, respectively.
There is no disclosure in those prior art references concerning prevention of soft error caused by alpha particles.
SUMMARY OF THE INVENTION
This invention has for its object to eliminate the disadvantages of the prior art, and to provide a semiconductor device which maintains a high reliability for moisture and heat exhibited by the ceramic encapsulation wherein the failure of an integrated circuit due to alpha-rays as previously stated, is prevented from occurring; as well as a method for producing such a semiconductor device.
In order to accomplish the object, a semiconductor device according to this invention has a coating film on at least a region of an element in a semiconductor substrate having at least one element, the coating film being made of a polyimide resin or a polyimide isoindoloquinazolinedione resin (hereinbelow, written “PII resin”) and being 10 &mgr;m or more thick, and it is encapsulated in a ceramic package.
The semiconductor device of this invention causes the polyimide resin or the PII resin to attenuate and absorb alpha particles which emit from impurities contained in a package material. Accordingly, the resin coating film to serve as an attenuating material and an absorbing material is required to be a film thick to the extent of preventing the alpha particles from penetrating therethrough. In order to avoid any fluctuation in the characteristics of the element, the thickness should preferably be at least 10 &mgr;m and more preferably be at least 30 &mgr;m. The capability of preventing the penetration of the alpha particles is not limited to the resin coating films, but it is generally possessed by insulating films. It is extremely difficult, however, that insulating films of silicon dioxide, phophosilicate glass, silicon nitride, aluminum oxide etc. having heretofore been employed in semiconductor devices are deposited on semiconductor substrates 10 &mgr;m or more. More specifically, these insulating films formed by the chemical vapor deposition undergo very high stresses and cause cracks when deposited several &mgr;m or more. With the sputtering process, the insulating films can be deposited under the condition under which the proportion of development of the cracks is held comparatively low. However, the deposition rate is as very low as several hundreds Å/min, and it is actually impossible to deposit the films 10 &mgr;m or more. In contrast, with the polyimide resin or the PII resin, the stress of the film is as very low as about 4 Kg f/mm
2
. In addition, the breaking distortion is about 30%, which is approximately one order greater as compared with those of the aforecited inorganic insulating films. Therefore, a thick film of several tens &mgr;m can be formed under the condition under which quite no crack develops. On the other hand, among high polymer resins, some possess film forming characteristics similar to those of the polyimide resin and the PII resin. Since, however, the sealing step of the ceramic package is ordinarily executed at high temperatures of around 450° C., a heat-resisting property enough to endure the temperatures is required, and no resin other than the aforecited ones satisfies this property.
More specifically, as exemplified in
FIG. 1
, when various high polymer resins are subjected to thermogravimetric analyses, decreases in weight begin at 200°-250° C. in case of a silicone resin
13
and in case of an epoxy resin
14
, whereas a decrease starts at 500° C. in case of the polyimide resin
12
. In case of the PII resin
11
, the heat resistance is more excellent, and the weight residue at 600° C. is approximately 70% which is the most excellent. In this manner, the polyimide resin or the PII resin has the heat-resisting property against the high temperature step described previously.
In the PII resin or the polyimide resin, the contents of impurities such as uranium and thorium functioning as alpha-ray generating sources are as very low as 0.1—several ppb or so (the impurity analyses resorted to radioactivation analyses). Accordingly, the PII resin or the polyimide resin stops the alpha-rays emitted from the ceramic package material, and simultaneously,
Harada Seiki
Mukai Kiichiro
Saiki Atsushi
Dickstein , Shapiro, Morin & Oshinsky, LLP
Hitachi , Ltd.
Ngo Ngan V.
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