A heat resisting property to withstand heat generation of semiconductor devices and LED in operation is required for a sealing material for semiconductor. Furthermore, transparency is also required in addition to the heat resisting property for a sealing material for semiconductor for optical components such as LED. As these sealing materials, epoxy resins or cured products prepared by curing silicones have been used. Furthermore, in the present description, “silicone” means a polymer compound having a main skeleton by siloxane bond.
However, conventional epoxy resins have insufficient heat resisting properties to be used for sealing materials for semiconductor of power semiconductor, or high-brightness light emitting elements such as a high-brightness LED for a headlight of automobile or for a backlight of liquid crystal television, or semiconductor laser, so it has been known that malfunctions such as a leakage of electric current by heat deterioration of sealing material or yellowing of sealing material occur.
Furthermore, for a sealing material for semiconductor, little transit of water to restrain a leakage of electric current or short circuit and good adhesion to substrates or light emitting elements, etc. have been required.
As a sealing material for semiconductor fulfilling such requirements, a cured product of silicone is used for a sealing material of LED. In sealing an LED with a sealing material for semiconductor, a curable composition containing a silicone as a precursor of sealing material is cured to seal. At that time, a method for curing the curable composition can be divided into the following two ways. The first method is a method that a silicone is cured by addition polymerization by hydrosilylation. The second method is a method that an alkoxysilane is heated and cured by dehydration condensation of Si—OH group (silanol group).
The first method is mainly used in using a silicone as an LED sealing material, etc. Curing by hydrosilylation reaction, which is the first method, is an addition polymerization reaction between Si—H group (hydrosilyl group) and Si—CH═CH2 (vinylsilyl group) group, and molding the cured product is easy. However, in the first method, the cured product has a low heat resisting property as compared with in the second method, so it is easy to cause yellowing by a long heating.
In the second method, a cured product of silicone having a higher heat resisting property can be obtained than in the first method because a siloxane bond (Si—O—Si) is formed to bridge it by dehydration condensation between Si—OH groups (hereafter it may be just referred to as “condensation”). Furthermore, as it contains a higher content of Si—OH group, adhesion to substrates such as glass and silicon is improved. However, in the second method, there have been problems that the curable composition to make Si—OH group react at normal temperature (it is defined as approximately 20° C. in the present description) is easy to gel, so there is a fear in storage stability, and that it is easy to foam in molding curing, so curing time is longer as compared with in the first method, etc.
For example, it is reported that a silicon-containing curable composition disclosed in Patent Publication 1 has an excellent storage stability and its cured product has an excellent heat resisting property and an excellent flexibility.
Furthermore, in Patent Publication 2, it is disclosed that a silicone resin obtained by dehydration condensation of a specific alkoxysilane has a high heat resisting property. It is easy to obtain a thin film by coating a base body with the present silicone resin. However, molding a bulk body by potting process is difficult. The reason is that water is generated as a by-product in the process of dehydration condensation of Si—OH group, and foaming is generated within the bulk body before curing by the water vapor, so the foam remains within the bulk body after curing.
Furthermore, in Patent Publication 3, a sealing agent for optical semiconductor devices which has an excellent defoaming property is disclosed. The sealing agent for optical semiconductor devices described in Patent Publication 3 contains a first silicone resin (however, excepting a silicone resin having a hydrogen atom bonded to a silicon atom) that is synthesized by subjecting an alkoxysilane compound to a hydrolysis polycondensation by a base catalyst, that has an aryl group and an alkenyl group, and that is substantially free from an alkoxy group; a second silicone resin that is synthesized by subjecting an alkoxysilane compound to a hydrolysis polycondensation by an acid catalyst, that has an aryl group and a hydrogen atom bonded to a silicon atom, and that is substantially free from an alkoxy group; and a catalyst for hydrosilylation reactions. In obtaining a silicone resin as a raw material of the sealing material by condensation of an alkoxysilane while using an acid or a base as a catalyst for hydrosilylation reaction, the alkoxysilane contained in the silicone resin is reduced as much as possible by adding a monofunctional alkoxysilane such as trimethylmethoxysilane. Thereby, in curing the sealing agent for optical semiconductor devices, foaming is inhibited by reducing an alcohol generated by condensation of the alkoxysilane and Si—OH group. However, an adjustment of the content of Si—OH group in a silicone resin is not described at all in Patent Publication 3, and it is not considered that adhesion is improved while inhibiting foaming in curing. In a silicone synthesized by condensation of alkoxysilane, there exists a problem that strictly controlling the content of Si—OH group is difficult.
In a curable composition containing a silicone, it is preferable that the molecular weight of the used silicone can be easily adjusted. Furthermore, if the obtained cured product has an excellent heat resisting property without foaming in curing, it can be regarded as a useful sealing agent for semiconductors. For example, in a sealing material of high-brightness light emitting elements such as LED and semiconductor laser, further high heat resisting property has been required.
Furthermore, in a sealing material for semiconductors, maintaining transparency without deterioration such as degeneration and coloring under high temperatures and low moisture permeability to protect a semiconductor chip are required. However, in a sealing material for semiconductors using a conventional epoxy resin or silicone resin, a sealing material satisfying both maintaining sufficient transparency and low moisture permeability even exposed to high temperatures of 150° C. or higher for a long time has not ever been known.
A silicone obtained by condensation of Si—OH group (hereinafter it may be just referred to as silicone) maintains transparency and has low moisture permeability even exposed to high temperatures for a long time, and besides its adhesion to substrate is high. However, this silicone has a problem that molding of a bulk body is difficult as foaming is generated in heating. In carrying out a condensation of Si—OH groups, it is possible to obtain a bulk body with no air bubbles by conducting a molding by gradually raising the temperature and selecting a condition to avoid the generation of air bubbles in the bulk body after water as a by-product forms bubbles as water vapor. However, to obtain a bulk body having no bubble, in forming the bulk body, it is necessary to very slowly raise the temperature from a low temperature and to finally heat at a high temperature for a long time, so the productivity is low. Therefore, there has been a problem that the use for a sealing material for semiconductors such as LED and semiconductor laser is difficult.
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