1. Field of the Invention
This invention relates to a process for forming a heat resistant resin film used in semiconductor integrated circuit devices, and more particularly to a process for forming a heat resistant resin film which can be used as an intermediate insulating layer.
2. Description of the Prior Art
These days, a multi-layer interconnection technique is essential to increase the packing density of an integrated circuit device. In the multi-layer technique, multi-leveled conductive layers are electrically insulated from one another by insulating layers. The insulating layer is necessarily subjected to a relatively high temperature of about 500.degree. C. for at least 10-20 minutes during a heating process. Therefore, it is required that the insulating layer material has its superior heat-resistant property and is chemically stable during a heating process. Well-known polyimide resins and chemically-produced silicon dioxide have been used as such insulating materials. For example, a polyimide resin is described in detail in Japanese Patent Publication No. 44871/1976.
In order to form a polyimide resin film, a polyamic acid solution is coated on the surface of a silicon substrate. The polyamic acid is a polyimide resin precursor and has the following molecular structure. ##STR1## When the polyamic acid is heated at a temperature of approximately 200.degree. C. to vaporize the solvent therein, it turns through dehydration into a polyimide resin film having the following ring closure structure. ##STR2## However, such polyimide resin film gradually decreases in weight and thickness when heated in an oxygen atmosphere at a temperature of 420.degree.-470.degree. C. At 530.degree. C., this resin film decomposes by being baked within about 10 minutes as shown as the line 1A in FIG. 1. Therefore, the polyimide resin film shows poor heat resistant properties.
In addition, the polyimide resin has a relatively large ion mobility, so that it is not proper as a passivation material for suppressing the leak current in a silicon substrate.
On the other hand, a chemically produced silicon dioxide is described in detail in Japanese Patent Publication Nos. 20825/1977 and 16488/1977. An organosilicic compound, for example, is obtained by dissolving a silicon acetate as a principal constituent into ethyl alcohol. When the silicon acetate solution obtained is coated on the surface of a silicon substrate, and heated at a temperature of approximately 400.degree. C. in an oxygen atmosphere, the organosilicic compound turns into a chemical SiO.sub.2 film. Such SiO.sub.2 film features a high heat resistant temperature of about 1000.degree. C. and can be used as a mask against impurities. However, it is difficult to form a thick film of 1-2 .mu.m from an organosilicic compound, because it shrinks by heat treatment and cracks.
As described above, both a polyimide resin and a chemically produced SiO.sub.2 are not enough as intermediate insulating materials for electrically insulating laminated conductive layers in semiconductive ICs.