Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2001-06-27
2002-07-30
Foelak, Morton (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S061000, C521S064000
Reexamination Certificate
active
06426372
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
The present invention relates a porous polyimide, which is a polyimide used as a molded article such as film, etc. and has a plurality of micropores which is uniformly present from the inside through the surface of the molded article. It is possible to effectively control physical properties such as dielectric constant, refractive index. etc. of the polyimide by introducing such a porous structure. A film of such a polyimide can be used, for example, as an interlayer insulation film for a semiconductor element such as ULSI (Ulira LSI), etc.
PRIOR ART
As the interlayer insulation film of the semiconductor element, use of a polyimide film has hitherto been studied. Because the polyimide sufficiently satisfies required characteristics for the above interlayer insulation film, that is heat resistance, mechanical strength. solvent resistance, etc. However, when using the polyimide film as the above interlayer insulation film, it was also necessary to reduce the dielectric constant of the polyimide.
As a means for reducing the dielectric constant of the polyimide, for example, a method of introducing a fluorine atom or an alkyl group into a polyimide molecule as a substituent, or a method of incorporating a fluororesin filler by dispersing it into a polyimide matrix has been made. The method of introducing the substituent into the polyimide molecule is disclosed, for example, in Unexamined Patent Publication (Kokai) No. 4-328126. The method using the fluororesin filler is disclosed, for example, in Unexamined Patent Publication (Kokai) No. 4-33394. However, when the above substituent is introduced into the polyimide molecule or the fluororesin filler is incorporated. introduction of a comparatively large amount of the substituent or the filler content was required so as to effectively reduce the dielectric constant. Introduction of a large amount of the substituent or increase in content of the fluororesin filler tends to deteriorate the heat resistance of the polyimide. That is, according to such a method, it is necessary to largely change the structure of the polyimide molecule or composition of the polyimide matrix so as to reduce the dielectric constant. Therefore, intrinsic performances of the polyimide were liable to be deteriorated.
On the other hand, as another method, a method of producing a porous article, which comprises containing hollow microspheres or microporous acrogels by dispersing in a polyimide matrix and thereby previously introducing pores in an uncalcined precursor has also been studied. Such a method is disclosed, for example, in Unexamined Patent Publication (Kokai) No. 5-182518. According to such a method, since it is not necessary to largely change the structure of the polyimide molecule or composition of the polyimide matrix. intrinsic performances of the polyimide are not liable to be deteriorated.
Furthermore, Japanese Patent No. 2531906 corresponding to U.S. Pat. No. 5,776,990 discloses a foamed polyimide film obtained by forming a block copolymer of (a) a polyimide and (b) a thermally decomposable polymer, which is thermally decomposed at the temperature lower than a decomposition temperature of the polyimide, and then heating a film of the block copolymer to decompose the above thermally decomposable polymer. As the thermally decomposable polymer, polypropylene oxide and polymethyl methacrylate are disclosed and the polymer film contains pores having a diameter of less than 1000 angstroms (100 nm). According to such a method, since it is not necessary to largely change the structure of the polyimide molecule or composition of the polyimide matrix, intrinsic performances of the polyimide are not liable to be deteriorated.
As other alternative methods, a method for manufacturing a porous polyimide by forming a precursor comprising a blowing agent dispersed in a polyimide matrix, and heating the precursor to expand and foam it (disclosed in European Patent Publication No. 0373402), a method for manufacturing a porous polyimide by forming a precursor comprising a polyimide matrix and an organic polymer dispersed in said matrix, and decomposing and removing the organic polymer (disclosed in U.S. Pat. Nos. 3,883,452, 4,940,733 and 5,135,959, and Japanese Unexamined Patent Publication (Kokai) No. 63-278943) are known. As the above organic polymers, formaldehyde homopolymer, polypropylene carbonate), polyurethane, melamine, polyvinyl alcohol, polyacetal and polymethyl methacrylate, etc. are used. In the latter method in which an organic polymer is decomposed, a dispersion of one component (for example, an organic polymer) finely dispersed in the solution comprising the other component (for example, a polyimide) is prepared, and then the dispersion: is dried to form a dispersion-type precursor consisting of said one component dispersed in the matrix comprising the other component. According to these methods since it is not necessary to largely change the structure of the polyimide molecule or composition of the structure of the polyimide matrix, intrinsic performances of the polyimide are not liable to be deteriorated.
PROBLEMS TO BE SOLVED BY THE INVENTION
However, according to the method of previously introducing pores in the uncalcined precursor, the pores were apt to be broken during the calcination and it was difficult to form micropores (e.g. diameter of pores is within the range from 1 to 700 nm). Further, according to a method using a blowing agent or using a dispersion-type precursor, it was also difficult to form relatively fine pores. In these methods, relatively large size pores (for example, diameter of the pores is no less than 1 micrometer) was apt to be formed, and therefore, it was not possible to use the resulting polyimide as an optical material.
According to the method of forming the block copolymer of the polyamide and thermally decomposable polymer, although it was easy to form relatively fine pores, the synthesis reaction of the block copolymer was comparatively difficult, and thus it was disadvantageous to enhance the productivity.
Accordingly, an object of the present invention is to provide a porous polyimide, which can effectively reduce the dielectric constant (e.g. 2.7 or less) and refractive index of the polyimide while preventing deterioration of intrinsic performances (e.g. heat resistance, etc.) of the polyimide by uniformly introducing micropores having a comparatively small size (e.a. diameter of pores is within the range from 1 to 700 nm) from the inside through the surface, and which can be easily formed (produced), which is advantageous to enhance the productivity, and which is highly transparent.
MEANS FOR SOLVING THE PROBLEM:
The present invention has been accomplished to solve the above problems, and provides a porous polyimide comprising polyimide-containing a matrix, and a plurality of micropores dispersed in said matrix, characterized in that said micropores are formed by removing a hydrophilic polymer from a precursor comprising a polyimide-containing matrix and the hydrophilic polymer dispersed in said matrix.
MODE FOR CARRYING OUT THE INVENTION OPERATION
The porous polyimide of the present invention is formed by removing a hydrophilic polymer from a precursor comprising a polyimide-containing matrix, and the hydrophilic polymer dispersed in the matrix. and it contains a plurality of micropores formed uniformly from the trace of the above removed hydrophilic polymer. Accordingly, pores having a very small size (diameter is normally within the range from 1 to 700 nm) are uniformly present from the inside through the surface, and the porous polyimide has the effectively controlled dielectric constant. According to the present invention, the dielectric constant can be controlled typically to 2.7 or less, suitably to 2.6 or less, particularly 1.9 to 2.5.
The main reason why the dielectric constant is lowered is that an air (pore) having a dielectric constant lower than that of the polyimide is introduced in the matrix. Since the air has a refractive index lower than that of the polyim
Kobayashi Mitsuaki
Minami Kazuhiko
3M Innovative Properties Company
Dahl Philip Y.
Foelak Morton
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