Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – From silicon reactant having at least one...
Reexamination Certificate
2000-03-31
2002-01-22
Dawson, Robert (Department: 1712)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
From silicon reactant having at least one...
C528S014000, C528S021000, C528S043000
Reexamination Certificate
active
06340735
ABSTRACT:
This invention relates to a coating solution capable of forming a dense dielectric film having improved crack resistance and heat resistance and a method for forming such a dielectric film.
BACKGROUND OF THE INVENTION
As LSI elements are increased in speed and integration, more attention is paid to the delay of signals caused by the capacitance between wiring lines or between layers. As a general rule, the wiring delay is in proportion to the square root of the dielectric constant of insulating material. Reducing the dielectric constant of the interlayer dielectric is effective for reducing the wiring delay.
A variety of materials are now used as the dielectric film, but they have their own problem. It is urgently required to solve the problems, which are briefly described below.
First, the currently used interlayer dielectrics include silicon dioxide film by CVD method, and boro-phospho-silicate glass (BPSG) doped with boron, phosphorus, etc. These interlayer dielectrics based on SiO
2
have a dielectric constant of about 4.
One known method for reducing the dielectric constant of such an interlayer dielectric is to admix fluoride gas during formation of an oxide film by the CVD method, to thereby form a SiOF film which is used as the interlayer dielectric film. The silicon oxide fluoride film is intended to reduce the dielectric constant by coupling the oxide film-constructing silicon atoms with fluorine atoms having a low polarizability. The interlayer dielectric film in the form of an SiOF film, however, has the drawback that it becomes more hygroscopic as the fluorine content increases. Because of a limit on the fluorine content, the dielectric constant can be reduced to about 3.5 at the best.
The multilayer wiring technology is requisite for the fabrication of VLSIs which are seeking for higher accumulation and higher functions. In the microfabrication process, steps are formed on the substrate when wiring patterns and dielectric films are formed.
The planarization method is considered important for solving this problem. As the planarization method, a spin-on-glass (SOG) method is generally used in practice. The SOG method uses a silicon resin soluble in an organic solvent. The silicon resin solution is spin coated onto a substrate surface to bury irregularities at steps, and heat treated to form a dielectric film for providing a planar surface.
The coating solutions of alkoxysilanes such as tetraethoxysilane and triethoxysilane and halogenated silanes such as trichlorosilane are known as inorganic SOG. Since these silanes do not possess organic groups directly attached to silicon atoms, heat curing results in dielectric films of completely inorganic nature which have reliable characteristics. A number of studies have been made on the inorganic SOG.
For example, JP-A 6-42477 discloses a method for covering an electronic device with a silica thin film involving the steps of applying a solution of a hydrogensilsesquioxane resin in a solvent onto a substrate, evaporating the solvent, and heat treating at a temperature of 150 to 1,000° C. to convert the resin to ceramic silica. However, the hydrogensilsesquioxane resin usually contains a non-negligible amount of volatile component. When the resin film is exposed to high temperature, there arises the problems including the thinning of the film and internal stresses induced within the film. Additionally, the volatile component scatters to contaminate the surrounding equipment.
To eliminate these problems, it was proposed to remove a low-molecular weight fraction from the hydrogensilsesquioxane resin. For example, JP-A 6-157760 discloses to add a solvent to a hydrogensilsesquioxane resin as synthesized to remove a low-molecular weight fraction. However, when the hydrogensilsesquioxane resin from which a low-molecular weight fraction has been removed by the above method is applied to a substrate, the resulting film lacks some planarity. For application to electronic devices of multilayer structure, the same resin is inferior in the coverage of steps in substrates. The resin is thus not satisfactory.
Further, inorganic SOG films generally have a dielectric constant as high as about 4.0 because of a relatively high content of silanol groups which are polar groups. It is thus considered to use instead organic SOG films and organic dielectric films having a low dielectric constant as the interlayer dielectric for very fine LSIs.
The organic SOG films are formed by curing solutions of silanes or siloxanes containing organic components such as methyl and phenyl groups. Since organic components are left in the films after heat curing, the films have a relatively low dielectric constant of about 3.
The organic SOG films are formed by spin coating the solution, heat curing the coating and repeating the coating and curing steps several times. Undesirably, this production process takes a long time and results in a low yield. Since the majority of the solution is wasted during spin coating, the cost increases. The films have a dielectric constant of about 3.5 which is unsatisfactory.
Another known method for reducing the dielectric constant of interlayer dielectric is to form fine pores within the film. For example, triphenylsilanol is added to and reacted with the SOG solution, which is applied to a semiconductor wafer and heat treated for silylation of the SOG. Further heating completes an interlayer dielectric film. Since bubbles are generated in the spin-coated film during the SOG silylation step, pores are eventually left in the interlayer dielectric film. The porous interlayer dielectric film thus formed has a dielectric constant of about 2.3, though the dielectric constant varies with the proportion of pores. However, the moisture absorption and other problems of the porous film prohibit its application to customary semiconductor devices and electronic circuit parts.
SUMMARY OF THE INVENTION
Therefore, an object of the invention is to provide a novel and improved coating solution capable of forming a dense dielectric film having improved crack resistance and heat resistance and a low dielectric constant. Another object of the invention is to provide a method for forming such a dielectric film.
It has been found that by spin coating a coating solution of a poly(phenylsilsesquioxane) ladder polymer having a relatively low molecular weight as demonstrated by a weight average molecular weight of 4,000 to 100,000 in a solvent, there is obtained a coating having a high planarity and uniformity by virtue of the high solubility of the ladder polymer in the solvent. By heating the coating, it is converted into a tough thin film having crack resistance, heat resistance, water resistance, adhesion and strength as well as a low dielectric constant. The invention is predicated on this finding.
The invention provides a dielectric film-forming coating solution comprising a poly(phenylsilsesquioxane) ladder polymer having a weight average molecular weight of 4,000 to 100,000.
In another aspect, the invention provides a method for forming a dielectric film, comprising the steps of applying the coating solution defined above and heating the coating at a temperature of 200 to 1,000° C.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The coating solution of the invention contains a poly(phenylsilsesquioxane) ladder polymer having a weight average molecular weight of 4,000 to 100,000 as a main component.
From the past, poly(phenylsilsesquioxane) resins are utilized as coating materials, sealants and interlayer dielectrics on account of their heat resistance and electrical insulation. These resins are generally synthesized by hydrolyzing silanes having hydrolyzable groups such as phenyltrichlorosilane and phenyltrialkoxysilanes. The thus obtained resins have a structure in which siloxane chains are randomly bonded.
The randomly bonded poly(phenylsilsesquioxane) resins are in an insufficient condensed state, have a considerable amount of silanol groups or alkoxy groups left on silicon atoms, and are soluble in organic solvents. If condensation is promoted in orde
Dawson Robert
Shin-Etsu Chemical Co. , Ltd.
Zimmer Marc S.
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