Method for forming porous film
After applying a film-forming composition containing a polysiloxane, a pore-forming agent, an onium salt, and a solvent onto a semiconductor substrate, the solvent is evaporated from the film-forming composition in a first heat treatment. Then, a second heat treatment is carried out in an inert-gas atmosphere to promote the polymerization of the polysiloxane and thus form a polysiloxane resin film. Thereafter, a third heat treatment is carried out in an oxidizing-gas ambient to form pores in the polysiloxane resin film.
1. A method of forming a porous film comprising:
applying a film-forming composition containing a polysiloxane, a pore-forming agent, an onium salt, and a solvent onto a substrate,
evaporating said solvent from said film-forming composition in a first heat treatment,
promoting polymerization of said polysiloxane in an inert-gas atmosphere in a second heat treatment, and
vaporizing said pore-forming agent in an oxidizing-gas ambient in a third heat treatment at a temperature that is lower than the temperature of the polymerization-promoting second heat treatment.
2. The method of forming a porous film according to claim 1 , including evaporating the solvent in the first heat-treatment in an inert-gas atmosphere at a temperature not exceeding 350° C.
3. The method of forming a porous film according to claim 1 , including promoting polymerization in the second heat-treatment at a temperature not exceeding 400° C.
4. The method of forming a porous film according to claim 1 , including promoting polymerization in the second heat-treatment at a temperature not exceeding 350° C.
5. The method of forming a porous film according to claim 1 , wherein said oxidizing-gas ambient includes oxygen.
6. The method of forming a porous film according to claim 5 , wherein said oxygen contains one of ozone and oxygen radicals.
7. The method of forming a porous film according to claim 1 , wherein said polysiloxane is a hydrolytic condensation product of a compound represented by the general formula (1):
R n SiX 4−n (1)
wherein R represents a hydrogen atom or an organic group having from 1 to 20 carbon atoms, X represents hydrolysable groups which may be the same as or different from each other, and n represents an integer from 0 to 2, with the proviso that when n is 2, R may be the same or different moieties.
8. The method of forming a porous film according to claim 7 , wherein the weight-average molecular weight of said polysiloxane ranges from 300 to 20,000.
9. The method of forming a porous film according to claim 1 , wherein said pore-forming agent is a polymer having an alkylene-oxide structure with a weight-average molecular weight of from 200 to 10,000.
10. The method of forming a porous film according to claim 1 , wherein said onium salt is an ammonium salt.
11. The method of forming a porous film according to claim 1 , wherein said solvent is selected from the group consisting of an alkylene glycol dialkyl ether and a dialkylene glycol dialkyl ether.
12. The method of forming a porous film according to claim 1 , wherein said substrate is a semiconductor substrate.