disclosed herein are an insulating film forming method for semiconductor device interconnection and a plasma treatment system for use in the method . the method comprises a step of forming an insulating film free of void , on a substrate having an interconnection pattern , in which a mixed gas of a film forming source gas and an etching gas comprising a fluorine compound is used to perform both deposition of an insulating film by plasma cvd and reactive etching of the insulating film , simultaneously , and a step of planarizing the surface of the insulating film formed by the step and comprised of , for example , silicon oxide , in which a gas of a material decomposable by a reactive gas capable of decomposing the insulating film is supplied onto the substrate so as to deposit a solid film of the material , e . g . si 4 , on the insulating film on the substrate , the temperature of the substrate is then raised to liquefy the deposited material film , thereby planarizing the surface of the material film , the substrate temperature is again lowered to solidify the planarized liquid material film , and thereafter the insulating film together with the si . sub . 4 solid film is subjected to plasma etching with , for instance , cf 4 gas as an etching gas . according to the invention , an insulating film which is planar and free of voids can be formed on an interconnection pattern of a substrate , without affecting the characteristics of devices provided on the substrate .