1. Field of the Invention
The present invention relates to thin film formation technology for integrated semiconductor circuit, and particularly to a method of forming a porous film having low dielectric constant for use as inter-layer insulation film for multi-layer wiring.
2. Description of the Related Art
Wiring widths and intervals have been constantly decreasing in recent years to respond to the needs for more highly integrated, faster semiconductor devices. As this trend accelerates, however, one problem has surfaced that pertains to signal delay caused by wiring resistance (R) and inter-wire capacity (C) in a multi-layer wiring structure constituting a semiconductor device (this delay is called “RC delay”). To reduce RC delay, the industry first adopted copper having lower resistance R in place of traditional aluminum. In 90-nm node applications, the industry also replaced silicon oxide film (SiO2 film) having a dielectric constant k of 4 used traditionally as inter-layer insulation film, with another class of film exhibiting low dielectric constant (low-k film), called SiOC film, produced by introducing methyl groups into silicon oxide film. Examples of SiOC film include siloxane polymer film (SiOC film) whose specific dielectric constant is less than 3.0, formed by the plasma CVD method using (CH3)2Si(OCH3)2 and other material gases as disclosed in U.S. Pat. No. 6,432,846.
Next-generation 65-nm and 45-nm nodes are demanding inter-layer insulation film exhibiting even lower levels of specific dielectric constant, and ITRS (International Technology Roadmap for Semiconductors) 2003 reported specific dielectric constant 2.4 for 65-nm nodes and 2.1 or less for 45-nm nodes as ideal levels of specific dielectric constant. To achieve these levels of specific dielectric constant, an attempt has been made to introduce air, which has a specific dielectric constant of 1, into film (i.e., to make pores in film). For example, porous silica film developed under the Semiconductor MIRAI Project has a specific dielectric constant of 2.0. Through X-ray measurement, this film has been shown to have many pores with a diameter of approx. 2 nm (e.g., Hata et al., J. J. Appl. Phys 43(2004), 1323, Fujii et al., Mat. Res. Soc. Symp. Proc. Vol. 812, F4.10.1).
This porous silica film is formed by means of coating a semiconductor substrate with polysiloxane solution containing pore-forming casting molecules, and then heat-treating the coated substrate. In the solution, multiple pore-forming casting molecules come together to form micelles. When heat is applied, polysiloxane constitutes a network and at the same time micelles break down and are removed from the film due to the effect of heat. As a result, polysiloxane film containing pores with a diameter of several nm is formed. The coating method allows for easy material design, and the specific dielectric constant of film as well as pore diameter can be adjusted by changing the size and quantity of pore-forming casting molecules contained in the solution.