ABSTRACT
Vapor phase deposition (VPD) or vapor deposition polymerization (VDP) is an alternative to the industrially important spin-coating (SC) method to prepare polyimide films [1, 2]. In the vapor phase deposition process the diamine and the dianhydride are coevaporated onto a substrate where they react to form a polyimide precursor, which converts to polyimide upon thermal curing. Vapor phase deposition is a dry process and does not require the use of solvents. This can be advantageous in some applications, where the use of solvents can have deleterious effects or where contaminations have to be strictly avoided. Vapor phase deposition can be used to prepare thin polyimide films on nonplanar substrates, which is not possible using the spin-coating method. However, vapor phase desposition is a slower and more expensive process compared to spin coating and this makes it less suitable for large-scale industrial applications. The preparation of polyimide films by vapor phase deposition was introduced independently by Iijnna et al. [1] in Japan and by Salem et al. in the United States [2] in 1985 and 1986, respectively. In 1987 Grunze and Lamb demonstrated its feasibility for in situ polyimide/metal interface studies [3, 4]. In most of the subsequent investigations of polyimide films prepared by vapor phase deposition oxydianiline (ODA) and pyromellitic dianhydride (PMDA) were used as diamine and dianhydride components, respectively [5–18]. Only in a few cases other diamine and/or dianhydride precursors, such as decamethylene diamine (DMDA) [19, 20], m- and p-phenylene diamine (m and p-PDA) [21, 22], 4, 4′-diaminobiphenyl disulfide (DAPS) [23–25], benzidine (BZD) [26], 2,2′-bis(trifluoromethyl)-4,4′-diaminobiphenyl (TFDB) [26], and 2,2-bis(3,4-dicarboxyphenyl)hexafluoropropane dianhydride (6FDA) [27] have been used. Recently, a polyimide film was prepared using the diimide precursor pyromellitic diimide (PMDI) instead of the dianhydride precursor PMDA [28]. The formulas of the precursor molecules are shown in Figure 1.
