Polyimide multilayer films

Mono- and multilayer films of polyimides were successfully prepared using Langmuir-Blodgett technique. Monolayer films of polyamic acid long alkylamine salts were prepared at the air-water interface. The mono-layer films were deposited on appropriate plates to produce multilayer films of the precursor to polyimide films. Finaly, the polyimide multilayer films were obtained by treatment of the multilayer films of the polyamic acid amine salts with acetic anhydride and pyridine. The polyimide multilayer films had excellent coating ability giving a very smooth surface. They also exhibited insulating characteristics as reliable as polyimide thick films. 

The present method consists of three steps as illustrated in Scheme 1. In the first step, monolayer films of polyamic acid long chain alkyl-amine salts 4 at the air-water interface are prepared.(9) Unexpectedly, the polyamic acid 3 itself, which possess hydrophilic carboxyl functions in the polymer backbone, did not afford a stable monolayer at the air-water interface. Introduction of a hydrophobic long alkyl chain into 3 was performed by mixing polyamic acids and longchain alkylamines. Polyamic acid salts 4, thus obtained, afforded very stable monolayer films at the air-water interface. In the second step,(10) the polyamic acid salt monolayer films are successfully deposited on appropriate plates such as glass, quartz, or silicon wafer. Finally polyimide multilayer films are obtained by treatment of polyamic acid salt multilayer films on the plates with a mixture of acetic anhydride and pyridine. 

Flexible PCBs are produced in large quantities, usually consisting of polyester, polyimide, and polyimide—Teflon films. Both foil-laminated and uncoated versions are available. A modem complex multilayer board after final fabrication is shown in Figure 2. 

Much subsequent work has been carried out on the formation and characterisation of polyimide multilayers ([259-63] and other papers of less relevance to the theme of this book). It has been established that there is substantial orientation of the polymer axes in the direction of dipping, an effect which increased with the length of the polymer chains. Thin films consisting of ten monolayers were far more defect-free than fatty acid films of comparable thickness. Progress has also been made... 

Copper/Polyimide Thin Film Multilayer Interconnect Structure... 

KAKIMOTO ET AL. Preparation of Polyimide Mono- and Multilayer Films 485... 

In this article, we describe the first successful preparation of polyimide mono- and multilayer films without any pendant long alkyl chains using LB technique. 

A multilayer film of obtained as above was immersed overnight in a mixture of acetic anhydride, pyridine, and benzene (1 1 3) to afford a polyimide film of 5a.(1)... 

The elemental analysis of multilayer films (50 layers) of polyamic acid salt and polyimide 5a was performed by ESCA (electron spectroscopy for chemical analysis), with the following results ... 

The surface of the polyimide film of was observed by SEM to be as flat as that of The multilayer film of was prepared on a silicon wafer which contained evaporated aluminum lines in order to examine the coating ability of the present LB films. Figure 3(a) exhibits the mechanical stylus probe (Talystep) pattern of aluminum lines (30 nm high and 2000 nm wide) and (b) shows the lines after 120 layers of were deposited. Since the depth was observed as 30 nm in both cases, it is concluded that the multilayer film can conformally coat the uneven surface. This feature can be directly observed in the photographs of Figure 4 where a 60 layer film of is coated on an aluminum line (200 nm of high and 2000 nm wide). 

With advancements in MCMs, where large chips (up to 500-mils square) are closely packed and where devices and interconnect substrates are fragile (for example, polyimide thin-film multilayer substrates used in MCM-D), new rework challenges have arisen. In these high-density circuits, spacings between the chips may be as small as 40 mils—too small... 

Zhao G, Ishizaka T, Kasai H, Oikawa H, Nakanishi H (2007) Preparation of Multilayered Film of Polyimide Nanoparticles for Low-k Applications. Mol. Cryst. Liq. 

A similar family of polyimides has been described by other workers [38]. The polyamic acid alkylamine salt (23) was used to form a surface monolayer, and this was then deposited into multilayers by the LB technique. By treating the deposited films with acetic acid, a highly stable polyimide multilayer (24) was produced. The major difference between these polyimides and those of structure (22) is the mode of preparation. In this case a chemical method is used, whereas thermal cyclization was used for (22). LB polyimide films of structure (24) have been found to be very stable in various organic solvents and exhibit insulating behaviour (electrical and thermal) comparable to conventional polyimide films. 

Figure 15.24 shows the fabrication process of the optical filter on a fluorinated polyimide substrate. First, the low-thermal-expansion-coefficient PMDA/TFDB poly(amic acid) solution was spin-coated onto a Si substrate and baked. Then alternate TiO2 and SiO2 layers were formed on the polyimide film by ion-assisted deposition. The multilayered polyimide film was diced and peeled off from the Si substrate. In this way, thin optical filters on a fluorinated polyimide substrate are easily fabricated. 

Figure 5.11. (a) A polyamic salt which will spread at the air/water interface and form LB films. (b) A polyimide which can be produced as a multilayer by heating a multilayer of the polyamic salt and driving oif the long chains associated with the salt. 

There remains one very curious feature of the polyimide films discussed above. Some of the side chains removed contain in excess of 16 carbon atoms and are therefore relatively massive. Neverthless, they appear to diffuse through a tightly packed polymer multilayer consisting of up to 200 monolayers. Undoubtedly this process takes place, but it is hard to understand the mechanism. 

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