Polyimides dielectric constant

Polyimides containing C—F bonds have been receiving strong attention (96—98). Fluorine-containing polyimides possess lower dielectric constant and dielectric loss because of reduced water absorption and lower electronic polarization of C—F bonds vs the corresponding C—H bonds. Fluorine-containing polyimides are often more soluble and readily processible without sacrificing thermal stabilities. The materials are appHed primarily iu... 

Alicyclic dianhydrides are interesting for electronic applications. The polyimides obtained from them are colorless with high transparency in the visible range, exhibit low birefringence,125 and have a low dielectric constant.126 The reactivity of the polycyclic aliphatic dianhydride has been investigated. For example, bicyclo-[2,2,2]-oct-7-ene tetracarboxylic dianhydride reacts quickly with an aromatic amine because the bicyclo-imide is less strained than the corresponding dianhydride.127... 

The sizes and concentration of the free-volume cells in a polyimide film can be measured by PALS. The positrons injected into polymeric material combine with electrons to form positroniums. The lifetime (nanoseconds) of the trapped positronium in the film is related to the free-volume radius (few angstroms) and the free-volume fraction in the polyimide can be calculated.136 This technique allows a calculation of the dielectric constant in good agreement with the experimental value.137 An interesting correlation was found between the lifetime of the positronium and the diffusion coefficient of gas in polyimide.138,139 High permeabilities are associated with high intensities and long lifetime for positron annihilation. 

Additional drawbacks to the use of polyimide insulators for the fabrication of multilevel structures include self- or auto-adhesion. It has been demonstrated that the interfacial strength of polyimide layers sequentially cast and cured depends on the interdiffusion between layers, which in turn depends on the cure time and temperature for both the first layer (Tj) and the combined first and second layers (T2) [3]. In this work, it was shown that unusually high diffusion distances ( 200 nm) were required to achieve bulk strength [3]. For T2 > Tj, the adhesion decreased with increasing T. However, for T2 < Tj and Tj 400 °C, the adhesion between the layers was poor irrespective of T2. Consequently, it is of interest to combine the desirable characteristics of polyimide with other materials in such a way as to produce a low stress, low dielectric constant, self-adhering material with the desirable processabiHty and mechanical properties of polyimide. 

An alternative means of reducing the dielectric constant of polyimides is to have a low dielectric constant component dispersed as a second phase within the rigid polyimide matrix. Two key approaches have been pursued to this end. The first involves the preparation of polyimide block copolymers with highly fluorinated coblocks and the second involves the generation of a polyimide foam. The main drawback to the first approach is the solubility of highly fluorinated blocks in organic media which will permit copolymerization with polyimides. This led to the investigation of new semi-fluorinated polymers derived from polyfaryl ethers). 

To obtain a measure of the dielectric constant and anisotropy of thin films, the refractive index of thin film samples was measured. It has been shown that the measured dielectric constant is approximately the square of the refractive index at 633 nm wavelength [the actual relationship is roughly e (refractive index) -i- 02.] and the anisotropy is obtained from the difference between the in-plane and out-of-plane refractive index [97]. The measured anisotropy of foamed polyimides is lower than that observed for non-foamed polyimides. In addition, a drop in refractive index of the samples was observed upon foaming. The polyimide PMDa/3FDA has a measured dielectric constant of ca. 2.9 at 70 °C. A foamed sample of PMDA/3FDA derived from copolymer 6f showed a drop in dielectric constant of 2.3 [97]. 

The present work is a report of the properties of polyimide which define functionality as an interlevel dielectric/passivant. Thus, the planarizing and patterning characteristics and electrical characteristics of current vs voltage, dissipation, breakdown field strength, dielectric constant, charge and crossover isolation are discussed in addition to the reliability-related passivation properties. 

Figure 6. Real part of the dielectric constant t, (1 MHz) and dissipation factor D (1 MHz) as a function of polyimide thickness.

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