Enhanced Processing of Poly(tetrafluoroethylene) for Microelectronics Applications

Abstract

Polymers having low-dielectric constants (ε r ) are of strategic importance to the electronics industry since this physical property largely determines a device's overall signal speed and wiring density (1). In general, a lower value of ε r allows higher wiring density and greater signal speeds. Although Teflon ® -type fluo ropolymers, such as poly(tetrafluoroethylene) (PTFE), offer the lowest dielectric constant values (ε r = 2.1) of commercially available materials, they have not been viewed as attractive candidates for microelectronic applications. This is due in part to processing difficulties, e.g., generation of high resolution features (µm range).One means of microstructuring materials is by laser ablation. Kawamura, et al.(2) and Srinivasan and Mayne-Banton (3) were first to identify the use of high-energy ultra-violet (UV) excimer laser radiation to photo-ablate polymers. These early investigations were performed using materials that inherently ab sorbed radiation at the wavelength studied. For instance, ablation of polyethyl ene terepthalate) (PET) (1) and poly(methyl methacrylate) (PMMA) (2) was investigated at 193 nm. Subsequent work included ablation of materials which absorbed at longer wavelengths, such as polyimide at 308 nm (3-6).Not all polymers have the requisite chemical functionality in their molecular structure to inherently absorb the high-energy UV photons as required for excimer laser ablation. For instance, PMMA and PTFE are not readily struc tured at 308 nm. However, several researchers have reported successful excimer ablation of UV-transparent materials using a technique known as "doping" (7-11). In these investigations, a low-molecular weight, highly conjugated or ganic compound that absorbs strongly at the excimer laser wavelength of interest is incorporated into the non-absorbing host matrix. These studies have been limited to polymers and dopants that have similar solubility charactecteristics, for example, PMMA doped with pyrene using 308 nm exposure (9).