Vapor Deposition of Parylene Films from Precursors
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VAPOR DEPOSITION OF PARYLENE FILMS FROM PRECURSORS L. YOU, G.-R. YANG, C.AI. LANG, P. WU, J. A. MOORE, J. F. MCDONALD, and T.-M. LU Center for Integrated Electronics, Rensselaer Polytechnic Institute, Troy, NY 12180 ABSTRACT Paxylene films, depending on the type, are thermally stable up to 530 *C and have low dielectric constants ranging from 2.35 to 3.15. One of the most interesting properties of this material is its vapor depositability. Conventional vapor deposition involves cracking the parylene dimers at temperatures from 600 to 730 *C and polymerizing the monomers at - 35 OC to RT. We have developed a simpler and less expensive technique that directly uses the precursors from which the dimers are made. This method requires the use of metal catalysts to produce parylene films. We have used the precursors a, a'-dibromo-p-xylene and dibromotetrafluoro-p-xylene to produce N-type and F-type parylene films. FTIR, XPS, thermal stability, and electrical studies show that the F-type parylene films grown from the precursors are comparable to, or sometimes better than, the films grown from dimer, and have potential microelectronics applications. INTRODUCTION For the future VLSI, a lower RC constant of interconnections becomes an essential requirement. Because of its superior properties among low dielectric materials, Parylene (poly-pxylylene) and its derivatives are being studied (1, 2]. Parylenes (PAs), depending on the type, have dielectric constants of 2.35 to 3.15. In addition to their lower dielectric constants and lower water take-up as compared to the polyimides (PIs) commonly used in VLSI, the most prominent advantage of PAs is their vapor depositability. This method, first formulated by Gorham [3], enables PAs both to deposit conformally onto micron and submicron dimensions (e.g., gaps), and to be free from solvent contamination and high temperature cures. PAs, therefore, carry a lower intrinsic stress in the films. In addition to microelectronics, other applications of parylenes have been studied, which include medical uses, fusion targets, artifact conservation, contamination and corrosion control, dry lubricant, and thin coating films for reliability without hermeticity [4]. The parylene (PA) family is composed of five different types: parylene-N (PA-N), paryleneC (PA-C), parylene-D (PA-D), parylene-E (PA-E) and parylene-F (PA-F) [4]. Fig. 1 shows the molecular structure of PA-N and PA-F. PA-N has the highest melting temperature and the lowest dielectric constant among the unfluorinated parylenes. The melting temperature of PA-N is about 420 *C. To use PA-N in VLSI interconnections and packaging applications, a low-temperature processing scheme was proposed [1, 2]. In general, the diffusion and adhesion of metals to polymer films, polymer film stress, and the thermal stability of the polymer film depend strongly on the melting temperature of the polymer. For instance, Cu diffusion into PA-N starts at an annealing temperature of 300 'C to 350 *C. Adhesion failure between Cu and PA-N also starts at 300 *C. The
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