Change in chemical state of fluorinated polyimides after the electroless Cu deposition solution treatment

Abstract

Integrated circuit (IC) miniaturization continues to scale down to fulfill the demand for higher speed and greater efficiency in the high performance ultra-largescale (ULSI) applications. At such reduced dimensions, long RC time delay, excess power dissipation, and undesirable crosstalk between adjacent metal lines will limit performance of devices [1–6]. Therefore it is desirable to introduce new materials for conductive layers and low-dielectric-constant (low-k) interlayer dielectrics (ILDs) [7, 8]. The combination of Cu-based metallization and the low-k materials is considered promising for future integrated circuit fabrication. The present work is directed to investigate the compatibility between promising low-k materials and the electroless Cu deposition developed for sub-quarter-micron interconnect applications [9, 10]. Fluorinated polyimides have been increasingly used in the microelectronic applications due to their low dielectric constant, high glass transition temperature, low moisture absorption and high thermal stability. In this work, the low-k materials selected are two fluorinated polyimides, FPI-45M and FPI-136M, whose dielectric constants are 2.8 and 2.64 respectively [11]. Fourier transform infrared (FTIR) spectroscopy was employed to examine the possible chemical reactions between the low-k materials and electroless Cu deposition solution under different treatment solution temperatures and times. The electroless Cu deposition solution contained cupric sulfate, ethylenediaminetetraacetic acid (EDTA) and formaldehyde. The pH of the solution was in the range of 12.3–12.7, adjusted by tetramethylammonium (TMAH). Stabilizers and surfactants were also added to the solution to increase the solution stability and to decrease the surface tension of the solution [10]. The detailed composition of the electroless Cu deposition solution is shown in Table I. Two low-k fluorinated polyimide polymers with different proprietary formulations (FPI-45M and FPI-136M) developed by DuPont [11] were used as samples for the investigation of their compatibility with the electroless Cu deposition solution. FPI-45M and FPI-136M are two copolymers which mainly consist of pyromellitic dianhydride (PMDA) and 2,2-bis(trifluoromethoxy)benzidine (TFMOB), p-phenylenediamine (PPD) and different flexibilizing comonomers which can improve the mechanical properties of copolymers. The generalized structures for FPI-136M and FPI-45M are schematically shown in Fig. 1 [11]. As shown in Fig. 1, the flexibilizing comonomers for FPI-136M and FPI-45M are, respectively, 6FDA and BPDA. Generally, the bath of the electroless Cu deposition solution employed in this work is operated at temperatures ranging between 50 and 80 ◦C for Cu deposition. Therefore 57 and 77 ◦C were selected as low and high experimental temperatures at which the FPI-45M and FPI-136M supported on Si substrates were investigated, specifically to investigate whether they would chemically react with the electroless Cu deposition. In the experiments, the sample films were immersed in the electroless Cu solution at two different solution temperatures, 57 and 77 ◦C, for 5, 10, and 15 min. During the immersion of sample films in the electroless Cu deposition solution under the real deposition conditions, copper deposition did not occur because there was no Cu seed layer initially deposited onto the polyimide films.