UV Curing Effect on Mechanical Stability of Flexible Dielectric Thin Films Fabricated by Plasma-Enhanced Chemical Vapor Deposition of tetrakis(trimethylsilyloxy)silane Precursor

Low-dielectric constant (low-k, k&lt;4.0) materials have replaced the traditional silicon oxide to reduce resistance-capacitance (RC) delay in the interconnects of semiconductor devices. To further reduce the k-value to 2.5 or less, porosity is introduced into the film by adding a sacrificial porogen precursor and removing the porogen in a subsequent treatment using ultraviolet (UV) irradiation curing or thermal annealing. On the other hand, there is an increasing interest in researching various flexible electronic materials and their integration to manufacture flexible electronics. Dielectric materials should have good compatibility with flexible substrates and excellent electrical and mechanical stability for the fabrication of flexible electronic devices.<br/>Flexible low-k SiCOH thin films were fabricated by plasma-enhanced chemical vapor deposition (PECVD) of tetrakis(trimethylsilyloxy)silane (TTMSS, C₁₂H₃₆O₄Si₅) precursor on the substrate of indium tin oxide on polyethylene naphthalate (ITO/PEN). The sheet resistance of ITO was about 12 Ω/square. The transparency of ITO was ≥ 75 % with a haze of 3 %. The PEN substrate was DuPont Teijin film. Thicknesses of ITO and PEN layers were &gt;180 nm and 0.125 mm, respectively. The TTMSS precursor consists of Si-O and methyl functional groups. One Si atom centered in a whole molecule is bonded to four oxygen atoms which are connected trimethylsilyl (Si(CH₃)₃). When the substrate was placed on the susceptor in a reactor, gas molecules were vaporized from the TTMSS precursor in a bubbler. Vaporized molecules carried by argon (Ar) with a purity of 99.999 % gas were delivered from the bubbler to the reactor. The films were deposited at room temperature of 25°C with an operating pressure of 200 mTorr (26.7 Pa). The flow rate of Ar gas was maintained at 18 sccm. The RF plasma power with 13.56 MHz was chosen from 20 to 100 W. After deposition of the SiCOH films, UV curing with 270-280 nm wavelength and optical power of 100×10^-6 W/cm² was conducted at room temperature for various exposure times. After UV curing, the effects of the UV curing conditions on the chemical, electrical, and mechanical properties of low-k SiCOH thin films were studied. To confirm the mechanical stability, bending tests with bending cycles up to 10000 were performed. The thickness and refractive index of the SiCOH films were measured by ellipsometer. Surface morphology of the films was observed by atomic force microscopy (AFM). The water contact angle for hydrophilicity of the film was determined by contact angle goniometer. The functional groups of the films were identified using Fourier transform infrared (FTIR) spectroscopy. The chemical composition of the films was determined by X-ray photoelectron spectroscopy (XPS). Electrical properties including the k-value and leakage current density were observed by the fabrication of metal-insulator-metal (MIM) structure by depositing aluminum (Al) on the SiCOH films. Mechanical properties including hardness and elastic modulus were measured using a nanoindenter.<br/>UV irradiation preferentially removed porogen-related CH_x groups and then modified Si-CH₃ and Si-O-Si bonds. The fractions of cage and network configurations in Si-O-Si made an impact on the degree of cross-linking and thus mechanical properties. The modification of chemical bonds also affected k-values. It was expected that the enhanced electrical and mechanical performance could be obtained if porogen was completely removed with a sufficient curing time. The mechanical strength was affected due to the change in chemical functional groups after UV curing contributed to the electrical and mechanical stability after bending tests.