Effect of UV wavelength on the hardening process of porogen-containing and porogen-free ultralow- k plasma-enhanced chemical vapor deposition dielectrics

The effect of narrow-band 172 nm and broad-band > 200 nm UV sources in the new curing scheme of the plasma-enhanced chemical vapor deposition (PECVD) dielectrics is studied. The new curing scheme is based on porogen removal (organic sacrificial phase introduced to generate open porosity) from PECVD dielectric before its final UV curing. The results are compared with the PECVD films fabricated in the conventional scheme in which porogen is still present during the UV curing. The same curing time of porogen-containing conventional PECVD films with 172 nm and > 200 nm UV sources results in only 10% difference in their Young's modulus (YM): 5.84 and 5.32 GPa, respectively. However, the porogen-free films cured with 172 nm UV source show a YM of 6.64 GPa ( k 100 kHz ∼ 2.2 , 44% open porosity), approximately twice as large as those cured with > 200 nm UV having a YM of 3.38 GPa ( k 100 kHz ∼ 2.0 , 48% open porosity). The mechanical properties, optical properties in the range of 150-800 nm, dielectric constants at 100 kHz and 4 GHz, porosities, pore size distributions, and bonding structure are evaluated. The impact of porogen on optical characteristics and, therefore, on the photochemical UV-hardening mechanism is discussed. The achieved mechanical properties are explained on a basis of the percolation of rigidity theory and random network concepts.