Plasma assisted deposition of low dielectric constant fluorocarbon materials for microelectronic applications

Further breakthrough in high-performance integrated circuit technology hinges critically upon the development of new insulators with dielectric constant (k) lower than conventional silicon dioxide (k = 4.1). Fluoropolymers, such as polytetrafluoroethylene (PTFE), offer great potential as interlevel dielectrics since they have the lowest dielectric constants (k = 2--2.1) among non-porous materials, good thermal stability, and excellent chemical resistance. However, PTFE has poor mechanical properties and is extremely difficult to process into thin films using typical polymer processing methods. In this work, we investigate the use of plasma-assisted deposition as a solventless technique to produce films with properties approaching that of PTFE. A parallel plate radio frequency plasma reactor was designed and used for deposition of the fluorocarbon films. Two different monomers, a pure fluorocarbon (octafluorocyclobutane, C4F8) and a relatively environmentally benign hydrofluorocarbon (pentafluoroethane, CF3CHF2) were investigated and compared in terms of deposition rates and final film properties. In-situ mass spectrometry, infrared spectroscopy and x-ray photoelectron spectroscopy were used to gain insight into the monomer plasma chemistry and the chemical bonding structures of the deposited films as a function of operating parameters. Pyrolytic mass spectrometry was performed to gain an insight into the degradation mechanism and correlate the thermal stability of the deposited films with structure and chemical composition. A tradeoff exists between the thermal stability and the electrical properties of the deposited films. Fluorocarbon films with dielectric constants between 2.18 and 2.55 and thermal stability in excess of 300°C (<1% weight loss) have been achieved. Temperature-humidity studies (85°C/85% RH, 100 hours) indicated that the deposited fluorocarbon films were chemically stable and exhibited extremely low moisture absorption (<0.13 wt%).