Ultraviolet absorption spectroscopy for the detection of CF₂ in high-density plasmas

Abstract

Ultraviolet absorption spectroscopy has been employed to measure the density of CF2 in a high-density discharge used for the selective etching of silicon dioxide relative to silicon films. In a plasma containing C2F4H2 and CF4, CF2 accounts for more than 10% of the gas in the reactor. The level of CF 2 in the discharge is strongly dependent on the operating pressure and the applied power. A comparison of the intensity of optical emission from CF*2 with the ultraviolet absorption signal and microwave interferometry measurements shows that the optical emission signal is limited more by the electron density than by the availability of ground state CF2. The UV absorption signal for CF2 closely follows the selectivity of etching SiO2 to silicon. Both neutral fluorocarbon fragments and ions are believed to play a role in the deposition of fluorocarbon films which give rise to this selectivity. The ability to measure a fundamental plasma parameter which closely correlates with etch selectivity is an important step toward the real-time determination of an etch characteristic which currently can only be determined from post-process measurements.