Mechanism of microwave plasma etching of polyimides in O₂ and CF₄ gas mixtures

Abstract

A mechanistic study of microwave plasma etching of polyimides in oxygen plus CF4 gas mixtures was carried out with several mutually complementary surface-sensitive characterization techniques. In-situ X-ray photoelectron spectroscopy was used to characterize the polyimide surfaces before and after etching while multiple internal reflectance and Fourier transform IR spectroscopy was employed to obtain additional information on molecular structure and chemical bonds on these surfaces. In addition, steady-state and temperature-programmed mass-spectrometric measurements were made to identify the etching products, their precursor states and desorbing species. When the experimental evidence obtained by these techniques is pieced together a coherent picture of the etching process emerges. Attachment of atomic oxygen produced in the plasma to selected carbon sites on the PI surface causes aromatic ring opening, forming precursors to volatile etching products. Addition of CF4 to oxygen plasma in small amounts increases the efficiency of oxygen production and concomitant flourination of aromatic carbon provides excess energy for ring opening and promotes imide and ether bond cleavage. At higher CF4 concentrations flourination exhausts the reaction sites for oxygen, thus inhibiting the etch rate. Thermal desorption studies indicate that the fluorine coverage desorbs in the first stage as fluorine molecules. © 1986.