Surface Fluorination of Polyimide Thin Films by CF 4 + O 2 Reactive Ion Beam Etching
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SURFACE FLUORINATION OF POLYIMIDE THIN FILMS BY CF4 + 02 REACTIVE ION BEAM ETCHING WILLIAM E. VANDERLINDE AND ARTHUR L. RUOFF Department of Materials Science and Engineering, Cornell University, Ithaca, NY 14853 ABSTRACT Surface fluorination of polyimide thin films during CF + 0 reactive ion beam etching (RIBE) was investigated. The removal of the fluorinated layer by a subsequent oxygen ion beam etch was also studied. Electron spectroscopy for chemical analysis (ESCA) and Rutherford backscattering spectrometry (RBS) detected a fluorinated surface layer on the order of 100 A thick. Fluorine atom concentration in the surface of the film (as measured by RBS) and the etch rate of the film were measured as a function of several experimental parameters: ion energy, ion current density, etch time, and gas composition. The results are compared with theoretical predictions of the total number of fluorine atoms retained in the film after etching. INTRODUCTION Polyimides have excellent thermal and dielectric properties with applications as planarizing insulators for multilevel interconnect systems. As device density of very large scale integrated (VLSI) circuits increases, patterning techniques for polyimide films will become crucial [1,2]. Reactive ion beam etching has been shown useful for control of wall profiles in etching microstructures into polyimide layers [3]. It is well known that polymer surfaces can be fluorinated by direct application of fluorine gas [4]. Recent work has shown that polymers exposed to CF4 + 0 plasmas may develop fluorination 1000 A or more into the polymer [5]. Plisma etching with dilute mixtures of CF in He produces fluorinated layers about+50 A thIck [6]. Very low currant density mass separated ion beams of F and CF have been shown to produce very thin (< 10 A) fluorinated layers on polymerý [7]. Work on etching of polyimide with CFA + 02 gas mixtures has included studies of fluorination during CF4 + O pTasma etching [8], and fluorination during 02-CF -Ar microwave plasma down~tream etching [9]. The prelent work investigates surface fluorination of polyimide films during reactive ion beam etching with CF + 0 gas mixtures. Non-massselected beams of sufficient current denfity ?0.3 mA/cm ) to produce appreciable etching (,100 A/min) are found to cause fluorination to intermediate depths (50-100 A). The process parameters (ion current density, ion energy, gas composition, etching time) are varied independently to determine their effect on the fluorination. In addition, pxperimpnts are conducted with a chamber pressure of CF and ion beams of 0 or Ar . This allows the separation of contributions from ion implantatioh and from recoil implantation of adsorbed molecules. EXPERIMENTAL Si wafers were prepared by sputtering 500 A of Cr onto the surface to improve subsequent adhesion of the polyimide film. The wafers were then spin coated with polyamic acid (DuPont 5878) dissolved 0in N-methyl pyrrolidone solvent. The film was precured for 15 min at 200 C to partially remove the solvent and then cured at 4
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