Paper PS-TuM10
Study of Hydrofluorocarbon Precursor Parameters for Plasma Etching of ULK Dielectric

Tuesday, November 11, 2014, 11:00 am, Room 308

Plasma etching of ultra-low k (ULK) dielectrics requires gas precursors that enable very high etching selectivity of ULK over the mask and underlayer while minimizing ULK sidewall damage. We report a systematic study aimed at evaluating the impact of the nature of hydrofluorocarbon gas precursors on plasma etching performance of a representative ULK material, Black Diamond II (BDII) coated onto Si. The work was performed in a dual frequency capacitively coupled plasma (CCP) reactor with real-time characterization by ellipsometry, optical emission spectroscopy (OES), and electrical measurements. The chemical composition of deposited films and post plasma low-k surfaces was examined by X-ray photoelectron spectroscopy (XPS). Etching selectivity of BDII over amorphous carbon (aC), SiO₂ and Si₃N₄ masking materials was evaluated. The impact on the ULK material when exposed under sidewall-like plasma conditions was also studied for various precursor gases using the dilute HF etching method. The precursor gases studied included fluorocarbon (FC) and hydrofluorocarbon (HFC) precursors whose molecular weights and chemical structures were systematically varied i.e. fluorine/carbon ratio, presence/absence of carbon double bonds, and ring versus linear structure. The plasma process conditions were Ar with FC (or HFC) and O₂ admixture at a fixed total pressure with varying ion energies. The etch rate (ER) and deposition rate (DR) were measured by in-situ ellipsometry. We found that the DR was greater for precursors with a larger degree of unsaturation. Linearly shaped gases with higher F/C ratio showed the best etching performance, evaluated by the maximum value of product of BDII ER and BDII/aC selectivity value for each gas. By relating measured CF₂ optical emission for varying precursor concentrations to simultaneously determined FC (or HFC) film DR, we distinguished three deposition behaviors with qualitatively different etching performance based on the precursor structure. A gap structure, blocking direct ion bombardment, was used to simulate the sidewall plasma environment. With this approach, the evolution of the physical and chemical sidewall profile can be studied in real time using in-situ measurements. We found that gases exhibiting a particular deposition behavior differed systematically in formation/absence of a protective fluorocarbon film on the ULK sidewall. The dependence of improvements in low-k etching performance on the FC chemical structures and the surface model for underlying mechanisms relating these properties will be reported.