Etching processes of tungsten in SF6-O2 radio-frequency plasmas

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Abstract
The reactive ion etching of chemical vapor deposited tungsten in SF6/O2 radio‐frequency plasma has been studied by means of optical emission spectroscopy, mass spectrometry, and in situ x‐ray photoelectron spectroscopy. Two etch products are detected: WF6 and WOF4. A correlation is found between their concentration in the gas phase and the amount of atomic fluorine and oxygen, as measured by actinometry. In an atomic F‐rich plasma, WF6 dominates over WOF4, the latter appearing as soon as oxygen is introduced in the plasma. After etching, the tungsten surface contains three chemical elements: sulfur, oxygen and fluorine; their concentration depends on the reactor parameters (gas mixture, cathode material). Various species have been observed on this surface: S—W (with S 2p at 162 eV), Ox—S—Fy (with S 2p at 170 eV, O 1s at 533 eV, F 1s at 686.4 eV). Two types of tungsten fluorides have been identified: chemisorbed WFn (F 1s at 684.5 eV) and physisorbed WFn species (F 1s at 687.7 eV). The latter are thought to be the precursors of WF6 and O■W—F4 etch products. The role of sulfur, oxygen and fluorine during the etching process of tungsten is discussed.