ETCHING PROCESSES OF TUNGSTEN IN SF6-O2 RADIOFREQUENCY PLASMAS

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), O(x)-S-F(y) (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 WF(n) (F 1s 684.5 eV) and physisorbed WF(n) 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.