Etching of low-k dielectrics is one of the most critical processes for the next ULSI fabrication. Etching mechanism of a poly arylene ether (PAE), which is one of the organic low-k dielectrics, was investigated by using a plasma beam irradiation apparatus. The apparatus can control ion flux density (@GAMMA@i), neutral flux density (@GAMMA@n) and ion energy (Ei), independently. The controlled plasma beam was irradiated to the PAE coated on Si substrate in an irradiation chamber, and etch rates (ERs) and desorbed products were measured. The beam was extracted from an ECR type plasma chamber with a N@sub 2@ gas. The @GAMMA@i was controlled by changing the ion beam diameter using an electrostatic focusing lens. The @GAMMA@n was changed by altering the distance between the plasma chamber and the substrate. The Ei was adjusted by DC bias applied to the plasma chamber. Pressures in the plasma and the irradiation chamber were 4 mTorr and 1 x 10@super -6@ Torr during the beam irradiation, respectively. The dominant species in the incident beam were N@sub 2@ and N@sub 2@@super +@. The ER measurements were performed at an incident angle of 0° to the substrate surface normal (@theta@ = 0°). The ERs increased with increasing @GAMMA@n at the constant @GAMMA@i and Ei. The increase of ERs could be attributed to that of the @GAMMA@n. When @GAMMA@i and @GAMMA@n were kept constant, higher ERs were obtained at higher Ei in the range from 500 to 700 eV. Desorbed species of m/e < 100 during the beam irradiation with @theta@ = 50° were measured by a quadruple mass spectrometer. Signals of m/e = 26 (CN), 27 (HCN) and 52 (C@sub 2@N@sub 2@) were observed as etched products directly desorbed from the substrate surface. Effects of H@sub 2@ gas addition on the desorbed products formation and the ERs will be discussed. @FootnoteText@ This work was supported by NEDO.