Characterization of Photosensitive Low-k Films Using Electron-Beam Lithography

Abstract

Characteristics of photosensitive low-k methylsilsesquioxane ͑MSQ͒ were investigated by the use of electron-beam lithography. Photosensitive low-k MSQ makes it possible to realize via and trench structures for Cu damascene technology without dry etching processes in the multilevel interconnect integration of ultralarge scale-integrated circuits. In this paper the dependences of critical dimensions of developed patterns on exposure dose and hold time after exposure were investigated. Consequently the aspect ratio of the developed trench structure of 3.9 and the minimal feature size of 90 nm were achieved.According to the scaling rule 1 for miniaturization of the ultralarge-scale integrated circuits ͑ULSI͒, the interconnect feature sizes of lines and spaces should be reduced. The increase of interconnect resistance-capacitance ͑RC͒ delay occur as the reverse effect due to the scaling down. Therefore, the interconnect technologies with low-resistance metal wire and low-dielectric constant ͑low-k͒ interlayer film are needed for ULSI. Furthermore, the application of low-k dielectric to the interlayer dielectric film can reduce power consumption of ULSIs.Methylsilsesquioxane ͑MSQ͒ has been developed as a low dielectric constant material; k = 2.7. 2,3 Methylsilsesquiazane ͑MSZ͒ is a precursor component of MSQ as shown in Fig. 1. When the photoacid generator ͑PAG͒ molecule is added to MSZ, it acquires photosensitivity. The photosensitive MSZ precursor has sensitivity to not only ultraviolet light, but also to an electron beam ͑EB͒. Then the lithography of photosensitive low-k MSQ was examined by using an ultraviolet light, KrF excimer laser, electron beam, and silicon-onruby ͑SOR͒ X-ray. [3][4][5] In these lithographies via and trench patterns were formed directly in the MSZ film without using dry etching, however the tapered shape of exposed patterns was formed. At the same time in the KrF lithography, the standing waves between the substrate and the MSZ surface caused the nonflat sidewalls in the trench patterns. 4 Thus improvement of the exposed pattern was required.The dry etch-less process eliminates the resist coating and ashing, so that it can reduce process steps. Furthermore, the reliability issues such as void formation in the film due to dry etching and ashing 6 can be eliminated. At the same time disuse of the hardmask and etch-stop layer enables us to reduce the effective dielectric constant of the multilevel interconnect interlayer. ExperimentalE-beam exposure experiments of photosensitive low-k films were carried out using triazine radical TAZ-100 PAG ͑Midori Kagaku͒ as shown in Fig. 2a. The possible chemical reaction for TAZ-100 in photosensitive MSZ is shown in Fig. 2b. 7 The photosensitive MSZ precursor was spin-coated to a thickness of 350 nm on 2 in. Si͑100͒ wafers at 2000 rpm for 20 s. It was prebaked at 90°C for 1 min. The humidification of the film was carried out after the EB lithography in the posthumidification process. The EB lithography was performed by use of an Hitachi HL-700 electron-beam s...