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Metallization of integrated circuits by laser evaporation of metal sandwich layers
The structure shrink of IC dimensions leads to a demand of improved metallization layers because conductivity and contact resistivity determine the RC constants and thus the speed of the circuits. Moreover the extreme current density causes electromigration of the wiring and field-enhanced metal redistribution in connection with chemical and even thermal forces. The advantages of laser evaporation are the absence of high electric fields (which are present in sputtering machines and could possibly damage thin oxides), the absence of x-rays (present in e-gun evaporation) and the very fast alteration of materials to be deposited due to the use of solid evaporation sources. With different kinds of pulse lasers we investigated the evaporation of pure metals as well as alloys and dielectrics. For this, we built up a laser evaporation system consisting of a deposition chamber (with a basic pressure better than 10 high -4 Pa) which can be connected to three different kinds of laser: 1.Nd:YAG, 70 W CW, Q-switched, 5 kHz, 150 ns, 1064 nm 2. Nd:YLF, 135 myJ, diode pumped solid state pulsed laser, 533 nm 3. Excimer, 300 mJ, 248 nm. The laser is focused onto a rotating target. The processing chamber contains a lamp heated chuck which is moveable by an x-y-table to ensure good lateral homogeneity and step coverage. Special precautions were made to avoid the dimming of the laser entrance window. With this system we have investigated the evaporation of different metals (Al, Ti, Wo, Pt, Au) and alloys (Ta-Si) with regard to deposition rate, homogeneity, adhesion, step coverage, surface roughness and sheet resistivity as a function of individual deposition parameters. In the case of alloys, congruent deposition resulting in very good stoichiometry was observed by RBS analysis.