Laser cleaning of etch-induced polymers from via holes

Abstract

The demand for new wafer cleaning technology after plasma etching increases as the industry enters into submicron processes. The success of low-resistance interconnecting high-density ultralarge-scale integrated devices depends on the cleanliness of via holes. A new approach in removing the sidewall and bottom polymers resulting from reactive ion etching of via holes, using a noncontact dry laser cleaning technique is reported and investigated in this study. Pulsed excimer laser irradiation at 248 nm is found to be capable of removing the polymers by subthreshold ablation, even at fluences limited by the damage threshold of the underlying Al–Cu metal film with titanium nitride (TiN) antireflective coating of 250–280 mJ cm−2. The various mechanisms possibly involved in the laser removal of the via-etch-induced polymer below its ablation threshold are discussed. Experimental results also show that the ablation rate when irradiating at an angle is not only comparable to that at vertical incidence, but even register higher values for most of the ablation rate data obtained. Thus the performance in terms of ablation rate does not slacken when the incident radiation is done at an angle to expose the sidewall polymers of the via holes to laser irradiation. Comparing ablation results obtained using Nd-YAG laser and excimer laser shows that although the shorter 7 ns Nd-YAG laser pulse gives a greater etch thickness than the 23 ns excimer laser pulse, it also tends to damage the metal films and the silicon substrates of the via wafers more easily. The damage threshold using the third and fourth harmonics of the Nd-YAG laser at 355 and 266 nm was just above 100 mJ cm−2 after 20 pulses.