LASER CLEANING OF ETCH-INDUCED POLYMERS FROM VIA HOLES

The demand for new wafer cleaning technology after plasma etching incr eases as the industry enters into submicron processes. The success of low-resistance interconnecting high-density ultralarge-scale integrate d devices depends on the cleanliness of via holes. A new approach in r emoving the sidewall and bottom polymers resulting from reactive ion e tching of via holes, using a noncontact dry laser cleaning technique i s reported and investigated in this study. Pulsed excimer laser irradi ation at 248 nm is found to be capable of removing the polymers by sub threshold ablation, even at fluences limited by the damage threshold o f the underlying Al-Cu metal film with titanium nitride (TiN) antirefl ective coating of 250-280 mJ cm(-2). The various mechanisms possibly i nvolved 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 mo st of the ablation rate data obtained. Thus the performance in terms o f ablation rate does not slacken when the incident radiation is done a t an angle to expose the sidewall polymers of the via holes to laser i rradiation. 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 th e via wafers more easily. The damage threshold using the third and fou rth harmonics of the Nd-YAG laser at 355 and 266 nm was just above 100 mJ cm(-2) after 20 pulses. (C) 1998 American Institute of Physics. [S 0021-8979(98)03203-4].