A fine process control on the via hole of multilevel interconnection

Via contact resistance degradation was evaluated by high-resolution transmission electron microscopy. An amorphous layer approximately 2.5 nm in thickness was found between the two levels of aluminum at the via hole. Aluminum, fluorine, oxygen, and silicon were detected in this amorphous layer by using electron probe microanalysis. In evaluating the via hole process, by-products formed during reactive ion etching (RIE) were observed. Such treatments as oxygen plasma and an organic solvent soak did not remove these by-product layers, which were on the order of 20~50 nm thick on the sidewalls and about 10-nm thick on the aluminum surface at the bottom of the via hole. Electron energy loss spectroscopy showed that the by-products consisted of aluminum fluoride (AlF₃, AlF₂) and aluminum oxide (Al₂O ₃). The addition of oxygen gas to trifluoromethane (CHF₃ ) gas in via hole etching has the effect of suppressing the formation of these by-products. RIE at a higher frequency of 13.56 MHz resulted in almost no by-products compared with etching at 380 kHz