LIE HomeIssue Contents

Monitoring Acoustic Emission in the Ablation of Photoresist Coatings from a Metallic Substrate During Excimer Laser Micromachining
B.P. Badgujar, Santosh Kumar, C.S. Viswanadham and G.L. Goswami

Excimer laser based projection micromachining process is a popular method of engraving miniaturized complex patterns on metallic/organic surfaces. Acoustic signals are generated during the process due to the interaction of a laser beam with the photoresist material. These acoustic signals are analysed to explore the possibility of using Acoustic Emission (AE) techniques for on-line monitoring of the ablation of the photoresist coating. On-line monitoring of the acoustic signals was accomplished using resonant sensors of frequencies 30 kHz &175 kHz,during laser ablation with various laser powers. The Root Mean Square (RMS)voltage value of each AE signal was analysed. The AE signals recorded by a 175 kHz resonant frequency sensor did not show any change during the entire ablation process. However,there was a considerable drop in the RMS voltage value for the 30 kHz sensor for all fluences (pulse energies), when the coating is removed, and this can be used to monitor and control the laser ablation process. There was a considerable fluctuation in the AE signal strength (RMS Voltage) at lower pulse energy values such as 90 mJ and 135 mJ, due to the large fluctuation in the laser pulse energy from one pulse to another when it was set at lower values. The variation in the number of laser pulses needed for the complete removal of the photoresist coating at various laser pulse energies was also studied. It was observed that the number of laser pulses needed for the complete removal of the photoresist coating of a given thickness decreases with increasing laser fluence until it attains saturation at the fluence of 0.63 J/cm 2. The frequency spectrum analysis of AE signals during different stages of the ablation process was done and frequency peaks in 2 –8 kHz range found disappeared when the removal of photoresist coating is over. The results of all these studies are discussed in this paper.

Full Text (IP)