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Tham khảo tài liệu 'micromachining techniques for fabrication of micro and nano structures part 6', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Laser Micromachining and Micro-Patterning with a Nanosecond UV Laser 89 2.3 Laser cutting of sapphire wafer As our scheme of micromachining is targeted at die separation of GaN-based LEDs sapphire wafers are used for testing the results as it is the typical substrate for the metalorganic chemical vapour deposition MOCVD growth of GaN. The quality of the cleave can be quantified by the width depth linearity and sidewall roughness of the trench formed by the laser beam. Each of these parameters will be investigated. Since the focal length of the focusing lens f 75 mm is much longer than the thickness of the sapphire wafer t 420 pm the depth of the trench mainly depends on the number of micromachining cycles. The number of cycles is controlled by configuring the translation stage to repeat its linear path over a number of times. Since the position repeatability of the stage is better than 5 pm increasing the number of cycles should not contribute significantly to the width of the feature. Figure 4 shows the cross-sectional optical image of a 420 pm thick sapphire wafer that has been micro-machined with an incident beam inclined at 45 with scan cycles ranging from 1 to 10. These incisions were carried out by setting the laser pulse energy to 54 pJ at a repetition rate of 2 kHz. The relationship between the inclined cutting depth and the number of passes of the beam are plotted in Figure 5. After the first pass of the beam a narrow trench with a width of 20 pm and a depth of 220 pm was formed. Successive scans of the beam along the trench results in further deepening and widening but the extent was increasing less. The depth of the trench depends on the effective penetration of the beam. From the second scan onwards the beam has to pass through the narrow gap before reaching the bottom of the trench for further machining. The energy available at this point is attenuated partly due to lateral machining of the channel causing undesirable widening absorption and .
