Liquid-immersion nanosecond-pulsed laser micromachining is introduced as an efficient way for device isolation and rapid prototyping on GaN-on-sapphire wafer. Using deionized water as an ambient medium for laser micromachining, smooth trenches that are free from redeposition can be formed in the GaN layer. Coupled with the large difference between the ablation thresholds and ultraviolet absorption coefficients of GaN and sapphire, the GaN/sapphire interface can be left undamaged after the ablation process. This technique overcomes the limitation of heat accumulation in nanosecond-pulse regime, and offers a cost-effective alternative to ultrashort-pulse laser micromachining. In this report, the advantages offered by liquid immersion are elucidated in terms of improved heat conduction, increased plasma-induced recoil pressure due to water confinement, weakened plasma shielding effect in water, and the collapse of cavitation bubbles. Simulation results show that the reduced fluctuation of temperature profile over time in water could be correlated with the reduced redeposition of Ga from thermal decomposition at the trench sidewalls.
The fabrication and operation of a monolithic InGaN alternating-current light-emitting diode ͑LED͒ based on the bridge rectifier design are demonstrated. The device consists of on-chip interconnected LED elements that have been isolated by direct-write laser micromachining, a powerful tool well-suited for rapid device prototyping. The effects of capacitors coupled to the dc path of the rectifier have been investigated. Although an increase of radiant flux can be achieved through capacitive voltage smoothening, the wall-plug efficiency drops as a result. The device can be applied to 12 V rms lighting applications.
Trench formation for device isolation on GaN light-emitting diode (LED) wafers via nanosecond ultraviolet laser micromachining is demonstrated. Trenches with smooth sidewalls and flat bottom surfaces are produced. Unlike wafer scribing with laser beams, the formation of trenches requires that the incident fluence is sufficient for laser ablation of GaN, yet low enough to prevent ablation of the sapphire substrate. Owing to the dissimilar ablation thresholds between GaN and sapphire, the etch process terminates automatically at the GaN/sapphire interface. The effect of the following parameters on the trench properties and quality has been investigated: focus offset, pulse energy, pulse repetition rate, scan speed, and the number of scan passes. It was found that optimal focus offset and pulse energy, a high pulse repetition rate, and single cycle of slow scanning are the key factors for obtaining a trench with tapered sidewall and smooth bottom surface, which is suitable for the laying of interconnects conformally across the trench for device interconnection. This technique has been successfully applied to the rapid prototyping of interconnected LED arrays on a single chip, where metal interconnects run continuously across the micromachined trenches to connect the individual LED devices.
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