Giuseppe Y. Mak

Precision laser micromachining of trenches in GaN on sapphire

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 con...

Interconnected alternating-current light-emitting diode arrays isolated by laser micromachining

The fabrication and operation of a monolithic InGaN alternating-current light-emitting diode (LED) 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 Vrms lighting applications.

Placement sensitivity to aberration in optical imaging

Theories are developed to quantify the shift of image intensity extremum (/spl Delta/x) due to aberration. The theory on real, one-dimensional mask spectrum has been extended to complex, two-dimensional mask spectrum under coherent and partially coherent imaging. Verification of the formulae was performed on alternating phase-shifting mask (PSM) and contact array. Balanced third-order coma was used to illustrate the validity of the theories. It is found that the image shift due to aberration of alternating PSM decreases when the partial coherence factor increases. In general, the theories can be applied to any mask spectra and aberration functions.

Plasmonically enhanced quantum-dot white-light InGaN light-emitting diode

The enhancement of white light emission from quantum-dot-coated InGaN light-emitting diodes (LEDs) via localized surface plasmon (LSP) resonance of metallic nanoparticles (MNPs) is investigated and demonstrated. With liquid-immersion laser ablation of metals, MNPs with a broad range of dimensions were synthesized in a single process. Since the LSP resonant wavelength depends strongly on the dimensions of MNPs, enhancement over a wide range of wavelengths in the visible spectrum is expected. MNPs of Ag, Au, Cu, Ni and Ti were experimented on. It is found that all MNPs result in an increase in the luminous flux and luminous efficacy of the quantum-dot-coated LEDs, with Ag NPs having the strongest effect (17.9%) amongst all metals tested. This observation is explained in terms of the resonance of the polarizability of the MNPs.

Optimization of photomask design for reducing aberration-induced placement error

In semiconductor manufacturing, the accurate placement of circuit components ensures the proper functioning of microelectronic circuits. This is often subject to photolithography, an optical technique that transfers circuit patterns from photomasks to silicon wafers. Sources of placement error include aberration and misalignment between different levels, and we focus on the former. Aberration is an optical phenomenon that often degrades imaging system performance. Since aberration differs from one imaging system to another, a photomask design that minimizes the aberration-induced placement error is desired. In this paper, we discuss the optimization process of a general one-dimensional mask pattern under a general illumination condition. The constraint is a known population mean of the root mean square aberrations for the imaging systems under consideration. To apply the theory, we search for the optimal parameters for two common mask designs: alternating phase-shifting masks (PSMs) and attenuated PSMs. The theoretical results are compared with those from a Monte Carlo analysis on a large set of imaging systems. These results are indicative to mask manufacturers and circuit designers of increasing manufacturability of circuits

Alternating phase-shifting mask design for low aberration sensitivity

Theories are developed to optimize the mask structure of alternating phase-shifting masks (PSMs) to minimize the average image placement error towards aberration under coherent imaging. The constraint of the optimization is a given mean value of RMS aberration, which corresponds to infinitely many sets of random Zernike coefficients. To begin the analysis, the image placement error is expressed as a function of the mask spectrum and the wave aberration. Monte Carlo analysis on the Zernike coefficients is then performed, which assures us that a global minimum of average image placement error is likely to occur at low phase widths. This result is confirmed by analytically considering the expected value of the square of the image placement error. By Golden Section Search, the optimal phase width is found to be 0.3707 (λ/NA) at 0.07 λ RMS aberration. This result is applicable to the design of all alternating PSMs.