(a) Sketch of a high-power flip-chip LED and (b) optical microscope image of the flip-chip LED mounted on a PCB by chip-on-board technology. The base point (0,0) is a benchmark for an Cartesian coordinate system, which defines the coordinates and the orientation of the inscribed structure. The quarters , , , and and the white lines define the individual structures and directions the optical cross sections on the light intensity distributions were recorded from (compare also Fig. 9).
Schematic of the experimental setup for the femtosecond laser direct structuring of the backside surface of the flip-chip LEDs. Groove lines are fabricated by an irradiation of the backside surfaces of the flip-chip LEDs with femtosecond laser pulses and a contemporaneous motion of the translation stage the LEDs are fixed on with a specific scanning velocity.
Optical microscope image of the test lines fabricated by femtosecond laser ablation on the backside surface of the sapphire substrate of a flip-chip LED. The lines were inscribed with laser powers varying from 120 to with an increment of , (from top to bottom) and a constant scanning velocity of . Note that the lowermost line, in turn, was written with the maximal laser power of for a better differentiation of the fabrication area.
AFM image of the groove lines inscribed on the backside surface of the sapphire substrate of a flip-chip LED with a laser power of and a scanning velocity of .
3D AFM images of the groove lines inscribed on the backside surfaces of the sapphire substrates of flip-chip LEDs with laser powers of 70 and , respectively.
AFM height profiles of the inscribed groove lines for laser powers of 70 , 80 , 90 , and .
Optical output power of a LED measured as a function of the dc forward current before and after (groove lines with a periodicity of were inscribed with a laser power of at a scanning speed of ) femtosecond laser structuring of the sapphire backside surface.
Enhancement of light extraction from the backside surface of the sapphire substrates vs laser power for groove line fabrication.
optical sections of the light intensity distribution from a partially structured LED. [see Fig. 1(b)]. (a) Left side: LED area structured with groove lines fabricated with a laser power of . Right side: nonstructured LED area. (b) Left side: LED area structured with groove lines fabricated with a laser power of . Right side: nonstructured LED area. The images were recorded by a confocal technique with a objective lens having a numerical aperture of 0.95. The values of the light intensity are normalized to a linear color spectrum scale, with the highest values represented by a red color and the lowest values by a blue color.
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