Method to Control Etching Rate and Homogeneity for Photo electro-less chemical etching with oxidiser (K2S2O8, NaClO4) for multi-junction LED, multi-die LED
Publication Date: 2014-Feb-17
The IP.com Prior Art Database
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ID Number: 2013ID02589
Title: Method to Control Etching Rate and Homogeneity for Photo electro-less chemical etching with oxidizer (K2S2O8, NaClO4) for multi-junction LED, multi-die LED.
The present concept proposes the use of oxidizer (K2S2O8, NaClO4) solution in photo electro- less chemical etching process to enhance epitaxial roughening for multi- junctions LED, multi die LED to achieve maximum light extraction. The etching rate and homogeneity among the dies and junctions are highly dependence on the exposed surface area of the metal pads where the cathodes of the die or junction are connected and the oxidizer concentration.
Photo-electro-chemical (PEC) etching is one of the most critical process to improve light output of LEDs. The current production version of a class of InGaN devices utilizes the following steps:
• Gold stud bumps (~80 µm diameter) are applied to the LED submount tile
• The LED die is flip chip bonded to the submount's stud bumps
• Underfill material is injected between the die and the substrate
• The 'wings' of leftover epoxy are removed by micro bead blasting
• The epi substrate is removed by laser lift-off (LLO)
After laser lift-off to remove the epi substrate, a photo-electro-chemical etching process is applied to roughen the surface of the die. This process applies UV exposure, bias voltage and at the same time submerges the submount LED tile into KOH bath for certain times to etch <1um and roughen the LED surface layer. The chemical reaction occurred during etching are as below:
(1) GaN + photon (UV) GaN + e− + h+
UV light with certain wavelength and intensity is applied to LED surface to excite electron-hole pairs on die surface.
(2) Bias Voltage applied to pull electrons (e− ) leaving free holes (h+) in GaN surface. Bias voltage only can be applied if there is tie-bar connecting the cathode of LED-junction to the sub-mount arm where voltage from power supply will be applied.
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(3) Holes on the die surface will start etching reaction:
2GaN + 6h+ + 6OH− Ga2O3 + 3H2O + N2
Ga2O3 + 6OH− 2(GaO3)3− + 3 H2O
The holes (OH−) oxidize the gallium nitride, then Ga2O3 is dissolved away from the epi surface and create rough surface on the LED surface. Roughen LED surface will aloow more light to escape and increase overall light output.
Picture 1. U-channel LED sub-mounted on tile with tie bar and
its schematic layout of a conventional PEC
With the optimized UV light intensity, KOH chemical concentration, bias voltage and etching time will provide good/uniform roughen die surface. The good die roughness and uniformity will allow more light extraction from the die and make the LED brighter
1. Problem: for multi-junction LED or Multi-die LED, where the junctions/LEDs are connected in series, tie-bar can't be connected to all junctions/LEDs, but maximum can be connected to only on 1 junction/LED at the cathode-pad side. With this design, bias voltage will only work on 1...