One of the major challenges in realizing fluorescence-based lab-on-a-chip or micro-total analysis systems (m-TAS) by heterogeneous integration of the necessary components such as light sources, microfluidic channels, optical filters, and photodetectors, is the direct integration of the light source into the system. The ability to integrate multi-color narrow-band sources that are spatially distributed so as to operate in the near field would provide functionality on a chip that is currently only feasible with benchtop instruments.
We have developed a novel pixel-to-point transfer process to integrate GaN light-emitting diodes with photodetector chips and thin-film band-edge filters. This integration process was enabled by the double transfer technique  (Figure 1) which we previously developed for integration of GaN light-emitting diode (LED) arrays with silicon substrates. The pixel-to-point transfer process solved problems of pixel pick-up from the source wafer and pixel registration to the target system. The transfer was accomplished by (1) temporarily bonding the LED pixel to a specially designed pick-up rod with sapphire substrates facing upward using Super Glue®, (2) removing the sapphire substrates using laser lift-off, and (3) permanently bonding the LED pixel to the designated area in the pre-fabricated silicon photodiodes using Pd-In transient-liquid-phase bonding.
Using the pixel-to-point transfer process, we are now fabricating a prototype light-source/detector chip based on fluorescence devices to evaluate the performance of the integrated biochips. A GaN LED with peak emission at 463 nm will be used to excite 515 nm fluorescence from FluoSpheres® carboxylate-modified yellow-green fluorescent microspheres (40 nm in diameter).
Figure 1: General scheme of double transfer process