Micro-VCSELs and silicon photodiodes into flexible micro-fluidics

June 16, 2016 // By Julien Happich
Researchers from the University of Southern California have designed fully encapsulated flexible opto-fluidic fluorescence sensors based on the heterogeneous integration of thin film micro-VCSELs and silicon photodiodes.

Only a few micron thin and measuring a few square centimetres (including the fluidic micro-channels polymer overlay), the flexible sensors were proven to perform multiplexed, real-time monitoring of fluorescent analytes flowing through the transparent-fluidic channels, at luminophore concentrations as low as 5×10 −5 weight %.

This heterogeneous co-assembly on a flexible PET substrate was only possible thanks to a transfer-printing method the scientists had developed in prior research, enabling them to lift-off micrometer-thin microscale VCSELs from their GaAs growth wafer as well as the silicon photodiodes (Si-PDs) from their SOI substrate before gluing the devices in a predetermined pattern to build the sensor.

Using this transfer-printing method, the researchers broke free of the limitations of traditional semiconductor substrates. They were able to design sensor arrays over a large area in a flexible, liquid-proof layered construction, each sensor including an 850nm-emitting micro-VCSEL surrounded by a U-shaped array of Si-PDs, the two being optically separated by metallised trenches.

The optical stack also included multilayer-based angle- and wavelength-selective spectral filters to reduce optical cross-talks between the co-integrated micro-VCSELs and Si-PDs, hence optimising the signal-to-noise ratio and detection threshold of the fluorescence sensor as luminophores circulated in the micro-fluidic channels and reservoirs laminated on top of the devices.

(a) Exploded- (left) and tilt-view (right) schematic illustrations of mechanically flexible integrated fluorescence sensors based on heterogeneously integrated micro-VCSELs and silicon photodiodes (Si-PDs) on a polyethylene terephthalate (PET) substrate. (b) Tilt-view colorized scanning electron microscope (SEM) image of an 850 nm-emitting micro-VCSEL co-integrated with a 3μm-thick Si-PD on a silicon substrate. The inset shows the detailed doping layouts of Si-PD including n+- and p+-doped regions in the n-type background. (c) Photographic image of a 2x4 array of the interconnected fluorescence sensor on PET wrapped on a cylindrical support (bending radius: 12 mm).

The whole laminated elastomeric fluidics and optical sensor assembly was shown to reliably perform fluorescence measurements even under repeated flexure at a bending radius