Micro-VCSELs and silicon photodiodes flex with micro-fluidics

Micro-VCSELs and silicon photodiodes flex with micro-fluidics

Technology News |
By Julien Happich

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 2×4 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 as small as 50mm, the researchers reported.

An interesting note is that rather than losing out on output power, the micro-VCSEL (with an aperture area of 22×22μm2) actually increased from circa 4.5mW as characterised on its GaAs source wafer to circa 5.3mW after polymer encapsulation. The researchers attribute this to the polymer layer on top of the laser aperture, resulting in an increase of the transmitted lasing output.

Such flexible sensor arrays could be designed to detect various luminophores simultaneously across large areas with distributed sensors. Ultimately, these flexible opto-fluidics could find their way into wearable diagnostic systems or even implantable devices for in vivo fluorescence sensing or imaging. Here, mechanical flexibility would make the sensors less obtrusive.

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