Flow-Through PIN Photodiodes for Integrated Micro Diagnostic Biosensors
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This study focuses on the design, fabrication, and evaluation of a trench Flow-Through PIN Photodiode (FTPD). Highly sensitive silicon PIN photodiodes embedded into the bottom as well as the sidewalls of a microfluidic channel can significantly improve the collection of photons emitted from particles in the channel. And a 360 degree angular field of view (FOV) of the sensor minimizes the optical loss. The microfluidic channel was made by a deep reactive ion etching (DRIE) process so that photo-luminescent particles can flow through the photodiodes. The geometry that has been investigated for the trench FTPD is surprisingly simple, and requires only three diffusions; one for n+, two for p+; a DRIE etching step to create the microfluidic channels, and metal contacts for the anode and cathode. However, the deep trenches requires specialized photolithography steps. The FTPDs characteristics of dark current, breakdown voltage, photocurrent versus wavelength, and responsivity to light were measured and compared against other test structures and commercial photodiodes. The trench FTPD exhibited a 114 nA background dark current at - 2 V and a maximum photocurrent of 256 nA from phosphorescent microspheres of 5.59 nW optical power. By comparison, a planar type PD of the same lateral area shows only 14.8 nA of photocurrent generated by phosphorescent emission from the same microspheres. The back ground dark current of planar type PD is 2.83 nA at - 2 V. The FTPD improves photon collection efficiency and responsivity to light over that of conventional planar type photodiodes due to the larger active area of photodiodes. This sensor can be used for biosensing applications such as disease detection, cell counting, laser induced fluorescence detection, and miniaturized total analysis systems.
- Electrical engineering