Near Infrared Spectroscopy Systems for Tissue Oximetry

We present exible silicon device platforms, which combine polyimide with polydimethylsiloxane in order to add flexibility and biocompatibility to the silicon devices. The device platforms are intended as tissue oximeters, using near infrared spectroscopy, but could potentially also be used for other medical applications. The tissue oximeters are realised by incorporation of pn-diodes into the silicon in order to form arrays of infrared detectors. These arrays can then be used for spatially resolved spectroscopy measurements, with the targeted end user being prematurely born infant children. Monte Carlo simulations have been performed on a model of a neonatal head and they show only weak changing signals as function of changes in cerebral oxygenation. A mechanical and electrical analysis of the device platforms, both by analytical expressions and numerical simulation, indicated that the proposed designs were feasible. A number of different devices were fabricated including devices with detectors formed as arrays (1D) and as matrices (2D). The mechanical testing of the devices showed that they could be bent to more than 90° over their full lengths and strained with 3 % without damage to the electrical interconnects. Furthermore, the devices could be repeatedly bent more than 10,000 times with no indication of fatigue. IV-measurements indicated fairly low reverse bias current densities in the order of 12-30 nA/cm² depending on the detector size and shape. By depositing a 30 nm thick layer of aluminium oxide, as passivation on the detectors, the reverse bias current could be reduced by 30 % for small devices. Quantum efficiencies of 80- 85 % were measured for the best detectors. By using black silicon nanostructures, the reflectance from the detector surfaces could be reduced for all angles of incidence. Thus, also minimising the drop in quantum efficiency for light incident at 38 from normal to only 5.2 % compared to a drop of 9.1 % for devices without the black silicon nanostructures. In conclusion both the flexible device platforms and infrared detectors were found to work.