In electronic circuits a common way to eliminate background

In electronic circuits a common way to eliminate background signal consists in using differential readout approaches [23]. Balanced structures are extensively used to reject large common signals and to amplify only their difference. The authors have presented a balanced photodiode based on amorphous silicon (a-Si:H)/amorphous silicon carbide (a-SiC:H) thin film photosensors [10,11], whose spectral response extends down to the ultraviolet range. The structure is a single detector, that, combining two sensors, gives, in a hardware way, as output signal directly the differential current. This permits a high dynamic range in the signal measurement without degrading the resolution of the single photosensor and gives the possibility to reveal very small variations of photocurrent in a large background current signal. In this paper we present the integration of the balanced photosensor with a microfluidic network to perform high-dynamic, high-sensitive on-chip detection. This kind of integration is a challenging issue in the scientific Child Health working on lab-on-chip systems for biomedical applications [32]. The paper is organized as follows: in Section 2 the structure and the operation mode of the integrated system are described; in Section 3 the fabrication process and the characterization of the sensor structure is reported together with the fabrication process of the microfluidic network and their integration; in Section 4 experimental results, carried out measuring the differential current of the balanced structure in several conditions, are described in details and discussed.
Structure and operation of the device The device combines on the same side of a glass substrate a differential photodiode aligned with two microfluidic channels. Fig. 1 reports its cross section and top view. From the cross section, we see that the single sensor of the balanced device is a n-doped/intrinsic/p-doped a-Si:H stacked structure and that the balanced device is achieved connecting in series two a-Si:H/a-SiC:H n-i-p sensors. A polymer film (SU-8) acts as insulating and passivation layer. The two microchannels are made in polydimethilsiloxane and are placed on top of the passivation layer. The inner walls of the microchannels have been properly treated to make them hydrophilic. Assuming photosensors perfectly matched and uniform light radiation distribution, differences in luminescence or absorption phenomena occurring inside the two channels produce differences in the light impinging on the two photosensors of the balanced device and give rise to a differential current flowing through the common electrode. It is worth noticing that, integrating the balanced photodiode and the microfluidic network on the same side of the glass substrate, the distance between the photosensors and the microfluidic channels is reduced to the few microns of the SU-8 thickness. This implies a maximization of the light impinging on the photosensors and a reduction of the inter-channel crosstalk. Furthermore, the n-i-p structure used in this work results in a device with very low dark current, that is useful to maximize the signal-to-noise ratio. Indeed in this structure, the deposition temperature of the three subsequent layers decreases through the whole process, thus reducing doping contamination of the i-layer, which would have lead to the increase of the dark current due to thermal carrier generation [34]. The corresponding electronic circuit and the read-out electronics are depicted in Fig. 2, where the differential readout approach eliminates the background signal. Indeed, thanks to the virtual ground of the operational amplifier, the two diodes are biased at the same reverse voltage, therefore if the two diodes are perfectly matched the output voltage of the amplifier is proportional only to the difference between the currents flowing through the two photosensors, which in turn is related to the difference between the light intensities impinging on the sensors.