Formation of porous silicon on silicon microcantilevers

At University Autonoma de Madrid the role of PSi on the silicon microcantilevers has been emphasized for the development of nanomechanical interfaces as biosensors. The new bimodal mechanical-optoplasmonic system for biosensing was demonstrated performing a sandwich assay for the detection of  a Prostate Specific Antigen (PSA).

Microcantilevers are the most simple and widely used nanomechanical biosensing systems. The resulting mechanical response to their interaction with a biological analyte is either a deformation (static mode) or a resonance frequency shift (dynamic mode) as a consequence to the added mass of the analyte on the sensor surface. The formation of porous structures on microcantilever sensors is of interest because of the influence of the increased surface area in a wide set of physical and chemical parameters. Indeed, the formation of PSi on silicon microcantilevers allows combining the advantages of both forms of Si. The crystalline Si provides excellent elastic and mechanical properties while PSi provides large adsorption surfaces and constitutes an excellent biofunctional material. Additionally, porosification implies an initial surface activation, which can be redirected to induce an organosilane functionalization of the surface.
In this work, we used p-doped crystalline Si chips with 8 cantilevers per chip (Fig. 1 SEM images of a Micromotive chip).

To generate PSi, electrochemical etching is the most widespread method. However, in the case of cantilevers, due to their fragility, small size, and the absence of metallic contact, we performed vapor phase stain etching. This technique does not require any technical equipment such as current source and consists of exposing the Si substrate to acid vapors issued from a mixture of HNO3 and HF (Fig.2). To start the process and initiate the formation of brown NOx vapors, a piece of sacrificial Si is added to the solution. The chip, stuck on a Teflon lid, is exposed to the vapors once the reactions reach a steady-state.


Figure 2. Vapor phase stain-etching formation of PS on Si cantilever.


Figure 3 shows the microcantilever surface modification after the vapor phase stain etching using a mixture HNO3:HF (1:1) and exposing the chip to the acid vapors for 20s. The microcantilevers have an initial width of 1 μm.

The role of PSi on the silicon microcantilevers has been emphasized for the development of nanomechanical interfaces as biosensors. Indeed, the significance of this new bimodal mechanical-optoplasmonic system for biosensing was demonstrated performing a sandwich assay for the detection of PSA (Prostate Specific Antigen), a prostate cancer biomarker.

Chloé Rodriguez