Chemical functionalization and device characterization of Nano-scale Field Effect Transistors (FETs) for Biomolecule Sensing

Biosensors have been developed since decades and for a wide range of applications, such as diagnostics, monitoring of diseases, drug discovery and food industry. One key demand for all the different fields is the highly specific detection of biomolecules. Various biosensing strategies have been investigated to obtain more sensitive and specific biological sensors. So far, the most interesting results in terms of low detection limit have been achieved with bottom-up fabricated biosensors. A bottom-up approach allows for more easy and economical fabrication of sensors for proof-of-concept studies compared to the photolithographic process which relies on a cleanroom. However, a top-down photolithographic approach remains the only choice to realize high-volume biosensor chips for high-throughput applications with good yield and reliability. For this reason, this thesis work is focused on the realization of Biological Field-Effect Transistor (bioFET) sensors fabricated in imec’s facilities. The established finFET technology process at imec is followed by a metal gate removal step forming a cavity and exposing the gate oxide to the environment. Afterward, the oxide surface is functionalized with a self-assembled monolayer (SAM) to obtain a biosensor that is specific to a certain target biomolecule. The use of the advanced silicon-based finFET technology allows for high-density integration and parallelization of bioFETs on chip. Moreover, finFETs can be made small while still maintaining good electrical parameters, which is critical to achieve the single-molecule detection limit with bioFET sensors. When biomolecules attach to the FET’s surface, the channel charge is redistributed and so a modulation of the FET drain current occurs. The molecule signal depends strongly on the nature of the bioFET’s surface. For this reason, two types of aminoalkylsilane SAMs have been characterized: 3-Aminopropyltrimethoxysilane (APTMS) and 11-aminoundecyltriethoxysilane (AUTES). The two functionalized layers have been characterized in terms of contact angle, thickness, surface charge and binding capability. Consistent results have been obtained for the different surface characterization techniques. Afterwards, the best SAM has been deposited on the bioFETs to proceed with the electrical characterization. At this stage, the good quality of the devices has been demonstrated in terms of electrical parameters like threshold voltage and subthreshold swing. These parameters have been compared with ideal values found in literature and with experimental results derived by previous works. The aminoalkylsilane functionalized bioFETs give subthreshold swing values like the uncoated devices. This means that the functionalized layer does not impede the good functioning of the FETs. The electrical characterization ends with the two-step binding of 2μM 50A50T DNA oligonucleotides to the bioFET surface using glutaraldehyde as a cross-linker. The experiments provide show a clear change in the FET current after the binding of glutaraldehyde with the aminoalkylsilane functionalization layer. However, the injection of DNA in the flow cell does not result in a DNA signal. Outcomes and possible reasons behind the results are analysed and discussed at the end of the electrical characterization part. Eventually, limitations of the presented study and outlook on biomolecule detection with Silicon-based FETs will conclude the thesis.