# Development of Highly Sensitive Temperature Microsensors for Localized Measurements
## 2 Materials and Methods
### Fabrication Process  
Considering the results obtained from the numerical simulations, the temperature microsensors were fabricated with the dimensions described in Table 1 and the simulated and experimental performances were compared. The fabrication process of these microsensors is based on physical vapor deposition (PVD) technologies for thin metallic films, and their standardization is carried out by photolithography techniques and lift-off process (Figure 5). The fabrication process started with the cleaning of a glass wafer using the PVA Tepla GIGAbatch 360M equipment comprising a microwave plasma, created from a mixture of O\({}_{2}\) and CF\({}_{4}\), operating at 2.45 GHz. Then, a double layer lift-off process was performed using standard photolithography and wet etching, using MicroChemicals AZ-family photoresist, developer, and solvents. For that purpose, lift-off resist (LOR1A-SB200) and positive photoresist AZ4110 are sequentially deposited in the substrate (Figure 5b,c, respectively) by spin coating and cured. A 405 nm high-resolution laser is used to expose the photoresist for substrate patterning (Figure 5d) by a direct writer laser process (DWL2000, Heidelberg Instruments, Heidelberg, Germany) and a development step using AZ400K solvent is performed (Figure 5e). After this stage, PVD was used for performing two sputtering processes. For this purpose, a Kenosistec ultra-high vacuum magnetron sputtering system consisting in a confocal geometry, was employed to deposit an adhesion layer of tantalum with a thickness of 5 nm and a layer of platinum with a thickness of 200 nm (Figure 5f). Once the above steps have been successfully carried out, the lift-off resist is removed using a 60 \({}^{\circ}\)C pre-heated solution of mr-REM500 (microresist technology GmbH, Berlin, Germany) in an ultrasonic bath to improve the etching rate (Figure 5g). Finally, photoresist residues in the wafer with the patterned sensors are removed by a high temperature plasma ashing process. An electrical insulator with high thermal conductivity is further deposited on top of the temperature microsensors for electrically isolation of the sensors from the fluidic medium (Figure 5h), increased chemical and mechanical resistance and to provide a rapid response to changes in temperature. For achieving this electrical insulation layer, a conformal layer of SiO\({}_{2}\), is deposited by chemical vapor deposition or atomic layer deposition, respectively. Then, a high-resolution mask aligner (MA6 BA6 Suss Microtec, Garching, Germany) is used to transfer a mask to the wafer. For that, photoresist AZ4110 is spin-coated on the wafer, which is then exposed at a wavelength of 365 nm and developed using AZ400K solvent. A dry etching of the SiO\({}_{2}\) using a reactive ion etching advanced plasma system (APTS SPTS Technologies, Newport, U.K.) or a wet etching process using aluminum etchant 16:1:2 W/AES (Fuifilim, Electronic Materials, Tokyo, Japan) is further performed in order to remove the deposited material merely in the regions where electrical contacts are made. At the end, a plasma ashing step is performed for removing the remaining photoresist. In Figure 6 the fabricated temperature microsensors is shown.