The fabrication of the devices was as follows: SOI layer was thinned down using oxidation and oxide removal. Several consecutive oxidation/oxide-removal steps took place in order to ensure a small thickness variation across the wafers. Eventually, several wafers with SOI thickness in the 10 �C 30 nm range were fabricated with with-in-wafer SOI thickness variation not greater than 10%. The SOI EISFET is fully depleted (FD) for the given SOI resistivity (doping) and SOI thickness. MESA-type isolation was used between the devices. Subsequent arsenic implant (15 keV/5 e14)) for the source and drain regions took place followed 100 nm SiO2 PECVD for inter layer dielectric (ILD) and opening of the contacts. Ti/Al/TiN was sputtered and patterned for interconnection purposes followed by 4,500 ? passivation layer of PECVD nitride.
The seed Ti/Au layer was sputtered followed by Au electroplating in the pad areas. The metal gate of the MOSFETs is located over a 100 nm PECVD SiO2 layer which is significantly thicker than the 30 nm LPCVD SiO2 of the FD EISFETs. The last step of the process was the actual opening of the passivation above the FD EISFETs’ active region. This was performed with dry etch followed by final wet etch in order to ensure no physical and/or electrical damage to the underlying active gate.In the overall die layout (Figure 1), the central gold circle defines the sensing area, and the location of the sealing O-ring of the liquid flow-cell. The lower part contains the test structures zone. On the left side, a chemical window (1.5 mm �� 1.
5 mm) is located that was used in order to perform detailed surface analysis (AFM and ellipsometry).Figure 1.Schematic illustration of die layout (17 mm �� 17 mm).2.2. Electronic MeasurementsI-V measurements were performed for both test structures and FD EISFETs. For the FD EISFETs, I-V measurements were performed both under dry and wet conditions. The electrical setting for both the test structures and the FD EISFETs wet measurements are presented in Figures 2A, B, respectively. In order to work in aqueous conditions we designed a unique liquid application apparatus – a flow cell (Figure 3). This unique design facilitated the work in aqueous GSK-3 environments without the need for contact isolation. The liquids were retained within the O-ring gasket while the thumb screws were tightened against the probe-station chuck.
The connecting pads were left out, providing easy approach for electrical testing (see also Figure 1). The apparatus included additional important features; ultra-low sample volume (30 ��L), fast and convenient way of die replacement and black material to prevent light induced currents. A homemade Ag/AgCl wire type reference electrode (VREF) was used for the wet I-V measurements that were performed with various solutions.