Periodic Reporting for period 1 - SiCWIRE (Silicon Carbide Nanowires for Electronic and biosensing applications)
Reporting period: 2016-04-01 to 2017-09-30
More Moore: Logic applications. SiC NWFETs have the potential of high temperature operation and eventually of efficient power dissipation and thus, they can address main issues in semiconductor device scaling. The area of nanoelectronics is a major one in what it concerns market size and impact on every-day life.
More than Moore: Biosensor applications. SiC exhibits superior to Si chemical stability and biocompatibility and SiC NWFET-based biosensors can thus, exhibit challenging performances. The main interest comes from the stability of SiC in aqueous solutions like the physiological ones, which is not the case for Si.
Previous studies of SiC NWFETs employed bottom-up SiC NW growth techniques and back gated geometry. The transistors exhibited weak gating effect and the device switching off was not achievable even for high negative gate voltages. This mediocre performance has been attributed to poor material properties of the grown materials and/or the basic technology employed for fabricating SiC NWFETs resulting mainly in poor quality interface with gate dielectrics. A possible solution for resolving material quality issues is to form the NWs by top-down (mainly plasma etching) techniques from epitaxial SiC material. Use of top gate or gate-all-around (GAA) geometries with thermal or deposited oxides would be the solution for addressing the poor dielectric/SiC interface.
SICWIRE main aim was to develop the technology of SiC NWFETs with top-gate or GAA device geometry incorporating NWs fabricated by top-down techniques. Then, devices suitable for the two areas of applications would be fabricated as demonstrators of the SiC NWs use advantages.
In addition, SICWIRE targeted to enhance the professional maturity of the fellow with a consequent effect in his career by reaching a higher grade (“Research Director”, the highest for Greek researcher) and higher academic positions.
A major conclusion of this effort was that the lift-off process results in inclined mask sidewalls and edge mask erosion results in the pyramidal shape. In order to obtain vertical metal mask sidewalls, a nanoimprint lithography (NIL) process has been adopted as solution. A special process has been applied to narrow the diameter of the resulting SiC NWs after the plasma etching. The process resulted in 100nm diameter NWs.
WP2: Ohmic contacts: the effect of annealing in various environments (vacuum 10-5 Torr, vacuum 10-2 with small Ar forming gas flow through needle valve, atmospheric pressure in Ar forming gas ambient) was found not so important as the annealing temperature and duration.
Separation of vertical SiC NWs from their substrate: The process is necessary for fabricating horizontal back-gated NWFETs from vertical top-down (plasma etching) formed long SiC NWs. The optimized cut has been performed by loading the nanopillars in an IPA bath in downright position and by performing a sonication process.
NW diameter narrowing by anisotropic oxidation: The SiC oxidation rate varies according to the crystal orientation being lower along polar faces. This fact has been exploited for lowering the diameter of the vertical to the basal plane SiC nanopilars with a minimum reduction of their length. Thermal oxidation (1150oC wet oxidation) has been employed towards this purpose. The process was successful with only limitation the stability of NWs after the oxide removal when their diameter is lower than 100nm.
Gate dielectrics: Initial experiments with HfO2 layer deposited by Atomic Layer Deposition (ALD) and subsequent top gate metal (Au) deposition resulted in non-conducting horizontal transistors. Therefore, it has been decided to limit the related effort in thermal oxides. Planar MOS capacitors fabricated on 4H-SiC layers exhibited a DIT~8•1011 cm².eV-1.
WP3: The fellow participated in the fabrication of similar devices with Si NWs and has now the know-how to apply this technology in SiC NWFETs.
In addition, a new photolithography mask setup has been designed in order to fabricate modules of 6 GAA SiC NWFETs. SiC NWFETs performance estimations showed the necessity of developing such modules.
WP4: The related work was conducted on three axes:
1) Further investigation of DNA functionnalization (grafting) of SiC NWs in a back-gated NWFET configuration. The formation of top-down (e-beam lithography and plasma etching) horizontal Si NWs and subsequent carbonization to SiC has been investigated by employing SOI substrates. The Si/SiC core /shell configuration would combine cheap Si substrates as well as mature process with SiC chemical stability. The process is under optimization as in the initial experiments the carbonization process resulted in important deterioration of the underneath oxide.
2) The stability of Si and SiC NWs in saline solutions has been studied. Contrary to Si NWs, no diameter reduction of SiC NWs has been observed, which constitutes a clear proof of chemical inertness of SiC NWs.
3) Vertical SiC NWs are investigated as a solution for increasing the signal of intercortical neural interface (INI) probes for improving longevity of brain-machine interfaces. Indeed, forming arrays of SiC NWs in the detecting region would increase the detecting surface and the corresponding electrical signal.
• It has been demonstrated that hard-mask sidewall angle is the main factor for obtaining pyramidal shape SiC NWs after plasma etching. Nanoimprint (NIL) lithography has been applied for the first time to address this issue.
• SiC NW diameter narrowing through oxidation and subsequent oxide removal has been demonstrated for the first time.