| In recent years,the semiconductor industry has already revolutionized our lives,but traditional devices on rigid substrate are having trouble meeting the requirements of working in some non-flat environments as in the field of medical care,telecommunications,and defense.Therefore,the demand for flexible devices with great bending ability is imperative.However,despite its great flexibility,the electrical properties of common organic electronics are much poorer than those made of common semiconductor materials.If silicon-based electronics can be combined with flexible substrates,flexible and bendable inorganic devices with high-performance will be realized,which would enable inorganic systems suitable for a wider range of applications in unconventional work environments.This thesis mainly focuses on Si nanomembranes(NMs)based flexible metal-oxide-semiconductor(MOS)capacitor,including the device structure design,key manufacturing process optimization and the capacitance characteristics under bending conditions.The detailed investigations are as follows:The design includes the device layout and transfer-printing related process of flexible MOS capacitor.The layout design mainly includes the design of etching holes and the adaptation of flip transfer-printing process.As far as manufacturing process is considered,Reactive ion etching(RIE)was employed to open etching holes on Si NMs.Subsequently,we used water diluted hydrofluoric acid with solution of 1:1 to selectively etch the sacrifice layer off and optimized the transfer-printing approach to combine the Si NMs onto the flexible substrate made of polyethylene terephthalate(PET).To confirm the successful transfer-printing of Si NMs,we used an atomic force microscope(AFM)and a scanning electron microscope(SEM)to characterize the sample surface.Lastly,we used a spectroscopic ellipsometer(SE)to determine the thickness of the gate dielectric layer.To quantitatively determine the doping concentration of the si functional layer,the silicon on insulator(SOI)sample was subjected to a mercury probe test.Afterwards the capacitance and current tests were performed on flexible MOS capacitors in both flat-state and curved conditons with different bending radius.In the flat state,the flat-band voltage of the capacitor is around-1.1V,which had significant difference from that in ideal conditions.This indicated the existence of the fixed oxide charges.As for MOS capacitors in the flat condition,we have analyzed relevant electrical properties based on the capacitance-voltage characteristic curve.In the convex condition,as the bending radius of the capacitors decreased,the tensile strain applied to the capacitors increased,we observed that threshold voltage and the flatband voltage had a significant negative drift.This was mainly caused by the tensile deformation of the gate-dielectric layer,which decreased its equivalent thickness.Meanwhile,the hysteresis voltage was found to decrease slightly.Based on that,we could speculate that the density of fixed oxide charge was not under heavy influence of the strain,the oxide trap charge decreased slightly and the interface trap charge would increase,which would have an impact on the stability of the flexible MOS capacitor;In the case of the concave states,as the compress stress gradually increased due to the compressive deformation of the gate-dielectric layer,the threshold voltage and the flat-band voltage shifted positively and the hysteresis voltage reduced.However,the non-ideal charge in the dielectric layer reduced compared with the capacitor in the flat-state,which may be due to the deviation of the calculation resulting from the decrease of the oxide capacitance value.Finally,taking sample #6 as an example,the leakage current density of flexible Si MOS capacitor was studied.It has been found that the leakage current of the flexible capacitor basically showed a decrease as the bending degree increased,which might mainly due to the reduction of the trap charge of the gate oxide layer. |
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