Fabrication And Characterization Of Vertical Channel Thin Film Transistors Based On InGaZnO Channel

During the post-Moore era,the movement to 3D integration is already inevitable in the IC filed.As a new device structure of thin film transistor(TFT),vertical channel thin film transistor(VTFT)has raised more and more concerns because of its ability to scale down the device size,to reduce the channel length,and to improve the flexibleness of devices.Furthermore,vertical channel charge trapping memory(VCTM)can be fabricated based on VTFT,which is important to the study of 3D NAND.This thesis focus on fabricating VTFT based on IGZO channel after the optimization of etching process of ITO/SiO2 stack with steep and smooth vertical sidewall,and the deposition of IGZO by regulating the oxygen content in the channel.Besides,the device performance of the fabricated VTFT was measured,analyzed and improved.This work lays the foundation for its application to flexible devices and 3D NAND.In this thesis,systematical research is accordingly made,with the following innovations:1.The etching of ITO and SiO2 films,and ITO/SiO2 stackThe deposition and etching process of ITO and SiO2 films were first studied respectively.Then,SiO2 film and ITO film were deposited sequentially on the substrate to form a stack.The results display that the ITO and SiO2 films both had good quality and the sheet resistance of ITO was about 100 Ω·cm/□.Besides,we used CH4 and H2 as etching gases to etch ITO films.CHF3 and O2 was used as etching gases to etched SiO2 films.Based on the above two etching processes,the ITO/SiO2 stack were etched to form a vertical sidewall.In this process,we found pillar-like remainders after etching which is caused by crystalloid ITO.Finally,the steep and smooth vertical sidewall were obtained by optimizing the etching process through increasing etching time,and improving other etching conditions.Thus,the foundation is laid for subsequent VTFT fabrication.2.The effect of the deposition of ALD Al2O3 on the performance of IGZO TFTsSince the VTFT to be prepared in this thesis is a top gate structure,and its preparation process requires depositing Al2O3 films after depositing IGZO films.Therefore,the effect of depositing Al2O3 films in ALD on the performance of IGZO channel layer should be investigated.Two bottom-gate TFTs were fabricated as controls,in which device A was not annealed and device B was annealed at 150℃ in air for one hour.Then,a 10 nm Al2O3 passivation layer was grown on the surface of devices A and B,and a rapid thermal annealing was carried out at 400℃ in an O2 atmosphere.Experimental results show that performance of TFT was degraded for both devices A and device B after the Al2O3 passivation layer is deposited.It should be noted that the device A cannot even be shut down after the passivation of Al2O3 which proves that the deposition of Al2O3 in ALD results in oxygen escape in the IGZO channel,and increases the oxygen vacancy in IGZO.In order to solve the above problems,the feasibility of oxygenation of the IGZO channel layer was studied experimentally.Five sets of TFTs were designed experimentally,which divided the IGZO channel layer into two layers,of which the lower layer grew in a pure Ar atmosphere,and the upper layer grew in an Atmosphere of Ar/O2 Mixture(oxygen content of 15%).So,the oxygen content in the channel could be regulated by changing the thickness of the upper and lower layers.The results show that the TFT with 10 nm IGZO(Ar/O2)/20 nm IGZO(Ar)channel layer has the best performance,and the performance is slightly improved after depositing the Al2O3 film,with a field effect mobility of 8.71 cm2V-1s-1,an Ion/Ioff ratio of 4.89×108,and a sub-threshold swing significantly reduced,which proves that oxygenation is fully feasible and effective when growing the IGZO channel layer.3.The fabrication and measurements of VTFTAfter optimizing the etching process of the vertical sidewall and the growth process of the IGZO channel layer,a VTFT device based on the IGZO vertical channel was fabricated experimentally.Through the electrical measurement of VTFT,the experimental VTFT has a good TFT performance.The threshold voltage of the prepared VTFT device with a channel width of 100 μm is 0.26 V,its mobility is 0.57 cm2V-1s-1,its current switching ratio is about 1.66×106,and its sub-threshold swing is 0.14 V/dec.The threshold voltage of the VTFT device with a channel width of 200 μm is 0.71 V,its mobility is 0.27 cm2V-1s-1,its current switching ratio is about 2.98×106 and its sub-threshold swing is 0.16 V/dec.Finally,the relevant factors that may affect VTFT performance analyzed.