Research On Flexible CMOS Integrated Circuits Based On N-type IGZO And P-type SnO Thin-Film Transistors

In the past few decades,flexible electronics developed rapidly and show great potential in wearable devices,flexible displays,RFID,electronic skins,advanced energy,etc.Flexible electronics fabricated on flexible substrates such as plastics,papers,flexible glass,etc.are an emerging science and technology which will completely change our daily lives.Compared with traditional amorphous silicon,polycrystalline silicon,and organic semiconductors,oxide semiconductors(OSs)have great advantages in high carrier mobility(?1-100 cm2/Vs),low cost,low deposition temperature(so as to fabricated on flexible substrates),high transparency in the visible light region,and large-area homogeneous processibility.Thus,OSs are ideal material candidates for flexible electronics.In fact,during device fabrication,many OS thin films require thermal annealing(e.g.above 100?)to improve thin-film quality,optimize electrical contact,and regulate the conductivity.Moreover,the photolithography process usually requires thermal baking.Due to the low glass transition temperature,large thermal expansion coefficient,and large surface roughness of the common flexible substrates,it highly restricts the real applications of OSs in flexible electronics.To overcome the above problems,in this thesis work,we use a "water-assisted peeling off and transfer method" fabrication technique,that is,fabricating devices on rigid substrates and then peeling off and transferring to flexible substrates,and realize flexible n-type InGaZnO(IGZO)based thin-film transistors(TFTs),p-type SnO TFTs,and complementary metal-oxide-semiconductor(CMOS)circuits.These flexible TFTs and CMOS circuits remain high electrical performances without obvious degradation under mechanical bending strains,and the fabricated CMOS inverters show the highest performances both on flat state and under strains in comparison with the same type flexible circuits.The results demonstrate that the fabrication method is an ideal technique for fabricating high-performance OS-based flexible electronics and circuits.The main contents of this thesis work are as follows:(1)Fabrication of flexible complementary OS-based TFTs.Metallic nickel(Ni)was evaporated firstly on the rigid SiO2/Si substrate,and then high-performance n-type IGZO TFTs and p-type SnO TFTs were fabricated on the Ni-covered SiO2/Si rigid substrate via common fabrication processes including photolithography,thermal baking,annealing,etc,with the highest process temperature of 225?.Then,an organic protective layer,also as the supporting layer,was covered on the rigid samples.After that,the samples were dipped into the deionized water,and in this process the Ni reacted with SiO2 and formed nickel silicate,which can significantly reduce the Ni/SiO2 interface adhesion energy.Thus,the TFTs can be easily peeled off mechanically,and transferred to the flexible polyethylene terephthalate(PET)substrates.This method has very good compatibility with common semiconductor processes(photolithography,thermal annealing,vacuum deposition,etc.),and is not limited by the substrate disadvantages such as low glass transition temperatures,large thermal expansion coefficients,large surface roughness,etc.(2)Performance study on the flexible complementary TFTs.In the flat state,the electron/hole mobility ?e/?h,on/off current ratio ION/IOFF,threshold voltage VTH,and subthreshold swing SS of the n-type IGZO TFTs and p-type SnO TFTs are 10.441 and 0.433 cm2/Vs,1.016 × 106 and 2.032×104,5.645 and-4.589 V,and 0.427 and 2.137 V/decade,respectively.Under the bending strain in the range of 0.33?1.25%,the ?e/?h VTH,and SS of these TFTs show no clear degradation in comparison with the TFTs in the flat state.The results indicate that in strain range of 0.33?1.25%,these flexible TFTs have excellent bending stress endurability.(3)Design,fabrication,and performance studies on the flexible CMOS inverters.Based on the above research results on flexible complementary TFTs,flexible CMOS inverters based on n-type IGZO and p-type SnO were design and fabricated.when the channel width/length ratio of SnO TFT and IGZO TFT N=5,VDD=10 V,in the flat state,the inverters show high performances with the maximum voltage gain of the inverters reaches-122.6 V/V(the highest value among the reported oxide CMOS inverters),the threshold voltage Vtr=4.83 V,and noise margin low NML and noise margin high NMH of 43.6%and 48.6%of VDD,respectively.Under bending tensile strains with the ? in the range of 0.50%-1.25%,and with VDD of 10 V,the gain,Vtr,NML,and NMH of the flexible CMOS inverters show no clear degradation comparing to those of the inverters in the flat state.With the maximum ? of 1.25%,the gain remains above-100 V/V,Vtr reaches 4.67 V,and NML and NMH reach 41.3%and 49.9%of VDD,respectively.To the best of our knowledge,this is the best performances among the reported flexible CMOS inverters based on complementary oxides.(4)Design,fabrication,and performance studies on the flexible CMOS logic gates.Flexible NAND and NOR gates based on n-type IGZO and p-type SnO were designed and fabricated.The performance characterization indicates that the input and output logic of these gates is correct in both flat state and with a strain ? of 0.50%,indicating great application potential of these flexible CMOS circuits prepared by this method.