Fabrication And Electrical Properties Of Flexible Polymer-based MIM Capacitors With High-k Nanolaminate Dielectrics By Atomic Layer Deposition

The rapid development of the Internet and the Internet of Thing is changing human life dramatically,as well as the artificial intelligence,big data,and cloud computing.Flexible electronic technology is also in progress along with the exponentially increasing demand for the wearable,implantable,mobile,and smart healthcare devices.The fabrication of electronic circuits on flexible substrates become more and more important so as to meet device requirements for light weight,bendability,and reliability.Capacitor,as one of the passive basic circuit components,also needs to satisfy these conditions.Compared to traditional capacitors,metal-insulator-metal?MIM?capacitors show evident advantages such as no parasitic capacitance and high figure of merit.Therefore,research of high-performance MIM capacitors on flexible substrates has also become a vital task for flexible electronics.At present,single high-k dielectric layer cannot meet the demands of the MIM capacitor as core functional layer.It is imperative to develop composite high-k oxide dielectric films with high comprehensive performances.Polymer is a competitive substrate for flexible MIM capacitors due to its low cost,easy bending,and mass production.However,its low high-temperature resistance limits its application in flexible MIM capacitors.Atomic layer deposition?ALD?is a kind of new material preparation technology compatible with semiconductor processing.It shows unusual advantages in controllable fabrication of nanolaminate/nanocomposite thin films due to its unique sequence self-limiting surface reaction mechanism.Plasma-enhanced atomic layer deposition?PEALD?can deposit inorganic dielectric films at low temperature,even room temperature because of using active plasma gas as a reactant.Therefore,this thesis mainly focuses on the preparation of several polymer-based flexible MIM capacitors at low temperature and room temperature by PEALD/thermal ALD method.A series of high-k nanolaminate systems,including Hf-Ti-O,Hf-Sn-TiO,Al₂O₃/Hf-Ti-O?Hf-Sn-Ti-O?/Al₂O₃,Al-Ti-O,Al-Ti-Sn-O,and Al-Ti-Zn-O,have been designed and obtained for optimal processing and electrical properties.Moreover,the effect of different bending radii on the electrical properties of polymer-based MIM capacitors has been investigated,and the dynamic bending tests have been performed in the optimal polymer-based MIM capacitor system.Main achievements are summarized as follows:1.Hf-Ti-O and Hf-Sn-Ti-O MIM capacitors were prepared on PET,PI and epoxy substrates at 80 ? using PEALD.The effect of inorganic TiCl₄ and organic amino titanium source on electrical properties were investigated deeply.The results show that for optimal Hf-Ti-O system,polymer-based MIM capacitors derived from organic amino titanium source exhibit significantly improved electrical properties with the increased capacitance density of 6.2-10 f F/?m2,the reduced leakage current density of 10-7 A/cm2,and the significantly-decreased voltage linearity of 480-770 ppm/V²,compared to from TiCl₄ source,due to the removal of Cl impurities.For Hf-Ti-Sn-O system,compared with from TiCl₄,the polymer-based MIM capacitors from organic amino titanium source have evidently increased capacitance density of 8.7-10.2 f F/?m2 and markedly reduced voltage linearity of 680-1290 ppm/V²,however the leakage current density increases by 1-2 orders of magnitude,which is related to the residual Cl impurities from inorganic Sn Cl4 source,and possibly relatively high C residue due to the organic Ti source.In order to reduce the leakage current density,Al₂O₃/Hf-Sn-TiO/Al₂O₃ composite dielectric layer capacitors were prepared at low temperature on polymer substrates by PEALD.The thickness influence of the Al₂O₃ ultrathin layer and the intermediate dielectric layer on the electrical properties was studied.Under optimal conditions,compared to the polymer-based Hf-Sn-Ti-O MIM capacitor,the introduction of 1 nm Al₂O₃ ultrathin layer can reduce the leakage current density to about 10^-8 A/cm2 by 4 orders of magnitude and the voltage linearity to 350-540 ppm/V²,although the capacitance density also reduces to ³ f F/?m2.Nevertheless,the insertion of Al₂O₃ ultrathin layer is still an effective method to improve the electrical performance of polymer-based Hf-Sn-Ti-O capacitors.2.Al-Ti-O,Al-Ti-Sn-O and Al-Ti-Zn-O MIM capacitors were prepared on Si and PET,PI,epoxy resin-based substrates at room temperature by a combination method of PEALD and thermal ALD.The effect of various Al,Sn,and Zn contents on electrical properties of MIM capacitors was investigated.Under optimal conditions,when the Al: Ti sub-cycle ratio is 4: 8 with total 300 cycles,the Si-based Al-Ti-O MIM capacitors show excellent electrical properties with the capacitance density of 5.5 f F/?m2,the leakage current density of 2.8 × 10^-6 A/cm2 and the voltage linearity of 172 ppm/V².When the Al: Ti: Sn sub-cycle is 4: 8: 1 with total 325 cycles,the polymer-based Al-TiSn-O MIM capacitors have better electrical properties such as the comparable capacitance density of 4.2-5.6 f F/?m2 and the best voltage linearity of 21,76 and 30 ppm/V² for PET,PI and epoxy resin,respectively,all of which are less than 100 ppm/V².The corresponding leakage current density rises by one order of magnitude due to the addition of Sn element.For the Al-Ti-Zn-O system,when the Al: Ti: Zn sub-cycle ratio is 4: 8: 3 with total 375 cycles,PET and epoxy-based MIM capacitors exhibit better electrical properties with the increased capacitance density of 6.2-7.4 f F/?m2,evidently reduced leakage current density of 10^-8 A/cm2 by two orders of magnitude.However the voltage linearity increases to 400 ppm/V² due to the addition of Zn element.3.For the above-mentioned optimal polymer-based high-k nanolaminates MIM capacitors,the influence of different bending radii on electrical properties was studied.For the Hf-Ti-O,Hf-Sn-Ti-O,Al₂O₃/Hf-Ti-O?Hf-Sn-Ti-O?/Al₂O₃ polymer-based MIM capacitors prepared at 80 ? by PEALD,when the bending radius ? 8.2mm,the electrical performances keep basically stable.The relative change of capacitance density is less than 10%,the leakage current density is relatively sensitive to the bending state with the increase by no more than one time,and the voltage linearity is less sensitive to the bending state with the relative change less than 25 %.Under the dynamic bending test,when the bending cycles are less than 40,000 to 50,000 times,the electrical properties of polymer-based MIM capacitors usually change little;when bending cycles reaches 70,000 times,the capacitance density drops suddenly and the leakage current density increases rapidly,leading to the broken devices.For Al-Ti-SnO and Al-Ti-Zn-O MIM capacitors prepared at room temperature by combination of PEALD and thermal ALD,the electrical properties of PET and epoxy-based MIM capacitors are basically stable when the bending radius is ? 8.2mm.The relative change in capacitance density is less than 15%.The leakage current density and voltage linearity are more sensitive to bending radii.The former usually increases by no more than 1.5 times,and the latter has a relative change of less than 1-5 times.Under dynamic bending test,when the bending cycles are less than 4,000 times,the electrical performances show less change.When the bending cycles reaches 8000 times,the capacitance density reduces sharply,the leakage current density and voltage linearity rise dramatically,and the device fails.Compared with the result of 40,000 times bending resistance of polymer-based MIM capacitors prepared at 80 ? by PEALD,the samples prepared at room temperature can only withstand about 4,000 fatigue tests.The difference of nearly 10 times may be related to the worse mechanical properties of the dielectric thin films deposited at room temperature.The strength and adhesion of the dielectric thin films in polymer-based MIM capacitors at room temperature still need further improving.In summary,the electrical properties of several high-k nanolaminated MIM capacitors prepared on polymer substrates at low temperature or room temperature by PEALD/thermal ALD have reached the level on Si substrate.Based on the performance and the bending resistance,PET-based high-k nanolaminated MIM capacitors undoubtedly show better comprehensive performances,followed by epoxy-based MIM capacitors.There is much room for the improvement of PI-based MIM capacitors due to the surface roughness problem of PI substrates.This work is of significance and value by exploring high-k materials and low-temperature ALD technology compatible with semiconductor processing in flexible polymer-based MIM capacitors.