Micro-nano Robotic Manipulation For Assembly Of A 3D Carbon Nanotube Field-Effect Transistor

To make Moore's law continue,fabrication of 3D carbon nanotube field effect transistor(CNTFET)is needed to significantly improve the speed of chips,reduce their power consumption and save costs.It has become an issue that needed to be solved in the current electronic industry.The implementation of its structure and function of the 3D CNTFET must rely on reliable assembly technologies of carbon nanotubes(CNTs).However,the existing assembly techniques,such as chemical vapor deposition(CVD),random distribution and dielectrophoresis(DEP),aim to process and manufacture the plane CNTFET,and it is difficult for them to deal with 3D devices with good flexibility and reliability.To address these problems,this dissertation first designs a novel 3D CNTFET based on a review of the research status at home and abroad.To realize the structure,we propose an assembly method based on micro-nano robotic nanomanipulation inside a scanning electron microscope(SEM).Through the integration of the assembly strategy and the study of the basic operations theoretically and experimentally,our assembly method can effectively complete the 3D assembly tasks of CNTs,and meet the requirements of a reliable electrical-contact.It also provides a new way to achieve the flexible 3D assembly of CNTs and the reliable manufacturing of nano devices,and can help to build a new theory on the fussion of micro-nano robotic manipulations and nano device fabrication.The main research issues and results are summarized as follows:Firstly,with reference of the FinFET structure,we propose a novel structure of 3D CNTFET,in which CNTs contact with a gate by three sides.Its increased contact area is conducive to enhance the control of gate voltage on CNTs.To achieve the structure,a micro-nano robotic system inside an SEM is introduced.One manipulator(with 4 degrees of freedom and the precision of 30 nm)of the system is used to integrate the 3D assembly strategy of CNTs,in order to meet the demand of a high precision and reliability in the fabrication of 3D CNTFET.Compared with multiple manipulators,the operation of single manipulator is simpler,easier to control and realize the automation.Secondly,specific operation methods for the assembly strategy are studied based on a single manipulator and mainly involve the pick-up,placement,fixing and cutting of a single CNT.The pick-up of a CNT through an AFM cantilever is realized by investigating the CNT dispersion method,mechanical and chemical adhesion mechanism between the CNT and metal surface.The success rate of pick-up was proved to be 100%,and the utilization rate was 79.2%.Through the theoretical study of a strategy on CNT placement,it is concluded that the cantilever must clockwise rotate an angle(between 40.4 ° and 90 °),and move in a zigzag motion with cooperation of a van der Waals force.Only in this way,we can achieve the CNT placement on the surface of the FET electrodes.The placed CNT needs a further reinforcement with EBID technology,to improve the mechanical and electrical characteristics of the CNT/metal contact.To release the cantilever from the fabricated structure for the next CNT pick-up,a method of electrical breakdown is used to cut CNT.The needed voltage was determined as 2.0 V by experimental tests.Thirdly,to quantitatively characterize the mechanical and electrical properties of the CNT/metal contact,we measure the elastic deformation of the cantilever to obtain the contact force between CNTs and FET electrodes(Au).Experimental results showed that the van der Waals force per unit length was 210.5nN/?m,and the fixing force of single EBID deposit was more than 151.1 nN.Then,we investigate a two-ternimal method to measure the contact resistance between CNT and Au.Experimental results showed a contact resistance of 7.5 k? after seven EBID-tungsten deposits,which verified the electrical reliability of our assembly strategy for 3D CNTFET.Furthermore,we analyze the impact factors on the contact resistance such as the contact force and EBID materials.A theoretical relationship between the contact force and the contact resistance is set up and verified with analysis of the above experimental results.Contact resistance was measured under different EBID materials,which proved that EBID is able to modulate the work function of the CNT/Au contact,and can be used to strengthen the contact between CNT and other metals.Finally,we design the size of 3D CNTFET electrodes and their outer leads,and fabricated them through lithography and FIB system.Two CNTs was successfully assembled to the front surface of the FET electrodes,which validated the proposed assembly strategy in this dissertation.Mass production of the 3D CNTFET needs to improve the assembly efficiency of CNT.Therefore,we will try to realize the automatic assembly in the future.