Fabrication Of Nano Tip For Electrohydrodynamics Printing Head Using Micro Electroforming Technique

The unique advantage of electrojet printing technique is that it can use large nozzle to obtain a much smaller jet,and the diameter of the jet is 1-2 orders of magnitude smaller than the nozzle diameter.Therefore,the related studies generally improve the print resolution by reducing the diameter of the orifice.However,there will be a great challenge to improve the electrojet printing resolution from submicron to nanoscale due to the physical scale limitation of current body focusing.Based on this,a new electrojet printing method using nano tip instead of hollow needle is studied in this paper,and it is expected that the print resolution will be improved from micron scale to nano scale by the principle of infiltration focusing on the surface of nano tip,which has the advantages of high printing resolution,good control precision,wide adaptability of materials,and so on.Focusing on the design and fabrication of nano tip,the main contents of this paper are as follows:?1?A new method of electrojet printing based on the nano tip infiltration focusing is explored.Next,the printing nozzle with an integrated structure of nano tip,ink capacity cavity and control electrode is designed based on the infiltration focusing principle and the jet process.Applying the Navier-Stokes equation of electrohydrodynamic,the n?merical model of printing principle process is constructed,and then the jet process is simulated by applying the COMSOL Multiphysics two-phase flow phase filed method.Finally,the dynamic printing process of liquid film climbing and covering nano tip,liquid cone pulling fracture and droplet ejection is successfully captured in the three-dimensional flow velocity simulation slice cloud image,and it preliminarily verifies the feasibility of the nozzle structure design and printing principle.The relationship between the structure parameters of nano tip and the volume of focused liquid film is further explored,which provides important theoretical guidance for the design and fabrication of nano tip.?2?Based on the above simulation results,the fabrication process of nano-tip,which is the key component of the nozzle,is discussed,and a fabrication method of self-stop etching of?100?silicon wafer combined with micro electroforming is developed,followed by the process design and fabrication of inverted pyramidal conical pit silicon template are emphasized.In this process,the relative error of template making can be reduced by about 9%by dry etching instead of HF etching to pattern the SiO₂ mask windows.The impurity particles generated in the KOH etching process of conical pit are analyzed by EDS,and the formation mechanism is also discussed.Finally,the template with average size of 140.250?m×140.060?m,bottom curvature radius?100nm and relatively flat sidewalls is prepared.Furthermore,the opening angle of the cone pit bottom is reduced from 70.42°to about 30°by the low temperature oxidation sharpening process,which provides the possibility for the preparation of the nano-nickel tip with more steep sidewalls.?3?The micro electroforming process of nano-nickel tips based on silicon template morphology replication technology is studied.During this process,the microelectroforming basic principle,the relationship between the cast layer thickness and the electroforming time,the influence of working conditions as well as parameters of the microelectroforming on the cast layer quality are analyzed.And the separation of seed layer from silicon template and casting layer in electroforming process is solved by stressing splitting and vacuum annealing process,which can ensure the electroforming will be carried out smoothly.Simultaneously,the formation cause of the Ni tip with different top shapes is analyzed,and the improvement measure is furthermore put forward to enhance the electroforming reliability.Experimental results indicate that the pyramidal Ni shells are exhibited with base dimensions?138?m,tip-curvature radius of?73nm,and the precision of microelectroforming is?99%.Finally,the top opening angle of positive V-shaped nickel pyramid was reduced from 60°to?32.3°by using oxidation sharpening process,which proves the effect of oxidation sharpening and builds a foundation for the preparation of sharper nano-nickel tip.