Preliminary Studies In Electrochemical Nanoimprint Lithography Directly On Crystalline Semiconductor Wafer

Materials with three-dimensional micro-nanostructures usually have excellent mechanical,electronic,optical and photoelectronic properties that enable micro/nano fabrication technology to play a significant role in multiple fields such as integrated circuits,integrated optics,microelectromechanical systems,nanotechnology,and precision machining.The semiconductor materials are widely used in the micro/nano devices,so micro/nano fabrication technologies on semiconductors are of vital importance.Electrochemical micro/nano fabrication technology has features of no thermal effect,precision control,high fabrication efficiency and environment-friendly,and has great potential in the fabrication of micro/nano structures on semiconductors.On the basis of the traditional micro/nano fabrication technology,we have developed a new type of electrochemical micro/nano fabrication technology.Combining the working mode of nanoimprint lithography,the principle of traditional metal assisted chemical etching(MacEtch)and the theory of confined etchant layer technology(CELT),we have developed a new electrochemical nanoimprint lithography(ECNL)that directly acts on semiconductor materials.The main work and innovation are summarized as follows:(1)When the Pt metallized imprint mold is in close contact with the semiconductor like n-Si or n-GaAs,electrons are transferred from the n-type semiconductor to Pt because they have different electronic work function,and holes are generated in the semiconductor.The space charge region is generated at the contact interface to produce a contact electric field and a contact potential,and the presence of the electric field makes it possible for bands bending.When electron transfer reaches equilibrium,the metal and semiconductors reach a new equivalent Fermi level.In the presence of an electrolyte,the potential of the Pt/electrolyte interface will shift because of the contact electricity and induce the spontaneous reduction of oxidant on the Pt surface.Because the equilibrium EF builds an electron tunnel between n-semiconductor and Pt,electrons will transfer from n-semiconductor through Pt to the oxidant.Thus,the accumulated positive holes at the n-semiconductor/electrolyte interface make n-semiconductor dissolve anodically along the Pt/n-semiconductor/electrolyte 3-phase interface,there will be three-dimensional structures on the semiconductor.(2)Electrochemical nanoimprint lithography directly on n-type crystalline silicon(111)wafer was achieved.Pt-metallized imprint mold was used as the cathode,and n-Si in open state was anode and H2O2/HF solution was reaction solution.H2O2 was reduced as an electron acceptor,HF used to dissolve the etching products.By optimizing the experimental conditions,the etching conditions were determined to be 0.3/0.4 M H2O2 and 5 M HF,the contact force was 5 N and the temperature was 25?.And the fabrication of three dimensional micro/nanostructures such as nanohole shape,microchamber and nanogroove was realized.At the same time,the electrochemical parameters(corrosion potential,corrosion current,exchange current density,Tafel slope,etc.)of the fabrication system were measured by electrochemical methods such as open circuit potential and Tafel curve.The exchange current density iOof n-Si anodic reaction was is 3.8×10-4 A/cm2,and that of Pt cathodic reaction is 2.5×10-3 A/cm2,and the degree of polarization of Si is greater than the degree of polarization of Pt.Anodic dissolution is the rate determination step of the fabrication reaction.The feasibility of electrochemical nanoimprint lithography is validated theoretically.(3)photoelectric synergistic electrochemical nanoimprint lithography directly on n-type crystalline GaAs(100)wafer was achieved.As the semiconductor has photoelectric effect,the absorption of light can increase the concentration of internal carrier and the number of electron-holes and improve their separation rate.Therefore,the use of photoelectric synergistic effect to improve the electron-hole separation can improve the rate of electrochemical nanoimprint lithography.Pt-metallized imprint mold was used as the cathode,and n-GaAs in open state was anode and KMnO4/H2SPO4 solution was reaction solution.KMnO4 was reduced as an electron acceptor,H2SO4 used to make the reduction product be Mn2+.By investigating the experimental conditions,the etching solution contained 40 mM KMnO4 and 1.84 M H2SO4,the contact force was 5 N and the temperature was 25?.Succeed in fabrication of the concave hemisphere microstructures on gallium arsenide semiconductor(n-GaAs).The effects of photoetching time and dark state etching time and light intensity on the effect of structures were studied.The removal rates of photoetching and dark-state etching are roughly linear with the processing time,and the linear relationship of dark-state etching is better than that of photoetching.And the processing rate of photoetching is much larger than that of dark-state etching.The removal volume increases with the increase of the optical power in a certain range.The etching depth of the structure exhibits a good linear relationship with the optical power.The electrochemical parameters of the processing system under photoetching and dark-state etching were measured by electrochemical methods such as open circuit potential?Tafel curve and Linear-scan voltammograms.By comparing the potential changes of the anode and cathode,and the exchange current density(3.9×10-8 A/cm2)of GaAs electrode is smaller than that(3.5×10-5 A/cm2)of Pt electrode,it was found that the polarization of GaAs electrode is greater than that of Pt electrode,which indicate that the anode dissolution of GaAs electrode is the rate determination step of processing.When being illuminated,the exchange current density of Pt and GaAs electrode both increase to 4.5×10-6 A/cm2 and 1.2×10-4 A/cm2,respectively,and the polarization degree of GaAs is much greater than that of Pt.Photoetching increases the rate of anodic dissolution reaction,which in turn increases the rate of fabrication.Using photoelectric synergistic effect to improve the rate of anodic reaction and thus improve the ECNL processing rate.Electrochemical nanoimprint lithography overcomes the shortcomings of the difficult of process complex multi-level structure of MacEtch,of direct processing semiconductor materials of NIL,of nano-precision resolution of CELT,and can fabricate three-dimensional micro/nano structures directly on the crystalline semiconductor wafer fast and efficiently.ECNL can maintain high processing resolution and accuracy,reduce operating cost and simplify the operation process,in the field of semiconductor micro/nano device processing has broad development prospects.