Fabrication Of Nanoelectrodes By Nanoimprint Lithograph And E-beam Lithograph

Metal nanoelectrodes have attracted a great deal of interest during the past decade because of theirunique application on the research of the basic properties of the one dimensional nanostructures, molecularelectronics and biosensors. With the device features are pushed towards the deep sub-100nm regime,nanoelectrodes with sub-100nm are corresponding needed for studying charge transport on a nanometerscale such as molecular conduction, single electron devices, and other photonic devices that are ofconsiderable interest due to their fundamentally new physical properties and potential applications in futureelectronics. However, basic researches on nanoelectronics are often limited by the lack of low cost andlarge scale method to fabricate electrodes with controlled gap size in nanometer scales. A number oflithography techniques such as electron beam lithography (EBL), nanoimprint lithography (NIL), focusedion beams (FIB) lithography and dip-pen nanolithography (DPN) have been used to achieve metalnanoelectrodes with the above requirements.Although these methods have the advantage to fabricate nanoelectrodes with the feature size down tosub-100nm，only the NIL has been considered as a promising nanofabrication technology in future whichhas the advantage in fabricating nanostructures with high resolution, low cost and large areas. EBL hasattracted a great deal of interest because of its high resolution ration and flexible design properties. In ourwork, both NIL and EBL were used to fabricate nanoelectrodes with the minimum feature size at about70nm. By introducing the bi-layer resist technique into the nanoimprint process，the gap controlednanoelectrodes was obtained, and the results showed that the gap of nanoelectrodes decreased with theincreasing of the development time in the case of keeping the same development concentration. What’smore, nanoelectrodes with various structures and gaps were obtained by e-beam lithograph. The maincontent is divided into three aspects:Firstly，nanoelectrodes fabricated by nanoimprint lithograph with single layer PMMA resist.The mold consists of four different gaps nanoelectrode with same line width of about300nm, and the gapsof the four nanoelectrodes are100,200,400and800nm, ranging from micro to nanometer scalesrespectively. In our work,200nm thickness PMMA resist layer is used for the nanoimprint experiment considering the height of the mold. Then, the nanoelectrodes with four periods and gaps were prepared byNIL with a series of subsequent experimental process (RIE, metal deposition and lift off process). In asingle layer lift off process, the line width and gap size of the electrodes are limited by the nanoimprintmold. Due to the thin sputtering metal films in the single layer lift off process large leakage current wasobserved in the following I-V curve tested experiment, thus double layer lift off technique was performedto not only control the gaps size but also increase the aspect ratio and reduce the leakage current of thenanoelectrodes.Secondly，SF5/PMMA duble-layer resist technique was used to fabricate nanoelectrode by NIL.In our double layer lift off technique, the gap size is instead defined by the under cut length of the lift offresists (LOR). The undercut length of the LOR can be controlled by the processing conditions just as resistthickness, opening size of the lithography patterns on the top PMMA layer, the concentration and thedissolution time in the TMAH solvent. In our experiment, the resist thickness is kept no change, typically200nm thickness of SF5(polydimethylglutarimide polymer) is coated on the Si substrate first, and thenfollowed by200nm thickness of PMMA coating. Here, SF5has high thermal stability with a glasstransition temperature at about195C which is greater than the nanoimprint processing temperatures (150C) required for PMMA in our work. Moreover, because of the chemical properties of LOR(SF5), nointermixing occurs with the subsequent PMMA coating. Next, pattern transfer is demonstrated bysputtering25nm of Ti and60nm of Au directly on the sample, and then removing the resist film bydissolving the SF5transfer layer in N-Methyl pyrrolidone (NMP) solution. Four different gaps ofnanoelectrodes which related to the opening size of the top PMMA layer are investigated by varying theparameter of the TMAH solvent, and the gap size of the nanoelectrodes can be controlled from micro tonanometer scales.Finally, various nanoelectrodes with defined line withth and structure were fabricated bye-beam lithograph (EBL). E-beam lithograph has the advantage to prepare various nanoelectrodes whichare not limited by the templates. Thus, differrent nanoelctrodes with various periods, line width and gapsare designed by EBL technique. The fabrication processes are divided into the following sections:substrates preparation (PMMA or SF5/PMMA resist layers), graphic design, exposure procedure,development process, RIE, metal deposition and metal lift off process. Similarly, well defined nanoelectrodes were obtained according to our requriements, and the linewidth of the nanoelectrode canadjust from900nm to500nm, and the gap of the nanoelectrode can change from micro scale to70nm.