09 Nanofabrication based on MEMS technology

Nanofabrication Based on MEMS Technology Yuelin Wang,Senior Member,IEEE,Xinxin Li,Tie Li,Heng Yang,and Jiwei JiaoAbstract—In this paper,a novel nanofabrication method that develops from the traditional microelectromechanical system (MEMS)technology of anisotropic etching,deep reaction ion etching,and sacrificial layer process has been reviewed based on our work.With such a technology,nano tips,nano wires,nano beams even nano devices can be fabricated in a batch process. Beams with thickness of only12nm,a nano tip with a heater on the beam,and a nano wire whose width and thickness is only 50nm are demonstrated.The scale effect of the Young’s modulus of silicon has been observed and the nano-electronic-mechanical data storage has been presented.Index Terms—Microelectromechanical system(MEMS), nanofabrication,nanoelectromechanical system(NEMS).I.I NTRODUCTIONI N THE PAST decade,nanotechnology has captured lotsof interests for their novel properties in nano materials and dramatic behaviors in nano devices.All these above improve-ments are based on the manufacturing method development in that scale.Currently,many nanofabrication methods have been developed aiming on them,for example,X-ray lithography, electron-beam lithography[1]–[4],nano-imprint lithography (NIL)[5]–[8],focused ion beam(FIB)nanolithography[9], [10],single atom manipulation[11],and quantum dot self-assembly process[12]–[14],etc.All these technologies can be generally classified into top-down or bottom-up methods.Roukes et al.[1]used focused electron-beam to scan point by point to the substrate coated with polymethylmethacrylate(PMMA)so that the PMMA is exposured and patterned to form the nano device structure they needed.Orloff reported kinds of applications of FIB technology[9],which is something like the electron-beam lithography.Zankovych achieved15-nm feature size using NIL technology[5].In this method,a stamp which is fabricated by electron-beam lithography and dry etching is needed to mask the substrate.It can be found that all these technologies are top-down methods and base on the electron lithography.In the meanwhile,manipulation of selected single atom using a near contact atomic force microscope working at low temperature is an atom-by-atom method to build a nano structure,which is a bottom-up method.Manuscript received November24,2004;revised March19,2005.This work was supported by the Major State Basic Research Program of China"Micro-to-nano scale fabrication based on top-down principle(Project2006CB300403). The associate editor coordinating the review of this paper and approving it for publication was Dr.Ai-Qun Liu.The authors are with the State Key Laboratory of Transducer Technology, National Laboratory of Microsystem Technology,Shanghai Institute of Mi-crosytem and Information Technology,Chinese Academy of Sciences,Shanghai 200050,China.(e-mail:ylwang@;xxli@; tli@;h.yang@;jiaojw@).Digital Object Identifier10.1109/JSEN.2006.874034Fig.1.(a)Scheme of the structure and(b)the microscopy photo of the nanoprobe.Although all these technologies have successfully formednanostructures,however,they all have drawbacks such as lowefficiency,low controllability,poor batch product ability,or lowcompatibility with the current semiconductor technology.Theymay not be a good candidate for the coming nano industry.As a powerful technology to make microdevices,MEMS hasbeen successfully used in amounts of sensors and actuators inthe past decades[15]–[22].To our knowledge,the key pointof MEMS is its high selectivity especially in the processesof anisotropic wet etching,sacrificial layer process,and dryetching[15],[16].Anisotropic wet etching is based on the factthat different crystalline plane of silicon has a different etchrate in alkaline solution such as KOH,TMAH,etc.[17]–[22].Therefore,silicon with a different plane will have differentetching performance,which makes silicon a good material inbulk micromachining.The sacrificial layer process is one of themost important technologies in surface machining.Generally,it hasfive basic steps.After the deposition and patterning ofthe sacrificial and structure layer as well as the removal of sac-rificial layer,a three-dimensional structure can be formed.The 1530-437X/$20.00©2006IEEEFig.2.SEM photos of the nano tip on a beam integrated with a heater and piezoresistances.success of this process is strongly dependent on the selectivity of etch rate in an etchant.Dry etching is another etching method to form the structure[23],[24].Compared to the wet etching, the etch reactants appear in gas or vapor phase,and in a lot of case it is ionized.In general,dry etching has lower etch rate and higher degree of anisotropy than wet pared to the wet etching process,its selectivity depends on the mask,and it is much easier to be started or stopped.Anisotropic or isotropic etching is dependent on the different etchants.Anisotropic dry etching always has a high aspect ratio and steep walls.It is known that all these processes have been applied on amounts of sensors such as accelerators,gyroscopes,infrared thermopiles,and so on.In this paper,we will review the nanofabrication method based on the MEMS technology in our work,and three different structures,nano probe,nano cantilever and nano wire,are presented.II.R ESULTS AND D ISCUSSIONA.Nano ProbeNano probe is famous not only because of the role as a nano tip to get the atomic photo in atomic force microscopy(AFM), but also a tool to manipulate the atoms in nano scale.In our paper,the nano probe is integrated with a heater and a piezore-sistor to demonstrate its ultrahigh data storage capability.The nano probe combined with a heater works as a write pen to make marks on a media when it is heated,while the piezore-sistor can provides a read process by detecting the slight force change on the cantilever when the nano tip contacts the marks. The scheme structure and a microscopy photo of our nano probe can be found in Fig.1.The fabrication is begun from an SOI wafer,which consists of an8-m-thick n-type top silicon layer.After thermal oxidation,Fig.3.Data storage demonstrated with the nano probe in different heating times.the tip region is patterned by buffered HF solution.And then,the tip is formed by KOH etching and sharpened by low-tempera-ture oxidation at950 C.The heater,piezoresistor,and wire con-nection hole is boron doped,respectively.Reaction ion etching (RIE)is used to shape the cantilever from the top side after the aluminum interconnection.Then deep reaction ion etching (DRIE)process is employed from the back side to etch through the wafer.After the SiO layer is stripped by BHF,the cantilever is released.A more detailed description can be found in[15]. In this structure,the most important part is the nano probe tip, as it describes above,it is formed by anisotropic wet etching and low temperature oxidation.As the(111)and(411)facet have very slow etch rate in KOH solution,a well-aligned mask can make a silicon pyramid.In the following fabrication process, the large stress on the tip will prevent the oxygen atoms pen-etrating into the tip and stop the oxidation;therefore,the tip of the pyramid will become sharper and sharper and becomes nano scale.Fig.2represents three scanning electron microscope (SEM)photos of the nano probe.The tip size is on the order of several tens of nanometers.As the MEMS technology can make a batch procedure of nano probes at the same time,the array is a promising result to make the tip a useful multiple read/write head.In our data storage experiments,the nano tip has been em-ployed to demonstrate its ability on a polymer.In this case, the data writing process depends on the temperature of the nano probe,i.e.,thetemperatureoftheheater.Furtherexperiments[15] indicate that the heater temperature increased to463K in3ms in a 4-V pulsed voltage,which determines the write speed of the nano probe on a microsecond level.Meanwhile,the piezoresistor canrespond to a force applied on the tip in the orderof N,which demonstrates the read-out ability of the nano probe. When the tip touches on the surface of the polymer,if it is hot enough,the polymer will expand to mark a“1”state on the surface.The write process can be controlled easily by applied voltage,time,and force.A demonstrated data storagecapability Fig.4.(a)Nano beams with thickness of12nm and(b)the scale effect of the nano silicon beam[16].in our experiment is shown in Fig.3.As shown in thefigure,the diameter of the storage point can be less than100nm,it indicates that a storage density as high as30GB/in can be achieved by our nano probe.B.Nano CantileverIn our previous work,an ultrathin silicon nano cantilevers has also been formed with thermal oxidation and XeF isotropic etching in a SOI wafer.The fabrication method can be found elsewhere[16].It also begun the structure from a(100)SOI wafer with P top silicon layer.The110-oriented cantilevers are patterned by photolithography.The dry oxidation is processed to consume up the remaining silicon in the surrounding areas,which deter-mines the thickness of the cantilever.Finally,the cantilevers are released by CO supercritical-point drying.It has been demonstrated that the thickness of the cantilevers can be only12nm as shown in Fig.4(a).Upon the cantilever, the scale effect of the Young’s modulus of the silicon has beenFig.5.Nano wire with a width and thickness of 60nm.Fig.6.Fabrication process of the nano wire by MEMS technology.presented as in Fig.4(b),which indicates that the Young ’s mod-ulus will decrease with the decreasing thickness of the cantilever [16].Besides,the ultrathin cantilever can also be used to detect the ultrasmall force when combined with Carbon nano tube.It has been used to detect the hydrogen storage in a resolution of510g.[25]C.Nano WireWith anisotropic wet etching and the sacri ficial layer process,a silicon nano wire has also been fabricated as shown in Fig.5.The fabrication process can be found in Fig.6.The first oxi-dation and photolithography on a (100)SOI wafer is followed by a KOH etching which de fined a step on the top silicon layer [Fig.6(a)].And then a silicon nitride is deposited on the wafer to cover the step,and a window is opened on the silicon nitride [Fig.6(b)].After the silicon oxide layer is moved as shown in Fig.6(c),a nano Si wire can be fabricated with a carefully con-trolled KOH etching [Fig.6(d)–(e)].Finally,the nano wire is released by the BHF solution.The SEM image indicates that the silicon wire has a cross section as small as6060nm .The–property of the nano wire implies that the fresh sample has different behavior to the same sample after 24h in the air,which indicates that the surface state of the wire may be unstable for the oxidation in the air.Further investigation is still undergoing now.D.DiscussionThe above three nano structures are all fabricated based on traditionally MEMS technology with smart design and care-fully controlled processes.As in a traditional bottom-up pro-cesses,each nano structure should be operated atom by atom or molecular by molecular,our nanofabrication technology pro-vides a well-controllable method to form nano structures in a batch product,which is one of the fundamental requirements for a low-cost pared to other top-down process which always relates to the electron lithography,our technology is more ef ficient and cheaper.Of course,the shape of our nano structures may be limited by the wet etching selectivity and the structure should be consist of some low etch rate crystalline planes,this nanofabrication method is still worthy to be recom-mended to the coming nano industry.III.S UMMARYUpon our nanofabrication work,we can say that nanofabrica-tion can be achieved by MEMS technology with smart design and carefully controlled processes.It can make nano structures and even nano devices conveniently with low cost and batch pro-ductivity,which may accelerate the application of nano devices in the future.R EFERENCES[1]A.N.Cleland and M.L.Roukes,“Fabrication of high-frequencynanometer-scale mechanical 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