纳米压印实验

Nanoimprint lithographyMario Bareiß, Bernhard Fabel, Giuseppe Scarpa and Paolo LugliInstitute fo Nanoelectronics, Department of Electrical and Information Engineering, Technical University of Munich, GermanyE-mail: bareiss@nano.ei.tum.deIntroductionNowadays, nanodevices like small fluidic channels, semiconductor lasers, nano contacts for solar cells, nano magnets or nano antennas for infrared detection (THz) are of great interest. The applications for these devices extend the fields of e.g. energy, computational or medical sciences. For example, by arranging nano dots consisting of a magnetic material, logical circuits can be designed which can lead to a novel computing device. The challenge which has to be solved is the design of low cost and fast process for the production of nano structures.State of the artThe fabrication methods of the state of the art in university and industry are the optical lithography (OL) and the electron beam lithography (EBL). The OL process is briefly shown in figure 1. A substrate (SiO2/Si) is covered by a photosensitive polymer which changes its chemical characteristics while exposing to ultraviolet (UV) light. A OL-mask which normally consists of glass covered with chromium is put on top of the substrate containing thephotoresist. The chromiumhave holes and structuresthrough which light can pass.By exposing the sample withthe mask to UV light, parts ofthe photoresist are illuminated.The illuminated photoresistcan be dissolved in a solutionand as a consequence, thephotoresist shows the samestructures than the chromiumon the OL-mask. Furtherprocess steps can include theetching of e.g. SiO2forpatterning the surface withstructures of a certain height.The resolution R is limited by Figure 1: Optical lithography processoptical diffraction according to the Reyleigh equation:whereas λ is the wavelength of the illuminating light, NA is the numerical aperture of the lens system and k is a constant that depends on the photoresist. The theoretical limit set by optical diffraction is here . That means that corresponding to the feature size which is fabricated, the wavelength of the light source has to be decreased and the lens system has to be improved. When using usual glass masks, another physical limit is the bandgap of glass. Glass is an insulator, but by decreasing the wavelength of the light source, the energy of the emitted photons can achieve values larger than the bandgap of glass and thus the photons will be absorbed by the glass before reaching the photoresist. Complete new lithography systems would have to be designed to overcome thesechallenges. This is normally not done at universitybecause of financial reasons. However, industriesinvest billions of dollars to develop new systemsfor sticking to the optical lithography technique.The company “ASML” developed a system calledNXE:3100 with an extreme ultraviolet light (EUV)source with a wavelength of 13.5 nm. With thissystem the fabrication of structures with aresolution of 22 nm and less will be achievable [1].The EBL offers, as another fabrication method, the possibility of fabricating nano structures down to a few tens of nanometer. In this process, a wafer is also covered by a polymer which is sensitive to an electron beam. By directly writing structures in the polymer with the electron beam, the written structures are solved in a solution and further etching in the substrate or evaporations are possible. The disadvantage of the process is its velocity and therefore the costs. It is extremely time-consuming fabricating a larger area of nano structures with an EBL process and therefore it is a relatively high cost method. The goal is the use a fast and low cost technique for the fabrication of nano devices.Nanoimprint lithographyNanoimprint lithography is a highly innovative technology based on a quite simple principle that has been well known in the macroscopic world for hundreds of years. A combination of conceptual simplicity and innovative potential has often been the fingerprint of groundbreaking technologies. NIL fits into that scheme very well, and both aspects, the basic technological idea as well as the evolving ecomomical and research related potential will be discussed in detail in the following section.Figure 2: NXE:3100 by ASML [2]Nanoimprint lithography - basic technological approachSeveral NIL methods were developed. The most commonly used are the following ones:Thermal NIL (T-NIL): This lithography method is the initial approach. The imprint polymer is heated above its glass transition temperature. In the following imprint step, the stamp is pressed against the substrate at a given pressure for a specific time. The sample is cooled down below the glass transition temperature of the imprint polymer while the stamp is still in contact, then the mold is removed, and the imprinted pattern remains in the polymer layer.Ultraviolet light-based NIL initially introduced as step-and-flash lithography (SFIL): In contrast to the previous approach, in SFIL no heat is applied to the polymer layer. Instead, polymer crosslinking induced by UV radiation is used during the imprint step to prevent deformation of the imprinted pattern after the mold is removed. The most common approach is to apply the UV-light through the mold, which therefore has to be made out of a UV-transparent material such as quartz or fused silica.Figure 3 illustrates both approaches, each consisting of four steps. (1) one layer of an imprint polymer is deposited on the substrate to be imprinted. When performing T-NIL the polymer is heated up at a temperature above its glass transition temperature T g . No heating of the polymer is necessary for SFIL processes. (2) Astamp comprising the structure of the finaldesired pattern is pressed into the polymerlayer, partly deforming, and partlycompressing it. (3) Before the stamp isremoved the imprint polymer layer is cured bycooling it down or exposing it to UV-radiation. (4) The stamp is then removed andsubsequent etching steps allow for transferringthe final pattern into the substrate. Note thatevery imprint cycle yields a residual imprintpolymer layer at the bottom of imprintedgrooves caused by the compression componentof the imprint step. Even with mold featuresbeing deeper than the thickness of therespective imprint polymer layer and whenpressing with maximum imprint force andduration one does not eliminate polymercompression, and hence the formation of a thinresidual layer cannot be avoided. Reactive-ion-etching (RIE) or oxygen plasma etchingprocesses - optionally in combination with ametal-evaporation step for lift-off purposes -are commonly performed to transfer theimprinted pattern into the substrate usingpolymer and metal layers as etch masks.Figure 3: Working principle of T-NIL (left) and SFIL (right) depicting (from top to bottom): alignment of mold and substrate, imprint step, cool down/UV-curing and mold removal.Advantages and potential of nanoimprint lithographyNIL is an entirely new approach to lithography as the pattern is generated by mechanical forces and not through chemical changes due to radiation exposure. Because of this fundamentally different working principle, NIL is not limited by diffraction as lithography based on (UV) light, scanning electron beam or X-ray lithography. Instead, resolution in NIL is influenced by mechanical factors involved in the imprint process (such as surface forces, elasticity and viscosity) allowing NIL to push the resolution limit down into the sub-10-nm region. In 2004 a group at the University of Chicago demonstrated the imprint of the shape of a single-walled carbon nanotube. The authors claim to be capable of reproducing objects of molecular dimensions [3]. The same year, Chou et. al. demonstrated the imprint of 5 nm structures. The mold was generated by cleaving a wafer with pre-deposited GaAs/AlGaAs-layers. These layers can be selectively etched to form the mold pattern. Imprint is performed by using the edge of the cleaved wafer [4]. While these experimental results show that imprint of features smaller than 10 nm is possible, the question of the theoretical limit of resolution arises. For thermal NIL the van der Waals bonds (on the order of 1 nm - 3 nm) are considered to be the limiting factor.Furthermore NIL provides the following process features:•No exposure of the device to electron-beam or other damaging radiation. This radiation exposure currently makes it difficult to form patterns on certain technologically relevant substrates, for example organic semiconductors and ultra-thin tunneling junctions. These materials are often object of current research, and thus lithography methods that do not harm them are necessary. A NIL system working analogously to e-beam lithography is highly interesting for the scientific community as no high energy radiation is involved in pattern generation.•no pattern-density-dependent biasing ("proximity") effect. Materials that show large backscattering effects (substrates consisting of atoms with high Z numbers) are difficult to pattern precisely using electron beam lithography, but can be readily patterned with nanoimprint.•no shot noise in the replication of devices (although a certain amount of shot noise is unavoidable in the template if it is generated using quanta such as electrons or photons). Nearly identical devices can thus be reproduced quickly and in large quantities.NIL technology therefore is a major improvement over electron-beam lithography, and any radiation-based lithography techniques.Areas of application of nanoimprint lithographyDue to the conceptual simplicity of the process and attractiveness from the economical point of view, NIL is today being adopted by the semiconductor industry for large-scale manufacturing and it was included into the International Technology Roadmap for Semiconductors (ITRS) in the year of 2003. It is considered as a feasible technology to achieve the 18 nm node, and the Massachusetts Institute of Technology's "TechnologyReview" listed NIL amongst the top 10 emerging technologies regarding their future impact on science worldwide. Leading industrial companies involved in NIL Technology are for example Obducat (Sweden), Süss MicroTec. (Germany), micro resist technology (Germany), Nanonex (USA), NIL Technology (Denmark), Molecular Imprints Inc. (USA), Intel Corp. (USA), IBM (USA), and Hewlett Packard (USA) to name just a few. NIL technology contributes significantly to research and development on many areas:•electronics: hybrid plastic electronics, organic electronics and photonics, nanoelectronic devices in Si and in GaAs;•photonics: organic lasers, conjugated and nonlinear optical polymer nanostructures, high-resolution organic light-emitting diode (OLED) pixels, diffractive optical elements, broadband polarizers;•magnetic devices: single-domain magnetic structure, high density patterned magnetic media and high-capacity disks;•nanoscale control of polymer crystallization;•biological applications: manipulating DNA in nanofluidic channels, nanoscale protein patterning, and the effect of imprinted nanostructures on cell cultures.NIL provides the basis for numerous technological solutions in nanofabrication. However, in order to meet the strict requirements of industrial semiconductor manufacturing with special focus on high-throughput fabrication the following compendium is mainly extracted from, important recent publications that appeared in 2007 have been added and complement cited references and defect control, NIL processes have to be further optimized. As opposed to contactless radiation-based lithography NIL processes incorporate bringing the mold into physical contact with the imprint resist layer causing a mold wearout effect over time. Thus for future work, mold lifetime and imprint quality have to be optimized by systematically collecting and analyzing experimental data on mold and imprint polymer materials as well as by designing NIL tools capable of efficient mold and substrate handling during imprint steps [5].NIL Process in the laboratory work using a thermal polymerThe NIL process consists of several steps which are briefly mentioned:•Cleaning of substrate•Spincoating of polymer on substrate•Nanoimprint process•Characterization of imprinted structuresAll process steps are here described in detail.Cleaning the substrate:In general, every flat surface can be used as a substrate, no matter of which material it consistsof. Normally, Si substrate is used as it is CMOS compatible and a low cost material. A usualtype of wafer is a 2” p-doped Si-wafer. The wafers are delivered in clean packages butnevertheless are cleaned before using them. Normally, a cleaning process consists in anacetone and isopropanol clean for 10 min in ultra-sonic bath. This removes all dust particleswhich were set on the wafer surface. In the lab, a “piranha clean” will be done instead of thecleans mentioned before which forms a hydrophilic wafer surface in addition to the removalof the dust particles. The chemical composition of that clean is:H2SO4 : H2O2 = 4 : 1A hydrophilic surface improves the following spincoating process. After the “piranha clean”,the wafer will be rinsed in de-ionized (DI) water.Spincoating of polymer on substratespincoater. The substrate is held by a vacuum pump. Before puttingthe polymer on top of the wafer, the surfaces must be inspectedanother time if some dust particles are meanwhile located in top ofthe wafer. If this is the case, the substrate has to blow clear with drynitrogen.Figure 4: Spincoater [6]In the unprocessed state, the polymer is a powder. By diluting the powder in a solvent, it is easy to put the polymer on a wafer. In the lab work, the polymer “mr-I 8020” is used. The polymer is sucked out of the bottle using a syringe. The prefilter “SRP 15 0,2 µm” is then placed on the syringe and the polymer is put on the surface of the wafer through the prefilter. The function of the filter is the absorbance of remaining dust particles in the solution. Once the whole surface is covered by the polymer-solvent-mixture a defined rotation speed is adjusted. By rotating the wafer with 3000 rpm for 30s a 200 nm thick polymer layer is formed. After that, the wafer is put on a hotplate at 140°C for 2 min. The object of this process step is the removing of remaining solvents.Nanoimprint processThe nanoimprinting machine used in the Institute forNanoelectronics is a Nanoimprinter NIL 2,5 fromObducat. It is remotely controlled by a labviewsoftware. The parameters which can be adjusted inthat machine are the temperature, the pressure, theimprint time and the use of UV light. The latterparameter is important in the case of using UVsensitive polymers. The wafer with the polymer ontop is placed on a metal plate in the nanopimprinter.A stamp which already consists of the desiredstructures is put on top of the substrate and theprocess parameters are entered. For the polymer “mr-I 8020” the parameters are shown in table. Thesubstrate and the polymer are heated up until theyreach a temperature of 190°C. This temperature isabove the glass transition temperature of the thermalpolymer. That means that the polymer is in a liquidstate. At this moment, a pressure of 50 bar is applied for 240 s. The stamp with the structures is pressed into the liquid polymer. After this period of time, the substrate is cooled down to 90°C and the pressure will be held on 50 bar. After the polymer crosses the glass transition temperature, the stamp can be demolded and the imprinted substrate comes out of the imprinter. For demolding purposes, the stamp is covered with a layer of self-assembled monolayers (SAM) which does not adhere to the polymer material. Temperature [°C]Pressure [bar] Time [s] UV 19050 240 0 900 00 00 0 0Characterization of imprinted structuresThe characterization of the imprinted structures is done by an atomic force microscope (AFM). The atomic force microscope used in the institute is a SPM-5200 from JEOL. The SPM-5200 is a multipurpose, high resolution scanning probe microscope offering ease of use with diverse measurement and sample environments. It can perform a wide range of applied measurements including the combination of image signals and instantaneous switching between operation modes. The open architecture of the SPM-5200 provides multiple access ports and easy access to the probe. A drift-free stage is implemented to provide an extremely stable imaging platform. The AFM consists of a cantilever with a sharp tip (probe) at its end Figure 5: Nanoimprinter NIL 2.5that is used to scan the specimen surface. The cantilever is typically silicon or silicon nitride with a tip radius of curvature on the order of nanometers. When the tip is brought into proximity of a sample surface, forces between the tip and the sample lead to a deflection of the cantilever according to Hooke's law. Typically, the deflection is measured using a laser spot reflected from the top surface of the cantilever into an array of photodiodes. If the tip was scanned at a constant height, a risk would exist that the tip collides with the surface, causing damage. Hence, in most cases a feedback mechanism is employed to adjust the tip-to-sample distance to maintain a constant force between the tip and the sample. The tip is mounted on a vertical piezo scanner while the sample is being scanned in X and Y using another piezo block. The resulting map of the area s = f(x,y) represents the topography of the sample.Figure 6: Measuring method of atomic force microscope [7]The most important thing when working in laboratory is the safety for yourself and for the environment. The students will get a safety introduction to the work in the lab. Some aspects will be pointed out here:•As the students will work with polymers and spincoating processes, the wear of glasses is obligatory•Al work with solvents have to be done under an extractor hood•When working with acids, the wear of special gloves may be necessary•In general, whenever one is not absolutely sure of a process step, especially when working with chemicals, one has always to ask before workingDuring the whole lab work, an advisor will be always assisting the group of students.The goal of the experiment is that the students learn how to work with organic materials and to learn a new fabrication method in semiconductor industries and nanotechnology. A nanoimprint lithography process will be done using a “DVD” stamp. The structures on the stamp are in the range of 800 nm x 1µm until 400 nm x 1µm. Afterwards, a characterization of the imprinted structures will be done either by using an optical microscope and an AFM. The whole process including the results, the discussions during the lab work and the answer of the questions shown below will be presented in a report.Questions•How can the stamp lifetime be increased?•How is the pressure applied during the imprint process and what is the reason for this special technique?•What is typical thickness a spincoated polymer?•Why does a polymer has a date of expirey?•What is a residual layer and what are the problems when removing it?•What is the difference between a Reactive-ion-etching and an oxygen plasma etching process?•Explain a lift-off process.•Explain the action of self-assembled monolayers.[1]Y. Nakajima et al., SPIE 26(8) (2010) 1-7[2]/blog/2010/7/6/asmls-pre-production-euv-tool-achieves-first-light.html[3]F. Hua et al., Nano Letters 4(12) (2004) 2467-2471[4]M.D. Austin, S.Y. Chou et al., Applied Physics Letters 84 (2004) 5299[5]S. Harrer, Next-generation nanoimprint lithography: Innovative approaches towardsimproving flexibility and resolution of nanofabrication in the sub-15 nm region, Doktorarbeit, Technische Universität München, 2008[6]/spincoater-ace-1020s-1.jpg[7]/~jchakhal/afm-draw-1-c-1.jpg。