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Nanoprinting

contact person: Susan (Yajing) Liu (yl2b@virginia.edu)

The nanoprinting project is part of Molecular-Level Large Area Printing (MLP) program funded by the Defense Advanced Research Projects Agency (DARPA). The main goal at University of Virginia is to develop fundamental materials and processing technologies to demonstrate feasible nanoscale patterning and printing techniques for both planar and curved surfaces. A non-photolithographic strategy based on focused ion beam (FIB) direct pattering has been explored for this purpose.  

A FIB is a very versatile tool in lithography, etching, deposition and doping and has been widely used for creating structures for online industrial point- to- point analysis or research device prototyping. A 30 keV Ga⁺ FIB (FEI 200) is used to create nano-scale topographic patterns onto Si and Poly(methyl methacrylate)(PMMA) thin films-master fabrication. The primary advantages of this mastering technique are: fabrication of high resolution (<50nm) features, rapid prototyping capability (patterns of up to 10⁵-10⁶ features can be taken from conception to realization in a few hours), high depth of focus (over 100mm can be achieved while maintaining a resolution of 100nm), real time inspection of masters during fabrication, and the ability to individually modify master features. This high throughput patterning technique shows great potential for higher throughout FIB fabrication of lithographic features, for example, through micro-contact printing (mCP) as developed by George Whitesides's Group at Harvard. It uses the relief pattern on the surface of an elastomeric stamp to form patterns of self-assembled monolayers (SAMs) on the target surface (Ag, Cu, Au, etc.) by contact. The schematic of the mCP technique is shown in Figure 1. In step 1, a topographic pattern is produced into a suitable surface (such as Si<100>) using the Ga+ FIB, which topographic surface is termed as a master. In step 2, a liquid elastomer (conventionally polydiemthylsioxane (PDMS)) is poured over the surface and cured. The solid elastomer is then peeled away from the master ( which has been silanized before molding so that the cured elastomer mold could be released easily). The elastomer surface then contains raised features corresponding to the recessed features in the master. The raised features on the mold are then coated with a self-assembled monolayer (SAM) of hexadecanethiol in step 3. The concentration is 1millimolar solution of 1-hexadecanethiol SAMs precursor in ethanol.), a self assembled monolayer of which forms an approximately 2 nm thick film of organic molecules when adsorbed onto the mold surface. The thiol “inked” sample then is manipulated using tweezers to conformally contact with the Ag thin films on the target surface, and the topography of the original master is replicated by thiol transfer onto the target surface in step 4. The thiol-patterned target is then wet etched (0.001 molar potassium ferrocyanide(Π)trihydrate (K₄Fe(CN)₆), 0.001 molar potassium ferricyanide (K₃Fe(CN)₆), 0.1 molar sodium thiosulfate pentahydrate (Na₂S₂O₃), in deionized water), with the thiol pattern serving as an etch barrier, such that the Si/PMMA pattern is finally transferred to the target. This technique circumvents the diffraction limitation of projection photolithography and generates patterns and structures on planar and nonplanar surfaces due to the conformal nature of the elastomeric stamps. This can be used with a wide variety of materials and surface chemistry.

Figure 1.  Schematic of Microcontact (mCP)  printing process

With FIB fabricated Si masters, features as small as 150 nm ( with PDMS mold, shown in Figure 2.) and 90 nm  ( with a higher modulus copolymer “APS-B” described by Shmid and Michel^[1]) have been transferred successfully by mCP to planar target substrate and features of order 500 nm have been transferred successfully to curved target substrates. Figure 2 shows PDMS contact printing pattern transfer in the 150-170 nm range. With The material sputtering rate of 0.5 mm³/nC for Si (about 30 of 100nm by 100 nm by 100 nm feature per second for 70pA beam current), it requires hours to fabricate 10⁶ features over 1mm² area. So it is impractical to extend the fabrication to the order of 1 cm² area due to the length of fabrication time. With our new discovery of high material removal rate for PMMA with the FIB, the patterning speed up to 5x10⁴ features/second for certain experimental conditions (corresponding to 10⁴ atoms per incident ion and this is extraordinary high value; for most materials,  material sputtering rate is in the order of 10.) (Figure 3a) and hundreds of printable features per second (Figure 4a) have been achieved. TEM observations (Figure 1c) recorded under “mass-thickness absorption contrast” conditions shows that materials have been removed away instead of compacting. The exploration to the mechanism of the extraordinary material removal rate is under the current investigation. These fabricated topographic PMMA patterns are then used as masters for the micro contact printing techniques. Patterns are successfully transferred from the original PMMA master to an elastomer mold (tens of nm at the top of the tips Figure 4d). The final transferred patterns onto a Si wafer by conformal contact are 100 nm - 200nm in diameter, and the high PMMA sputtering yield allows master patterns to be fabricated at least an order of magnitude faster than for previous masters sputtered directly into Si. Thus we have accelerated mCP throughput with all polymer, i.e. polymer mastering and polymer molding processes.

                        (a)                                (b)                                      (c)

 Figure 2 AFM image of (a) FIB master in planar (100) Si, 60-nm lines, vertical scale 0.75 mm /division. (b) PDMS mold cast thereform, 150 nm lines, vertical scale 1.25 mm /division, and (c) etched Ag surface following SAMs transfer from PDMA mold, 170 nm lines, vertical scale 0.3 mm /division.

 Figure 3 Atomic force microscope images of topography in PMMA following FIB exposure at (a) 1pA beam current and dwell time a total irradiation time of 20 is 10ms / feature, (b) 11pA beam current and dwell time is 100msa total irradiation time of 500ms/feature.  (c) Transmission electron microscope image (recorded at 200 kV, using mass-thickness contrast with an objective aperture diameter including just the transmitted beam of an array of sputtered features created using a 11 pA beam and a total irradiation time of 5 ms per feature). Samples were prepared by direct spinning of PMMA films onto TEM grids coated with thin (5 nm) amorphous C films.

Figure 4 Atomic force images of (a) A printhead topographical master fabricated with an ion beam current of 11pA and a dwell time of 5ms/feature, (b) APS-B Elastomer replica therefrom, the features are about 80nm at the top of the tips, (c) Pattern transfer by microcontact printing, final transferred patterns are about 200nm due to grain limitation.