Micromachined inking chip for scanning probe nanolithography using local thermalvapor inking methodShifeng Li, Kashan A. Shaikh, Sandra Szegedi, Edgar Goluch, and Chang Liu Citation: Applied Physics Letters 89, 173125 (2006); doi: 10.1063/1.2364881 View online: http://dx.doi.org/10.1063/1.2364881 View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/89/17?ver=pdfcov Published by the AIP Publishing Articles you may be interested in Improvement of scanning probe microscopy local oxidation nanolithography J. Vac. Sci. Technol. B 27, 948 (2009); 10.1116/1.3093907 Optical nanolithography using a scanning near-field probe with an integrated light source Appl. Phys. Lett. 93, 213103 (2008); 10.1063/1.3032912 Submicron patterning of a catalyst film by scanning probe nanolithography for a selective chemical vapordeposition of carbon nanotubes J. Appl. Phys. 101, 066101 (2007); 10.1063/1.2711144 Publisher’s Note: “Integrated microfluidic inking chip for scanning probe nanolithography” [Appl. Phys. Lett.85,136 (2004)] Appl. Phys. Lett. 86, 189901 (2005); 10.1063/1.1925280 Integrated microfluidic linking chip for scanning probe nanolithography Appl. Phys. Lett. 85, 136 (2004); 10.1063/1.1771453

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Micromachined inking chip for scanning probe nanolithography using localthermal vapor inking method

Shifeng Li, Kashan A. Shaikh, Sandra Szegedi, Edgar Goluch, and Chang Liua�

Micro and Nanotechnology Laboratory, University of Illinois at Urbana-Champaign, 208 North WrightStreet, Urbana, Illinois 61801

�Received 3 July 2006; accepted 14 September 2006; published online 26 October 2006�

Here the authors show a method for inking scanning probes based on local thermal evaporativeinking transfer. This method results in low loss, rapid, and parallel inking action. Scanning probesare accommodated at openings of ink reservoirs. A resistive heater is used to initiate vaporization ofinks to uniformly coat probes. 1 mM 16-mercaptohexadecanoic acid in ethanol solution is used tocharacterize this inking chip. Moreover, an array of scanning probes can be inked simultaneouslyusing an array of inking reservoirs. During the inking process, each probe is tightly sealed insideindividual reservoir so cross contamination is limited. © 2006 American Institute of Physics.�DOI: 10.1063/1.2364881�

Scanning probe nanolithography �SPN� has been widelyused to generate nanometer scale patterns made of a varietyof materials, including small organic molecules, peptides,proteins, oligonucleotides, and inorganic sol-gel.1–7 Thechemicals �inks� must be deposited on scanning probe tipsfor subsequent writing onto a substrate.8,9 This step of probecoating is referred to as inking, which is very critical andchallenging. Ideally, the inking procedure should be fast andefficient. It should allow paralleled ink transfer onto an arrayof SPN probes.10

Existing methods of inking include liquid phase inking,inkpad transfer, and vapor phase inking. These methods allface severe limitations. The liquid phase dip inking is simpleand popular, but it provides nonuniform, uncontrollable ink-ing. It also suffers from high rate of evaporative ink lossfrom reservoirs as well as cross contamination when inkingan array.11–13 Inkpad-based probe inking method has beenproposed to overcome this issue. It employs a porous mem-brane �e.g., polydimethlysiloxane �PDMS�� for containing/capping the inking solution. However, it takes long time��6 h� for thiol molecules to diffuse through the thin mem-brane from ink-delivery channels.14 Vapor phase inking isuniform, reliable, and much faster than liquid phase inking.Traditionally, the vapor phase inking is conducted by placingprobes in a container filled with liquid chemical solutions orcrystallized chemical compounds. Unfortunately, this methoddoes not support multiprobe and multi-ink delivery.15–17

We report design, fabrication, and testing of a microma-chined chip for inking SPN probe array and validation of itsperformance. The method, based on local thermal evapora-tive inking transfer, results in low loss and rapid parallelinking action �within minutes�. Inks are delivered to closelyspaced ink sites that provide local, on-demand vaporizationand ink transfer to scanning probes.

The structure of an individual inking site is illustrated inFig. 1. Each site consists of an ink reservoir with a controlledaperture opening. The size of the opening matches that of thebase on SPN tips. The reservoir contains liquid or crystal-lized form of the chemical ink. A SPN probe is brought to the

inking site and forms a tight sealing. Upon local heating��60 °C�, the ink molecules inside the reservoir vaporize tocoat the SPN probe tip uniformly. Ink-coated SPN probes areremoved after the chip is cooled. The inks are supplied to thereservoir using microchannels, with the walls made of hydro-philic silicon nitride. Due to the dominant surface tensionforce inside hydrophilic silicon nitride microchannels, thiolsolution would be automatically pumped into the individualinking reservoir.

This inking chip consists of two parts: silicon inkingreservoir array and ink transport network. In order to keepink solution from overflowing the reservoirs, a 2-�m-thickhighly hydrophobic parylene layer is deposited on the top ofsilicon nitride layer to establish a barrier against liquidspreading. A thin film heater is located underneath the siliconchip to locally heat up thiol molecules inside reservoirs�Fig. 1�.

There are several advantages associated with this inkingchip: �1� it does not require active pumping or valving totransport ink solution, as ink transport in the microchannel ismainly due to surface tension force, �2� it involves low lossof ink and rapid parallel uniform inking, �3� it incurs mini-mal cross contamination of adjacent ink reservoir because ofthe seal formed between the SPN probes and the apertures,and �4� it is scalable to high density parallel inking.

The fabrication process to realize the inking chip isshown in Fig. 2�a� A 2600-Å-thick silicon dioxide layer isthermally grown on the 2 in. diam �100�-oriented silicon wa-fer �International Wafer Service, Portola Valley, CA, USA�.A Cr/Au thin film resistive heater is deposited on the sub-

a�Author to whom correspondence should be addressed; electronic mail:changliu@uiuc.edu FIG. 1. �Color online� Mechanism of local vapor inking for SPN.

APPLIED PHYSICS LETTERS 89, 173125 �2006�

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strate using thermal evaporation �Cr/Au=50 Å/5000 Å�. �b�Using photolithography with back side alignment, the SiO2layer at the front side is wet etched to open square windows,which are subsequently used as masks for etching5-�m-deep reservoir cavities using ethylenediamine pyrocat-echol �EDP� silicon etchant. �c� We deposit a 1-�m-thickzinc oxide �ZnO� layer on the substrate as sacrificial layerand photolithographically pattern it to form ink channels. Wethen deposit a 2-�m-thick silicon nitride layer followed by a2 �m parylene layer to cover the ink microchannels. A1200-Å-thick aluminum thin film is deposited as etchingmask for subsequent reactive ion etching to etch inking load-ing holes and ink reservoir windows. �d� The whole chip isreleased using diluted hydrochloride acid �HCl� solution�38% HCl:H2O=2 ml:400 ml�. This step requires 48 hwhen performed with an orbit shaker set at 60 rpm. �e� Asilicon mold is etched using deep reactive ion etching. Theresultant depth of the mold pattern is 60 �m. �f� The siliconmold is covered with a thin carbon layer to facilitate moldrelease. We pour a PDMS prepolymer �10:1 mixing ratiowith curing agent, Dow Corning Sylgard 184, Midland, MI,USA� on the silicon mold and cure the polymer in situ. �g�After 30 min curing at 90 °C, the PDMS layer is peeledaway from the silicon piece manually. We punch holes on thePDMS piece to provide fluid access and then cut the PDMSpiece using a blade. �h� Finally, the PDMS layer is alignedand assembled with silicon chip resultant from step �d�. Fig-ure 3 is an optical picture of the final assembled inking chip.The inset is the magnified view of a ten-inking reservoirarray.

Thiol chemical mercaptohexadecanoic acid �MHA� isused to validate and characterize performances of the inkingchip. At first, 1 mM ethanolic MHA is filled into PDMSchannels. We load the ink solution at the beginning of thesilicon nitride microchannels. Due to the dominant surfacetension force inside hydrophilic silicon nitride microchan-nels, the ink solution is automatically pumped to individual

ink reservoir. The MHA ink is deposited inside ink reservoirs�Fig. 4�.

The measured resistance of the thin film heater is around150 �. After applying a 20 V dc bias, we find that the localtemperature at the ink reservoirs reaches the melting tem-perature of MHA �64 °C� within 2 min. After 2 min heatingand followed by 2 min cooling, the inked probe �type A,NanoInk Inc. Chicago, USA� is loaded on scanning probenanolithography instrument �Nscriptor, Nanoink Inc, Skokie,IL, USA�.

MHA patterns have been successfully written on thefresh gold-coated silicon substrate �Au/Cr=30 nm/5 nm� at25 °C room temperature and 30% relative humidity environ-ment �Fig. 5�a��. The minimum feature can be less than60 nm. Using the local thermal vapor inking method, a ten-probe array was inked and the inked probe array can success-fully write the same pattern on the substrate at the same time�Fig. 5�b��. The linewidth of the pattern is 130 nm.

In order to prove that the ink transfer is truly due tothermal evaporation, the above-mentioned procedure is re-peated with the heater turned off. After 15 min inking, theprobe was loaded on Nscriptor to write the pattern on thefresh gold-coated substrate under the same working tempera-ture and humidity. The same patterns were at the least tentimes slower to write on the substrate even after 15 mininking.

In order to limit the ink cross contamination problem,the ink reservoir is specially designed �Fig. 1�. Because each

FIG. 2. �Color online� Inking chip fabrication flow: �a� Au thin film heaterfabrication on oxide wafer; �b� flat bottom cavity array EDP etching; �c�scarification layer ZnO deposition and patterning and function layer nitride/parylene deposition, then etching windows; �d� hydrochloride acid solutionchip releasing; �e� silicon molding surface treatment; �f� PDMS liquid poly-mer heat curing; �g� loading hole punching and cutting; �h� PDMS layeralignment with silicon chip.

FIG. 3. �Color online� Assembled thermal vapor inking chip.

FIG. 4. Chemical thiol was deposited inside individual ink reservoir.

173125-2 Li et al. Appl. Phys. Lett. 89, 173125 �2006�

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SPN probe is sealed inside individual reservoir, the ink crosscontamination is expected to be limited. A cross contamina-tion test was performed by filling the ink solution in thespecific ink channel, as shown in Fig. 3, and leaving theadjoined ink reservoirs empty. After the aforementionedmethod to finish inking, we performed writing on fresh goldsubstrates. But only the probe that was inked from the filledchannel was able to write the patterns on the substrate. Forother probes, no MHA patterns were detected. We can con-clude that no appreciable cross contamination occurs duringthe local thermal vapor inking. If the different inks are de-livered to the different ink reservoirs, the different probescan be simultaneously coated with the different inks usingthe local thermal vapor inking method.

This local vapor inking chip can be reused after twocleaning procedures. First, the inking chip need rinsing usingethanol and acetone solution to wash away most of the inkresidue inside ink reservoirs and ink channel, and then Pira-nha solution �H2O2:H2SO4=1:3� is needed to further cleanand treat silicon nitride channel surfaces to make themhydrophilic.

In this letter, we report a method to ink scanning probesfor nanolithgraphy based on local thermal vapor ink transfermethod. This method can finish low loss and parallel inkingscanning probes within several minutes. At the same time,the special design ink reservoir can accommodate probetip into individual reservoir to limit the possible inkcontamination.

The authors thank Defense Advanced Research ProjectsAgency �DARPA� Advanced Lithography program and Na-tional Science Foundation �NSF� Center for Nanoscale Sci-ence and Engineering �Northwestern University� for support.

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FIG. 5. �Color online� Characterization of local vapor inking chip �a� re-verse image of MHA dots array writing �3.58�3.58 �m2 scanning size� at25 °C and relative humidity of 30% and �b� the same pattern generated bya ten-pen array.

173125-3 Li et al. Appl. Phys. Lett. 89, 173125 �2006�

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