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Microelectronic Engineering 84 (2007) 904–908

Improved mold fabrication for the definition of high qualitynanopatterns by Soft UV-Nanoimprint lithography using diluted

PDMS material

Namil Koo *, Markus Bender, Ulrich Plachetka, Andreas Fuchs, Thorsten Wahlbrink,Jens Bolten, Heinrich Kurz

AMO GmbH, AMICA (Advanced Microelectronic Center Aachen), Otto-Blumenthal Strasse 25, D-52074 Aachen, Germany

Available online 25 January 2007

Abstract

An improved mold fabrication process that utilizes toluene diluted polydimethylsiloxane (PDMS) as flexible mold material was devel-oped. Various toluene concentrations and their implication on the pattern definition using the Soft UV-Nanoimprint process were ana-lyzed and discussed. Dots with a resolution of 50 nm are well replicated and an excellent imprint homogeneity across a 4 in. wafer withone imprint step only is demonstrated.� 2007 Elsevier B.V. All rights reserved.

Keywords: Soft UV-Nanoimprint; High resolution; Flexible molds; PDMS mold fabrication; Wafer scale imprint; UV-curable resist

1. Introduction

The definition of high resolution features on wafer scaleat a reasonable cost model is one of the major issues for theexisting as well as for future trends in nanotechnology. UV-based nanoimprint techniques have demonstrated theirpotential to be an attractive low cost method for high res-olution nanostructure definition [1,2]. Up to now, the reso-lution is not limited by the imprint process or the resistmaterial applied but by the mold fabrication process [3].

We have developed the Soft UV-Nanoimprint tech-nique, featuring transparent flexible molds and low viscoseUV-curable resist, which enables the definition of highaspect nanostructures on wafer scale with one imprint steponly [4,5]. Here, the fabrication of the flexible molds is car-ried out by a cast moulding process where an appropriateliquid mold material is deposited on a structured master,followed by thermal curing of the material. Poly-dimethylsiloxane (PDMS) is used as standard mold mate-

0167-9317/$ - see front matter � 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.mee.2007.01.017

* Corresponding author. Tel.: +49 241 8867 218; fax: +49 241 8867 571.E-mail address: koo@amo.de (N. Koo).

rial due to its favourable properties concerning flexibility,UV-transparency and low surface energy. The main draw-back of PDMS materials is the high viscosity, which is forSylgard 184 (Dow Corning) 3900 mPa s. As a consequence,the profiles of nanopatterns on the mold are not completelydefined, resulting in a loss of pattern height. Thus, the res-olution of the mold fabrication process is limited by aninappropriate material flow for pattern geometries withinthe sub-100 nm regime [6,7]. To overcome this problemother groups have reported the lowering of the viscosityof h-PDMS [6] using triethylamine, toluene and hexaneas solvent and demonstrated the imprinting of 75 nm lineswith a pitch of 150 nm [8]. However, due to the fragility ofh-PDMS the usage for large scale imprint process and thereproducibility seems to be limited.

To realise both requirements, definition of nanostruc-tures and wafer scale imprinting within one step we haveinvestigated an improved mold fabrication process usingSylgard 184 diluted with toluene. For a detailed study thePDMS base material was diluted with five concentrationsof toluene before the mixture was deposited onto a 6 in. sil-icon master featuring micro- and nanometerstructures.After curing the quality of the fabricated molds were inves-

Fig. 1. Influence of the material viscosity on the penetration depth intothe master cavities for nanostructures.

N. Koo et al. / Microelectronic Engineering 84 (2007) 904–908 905

tigated via SEM- and AFM-analysis of the imprinted resiststructures. Furthermore, the results obtained were trans-ferred to perform imprinting of high resolution nanostruc-tures on 4 in. wafer scale. The high degree of dimensionstability of the mold fabrication process is demonstratedvia SEM-analysis of five different fields distributed overan imprinted 4 in. silicon wafer. The excellent homogeneityof the residual resist thickness gives evidence of the unifor-mity of both, the mold fabrication as well as the imprintprocess.

2. Experimental

For the investigation of the mold fabrication process anappropriate 6 in. silicon master for the cast moulding pro-cess had to be fabricated. Here, in a first step five fields withnanopatterns containing various geometries and structuresizes from 500 down to 50 nm as well as alignment markswere defined in hydrogensilsesquioxane (HSQ) resist byan EBPG-5000 TFE E-beam writer. Subsequently, thedefined HSQ-patterns were transferred into the siliconmaster via high anisotropic reactive ion etch process, fol-lowed by the HSQ mask removal. The definition ofmicrometer structures with feature sizes from 100 lm downto 1 lm was carried out via contact lithography using thealready defined alignment marks. The pattern transferwas carried out with the identical RIE-process guarantee-ing a uniform pattern depth of all structures. After theresist mask removal the master was modified with a thinanti-adhesion layer to lower the surface energy and there-fore eases the separation of the PDMS molds.

To investigate the influence of different concentrationsof toluene on the mold fabrication process the materialwas prepared as follows. The base material, PDMS (Syl-gard 184, Dow Corning) was mixed with its curing agent.The mold fabrication process was investigated with theundiluted PDMS basis, as well as diluted mixtures withfour different toluene weight concentrations (10%, 20%,40%, and 60%). These material mixtures were spin coatedat 3000 rpm for 30 s onto the silicon master in order toachieve a thin uniform layer. Subsequently, the mixturewas degassed and cured at 120 �C for 15 min.

To guarantee identical mold thicknesses a metal spacerwith 3 mm height was placed onto the master and a thickPDMS cushion layer was cast onto the already cured thinlayer. Finally a glass carrier with an appropriate primerwas placed onto the material stack and the sandwich curedon a hot plate at 120 �C for 30 min. The quality of the fab-ricated molds was investigated by the SEM- and AFM-analysis of imprinted resist structures. The imprint resistused for all investigations was AMONIL, a low viscosityUV-curable imprint resist distributed by AMO GmbH.The resist basis is a mixture of inorganic and organic com-pounds which is modified with a fluorine additive whichmigrates to the resist free surface during spin coating pro-cesses and creates a asymmetric adhesion behaviour withan high adhesion to the substrate and a lower adhesion

at the resist surface. The main advantage of this materialis that no interaction between PDMS-based molds andthe resist occurs even for contact times longer than10 min. In the experiments presented here AMONILMMS 4 with an initial film thickness of 180 nm was chosen.The imprint process was carried out on the EV 620 imprinttool from EV Group, Austria. As imprint process condi-tion a pressure of 600 mbar and a contact time of 2 minwere defined before UV-polymerization of the resistthrough the transparent mold was carried out. Subse-quently the mold was detached from the cured resist.

3. Results and discussion

The material flow for the PDMS base material as well asfor the diluted material is schematically shown in Fig. 1.The viscosity limits the penetration depth of the materialinto the master cavities, an effect which depends stronglyonto the master geometry. Materials with a lower viscosityenable a higher penetration depth thereby the fabricationof molds with a higher aspect ratio is feasible (position Ein Fig. 1).

The different mold preparation processes and their influ-ence on pattern generation were investigated by SEM- andAFM-analysis of imprinted resist structures. We choseperiodic dot arrays with pillar diameters of 50, 100, 250,500, and 1000 nm and the period of three times the struc-ture diameter to verify the dependency of the master geom-etry onto the mold fabrication process. The structures werearranged in 100 · 100 lm2 arrays on the master surface.The master depth for all structures was measured to110 nm. Deformation effects of the flexible mold can beneglected due to the fabrication of molds with a constantthickness and the usage of identical imprint process condi-tions. The imprinted resist structures were analysed withregard to the lateral and vertical dimensions. It turns outthat the lateral dimensions are nearly the same for all moldpreparation variations and master geometries. Even struc-tures in the sub-50 nm range match well with the corre-sponding master structure. In Fig. 2 exemplary SEMpictures of an imprinted 50 nm dot array using differenttoluene dilutions are shown. The pictures indicate clearlythat the pattern quality depends strongly on the concentra-tion of dilution.

To verify this phenomena the height of the imprinteddots was analysed via AFM. In Fig. 3 the height of the

Fig. 2. Exemplary SEM pictures of an 50 nm dots array imprinted with diluted PDMS molds prepared with toluene concentration of 10, 20, 40, and60 wt%.

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imprinted resist pattern in dependency of the toluene con-centration for five different feature sizes or rather geome-tries are displayed. One can clearly see that for structuresin the lm-range the height of the fabricated structures cor-responds well to the master height, whereas for structureswith a smaller size the pattern height and therefore themold depth are significantly lower than the master height.

For mold fabrication without any toluene dilution theresist pattern height decreases from 90 nm for 500 nm dotsdown to 5 nm for 50 nm dots due to the high viscositywhich limits the penetration of the material into the cavi-

Fig. 3. Analysis of the resist pattern height in dependency of the tolueneconcentration for various master geometries. The master depth for allstructures was determined to 110 nm.

ties. The analysis of the pattern height for toluene dilutedmolds show an improvement for the definition of structuresespecially for toluene concentrations of 60 wt%. Dots witha diameter of 500 nm and 250 nm show a pattern depthincrease of only a few nanometers. Yet, a vast improve-ment for sub-100 nm features is obvious. For the 50 nmdots the pattern height increases from 10 nm up to 70 nmfor a toluene concentration of 10 wt% and 60 wt%, respec-tively. Higher toluene concentration may lead to furtherimprovement of structure definition but dewetting effects

Fig. 4. Photograph of a patterned 4 in. wafer imprinted with a PDMSmold prepared with 60 wt% toluene diluted PDMS. The 5 · 5 matrixmarks the measurement points for the analysis of the residual resistthickness.

Fig. 5. SEM-analysis of the five imprinted resist fields distributed over an imprinted 4 in. silicon wafer.

N. Koo et al. / Microelectronic Engineering 84 (2007) 904–908 907

during the spin coating process inhibit the fabrication ofsuch molds. As a consequence of these experiments wecan estimate that the lowering of the viscosity of the moldmaterial enables a higher penetration depth which resultsin a higher pattern height. The pattern height wasimproved by a factor of 15 for 50 nm structures and a fac-tor of 2 for 100 nm structures.

These results were transferred to the imprinting of nano-structures on wafer scale. A 4 in. flexible mold with a tolu-ene concentration of 60 wt% was fabricated using theprocess described in Section 2 . To assess the mold fabrica-tion process further we imprinted 100 mm silicon wafersand determined the homogeneity and the dimensional sta-bility of the resist pattern. In Fig. 4 a photograph of an

Fig. 6. (a) SEM picture of an expemplary 45 nm dot array in Field 3 on the siimprinted with a 60 wt% toluene diluted PDMS mold.

imprinted wafer gives evidence of the homogeneity of theimprint process and therefore the mold fabrication process.The residual resist thickness, an essential indicator for theimprint homogeneity, has been measured with an ellipsom-eter at 25 data points, spread in a 5 · 5 matrix as schemat-ically shown in Fig. 4. The mean value was estimated to140 nm with a small deviation of less than 1.4 nm. Thisexcellent uniformity demonstrates the high quality of theimprint process on wafer scale and the suitability using tol-uene diluted mold material for the fabrication of high res-olution flexible molds.

The dimension stability of the imprint process on waferscale was investigated by SEM-analysis of the five fields(F1–F5) containing the nanostructures. Exemplary SEM

licon master; (b) SEM picture of the corresponding 43 nm resist dot array

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pictures of the resist structures are shown in Fig. 5. Thegood quality of the imprinted resist structures demonstratethe capability of imprinting nanostructures in the 50 nmregime on wafer scale using flexible molds. The dimen-sional stability of the process is exemplary demonstratedby SEM-analysis of the master and the corresponding resiststructure of Field 3 in Fig. 6. Taken additional deformationeffects of the flexible mold into account the reduction of thedot diameter from 46.1 nm for the silicon master to 43.1 forthe resist structure is nearly neglectable.

4. Conclusion

The definition of high resolution features on wafer scaleat a reasonable cost model is one of the major issues for theexisting as well as for future trends in nanotechnology. SoftUV-Nanoimprint technology enables the imprint of nano-structure on wafer scale but the resolution is limited by thefabrication processes of the flexible molds. In this paper wepresent an improved mold fabrication process using tolu-ene diluted PDMS as mold material. We clearly demon-strated the capability of the technique to define patternswith 50 nm resolution over 4 in. with one imprint step only.The excellent homogeneity on wafer scale as well as thehigh degree of dimensional stability of both, mold fabrica-tion and the imprint process is demonstrated.

Acknowledgments

The partial support of the EC-funded project NaPa(Contract No. NMP4-CT-2003-500120) and a Grant(06K1401-00210) from Center for Nanoscale Mechatronicsand Manufacturing, one of the 21st Century Frontier Re-search Programs, which are supported by Ministry of Sci-ence and Technology, Korea are gratefullyacknowledged. The content of this work is the sole respon-sibility of the authors.

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