Fabrication Approaches for Generating Complex Micro- and Nanopatterns onPolymeric Surfaces
Aranzazu del Campo,* and Eduard Arzt
Max-Planck-Institut fur Metallforschung, Heisenbergstrae 3, 70569 Stuttgart, Germany
Received June 19, 2006
Contents1. Introduction 9112. Advanced Photolithography 912
2.1. High Aspect Ratio Patterns 9122.1.1. Photoresist Materials 9122.1.2. Particularities of HAR Processing 9142.1.3. Examples and Actual Limits 916
2.2. Hierarchical Patterns by Layer-by-LayerExposure
2.3. Tilted Patterns by Inclined/RotatedLithography
2.4. 3D Patterns by Modulated Exposure 9172.5. Periodic 3D Patterns by Holographic
3. Laser Scanning 9183.1. Stereolithography by Scanning Resist
3.2. Two-Photon Lithography (TPL) 9194. Serial Writing with Charged Particles 920
4.1. Electron Beam Lithography 9204.2. Ion Beam Lithography 9204.3. Scanning Probe Resist Lithography 921
5. Micro- and Nanomachining 9215.1. Focused Ion Beam 9215.2. Scanning Probe Nanomachining 921
6. Direct Writing and Material Deposition 9217. Moulding 922
7.1. Mould Fabrication 9227.2. Demoulding: Mould Treatment 9247.3. Embossing Thermoplastic Materials:
Nanoimprint Lithography (NIL)924
7.3.1. HAR Structures Produced by NIL 9257.3.2. Secondary Structures by NIL 9257.3.3. 3D Patterns with NIL 926
7.4. Moulding UV-Sensitive Materials: UV NIL 9267.5. Soft Lithography 9277.6. Solvent-Assisted Moulding 928
8. Transfer Printing 9298.1. 3D Patterns by Multistep Transfer Printing 930
9. Filling Mesoporous Matrices 9309.1. With Polymer Precursors 9319.2. With Polymeric Melts 931
10. Surface Instabilities 932
10.1. Electric Field-Induced Instabilities 93210.2. Temperature-Induced Instabilities 935
11. Patterning Through Self-Assembly of BlockCopolymers
11.1. BC Morphology in Thin Film Geometry 93511.2. Hierarchical Patterns 936
12. Comparison of Patterning Methods 93813. Applications 939
13.1. Biosensors with Increased Sensitivity,Miniaturization, and Throughput
13.2. Drug Delivery 93913.3. Force Sensors 93913.4. Tissue Engineering and Implant Fabrication 93913.5. Biofouling 94013.6. Biomimetic Surfaces 94013.7. Photonic Structures 940
14. Conclusions 94015. References 940
1. IntroductionRecent innovations in the area of micro- and nanofabri-
cation have created a unique opportunity for patterningsurfaces with features with lateral dimensions spanning overthe nano- to millimeter range. The microelectronics industryand need for smaller and faster computing systems havepushed this development during the last two decades, mainlyfocused on obtaining patterns with the smallest possiblelateral dimensions via optical lithography in its multiplevariants.1 In parallel, new application fields for miniaturizeddevices have emerged, analytical chips or lab-on-a-chipdevices for application in biochemistry being the mostnotable example. The interest in light and low-cost deviceshas caused the development of alternative patterning tech-nologies more suited for plastics manufacturing.2-4
Increased effort has been paid in the last 10 years toestablishing fabrication technologies which allow productionof structured surfaces with greater geometrical complexityat reduced operation time and cost. These include patternsmade of polymer materials possessing elongated features inthe vertical dimension (aspect ratio > 3), exhibiting severalhierarchy levels, or in intricate tilted, suspended, or curvedthree-dimensional (3D) arrangements. Such surface structuresfind applications in emerging fields like biosensors withincreased sensitivity and throughput due to higher effectivesurface area; fibrillar surfaces with controlled adhesion asbetter scaffolds for tissue engineering, nonbiofouling coatingsfor undersea pipelines, high adherence wheels, and hapticdevices; or high-luminosity lighting panels and photonicstructures for the visible spectrum.
* To whom correspondence should be addressed. Phone: +49 (0)7116893416. Fax: +49 (0)711 6893412. E-mail: firstname.lastname@example.org. Current address: Leibniz Institut fur Neve Materialien, Campus D2 2,66123 Saarbrucken, Germany.
911Chem. Rev. 2008, 108, 911945
10.1021/cr050018y CCC: $71.00 2008 American Chemical SocietyPublished on Web 02/26/2008
In this review we present available processing methodssuitable for the fabrication of such complex micro- andnanostructured surfaces made out of polymeric materials. Thesteps and polymeric materials involved and the achievedstructures will be described. With this work, we intend tocomplement recent reviews on surface patterning,2-4 mainlyconcentrated on less complicated surface designs and ap-plications in microelectronics, and provide the reader witha critical understanding of this topic at an early stage of itsdevelopment.
2. Advanced PhotolithographyLithography is the technique used to transfer a pattern onto
a substrate by means of an etching process. Resist lithographymakes use of an irradiation source and a photosensitivepolymer material to perform the pattern transfer (Figure 1a).This process starts with the coating of a planar substrate(typically a silicon wafer) with the photoresist in liquid form.After the coating process, the substrate is soft baked inorder to remove the solvents from the resist and improveresist-substrate adhesion. In a subsequent step, selected partsof the photoresist film are exposed to a light source (typicallya UV lamp, electron beam, or X-rays). Light initiates a seriesof photochemical processes in the resist which alter thephysical and chemical properties of the exposed areas suchthat they can be differentiated in a subsequent imagedevelopment step. Most commonly, the solubility of the filmis modified, either increasing the solubility of exposed areas(yielding a positive image after development) or decreasingthe solubility to yield a negative-tone image. After develop-ment, substrates patterned with negative-tone photoresists canbe baked a second time (hard bake) at higher temperaturesto further activate cross-linking processes and improve themechanical stability of the pattern; however, this process maycause thermal flow of the patterned features and consequentlydistortion of their initial geometry.
During the last three decades, most efforts and develop-ments in lithography have been directed at shrinking thelateral dimensions of the imaged features.1,5-10 The applica-tion of different resolution enhancement approaches (il-lumination sources with shorter irradiation wavelength,projection and immersion optics, phase-shifting masks, etc.)and development of advanced photoresist materials (e.g.,chemically amplified resists11) have permitted a reductionof lithographic structures down to sub-100 nm dimensions.Current trends have been predicted to improve shrinking toat least 45 nm in the year 2010.5 Only recently havelithographic approaches been extended to the fabrication ofmore complex patterns including high aspect ratio (HAR),tilted, suspended, or curved geometries. Initially, suchstructures were fabricated in layer-by-layer processes wheremultiple coating/irradiation steps are concatenated or iterated,each of them defining structures at different levels. Toovercome the complexity of the layer-by-layer approach andminimize the number of process paths required to obtain thefinal structure, new fabrication methodologies are trendingtoward lithography techniques with intrinsic 3D structuringcapability. Some of the available approaches will be dis-cussed in this section.
2.1. High Aspect Ratio Patterns
2.1.1. Photoresist MaterialsResist materials must satisfy stringent requirements as the
lateral feature size shrinks and the aspect ratio increases.11
Aranzazu del Campo was born in 1972 in Coomonte (Spain) and studiedchemistry at the Universidad Complutense and Polymer Science andEngineering at the Instituto de Ciencia y Tecnologa de Polmeros (Madrid,Spain). She received her Ph.D. degree in 2000 working in the field ofliquid crystalline polymers. She then joined the Max-Planck-Institut furPolymerforschung in Mainz (Germany) as Marie Curie Fellow and startedto work in the field of surface chemistry and nanotechnology. In 2003she moved to the Universita degli Studi di Urbino (Italy), and since 2004she has been leading the group Complex, Multifunctional Surfaces atthe Max-Planck-Institute in Stuttgart (Germany). Her group is mainlyengaged in developing novel synthetic approaches for manufacturinghierarchical, chemically, and topographically patterned surfaces. Theseare based on challenging organic chemistry concepts and conceived tounderstand adhesion phenomena at different levels.
Eduard Arzt was born in 1956 in Linz, Austria, and studied physics andmathematics at the University of Vienna. He completed his Ph.D. degreethere in 1980 with a metallurgical project carried out at the University ofLeoben. Following a period as a research associate at CambridgeUniversity, U.K., he joined the Max Planck Institute for Metals Researchas a group leader in 1982. After a tenure as Visiting Professor at StanfordUniversity, he was appointed in 1990 to Professor of Physical Metallurgy/Metal Physics at the University of Stuttgart and jointly Director at theMax Planck Institute for Metals Research. He is the recipient of severalnational and international awards, among them the Acta MetallurgicaOutstanding Paper Award (1990), the highest German science awardGottfried Wilhelm Leibniz Prize (1996), and inclusion among the highlycited materials scientists by ISI (2003). He has been Visiting Professor atthe Massachusetts Institute of Technology, Cambridge, MA (1996), anddelivered distinguished lectures there and at the University of Minnesota(2005).