Patroonvorming is alomtegenwoordig in de natuur: van menselijke huid tot bladeren, vruchten en groenten. Dergelijke patronen, met name netwerken van kanaaltjes, zijn ook interessant voor veel moderne technologische toepassingen, zoals flexibele elektronica, optische roosters en lab-on-a-chip systemen. De fabricage van deze structuren op zeer kleine schaal (orde nano- en micrometer) is echter niet eenvoudig. Ratna Kumar Annabattula verrichtte theoretisch onderzoek naar de vorming van micro- en nanokanaaltjes op basis van een recent ontwikkelde techniek, de ‘release and bondback’-methode. Deze methode bestaat uit drie stappen: (i) het groeien van een dunne film met voorspanning op een substraat (ii) het relaxeren van de film door het verminderen van de hechtsterkte (door chemisch etsen of elektrolyse) en tenslotte (iii) bondback, het terugvallen van de vrijgekomen film op het substraat als gevolg van de cohesiekracht tussen de film en het substraat. Annabattula ontwikkelde een nieuwe eindige-elementen-methode om het proces van delaminatie en bondback te bestuderen. De uiteindelijk gevormde kanaalafmetingen en structuren van verschillende systemen zijn onderzocht als functie van de verschillende systeemparameters, zoals laagdikte, laaggrootte, elastische eigenschappen en cohesie-energie tussen de dunne laag en het substraat. In het proefschrift is het gehele proces van kanaalvorming vastgelegd. Er zijn vier verschillende systemen onderzocht: twee- dimensionale lijnpatronen, lineaire kanalen, polygoon-vormige patronen en netwerken van nanokanaaltjes. De resultaten laten zeer goede overeenkomsten zien met experimentele kanaalstructuren, waardoor de resultaten gebruikt kunnen worden als designtool voor toekomstige kanaalsystemen

Annabattula, Veera Venkata Satya Durga Ratna Kumar

English

ARTS repository - University of Groningen

   University of GroningenBuckling-driven self-formation of microchannelsAnnabattula, Veera Venkata Satya Durga Ratna KumarIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Annabattula, V. V. S. D. R. K. (2011). Buckling-driven self-formation of microchannels. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 30-10-20231 INTRODUCTION1.1 Nano and microfabrication technologyMiniaturization is the continuous downsizing of components and devices for applicationsin micro-electronics, optoelectronics and micro/nano-electro mechanical systems (NEMS,MEMS). The beginning of the twentieth century was very important in modern human historywith the advent of electronics and computers. Exceptional developments in micro-electronicsenabled computers to be generally accessible and thus laying the foundation for modern tech-nology. The tendency towards building small-scale components and products in all fields ofscience (mechanical, optical, electronic, chemical or even biological) has become increas-ingly desirous, allowing products and devices to be easier to carry, easier to store, and muchmore convenient and efficient to use. As a result of the quest to build smaller structures, thefield of nano- and microfabrication technology emerged. Specialized fabrication strategieshave been developed in the last three decades to realize micro- and nanosystems, such asphoto-lithography [Okazaki, 1991], focused ion beams [Moon et al., 2007] and soft lithog-raphy [Xia and Whitesides, 1998]. The continuous downsizing of consumer electronics (e.g.smart phones) is the outcome of the considerable progress made in this field.Much attention has been focused on generating ordered surface patterns, wrinkles andchannels at the nano- and micron scale with controllable dimensions. Recent developmentsin this area has lead to a wide range of fascinating applications such as tunable optical grat-ings [Edmondson et al., 2006; Edmondson and Huck, 2004; Xia et al., 1996], flexible elec-tronics [Baca et al., 2008; Hsu et al., 2002a; Jiang et al., 2007b; Kim et al., 2008; Lacouret al., 2003; Sun et al., 2006], micro-reactors [Watts and Wiles, 2007], micro- and nanoflu-idic channels [Malachias et al., 2008; Mei et al., 2007], actuation/sensing devices [Comrieand Huck, 2008], particle separators [Efimenko et al., 2005], templates for microstructurefabrication [Huck et al., 2000; Peng et al., 2004; Schäffer et al., 2000], scaffolds for tissueengineering [Langer and Vacanti, 1993] and surfaces for marine anti-fouling [Efimenko et al.,2009].2 1. Introduction1.2 Surface patterning by bucklingAlmost all methods employed in generating patterns/order at small length scales rely on thethree step process of conventional lithography, thin film deposition and etching. In the searchfor alternative approaches, a new method based on the buckling of pre-stressed thin films hasemerged with promising features compared to conventional lithography based techniques.Buckling, wrinkling and other mechanical instabilities have long been considered as beset-ting phenomena to be avoided for better performance of structures. However, buckling canalso be exploited for the well-controlled generation of regular surface patterns in the micro-and nanometer range [Bowden et al., 1998]. The spontaneous formation of ordered structuresthrough buckling eliminates the necessity of pre-patterning in contrast to lithography-basedmethods, thus evolving as a promising cost and time-efficient alternative approach. It is worthnoting that the self-assembly or self-organization of order at length scales much smaller thanthe induced feature sizes is an appealing feature of this technique. Many different ways ofbuckling-driven pattern and channel formation have been explored in the literature. Blis-ters of various morphologies (straight, circular and telephone-cord) can be formed throughbuckling-driven delamination of pre-stressed thin films from a rigid substrate [Gille and Rau,1984; Moon et al., 2004; Ortiz and Gioia, 1994]. Wrinkle formation can also be achievedwhen the film does not delaminate “from”, but buckle “into” the substrate. In this case,the substrate is compliant and pre-strained by heating or stretching before the stiff film isattached [Bowden et al., 1998; Huck et al., 2000; Jiang et al., 2007a], thus inducing com-pressive stresses in the film. The generation of anisotropy in the pre- or postbuckling stressstate allows for a preferred direction of buckling, causing ordered ridging patterns. A wealthof studies has been devoted to understand the mechanism of wrinkle/pattern formation in theabove two methods [see e.g., Hutchinson and Suo, 1992; Ortiz and Gioia, 1994; Song et al.,2008; Suo and Hutchinson, 1990]. Recently, a new method of buckle-driven channel forma-tion is reported, which is based on the partial release of pre-stressed thin films from a rigidsubstrate by chemical etching [Malachias et al., 2008; Mei et al., 2007] or electrolysis [Ed-mondson et al., 2006; Edmondson and Huck, 2004], causing the films to buckle-up, followedby bond back of the film onto the substrate. In this method a thin film is grown on a substrateduring which eigenstrains are induced, causing compressive film stresses to develop. Next,the film-substrate system is exposed to an external stimulus (chemical etching or an electricpulse) which reduces the interface strength between the film and the substrate, allowing thefilm to release its eigenstrains through buckling from the substrate. Finally, the system isleft for drying, during which the partially buckled-up film bonds back to the substrate dueto cohesive forces of attraction between the film and the substrate, freezing in the final ge-ometry of the channel. Despite its success in generating well-controlled ordered channelsof (sub)micrometer/nanometer dimensions, the underlying mechanics of pattern formation isnot yet well understood. This will be the focus of this thesis.1.3. Objective and outline of the thesis 31.3 Objective and outline of the thesisThe objective of this thesis is to develop a fundamental understanding of the mechanism ofchannel formation based on the release and bond back of pre-stressed thin films on rigid sub-strates. We investigate how various system parameters such as the film geometry (film size,thickness and shape), film properties (elastic modulus E, Poisson’s ratio ν and eigenstrainε∗), interface energy Γ and the method of interface strength reduction (uniform electrolysisversus directional etching) control the final channel geometry (channel morphology, heightand width). We will carry out a dimensional analysis to identify the key dimensionless param-eters that fully capture the channel formation process in order to provide insight for design.The thesis is organized as follows.Chapters 2 and 3 deal with the formation of line buckles in polymer films by the electrol-ysis process. In this chapter a two-dimensional finite element model is developed to describethe mechanics of buckling-driven delamination of the film from the substrate and the processof draping back. A cohesive zone model is implemented to mimic the interface cohesionbetween the film and the substrate during delamination and bond-back. Buckling-up will bestudied through a ‘2-step’ process in which first the eigenstrain is applied and subsequentlythe interface strength reduced. The results will be compared to the well-known process ofbuckling-driven delamination (a ‘1-step’ process) in which the interface delaminates instan-taneously when eigenstrains are applied. Finally, the bond-back process is analyzed and thegoverning dimensionless parameters are identified.In chapter 4 we describe the self-organization of linear nanochannel formation by releas-ing the eigenstrains through directional under-etching of a sacrificial layer between the filmand substrate. Analytical calculations are carried out to obtain the critical buckling strain andwavelength at the onset of buckling. We use three-dimensional finite element simulations tostudy the evolution of the buckling wavelength and height as a function of eigenstrain fordifferent etch depths. Finally, bond-back simulations are performed to obtain the final chan-nel morphologies in terms of channel spacing, channel width and height. The results of themodel are compared with the experimental observations, showing good agreement.In chapter 5 we study the mechanics of pattern formation in pre-stressed polygonal films.Three-dimensional simulations are carried out to study the evolution of the different deflec-tion patterns as a function of eigenstrain during buckling-up and as a function of interfacestrength during bond-back. For rectangular films the results are compared to analytical solu-tions based on minimization of energy. Films of different shape (square, rectangular, pentag-onal and hexagonal) are analyzed and the results are captured in scaling relations.Chapter 6 describes the mechanics of nanochannel network formation for the situationin which the etchant solution enters the system through periodically-spaced pre-defined etchpits. First, we study the evolution of deflection patterns around a single etch-pit. The evolu-tion of wavelength is compared with the master curve of linear channel simulations studiedin chapter 4, showing close correspondence. A semi-analytical method has been developedto describe the transition of channel morphology from edge channels to corner channels as afunction of film geometry, thickness and eigenstrain. Next, we study the channel evolution ina 3 × 3 etch-pit array. The networks are formed in InGaAs films, which have a very strong4 1. Introductioninteraction with the substrate during the etching process unlike the Si-Ge systems as studiedin chapter 4. We show that the presence of film-substrate interaction is essential to accountfor the complex nanochannel networks reported in the experiments.Finally, in chapter 7 we summarize the results obtained in this thesis, draw conclusionsand present an outlook for future developments.

Buckling-driven self-formation of microchannels

   University of GroningenBuckling-driven self-formation of microchannelsAnnabattula, Veera Venkata Satya Durga Ratna KumarIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Annabattula, V. V. S. D. R. K. (2011). Buckling-driven self-formation of microchannels. s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 08-06-20221 INTRODUCTION1.1 Nano and microfabrication technologyMiniaturization is the continuous downsizing of components and devices for applicationsin micro-electronics, optoelectronics and micro/nano-electro mechanical systems (NEMS,MEMS). The beginning of the twentieth century was very important in modern human historywith the advent of electronics and computers. Exceptional developments in micro-electronicsenabled computers to be generally accessible and thus laying the foundation for modern tech-nology. The tendency towards building small-scale components and products in all fields ofscience (mechanical, optical, electronic, chemical or even biological) has become increas-ingly desirous, allowing products and devices to be easier to carry, easier to store, and muchmore convenient and efficient to use. As a result of the quest to build smaller structures, thefield of nano- and microfabrication technology emerged. Specialized fabrication strategieshave been developed in the last three decades to realize micro- and nanosystems, such asphoto-lithography [Okazaki, 1991], focused ion beams [Moon et al., 2007] and soft lithog-raphy [Xia and Whitesides, 1998]. The continuous downsizing of consumer electronics (e.g.smart phones) is the outcome of the considerable progress made in this field.Much attention has been focused on generating ordered surface patterns, wrinkles andchannels at the nano- and micron scale with controllable dimensions. Recent developmentsin this area has lead to a wide range of fascinating applications such as tunable optical grat-ings [Edmondson et al., 2006; Edmondson and Huck, 2004; Xia et al., 1996], flexible elec-tronics [Baca et al., 2008; Hsu et al., 2002a; Jiang et al., 2007b; Kim et al., 2008; Lacouret al., 2003; Sun et al., 2006], micro-reactors [Watts and Wiles, 2007], micro- and nanoflu-idic channels [Malachias et al., 2008; Mei et al., 2007], actuation/sensing devices [Comrieand Huck, 2008], particle separators [Efimenko et al., 2005], templates for microstructurefabrication [Huck et al., 2000; Peng et al., 2004; Schäffer et al., 2000], scaffolds for tissueengineering [Langer and Vacanti, 1993] and surfaces for marine anti-fouling [Efimenko et al.,2009].2 1. Introduction1.2 Surface patterning by bucklingAlmost all methods employed in generating patterns/order at small length scales rely on thethree step process of conventional lithography, thin film deposition and etching. In the searchfor alternative approaches, a new method based on the buckling of pre-stressed thin films hasemerged with promising features compared to conventional lithography based techniques.Buckling, wrinkling and other mechanical instabilities have long been considered as beset-ting phenomena to be avoided for better performance of structures. However, buckling canalso be exploited for the well-controlled generation of regular surface patterns in the micro-and nanometer range [Bowden et al., 1998]. The spontaneous formation of ordered structuresthrough buckling eliminates the necessity of pre-patterning in contrast to lithography-basedmethods, thus evolving as a promising cost and time-efficient alternative approach. It is worthnoting that the self-assembly or self-organization of order at length scales much smaller thanthe induced feature sizes is an appealing feature of this technique. Many different ways ofbuckling-driven pattern and channel formation have been explored in the literature. Blis-ters of various morphologies (straight, circular and telephone-cord) can be formed throughbuckling-driven delamination of pre-stressed thin films from a rigid substrate [Gille and Rau,1984; Moon et al., 2004; Ortiz and Gioia, 1994]. Wrinkle formation can also be achievedwhen the film does not delaminate “from”, but buckle “into” the substrate. In this case,the substrate is compliant and pre-strained by heating or stretching before the stiff film isattached [Bowden et al., 1998; Huck et al., 2000; Jiang et al., 2007a], thus inducing com-pressive stresses in the film. The generation of anisotropy in the pre- or postbuckling stressstate allows for a preferred direction of buckling, causing ordered ridging patterns. A wealthof studies has been devoted to understand the mechanism of wrinkle/pattern formation in theabove two methods [see e.g., Hutchinson and Suo, 1992; Ortiz and Gioia, 1994; Song et al.,2008; Suo and Hutchinson, 1990]. Recently, a new method of buckle-driven channel forma-tion is reported, which is based on the partial release of pre-stressed thin films from a rigidsubstrate by chemical etching [Malachias et al., 2008; Mei et al., 2007] or electrolysis [Ed-mondson et al., 2006; Edmondson and Huck, 2004], causing the films to buckle-up, followedby bond back of the film onto the substrate. In this method a thin film is grown on a substrateduring which eigenstrains are induced, causing compressive film stresses to develop. Next,the film-substrate system is exposed to an external stimulus (chemical etching or an electricpulse) which reduces the interface strength between the film and the substrate, allowing thefilm to release its eigenstrains through buckling from the substrate. Finally, the system isleft for drying, during which the partially buckled-up film bonds back to the substrate dueto cohesive forces of attraction between the film and the substrate, freezing in the final ge-ometry of the channel. Despite its success in generating well-controlled ordered channelsof (sub)micrometer/nanometer dimensions, the underlying mechanics of pattern formation isnot yet well understood. This will be the focus of this thesis.1.3. Objective and outline of the thesis 31.3 Objective and outline of the thesisThe objective of this thesis is to develop a fundamental understanding of the mechanism ofchannel formation based on the release and bond back of pre-stressed thin films on rigid sub-strates. We investigate how various system parameters such as the film geometry (film size,thickness and shape), film properties (elastic modulus E, Poisson’s ratio ν and eigenstrainε∗), interface energy Γ and the method of interface strength reduction (uniform electrolysisversus directional etching) control the final channel geometry (channel morphology, heightand width). We will carry out a dimensional analysis to identify the key dimensionless param-eters that fully capture the channel formation process in order to provide insight for design.The thesis is organized as follows.Chapters 2 and 3 deal with the formation of line buckles in polymer films by the electrol-ysis process. In this chapter a two-dimensional finite element model is developed to describethe mechanics of buckling-driven delamination of the film from the substrate and the processof draping back. A cohesive zone model is implemented to mimic the interface cohesionbetween the film and the substrate during delamination and bond-back. Buckling-up will bestudied through a ‘2-step’ process in which first the eigenstrain is applied and subsequentlythe interface strength reduced. The results will be compared to the well-known process ofbuckling-driven delamination (a ‘1-step’ process) in which the interface delaminates instan-taneously when eigenstrains are applied. Finally, the bond-back process is analyzed and thegoverning dimensionless parameters are identified.In chapter 4 we describe the self-organization of linear nanochannel formation by releas-ing the eigenstrains through directional under-etching of a sacrificial layer between the filmand substrate. Analytical calculations are carried out to obtain the critical buckling strain andwavelength at the onset of buckling. We use three-dimensional finite element simulations tostudy the evolution of the buckling wavelength and height as a function of eigenstrain fordifferent etch depths. Finally, bond-back simulations are performed to obtain the final chan-nel morphologies in terms of channel spacing, channel width and height. The results of themodel are compared with the experimental observations, showing good agreement.In chapter 5 we study the mechanics of pattern formation in pre-stressed polygonal films.Three-dimensional simulations are carried out to study the evolution of the different deflec-tion patterns as a function of eigenstrain during buckling-up and as a function of interfacestrength during bond-back. For rectangular films the results are compared to analytical solu-tions based on minimization of energy. Films of different shape (square, rectangular, pentag-onal and hexagonal) are analyzed and the results are captured in scaling relations.Chapter 6 describes the mechanics of nanochannel network formation for the situationin which the etchant solution enters the system through periodically-spaced pre-defined etchpits. First, we study the evolution of deflection patterns around a single etch-pit. The evolu-tion of wavelength is compared with the master curve of linear channel simulations studiedin chapter 4, showing close correspondence. A semi-analytical method has been developedto describe the transition of channel morphology from edge channels to corner channels as afunction of film geometry, thickness and eigenstrain. Next, we study the channel evolution ina 3 × 3 etch-pit array. The networks are formed in InGaAs films, which have a very strong4 1. Introductioninteraction with the substrate during the etching process unlike the Si-Ge systems as studiedin chapter 4. We show that the presence of film-substrate interaction is essential to accountfor the complex nanochannel networks reported in the experiments.Finally, in chapter 7 we summarize the results obtained in this thesis, draw conclusionsand present an outlook for future developments.

University of Groningen

   University of GroningenBuckling-driven self-formation of microchannelsAnnabattula, Veera Venkata Satya Durga Ratna KumarIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2011Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Annabattula, V. V. S. D. R. K. (2011). Buckling-driven self-formation of microchannels. Groningen: s.n.CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 12-11-2019SAMENVATTINGPatroon- en rimpelvorming op oppervlakken is alomtegenwoordig in de natuur: van mense-lijke huid tot bladeren, vruchten en groenten. Dergelijke patronen zijn ook interessant voorveel moderne technologische toepassingen, zoals in flexibele elektronica, optische roosters,microfluidische devices en lab-on-a-chip systemen. Echter, de fabricage van deze structurenop zeer kleine schaal (orde nano- en micrometer) is een uitdagende taak. Recentelijk is ereen nieuwe techniek ontwikkeld, gebaseerd op het release and bondback mechanisme. Dezemethode bestaat uit drie stappen: (i) het groeien van een dunne film met voorspanning opeen substraat (ii) het relaxeren van de film door het verminderen van de hechtsterkte (doorchemisch etsen of elektrolyse) en tenslotte (iii) bondback, het terugvallen van de vrijgekomenfilm op het substraat als gevolg van de cohesiekracht tussen de film en het substraat.In dit proefschrift is theoretisch onderzoek gedaan naar de vorming van micro- en nano-kanaaltjes op basis van deze release and bondback methode. Er is een nieuwe eindige-elementen-methode ontwikkeld om het proces van delaminatie en bondback te bestuderen.De uiteindelijk gevormde kanaalafmetingen en structuren van verschillende systemen is on-derzocht als functie van de verschillende systeemparameters, zoals laagdikte, laaggrootte,elastische eigenschappen en cohesie-energie tussen de dunne laag en het substraat. Eendimensie-analyse van de systeemvergelijkingen resulteert in een reeks dimensieloze param-eters die het gehele proces van kanaalvorming vastleggen in de vorm van master-krommen.In dit proefschrift zijn vier verschillende systemen onderzocht: twee-dimensionale lijnpa-tronen (hoofdstukken 2 en 3), lineaire kanalen (hoofdstuk 4), polygoon-vormige patronen(hoofdstuk 5) en netwerken van nanokanaaltjes (hoofdstuk 6). De resultaten laten zeer goedeovereenkomsten zien met experimentele kanaalstructuren, waardoor de resultaten gebruiktkunnen worden als design-tool voor toekomstige kanaalsystemen.

https://pure.rug.nl/ws/files/14445620/01c1.pdf

Buckling-driven self-formation of microchannels

Abstract

Similar works

Full text

Available Versions

ARTS repository - University of Groningen

ARTS repository - University of Groningen

University of Groningen