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Journal Club Theme of March 2007: Mechanics of Flexible Electronics

Teng Li's picture

Flexible electronics is an emerging technology with an exciting array of applications, ranging from paper-like displays, skin-like smart prosthesis, organic light emitting diodes (OLEDs), to printable solar cells. These potential applications will profoundly impact various facets of our daily life, and excite our curiosity on: what's the future of newspapers and books? Will OLEDs replace light bulbs and fluorescent lamps, and emerge as future lighting source? Can we power electronic devices everywhere cordlessly? Significant progress has been made in the past several years, especially as sizable investments flux in. For example, Polymer Vision just released the first commercial product of rollable display (as shown in the figure) after secured $26M investment in January 2007. The future success of this emerging technology largely relies on:

  • New architecture design and new materials choice, to enable the functionality and improve the reliability of flexible devices.
  • Revolutionary fabrication process, to reduce the manufacturing cost for massive production of flexible devices.

New architecture design and new materials choice
The electronic materials used in the current microelectronics technology (e.g., Si, SiO2, Cu) are inorganic. These inorganic materials are excellent in electrical performance, but are poor in mechanical deformability. The discovery of conductive and semiconductive polymers provokes the enthusiasm to build flexible devices entirely out of organic materials. So far, however, the performance of such organic electronics are still unsatisfactory. For example, the best available conductive polymer is still two orders of magnitude less conductive than typical metals. Therefore, a suitable solution for flexible electronics will be the organic/inorganic hybrid architecture. For example, thin films of indium tin oxide (ITO ) deposited on polyethyleneterephthalate (PET) are commonly used as transparent conductors in flexible display design.
Flexible electronic devices will be subject to large, repeated deformation during manufacturing and use (for example, a cell phone with rollable screen). While organic materials are compliant, and can recover from large strains, most inorganic electronic materials are stiff, and fracture at small strains (typically < 1%). How to use these materials to make electronic devices with reliable deformability under cyclic loadings remains uncertain. Furthermore, the organic/inorganic hybrids exhibit rich mechanical responses under loading, many of which have not been well understood.

Revolutionary fabrication process

Current IC manufacturing is a batch process: one component at a time. Many of the fabrication steps involve the use of chambers in the billion dollar fab facility. Such a manufacture process is not suitable for making flexible electronics. For example, the processing temperature in current fabrication steps is often too high for organic materials; the size of the resulting device is limited by the chamber size, while flexible electronics, such as thin film solar cells, require the distribution of electrical components over large area. Therefore, novel fabrication processes are desirable to manufacture rugged, large area, and flexible electronic devices in a cost-effective and time-efficient way. There has been a surge of interest in developing a roll-to-roll process in which multiple functional layers of inorganic electronic materials are patterned and printed on a plastic substrate, resulting in lightweight, rugged and low-cost devices. Still, many issues need to be addressed, such as layer-to-layer registration, damage due to handling, and adhesion quality. Scientists are also exploring other innovative processes to fabricate flexible electronics through direct growth, or self assembly.

Promising opportunities and great challenges co-exist for the flexible electronics technology. Many of such challenges find their origins in the mechanical response of new architecture made of hybrid materials. More opportunities will emerges as the understanding of such mechanical response advances. The March issue of jClub includes three papers to reflect various aspects of the challenges and opportunities in the emerging field of the mechanics of flexible electronics. The three papers under discussion are:

1. Electromechanical properties of transparent conducting substrates for flexible electronic displays, Cairns, D.R. and Crawford, G.P. Proc. IEEE, 93, 8, 1451- 1458 (2005)

The paper starts with a nice brief introduction to the flexible display technology and then focuses on the electromechanical properties of the flexible anodes (e.g., ITO-coated PET) in flexible displays. The thin coatings of ITO (~100 nm thick) are brittle and crack at a tensile strain of ~2.3%. Of particular interest is Section V of the paper. Under cyclic loading, even when the strain is much lower than the ITO virgin cracking threshold , the brittle ITO films on PET substrates show fatigue fracture behavior. For example, SEM images after 100K cycles clearly show the fatigue cracks in the ITO films (Fig. 8). An open question worth of discussion is that, what is the deformation mechanism of the fatigue of a brittle ITO film on a compliant PET substrate?

2. Calculation of adhesive and cohesive fracture toughness of a thin brittle coating on a polymer substrate, N.E. Jansson, Y. Leterrier, L. Medico and J.-A.E. Manson, Thin Solid Films, 515, 4, 2097-2105 (2006)

A typical inorganic/organic hybrid structure in flexible electronics often consists of a thin film of inorganic materials (e.g. SiNx) on a relatively thick organic substrate (e.g., polymers). The thin film fracture toughness as well as the film/substrate interfacial adhesion are important properties that govern the durability of the film (often the functional layer in a device) under mechanical loads. Determination of these fracture parameters for thin films with a sub-micron thickness is rather challenging for both experiments and modeling. Often the fracture of the film and the interfacial debonding co-evolve during the deformation. The polymer substrate deforms plastically under large deformation, an important effect on the fracture process that is not well studied. In this paper, the fragmentation test and finite element method are combined to simultaneously derive both the adhesive fracture toughness of the interface between a thin brittle coating and a polymer substrate and the cohesive fracture toughness of the thin film coating. A cohesive zone model is used to simulated the debonding process. The approach adopted in this paper may be of interest of many fellow iMechanicians for further discussion.

3. Self-assembled single-crystal silicon circuits on plastic, Stauth, S.A., Parviz, B.A., Proc. Nat. Acad. Sci., 103, 38, 13922 -13927 (2006)

The authors demonstrate a new way to fabricate circuits on a plastic substrate by self-assembly. Thousands of single-crystal silicon transistors and resistors are integrated onto flexible plastic substrates. The micron-scale components self-assemble onto etched channels in the plastic substrates to form circuits. The assembly process is driven by the capillary forces and controlled by using differently shaped components, including circles, triangles, squares, and rectangles, that selectively assemble in matching substrate channels. A mechanics model is set up to examine the role of both capillary and fluidic forces during the self-assembly. The possible interests of discussion can be either the optimization of materials properties and experimental designs to improve the efficiency of this specific assembly process, or other potential fabrication processes enabled by self-assembly.

While some discussion topics are proposed above, we welcome discussions on any aspects of these papers, or the general field of the mechanics of flexible electronics.

(Image credit: Polymer Vision)

Zhigang Suo's picture

A brittle material like ceramics is less susceptible to fatigue crack growth than a ductile material.  It is possible that in this case polymeric substrate may undergo ratcheting deformation under cyclic loading.  A few years ago, in collaboration with a group at Intel, we worked out a particular process of ratcheting-induced stable crack (RISC) (M. Huang, Z. Suo, and Q. Ma, "Plastic ratcheting induced cracks in thin film structures". Journal of the Mechanics and Physics of Solids, 50, 1079-1098, 2002)

To study this phenomenon identified in Paper 1 in this Theme, one might

  • Experimentally determine if the polymer or the interface does undergo ratcheting deformation.
  • How the fatigue crack relate to ratcheting deformation.

Also, from an image of this work, much damage occurs near the crack.  This crack does more than just breaking a plane of atomic bonds.

Teng Li's picture

Zhigang,

Thanks for pointing out the ratcheting paper. 

In Fig. 8 of the Cairns and Crawford paper, the close-up view of the damage along a fatigue crack shows severe microcracking in a stripe of ITO film with a width about 1 micron (tens of film thickness).  No close-up image of the cracks in ITO under large uniaxial tension is available for comparison in this paper.  Figure 1 of the Jansson et al paper shows the cracked SiNx thin film on a polyester substrate under uniaxial tension. The crack surfaces of the SiNx are rather sharp, showing typical brittle fracture behavior.

Are there detail observation of the damages near a RISC? Do they look similar with those in the ITO films? 

Zhigang Suo's picture

Teng:  The detailed morphology of the crack in Fig. 8 of the Cairns and Crawford paper looks different from the RISC in the paper by Huang, Suo and Ma. Although our paper did not contain a high resolution image, I have seen other images from Intel and elsewhere, which clearly showed a clean tensile crack. 

Fig. 8 of the C & C paper, however, looks like the damage has rubbed against each other.  Hopefully some experimentalist can interpret this image.

Rui Huang's picture

Teng:

Thanks for the nice summary. I guess you intentionally left out the works you have been involved in this area with the Princeton Group led by Sigurd Wagner. Also worthy of mentioning may be recent works on stretchable Si and other semiconductors by John Rogers' group at University of Illinois, in collaboration with Young Huang. Both Wagner and Rogers are non-mechanicians. By working with groups of mechanicians, their works are quite inspiring. This might be the way how mechanicians would contribute to this exciting area.

RH

Henry Tan's picture

Dear Rui,

Can you provide summarize of their work, or some links to their publications?

Thanks.

Henry.

Teng Li's picture

Dear Rui and Henry,

Thanks for bringing this up. Both Wagner's group and Rogers' group are pioneering in the research field of flexible macroelectronics. Both groups collaborate closely with mechanicians on the mechanical issues related to the flexible macroelectronics.

Zhigang Suo has been collaborating with Wagner's group since 1997. I got involved in this collaboration since 2002. The research efforts we've been focusing on include:

(1) the deformability of thin metal films on polymer substrates, a representative inorganic/organic hybrid structure in flexible electronics. The related publications are available here, here and here. You may also find some discussions on this topic in iMechanica helpful.

(2) a general principle to make thin film of stiff materials compliant by suitably patterning. Such compliant patterns can serve as general platforms for flexible electronics. For example, by fabricating the whole circuit on such a platform, the resulting device can sustain large, cyclic deformation while remaining electrical functions. The related publications include this and this.

Rogers' groups at UIUC has been collaborating with Young Huang's group on stretchable Si and other semiconductors. Thin semiconductor films are fabricated on pre-stretched elastomer substrates. Upon releasing the prestretch, the stiff films buckle up. Under tension, such buckled films flatten to accommodate the elongation, the resulting strain in the films is mainly due to bending, thus is small. The related publications are available here and here.

Hope this helpful.

-Teng

Henry Tan's picture

Teng,

What’s the bonding law between the metal film and the polymer substrate in your study published in the paper

Li T, Huang ZY, Xi ZC, Lacour SP, Wagner S, Suo Z (2005) Delocalizing strain in a thin metal film on a polymer substrate, Mech. Mater. 37: 261-273.

Thanks,
Henry

Teng Li's picture

Henry,

In the Mech. Mater paper,  the metal film and the polymer substrate were assumed to be perfectly bonded. The study was focused on the constraint of the substrate to the strain localization in the metal film.  The debonding at the interface was pre-introduced in the simulation model and no further propagation of the debonding was considered.  

In real structures the film/substrate interface is never perfect. Under a modest tensile strain, debonding may initiate and propagate as strain localization (e.g., necking) occurs in the film. The debonded part of the film becomes freestanding, thus easy to rupture.  Therefore, the deformability of the thin metal film on the polymer substrate is also sensitive to its adhesion to the substrate.  This has been observed in a recent experiment.  To further quantify the effect of the interfacial adhesion, we further modeled the co-evolution of the interfacial debonding and the film necking. We found that the film necking is mainly accomodated by the interfacial sliding, rather than interfacial opening.  Consequently, a high interfacial shear strength is critical to achieve better ductility of a metal film on a polymer substrate. In this simulation, the interface was modeled by cohesive zone elements.  A simple triangle shape of cohesive law was used. Other cohesive law (i.e., Xu-Needleman model) was also used, which led to the similiar results. This study has been reported in the following paper:

Ductility of thin metal films on polymer substrates modulated by interfacial adhesion, Teng Li and Z. Suo, International Journal of Solids and Structures, 44, 1696-1705 (2007).  

-Teng 

Henry Tan's picture

Teng,

Seems you didn't mention the work of Nanshu Lu, a PhD student in Suo’s group.

She is studying the size effect of stiff islands in strechable electronics (http://imechanica.org/node/149#comment-1737). Any new progress coming along, any future plan?

One of their recent paper, first authored by Dr. Juil Yoon, is interesting.
The Effect of coating in increasing the critical size of islands on a compliant substrate. http://imechanica.org/node/1040

Teng Li's picture

Nanshu's recent work on the delamination of stiff islands patterned on stretchable substrate is available in a separate post here. Of interest is also the instructive discussion following her post.

Henry Tan's picture

Teng,

Viscosity of the polymeric substrate was not considered in you modeling.

What kind of difference would you expect to see if that is considered?

Henry

Zhigang Suo's picture

Dear Henry: 

In collaboration with Rui Huang and several other colleagues, we have studied the effect of viscoelasticity of a polymer substrate on the cracking in a brittle film. 

The main effect is that the time-dependent deformation in the substrate gradually reduces the constraint of the substrate on the crack opening in the film.  Here is one of the papers which contains a review of our earlier papers: 

Z. Suo, J.H. Prévost, J. Liang, Kinetics of Crack Initiation and Growth in Organic-Containing Integrated Structures, .Journal of the Mechanics and Physics of Solids, 51, 2169-2190 (2003).

It is likely that the viscoelasticity will have a similar effect on the necking of a metal film, but I have not seen an analysis on the effect yet. 

Henry Tan's picture

Say if I close a flexible displayer slowly, the stretchability  may be increased; and if quickly, the stretchability  may be decreased.

Teng Li's picture

Henry: 

Let me further elaborate Zhigang's argument. The conjecture is, over the time, the constraint of the substrate to the film necking reduces due to the creeping relaxation of the substrate.  When such a constraint is too weak, deformation localization (e.g., necking) is more likely to occur in the film.  As a result, the deformability of a thin metal film on a viscoelastic substrate may decrease over the time.  The time scale of the decay in deformability is, of course, related to the viscosity of the substrate.

Above said, what could possibly happen may be the other way rather than what you said.  But it's likely that the time scale of the decay in the deformability is so long that you won't experience an appreciable change when you close a flexible display slowly or quickly.

-Teng 

Rui Huang's picture

As pointed out by a few comments above, the compliance and viscoelasticity of polymer substrates lead to some interesting mechanics of thin films, with implications for the design and reliability of flexible electronics. I have been involved in and aware of some recent studies in this area, and dare to give a brief summary as follows.

(1) Under tension, the necking of an elastic-plastic ductile film (e.g. metal) bonded to a polymer substrate is constrained. The level of constraint depends on the elastic compliance of the substrate and the interfacial adhesion, which has been studied by Teng Li , Zhigang Suo , and their collaborators. As mentioned by Zhigang above, the effect of substrate viscoelasticity on the necking and rupture of metal films has not been analyzed, at least to my knowledge.

(2) Also under tension, an elastic, brittle thin film (e.g., Si or SiN) may fracture by forming channeling cracks (parallel or random mud cracks, depending on the stress state). The effect of substrate constraint on channel cracking has been extensively studied, including elastic substrates, elastic-plastic substrates, and viscous/viscoelastic substrates. The effect of interfacial adhesion on channel cracking is currently under investigation and will be reported soon.

(3) Under compression, buckling of thin films (including metal, polymer, SiGe, DLC, etc.) have been observed. On a hard/stiff substrate, blisters of buckle delamination occur. On a soft/compliant substrate, wrinkling occurs. The effects of substrate compliance on both buckle-delamination and wrinkling have been examined. A recent study has also unified the two buckling phenomena to give a criterion for the selection of bucking modes. The effects of viscous/viscoelastic substrates on wrinkling have also been analyzed. I am not aware of any studies on buckle-delamination with viscous or viscoelastic substrates.

(4) Under cyclic loading (e.g., fatigue tests, thermal cycling), with a elastic-plastic substrate, ratcheting induces cracking under tension and wrinkling under compression. Some simple models have been proposed for RISC (ratcheting-induced stable cracks) and RIW (ratcheting-induced wrinkling).

If interested, I would provide references to the above-mentioned studies in separate posts.

RH

Juil Yoon's picture

Hello Rui.

Thank you for your summary.

Now I have a interest about viscoelasticity of polymer. Can you give me those references which you mentioned above?

Thank you

Juil

Rui Huang's picture

Juil: thanks for your interest. Here is my list.

For brittle fracture under tension:

For wrinkling under compression:

 

RH

Hanqing Jiang's picture

Young Huang started the collaboration with Rogers’ group in 2004 on the mechanics issues on photolithography which persists today. They are primarily interacting on the following two focuses:

(1) Mechanics issues associated with photolithography, including stamp collapse in soft lithography, transfer printing on patterned soft materials. They found a scaling law that controls the unwanted stamp collapse in many micro/nano-fabrication applications, such as micro-fluidic channel made of soft materials. Transfer printing method provides robust capabilities for generating microstructured hybrid materials systems and device arrays with the applications in optoelectronics, photonics, non-planar fabrication and biotechnology. Their publications are:

a) Huang YGY, Zhou WX, Hsia KJ, et al., Stamp collapse in soft lithography, LANGMUIR 21 (17): 8058-8068 AUG 16 2005;

b) Hsia KJ, Huang Y, Menard E, et al., Collapse of stamps for soft lithography due to interfacial adhesion, APPLIED PHYSICS LETTERS 86 (15): Art. No. 154106 APR 11 2005;

c) Zhou W, Huang Y, Menard E, et al., Mechanism for stamp collapse in soft lithography, APPLIED PHYSICS LETTERS 87 (25): Art. No. 251925 DEC 19 2005;

d) Meitl MA, Zhu ZT, Kumar V, et al., Transfer printing by kinetic control of adhesion to an elastomeric stamp, NATURE MATERIALS 5 (1): 33-38 JAN 2006.

Dr. Xue Feng and Mr. Weixing Zhou got involved.

(2) Mechanics analysis of stretchable electronics. They started this project since 2005 summer. As introduced by Teng, the basic idea to achieve the stretchability of brittle materials, such as Si and GaAs, is to buckle the brittle thin films by relaxing the pre-stretching of compliant substrate, usually PDMS, which leads to periodic wavy structures of thin films. By changing the wavelength and amplitude, the wavy thin films can accommodate large tensile and compressive strain, thus avoid fracture the thin film and realize the stretchability. A new patterned PDMS substrate was introduced recently to generate a controlled buckling of thin films. This controlled buckling can extremely improve the stretchability, up to 70% for GaAs whose fracture strain is only 1%. The main mechanics issue here is how to relate the strains (pre-strain and applied strain) to the buckling profile (wavelength and amplitude), and how to optimize the shape of the patterned PDMS to improve the stretchability. The publications are already pointed out by Teng. I got involved in at the beginning of this project. After I left UIUC and joined ASU, other two students may work on this project now.

Teng Li's picture

Hanqing,

Thank you for the nice summary of Young Huang's research in this field. 

Teng 

Henry Tan's picture

Hanqing:

The links you provided can only be accessible from Arizona State University.

Ying Li's picture

In Jan., I attended a lecture given by Huang at Tsinghua.  He summarized his works on extending the Stoney Formula. It is very interesting to find that the Stoney Formula is also validate for no uniform thin films.

1)      Thin film/substrate systems featuring arbitrary film thickness and misfit strain distributions. Part I: Analysis for obtaining film stress from non-local curvature information  International Journal of Solids and StructuresVolume 44, Issue 615 March 2007Pages 1745-1754

2)      Thin film/substrate systems featuring arbitrary film thickness and misfit strain distributions. Part II: Experimental validation of the non-local stress/curvature relations     International Journal of Solids and StructuresVolume 44, Issue 615 March 2007Pages 1755-1767

Henry Tan's picture

Are there any military of anti-terrorist applications for flexible electronics?

Come on, give some hints.

Teng Li's picture

One possible application is the large area, foldable antenna that can be used for soldiers in the battle field.  Another application that are being more and more widely used is the flexible radio frequency idenfications (RFIDs). RFIDs can be used for many purposes, such as product tracking, patient identification, e-passport, etc.  In world cup 2006, all 3.7 million tickets were tagged with RFIDs

-Teng

 

Henry Tan's picture

How capillary forces control the adhesion between the metal film and the polymer substrate?

Teng Li's picture

Henry,

Not quite sure what you're asking. Are you talking about the capillary force at the interface between a solid film and a viscous substrate?  For a metal film on a plastic substrate, the effect of such capillary forces on adhesion might be negligible. 

-Teng 

It seems to me that we should include graphene, given its compliance, as a reasonable candidate material for flexible electronics.

Just to toss that on the table.

Teng Li's picture

Dear Rod,

Thanks for bringing this up. Actually there have been efforts to use carbon nanotube mats for conductive transparent coatings, transparent thin-film transistors, etc. I'm not aware of any work using graphene as candidate materials for flexible electronics, and I'll be happy if someone can point out some if available.

-Teng 

jhchen's picture

Hi Dr. Li,

This is quite true that graphene could be made into transparent/flexible conductive coating and TFTs. There will be a paper coming out pretty soon reporting single layer graphene device made on PET substrate. I will update the reference when it is ready.

The problem for graphene-based electrode is that it has a special density of states which is not metallic, and the problem for graphene transistor is that it could not be completely turned off. People are already tackling the later problem by basically band gap engineering (see Zhihong Chen et al.,http://lanl.arxiv.org/abs/cond-mat/0701599 and M. Han et al., http://lanl.arxiv.org/abs/cond-mat/0702511), and there will probably be other solutions coming out soon.

For Carbon Nanotube Mat transparent electrode, the best result I know is 87% transparency with 160 ohm/sq, by Daihua Zhang et al., Nano. Lett. 6, 1880(2006), comparing with ITO(10 ohm.sq with >80% transparency or >100 ohm/sq with 90% transparency). It is getting closer.

 

-jianhao

Henry Tan's picture

Graphene sheet is stable, highly flexible and strong and remarkably conductive (http://imechanica.org/node/998).

Maybe it can be a better candidate to replace metal film on the polymer substrate.

Juil Yoon's picture

Teng:

Thanks for the nice summary.

As you know, the thin-film organic/inorganic composite barrier layers have been reported to achieve a certain water vapor permeation rates.

Our recent paper calculates the critical strains for various configurations of channel cracks in a coating consisting of organic and inorganic layers. We show that the coating can sustain the largest strain when the organic layer is of some intermediate thicknesses.

Teng, do you have any other idea about permeation problem in flexible electronics?

Thank you in advance.

Nanshu Lu's picture

MSE 542 Flexible Electronics @ Cornell 

Complete lecture notes and handouts are available here.

Rui Huang's picture

Nice notes. But I don't see any coverage on mechanics issues. It seems strange when you talk about anything "flexible" without mentioning mechanics. 

RH

Henry Tan's picture

Engineers can develop new technology without knowing mechanics.Mechanics is luxury stuff for developing new technology.

Zhigang Suo's picture

Thermodynamics benefited more from steam engines than steam engines benefited from thermodynamics.

I saw a statement like this a long time ago, but cannot recall who said it.  I suppose science and technology will always have this uneasy relationship. 

While the early development of steam engines did play significant roles in creating thermodynamics, we are still delighted that thermodynamics was created, and find its applications far away from steam engines.

Another way to picture this.  Suppose steam engines did not lead to thermodynamics, it would be hard to imagine how lessons learned in developing steam engines would shed light on, say, fluctuation of a DNA.  Steam engines would be just another invention, functional and important, but rather dull by now.

Flexible electronics does pose mechanics problems not well understood.  While it is rare that a technology leads to a subject as profound and as fun as thermodynamics, we might as well enjoy doing what we are good at, and strive for insight that helps advancing the technology.  We might even hope that what we learn may transcend the immediate applications.  We will never know unless we try.  

Teng Li's picture

Although bulk silicon is immune to fatigue failure,  thin micron-scale films of silicon apprear to be highly susceptible.  Ritchie et al showed that the fatigue in such silicon thin films is due to subcritical cracking within the oxide layer on the films. High stresses induce a thickening of the post-release oxide at stress concentrations such as notches, which subsequently undergoes moisture-assisted cracking.  Related publication is available here.

These polysilicon films were freestanding, and failed after 10^6+ cycles. The ITO films in Cairns and Crawford paper were bonded to polymer substrates and failed within <10^5 cycles. The proposed fatigue mechanism for thin silicon films may not find its counterpart for the ITO films on polymer, but is included here for reference and any possible discussion.

I'd like to point out an alternative way of fabricating compliant electrodes as described in the following paper:

R. Delille, M. Urdaneta, K. Hsieh and E. Smela, "Compliant electrodes based on platinum salt reduction in a urethane matrix," Smart Materials and Structures, vol. 16, pp. 272-279, 2007.

Weblink: http://www.iop.org/EJ/abstract/0964-1726/16/2/S11/

The authors demonstrate the fabrication of a compliant electrode by mixing a platinum salt into a UV-curable polymer. The polymer/salt-mixture can be photopatterned because the salt is transparent while the metal is not. Finally, the salt is chemically reduced by immersing the structure in sodium borohydride. As a result, a conductive platinum film is formed on the surface of the polymer substrate. The Young's modulus of the composite structure was reported to be 10 MPa while the conductivity had maximum values of nearly 1 S/cm. Strains of up to 30 % were reported.

The fabrication method is not close to being mature yet and certainly requires a lot of additional work to reach a stage where it could be employed for flexible macroelectronics. However, the fact that the electric leads can be photopatterned could prove to be beneficial as it circumvents the traditional way of deposition and patterning and can be carried out without the use of expensive cleanroom facilities.

Farbod A. Farahani's picture

I’d like share this paper that I just read on fabrication of integrated circuits on electrometric substrates. Such circuits will stretch and relax reversibly, similarly to the human skin.

 

This paper demonstrates all components required for an elastically stretchable active matrix backplane of amorphous silicon thin film transistors. It is also shown that discrete transistors made on an elastomeric substrate can be integrated to a circuit by interconnecting with elastically stretchable metallization.

Stéphanie P. Lacour and Sigurd Wagner, "Thin Film Transistor Circuits Integrated onto Elastomeric Substrates for Elastically Stretchable Electronics",  http://ieeexplore.ieee.org/iel5/10701/33791/01609278.pdf

Anand_Pillarisetti's picture

I came across an article, which might be of interest to "imechanica" community. The article summarizes the current knowledge of the mechanics of thin film electronics on flexible substrates.

The paper focuses on the mechanical response of amorphous silicon thin-film transistors and solar cells subjected to externally applied tensile strain. Under compressive strains, there is no appreciable difference in their response. The a-Si:H TFT's does not undergo mechanical failure when subjected to compressive strain of up to 2%, but crack formation starts at a tensile strain of 0.3%. Between 0.3% and 0.5% tensile strain, the TFT fails but the electrical function is restored when the strain is eliminated. Beyond 0.5% tensile strain, the crack becomes permanent and mechanical failure occurs. For amorphous silicon germanium solar cells, there is no change in the electrical performance for tensile strain of up to 0.7% and compressive strains of up to 1.7%. For tensile strain larger than 0.7%, a decrease in electrical performance occurred. Surprisingly even at tensile strain of 2%, the solar cell efficiency was maintained at 50% of the original value.

The authors also point out that the change in radius of curvature of the flexible device due to mismatch strain in the device structure should be minimized through out the fabrication process. The mismatch strain is caused due to thermal mismatch strain between the metal film and substrate as well as the built in strain of the metal film. The possible solutions to reduce undesired curving are: (i) by compensating the CTE mismatch with built in strain, (ii) by attaching the flexible substrate to a rigid carrier for the duration of the fabrication and (iii) by patterning of continuous layers into islands that relieves the global mismatch strain.

Helena Gleskova, I-Chun Cheng, Sigurd Wagner, James C. Sturm, Zhigang Suo, “Mechanics of thin film transistors and solar cells on flexible substrates”, Solar Energy, vol 80, pp. 687-693, 2006.

Weblink: http://linkinghub.elsevier.com/retrieve/pii/S0038092X05003683  

Nathan_Vickey's picture

A method to evaluate mechanical performance of thintransparent films for flexible displays, Sonia Grego, Jay Lewis, Erik Vick, Dorota Temple,  Thin Solid Films, Volume 515, Issue 11 , 9 April 2007, Pages 4745-4752 

In relation to discussion topic one, I would like to point out this paper.  It may be useful to those interested in the technology of organic light emitting diodes on flexible substrates.  The operating conditions for OLEDS are very stringent and are especially sensitive to moisture permeation.  As a result, effective transparent barrier layers must be developed to protect the OLEDS from outside moisture and contaminants.  However these barrier layers may crack and allow moisture to permeate into the device thereby rendering it useless.  The authors of this paper present a technique for the rapid optical evaluation of the performance of these barrier layers using a conventional dry etching process as a basis for highlighting cracking.  The technique is demonstrated for both single-layer barrier layers and multilayer organic/inorganic structures.

Lei Nie's picture

I quite agree that Mechanics is very powerful and necessary for us to understand the new technology. However we have to admit that we can not simply use the current knowledge of Mechanics to solve the new problems in the new technology.In another word, the new challenge in the new research field can help us understand the Mechanics more by another view/method.

I'd like to point out an paper about the Mechanics of thin films:

Chasiotis, I. Mechanics of thin films and microdevices, Device and Materials Reliability, IEEE Transactions on On page(s): 176- 188, Volume: 4, Issue: 2, June 2004

In Chasiotis's paper, he introduced an integrated AFM/DIC (Atomic Force Microscopy/Digital Image Correlation) technique to directly and full-filed measure Young's modulus, Poisson's ration, and fracture toughness from micron-sized specimens. By this technique, he indicated that the mechanical reliability of thin films is dictated by the fabrication paremeters (deposition, DRIE, sacrifical etching) and the device geometry (dimensions, stress concentrations, surface conditions).

I think this paper is helpful for students to have a rough idea about the Mechanics of thin films.

Hongbo Bi's picture

Thanks Lei for pointing this out. It is a really hot topic of measuring the mechanical properties of new materials or new architecture in flexible electronics. This information can be used to guide the design and materials selection. Digital Image Correlation is a strong tool to do this job. However, conventional DIC is designed of using extremely high signal/noise ratio optical digital camera which can provide almost no noise and stable intensity of background in the digital images. When DIC is extended into micron or beyond scale measurement using AFM as imaging system, it encounters some new challenges. During imaging, the AFM tip is only about 10 Å away from the specimen surface so the contact between tip and surface can occur. This produces random noises in the images before and after loading; and it may induce force on thin film surface that results in inaccuracy of mechanical property measurement. Drifting associated with the piezoelectric material used in the AFM also contributes to the random noise generating and image distortion that is not acceptable in the measurement. To cope with this problem, a hybrid method is proposed to achieve the goal. The method uses regularly oriented nano-scale structures that are fabricated on the surface of the specimen. After obtaining the SEM pictures of patterns on the region of interest before and after loading (deformation), the conventional low-pass filter combined with a de-blur filter (Wiener Filter) are applied to eliminate the noise during SEM imaging effectively. The fundamental concepts of pattern recognition and correlation are subsequently employed to determine the deformation field that will eventually convert to the mechanical properties measurement. A link here about N-PRCT describes the basic idea of this technique.

James Wang's picture

In the class, intrinsic surface buckling pattern of a metal thin film on the polymer substrate is taught. In the paper, 'Wrenching of a metal thin film on a structured polyemer substrate', the wrenching pattern of the film due to the annealing is studied. The author considered the interaction between the wrenching wave of corrugated structures and intrinsic buckling wave, and showed the influence of corrugation geometries, annealing temperature, material properties on the wrenching patterns. The results are meaningful in the control of surface wrinkling in the multi-layer thin films processing.

Since the modeling calculates the pattern with the pre-given polymer substrate modulus and the pattern is found to be closely related to this parameter, it is not very clear to use what modulus value in the calculation for polymer modulus is highly temperature dependent especially at high temperature as will happen during the annealing. Also, the modulus will change dramatically with the time at high temperature, evolution of the pattern could happen with the time. More rigorous modeling instead of the pure elastic modeling is needed in order to improve.

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