iMechanica - Journal Club Theme of March 2007: Mechanics of Flexible Electronics - Comments

Graphene flexible electronics

jhchen — Wed, 04 Apr 2007 18:15:37 -0400

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

The limitations of AFM/DIC and a new technique

Hongbo Bi — Tue, 03 Apr 2007 14:19:07 -0400

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.

Wrenching pattern of a metal thin film on the polymer substrate

James Wang — Tue, 03 Apr 2007 12:02:56 -0400

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.

Thin Film Electronics on Flexible Substrates

Anand_Pillarisetti — Mon, 02 Apr 2007 12:34:53 -0400

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

Mechanics of thin films

Lei Nie — Mon, 02 Apr 2007 11:23:58 -0400

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.

Evaluating the Performance of Transparent Films

Nathan_Vickey — Mon, 02 Apr 2007 11:06:12 -0400

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.

Elastically Stretchable Electronics

Farbod A. Farahani — Sun, 01 Apr 2007 22:15:01 -0400

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

Alternative fabrication technique

Matthias Irmscher — Sun, 01 Apr 2007 18:21:46 -0400

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.

Also extend the Stoney formula

Ying Li — Thu, 29 Mar 2007 10:06:48 -0400

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 Structures, Volume 44, Issue 6, 15 March 2007, Pages 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 Structures, Volume 44, Issue 6, 15 March 2007, Pages 1755-1767

Fatigue failure mechanism in thin polysilicon films

Teng Li — Tue, 27 Mar 2007 13:01:52 -0400

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.

Delamination of stiff islands patterned on stretchable substrate

Teng Li — Mon, 26 Mar 2007 17:09:48 -0400

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.

Mechanics and new technology

Zhigang Suo — Sat, 24 Mar 2007 12:42:29 -0400

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.

Mechanics is luxury stuff for developing new technology

Henry Tan — Fri, 16 Mar 2007 14:16:33 -0400

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

where is mechanics?

Rui Huang — Fri, 16 Mar 2007 13:46:12 -0400

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

A nice Flexible Electronics course taught in Cornell

Nanshu Lu — Fri, 16 Mar 2007 13:09:49 -0400

MSE 542 Flexible Electronics @ Cornell

Complete lecture notes and handouts are available here.

Journal Club Theme of March 2007: Mechanics of Flexible Electronics

Teng Li — Fri, 02 Mar 2007 23:31:12 -0500

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)