EMERGING TRANSPARENT CONDUCTORS – Advanced measurement tools needed

High-resolution sheet resistance image

Fig. 1: High-resolution sheet resistance image acquired at a metal mesh covered glass wafer fabricated through Rolith, Inc..

Following recent economic predictions the market for transparent conductor technologies excluding the yet standard material indium tin oxide (ITO) will see a significant increase from $206 million in 2013 to $4 billion by 2020 [1]. The applications requiring transparent conductors are manifold ranging from touch sensors, displays, lighting, thin-film solar cells to smart windows and others. Since ITO has deficits in terms of cost, mechanical flexibility and sheet resistance many companies like Atmel, Fujifilm, Cambrios, Rolith, Unipixel as well as research institutes are working on (and have already brought to market) alternative solutions such as graphene layers, silver nanowire dispersions or metal mesh nanostructures. Especially the latter approach appears to be very promising by offering superior conductivity, transparency and flexibility.However, the efficient development of non-ITO technologies also relies on the availability of powerful analysis tools. High-resolution measurements of sheet resistance distributions on large-scale areas, for example, have been a major problem so far and emerging transparent conductor technologies have even raised the analytic requirements. Metal mesh structures – typically consisting of sub-µm-wide wires – require time-consuming and destructive application of sufficiently large contact pad structures to enable contact based measurements. Existing contactless methods on the other hand (e.g. Eddy current based systems) are limited to only mm-scale spatial resolution – too low to visualize any local defects or inhomogeneity.

A new measurement tool recently developed by AMO GmbH, Germany, employing THz radiation in combination with the highly-resolving contactless microprobes (TeraSpike) [2] represents an important step towards the elimination of this lack. Offering quantitative sheet resistance measurements with up to 10 µm resolution the system has been applied recently to structured graphene layers from the Korean manufacturer Samsung Techwin [3]. Now the system has been used to demonstrate the prime performance of the metal mesh technology available from the Californian start-up company Rolith. Metal structures were fabricated in the form of submicron-width nanowires completely invisible to the human eye, lithographically placed in a regular 2-dimentional grid pattern with a period of tens of microns and thickness of a few hundreds of nanometers [4].

The highly detailed sheet resistance image (Fig. 1) acquired with the new THz microprobe scanner system from AMO using a scanning speed of 5 ms/Pixel reveals the low resistivity (<14 Ohm/☐) of the metal mesh fabricated by Rolith. Together with a transparency of >94% and very low haze (~2%) the manufacturer now considers his “technology above all major competition for ITO-alternative technologies” [4].

[1] http://touchdisplayresearch.com/?page_id=358

[2] https://www.amo.de/thz_tip.0.html?&L=1&L=2

[3] http://amoprotemics.wordpress.com/2013/04/19/terahertz-microprobes-efficiently-fostering-the-development-of-graphene-based-touch-screen-displays/

[4] http://www.rolith.com/press-releases/rolith-inc-demonstrates-superior-performance-of-ito-alternative-transparent-metal-grid-electrodes-fabricated-using-proprietary-nanolithography-technology

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TERAHERTZ MICROPROBES: Efficiently fostering the development of Graphene-based touch-screen displays

 

Touch-screen displays are still among the most expensive parts in a mobile device. The replacement of indium tin oxide (ITO) which is used as optically transparent conduction layer by Graphene is widely considered as a promising route to lower the costs of this component. Being better suited for future flexible touch-screens is a further advantage of Graphene against ITO. However, a hitherto existing problem is given by the lack of suitable measurement tools for the quality and process analysis of large-scale graphene layers. Photoconductive (PC) Terahertz microprobes developed at AMO GmbH (Aachen, Germany) have now proved as a key component for contact-free high-resolution mapping of graphene layer sheet conductance [1]. In comparison to standard contact-based four-point probing up to 1000-fold increased measurement speed (5 ms/pixel) has already been achieved.

THz microprobe and graphene conductivity measurement data

Fig. 1: (a) THz microprobe tip-structure (b) Measurement data example taken from a sheet conductance sample study conducted for Samsung Techwin, South Korea. The plot is showing the sheet conductivity of a structured Graphene layer.

The microprobes are used as THz near-field detectors triggered by femtosecond laser pulses for the spatially resolved mapping of pulsed THz radiation transmitted through the device under test. Absolute sheet conductivity values can be directly determined from the obtained transmission data.

In contrast to earlier diffraction-limited THz transmission systems [2, 3] – the new microprobe-equipped system is able to achieve substantially higher (deep sub-wavelength) resolution as required for the inspection of the typically micro-structured graphene layers in touch screen devices. At the same time the measurement area size can be freely selected from small cut-outs to full display mappings.
In addition to sheet conductivity mappings the microprobes are also used for further analytic applications on active graphene-based devices as they have already been applied for THz emission measurements at optically excited graphite and graphene samples [4].

[1] http://www.amo.de/thz_tip.0.html?&L=1&L=2
[2] J. L. Tomaino et al. , “Terahertz imaging and spectroscopy of large-area single-layer graphene,“ Opt. Express, 19, 1, 141-146 (2011)
[3] J. D. Buron, D. H. Petersen, P. Bøggild, D. G. Cooke, M. Hilke, J. Sun, E. Whiteway, P. F. Nielsen, O. Hansen, A. Yurgens, and P. U. Jepsen, “Graphene Conductance Uniformity Mapping,” Nano Lett. 12 (10), 5074–5081 (2012).
[4] M. Nagel, A. Michalski, T. Botzem, and H. Kurz, “Near-field investigation of THz surface-wave emission from optically excited graphite flakes,“ Opt. Express 19 (5), 4667-4672 (2011).