TERAHERTZ PROBING – From sub-nanometers to micrometers

Contactless Terahertz microprobes can measure the sheet resistivity and thickness of large-area conductor films at unprecedented speed and resolution.

Emerging terahertz technology is on course to define the next state-of-the-art for thin-film conductor inspection [1]. A recent example is given by a novel instrument employing miniaturized terahertz near-field detectors [2]. Non-destructive high-resolution inspection of various conduction layers as used in touch-screens, electronic paper, displays, solar cells or OLED devices is efficiently accomplished by this new technology.

Background

While THz radiation can virtually not be transmitted through highly conducting bulk materials it penetrates fairly well through thin conductor layers with a thickness below skin-depth [3]. This property can be used to measure the absolute sheet-resistance and (indirectly) also the thickness of many technically relevant conductor layers under the usually satisfied assumption of constant bulk conductivity. THz radiation is sharing this property with microwave or even longer wavelength radiation. However, high-resolution measurements are much more difficult in these low-frequency regimes and thus have only been available for small measurement areas on the order of 100 µm x 100 µm using atomic-force-microscope-type equipment. Other methods which can be used for full-wafer mapping (like Eddy-current measurements) suffer from very low mm-scale spatial resolution. The THz microprobe-based technology is now enabling micron-scale resolution and high-speed full wafer mapping which has not been possible up to now. This increased performance is supplemented by capabilities to measure layers buried under isolating capping layers and generally contact-free probing.

Measurement results

In Fig. 1 the dependency of THz transmission against layer thickness is shown for some selected conductor materials. The accessible thickness range extends from sub-nanometers for a single layer of graphene to the micrometer range for indium-tin-oxide (ITO). The corresponding sheet resistance range is from sub-Ohm to some k-Ohm per square. For optical measurement systems such a large range of thickness values is usually inaccessible. In case of ellipsometric measurements a pre-knowledge of the approximate thickness value may allow the execution of measurements, but only in a small few-nm-range of thickness deviation.
Fig. 1

Fig. 1: THz transmission amplitude versus layer thickness of selected conductor layers.

In Fig. 2 an exemplary sheet resistance plot measured with a THz microprobe is shown. The investigated sample is a glass wafer covered with TiN and Ti layers of various thicknesses. The measurement speed is up to 5 ms/data-point which is sufficiently high to enable high-resolution full wafer mappings in a few minutes and up to three orders of magnitude higher compared to standard methods like four-point-probing or spectroscopic ellipsometry. The observed sheet resistance values range from 6 Ohm to 400 Ohm per square corresponding to 7 nm – 230 nm of TiN. The well visible radial increase of sheet resistance refers to a thickness decrease of up to 20% caused by a sputtering process inhomogeneity.
Fig. 2

Fig. 2: (Right) wafer-scale mapping of sheet resistance values measured at a glass wafer covered with differently thick TiN and Ti layers. The black area refers to uncovered glass. (Left) Colour-scale plot high-lighting the sheet resistance inhomogeneity within the marked wafer area.

References:
[1] M. Nagel, A. Safiei, S. Sawallich, C. Matheisen, T.-M. Pletzer, A.A. Mewe, N.J.C.M. van der Borg, I. Cesar, H. Kurz, “THz Microprobe System for Contact-Free High-Resolution Sheet-Resistance Imaging,” 28th EU PVSEC conference, 30 September 2013 – 4 October 2013, Paris.

[2] http://www.amo.de/?id=798

[3] M. Walther, D. G. Cooke, C. Sherstan, M. Hajar, M. R. Freeman, and F. A. Hegmann, “Terahertz conductivity of thin gold films at the metal-insulator percolation transition,” Phys. Rev. B 76, 125408 (2007).

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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