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).

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).