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 . A recent example is given by a novel instrument employing miniaturized terahertz near-field detectors . 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.
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 . 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.