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EETE NOV 2014

Fraunhofer ENAS shrinks 2D e-compasses By Julien Happich Offering an order of magnitude better sensitivity than Hall-based magnetic field sensors while about a hundred times smaller, the monolithically integrated 2D Spin-valve sensor developed at Fraunhofer ENAS by Dr. Olaf Ueberschär’s research team has the potential to displace numerous existing compass technologies. Spin valves (SV) consist of two ferromagnetic layers separated by a non-magnetic conducting spacer – see figure 1 and their field sensing principle relies on a so-called giant magneto-resistance effect (GMR). The magnetization of one of the ferromagnets is fixed and serves as a reference layer via exchange coupling with an anti-ferromagnetic layer, while the other ferromagnet is allowed to respond to an external magnetic field (free layer). A giant magneto-resistance effect (GMR) originates from the asymmetry in the spin-dependent scattering of electrons at the magnetic/nonmagnetic interfaces for spin-up and spindown electrons. This effect leads to a state Fig. 1: Spin-valve multilayer stack with a coupling to an antiferromagnet. of maximum resistance for an antiparallel alignment of the two ferromagnetic layer magnetizations, and to a state of minimum resistance in the case of parallel alignment. For a maximum signal-to-noise ratio, the researchers have crafted, layer by layer a double full-bridge layout with an antiparallel alignment of the pinned layer magnetization for neighbouring meander structures – see figure 2. Even producing the GMR sensors at wafer level, they were able to precisely tune the magnetic alignment of the ferromagnetic layers of these multi-layered meander resistors through localized laser heating and in-field cooling (at a microscopic resolution). The full design only measures 0.8x0.5mm, and because the sensing material layers are only a few nanometres thick and Fig. 2: Optical microscopy image of the double full-bridge sensor showing the alignment of the exchange bias defined by laser heating and in-field cooling for each individual meander (yellow arrows). the laser heating so shallow, the 2D monolithic compass is fully CMOS- compatible, that is, the sub-micrometre thick sensing meanders could be manufactured as a post-processing step on top of another functional IC, claims Ueberschär. In effect, you could probably add magnetic sensing on top of the chip processing the signal for gyroscope and accelerometer MEMS. The design enables full 360° sinusoidal signal resolution in the geomagnetic field, with an output signal of around 250μV for 50μT and a very high spatial and temporal resolution (1mm and 1ms respectively). “This 2D monolithic compass integration could not only be very cost-effective, low power and better performing for navigation or industrial sensing applications”, told us Dr. Olaf Ueberschär during a demo at SEMICON Europa, “but it could yield other interesting applications such as the magnetic field camera for which we have a patent pending”, he added. “You could build a large array of 2D spin-valve sensors to operate as pixels in a magnetic field camera, to check the homogeneity of magnets, or to determine the magnetic field of a machine part and highlight invisible defects”, Ueberschär continued. What’s more, this non-destructive observation technique would be completely biocompatible for use in medical applications. Ueberschär and his team are hard-working on developing such a camera and hope to have a first prototype ready in the spring of 2015. Cree patent integrates LED and OLED technologies By Paul Buckley Cre’s patent aplication which integrates LED with OLED technology to generate white light has been approved by the United States Patent and Trademark Office (USPTO). The hybrid LED-OLED patent application could be an early sign of Cree’s intention to change the company’s product strategy. The patent application describes a solid state lighting system and or a luminaire that can generate white light by combining OLED lights with another solid state emitter, for instance a conventional LED. The OLED and other emitters are arranged spaced apart in a mixing chamber of the luminaire to minimize color hot spots that can be found in typical LED emitters when using two different colors in close proximity in a luminaire. Cree offers four types of examples for how the proposed hybrid OLED-LED technology would function. In the first case the solid state emitter is a conventional LED packaged with phosphor to emit blue-shifted yellow light (BSY) as the first colour, while the OLED is operable to emit red light as the second colour of light. The two are combined to create white light with a Colour Rendering Index (CRI) of 90. In some cases, the diffuser is placed adjacent to the substantially transparent substrate. In the second example, the solid state emitters that emit the first colour of light are phosphor free LEDs that are used in combination with a remote phosphor. The remote phosphor is placed near the OLED substrate. LEDs that emit the first colour of light might be blue light, and the remote phosphor is used in combination with the LEDs to produce BSY light mixed with red light from OLED at the opening of the mixing chamber. 18 Electronic Engineering Times Europe November 2014 www.electronics-eetimes.com


EETE NOV 2014
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