S.X. Wang

Stanford University, Palo Alto, CA, United States

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Publications (95)197.1 Total impact

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    ABSTRACT: Magnetic nanotechnologies have shown significant potential in several areas of nanomedicine such as imaging, therapeutics, and early disease detection. Giant magnetoresistive spin-valve (GMR SV) sensors coupled with magnetic nanotags (MNTs) possess great promise as ultra-sensitive biosensors for diagnostics. We report an integrated sensor interface for an array of 256 GMR SV biosensors designed in 0.18 µm CMOS. Arranged like an imager, each of the 16 column level readout channels contains an analog front-end and a compact ΣΔ modulator (0.054 mm$^{2}$) with 84 dB of dynamic range and an input referred noise of 49 nT/$surd$Hz. Performance is demonstrated through detection of an ovarian cancer biomarker, secretory leukocyte peptidase inhibitor (SLPI), spiked at concentrations as low as 10 fM. This system is designed as a replacement for optical protein microarrays while also providing real-time kinetics monitoring.
    IEEE Journal of Solid-State Circuits 05/2013; 48(5):1290-1301. DOI:10.1109/JSSC.2013.2245058 · 3.11 Impact Factor
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    ABSTRACT: Resonances are observed in the transmission between two coplanar waveguides coupled by ferromagnetic Co90Ta5Zr5 tubes that wrap around their shorted ends. The resonances are assigned to the magnetostatic surface waves that counter propagate along the tube perimeter. We use a model based on an infinite ferromagnetic tube, with elliptical cross section of roughly the same dimensions as the studied structure. Additional theoretical analysis of the fundamental precession mode observed in experiment is carried out. Periodic boundary conditions dictated by the tube perimeter and applied to magnetostatic surface waves quantitatively account for the experimentally observed bandwidth of excited modes, despite the contorted tubular shape. The tubular topology appears to be more important than the shape details.
    Journal of Applied Physics 01/2012; 111(1):013905-013905-6. DOI:10.1063/1.3672835 · 2.19 Impact Factor
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    ABSTRACT: We have characterized the magnetic, mechanical, and optical properties of SU-8 polymer with embedded nickel nanoparticles (SU8-Ni) of concentrations ranging from 0% to 12.5% Ni by weight. Magnetic characterization was performed using a micropolysilicon torsional actuator as well as via alternating gradient magnetometry. Mechanical properties were measured using nanoindentation, and optical measurements were acquired via spectrophotometry. This magnetic polymer offers several advantages as a micromechanical structural material, including biocompatibility, chemical resistance, thermal stability, low cost, and mechanical compliance for large deflections. It is suitable for fluidics and biomedical applications where remote low-power actuation is desired.
    Journal of Microelectromechanical Systems 03/2011; 20(1-20):65 - 72. DOI:10.1109/JMEMS.2010.2093560 · 1.92 Impact Factor
  • C. Fery · W.E. Bailey · K. Yamada · S.X. Wang
    MRS Online Proceeding Library 01/2011; 569. DOI:10.1557/PROC-569-185
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    ABSTRACT: Micron scale ferromagnetic tubes placed on the ends of ferromagnetic CoTaZr spin waveguides are explored in order to enhance the excitation of Backward Volume Magnetostatic Spin Waves. The tubes produce a closed magnetic circuit about the signal line of the coplanar waveguide and are, at the same time, magnetically contiguous with the spin waveguide. This results in a 10 fold increase in spin wave amplitude. However, the tube geometry distorts the magnetic field near the spin waveguide and relatively high biasing magnetic fields are required to establish well defined spin waves. Only the lowest (uniform) spin wave mode is excited.
    Journal of Applied Physics 09/2010; 109(7). DOI:10.1063/1.3559475 · 2.19 Impact Factor
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    ABSTRACT: The effect of a closed magnetic core structure on an integrated inductor was investigated by winding two series connected inductors around a stretched hexagonal ring of CoTaZr. The 8.5 and 17.5 turn inductors were fabricated using standard silicon process, and simulations were performed using Ansoft HFSS software. The resulting inductance of the device showed no improvement over two inductors without the closed magnetic cores, indicating that the closed core structure was insufficient to enhance the mutual inductance between the two devices. This was attributed to the uniaxial anisotropy of the magnetic film, which prevented the film from routing the generated flux circularly around the core. Therefore, while the physical shape of the magnetic film was closed, the effective magnetic shape maintained an air gap.
    IEEE Transactions on Magnetics 07/2010; 46(6-46):2387 - 2390. DOI:10.1109/TMAG.2010.2044982 · 1.21 Impact Factor
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    D.A. Hall · R S Gaster · T Lin · S J Osterfeld · S Han · B Murmann · S X Wang
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    ABSTRACT: Giant magnetoresistive biosensors are becoming more prevalent for sensitive, quantifiable biomolecular detection. However, in order for magnetic biosensing to become competitive with current optical protein microarray technology, there is a need to increase the number of sensors while maintaining the high sensitivity and fast readout time characteristic of smaller arrays (1-8 sensors). In this paper, we present a circuit architecture scalable for larger sensor arrays (64 individually addressable sensors) while maintaining a high readout rate (scanning the entire array in less than 4s). The system utilizes both time domain multiplexing and frequency domain multiplexing in order to achieve this scan rate. For the implementation, we propose a new circuit architecture that does not use a classical Wheatstone bridge to measure the small change in resistance of the sensor. Instead, an architecture designed around a transimpedance amplifier is employed. A detailed analysis of this architecture including the noise, distortion, and potential sources of errors is presented, followed by a global optimization strategy for the entire system comprising the magnetic tags, sensors, and interface electronics. To demonstrate the sensitivity, quantifiable detection of two blindly spiked samples of unknown concentrations has been performed at concentrations below the limit of detection for the enzyme-linked immunosorbent assay. Lastly, the multiplexing capability and reproducibility of the system was demonstrated by simultaneously monitoring sensors functionalized with three unique proteins at different concentrations in real-time.
    Biosensors & Bioelectronics 02/2010; 25(9):2051-7. DOI:10.1016/j.bios.2010.01.038 · 6.45 Impact Factor
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    ABSTRACT: Giant magnetoresistive biosensors possess great potential in biomedical applications for quantitatively detecting magnetically tagged biomolecules. Magnetic sensing does not suffer from the high background levels found in optical sensing modalities such as the enzyme linked immunosorbent assay translating into a technology with higher sensitivity. However, to reveal the full potential of these sensors and compensate for non-idealities such as temperature dependence, digital correction and calibration techniques are not only useful but imperative. Using these calibration techniques to correct for process variations and dynamic changes in the sensing environment (such as temperature and magnetic field), we are able to obtain extremely sensitive and, more importantly, reproducible results for quantifiable biomolecular reorganization. The reproducibility of the system was improved by over 3 x using digital correction techniques and the sensors are made temperature independent by using a novel background correction technique.
    Biosensors & Bioelectronics 02/2010; 25(9):2177-81. DOI:10.1016/j.bios.2010.01.039 · 6.45 Impact Factor
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    ABSTRACT: The depinning field of a domain wall in a permalloy nanostructure can be used to detect the presence of a magnetic particle. In this device the displacement of the domain wall in a sweeping magnetic field produces a variation of the voltage drop across a corner due to the anisotropic magnetoresistance effect and hence an electrical signal. In this paper we use micromagnetic simulations to calculate the output signal of a particularly shaped device in the presence of a single synthetic antiferromagnetic nanoparticle. The calculated magnetoresistive signal is in good agreement with corresponding experimental data
    Journal of Physics Conference Series 01/2010; 200(12):122001. DOI:10.1088/1742-6596/200/12/122001
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    ABSTRACT: We have designed and fabricated both single-coil and parallel-coil magnetic integrated inductors with extremely small resistances and high quality factors on an 8-in-round printed circuit board (PCB) substrate for microprocessor power delivery applications. The dc resistances of these inductors are less than 12 mOmega. Soft magnetic material CoFeHfO was successfully integrated into the inductor fabrication to increase the inductance. The quality factors are more than 80 in a frequency range of 1.5-2 GHz for air-core inductors and more than 23 in a range of 200-300 MHz for magnetic inductors. The net inductance improvement of the magnetic inductor over air-core inductor is about 12%, which could be further enhanced with a thicker magnetic core, according to our theoretic calculation and HFSS simulation. We also characterized the permeability spectra of CoFeHfO material on the PCB substrate, simulated the high-frequency performance of the magnetic integrated inductor by HFSS, and, for the first time, reached a good agreement with the experimental data. The experimental and simulation results of the magnetic inductors as compared to those of the air-core inductors point out the future direction to further optimize magnetic integrated inductors.
    IEEE Transactions on Advanced Packaging 12/2009; DOI:10.1109/TADVP.2009.2019845 · 1.28 Impact Factor
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    ABSTRACT: The propagation of spin waves in single and multilayered ferromagnetic Co-Ta-Zr films is experimentally investigated in this work. Two microstrip antennas placed on top of the magnetic core are used for signal transmission and reception. A metallic ground layer placed on one side of the magnetic film leads to nonreciprocal spin wave dispersion, resulting in a direction-dependent coupling of the antennas. Compared to previous experiments with Ni-Fe films, the higher resistivity of the Co-Ta-Zr reduces the energy dissipation in the device and allows for the incorporation of a thicker magnetic film, thereby increasing the nonreciprocity and the spin wave propagation frequency range. In the case of the Co-Ta-Zr/SiO<sub>2</sub> multilayer, the spin wave-mediated coupling of the antennas is strongly dependent on the relative orientation of the magnetizations in the individual layers, and on the application of an external magnetic field.
    IEEE Transactions on Magnetics 11/2009; DOI:10.1109/TMAG.2009.2022491 · 1.21 Impact Factor
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    ABSTRACT: Magnetostatic spin-wave modes were excited and detected in micron size ferromagnetic Co<sub>90</sub>Ta<sub>5</sub> Zr<sub>5</sub> rectangular tubes that wrapped around the shorted ends of coplanar waveguides. The observed modes can be assigned to surface spin waves of an infinite ferromagnetic film but with periodic boundary conditions and quantized wave vector imposed by the finite circumference of the tube.
    IEEE Transactions on Magnetics 11/2009; DOI:10.1109/TMAG.2009.2023919 · 1.21 Impact Factor
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    ABSTRACT: Resonant coupling of coplanar waveguides is explored by wrapping proximate shorted ends of the waveguides with micron size ferromagnetic Co90Ta5Zr5 tubes. Ferromagnetic resonance and up to 7 outer surface modes are identified. Experimental results for these contorted rectangular tubes are in good agreement with micromagnetic simulations and model calculations of magnetostatic modes for an elliptical ferromagnetic tube. These results indicate that the modes are largely determined by tube topology and dimensions but less so by the detailed shape. Comment: 3 pages, 5 figures
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    ABSTRACT: Emerging applications in microwave communication, RFIC, and power delivery system are driving the need for the miniaturization, cost-reduction and quality optimization of inductors. In our prior work, high-quality-factor and low-resistance embedded inductors were fabricated on low-cost printed circuit board (PCB) for system-on-package applications. The inductor with a CoFeHfO magnetic core patterned by sand-blasting experimentally produced an inductance gain of ~12% over the air-core inductor. In order to further improve the inductance gain, we looked at alternative processes and materials to use in the fabrication of magnetic cores. In this paper, we investigated and compared the magnetic properties of CoFeHfO magnetic cores on the PCB substrate patterned by both wet-etching and sandblasting. The magnetic domain images by Kerr microscopy, hysteresis loops and permeability spectra clearly showed that the CoFeHfO core patterned by wet-etching had a 3times larger permeability value and softer magnetic properties than the core patterned by sand-blasting. With the implementation of the CoFeHfO core by wet-etching, the theoretic calculation predicted an inductance gain of 32% for our embedded inductor. Besides CoFeHfO, we also explored the magnetic properties of CoTaZr film deposited on the same PCB substrate and measured its permeability value to be ~600, which theoretically indicated a gain of 128% for the inductor we designed.
    Electronic Packaging Technology & High Density Packaging, 2009. ICEPT-HDP '09. International Conference on; 09/2009
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    ABSTRACT: Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic, metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion. The measured attenuation lengths, of the order of 3 mu m, are several of orders of magnitude shorter than that predicted from eddy currents in these thin wires. Spin waves effectively "tunnel" through air gaps, produced by focused ion beam etching, as large as 1.5 mu m. (c) 2009 American Institute of Physics. [DOI: 10.1063/1.3079767]
    Journal of Applied Physics 05/2009; 105(7-105):07D311 - 07D311-3. DOI:10.1063/1.3079767 · 2.19 Impact Factor
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    ABSTRACT: Magnetostatic spin wave dispersion and loss are measured in micron scale spin waveguides in ferromagnetic metallic CoTaZr. Results are in good agreement with model calculations of spin wave dispersion and up to three different modes are identified. Attenuation lengths of the order of 3 mum are several orders of magnitude shorter than that predicted from eddy currents in these thin wires.
    Applied Physics Letters 01/2009; 94(1). DOI:10.1063/1.3063124 · 3.52 Impact Factor
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    ABSTRACT: We developed a new process for laser-printing a thin-film magnetic-polymer composite microstructural material directly onto planar silicon substrates and characterized its magnetic and mechanical properties. Using this technique we fabricated cantilever beams 100-2000 mum long, 100-500 mum wide, and 6-8 mum thick. An external magnet was used to demonstrate actuation via magnetic field gradient forces, and a scanner was constructed able to deflect a laser beam 3deg. This technique takes advantage of advances in commercial laser printing technology and is suitable for low-cost, low-temperature, large-area MEMS devices and rapid MEMS fabrication on a variety of substrates.
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    ABSTRACT: Giant magnetoresistive (GMR) sensors are developed for a DNA microarray. Compared with the conventional fluorescent sensors, GMR sensors are cheaper, more sensitive, can generate fully electronic signals, and can be easily integrated with electronics and microfluidics. The GMR sensor used in this work has a bottom spin valve structure with an MR ratio of 12%. The single-strand target DNA detected has a length of 20 bases. Assays with DNA concentrations down to 10 pM were performed, with a dynamic range of three logs. A double modulation technique was used in signal detection to reduce the 1/f noise in the sensor while circumventing electromagnetic interference. The logarithmic relationship between the magnetic signal and the target DNA concentration can be described by the Temkin isotherm. Furthermore, GMR sensors integrated with microfluidics has great potential of improving the sensitivity to 1 pM or below, and the total assay time can be reduced to less than 1 h .
    IEEE Transactions on Magnetics 12/2008; 44(11-44):3989 - 3991. DOI:10.1109/TMAG.2008.2002795 · 1.21 Impact Factor
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    ABSTRACT: Microstrip transmission lines with ferromagnetic Co-Ta-Zr cores are investigated in this paper. Compared to nonmagnetic devices, an increase in both inductance (times11) and quality factor (times6) is achieved in the transmission lines. The role of the magnetic material's electrical resistivity and the effect of the shape-induced anisotropy field (created by patterning the magnetic film into narrow stripes) are discussed. It is shown that the nonuniform distribution of the shape anisotropy inside the Co-Ta-Zr pattern can be used to advantage by proper placement of the signal line, thereby increasing the ferromagnetic resonance (FMR) frequency of the magnetic core. Inductance enhancement is achieved at frequencies up to ~ 10 GHz.
    IEEE Transactions on Magnetics 12/2008; DOI:10.1109/TMAG.2008.2002432 · 1.21 Impact Factor
  • Dok Won Lee · Kyu-Pyung Hwang · S.X. Wang
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    ABSTRACT: Integrated solenoid inductors with magnetic core were fabricated and analyzed. An inductance above 70 nH was achieved while keeping the coil resistance below 1 Omega and the device area below 1 mm<sup>2</sup> using a solenoid design with a single magnetic layer. The inductance of the magnetic inductor was more than 30 times that of the air core inductor of the identical geometry, and the quality factor of the magnetic inductor was >5. Novel inductor designs and the scalability were also examined, and an inductance density higher than 200 nH/mm<sup>2</sup> was obtained. The measured device properties and engineering tradeoffs were well explained by analytical models we developed.
    IEEE Transactions on Magnetics 12/2008; DOI:10.1109/TMAG.2008.2003398 · 1.21 Impact Factor

Publication Stats

1k Citations
197.10 Total Impact Points

Institutions

  • 1995–2012
    • Stanford University
      • • Department of Materials Science and Engineering
      • • Department of Electrical Engineering
      Palo Alto, CA, United States
  • 2009
    • Tsinghua University
      • School of Materials Science and Engineering
      Beijing, Beijing Shi, China
    • University of California, Los Angeles
      • Department of Electrical Engineering
      Los Angeles, CA, United States
    • University of California, Santa Barbara
      Santa Barbara, California, United States
  • 2004
    • Delft University of Technology
      • Section of Electronic Components, Technology and Materials
      Delft, South Holland, Netherlands
  • 1997
    • San Jose State University
      San Jose, California, United States