Electrochemical and galvanic fabrication of a magnetoelectric composite sensor based on InP
© Gerngross et al.; licensee Springer. 2012
Received: 25 April 2012
Accepted: 12 June 2012
Published: 9 July 2012
A process chain for a magnetoelectric device based on porous InP will be presented using only chemical, electrochemical, photoelectrochemical, photochemical treatments and the galvanic deposition of metals in high-aspect-ratio structures. All relevant process steps starting with the formation of a self-ordered array of current-line oriented pores followed by the membrane fabrication and a post-etching step, as well as the galvanic metal filling of membrane structures are presented and discussed. The resistivity of a porous InP structure could be drastically increased and, thus, the piezoelectric performance of the porous InP structure. The developed galvanic Ni filling process is capable to homogeneously fill high aspect-ratio membranes.
The aim of this work is to develop small and cheap magnetoelectric sensors that are capable to sense biomagnetic signals in the range of pico tesla with a high sensitivity. In principle, this can be achieved with multiferroic materials, such as Cr2O3, that show magnetoelectric behavior. The drawbacks of these materials are their small effect magnitude and a Curie temperature far below room temperature. Magnetoelectric composites overcome these problems and are very promising candidates for biomagnetic sensing applications. Magnetoelectric composites consist of a piezoelectric and magnetostrictive component in various geometrical arrangements.
In this paper, a production chain for such a device will be presented using only chemical, electrochemical, photoelectrochemical, and photochemical etching of InP and galvanic deposition of metals. A 1-3 composite geometry is chosen because it allows for very high contact areas between the piezoelectric InP matrix (3-D) and the magnetostrictive wires (1-D). This is another prerequisite for a good magnetoelectric sensor performance. InP as a III-V compound semiconductor belongs to the cubic 4macro3m crystal structure and, thus, in principle allows for strong piezoelectric behavior. Due to its cubic crystal structure, the only non-vanishing piezoelectric coefficient is the d 14coefficient. Unfortunately, no intrinsic, i.e. insulating, InP can be produced, and thus, any piezoelectric voltage is short circuited by free charges inherent in the material. This problem has been overcome by etching an almost hexagonally close-packed array of current-line oriented pores (curro-pores) in <100> oriented InP wafers, leaving a porous structure with completely overlapping space charge regions (SCR), at least after further chemical etching. This allowed for increasing the piezoelectric response by a factor of 30 in comparison to bulk InP [2, 3]. Galvanic filling of the pores with a magnetostrictive metal – such as Ni – allows for filling of high aspect-ratio geometries. This enables the use of a several hundred microns thick piezoelectric layer, resulting in a higher magnetoelectric voltage coefficient. Galvanic filling of a membrane structure is much simpler than galvanic pore filling and a well established technique [4–6]. From basic principles, it is clear that it is not possible to etch curro-pores through the complete InP wafer. Thus, the bulk back-side of the InP pore array is opened photoelectrochemically/photochemically after the curro-pore formation. The individual steps of the sensor device production will be discussed as well as some first results of the piezoelectric and electric properties.
In this work, single-crystalline (100) InP wafers, doped with S at a doping concentration of 1.1 × 1017 cm-3 and a resistivity of 0.019 Ω cm are used. The InP wafers are double-side polished and epi-ready. Two different wafer thicknesses were used, 400 and 500 µm. The curro-pores were etched in an electrochemical double cell with a 6 wt.% aqueous HCl electrolyte at 20 °C [7, 8]. To achieve a homogenous pore nucleation, a voltage pulse was applied followed by a constant anodic potential.
In the second step, the membrane was produced by galvanostatic etching of crystallographically oriented pores (crysto-pores) in the wafer back-side until the previously etched curro-pore array is reached. During formation, this mesoporous layer was simultaneously dissolved photochemically by high power blue illumination. To avoid underetching and to have a good selectivity between the curro-pore array and the bulk back-side, blue light assisted photoelectrochemical etching is used. The etching was carried out in the same electrochemical double cell so that a misalignment due to a cell change is avoided. The etching is done in the same HCl-containing electrolyte at 20 °C. The high power blue illumination is provided by an Enfis UNO Tag LED array (ENFIS LIMITED, Swansea, UK) with a mean wavelength of 470 nm. To control the whole membrane etching process, the voltage was monitored and the Fast Fourier Transform impedance spectroscopy data were recorded, which will not be discussed in this paper.
The third process step is the post-etching of the InP membrane. This step is necessary because the width of the SCR is much larger during the anodic pore formation than at open-circuit conditions . Thus, only anodic pore etching does not result in a porous InP layer with completely overlapping SCR. Therefore, the InP membrane is post-etched in an HF : HNO3 : EtOH : HAc containing electrolyte under a cathodic potential for 48 h at 20 °C. The electrolyte was optimized to show isotropic etching behavior – especially along the full length of the high aspect-ratio pores – and to be self-limiting at the SCR surrounding each pore. A cathodic potential was chosen to artificially shrink the SCR around each pore and to expose more area unguarded by the SCR to the electrolyte for dissolution. As the post-etching electrolyte is self-limiting at the SCR, a strongly improved overlapping of the SCR is obtained under open-circuit conditions. The electrolyte is pumped through the complete membrane to ensure homogenous etching over the whole membrane thickness.
In the fourth step, the post-etched membrane shall be galvanically filled with a magnetostrictive material. To prevent possible leakage currents between piezoelectric matrix and magnetostrictive filler, a thin dielectric interlayer consisting of Al2O3 has to be deposited by atomic layer deposition (ALD). Therefore, the galvanic filling process is developed in an anodically oxidized aluminum (AAO) membrane that is commercially available at Whatman GmbH (Dassel, Germany), with a nominal pore size of 200 nm, which is similar to the average pore size obtained in the here presented InP membranes. A plating base consisting of 25 nm of Ti and 450 nm of Au was sputtered on the back-side of the membrane. The AAO membranes are galvanically filled in a single cell. The working electrode is a Pt sheet, on which the back-side of the membrane is mounted with InGa providing a good electrical contact. A Watt’s type based Ni electrolyte adjusted to a pH value of 2 by H2SO4 has been used. The electrolyte temperature was held constant at 35 °C. The galvanic deposition was carried out under galvanostatic conditions at a current density of 17 mA/cm2.
The etched and galvanically filled nanostructures were investigated using a HELIOS D477 SEM (FEI Co., Hillsboro, OR, USA). The piezoelectric properties of the InP nanostructures were characterized by a double beam laser interferometer from aixACCT Systems GmbH (Aachen, Germany) and the electric properties by a four-point measurement with an Elypor-01 from ET&TE GmbH (Kiel, Germany).
Results and Discussion
For the magnetoelectric composite device, the galvanic filling with a magnetostrictive metal, such as Ni, is needed. The post-etched InP membranes can be (and already have been) coated with a thin Al2O3 interlayer by ALD to prevent ohmic contacts between the piezoelectric and magnetostrictive component. Therefore, the galvanic deposition process of Ni can be developed and optimized in AAO membranes with similar pore dimensions because they are cheap and commercially easily available. Afterwards, this Ni deposition process is most probably directly applicable to Al2O3 coated InP membranes, because the interface electrolyte/Al2O3 is identical, and the pore dimensions are similar. The ALD deposition process of Al2O3 is capable to coat membrane structures with an aspect-ratio larger than 1,500.
A self-ordered membrane structure in InP with strong piezoelectric behavior has been fabricated using only chemical, electrochemical, photoelectrochemical, and photochemical treatments. By cathodic post-etching of this pore structure, the resistivity could be drastically increased by a factor of 3,000 allowing for a strong piezoelectric effect being 30 times larger than for bulk InP. A photoelectrochemical/photochemical process for fabricating InP membranes with a freely adjustable thickness and high surface homogeneity is presented. The galvanic Ni filling process, which has been presented in this work, will be transferred to Al2O3 coated InP membranes, and after galvanic Ni filling, the device is expected to show a good magnetoelectric performance.
HF received his PhD degree in Physics in 1976 from the University of Stuttgart in conjunction with the Max-Planck-Institute for Metal Research in Stuttgart. After post-doctoral work at the Department of Materials Science and Engineering at Cornell University (1977 to 1978) and a position as guest scientist at the T.J. Watson Res. Center of IBM in Yorktown Heights (1979 to 1980), he joined Siemens in 1980, working in the newly founded Solar Energy Department of Central Research in Munich. In 1991, after holding various senior positions in microelectronics development at Siemens, he accepted an offer of the Christian-Albrechts-University of Kiel to become the founding dean of the newly established Faculty of Engineering, where he also holds the chair for Materials Science. Since 1998, he is back to research as a tenured full professor, with particular interest in solar cell technology and the electrochemistry of semiconductors. He is one of the pioneers in the field of porous semiconductors, established the so-called ‘CELLO’ technique for characterizing solar cells, and has coauthored more than 250 papers and about 30 patents. He is also well known for his ‘Hyperscripts’ in the Internet (about 1.5 Mio requests per month).
JC began his studies in physics at the Christian-Albrechts-University of Kiel 1984 and received his PhD from the Institute of Theoretical Physics in 1993 for work on superconductivity. Subsequently, he joined the group of Prof. HF at the Institute for Materials Science at the Christian-Albrechts-University of Kiel as a research associate. He began his work employing semiconductor electrochemistry for the mapping of minority carrier life-time in solar cell substrates and in the emerging field of porous semiconductors and related self-organization phenomena. He was instrumental in developing the unique CELLO and ‘CELLOplus’ measurement techniques capable of mapping nearly all of the important solar cell parameters with good local resolution. He has coauthored more than 120 papers.
MDG received his MSc degree in Materials Science in 2010 from the Christian-Albrechts-University of Kiel. Subsequently, he joined the the group of Prof. HF at the Institute for Materials Science at the Christian-Albrechts-University of Kiel as a PhD student in 2010 working on the development of magnetoelectric composites based on porous semiconductors.
This work was funded by the Collaborative Research Center 855 ‘Magnetoelectric Composites - Future Biomagnetic Interfaces’ by the German Science Foundation (DFG) under grant number SFB 855/1.
Additionally, the authors thank J. Bahr for his advice in improving the post-etching setup and for the perfect implementation into the existing setup.
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