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All-Diamond Microelectrode Array Device Markus Pagels,† Clive E. Hall,‡ Nathan S. Lawrence,† Andrew Meredith,† Timothy G. J. Jones,† Herman P. Godfried,‡ C. S. James Pickles,§ Jonathan Wilman,§ Craig E. Banks,| Richard G. Compton,| and Li Jiang*,†
Schlumberger Cambridge Research, Madingley Road, High Cross, Cambridge, CB3 0EL U.K., Element Six B.V., Beversestraat 20, 5431 SH Cuijk, The Netherlands, Element Six Ltd., King’s Ride Park, Ascot, Berkshire, SL5 8BP U.K., and Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford, OX1 3QZ U.K.
Diamond possesses a unique combination of physical and chemical properties, including the highest known hardness, thermal conductivity at ambient temperature and elastic modulus, a low dielectric constant, chemical inertness, and optical transmisivity from UV to far-infrared and beyond. These superior intrinsic properties make diamond particularly attractive for highperformance optoelectronic devices at micro- and nanoscales that are suited for critical applications in extreme conditions.1 Rapid technical advances now allow resolutions of 0.02 nm in depth, 50 nN in load,2 and 50 nm in spherical diamond tip radius.3 Recently, Niedermann et al.4 developed chemical vapor deposited (CVD) diamond STM and AFM probes demonstrating outstanding robustness and longevity. Isberg et al.5 reported the development of high-purity single-crystal diamond that has extremely fast drift mobilities for both electrons and holes, which lays down the foundation for high-power and wide-frequency bandwidth elec-
tronic devices. An emerging field is CVD diamond microelectromechanical systems (MEMS) that outperform siliconsthe current material of choicesin a number of critical aspects. Several fabrication technologies have been developed for diamond MEMS, such as conformal diamond coatings, multiple diamond depositions with different dopants in individual layers, selective area deposition of diamond, sacrificial release layers, and photolithography/etching cycles, using diamond layers ranging from ultrananocrystalline to highly oriented CVD diamond films.6-8 These techniques have resulted in a number of exceptional devices, such as a microturbine, a shaft assembly,6 an acceleration sensor,7 a coplanar microwave relay,8 and a movable microgripper.9 Introduction of a light, p-type dopant, such as boron, during growth opens up the exciting prospect of realizing novel electronic devices in a unique, wide band gap elemental semiconductor material. The transformation from insulator to semiconductor with boron doping is achieved by the creation of discrete acceptor states some 0.35 eV above the valence band that form a pseudoconduction band.10 By using conventional photolithographic patterning coupled with wet etching, Fujishima et al.11 deposited a thin layer of BDD onto silicon substrates. Subsequently, a spincoated thin layer of polyimide was then mechanically polished to expose the diamond tips leading to an array of microdiamond electrodes with 20-30-µm domain size protruding from the substrate. Swain et al.12 also produced BDD microelectrodes by chemical vapor depositing a 4.5-µm-thick BDD film onto platinum tips. Rychen et al.13 reported a recessed BDD-microelectrode array by depositing a 0.5-µm-thick Si3N4 layer on a BDD film, which in turn sits on a silicon substrate. Subsequently, the conducting areas were opened using conventional photolithography plus dry etching. Here we present an all-diamond MEA, fabricated using advanced chemical vapor deposition in a microwave-induced
* To whom correspondence should be addressed. Current address: Schlumberger Doll-Research, Ridgefield, CT 06877. Tel: 203 431 5249. Fax: 203 438 3819. E-mail:
[email protected]. † Schlumberger Cambridge Research. ‡ Element Six B.V. § Element Six Ltd. | University of Oxford. (1) Amaratunga, G. A. J. Science 2002, 297, 1657-1658. (2) Gates, T. S.; Hinkley, J. A. http://techreports.larc.nasa.gov/ltrs/PDF/2003/ tm/NASA-2003-tm212163.pdf. (3) Yu, N.; Polycarpou, A. A. J. Vac. Sci. Technol. B 2004, 22, 668-672. (4) Niedermann, P.; et al. Appl Phys A 1998, 66, (Suppl. 1), S31-S34. (5) Isberg, J.; et al. Science 2002, 297, 1670-1672.
(6) Auciello, O.; et al. J. Phys.: Condens. Matter 2004, 16, R539-R552. (7) Kohn, E.; et al. J. Phys. D: Appl. Phys. 2001, 34, R77-R85. (8) Kohn, E.; Gluche, P.; Adamschik, M. Diamond Relat. Mater. 1999, 8, 934940. (9) Shibata, T.; Kitamoto, Y.; Unno, K.; Makino, E. J. Microelectromech. Syst. 2000, 9, 47-51. (10) Saguy, C. Defects Diffus. Forum 2004, 226-228, 31-48. (11) Tsunozaki, K.; Einaga, Y.; Rao, T. N.; Fujishima, A. Chem. Lett. 2002, 31, 502-503. (12) Cvae`ka, J.; Quaiserova, V.; Park, J.; Show, Y.; Muck, A., Jr.; Swain, G. M. Anal. Chem. 2003, 75, 2678-2687. (13) Provent, C.; Haenni, W.; Santoli, E.; Rychen, P. Electrochem. Acta 2004, 49, 3737-3744.
We report the development of all-diamond microelectrochemical devices, namely, a microelectrode array (MEA), in which a periodic array structure with well-defined diameters, distance, and hexagonal unit cell pattern is micromachined using a combination of state-of-the-art microwave-induced plasma growth and laser ablation shaping techniques to prepare and coat a patterned borondoped diamond (BDD) substrate with an intrinsic diamond insulating layer. The active BDD element can be tuned to between 10 and 50 µm in diameter with a 10 times diameter center-to-center distance between two adjacent conducting elements, which are exactly coplanar to the dielectric surroundings. This type of device should enable applications in harsh conditions such as high temperature, high pressure, and resistive media under dynamic flow regimes.
10.1021/ac0502100 CCC: $30.25 Published on Web 04/29/2005
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plasma coupled with high-precision laser ablation micromachining. The device comprises both insulating and conducting diamond structures, which at the surface is in the form of hexagonally distributed, electrically connected boron-doped diamond microdomains, separated with their insulating next of kin in a coplanar structure. The surface distribution pattern is designed such that molecular diffusion toward the conducting domainssin an electrochemical processsdoes not overlap in the experimental time scale; hence, the overall output of current is simply the sum of the steady-state redox currents on individual elements of the array.14 EXPERIMENTAL SECTION Fabrication of All-Diamond Microelectrode Arrays. A highly boron doped CVD diamond layer was grown in a microwaveinduced methane/hydrogen plasma to a thickness in excess of 500 µm using B2H6 as the doping gas. The diamond used in this set of experiments was controlled to yield an average specific electrical resistance of 0.75 Ω‚m. The growth side of this wafer was then mechanically processed flat to a mirror finish, and a diamond plate of 10 × 10 mm in size was cut out of the wafer by laser and subsequently mounted on an X-Y table of a pulsed UV laser system (Exitech M5000) operating at a wavelength of 248 nm. The homogenized optics train projected the UV laser beam through a 20× reducting projection lens onto the surface of the diamond plate. A mask was placed just prior to this lens composed of an AR coated silica plate partially coated with titanium. In the middle of the mask was a transparent diamond-shaped area 17.3 mm long and 10 mm wide with four titanium-coated circles of 0.5 mm in diameter positioned to form the primitive lattice of a hexagonal array. Using a step and repeat procedure, the UV laser15 ablated the complete diamond surface to create a hexagonal array of tapered conducting diamond columns with diameters of 1025 µm at the top, as defined by the magnification of the projection lens, and heights of 15-50 µm depending on the overall energy of laser pulses applied to each area. The nearest-neighbor separation was 10× the top diameter of the diamond column. The dimensions of the diamond columns and center to center distance can be changed by using a different mask and/or magnification projection lens. At a laser repetition rate of 100 Hz and a power of 300 mJ/pulse, 100 pulses were needed to remove 1-µm depth of a 25-µm-diameter diamond column. After the laser ablation stage, a layer of intrinsic diamond just thicker than the boron-doped column height was grown over the bare array at ∼950 °C. This nonconducting layer was then partially removed by a lapidary technique16,17 to expose the tips of the boron-doped diamond columns, creating a hexagonal array of electrically interconnected, 10-50 µm in diameter disks of conducting diamond in a nonconducting diamond matrix. The size of the conducting diamond disks is controllable to a certain extent by overpolishing and revealing a larger diameter of the tapered conducting diamond columns. (14) Heinze, J. Angew. Chem., Int. Ed. Engl. 1993, 32, 1268-1288. (15) http://www.logitech.uk.com/. (16) Wilkes, J.; Wilkes, E. In Properties and Application of Diamond; Butterworth Heineman: Oxford, 1991. (17) Newbury, D. E.; Joy, D. C.; Echlin, P.; Fiori, C. E.; Goldstein, J. I. Advanced Scanning Electron Microscopy and X-ray Microanalysis; Plenum: New York, 1986.
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The surface of the polished arrays was examined using secondary electron channeling contrast in a scanning electron microscopy (SEM) (JEOL, JSM6100, 5KV) or with Nomarski optical microscopy (Leitz Ortholu) and laser interferometery (ZYGO GPI phase shifting 633-nm Fizeau-type) after giving the array surface a light oxygen plasma ash (pressure 1.4 mbar O2, 50 W at 30 duty cycles, 2.4 GHz for 10 min) to remove a nominal 5 nm of diamond. This nonisotropic etching process generates a slight height difference between crystallites that can be imaged using these height-sensitive techniques. Atomic Force Microscopy. For the contour plot, an easyScan AFM system (Nanosurf AG) was used with a microfabricated silicon cantilever with an integrated tip and the easyScan AFM system software v1.0f. The maximum scan range in x- and y-direction is 82.732 µm and in z-direction 19.460 µm. The force load on the tip was 50.03 nN. Reagents. All reagents were obtained from Aldrich, were of the highest grade available, and were used without further purification. All aqueous solutions and subsequent dilutions were prepared using deionized water from an Elgastat (Elga) UHQ grade water system with a resistivity of 18 MΩ‚cm. The electrochemical experiments were typically conducted at 20 ( 2 °C. Electrochemical Apparatus. Electrochemical measurements were conducted using an Autolab PGSTAT 30 computer-controlled potentiostat (Eco-Chemie) with a standard three-electrode configuration. A platinum wire provided the counter electrode and a saturated calomel electrode (SCE, Hg/HgCl2, Radiometer, Copenhagen, Denmark) was used as the reference electrode. The BDDMEA or a random carbon fiber microelectrode array (CSIRO, Sydney, Australia) served as the working electrode. Each BDD MEA was mounted on a brass pin and attached to a connecting wire using silver conducting paste. The electrode assembly was housed in a PEEK tube sealed by epoxy (Koford, E-109). The BDD MEA, or carbon fiber MEA, was polished using 1-µm alumina slurry on a polishing cloth prior to each experiment. RESULTS AND DISCUSSION When the binary diamond-type structure is examined using optical microscopy coupled to laser interferometery, the polished array surface appears to be quite flat and featureless except for a few polishing scratches with no height difference between the conducting and nonconducting diamond areas (Figure 1A). AFM contouring demonstrates that the BDD elements are at exactly the same height as the surrounding dielectric medium (Figure 1B). Figure 2 exhibits the side (A) and top (B) views of an individual BDD array element taken by SEM using secondary electron channeling contrast. The side view (A) was obtained by carefully polishing a cross section of the matrix to reveal from the center of a BDD diskslocated at the center of a hexagonal unit cells the columnar features of different single crystallites arising from the growth of the BDD substrates. The columnar features are individual diamond grains characteristic of the growth process, and in this image, the lighter area is boron-doped diamond and the darker area is intrinsic (insulating) diamond. The two subsurface peaks are caused by the mask step and the repeat distance being set just slightly too large, resulting in a ridge of unetched diamond between the individual mask imprints in the diamond surface. The top view (B) exhibits the heterogeneous
Figure 3. (A) Cyclic voltammograms of 0.2 mM (NH3)6RuCl3 in aqueous solution without added supporting electrolyte, recorded with a BDD MEA at potential scan rates of 0.01, 0.05, 0.1, 0.5, 1, 5, and 10 V‚s-1, respectively. (B) demonstrates the substantially wider electrochemical window on BDD MEA (a) than random carbon fiber microelectrode array (b). Figure 1. (A) Nomarski contrast optical image of an oxygen plasma etched MEA surface showing a well-defined hexagonal unit cell and its polycrystalline nature. The diameter of each conducting BDD disk is 15 µm with a 10× distance between two adjacant conducting elements. (B) shows an AFM contour exhibiting essentially no height difference at the nanometer scale between the BDD elements and the surrounding medium.
Figure 2. SEM images of an individual BDD conducting disk of 15 µm in diameter in the side (A) and top (B) views.
nature of the conducting surface, where the contrast in the subdomains is due to discrete crystallites having slightly different levels of boron doping. The actual boron concentration of the individual crystallites varies depending upon how the crystal-
lographic faces were presented by each crystallite to the plasma during growth. For example, it is known that the amount of incorporated boron varies with growth sector since (111) surfaces can incorporate some 10 times more boron than (100) surfaces.18,19 It is of particular interest to note the clear-cut boundaries between different BDD single crystallites, as well as between BDD and intrinsic diamond, as there is negligible boron diffusion across the interface under the prevailing experimental conditions, which results in an exceptionally high structural definition on a microscopic scale. This may be attributed to the fact that, in the circumstance of extremely tight binding of C atoms in the highly compressed diamond crystal, foreign species, such as boron, are only shallowly incorporated in the substitutional sites as their diffusion coefficients are of the order of 3.5 × 10-14 cm2 s-1 at a temperature in the region of 1000 °C.10 The growth process of the structured diamonds takes place at 950 °C for a period up to several days, and the diffusion distance of boron dopants is much less than 1 µm, resulting in very high definition of the architecture. The electrochemical features of the device are examined under a number of challenging conditions in order to evaluate its outstanding performance. Figure 3A displays a series of cyclic voltammograms of (NH3)6RuCl3 with potential scan rates varying from 0.01 to 10 V‚s-1 in water in the absence of any intentionally added supporting electrolyte. They all show characteristic sigmoidal shape with a constant steady-state limiting current of 1.5 µA across 3 orders of magnitude in potential scan rates. One of the perceived advantages of the BDD electrode is its wide potential (18) Bachman, P. K. In Handbook of Industrial Diamonds and Diamond Films; Prelas, M. A., Popovici, G., Bigelow, L. K., Eds.; Marcel Dekker: New York, 1998. (19) Ushizawa, K.; Watanabe, K.; Ando, T.; Sakaguchi. I.; Nishitani-Gamo, M.; Sato, Y.; Kanada, H. Diamond Relat. Mater. 1998, 7, 1719-1722.
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window. It can therefore be anticipated that the BDD MEA would also exhibit such behavior. The voltammetric responses at a potential scan rate of 0.1 V‚s-1 obtained at the 15-µm BDD MEA (a) and a commercially available MEA (b) composed of random carbon fibers of ∼7 µm in diameter20 are displayed in Figure 3B. An operational potential window is achieved for the BDD MEA with no appreciable redox waves in the potential range -1.50 to +1.70 V. In contrast, the random carbon fiber microelectrode array has a limited potential range in the cathodic region due to oxygen reduction at -0.40 V caused by the copious oxygen evolution at the positive end of the scan, though the cathodic region can be extended to -1.00 V when the anodic potentials are cut at +1.50 V.20 These results demonstrate the significant advantage in terms of functional potential window associated with the BDD MEA. In summary, we have developed a microelectrochemical device based entirely on diamond materials. The boron-doped diamond
conducting domains, in hexagonal unit cells, are separated by intrinsic diamond in a coplanar configuration. This device has demonstrated a number of remarkable structural characteristics with a high level of definition and is expected to find important applications in harsh environments such as elevated temperature and pressure and resistive medium with dynamic flow regimes.
(20) Fletcher, S.; Horne, M. D. Electrochem. Commun. 1999, 1, 502-512.
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SUPPORTING INFORMATION AVAILABLE Figure S1: a typical SEM image showing both the side and growth surface of an unprocessed boron doped CVD diamond wafer. The columnar growth is clearly visible as well as the crystallinity of the individual crystallites. This material is available free of charge via the Internet at http://pubs.acs.org. Received for review February 2, 2005. Accepted March 3, 2005.
