Silicon-on-Insulator Microfluidic Device With Monolithic ... - IEEE Xplore

13 downloads 0 Views 1MB Size Report
Silicon-on-Insulator Microfluidic Device With. Monolithic Sensor Integration for TAS Applications. Sanjiv Sharma, Karin Buchholz, Sebastian M. Luber, Ulrich ...
308

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 2, APRIL 2006

Silicon-on-Insulator Microfluidic Device With Monolithic Sensor Integration for TAS Applications Sanjiv Sharma, Karin Buchholz, Sebastian M. Luber, Ulrich Rant, Marc Tornow, and Gerhard Abstreiter

Abstract—A novel concept for the integration of liquid phase charge sensors into microfluidic devices based on silicon-on-insulator (SOI) technology is reported. Utilizing standard silicon processing we fabricated basic microfluidic cross geometries comprising of 5–10-mm-long and 55- m-wide channels of 3 m depth by wet sacrificial etching of the buried oxide of an SOI substrate. To demonstrate the feasibility of fluid manipulation along the channel we performed electroosmotic pumping of a dye-labeled buffer solution. At selected positions along the channel we patterned the 205-nm thin top silicon layer into freely suspended, 10- m wide bars bridging the channel. We demonstrate how these monolithically integrated bars work as thin-film resistors that sensitively probe changes of the surface potential via the field effect. In this way, a combination of electrokinetic manipulation and separation of charged analytes together with an on-chip electronic detection can provide a new basis for the label-free analysis of, e.g., biomolecular species as envisaged in the concept of micrototal [1484] analysis systems ( TAS) or Lab-on-Chip (LOC). Index Terms—Field-effect transistor (FET), lab-on-chip (LOC), microfluidics, micrototal analysis system ( TAS), silicon-on-insulator (SOI).

I. INTRODUCTION

T

HE concept of micrototal analysis systems or lab-on-a-chip (LOC) was put forth by Manz and Widmer in 1990 [1]. Driven by applications in (bio-) chemical analysis, medical research and diagnostics such systems have been envisaged to integrate the preparation and separation of analyte molecules, monitoring, specific detection, and quantification on a single device, preferably with all-electronic in- and output signal processing. The simplest microfluidic network design underlying most LOC architectures includes a set of orthogonally intersecting microchannels. These channels provide for the controlled handling of smallest volumes of aqueous analyte solutions on a planar substrate “chip.” Commonly, one of the channels functions as the injection unit while the other usually longer microchannel provides the necessary conditions to facilitate separation of the analytes present in the mixture by electroosmosis and electrophoresis [2]. Within this channel, the detection units are integrated.

Manuscript received December 11, 2004; revised June 7, 2005. This work was supported by the DFG via SFB 563 (B13) and by the Fujitsu Laboratories of Europe. The work of M. Tornow was supported by the BMBF by Grant 03N8713 (Junior Research Group “Nanotechnology”). Subject Editor A. J. Ricco. S. Sharma is with the Institute of Biomedical Engineering, Imperial College of Science and Technology, London SW7 2AZ, U.K. K. Buchholz, S. M. Luber, U. Rant, M. Tornow, and G. Abstreiter are with the Walter Schottky Institut, Technische Universitaet Muenchen, 85748 Garching, Germany (e-mail: [email protected]). Digital Object Identifier 10.1109/JMEMS.2006.872222

this field has seen an enorSince the introduction of mous development in microfabrication [3], separation mode [4], detection schemes, analysis of biological species [5], bio- and chemical reactors [6], [7]. While polymer substrates have been widely used due to their ease in fabrication [8] (molding techniques), optical properties (transparency) and low unit costs, semiconductor substrates, in particular silicon have gained a growing interest for specific applications [9]–[13]. Here, the fabrication of microfluidic components benefits from the compatibility to existing (MEMS) standard microelectronic and micromachining processing technologies. In particular, the direct monolithic integration of electronic components for signal processing (e.g., amplifier circuits) becomes feasible. In this paper, we present a novel microfluidic structure concept based on silicon-on-insulator (SOI) substrates, with applimonolithic integration of the sensor elements for cations. These microstructured sensors are located at the top level of the microfluidic channel in the form of freely suspended silicon bars. The sensors function as surface potential sensitive thin film resistors, based on the transducing mechanism that has been widely employed in ion-sensitive-field-effect-transistor (ISFET) devices [14] and has recently been transferred successfully to SOI substrates [15]. Charged molecule bands or zones separated due to differential electrophoretic migration along the microchannel would pass underneath the sensor elements and thereby be identified with a spatial resolution mainly limited by lithography and the mechanical stability of silicon. II. DESIGN AND FABRICATION In first experiments, we processed prototype devices with a microfluidic cross geometry out of SOI substrates as illustrated in Fig. 1. The SOI substrate (SOITEC, France) consisted of a 3buried oxide layer (BOX) separating a 205-nm thin top silicon layer (boron p-doped, ) from the bulk silicon substrate. The samples were patterned by a combination of standard photolithographic and (selective) wet etching techniques. As illustrated in Fig. 1(b), at first the top silicon layer is opened in the ratio 30:70) by either wet etching (1.5% HF: 69% or in an inductive coupled plasma (ICP) enhanced reactive ion-etching process (RIE, Oxford Instruments, UK) based on a chemistry. Subsequently, the BOX layer was mixed etched in diluted hydrofluoric acid (HF) with high selectivity to Si (5%HF, 90 min), thereby significantly underetching the top Si layer. In a last process step some of the microfabricated structures were passivated by thermally growing 50–200 nm ( atmosphere, 1000 , 2.5–9 h) dry silicon dioxide in order to facilitate electrokinetic experiments.

1057-7157/$20.00 © 2006 IEEE

SHARMA et al.: SOI MICROFLUIDIC DEVICE WITH MONOLITHIC SENSOR INTEGRATION

309

(a)

(a)

(b) Fig. 1. (a) Schematic showing the SOI substrate with basic channel geometry: The injection channel between sample (1) and sample waste (2) reservoir is crossed by the longer separation channel terminated by buffer (3) and buffer waste (4) reservoir. A PMMA cover plate with drilled access holes for electrode insertion seals the chip. Potentials are applied between Pt electrodes inserted into the electrolyte solution. (b) Left: Basic process steps for channel fabrication from SOI substrates using different (selective) etching methods. In this schematic cross-section three different main parts of the channel can be distinguished: Underetched, 205-nm-thick top Si layer (a). Selectively etched, 3  buried oxide (b). Channel bottom forming Si substrate (c). Right: Tilted SEM micrograph of a processed channel cross geometry with part of a reservoir. The smaller SEM picture shows a close up of the channel intersection region.

m

The device layout as illustrated in Fig. 1(a) consists of two channels, 5.5 and 7.5 mm in length that intersect at right angle and terminate into 1 mm diameter reservoirs. The lithographic (55 after underetching). The width of the channels is 30 deep, as determined by the BOX thickness. channels are 3 Such considerably long selective etching is required to ensure the complete release of the sensors bridging the channels, which will be described in the following paragraph. Finally, a 5-mmthick PMMA cover plate having 1-mm diameter access holes [cf. Fig. 1(a)] was physically clamped onto the SOI chip sealing the top of the microchannels. Field-effect-based sensors were monolithically integrated at wide bridge-like, the channel top level in the form of 10 freely suspended Si bars, (cf. Fig. 2) by selective etching of the sacrificial BOX layer. Although only of 205-nm thickness these structures were sufficiently mechanically stable to endure the entire process. Subsequently, Ti/Au (10 nm/200 nm) ohmic contacts were deposited by standard high vacuum evaporation to contact the

(b) Fig. 2. (a) Bottom: Schematic of the chip illustrating the Si bar sensor structures for analyte detection (not to scale). The Si sensors are electrically insulated from each other by etched trenches. Top: Optical microscope picture of one silicon bar bridging the microchannel. (b) Tilted SEM micrograph showing a 205-nm thick, freely suspended silicon bar crossing a microchannel that connects to a reservoir (left), with close-up (right), before etching final isolation trenches.

source and drain of the Si bar. Electrical insulation of the leads from the surrounding Si plane was achieved by etching trenches (RIE) as shown in Fig. 2. III. EXPERIMENTAL SECTION We performed three experiments to characterize the electrical functionality of our devices: determination of the breakdown voltage after oxide passivation, demonstration of electroosmotic pumping and transconductance measurements of the integrated field-effect sensors. A. Breakdown Voltage Experiment A semiconductor parameter analyzer (Hewlett Packard, model 4145B) was used to investigate the current–voltage (I-V) characteristics of the device along the electrolyte-filled

310

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 2, APRIL 2006

surface potential is set by a biased metal gate the Si oxide surface of our sensing bars is in direct contact to the electrolyte solution [electrolyte-oxide-semiconductor structure (EOS) [17]]. To enable the contact to the electrolyte, the chip was mounted in a custom made liquid flow chamber filled with phosphate buffer solution (pH 6.8, 10 mM, with 10 mM NaCl added). While in this arrangement the Si bar under study including part of its leads is exposed to the solution, all metal contacts are kept dry. A Ag/AgCl reference electrode and a Au counter electrode were inserted in the flow chamber to allow the setting of the electrolyte potential versus the Si sensor source contact using a potentiostat (Autolab, Ecochemie, The Netherlands) [18], cf. Fig. 5(a), inset. We characterized the transducing operation of one Si bar by directly gating it via the reference potential of the electrolyte.

SiO

Fig. 3. Effect of the passivation layer thickness (thermally grown ) on the breakdown characteristics of the device. Channels were filled with PBS solution and a voltage was applied between reservoirs 2 and 4 by inserting Pt-electrodes as shown in the inset. The breakdown voltage is rising with increasing passivation layer thickness (unpassivated, 50, 100, and 200 nm). We assign the measured lower resistance (linear slope below breakdown) of the 100-nm sample to leakage of the electrolyte in-between the top Si layer and the cover plate, which would not lead to a change in breakdown voltage.

SiO

channels, allowing for a study of the oxide breakdown characteristics. Channels and reservoirs of the covered device were completely filled with phosphate-buffered saline (PBS) solution (pH 7.5, 10 mM). Platinum electrodes were introduced in the liquid through the access holes in the cover plate to establish electrical contact to the solution (inset of Fig. 3). The voltage was applied between two adjacent reservoirs, ramped from 0 V up to a maximum of 200 V while the current was monitored. B. Electrokinetic Fluid Manipulation Electroosmotic pumping of a buffer solution (10 mM borate buffer, pH 9.2) was studied in devices coated with 200 nm silicon oxide by applying an electric field of 330 V/cm (corresponding to 182 V between reservoirs 1 and 2, see Fig. 1 for numbering). To monitor the bulk fluid movement the organic , elecfluorescent dye rhodamine B (concentration: trically neutral at pH 9.2) was added to the buffer in reservoir 1. The weak fluorescence signal of this optical marker was observed in a top-view fluorescence microscope with bandpass filtering (Leitz Ergolux) and mounted CCD camera. Pumping of the dye-labeled buffer through the microchannel from reservoir 1 to 2 was investigated. By applying counter potentials to reservoirs 3 and 4 with respect to reservoir 1 the diffusion of the dye into channels leading to reservoirs 3 and 4 was controlled. C. Electrical Characterization of Integrated Field-Effect Sensors To characterize the integrated sensor the contact pads on the source and drain side of the bridge were connected to a sourcemeter (Keithley 2400) measuring the drain current as a function of applied source drain voltage. The charge carrier density (and thereby the drain current and the resistance) of the weakly doped Si thin film is mainly determined by the surface potential induced band bending, analogous to thin film metal–oxide–semiconductor field-effect transistors (MOSFET) [16]. However, unlike in those devices where the

IV. RESULTS AND DISCUSSION A. Breakdown Voltage Experiment Fig. 1(b). shows a scanning electron micrograph of the device region close to one reservoir, together with a further enlarged detail of the channel intersection. As can be clearly seen, the thin Si top layer is underetched with a high selectivity thereby increasing the volume of the enclosed fluid. The channel intersection has a total volume of 9.0 pl. We measured the breakdown characteristics of our devices as passivation thickness, with voltage applied a function of along the microchannels between two reservoirs filled with electrolyte solution. Such determination of the breakdown characteristics is important as it provides the limits of voltage that can be applied at the microchannels without that leakage current is bypassed through the Si substrate. Test devices with three different dry oxide thicknesses (50, 100, and 200 nm) were measured in comparison to an unpassivated sample (native oxide only). Fig. 3 shows the I–V characteristics obtained when up to 200 V were applied between the Pt electrodes inserted in two reservoirs of the device. Up to a certain critical voltage, the current increases linearly with the voltage. Above this critical voltage, the current starts to rise rapidly. We assign the low voltage linear regime to ionic currents in the electrolyte only breakdown for our particular deand the transition to the vice setup and choice of parameters. We anticipate that above breakdown, most current is bypassed through the Si substrate layer. As expected, the device stability against across the breakdown increases with increasing oxide thickness. For silicon covered by native oxide only we obtain breakdown volt: 140 V, for 100 ages less than 10 V, for 50 nm thermal , respectively. nm: 190 V, and for For sensor-integrated structures the oxide thickness has to be limited to a maximum of 210 nm (equivalent to a consumption of 90 nm Si during growth) to ensure that a minimum Si bar will be retained as thickness of at least the sensing layer. B. Electrokinetic Fluid Manipulation We carried out electroosmotic flow experiments on a microfluidic network passivated with 200 nm dry oxide. Fig. 4(a)

SHARMA et al.: SOI MICROFLUIDIC DEVICE WITH MONOLITHIC SENSOR INTEGRATION

(a)

311

at 330 V/cm. Movement toward the negative electrode is in accordance with the expectation. The positive counterions in the solution adjacent to the negatively charged wall (deprotonated -OH groups) are drawn toward the negative electrode when subjected to an electric field resulting in a bulk flow of liquid toward the negative electrode. To distinguish the bare electroosmotic flow from thermal diffusion spreading we also reversed the potential along the shorter channel, which lead to a bulk reverse movement as expected (data not shown). Observed electroosmotic flow velocities were between and 5 at a field strength of 330 V/cm for the rho3 damine B dye in pH 9.2 borate buffer, depending on conditions. Driving fields and flow rates were comparable to those reported in earlier work on silicon substrates [19]. A maximum rate of was observed when the structures were charged with 5 100-mM NaOH solution before introduction of buffer. This can be attributed to a higher negative charge on the wall leading to higher electroosmotic flow velocities. A further increase of the flow velocity could be achieved by applying larger driving voltages. This, however, would require an enhanced thickness of the oxide passivation layer, or alternatively a passivation material with higher breakdown voltages. When electroosmotic pumping was carried out leaving the reservoirs of the perpendicularly crossing channel electrically floating we observed that the dye diffused into the floating channel parts, an effect often termed as “bleeding.” In order to overcome bleeding, we applied “pinching,” or “counter” potentials [20] at the reservoirs of the perpendicular, longer channel. This way, we investigated the optimal conditions for an injection mode that would enable the introduction of reproducible trapezoidal liquid plugs into the separation channel, as can be seen in Fig. 4(b). Here, besides the 78 V applied to drive the dye toward the sample waste reservoir, 57 and 67 V were applied to the reservoirs connected to the perpendicular channel thereby efficiently preventing bleeding. C. Electrical Characterization of Integrated Field Effect Sensors

(b) Fig. 4. (a) Top-view fluorescence microscope images of the channel intersection region taken after t = 28, 30 and 40 min. (top to bottom, respectively) after starting an electroosmotic pumping experiment at t = 0. Throughout the entire experiment the channels were completely filled with a borate buffer solution and a driving voltage of 182 V was applied between the Pt-electrodes inserted in the left and right reservoirs 1 (“Res.1”) and 2 (“Res.2”). The upper and lower reservoirs “Res. 3” and “Res. 4” were left electrically floating (not connected). The light color gradually emerging from the channel that connects to Res.1 originates from the fluorescence of dyes (Rhodamine B) that had been added to the solution of Res. 1 before applying the driving voltage. It indicates a bulk electroosmotic movement of the borate buffer toward the relatively negative electrode. Note the diffusion into the perpendicular channels leading to Res. 3 and 4. (b) Fluorescence microscope image of a pinching injection mode to introduce trapezoidal reproducible plugs in the separation channel by application of counter potentials. A trapezoid is drawn to guide the eye. Voltages applied to reservoirs 1 to 4 were 78, 0, 57, and 67 V, respectively.

demonstrates the gradual movement of the dye-labeled buffer with time, along the shorter channel (between reservoirs 1 and 2)

Freely suspended silicon bars bridging the microchannels as they are characterized in this section are depicted in Fig. 2. It should be noted that in our first electrical characterization of the sensor structures the Si surface was covered by the thin natural oxide only ( 2 nm), i.e., not further passivated by thermal oxidation. Under ambient (dry) conditions, the resistance of the bar ) was three orders of magnitude lower including leads (10 than the values between two neighboring bar structures confirming a good mutual insulation. I–V characteristics of the Si bar as a function of the electrolyte reference potential are displayed in Fig. 5(a). In this range of positive reference (or gate-) potential the device is operated as a field effect transistor in the n-channel enhancement mode. An electron inversion layer is formed within the weakly p-doped Si film close to the Si/SiO interface [16], at all sides of the bar. The is saturating at larger positive drain voltages. drain current The sensitivity of our integrated sensor is determined by the change of current flow along the Si bar as a function of its ). It has to be taken into surface potential (transconductance account that Fig. 5(a) reflects the characteristics of the entire

312

JOURNAL OF MICROELECTROMECHANICAL SYSTEMS, VOL. 15, NO. 2, APRIL 2006

For a planar SOI sensor device sensitivities of the same order of magnitude have been reported earlier. These corresponded to detectable changes in surface charge density down to the order of one electronic charge per 100 nm , neglecting screening effects [15]. Hence, in a rough approximation zones of molecules migrating beneath the Si sensing bar could be discriminated by such difference in their layer charge density measured directly at the sensor surface. V. CONCLUSION

(a)

(b) Fig. 5. (a) I–V characteristics of an electrolyte-gated Si-bar that bridges the separation channel. For reference potentials ranging from 200 to 800 mV typical characteristics of a field effect transistor operating in the electron inversion regime are observed. Inset: Schematic sketch of the measurement setup: A test flow chamber exposes the central chip area including the channel at the detection area to a PBS solution, but excludes the TiAu Ohmic source and drain contacts. The potential of the electrolyte is controlled versus source by a potentiostat in a standard three-electrode setup (counter electrode not drawn). (b) Drain current as function of reference potential at a constant source-drain voltage of , extracted from (a). The line is a linear fit to the data points j :  . above 650 mV with slope @I =@

I V = 100 mV

9

= 0 352 S

system (Si bridge including leads) which was gated by the electrolyte. In the linear operation regime of a MOSFET (well below saturation) one can separate the individual contributions by approximating the whole device as a series resistance. As deduced from simple geometrical considerations the Si bar here contributes 18% to the entire resistance. To estimate the maximum transconductance below the onset of saturation we limit our analysis to the regime of drain voltages below and reference potentials . Here, the transconductance is found to be independent of , as expected for the linear regime. From Fig. 5(b), we . Given extract that in our measurement setup lowest changes of current of the order of 0.5 nA can be resolved this transforms into smallest detectable surface potential changes of the Si sensor bridge of within the chosen electrolyte buffer system.

In this paper, we investigated SOI as a new functional substrate for microfluidic lab-on-chip or micrototal analysis system ( TAS) devices thereby focussing on two central aspects, namely the electrokinetic liquid sample handling and integrated sample detection. Basic microfluidic channel geometries allowed for an electroosmotic flow of a sample aqueous after passivation of the silicon buffer solution up to 5 surface with 200 nm thermal SiO . At the intersection with the separation channel the application of counter potentials could successfully prevent the electrically driven liquid from diffusing into the perpendicular separation channels—a requirement for the injection of reproducible sample plugs. During the microfabrication process using the initial substrate, field-effect-based sensors were monolithically integrated into the microfluidic network successfully. Using these freely suspended silicon bars bridging the channels, surface potential changes in the were detected. In summary, electrolyte down to we demonstrated the potential of SOI as novel base substrate for advanced TAS integrating fluid handling and analysis of charged molecule bands on the same device chip. ACKNOWLEDGMENT The authors are most grateful to K. Brunner, K. Arinaga, M. Aki, S. Fujita, and N. Yokoyama for useful discussions. They also acknowledge the assistance of A. Baumer, C. Lin, A. Solovev, S. Strobel, and P. Truman in semiconductor processing and device characterization. REFERENCES [1] A. Manz, N. Graber, and H. M. Widmer, “Miniaturized total chemical analysis systems: a novel concept for chemical sensing,” Sens. Actuators B, Chem., vol. 1, pp. 244–248, 1990. [2] S. C. Jacobson, R. Hergenröder, L. B. Koutny, and J. M. Ramsey, “Highspeed separations on a microchip,” Anal. Chem., vol. 66, p. 1114, 1994. [3] J. Voldman, M. L. Gray, and M. A. Schmidt, “Microfabrication in biology and medicine,” Ann. Rev. Biomed. Eng., vol. 1, pp. 401–425, 1999. [4] M. Szumski and B. Buszewski, “State of the art in miniaturized separation techniques,” Crit. Rev. Anal. Chem., vol. 32, no. 1, pp. 1–46, 2002. [5] S. C. Jakeway, A. J. de Mello, and E. L. Russell., “Miniaturized total analysis systems for biological analysis,” Fresenius’ J. Anal. Chem., vol. 366, pp. 525–539, 2000. [6] D. R. Reyes, D. Iossifidis, P. A. Auroux, and A. Manz, “Micro total analysis systems. 1. Introduction, theory, and technology,” Anal. Chem., vol. 74, pp. 2623–2636, 2002. [7] P.-A. Auroux, D. Iossifidis, D. R. Reyes, and A. Manz, “Micro total analysis systems. 2. Analytical standard operations and applications,” Anal. Chem., vol. 74, pp. 2637–2652, 2002. [8] H. Becker and C. Gärtner, “Polymer microfabrication methods for microfluidic analytical applications,” Electrophor., vol. 21, pp. 12–26, 2000.

SHARMA et al.: SOI MICROFLUIDIC DEVICE WITH MONOLITHIC SENSOR INTEGRATION

[9] G. Blankenstein and U. D. Larsen, “Modular concept of a laboratory on a chip for chemical and biochemical analysis,” Biosens. Bioelectron., vol. 13, no. 3–4, pp. 427–438, 1998. [10] D. Erickson and D. Li, “Integrated microfluidic devices,” Anal. Chim. Acta, vol. 507, pp. 11–26, 2004. [11] R. W. Tjerkstra, J. G. E. Gardeniers, J. J. Kelly, and A. van den Berg, “Multi-walled microchannels: free-standing porous silicon membranes for use in mu TAS,” J. Microelectromech. Syst., vol. 9, no. 4, pp. 495–501, 2000. [12] M. J. de Boer, R. W. Tjerkstra, J. W. Berenschot, H. V. Jansen, G. J. Burger, J. G. E. Gardeniers, M. Elwenspoek, and A. van den Berg, “Micromachining of buried micro channels in silicon,” J. Microelectromech. Syst., vol. 9, no. 1, pp. 94–103, Mar. 2000. [13] J. D. Zahn, A. A. Deshmukh, A. P. Papavasiliou, A. P. Pisano, and D. Liepmann, “An integrated microfluidic device for the continuous sampling and analysis of biological fluids,” in Proc. 2001 ASME Int. Mechanical Engineering Congress and Exposition, New York, Nov. 2001. [14] P. Bergveld, “Thirty years of ISFETOLOGY—What happened in the past 30 years and what may happen in the next 30 years,” Sens. Actuators B, Chem., vol. 88, pp. 1–20, 2003. [15] M. G. Nikolaides, S. Rauschenbach, S. Luber, K. Buchholz, M. Tornow, G. Abstreiter, and A. R. Bausch, “Silicon-on-Insulator based thin-film resistor for chemical and biological sensor applications,” Chem. Phys. Chem., vol. 4, pp. 1104–1106, 2003. [16] S. M. Sze, Physics of Semiconductor Devices, 2nd ed. New York: Wiley-Interscience, 1981. [17] P. Bergveld and A. Sibbald, “Analytical and biomedical applications of ion-sensitive field-effect transistors,” in Analytical Chemistry, G. Svehla, Ed. New York: Elsevier, 1988, vol. XXIII. [18] A. J. Bard and L. R. Faulkner, Electrochemical Methods, 2nd ed. New York: Wiley, 2001. [19] D. J. Harrison, A. Manz, and P. G. Glavina, “Toward miniaturized electrophoresis and chemical analysis systems on silicon—an alternative to chemical sensors,” Sens. Actuators B, Chem., vol. B10, pp. 107–116, 1993. [20] J. P. Alarie, S. C. Jacobson, C. T. Culbertson, and J. M. Ramsey, “Effects of the electric field distribution on microchip valving performance,” Electrophor., vol. 21, pp. 100–106, 2000.

Sanjiv Sharma was born on June 30, 1973, in Bareilly, India. He received the M.S. degree in chemistry from Dr. Hari Singh Gour University Sagar (M.P), India, in 1995 and the Ph.D. degree from Barkatullah University, Bhopal, India, in 2001. Since February 2001, he has worked in multidisciplinary projects related to DNA sequencing, proteomics, and silicon-on-insulator-based micrototal analytical systems (TAS) with the Institute for Nuclear Physics, Orsay, France, the Hospital St. Antoine, Paris, France, and the Walter Schottky Institute, Technische Universität München (TUM), Munich, Garching, Germany, respectively. He has been a Chevening Technology Fellow and a Postdoctoral Research Associate with the Department of Chemistry before taking up the position of a Research Officer in the Institute of Biomedical Engineering in Imperial College, London. His current interests are in the areas of bionanotechnology and biosensors.

Karin Buchholz was born on March 9, 1974, in Starnberg, Germany. She received the Dipl.-Ing. degree in electrical engineering and information technology from the Technische Universität München (TUM), Munich, Germany, in 2001. After graduating, she joined the Walter Schottky Institute, Bio-Nanostructures Group, Physics Department of the TUM as a Ph.D. degree student. Her main research area is the microstructuring of silicon-oninsulator substrates and the biofunctionalization of silicon dioxide surfaces.

313

Sebastian M. Luber was born on May 14, 1976, in Munich, Germany. He received the Diplom (German M.Sc.) degree in applied physics from the Technische Universität München (TUM), Munich, in 2002. Since then, he has been a postgraduate researcher with the Semiconductor Nanostructures for Molecular Bioelectronics group at the Walter Schottky Institute, TUM. His main research area is the development of novel III–V semiconductor devices for molecular electronics and biosensing applications.

Ulrich Rant was born on January 22, 1975, in Graz, Austria. He received the degree as Diplom Ingenieur in physics from the Technische Universität Graz in 2000. In 2001, he joined the group of Prof. G. Abstreiter at the Walter Schottky Institute, Technische Universität Munich, Germany, for his Ph.D. degree studies.

Marc Tornow received the physics diploma degree from the University of Heidelberg, Germany, in 1993. He received the Ph.D. degree from the University of Stuttgart, Stuttgart, Germany, in 1997. In 1993, he joined the Max-Planck-Institute for Solid State Research, Stuttgart. He continued his research on mesoscopic semiconductor structures as a Postdoctoral Researcher with the Weizmann Institute of Science, Israel, before joining Infineon Technologies, Munich, Germany, in 1999. Since August 2001, he has been working as a group leader at the Chair of Experimental Semiconductor Physics, Prof. G. Abstreiter, at the Walter Schottky Institute of the Technical University Munich. Here, his research mainly focuses on functionalized semiconductor nanostructures for biosensing and molecular bioelectronics. Dr. Tornow was awarded an independent Junior Research Group “Nanotechnology” funded by the German Ministry of Science BMBF in 2004.

Gerhard Abstreiter received the Ph.D. degree in physics from the Technical University Munich (TUM), Germany, in 1975. During his employment as a group leader with the Physics Department, TUM, from 1979 until 1986, he finished his “Habilitation” in 1984 in the field of optical spectroscopy of semiconductor heterostructures. From 1975 to 1979, he worked as a Scientific Staff Member at the Max-Planck-Institute for Solid State Research, Stuttgart, Germany, and Grenoble, France. Since 1987, he has been a full Professor with the Physics Department and the Walter Schottky Institute of TUM. His main current areas of research include the physics and technology of semiconductor nanostructures and development of novel semiconductor devices for optoelectronics, nanoelectronics, quantum information technology, and chemical/biological sensing. Dr. Abstreiter received numerous honors among which are the Walter Schottky Prize (DPG) in 1986, the Gottfried Wilhelm Leibniz Award (DFG) in 1987, a Fellow of the American Physical Society in 1992, and the Max Born Prize and Medal (DPG and IoP) in 1998.