[show abstract][hide abstract] ABSTRACT: We report on investigations of nonlinear radiofrequency responses of electrolytes with Na(+) and Cl(-) ions placed within gold electrodes of a capacitor. The sample was part of a frequency-adjustable inductance-capacitance-resistance (LCR) parallel resonant circuit, and measurements were carried out using the two frequencies intermodulation distortion technique. We employed double layer model to analyze the observed nonlinearities and their dependence on ionic concentration. Electrode-electrolyte interface polarization was found to be a predominant cause of this intrinsic nonlinearity and to be dependent on electrolytic ion concentration. We also measured and calculated coefficients of resistive and capacitive components of the observed nonlinearity.
[show abstract][hide abstract] ABSTRACT: Dielectric measurements of biological samples are obscured by electrode polarization, which at low frequencies dominates over the actual sample response. Reduction of this artifact is especially necessary in studying interactions of electric field with biological systems in the α-dispersion range. We developed a method to reduce the influence of electrode polarization by employing mesh instead of solid electrodes as sensing probes, thereby reducing the area of the double layer. The design decreases the electrode-electrolyte contact area by almost 40% while keeping the bulk sample capacitance the same. Interrogation electric fields away from the electrode surface and sensitivity are unaffected. Electrodes were microfabricated (600μm×50μm, spacing of 100μm) with and without mesh holes 7.5μm×7.5μm in size. Simulations of electric field performed using Comsol Multiphysics showed non-uniformity of the electric field within less than 1.5μm from the electrode surface, which encompasses the double layer region, but at greater distance the solid and mesh electrodes gave the same results. Mesh electrodes reduced capacitance measurements for water and KCl solutions of different concentrations at low frequencies (<10kHz), while higher frequency capacitance remained the same for both electrode types, confirming our hypothesis that this design leaves the electric field mainly unaffected. Impedance measurements at low frequencies for water and mice heart mitochondrial suspension were lower for mesh than for solid electrodes. Comsol simulations confirmed these results by showing that mesh electrodes have a greater charge density than solid electrodes, which affects conductance. These electrodes are being used for mitochondrial membrane potential studies.
[show abstract][hide abstract] ABSTRACT: A film residue obtained by evaporating surfactant-stabilized single-walled carbon nanotube (SWNT) suspension was characterized at 12 GHz using a scanning-sample dielectric resonator technique. Resonant frequency and quality factor changes were measured and cavity perturbation method was used to calculate the SWNT complex permittivity. The effective permittivity of the SWNT was determined as (3516-j316.5) , which provided an average dielectric constant and conductivity for a single SWNT to be 8.1×10<sup>5</sup> and 8.4×10<sup>6</sup> S / m , respectively. Microwave induced losses originated only from the electric field, not from the magnetic field, thus indicating an absence of direct electrical contact between nanotubes and a below percolation-limit configuration.
[show abstract][hide abstract] ABSTRACT: We designed planar electrodes, for dielectrophoretic manipulation of single-walled carbon nanotubes (SWNTs), built as metal-oxide-semiconductor nanogap capacitors with common substrate and oxide thicknesses of 17 and 150 nm. Such design generates high electric fields (10(9) V m(-1)) and also the fringing field is curved due to the conducting substrate, unlike fields generated by conventionally used planar electrodes. Scanning electron microscopy images showed SWNTs aligned parallel and perpendicular to the electrodes. Raman spectroscopic mapping was used to produce separate images of the metallic (m-SWNT) and semiconducting (s-SWNT) nanotube density distributions. As expected, parallel alignment of the m-SWNTs with the E-field was found; however, also a perpendicular alignment of s-SWNTs was observed. Such orthogonal alignment of s-SWNTs is a rare observation and has not been experimentally reported before in detail with Raman images. Due to the unique electrode design, we were able to obtain substantial separation of m-SWNTs and s-SWNTs. Numerical modeling of the electric field factor of the dielectrophoresis force was done, and it matched perfectly with the experimental results. The orthogonal alignment of s-SWNTs results from comparable values of parallel and perpendicular polarizability to the nanotube axis.
[show abstract][hide abstract] ABSTRACT: We developed a BioMEMS device to study cell- mitochondrial physiological functionalities. The pathogenesis of many diseases including obesity, diabetes, heart failure as well as aging has been linked to functional defects of mitochondria. This is understandable as the mitochondria produces up to 90% of ATP, and plays a critical role in cell signaling and apoptosis. The synthesis of ATP is determined by the electrical potential across the inner mitochondrial membrane (IMM) and by the pH difference due to proton flux across it. Therefore, electrical characterization by E-fields with complementary chemical testing was used here. Mitochondrial ion channels present in the IMM control specific ion fluxes, and maintain ion homeostasis, matrix volume, IMM potential etc and thus serve a central role in cell growth and death related processes. Defects in ion channels (Channelopathies) are being attributed to many diseases like cancer, neurodegeneration, etc. Complete physiological roles of various ion channels and their interactions are still unknown, hindering the development of targeted therapeutic agents. The BioMEMS device was fabricated as an SU-8 based microfluidic system with gold electrodes on SiO2/Si wafers for electromagnetic interrogation. Ion Sensitive Field Effect Transistors (ISFETs) were incorporated for proton studies important in electron transport chain, together with monitoring Na+, K+, Ca++ions for ion channel studies. ISFETs are chemically sensitive MOSFET devices, their threshold voltage is directly proportional to the electrolytic H+ ion variation. These ISFETs (sensitivity ˜55 mV/pH for H+) were further realized as specific ion sensitive CHEMFETs by depositing a poly-HEMA layer sandwiched between the gate and a final specific ion sensitive membrane. Electrodes for dielectric spectroscopy studies of mitochondria were designed as 2- and 4-probe structures for optimized operation over a wide frequency range. In addition, to limit polarization effects (which masks actual impedance for high conductivity solutions at low frequencies), a 4-electrode set-up with unique meshed pickup electrodes (7.5×7.5 μm2 loops with 4 μm wires) was fabricated. An electrical model was developed for the mitochondrial sample, and its frequency response correlated with impedance spectroscopy experiments of sarcolemmal mitochondria. Using the mesh electrode structure, we obtained a reduction of 83.28% in impedance at 200 Hz. COMSOL simulations of selected electrical structures in this sensor were compared with experimental results to better understand the physical system. The simultaneous measurement of membrane potential, ion concentrations and pH would enhance diagnostics and studies of mitochondrial diseases.
[show abstract][hide abstract] ABSTRACT: Diborides of Ti, Hf and Zr are thermally, mechanically and chemically stable with good thermal and electrical conductivity. We tested their properties in front-end processes used in Si integrated circuits (IC). Films were deposited by e-beam evaporation either on Si, for the formation of contacts to the source/drain (S/D) regions, or on Si oxides, for the formation of metal gates in p-type metal-oxide-semiconductor (PMOS) transistors. We focused on their crystallization caused by rapid thermal processing (RTP) at temperatures up to 1100 degrees C. Transmission electron microscopy was used for identification of nanocrystallites of TiB(2), ZrB(2), and HfB(2). The grain growth was correlated with temperature and time of RTP. Of all borides, HfB(2) resulted in the most complete crystallization with little amorphous phase left. There was no crystallographic degradation of the interface with Si or dielectrics, except for extreme thermal budgets. Complementary techniques were used for monitoring chemical stability and electrical parameters of test structures to assess the role of recrystallization in device behaviour.
Journal of Microscopy 10/2006; 223(Pt 3):227-30. · 1.63 Impact Factor
[show abstract][hide abstract] ABSTRACT: Nano-gap metal oxide semiconductor (MOS) capacitors were studied to evaluate their limitations in applications of dielectric spectroscopy in living cells. The purpose was to optimize the design of a transducer to avoid interfacial polarization at the electrodes. Silicon IC technology was selected for designing processes in which we could limit electric double layer impedance by precisely controlling dielectric thickness of the capacitors in the range of 17 to 150 nm. The working capacitance was defined by lateral oxide etching of capacitor structures of various configuration to ensure high perimeter to area ratio. Highly doped n+ polysilicon and n+ implanted Si substrate were acting as capacitor's electrodes. Restrictions known from CMOS circuits regarding oxide leakage current, which depends on geometry and increases with the gate area were taken into account. To allow for testing cells (yeasts), which have larger dimensions than nano structures it was necessary to include cell manipulation using dielectrophoresis (DEP). Entrapment of cells at the electrode perimeter preceded electrical measurements. Our focus in analyses was on the frequency dependence of impedance parameters.
[show abstract][hide abstract] ABSTRACT: Titanium boride is used here for contact formation in scaled down devices. Properties of deposited films show strong dependence on electron beam deposition and annealing conditions. Thin TiB<sub>2</sub> films show stable composition in high temperature RTP. Ohmic contacts are formed on p type silicon both by using as deposited and annealed layers. On n-type Si, large thermal budget processes result in dopant outdiffusion and formation of junctions significantly more shallow than in implantation. Recrystallization of TiB<sub>2</sub>, observed after high temperature annealing, depends on films deposition conditions and interface perfection. Material characterization techniques including RBS, XPS, SIMS, and TEM are used to explain results of electrical measurement of test structures.
Advanced Thermal Processing of Semiconductors, 2003. RTP 2003. 11th IEEE International Conference on; 10/2003
[show abstract][hide abstract] ABSTRACT: We address issues of process integration for source and drain (S/D) regions, which include formation of junctions and contacts for future scaled down MOSFETs. Computer simulation using Silvaco and material studies, were used to consider constraints originating in these regions and associated with both device operation and fabrication. In simulations, we focused on the role of doping profiles in the S/D regions on current drivability while devices are being scaled down to the sub. 0.1 μm range. Fabrication and material analysis were oriented on shallow and heavy doped junctions produced using boride layers (TiB<sub>2</sub>), which also acted as a contact material. High thermal stability of the stochiometric diboride ensures significantly restricted dopant outdiffusion to silicon during Rapid Thermal Processing. That should allow for high surface concentrations required for small contact and series resistance. Ohmic contacts are obtained for p-type wafers after annealing, while for n-type wafers, well defined non-leaky p-n diodes are formed. Techniques used in material studies (RBS, XPS and SIMS) did not reveal convincing dopant outdiffusion or significant changes in composition of the films.
Advanced Thermal Processing of Semiconductors 9th Internationa Conference on RTP 2001; 02/2001
[show abstract][hide abstract] ABSTRACT: We investigated the process integration of shallow junctions' formation and titanium silicidation. Junctions were doped with boron or arsenic during diffusion in rapid thermal processing (RTP). Dopant sources were in the form of pure layers of B or As deposited on the silicon substrate by e-beam evaporation or molecular beam epitaxy (MBE), respectively. Contact formation to the junctions was preceded by Ti deposition, either on a sacrificial layer of amorphous silicon or directly on the dopant layer. We studied the role of dopant–metal compound formation as a diffusion barrier to prevent junction degradation during the silicidation process. Such a barrier completely stops silicide formation in the case of B but is less efficient for the As doping. It also impedes boron diffusion into the silicon substrate much more than arsenic diffusion. A standard process sequence, where the junction is formed first and silicidation follows, and a modified process flow, where silicidation is done in situ together with the junction doping lead to differences in the fabricated structures. Various analytical techniques such as secondary ion mass spectroscopy (SIMS), Auger electron spectroscopy (AES), Rutherford backscattering spectroscopy (RBS), spreading resistance profiling (SRP), cross-section transmission electron microscopy (XTEM) and four-point probe were used for junction characterization.
Materials Science in Semiconductor Processing - MATER SCI SEMICOND PROCESS. 01/1998; 1(3):243-247.
[show abstract][hide abstract] ABSTRACT: Diffusion via rapid thermal processing (RTP) has been investigated for fabrication of shallow p-type layers doped with boron. The authors used dopant sources deposited by electron beam evaporation in the form of thin boron layers with or without in situ deposited silicon capping films. The deposition process is compatible with the resist mask due to low temperatures and poor step coverage, which facilitate dopant removal via a lift-off process. Sheet resistance measurements together with secondary ion mass spectroscopy and spreading resistance profiling analyses indicate that doping efficiency is high for both types of sources in the temperature range of 900 to 1,050 C for 10 to 30 s. Full dopant activation in the silicon substrate, except for the surface region, has been recorded for all process conditions. High surface concentrations observed in the processed samples were attributed to a residual boron layer. Oxidation during doping via RTP results in diffusion enhancement and in consumption of the boron source. Results of cross-sectional transmission electron microscopy (TEM) analyses confirm fast oxide growth rates during the diffusion processes in an oxygen ambient. No defects within the doped layers have been found for the process conditions used in the experiments.
Journal of The Electrochemical Society 01/1996; 143(9):2981-2989. · 2.59 Impact Factor
[show abstract][hide abstract] ABSTRACT: Various techniques used in fabrication of deep submicron junctions
are reviewed with respect to their advantages and disadvantages in
silicon very large scale integration (VLSI) circuits technology.
Proximity rapid thermal diffusion is then presented as an alternative
process which results in very shallow junctions with high dopant
concentrations at the surface. The feasibility of Si doping with B, P,
and As for both planar and 3-D structures such as trench capacitors used
in high density DRAM memories is shown based on sheet resistance
measurements, secondary ion mass spectroscopy and scanning electron
micrographs. Retardation effect of arsenic diffusion similar to the well
known inhibition of silicon or SiO<sub>2</sub> deposition in chemical
vapor deposition (CVD) processes is identified and discussed
IEEE Transactions on Electron Devices 01/1995; · 2.06 Impact Factor
[show abstract][hide abstract] ABSTRACT: A new rapid thermal diffusion process for shallow, heavily doped trench junctions in high density dynamic RAMs is described. Planar dopant sources are formed by spin-coating rigid substrates, such as silicon wafers or solid dopant sources, with liquid dopants. Diffusion takes place at high temperatures when the source, placed in proximity to the silicon wafer, releases dopant via evaporation followed by diffusion to the silicon surface. Well-controlled, heavily doped shallow junctions are readily obtained for B, P, and As. The doping process is shown to provide uniform doping of high-aspect-ratio trenches. Process control is achieved by controlling the wafer temperature and duration of the process. Junction depths near 0.1 mu m have been demonstrated over the entire surface of trenches 0.7 mu m in diameter and 6 mu m in depth.< >
IEEE Electron Device Letters 07/1991; · 2.79 Impact Factor