Showing papers by "Soumen Das published in 2012"

Study of hydrophilicity and stability of chemically modified PDMS surface using piranha and KOH solution

Abstract: Successful realization of various BioMEMS devices demands effective surface modification techniques of PDMS elastomer. This paper presents a detailed report on a simple and cost effective approach for surface modification of PDMS films involving wet chemical treatment in two-step processes: primarily involving piranha solution followed by KOH dip to improve hydrophilicity and stability of PDMS surface. Chemical composition of the solution and surface treatment condition have been varied and optimized to significantly increase the surface energy. The effect of surface modification of the elastomer after wet chemical treatment is analyzed using contact angle measurement and FTIR-ATR study. PDMS surface treated in piranha solution with H2O2 and H2SO4 in the ratio of 2:3 followed by a dip in KOH solution for 15 min duration each, demonstrated a maximum reduction of contact angle to ∼27° as compared to untreated sample having a contact angle of ∼110°. The removal of hydrophobic methyl group from elastomer surface and subsequent hydrophilization of surface by wet chemical process was confirmed from FTIR-ATR spectra. This result is also supported by improved adhesion and electrical continuity of deposited aluminum metal film over the modified PDMS surface. Copyright © 2011 John Wiley & Sons, Ltd.

Design, fabrication and performance evaluation of a vaporizing liquid microthruster

Abstract: A recent application domain of MEMS technology is in the development of microthrusters for micro-/nanosatellites. Among the various types of MEMS microthruster developed so far, the vaporizing liquid microthruster (VLM) has been widely explored for its capability to produce continuously variable thrust in the micro-Newton (µN) to mili-Newton (mN) range. This paper reports the design and experimental validation of silicon MEMS VLM consisting of a microcavity, inlet channel and converging–diverging (C-D) in-plane exit nozzle integrated in two micromachined bonded chips and sandwiched between two p-diffused microheaters, located at the top and bottom surface of the device. Structural configuration was designed using simple analytical equations to achieve maximum thrust force by controlling the inlet propellant flow and heater power of VLM in an efficient way. In addition, a 3D model using a computational fluid dynamics technique was constructed to simulate the aft section of VLM for the investigation of its aerodynamic behavior. The device fabrication and testing have been briefly described. The fabricated VLM is capable to produce 1 mN thrust using maximum heater power of 3.6 W at a water flow rate of 2.04 mg s−1 using an in-plane C-D exit nozzle of throat area 130 µm × 100 µm. A detailed thrust force measurement was carried out with the variation of input heater power for different mass flow conditions and exit to throat area ratio of the exit nozzle, and the results were interpreted with the theoretical model. The model gives considerable physical insight in the operation of the VLM. Finally, a performance comparison with other published VLM results indicates that the present design can yield comparatively more thrust force with much less input power. A performance comparison with other published VLM results indicates that the present design can achieve improved performance by integrating two heaters with appropriate chamber volume in respect of propellant flow rate and input power for obtaining a supersaturated dry stream.

Impedimetric characterization of human blood using three-electrode based ECIS devices

Abstract: In this study, three-electrode based electric cell-substrate impedance sensing (ECIS) devices were used to study the electrical properties of blood and its constituents using electrochemical impedance spectroscopy. The three-electrode based ECIS devices were fabricated using micromachining technology with varying sizes for working, reference, and counter electrodes. The blood and its constituents such as serum, plasma, and red blood cells (RBCs) were prepared by conventional methods and stored for impedance measurement using fabricated microdevices. Equivalent circuits for blood, serum, plasma, and RBCs were proposed using the software package ZSimpWin to validate the experimental data. The proposed equivalent circuit models of blood and its components have excellent agreement up to a frequency of 1 MHz. It is evident from the experimental results that blood and its components have specific impedance signatures that decrease with the increase of frequency. Blood shows higher impedance than the other samples in the lower frequency range (<50 kHz). It was also found that above 50 kHz, the impedance value of RBCs is nearly the same as whole blood. The impedance of serum and plasma steadily decreases with the increase of frequency up to 100 kHz and flattens out after that. The minimum impedance value achieved for serum and plasma is much less than the value obtained for whole blood.

Design, fabrication and testing of a high performance silicon piezoresistive Z-axis accelerometer with proof mass-edge-aligned-flexures

Abstract: This paper presents design, fabrication and testing of a quad beam silicon piezoresistive Z-axis accelerometer with very low cross-axis sensitivity. The accelerometer device proposed in the present work consists of a thick proof mass supported by four thin beams (also called as flexures) that are connected to an outer supporting rim. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high performance applications. In the present study, low cross-axis sensitivity is achieved by improving the device stability by placing the four flexures in line with the proof mass edges. Various modules of a finite element method based software called CoventorWare™ was used for design optimization. Based on the simulation results, a flexure thickness of 30 μm and a diffused resistor doping concentration of 5 × 1018 atoms/cm3 were fixed to achieve a high prime-axis sensitivity of 122 μV/Vg, low cross-axis sensitivity of 27 ppm and a relatively higher bandwidth of 2.89 kHz. The designed accelerometer was realized by a complementary metal oxide semiconductor compatible bulk micromachining process using a dual doped tetra methyl ammonium hydroxide etching solution. The fabricated accelerometer devices were tested up to 13 g static acceleration using a rate table. Test results of fabricated devices with 30 μm flexure thickness show an average prime axis sensitivity of 111 μV/Vg with very low cross-axis sensitivities of 0.652 and 0.688 μV/Vg along X-axis and Y-axis, respectively.

Characterization of Electrode/Electrolyte Interface of ECIS Devices

Abstract: Electric cell-substrate impedance sensing requires low electrode/electrolyte interface impedance for effective biomedical and biophysical applications. Thus a complete understanding of physical processes involved in the formation of an electric double layer is required to design a low interface impedance device. This paper presents the numerical simulation of the impedance for the electrode/electrolyte interface of three-electrode devices along with the practical realization for the effective workout of impedance sensing devices. The three-electrode based impedance sensing devices along with phosphate buffered saline as electrolyte is simulated using COMSOL Multiphysics to evaluate the impedance of the electrode/electrolyte interface. Microfabrication technology is used to realize threeelectrode impedance sensing devices with diverse configuration which are used to measure the electrode/electrolyte interface impedance. The measured impedance data were then compared with the COMSOL simulated results and it is found that both the data sets fitted well with less than 5 % RSE. The results obtained from simulation and experiments indicate that the impedance due to double layer diffusion dominates in the low frequency region up to few kHz whereas electrolytic bulk resistance plays a major role in the higher frequency range. The experimental impedance data were further interpreted by electrochemical impedance spectroscopy analysis software to model the equivalent circuit of the electrochemical system.

Realisation of silicon piezoresistive accelerometer with proof mass-edge-aligned-flexures using wet anisotropic etching

Abstract: This Letter presents simulation, fabrication and testing of a high-performance quad-beam silicon piezoresistive accelerometer with very low cross-axis sensitivity. Cross-axis sensitivity in piezoresistive accelerometers is an important issue particularly for high-performance applications. In the present Letter, low cross-axis sensitivity is achieved by improving the device stability by placing four flexures in line with the proof mass edges. The accelerometer device is realised in a single-step double-sided bulk micromachining technique using a 5% dual-doped tetra methyl ammonium hydroxide solution as an anisotropic etchant. Test results of four fabricated devices show an average prime-axis sensitivity of 559.5 µV/g/5 V, a maximum cross-axis sensitivity of 0.62% full scale (FS acceleration=13 g) of the prime-axis sensitivity and nonlinearity at a level of 0.5% of FS which are comparatively better than already reported devices and commercially available piezoresistive sensors.

Compatibility study of thin passivation layers with hydrazine for silicon-based MEMS microthruster

Abstract: In this work, the compatibility studies of silicon and its different multilayer structures with hydrazine for possible applications to MEMS have been reported. Grazing incidence x-ray diffraction patterns of the r.f. sputtered Si/SiO2/Si3N4 stack layer show preferably oriented crystalline structure after hydrazine treatment at different temperatures. The Fourier transform infrared spectroscopic measurement reveals that local bonding of the constituent atoms of the surface layers, where Si–O bond is replaced partially by Si–N bond while treated. Further, the surface morphology carried out by atomic force microscopy exhibits the tendency of reducing surface roughness with the increase in temperature during hydrazine treatment. From the axisymmetric drop shape analysis (ADSA), it is observed that static contact angle changes slightly for different wettability nature of solid surface due to aggregation of crystallites in the valley of the surface fluctuation and anisotropic modification in preferred orientation of the film surface. On the basis of equation of state theory with approximation of solid surface–liquid, interfacial energy was applied to determine the solid surface free energy providing the limited variation in different stack layers. Lastly, the J–V characteristic of the stack layer treated by hydrazine at different temperatures shows multiple current conduction regions with the same current density for varying electric field. Therefore, among various single or multilayer silicon-based thin film combinations, the Si/SiO2/Si3N4 stack layer is the most promising passivation layer for hydrazine-based MEMS applications.

Electrical characterization of suspended HeLa cells using ECIS based biosensor

Abstract: Impedance spectroscopy of biological cells has been used to monitor cell status, e.g. cell proliferation, viability, etc. It is also a fundamental method for the study of the electrical properties of cells which has been utilised for cell identification in investigations of cell behaviour under an applied electric field. Impedance measurement of cell suspensions i.e. a group of cells within a culture medium provides better information about the cell properties than that of single cell measurement. This paper presents electrical characterization of cervical carcinoma cell lines (HeLa) suspending in PBS medium using electric cell-substrate impedance sensing based biosensor. The impedance data are measured using commercially available ECIS device for without and with HeLa cells in the PBS culture medium over a frequency sweep of 100 Hz to 10 MHz. An electrical equivalent circuit of ECIS system with cells suspending in medium is developed and the corresponding electrical parameters are extracted by fitting the experimental data in fitting software. The result also shows that impedance data becomes constant after a certain time for a particular frequency and this response can be used to monitor the cell attachment and confluence.

Extended Stern Model

Abstract: In this paper, a theoretical approach of extended Stern model is formulated to represent the electric double layer (EDL) for biochemical as well as biological samples. The existing Stern model is used for several decades to describe the phenomena of electric double layer of electrode/electrolyte interface. In the conventional stern model the double layer which is formed between the electrode and electrolyte interface is described by double layer capacitance. Using the existing Stern model, the equivalent circuit model is not valid for electrical double layer capacitance of electrode/electrolyte interface in I² dispersion range. The protein molecules form chemical coupling and chemical adsorption along with classical ionic bonding with gold electrodes. Thus, the compactness of EDL decreases and the double layer capacitance is replaced by a constant phase element (CPE). In the present paper, a three-electrode based ECIS device was used to measure the impedance of various enzymatic solutions for practical realization of theoretical approach. The results obtained from experimental work, were simulated by equivalent circuit simulator, ZsimpWin to validate the extended Stern model by comparing I‡ 2 value. Finally the electrical parameters were extracted and compared for Stern model and extended Stern model. The results obtained by practical experiment and equivalent circuit simulation showed the effectiveness of extended Stern model over Stern model.

Pyrex/Si-bonded microvalve performance for precision applications in micropropulsion systems

Abstract: Reduction in power consumption while maintaining trajectory alignment and ensuring precise attitude control in microsatellites has been a global bottleneck in the coupled fields of inertial navigation and micropropulsion systems research. This paper presents the development of a novel microelectromechanical Pyrex/Si binary valve with piezoelectric stack-actuation for applications in microsatellite propulsion systems. The Pyrex 7740 and silicon wafers have been processed in parallel with two different structures being microfabricated and bonded eutectically to realize the microvalve. Electrochemical spark erosion method has been used to realize the inlet/outlet holes in Pyrex 7740 and KOH bulk micromachining to fabricate the top membrane structure in silicon. The device has been subjected to extensive analyses and characterization and has been shown to comply with the requirements of generic ion thrusters used for microsatellite propulsion.

Fabrication of polymer micro-tip by optical lensing technique

Abstract: Fabrication of polymer micro-tips using SU-8 negative photoresist for bio-applications is reported. The SU-8 processing technology and isotropic glass etching process have been developed and utilized to fabricate micro-tips on glass substrate by applying optical lensing effect during photolithography. Experimentally, micro-tips of 25 μm base diameter, ~1 μm tip diameter, and ~250 μm height, have been demonstrated.