Michael Gaitan

Tin oxide gas sensor fabricated using CMOS micro-hotplates and in-situ processing

A monolithic tin oxide (SnO/sub 2/) gas sensor realized by commercial CMOS foundry fabrication (MOSIS) and postfabrication processing techniques is reported. The device is composed of a sensing film that is sputter-deposited on a silicon micromachined hotplate. The fabrication technique requires no masking and utilizes in situ process control and monitoring of film resistivity during film growth. Microhotplate temperature is controlled from ambient to 500 degrees C with a thermal efficiency of 8 degrees C/mW and thermal response time of 0.6 ms. Gas sensor responses of pure SnO/sub 2/ films to H/sub 2/ and O/sub 2/ with an operating temperature of 350 degrees C are reported. The fabrication methodology allows integration of an array of gas sensors of various films with separate temperature control for each element in the array, and circuits for a low-cost CMOS-based gas sensor system.<<ETX>>

Microfluidic Mixing and the Formation of Nanoscale Lipid Vesicles

We investigate the formation of unilamellar lipid vesicles (liposomes) with diameters of tens of nanometers by controlled microfluidic mixing and nanoparticle determination (COMMAND). Our study includes liposome synthesis experiments and numerical modeling of our microfluidic implementation of the batch solvent injection method. We consider microfluidic liposome formation from the perspective of fluid interfaces and convective-diffusive mixing, as we find that bulk fluid flow parameters including hydrodynamically focused alcohol stream width, final alcohol concentration, and shear stress do not primarily determine the vesicle formation process. Microfluidic device geometry in conjunction with hydrodynamic flow focusing strongly influences vesicle size distributions, providing a coarse method to control liposome size, while total flow rate allows fine-tuning the vesicle size in certain focusing regimes. Although microfluidic liposome synthesis is relatively simple to implement experimentally, numerical simulations of the mixing process reveal a complex system of fluid flow and mass transfer determining the formation of nonequilibrium vesicles. These results expand our understanding of the microfluidic environment that controls liposome self-assembly and yield several technological advances for the on-chip synthesis of nanoscale lipid vesicles.

Characterization of broad-band transmission for coplanar waveguides on CMOS silicon substrates

This paper presents characteristics of microwave transmission in coplanar waveguides (CPWs) on silicon (Si) substrates fabricated through commercial CMOS foundries. Due to the CMOS fabrication, the metal strips of the CPW are encapsulated in thin films of Si dioxide. Many test sets were fabricated with different line dimensions, all on p-type substrates with resistivities in the range from 0.4 /spl Omega//spl middot/cm to 12.5 /spl Omega//spl middot/cm. Propagation constant and characteristic impedance measurements were performed at frequencies from 0.1 to 40 GHz, using a vector-network analyzer and the through-reflect-line (TRL) deembedding technique. A quasi-TEM equivalent circuit model was developed from the available process parameters, which accounts for the effects of the electromagnetic fields in the CPW structure over a broad frequency range. The analysis was based on the conformal mapping of the CPW multilayer dielectric cross section to obtain accurate circuit representation for the effects of the transverse fields.

Fast temperature programmed sensing for micro-hotplate gas sensors

We describe an operating mode of a gas sensor that greatly enhances the capability of the device to determine the composition of a sensed gas. The device consists of a micromachined hotplate with integrated heater, heat distribution plate, electrical contact pads, and sensing film. The temperature programmed sensing (TPS) technique uses millisecond timescale temperature changes to modify the rates for adsorption, desorption, and reaction of gases on the sensing surface during sensor operation. A repetitive train of temperature pulses produces a patterned conductance response that depends on the gas composition, as well as the temperature pulse width, amplitude, and specific sequence of pulses. Results are shown for the vapors of water, ethanol, methanol, formaldehyde, and acetone.

Micromachined thermal radiation emitter from a commercial CMOS process

Fabrication of thermally isolated micromechanical structures capable of generating thermal radiation for dynamic thermal scene simulation (DTSS) is described. Complete compatibility with a commercial CMOS process is achieved through design of a novel, but acceptable, layout for implementation by the CMOS foundry using its regular process sequence. Following commercial production and delivery of the CMOS chips, a single maskless etch in an aqueous ethylemediamine-pyrocatechol mixture is performed to realize the micromechanical structures. The resulting structures are suspended plates consisting of polysilicon resistors encapsulated in the field and CVD (chemical-vapor-deposited) oxides available in the CMOS process. The plates are suspended by aluminum heater leads that are also encapsulated in the field and CVD oxides. Studies of the suitability of these structures for DTSS have been initiated, and early favorable results are reported.<<ETX>>

Micromachined microwave transmission lines in CMOS technology

Coplanar waveguides were designed and fabricated through a commercial CMOS process with post-processing micromachining. The transmission-line layouts were designed with commercial computer-aided design (CAD) tools. Integrated circuits (ICs) were fabricated through the MOSIS service, and subsequently suspended by top-side etching. The absence of the lossy silicon substrate after etching results in significantly improved insertion-loss characteristics, dispersion characteristics, and phase velocity. Two types of layout are presented for different ranges of characteristic impedance. Measurements of the waveguides both before and after micromachining were performed at frequencies from 1 to 40 GHz using a vector network analyzer and de-embedding techniques, showing improvement of loss characteristics of orders of magnitude. For the entire range of frequencies, for the 50-/spl Omega/ layout, losses do not exceed 4 dB/cm. These losses are mainly due to the small width and thickness of the metal strips. Before etching, losses are as high as 38 dB/cm due to currents in the underlying substrate. Phase velocity in the micromachined transmission lines is close to that in free space.

Optimized temperature-pulse sequences for the enhancement of chemically specific response patterns from micro-hotplate gas sensors

Abstract Microfabricated solid-state gas sensors have been of continuing interest because of the potential for arrays of devices with low power consumption. Devices based on a micromachined ‘hotplate’ offer the additional advantage of a wide operating temperature range with a rapid thermal time constant of order 1 ms. By operating the device in a temperature-programmed mode, reaction kinetics on the sensing film surface are altered, producing a time-varying response signature that is characteristic of the gas being sensed. Approaches to optimizing such temperature programs to maximize the differences in response signatures for gases of interest or to enhance the sensitivity of the device are discussed.

Micromachined Convective Accelerometers in Standard Integrated Circuits Technology

This letter describes an implementation of micromachined accelerometers in standard complimentary metal–oxide–semiconductor technology. The devices operate based on heat convection and consist of microheaters and thermocouple or thermistor temperature sensors separated by a gap which measure temperature difference between two sides of the microheater caused by the effect of acceleration on free gas convection. The devices show a small linearity error of <0.5% under tilt conditions (±90°), and <2% under acceleration to 7g(g≡9.81 m/s2). Sensitivity of the devices is a nearly linear function of heater power. For operating power of ∼ 100 mW, a sensitivity of 115 μV/g was measured for thermopile configuration and 25 μV/g for thermistor configurations. Both types of devices are operable up to frequencies of several hundred Hz.

Hybrid postprocessing etching for CMOS-compatible MEMS

A major limitation in the fabrication of microstructures as a postCMOS (complimentary metal oxide semiconductor) process has been overcome by the development of a hybrid processing technique, which combines both an isotropic and anisotropic etch step. Using this hybrid technique, microelectromechanical structures with sizes ranging from 0.05 to /spl sim/1 mm in width and up to 6 mm in length were fabricated in CMOS technology. The mechanical robustness of the microstructures determines the limit on their dimensions. Examples of an application of this hybrid technique to produce microwave coplanar transmission lines are presented. The performance of the micromachined microwave coplanar waveguides meets the design specifications of low loss, high phase velocity, and 50 /spl Omega/ characteristic impedance. Various commonly used etchants were investigated for topside maskless postmicromachining of silicon wafers to obtain the microstructures. The isotropic etchant used is gas-phase xenon difluoride (XeF/sub 2/), while the wet anisotropic etchants are either ethylenediamine-pyrocatechol (EDP) or tetramethylammonium hydroxide (TMAH). The advantages and disadvantages of these etchants with respect to selectivity, reproducibility, handling, and process compatibility are also described.

An experimental comparison of measurement techniques to extract Si-SiO2 interface trap density

Abstract For the first time, five methods of measuring Si-SiO 2 interface trap densities were compared experimentally on three otherwise identical MOSFETs which were radiation-stressed so as to induce different levels of interface trap densities. The results show that when sources of error and limitations are taken into account, these methods are capable of yielding interface trap density estimates which are in good quantitative agreement. Furthermore, the change in measured interface trap densities with radiation is independent of the method used. A comprehensive review of the methods is presented.