Mona E. Zaghloul

A monolithic CMOS microhotplate-based gas sensor system

A monolithic CMOS microhotplate-based conductance-type gas sensor system is described. A bulk micromachining technique is used to create suspended microhotplate structures that serve as sensing film platforms. The thermal properties of the microhotplates include a 1-ms thermal time constant and a 10/spl deg/C/mW thermal efficiency. The polysilicon used for the microhotplate heater exhibits a temperature coefficient of resistance of 1.067/spl times/10/sup -3///spl deg/C. Tin(IV) oxide and titanium(IV) oxide (SnO/sub 2/,TiO/sub 2/) sensing films are grown over postpatterned gold sensing electrodes on the microhotplate using low-pressure chemical vapor deposition (LPCVD). An array of microhotplate gas sensors with different sensing film properties is fabricated by using a different temperature for each microhotplate during the LPCVD film growth process. Interface circuits are designed and implemented monolithically with the array of microhotplate gas sensors. Bipolar transistors are found to be a good choice for the heater drivers, and MOSFET switches are suitable for addressing the sensing films. An on-chip operational amplifier improves the signal-to-noise ratio and produces a robust output signal. Isothermal responses demonstrate the ability of the sensors to detect different gas molecules over a wide range of concentrations including detection below 100 nanomoles/mole.

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.

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.

Electrostatically actuated resonant microcantilever beam in CMOS technology for the detection of chemical weapons

The design, fabrication, and testing of a resonant cantilever beam in complementary metal-oxide semiconductor (CMOS) technology is presented in this paper. The resonant cantilever beam is a gas-sensing device capable of monitoring hazardous vapors and gases at trace concentrations. The new design of the cantilever beam described here includes interdigitated fingers for electrostatic actuation and a piezoresistive Wheatstone bridge design to read out the deflection signal. The reference resistors of the Wheatstone bridge are fabricated on auxiliary beams that are immediately adjacent to the actuated device. The whole device is fabricated using a 0.6-/spl mu/m, three-metal, double-poly CMOS process, combined with subsequent micromachining steps. A custom polymer layer is applied to the surface of the microcantilever beam to enhance its sorptivity to a chemical nerve agent. Exposing the sensor with the nerve agent simulant dimethylmethylphosphonate (DMMP), provided a demonstrated detection at a concentration of 20 ppb or 0.1 mg/m/sup 3/. These initial promising results were attained with a relatively simple design, fabricated in standard CMOS, which could offer an inexpensive option for mass production of a miniature chemical detector, which contains on chip electronics integrated to the cantilever beam.

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.

Thermoelectric power sensor for microwave applications by commercial CMOS fabrication

This work describes an implementation of a thermoelectric microwave power sensor fabricated through commercial CMOS process with additional maskless etching. The sensor combines micromachined coplanar waveguide and contact pads, a microwave termination which dissipates heat proportionally to input microwave power, and many aluminum-polysilicon thermocouples. The device was designed and fabricated in standard CMOS technology, including the appropriate superimposed dielectric openings for post-fabrication micromachining. By removing the bulk silicon located beneath the device through micromachining, thermal and electromagnetic losses are minimized. The sensor measures signal true RMS power in the frequency range up to 20 GHz with input power in the -30 dBm to +10 dBm range. Over this 40 dB dynamic range, output voltage versus input power is linear within less than /spl plusmn/0.16%. Automatic network analyzer data show an acceptable input return loss of less than -30 dB over the entire frequency range.

CMOS foundry implementation of Schottky diodes for RF detection

Schottky diodes for RF power measurement were designed and fabricated using a commercial n-well CMOS foundry process through the MOSIS service. The Schottky diodes are implemented by modifying the SCMOS technology file of the public-domain graphics layout editor, MAGIC, or by explicitly implementing the appropriate CIF layers. The modifications allow direct contact of first-layer metal to the low-doped substrate. Current-voltage measurements showed that only the n-type devices had rectifying properties with a barrier height of 0.78 eV. The I-V results were verified by performing capacitance-voltage measurements on diodes of different contact-areas. The diodes were tested in an RF detector circuit. The cut off frequency of the detector was shown to be 600 MHz.

An enhancement-mode MOS voltage-controlled linear resistor with large dynamic range

It is shown that the depletion-mode linear resistor of Babanezhad and Temes (IEEE J. Solid-State Circuits, vol. SC-19, p.932-8, 1984) can be implemented in enhancement-mode devices. This allows a large increase in the dynamic range of the resistors. By inserting a bias source, the linearity can also be improved. A layout and experimental results on the resulting IC are included. >

SYNCHRONIZATION OF CHAOTIC NEURAL NETWORKS AND APPLICATIONS TO COMMUNICATIONS

Methods for synchronizing discrete time chaotic neural networks are presented with possible applications in single- or multi-user private communications. Chaotic neurons, characterized with a piecewise-linear N-shaped transfer function, are connected into Hopfield-like networks with parameters set for chaos. The networks are used as transmitter and receiver circuits in chaotic communications schemes. The first algorithm is a modification of simple chaotic masking which makes synchronization robust and insensitive to the perturbation from the added information signal. A mathematical proof and simulation results of the scheme are shown for small networks. We have verified the method experimentally, using single- and two-neuron circuits. The second algorithm utilizes modulation of the transmitting chaotic network by a binary bit stream and detection of the corresponding synchronization error at the receiver. A method for multiple-user chaotic communication is also presented, utilizing chaotic neurons and spread spectrum techniques. The effects of additive noise in the proposed communication schemes are considered and simulated. Synchronization of larger networks and possible applications are also discussed.