Showing papers by "Mario Kupnik published in 2012"

Mesoporous Thin-Film on Highly-Sensitive Resonant Chemical Sensor for Relative Humidity and CO2 Detection

Abstract: Distributed sensing of gas-phase chemicals is a promising application for mesoporous materials when combined with highly sensitive miniaturized gas sensors. We present a direct application of a mesoporous silica thin film on a highly sensitive miniaturized resonant chemical sensor with a mass sensitivity at the zeptogram scale for relative humidity and CO2 detection. Using mesoporous silica thin-film, we report one of the lowest volume resolutions and a sensitive detection of 5.1 × 10–4% RH/Hz to water vapor in N2, which is 70 times higher than a device with a nontemplated silica layer. In addition, a mesoporous thin-film that is functionalized with an amino-group is directly applied on the resonant sensor, which exhibits a volume sensitivity of 1.6 × 10–4%/Hz and a volume resolution of 1.82 × 10–4% to CO2 in N2.

Functionalization layers for CO2 sensing using capacitive micromachined ultrasonic transducers

Abstract: Sensing of carbon dioxide (CO 2 ) using inexpensive, miniaturized, and highly sensitive sensors is of great interest for environmental and consumer applications. In this paper, we present four functionalization layers that are suitable for resonant sensors based on mass-loading for CO 2 detection. We compare the volume sensitivities of these layers to CO 2 and relative humidity (RH) by using a highly sensitive 50-MHz capacitive micromachined ultrasonic transducer (CMUT) as a resonant sensor. Among the four functionalization layers, the layer based on a guanidine polymer exhibits the highest volume sensitivity to CO 2 of 1.0 ppm/Hz in N 2 and 3.8 ppm/Hz in air (∼45%RH). Furthermore, we report on other important characteristics of the guanidine polymer for sensing applications, including polymer saturation, regeneration, and repeatability.

Long-term measurement results of pre-charged CMUTs with zero external bias operation

Abstract: We present long-term measurement results (>1.5 years) of CMUTs, which have been pre-charged for zero external bias operation. The fabrication is based on a direct wafer bonding process with a thick-buried-oxide-layer, which allows the realization of only partially connected, donut-shaped bottom electrodes. The only partially connected bottom electrode has a central portion that is completely encapsulated by 3-μm-thick thermally-grown silicon dioxide, and, thus, electrically floating. The devices are pre-charged by applying a dc voltage higher than the pull-in voltage, which injects charges into the electrically floating portion and creates a sufficiently strong intrinsic electric field in the gap. Measurements of resonant frequency at various bias voltages show that the charges have completely remained in the floating portion for the last 19 months. We prove the zero-external-bias operations with the pre-charged CMUTs by measuring the electrical input impedance, the ac signal displacement, and pitch-catch measurements under zero external dc bias voltage. Our results show that pre-charging CMUTs is feasible, and that the devices are capable of long-term, zero external bias voltage operation.

Method for operating CMUTs under high and varying pressure

Abstract: Capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode are provided. Such a CMUT always has its plate in contact with the substrate, even for zero applied electrical bias. This contact is provided by the pressure difference between the environment, and the pressure of the evacuated region between the CMUT plate and substrate. Due to this permanent contact, the electric field in the gap for a given DC bias voltage will be larger, which provides improved coupling efficiency at lower DC bias voltages. Furthermore, in an environment with high and varying pressure, the plate will not shift between the conventional mode and the collapsed mode, but will only be pushed down with varying contact radius. In some embodiments, an electrode shaped as an annulus is employed, so that only the active vibrating part of the CMUT plate sees the applied DC and AC voltages.

Wide pressure range operation of air-coupled CMUTs

Abstract: We present measurement results of capacitive micromachined ultrasonic transducers (CMUTs) in permanent contact mode over a wide pressure range (1 - 8 atm). The CMUT plates are in contact with the bottom of the cavities due to atmospheric pressure, even without any dc bias voltage. The electrical input impedance at various dc bias voltages are measured at elevated pressure to characterize individual devices. The open and short circuit resonant frequencies are extracted from the impedance data, and the acoustic performance of pairs of devices is evaluated by performing pitch-catch measurements. A frequency matching method is proposed and used to determine the optimal dc bias voltages for the transmitting and receiving CMUTs individually. Our electrical impedance results show good agreement with the finite element model results (modal and harmonic analysis performed with ANSYS) over the entire pressure range. Moreover, the pitch-catch measurement results validate the proposed frequency matching method for an optimal biasing scheme, and a received signal with good signal-to-noise ratio of 45 dB was observed at a pressure of 7 atm. In conclusion, the behavior of CMUTs in permanent contact mode can be predicted well with our FEA, and they are indeed a promising solution in providing ultrasonic transducers that can operate over a wide pressure range.

Pre-charged CMUTs for zero-external-bias operation

Abstract: Capacitive micromachined ultrasonic transducers (CMUTs) having a pre-charged floating electrode are provided. Such CMUTs can operate without an applied DC electrical bias. Charge can be provided to the floating electrode after or during fabrication in various ways, such as injection by an applied voltage, and injection by ion implantation.