MEMS Microhotplate Research Articles

Novel screen-printed ceramic MEMS microhotplate for MOS sensors

We developed a new approach to the fabrication of MEMS substrates for MOS gas sensors. This full screen-printing process is based on the application of sacrificial material, which is solid at the near-room temperature of printing and turns to powder after the firing of the elements of the sensor. Therefore, this sacrificial material can be removed from under the suspended elements of the MEMS structure in ultrasonic bath. The glass-ceramic MEMS is a cantilever structure equipped with a Pt-based microheater made of Pt resistive material with sheet resistance of about 4 Ohm/square fabricated using core-shell technology. It is located at the end edge of the cantilever and is isolated from the contacts to the sensing layer by glass-ceramic insulation. Screen-printing provides cheap fabrication, robustness and low power (∼120 mW@450°C) of the sensing element. The functionality of the microhotplate was checked using ZnO nanomaterial deposited by microextruder, it demonstrated high response and selectivity of ZnO material to NO2 (response 41.6 at 200°C for 10 ppm).

Acceleration and drift reduction of MOX gas sensors using active sigma-delta controls based on dielectric excitation

The objective of this paper is to apply a closed-loop control based on dielectric excitation to MOX gas sensors in order to improve their response time. The control implements a feedback loop in which temperature modulations keep constant the sensor reactance, measured at constant temperature. The required fast temperature switching has been implemented on MEMS microhotplates. The mean temperature generated by the control is the new output signal. This technique is applied to an in-house sensor made of WO3 nanowires decorated with gold nanoparticles to detect NH3 and to a commercial MEMS MOX sensor (CCS801).

Flame-Made La2O3-Based Nanocomposite CO2 Sensors as Perspective Part of GHG Monitoring System.

Continuous monitoring of greenhouse gases with high spatio-temporal resolution has lately become an urgent task because of tightening environmental restrictions. It may be addressed with an economically efficient solution, based on semiconductor metal oxide gas sensors. In the present work, CO2 detection in the relevant concentration range and ambient conditions was successfully effectuated by fine-particulate La2O3-based materials. Flame spray pyrolysis technique was used for the synthesis of sensitive materials, which were studied with X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), diffuse reflectance infrared Fourier transform spectroscopy (DRIFTs) and low temperature nitrogen adsorption coupled with Brunauer–Emmett–Teller (BET) effective surface area calculation methodology. The obtained materials represent a composite of lanthanum oxide, hydroxide and carbonate phases. The positive correlation has been established between the carbonate content in the as prepared materials and their sensor response towards CO2. Small dimensional planar MEMS micro-hotplates with low energy consumption were used for gas sensor fabrication through inkjet printing. The sensors showed highly selective CO2 detection in the range of 200–6667 ppm in humid air compared with pollutant gases (H2 50 ppm, CH4 100 ppm, NO2 1 ppm, NO 1 ppm, NH3 20 ppm, H2S 1 ppm, SO2 1 ppm), typical for the atmospheric air of urbanized and industrial area.

Miniaturized ceramic platform for metal oxide gas sensors array

In work is developing an ultra-fast, low cost and technology flexible process for production array of ceramic MEMS microhotplates for using in semiconductor gas sensors orientated to small series applications, where is sufficient to produce 10-100 samples with a different layout of heaters and membrane per day.

A new method to integrate ZnO nano-tetrapods on MEMS micro-hotplates for large scale gas sensor production

A new method, which is easily scalable to large scale production, has been developed to obtain gas sensor devices based on zinc oxide (ZnO) nanostructures with a ‘tetrapod’ shape. The method can be easily extended to other kinds of nanostructures and is based on the deposition of ZnO nanostructures through polymeric masks by centrifugation, directly onto properly designed MEMS micro-hotplates. The micromachined devices, after the mask is peeled off, are ready for electrical bonding and sensing test. Sensor response has been successfully measured for some gases and volatile organic compounds with different chemical properties (ethanol, methane, nitrogen dioxide, hydrogen sulfide).

Thermal characterization of carbon nanotube foam using MEMS microhotplates and thermographic analysis

Thermal material properties play a fundamental role in the thermal management of microelectronic systems. The porous nature of carbon nanotube (CNT) arrays results in a very high surface area to volume ratio, which makes the material attractive for surface driven heat transfer mechanisms. Here, we report on the heat transfer performance of lithographically defined micropins made of carbon nanotube (CNT) nanofoam, directly grown on microhotplates (MHPs). The MHP is used as an in situ characterization platform with controllable hot-spot and integrated temperature sensor. Under natural convection, and equivalent power supplied, we measured a significant reduction in hot-spot temperature when augmenting the MHP surface with CNT micropins. In particular, a strong enhancement of convective and radiative heat transfer towards the surrounding environment is recorded, due to the high aspect ratio and the foam-like morphology of the patterned CNTs. By combining electrical characterizations with high-resolution thermographic microscopy analysis, we quantified the heat losses induced by the integrated CNT nanofoams and we found a unique temperature dependency of the equivalent convective heat transfer coefficient, Hc. The obtained results with the proposed non-destructive characterization method demonstrate that significant improvements can be achieved in microelectronic thermal management and hierarchical structured porous material characterization.

Implementation of high-performance MEMS platinum micro-hotplate

Purpose – One of the key components of the micro-sensors is MEMS micro-hotplate. The purpose of this paper is to introduce a platinum micro-hotplate with the proper geometry using the analytical model based on the heat transfer analysis to improve both heating efficiency and time constant. Design/methodology/approach – This analytical model exhibits that suitable design for the micro-hotplate can be obtained by the appropriate selection of square heater (LH) and tether width (WTe). Based on this model and requirements of routine sample loading, the size of LH and WTe are chosen 200 and 15 μm, respectively. In addition, a simple micro-fabrication process is adopted to form the suspended micro-heater using bulk micromachining technology. Findings – The experimental results show that the heating efficiency and heating and cooling time constants are 21.27 K/mW and 2.5 ms and 2.1 ms, respectively, for the temperature variation from 300 to 400 K in the fabricated micro-hotplates which are in closed agreement with the results obtained from the analytical model with errors within 5 per cent. Originality/value – Our design based on the analytical model achieves a combination of fast time constant and high heating efficiency that are comparable or superior to the previously published platinum micro-hotplate.

Technology for Fast Fabrication of Glass Microhotplates Based on the Laser Processing

In this paper, we describe a novel cost-effective and simple technology for the production glass MEMS applied as microhotpalte platform for metal oxide gas sensors. The basis of the technology is magnetron sputtering of platinum heating layer followed by precise laser engraving and cutting used for heater patterning. As a result of the technology, we demonstrate the glass microhotplate cantilever with thickness of 30μm equipped with platinum microheater with dimension of about 500×500μm. The cantilever type MEMS microhotplate demonstrate very high stability at working temperatures up to 600 0C, which gives possibility to use it for the low-scale fabrication microhotplate of metal oxide gas sensors.

Design and Optimization Analysis of a Novel MEMS Micro-Hotplate for Gas Sensor

The paper presents a novel Si-substrated micro-hotplate for gas sensor. Use platinum filament as heater, SiO2 as thermal and electricity insulation. Through optimizing analysis, in order to make the gas sensor obtain perfect properties of higher temperature and uniform temperature distribution, the thickness of SiO2, thickness of Si substrate, electrode width and electrode space are designed to be 50, 250, 20 and 500 μm, respectively. Via the analysis of magnetic field distribution of the new gas sensor, the new electrode structure can eliminate the interference on the measurement signal from magnetic field, which is benefit for the improvement of sensor performance.

MEMS-microhotplate-based hydrogen gas sensor utilizing the nanostructured porous-anodic-alumina-supported WO3 active layer

Practical microsensors for fast, highly sensitive hydrogen gas detection were fabricated by combining silicon integral technology for MEMS microhotplate platform with newly developed technological, electrical, and electrolytic conditions for forming nanostructured porous-anodic-alumina-templated WO3 layer as the sensing material. The morphology–structure–property relationship for the nanostructured sensing layer was determined by scanning electron microscopy, X-ray diffraction, and through systematically investigating the sensor performance at various H2 concentrations (5–1000 ppm) and operating temperatures (20–350 °C). The sensors showed superior sensitivity to hydrogen gas, with the lowest detection limit ever reported for WO3 semiconductors (5 ppm), the fast response and recovery times (2–3 min), and the best sensitivity at 150 °C, which was 100 times higher than that of a reference sensor having a smooth WO3 active film. The technology developed enables high-volume, low-cost, and low-power sensor-on-a-chip solution for a hydrogen-based energy economy where the use of highly sensitive and low-power-consuming devices is encouraged.

A molybdenum MEMS microhotplate for high-temperature operation

This paper presents a fabrication process for high-temperature MEMS microhotplates that uses sputtered molybdenum as a conductive material. Molybdenum has a high melting point (2693°C, bulk) and is simpler to deposit and pattern in larger series than platinum. Molybdenum is sensitive to oxidation above 300°C, so during fabrication it is protected by PECVD silicon oxide. The electrical resistivity is linear with the temperature up to 700°C at least. Molybdenum microhotplate has a higher maximum operating temperature than platinum which is demonstrated by the observation of the boiling of barium carbonate (BaCO3) microcrystals at 1360°C. Annealing at 1100°C is effective in extending the operating range. The molybdenum microhotplate performs far better than platinum also in terms of long-term resistance drift. The material properties of the sputtered molybdenum are studied and the overall performances of the microhotplate are tested, with regard to power consumption, temperature uniformity and dynamic behavior.

Fabrication of a MEMS micro-hotplate

With the development of microelectronics and micromachining technology, micro-hotplate as a micro-heater is a very important component in micro-sensor. So far, it has wide applications in pressure detection, gas detection and so on. In this paper, a micro-hotplate based on micro-bridge structure was designed and fabricated. Here we present a micro-hotplate with SOI (silicon-on-insulator) as the substrate and metal Au as the heating electrode material. It shows low thermal conductivity coefficient, good electrical insulation, low power consumption, and excellent resistance to high temperature and low pressure. Some IC (integrated circuit) processes such as magnetron sputtering of Ni/Cr, ion beam sputtering of Au, dry and wet etching processes are employed to fabricate the device. Anisotropic wet etching of silicon and thin film deposition were mainly studied. To protect the front side micro structure during the wet etching process, a special apparatus was designed and fabricated. The optimization process for anisotropic wet etching was obtained for releasing the micro-bridge after a large number of experiments and the micro-hotplate was successfully fabricated. The electrical, thermal characteristics of the micro-hotplate were tested and approved it a well functional device.

Electro-thermal analysis of MEMS microhotplates for the optimization of temperature uniformity

This paper presents a microhotplate working up to 1200°C with improved temperature uniformity by optimizing the geometry of the thin-film resistor. By varying the linewidth of meandering resistive tracks, heat is generated in such a way to have more homogenous temperature distribution. The microhotplates are fabricated using molybdenum as conductive material for the heater. Infrared thermal mapping shows that the temperature variation over the heated area is reduced from an initial 13% to 4%.

Effects of O 2 plasma treatment on NH 3 sensing characteristics of multiwall carbon nanotube/polyaniline composite films

The effects of radio frequency oxygen plasma treatment on the NH 3 gas sensing characteristics of multiwall carbon nanotube (MWCNT)/polyaniline (PANI) composite films are investigated using sensor devices fabricated onto a MEMS micro-hotplate. The Raman and XPS spectra of O 2 plasma-treated MWCNTs indicate that oxygenated groups are created on the surface of CNTs after the plasma treatment. These oxygen-containing defects at the surface of plasma-functionalized multiwall carbon nanotubes (pf-MWCNTs) provide a binding site for the PANI, facilitate the CNT–PANI bond, and promote conduction between the MWCNTs and the PANI. The pf-MWCNT/PANI films are found to have a much better linear response and a sensitivity about three times higher than that of a corresponding pristine MWCNT for ammonia concentrations ranging from 0 to 100 ppm. This dramatic improvement in sensing performance may be attributed to the resistance change of the PANI due to deprotonation and the resistance increase of the MWCNTs due to electron transfer from the PANI to the MWCNTs. The pf-MWCNT/PANI films also exhibit a much lower sensitivity to humidity compared with MWCNTs alone. This may be partially due to the reduction of defects on the pf-MWCNT surface available for binding with water molecules, and partially due to opposite sign slope cancellation.

High-Speed, High-Sensitivity Polyimide Humidity Sensors Based on MEMS Microhotplates

ABSTRACTPolyimides thin films, which are the most commonly used group of sensing materials for capacitive humidity sensors, were cured locally using MEMS microhotplates. The polyimide locally cured at temperature over 350 °C for 1 hour was fully cured. There were no significant differences in the polyimide thin films between cured in convection ovens and locally cured on microhotplate. The locally cured polyimide humidity sensor showed a linearity of 0.9995, a sensitivity of 0.766pF/%RH, a hysteresis of 0.6%RH, and a time response of 3 s. These results show the possibility of locally-cured polyimide films as high speed, high-sensitivity humidity sensors.

Coupling nanowire chemiresistors with MEMS microhotplate gas sensing platforms

Recent advances in nanotechnology have yielded materials and structures that offer great potential for improving the sensitivity, selectivity, stability, and speed of next-generation chemical gas sensors. To fabricate practical devices, the “bottom-up” approach of producing nanoscale sensing elements must be integrated with the “top-down” methodology currently dominating microtechnology. In this letter, the authors illustrate this approach by coupling a single-crystal SnO2 nanowire sensing element with a microhotplate gas sensor platform. The sensing results obtained using this prototype sensor demonstrate encouraging performance aspects including reduced operating temperature, reduced power consumption, good stability, and enhanced sensitivity.

Design and Simulation of Electro-fabricated MEMS Microhotplate for Gas Sensor Applications

In this paper, we present the simulation results of a new MEMS micro-hotplate design for gas sensor applications. The structure is designed to be fabricated on a glass substrate by the low cost process including physical vapor deposition, photolithography, electroplating, and photoresist-sacrificed process. The electro-thermo-mechanical behaviors of the proposed structure have been simulated by CoventorWareTM. In the simulation, the effects of thickness of the NiCr heater layer temperature, mechanical deflection, stress, and power consumption are evaluated. The 3D simulation on results show that the temperature, displacement, stress and current density are pronounced at the center of the beam as desired. At the nominal sensor temperature of ∼400 °C, the power consumption and Mises stress are greatly reduced from 126 to 7 mW and 1280 to 472 MPa as the thickness decreases from 5 to 0.25 µm. In contrast, the compressive beam deflection is not monotonically increased as thickness increases and it is maximum at the thickness of ∼0.5 µm. Therefore, the simulation results indicate that the proposed design with 0.25 m-thick NiCr heater requires very low power consumption of 7 mW with low stress suitable and acceptable membrane deflection for gas sensor applications.

Mesoporous nanoparticle TiO 2 thin films for conductometric gas sensing on microhotplate platforms

Mesoporous TiO 2 nanoparticle thin films were prepared on MEMS microhotplate (μHP) platforms and evaluated as high-sensitivity conductometric gas sensor materials. The nanoparticle films were deposited onto selected microhotplates in a multi-element array via microcapillary pipette and were sintered using the microhotplate. The films were characterized by optical and scanning electron microscopies and by conductometric measurements. The thin films were evaluated as conductometric gas sensors based on the critical performance elements of sensitivity, stability, speed and selectivity. The nanoparticle films were compared with compact TiO 2 films deposited via chemical vapor deposition (CVD) and the nanoparticle films were found to demonstrate higher sensitivity to target analytes. The nanoparticle films were also stable with regard to both baseline conductance and signal response over 60 h of continuous operation at high temperatures (up to 475 °C). Sensor response times were evaluated and the TiO 2 nanoparticle films showed fast responses to the presence of analyte (≈5 s) and a response-time dependence on the analyte concentration. Control of the sensor operating temperature, an inherent benefit of the microhotplate platform, was employed to demonstrate the selectivity of the nanoparticle films.

The Development and Evaluation of TiO2 Nanoparticle Films for Conductometric Gas Sensing on MEMS Microhotplate Platforms

ABSTRACTOver the past decade, MEMS microhotplate devices have been developed at the National Institute of Standards and Technology to support semiconductor metal oxide films for use in conductometric gas sensor arrays. In most cases, the materials have been based on compact thin films of SnO2 or TiO2 deposited by single-source precursor chemical vapor deposition. Of particular interest to our group is the enhancement of the sensitivity of the microsensors to trace gas species by inducing nanostructured porosity and large internal surface areas in the films. In this presentation, we discuss the development of nanostructured sensor materials based on porous TiO2 nanoparticle thin films. The preparation and evaluation of pure and Nb-doped TiO2 nanoparticle films are described. The films on the MEMS microhotplate substrates are evaluated as conductometric gas sensors based on the critical performance elements of sensitivity, stability, speed and selectivity. The sensor performance, and specifically the sensitivity, of the novel nanoparticle TiO2 films is compared with that of traditional compact CVD-derived films.