Jun Yu

Thermal Characterization of

In this paper, we measure the thermal conductivities (TCs) of Si₃N₄ thin films prepared by lower pressure chemical vapor deposition with thickness ranging from 37 to 200 nm. The measurements were made at room temperature using a transient thermoreflectance technique. A three-layer model based on the transmission-line theory and the genetic algorithms were applied to obtain the TC of thin films and the interfacial thermal resistance (ITR). The results show that the value of the TC is 1.24-2.09 Wmiddotm^-1middotK^-1. The ITR between the metal layer and the thin film is about 1.2 times 10^-8 m² ldr K ldr W^-1. The estimated uncertainty of the TC is less than 18%.

\hbox{Si}_{3}\hbox{N}_{4}

Molecular dynamics simulations were performed to evaluate temperature-dependent thermal conductivity of bent carbon nanotubes.Thermal conductivities of bent nanotubes are predicted to be smaller than those of straight nanotubes. This is due to the suppression of high frequency phonons from the density of states calculations. It was found that for the defect-free bent nanotubes, the ratio of thermal conductivity of bent nanotubes to that of the straight ones are temperature and diameter independent, while significantly relies on the bent characteristic size. The more is the nanotube bent, the smaller is thermal conductivity obtained. For the larger nanotubes, the buckled defects were observed after bending and the ratio decrease rapidly. The ratios of thermal conductivity of the buckled nanotubes to that of the straight ones increase with the increasing temperatures until a maximum is obtained.

Thin Films Using Transient Thermoreflectance Technique

A novel micro-hotplate (MHP) gas sensor is designed and fabricated with a standard CMOS technology followed by post-CMOS processes. The tungsten plugging between the first and the second metal layer in the CMOS processes is designed as zigzag resistor heaters embedded in the membrane. In the post-CMOS processes, the membrane is released by front-side bulk silicon etching, and excellent adiabatic performance of the sensor is obtained. Pt/Ti electrode films are prepared on the MHP before the coating of the SnO2 film, which are promising to present better contact stability compared with Al electrodes. Measurements show that at room temperature in atmosphere, the device has a low power consumption of ∼19 mW and a rapid thermal response of 8 ms for heating up to 300 °C. The tungsten heater exhibits good high temperature stability with a slight fluctuation (<0.3%) in the resistance at an operation temperature of 300 °C under constant heating mode for 336 h, and a satisfactory temperature coefficient of resistance of about 1.9‰/°C.

Temperature-dependent thermal conductivity of bent carbon nanotubes by molecular dynamics simulation

The CuO-In2O3 core-shell nanowire was fabricated by a two-step method. The CuO nanowire core (NWs) was firstly grown by the conventional thermal oxidation of Cu meshes at 500°C for 5 hours. Then, the CuO nanowires were immersed into the suspension of amorphous indium hydroxide deposited from the In(AC)3 solution by ammonia. The CuO nanowires coated with In(OH)3 were subsequently heated at 600°C to form the crystalline CuO-In2O3 core-shell structure, with In2O3 nanocrystals uniformly anchored on the CuO nanowires. The gas sensing properties of the formed CuO-In2O3 core-shell nanowires were investigated by various reducing gases such as hydrogen, carbonmonoxide, and propane at elevated temperature. The sensors using the CuO-In2O3 nanowires show improved sensing performance to hydrogen and propane but a suppressed response to carbon monoxide, which could be attributed to the enhanced catalytic properties of CuO with the coated porous In2O3 shell and the p-n junction formed at the core-shell interface.

Design and fabrication of a CMOS-compatible MHP gas sensor

With the development of sensors, it is possible to embed many sensors within a certain space, which makes the monitor and alarm system with multisensor possible. There are two important parameters in a monitor and alarm system, namely, the false alarm rate and the missed alarm rate. In this work, a method for selecting optimal number of sensors in the sensor array is presented to improve the credibility. The influence factors of the weights and the false alarm rate and the missed alarm rate of one sensor and total number of sensors are discussed. An experimental setup was developed. The monitoring methods of common strategies and the proposed optimal number of sensors strategy are compared graphically by the receiver operating characteristic curves and the area under receiver operating characteristic curve values. The receiver operating characteristic curves graphically prove that the optimal number of sensors’ method presents the best performance, and it is shown that the optimal number of sensors’ method has the highest area under receiver operating characteristic value (0.9631). This method may aid future users of the monitor and alarm system by providing an optimal number of sensors strategy for high credibility.

CuO-In 2 O 3 core-shell nanowire based chemical gas sensors

Because the sensor response is dependent on its operating temperature, modulated temperature operation is usually applied in gas sensors for the identification of different gases. In this paper, the modulated operating temperature of microhotplate gas sensors combined with a feature extraction method based on Short-Time Fourier Transform (STFT) is introduced. Because the gas concentration in the ambient air usually has high fluctuation, STFT is applied to extract transient features from time-frequency domain, and the relationship between the STFT spectrum and sensor response is further explored. Because of the low thermal time constant, the sufficient discriminatory information of different gases is preserved in the envelope of the response curve. Feature information tends to be contained in the lower frequencies, but not at higher frequencies. Therefore, features are extracted from the STFT amplitude values at the frequencies ranging from 0 Hz to the fundamental frequency to accomplish the identification task. These lower frequency features are extracted and further processed by decision tree-based pattern recognition. The proposed method shows high classification capability by the analysis of different concentration of carbon monoxide, methane, and ethanol.

A Method for Selecting Optimal Number of Sensors to Improve the Credibility

Nanoporous TiO2 powders were prepared following a two-step method. The highly ordered uniform TiO2 nanotube arrays were first grown by the electrochemical anodization of a Ti metal sheet and subsequently, mechanically milled to obtain the porous TiO2. The obtained nanoporous TiO2 sample contains nanotube-like particles with a length around 400 nm and inner diameter ~100 nm and also some near-spherical TiO2 single crystals with a diameter around 10 nm. The sensing properties of assynthesized nanoporous TiO2 based resistive-type chemical sensor to formaldehyde were demonstrated. The sensor exhibits good sensitivity to formaldehyde at room temperature under UVirradiation. The response of the sensor increased almost linearly as a function of the concentration of formaldehyde from 10-50 ppm.

Short-Time Fourier Transform and Decision Tree-Based Pattern Recognition for Gas Identification Using Temperature Modulated Microhotplate Gas Sensors

The CMOS-compatible MHP with tungsten as the heater material shows excellent stability at 300 degrees Celsius, yet its thermal reliability at higher temperatures and failure mechanism have not been reported previously. In this paper, the fluctuation of the tungsten heaters of the MHPs were recorded when the MHPs were heating at temperatures higher than 400 degrees Celsius for 1 hour. For the MHP with Al electrodes for gas sensor application, failure occurs at around 650 degrees Celsius when the Al material begins to melt. Without Al, the W-based MHP worked stably with variation of heater resistance around 0.5% at 700 degrees Celsius before it broke down at 750 degrees Celsius. The microscope observations show that, unlike the electro-stress migration failure of Pt-based MHPs or the resistance drift problem of poly-Si-based MHPs, the W-based MHP breaks down because of the delamination of the membrane.

8.3.3 UV Assisted Chemical Gas Sensing of Nanoporous TiO2 at Low Temperature

For crystal materials, thermal conductivity (TC) is proportional to T3 at low temperatures and to T−1 at high temperatures. TCs of most amorphous materials decrease with the decreasing temperatures. If a material is thin film, boundary will influence the TC and then influence the temperature dependence. In this paper, we calculate the TC of crystal and amorphous SiO2 thin films, which is a commonly used material in micro devices and Integrated Circuits, by NEMD simulations. The calculation temperatures are from 100K to 700K and the thicknesses are from 2nm to 8nm. TCs of crystal thin films reach their peak values at different temperatures for different thicknesses. The smaller thickness the larger peak values obtained. But for amorphous thin films, the results show that the temperature dependence of thin films is the same as bulk materials and not relative to their thicknesses. The obtained temperature dependence of the thin films is consistent with some previous measurements and the theory predictions.Copyright

High temperature reliability and failure of W-based microhotplates

A high-performance low-power micro hotplate (MHP) hydrogen sensor was fabricated through electrohydrodynamic (EHD) inkjet printing technique. Electrospun Pd loaded SnO2 nanofibers with lengths of 250-850 nm were precisely printed on the suspended central part of an MHP with an area of 100 um × 100 um. The printhead in the printing system was a low-cost metallic needle with an inner diameter of 110 um, which was large enough to prevent clogging by the nanofibers. The printing process was observed by a high-speed camera. Small droplets with diameters of 50-80 um were produced at each ejection by providing a high voltage to the metallic needle. It was found that the bridge-type MHPs used in our experiment can promote the positioning precision due to its bound effect to the droplet. In the gas sensing measurement, the Pd loaded SnO2 MHP gas sensor showed a remarkable response to H2 with a low power of only 9.1 mW. The experiment results demonstrate the excellent adequacy of EHD inkjet printing technique to realize effective mass fabrication of MHP gas sensors or sensor arrays.