Zhenan Tang

An integrated gas sensor technology using surface micro-machining

We report the silicon based integrated gas sensor technology using surface micro-machined micro-hotplate. Using this technology, an integrated gas sensor sensitive to 1 ppm of carbon monoxide was demonstrated. This approach was extended to integrated gas sensor array application.

Thermal analysis and design of a micro-hotplate for integrated gas-sensor applications

Abstract The application of commercial mechanical computer-aided engineering (MCAE) software to the design and analysis of micro-hotplate (MHP) structures is presented. The simulation provides an estimation of heating efficiency and temperature distribution on the hotplate. The analysis is applied to a newly proposed MHP structure during layout design. Novel design results in a hotplate with high heating efficiency, good temperature uniformity and ease of temperature sensing. The simulation result has been compared with experimental measurement. For the first time, liquid crystal thermography is used to visualize the temperature profile on the hotplate.

A low-power CMOS compatible integrated gas sensor using maskless tin oxide sputtering

Abstract This paper describes a CMOS compatible integrated gas sensor. The device was designed so that the front-end fabrication is fully compatible with the standard CMOS process. The non-CMOS compatible fabrication steps were carried out as post-processing steps. This included the silicon anisotropic etch to create the thermally isolated micro-hotplate (MHP) and the deposition of gas-sensitive thin films using maskless r.f. SnO 2 sputtering. The sensors exhibited high sensitivities to gases, such as ethanol and hydrogen.

Investigation of stability and reliability of tin oxide thin-film for integrated micro-machined gas sensor devices

Abstract The stability and reliability of SnO 2 /Pt, SnO 2 –Cu/Pt and SnO 2 /Pt/SnO 2 /Pt sensitive thin-films used for CO gas sensor device are discussed in this paper. The stability and reliability are very important for the tin dioxide-based gas sensor devices when these devices are to be integrated with standard CMOS circuitry. The drift in output voltage of tin dioxide sensing thin-film device must be minimized. The stability in the output of the tin dioxide thin-film sensing resistor is very essential to implement reliable integrated sensor device, because a small drift in baseline led to large change in the biasing current in the subsequent signal processing circuit. Here, we define the baseline as the output voltage across the two-terminal sensor in the absence of the signal. Another important parameter is the sensitivity of the sensor. The sensitivity of the device should be repeatable over large number of operation cycles. The drift in baseline and sensitivity of devices which are fabricated using SnO 2 /Pt, SnO 2 –Cu/Pt and SnO 2 /Pt/SnO 2 /Pt sensitive thin-films as gas sensitive material were studied. The sensor device based on SnO 2 –Cu/Pt thin-film shows good sensing characteristics. It was observed that the tin dioxide thin-film cracks during large number of operation cycles. The annealing of these thin-film at higher temperature or longer time induced the cracks in the sensing thin-film. These cracks are responsible for the drift in baseline and sensitivity of the thin-film. The cracks change the resistivity of the sensitive thin-film, which directly impact the device performance and reliability of the device. Our result shows, that SnO 2 –Cu/Pt thin-film is more stable as compared to SnO 2 /Pt and SnO 2 /Pt/SnO 2 /Pt thin-film. The change in surface structure during the operation of device was investigated using SEM technique. The accelerated thermal tests on the devices were performed to estimate the lifetime and evaluate the stability of the sensor device.

An integrated gas sensor based on tin oxide thin-film and improved micro-hotplate

Abstract The fabrication and characterization of an integrated gas sensor based on tin oxide thin-film and improved micro-hotplate (MHP) are presented in this paper. Tin oxide thin-films are sputtered onto the MHP from tin dioxide target or pure tin metal target under different sputtering conditions. The properties of the films are analyzed by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The sensors are characterized in an automated precision characterization system. Sensors are characterized in both dynamic and static modes. In the dynamic mode, a gas analyzer is used to extract the gas out of the measurement chamber. In the static mode, the gas analyzer is replaced by a gas chromatography port.

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%.

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Low-concentration formaldehyde (HCHO) together with ethanol/toluene/acetone/?-pinene (as an interference gas of HCHO) is detected with a micro gas sensor array, composed of eight tin oxide (SnO2) thin film gas sensors with Au, Cu, Pt or Pd metal catalysts. The characteristics of the multi-dimensional signals from the eight sensors are evaluated. A multilayer neural network with an error backpropagation (BP) learning algorithm, plus the principal component analysis (PCA) technique, is implemented to recognize these indoor volatile organic compounds (VOC). The results show that the micro gas sensor array, plus the multilayer neural network, is very effective in recognizing 0.06 ppm HCHO in single gas component and in binary gas mixtures, toluene/ethanol/?-pinene with small relative error.

Thin Films Using Transient Thermoreflectance Technique

Abstract In this paper, the role of microcracks in tin oxide gas sensing thin-films caused by high temperature annealing is discussed. In our research, cracks in rf sputtered tin oxide thin-films annealed at different conditions were investigated with scanning electron microscopy (SEM). Two techniques were used to reduce the cracking. The first was the use of tin oxide thin-films with copper-doping. The second method involved smoothing the underlying surface through the use of phosphorus-doped silicon glass (PSG). The role of the cracks for enhancing diffusion of moisture and oxygen is discussed using a revised short-circuiting pathway model.

Recognizing indoor formaldehyde in binary gas mixtures with a micro gas sensor array and a neural network

Abstract Material and gas sensing properties of tin oxide thin-films are described in this paper. The thin-films are prepared by r.f. sputtering using tin dioxide target and tin metal target under different sputtering conditions. Rutherford back scattering (RBS) was used to analyze the concentrations of Sn 4+ and Sn 2+ species in the sputtered thin-films. The gas sensing properties of the thin-films are interpreted according to the concentration of Sn 4+ and Sn 2+ and a mutual transition model.

Investigation and control of microcracks in tin oxide gas sensing thin-films

Abstract In this paper, we discuss the integration problems associated with tin dioxide thin film to meet the requirement of silicon IC processing. Since aluminum is being used as interconnection metal lines in various IC devices, the post annealing of tin dioxide thin film cannot be performed at high temperatures. At higher annealing temperature, the aluminum diffuses into the silicon and forms a metal alloy, which reduces the performance of the device. To overcome this problem, we propose an annealing process for the tin dioxide thin film at 450°C in oxygen ambience for shorter time (15 min) and then quenching at room temperature. The performance, such as sensitivity and response time of the quenched device, is better than that of the device cooled at 50°C/h. Sensor measurements were performed at 150–300°C at 400–1000 ppm CO gas concentration, and stability of the devices was measured up to 720 h. The device was stable during long operation hours. Hydrogen gas was used to investigate the selectivity of the device. The response indicated that the quenched device was selective towards CO gas. The response of the quenched device was quite different towards CO and hydrogen gas. The morphology of the devices was investigated using SEM.