I. Gràcia

Thermal and mechanical analysis of micromachined gas sensors

In this paper, we present a complete thermomechanical study of a micromachined gas sensor substrate. The work has been carried out combining coupled electrothermomechanical three-dimensional finite element modelling simulations with electrical, infrared thermography and interferometric microscopy experimental measurements. The performances predicted by simulations, such as the power consumption (heating efficiency in air of 5.7 °C mW−1), the time response (19 ms), the membrane deflection during operation and the preferential failure sites in the micromachined substrate have been confirmed by experience. Their good agreement validates the model, and allows us to consider the adaptability of this design as a micromachined substrate for integrated gas sensors.

Microtechnologies for PH ISFET chemical sensors

A review of different microtechnologies for the fabrication of pH ion sensitive field effect transistor (ISFET) sensors is presented. Integrated ISFETs are of interest due to the advantages of low price, fast response and small dimensions that they present compared to ISE electrodes. ISFETs can be also applied to the detection of different ions, using the proper sensitive membranes. A lot of work has been done during the last few decades to obtain commercial devices, and many technologies and structures can be found in the literature. In this paper, both front-side and back-side contacted devices are studied, in order to determine the compatibility of different processes, devices and materials with standard CMOS technologies, which seems to be a goal for present and future applications.

Test structures for ISFET chemical sensors

Simple test structures and measurements were studied for determining their usefulness for the online monitoring of the aging effects on pH ion sensitive field effect transistor (ISFET) chemical sensors used in automatic systems. Devices consisted of long drain and field transistors and metallic meanders. Current and capacitance measurements were carried out with standard instruments. Results showed that not all the test structures and their related parameters were equally sensitive, and in some cases tests at a high temperature were necessary to accelerate the desired effects. The metallic meander structure proved to be quite sensitive to epoxy-coating degradation. For dielectric passivating layers, the parasitic transistors also showed sensitivity, especially when electrolyte was heated.<<ETX>>

Thermal and mechanical aspects for designing micromachined low-power gas sensors

Metal oxide gas sensors have to be heated during operation. Ideally they should have low power consumption and a uniform temperature distribution over the sensitive area. Thermal isolation is achieved by locating the sensitive element on thin free-standing membranes fabricated by means of micromachining of silicon. Uniformity of the temperature is helpful for discriminating different gases and, thus, increasing the selectivity. This can be achieved by careful design of the heater or by additional structures. Micromachined test structures have been fabricated, their mechanical strength and thermal behaviour have been determined and the results have been correlated with extensive FEM studies.

A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons

Abstract Preliminary studies to obtain an integrated solid state gas microsensor are presented in this work. FEM thermal simulations have been carried out to optimize the membrane and heater layouts with the aim of increasing temperature uniformity on the sensitive area and decreasing power consumption. Heater materials and full process flow have been defined in order to achieve compatibility with a CMOS technology with minor changes. On the other hand, first tests of sensitivity with pure tin oxide and Pt-doped tin oxide deposited by a sputtering process on alumina substrates, for benzene and toluene, have given good results for low concentrations (100 ppm) at 400 and 300°C respectively.

Digital image correlation of nanoscale deformation fields for local stress measurement in thin films

In this paper, the application of an in situ stress measurement technique to a silicon nitride thin film deposited onto a thick silicon substrate is presented. The method is based on the measurement of the displacement field originated when a slot is milled into the material under study. Displacements are obtained by digital correlation analysis of scanning electron microscope (SEM) images, whereas milling is performed by ion milling in focused ion-beam equipment. Due to the mechanical constraint introduced by the substrate and the small thickness of the tested layer, the displacements generated by the milling process are in the range 0?5?nm, which is one of the smallest displacement ranges measured up to now in a relaxation-based measurement technique. The local stress value determined with this new method is in good agreement with values obtained by a classical method like the wafer bending test.

Detection of low NO2 concentrations with low power micromachined tin oxide gas sensors

Abstract Semiconductor gas sensors integrated on silicon substrates with thermally isolated structures are presented and technological processing steps of their fabrication are described. Tin oxide sensitive layers have been deposited by reactive sputtering technique due to the compatibility with IC fabrication. The active area has a size of 500×500 μm2 and is supported by a membrane of silicon nitride. Polysilicon is used as heating material and the power consumption is below 50 mW at the operating temperature of 350°C for every sensor prepared. Good isolation among chip devices was guaranteed from FEM thermal simulations [A. Gotz, I. Gracia, C. Cane, E. Lora-Tamayo, M.C. Horrillo, J. Getino, C. Garcia, J. Gutierrez, A micromachined solid state integrated gas sensor for the detection of aromatic hydrocarbons, Sensors and Actuators B 44 (1997) 483–487.]. Very low concentrations of NO2 have been detected with such type of device obtaining good sensitivity and short response time for various thin-film thicknesses of tin oxide.

Measurement of residual stress by slot milling with focused ion-beam equipment

In this paper, the authors present a new approach to residual stress measurement that takes advantage of the combined imaging–milling capabilities of focused ion-beam equipment. The method is based on the measurement of the displacement field originated when a slot of a few microns is milled on the material under study. The fitting of the experimental results with an analytical model together with the independent determination of Youngs modulus allows us to find the residual stress of the layer under study. The complete experimental procedure is described and its feasibility is demonstrated on a LPCVD silicon nitride micromachined membrane. Values obtained by this new method show a good agreement with values obtained by a classical method such as the bulge test.

Residual Stress Measurement on a MEMS Structure With High-Spatial Resolution

A new approach to the local measurement of residual stress in microstructures is described in this paper. The presented technique takes advantage of the combined milling-imaging features of a focused ion beam (FIB) equipment to scale down the widely known hole drilling method. This method consists of drilling a small hole in a solid with inherent residual stresses and measuring the strains/displacements caused by the local stress release, that takes place around the hole. In the presented case, the displacements caused by the milling are determined by applying digital image correlation (DIC) techniques to high resolution micrographs taken before and after the milling process. The residual stress value is then obtained by fitting the measured displacements to the analytical solution of the displacement fields. The feasibility of this approach has been demonstrated on a micromachined silicon nitride membrane showing that this method has high potential for applications in the field of mechanical characterization of micro/nanoelectromechanical systems

Measurement of residual stresses in micromachined structures in a microregion

The use of a focused ion beam equipment is reported to find out the in-plane residual stress value of a microelectromechanical system structure by performing a local stress release. The ion beam column is used to mill stress-release slots of a few microns, whereas the scanning electron column captures micrographs of the milled area before and after the stress release process. The displacement component perpendicular to the slot is obtained from digital image correlation analysis of the captured high-resolution micrographs. The fitting of the experimental results with an analytical model together with the independent determination of the Young’s modulus allows one to find the residual stress value of the layer under study to a very good accuracy.