E. Castaño

Characterization of tantalum oxynitride thin films as high-temperature strain gauges

Abstract Sputtered tantalum oxynitride thin films on silicon substrates have been studied for use as high-temperature strain gauges. Thin-film depositions have been carried out by r.f. reactive magnetron sputtering in an argon-nitrogen-oxygen atmosphere. A cermet structure, tantalum oxide dielectric islands embedded in a tantalum nitride metallic matrix, has been identified in the sputtered thin films. Tantalum oxynitride thin films with a linear gauge factor (GF ≈ 3), high electrical resistivity (ϱ ≈ 400–1000 μΩ cm) and TCR

A design tool for pressure microsensors based on FEM simulations

Abstract The main features involved in the design of a pressure sensor are the maximum non-destructive pressure and the sensitivity. In this work, these two characteristics are related to the following design variables: dimensions of the membrane and mechanical properties of the selected material. Von Misses stress and strain distributions have been calculated by the finite-element method (FEM). The knowledge of these distributions is a good design guideline for an accurate location of the piezoresistors. The results obtained have been applied to the design of silicon microsensors for biomedical and domestic applications.

Strain sensitivity and temperature influence on sputtered thin films for piezoresistive sensors

Abstract The use of Ni-Cr thin-film resistors as strain-sensitive gages has been studied. Composition and thermal treatments are optimized for the following parameters: gage factor ( FG ) and temperature coefficient of resistance ( TCR ). TCR has been determined using an automatic data acquisition system with implemented software. FG has been determined by the cantilever method. The optimum thin-film composition (Ni 50%-Cr 50%) has been determined. Low stable TCR values (less than 20 ppm in the 30–150 °C temperature range) are obtained. Resistance stability is achieved with a 24 h heat treatment at 150 °C. The desired 40.Ω/square sheet resistance has been attained for 50 nm thick films. A longitudinal gage factor of 2.3 is obtained for this composition (and a lower transversal gage factor of 0.83). With the established process, Ni-Cr thin-film strain gages satisfy the main properties required for their application to piezoresistive pressure transducers: adequate strain sensitivity, high sheet resistance, low thermal sensitivity and good thermal stability.

Metallic thin-film thermocouple for thermoelectric microgenerators

Abstract A new metallic thin-film thermocouple orientated towards thermoelectric microgenerators has been developed. It consists of a 3 μm thick NiCr/SiO 2 /Sb multilayer structure sputter deposited onto a thermally oxidized silicon substrate. A relative Seebeck coefficient of α ab = 76 μ V K −1 and an optimal figure of merit of z ab = 0.08 × 10 −3 K −1 have been measured for this material combination. Both parameters are very close to the theoretical values.

High-temperature Polysilicon Pressure Microsensor

Abstract A thin-film polysilicon on insulator microsensor for low-pressure/high-temperature measurements has been developed. The microsensor is constituted by a micromachined silicon diaphragm, a thin silicon dioxide layer, an optimized sputtered boron-doped polysilicon layer, photolithographically patterned on a Wheatstone bridge configuration, and an aluminium interconnection layer. The complete fabrication process is described. The sensors manufactured in this way exhibit low temperature coefficients of resistance and sensitivity over the range 25–250 °C and a good long-term stability. The sensitivities measured are up to 3 mV V−1 bar−1. The deviation from linearity and hysteresis observed in the output characteristics is measured in the pressure range 0–10 bar.

High-temperature ceramic pressure sensor

Abstract A pressure microsensor for working at high temperature has been developed. The device consists of a tantalum nitride thin film, patterned on a Wheatstone bridge configuration, sputter-deposited onto thermally oxidized silicon wafers with an aluminium interconnection layer and a silicon dioxide passivation. The microsensors present a low temperature coefficient of resistance and good long-term stability. The sensitivity is 0.15 mV (V bar) −1 with low sensitivity drift and low combined non-linearity and hysteresis in the pressure range 0–10 bar.

A micromachined pressure sensor for biomedical applications

A silicon pressure microsensor fabrication process is reported. The microsensor has been designed as a low-cost disposable device for invasive blood pressure measurements. Results obtained from static, dynamic and leakage pressure tests are presented. A sensitivity of has been measured. The combined linearity and hysteresis is less than 1.5% FSO. The dynamic response is fast enough to reproduce the blood pressure waveform of the human heart. Results based on a material biocompatibility study are also included.

Ceramic pressure sensor based on tantalum thin film

Abstract The properties of piezoresistive tantalum nitride thin films on silicon substrates have been investigated for use in a pressure-sensing element. The thin-film deposition has been carried out by reactive sputtering techniques under d.c. and r.f. power conditions. An r.f. reactive sputtered tantalum nitride thin film in an Ar-9%N2 deposition atmosphere has been selected as the ideal piezoresistive material for the strain gauge. This piezoresistive strain gauge combines a high resistivity, ϱ = 230 μΩ cm, a low temperature coefficient of resistance, TCR= − 80 ppm/°C and a high temporal stability with a good longitudinal gauge factor, GF = 3.5.

Thin film technology applied to the development of a multilayer pressure sensor device

Abstract Thin film technology has been applied to the development of high-performance sensing devices based on nickel-chromium piezoresistive thin films. These sensing devices combine the high stability piezoresistive properties of an optimized NiCr metallic alloy with the high dielectric performance of a silicon dioxide thin layer, coating a stainless steel substrate. A complete production process, based on sputtering and photolithographic techniques, is proposed. The mechanical and electrical properties of the sensing device have been simultaneously optimized in order to obtain a highly reliable transducer.

An optimized preparation process of stainless-steel substrates and their application to thin-film high pressure sensors

Abstract An optimized preparation process of 17.4 PH stainless-steel substrates for thin-film pressure sensors (with an SiO 2 insulator interlayer) has been established. It includes a precipitation heat treatment, polishing and cleaning processes, and a chemical surface preparation. Several cleaning steps have been studied: decreasing (both ultrasonical and vapour phase), alkaline cleaning (both ultrasonical and electrolytical), and acid cleaning (passivation). Chloride particles (with alkaline metals and sulfur) have been identified as the first dielectric failure cause. They are eliminated mainly during alkaline cleaning steps. Although chloride particles are eliminated, their previously corrosion-induced holes in the stainless-steel surface act as a secondary—less critical—dielectric failure cause. With the established substrate preparation and SiO 2 sputtering deposition processes, films 2.4 μm thick resist breakdown voltages up to 50 V, with negligible leakage currents (less than 10 nA).