Angelique Tetelin

Fast response humidity sensors for a medical microsystem

A fast response humidity sensor was fabricated to equip a medical microsystem for diagnosis of pulmonary diseases. Its main characteristics are reported in this paper. The sensor is based on a capacitor made of a divinyl siloxane benzocyclobutene (BCB) thin film in between parallel plate electrodes. It was fabricated with compatible CMOS technology. It exhibits good linearity, good sensitivity, and a short response time. Equilibrium capacitance variations versus humidity can be considered linear with a linearity error less than 2% of the humidity level. The sensitivity is 0.1 pF by per cent of the humidity level. The device displays a typical adsorption time of 650 ms, a minimum adsorption time of 400 ms, and a desorption time of a few seconds at ambient temperature. Its performances were compared to other types of capacitive humidity sensors fabricated for the same medical use. The static behavior and the dynamic behavior of the device are reported. They are interpreted according to physical processes of gas adsorption and diffusion in and through glassy polymers. The suitability of the conventional Dual-Mode model to explain water sorption in BCB is discussed. This investigation is a starting point in a modeling process to improve the design of the sensor interface circuitry.

Modeling and optimization of a fast response capacitive humidity sensor

This paper presents a model which predicts the temporal response of capacitive humidity sensors made of parallel electrodes and a polymer sensitive coating. This model is used for the simulation of the response of the sensor subjected to specific conditions. The model includes electrostatic and molecular diffusion calculation. As a validation, the model is confronted with finite-element simulation and experimental results. Experiments were carried out for divinyl siloxane benzocyclobutene (DVS-BCB) polymer films with different film thicknesses and upper electrode dimensions. As an application example, the model is used to simulate the response of several humidity sensor structures exposed to the conditions of a medical application related to breath analysis. The model may also be helpful to predict the effects of fabrication process uncertainties. Because the model used to describe water diffusion in DVS-BCB is based on Henrys law and Ficks law, it could be extended to the optimization of capacitive sensors for other vapors

Love Wave Characterization of the Shear Modulus Variations of Mesoporous Sensitive Films During Vapor Sorption

Anomalous responses of acoustic humidity sensors coated with mesoporous titania sensitive films have been observed at high humidity levels, due to capillary contraction that alters the film mechanical behavior. As the methods commonly used to assess the elasticity of thin films are difficult to apply during sorption, a dedicated method for the characterization of the variations of the elastic shear modulus of thin films under humidity exposure has been developed. The method combines a Love wave platform with environmental ellipsometric porosimetry (EEP). In the presented approach, EEP measures the thickness of the film and the adsorbed humidity volume fraction under vapor exposure, while the Love wave platform provides the phase velocity shifts induced by water sorption. These parameters then feed an accurate model of Love wave propagation in the multilayered platform for the derivation of the shear modulus of the sensitive film. The method was applied to 100 ± 10 nm thick mesoporous titania films, with 25 ± 1% porosity, under relative humidity exposure in the 3%-95% range. It successfully determined decreases of 39% and 67% of the shear modulus during adsorption and desorption, respectively, from a 3.1 GPa initial value.

Mesoporous thin films as versatile sensitive matrices on Love wave sensors for fast sub-ppm vapor detection

Love wave gas sensors with mesoporous thin film as sensitive layers are presented. They are dedicated to volatile organic compound vapor detection. Mesoporous films of SiO2 were prepared using the Sol-Gel process and deposited on the devices acoustic path. The obtained films have been characterized by ellipsometry resulting in the determination of porosity and pore size distribution in the film and of water adsorption isotherms. Ethanol (130–2000ppm) and toluene (70–1000ppm) exposures resulted in frequency shifts up to 20 kHz, showing the high sensitivity of the mesoporous-based Love wave devices.

Capacitive humidity sensors based on oxidized PhotoBCB polymer films: enhanced sensitivity and response time

A fast response capacitive humidity sensor was previously designed with a PhotoBCB film, cured in inert atmosphere with O2 plasma surface modification. Satisfactory sensitivity (0.15 pF/%RH) and response time (less than 2 s) were due to the oxidized film surface which improved water adsorption. In this paper, full oxidation of the film has been considered to design a new fast response humidity sensor with enhanced sensitivity and response time. The sensitivity of the new sensor is 1.3 times higher, and its response time is less than 350 ms at ambient temperature, thus being more than four times faster than the previous sensor. The enhancement of the sensitivity is particularly noticeable under high humidity levels, and this new sensor could be suitable for medical applications. Future improvements consider decreasing the thickness of the film, with support of predictive simulations

Guided SH-SAW characterization of elasticity variations of mesoporous TiO 2 sensitive films during humidity sorption

Anomalous responses of acoustic humidity sensors coated with mesoporous titania have been observed at high humidity levels, due to capillary contraction that alters the film mechanical behavior. As the methods commonly used to assess the elasticity of thin films are difficult to apply during sorption, a dedicated method for the characterization of the variations of the elastic shear modulus of thin films under humidity exposure has been developed. The method combines a guided shear-horizontal surface acoustic wave (guided SH-SAW or Love wave) platform with environmental ellipsometric porosimetry (EEP). In the presented approach, EEP measures the thickness of the film and its adsorbed humidity volume fraction under vapor exposure, while the guided SH-SAW platform provides the phase velocity shifts induced by water sorption. These parameters then feed an accurate model of Love wave propagation in the multilayered platform for the derivation of the shear modulus of the sensitive film. The method was applied to 100 ± 10 nm thick mesoporous titania films, with 25 ± 1% porosity, under relative humidity exposure in the 3%-95% range. It successfully determined decreases of 39% and 67% of the shear modulus during adsorption and desorption, respectively, from a 3.1 GPa initial value.

Guided SH-SAW toluene sensors with mesoporous silica sensitive coatings: Increased sensitivity through mesostructuration control

Thanks to high specific surface areas, mesoporous silica films have been considered as sensitive coatings for guided shear-horizontal surface acoustic wave toluene sensors. In order to increase their sensitivity, the influence on toluene detection of the structuring agent used in the evaporation-induced-self-assembly fabrication process of these inorganic films has been studied. In this paper, F127 block copolymer is compared to cethyltrimethylammonium bromide (CTAB). Sensors coated with 100 nm thick films structured with CTAB show an average sensitivity of 14 Hz.ppm−1 for toluene concentrations in the 130–1040 ppm range at room temperature, which is 4.5 times higher than the sensitivity obtained with films of similar thicknesses and porosities that were structured with F127.

Mesoporous TiO2 Sensitive Films for Love Wave Humidity Detection: Origins of Stress Release Induced by Sorption

Mesoporous titania-coated acoustic humidity sensors show strongly non-linear responses at high exposure, not fully explained by water condensation. Three possible sources of stresses that alter the film mechanical behavior have been considered in this work: increased swelling, unreleased sol-gel stress, and capillary contraction. If the latter is linked to the film porous nature and cannot be avoided, the two other are mainly side effects of the specific fabrication process developed for the deposition of mesoporous oxides on quartz piezoelectric substrates: partial crystallization and a large pore size distribution.

Hermeticity Assessment of MEMS Micro Packages: Leak Rate Measurements Based on Infrared Spectroscopy

This paper presents an alternative method for the hermeticity assessment of MEMS micro-packages, and the first experiments performed using this technique. The method consists in monitoring the internal gas pressure of the cavity by Fourier Transform Infra Red (FTIR) spectroscopy. The package’s hermeticity is then deduced from the pressure variations. In order to carry out the IR transmission measurements, nitrous oxide was chosen as bombing gas due to its excellent absorption properties for mid-IR radiations. The method was applied to BCB-sealed silicon micro-packages and allowed to study the influence of different parameters like the sealing ring dimensions or the internal volume on their hermeticity. Additional measurements were carried out on hermetic packages, with an additional organic coating. These experiments validated the method for micro-packages with a volume of 5 mm3 or more.Copyright