Micro-hotplates are MEMS structures of interest for low-power gas sensing, lab-on-chips and space applications, such as micro-thrusters. Micro-hotplates usually consist in a Joule heater suspended on a thin-film membrane while thermopiles or thermodiodes are added as temperature sensors and for feedback control. The implementation of micro-hotplates using a Silicon-On-Insulator technology makes them suited for co-integration with analog integrated circuits and operation at elevated environmental temperatures in a range from 200 to 300 °C, while the heater allows thermal cycling in the kHz regime up to 700 °C, e.g. necessary for the activation of gas sensitive metal-oxide on top of the membrane, with mWrange electrical power. The demonstrated resistance of micro-hotplates to gamma radiations can extend their use in nuclear plants, biomedical sterilization and space applications. In this work, we present results from electrical tests on micro-hotplates during their irradiation by Cobalt-60 gamma rays with total doses up to 18.90 kGy. The micro-hotplates are fabricated using a commercial 1.0 μm Silicon-On-Insulator technology with a tungsten Joule heater, which allows power-controlled operation above 600 °C with less than 60 mW, and temperature sensing silicon diodes located on the membrane and on the bulk. We show the immunity of the sensing platform to the harsh radiation environment. Beside the good tolerance of the thermodiodes and the membrane materials to the total radiation dose, the thermodiode located on the heating membrane is constantly annealed during irradiation and keeps a constant sensitivity while post-irradiation annealing can restore the thermodiode.
The impact of different types of radiation on the electromechanical properties of materials used in microfabrication and on the capacitive and piezoresistive transduction mechanisms of MEMS is investigated. MEMS technologies could revolutionize avionics, satellite and space applications provided that the stress conditions which can compromise the reliability of microsystems in these environments are well understood. Initial tests with MEMS revealed a vulnerability of some types of devices to radiation induced dielectric charging, a physical mechanism which also affects microelectronics, however integration of novel functional materials in microfabrication and the current trend to substitute SiO2 with high-k dielectrics in ICs pose new questions regarding reliability in radiation environments. The performance of MEMS devices with moving parts could also degrade due to radiation induced changes in the mechanical properties of the materials. It is thus necessary to investigate the effects of radiation on the properties of thin films used in microfabrication and here we report on tests with γ, high energy protons and fast neutrons radiation. Prototype SOI based MEMS magnetometers which were developed in UCL are also used as test vehicles to investigate radiation effects on the reliability of magnetically actuated and capacitively coupled MEMS.
Gas sensing can be achieved by fingerprinting the ionization characteristics of distinct species. In this study, the
fabrication of a miniaturized gas ionization sensor using polyimide as sacrificial layer is reported. The sensor consists of
two planar metallic electrodes with a gap spacing obtained by the polyimide under-etching. This known sacrificial layer
has the advantage besides a high planarization factor, to be CMOS compatible. Furthermore, its chemical resistance up to
high temperatures, high resistance to radiation from both electrons and neutrons, and low outgassing are of primary
importance to avoid interferences with the ionization gas sensing. A suspended micro-bridge with dimensions 20 μm
width and 220 μm length has been developed and released by using etching holes in the membrane. The ionization
characteristics of air at controlled temperature, humidity and pressure (21°C, 40% humidity and 1 atm) have been
obtained during non-destructive electrical characterizations, with a breakdown voltage of 350 V for a 6 μm gap. The
growth of metallic nanowires templated in ion track-etched polyimide on the electrode is envisioned in order to enhance
the ionization field and to reduce the required measurement power of the sensor.
Nowadays, a lot of applications including nanoelectronics, spintronics or miniaturized sensors are using nanowires.
Unfortunately, current techniques used for local synthesis of nanowires are still not fully compatible with common
microfabrication techniques. In this study, we focus on the synthesis of patterned metallic nanowires by
electrodeposition within nanoporous polyimide membranes integrated on 3 inch Si bulk wafers. Known to have a high
planarization factor, a good resistance to most non-oxidizing acids and bases and to be CMOS compatible, polyimide is
increasingly used in microsystems. Furthermore, like polycarbonate or polyester, nanoporous polyimide can be obtained
by ion track-etching process. This polymer shows then a great interest to be used as a mold for nanowires growth.
Patterned freestanding Nickel nanowires have been synthesized over a 100 nm thickness gold layer evaporated onto a
SiO2/Si substrate, with diameters of 20 and 60 nm, and length between 2 and 2.5 μm, depending on the electrodeposition
time. Such fabrication process is promising to achieve more complex microelectromechanical systems incorporating
One significant challenge facing biosensor development is packaging. For surface acoustic wave based biosensors, packaging influences the general sensing performance. The acoustic wave is generated and received thanks to interdigital transducers and the separation between the transducers defines the sensing area. Liquids used in biosensing experiments lead to an attenuation of the acoustic signal while in contact with the transducers. We have developed a liquid cell based on photodefinable epoxy SU-8 that prevents the presence of liquid on the transducers, has a small disturbance effect on the propagation of the acoustic wave, does not interfere with the biochemical sensing event, and leads to an integrated sensor system with reproducible properties. The liquid cell is achieved in two steps. In a first step, the SU-8 is precisely patterned around the transducers to define 120 μm thick walls. In a second step and after the dicing of the sensors, a glass capping is placed manually and glued on top of the SU-8 walls. This design approach is an improvement compared to the more classical solution consisting of a pre-molded cell that must be pressed against the device in order to avoid leaks, with negative consequences on the reproducibility of the experimental results. We demonstrate the effectiveness of our approach by protein adsorption monitoring. The packaging materials do not interfere with the biomolecules and have a high chemical resistance. For future developments, wafer level bonding of the quartz capping onto the SU-8 walls is envisioned.
Conference Committee Involvement (2)
Bioinspiration, Biomimetics, and Bioreplication IX
4 March 2019 | Denver, Colorado, United States
Sensors for Extreme Harsh Environments II
22 April 2015 | Baltimore, Maryland, United States