Thomas Walewyns

Polyimide as a versatile enabling material for microsystems fabrication: surface micromachining and electrodeposited nanowires integration

The interest of using polyimide as a sacrificial and anchoring layer is demonstrated for post-processing surface micromachining and for the incorporation of metallic nanowires into microsystems. In addition to properties like a high planarization factor, a good resistance to most non-oxidizing acids and bases, and CMOS compatibility, polyimide can also be used as a mold for nanostructures after ion track-etching. Moreover, specific polyimide grades, such as PI-2611 from HD Microsystems™, involve a thermal expansion coefficient similar to silicon and low internal stress. The process developed in this study permits higher gaps compared to the state-of-the-art, limits stiction problems with the substrate and is adapted to various top-layer materials. Most metals, semiconductors or ceramics will not be affected by the oxygen plasma required for polyimide etching. Released structures with vertical gaps from one to several tens of μm have been obtained, possibly using multiple layers of polyimide. Furthermore, patterned freestanding nanowires have been synthesized with diameters from 20 to 60 nm and up to 3 μm in length. These results have been applied to the fabrication of two specific devices: a generic nanomechanical testing lab-on-chip platform and a miniaturized ionization sensor.

Fabrication of a miniaturized ionization gas sensor with polyimide spacer

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.

A novel MEMS tunable ionization sensor based on patterned freestanding Nickel nanowires and moving electrode

The ionization of gases results in a unique fingerprint depending on their nature and concentration. Moreover, according to Paschens law, the measurement of distinct species is possible by modifying the distance between the electrodes. In order to analyze gaseous compounds within a mixture, a novel MEMS tunable sensor incorporating freestanding nanowires is proposed. This device allows controlling the ionization gap by capacitive actuation. Mechanical and electrical characterizations, together with finite elements simulations have been conducted in order to evaluate electric field interactions and minimize interferences. Such system should allow the development of a universal gas sensor with pattern recognition.

Development of a Miniaturized Gas Ionization Sensor for Harsh Environments by Using Polyimide Spacer

Gas sensing can be performed by fingerprinting their field ionization characteristics. This paper presents the development of a miniaturized ionization sensor using ion-track etched polyimide as structural layer and template for Ni nanowires synthesis. The device consists in two parallel plate electrodes with gaps varying from 5 to 12 μm. The nanowires impact on breakdown voltage has been analyzed during first electrical characterizations and I-V curves measurements. For a 5.5 μm-gap, breakdown voltage is reduced from 320 to 80 V with a corresponding current at least three order of magnitude lower. Using the sensor in harsh environments such as space applications is also discussed. Miniaturized ionization sensors are powerful candidates as integrated universal gas sensor based on pattern recognition for environmental monitoring. Such a system should be easily integrated in picosatellites such as CubeSats dedicated to the physical analysis of low thermosphere composition.

Synthesis of patterned freestanding nickel nanowires by using ion track-etched polyimide

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 nanostructures.

Ultra-low-power chemiresistive microsensor array in a back-end CMOS process towards selective volatile compounds detection and IoT applications

We describe an ultra-low-power volatile compounds microsensor array towards increased selectivity and sensitivity. The chemiresistive transducers are 100 nm-thick interdigitated gold microelectrodes coated with polypyrrole-based polymer. Two sensors arrays were implemented with respectively 3 × 3 and 2 × 2 pixels2, showing a surface per pixel down to 400 × 200 μm2. The fabrication is fully CMOS-compatible and the polymer coating is performed at wafer level by electropolymerization, using a differential pulse method from 1.1 to 1.5 V. The polymer film thickness varies from 1.2 to 1.5 μm. Looking at ammonia detection, a sensitivity up to 80 % at 5 ppm in nitrogen is achieved, while consuming below 20 μW continuously. Finally, temperature and humidity effects are analyzed at 25 and 45 °C, from 45 to 95 % RH. Such devices are very promising for remote environmental monitoring applications requiring low-cost low-power sensors associated with dedicated electronics.

A highly selective MEMS transducer for hydrogen sensing based on stress modification in palladium thin films

We developed and integrated a MEMS capacitive transducer based on aluminium/palladium bimorph configured as clamped-clamped beams, performing fast and highly selective hydrogen detection. The bimorph is obtained by combining evaporation and sputtering deposition techniques and acts as actuator of the structure. The stress control in the beams has been investigated in order to gain control over the sensing dynamics. The transducer response time is less than 4 s at 0.2 % vol. H2/N2 concentration. The related kinetic shows good correlations to previous in situ experiments of stress modifications in palladium thin films with controlled initial stress. The device is interfaced by an AD774x capacitance-to-digital converter and a CC253x microcontroller within a dedicated housing including a Porex® microporous polymer membrane for IP66 compatibility. Finally, the ultra-low power consumption (<; 10 μW) of such devices is very promising to ease their integration in emerging interconnected sensor nodes within the Internet-of-Things vision.

A hybrid graphene-metal oxide sensor for formaldehyde detection at room temperature

The formaldehyde detection is important for protecting human health and controlling environment pollution. Many metal oxide sensors have been developed for the formaldehyde detection in the last decade. The NiO sensor is considered as the most sensitive one, which is able to detect very low concentration of formaldehyde (< 1 ppm). But it needs a high operating temperature. Presently, graphene has attracted much attention for sensor applications. Its high surface-to-volume ratio possesses the potential ability to detect the presence of a single interacting molecule and its high carrier mobility ensures low electrical noise and low power consumption. However, pristine graphene is chemically inert and shows weak adsorption of formaldehyde molecules. To obtain stronger adsorption ability, thereby a higher sensitivity, it is necessary to functionalize or pretreat graphene. In this study, we design a new hybrid sensor, in which graphene acts as a highly conductive network and NiO as a sensitive layer for formaldehyde. This hybrid sensor combines the advantages of both materials. Comparing to the pure graphene sensor and the usual NiO sensor, the hybrid sensor demonstrates better sensing performances, such as faster response and recovery, importantly, operating at room temperature. This sensor could be easily integrated with complementary metal oxide semiconductor (CMOS) chip in the future.

Live demonstration: Microsystem integration of a palladium-based MEMS hydrogen gas sensor

In this live demonstration, we present the microsystem integration of a palladium-based MEMS capacitive hydrogen gas sensor and its sub-components. The system includes all the elements of the data acquisition chain. The MEMS device is interfaced by a capacitance-to-digital converter for signal conditioning, and a microcontroller associated to a radio for both data processing and transmitting. Moreover a specific housing has been designed with trade consideration, including a Porex® microporous polymer membrane for moisture protection. Finally, a dedicated graphic user interface has been implemented with assisted calibration or recording procedures, and an oscilloscope for data monitoring. Thanks to compatibilities with ZigBee or Bluetooth low energy protocols, this system is directly dedicated to interconnected sensor nodes applications within the Internet-of-Things vision.