Characterization of Materials

© Fraunhofer IPMS
Wafer with OFET segments

Material characterization of inks and pastes for electrical applications involves analyzing their electrical properties to ensure optimal performance. Key aspects include measuring conductivity and resistivity to determine how well the material can conduct electricity. Dielectric properties, such as dielectric constant and loss tangent, are assessed to understand the material's insulating capabilities. Additionally, the examination of the percolation threshold helps identify the minimum concentration of conductive particles needed for effective conductivity. Thermal and electrical stability are also evaluated under various environmental conditions to ensure reliability. This comprehensive electrical characterization ensures that inks and pastes meet the stringent requirements for use in electronics, printed circuits, and other high-tech applications.

The chip substrates produced at Fraunhofer IPMS offer high-precision structures and high-performance materials, providing a promising basis for reproducible material evaluation in the context of R&D questions and qualification. The substrates offer different customized designs of the electrode structures, e.g. different channel widths and lengths on one chip, so that the ideal parameters can be used for specific applications.

The substrates are regularly fabricated in a clean room on silicon wafers with thermal silicon dioxide (SiO₂), with other oxides such as hafnium dioxide (HfO₂) available as dielectrics.

In sensor development, the sensitive materials determine the performance of the entire sensor. Material and process developers can apply semiconductor layers to the substrates by solution, chemical vapor deposition (CVD), or physical vapor deposition (PVD). Subsequent electrical characterization allows for characterization and evaluation based on conductivity, carrier mobility, and other performance parameters. As soon as a gas-sensitive material comes into contact with the analyte, this leads to a change in the electrical properties.

The substrates from Fraunhofer IPMS offer a simple way to record these changes. The sensor materials can be evaluated with respect to sensitivity and drift and then optimized, e.g. by adjusting the deposition parameters. Due to the wafer-level manufacturing technology, the substrates are also an interesting basis for product-oriented development.

The test structures can be easily contacted and measured with the specially developed prober.

Gas sensors

The Fraunhofer Institute for Photonic Microsystems IPMS develops and manufactures individual heatable test chips for the characterization of new gas sensor materials. Deposited sensing layers and their application-specific parameters, such as sensitivity and selectivity, can thus be specifically evaluated. Customized chip designs allow the optimal and highly accurate characterization of these thin films.

The detection of gases such as NO2, NH3, CO, H2S or volatile organic compounds (VOC) such as acetone, formaldehyde and methanol is of great importance for the assessment of potential health risks. Gas sensors based on single-component metal oxides and carbon-based materials currently suffer from limitations such as low sensitivity in the lower ppm and ppb range as well as limited lifetime, which prevents their widespread use as high-performance gas sensors. Therefore, further developments are needed to achieve electrical and thermal properties in combination with high sensitivity, fast response, high selectivity and fast repeatability.

In the context of such research, the characterization of sensitive layers plays a decisive role in order to be able to produce and use the materials in a targeted manner. Fraunhofer IPMS develops and produces conductivity and single transistor structures for the evaluation of novel materials. These substrates can be used, for example, to characterize the electrical properties of thin-film gas sensors. These substrates can also be used as a basis for further product developments.

Gas sensors often need to be operated at defined temperatures. Our substrates make it possible to control the temperature of the layers, allowing materials to be studied easily and effectively. This includes studying stability and drift over different time periods. In addition, the behavior in processes can be investigated already during the deposition. We are looking for partners to further develop the technology and can also provide chips for measurements.

Substrates for deposition of sensitive materials

Organic Field-Effect Transistor (OFET) substrates serve as the foundational layer on which the device components, including electrodes and sensing materials, are built.

In OFET-based gas sensors, electrodes are typically designed to ensure efficient charge injection and collection. They are often made from materials like gold, silver, or conductive polymers, which are patterned onto the substrate using photolithography. The precise design and placement of these electrodes are crucial for the sensor's sensitivity and response time.

The sensing materials in OFET gas sensors are usually organic semiconductors that can interact with specific gas molecules. These materials include small molecules like pentacene or polymers like polythiophene derivatives. When the target gas interacts with the organic semiconductor, it alters the charge carrier mobility, leading to a measurable change in the transistor's electrical properties. This change can be correlated to the concentration of the gas, allowing for sensitive detection.

The use of OFET substrates for gas sensors offers several advantages, including flexibility in design, the potential for low-cost production, and the ability to create lightweight and portable sensing devices. Additionally, the compatibility of OFETs with various organic materials allows for the customization of sensors for specific gases, enhancing their applicability in environmental monitoring, industrial safety, and healthcare diagnostics. Overall, OFET substrates provide a versatile and efficient platform for developing advanced gas sensors, with carefully designed electrodes and tailored sensing materials contributing to their high sensitivity and specificity.

The silicon based fabrication method allows for the integration of multiple OFETs (array) on one chip with different sensing materials, each tailored to detect specific gases. This enhances the overall sensitivity and selectivity of the sensor system, enabling the detection of a wide range of gases simultaneously. It can also be used for redundancy and increased reliability.

Check out the design options.