Illustration of the self-sensing multilayer AFM cantilever. (a) Schematic of the polymer core and self-sensing electronics (an active piezoresistor) sandwiched between two ceramics. Due to the polymer core, the cantilever can be thick with a low spring constant. (b) Optical image of a multilayer cantilever with two active piezoresistors. Credit: Hosseini et al.
Microelectromechanical systems (MEMS) are tiny devices that integrate various components, such as miniature sensors, electronics, and actuators, onto a single chip. These small devices have shown great promise for accurately detecting biological signals, acceleration, force, and other measurements.
Most MEMS devices developed to date are made of silicon and silicon nitride. While some of these devices have shown promising results, their composition and design limit their sensitivity and versatility, for example limiting their use in humid environments.
In a recent Natural electronics In a paper published in the journal Science, researchers at the Ecole Polytechnique Fédérale de Lausanne (EPFL) have presented an innovative cantilever design for MEMS, based on a polymer, a semiconductor and ceramic. Cantilever beams are tiny flexible beams that can adapt their shape in response to external forces or molecular interactions, and can thus serve as sensors or actuators.
“Our team has previously worked on polymer cantilevers for high-speed atomic force microscopy (AFM) and developed MEMS-based self-sensing AFM cantilevers for industrial and biological applications,” Dr. Nahid Hosseini, lead author of the paper, told Tech Xplore.
“However, self-sensing cantilevers have traditionally faced challenges, including achieving high force sensitivity and ensuring biocompatibility, because the strain sensors are typically placed on the outer surface of the MEMS cantilever.”
The recent study by Dr. Hosseini and colleagues aimed to develop a new self-sensing cantilever that still performs well in harsh environments, such as in fluids. Such a cantilever could prove particularly useful for biomedical applications, enabling the development of new miniature biosensing technologies.
The cantilever designed by the researchers features a unique layered design incorporating three different materials.
“The polymer layer was chosen for its relatively low Young’s modulus, which allows the cantilever to be thick while remaining flexible enough for high deflection sensitivity,” explains Dr. Hosseini. “In addition, polymer-based cantilevers exhibit much faster dynamic responses than those made of silicon or silicon nitride.”
For the semiconductor layer of the cantilever, the team used doped polysilicon. This layer contributes to the device’s sensing capabilities, improving its ability to detect small deflections (i.e., applied force or displacements).
Self-detecting multilayer cantilevers are a platform for various scanning probe techniques, such as magnetic force microscopy (MFM). (a) Schematic showing a multilayer cantilever modified for MFM measurements by coating the cantilever tip with 70 nm Ni81Fe19. Measurements were performed in vacuum with a hybrid SEM-AFM system. (b) Superposition of topography and phase data showing the magnetic field intensity created by the separated Ni81Fe19 nanorods. The inset is the SEM image. Credit: Hosseini et al.
Finally, the device’s ceramic outer layer encapsulates the polymer core and its underlying electronics. The ceramic improves the device’s mechanical and chemical stability, allowing it to operate safely in a variety of environments.
“Our hermetically sealed multi-layer design enables rapid measurement of small forces and works even in aggressive and opaque fluids,” said Dr. Hosseini.
“This also extends the application of self-detecting AFM cantilevers to a wider range of surface characterization techniques, such as magnetic force microscopy (MFM) or Kelvin probe force microscopy (KPFM), where the cantilever surface needs to be coated with functional layers.”
As part of their recent work, Dr. Hosseini and his colleagues used their design to build a prototype MEMS device. Early tests showed that the device performed remarkably well, consistently detecting force and deflection in different environments.
“A major achievement of this work is the fabrication of a MEMS device that combines high deflection sensitivity with mechanical robustness,” said Dr. Hossaini. “The combination of a polymer core and doped polysilicon strain sensors allows the cantilever to detect very small forces.”
The newly designed cantilever proved to be extremely robust and adaptable, which would allow it to find various practical applications. For example, it could be used to detect mass changes in chemical and biological samples, facilitating their characterization at the nanoscale.
In the healthcare field, the device could enable high-precision diagnostics and detailed monitoring of biological signals. In addition, the cantilever could be used to monitor natural environments, detecting small but significant changes in pollution.
“In the future, we plan to continue optimizing the performance of these cantilevers by exploring new material combinations and refining their sensitivity and durability,” added Dr. Hosseini. “A key goal will be to integrate them into more complex systems, such as microfluidic platforms, to expand their real-time diagnostic and monitoring capabilities.”
“Prototypes of our multi-layer cantilevers have already attracted interest from international companies, and I am actively manufacturing these devices for use in various industrial sectors.”
Dr. Hosseini is making the cantilever presented in his paper available to engineers and manufacturers around the world. Over the next year, the researchers plan to launch a spin-off company based on their patented design so that it can be used by semiconductor manufacturers and engineers developing medical technologies.
More information:
Nahid Hosseini et al, A polymer-semiconductor-ceramic cantilever for high-sensitivity fluid-compatible microelectromechanical systems, Natural electronics (2024). DOI: 10.1038/s41928-024-01195-z
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