What Materials Are Used for MEMS Sensors?

By JanakyReviewed by Lexie CornerJul 24 2024

Micro-electro-mechanical systems (MEMS) based sensors are microscopic devices that incorporate mechanical and electronic components on a single chip to measure physical quantities such as pressure, temperature, acceleration, and magnetic fields.¹

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MEMS devices can convert electrical signals into mechanical force or vice versa. These sensors are essential in modern technology, providing precise measurements and enabling various applications, including motion sensors, pressure and force sensors, small actuators, acoustic and ultrasonic devices, resonators, and fluid sensors in medical treatments.²

Material Selection for MEMS Sensors

Material selection is crucial for MEMS sensor performance due to the wide range of materials available, complicating the selection process and traditional design methods. The chosen material greatly influences crucial properties like density, Young's modulus, and ultimate tensile strength, which are key to accuracy and reliability.³

Modern MEMS sensors are typically made from silicon, polymers, metals, or ceramics.¹

Semiconductor Materials for MEMS Sensors

Semiconductor engineers leveraged manufacturing techniques developed for computer chips to create microscopic MEMS devices. Fabrication methods for MEMS sensors, such as layer deposition, photolithography, and etching, are similar to those used for basic semiconductor devices like transistors and integrated circuits.⁴

Silicon is popular for MEMS because it is inexpensive, widely available, and easily integrates electronic functions. Its near-perfect flexibility allows it to endure billions to trillions of movements without breaking, making it highly reliable and durable for MEMS applications.⁵

Silicon can be precisely shaped using different etching techniques to create complex 3D structures. Additionally, silicon dioxide, derived from silicon, protects these tiny devices from contamination and maintains their performance.⁷ Polysilicon is also notable for its low-loss, extremely stable mechanical properties, making it widely used in MEMS.⁸

Piezoelectric and Pyroelectric Materials for MEMS Sensors

Piezoelectric materials such as quartz and lead zirconate titanate (PZT) produce charge when pressure changes their unit cell dimensions due to the separation between positive and negative charges in the material.⁹ Thin films of piezoelectric material wafers are used in MEMS devices with specific sensing applications.

The most established piezoelectric material-based MEMS technology uses zinc oxide (ZnO) thin films, which are widely used in film-bulk acoustic-wave resonators, surface acoustic wave resonators, acousto-optic devices, and acousto-electric devices.¹⁰ PZT is also used for several MEMS applications, including ultrasonic transducers, acoustic sensors, pressure microsensors, and accelerometers.¹⁰

Pyroelectric materials convert temperature changes into electrical energy by shifting their internal structure. Polycrystalline aluminum nitride is widely used in pyroelectric MEMS devices for applications such as thermal imaging, missile targeting and guidance, gas leakage detection, fire monitoring, and medical diagnostics.¹¹

Pyroelectric detectors are ideal for these applications due to their quick response and high sensitivity to changes in infrared radiation, operating without the need for sensor cooling.¹²

MEMS pyroelectric infrared sensors are often fabricated with suspended ZnO pyroelectric films on thermally insulated silicon substrates, using standard techniques like thin film deposition, photolithography, and etching.

Role of Metal and Polymeric Materials in MEMS Technology

Metals are used to create electrodes, interconnects, and structural components of MEMS sensors due to their excellent conductivity, elastic properties, and reliability under mechanical stress.¹³ Metals like gold, aluminum, nickel, and copper are coated on MEMS parts by electroplating, evaporation, or sputtering.

In MEMS flow sensors, thin layers of gold construct heat sinks and flow guides, enabling precise measurement of fluid flow rates.¹⁴ Additionally, metals like platinum, nickel, and copper are used in heat sensing elements within MEMS sensors, where their temperature-resistance ratios help accurately detect changes in environmental conditions like airflow or fluid flow.

While silicon remains prevalent in the electronics industry, polymers offer distinct advantages in MEMS technology due to their ability to be mass-produced with diverse material properties. Polymers are particularly useful in biomedical MEMS (bioMEMS) applications.¹⁵ They can serve as both structural components and flexible substrates that house other microdevices.

Polymers like SU-8 and polydimethylsiloxane offer a simpler manufacturing process than silicon, avoiding complex etching and lithography.¹⁶ The primary benefit of using polymers in MEMS is their cost-effectiveness for making micro and nanostructures.

Typical methods for making MEMS devices from polymers include injection molding, embossing, and stereolithography. These techniques are well-suited for applications like disposable blood testing cartridges in healthcare.

The flexibility of polymers and their compatibility with living tissues are essential for accurate medical diagnostics. MEMS sensors in this area can quickly and accurately detect biological substances using tiny structures like microcantilevers, nanopores, or microchannels to translate biological interactions into measurable electrical signals.¹⁶

Future Outlook

New materials such as graphene, nanowires, and advanced polymers are set to transform MEMS sensors, boosting their performance and capabilities. These materials bring unique benefits, such as greater flexibility, strength, and conductivity, all of which improve sensor performance significantly. Advances in MEMS fabrication methods are paving the way for more efficient and durable devices.

A recent review in ACS Applied Electronic Materials highlights that emerging materials could make MEMS sensors more reliable and capable, driving advancements in healthcare, automotive, and electronics. Research is focusing on materials that work better inside the human body, stay stable, and enhance signal detection.¹⁷

While silicon and glass will remain key in MEMS technology, there is a trend toward polymer alternatives. New liquid and dry film polymers with properties like shape memory, photosensitivity, or piezoelectricity are being developed. These smart materials open exciting possibilities, especially in medicine, such as invasive surgeries, tissue and bone growth stimulation, and mini-labs on computer chips.⁵

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References and Further Reading

1. ESSystems (2023). A Comprehensive Guide to MEMS Sensors. [Online] ESSystems. Available at: https://esenssys.com/comprehensive-guide-to-mems-sensors/
2. Choudhary, V., Iniewski, K. (2017). Mems: fundamental technology and applications. CRC Press. https://doi.org/10.1201/b14970
3. Mehmood, Z., Haneef, I., Udrea, F. (2018). Material selection for micro-electro-mechanical-systems (MEMS) using Ashby's approach. Materials & design. DOI: 10.1016/j.matdes.2018.07.058
4. Hajare, R., Reddy, V., Srikanth, R. (2022). MEMS based sensors–A comprehensive review of commonly used fabrication techniques. Materials Today: Proceedings. DOI: 10.1016/j.matpr.2021.05.223
5. Hussain, H. (2019). MEMS Evolution: from Silicon to Polymers. [Online] The Technology Partnership. Available at: https://www.ttp.com/insights/mems-evolution-from-silicon-to-polymers/
6. Angell, JB., Terry, SC., Barth, PW. (1983). Silicon micromechanical devices. Scientific American. DOI: 10.1038%2Fscientificamerican0483-44
7. Helvajian, H. (1999). Microengineering aerospace systems. The Aerospace Press. ISBN 1884989039, 9781884989032
8. Ando, T., Fu, XA. (2019). Materials: Silicon and beyond. Sensors and Actuators A: Physical. DOI: 10.1016/j.sna.2019.07.009
9. Zhu, R., Wang, Z. (2020). Piezoelectric one-to two-dimensional nanomaterials for vibration energy harvesting devices.  Emerging 2D materials and devices for the internet of things. DOI:10.1016/B978-0-12-818386-1.00009-6
10. Ali, WR., Prasad, M. (2020). Piezoelectric MEMS based acoustic sensors: A review. Sensors and Actuators A: Physical. DOI:  10.1016/j.sna.2019.111756
11. Gaur, SP., Rangra, K., Kumar, D. (2019). MEMS AlN pyroelectric infrared sensor with medium to long wave IR absorber. Sensors and Actuators A: Physical. DOI: 10.1016/j.sna.2019.111660
12. Lee, CY., Yu, CX., Lin, KY., Fu, LM. (2021). Effect of substrate-thickness on voltage responsivity of MEMS-based ZnO pyroelectric infrared sensors. Applied Sciences. DOI:10.3390/app11199074
13. Aabachy. (2022). Materials. [Online] Aabachy. Available at: https://abachy.com/catalog/mems/materials
14. Ejeian, F., Azadi, S., Razmjou, A., Orooji, Y., Kottapalli, A., Warkiani, ME., Asadnia, M. (2019). Design and applications of MEMS flow sensors: A review. Sensors and Actuators A: Physical. DOI: 10.1016/j.sna.2019.06.020
15. Chircov, C., Grumezescu, AM. (2022). Microelectromechanical systems (MEMS) for biomedical applications. Micromachines. DOI: 10.3390/mi13020164
16. Kim, BJ., Meng, E. (2015). Review of polymer MEMS micromachining. Journal of Micromechanics and Microengineering. DOI: 10.1088/0960-1317/26/1/013001
17. Hossain, N., Al Mahmud, MZ., Hossain, A., Rahman, MK., Islam, MS., Tasnim, R., Mobarak, MH. (2024). Advances of Materials Science in MEMS Applications: A Review. Results in Engineering. DOI: 10.1016/j.rineng.2024.102115

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