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July 16, 2024

Micromirror control in flexible microsystems using 3D printed MEMS

By utilizing Two-Photon Polymerization (2PP) 3D printing and flexible printed circuit boards (FPCB), researchers at Carnegie Mellon University have developed small-scale, lightweight and flexible microsystems with electrostatic microactuators. Demonstrated in an array of movable micromirrors, these systems exhibit precisely controllable actuation capabilities, even when deformed. The challenge was to activate the actuators via the electrically conductive 3D structures of micro-electro-mechanical systems (MEMS) by integrating metal sputtering into the fabrication process. Printing on FPCBs is a particular challenge due to their flexibility and uneven surface, as well as the variable reflectivity of the materials used. This innovation opens new perspectives for applications in adaptive optics and wearable devices.

Micro-Electro-Mechanical Systems (MEMS) are used in a wide range of applications due to their small size, high precision and ability to be integrated into electronic systems. MEMS are deployed as accelerometers, gyroscopes and magnetometers in smartphones, tablets and gaming or virtual reality applications, as sensors in wearable devices and much more. The ability to produce complex, microscale designs with high shape accuracy on a variety of substrates makes 2PP-based 3D printing the first choice for MEMS manufacturing, eliminating the need for multiple assembly steps.
While 3D printing on rigid substrates such as a glass slide or silicon wafer is straightforward, printing on flexible printed circuit boards (FPCBs) is a significant challenge due to the flexible and non-flat surface of the substrate, which consists of different materials such as polyamide and copper at different heights, making it difficult to find the interface and print the boundary layer.

MEMS actuator fabrication on flexible circuit boards

The integration of microactuators on flexible substrates presents numerous challenges, particularly in maintaining functionality under deformation. The researchers at Carnegie Mellon University, Sukjun Kim, Regan Kubicek, and Sarah Bergbreiter, addressed these issues by utilizing Nanoscribe’s 3D printing technology with Two-Photon Polymerization. This innovative method enabled the precise fabrication of electrostatic microactuators directly onto off-the-shelf flexible printed circuit boards (FPCBs). The result is a robust and high-performance flexible microsystem that maintains actuation capabilities even under significant deformation. This capability is particularly evident in the flexible micromirror array, where the actuators can precisely control mirror movements to alter the direction of reflected light. A large number of micromirrors can be rapidly fabricated and integrated over a large area by leveraging automated 3D printing in the fabrication process. In this research project, a 3×9 micromirror array has been successfully demonstrated.

Aligned 3D printing on complex substrates

3D printing on FPCBs is particularly challenging due to the unconventional substrate, which includes an existing topography and multiple materials such as polyamides and copper. The researchers developed a fabrication strategy to 3D print the microactuators on the prefabricated, non-planar surfaces of the flexible substrates. In particular, the topography with varying heights of copper traces and other structures on the foil was a challenge that was successfully overcome with customized buffer layers.
The different reflectivity presents a challenge for identifying the surface to be printed on. Additionally, the adhesion properties of the FPCB structures must be addressed to ensure the firm placement of the MEMS structures. The electrical integration of the 3D printed microactuators on the FPCBs also requires a high level of precision in the manually aligned 3D printing and metal deposition step.

Applications with flexible microsystems

The researchers have successfully demonstrated that FPCBs are well suited as a platform for micro-electro-mechanical systems (MEMS) with a high degree of precision and control. This opens the door to a whole new world of possible applications. Other types of microactuators, such as thermal or liquid crystal elastomers, can also be integrated, as can all types of electrically connected MEMS sensors, such as novel capacitive sensing architectures. Utilizing the integration capability of the FPCB through embedded metal layers, untethered flexible microsystems with on-board electronics can pave the way to smart flexible microsystems with power and control autonomy.

Next generation 3D printing opportunities

The research project used Nanoscribe's Photonic Professional GT+ for 3D printing, a highly accurate and mature microfabrication system. However, many of the challenges described in the project can be solved much faster, more elegantly, and with higher quality and accuracy using the next generation of microfabrication.

Fabrication of a flexible printed circuit board (FPCB) with an integrated micromirror
Fabrication of a flexible printed circuit board (FPCB) with an integrated micromirror. A) FPCB is attached to a glass substrate using SU-8 as temporary adhesive. B) 3D structures are 3D printed directly onto the FPCB using 2PP. C) The printed structures are sputter coated with aluminum. D) Electronic components are mounted and connected to the actuator through copper traces embedded in the FPCB. Pictures: Carnegie Mellon University
a complex micro-electro-mechanical system (MEMS) that can be 3D printed using Two-Photon Polymerization (2PP)
Each micromirror is driven by an electrostatic actuator. The result is a complex micro-electro-mechanical system (MEMS) that can be 3D printed using Two-Photon Polymerization (2PP). The flat and square micromirror was printed with the movable comb so that the direction of reflected light changes when voltage is applied to the actuator. SEM image: Carnegie Mellon University
Schematic of a flexible printed circuit board (FPCB) with an integrated 2x2 micromirror array
Schematic of a flexible printed circuit board (FPCB) with an integrated 2x2 micromirror array. To demonstrate the flexibility and the integration capability of the FPCBs, micromirrors were tested while the FPCBs were bent over a curved substrate. Thus, new applications like adaptive and wearable devices can be realized. Picture: Carnegie Mellon University
Functional prototype of a flexible printed circuit board (FPCB) with an integrated 2x2 micromirror array
Functional prototype of a flexible printed circuit board (FPCB) with an integrated 2x2 micromirror array. Each micromirror with an electrostatic actuator is a complex electro-mechanical system (MEMS) that is 3D printable by Two-Photon Polymerization (2PP). Picture: Carnegie Mellon University

Nanoscribe's Quantum X systems feature up to three different interface detection methods. This significantly improves detection accuracy for a wide range of substrate characteristics. In addition, Quantum X align simplifies tedious placement and active alignment of the various parts and elements to each other. Equipped with Aligned 2-Photon Lithography A2PL®, the system automatically detects interfaces on complex substrates and their spatial orientation with nanoprecision. Microscale parts are defined using nanoPrintX, a software for complex objects that require precise alignment on a prefabricated substrate and printed directly in place with correct orientation and tilt compensation. This reduces the complexity of the process chain, relaxes assembly tolerances, and enables further miniaturization of devices.
 

Would you like to find out more about this inspiring project? Then read the open-access publication here: “3D-printed electrostatic microactuators for flexible microsystems

This and other scientific publications on more than 1,800 research projects by Nanoscribe customers and system users can be found in a powerful keyword-searchable database in the Premium Resources section. Register for free to see for yourself the potential of Nanoscribe's 3D microfabrication technology for innovative applications and fundamental innovations in many fields, and to evaluate its suitability for your project.

This and further scientific publications on more than a thousand research projects from Nanoscribe customers and system users can be found in a powerful database with a keyword search in the Premium resources section. Register for free to see for yourself the potential of Nanoscribe’s 3D Microfabrication technology for innovative applications and fundamental innovations in many areas, and to evaluate its suitability for your project.

 

Video information

Flexible printed circuit boards (FPCB) with an integrated 2x2 micromirror array. Each micromirror with an electrostatic actuator is a complex micro-electro-mechanical system (MEMS) that is 3D printable by Two-Photon Polymerization (2PP). New applications such as adaptive or wearable devices become possible. Pictures, SEM image and video: Carnegie Mellon University

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