Fabrication of Polymer Membrane-Suspended Microstructures on Printed Circuit Boards

When microelectromechanical systems are manufactured in high densities, the necessary routing layer structures can become complex and thus expensive if executed in silicon technology. If, additionally, the microstructures are also to be fabricated suspended on membranes, complex thinning processes become necessary to form these often rather fragile membranes. Here, a novel fabrication concept is demonstrated that allows for the fabrication of membrane-suspended microstructures using standard microfabrication techniques directly integrated with printed circuit boards (PCB) by laminating polyimide (PI) films over cavities in the PCB. This results in cost-effective routing layer fabrication and direct compatibility with common electronic component standards such as SMT (surface-mount technology) and THT (through-hole technology). To illustrate the capabilities of the fabrication process, we fabricated an array of membrane suspended microheaters. Individual platinum microheater elements are suspended on PI membranes that are 25 $\mu \text{m}$ thick and 500 $\mu \text{m}$ in diameter. The fabricated PCB carries a $10\times 10$ array of suspended microheaters on an area of 26mm $\times\,\,26$ mm. The characterization of the microheater array shows that a yield of 86% functional microheaters was achieved. An individual microheater shows an almost linear heating characteristic at least up to 200°C demonstrating the viability of the technique for creating functional membrane-suspended microstructures on a PCB. [2021-0226]

I. Introduction

Microstructures suspended on membranes are essential parts of various microsystems, such as pressure sensors [1]–[3], thermal flow sensors [4]–[6], gas sensors [7], [8], audio frequency filters [9], optical filters [10], microactuators [11], [12] and infrared emitters [13]. Depending on the application, these thin membranes serve different purposes, ranging from thermal isolation of components to improving sensor performance parameters such as sensitivity, response time and low power consumption as well as high positioning accuracy of moveable structures. These thin membranes can be fabricated from various materials, such as silicon [14], silicon dioxide [15], [16], silicon nitride [17], [18], silicon carbide [19] or a variety of polymers [20]. In particular, the polymer polyimide (PI) has been used as a membrane material in several works to suspend microstructures such as microheaters, using either silicon-based substrates [21], [22] or even without a solid substrate [23]. Polymers allow membrane-suspended structures to be also implemented on non-silicon-based substrates. One of the most common wiring carriers for electric components are printed circuit boards (PCB). With their high component compatibility, multilayer technology for advanced routing and low-cost production, they offer many advantages, which is why they appear very attractive as a carrier board for microelectromechanical structures. The direct fabrication of MEMS devices with PCB technology has been the subject of research for some time [24]–[26]. Such devices are often referred to as PCB-MEMS [27]. Among the numerous PCB-MEMS devices presented in the past, there are also devices with membrane-suspended structures in particular such where a PI layer is used as the membrane material. A structured PI layer, which carries copper electrodes, is, e.g., used in an RF-MEMS capacitive switch [28]. In another application, a PI membrane is used to suspend a mass in a microelectromechanical accelerometer [10], [29].