Non-Cleanroom Fabrication of Carbon Nanotube-Based MEMS Force and Displacement Sensors

Abstract

Traditional microelectromechanical MEMS fabrications such photolithography and deep reactive ion etching (DRIE) are expensive and time consuming. This limits the types and designs of MEMS devices that can be produced cost effectively since in order to overcome the high startup costs and times associated with traditional MEMS fabrication techniques tens of thousands of each type of MEMS device must be produced and sold. In this paper, we will present a method for placing carbon nanotube (CNT) based piezoresistive sensors onto metallic flexural elements that are created via micromachining. This method reduces the fabrication time from over 3 months to less than 3 days. In addition, the fabrication cost is reduced form over $500 per device to less than $20 per device. This flexible, low cost fabrication method enables rapid prototyping of MEMS devices which is an important step in the design and development process for electromechanical systems. Also, the development of this type of low cost fabrication method will help to make low volume manufacturing of MEMS devices feasible from a cost prospective. In this fabrication method, a micromill is used to fabricate the flexure beams. Electron beam evaporation is then used to deposit (1) an insulating ceramic thin film layer and (2) metal traces on the flexure. A shadow mask is used to define the wire patterns. Either a tungsten wire or a focused ion beam (FIB) is used to define a 1–5 μm gap in the wire traces. Dielectrophoresis is then used to orient/position the CNT sensors across the gap. Finally, the structure is coated with a thin ceramic layer to protect the sensor and mitigate noise. When the flexure element is deflected, the CNTs strain which results in a measurable change in resistance. Several meso-scale test devices were produced using this fabrication method. The devices that were fabricated using a FIB to create the gap in the wire traces have the same strain sensitivity as devices fabricated using traditional cleanroom based techniques. However, the devices that were fabricated using the tungsten wire have a strain sensitivity that is almost 7 times lower than the devices fabricated using traditional cleanroom based fabrication techniques. This is because the gap size for the tungsten wire fabrication method is about an order of magnitude larger than for the FIB cut or lithography based gap fabrication methods. Therefore, the CNT are not able to stretch across the entire gap. This creates CNT-CNT junctions in the electrical pathway of the sensors which significantly increases the sensor resistance and decreases the strain sensitivity of the sensor. Overall, these results show that functional CNT-based piezoresistive MEMS sensors may be fabricated without conventional integrated circuit (IC) microfabrication technologies but that tight control over the gap size is needed in order to ensure that the sensor performance is not degraded.