2010 12 th Electronics Packaging Technology Conference Force Transmitting Element Fabrication and Characterization for MEMS Tri-axial Force Sensor Application Muhammad HAMIDULLAH* 1 , Li Shiah LIM 1 , Shimul Jennifier NOOR 1,2 , Daquan YU 1 , Woo Tae PARK 1 , Lichun SHAO 1 , Kotlanka RAMAKHRISNA 1 , Hanhua FENG 1 1 Institute of Microelectronics, A*STAR (Agency for Science, Technology and Research) 11 Science Park Road, Singapore Science Park II, Singapore 117685 2 National University of Singapore *Tel : (65) 6770 5422 Fax : (65) 6774 5747 Email : muhammad@ime.a-star.edu.sg Abstract This paper presented the design, fabrication, and characterization of force transmitting element for MEMS tri- axial force sensor application. The transmitting element consists of two main parts: integrated silicon rod and mechanical stopper. The advantages of the design are the simplicity of fabrication process, the ability to detect force in all direction and to protect the sensing element from excessive force, thus increase the robustness of the devices. Introduction From conversations with experienced vascular surgeons in minimally invasive endovascular surgery, up to 20% of the procedures are abandoned due to failed wire traversal. Higher number of procedures is prolonged due to repeated attempts to manipulate the guidewire across the lesions. This is because; current procedure relies mainly on surgeon’s skill in manipulating the guidewire and determining the location of the blockage of the vessel lumen. To help resolve above problems, sensorised guidewires with MEMS tri-axial force sensor has been designed and currently is being fabricated [1]. The basic structure of the MEMS triaxial force sensor was demonstrated for bigger dimensions for surgical knife application [2,3]. In this work, we are leveraging the knowledge of silicon nanowire force sensors [4], integrated silicon rod structure and a novel mechanical stopper concept. Basically, a tactile sensor comprises four layers––a sensing layer, an electronics layer, a protective layer and a support layer [5]. Design and material of protective layer is important because it will also be a part of force transmitting element. For guidewire application, it will have an important role for the overall functionality of tactile sensor because it will be in direct contact with the blood and blood vessel lumen and must be able to limit the sensing range, therefore it will protect the sensing element from excessive force and increase the reliability of the devices. Elastic polymer is commonly used as protective layer because it will not reduce the sensitivity of the sensing element due to low young's modulus and high yield strain properties [6,7]. However, despite giving a good normal force transfer to the sensing element, Gábor Vásárhelyi shows that the ability to transfer shear force could be limited and they have demonstrated the use of plastic rod structure as force transmitting element [8]. Significant improvement of shear force sensitivity is achieved in comparison with direct elastomer covering without rod structure. However, the placement of the plastic rod into the hole on sensing element requires a precise positioning and there is no protection for the sensing element from excessive external force. This paper presents the design, fabrication, and characterization of force transmitting element to achieve robust structure of the tactile sensor. It will begin with design requirement continued by the fabrication process and characterization of force transmitting element. Finally, it will discuss the possible failure mechanism and summaries the feasibility for guidewire application in minimally invasive endovascular surgery. Design Requirement For sensorised guidewire application, the final device size is the important factor. As the device will be placed in the tip of guidewire, the maximum width of the device must be less than 360um, which is the diameter size of the guidewire. It has to be able to sense three dimension forces and must be able to limit the displacement of sensing element because too much displacement will break the sensing element. The whole device must be able to sustain the tip load of guidewire. The tip load is defined by measuring the buckling load of the guidewire. The amount of tip load depends on the type of guidewire, which is mainly categorized on severity of blood vessel blockage. An example is the commercial guidewire from Asahi Intecc, which in general has two different type of guidewire based on the ranges of tip load [9]. For front line cases, the tip load range is from 0.5 gram to 1 gram and for chronic occlusion case, the tip load range is from 3 gram to 12 gram. Finally, the biocompability of the material used is the most important factor for this application. Figure 1(a) shows the design of the sensorized guidewires with MEMS tri-axial force sensor and the expected dimension of the MEMS force sensor chip consist of sensing and force transmitting element in figure 1(b). There will be two main part of force transmitting element, the silicon rod and ball mechanical stopper. Integrated silicon rod structure will simplify the overall fabrication process as the structure is integrated with the sensing element. The mechanical stopper structure will enable the tactile sensor to detect forces in three dimensions, and at the same time is also functioning as 98 978-1-4244-8562-8/10/$26.00 ©2010 IEEE