P8.3 - Design, Fabrication and Characterization of a Novel 3D Force Sensor Made of SU-8

SENSOR+TEST Conferences 2011
2011-06-07 - 2011-06-09
Proceedings SENSOR 2011
P8 - Mechanical
A. Jordan, A. Phataralaoha, A. Tibrewala, S. Büttgenbach - Technische Universität Braunschweig (Germany)
854 - 857


This work presents an innovative approach to develop highly sensitive piezoresistive 3D force sensors made of SU-8 photoresist and diamondlike carbon (DLC). Therefore a low cost fabrication process is developed accompanied by design studies and an estimation of mechanical properties of the deforming elements. The results of this study provide a comparison between characteristics of 3D force sensors made of SU-8 and silicon.
The design of the SU-8 force sensor is based on a boss-membrane. It is chosen in order to compare the sensor characteristics with a conventional force sensor made of silicon with diffused piezoresistors. The intention is to apply thin films of sputter deposited amorphous carbon and plasma assisted chemical vapour deposited hydrogenated amorphous carbon as strain sensitive material. As tactile element, a 5 mm long stylus with a ruby probing ball of 300 μm diameter is used.
To obtain comparably results in bending stiffness and displacement of the stylus, a higher membrane thickness for the SU-8 deforming elements is to be used. Breaking points as test and numerical results for silicon and SU-8, respectively, are presented. Regarding the achievable maximum force at all spatial directions, it is considerably higher for SU-8. In the event of a lateral load of 1 mN, distinguishing values of about 1 μm displacement at 26 μm thickness (silicon) and of 8.7 μm displacement at 50 μm thickness (SU-8) can be found. The mechanical stress along the membrane axis is analysed. The standardized curve mapped for SU-8 reveals deviations from the ideal linear shape at a membrane thickness lower than 35 μm, affected by the high displacement. In the event of an equal lateral load, the stress maxima reach 10 times higher values compared to the stress at vertical load.
The combination of SU-8 and DLC, due to low Young´s modulus and high gauge factors, respectively, promises sensitivity much higher than what is obtained using conventional silicon force sensors. The presented results provide great opportunities to adjust the characteristics of the innovative SU-8 force sensor to obtain an ideal sensor design.