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Influence of photo-initiator concentration on residual mechanical stress in SU-8 thin films

S.D. Psoma, D.W.K. Jenkins, G.E. Derbyshire and R. Stevens
CCLRC RUTHERFORD APPLETON LABORATORY, UK

Keywords:
SU-8 films, residual stress, photo-initiatiror, fabrication

Abstract:
The negative photoresist SU-8 is widely used in the development and fabrication of MEMS devices. The structures of the resist are the product of photochemical and thermal cationic processes and result in vertical sidewalls and high aspect ratio feature, which are among SU-8’s most desirable attributes (1,2). However, during the fabrication process considerable internal stress is generated in SU-8 films, which causes the appearance of cracked features. Therefore, it is important to investigate each process step in relation to the internal stress formulation during the SU-8 fabrication process in order to attenuate stress cracking and distortions without significantly affecting lithography. In the literature the most common tool of investigating this topic is the utilisation of simulation tools like Finite Element Analysis (FEA) (3).
In this paper, experimental results on the residual mechanical stress generated during the different steps of the photolithography process of SU-8 thin films using different photo-initiator percentages are presented. The main aim is to investigate the causes and optimise the fabrication process in order to minimise the residual stresses. It was found that the major parameters that influence the generation of internal stresses were the exposure time, the post-baking temperature and time and the concentration of the photo-initiator used (Cyracure UVI).

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