Ultra-deep x-ray lithography of densely packed SU-8 features: I. An SU-8 casting procedure to obtain uniform solvent content with accompanying experimental results

Although the benefits of SU-8 as a thick photoresist material for MEMS applications have been shown by various researchers, its use for ultra-thick applications (> 1 mm) has been hampered by the difficulties associated with the high solvent gradient of the resultant layers. In this paper, the lithographic performance of the traditional methods of thick resist formation based on the casting of high solvent content SU-8 is presented. As an extension of the work, a new photoresist preparation method called 'dry chip casting' was developed. The new method involves the reduction of the solvent content of a given photoresist until the desired content is reached, the collection of the 'dry' chips and re-constitution on a substrate with the aid of heat and vacuum. The dry chip casting method yielded average solvent contents of about 7% (+/- 0.5%) throughout the thickness of the SU-8 layers, with excellent x-ray lithographic characteristics. Results of ultra-deep x-ray lithography experiments in SU-8 layers as thick as 3 mm are presented in this paper.