PMMA to SU-8 bonding for polymer based lab-on-a-chip systems with integrated optics

Abstract

An adhesive bonding technique for wafer-level sealing of SU-8 based lab-on-a-chip microsystems with integrated optical components is presented. Microfluidic channels and optical components, e.g. waveguides, are fabricated in cross-linked SU-8 and sealed with a Pyrex glass substrate by means of an intermediate layer of 950k molecular weight poly-methylmethacrylate (PMMA). Due to a lower refractive index of PMMA (n = 1.49 at λ = 600–900 nm) this bonding technique preserves waveguiding in the cross-linked SU-8 structures (n = 1.59 at λ = 633 nm) in combination with good sealing of the microfluidic channels. The bonding strength dependence on bonding temperature and bonding force is investigated. A maximum bonding strength of 16 MPa is achieved at bonding temperatures between 110 °C and 120 °C, at a bonding force of 2000 N on a 4 inch wafer. The optical propagation loss of multi-mode 10 µm (thickness) × 30 µm (width) SU-8 waveguides is measured. The propagation loss in PMMA bonded waveguide structures is more than 5 dB cm−1 lower, at wavelengths between 600 nm and 900 nm, than in similar structures bonded by an intermediate layer of SU-8. Furthermore 950k PMMA shows no tendency to flow into the bonded structures during bonding because of its high viscosity.