Encapsulation of a liquid-sensing microchip using SU-8 photoresist

This paper presents a method for packaging sensor microchips to be used in a liquid environment. The chip is inserted into an accurately machined recess in a printed circuit board (PCB) before wire bonding. A 150 µm thick layer of SU-8 photoresist is used as the encapsulating material. Wells in the resist layer are defined lithographically above the sensing areas of the chip, using a standard mask aligner. These wells contain the solvent-cast polymers used to functionalise the sensor. An additional thin layer of SU-8 is used to prevent the formation of bubbles and to allow the definition of smaller wells. The working lifetime of a pH sensor microchip, encapsulated using this method, is shown to be longer than one week.