Abstract
The requirement to make low profile ohmic contacts to a piezo-resistive MEMS pressure sensor has highlighted limitations of ultrasonic wire bonding technology. Wire bonding typically uses 25-50 mu m diameter gold or aluminium wire and ultrasonic welding to the contact pads of micro-electronic devices results in a contact wire proud of the pad surface. If the application involves the MEMS pressure sensor and contacts being encapsulated, then repetitive changes in pressure flexing the contact wires can lead to fracture.
A possible solution is to scale down laser welding technology to fuse materials at the micron scale. For this purpose a precision ophthalmic surgical laser system has been modified to investigate optimum conditions for laser welding, both at the micron scale and for the typical geometries involved. Typical-requirements involve a cylindrical contact wire to be bonded to a thin contact pad on the MEMS device. Since the pad size is of similar dimension to the wire, and the requirement for a low profile stable configuration, a keyhole welding strategy is required.
The Nd:YAG based ophthalmic laser has been modified, the Q-switch removed and the output pulse width and energy controlled principally via control of the flashlamp.