In situ measurement of mechanical properties of polyimide films using micromachined resonant string structures

Two irt situ measurement schemes, using micromachined resonant string struc tures, for the measurement of the polyimide residual stress and polyimide/m etal adhesion durability have been developed. The residual stress of polyim ide films, DuPont PI-2555 and PI-2611, have been measured using a bulk micr omachined string structure. According to the Rayleigh's method, the resonan t frequency of a polyimide string can be related to the film stress. By mea suring the resonant frequency of these polyimide strings, the residual stre sses have been calculated. The measurement results of various strings have been compared with conventional measurement results, which shows that they are in good agreement, Also, a noble scheme to quantize the adhesion durability between a polyimid e film and a metal film has been developed. This scheme is based on a polyi mide/metal bimorph string structures, fabricated using a surface micromachi ning technique, vibrating with an alternating potential, The change of reso nance profile of this string structure can be related to the degradation of adhesion strength at the polyimide/metal interface. Various polyimide/gold string structures have been fabricated using a surface micromachining with Cu sacrificial layers, and the resonant qualities have been monitored. Not able changes of resonant Q-factor and resonant frequency, due to the degrad ation of adhesion between the metal and polyimide, have been observed after 10(8) cycles (string vibration) for the polyimide/gold bimorph strings. Th e changes of resonant Q-factor and resonant frequency over a time period (v ibration cycles) have been monitored.