On-line monitoring of the micro welding process of Si parts in MEMS
Micro-welding processes are applied in special MEMS components in order to permanently fix the position of initially moving parts in a new position and thus, achieve structures and dimensions that cannot solely be produced by etching processes. For this purpose, electric current impulses are fed through the welding points in order to melt them for a short time. This results in a material-locked connection of the parts to be joined. Due to the small dimensions of the weld spots, which are often only 2 µm x 1 µm, a visual or microscopic inspection is not possible or too imprecise. Therefore, a method has been developed which tests the strength of the welding points immediately after welding or at a later fabrication stage. For this purpose, the resonance frequency of the structure with the weld spot is measured, which gives information about the rigidity. While thermally stimulating the mechanical structure to oscillations by means of laser pulses at a suitable location, these oscillations are detected by means of a laser doppler vibrometer. Due to the applied broadband excitation, the frequency response of the mechanical structure and its resonant frequencies are determined by evaluating the excitation and vibration signals. In a learning process, typical frequency values of structures with intact welds are determined, so that defective welds can be easily identified on the basis of lower resonance frequencies. The use of the laser excitation together with an infrared laser doppler vibrometer is particularly advantageous for subsequent testing of sealed chips, since the strength of the welding site can also be assessed in a fabrication stage in which the systems are not freely accessible but hidden under a silicon lid wafer. This can only be achieved by using the IR wavelength for excitation and detection.