Diffraction Interferometer for Mapping the Surface Deformation of Fine-pitch Grating Structures
Michael Mello (Georgia Institute of Technolog), Bongtae Han (University of Maryland), Zhaoyang Wang (The Catholic University of America)
Eringen Medal Symposium in honor of G. Ravichandran
Mon 4:20 - 5:40
Salomon 001
A whole field interferometer technique is presented which uses a collimated, infrared (=10.6m) laser beam to map the surface profile of fine-pitch grating structures. The technique is demonstrated by mapping the 3D surface profile of a high-density solder bump pattern. In this instance the solder bumps act as a periodic array of scattering sources which collectively diffract the incident wave front into a 2D array of diffraction orders. By precisely tilting the substrate, a particular diffraction order is directed to interfere with a planar reference beam. Oriented in this fashion, any background radiation reflected from the lower underlying substrate is diverted from the interference path. A “spatially filtered” interference pattern emerges which is then imaged by an IR CCD camera. The resulting fringe pattern representing the distorted optical wave front is directly correlated to the to surface topology of the fine-pitch solder bump pattern. Fringe sensitivity analysis reveals that for shallow diffraction angles the interferometer is extremely insensitive to in-plane deformation of the solder bump pattern and thus behaves as a normal displacement interferometer with a fundamental sensitivity of ~/2 [m/fringe]. Resulting fringe patterns thus provide a whole field topographical contour map of the bump pattern top surface profile at a sensitivity of 5.3 [m/fringe]. Four-step /2 phase shifting interferometry is used to generate a phase map, which is subsequently scaled to generate a 3D whole-field plot of the bump pattern surface profile. Details of the optical arrangement are presented along with a derivation of interferometer sensitivity. Applications of the technique to solder bump profiles are presented and compared to 3D profiles of the same units generated by an industry standard Wyko white light scanning profilometer.