|
An innovative moiré technique for full-field wafer warpage measurement is proposed in this study. The wafer warpage
measurement technique is developed based on moiré method, Talbot effect, scanning profiling method, stroboscopic,
instantaneous phase-shift method, as well as four-step phase shift method, high resolution, high stability and full-field
measurement capabilities can be easily achieved. According to the proposed full-field optical configuration, a laser beam
is expanded into a collimated beam with a 2-inch diameter and projected onto the wafer surface. The beam is reflected
by the wafer surface and forms a moiré fringe image after passing two circular gratings, which is then focused and
captured on a CCD camera for computation. The corresponding moiré fringes reflected from the wafer surface are
obtained by overlapping the images of the measuring grating and the reference grating. The moiré fringes will shift when
wafer warpage occurs. The phase of the moiré fringes will change proportionally to the degree of warpage in the wafer,
which can be measured by detecting variations in the phase shift of the moiré fringes in each detection points on the
surface of the entire wafer. The phase shift variations of each detection points can be calculated via the instantaneous
phase-shift method and the four-step phase-shift method. By adding up the phase shift variations of each detection points
along the radii of the circular gratings, the warpage value and surface topography of the wafer can be obtained.
Experiments show that the proposed method is capable of obtaining test results similar to that of a commercial sensor, as
well as performing accurate measurements under high speed rotation of 1500rpm. As compared to current warpage
measurement methods such as the beam optical method, confocal microscopy, laser interferometry, shadow moiré
method, and structured light method, this proposed technique has the advantage of full-field measurement, high
resolution, stability and adaptability.
|
