In this study, a heterodyne grating interferometer based on the sinusoidal phase modulation method for displacement
measurements was proposed. The interference beams were modulated using a sinusoidal oscillating grating, and the
proposed frequency-domain quadrature detection method was used to detect the optical phase of the interferometer
and determine the displacement. Experimental results were consistent with the strain gauge results for several
displacement ranges. When only high-frequency noise was considered, our method achieved a measurement
resolution of approximately 2 nm.
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.
In this research, a novel heterodyne laser encoder for 6-DOF displacement and angle measurements is proposed. The technique combines the advantages of heterodyne interferometry, grating shearing interferometry, and Michelson interferometry. When a heterodyne light beam with two orthogonally polarized directions is used to focus on a semi-transmission grating, two detection configurations for in-plane and out-of-plane will be obtained. By means of measuring the phase variations of the interfering signals from the moving grating, the in-plane displacement can be acquired. Besides, the out-of-plane displacement can be obtained by detecting the optical path difference between the reference beam and the reflection beam. Furthermore, 6-DOF displacement and angle information can be measured simultaneously by using the beam dividing method. According to the experimental results, the measurement resolution is about 2 nm. The experimental results show that our proposed method has the ability to measure 6-DOF displacement and angle information with high system stability. Comparing with other commercial measurement instructions, this laser encoder has the advantages of high resolution, high stability, and high flexibility.