The laser interferometer is widely used in various fields because of its high resolution, high stability, high measurement speed and large-scale measurement capabilities, thus many research groups and equipment manufacturers have devoted time and resources to its development. This study presents an innovative symmetrical double diffraction laser encoder for precision displacement measurement. The system has the advantages of not defocusing during measurement, and can provide long range dual-axis linear displacement and rotation angle measurement. The system consists of two detection configurations, each composed of a double diffraction optical configuration, grating interferometer and phase demodulation system. The light source is passed through a non-polarized beam splitter, diffraction grating and mirror to form a grating interferometer system. The positive and negative first order beams formed from grating diffraction are reflected back through the grating by mirrors, forming a symmetrical double diffraction optical configuration to effectively enhance the system resolution. When the grating moves a corresponding phase shift will be introduced into the signal. Finally, a photodetector receives the signal and the data is analyzed with a self-developed phase demodulation program to obtain the displacement information. By comparing the displacement information of the two axes, rotation information can be obtained via trigonometric calculation. It can be inferred from the measurement principles that the theoretical resolution can be as high as 15 pm. Experimental results demonstrate that for displacement and rotation measurement, the repeatability of the symmetrical double diffraction laser encoder is 5 nm and 35 nrad, respectively. The system has excellent measurement performance, and its simple structure lends to easy setup and calibration.
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.