## 3 Experimental Design and Result Analyses  
To demonstrate the feasibility and measurement performance of the proposed technique, a 3-D measurement system consisting of two sensing cameras, a single-frequency transmission grating with a period of 24 line pairs per millimeter, and a step motor with positioning accuracy less than 1 \(\upmu\)m was developed. Two telecentric lenses with the same optical specification are integrated in front of the two cameras as the optical objective to collimate the reflected diffuse and specular light from the measured object surface. The light module was powered by a 30-Watt white light LED, in which the LED is mounted on a specially designed mechanical radiator for good heat dissipation. A SolidWorks design and hardware of the housing and optical system were implemented and shown in Figure 10a,b, respectively. The measuring FOV can reach 1.0 \(\times\) 1.0 mm for every measurement. A maximum measuring depth of the system can reach 1.0 mm which is sufficient for most microstructure profile inspection. Figure 10: Developed system: (**a**) SolidWorks design; (**b**) system hardware. Figure 9: Phase-shifted images acquired from four-step phase shifting operation and image summing algorithm: (**a**) Phase-shift 0\({}^{\circ}\); (**b**) Phase-shift 90\({}^{\circ}\); (**c**) Phase-shift 180\({}^{\circ}\); (**d**) Phase-shift 270\({}^{\circ}\); and (**e**) Image result of average 4 channels. Figure 11a,b shows the fringe images of the tested wafer surface captured using the tilted and vertical cameras, respectively. Figure 11c,e illustrates the phase map and one height cross-section of the specular wafer surface measured using the tilted camera. Similarly, the phase map and one height cross-section for the laser-etched groove surface regions are measured using the vertical camera simultaneously and shown in Figure 11d,f. The 3-D profile of groove surface region can be well measured and reconstructed (see Figure 11h). It is important to note that the 3-D measured shape area of the groove region has random noises due to the non-detectable diffuse light from the laser groove surface region. Nevertheless, this region can be totally recovered and reconstructed by another vertical camera which can capture the diffuse light from the groove region. Two detected 3-D profiles, shown in Figure 11g,h, are extracted from their respective original images by the image summing algorithm and further mapped by the camera calibration technique precisely to reconstruct a full-field 3-D profile of the measured wafer, shown in Figure 11i. These results indicate that the full-field 3-D wafer surface can be measured and reconstructed accurately using the developed technique. Figure 11: _Cont._  
Figure 12 exhibits the measured height cross-section along one lateral section on the tested wafer being captured by the tilted and vertical cameras. The standard deviation of the wafer surface region and groove area are evaluated for measurement precision. In a 30-time repeatability test, the standard deviation in the measured data region, shown in Figure 12a,b, are 0.265 \(\upmu\)m and 1.937 \(\upmu\)m, respectively. The results obtained show that the measurement repeatability can be achieved for less than 2.0 \(\upmu\)m when measuring a rough laser-machined wafer surface. With the proposed technique, the measured average step height of the cutting groove on the wafer is 19.01 \(\upmu\)m.  
To check the measurement accuracy, the measured result was compared with a pre-calibrated laser confocal microscope.