Calvin C. Chang

Development of dynamic 3-D surface profilometry using stroboscopic interferometric measurement and vertical scanning techniques

The main objective of this technical advance is to provide a single optical interferometric framework and methodology to be capable of delivering both nano-scale static and dynamic surface profilometry. Microscopic interferometry is a powerful technique for static and dynamic characterization of micro (opto) electromechanical systems (M (O) EMS). In view of this need, a microscopic prototype based on white-light stroboscopic interferometry and the white light vertical scanning principle, was developed to achieve dynamic full-field profilometry and characterization of MEMS devices. The system primarily consists of an optical microscope, on which a Mirau interferometric objective embedded with a piezoelectric vertical translator, a high-power LED light module with dual operation modes and light synchronizing electronics unit are integrated. A micro cantilever beam used in AFM was measured to verify the system capability in accurate characterization of dynamic behaviours of the device. The full-field second-mode vibration at a vibratory frequency of 68.60 kHz can be fully characterized and 3–5 nm of vertical measurement resolution as well as tens of micrometers of vertical measurement range can be easily achieved.

A white-light profiling algorithm adopting the multiwavelength interferometric technique

A new profiling algorithm is proposed for the scanning white-light interferometry. A series of white-light interferograms are acquired by traditional vertical scanning process. The collected intensity data of the interferograms are then Fourier-Transformed with respect to the ordinate, or the scanning axis, into the wave number domain, where two or more wave numbers are selected for further calculation. The multi-wavelength phase-unwrapping technique is then used to solve for the surface profile. Preliminary experiment has been carried out with a Mirau-type white-light interferometer on two sets of step-height standards. The proposed algorithm works as well even when the spectrum of the white-light source is not Gaussian distributed, while the conventional peak sensing algorithms do not.

3D surface profilometry for both static and dynamic nanoscale full field characterization of AFM micro cantilever beams

A static and dynamic 3-D surface profilometer with nano-scale measurement resolution was successfully developed using stroboscopic illumination and white-light vertical scanning techniques. Microscopic interferometry is a powerful technique for static and dynamic characterization of micro electromechanical systems (MEMS). As MEMS devices move rapidly towards commercialization, the issue of accurate dynamic characterization has emerged as a major challenge in design and fabrication. In view of this need, an interferometric microscopy based on white-light stroboscopic interferometry using vertical scanning principle was developed to achieve static and dynamic full-field profilometry and characterization of MEMS devices. A micro cantilever beam used in AFM was characterized using the developed instrument to analyze its full-field resonant vibratory behavior. The first five mode resonant vibration can be fully characterized and 3-5 nm of vertical measurement accuracy as well as tens micrometers of vertical measurement range can be achieved. The experimental results were consistent with the theoretical simulation outcomes from ANSYS. Using white-light stroboscopic illumination and white-light vertical scanning techniques, our approach has demonstrated that static and dynamic 3-D nano-scale surface profilometry of MEMS devices with tens-micrometer measurement range and a dynamic bandwidth up to 1MHz resonance frequency can be achieved.

Innovative automatic resonant mode identification for nano-scale dynamic full-field characterization of MEMS using interferometric fringe analysis

A dynamic 3D nano-scale surface profilometer was successfully developed for novel automatic resonant frequency identification using stroboscopic interferometric principle. With rapid increase in the application of micro electromechanical systems (MEMS) to industries, the needs of accurate dynamic characterization have become a major challenge in design and fabrication. In view of such, an interferometric microscopy was developed using LED stroboscopic interferometry to achieve dynamic full-field profilometry and characterization of MEMS with a measurement bandwidth exceeding 1 MHz. Most importantly, a novel detection algorithm was also developed employing interferogram fringe density measure for automatic resonant frequency identification. Natural resonant modes of a series of microstructures can be accurately detected, giving values consistent with theoretical ones. To verify the effectiveness of the developed methodology, an AFM cantilever microbeam and a cross-bridge microbeam were measured to analyze their full-field resonant vibratory shapes. Our experimental results confirmed that the resonant vibration of the tested beams can be fully characterized while achieving an accuracy in vertical measurement of 3–5 nm with a vertical measurement range of tens of micrometers.

Dynamic Nano-scale Surface Profilometry Using Stroboscopic Interferometry

Dynamic 3-D nano-scale surface profilometry using stroboscopic interferometry was successfully developed. An optical microscopy based on stroboscopic interferometry was developed to achieve full-field vibratory out-of-plane surface profilometry and system characterization. To increase the measurement bandwidth of the developed system, an innovative image processing algorithm based on deconvolution principle was developed to improve the signal to noise ratio of the detected interferometric data. The method provides an excellent way to increase the measurement bandwidth without adding any significant hardware in a stroboscopic interferometric framework. Meanwhile, an innovative detection algorithm based on image contrast measure was developed for automatic identification of accurate resonant modes. To verify the effectiveness of the developed methodology, a cross microbeam was measured to analyze the full-field resonant vibratory modes and dynamic characteristics. The experimental results confirm that the dynamic behavior of the tested microcantilever beams can be accurately characterized and 5 nm of vertical measurement accuracy as well as tens micrometers of vertical measurement range can be achieved. The measured results were satisfactorily consistent with the theoretical simulation outcomes from ANSYS.

Fast surface profiling using monochromatic phase and fringe order in white-light interferometry

The results of combining the wrapped phase with the fringe order of this phase to increase the precision of white-light interferometry at high scanning speed are presented. Monochromatic phase data are calculated using the Fourier method and the fringe order is determined using a general coherence peak sensing method. A wide scanning interval of 5λ/8 and a narrow-band color filter with a bandwidth of 70 nm are adopted to acquire interferograms. Experiments with an rms repeatability of step height measurement of below 1 nm and a scanning speed of 40 μm/s are performed.

Auto-scanning white-light interferometer

This study proposes the auto-focusing procedure and the scan-range determining algorithm for white-light scanning interferometry. During white-light scanning interferometry, the interference fringe must be located and to the best-focus interferogram identified. The vertical-scan range must also be determined prior to the scanning procedure. A series of images, either in-focus or out-of-focus, are collected in a proposed interference-fringe searching step. The contrast and the sharpness indices of each image are calculated and applied in the auto-focusing scheme, and the vertical-scan range is determined accordingly. Some preliminary experiments are performed to demonstrate that the best-focus interferogram can be located precisely and the vertical-scan range can be determined.

A hybrid phase unwrapping method for correction the error

Phase unwrapping is a very important processing step in phase shift interferometry. In this work, we propose a new method which combines the branch-cut method with error correcting. The method can avoid the propagation of the phase errors and have higher reliability. The experiment proves the proposed method is feasible and effective.

A Multifunctional Scanning White-Light Interferometer (SWLI): Construction of 2-D and 3-D Measurements

It is necessary and important to find the focal plane in order to locate where the interference fringe is, and to determine the vertical-scan range prior to the scanning operation for an SWLI. A novel scheme is proposed in this paper for simultaneously carrying out the auto-focusing and the real-time 2-D measurement. Conventionally in a SWLI, it always happens that the interference fringe appears across the focal plane, and therefore the in-focus image is blurred by the interference fringe, which makes it difficult to carry out the auto-focusing and the 2- D measurement with the in-focus image. A multi-function SWLI has been developed such that the 2-D and 3-D measurements can be done simultaneously. Although the 3-D profile reconstruction is basic to an SWLI, the real-time 2-D measurement is not trivial. The proposed scheme is based on clarifying the out-of-focus images where interference fringes do not exist. The proposed scheme is applied on some preliminary experiments and it shows that the real-time 2-D measurement can be realized, and moreover, the accuracy of the 3-D profile reconstruction can also be improved.

Dynamic out-of-plane profilometry for nano-scale full field characterization of MEMS with automatic detection of vibratory modes and MHz-scale measurement bandwidth

A dynamic 3-D nano-scale surface profilometer using stroboscopic white light interferometry with novel image deconvolution and automatic identification of structure resonant modes was successfully developed. As micro electromechanical systems (MEMS) increase rapidly towards industrial application, the needs of accurate dynamic characterization are extremely important to optimal design and fabrication. To meet the demands, an optical microscopy based on stroboscopic interferometry was developed to achieve full-field vibratory out-of-plane surface profilometry and system characterization. A novel deconvolution strategy with correction of the light response function was established to remove the potential image blurs caused by the unavoidable vibration of the tested parts. With this technical advance, the bandwidth of dynamic measurement can be significantly increased up to 10 MHz without sacrificing measurement accuracy. Meanwhile, an innovative detection algorithm based on image contrast measure was developed for automatic identification of accurate resonant modes. The detection method provides the simplest and most economic way to detect accurate resonant peaks without adding any significant hardware in a stroboscopic interferometric framework. To verify the effectiveness of the developed methodology, AFM cantilever beams were measured to analyze the full-field resonant vibratory modes and dynamic characteristics. The experimental results confirm that the resonant vibration behavior of the tested microcantilever beams can be accurately characterized and 5 nm of vertical measurement accuracy as well as tens micrometers of vertical measurement range can be achieved. The measured results were satisfactorily consistent with the theoretical simulation outcomes from ANSYS.