Yi Zhou

Topography Measurement of Large-Range Microstructures through Advanced Fourier-Transform Method and Phase Stitching in Scanning Broadband Light Interferometry

Scanning broadband light interferometry (SBLI) has been widely utilized in surface metrology due to its non-contact and high-accuracy method. In SBLI, phase evaluation through Fourier Transform (FT) is a prevalent and efficient technique, where the topography measurement can often be achieved through one interferogram. Nevertheless, the accuracy of the FT method would be significantly influenced by intensity modulation depth: “the lower the modulation of the pixel, the higher the error probability of its phase assignment”. If the structure has a large enough range along the z-axis, several areas in an individual interferogram would be weakly modulated due to the limited depth of focus (DOF). In this paper, we propose an advanced FT-based method when it comes to large-height structures. Spatial modulation depth is first calculated for each interferogram independently. After that, a binary control mask is reasonably constructed to identify the pixels that are valid for phase unwrapping. Then, a phase stitching method along the z-axis is carried out to conduct the large-height topography measurement within a giving field of view. The theoretical principle, simulation, and experimental validation are elaborated to demonstrate that the method can achieve an improved robustness for the reconstruction of large-range microstructures, the advantages of which include the elimination of stepping errors, the suppression of light fluctuations, and the freedom of a limited DOF.

Fabrication of Micro-Optics Elements with Arbitrary Surface Profiles Based on One-Step Maskless Grayscale Lithography

A maskless lithography method to realize the rapid and cost-effective fabrication of micro-optics elements with arbitrary surface profiles is reported. A digital micro-mirror device (DMD) is applied to flexibly modulate that the exposure dose according to the surface profile of the structure to be fabricated. Due to the fact that not only the relationship between the grayscale levels of the DMD and the exposure dose on the surface of the photoresist, but also the dependence of the exposure depth on the exposure dose, deviate from a linear relationship arising from the DMD and photoresist, respectively, and cannot be systemically eliminated, complicated fabrication art and large fabrication error will results. A method of compensating the two nonlinear effects is proposed that can be used to accurately design the digital grayscale mask and ensure a precise control of the surface profile of the structure to be fabricated. To testify to the reliability of this approach, several typical array elements with a spherical surface, aspherical surface, and conic surface have been fabricated and tested. The root-mean-square (RMS) between the test and design value of the surface height is about 0.1 μm. The proposed method of compensating the nonlinear effect in maskless lithography can be directly used to control the grayscale levels of the DMD for fabricating the structure with an arbitrary surface profile.

Surface and thickness measurement of transparent thin-film layers utilizing modulation-based structured-illumination microscopy

In this research, an approach called modulation-based structured-illumination microscopy (MSIM) is proposed to measure the surface and thickness profile of thin film layers. With this method, a sinusoidal fringe pattern generated by digital micro-mirror devices (DMD) is projected on the sample. The modulation estimation of the reflected patterns is implemented for characterizing the surface and thickness profile of the sample. The measurement system is relatively simple and only an ordinary objective is enough to achieve imaging of the sample. In addition, the reflected signals come from the back surface of the film create less disturbance to the front surface compared with white-light interferometry. Consequently, they can be easily distinguished and achieve a successful measurement precisely. Both simulation and experiments are carried out to demonstrate the availability of this MISM method. The results are in excellent agreement with commercial stage profiler and the relative uncertainty is less than 10 nm.

Dimensional metrology of micro structure based on modulation depth in scanning broadband light interferometry

Three-dimensional measurement and inspection is an area with growing needs and interests in many domains, such as integrated circuits (IC), medical cure, and chemistry. Among the methods, broadband light interferometry is widely utilized due to its large measurement range, noncontact and high precision. In this paper, we propose a spatial modulation depth-based method to retrieve the surface topography through analyzing the characteristics of both frequency and spatial domains in the interferogram. Due to the characteristics of spatial modulation depth, the technique could effectively suppress the negative influences caused by light fluctuations and external disturbance. Both theory and experiments are elaborated to confirm that the proposed method can greatly improve the measurement stability and sensitivity with high precision. This technique can achieve a superior robustness with the potential to be applied in online topography measurement.

Thickness measurement of transparent film by white-light interferometry

White-light scanning interferometry plays an important role in precise profile metrology of microstructure. However, applying this approach may also be limited because of the optical reflection behavior of the surface. While there is a thin film on the surface, the reflection behavior of top and bottom of the thin-film will cause severer phase errors. Recently, the method by combining both reflectometry and white-light scanning interferometry is proposed to measure the film thickness and surface profile. This article firstly explains the principle of the proposed method and then verifies the feasibility of the thickness-measurement method for transparent film on a Silicon surface. Both of the algorithm and the experiment system have been optimized to measure the film thickness with high precision.

Topography measurement of micro structure by modulation-based method

Dimensional metrology for micro structure plays an important role in addressing quality issues and observing the performance of micro-fabricated products. Different from the traditional white-light interferometry approach, the modulation-based method is expected to measure topography of micro structure by the obtained modulation of each interferometry image. Through seeking the maximum modulation of every pixel respectively in Z direction, the method could obtain the corresponding height of individual pixel and finally get topography of the structure. Owing to the characteristic of modulation, the proposed method which is not influenced by the change of background light intensity caused by instable light source and different reflection index of the structure could be widely applied with high stability. The paper both illustrates the principle of this novel method and conducts the experiment to verify the feasibility.

Moiré-Based Interferometry for Magnification Calibration of Bitelecentric Lens System

The bitelecentric lens system is widely used in many domains, such as 3-D measurements and tube inspection. The magnification calibration for such a system is a crucial problem for its further application in achieving precise measurements. However, it is difficult to obtain an accurate result using a general magnification calibration method. In this paper, the Moiré-based interferometry is demonstrated to accurately calibrate the magnification of bitelecentric lens system that increases the speed and precision of the measurement. Two special grating marks containing upper and lower parts with slightly different periods are designed to generate Moiré fringes. By analyzing the finally obtained Moiré fringes, the magnification could be determined through Fourier-based phase analysis and a phase unwrapping algorithm, and the further the deviation is from the theoretical value, the more obvious the differences between upper and lower periods will be. Both simulations and experiments are conducted to verify the feasibility and effectiveness of the proposed approach. Results indicate that the magnification could be calibrated at the accuracy of 0.4% with extraordinary sensitivity.