Yi-Sha Ku

Scatterometry-based metrology with feature region signatures matching

Scatterometry takes advantage of the sensitivity exhibited by optical diffraction from periodic structures, and hence is an efficient technique for lithographic process monitoring. A feature region measurement algorithm has been developed to extract accurately and quickly the relevant constitutive parameters from diffraction data. It is a method for efficiently determining grating structure by seeking the reflectance at some angles contains more information about the structure of the surface relief profile than the reflectance at other angles in a library data match process. The number of measurements and size of signature matching library will be reduced in a great percentage by performing the feature region algorithm.

In-chip overlay measurement by existing bright-field imaging optical tools

We have developed a target design for overlay measurement which is small enough (3x3μm) that it could be positioned within the active area of integrated circuit devices. These targets have been measured using an unmodified overlay tool. The targets are too small for the image to be fully resolved using visible wavelengths, and so measurement using the normal methods based on determining the relative positions of features in the image does not produce acceptable levels of measurement uncertainty. Instead, we show that the symmetry of the image can be used to determine the overlay error. We report initial results which show measurement uncertainty using this technique approaching the levels needed for overlay control at design rules under 100nm. These results are limited by the process used to create our test structures, and even better results may be possible with state-of-the-art lithography and processing techniques.

Through-focus technique for nano-scale grating pitch and linewidth analysis

We report results of experimental investigations into a through-focus method relevant to sub-wavelength feature dimension measurement. The method linearizes the partial derivative values of a focus indicator with respect to minimum intensity order, and hence permits determination of pitch using a classical linear method. By evaluating the variations in focus indicator of the different captured images obtained at various focal positions, the through-focus curves show a response to sub-resolution changes in the grating structure. The results suggest that sub-wavelength feature dimensions can be evaluated using regular optical microscopes by implementing the through focus method.

Comparisons of overlay measurement using conventional bright-field microscope and angular scatterometer

As overlay tolerances of microlithographic technology become increasingly severe, conventional bright-field metrology systems are limited by image resolution and precision. Scatterometer (angular scatterometer or spectroscopic reflectometer, for example) has the advantages of good repeatability and reproducibility, and is proposed as an alternative solution for overlay metrology. Previous studies have applied a spectroscopic reflectometer, which is as function of incident wavelength, to overlay measurement. This work investigated overlay measurement by using an angular scatterometer, which is as function of incident angle. A focused laser spot was incident on linear grating, an overlay target. An angular signature, a 0th-order reflective light beam, scattered from linear grating was measured when the incident and reflective angles were changed simultaneously. The overlay target consists of two linear gratings located on two different layers of a stacked structure, and the overlay error is the misalignment between these two different layers. The measured results using angular scatterometer (also known as the diffraction-based method) are compared with using the bright-field microscope (also known as the image-based method), which use a bar-in-bar target as an overlay target. Statistical data sets demonstrate that angular sctterometer has nearly one order better of repeatability and tool induced shift than conventional bright-field microscope. Additionally, a series of different parameters of overlay targets, such as different pitches, line-to-space ratios, and stacked structures is designed and manufactured. The sensitivity of overlay measurement of various linear grating targets is also measured and discussed.

Angular Scatterometry for Line-Width Roughness Measurement

We propose using angular scatterometry as a means to investigate LWR (line-width roughness) and CD (critical dimension). The grating target is illuminated by a single wavelength light source which has large angular aperture both in incidence angle θ and azimuth angle φ. A preliminary scatterometry model was first built by assuming perfect critical dimension printed without any line-width roughness. The difference between the model prediction and actual measurement is cased by line-width roughness contribution. We developed a calibration curve as a function of line-width roughness based on the statistical quantity of the incidence and azimuth angle dependence. The results demonstrate that scatterometry can indeed be used to extract line-width roughness and critical dimension information in production line with nano-scale resolution.

Through-focus technique for grating linewidth analysis with nanometer sensitivity

We present a new algorithm for determining nano-scale feature dimensions of grating structures with a bright-field imaging tool. The algorithm is based on the intensity and focus quality of images obtained with varying amounts of defocus. Analysis of the intensity of optical images obtained at various focal positions demonstrates nanometer sensitivity with grating structures. An empirical quadratic model was developed to fit the experimental results of image intensity versus critical dimension.

Through-focus algorithm to improve overlay tool performance

Current optical overlay measurement tools utilize visible light and operate with optical resolution of approximately 0.5-1.0 μm. Such tools cannot resolve the targets based on the design rule features. Hence, reliable theoretical model-based measurements and enhanced algorithms are required to address this problem. The test targets, with features similar to those specified by the design rule, were fabricated by a high precision E-beam writer. An optical bright field overlay metrology tool was applied to acquire the optical images of the test targets. The best focus position of test target is selected using an auto focus algorithm. The focus offset is specified relative to the best focus position and the optical image data is measured with a full field-of-view CCD array. The through focus image data are analyzed to obtain the relationship between the intensity profile and the structural parameters of the test targets. These structural parameters are also verified with the CD-SEM. This work experimentally analyzes the through-focus behavior of the test targets. These targets are based on grating patterns, and while they provide more information than traditional targets, they are more sensitive to the focus position. The through focus image formed at the image plane depends on the relative focus position between the target and the optical system. An appropriate design for the optical configuration and target geometry produces a unique image at each focus position, for a specific physical feature.

Angle-resolved scatterfield microscope for linewidth measurement

Angle-resolved scatterfield microscope (ARSM) is developed for several years. It combines the optical microscope and angle-resolved scatterometer with a relay lens and an aperture. In our research, the spatial light modulator (SLM) is used to instead of the relay lens and the aperture. In the SLM, the phase modulation is used to simulate the Fresnel lens, and then an incident plane wave is modulated and focused on the back focal plane of the objective lens. A plane wave with an angle which is according to the position of focused point on the back focal plane is emitted from the entrance pupil of the objective lens. By modulating the SLM, the angle of plane wave from the objective lens can be changed. In our system, an objective lens with NA 0.95 and the magnification of 50 is used for wide angle scan. A bare silicon wafer and a grating with the pitch of 417nm are measured with full-angle scan. By using the SLM, the advantage is full-optical modulation, that is, the mechanical motion is not needed in the ARSM. Thus, the system will have higher throughput and stabilization.

Accuracy of diffraction-based overlay metrology using a single array target

We focus on the capability and theoretical limits of a model-based scatterometry method to determine overlay using a single two-dimensional array target. We use our modeling capability to design an optimized test target for scatterometer-based overlay measurements in a range of semiconductor films. We propose a methodology to measure the overlay using a single two-dimensional array target designed with intentional offsets, Δx and Δy, between the top and bottom grid arrays along the X and Y directions. This method allows extraction of the two-dimensional overlays from first diffraction order measurements through bi-azimuth angle analysis (0 and 90 deg with respect to the incidence plane), and includes a simple linear response algorithm. Two critical issues are taken into account: correlation of Δx and Δy and lithography process errors. We have simulated the diffraction signatures of a two-dimensional target with a pitch of 400 nm and linewidth of 100 nm, and optimized the overlay target design to maximize the measurement sensitivity and minimize the correlation of two axial measurements. We also investigate the influence of parameter variations on overlay measurement error

Performance study of CD mark size for angular scatterometry

We report a study of the effect of target size on CD measurement by angular scatterometry in two ways. One is reducing the spot size (say to 40 mm) to permit the use of a smaller target; another is to overfill the target. Starting with standard grating targets of 80 x 80 mm size, with fixed CD 400 nm and LS (Linewidth to Spacing) ratio 1:1, test gratings have been designed with X and Y dimensions varied from 80 to 10 mm in 10 mm intervals. We show how the scattering signatures are influenced by the varying target sizes and spot sizes especially when the target grating is overfilled. The errors in CD measurements caused by the target and spot size variations are also quantified. Working with an overfilled small grating target and filtering out the specular noise offers a promising way to present the scattering signal from diffraction. An empirical model to predict the scattering signatures as a function of target size is under development.