Angle-resolved Scatterometer Research Articles

In-situ calibration of the objective lens of an angle-resolved scatterometer for nanostructure metrology.

Due to the advantages of being non-contact, non-destructive, highly efficient, and low in cost, scatterometry has emerged as a powerful technique for nanostructure metrology. In this paper, we propose an angle-resolved scatterometer composed of a scattered light acquisition channel and a spatial imaging channel, which is capable of detecting multi-order diffracted light in a single measurement. Since the high numerical aperture objective lens is usually employed in an angle-resolved scatterometer, the polarization effect of the objective lens introduced by the non-normal incidence and installation stress should be considered. An in-situ calibration method for the objective lens's polarization effects is proposed, in which a known analyzer is appended to the output light path to enable the extraction of the ellipsometric parameters of isotropic samples. Then the polarization effect of the objective lens can be determined in-situ by fitting the measured ellipsometric parameters to the calculated ones. With the objective lens polarization effect being considered, significant improvements in the accuracy and repeatability precision can be achieved in the metrology of the film thickness and grating topography parameters.

Low scatter and ultra-low reflectivity measured in a fused silica window

We investigate the reflectivity and optical scattering characteristics at 1064 nm of an antireflection coated fused silica window of the type being used in the Advanced LIGO gravitational-wave detectors. Reflectivity is measured in the ultra-low range of 5-10 ppm (by vendor) and 14-30 ppm (by us). Using an angle-resolved scatterometer we measure the sample's bidirectional scattering distribution function (BSDF) and use this to estimate its transmitted and reflected scatter at roughly 20-40 and 1 ppm, respectively, over the range of angles measured. We further inspect the sample's low backscatter using an imaging scatterometer, measuring an angle resolved BSDF below 10(-6) sr-1 for large angles (10°-80° from incidence in the plane of the beam). We use the associated images to (partially) isolate scatter from different regions of the sample and find that scattering from the bulk fused silica is on par with backscatter from the antireflection coated optical surfaces. To confirm that the bulk scattering is caused by Rayleigh scattering, we perform a separate experiment measuring the scattering intensity versus input polarization angle. We estimate that 0.9-1.3 ppm of the backscatter can be accounted for by Rayleigh scattering of the bulk fused silica. These results indicate that modern antireflection coatings have low enough scatter to not limit the total backscattering of thick fused silica optics.

Holistic optimization architecture enabling sub-14-nm projection lithography

Parallel with the introduction of EUV lithography, immersion lithography is being extended to the 14- and 10-nm node, and the lithography performance requirements need to be tightened further to enable this shrink. Next to generic scanner system improvements, application-specific solutions are needed to follow the requirements for critical dimension (CD) control and overlay. The application-specific solutions need a holistic optimization approach for the scanner, the mask, and the patterning process. We will describe the holistic lithography systems architecture that enables dynamic use of high-order scanner optimization based on advanced actuators of projection lens and scanning stages. Next to the scanner system, key components of this architecture are an angle-resolved scatterometer to measure CD, overlay, and focus, and an off-tool computation server to calculate application-specific recipes for the scanner. Based on real production wafer data, we will show the benefit for CD control, focus control, and overlay control, and demonstrate lithography performance levels required for 14- and 10-nm node production.

Novel technique for characterizing feature profiles in photolithography process

A novel angle-resolved scatterometer based on pupil optimization for feature profile measurement in a photolithography process is proposed. The impact of image sensor errors is minimized by optimizing the intensity distribution of the incident light using a spatial light modulator. The scatterometry sensitivity of feature measurement at different polarization conditions is calculated using the rigorous coupled-wave and first-order analyses, and the reproducibility of the scatterometer is evaluated. The results show that the sensitivity and reproducibility of the angle-resolved scatterometer increase by 90% and 40% with pupil optimization, respectively.

A data driven model of laser light scattering in metallic rough surfaces

A new model of laser light scattering was obtained from experimental profilometer data and ray casting technique. Modelling was based on variables common to an angle resolved scatterometer and geometric optics. With this model, it was possible to obtain the scattering patterns, masking and shadowing effects, multiple lobes and backscattering. As advantage of the model, it was unnecessary to make statistical assumptions about the slope distributions. Satisfactory results were obtained from very small Ra values (0 mm) to relatively large values (7.53 mm). With no modifications or additions to the model, for different surface profiles, off-specular and multiple lobes were obtained. A complete set of equations is given, detailing the construction of the model. In addition to reproducing the scattering patterns reported in the literature, it was found that, if measured profilometer data are used, for large roughness values and large incidence angles, the scattering pattern does not hold the Helmholtz principle. Key words: Modelling, geometric optics, roughness, reciprocity principle.

Multiwavelength (045–106 μm) angle-resolved scatterometer or how to extend the optical window

An apparatus to record scattered light in whole space over a large range of visible and infrared wavelengths (0.45-10.6 µm) is described. Parasitic light, calibration, and dynamic range are discussed to point out performances and limits of the experimental setup. Angular measurements at several wavelengths give access to bidimensional roughness spectra of polished samples in different frequency bandwidths. The results show overlap of the spectra at the intersection of the bandwidths, which provides an extended view of surface microroughness. In the midinfrared, measurements are more difficult, and specific problems such as thermal emission are analyzed.