Through-focus scanning optical microscopy

Abstract

Through-focus scanning optical microscopy (TSOM) is another 'scanning' based method that provides threedimensional information (i.e. the size, shape and location) about micro-and nanometer-scale structures. TSOM, based on a conventional optical microscope, achieves this by acquiring and analyzing a set of optical images collected at various focus positions going through focus (from above-focus to under-focus). The measurement sensitivity is comparable to what is possible with typical light scatterometry, SEM and AFM. One of the unique characteristics of the TSOM method is its ability to separate different dimensional differences (i.e. ability to distinguish, for example, linewidth difference from line height difference), and hence is expected to reduce measurement uncertainty. TSOM holds the promise of high-throughput, through comparative measurement applications for wide variety of application areas with potentially significant savings and yield improvements.

Through-focus scanning optical microscopy with the Fourier modal method

“…For successful TSOM metrology, the parameters of the optical microscope for TSOM sensitivity such as the illumination and collection numerical apertures (NAs) and the illumination angle must be exactly quantified [1][2][3]. The meanings of illumination NA (INA) and collection NA (CNA) are schematically illustrated in Fig.…”

“…Conventional high-NA microscopes are limited in their transversal resolution and depth-offocus (DOF) when measuring nanoscale high-aspect ratio structures. The development of TSOM is motivated for application to in-line non-destructive inspection and measurement of such high-aspect ratio and subwavelength nano-scale structures in state of the art semiconductor devices [1,2]. The TSOM method differs from conventional optical microscopy in that it indirectly measures deep-subwavelength structures based on computational interpretation of far-field TSOM images [3].…”

“…The structural difference that induces the differential TSOM image features can be determined by numerically classifying the target samples with respect to nanometer scale structural variations in the prepared TSOM image database. To quantify this TSOM analysis, several quantitative measurement factors have been developed including optical intensity range (OIR), different TSOM image (DTI), and mean square difference (MSD) [1,3]. In practice, the construction of the numerical TSOM reference database is the essential part, so TSOM image modeling is of paramount importance.…”

“…This paper shows that careful consideration of the coherence of the illumination light, the input numerical aperture and the collection numerical aperture of the microscope objective, and the illumination angle are the critical factors for successful TSOM metrology. For this purpose, several tools such as finite-difference time-domain (FDTD) and finite element method (FEM) can be used [1][2][3][4]. The TSOM technique preserves all the optical information by capturing in and around the focus position the scattered field reflected off the target in the form of an image containing the target features and moves the target sample to build an overall intensity map of the target.…”

Multiple parametric nanoscale measurements with high sensitivity based on through‐focus scanning optical microscopy

“…Theoretically, there are at least 15 approaches to defocusing, and these approaches can be summarised into two major categories: extracting images directly or computationally (Attota, ). Most of the current literatures are on stage scanning methods (Attota et al ., ; Attota & Dixson, ; Attota et al ., ,b). Nevertheless, TSOM based upon stage scanning is strongly affected by mechanical noise, especially has issues in in‐line and in situ measurements (Attota, ).…”