TSV reveal height and dimension metrology by the TSOM method

Abstract

This paper reports on an investigation to determine whether through-focus scanning optical microscopy (TSOM) is applicable to micrometer-scale through-silicon via (TSV) reveal metrology. TSOM has shown promise as an alternative inspection and dimensional metrology technique for FinFETs and defects. In this paper TSOM measurements were simulated using 546 nm light and applied to copper TSV reveal pillars with height in the 3 µm to 5 µm range and diameter of 5 µm. Simulation results, combined with white light interferometric profilometry, are used in an attempt to correlate TSOM image features to variations in TSV height, diameter, and sidewall angle (SWA). Simulations illustrate the sensitivity of Differential TSOM Images (DTI's) using the metric of Optical Intensity Range (OIR), for 5 µm diameter and 5 µm height TSV Cu reveal structures, for variation of SWA (∆ = 2°, OIR = 2.35), height (∆ = 20 nm, OIR = 0.28), and diameter (∆ = 40 nm, OIR = 0.57), compared to an OIR noise floor of 0.01.In addition, white light interferometric profilometry reference data is obtained on multiple TSV reveal structures in adjacent die, and averages calculated for each die's SWA, height, and diameter. TSOM images are obtained on individual TSV's within each set, with DTI's obtained by comparing TSV's from adjacent die. The TSOM DTI's are compared to average profilometry data from identical die to determine whether there are correlations between DTI and profilometry data.However, with several significant TSV reveal features not accounted for in the simulation model, it is difficult to draw conclusions comparing profilometry measurements to TSOM DTI's when such features generate strong optical interactions. Thus, even for similar DTI images there are no discernible correlations to SWA, diameter, or height evident in the profilometry data. The use of a more controlled set of test structures may be advantageous in correlating TSOM to optical images.Keywords: TSOM, Through-focus scanning optical microscopy, metrology, TSV, through silicon via, 3D interconnect critical dimension, optical microscopy INTRODUCTIONThrough-focus scanning optical microscopy (TSOM) [1-8] allows conventional optical microscopes to collect dimensional information by combining 2D optical images captured at several through-focus positions, transforming a conventional optical microscope into a 3D metrology tool. TSOM is not a resolution enhancement method but an image comparison method and has been demonstrated through simulations to provide lateral and vertical measurement sensitivity of less than a n anometer. This performance is comparable to the dimensional measurement sensitivity of critical dimension (CD) metrology tools such as critical dimension scanning electron microscopy (CD-SEM) and optical critical dimension (OCD). TSOM has shown promise as an inspection and dimensional metrology technique for FinFETs and defects [9], where CD-SEM or bright-field inspection, respectively, are currently used. These simulations indicate that the technique is capable of meas...

Parameter optimization for through-focus scanning optical microscopy

“…For this reason, it needs only a fraction of the target size compared to a scatterometry target [42], and hence has the potential for indie metrology. TSOM can analyze truly 3-D, deep structures such as through silicon vias and high aspect ratio trenches or vias [46,47]. In addition, it can reduce the effects of optical cross-correlation, making it less ambiguous to identify the dimensional variations that are leading to the observed optical signal [37,38].…”