Brennan Peterson

Mueller based scatterometry measurement of nanoscale structures with anisotropic in-plane optical properties

The uses of strained channel became prevalent at the 65 nm node and have continued to be a large part of logic device performance improvements in every technology generation. These material and integration innovations will continue to be important in sub-22nm devices, and are already being applied in finFET devices where total available in-channel strains are potentially higher. The measurement of structures containing these materials is complicated by the intrinsic correlation of the measured optical thickness and variation of optical properties with strain, as well as the dramatic reduction in total volume of the device. Optical scatterometry has enabled characterization of the feature shape and dimensions of complex 3D structures, including non-planar transistors and memory structures. Ellipsometric methods have been successfully applied to the measurement of thin films of SiGe and related strained structures. A direction for research is validating that the thin film stress results can be extended into the much more physically complex 3D shape. There are clear challenges in this: the stress in a SiGe fin is constrained to match the underlying Si along one axis, but the sides and top are free, leading to very large strain gradients both along the fin width and height. Practical utilization of optical techniques as a development tool is often limited by the complexity of the scatterometry model and setup, and this added material complexity presents a new challenge. In this study, generalized spectroscopic ellipsometric measurements of strained grating was undertaken, in parallel with reference cross sectional and top down SEM data. The measurements were modeled for both anisotropy calculations, as well as full scatterometry calculations, fitting the strain and structure. The degree to which strain and CD can be quickly quantified in an optical model is discussed. Sum decomposition method has been implemented to extract the effective anisotropic coefficients and a discussion on the effect of anisotropy toward modeling is presented. Finally, errors in the scatterometry measurement are analyzed, and the relative strengths and limitations of these optical measurements compared.

Characterization of e-beam patterned grating structures using Mueller matrix based scatterometry

Abstract. Scatterometry is one of the most useful metrology methods for the characterization and control of critical dimensions and the detailed feature shape of periodic structures found in the microelectronics fabrication processes. Spectroscopic ellipsometry (SE) and normal incidence reflectometry (NI)-based scatterometry are widely used optical methodologies for metrology of these structures. Evolution of improved optical hardware and faster computing capabilities led to the development of Mueller matrix (MM)-based scatterometry (MMS). Unlike SE and NI, MM data provides complete information about the optical reflection and transmission of polarized light interacting with a sample. This gives MMS an advantage over traditional SE scatterometry due to the ability to characterize samples that have anisotropic optical properties and depolarize light. In this paper, we present the study of full MM (16-element) scatterometry over a wide spectral range from 245 to 1700 nm on a series of one-dimensional, two-dimensional symmetric, and asymmetric grating structures. A series of laterally complex nanoscale structures were designed and fabricated using a state-of-the-art e-beam patterning. Spectroscopic MM and SE data were collected using a dual rotating compensator ellipsometer. Commercial modeling software based on the rigorous coupled-wave approximation was used to precisely calculate the critical dimensions. Results from MMS were compared with scanning electron microscopy.

Enhancing one dimensional sensitivity with plasmonic coupling.

In this paper, we propose a cross-grating structure to enhance the critical dimension sensitivity of one dimensional nanometer scale metal gratings. Making use of the interaction between slight changes in refractive index and localized plasmons, we demonstrate sub-angstrom scale sensitivity in this structure. Compared to unaltered infinite metal gratings and truncated finite gratings, this cross-grating structure shows robust spectra dependent mostly on the dimension of the smaller line width and pitch. While typical scatterometry simulations show angstrom resolution at best, this structure has demonstrated picometer resolution. Due to the wide range of acceptable specifications, we expect experimental confirmation of such structures to soon follow.

Metrology for block copolymer directed self-assembly structures using Mueller matrix-based scatterometry

Abstract. Patterning based on directed self-assembly (DSA) of block copolymer (BCP) has been demonstrated to be a cost-effective manufacturing technique for advanced sub-20-nm structures. This paper describes the application of Mueller matrix spectroscopic ellipsometry (MMSE) based scatterometry to optically characterize polystyrene-b-polymethylmethacrylate patterns and Si fins fabricated with DSA. A regression-based (inverse-problem) approach is used to calculate the line-width, line-shape, sidewall-angle, and thickness of the DSA structures. In addition, anisotropy and depolarization calculations are used to determine the sensitivity of MMSE to DSA pattern defectivity. As pattern order decreases, the mean squared error value increases, depolarization value increases, and anisotropy value decreases. These specific trends are used in the current work as a method to judge the degree of alignment of the DSA patterns across the wafer.

Fin Stress and Pitch Measurement using X-ray Diffraction Reciprocal Space Maps and Optical Scatterometry

Although fin metrology presents many challenges, the single crystal nature of the fins also provides opportunities to use a combination of measurement methods to determine stress and pitch. While the diffraction of light during a scatterometry measurement is well known, X-ray diffraction from a field (array) of single crystal silicon fins can also provide important information. Since some fins have Si1-xGex alloys at the top of the fin, determination of the presence of stress relaxation is another critical aspect of fin characterization. Theoretical studies predict that the bi-axially stressed crystal structure of pseudomorphic alloy films will be altered by the fin structure. For example, one expects it will be different along the length of the fin vs the width. Reciprocal space map (RSM) characterization can provide a window in the stress state of fins as well as measure pitch walking and other structural information. In this paper, we describe the fundamentals of how RSMs can be used to characterize the pitch of an array of fins as well as the stress state. We describe how this impacts the optical properties used in scatterometry measurement.

Investigation of E-beam patterned nanostructures using Mueller Matrix based Scatterometry

Scatterometry is one of the most useful metrology methods for the characterization and control of critical dimensions (CD) and the detailed topography of periodic structures in microelectronics fabrication processes. Spectroscopic Ellipsometry (SE) and Normal Incidence Reflectometry (NI) based Scatterometry are the most widely used methodologies for metrology of these structures. Evolution of better optical hardware and faster computing capabilities led to the development of Mueller Matrix (MM) based Scatterometry (MMS). In this paper we present the first study of dimensional metrology using full Mueller Matrix (16 element) Scatterometry in the wavelength range of 245nm- 1000nm. Unlike SE and NI, MM data provides complete information about the optical reflection and transmission of polarized light through a sample. MM is a 4x4 transformation matrix (16 elements) describing the change in the intensities of incident polarized light expressed by means of a Stokes Vector. The symmetry properties associated with MM provide an excellent means of measuring and understanding the topography of the periodic nanostructures. Topography here refers to uniformity of the periodic structure. The advantage of MMS over traditional SE Scatterometry is the ability of MMS to measure samples that have anisotropic optical properties and depolarize light. The present study focuses on understanding the precision and accuracy of Mueller based Scatterometry with respect to other methodologies by a systematic approach. Several laterally complex nanoscale structures with dimensions in the order of nanometers were designed and fabricated using a state of the art E-beam pattering tool (VISTEC [R] 300). Later, Spectroscopic Mueller matrix (all 16 elements) and SE data were collected in planar diffraction mode for the samples using J.A. Woollam RC2 [TM] Spectroscopic Ellipsometer. NanoDiffract [TM] (Scatterometry software provided by Nanometrics Inc.) was used to model the nanostructures to precisely calculate the critical dimensions. Complementary techniques like SEM were used to compare the results obtained from Scatterometry. Finally, Mueller and SE based Scatterometry techniques were compared commenting on reliability of MM based Scatterometry.

New interferometric measurement technique for small diameter TSV

High aspect ratio through silicon vias (TSV) present a challenge for measurement of bottom critical dimension (BCD) and depth. TSVs smaller than 5 micron diameter with greater than 12:1 depth to BCD aspect ratio have particularly poor signal to noise ratio in the measured signal. This paper proposes a method for improving the interferometric measurement of these very small and high-aspect ratio TSVs with data showing the feasibility of measuring both BCD and depth of 19:1 aspect ratio TSVs. This work demonstrates the capability to analyze the scanning white-light interferometry (SWLI) signal for such high aspect ratio TSV BCD and depth measurements with >0.95 R2 correlation to reference metrology obtained through cross section SEM. Precision of within 2.5% of nominal BCD and within 0.1% of nominal depth was demonstrated for 10x repeatability measurements.

Metrology for directed self-assembly block lithography using optical scatterometry

Directed self-assembly (DSA) shows considerable promise as a cost-effective manufacturing technique for advanced sub-20 nm patterning. Along with continued progress, the patterning process requires advances in both CD metrology and high-speed characterization of DSA defectivity. This work is a report on the study of Mueller matrix spectroscopic ellipsometry (MMSE) scatterometry measurements of 28 nm pitch DSA line/space patterns consisting of polystyrene-block- polymethylmethacrylate (PS-b-PMMA) block copolymer sample fabricated using a chemical epitaxy process. Generalized ellipsometric data (all 16 Mueller elements) is collected over a spectral range from 245 to 1700 nm for various different pre-pattern pitch/guide strip combinations created by modulating the pre-pattern photoresist CD. Scatterometry is used to evaluate and calculate the CD, line shapes, and thicknesses of the plasma developed PS patterns (PMMA removed). Likewise, spectral comparisons based on anisotropy and depolarization are used to determine the DSA pattern defectivity. CD-SEM metrology and imaging is also conducted as a comparative metric for scatterometry. The sensitivity of MMSE to pre-pattern pitch and pitch multiplication on PS line CD and defectivity is demonstrated. Slight imperfections in the line/space pattern as well as fingerprint like patterns (undirected assembly) can be distinguished from aligned patterns using MMSE scatterometry.

Size matters in overlay measurement

We report an analysis of measurements of overlay targets fabricated in a single lithography step in a resist film. With patterns printed in a single step the expected result is zero, providing a rare opportunity to qualify the measurement error completely. Our results allow validation of a complete model for this error, which includes precision and TIS terms that vary as 1/√L, where L is the total pattern length in each target. Removing the precision and TIS effects shows that the remaining errors, which are usually undetectable, also vary as 1/√L. This residual error is very small (0.1nm or less) for the very large targets normally used in scanner qualification, but exceed the ITRS goals for overlay measurement uncertainty in traditional image-based overlay targets smaller than 25x25μm. As the industry drives towards smaller overlay targets it is important to consider the impact on complete measurement uncertainty, and to maximize the information content of each target. We show that acceptable uncertainty in 10x10μm targets is possible using targets patterned with dense gratings that can be measured by both diffraction and imaging tools.

Mueller matrix optical scatterometry of Si fins patterned using directed self-assembly block copolymer line arrays

Directed self-assembly (DSA) based block copolymer (BCP) lithography shows promise as a cost-effective manufacturing technique for advanced sub-20 nm patterning. Characterization of types of defects and their levels in the sub-20 nm DSA patterns is a challenge for the current state-of-the-art metrology. Scatterometry has the capability of measuring important feature dimensions of DSA structures quickly and non-destructively at multiple steps in the process. This work is a report on the use of Mueller matrix spectroscopic ellipsometry (MMSE) based measurements of 28 nm pitch Si fins fabricated from DSA line patterns consisting of polystyrene-block-polymethylmethacrylate (PS-b-PMMA) fabricated using a chemical epitaxy process. A regression-based (inverse-problem) scatterometry approach was used to calculate the CD, line shapes, and thickness of the developed Si fin patterns. Sensitivity of MMSE to defectivity and line edge roughness (LER) in Si fin samples is discussed. At DSA dimensions the characterization of LER is crucial due to increase in the ratio between LER and target line width. HR-XRD and SEM imaging is also conducted as a comparative metric for scatterometry. This paper emphasizes the effectiveness of MMSE based scatterometry as a technique for optical characterization of Si fins fabricated with DSA lithography.