Multiple electron beam direct write lithography is an emerging technology promising to address new markets, such as truly unique chips for security applications. The tool under consideration, the Mapper FLX-1200, exposes long 2.2 μm-wide zones called stripes by groups of 49 beams. The critical dimensions inside and the registration errors between the stripes, called stitching, are controlled by internal tool metrology. Additionally, there is great need for on-wafer metrology of critical dimension and stitching to monitor Mapper tool performance and validate the internal metrology.
Optical Critical Dimension (OCD) metrology is a workhorse technique for various semiconductor manufacturing tools, such as deposition, etching, chemical-mechanical polishing and lithography machines. Previous works have shown the feasibility to measure the critical dimension of non-uniform targets by introducing an effective CD and shown that the non-uniformity can be quantified by a machine learning approach. This paper seeks to extend the previous work and presents a preliminary feasibility study to monitor stitching errors by measuring on a scatterometry tool with multiple optical channels.
A wafer with OCD targets that mimic the various lithographic errors typical to the Mapper technology was created by variable shaped beam (VSB) e-beam lithography. The lithography process has been carefully tuned to minimize optically active systematic errors such as critical dimension gradients. The OCD targets contain horizontal and vertical gratings with a pitch of 100 nm and a nominal CD of 50 nm, and contain various stitching error types such as displacement in X, Y and diagonal gratings.
Sensitivity to all stitching types has been shown. The DX targets showed non-linearity with respect to error size and typically were a factor of 3 less sensitive than the promising performance of DY targets. A similar performance difference has seen in nominally identical diagonal gratings exposed with vertical and horizontal lines, suggesting that OCD metrology for DX cannot be fully characterized due to lithography errors in gratings with vertical lines.
Future of logic silicon extension lies at the heart of gate all around developments (1). Due to the increasing limitations in further FinFET flow extension, teams around the globe are researching with vertical and horizontal nanowires flavors. Horizontal NW are of great interest due to their integration similarity to the existing FinFET integration flow (2). This in turn allows to extend the usage of existing process and metrology platforms, and reduce the cost of shifting to a new technology. Even though the integration changes seem limited, it springs many new obstacle for fab metrology. New parameters of interest take place, and the metrology capability needs to reach higher performance, and develop new solution methods (3,4).
The current paper will focus on one of the new rising metrology challenges, which exist at the nanowire release process step. The nanowire release step, a SiGe dummy layer is being removed by dry etching, to leave behind the active Silicon nanowires, for nfet device. A detailed metrology of these nanowire profile and thickness is required to verify the device can perform to the expected specifications. To examine the scatterometry performance at this application, a specific design of experiment was set, at multiple process step. at fin formation we begin with split condition, on the silicon-silicon germanium (Si-SiGe) multi layer deposition, where SiGe , are being varied between wafers, by increasing the SiGe layer thickness, thus different amount of SiGe material will be released (figure 1a). Scatterometry and X-ray reflectivity (XRR) are verifying this split condition (figure 1b). We continue with additional split condition for the fin reveal, allowing the lower SiGe layer to be more or less revealed to the SiGe release etching step (figure 2a). To confirm the fin height for the different splits condition we use atomic force microscope (AFM), and scatterometry (figure 2b). the last process DoE we report is the etching method in which the SiGe is released. The two etch methods, we address in this paper, provides different nanowire profile (figure 3), a circular or rectangular shapes, respectively. The last part of this paper will highlight how scatterometry nanowire profile accuracy, at the SiGe release step, is improved by incorporating the complete GAA steps scatterometry solutions, and the combination of Transmission electron microscopy (TEM) rich sampling.
Multi-channel gate all around (GAA) semiconductor devices march closer to becoming a reality in production as their maturity in development continues. From this development, an understanding of what physical parameters affecting the device has emerged. The importance of material property characterization relative to that of other physical parameters has continued to increase for GAA architecture when compared to its relative importance in earlier architectures. Among these materials properties are the concentration of Ge in SiGe channels and the strain in these channels and related films. But because these properties can be altered by many different process steps, each one adding its own variation to these parameters, their characterization and control at multiple steps in the process flow is crucial. This paper investigates the characterization of strain and Ge concentration, and the relationships between these properties, in the PFET SiGe channel material at the earliest stages of processing for GAA devices. Grown on a bulk Si substrate, multiple pairs of thin SiGe/Si layers that eventually form the basis of the PFET channel are measured and characterized in this study. Multiple measurement techniques are used to measure the material properties. In-line X-Ray Photoelectron Spectroscopy (XPS) and Low Energy X-Ray Fluorescence (LE-XRF) are used to characterize Ge content, while in-line High Resolution X-Ray Diffraction (HRXRD) is used to characterize strain. Because both patterned and un-patterned structures were investigated, scatterometry (also called optical critical dimension, or OCD) is used to provide valuable geometrical metrology.
Scaling of interconnect design rules in advanced nodes has been accompanied by a reducing metrology budget for BEOL process control. Traditional inline optical metrology measurements of BEOL processes rely on 1-dimensional (1D) film pads to characterize film thickness. Such pads are designed on the assumption that solid copper blocks from previous metallization layers prevent any light from penetrating through the copper, thus simplifying the effective film stack for the 1D optical model. However, the reduction of the copper thickness in each metallization layer and CMP dishing effects within the pad, have introduced undesired noise in the measurement. To resolve this challenge and to measure structures that are more representative of product, scatterometry has been proposed as an alternative measurement. Scatterometry is a diffraction based optical measurement technique using Rigorous Coupled Wave Analysis (RCWA), where light diffracted from a periodic structure is used to characterize the profile. Scatterometry measurements on 3D structures have been shown to demonstrate strong correlation to electrical resistance parameters for BEOL Etch and CMP processes. However, there is significant modeling complexity in such 3D scatterometry models, in particlar due to complexity of front-end-of-line (FEOL) and middle-of-line (MOL) structures. The accompanying measurement noise associated with such structures can contribute significant measurement error. To address the measurement noise of the 3D structures and the impact of incoming process variation, a hybrid scatterometry technique is proposed that utilizes key information from the structure to significantly reduce the measurement uncertainty of the scatterometry measurement. Hybrid metrology combines measurements from two or more metrology techniques to enable or improve the measurement of a critical parameter. In this work, the hybrid scatterometry technique is evaluated for 7nm and 14nm node BEOL measurements of interlayer dielectric (ILD) thickness, hard mask thickness and dielectric trench etch in complex 3D structures. The data obtained from the hybrid scatterometry technique demonstrates stable measurement precision, improved within wafer and wafer to wafer range, robustness in cases where 3D scatterometry measurements incur undesired shifts in the measurements, accuracy as compared to TEM and correlation to process deposition time. Process capability indicator comparisons also demonstrate improvement as compared to conventional scatterometry measurements. The results validate the suitability of the method for monitoring of production BEOL processes.
The evaluation of scatterometry for monitoring intended variations in innovative scatterometry targets that mimic nonuniformities potentially caused by multibeam Maskless Lithography (MEB-ML2) is presented. Specialized scatterometry targets consisting of lines and spaces were produced that have portions exposed using the nominal, or POR (Process of Record), dose, and portions exposed with a slightly different dose. These exposure plans created targets with different line CDs (critical dimensions). Multiple target designs were implement, each with a different combination of magnitude of CD shift and size of the region containing lines with a shifted CD. The scatterometry, or OCD (Optical Critical Dimension), spectra show clear shifts caused by the regions with shifted CD, and trends of the scatterometry results match well with trends of the estimated CD as well as the trends produced by measurements using a critical dimension scanning electron microscope (CD-SEM) system. Finally, the OCD results are correlated to the CD-SEM measurements. Taking into account resist morphology variations across the wafer, correlations between OCD and CD-SEM of the weighted average CD across the various targets are shown to be very good. Correlations are done using the rigorous TMU analysis methodology. Due to the different targeted CD values within each scatterometry structure, a new methodology for estimating the error of the CD-SEM measurements for nominally non-uniform targets is presented.
Work using the concept of a co-optimization-based metrology hybridization is presented. Hybrid co-optimization involves the combination of data from two or more metrology tools such that the output of each tool is improved by the output of the other tool. Here, the image analysis parameters from a critical dimension scanning electron microscope (CD-SEM) are modulated by the profile information from optical critical dimension (OCD, or scatterometry), while the OCD-extracted profile is concurrently optimized through addition of the CD-SEM CD results. The test vehicle utilized is the 14-nm technology node-based FinFET high-k/interfacial layer (HK/IL) structure. When compared with the nonhybrid approach, the correlation to reference measurements of the HK layer thickness measurement using hybrid co-optimization resulted in an improvement in relative accuracy of about 40% and in R2 from 0.81 to 0.91. The measurement of the IL thickness also shows an improvement with hybrid co-optimization: better matching to the expected conditions as well as data that contain less noise.
In recent years Hybrid Metrology has emerged as an option for enhancing the performance of existing measurement toolsets and is currently implemented in production<sup>1</sup>. Hybrid Metrology is the practice to combine measurements from multiple toolset types in order to enable or improve the measurement of one or more critical parameters. While all applications tried before were improved through standard (sequential) hybridization of data from one toolset to another, advances in device architecture, materials and processes made possible to find one case that demanded a much deeper understanding of the physical basis of measurements and simultaneous optimization of data. This paper presents the first such work using the concept of co-optimization based hybridization, where image analysis parameters of CD-SEM (critical dimensions Scanning Electron Microscope) are modulated by profile information from OCD (optical critical dimension – scatterometry) while the OCD extracted profile is concurrently optimized through addition of the CD-SEM CD results. Test vehicle utilized in this work is the 14nm technology node based FinFET High-k/Interfacial layer structure.
The accelerated pace of the semiconductor industry in recent years is putting a strain on existing dimensional metrology
equipments (such as CDSEM, AFM, Scatterometry) to keep up with ever-increasing metrology challenges. However, a
revolution appears to be forming with the recent advent of Hybrid Metrology (HM) - a practice of combining
measurements from multiple equipment types in order to enable or improve measurement performance. In this paper we
extend our previous work on HM to measure advanced 1X node layers - EUV and Negative Tone Develop (NTD) resist
as well as 3D etch structures such as FinFETs. We study the issue of data quality and matching between toolsets
involved in hybridization, and propose a unique optimization methodology to overcome these effects. We demonstrate
measurement improvement for these advanced structures using HM by verifying the data with reference tools (AFM,
XSEM, TEM). We also study enhanced OCD models for litho structures by modeling Line-edge roughness (LER) and
validate its impact on profile accuracy. Finally, we investigate hybrid calibration of CDSEM to measure in-die resist line
height by Pattern Top Roughness (PTR) methodology.