Overlay metrology tool calibration

The objective of this work is to determine a guideline for defining practical overlay metrology requirements for a given design rule. Total measurement uncertainty (TMU) of overlay metrology is defined as the square root of square sum of the following items: the mean of the tool-induced shift (TIS), TIS 3-sigma, dynamic precision, and tool-to-tool matching. The TMU is dependent upon process conditions, so TMU is different by process layer. In this study, the impact of TMU on overlay error correction, which includes process and measurement noise, is investigated in terms of the stability of high-order overlay correction parameters. By evaluating the variation range of correctable parameters as a figure of merit, the corresponding TMU is determined for a given design rule. Utilizing this methodology, we determined that 2 nm of TMU value is the allowable limit for 45 nm of dynamic random access memory (DRAM) half pitch based upon simulation results. Similarly, we recommend 1 nm of TMU for 36 nm of DRAM half pitch. Our methodology is described and our simulation results are discussed.

Overlay control for recording head manufacturing is becoming increasingly challenging as design geometry specifications tighten in the transition to advanced Heated Assisted Magnetic Recording (HAMR) devices. As overlay control requirements for critical patterning layers tighten below 5nm, the need for accurate and robust overlay metrology is key to enabling the patterning roadmap and improving yields. This work focuses on improving Imaged Based Overlay (IBO) metrology to ensure accurate overlay measurements for critical lithography steps in the recording head fabrication process. Selection of the optimal settings for color filter and Numeric Aperture (NA) parameters can have a significant impact on overlay measurement accuracy and Total Measurement Uncertainty (TMU) when setting up a new overlay metrology recipe1-2. These overlay metrology recipe parameters can be selected to minimize the influence of process induced overlay target imperfections on the measured overlay 3. This paper reports our revised workflow for selecting IBO measurement conditions and evaluating pre-existing recipes for robustness. We utilize metrics such as the overlay model residuals, Tool Induced Shift (TIS), and Qmerit error in identifying recipes requiring improvement. The new “Train Log” feature available on KLA’s Archer™ IBO metrology tool platform can be used to compare the through focus contrast precision of different illumination conditions at the time of recipe creation. In this study, we show that an optimization workflow that utilizes the “Train Log” collection of the contrast precision metric and imageless recipes in selecting color filter and numeric aperture settings can significantly improve the overlay measurement repeatability and tool to tool matching.

The integrated circuit (IC) manufacturing factories have measured overlay with conventional "box-in-box" (BiB) or "frame-in-frame" (FiF) structures for many years. Since UMC played as a roll of world class IC foundry service provider, tighter and tighter alignment accuracy specs need to be achieved from generation to generation to meet any kind of customers' requirement, especially according to International Technology Roadmap for Semiconductors (ITRS) 2003 METROLOGY section1. The process noises resulting from dishing, overlay mark damaging by chemical mechanism polishing (CMP), and the variation of film thickness during deposition are factors which can be very problematic in mark alignment. For example, the conventional "box-in-box" overlay marks could be damaged easily by CMP, because the less local pattern density and wide feature width of the box induce either dishing or asymmetric damages for the measurement targets, which will make the overlay measurement varied and difficult. After Advanced Imaging Metrology (AIM) overlay targets was introduced by KLA-Tencor, studies in the past shown AIM was more robust in overlay metrology than conventional FiF or BiB targets. In this study, the applications of AIM overlay marks under different process conditions will be discussed and compared with the conventional overlay targets. To evaluate the overlay mark performance against process variation on 65nm technology node in 300-mm wafer, three critical layers were chosen in this study. These three layers were Poly, Contact, and Cu-Metal. The overlay targets used for performance comparison were BiB and Non-Segmented AIM (NS AIM) marks. We compared the overlay mark performance on two main areas. The first one was total measurement uncertainty (TMU)3 related items that include Tool Induced Shift (TIS) variability, precision, and matching. The other area is the target robustness against process variations. Based on the present study AIM mark demonstrated an equal or better performance in the TMU related items under our process conditions. However, when non-optimized tungsten CMP was introduced in the tungsten contact process, due to the dense grating line structure design, we found that AIM mark was much more robust than BiB overlay target.

This paper is a continuation of our earlier work which investigated means of reducing Tool Induced Shift (TIS) in overlay tools. In this paper we show that there can be significant errors in the measurements from a tool that shows zero TIS. These errors arise from proximity effects within the measurement target. Improving the resolution of the instrument reduces these errors, although this is counter to the current trend of the industry. We present the results of detailed comparisons of two measurement objectives. These have the same magnification but different numerical aperture so that they differ in resolution and depth of field. We will show that the lowest measurement error are achieved by acquiring all images from a common focal plane, even when the target marks are displaced vertically by more than 2 micrometers . Both objectives perform equally well, although we had expected that increasing the depth of field would provide benefits. The data shows that increasing the resolution of the microscope will allow the high accuracy and precision required for the future to be achieved without sacrificing an ability to measure the deepest structures likely to be met in these processes.

The appropriate definition and identification of total measurement uncertainty from a group of metrology tools is becoming ever important as process tolerances continue to shrink in today's data storage and semiconductor manufacturing environments. The precision-to-tolerance ratio needs to be properly defined and minimized in order to maintain capable process control. The task of identifying components contributing the total measurement uncertainty therefore poses a major challenge for both the metrology tool manufacture's and the system owners on the customer side. In this paper, two models are proposed to perform the estimation of total measurement uncertainty component and the corresponding precision-to-tolerance ratio estimation with the methodology of the analysis of variance. Two models are developed to suit the measurement characteristics difference. The first is a crossed model designed for the nondestructive measurements and the second is a nested model developed for the measurement environments where sample destruction is unavoidable. The models analyze precision components from an individual tool as well as the entire tool group so that the error from matching is accounted for. Optical overlay and CDSEM tools were both selected for study and the measurement data were used for precision analysis. The error contribution from the bias identification was performed using a CDAFM as a reference and a CDSEM as tool under test. The methodologies developed in this paper serve as a guide for the metrology tool manufacturers and tool users to systematically estimate the total measurement uncertainty and the related improvement for precise process control.

This Performance Testing and Validation Evaluation (PTVE) reviews and analyzes the data collected with the Neutron Slab Counter (NSC) according to the Performance Test and Validation Plan (PTVP, UCOR-4443) and reported in the NSC Performance Test and Validation Report (PTVR, NDA-13-0622). The NSC was designed for quantitative Nondestructive Assay (NDA) measurements of compressor volutes from the K25/K27 Buildings. The NSC PTVP (UCOR-4443) prescribed a methodology and set of field measurements to determine the NSC system efficiency, calibrate the system, perform verification measurements, define the components of the system's total measurement uncertainty, and perform a calibration confirmation measurement. All of the required measurements of the NSC PTVP (UCOR-4443) were completed according to the Calibration of NDA Equipment Procedure (PROC-DE-0714) and the Quantitative Measurements Using the Neutron Slab Counter Procedure (PROC-DE-0723). Following the completion of this PTVE, if the data quality objectives (DQO) of the NSC PTVP (UCOR-4443) are met, a commissioning report will be prepared which specifies the limits of use of the NSC.

Overlay performance will be increasingly important for Spacer Patterning Technology (SPT) and Double Patterning Technology (DPT) as various Resolution Enhancement Techniques are employed to extend the resolution limits of lithography. Continuous shrinkage of devices makes overlay accuracy one of the most critical issues while overlay performance is completely dependent on exposure tool. Image Based Overlay (IBO) has been used as the mainstream metrology for overlay by the main memory IC companies, but IBO is not suitable for some critical layers due to the poor Tool Induced Shift (TIS) values. Hence new overlay metrology is required to improve the overlay measurement accuracy. Diffraction Based Overlay (DBO) is regarded to be an alternative metrology to IBO for more accurate measurements and reduction of reading errors. Good overlay performances of DBO have been reported in many articles. However applying DBO for SPT and DPT layers poses extra challenges for target design. New vernier designs are considered for different DPT and SPT schemes to meet overlay target in DBO system. In this paper, we optimize the design of the DBO target and the performance of DBO to meet the overlay specification of sub-3x nm devices which are using SPT and DPT processes. We show that the appropriate vernier design yields excellent overlay performance in residual and TIS. The paper also demonstrated the effects of vernier structure on overlay accuracy from SEM analysis.

Overlay performance has been a critical factor for advanced semiconductor manufacturing for years. Over time these requirements become more stringent as design rules shrink. Overlay mark design and selection are the first two steps of overlay control, and it is known that different overlay mark designs will have different responses to process conditions. An overlay mark optimized for traditional process might not be suitable for SADP (self-aligned double patterning) technology due to changes in lithography and etching process conditions. For instance, the traditional BIB (box-in-box) target defined by the core mask becomes a template structure in SADP flow, the pitch and cycle of the overlay mark is further changed after spacer formation and core film removal hence the mark recognition and robustness have been challenging for the subsequent process layers. The comprehensive study on the methodology of overlay mark design and selection is still not available for SADP process. In this paper, various types of overlay marks were designed to comply with the SADP process to get rid of the weaknesses of traditional targets. TMU (total measurement uncertainty) performance was adopted to determine the optimal overlay marks for meeting production overlay control requirements in SADP process flow. The results have suggested the segmented marks outperform to non-segmented marks on image contrast as well as TMU.

The paper considers the features of the evaluation of the component of standard measurement uncertainty of type B as a result of a correction due to the finite resolution of the readings of measuring equipment during calibration. The problem of carrying out the procedure for establishing the compliance of the measuring equipment with the technical specification based on the results of its calibration is stated. It is emphasized that the procedure shall necessarily account for the measurement uncertainty. In this regard, the metrological characteristics of the measuring equipment after calibration may not meet the requirements of the technical specification. The calibration and measurement capabilities of accredited laboratories when calibrating a smooth micrometer MK25 for compliance with the technical specification according to the results of its calibrations are analysed. It is shown that the component of standard measurement uncertainty of type B as a result of a correction due to the resolution of the readings of analogue measuring instruments cannot be dominant in the uncertainty budget. It is proposed to consider the establishment of a target uncertainty during calibration for the analysis of the calibration and measurement capabilities of measuring equipment in the laboratory. At the same time, the reliability of the evaluation of measurement uncertainty during the calibration of measuring equipment depends on the correct choice of appropriate procedures. Therefore, at the legislative level, namely in the Law of Ukraine “On Metrology and Metrological Activities”, it is established that the measurement equipment results are calibrated and registered in accordance with national standards harmonized with relevant international and European standards, and documents approved by international and regional metrology organizations.

An apparatus incorporating a magnetic suspension technique has been developed for density measurements on liquids and liquid mixtures, particularly at saturation, at temperatures between 90 and 300 K and at pressures to 5 MPa (approximately 50 atm). The feasibility of adapting this method, previously used at room temperature, for low temperature use had been demonstrated in an earlier study with a density measurement on saturated liquid nitrogen near its normal boiling point. The present apparatus, which is significantly improved, and in most respects different from the earlier model, is described in detail. It includes a cryostat for continuous wide-range temperature control, a windowed equilibrium cell particularly suited for studies of liquid mixtures, and a new electronic servocircuit with a linear differential transformer for position control of the magnetic buoy. Extensive tests and density measurements have been carried out to evaluate the performance of this apparatus. Densities of saturated liquid nitrogen between 95 and 120 K and saturated liquid methane between 105 and 160 K are reported. The estimated standard deviation of a single density measurement is less than 0.02%. The total systematic error in the measurement process from known sources is approximately 0.05%. The total uncertainty of a single density measurement, which is taken as three times the standard deviation plus the systematic error, is approximately 0.1%. Comprehensive comparisons of the present results with previous experimental data are presented.

In practice there are three aspects that need to be considered in order to achieve the required traceability according to its definition: the 'stated reference', the 'unbroken chain of calibrations' and the "stated uncertainty". For a certain chemical result, each of these aspects highly depends on the measurement uncertainty, both on its magnitude and how it was estimated. Therefore, the paper describes the experience of the Romanian National Institute of Metrology in estimating measurement uncertainty during the certification of reference materials (RMs), in metrological activities (calibration, pattern approval, periodical verification, etc.), as well as during the analytical measurement process. Practical examples of estimation of measurement uncertainty using RMs or certified reference materials are discussed for their applicability in spectrophotometric and turbidimetric analysis. Use of the analysis of variance to obtain some additional information on the components of measurement uncertainty and to identify the magnitude of individual random effects is described.

In a previous editorial we concluded that neither measurement repeatability nor measurement reproducibility are synonyms for measurement uncertainty, because they are only parts of the measurement uncertainty of a measurement result. We also concluded that an important part of the evaluation of measurement uncertainty does not require statistical analysis of the measured quantity values alone, but professional skill and judgement, both according to VIM [1] and GUM [2]. It should be remembered that Type A and Type B evaluations each lead to at least one component of measurement uncertainty where Type B evaluation usually leads to several Type B components. Two observations seem obvious:

Metrology target quality highly impacts overlay measurement accuracy and robustness. Factors that can affect target quality include target pattern symmetry, pattern uniformity over one target, and target quality variation within wafer, wafer to wafer and lot to lot. Historically, multiple studies have revealed how metrology pattern asymmetry causes overlay measurement inaccuracy issues. Target design can address pattern asymmetry induced overlay error, to some extent, by providing a process compatible design. Metrology targets synchronously consider process compatibility, metrology system capability and target design basic rule. In some cases, the minimum design rule is constrained by process condition. Staircase loop layers of the 3D-NAND integration process are the typical case where resist thickness limits the accessible minimum CD/pitch. The required larger CD/pitch sometimes even violates the basic design rule of the metrology target. In this study, a novel target design is applied to address thick photo-resist processes. Compared to the baseline, the new target design helps address overlay error induced by pattern asymmetry, thus improving thick resist layers overlay measurement accuracy.

For improved luminous flux measurements, the Physikalisch-Technische Bundesanstalt (PTB) was equipped with a new robotic goniophotometer. In order to characterize this instrument, to reduce the combined measurement uncertainty, and to determine the respective uncertainty components, the influence of stray light on the measurement results was investigated. Considering the specific structure of the robotic goniophotometer, stray light was measured on-line by means of a purpose-built “back-looking” photometer. By this means, the influence of the luminous intensity distribution of each individual light source with regard to its stray light could be considered. Before the first experimental application, the invented procedure was tested by means of calculations using mathematical models. In this paper, the mathematical model for the calculation of stray light in the goniophotometer room is presented together with the calculation results and the results of the first measurements. The achieved results indicate that the recommended stray light measurements can decrease the total relative standard measurement uncertainty of luminous flux measurements down to 2 × 10−3.

As the most critical component of a deep manned submersible, the manned pressure hull provides a safe living space for pilots and scientists under the deep sea pressure, and its weight occupies almost 1/3 total weight of the manned submersible. So the pressure hull should be designed to have enough strength and should be as light as possible. However, a comparison of the current available design rules from various classification societies indicated significantly different results among these design rules and many existing spherical pressure hulls are found to be not in compliance with most of the current design rules. This indicates that current design rules need to be updated and unified, similarly to the Common Structural Rules for tankers and bulk carriers. To update the design rules, a systematic study is carried out. Then, a new design standard based on this study is proposed to the China Classification Society (CCS) for consideration of updating their rules. The main purpose of this paper is to briefly summarize the study on this new design standard and to present the structural optimization of the manned spherical pressure hull of a new 4500-m rating manned submersible based on this new standard.