Dieter Van den Heuvel

Defect source analysis of directed self-assembly process (DSA of DSA)

As design rule shrinks, it is essential that the capability to detect smaller and smaller defects should improve. There is considerable effort going on in the industry to enhance Immersion Lithography using DSA for 14 nm design node and below. While the process feasibility is demonstrated with DSA, material issues as well as process control requirements are not fully characterized. The chemical epitaxy process is currently the most-preferred process option for frequency multiplication and it involves new materials at extremely small thickness. The image contrast of the lamellar Line/Space pattern at such small layer thickness is a new challenge for optical inspection tools. In this investigation, the focus is on the capability for optical inspection systems to capture DSA unique defects such as dislocations and disclination clusters over the system and wafer noise. The study is also extended to investigate wafer level data at multiple process steps and determining contribution from each process step and materials using ‘Defect Source Analysis’ methodology. The added defect pareto and spatial distributions of added defects at each process step are discussed.

Defect reduction and defect stability in IMEC's 14nm half-pitch chemo-epitaxy DSA flow

Directed Self-Assembly (DSA) of Block Co-Polymers (BCP) has become an intense field of study as a potential patterning solution for future generation devices. The most critical challenges that need to be understood and controlled include pattern placement accuracy, achieving low defectivity in DSA patterns and how to make chip designs DSA-friendly. The DSA program at imec includes efforts on these three major topics. Specifically, in this paper the progress in DSA defectivity within the imec program will be discussed. In previous work, defectivity levels of ~560 defects/cm2 were reported and the root causes for these defects were identified, which included particle sources, material interactions and pre-pattern imperfections. The specific efforts that have been undertaken to reduce defectivity in the line/space chemoepitaxy DSA flow that is used for the imec defectivity studies are discussed. Specifically, control of neutral layer material and improved filtration during the block co-polymer manufacturing have enabled a significant reduction in the defect performance. In parallel, efforts have been ongoing to enhance the defect inspection capabilities and allow a high capture rate of the small defects. It is demonstrated that transfer of the polystyrene patterns into the underlying substrate is critical for detecting the DSA-relevant defect modes including microbridges and small dislocations. Such pattern transfer enhances the inspection sensitivity by ~10x. Further improvement through process optimization allows for substantial defectivity reduction.

Natural EUV mask blank defects: evidence, timely detection, analysis and outlook

A combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) is used to find as many as possible printing defects on two different EUV reticles. These multiple inspections result in a total population of known printing defects on each reticle. The printability of these defects is first confirmed by wafer review on wafers exposed on the full field ASML Alpha Demo Tool (ADT) at IMEC. Subsequently reticle review is performed on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology allows to separate absorber related mask defects and multi layer (ML) related mask defects. In this investigation the focus is on ML defects, because this type of reticle defects is EUV specific, and not as evolutionary as absorber defects which can be mitigated in more conventional ways. This work gives evidence of critical printing ML defects of natural origin, both pits as shallow as 3nm and bumps just 3nm high at the surface. Wafer inspection was the first inspection technique to detect these ML-defects with marginal surface height distortion, because both state-of-the-art PMI and especially standard BI on the Lasertec M1350 had failed to detect these defects. Compared to standard BI, the more advanced Lasertec M7360 is found to have much better sensitivity for printing MLdefects and our work so far shows no evidence of printing ML defects missed by this tool. Unfortunately it was also observed that this required sensitivity was only achieved at the cost of an unacceptable nuisance rate, i.e., with a too high number of detections of non-printing defects. Optical blank inspection is facing major challenges : It needs not only to find ML defects with height distortions of 3nm and less (and in theory maybe even 0nm), but also it must be able to disposition between such likely-printing and non-printing defects.

Evidence of printing blank-related defects on EUV masks missed by blank inspection

In this follow-up paper for our contribution at BACUS 2010, first evidence is shown that also the more advanced Lasertec M7360 has missed a few printing reticle defects caused by an imperfection of its EUV mirror, a so-called multilayer defect (ML-defect). This work continued to use a combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) to find as many as possible printing defects on EUV reticles. The application of more advanced wafer inspection, combined with a separate repeater analysis for each of the multiple focus conditions used for exposure on the ASML Alpha Demo Tool (ADT) at IMEC, has allowed to increase the detectability of printing MLdefects. The latter uses the previous finding that ML-defects may have a through-focus printing behavior, i.e., they cause a different grade of CD impact on the pattern in their neighborhood, depending on the focus condition. Subsequent reticle review is used on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology has allowed achieving clear evidence of printing ML defects missed by this BI tool, despite of an unacceptable nuisance rate reported before. This is a next step in the investigation if it is possible to avoid actinic blank inspection (ABI) at all, the only presently known technique that is expected to be independent from the presence of a (residual) topography of the ML-defect at the top of the EUV mirror, in detecting those defects. This is considered an important asset of blank inspection, because the printability of a ML-defect on the EUV scanner and its detectability by ABI is determined by the distortion throughout the multilayer, not that at the surface.

Immersion specific defect mechanisms : Findings and recommendations for their control

Defectivity has been one of the largest unknowns in immersion lithography. It is critical to understand if there are any immersion specific defect modes, and if so, what their underlying mechanisms are. Through this understanding, any identified defect modes can be reduced or eliminated to help advance immersion lithography to high yield manufacturing. Since February 2005, an ASML XT:1250Di immersion scanner has been operational at IMEC. A joint program was established to understand immersion defectivity by bringing together expertise from IMEC, ASML, resist vendors, IC manufactures, TEL, and KLA-Tencor. This paper will cover the results from these efforts. The new immersion specific defect modes that will be discussed are air bubbles in the immersion fluid, water marks, wafer edge film peeling, and particle transport. As part of the effort to understand the parameters that drive these defects, IMEC has also developed novel techniques for characterizing resist leaching and water uptake. The findings of our investigations into each immersion specific defect mechanism and their influencing factors will be given in this paper, and an attempt will be made to provide recommendations for a process space to operate in to limit these defects.

Investigation of EUV mask defectivity via full-field printing and inspection on wafer

Full-field printing on the ASML Alpha Demo Tool, followed by wafer inspection on a KLA-T 2800, is used to qualify typical defectivity levels of EUV reticles. Mask defects are found as repeaters among multiple dies on wafer. The uniform pattern consists of dense lines and spaces. In a first reticle with 40nm linewidth, high levels of natural defects have been found of which a relatively large share was considered as multilayer (ML) type defects, because they printed as rings. Simulation of ML defects could explain this printing behavior as a function of height, size and slope. The main parameter determining the printing behavior of a ML defect is its height. A local distortion of the upper part of the ML, as thin as ~2nm can already print. On-reticle analysis of the ring defects by SEM showed that the defects are present on the absorber, which already explains the printing result. Yet, still several other defects were found to print on the wafer, whereas they were not visible on the reticle and considered local distortions of the ML. Printing results with a second version of the mask that additionally includes programmed multilayer defects with 3nm height confirmed the pronounced printing impact of ML defects as they were simulated. Encouragingly low numbers of natural defects have been found on a third reticle. With this reticle also a first correlation has become possible between the defect maps obtained from wafer inspection, (direct) mask inspection, and blank inspection. This is a viable method to highlight potential gaps between the capability of these tools and printability of defects.

Additional evidence of EUV blank defects first seen by wafer printing

First experimental evidence is given that a second generation blank inspection tool has missed a number of printing reticle defects caused by an imperfection of its EUV mirror, i.e., so-called multi-layer defects (ML-defects). This work continued to use a combination of blank inspection (BI), patterned mask inspection (PMI) and wafer inspection (WI) to find as many as possible printing defects on EUV reticles. The application of more advanced wafer inspection, combined with a separate repeater analysis for each of the multiple focus conditions used for exposure on the ASML Alpha Demo Tool (ADT) at IMEC, has allowed to increase the detection capability for printing ML-defects. It exploits the previous finding that ML-defects may have a through-focus printing behavior. They cause a different grade of CD impact on the pattern in their neighborhood, depending on the focus condition. Subsequent reticle review is done on the corresponding locations with both SEM (Secondary Electron Microscope) and AFM (Atomic Force Microscope). This review methodology has allowed achieving clear evidence of printing ML defects missed by this BI tool, despite of a too high nuisance rate, reported before. This establishes a next step in the investigation how essential actinic blank inspection (ABI) is. Presently it is the only known technique whose detection capability is considered independent from the presence of a (residual) distortion of the multi-layer at the top surface. This is considered an important asset for blank inspection, because the printability of a ML-defect in EUV lithography is determined by the distortion throughout the multilayer, not that at the top surface.

Defect mitigation and root cause studies in IMEC's 14nm half-pitch chemo-epitaxy DSA flow

High defect density in thermodynamics driven DSA flows has been a major cause of concern for a while and several questions have been raised about the relevance of DSA in high volume manufacturing. The major questions raised in this regard are: 1. What is the intrinsic level of DSA-induced defects, 2. Can we isolate the DSA-induced defects from the other processes-induced defects, 3. How much do the DSA materials contribute to the final defectivity and can this be controlled, 4. How can we understand the root causes of the DSA-induced defects, their kinetics of annihilation and finally, 5. Can we have block co-polymer anneal durations that are compatible with standard CMOS fabrication techniques (in the range of minutes) with low defect levels. This manuscript addresses these important questions and identifies the issues and the level of control needed to achieve a stable DSA defect performance.

The use of unpatterned wafer inspection for immersion lithography defectivity studies

The switch from dry to immersion lithography has important consequences regarding wafer defectivity. It has been shown that for successful and efficient defect reductions related to immersion lithography the capability to distinguish immersion/patterning related defects from stack related defects is very useful during process control. These stack related defects can be observed after careful partitioning of individual layer inspections and the analysis of this data through DSA in Klarity. The optimisation of the dark field inspection SP2 tool, central in this paper, shows that improved sensitivity at adequate signal to noise ratio can be obtained on the resist stacks by using the smaller wavelength as the UV-laser light present in the SP2. For bare Si and BARC oblique incidence illumination gives the best sensitivity and captures the most defects. However monitoring of the resist and stacks with resist requires normal incidence illumination since the nature of defects and film result in a higher scattering intensity using normal illumination. The use of an optical filter and a 10% laser power also contributed to establishing a lower and stable background signal for each inspection scan. As immersion tool development is improved and immersion specific defectivity is reduced, the proportion of the stack related defects will become a significant fraction of the overall target for further defect reduction. This includes point defects (embedded particles) or flow defects (streaks) identified and classified using SURFimage. Finally this information is to be used to identify the defect origin(s) for ultimate elimination of defects in the stacks.

Ebeam based mask repair as door opener for defect free EUV masks

The EUV-photomask is used as mirror and no longer as transmissive device. In order to yield defect-free reticles, repair capability is required for defects in the absorber and for defects in the mirror. Defects can propagate between the EUV mask layers, which makes the detection and the repair complex or impossible if conventional methods are used. In this paper we give an overview of the different defect types. We discuss the EUV repair requirements including SEM-invisible multilayer defects and blank defects, and demonstrate e-beam repair performance. The repairs are qualified by SEM, AFM and wafer prints. Furthermore a new repair strategy involving in-situ AFM is introduced. This new strategy is applied on natural defects and the repair quality is verified using state of the art EUV wafer printing technology.