Still acknowledged as a peculiarly challenging module, the Dual Damascene (DD) in via-first approach is constantly the object of an improvement effort, through many ways: I) testing new materials, II) tuning the coating/baking recipes and III) varying the scanner illumination conditions. The extensive characterization involved, in this case, a KrF 4-layer stack, in which the under-layer gained the role of both vias-filler and flattening material, thanks to the evaluation of many parameters like the introduction of a multi-bake route and the variation of the film thickness. The stack, customized to guarantee the etch feasibility, implied: the usage of a hard mask to allow the digging selectivity, a Bottom-Anti-Reflective-Coating (BARC) to avoid any back-reflection coming from the Silicon oxide substrate and the PhotoResist (PhR) on top. Evaluating different illumination modes, this study demonstrated how the tuning of the Numerical Aperture (NA) and of the σ values can be a feasible solution to decrease the swing effect entity; being it responsible, in most cases, for large dimensional variations when encountering small film thickness intra-die biases. Process Window (PW) and Depth of Focus (DoF) were analyzed thanks to the usage of properly designed test vehicles, to check the definition of the expected structural dimensions. This gave the opportunity to improve the Critical Dimension Uniformity (CDU), always remaining a key topic, especially when dealing with devices addressed to the automotive market.
In the field of semiconductor manufacturing, there is still a continuous search for techniques to improve the Critical Dimension Uniformity (CDU) across the wafer. CDU improvement and general defectiveness reduction increase the industrial yield and guarantee high reliability standards. In the KrF Dual-Damascene module integration, at a lithographic level, deep trench planarization is mandatory to minimize interference patterns of the photoresist known as the swing curve effect. Swing curve models explain why small changes in the film thickness of the photosensitive material can create wide critical dimensions changes. Different approaches have been developed to improve the CDU, like the etch-back approach and partial via filling. Within the plethora of materials studied to improve the performances, the Marangoni-effect based underlayers are now showing their potential. In the past, an extensive testing of a double-layer solution comprising one Marangoni-effect-based material and one standard underlayer has been carried out with success. Despite this, double-layer spinning brings drawbacks at manufacturing level, increasing the cost and limiting the tools’ throughput. A new solution, resulting from the chemical tuning of the material exploiting the Marangoni effect has brought to the synthesis of a new single-layer planarization material, with properties similar to the previously tested double-layer approach. This advanced material opens the pathway to an alternative and cost-effective way to solve the issues typical of this module integration.
One of the main challenges of a Dual Damascene (DD) via-first process is the control of the Critical Dimensions (CDs) in the lithography of the trenches. The PhotoResist (PhR) thickness presents variations from the via arrays to the open areas, which cause the variation of CDs: the swing effect.
The planarization of a DD via-first process is reported. A dual-layer solution is used to demonstrate the complete filling of deeply etched structures and the advanced planarization in a multilayer technique. The first material coating the substrate exhibits a Marangoni effect; the second material owns the same physico-chemical properties of the first one, except for the Marangoni properties.
The Marangoni effect of the first coating produces a thicker film in the via array than in the open area, resulting in a negative Film Thickness (FT) bias. Then, the coating with second material is performed and having it standard planarization properties, a positive thickness bias occurs. The addition of these two coatings results in a thickness bias intra-die and on complex topographies, ranging from 0 nm to 30 nm across the wafer. Additionally, the similar physicochemical nature of these two planarization materials enables this dual layer system to be treated as a single homogeneous layer as far as the etching process is concerned.
In the perspective of a KrF lithographic process, this dual-layer offers a planar substrate to be coated with the PhR and allows a strong reduction of the intra-die swing effect, providing a scalable solution.
In the preparation of Integrated Circuits (ICs), employing the Dual Damascene (DD) via-first approach, the fulfilling of extremely deep via and, in general, of highly etched structures, still remains a challenging task. Especially, if this is combined with the need to obtain a planar surface with a thickness bias proximal to zero between flat zones and highly via-rich parts of the die or deep trenches. Herein, we report a full analysis of the impact of the nature of solvent and polymer composing the Hard-Mask (HM) precursor on the filling of via and long trenches. The analysis is carried out by means of optical microscopy (OPT) and scanning electron microscopy (SEM) on standard silicon wafers processed with the HM, changing different variables; from the use of materials comprising different solvents and polymers with various C-contents, up to the variation of the coating parameters such as: the spin speed, the bake temperature and the primer vaporization step before dropping the via-filler material. Interestingly, the solvent is demonstrated to play a crucial role in the formation of macro-defectiveness on long deep trenches surrounded by a flat area: PropyleneGlycolMonomethylEtherAcetate (PGMEA) uniquely-based materials can bring to a peculiar halation-effect, partially avoided with the introduction of several pre-bake steps of the under-layer on a HexaMethylDiSilazane (HMDS)-presprayed Silicon surface.
Line patterning, in via first dual damascene approach, is conditioned by vias density: bottom anti–reflective coating (BARC), used to minimize thin film interference effects by reducing reflected light, and photoresist reflow into vias, leading to materials thickness variation, and so to unwanted modification of metal lines critical dimension (CD), due to local reflectivity change and to swing effect. Aim of the work is to assess CD variations to be expected at device level when applying via first integration scheme, in order to compensate them, where and when feasible, or to setup restrictions to vias density at design level, forbidding critical configurations that might lead to patterning failures. The paper presents an experimental characterization of metal line CD variation as a function of vias density based on the study of a test pattern, designed to explore a wide variety of vias and metals respective configurations, and investigates different approaches to model and predict CD deviations from expected targets. Vias densities, or their convolution with specific kernels, are extracted using conventional design rule check (DRC) tools, and are used as predictors to model metal lines CD variation behavior. Simple via density computation is not able to capture the effect, so we propose a flow, based on machine learning artificial neural network algorithms, able to predict metal line width variations to be expected on product devices as a function of the vias pattern underneath.
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