Journal of Micro/Nanopatterning, Materials, and Metrology
VOL. 20 · NO. 1 | January 2021
ISSUES IN PROGRESS
SPIE publishes accepted journal articles as soon as they are approved for publication. Journal issues are considered In Progress until all articles for an issue have been published. Articles published ahead of the completed issue are fully citable.
Abbe’s reasoning behind his expression of the resolution limit can be seen from his 1876 letter to J. W. Stephenson. Helmholtz came up with the same resolution limit expression almost at the same time as Abbe in another way. The numerical aperture (NA ≡ n sin θ) part of the expression is a consequence of the sine condition, which must be fulfilled for aplanatic imaging.
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Linewidth roughness (LWR) remains a difficult challenge for improvement in resist materials. We intend to review work that focused on the impact of key components of LWR by analyzing the unbiased power spectral density (PSD) curves. We studied systematic changes to ArF resist formulations and correlated these changes to the overall PSD curves. In this manner, we could extract LWR 3σ values and resist correlation length and the low/high-frequency roughness components. We also investigated the relationship between PSD and LWR through lithographic/etch processing and demonstrated which PSD components correspond to the largest impact on LWR. This work was extended further to investigate how frequency components are influenced by basic resist properties such as diffusion and aerial image properties such as normalized image log-slope (NILS). Particular attention was given to how changes in correlation length affected LWR as feature size decreases. We also looked at the impact of diffusion or resist blur on PSD(0) as a function of NILS. Finally, we will review how LWR improvement can be achieved by several strategies that focus on both PSD(0) and correlation length (ξ) and not a single LWR number. The trends presented highlight the true nature of LWR with respect to its high and low-frequency components. It also shows the benefits of measuring and developing resists as a function of roughness power spectral density and not as a function of a single LWR measurement.
In the past years, EUV lithography scanner systems have entered high-volume manufacturing for state-of-the-art integrated circuits (IC), with critical dimensions down to 10 nm. This technology uses 13.5-nm EUV radiation, which is transmitted through a near-vacuum H2 background gas, imaging the pattern of a reticle onto a wafer. The energetic EUV photons excite the background gas into a low-density H2 plasma. The resulting plasma will locally change the near-vacuum into a conducting medium and can charge floating surfaces and particles, also away from the direct EUV beam. We will discuss the interaction between EUV-induced plasma and electrostatics, by modeling and experiments. We show that the EUV-induced plasma can trigger discharges well below the classical Paschen limit. Furthermore, we demonstrate the charging effect of the EUV plasma on both particles and surfaces. Uncontrolled, this can lead to unacceptably high voltages on the reticle backside and the generation and transport of particles. We demonstrate a special unloading sequence to use the EUV-induced plasma to actively solve the charging and defectivity challenges.
Background: Physical modeling of grayscale lithography processes for the prediction of photoresist heights leads to complex mathematical algorithms. A promiment example is the numerical simulation of the photoresist shape after development through Dill’s equations. These grayscale lithography models exhibit accurate prediction quality but can not directly implemented into mask layout tools to simplify the layout procedure. Limited process windows, changes in the mask design, variations of the used materials or manufacturing tools lead to time-consuming and cost-intensive test procedures to adjust the photoresist model for sufficient results.
Aim: The focus of this work is to enhance current grayscale lithography models for a straightforward method with the same precise prediction of remaining photoresist heights to simplify the mask layout process. Moreover, we aim for an uncomplicated optimization of the model to minimize the empirical analysis necessary for its use.
Approach: Based on experimental results, we deploy a sectionally defined mathematical expression that includes the theory of Fraunhofer diffraction and illumination-dependent activation of the photo-sensitive component and its solubility in developer.
Results: We produced pyramidal, spherical and chess field structures with exposure doses of 3000 and 15 , 000 J / m2 on bare silicon substrates with 100-nm resolution and on silicon substrates with anti-reflective coatings, with accuracy as fine as 20 nm.
Conclusion: The proposed three-state lithography model has been verified by experimental evaluation. It is able to operate in a wide process window and can be directly implemented in existing mask layout software. This model ensures a cost-efficient and precisely controlled production of three-dimensional topographies using grayscale lithography processes.
Background: Negative-tone development (NTD) photoresists are prone to shrinkage effects during lithographic processing. Along with deformation seen during the postexposure bake (PEB), there are additional effects during the development that cannot be fully explained by a conventional PEB shrinkage model alone.
Aim: Understand the impact of PEB shrinkage on the development rate. Develop a model that can help predict resist profiles after chemical development.
Approach: A PEB shrinkage model for NTD resists is introduced, which uses the thermal properties of the resist material to help simulate shrinkage. The deformed state of the resist is used as an input to the development rate equation to predict the final feature dimensions observed in experiments.
Results: The strain concentration within the resist bulk can have an influence on the stability of the resist during the development. The strain influences the development rate depending on the resist feature shape and contours.
Conclusions: The results from this study can help improve optical proximity correction (OPC) modeling performance and help better understand the deformation characteristics of NTD resist materials. The model also shows that the development shrinkage has an influence on the edge placement error.